Friction and Wear Performance of Experimentally...

Friction and Wear Performance of

Experimentally Developed

Self-lubricating PPS/PTFE Composites

Kimberly Rose Lagrama

Mechanical Engineering, master's level (120 credits)

2019

Luleå University of Technology

Department of Engineering Sciences and Mathematics

Acknowledgements

This master’s thesis project has been carried out at the Division of Machine Elements at theLuleå University of Technology as part of the Erasmus Mundus Joint European Master inTribology of Surfaces and Interfaces (TRIBOS) programme.

I would first like to thank my thesis advisor Prof. Nazanin Emami for the patience, men-torship, and guidance throughout the development of this thesis. She consistently allowedme to be creative in this work but steered me in the right direction whenever she thought Ineeded it.

I would also like to thank Julian Somberg and Hari Vadivel for the tremendous help thatthey provided for the development of this thesis. To the staff in the Engineering MaterialsLab as well to Zainab Al-Maqdasi, thank you very much for your support.

To my parents who have always been supportive of my adventures, thank you for lettingyour eldest daughter chase her dreams - down from the tropics up to the Arctic.

Finally, I would like to thank the entire team behind the TRIBOS programme for thiswonderful opportunity as well as my classmates who became my closest friends and familyhere in Europe.

Abstract

Demanding applications, as well as the push to eliminate the need for fossil-fuel basedlubricants, create the need for the development of high-performance polymers.

Polyphenylene Sulfide (PPS) is an example of a high-performance polymer and has a highservice temperature, good dimensional stability, and excellent chemical resistance. However,it has a low impact strength and is very brittle in neat form. Another high-performancepolymer, Polytetrafluoroethylene (PTFE), provides low friction in dry sliding conditions andcan deposit polymer transfer films onto the counterface but exhibits high wear rates in neatform.

To take advantage of the desirable characteristics of both polymers, PPS/PTFE-basedcomposites were produced through the Injection Molding process. The individual disad-vantages of these polymers were further improved by incorporating the following fillers:SCF, GO and CNT. The tribological performance under dry sliding conditions and twodifferent loads were investigated as well as the microhardness and degree of crystallinity ofthe materials.

The DSC results showed that the incorporation of reinforcements did not significantlyalter the total degree of crystallinity in the material. PPS/PTFE and the composites havesignificantly lower specific wear rates and coefficient of friction values compared to neatPPS and PTFE. The composites have higher microhardness and friction coefficient values(60N and 100N) compared to PPS/PTFE. For both loads, composites SCFCNT, 5SCF and10SCF had the lowest specific wear rates recorded where a synergistic effect between SCFand CNT has been observed.

The filler loading content did not affect the friction performance of the composites inboth loads. However, for composites with SCF as the only reinforcement, the increase inwt% content of SCF increased the specific wear rate at 60N and decreased the specific wearrate at 100N.

SEM images of the pin surfaces show that the governing wear mechanisms in the polymerblend and composites are abrasive and adhesive wear. The reduction of the specific wear ratevalues is also accompanied by the improvement in the uniformity of the observed transferfilm formation.

Table of contents

List of figures ix

List of tables xi

1 Introduction 11.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.2 Manufacturing Methods . . . . . . . . . . . . . . . . . . . . . . . 41.1.3 Tribological Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 PPS/PTFE Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.1 Polyphenylene Sulfide (PPS) . . . . . . . . . . . . . . . . . . . . . 61.2.2 Polytetrafluorethylene (PTFE) . . . . . . . . . . . . . . . . . . . . 81.2.3 Reinforcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.3 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.4 Aims and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 Methodology 132.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.1 Microhardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.2 Differential Scanning Calorimetry . . . . . . . . . . . . . . . . . . 152.2.3 Pin-on-Disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.2.4 SEM/EDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3 Results and Discussion 193.1 Microhardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2 Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . . . . . 203.3 Tribological Characterization and Observations . . . . . . . . . . . . . . . 21

3.3.1 Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table of contents

3.3.2 Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3.3 Wear Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.3.4 Transfer Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Conclusion 33

References 35

viii

List of figures

1.1 High-performance Polymers . . . . . . . . . . . . . . . . . . . . . . . . . 41.2 Abrasive Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.3 Poly(1,4-phenylene sulfide) . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4 Poly(tetrafluoroethylene) . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.5 Multi-walled CNT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2 Schematic of a Pin-on-Disc configuration . . . . . . . . . . . . . . . . . . 16

3.1 Microhardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2 Total Degree of Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . 203.3 Coefficient of Friction: load comparison . . . . . . . . . . . . . . . . . . . 213.4 Coefficient of Friction: a) 60N and b) 100N . . . . . . . . . . . . . . . . . 223.5 Specific Wear Rate: a) 60N and b) 100N . . . . . . . . . . . . . . . . . . . 243.6 SEM images of PPS, PTFE and PPS/PTFE pin surfaces . . . . . . . . . . . 253.7 SEM image of SCFCNT pin surface . . . . . . . . . . . . . . . . . . . . . 263.8 SEM images of 0.5CNT and 1CNT pin surfaces . . . . . . . . . . . . . . . 263.9 SEM images of 5SCF and 10SCF pin surfaces . . . . . . . . . . . . . . . . 273.10 SEM images of 0.5GO, 1GO and GOCNT pin surfaces . . . . . . . . . . . 283.11 SEM images of SCFGOCNT and SCFGO pin surfaces . . . . . . . . . . . 293.12 SEM images of counterface discs post-tribotest . . . . . . . . . . . . . . . 303.13 SEM image of the counterface disc post-tribotest used for PPS/PTFE . . . . 31

ix

List of tables

1.1 Properties of Poly(phenylene sulfide) . . . . . . . . . . . . . . . . . . . . . 71.2 Properties of Polytetrafluoroethylene . . . . . . . . . . . . . . . . . . . . . 8

2.1 Composite Constituents in wt% . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Tribotest Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

xi

Chapter 1

Introduction

Demanding applications and environments that involve high and low temperatures, vacuum,radiation, and corrosive chemicals create the need for developing high-performing materials.They have been increasingly developed for different industrial applications because of theiroverall structural integrity and excellent strength to density ratio. They are used in seals,bearings, and also in intensive applications under non-lubricated conditions [1]. The drive toincrease the understanding of polymers and its composites has also been increasing due tothe shift towards the use of eco-friendly materials in different industries, the need to reducethe dependence on fossil-fuel based lubricants. Hydropower plants, for example, are nowgearing towards the use of self-lubricated bearings [2].

The use of high-performance polymers in components is also now preferred over metalsin many bearings and gear applications due to it being lighter and having a low maintenancerequirement [3]. Aside from these advantages, most high-performance polymers are alsoreadily formed through injection moulding, casting, or machining with minimal waste [4].

Polyphenylene Sulfide (PPS), an example of a high-performing polymer, is a semi-crystalline thermoplastic and is regarded as a good polymer matrix for tribological appli-cations for having properties such as high service temperature, good dimensional stability,excellent chemical resistance, and cohesive strength. Neat PPS, however, has low impactstrength and is very brittle [5].

Polytetrafluoroethylene (PTFE) is another high-performing polymer and an exampleof a solid lubricant. It is widely used due to its ability to provide low friction in drysliding conditions on several counterface materials including stainless steel [6, 7]. Theself-lubricating property is attributed to the ability of PTFE to deposit polymer transfer filmsonto the counterface. One way to use PTFE as a solid lubricant is to incorporate it as acomponent of a polymer blend or composite to produce a self-lubricating material. PTFEalso has excellent dimension stability due to its chemical inertness and it does not absorb

1

Introduction

water. All of these characteristics make PTFE a very good candidate for a matrix polymerof polymer-based composites used in sliding applications [8] however in neat form, PTFEexperiences high wear rates.

Previous experiments have been conducted before this thesis project to investigate theinfluence of PTFE on the tribological characteristics of the polymer blend. Polymer blendsamples with three different wt% content of PTFE (15, 30 and 40) were produced, char-acterized and compared with neat PPS. The results of the experiments showed that theincorporation of PTFE in the polymer blend significantly reduced the specific wear rateand coefficient of friction - effectively making the polymer blend self-lubricating. Similarobservations regarding the influence of PTFE in PPS-based composites have been reportedin literature [9, 8, 10].

Increasing the wt% content from 15 to 40 did not yield any significant change in thefriction coefficient values measured. However, the increase in PTFE wt% content reduced thepolymer blend’s specific wear rate significantly. The governing wear mechanism observedin pure PPS is abrasive wear and upon the addition of PTFE, adhesive wear became moredominant but signs of abrasive wear are still visible. PTFE content was limited to 40% dueto processing limitations when using the injection moulding machine.

1.1 Polymers

1.1.1 Properties

Polymers are classified into three categories: elastomers, thermosets, and thermoplastics,and the difference between these classes are best described by how they behave under theapplication of heat.

Elastomers are the type of polymers that can be extremely deformed elastically withoutcausing any permanent change or damage to their shape. Natural and synthetic rubbers areexamples of elastomers and these are used in various applications due to their durability,resiliency, high resistance to abrasion, and most importantly, high elasticity. Elastomers arealso stable under heat and pressure. They are used as gaskets, tires, seals, electrical insulation,hoses, tubes, and bridge bearings [11].

Thermosets can be formed by the application of pressure and heat and due to chemicalreactions, form permanent cross-links. These types of polymeric materials cannot be reformedupon the application of additional heat or pressure [12]. These kinds of polymers are toughand heat-resistant due to the curing process it goes through. This specific property of

2

1.1 Polymers

thermosets makes it an ideal candidate for high-temperature applications such as in appliancesand electronics [11].

Meanwhile, thermoplastics are rigid and solid at normal temperatures but at elevatedtemperatures, they soften and melt. For this type of polymer, the curing process it goesthrough during formation is completely reversible because no chemical bonding occurs. Thisallows for it to be melted again and remoulded without causing any negative effects on thematerial’s physical properties [13].

Thermoplastics can either be in a hard state as a glassy material or partially crystalline, orin a rubber-like liquid state. Examples of widely used thermoplastic polymers are Polyethy-lene (PE), Polyvinyl Chloride (PVC), Polypropylene (PP), Polyamides (PA), and Polytetraflu-oroethylene (PTFE) [13]. Another characteristic of thermoplastic materials is its excellentresistance to acids, lubricants, and oil. This ability is needed for many applications becauseaside from mechanical stresses an environmental attack is also another case that must betaken into consideration.

Thermoplastics can either have an amorphous or semi-crystalline structure. Amorphouspolymers have a random molecular structure that does not exhibit a melting point unlikecrystalline substances and the crystalline phase of a semi-crystalline material. Instead,amorphous polymers soften gradually as temperature rises. The polymer chains within anamorphous polymer are random and have no specific order or alignment.

A semi-crystalline polymer has both amorphous and crystalline regions. The crystallineregions are called spherulites and these vary in shape and size, with amorphous areas inbetween. In the amorphous region of the polymer, the molecules are in a frozen state at lowertemperatures. During this “glassy state”, the molecules can still vibrate but it will not be ableto move significantly. The polymer will also be brittle, hard, and rigid akin to glass. Thetemperature at which the polymer transitions from glassy state to a rubbery state when heatedsignifies the glass transition temperature (Tg). The crystalline region remains unaffectedduring the glass transition as it only occurs in the amorphous regions.

The value of glass transition temperature varies because the glassy state is not in equilib-rium and this depends on several factors such as molecular weight, measurement method,and the rate of heating or cooling [14].

The glass transition temperature (Tg) is an important intrinsic character in semi-crystallinepolymers as this influences the material properties of a polymer and its potential applications.In the glass transition state, the mechanical properties (such as deformation modulus, etc.)and physical properties (such as volume, density, and specific heat) of the polymer willchange. However, the influence of glass transition temperature on the friction and wearbehavior of polymers has not been investigated yet [15].

3

Introduction

In high-performance thermoplastics, the degree of crystallinity is important because thisproperty has a strong influence on the mechanical and chemical properties of the material.The crystalline phase contributes to the increase in stiffness and tensile strength and onthe other hand, the amorphous phase is more suited to absorb impact energy [16]. Thesepolymers can be arranged in a pyramid according to their physical properties, cost andoperating temperature ranges (Figure 1.1).

Fig. 1.1 High-performance Polymers [17]

1.1.2 Manufacturing Methods

There are several methods to process thermoplastics such as extrusion, compression moulding,casting, and injection moulding. In extrusion, the plastic material is loaded as a granule,powder, or a pellet into a hopper and then fed into a long-heated chamber. The molten plasticmaterial is then moved by the action of a continuously revolving screw inside the cylindricalchamber which is referred to as the extruder. This process melts the plastic and it forcedthrough a small opening when it reaches the end of the heated chamber. A die is located inthis small opening that forms the shape of the finished product.

In compression moulding, a prepared volume of plastic is placed into a mould cavity andthen, a second mould will be applied to squeeze the plastic in the desired shape. This type ofmoulding can either be automated or performed through considerable hand labour [18].

Injection Moulding, on the other hand, is a process that can produce complex geometriesthat are reproducible. A process that is mostly used for thermoplastics, it requires for amaterial to be fed into a hopper which subsequently feeds it into an extruder. This extruder

4

1.1 Polymers

then pushes the plastic through the heating chamber where the material will be melted. Themolten plastic is then forced at high pressure into a closed cold mould with the desired shape.The high pressure is important, especially for viscous materials, to ensure that the mould isfilled [18].

It is important to discuss the effect of various parameters involved in the processingas these influence the manufactured parts shrinkage and dimensional stability [19] andconsequently, the parts’ mechanical properties. Bowman et al. investigated the relationshipbetween the processing conditions, microstructure, and mechanical properties of injectionmoulded semi-crystalline thermoplastics. In this study, they produced semi-crystallinethermoplastic TPX using different barrel temperatures, mould temperatures, and injectionpressures. The three-point bend tests performed on the injection moulded parts indicateda strong correlation between the microstructure and crystalline texture, and processingconditions [18]. Rapid cooling of the polymer during processing often decreases the amountof crystallinity because there is not sufficient time to allow the long chains to organizethemselves into more ordered structures [20]. Another factor that influences the crystallizationof thermoplastics during polymer processing is the temperature changes that occur due to theheat conduction to the mould walls.

In a study conducted by Renterghem et al., the impact of the injection mould temperatureon the polymer crystallization was investigated. They found that more perfect polymer crys-tals are obtained with higher mould temperatures [21]. However, the increase in crystallinityalso meant a decrease in toughness.

1.1.3 Tribological Behaviour

Despite the advantages, polymers in the neat form are still limited by their thermal stabilityand properties such as toughness, stiffness, and strength of most polymers in the neatform are still lower than that of most metals. Most thermoplastics degrade at relativelylow temperature and this restricts the use of most unreinforced thermoplastic materials intribological applications at elevated temperatures.

Since the wear rate of most base polymer materials is poor, different approaches areemployed to improve the tribological behaviour of the polymers. One approach is byreinforcing it with suitable filler materials and another is through blending it with otherpolymers [22].

Wear is defined as the loss and progressive damage of a material that occurs on the surfaceof a machine component. It can occur when two surfaces undergo a motion relative to oneanother and when a load is applied such as during sliding, rolling, and impact. Wear causes achange in geometry of the affected body as well as a change in its surface roughness. As

5

Introduction

of today, there are several wear mechanisms in polymers that have been defined in differentliterature and the two of the most common types of wear are abrasive and adhesive wear.

Adhesive wear is regarded as the most common form of wear that occurs when one solidmaterial slides over the surface of another material or is pressed against it. During the slidingof the contacting surfaces, attractive forces form between them and when the adhesive forcesover the interface are strong and large enough, particles can be detached from either or bothof the surfaces [23].

Another common type of wear is abrasive wear that occurs between two surfaces withdifferent hardness and can be divided further into two types: two-body and three-bodyabrasive wear. In two-body abrasive wear, the asperities of the harder surface plow out thesurface of the softer material as they move against each other [23]. In three-body abrasivewear, the presence of trapped hard particles within the contact causes the removal of materialin the surfaces which are softer than the trapped particles [23, 24]. These trapped hardparticles can either be contaminants or by-products from adhesion or oxidation [23]. Thecharacteristic surface appearance of abrasive wear consists of long parallel grooves runningalong the direction of the sliding (Figure 1.2).

Fig. 1.2 Example of a worn surface due to abrasive wear

1.2 PPS/PTFE Composites

1.2.1 Polyphenylene Sulfide (PPS)

6


Density 1.36 g/cm3Tensile Strength 195 MPaHardness, Rockwell R 125Melting Temperature, Tm 285 °CGlass Transition Temperature, Tg 85 °CDegradation Temperature (onset), Ti 550 °CDH(100% crystalline) 76.5 J/g

Table 1.1 Properties of Poly(phenylene sulfide) [25–29]

Polyphenylene Sulfide (PPS) is a semi-crystalline thermoplastic that has excellent chemi-cal and thermal resistance (Figure 1.3). It is suitable for applications where elevated tempera-ture or corrosive environments are likely to be encountered [30]. Table 1.1 summarizes someof the properties of PPS.

Fig. 1.3 Poly(1,4-phenylene sulfide)

PPS (Figure 1.3) is regarded as a proper tribo-matrix for polymer composites in tribologi-cal applications for having properties such as high service temperature, good dimensionalstability, excellent chemical resistance, and cohesive strength [31]. Neat PPS, however, haslow impact strength and is very brittle. To address this limitation, PPS composites havebeen developed by incorporating various fillers [5]. PPS has a low melt viscosity value of200 Pas [16] when melted which allows it to be molded with a high loading of fillers andreinforcements.

7

Introduction

Fig. 1.4 Poly(tetrafluoroethylene)

1.2.2 Polytetrafluorethylene (PTFE)

Polytetrafluoroethylene (PTFE) is a kind of thermoplastic that has a high degree of crys-tallinity. This linear polymer (Figure 1.4) is tough and flexible and can be used in temperaturesranging from 0°C to 250°C without losing its ability to resist chemical attacks from anyreagent or solvent [13]. PTFE solids, in summary, have low-load carrying capacity, low coef-ficient of friction at low loads, good chemical resistance and good sliding-friction reduction[32]. Table 1.2 summarizes some of the important properties of PTFE.

Density 2.2 g/cm3Tensile Strength 24 MPaHardness, Rockwell R 58Melting Temperature, Tm 327 °CGlass Transition Temperature, Tg 114.85 °CDegradation Temperature (onset), Ti 467 °CDH(100% crystalline) 82 J/g

Table 1.2 Properties of Polytetrafluoroethylene [27, 29, 22, 33, 34]

Aside from its low coefficient of friction and chemical stability, PTFE is almost impossiblefor other materials to adhere to due to its low surface energy, which makes it one of themost slippery manmade materials known. Similar to lamellar structures, the macromoleculeswithin PTFE slip easily along each other [2].

Active groups are created when the chains of PTFE undergo scission during rubbingagainst a hard surface. These active groups chemically interact with the countersurface thatresults in a strongly adhered transfer film.

Tabor and Makinson attributed this low friction of PTFE with their observation of stronglyadhered and molecularly thing transfer films after low friction sliding. They concluded that

8


the low friction observation was due to the easy shear of the PTFE lamellae under low shearrates [35].

However, while it exhibits a low coefficient of friction, neat PTFE experiences a highdegree of wear [36].

1.2.3 Reinforcements

Short Carbon Fiber (SCF)

Short Carbon Fibers (SCF) are used as reinforcements in engineering polymer compositesand these fibers increase the strength and stiffness of the material. Stress is transferred fromthe fiber to the polymer matrix and vice versa. However, the compatibility between thepolymer matrix and the SCF must be achieved. SCFs are lightweight and have excellentthermal and mechanical stability [37]. Several studies have shown that the incorporation ofSCF in PPS composites improved the tribological behavior (reduced wear rate and coefficientof friction) of these materials in both dry and lubricated conditions [9, 38–40, 34].

Carbon Nanotubes (CNT)

Multi-walled carbon nanotubes (Figure 1.5) are stacked concentric layers of several rolled-up graphene layers (cylinders) with an interspacing of 0.34nm [41]. Carbon nanotubeproperties are highly dependent on size, morphology, and diameter. CNTs are producedusing various methods such as arc evaporation method, laser ablation, chemical vapordeposition, electrolysis, etc. Carbon nanotubes are very promising as fillers for polymer-based composites due to their overall better structural and functional properties includinghigh aspect ratio, high mechanical strength, and high electrical properties [41, 42].

The dispersion of CNTs in the matrix, as well as the interfacial interactions between theCNT and the polymer, affect the performance of a CNT-based polymer nanocomposite. Thecarbon atoms located on the CNT walls are chemically stable due to the aromatic nature ofthe bond and because of this, CNT reinforcements are inert and interact mainly through vander Waals interactions with the surrounding matrix. As a result, an efficient load transferacross the CNT/matrix interface is not achieved. To counter this, surface properties of CNTsare modified through functionalization. Functionalization of CNTs can either be chemical,which is based on the covalent linkage of functional groups onto the CNT surface, or physical,where non-covalent interactions govern between the CNT surface and molecules used tomodify the interfacial properties such as surfactants and inorganic particles [43]. In a studyconducted by Schulte et al. [44] on surface-modified multi-walled CNTs in CNT/epoxy

9

Introduction

composites, the functionalized CNTs led to a reduced agglomeration and improved interfacialinteraction between the CNTs and the epoxy resin.

Fig. 1.5 Multi-walled CNT [42]

Graphene Oxide (GO)

Graphene Oxide is another type of nano-reinforcement and is a chemically modified graphenecontaining oxygen that comes in the form of single-layer sheets of graphite oxide [45]. Ithas excellent mechanical properties such as high Young’s modulus, hardness, and flexibility.Compared to CNT, it is cheaper to produce and has been considered to be effective asreinforcement to high-performance composites [46].

1.3 State of the Art

Friction and wear mechanisms of polymer-based composites are less well-understood com-pared to ceramics and metals. And aside from this, there is little evidence available regardingthe influence of the injection moulding processing parameters on the tribological performanceof these types of materials as well as other processing parameters involved in the fabricationof parts.

As for the influence of PTFE on the friction and wear behavior of polymer compositesand polymer blends, several studies have been conducted on this topic and showed thatincorporation of PTFE in polymer blends and composites (based on PPS, PMMA, PA66and PEEK) reduced the Coefficient of Friction and improve the wear rates [47, 9, 8, 48–51, 10, 36].

In a study conducted by Gu et al. [6], it was observed that the incorporation of PTFEinto the PMMA matrix significantly reduced the coefficient of friction and wear rate of thepolymer blend. Qu et al. [51] investigated the effect of PTFE in the sliding friction and wearbehavior of the PTFE/PEEK based composites. The results show that the PTFE particulates

10

1.3 State of the Art

in the PEEK matrix decreased the friction coefficient of the composite as well as the wearrate. In this study, the decrease in wear rate is associated with the formation of PTFE transferfilms over the surface of the steel counterface used in the pin-on-disk tribometer. The mainfriction and wear mechanisms of the composite were attributed to abrasive friction and wearof the PEEK matrix, PTFE adhesion and pull-out, the formation of low-friction-coefficientsolid-state transfer films, and shear flow of solid transfer films under combined compressionand surface traction [51].

In a study conducted by Jiang et al. [31] on PPS reinforced with SCF and sub-micronTiO2, they investigated the effects of these fillers on the tribological performance of the com-posite. The influence of SCF in the tribological behavior, as well as PTFE as a solid lubricantin PPS-based composites, have been reported in literature [10, 9, 8, 34]. Present studiesindicate that nanoparticles as reinforcements can also improve the tribological performanceof materials.

The tribological applications of carbon nanotubes (CNT) have been investigated inseveral studies where CNT was used as a reinforcement in polymer matrix materials such aspolyamide, polyamide 6, PTFE, polystyrene and epoxy [52, 53]. In these studies, increasedwear resistance and lower friction coefficient were observed in the CNT reinforced polymers.However, the usage of CNT in the industry faces a problem due to its dispersity in the polymermatrix. The strong molecular inter-atomic forces between the tubular structures of CNT areprone to agglomeration [54] but this has already been addressed through functionalization.

In another study performed by Min et al. [55], the influence on the tribological and me-chanical performance of GO and CNT on polyimide-based nanocomposites was investigatedand it has been observed that the addition of GO and CNT increased the tensile strengthby 25.1% and reduced the friction coefficient by 41% compared to pure PI. Golchin et al.[56] also investigated the tribological behavior of GO and CNT reinforced UHMWPE inwater-lubricated contacts and the improvement in tribological performance was attributed tothe lubricating action of carbon nano-fillers.

Even though polymers and its composites are widely used in the industry, knowledge onthis subject is still limited and most literature in this subject is mostly empirical. The gap inknowledge of the wear and frictional behavior of polymers and its composites consequentlylimits the prediction of the polymer’s reliability for long-term applications.

11

Introduction

1.4 Aims and Objectives

In this thesis project, PPS-PTFE based composites reinforced with various fillers of differentscales (SCF, GO, CNT) will be produced using the Injection Moulding process and thefollowing will be investigated and discussed:

• The effect of SCF, CNT and GO on the microhardness, degree of crystallization andtribological performance (under dry sliding conditions) as well as the synergistic effectsbetween the reinforcements.

• The effect of loading on the performance of the composites.

• The governing wear mechanism(s) in the composites.

• The effect of fillers on the formation of transfer film.

12

Chapter 2

Methodology

2.1 Sample Preparation

Table 2.1 lists the samples prepared in this thesis project and their corresponding constituentsin weight percentages. The allocation of 60wt% PPS and 40wt% PTFE as base polymersis based on a previous project conducted where this formula yielded the best results. Thepreparation of the composites follows the steps indicated in Figure 2.1.

Sample Number Samples GO CNT SCF

1 PPS+40PTFE - - -2 5SCF - - 53 10SCF - - 104 0.5GO 0.5 - -5 1GO 1 - -6 SCF-GO 0.5 - 107 0.5CNT - 0.5 -8 1CNT - 1 -9 SCF-CNT - 0.5 10

10 GO-CNT 0.5 0.5 -11 SCF-GO-CNT 0.5 0.5 10

Table 2.1 Composite Constituents in wt%

The PTFE and PPS powders used in this experiment were both obtained from Sigma-Aldrich Co. (Stockholm, Sweden). As for the reinforcements: GO was obtained fromNanoInnova Technologies (Madrid, Spain), the carboxylated MWCNTS from Nanocyl (Sam-breville, Belgium) and the SCFs from Teijin Carbon Europe GmbH (Wuppertal, Germany).

13

Methodology

Fig. 2.1 Sample Preparation

PPS and PTFE were dried separately in an oven for 5 hours at 160�C to remove theresidual moisture in the powders. The presence of moisture promotes the formation of defectsin the injection molding part since small bubbles of water vapor will be trapped during themelting of the powder [57].

The GO and CNT fillers were placed inside a beaker with 60mL ethanol and thenultrasonicated for an hour. This process is done to prevent agglomeration in the particles.After this process, the degassed PPS and PTFE powder were added to the beaker with thenano-reinforcements and ethanol. The mixture is ultrasonicated again for an hour to improvethe dispersion of the reinforcements.

The slurry mixture is then ball milled (wet milling) for 2 hours under 400RPM using aPM 100 planetary ball mill (Retsch, Haan, Germany) to ensure homogeneity. After this step,the slurry mixture is dried in an oven for 24 hours at 160�C.

14

2.2 Characterization Techniques

For samples 6, 9 and 11, the dried powder from the previous step is then mixed with SCFand ball milled for five minutes at 100RPM. The SCF used in this experiment has a lengthof 100 µm and a diameter of 7 µm. The addition of SCF at the last step and the use of lowrotational speed for the dry ball milling are to ensure that the mechanical integrity of thefibers is kept. For samples 2 and 3, the same ball milling parameters were used to mix PPS,PTFE, and SCF.

The blended powders are then consolidated using the MiniJet Pro Injection Moulder(Haake, Karlsruhe, Germany) and sample bars with the dimension of 80mm x 10mm x 4mmare produced. Due to the brittle characteristic of the consolidated materials, the injection-molded bars were cut into 4.2mm x 4.2mm x 4.2mm pins using a multi grinder tool with adiamond cutting wheel.


2.2.1 Microhardness

Microhardness is shown to be a useful complementary technique of polymer characterizationproviding information on microscopic mechanical properties. The microhardness values ofthe consolidated samples, in the HV scale, were measured using the Matsuzawa MXT-amachine. After several trials of different microhardness test parameters yielding repeatableresults, the measurement was done under 200gf of applied load for 25 seconds. Fivemeasurements of microhardness tests were performed on different locations to account forthe non-homogeneity in the sample’s surface.

2.2.2 Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) is a common thermo-analytical technique used tocharacterise and investigate the thermal properties of polymers. A sample and an emptyreference pan are heated together at a fixed heating rate and the difference in the amountof heat needed to raise the temperature of the sample and the reference is measured as afunction of temperature. This difference in energy gives information regarding the thermaltransitions experienced by the sample [58][37].

Properties such as glass transition and melting temperatures can be determined by observ-ing the peaks in the resulting DSC plot. The degree of crystallinity (Xc) of the compositescan be calculated using Equation 2.1 where DH100%crystalline is the enthalpy of fusion for acompletely crystalline polymer (provided in Tables 1.1 and 1.2 for PPS and PTFE respec-tively) and DHsample is the sample’s latent heat of fusion derived from the DSC plot. The

15

Methodology

latent heat of fusion is obtained by integrating the areas under the curves for the respectivetransitions. The total degree of crystallinity of the composites and polymer blend is obtainedby adding the degree of crystallinity values calculated for both PTFE and PPS.

Xc = (1

1�wt%)(

DHsample

DH100%crystalline)⇥100 (2.1)

DSC measurements were carried out using the Mettler Toledo differential scanningcalorimetry machine. A test specimen weighing between 8mg and 10mg was cut from thesample and placed inside an aluminum crucible. The sample was then heated from 25�C to360�C (above the melting temperatures but below the decomposition temperatures of PPSand PTFE) at a rate of 10�C/min. It is then maintained at 360�C for five minutes to allowsufficient time for the crystalline material to melt before cooling it back to 25�C at a rate of20�C/min. By subjecting the sample to this controlled heating-up and cooling-down cycle,any prior thermal history is erased thus making the sample standardized for thermal analysis[15]. The sample is then heated from 25�C to 600�C (above the decomposition temperatureof PTFE and PPS) at a rate of 10�C/min and then cooled back to 25�C at a rate of 20�C/min.An inert environment was maintained in the furnace through a continuous purge of nitrogengas (80mL/min) and liquid nitrogen was used to accelerate the cooling process.

2.2.3 Pin-on-Disc

Fig. 2.2 Schematic of a Pin-on-Disc configuration

Dry sliding tests were performed on a pin-on-disc tribometer (TE67 Pin-on- Disc Tri-bometer, Phoenix Tribology, UK) to evaluate the friction and wear behaviour of the preparedcomposites and polymer blend. The prepared 4.2x4.2 mm2 pin is mounted on a sample holderas shown in Figure 2.2 and the counterface material used for the discs is 316L Stainless Steel.

Stainless steel was chosen as a counterface material since it is a common bearing materialfor tribological applications and also due to its anti-corrosive properties. Before each tribo-

16


test, the counterface discs were ground to maintain the same surface roughness used in eachtrial. The discs were ground using #240P Al2O3 abrasive papers on the Buehler MetaServ250 twin grinder-polisher and then manually ground on #240P Al2O3 abrasive papers. Forthe manual grinding, the discs were attached to a heavy metal block to maintain an equaland uniform load applied during each stroke. Each grinding stroke was done in the samedirection and for every disc, the same number of strokes was used. This method ensuredthat the surface lay is unidirectional and that the surface roughness measured in the opticalprofilometer after each grinding process is 0.2µ Ra.

Calibration of the tribometer was performed before the tribotesting and the same calibra-tion file was used for all the tests. For a summary of the parameters used for the pin-on-disctest, the reader is referred to Table 2.2. The dry sliding tests were performed using a speed of0.15 m/s and under two different loads, 60N and 100N. The variation in load was introducedto investigate the effect of the applied load on the transfer film formation and tribologicalbehavior of the composites and polymer blend.

Each test was run for an hour except for samples containing SCF (2 hours) and neat poly-mer samples (30 minutes). The wear of pins was measured using a linear variable differentialtransformer (LVDT) sensor in the tribometer that detects and records the displacement ofthe pin over time. The friction and displacement values are then recorded by the tribometerfor the entire duration of the test with one-second intervals between each point. The test isrepeated three times for each sample.

Parameters Values

Speed 0.15 m/sLoad 60N & 100NCounterface SS 316L; Ra = 0.2 micro meterTest Time 1 hour,

2 hours (w/ SCF),30 mins (PPS, PTFE)

Table 2.2 Tribotest Parameters

2.2.4 SEM/EDS

Obtaining images at much smaller scales through the use of scanning electron microscopy(SEM) allows the investigation of the surface of the pins and discs before and after tribo-testing. The analysis is performed using the JEOL JCM 6000 SEM (JEOL, Peabody, MA,USA) and the images were obtained through the analysis of signals resulting from the

17

Methodology

interaction of the electron beam with the test specimen. Surface analysis through SEMgives a piece of valuable information regarding the wear mechanism experienced by thecomposites. Supporting energy dispersive x-ray spectroscopy (EDS) analyses were alsoperformed on the counterface discs before and after tribotesting to investigate the transferfilm material on the surface.

18

Chapter 3

Results and Discussion

3.1 Microhardness

Fig. 3.1 Microhardness

As shown in Figure 3.1, the PPS/PTFE polymer blend has a lower microhardness valuecompared to the neat PPS. However, the addition of reinforcements to the polymer blendincreased its microhardness and composites with only GO as fillers have relatively similarmicrohardness values with that of the neat PPS. An increase in the microhardness is alsoobserved when the wt% content of the reinforcement (SCF, GO and CNT) in the sample is

19


increased. Reinforcements are known to improve a composite’s load-bearing capacity hencethe increase in the observed microhardness values is expected in the composite materials.

3.2 Differential Scanning Calorimetry (DSC)

Fig. 3.2 Total Degree of Crystallinity

All composites with SCF, specifically those containing 10wt%, exhibited a slightly lowertotal degree of crystallinity compared to PPS/PTFE but the difference is minimal with respectto standard deviation. As for the composites containing solely nano-reinforcements, thedifference in the total degree of crystallinity from PPS/PTFE is also minimal as shown inFigure 3.2. The slight decrease in the total degree of crystallinity observed in SCF-containingcomposites could be attributed to the particle size of the constituents, particularly that ofthe SCF. SCF has larger particles compared to PTFE and PPS and these particles may haveimpeded the crystallization of PTFE and PPS. On the other hand, the particle size of thenano-reinforcements is significantly smaller compared to the size of the polymers. Thenano-reinforcements did not cause any significant effects on the crystallization of PPS andPTFE (as seen in the minimal change in the total degree of crystallization). It is knownthat for semi-crystalline polymer composites, reinforcements or fillers have the potential toalter polymer crystallization [16]. Overall, the addition of reinforcements did not affect theintegrity of the material.

20

3.3 Tribological Characterization and Observations


3.3.1 Friction

Fig. 3.3 Coefficient of Friction: load comparison

The coefficient of friction values measured for the samples are presented in Figures 3.3and 3.4. The observed friction coefficients under the two loads are relatively similar for allsamples except for 5SCF and SCFCNT (Figure 3.3). At 100N, the friction coefficient of5SCF is 31% higher and the friction coefficient of SCFCNT is 26% lower compared to thefriction coefficient values of these composites at 60N.

Compared to PPS/PTFE, the composites had higher friction coefficient values. PTFE’sability to lower the coefficient of friction is attributed to the formation and the quality of thepolymer film on the countersurface. The presence of reinforcements in the composites, in adry sliding condition, could have affected the formation of the polymer fon the countersurface.For both 60N and 100N, composites containing SCF have the highest friction coefficientvalues measured. For samples 0.5GO, 1GO, 0.5CNT, 1CNT, 5SCF and 10SCF, the increasein filler loading did not have any significant effects on the friction performance.

21


(a) 60N

(b) 100N

Fig. 3.4 Coefficient of Friction: a) 60N and b) 100N

22


3.3.2 Wear

The specific wear rate (SWR) of the composites and the polymer blend PPS/PTFE is presentedin Figure 3.5. For both 60N and 100N, the samples containing only SCF as reinforcementsas well as SCFCNT had the lowest SWR measured among all samples and performed betterthan PPS/PTFE. SCFs prevent the removal of material from a composite by interruptingsubsurface crack propagation [35]. These results are similar to the outcome of a studyconducted by Noll [59] on SCF and CNT reinforced PPS composites in which the SCF andCNT showed synergistic effects and improved the specific wear rate of the composites whenused together.

The specific wear rates of 0.5CNT and 1CNT did not have a significant difference withrespect to PPS/PTFE under 60N. However, under 100N, these composites have higher specificwear rates compared to PPS/PTFE by 281% and 71% respectively.

SCFGOCNT, 1GO and 0.5GO had the highest specific wear rates under both loads.For both 60N and 100N, 0.5GO and 1GO had higher specific wear rates compared to thatof PPS/PTFE by 524% and 670% respectively. SCFGOCNT, the sample containing allthe reinforcement types, had higher SWR in both 60N and 100N compared to that of thePPS/PTFE by 572% and 1300% respectively. 1GO, 0.5GO and SCFGOCNT also have thehighest microhardness values among all samples.

1GO has a higher specific wear rate than 0.5GO under both 60N and 100N. At 60N,there’s a minimal difference between the specific wear rates of 0.5CNT and 1CNT but at100N, the SWR of 0.5CNT is twice higher than that of 1CNT. The SWR of 10SCF is 1.5xhigher than the SWR of 5SCF at 60N but at 100N, the SWR of 5SCF is 4x the SWR of10SCF.

The SWR of PPS/PTFE at 100N is lower by 57% compared to the SWR of the polymerblend at 60N. As the load or contact pressure is increased, the deposition of polymer materialon the countersurface also increased [60, 61]. This results in an improved transfer film layerthat protects the polymer from further wear. All composites also showed a similar behaviorexcept for SCFGOCNT which showed a significant increase (135%) in SWR at 100N. Athigher contact pressures, the contact temperatures are much higher and this results in adecrease in the stiffness of the matrix [62]. This decrease in the stiffness could have led toan increase in stress concentration on the fillers and promoted debonding - releasing moreabrasive particles on the contacting surface leading to more wear.

23


(a) 60N

(b) 100N

Fig. 3.5 Specific Wear Rate: a) 60N and b) 100N

24


Blanchet and Kennedy [63] described two determining factors for the general wear rateof PTFE based composites - prevention of initial removal of material from the compositeand the secondary removal of material from the transfer film. For the first factor, this can beinvestigated using the characterization of the pin surface after tribotesting and the secondone, by investigating the formed transfer film on the counterface disc.

3.3.3 Wear Mechanism

Fig. 3.6 SEM images of PPS, PTFE and PPS/PTFE pin surface: a) Neat PPS, b) Neat PTFE,and c) PPS + 40 wt% PTFE

As shown in Figures 3.6A and 3.6B, the governing wear mechanism for neat PPS isabrasion and for neat PTFE is adhesion. The polymer blend PPS/PTFE (Figure 3.6C) has agoverning wear mechanism of adhesive wear but signs of abrasive wear are still visible.

25


Fig. 3.7 SEM image of SCFCNT pin surface

As previously stated, SCFCNT had the lowest specific wear rate and one of the highestfriction coefficients at 60N. Based on the SEM image of the pin surface (Figure 3.7), mostof the pin surface is smoothened out which indicates that the removal of material from thecomposite is not severe. Short carbon fibers are also visible in the SEM image of the pinsurface. The governing wear mechanism observed for SCFCNT is a combination of adhesiveand abrasive wear.

Fig. 3.8 SEM images of 0.5CNT and 1CNT pin surfaces: a) 0.5CNT, b) 1CNT

The wear features on the surfaces of 0.5CNT (Figure 3.8A) and 1CNT (Figure 3.8B)pins are similar to that of the PPS/PTFE polymer blend – majorly adhesive wear but signsof abrasive wear are still visible. At 60N, the specific wear rate of 1CNT and 0.5CNT is

26


relatively similar to that of the PPS/PTFE. The increase in the wt% content of CNT in thecomposite seems to have no visible effect on the governing wear mechanism.

The SCF composites, 5SCF (Figure 3.9A) and 10SCF (Figure 3.9B), have similar wearfeatures on the surface but the adhesive wear characteristics are more pronounced on 5SCFpin surface compared to that of the 10SCF. Removal of material from the composite isreduced upon the increase of SCF content however, as mentioned earlier, 5SCF has a lowerSWR than 10SCF at 60N. The removed material could have contributed to the formation oftransfer film hence a lower specific wear rate recorded for 5SCF. After SCFCNT, these twosamples had the lowest SWR recorded among the samples and similar to SCFCNT, the shortcarbon fibers are visible on the SEM images of the pin surfaces.

Fig. 3.9 SEM images of 5SCF and 10SCF pin surfaces: a) 5SCF, b) 10SCF

At 60N, samples 0.5GO (Figure 3.10A), 1GO (Figure 3.10B) and GOCNT (Figure 3.10C)have relatively similar specific wear rates and have the highest specific wear rates amongall the samples. The SEM images of the pin surfaces of these samples show similar wearfeatures as well where signs of both adhesive and abrasive wear are present. Grooves alongthe surfaces seem to be deep and the shape of the wear particles present is angular instead ofthe flat wear debris observed in those with lower SWR.

27


Fig. 3.10 SEM images of 0.5GO, 1GO and GOCNT pin surfaces: a) 0.5GO, b) 1GO, and c)GOCNT

The SEM image of SCFGOCNT pin surface (Figure 3.11A) indicates that abrasive wearis more prominent than adhesive wear - similar to other composites with high microhardnessvalues (1GO and 0.5GO). However, the abrasions on SCFGOCNT pin surface are not as deepas those observed in the other GO-containing composites (except SCFGO). The compositeSCFGO (Figure 3.11B), on the other hand, has a smoother profile but still has visible signsof abrasion and adhesive wear.

28


Fig. 3.11 SEM images of SCFGOCNT and SCFGO pin surfaces: a) SCFGOCNT, b) SCFGO

3.3.4 Transfer Film

SEM images of the counterface discs post tribotest (60N) are presented in Figure 3.12 andbased on these images, transfer film formation on all discs used was evident. The transferfilm formed on the counterface used for the samples with the lowest SWR (SCFCNT, 5SCF,and 10SCF) is relatively uniform compared to that of the samples with the highest SWR(GOCNT, 1GO and 0.5GO). In a study conducted by Sebastian et al., a similar finding wasreported that SCF fillers in PPS based composites contribute to the formation of a regulartransfer film [34]. They attributed this to the load carrying-effect of SCF where it reduces theloading stress on the polymer’s surface layer thereby changing load transfer behaviors withinthe matrix and the internal solid lubricants.

The transfer film formed on the counterface discs for samples GOCNT, 1GO and 0.5GO,on the other hand, are non-uniform and patchy.

29


Fig. 3.12 SEM images of counterface discs post-tribotest: a) 0.5CNT, b) 1CNT, c) 0.5GO, d)1GO, e) 5SCF, f) 10SCF, g) GOCNT, h) SCFGO, i) SCFCNT, and j) SCFGOCNT

30


All GO-containing composites, except for SCFGOCNT, have a patchy and non-uniformtransfer film formed onto their countersurface discs. The transfer film formed by the SCF-containing composites have marks aligned with the direction of the sliding. These marksare due to the protruding fibers on the pin’s surface as well as the fibers released from thesurface, acting as a third-body abrasive. In a study conducted by Ye et al. [35] on transferfilm evolution in PTFE nanocomposites, it was mentioned that large hard fillers, such ascarbon fibers, can abrade protective transfer films and based on the results mentioned earlier,SCF-containing composites yielded the highest friction coefficient values at 60N.

Fig. 3.13 SEM image of the counterface disc post-tribotest used for PPS/PTFE

31

Chapter 4

Conclusion

In this thesis project, PPS-PTFE based composites reinforced with GO, CNT, and SCF weresuccessfully prepared using the Injection Moulding process. Based on the experimentalresults and discussion, it is concluded that:

• The addition of reinforcements to PPS/PTFE increased the microhardness and did notsignificantly alter the total degree of crystallinity in the material.

• The PPS/PTFE based composites and polymer blend have a lower coefficient of frictionand specific wear rate compared to pure PPS and PTFE.

• Compared to the PPS/PTFE polymer blend, the composites have a higher coefficientof friction values measured under both loads 60N and 100N.

• The addition of SCF to the polymer blend improved its specific wear rate. For bothloads, composites SCFCNT, 5SCF and 10SCF had the lowest specific wear ratesrecorded and in this case, SCF and CNT showed a synergistic effect.

• The filler loading content did not influence the friction performance of the composites.However, it had an effect on the wear performance of the composites. IncreasedGO content is accompanied by a slight increase in the SWR measured in both loadconditions. At 100N, the SWR of 0.5CNT is twice higher than that of 1CNT and theSWR of 5SCF is 4x higher than that of 10SCF.

• The formation of transfer film on the counterface discs is evident and the reduction ofthe friction coefficient and specific wear rate values is accompanied by the improvementof the observed transfer film formation.

33

Conclusion

• The transfer film formed by the samples with lowest SWR (SCFCNT, 5SCF, and10SCF) are relatively uniform compared to that of the samples with the highest SWR(GOCNT, 1GO and 0.5GO).

34

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