Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless...
Transcript of Electroless deposition of NiP alloys...1. CHAPTER 1 1.1 From electroplating to electroless...
POLITECNICO DI MILANO
School of Industrial and Information engineering
Master of Science in
Materials Engineering and Nanotechnology
Electroless deposition of NiP alloys
Author: Supervisor:
Filippo Mariani Prof. Ing. Luca Magagnin
ID: 837140 Advisor:
Simona IEFFA
Academic year 2016/2017
Contents
Abstract……………………………………………………………………………………………...4
Abstract (Italiano)…………………………………………………………………………………..5
1. Chapter 1………………………………………………………………………………………….6
1.1 From electroplating to electroless deposition…………………………………………6
1.2 Differences between the two techniques……………………………………………….7
1.3 Typology of Nickel baths………………………………………………………………..8
1.4 Electroless Nickel baths……………………………………………………………...….8
1.4.1 Acidic baths…………………………………………………………………....9
1.4.2 Alkaline baths………………………………………………………………...10
1.5 Components of the bath……………………………………………………………….11
1.5.1 Reducing agents……………………………………………………………...13
1.5.1.1 Sodium hypophosphite…………………………………………….13
1.5.1.2 Aminoborane baths………………………………………………...14
1.5.1.3 Sodium borohydride baths………………………………………...14
1.5.2 Complexing agents…………………………………………………………...14
1.6 Deposit characteristics…………………………………………………………………15
1.7 Applications…………………………………………………………………………….18
1.8 Electroless codeposition of particles in NiP matrix…………………………………..19
1.8.1 Carbides………………………………………………………………………20
1.8.1.1 Silicon carbide……………………………………………………...20
1.8.1.2 Boron carbide………………………………………………………22
1.8.1.3 Tungsten carbide…………………………………………………...23
1.8.2 Ceramics……………………………………………………………………...24
1.8.2.1 Titanium oxide……………………………………………………...24
1.8.2.2 Zirconia……………………………………………………………..28
1.8.2.3 Aluminum oxide……………………………………………………28
1.8.2.4 Silica…………………………………………………………………30
1.8.2.5 Ceria………………………………………………………………...32
1.8.3 Lubricants…………………………………………………………………….33
1.8.3.1 PTFE………………………………………………………………...33
1.8.3.2 Molibdenum sulfide………………………………………………...35
1.8.3.3 Tungsten sulfide…………………………………………………….36
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1.8.4 Allotropics form of carbon…………………………………………………..37
1.8.4.1 Graphite…………………………………………………………….37
1.8.4.2 Carbon nanotubes (CNTs)…………………………………………37
1.8.4.3 Diamonds…………………………………………………..………..39
1.8.5 Rare Earth elements………………………………………………..………..41
2. Chapter 2………………………………………………………………………………………...42
2.1 Characterization techniques…………………………………………………………..42
2.1.1 Mechanical Polishing for Optical Microscopy (OM)………………………42
2.1.2 Optical Microscope……………………………………………..……………42
2.1.3 Scanning electron microscope (SEM) and EDX………………...………….43
2.1.4 XRF………………………………………………………………..………….47
2.1.5 XRD…………………………………………………………………….……..48
2.1.6 Microdurometer………………………………………………...……………49
3. Chapter 3………………………………………………………………………………………...50
3.1 Procedures……………………………………………………………………………...50
3.1.1 Brass pretreatment…………………………………………………………..50
3.1.2 Preparation of CTAB and DTAB solutions………………………………...50
3.1.3 Preparation of the bath for CTAB and DTAB tests………………………..50
3.1.4 Addition of particles………………………………………………………….51
3.2 Results…………………………………………………………………………..………51
3.2.1 SEM-EDX, XRD and Optical Microscope analysis………………..………52
3.2.1.1 NiP first formulation (NiP0)……………………………..…….…..52
3.2.1.2 NiP0(CTAB)……………………………………………….………..53
3.2.1.3 NiP0(DTAB)………………………………………………….……..55
3.2.1.4 NiP0/W………………………………………………………………56
3.2.1.5 NiP0/W(CTAB)……………………………………….…………….58
3.2.1.6 NiP0-SiC………………………………………………………...…..59
3.2.1.7 NiP0-SiC/B4C………………………………………………….……61
3.2.1.8 NiP second formulation (NiPA)………………………………..…..63
3.2.1.9 NiPA/W (4g/L)………………………………………………...…….65
3.2.1.10 NiPA/W (6 g/L)…………………………………………………….67
3.2.1.11 NiPA/W(18 g/L)…………………………………………..………..69
3.2.1.12 NiP third formulation (NiPB)…………………………….………71
3.2.1.13 NiPB(CTAB) ………………………………………………………73
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3.2.1.14 NiPB(DTAB)…………………………………………………….....75
3.2.1.15 NiPB/W……………………………………………………...……..77
3.2.1.16 NiPB/W(CTAB)………………………………………..…………..79
3.2.1.17 NiPB/W(DTAB)……………………………………………………81
3.2.1.18 NiPB/W(CTAB)-SiC………………………………………………83
3.2.1.19 NiPB/W(DTAB)-SiC………………………………………………85
3.2.1.20 NiPB/W(CTAB)-TiO2......................................................................87
3.2.1.21 NiPB/W(DTAB)-TiO2…………………………………………..…89
3.2.1.22 NiPB/W(CTAB)-SiC/B4C………………………………………....91
3.2.1.23 NiPB/W(DTAB)-SiC/B4C…………………………………………93
3.2.2 Microhardness………………………………………………………………..95
4. Chapter 4……………………………………………………………………………………...…97
4.1 Conclusions and future developments………………………………………………..97
Index of figures………………………………………………………………………………...…..98
Index of tables…………………………………………………………………………………….103
References…………………………………………………………………………...…………….104
Acknowledgments……………………………………………………………………………..….110
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Abstract
This thesis presents the electroless deposition of Nickel-Phosphorus (NiP) with tecnoplate3000®
solutions on brass.
The aim is improving and enhancing the mechanical properties of some mechanical parts.
After the pretreatment of the substrate, we tried three formulations to obtain the best performance:
NiP0, NiPA, NiPB; they vary mainly for the different concentration of additives. The selected one is
NiPB a stable, monophase solution.
At the end we added in this solution particles of carbides and ceramics to produce a hard and with
high wear resistance coating.
A characterization of the samples obtained was performed by optical microscope analysis, scanning
electron microscope analysis (SEM), X-ray diffraction technique (XRD) and microhardness test.
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Abstract (Italiano)
In questa tesi viene presentata la deposizione electroless di Nichel-Fosforo (NiP) su ottone avvenuta
grazie alla soluzione tecnoplate3000®.
L’obiettivo è quello di aumentare le proprietà meccaniche e quindi le prestazioni di alcuni
componenti utilizzati in meccanica.
Dopo il pretrattamento del substrato, abbiamo provato tre diverse formulazioni per ottenere le
migliori perfromance: NiP0, NiPA, NiPB; esse variano soprattuto per la concentrazione degli additivi.
La soluzione migliore selezionata è NiPB che risulta stabile e monofasica. Alla fine in questa
soluzione abbiamo aggiunto particelle di carburi e ceramici per produrre un rivestimento che fosse
duro e resistente all’usura.
Sono state infine effettuate prove di caratterizzazione sui provini ottenuti attraverso analisi al
microscopio elettronico (SEM), diffrazione ai raggi X (XRD) e test di microdurezza.
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1. CHAPTER 1
1.1 From electroplating to electroless deposition
Surface engineering is a very large and important sub-discipline of material science and deals with
the surface of solid matter. In particular, it tries to alter and improve the properties of the surface
phase in order to reduce the degradation over time. This aspect is reached thanks to some useful
techniques such plating and emerging nanotechnologies.
Nowadays one of the most used process to ensure a long life to a material surface is
electrodeposition.
This technique needs the use of an external current (direct, alternate or pulsed), applied on an
electrolytic cell, to reduce some metal cations to form a coherent thin coating on an electrode.
The electrolytic cell is formed by a soluble or insoluble anodes, a cathode which acts like a
substrate all immersed in an electrolytic solution where metal salts, that we want to deposit, are
dissolved.
The technique was developed in the XIX century, although undergoing subtle changes through the
time, to improve environment sustainability replacing toxic chemical compounds.
The most important researches were about the efficiency and the way to ensure the metallization of
other substrates, for example polymers. In 1844 Wurtz firstly reported an autocatalytic substrate
surface[1]
. This discovery led to the most important innovation in electrochemistry field: electroless
deposition[2]
. This new technique didn’t need an external current to deposit metal ions on a
substrate[3]
. In an electroless deposition system there is no generator or anode and so the depositions
are made only under the controlled chemical reduction reactions.
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1.2 Differences between the two techniques
There are some differences between the two processes: as we have just seen the main difference is
the lack of the external power in electroless deposition; in electroplating it isn’t necessary to use
reducing agents or surfactants; electroless can deposit also onto non metallic substrates, wherever
the plates touch the solution; by the proper choice of the solution composition, pH, and the
operating temperatures, the rate of deposition can be seen to be as high as 20 to 25 mm/h, which is
sufficiently fast for industrial applications, finally electroless is more expensive.
Table1. 1 Differences between electrodeposition and electroless deposition
[5]
Electroplating Electroless deposition
Need external power No external power
Reducing agents not necessary Need reducing agents
Only conductive substrates Also non conductive substrates
Medium costs High costs
Other more important peculiarities of the two processes: electroless deposition has more uniform,
less porous and better abrasion and wear resistance deposit, but it is more brittle, the temperature is
higher and the life of the bath is shorter than electrodeposition[4]
. Below a table which summarizes
advantages and disadvantages of this technique.
Table1. 2: Advantages and disadvantages of electroless deposition
[5]
Advantages Disadvantages
More uniform deposit More brittle deposit
Less porous Shorter bath life
Better abrasion and wear resistance Higher bath temperature
Plates where the part is wetted More chemical control
Our main porpurs is obteining a better abrawion and wear resitence, thus we select to procede with
the electroless deposition. In particular we focus on the NiP electroless deposition baths, being
higkly stabile, easy to reproduce and ablee to make amorphous and hard coatings.
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1.3 Typology of electroless baths
Most applications of the electroless coating are based on their wear and corrosion resistance.
However, characteristic like luminescence has a great potential in defense and aerospace
applications[3]
.
Electroless coatings can be divided, essentially, into three main categories:
Alloy coatings
Composite coatings
Metallic coatings
In particular NiP with or without particles is considered alloy or composite coating.
1.4 Electroless Nickel Baths
Electroless nickel plating is carried out by the immersion of objects, with a surface wetted and
activated by a catalyst, in a solution containing nickel ions and a suitable reducing agents, which
may include hypophosphite, borohydride, aminoboranes, hydrazine, etc. at temperatures above
90 °C. Further, some organic complexing agents for nickel ions, buffers, stabilizers, accelerators,
etc. are also present.
The compositions of chemical nickel-plating solutions used by Brenner and Riddell[2]
have certain
advantages over other formulations and, hence, are most popular. The compositions are more stable
since there is no loss of the complexants by evaporation. The properties of the electroless nickel-
phosphorous alloy can be regulated easily by controlling the amount of phosphorous in the deposit.
Hence, the acid solutions are generally preferred in many applications. Reactions occurring in
electroless nickel deposition with hypophosphite ion as the reducing agent may be represented as[4]
:
Ni2+
+ H2PO2- + H2O Ni + 2H
+ + H(HPO3)
-
HPO2- + H2O H(HPO3)
- + ½H2
Mainly two types of baths have been used for depositing alloys, These also include acidic and
alkaline baths[3]
.
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Catalytic active
Catalytic active
surface
surface
1.4.1 Acidic Baths
Firstly acidic baths, that represents almost the whole of the process used nowadays, are described.
Once the substrate is immersed into the bath, the reaction proceeds forward due to the following
factors: reduction in nickel ion concentration, conversion of the hypophosphite to phosphate, then
the consequence is the increase in hydrogen ion concentration, and adsorption of this gas by the
deposit.
Aminoboranes are also used as reductants instead of hypophosphite in electroless nickel deposition
from acid solutions[4]
.
The coatings obtained from this formulation have a better quality.
Table1. 3 Example of acid bath (all the values represent the concentration of the compounds)
[3]
Component Parameter 1 Parameter 2 Parameter 3 Parameter 4
Nickel chloride 30 g/L 30 g/L 30 g/L -
Nickel sulphate - - - 30 g/L
Sodium
hypophosphite
10 g/L 10 g/L 10 g/L 10 g/L
Sodium glycolate 50 g/L 10 g/L - -
Sodium acetate - - - 10 g/L
Sodium citrate - - 10 g/L -
General
appearance of the
coating
Semi-bright Semi-bright Semi-bright Coarse uneven
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Figure1. 1 Some mechanical parts in an electroless bath of NiP[6]
1.4.2 Alkaline baths
The second and less used type of electroless nickel baths consists in solutions with alkaline features.
The reduction of nickel in these solutions follows the same pattern as in acid ones unfortunately
they have some problems.
The main disadvantage of the alkaline solutions are their high instability at higher temperature, in
particular greater than 90°C, due to the loss of ammonia, which is useful to raise the bath pH at that
temperature. Another difference from the previous solutions is that the nickel’s rate of deposition
increases with hypophosphite concentrations; unfortunately very high concentrations of this element
make the bath unstable due to homogeneous deposition in the bulk. Also temperature influences the
rate of deposition in the same way as in acid solutions. The control of pH is very difficult due to
higher temperature than 90°C. Borohydrides are used as reducing agents in alkaline electroless
nickel-plating baths. They operate at low temperatures, i.e., from 20 °C to 97 °C, the rate of
deposition being lower at low temperature. In table 4-5 examples of alkaline are shown[7-22]
.
Table1. 4 First example of alkaline bath (all the values represent the concentration of the compounds)
[4]
Component Parameter 1 Parameter 2
Nickel chloride NiCl2 * 6H2O 20 g/L 24 g/L
Ethylene diamine (98 pct) 45 g/L -
Sodium hydroxide NaOH 40 g/L 120 g/L
Sodium borohydride NaBH4 0.67 g/L 0.4 g/L
pH 11 to 12 11 to 12
Temperature 97 °C 60 °C
Rate of deposition 7 to 8 to 9.2 mgm/cm2/h 0 to 12 mgm/cm
2/h
Table1. 5 Another example of alkaline bath (all the values represent the concentration of the compounds)
[3]
Components Parameter 1 Parameter 2 Parameter 3
Nickel chloride 30 g/L 30 g/L 30 g/L
Sodium hypophosphite 10 g/L 10 g/L 10 g/L
Ammonium chloride 50 g/L 100 g/L -
Sodium citrate 100 g/L - 100 g/L
Appearance of the
coating
Medium dark Bright Bright
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1.5 Components of Ni bath
Firstly the most important component is represented by the metal ions which constitutes the source
of metal that we want to deposit. The reducing agents are another fundamental element because
they reduce metal ions and allow them to deposit on the substrate. Complexant agents are used in
order to prevent excess of free Nickel ions concentration, so they stabilize the Nickel phosphate
precipitation and act also like pH buffers. Exultants, known as accelerators, are useful to increase
the deposition rate and are the exact opposite of complexants and stabilizers. These last prevent the
breakdown of the solution by shielding catalytically eventual active nuclei.
Buffers are used for long-term pH control instead pH regulators are used for subsequent pH
adjustment. Finally wetting agents increase the wettability of the surfaces that has to be coated.
It follows a list where are described the components, seen before, and some examples of them.
Metal ions: nickelchloride, nickel sulfate, nickel acetates
Reducing agents: sodium hypophosphite, amineborane, sodium borohydride, and hydrazine
Complexants: monocarboxylic acids, dicarboxlyic acids, hydroxycarboxylic acids,
ammonia, alkanolamines, etc.
Accelerators or exultants: anions of some mono- and di-carboxlyic acids, fluorides, borates
Stabilizers: lead, tin, arsenic, molibdenum, cadmium or thorium ions, thioures, etc.
Buffers: Sodium salt of certain complexants, choice depends on pH range used
pH regulators: sulfuric and hydrochloric acids, soda, caustic soda, ammonia
Wetting agents: ionic and nonionic surfactants
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Table1. 6 General components of a standard Ni bath[3]
Component Function Example
Metal ions Source of metal Nickel Chloride, Nickel Sulfate,
Nickel acetates
Hypophosphite ions Reducing agent Sodium hypophosphite
Complexants From Ni complexes, prevent
excess free Ni ion
concentration so stabilizing and
preventing
Ni phosphate precipitation; also
act as pH buffers
Monocarboxylic acids,
Dicarboxlyic acids,
Hydroxycarboxylic acids,
Ammonia,
alkanolamines, etc.
Accelerators (exultants) Active reducing agent and
accelerate deposition;
mode of action opposes
stabilizers and complexants
Anions of some mono- and di-
carboxlyic acids,
fluorides, borates
Stabilizers (inhibitors) Prevent solution breakdown by
shielding catalytically
active nuclei
Pb, Sn, As, Mo, Cd, or Th ions,
thioures, etc.
Buffers For long-term pH control Sodium salt of certain
complexants, choice depends
on pH range used
PH regulators For subsequent pH adjustment Sulfuric and hydrochloric acids,
Soda, caustic soda,
ammonia
Wetting agents Increase wettability of surfaces
to be coated
Ionic and nonionic surfactants
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1.5.1 Reducing agents
Several reducing agents have been used in electroless coating of alloys. Four types of reducing
agent have been used for electroless nickel bath including sodium hypophosphite, aminoboranes,
sodium borohydride, and hydrazine.
1.5.1.1 Sodium hypophosphite
More than 70% of electroless nickel is deposited from solutions reduced by this compound. The
main advantage of these solutions over those reduced by borohydride or hydrazine includes lower
costs, greater ease of process control. A lot of researches have been made to explain the mechanisms
involving the chemical reactions that occur in hypophosphite reduced electroless nickel plating
solutions. Most widely accepted theories are illustrated by the following equations[23,24]
.
1) Electrochemical mechanism, where catalytic oxidation of the hypophosphite yields electrons at
the catalytic surface which in turn reduces nickel and hydrogen ions is illustrated below:
H2PO2- + H2O → H2PO3
- + 2H
+ + 2e
- (1)
Ni2+
+ 2e- → Ni, (2)
2H+ + 2e
- → H2 (3)
H2PO2- + 2H
+ → P + 2H2O (4)
2) Atomic hydrogen mechanism, where atomic hydrogen is released as the result of the
catalytic dehydrogenation of hypophosphite molecule adsorbed at the surface is illustrated
below:
H2PO2- + H2O → HPO3
2- + H
+2Hads (5)
2Hads + Ni2+
→ Ni + 2H+
(6)
H2PO2 + Hads → H2O + OH- + P (7)
The adsorbed active hydrogen, (6) then reduces nickel at the surface of the catalyst.
(H2PO2)2-
+ H2O → H+ + (HPO3)
2- + H2 (8)
Simultaneously, some of the absorbed hydrogen reduces a small amount of the hypophosphite
at the catalytic surface to water, hydroxyl ion and phosphorus (7). Most of the hypophosphite
present is catalytic, which is oxidized to orthophosphite and gaseous hydrogen, (8), causing
low efficiency of electroless nickel solutions for alloy coating while the deposition of nickel
and phosphorus continues. For example, usually 5 kg of sodium hypophosphite are required to
reduce 1 kg of nickel, for an average efficiency of 37%[25,26]
. The amount of hypophosphite varies a
little with the nature of buffer additive; the highest one was observed in the solution containing
sodium acetate, the lowest is the one with sodium citrate. The main characteristic of the process is
the change in the composition of the solution. The physical process that occurs is that the
concentration of nickel salt and hypophosphite decreases and the concentration of acid increases
during the progress of deposition. This is the main factor that causes the lowering of deposition
rate[25]
.
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1.5.1.2 Aminoborane baths
Use of aminoboranes in electroless Ni plating solutions have been limited to two compounds:
N-dimethylamine borane (DMAB)–(CH3)2NHBH3, and H-diethylamine borane
(DEAB)–(C2H5)2NHBH3[26-28]
. DEAB is used primarily in European establishments, whereas
DMAB is used generally in USA. DMAB is readily soluble in aqueous solutions while DEAB
should be mixed with aliphatic alcohol such as ethanol, before being added to plating bath.
The aminoboranes are effective reducing agents over a wide range of pH but due to evolution
of hydrogen, although a lower limit of pH for the plating process exists[27]
. Nickel in the deposit
increases following the increasing of the bath’s pH. Usually, the aminoborane baths have a pH
range from 6 to 9. Operating temperatures for these baths range from 50 to 80°C, however, they can
be used at temperature as low as 30°C. Accordingly, aminoborane baths are very useful for plating
non-catalytic surfaces such as plastics, nonmetals, which are their primary applications. The rate of
depositions varies with pH and temperature, but it usually varies from 7 to 12 µm/h.
1.5.1.3 Sodium borohydride baths
The borohydride ion is the most powerful reducing agent available for electroless nickel plating.
Any water-soluble borohydride can be used; however, for optimum results sodium borohydride is
preferred[25]
. In acid or neutral solutions, hydrolysis of borohydride ions is very rapid. In the
presence of nickel ions nickel boride may form spontaneously. If the pH of the solutions is
maintained between 12 and 14 the formation of nickel boride is suppressed and the reaction product
is principally elemental nickel.
1.5.2 Complexing agents
One of the difficulties of reduction reactions or chemical plating is the maintenance of the bath
composition. As the plating proceeds, continuous lowering of the rate of reduction of nickel occurs.
The solutions cannot be replenished due to the formation of nickel phosphite. If nickel phosphite is
diminished in the bath, the surface quality of coating deteriorates resulting in rough and dark
coatings. Moreover, the nickel concentration in the solution also decreases and the bath goes to the
verge of total decomposition. Sodium citrate reduces the formation of nickel phosphite and reduces
the rate of deposition[29,30]
. The ability to form nickel complexes has been attributed to some of the
proposed additives like salts of glycolic, succinic or malonic acids, however, these fail to stop the
precipitation of nickel phosphite.
The best results are obtained when the sodium citrate concentration is about 30 g/l. It helps
in checking the coating from becoming porous and dull. Because of the reduction in rate
of deposition, accelerators like salts of carbonic acids, soluble fluorides and inhibitors like
thiourea can also be added to avoid the total decomposition of the bath.
Bi, Pb, Cd and Te additions act as bath stabilizers[31]
. Bismuth and Tellurium seem to be less
effective than Pb and Cd in bath stabilization. These stabilizers are added in concentrations of only
a few parts per million.
14
1.6 Deposit characteristic
Nickel is one of the most used metals in the electroless deposition; usually its properties are
increased through adding a certain number of other elements, phosphorus is added to the solution in
order to form also a composite matrix of NiP. We will discuss how the amount of phosphorus,
mixed to the solution as phosphoric acid, can influence the crystalline structure.
During the deposition of electroless nickel films, the growth of the film starts at isolated
locations on the substrate. Then the whole surface is covered by lateral growth. The alloys
containing lower phosphorus concentrations (low P, 1-4%) are characterized by the presence of
crystalline and microcrystalline nickel, which indicates that the numbers of phosphorus atoms are
not sufficient to distort the nickel lattice.
However, some very small pockets of amorphous nickel have been observed in coatings having
10.8% P (medium-high P)[32-34]
. As the phosphorus content increases to 23.4% P (high P), the
amorphous nickel region increases, due to larger lattice distortion caused by interstitial phosphorus
atoms position[35,36]
. In such deposited films, several crystalline non-equilibrium phases like Ni,
Ni3P, Ni5P12 and others have been observed.
The most stable of these phases is Ni3P which acts like the others as a reinforcement of the structure,
giving higher hardness values than the original film.
The variation of the reinforcement mechanisms shows a direct dependence of the annealing
temperature as we can see in the figures[37]
.
However, the precipitates other than Ni3P are metastable transition phases because on continued
heating, they disappear or become balances Ni3P[38-40]
.
The presence of inhomogenities in electroless films may, energetically as well as kinetically, favor
different local balances in different regions of the film.
The width and the shift of the rings, obtained for different polycrystalline electroless NiP
deposit with varying phosphorus contents, have been determined from the selected area diffraction
(SAD) patterns of transmission electron microscope (TEM) by the travelling microscope and are
reported in the table below along with the shift of these rings compared to pure nickel[32,41]
.
Table1. 7 SAD patterns of TEM
[32,41]
% P content in the
sample
Δ2θ θ shift % shift
14,3 0.07 0.07 14.92
15,3 0.09 0.08 17.63
17,8 0.12 0.07 15.59
19,8 0.16 0.07 15.13
22,4 0.21 0.11 22.01
23,4 0.22 0.1 21.91
15
Figure1. 2 Film reinforcement mechanism evolution vs. phosphorus content
[37]
Figure1. 3 NiP film Hardness variation vs. phosphorus content
[37]
The relationship between wear resistance and friction coefficient vs. phosphorus content have the
following values[42]
.
16
Figure1. 4 Friction coefficient in a high phosphorus content film (b) and low content (c)
[42]
Figure1. 5 Wear resistance vs. phosphorus content
[42]
17
1.7 Applications
There are a lot of NiP coating applications due to its high wear and corrosion resistance; we
concentrate our studies onto the improvement and enhancement of mechanical properties of some
parts. Below you can see a table where some of the most important applications of NiP are listed.
Table1. 8 Example of NiP coatings applications
[3]
Application avenue Components Coating
thickness (μm)
Automotive Heat sinks, carburetor components, fuel injection, ball
studs, differential pinion ball shafts, disk brake pistons and
pad holders, transmission thrust washers, synchromesh
gears, knuckle, pins, exhaust manifolds and pipes,
mufflers, shock absorbers, lock components, hose
couplings, gear and gear assemblies. Fuel
pump motors, aluminum wheels, water pump,
components, steering column wheel components, air bag
hardware, air conditioning, compressor components,
decorative plastics and slip yokes
2-38
Air craft/aerospace Bearing journals, servo valves, compressor blades, hot
zone hardware, pistons heads, engine main shafts and
propellers, hydraulic actuator splines, seal snaps and
spacers, landing gear components, pilot tables, gyro parts,
engine mounts, oil nozzle components, turbine front
bearing cases, engine mount insulator housing, flanges,
sun gears, breech caps, shear bolts, engine oil feed
tubes, flexible bearing supports, break attach bolts,
antirotational plates, wing flap universal joints and
titanium thruster tracks
10-50
Chemical &
petroleum
Pressure vessels, reactors, mixer shafts, pumps and
impellers, heat exchangers, filters and components, turbine
blades and rotor assembles, compressor blades and
impellers, spray nozzles, valves: ball, gate, plug, check
and butterfly, stainless steel valves, chokes and control
valves, oil field tools, oil well packers and equipment, oil
well turbine and pumps, drilling mud pumps, hydraulic
systems actuators and blowout preventers
25-125
Electronics Head sinks, computer drive mechanisms, chassis memory
drums and discs, terminals of lead wires, connectors, diode
and transistor, cans, interlocks, junction fittings and PCB
2-25
Material handling Hydraulic cylinders and shafts, extruders, link drive belts,
gears and clutches
12-75
Medical &
pharmaceutical
Disposable surgical instruments and equipment, sizering
screens, pill sorters and feed screws and extruders
12-25
Mining Hydraulic systems, jetting pump heads, mine engine
components, piping connections, framing hardware
30-60
18
1.8 Electroless codeposition of particles in NiP matrix
In addition to the classical electrodeposition or electroless deposition, there is the codeposition
process which forms faster a film, giving new and better characteristics; for example if we
separately process a substrate with the two methods for the same time we’ll obtain a higher
thickness of the coating for the one processed with the codeposition. Furthermore the codeposition
method can be a very effective alternative to the post-electroless deposition treatments for the
optimization of the surface characteristics. Adding micro or nanoparticles in the NiP matrix,
increases the hardness, wear and corrosion resistance and the general structural cohesion. There is a
large amount of materials that can be codeposited, the most used are divided as:
1) Carbides
2) Ceramics
3) Allotropics form of carbon
4) Lubricants
5) Rare earth elements
19
1.8.1 Carbides
Carbides are used to reinforce NiP matrix thanks to their very high hardness and very high wear
resistance. Another important property is the very good thermal and chemical stability because of
their very high melting point. All these properties make carbides the most used in electroless
codeposition with NiP; in literature, the most selected are: silicon carbide (SiC, or Carborundum),
tungsten carbide (WC), boron carbide (B4C) or a mixture of them.
1.8.1.1 Silicon carbide
Silicon carbide are codeposited to strenghten NiP matrix and to improve its mechanical peculiarities,
in particular wear resistance and hardness and the corrosion resistance of the coating that is
influenced by the inglobation of SiC particles. After the codeposition, an annealing treatment is
necessary to reach very high values of all the properties, and obtain the best performances.
The mechanical properties of the particles, hardness, work as a rigid bond which increases the
elastic response of the material and delays the plastic deformation. Both annealed and not annealed
coatings experiment the optimal concentration of the particles in the bath near the agglomeration
limit; if this is exceeded there’ll be a very bad dispersion causing a decreasing of performances[43]
.
Figure1. 6 SiC content effect on (1) micro-hardness and (2) wear loss of mass[43]
Table1. 9 Micro-hardness vs. annealing temperature of NiP-SiC[44]
Type of NiP-SiC coating Micro-hardness (HV)
As-plated 957.4
Heat treatment at 200°C 1017.7
Heat treatment at 400°C 1289.3
Heat treatment at 600°C 1325.3
Thermal treatment leads to the formation of secondary precipitated phases, different from
traditional NixPy, which contributes to the matrix reinforcement as shown by XRD analysis[44]
(figure1. 7)
20
Figure1. 7 NiP-SiC XRD spectrum variation vs. annealing temperature
[44]
The presence of SiC in the NiP matrix improves the corrosion resistance of the composite coating,
although the durability considerably decreases when silicon carbide exceeds 9%, and the corrosion
resistance is worse or equal as having only NiP coating[44]
.
21
1.8.1.2 Boron carbide
This compound is very important due to its extreme hardness (9.3 in the Mohs Scale), high
chemical stability, high melting point, low density and its very high neutrons absorption coefficient
which gives an optimal shielding ability to the coatings[45]
.
After the thermal treatment, NiP-B4C composites show remarkable mechanical characteristics
which attest how the incorporation of this carbide nanoparticles is useful to the microstructure
without changing the performance. Specifically, hardness values higher than 1000 HV can be
obtained[46]
.
Figure1. 8 Annealing temperature and B4C concentration influence on micro-hardness of NiP-B4C
composite[45]
Adding B4C particles into the metallic matrix of NiP increases the wear resistance in terms of
removed mass and friction coefficient, obtaining a coating with high performances like
electrodeposited chromium[47,48]
.
Figure1. 9 Variation of wear mass loss and friction coefficient of NiP-B4C film
[45]
22
1.8.1.3 Tungsten carbide
This compound is one of the hardest material that we can select and is used into the codeposition of
NiP to improve some mechanical properties like: hardness, friction coefficient, wear and corrosion
resistance. The presence of the WC particles doesn’t influence the physical phases, before or after
annealing, but they make thinner grains due to the increasing of the available nucleation sites[49]
. As
for SiC particles, an annealing treatment can bring the performances to very competitive values.
We can see typical values of hardness in the figure below.
Figure1. 10 Microhardness of (a) NiP, (b) NiP-WC and NiP-WC annealed at 200, 400 and 600 °C (c)-(d)
[50]
Regarding the wear resistance, WC plays a fundamental role: these types of nanoparticles can be
anchored very well to the metallic matrix and increase the cohesion forces of the coating. As we
have seen before, WC also prevents the plastic deformation of the film, it is capable of resisting to
high stress values and the high stripping coefficient of the particles makes the coating very resistant
to slip wear. After removing WC, the wear mechanism becomes mainly abrasive[50]
.
Figure1. 11 Friction coefficient evolution vs. wear test duration[50]
23
1.8.2 Ceramics
Micro or nanoparticles of ceramics, metal oxides, are used as reinforcement for the matrix of NiP
due to their hardness and their capacity of improving the wear resistance of the coating. Particles
are introduced in NiP through the codeposition after being added to the bath which can be
electroplating or electroless. The mainly used materials are titanium oxide (TiO2), zirconia (ZrO2),
alumina (Al2O3) and quartz (SiO2).
1.8.2.1 Titanium oxide
Titania, like other ceramic materials, are used as reinforcement phase in NiP matrix obtained
through classic electroplating or electroless deposition in order to improve mechanical
properties.The insertion of TiO2 to the baths produces advantages in terms of increasing in
hardness[51]
, which reaches and passes 1000 HV after thermal treatment, better wear resistance
capacity, showing a less damaged surface, reduction of surface friction coefficient, which depends
on the concentration of the particles dispersed in the coating. However, we obtain different
characteristics of the films through codeposition which can be caused by the micro or nano particles
synthesis method. As a matter of fact, dimensions and the capacity of being dispersed into the
matrix directly influence mechanical properties which are optimized by the presence of particles
themselves. For example, if we make the synthesis of the particles using the precursor TiCl3 we’ll
have particles with very small dimensions, about 12 nm, and after the codeposition these ones
produce an enhancement of the hardness, over 900 HV, and a very sensible improvement of the
friction coefficient[51]
. The obtained peculiarities can be summarized in the figure.
Figure1. 12 Hardness and friction coefficient evolution of NiP-TiO2 with chemical reduced Titanium [51]
24
If we consider Titania powder obtained by ball milling after 40 hours we’ll have particles with
higher dimensions than before, between 33 and 35 nm. These method produces a coating with
hardness above 1000HV, very good friction coefficient and wear resistance. In general, the
mechanical properties will be higher than those samples which have TiO2 precipitates[52]
.
Figure1. 13 Microhardness, wear and friction coefficient of NiP-TiO2 with ball milling Titania[51-52]
25
As shown the use of smaller particles for the reinforcement leads to better final results.
The main problem is that the smaller dimensions gets the more superficial area of particles becomes,
consequently, the ability of each other forming clusters that prevents good dispersion of the
reinforcement elements of the matrix. A proposed solution for this problem is represented by an
alternative synthesis method of TiO2 nanoparticles, which exploit a sol-gel process in order to have
a reduction of the dimensions and a very good dispersion caused by the impossibility of the
interactions[53]
.
This is possible due to the presence of polymeric elements which acts like hindrance for the
formation of the unwanted clusters during the codeposition. A properly solution is added to the bath
instead of micro or nanopowders. This type of treatment leads to higher crystallinity of the coating
structure which means higher hardness[54]
.
Figure1. 14 Microhardness of NiP-TiO2 film. (a) NiP, (b) conventional NiP-TiO2, (c) NiP-TiO2 with Titania
obtained from sol-gel[54]
A further characteristic is that the Titania codeposition can give to NiP coatings the antibacteric
ability. In fact antiseptic properties of TiO2 are largely known, when it is exposed to UV light, and
also in nanocomposites of NiP-TiO2 this kind of peculiarity is present. So, these coatings can have
new fields of application that we don’t consider before[55]
.
26
Figure1. 15 Comparison among antibacterial properties of stainless steel, NiP and NiP-TiO2[55]
27
1.8.2.2 Zirconia
The reinforcement of the NiP matrix through zirconia leads an improvement of a lot of
characteristic rather than film without particles: these coatings show a better hardness and wear
resistance also deposition rate is higher, without negatively influencing the structural composition
of the deposit. The amount of P in the matrix is slightly influenced by the concentration variations
of the zirconia leaving the properties related to the percentage of P unchanged, as corrosion, fatigue
and abrasion resistance. Moreover, useful for our research is the fact that the zirconia particles in
electroless deposition show catalyst properties also on substrates without the PdCl2 activator
pretreatment[53,56]
.
Table1. 10 Composition and physical-chemical properties of the composite NiP-ZrO2
[53]
1.8.2.3 Aluminum oxide
Aluminum oxide (Al2O3) shows similar characteristics to the other described metallic oxides in
terms of improvement of hardness ad wear resistance[57,58]
. As well as the other codeposited
particles, the process influences superficial morphology of the coating, which becomes more rough,
and the crystallinity without Al2O3 particles being involved to the phase transformation during the
thermal treatment[59]
. In the figure1. 16 and figure1. 17 show the hardness and wear resistance
behaviour.
Figure1. 16 Annelaing treatment effect on microhardness of NiP and NiP-Al2O3 coatings[59]
28
Figure1. 17 Annealing treatment effect on wear resistance of NiP-Al2O3
[59]
Similarly to data for the other ceramic materials, the annealing is the fundamental step which allows
very good improvements and in the case of alumina it is found that temperature of annealing and
corrosion resistance are closely related, as shown in the diagram[60]
.
Figure1. 18 Corrosion current density variation vs. annealing temperature for NiP and NiP-Al2O3 at
different pH[60]
29
1.8.2.4 Silica
The codeposition of SiO2 shows an excellent potential to reinforce classic NiP coatings due to its
ceramic characteristic. As we said before, the most important data on mechanical properties are
collected after thermal treatment which allows the modification of NiP structures from amorphous
to crystalline thanks to the precipitation of Ni3P which reinforces considerably the coating giving it
a very thin and coherent grains structure.
In the case of silica deposition if the annealing is done in oxidant atmosphere we’ll see the
formation of a new phase (NiO) in the matrix.
Also for quartz particles dimensions added to the solution and their dispersion coefficient
considerably influence the properties of the coating.
In fact making a comparison between silica particles’ data with an average diameter of 10-20 nm
and particles of 40-50 nm we can observe how smaller dimensions cause batter mechanical
performances[61,62]
.
Figure1. 19 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 10-20 nm
[62]
Figure1. 20 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 40-50 nm[61]
30
As we have seen for alumina, also silica contributes to improve film behavior in corrosive
environment in function of the concentration in bath, figure1. 21 refers to corrosion current of 40-50
nm SiO2 particles[63]
.
Figure1. 21 Corrosion current density variation vs. SiO2 content
[63]
31
1.8.2.5 Ceria
An alternative material to the more classical ceramics already considered is cerium oxide (CeO2)
which is used to increase the resistance of the NiP coating in a corrosive environment due to the
structural changes caused by the codeposition.
The presence of ceria in the bath influences the structural morphology of the NiP matrix increasing
the amorphous character. This is caused by the cerium ions inclusions which prevents the formation
of a well-ordered lattice.
Differently from the use of rare earth in chloride form, actually cerium oxide is codeposited to
modify the film structure. Ceria is scarcely used in NiP nanocomposite so its behavior in the matrix,
increasing the temperature, is investigated through differential calorimetry scansion (DSC),
showing that above 665°C occurs a solid phase reaction between nickel and cerium leading to a
formation of a new phase NiCe2O4[64]
.
The effects of ceria addition, on the corrosion resistance, are visible from the SEM images which
shows that structural integrity is maintained[65]
.
Figure1. 22 SEM morphology of (a) NiP and (b) NiP-CeO2 under corrosion for 48h in acid solution
[64]
32
1.8.3 Lubricants
Another class of interesting materials useful for codeposition in NiP matrix is chemical compounds
which have lubricating properties. One of the requested parameter for NiP coatings is to have a low
friction coefficient. This aspect is very important when the coating is functionally used so the
metallic surface treatments aimed to industrial use as machinery parts.
Main used materials for this purpose are PTFE (polytetrafluoroethylene better known as Teflon®),
MoS2 (molibdenum sulphide) and WS2 (tungsten sulphide).
1.8.3.1 PTFE
Polytetrafluoroethylene (PTFE or Teflon®) is a great material to have been codeposited in NiP
matrix to improve mechanical performance for its idrophobic characteristics, low friction
coefficient and low adhesion. Being a polymeric material, its incorporation during the deposition
shows some problems. Due to water repellency when it is introduced in the aqueous solution, it
tends to form clusters and precipitate. This fact affects the good dispersion of PTFE particles and
leads to their matrix incorporation into too high dimensions.
The result is an inhomogeneous coating with too rough superficial morphology, opposing the first
aim of the PTFE codeposition[66]
.
The solution is represented using surfactants which leads to steric and ionic interactions among
PTFE particles causing a well-done dispersion without the formation of clusters. Cationic
surfactants are preferred because they increase the charge of the particles towards the positive zone
stimulating the incorporation at the cathode. (figure1. 23)
Figure1. 23 Codeposited volumetric percentage of PTFE vs. concentration and type of surfactant
[66]
The presence of well dispersed particles in the coating leads to friction advantages and
improvement of corrosion resistance after a thermal treatment. (figure1. 24) This last feature seems
to be due to that the codeposition of PTFE modifies the NiP structure making it more crystalline,
which should favor the corrosion especially at low P content.
However, a homogeneous superficial layer is created which prevents from the corrosive ions. That
layer improves the porosity of the composite NiP-PTFE because it allows to expel rapidly hydrogen
formed during the deposition[67]
.
33
Figure1. 24 Evolution of friction coefficient
[68]
As the diagram shows (figure1. 25), NiP-PTFE-SiC quaternary compounds can be considered
because the addition of PTFE influences system microhardness, which results considerably lower
than traditional NiP films[68]
.
Figure1. 25 Film NiP-PTFE microhardness vs. annealing temperature
[68]
34
1.8.3.2 Molibdenum sulfide
Transition metal sulphides are known for their lubricant characteristics at solid state when they are
disposed with a multiple layer structure, known as 2H. However, due to dangling bonds, atoms of a
materials not stabilized by surface bonds, atmospheric conditions influence characteristics of 2H-
MoS2 going to smother lubricant properties in oxidant environment.
So, a form of molibdenum sulphide has been codeposited in NiP matrix: IF-MoS2 (inorganic
fullerene-like MoS2). This type of particles has a spherical structure, without dangling bonds, and
more indicated for NiP reinforcement applications. The incorporation of that particles have a
positive effect on the deposition rate and micro hardness, so we can obtain higher values than
1300HV for concentrations equal to 2 g/L corresponding to 5% of MoS2 in the coating[69]
(table1.
11).
Table1. 11 Chemical and physical properties of NiP-MoS2
[69]
Concentration of IF-
MoS2 (g/L)
Concentration of IF-
MoS2 (%)
Plating speed (μm/h) Microhardness of
coating (50HV)
0 0 12.0 795
0.4 2.2 13.3 1117
0.8 2.9 13.5 1229
1.2 3.4 13.8 1304
1.6 4.9 14.1 1320
2.0 5.0 15.0 1335
To evaluate the difference between 2H-MoS2 and IF-MoS2 we can compare the data of the friction
coefficient and wear rate when there is 5% of molibdenum sulphide in aqueous environment under
vacuum (table1. 12)
Table1. 12 Chemical and physical properties of NiP-MoS2 film
[69]
As we can notice from the previous table both MoS2 forms have a positive effect on the friction
coefficient and on wear resistance, however the IF form is less influenced by environmental
conditions, making this material more desirable to codeposit in NiP matrix. Although positive
results obtained with the use of this type of solid lubricant, there is the possibility that the thermal
treatment, made to achieve a crystalline coating, has a negative effect on the mechanical
performance due to the generation of a MoO3 phase.
35
1.8.3.3 Tungsten sulfide
The structure of WS2 is almost the same as MoS2, but the fullerene-like arrangement is more stable.
Also, tungsten sulphide has more stability at high temperature than MoS2 so this fact makes
impossible the undesired secondary compounds during the annealing after the deposition[70]
.
the codeposition of WS2, differently from molibdenum sulphide, has a negative effect on
microhardness, which results considerably inferior than in only NiP. So, as for PTFE, there must be
quaternary composites with very hard particles like carbides. (figure1. 26)
Figure1. 26 Thermal treatment effect on microhardness of NiP and NiP-WS2 film
[70]
Although hardness reduction of about 60%, this material improves friction coefficient and loss mass
rate during the wear test, making the surface suitable for abrasion and adhesion resistance. However,
wear resistance is slightly less than traditional NiP, graphics show the evolution of the discussed
values[70]
.
Figure1. 27 Evolution of friction coefficient and mass loss vs. creep distance, before and after thermal
treatment[70]
36
1.8.4 Allotropics form of carbon
In addition to other materials that we’ve said before, also carbon in its various forms is used as
reinforcement element in metallic NiP matrix. The effects of the addition are different due to the
nature of the used compounds, we can distinguish: graphite, carbon nanotubes (CNTs) and diamond.
1.8.4.1 Graphite
For the use of carbon as graphite as reinforcement particles, we’ll have the same observation made
for the other “soft” materials like PTFE and WS2. We’ll have a performances improvement in wear
resistance and friction coefficient, but lower microhardness of the composite coating rahter than
simple NiP. Similarly, to other libricants, it is desirable the synthesis of a quaternary film compound
with hard particles which make up the lack of NiP-Cg coatings[71-73]
.
Figure1. 28 Evolution of the friction coefficient and microhardness for NiP-Cg film
[71]
1.8.4.2 Carbon nanotubes (CNTs)
The use of carbon nanotubes, with multiple walls (MWNTs), it is desirable for tribological
properties and corrosion improvement for many reasons, among them we have hardness, toughness,
flexibility, resiliency, conductivity and chemical stability[74]
.
Optimal dilution is very difficult due to the high length/diameter rate which can lower the film
properties after the deposition. Therefore, CNTs powders are subjected to a ball milling treatment
which allows the reduction of rate and obtain a better dispersion during the codeposition[75]
.
The incorporation of CNTs in NiP makes a variation of film microstructure which results more
crystalline due to pauperization of P and a different superficial morphology. (figura1. 29)
37
Figure1. 29 NiP surface (a) and NiP-CNTs (b) taken with AFM (atomic force microscope)
[74]
After annealing treatment, the most important mechanical properties values are found; these are
generally improved as shown in the graphs below. (figure1. 30)
Figure1. 30 Microhardness of NiP and NiP-CNTs before and after annealing and variation of friction
coefficient[74]
The improvement of wear resistance is principally due to that the addition of CNTs contrasts the
brittle nature of Ni3P, which is formed after thermal treatment, in fact, nanotubes and its debris
contribute to create a lubricant layer which maintains the friction coefficient low and prevents the
excessive embrittlement due to mechanical stresses cracks[76]
.
A furthermore result is that NiP-CNTs has better corrosion resistance due to low chemical reactivity
and so are less susceptible to the activation of the corrosion process[77]
.
38
1.8.4.3 Diamonds
The use of diamond, as reinforcement element for metallic matrix in codeposition, is justified by its
incredible mechanical properties and chemical stability. The incorporation of diamond nanoparticles
(Nds) to form a nanocomposite (NiP-ND) leads to a very remarkable improvement of the
performance of the traditional NiP coating, both for mechanical and corrosion tests.
As well as the other “hard” particles, hardness, corrosion resistance and friction coefficient values
are higher than NiP coatings. The friction behaviour is influenced by the partial release, in time, of
nanoparticles at the surface, which have lubricant effect due to high hardness and spheric form.
Similarly, to the previous cases, the most remarkable data are described below (table1. 13) and are
referred to annealed films[78]
.
Table1. 13 Annealing temperature effect on micro-hardness, wear rate and friction coefficient
[78]
Composite coatings Hardness (HV)
Carbon steel (base) 424.23
Ni-P 673 K 873.52
As-prepared Ni-P-ND 614.79
Ni-P-ND 573 K 983.42
Ni-P-ND 673 K 1315.81
Ni-P-ND 773 K 1203.62
Composite coatings Wear volume/ x 10-4
mm3 Friction coefficient
Carbon steel (base) 21.6 0.64
Ni-P 10.2 0.58
As-prepared Ni-P-ND 5.6 0.46
Ni-P-ND 200°C 5.8 0.47
Ni-P-ND 400°C 3.4 0.36
Ni-P-ND 600°C 3.9 0.39
The presence of NDs modifies the film structure making thinner grains and more compact due to
nanoparticles in intergranular spaces. Moreover, the good dispersion of the particles into the matrix
influences both mechanical properties isotropy and corrosion mechanisms[79]
.
Infact, although particles can give surperficial preferential sites for corrosion phenomena,
proceeding with the codeposition and the incorporation of NDs in the NiP structure corrosion
resistance improves because the nanoparticles inside the film break the linearity of the corrosive
paths which are more hindered and disadvantaged[80]
.
39
Figure1. 31 Inglobation effect of diamond nanoparticles on corrosion preferential paths
[80]
40
1.8.5 Rare earth elements
The use of rare earth as reinforcement elements in NiP matrix shows a very good potential in terms
of compatibility and improvement of deposited coating characteristics. These materials are
particularly studied for their large atomic radius and quite low electronegativity which makes them
very active in the process without being co-deposited and improving considerably corrosion
resistance. Specifically, it has been noticed that there are bonds between phosphorus and RE
elements which allows high structural cohesion. Then supported by the fact that ionic behaviour
shown by solution compounds like Ce-P or Nd-P (Δχ ≈ 1). The most used rare earth elements (RE)
are cerium (Ce), lanthanum (La), neodimium (Nd) and ytterbium (Yb).
Table1. 14 Chemical and physical properties of rare earth elements used in deposition of composites
Cerium Lanthanum Neodimium Ytterbium
Atomic radius 181.8 pm 187 pm 181 pm 176 pm
Electronegativity
(χ)
1.12 1.1 1.14 1.1
Configuration [Xe]4f15d
16s
2 [Xe]5d
16s
2 [Xe]4f
46s
2 [Xe]4f
146s
2
In the case of medium-low phosphorus coatings (P<8%) chlorates Ce and La precursors generate
substantial modify of the whole film structure. As a matter of fact, XRD analysis shows how NiP-
RE peaks are considerably larger than only NiP, this fact giustifies the presence of a higher
percentage of amorphous phase. The importance of amorphous phase increase is that, not being the
structure partially composed by thin crystalline grains with particles precipitates, preferential sites
of corrosion have been removed, giving to the deposit a better resistance in corrosive
environment[81]
.
Some experimental results show that RE elements, used for NiP film treatment, don’t take part in
the codeposition and don’t influence the amount of P, otherwise other researches tell how the RE
action can be compatible with electrodeposition and electroless and can modify phosphorus
percentage.
The increase of P was found because of amorphous phase increase, these materials can be a very
good promoter of this process, allowing to create a classification of RE based on the amount of
amorphous structure is caused by each of them can create acting on NiP matrix[82]
.
For what concern ytterbium (Yb), its use can be very interesting due to its capacity to make thinner
crystalline grains and low possibility to be codeposited, this is caused by higher atomic radius and
lower electronegativity than other RE considered before. Some analysis on the concentration of
ytterbium show how there are saturation limit for deposition rate and for corroded surface
appearance time[83]
.
41
CHAPTER 2
2.1 Characterization techniques
2.1.1 Mechanical Polishing for OM analysis
In order to do the right observation at the microscope, samples must be treated as showed below.
Firstly they are cut by a circular saw then we obtain the cross sections, these are embedded inside a
transparent polyurethane resin (PRESI – Métallographie ™)[84]
which guarantees more stability for
the mechanical polishing.
Passages are listed below:
sandpaper 100 to sandpaper 1200
diamond paste from 6 to 1 μm
After this treatment samples can be observed with OM.
2.1.2 Optical Microscope
XRD analyzer has a thickness limit, so to eventually measure thickness and observe the surface of
the samples, it has been used pictures taken by optical microscope. Such device exploits the light
refraction to reach magnification till 1000X, this value is obtained multiplying the ocular and the
lens magnification. However, the most important value of the OM is the resolution that is the ability
to distinguish two points as close as possible to each other.
The following law shown how varies this parameter:
λ is the wavelength, n is the refraction index of the medium, α is the angle between the lens and the
sample.
Figure2. 1 Optical microscope in the laboratory
42
To obtain a good image is important the focus of the device, which occurs through two small
levers[85]
.
The microscope used in laboratory is a LEICA DLMN; linked to OM there’s a camera LEICA
model DFC290 as shown in figure.
After centering the sample, thanks to other two levers which allow the movement of the observation
plate along the x and y axis, pictures of the surface, and if we want pictures of the thickness, are
taken.
2.1.3 Scanning electron microscope (SEM) and EDX analysis
SEM is an electronic microscope used to analyze many kinds of materials. It uses a focused beam of
high-energy electrons to generate a variety of signals at the surface of the specimen. These signals
can give us some information:
- the topography which includes surface peculiarity of an object and textures which have a direct
correlation with the main properties of materials.
- the morphology, namely the shape and dimensions of the particles which make up the sample in
direct relation with ductility, strength, reactivity, defects, etc.
- the composition that is the elements and compounds making up the object and their relative rates
tightly related to melting point, reactivity, hardness, etc. (it is used in the study of biphasic systems:
metallic alloys or composites).
This type of tool was developed in the early ‘40 to overcome the limitations of the optical
microscope (OM), it has a very huge depth of field, which manages to focus at the same time almost
all the sample and gives us a characteristic three-dimensional appearance useful for understanding
the surface of the object. The resolution is better than a simple optical microscope as a matter of
fact two very near spots are distinguished also with very high magnification. All these elements
make SEM the most used tool in research and in industry.
Appendix: magnification, resolution and depth of field/focus of SEM
Firstly, any type of microscope must have the possibility to enlarge an object. Magnification is
defined as follows:
However, magnification can sometimes compromise another fundamental parameter that is the
resolution. This one is defined as the minimal distance between two points, which are seen through
the optical system as two different entities, direct consequence of the Rayleigh’s criterion:
where λ is the wavelength of the sources, μ is the index of refraction of the medium between the
lens and sample, α is the aperture half angle and NA is the numerical aperture.
43
The numerical aperture, referred to objective, is the measure of the capacity of collecting light and
resolving the small details of the sample at a fixed distance.
Since the aperture limits the resolution, it is demonstrated that an object can be also put not exactly
in the microscope focus, to be resolved. So are defined depth of field and focus:
Depth of focus increases decreasing the aperture, increasing the distance from the object and finally
increasing focal length. Approximately, to obtain this measure is necessary to multiply focal length
and the f-stop value (diaphragm) then divided by 1000. The formula to calculate depth of field,
considering the numerical aperture (A), the magnification (M) and the work distance (W), is:
Figure2. 2 Scanning electron microscope (SEM) of the laboratory
44
The experimental apparatus is composed firstly by an electrons source; they can be extracted
principally in two ways: thermoionic effect or field effect emission.
In the first case a low work function metallic wire, usually tungsten (W) or lanthanum boride
(LaB6), is heated at high temperature (2700K for W and 2000K for LaB6). This process permits to
overcome the electrons work function that comes out from the wire and they are accelerated by a
voltage around 1-50kV. To avoid an excessive oxidation a pressure between 10-3
-10-4
Pa is applied.
The second way uses a very tiny tungsten tip (radius < 0.1 μm, sometime coated by low work
function material as zirconium oxide) that in presence of strong electric field (> 107 V/cm),
intensified by the tip itself, causes the charges to be escaped. This operation must be under UHV
(Ultra High Vacuum, pressure < 10-6
Pa) conditions to avoid contaminants; very concentrated beam
can be obtained (diameter < 1nm).
The second part of the microscope is formed by the lenses system: a capacitor determining the
current which flows on the sample and a lens determining the beam dimensions and so the
resolution; between them there is a diaphragm controlling the aperture.
Moreover, to decrease the beam, electromagnetic lenses are used (cylindrical symmetry coil
developing a magnetic field which deflects the charges) and electrostatic lenses (Wehnelt cylinder).
The resulting beam proceeds to scan with an alternating movement along parallel and equidistant
lines so to cover a small part of the sample. Then the signal, modulated by an amplifier, is
transmitted to a pc monitor.
Figure2. 3 SEM working scheme
The resulting images from this particulr microscopy technique can be of two types, according to
which kind of electrons are used: back scattered electrons (BSE) and secondary (SE). The first ones
have an energy very similar to the source and they are sensible to the variations of the atomic
weight (Z) for this reason heavier elements will be brighter than lighter ones.
Instead the secondary electrons are dependant by the sample topography, due to low energy, and
give a tridimensional view of the image.
The last part is the detector (Everhart-Thornley is the standard) made by a material able to emit
electrons, a light guide and a photo-multiplier channel. It is equipped by a polarizable grid (from -
50 to +300 V) which allows to select the type of electrons.
45
Another way is a solid-state detector, formed by one or more silicon diodes, in which electron-hole
pairs are generated from BSE having the right energy for this passage.
One of the advantages is that can be very thin and can be assembled onto the lens with a good
collecting angle; it allows also to compare the signal among detectors in order to distinguish
contrast contributions.
Table2. 1 A comparison among the microscopy techniques
OM SEM
Magnification 1-103 10-10
5
Resolution: ordinary/limit 5μm/0.1μm 50nm/<1nm
Depth of field 1 mm at 10x
0.1 mm at 100x
10 mm at 10x
1 mm at 100x
Technics Transmission (T), Reflection
(R) and others
T, R and others
Sample:
Preparation
Type
Dimensions
Environment
Simple
Almost no limits
Tens of cm
Atmosphere
Simple
Conductors
Tens of cm
Vacuum
Cost of tool Intermediate Intermediate-high
A technique implemented in the SEM apparatus is the energy-dispersive X-ray spectroscopy (EDS
or EDX). It is used to analyze chemical composition of the samples and it is based on the
characteristic emission of some determined X-rays. To stimulate this characteristic emission a
source of high energy electrons or photons or X-rays themselves is used; this source excites one of
the core electrons of the sample which are at ground state; this electron comes out and an outer
electron fills the hole left from the previous one and the difference in energy between the higher-
energy shell and the lower energy shell may be released in the form of X-ray.
The number and the energy of the X-rays emitted from the specimen can be measured by an energy-
dispersive spectrometer. This difference is peculiar for each atomic structure of emitting element;
EDS allows the elemental composition of the specimen to be measured.
The equipment is composed by: the excitation source, an X-rays detector, the pulse processor and
finally the analyzer.
Moreover, there are technological variants: maybe there is an excess of energy resulting from the
decay of the outer electron is transferred to a third electron from a further outer shell, prompting its
ejection. This kind of species is called Auger electron.
46
2.1.4 XRF
An X-ray fluorescence (XRF) is a useful device used to determine the thickness and a rough
approximation of the composition of materials (rocks, minerals, alloys, metals and fluids). This
technique is based on the analysis of the x-rays emitted by the sample, placed in the machine, after
the irradiation with other energy known x-rays. After it starts, on the screen of a linked PC is
possible to see a diagram which represents the intensity peaks of the elements in correspondence
with values of energy. Studying this spectrum, it is possible to estimate the elements in the sample.
In our case, it was visible the peaks of Silicon wafer substrate and the Ruthenium layer. The
thickness is measured by the system making a comparison between the deposited layer spectrum
and the substrate spectrum only[86]
.
Figure2. 4 XRF apparatus in the laboratory
47
2.1.5 XRD
This technique uses the X-ray diffraction and it is on of the most important tools to measure the
crystallinity of the thin films. A beam of x-rays is sent to the sample surface varying the incident
angles; if the resulting pattern is a large peak the phase is amorphous while there are a very large
number of peaks, one larger than the others, we’ll have crystalline phase. The diffraction technique
is the key in the study of thin films crystallinity determined by the Bragg’s law.
with n= 0, 1, 2, …
d is the distance between crystalline planes
θ is the diffraction angle
Figure2. 5 XRD apparatus of the laboratory
48
2.1.6 Microdurometer
Another important instrument useful for our studies is the microdurometer. In the laboratory is
present a FISCHERSCOPE 57 HCV with a Vickers tip made with diamond.
Through this device is possible to determine the hardness of the film, defined as the resistance that
the material opposes to a penetration of unbendable object with a determined shape.
Then a microscope, which helps the positioning of the tip and measures the mark left on the sample,
is implemented in the laboratory apparatus.
Per standards, coating thickness can’t be less than 1.5 times the diagonal length of the tip, such
dimension allows the measure won’t be influenced by the substrate.
The diamond tip used for our purpose, has a square pyramidal shape with the top angle of 136°.
The law to determine the hardness for an instrumentation, such as in the laboratory, is the following:
with P equal to the applied load and d the mark diagonal.
The hardness measure must be independent from the applied load and it is obtained doing the
average of enough measurement made at a minimal distance each other[82]
.
Using the load-unload curve is possible to estimate the elastic modulus E of the thin film.
Figure2. 6 Microdurometer apparatus of the laboratory
49
CHAPTER 3
3.1 Procedures
3.1.1 Brass pretreatment
Clean and prepare the substrates is the first important passage of the whole experiment: it allows to
obtain a perfect coating and avoid any undesired difects. First of all brass is rinsed in demineralized
water, cleaned with acetone in order to remove any organic trace, then rinsed again in demi water;
after these passages the substrate is immersed in a solution (1:1 sulphuric acid (98%), hydrogen
peroxide) for about one minute, to remove inorganic residues and again rinsed.
Later it is immersed in a solution with PdCl2, which acts as an activator of the surface (it can be
observed a slight blackening of the brass surface), another rinse with demi water finally immersion
in the NiP solution (Tecnoplate 3000 ® provided by Tecnochimica S.p.A.).
3.1.2 Preparation of CTAB and DTAB solutions
In order to have the maximal control of the bath, we need to prepare a solution with surfactants:
cetyl trimethylammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB).
The concentration of both surfactants must be 1mM.
3.1.3 Preparation of the bath for CTAB and DTAB tests
Two kind of components were provided: part A, containing the nickel salts and thus displaying their
distinctive green color; part B, a transparent solution containing the reducing agents. Moreover both
A and B solutions could be with and without additives.
Firstly we prepare the solution all without additives: NiP0. Although the results derived from this
kind of formulation weren’t satisfactory, so we have decided to take into account also part A and
part B with additives.
The second formulation (NiPA) was prepared with half amount of additives.
With this solution, the experiments with CTAB and DTAB results very bad: after about 45 minutes,
the solution in both cases became turbid due to tiny particles of nickel in suspension.
Finally the solution NiPB was formed adding ¼ amount.
All baths should have a desired pH around 4.8-5: according to the effective measured pH we added
a solution 1:4 sulfuric acid (98%) and water to lower the pH or an ammonia solution to increase the
pH.
The brass substrate, after the pretreatment explained before, is immersed in one of the above
depositing solutions and the temperature is brought to 88° C through a heater and then kept constant.
This thermal condition is crucial because at that value the autocatalysis of the electroless deposition
starts.
Table3. 1 Three different bath formulations used in the experiments
Baths A without
additives
A with additives B without
additives
B with additives
NiP0 60 ml/L / 180 ml/L /
NiPA 30 ml/L 30 ml/L 90 ml/L 90 ml/L
NiPB 45 ml/L 15 ml/L 135 ml/L 45 ml/L
50
3.1.4 Addition of particles
Solution NiPB is chosen as the best because after some attempts, it results very stable. Then
tungsten has been added to this solution (9 g/L in form of sodium tungstate), measured the pH and
brought again between 4.8 and 5 (the change is caused by alkalinity of the salt). The codeposition
didn’t give notable problems so it is decided to make solutions with silicon carbide, titania and
mixed carbides (silicon and boron carbide), firstly only with CTAB then with only DTAB.
3.2 Results
The following results represent the studies of the samples, both morphological and chemical-
physical point of view. For what concern morphology studies it is used a scanning electron
microscope (SEM), X-rays diffraction (XRD) and an optical microscope. Chemical-physical
analysis was made by micro-hardness test and SEM-EDX technique.
51
3.2.1 SEM-EDX, XRD and Optical Microscope analysis
First of all it will be shown pictures of all sample with the first formulation: A plus B without
additives.
3.2.1.1 NiP first formulation (NiP0)
Figure3. 1 Optical microscope images of NiP0: 20X of magnification (left), 100X of magnification (right)
In this case we can see a small globular growth for NiP especially in figure3. 1 with 100X of
magnification.
Figure3. 2 XRD spectrum of the NiP0
52
3.2.1.2 NiP0(CTAB)
Figure3. 3 Optical microscope images of NiP0(CTAB): 20X of magnification (left), 100X of magnification
(right)
More evident the regular globular growth is shown for this sample due to the CTAB surfactant.
Figure3. 4 SEM image of NiP0(CTAB)
This sample has a Nickel content of 87.3% and 12.7% of Phosphorus with an average thickness of
about 11 μm.
53
Figure3. 5 XRD spectrum of the NiP0(CTAB)
54
3.2.1.3 NiP0(DTAB)
Figure3. 6 Optical microscope images of NiP0(DTAB): 20X of magnification (left), 100X of magnification
(right)
The sample with DTAB surfactant has less regular gstructure than the previous one with CTAB,
infact we can see thinner texture more evident in figure3. 6.
Figure3. 7 XRD spectrum of the NiP0(DTAB)
55
3.2.1.4 NiP0/W
Figure3. 8 Optical microscope images of NiP0/W: 20X of magnification (left), 100X of magnification (right)
The content of tungsten in this sample is 9 g/L. Growth is quite regular, globular structure is almost
absent.
Figure3. 9 SEM image of the NiP0/W (9 g/L)
This sample has 91% of Nickel content, 8,4% of Phosphorus and 0,6% of tungsten with an average
thickness of 6 μm.
56
Figure3. 10 XRD spectrum of the NiP0/W
57
3.2.1.5 NiP0/W(CTAB)
Figure3. 11 Optical microscope images of NiP0/W(CTAB): 20X of magnification (left), 100X of
magnification (right)
Also in this situation tungsten concentration is 9 g/L. The growth is quite regular and the globular
structure is thin (see figure3. 11)
Figure3. 12 XRD spectrum of the NiP0/W(CTAB)
58
3.2.1.6 NiP0-SiC
Figure3. 13 Optical microscope images of NiP0-SiC: 20X of magnification (left), 100X of magnification
(right)
We notice immediately that the globular structure is very thin at the surface due to the incorporation
of the carbides; this fact produce a very high hardness NiP coatings rather than the others without
carbides.
Figure3. 14 SEM image of the NiP0-SiC
In this case the content of nickel is about 80%, 7.8% of phosphorus and 11.02% of silicon; the
average thickness is about 15 μm.
59
Figure3. 15 XRD spectrum of the NiP0-SiC
60
3.2.1.7 NiP0–SiC/B4C
Figure3. 16 Optical microscope images of NiP0-SiC/B4C: 20X of magnification (left), 100X of magnification
(right)
Also in this case the globular structure at the surface is thin, anyway less than the previous one. The
nickel content of this sample is about 89%, phosphorus 10% and silicon about 1%; the average
thickness is about 16 μm.
Figure3. 17 SEM image of the NiP0-SiC/B4C
61
Figure3. 18 XRD spectrum of the NiP0-SiC/B4C
62
3.2.1.8 NiP second formulation (NiPA)
Figure3. 19 Optical microscope images of NiPA: 20X of magnification (left), 100X of magnification (right)
It is evident that in this sample the globular growth is very regular and well-distributed, not thin due
to the presence of the additives. The content of nickel in this sample is 89%, phosphorus is 10% and
1% represents silicon; the average thickness is about 12 μm.
Figure3. 20 SEM image of the NiPA
63
Figure3. 21 XRD spectrum of the NiPA
64
3.2.1.9 NiPA/W (4 g/L)
Figure3. 22 Optical microscope images of NiPA/W (4 g/L): 20X of magnification (left), 100X of
magnification (right)
With a different formulation we can see a more regular NiP growth due to the presence of additives.
Figure3. 23 SEM image of the NiPA/W (4 g/L)
In this case the content of Nickel is about 89.4%, Phosphorus is 10 % and Tungsten is 0,6% and the
average thickness is 11 μm.
65
Figure3. 24 XRD spectrum of the NiPA/W (4 g/L)
66
3.2.1.10 NiPA/W (6 g/L)
Figure3. 25 Optical microscope images of NiPA/W (6 g/L): 20X of magnification (left), 100X of
magnification (right)
The addition of tungsten changes drastically the structure: it is thinner and less regular.
Figure3. 26 SEM image of the NiPA/W (6 g/L)
This sample presents a content of nickel about 91%, phosphorus 8% and tungsten 1% with an
average thickness of about 10 μm.
67
Figure3. 27 XRD spectrum of the NiPA/W (6 g/L)
68
3.2.1.11 NiPA/W (18 g/L)
Figure3. 28 Optical microscope images of NiPA/W (18 g/L): 20X of magnification (left), 100X of
magnification (right)
The second formulation with a more tungsten concentration seems to be more regular and globular
growth but thinner than the coating without sodium tungstate.
Figure3. 29 SEM image of the NiPA/W (18 g/L)
The content of nickel is about 90%, phosphorus 9% and tungsten 1% with an average thickness of
about 17 μm.
69
Figure3. 30 XRD spectrum of the NiPA/W (18 g/L)
70
3.2.1.12 NiP third formulation (NiPB)
Figure3. 31 Optical microscope images of NiPB: 20X of magnification (left), 100X of magnification (right)
This sample presents a globular structure but thinner than NiPA (see figure3. 31 the right one). The
content of nickel is about 88%, phosphorus 11% and silicon 1%; the average thickness is about 10
μm.
Figure3. 32 SEM image of the NiPB
71
Figure3. 33 XRD spectrum of the NiPB
72
3.2.1.13 NiPB(CTAB)
Figure3. 34 Optical microscope images of NiPB(CTAB): 20X of magnification (left), 100X of magnification
(right)
The sample with CTAB has a globular structure like the previous one but larger than the sample
with DTAB. The content of nickel is 87.5% and phosphorus 12.5%; the average thickness is about 9
μm.
Figure3. 35 SEM image of NiPB(CTAB)
73
Figure3. 36 XRD spectrum of the NiPB(CTAB)
74
3.2.1.14 NiPB(DTAB)
Figure3. 37 Optical microscope images of NiPB(DTAB): 20X of magnification (left), 100X of magnification
(right)
The sample with DTAB presents a thinner globular structure maybe due to the presence of the
shorter chain of the surfactant compared with CTAB. The nickel content is 86% and phosphorus is
14% with an average thickness of about12 μm.
Figure3.38 SEM image of the NiPB(DTAB)
75
Figure3. 39 XRD spectrum of the NiPB(DTAB)
76
3.2.1.15 NiPB/W (9 g/L)
Figure3. 40 Optical microscope images of NiPB/W (9 g/L): 20X of magnification (left), 100X of
magnification (right)
This sample has one of the thinnest globular structure. Nickel content is about 96%, phosphorus is
6% and tungsten 1%; it has an average thickness of about 8 μm.
Figure3. 41 SEM image of the NiPB/W (9 g/L)
77
Figure3. 42 XRD spectrum of the NiPB/W (9 g/L)
78
3.2.1.16 NiPB/W(CTAB)
Figure3. 43 Optical microscope images of NiPB/W(CTAB): 20X of magnification (left), 100X of
magnification (right)
This is another example of thinnest globular structure, it has a nickel content of about 86% and 14%
of phosphorus. The average thickness is about 8.4 μm.
Figure3. 44 SEM image of the NiPB(CTAB)
79
Figure3. 45 XRD spectrum of the NiPB/W(CTAB)
80
3.2.1.17 NiPB/W(DTAB)
Figure3. 46 Optical microscope images of NiPB/W(DTAB): 20X of magnification (left), 100X of
magnification (right)
Another sample with a very large globular growth, the nickel content is about 87% and phosphorus
is 13% with an average thickness of about 12 μm.
Figure3. 47 SEM image of the NiPB/W(DTAB)
81
Figure3. 48 XRD spectrum of the NiPB/W(DTAB)
82
3.2.1.18 NiPB/W(CTAB)-SiC
Figure3. 49 Optical microscope images of NiPB/W(CTAB)-SiC: 20X of magnification (left), 100X of
magnification (right)
The structure of this sample is considerably less thin than the others with particles. The content of
nickel is about 86.42%, phosphorus is 13.19% and silicon 0.27%; the average thickness is about 8
μm.
Figure3. 50 SEM image of the NiPB/W(CTAB)-SiC
83
Figure3. 51 XRD spectrum of the NiPB/W(CTAB)-SiC
84
3.2.1.19 NiPB/W(DTAB)-SiC
Figure3. 52 Optical microscope images of NiPB/W(DTAB)-SiC: 20X of magnification (left), 100X of
magnification (right)
The structure is thinner than the previous sample with CTAB, the content of nickel is about 72%,
phosphorus is 10.86% and silicon is 17.12%; average thickness is about 15 μm
Figure3. 53 SEM image of the NiPB/W(DTAB)-SiC
85
Figure3. 54 XRD spectrum of the NiPB/W(DTAB)-SiC
86
3.2.1.20 NiPB/W(CTAB)-TiO2
Figure3. 55 Optical microscope images of NiPB/W(CTAB)-TiO2: 20X of magnification (left), 100X of
magnification (right)
Also in this sample the structure is very thin due to the presence of TiO2 particles. The average
thickness is about 15 μm.
Figure3. 56 SEM image of the NiPB/W(CTAB)-TiO2
87
Figure3. 57 XRD spectrum of the NiPB/W(CTAB)-TiO2
88
3.2.1.21 NiPB/W(DTAB)-TiO2
Figure3. 58 Optical microscope images of NiPB/W(DTAB)-TiO2: 20X of magnification (left), 100X of
magnification (right)
This sample presents a slightly larger structure of the surface than the one with CTAB. The content
of nickel is about 89%, phosphorus 10% and titanium 1%; the average thickness is about 17 μm.
Figure3. 59 SEM image of the NiPB/W(DTAB)-TiO2
89
Figure3. 60 XRD spectrum of the NiPB/W(DTAB)-TiO2
90
3.2.1.22 NiPB/W(CTAB)-SiC/B4C
Figure3. 61 Optical microscope images of NiPB/W(CTAB)-SiC/B4C: 20X of magnification (left), 100X of
magnification (right)
Also in this case the structure is very thin due to the mix of carbides added to the bath. The content
of nickel is about 86%, phopshorus is 12.52% and silicon 1.48%; the average thickness is about 6
μm.
Figure3. 62 SEM image of the NiPB/W(CTAB)-SiC/B4C
91
Figure3. 63 XRD spectrum of the NiPB/W(CTAB)-SiC/B4C
92
3.2.1.23 NiPB/W(DTAB)-SiC/B4C
Figure3. 64 Optical microscope images of NiPB/W(DTAB)-SiC/B4C: 20X of magnification (left), 100X of
magnification (right)
For what concern the structure of the surface, this sample with DTAB is very similar to the previous
one. The content of nickel is about 88.56%, phosphorus is 8.59% and silicon 2.96%; the average
thickness is about 20 μm.
Figure3. 65 SEM image of the NiPB/W(DTAB)-SiC/B4C
93
Figure3. 66 XRD spectrum of the NiPB/W(DTAB)-SiC/B4C
94
3.2.2 Microhardness
Finally we did microhardness test on all the samples obtained: some results are quite similar to tool
steels hardness (690-840 HV).
Figure3. 67 Microhardness values for the first formulation NiP0
Figure3. 68 Microhardness values for the second formulation NiPA
95
Figure3. 69 Microhardness values for the third formulation NiPB
For the first formulation (figure3. 67) the highest value is reached for samples containing silicon
carbides as we expect; otherwise the remaining samples are considerably lower.
Due to the presence of more compact globular structure, the samples obtained from the second
formulation are harder (figure3. 68) but there is a minimum represented by NiPA/W where the
concentration of tungsten is 4 g/L.
In the last case (figure3. 69) the highest value is represented by NiPB(DTAB) due to the presence of
globular thinner structure than the others that we have seen because DTAB leads to a more regular
and controlled growth. FInally there is a remarkable decrease in values for the samples with
particles.
96
4. CHAPTER 4
4.1 Conclusions and future developments
According to the general analysis of the properties of NiP we can say that electroless deposition of
this type of coating can be a valid substitute to electrodeposition one. In particular some coatings
obtained by NiPA, NiPA/W (18 g/L), NiPB(DTAB) and NiPB/W(DTAB) present hardness and other
interesting properties. However the procedure to make this kind of coatings is very laborious and it
is necessary the presence of qualified operators, thus affecting potential application on a large
industrial scale. Anyway it can be a very effective method in a small industrial production.
In these years Significant forward steps in the formulation of solutions are taking place: more
efficient electrolytic solutions which are able to incorporate countless materials as an additional
particulate.
Finally the electroless coating chemistry has emerged as one of the leading areas in surface
engineering, metal finishing etc. and is estimated to grow at a rate of beyond fifteen percent per
annum: no other chemistry is growing at this rate[3]
.
97
Index of figures
Figure1. 1 Some mechanical parts in an electroless bath of NiP…………………………...………..9
Figure1. 2 Film reinforcement mechanism evolution vs. phosphorus content……………...………16
Figure1. 3 NiP film Hardness variation vs. phosphorus content…………………………..………..16
Figure1. 4 Friction coefficient in a high phosphorus content film (b) and low content (c)………...17
Figure1. 5 Wear resistance vs. phosphorus content…………………………………………………17
Figure1. 6 SiC content effect on (1) micro-hardness and (2) wear loss of mass……………………20
Figure1. 7 NiP-SiC XRD spectrum variation vs. annealing temperature…………………………...21
Figure1. 8 Annealing temperature and B4C concentration influence on micro-hardness of NiP-B4C
composite……………………………………………………………………………………………22
Figure1. 9 Variation of wear mass loss and friction coefficient of NiP-B4C film…………………..22
Figure1. 10Microhardness of (a) NiP, (b) NiP-WC and NiP-WC annealed at 200, 400 and 600 °C
(c)-(d)………………………………………………………………………………………………..23
Figure1. 11 Friction coefficient evolution vs. wear test duration…………………………………...23
Figure1. 12 Hardness and friction coefficient evolution of NiP-TiO2 with chemical reduced
Titanium……………………………………………………………………………………………..24
Figure1. 13 Microhardness, wear and friction coefficient of NiP-TiO2 with ball milling Titania….25
Figure1. 14 Microhardness of NiP-TiO2 film. (a) NiP, (b) conventional NiP-TiO2, (c) NiP-TiO2 with Titania
obtained from sol-gel…………………………………………………………………………………………26
Figure1. 15 Comparison among antibacterial properties of stainless steel, NiP and NiP-TiO2.........27
Figure1. 16 Annelaing treatment effect on microhardness of NiP and NiP-Al2O3 coatings………..28
Figure1. 17 Annealing treatment effect on wear resistance of NiP-Al2O3.........................................29
Figure1. 18 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 10-
20 nm………………………………………………………………………………………………..29
Figure1. 19 Corrosion current density variation vs. annealing temperature for NiP and NiP-Al2O3 at
different pH………………………………………………………………………………………….30
Figure1. 20 Microhardness and friction coefficient of NiP-SiO2 with particles dimension about 40-
50 nm………………………………………………………………………………………………..30
Figure1. 21 Corrosion current density variation vs. SiO2 content…………………………………..31
Figure1. 22 SEM morphology of (a) NiP and (b) NiP-CeO2 under corrosion for 48h in acid
solution……………………………………………………………………………………………...32
Figure1. 23 Codeposited volumetric percentage of PTFE vs. concentration and type of
surfactant……………………………………………………………………………………………33
98
Figure1. 24 Evolution of friction coefficient………………………………………………………..34
Figure1. 25 Film NiP-PTFE microhardness vs. annealing temperature…………………………….34
Figure1. 26 Thermal treatment effect on microhardness of NiP and NiP-WS2 film………………..36
Figure1. 27 Evolution of friction coefficient and mass loss vs. creep distance, before and after
thermal treatment……………………………………………………………………………………36
Figure1. 28 Evolution of the friction coefficient and microhardness for NiP-Cg film……………...37
Figure1. 29 NiP surface (a) and NiP-CNTs (b) taken with AFM (atomic force microscope)………38
Figure1. 30 Microhardness of NiP and NiP-CNTs before and after annealing and variation of
friction coefficient…………………………………………………………………………………..38
Figure1. 31 Inglobation effect of diamond nanoparticles on corrosion preferential paths………….40
Figure2. 1 Optical microscope in the laboratory……………………………………………………42
Figure2. 2 Scanning electron microscope (SEM) of the laboratory………………………………...44
Figure2. 3 SEM working scheme…………………………………………………………………...45
Figure2. 4 XRF apparatus of the laboratory………………………………………………………...47
Figure2. 5 XRD apparatus of the laboratory………………………………………………………..48
Figure2. 6 Microdurometer apparatus of the laboratory……………………………………………49
Figure3. 1 Optical microscope images of NiP0: 20X of magnification (left), 100X of magnification
(right)………………………………………………………………………………………………..52
Figure3. 2 XRD spectrum of the NiP0………………………………………………………………52
Figure3. 3 Optical microscope images of NiP0(CTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...53
Figure3. 4 SEM image of NiP0(CTAB)……………………………………………………………..53
Figure3. 5 XRD spectrum of the NiP0(CTAB)……………………………………………………..54
Figure3. 6 Optical microscope images of NiP0(DTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...55
Figure3. 7 XRD spectrum of the NiP0(DTAB)……………………………………………………..55
Figure3. 8 Optical microscope images of NiP0/W (9 g/L): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...56
Figure3. 9 SEM image of the NiP0/W (9 g/L)………………………………………………………56
Figure3. 10 XRD spectrum of the NiP0/W (9 g/L)………………………………………………….57
Figure3. 11 Optical microscope images of NiP0/W(CTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...58
99
Figure3. 12 XRD spectrum of the NiP0/W(CTAB)…………………………………………………58
Figure3. 13 Optical microscope images of NiP0-SiC: 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...59
Figure3. 14 SEM image of the NiP0-SiC……………………………………………………………59
Figure3. 15 XRD spectrum of the NiP0-SiC………………………………………………………..60
Figure3. 16 Optical microscope images of NiP0-SiC/B4C: 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...61
Figure3. 17 SEM image of the NiP0-SiC/B4C………………………………………………………61
Figure3. 18 XRD spectrum of the NiP0-SiC/B4C…………………………………………………...62
Figure3. 19 Optical microscope images of NiPA: 20X of magnification (left), 100X of magnification
(right)………………………………………………………………………………………………..63
Figure3. 20 SEM image of the NiPA………………………………………………………………..63
Figure3. 21 XRD spectrum of the NiPA…………………………………………………………….64
Figure3. 22 Optical microscope images of NiPA/W (4 g/L): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...65
Figure3. 23 SEM image of the NiPA/W (4 g/L)……………………………………..……………..65
Figure3. 24 XRD spectrum of the NiPA/W (4 g/L)…………………………………………………66
Figure3. 25 Optical microscope images of NiPA/W (6 g/L): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...67
Figure3. 26 SEM image of the NiPA/W (6 g/L)…………………………………….……………....67
Figure3. 27 XRD spectrum of the NiPA/W (6 g/L)…………………………………………………68
Figure3. 28 Optical microscope images of NiPA/W (18 g/L): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...69
Figure3. 29 SEM image of the NiPA/W (18 g/L)………………………………………………...…69
Figure3. 30 XRD spectrum of the NiPA/W (18 g/L)………………………………………………..70
Figure3. 31 Optical microscope images of NiPB: 20X of magnification (left), 100X of magnification
(right)………………………………………………………………………………………………..71
Figure3. 32 SEM image of the NiPB………………………………………………………………..71
Figure3. 33 XRD spectrum of the NiPB…………………………………………………………….72
Figure3. 34 Optical microscope images of NiPB(CTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...73
Figure3. 35 SEM image of NiPB(CTAB)…………………………………………………………..73
Figure3. 36 XRD spectrum of the NiPB(CTAB)………………………………………………….74
100
Figure3. 37 Optical microscope images of NiPB(DTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...75
Figure3. 38 SEM image of the NiPB(DTAB)……………………………………………………….75
Figure3. 39 XRD spectrum of the NiPB(DTAB)……………………………………………………76
Figure3. 40 Optical microscope images of NiPB/W (9 g/L): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...77
Figure3. 41 SEM image of the NiPB/W (9 g/L)…………………………………...……...………...77
Figure3. 42 XRD spectrum of the NiPB/W (9 g/L)……………………………………………...….78
Figure3. 43 Optical microscope images of NiPB/W(CTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...79
Figure3. 44 SEM image of the NiPB(CTAB)…………………………………..…………………...79
Figure3. 45 XRD spectrum of the NiPB/W(CTAB)………………………………………...………80
Figure3. 46 Optical microscope images of NiPB/W(DTAB): 20X of magnification (left), 100X of
magnification (right)………………………………………………………………………………...81
Figure3. 47 SEM image of the NiPB/W(DTAB)…………………………………...………………81
Figure3. 48 XRD spectrum of the NiPB/W(DTAB)………………………………………………...82
Figure3. 49 Optical microscope images of NiPB/W(CTAB)-SiC: 20X of magnification (left), 100X
of magnification (right)……………………………………………………………………………..83
Figure3. 50 SEM image of the NiPB/W(CTAB)-SiC……………………………………………….83
Figure3. 51 XRD spectrum of the NiPB/W(CTAB)-SiC……………………………………………84
Figure3. 52 Optical microscope images of NiPB/W(DTAB)-SiC: 20X of magnification (left), 100X
of magnification (right)……………………………………………………………………………..85
Figure3. 53 SEM image of the NiPB/W(DTAB)-SiC……………………………………………….85
Figure3. 54 XRD spectrum of the NiPB/W(DTAB)-SiC……………………………………………86
Figure3. 55 Optical microscope images of NiPB/W(CTAB)-TiO2: 20X of magnification (left), 100X
of magnification (right)……………………………………………………………………………..87
Figure3. 56 SEM image of the NiPB/W(CTAB)-TiO2……………………………………………..87
Figure3. 57 XRD spectrum of the NiPB/W(CTAB)-TiO2……………………………………..…...88
Figure3. 58 Optical microscope images of NiPB/W(DTAB)-TiO2: 20X of magnification (left), 100X
of magnification (right)……………………………………………………………………………..89
Figure3. 59 SEM image of the NiPB/W(DTAB)-TiO2……………………………………………..89
Figure3. 60 XRD spectrum of the NiPB/W(DTAB)-TiO2…………………………………………..90
Figure3. 61 Optical microscope images of NiPB/W(CTAB)-SiC/B4C: 20X of magnification (left),
100X of magnification (right)………………………………………………………………………91
101
Figure3. 62 SEM image of the NiPB/W(CTAB)-SiC/B4C…………………………………………92
Figure3. 63 XRD spectrum of the NiPB/W(CTAB)-SiC/B4C………………………………………92
Figure3. 64 Optical microscope images of NiPB/W(DTAB)-SiC/B4C: 20X of magnification (left),
100X of magnification (right)………………………………………………………………………93
Figure3. 65 SEM image of the NiPB/W(DTAB)-SiC/B4C………………………………………….93
Figure3. 66 XRD spectrum of the NiPB/W(DTAB)-SiC/B4C………………………………..…..…94
Figure3. 67 Microhardness values for the first formulation NiP0………………………………….95
Figure3. 68 Microhardness values for the first formulation NiPA…………………………………95
Figure3. 69 Microhardness values for the first formulation NiPB…………………………………96
102
Index of tables
Table1. 1 Differences between electrodeposition and electroless deposition………………………...7
Table1. 2 Advantages and disadvantages of electroless deposition…………………………………..7
Table1. 3 Example of acid bath………………………………………………………………………9
Table1. 4 Example of alkaline bath…………………………………………………………………10
Table1. 5 Another example of alkaline bath………………………………………………………...10
Table1. 6 General components of a standard Ni bath……………………………………………….12
Table1. 7 SAD patterns of TEM…………………………………………………………………….15
Table1. 8 Example of NiP coatings applications……………………………………………………18
Table1. 9 Micro-hardness vs. annealing temperature of NiP-SiC…………………………………..20
Table1. 10 Composition and physical-chemical properties of the composite NiP-ZrO2…………....28
Table1. 11 Chemical and physical properties of NiP-MoS2………………………………………..35
Table1. 12 Chemical and physical properties of NiP-MoS2 film…………………………………...35
Table1. 13 Annealing temperature effect on micro-hardness, wear rate and friction coefficient…...39
Table1. 14 Chemical and physical properties of rare earth elements used in deposition of
composites…………………………………………………………………………………………..41
Table2. 1 A comparison among the microscopy techniques……………………………………..….46
Table3. 1 Three different bath formulations used in the experiments………………………….…...50
103
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Acknowledgments
In questa sezione vorrei ringraziare tutte le persone che mi sono state vicine e mi hanno
accompagnato in questo lungo e faticoso viaggio accademico.
In primis il mio ringraziamento va ai miei genitori che mi hanno sempre supportato e incoraggiato
nei momenti difficili. È soprattutto grazie a loro se oggi ho raggiunto questo importante traguardo.
Continuando, un ringraziamento speciale va al prof. Magagnin e a Simona che mi hanno seguito
durante le esperienze di laboratorio.
Un grazie va anche alla mia compagna di vita, Rita, conosciuta proprio tra le mura dell’ateneo e ai
suoi genitori che mi hanno spronato al raggiungimento dell’obiettivo.
Infine vorrei omaggiare tutti i miei familiari e gli amici che ho conosciuto dentro e fuori i corsi di
studi in tutti questi anni.
110