Low Friction Layers

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7 8 November 2005, Srni - Czech Republic

LOW FRICTION LAYERS

M.Sosnov, J.Hjek, P. Podan, M.Zetek, P. Bene, A. K

Keywords: Low friction layers, DLC, PLC.

1. Introduction

Low friction layers have lower hardness than common wear resistant layers. Their mainfunction is to create the sliding surface. To this group belong soft layers, which are appliedonly in combination with hard coatings like TiN, TiAlN, TiCN and layers Diamond-like carbon(DLC); which appropriately combine excellent sliding properties with good hardness.There are many varieties of carbon layers in the market. They have in common - low frictioncoefficient [Holub 2005].

2. Low friction layers

Low friction layers represent important group of tools industrial surface modification.Their technical benefit in machining and forming is possible to summarize [Zindulka 2005]:

better sliding properties

significant decrease in adhesion between layer and workpiece

tools steady run-in

decrease of cutting forces and their fluent course

limitation of built-up edge effect, especially during machining of non-ferrous materials

Carbon forms a great variety of crystalline and disordered structures because it is ableto exist in three hybridisations, sp3, sp2 and sp1 (see fig.1) [Robertson 2002].

Carbon coatings have attracted a rapidly increasing number of researches sincethe middle of 1980s and these layers are today probably the most intensively researchedsurface coatings. Diamond has a cubic crystal lattice with a four fold sp3 covalent bondstructure. Graphite has a hexagonal lattice structure with sp2-bonded two-dimensional planesinterconnected by weak Van der Waals forces. Amorphous carbon is a transitional formof carbon which has no long-range order and the local structure varies dependingon the growth procedure [Holmberg 1998].

Fig. 1 The sp3, sp2, sp1 hybridised bonding.
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Amorphous hydrogenated carbon films (a-C:H) are usually deposited in lowtemperature plasmas from a hydrocarbon precursor gas [Keudell 2002]. R.f. plasmadeposition is the most popular preparation method of a-C:H. A number of different sourcegases can be used, such as methane, acetylene or benzene. The feed gas is dissociatedand ionized in the plasma, and radicals and ions impinging onto the surface leading to filmgrowth. Final stoichiometry and material properties depend strongly on composition, flux,and energy of the film forming species. Depending on process parameters, film propertiesrange from polymer-like and soft films to hard and wear resistant coatings [Keudell 2002].

Hydrogenated amorphous carbon (a-C:H) has various phases such as diamond-like(DLC), graphite-like and polymer-like carbon (PLC). Structure and the properties of a-C:Hdepend on the preparation conditions, therefore the source gas and hydrogen are importantfor control of structure and properties of a-C:H [Saito 2000].

3. DIAMOND-LIKE CARBON (DLC)

Diamond-like carbon (DLC) is the name commonly accepted for hard carbon coatings whichhave similar mechanical, optical, electrical and chemical properties to natural diamond,

but which do not have a dominant crystalline lattice structure. They are amorphousand consist of a mixture of sp3 and sp2 carbon structures with sp2-bonded graphite-likeclusters in an amorphous sp3-bonded carbon matrix [Holmberg 1998].

To produce DLC, the deposition gasshould be a hydrocarbon, suchas methane, acetylene or propane.Adding to the hydrocarbon othergases such as hydrogen, oxygen,nitrogen or others (doping, alloying)results in a change of the chemicalcomposition and atomic structure

of the DLC, and is a very effectivemeans of tailoring its properties suchas friction, hardness and elasticity.

The most interesting properties of DLC layers, however, are their mechanical ones[Enke 1999]:

plastic hardness between 2000 HV (pure a-C:H) and 3000 HV (dopedor alloyed a-C:H);

dry friction coefficient against metals for a relative humidity of 50%, =0.1;

dry friction coefficient against SiC for a relative humidity of 50%, =0.05;

elastic recovery (resilience) 90%; adhesive interaction with other materials nearly none;

maximum endurable temperature 600C for a steel substrate and >800Cfor ceramic substrate;

Diamond-like carbon (DLC) coatings are promising candidates for dry machiningof aluminium alloys since in ambient conditions aluminium has much less tendency to adhereto the DLC coating surfaces and the formation of a built-up edge compared to other hardcoatings such as TiN, TiAlN and CrN [Koncaa 2005].

Diamond-like carbon (DLC) layers are characterized by high wear resistance, lowfriction coefficients and chemical inertness, thus high-corrosion resistance, appear to bean ideal material for protecting implants, such as hip and knee joints. These properties make

the films good candidates as biocompatible coatings for biomedical devices and tools[Grill 2003].

Fig. 2 Bonding in DLC layers.


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4. POLYMER LIKE CARBON - PLC

Polymer-like carbon (PLC) layers have similar sliding properties like above-mentioned DLClayers. Their deposition is done only on experimental basis, which have not been examinedsubstantially. Deposition is done in chamber with the presence of hydrocarbon gas, whichis bearer of carbon, most commonly used is C2H2. Carbon thanks to its reactivity

and presence of hydrogen forms polymers. Compared to DLC layers Polymer-like carbon(PLC) layers do not achieve such high hardness values; however their sliding propertiesare thanks to their character steadier. Lower hardness values result from the absence of sp3

bonds. Due to that, PLC layer is never applied individually, but only in presence of otherlayer, which ensures the systems mechanical properties [K 2005].

5. SCRATCH TEST

Scratch test is nowadays the most commonly method used to evaluate the adhesionof the coating substrate interface. The method is repeatable and capable of generatingstresses, which exceed the interfacial bond strength of thin layers. In the scratch test (fig.3)a diamond stylus (Rockwell tip radius 200 m) is drawn over the sample surface undera stepwise or continuously increasing normal force until a particular failure of the coatingoccurs and the coating detaches. The normal load at which this happens is called the criticalload Lc (fig.4) expressed in Newton (N).To determine the critical load is used light (LM) or scanning electron microscopy (SEM),acoustic emission (AE) and frictional force measurement. Coating detachment at the criticalload is a measure for the adhesion.

LC1[N] represents the load at which the first failure occurs at the borders of the scratchtrack.LC2 [N] represents the load at which the cracks are accompanied by interfacial spallation.LC3[N] represents the load at which the substrate is firstly revealed.

LS [N] represents the load at which the substrate is totally revealed. [Jacobs, R. 2003,Precht, W. 2003]

Fig. 3. Scheme of the scratch test. Fig. 4. Critical load LC LC1, LC2, LC3.

POLYMER LIKE CARBON (PLC) LAYER

PLC layer excellently accumulates plastic deformation as is obvious from the light

microscopy. First failure of the coating occurs at critical load LC1 ~ 35N and sudden inburst

to the substrate follows afterwards. Values of critical loads LC2 (~ 38N), LC3 (~ 44N) and LS(~ 46N) do not differ much. That is the consequence of layers qualities. Followingthe theoretical information PLC layers belong to the soft low friction layers, with presenceof sp2-bonded carbon => qualities of the layer approach the graphite qualities (graphite has

hexagonal crystal lattice). These qualities cause good accumulation of plastic deformation,which is developed by indenter during the scratch test. Critical load values shown in figure 5are summarized in table 1.


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Fig. 5 Detail of thin layer failure - continuous ductile perforation of the coating (LM).

DIAMOND-LIKE CARBON (DLC) LAYERDiamond like carbon layer has good adhesive cohesive properties. First failure of the coating

appeared at critical load LC1 ~ 24N and LC2~ 26N. Spallation along the scratch track bordersoccurs by all scratches, causing the layer to delaminate into the substrate (fig 6). Substrate

is totally revealed at LS ~ 50N. Critical load values shown in figure 6 are summarizedin table 1.

Fig. 6 Detail of thin layer failure - spallation along the scratch track borders (SEM).


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Fig. 7 End of the scratch total substrate exposure (SEM).

Table 1. Critical loads of thin layers.

LayerCritical load Lc

LC1 [N] LC2 [N] LC3 [N] LS [N]

PLC 35 15 38 12 44 3 46 2

DLC 24 4 26 5 44 6 50 11

6. SUMMARY

Results from Scratch test showed, that values of the critical loads are not so high comparedto common wear resistant layers, but PLC and DLC layers have excellent friction propertiesand great potential for dry cutting operations.

Low friction layers are considered as a perfect coating for aluminium alloy drymachining since in ambient conditions aluminium has much less tendency to adhere

to the DLC coating surfaces and the formation of a built-up edge. However, if the workmaterial sticks strongly to the coating it could result in severe cohesive delaminationor detachment of the coating [Koncaa 2005].

Acknowledgement

This contribution was supported by internal grant FST-ZCU in Plzen and project GACR - 106/03/P092.Whole version of this contribution, as well as presentation, will be available during the conferenceat www.ateam.zcu.cz.
http://redir.netcentrum.cz/?noaudit&url=http%3A%2F%2Fwww.ateam.zcu.czhttp://redir.netcentrum.cz/?noaudit&url=http%3A%2F%2Fwww.ateam.zcu.cz


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References

Enke K.: Dry machining and increase of endurance of machine parts with improved dopedDLC coatings on steel, ceramics and aluminium. Surface and Coatings Technology 116119.1999. Str. 488491.

Grill A.: Diamond-like carbon coatings as biocompatible materials-an overview. Diamond andRelated Materials 12. 2003. Str.166170.

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www.shm.-cz.cz. Jacobs, R. ET AL: Surface and coatings technology, 173-174, 2003, 1008-1013. Keudell A.: Formation of polymer-like hydrocarbon films from radical beams of methyl and

atomic hydrogen. Thin Solid Films 402. 2002.137. Koncaa E., Chengb Y.-T., Weinerb A.M., Daschc J.M., Alpasa A.T.: Elevated temperature

tribological behavior of non-hydrogenated diamond-like carbon coatings against 319aluminum alloy. Surface&Coatings Technology. 2005.

K, A.: Komplexn vlastnosti eznch nstroj s tenkmi vrstvami nitrid kov. Habilitanprce. ZUPlze. 2005.

Precht, W. Cyzyniewski, A.: Surface and coatings technology, 174-175, 2003, 979-983. Robertson J.: Diamond like amorphous carbon. Materials Science and Engineering R 37.

2002. 129-281. Saito K., Itoh T., Katoh Y., Okada N., Hatta A., Inomoto H., Nitta S., Nonomura S., Hiraki A.:

Properties and electron field emission of highly resistive and transparent polymer-like a-C:H.Journal of Non-Crystalline Solids 266269.2000. Str. 788792.

Zindulka O., Holub P., Jlek M., Jank R.: Nov systm kluznch PVD povlak.MM Prmyslov spektrum. 4/2005.

Sosnov Martina, Ing.

University of West Bohemia in Pilsen, Department of Material Engineering and MetallurgyUniverzitn 22, 306 14 Plze+420 377 63 83 15, [email protected]
mailto:[email protected]:[email protected]

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