Harishchandra A. Lanjewar. Application - “PhD Scholarship in Polymer Tribology” Page 1

Annexure II: Polymer Tribology

Polymers are used in various tribological applications such as seals, gears, bearings, brakes and

clutches, transmission belts, rollers, tank track pads, artificial joints, grinding mills, engines, space instruments,

office automation machinery, and audio-visual machinery [1]. In addition to the applications involving friction

and wear loading, new breed of polymers have found use in special tribological applications such as those

requiring high service temperatures [2]. Thus with the advent of new technologies owing to operational

conditions, automation and computerization, and miniaturization; more stringent requirement are laid on

tribological performance of each and every material [3].

Polymers have very specific structure and mechanical behaviour and hence are sensitive to the

factors such as mechanical stresses, temperature and chemical reactions, which are capable of causing a

change in polymer surface layers in contact region. More importantly, temperature at the contact region are

generally higher due to friction and also transient flashes or hot-spots at asperity level contacts, further

deteriorates the wear resistance of polymers [4].

Wear of Polymers

Wear debris formed during severe friction was studied and friction extrusion at the contacting region

was concluded to be the mechanism of wear debris generation in polymer composites [5]. In composites with

good ductility wear debris was in the form of complete unbroken wavelike ribbons and in flake form for

composites with poor ductility. In steel counterfaces, it has been observed that formation of transfer layer is a

mechanical process in which abraded fragments of material are deposited in the crevices between the

asperities on steel surface [1]. During and after the transfer layer deposition, compounds such as FeF2, FeSO4

and FeS, which are formed near the interface, enhance the bonding between the transfer layer and the steel

counterface. A thin, uniform and tenacious transfer layer was found to be the reason to enhance the

tribological behaviour of PEEK composites filled with nano-meter sized Si3N4 and SiO2 particles during sliding

against a carbon steel ring [6], [7]. In case of polymers, general wear modes can be classified as follows;

(A) Abrasive Wear – it can be either two-body or three-body in nature with microploughing (ridge formation

and no material loss) and microcutting (cutting of ridges as microchips) as the micromechanisms of the

wear process. Wear is manifested as scratches, gouges, scoring marks, or fine cutting chips at much finer

scale. In two-body abrasion, the attack angle of asperity and the interfacial shear strength (the ratio of

shear stress at the interface and the shear yield stress of plastically deformed material) decides the

effective wear micromechanisms: ploughing or cutting. One approach for modelling abrasive wear

considers geometric aspects of asperity while another considers that wear rates are proportional to

1/𝜎u𝜀u (Lancaster and Ratner model) [4], [8] where 𝜎u is the ultimate tensile stress and 𝜀u is the

corresponding strain.

Harishchandra A. Lanjewar. Application - “PhD Scholarship in Polymer Tribology” Page 2

(B) Adhesion Wear – consists of formation of junction, material transfer across the contacting solids, its

growth and fracture. It may be accompanied by other wear types such as abrasion, fatigue [9]. Though

the mechanism of friction transfer is observed in almost all the cases, e.g. metals, ceramics, polymers,

etc., in polymers it is significantly distinct [10]. The transfer layer formed on contacting steel surfaces may

either be held in place which is desirable or is carried away and a new layer is formed, resulting in

increased wear. Transfer of hard particles such as bronze particles is also known to occur in case of

polymers causing increased wear of polymers. The growth and steady condition for transfer layers in

polymer tribology is found to be dependent on test variables such as load and sliding velocity [11].

(C) Fatigue Wear (Friction fatigue) – like in fatigue, presence of frictional contact in polymers results in cyclic

stress during rolling and reciprocal sliding [9]. As a result asperities from friction surface experience

sequential loading from the asperities of counterface, generating stress fields. These stress fields causes

fatigue degradation of surface as well as subsurface layers and finally cracking at the areas experiencing

maximum tangential stress, generates wear particles. Fatigue cracks are also initiated at defects such as

scratches, dents, marks, pits.

Harishchandra A. Lanjewar. Application - “PhD Scholarship in Polymer Tribology” Page 3

Influencing Factors in Polymer Tribology Based on the detailed discussion and deliberations covered in previous paragraphs and following

sections, Fig. 1 gives an outline for the significant variables and factors which needs to be given a consideration

in order to arrive at definitive inter-relationships between the working conditions and desired combination of

properties in polymer composites for improved tribological performance.

Fig. 1: Significant variables and factors affecting wear polymer composites.

Views on the Project:

In light of the available previous studies discussed and above schematic, it can be seen that in

polymer tribosystem the wear response is material intensive, i.e. wear response of the polymer materials can

be significantly improved via polymer composite structure and ingredient modification/alteration. However, in

order to make a judicious decision on the structural configuration of a polymer composite for a specific

tribological application, it is extremely vital to know the performance characteristics of the decided composite

material in intended application environment and working conditions or an extensive study is to be

undertaken to define the limits and design the enhanced composite based on the experimental evidence thus

obtained. Help of wear modelling is of limited importance in case of polymer composites because of complex

nature of thermo-mechanical and chemical interactions occurring between the composite material and

counterfaces as well as amongst the ingredients (matrix-fibre-filler) of composite material.

Polymer Material

Structure

Mechanical Properties

Ductility

Toughness

Mechanical, Chemical & Physical Compatibility with Filler-Fibre

Surface Treatment

Transfer Layer Characteristics

Microstructure

Fibre Reinforcement

Mechanical Properties

Interface Strength

Weaving Structure

Solid Fillers

Type of Lubricant

Amount of Lubricant

Filler Critical Packing

Chemical Compatibility

Process Characteristics

Load

Sliding velocity

Temperature

Wear Mode

Chemical Reactions

Surface Roughness

Micro-roughness of Asperities

Transfer Layer Bonding with Counterface

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Structure and Properties of Polymer Composites

Like metallic materials, properties of polymer materials can be tailor made either through modifying

the structure of the polymer itself or introduction of additives such as solid lubricants and fibres, to aim for

certain desired properties. For example with increase in molecular weight depth wear rates are enhanced [12]

or by varying the volume proportions of ether and ketone groups in polymer chains, polyaryletherketones can

be altered to have higher glass transition and melting range [13]. In order to improve wear resistance of

polymers other alterable properties include degree of crystallinity, and preorientation [14]. Also tough matrix

polymers like PEEK perform better in wear when compared with brittle epoxy resins. Zhang et. al. [15] studied

the effect of plasms surface treatment on tribological behaviour of carbon fibre reinforced PEEK polymers.

Owing to enhancement in interface strength between the matrix and polymer, improved tribological

behaviour was observed. Yan et. al. [16] studied the inter-relationship between microstructure and wear

performance of graphite/PTFE composites. Presence of microscopic imperfections were analysed and wear

response corresponded well with the imperfections at a macro- and microscopic scale level and reflected by

the interfacial properties of crystalline and amorphous regions.

In polymers, addition of solid lubricants such as PTFE, graphite, and MoS2, reduces friction coefficient.

However, the role of fillers is slightly intricate than fibres in polymer composites. For example, addition of PTFE

powders in PEEK composites reduced both wear and friction coefficients when slid against tool steel, whereas

addition of CuS powders resulted in increase in coefficient of friction [17], [18]. Addition of solid lubricants

results in degradation in mechanical properties too. Bahadur and Gong [19] formulated a model for a critical

packing of inorganic filler powder proportion in polymer-based composites, giving maximum mechanical

strength and highest abrasion resistance. The degradation in mechanical properties due to the addition of

filling material can be compensated via fibre reinforcement which in addition enhance properties such as

stiffness, impact resistance, thermal conductivity, and creep resistance at elevated temperature [20]. Specific

wear rates are also significantly reduced with almost any type of fibre reinforcements when compared to

unreinforced polymers. When introduced in same volume fractions, carbon improves material wear resistance

by almost 5 times to that of glass [21], whereas performance of aramid fibres lies somewhere between the

above two [22], [23], [24]. Carbon fibres have been found to lower the friction coefficients in PEEK polymers

[1]. In another study [25], in a poly(vinyl butyral)-modified phenolic resin-matrix composites reinforced with

three different fibres (E-glass, high-strength carbon and Kevlar-49), Kevlar-49 exhibited the best wear

performance.

On the part of fibres, they should have less sensitivity with respect to the fibre-fibre friction and

hinder early breakage. Strong bonding with the matrix polymer also helps to preserve the broken fibres in the

matrix and thus avoid formation of third bodies between the contacting surfaces [26]. Fibre orientation with

respect to the sliding direction also plays an important role [27]. Carbon fibres parallel to the sliding direction

in PEEK matrix, offers better wear resistance than in normal position. However such an arrangement may work

well in unidirectional sliding. In case of multidirectional sliding conditions, additional fibres in antiparallel

Harishchandra A. Lanjewar. Application - “PhD Scholarship in Polymer Tribology” Page 5

direction like a woven structure, causes multifold improvement in wear resistance probably on account of fibre

interlocking in the contact area. 3D hybridization has also been found to have extremely good wear response.

Kery et. al. [28] when reinforced amorphous polyamide with carbon and aramid fibres; and used them in

sliding against rotating steel rings, phenomenally superior wear resistance was observed. Addition of

lubricants as inclusions in such 3D fibre arrangement also has shown promising results [29].

Deformation and Friction in Polymer Tribosystems

In recent times, a review was performed on tribological behaviour of polymers covering adhesion at

interface, friction, wear and subsequent mass transfer [9]. It was suggested that friction involves three basic

elements;

(i) interfacial adhesive bonds, their type and strength,

(ii) deformation and fracture within and around interacting layer,

(iii) real contact area.

Attraction and repulsive forces are generated between the atoms and molecules of the material,

when brought in contact. The interfacial bonds are followed by junctions at contact spots, which are sheared

on application of tangential forces, resulting in the frictional force. The nature of surfaces in contact, surface

chemistry, and loading conditions in the interacting layer governs the formation, growth and fracture of

interfacial junctions.

Deformation in the interaction volume of the contacting materials is another source of frictional

force. Mechanical energy expended is dissipated including deformation in various forms depending on

deformation mode, sliding conditions, rubbing materials, scale level of mechanical properties, and

environment [9]. The deformed layers and fracture products are regarded as the ‘third body’ owing to their

properties in variance with the bulk polymer properties [30], [31]. Ploughing contributes to friction when hard

surface asperities plough the softer body in sliding friction condition [32]. Though very small; this component

often increases friction when combined with adhesion and subsequent microcutting. Other forms of energy

dissipation include hysteresis loss in case of viscoelastic polymers [32], [33]; and in the form of elastic waves

owing to nucleation and development of microcracks in material [34], [35].

When two surfaces approach each other closely, their asperities of maximum height come into

contact forming contact spots and overall area of these spots is called as real contact areas (RCA) [9].

Temperature of the contacting surfaces affects the RCA [9] and using Greenwood-Williamson’s approach [36]

RCA was found to decrease with temperature. Thus temperature-dependent RCA may pass through a

minimum with rise in temperature. However its existence and minimum value depends on the thermal and

mechanical properties of the contacting materials.

Harishchandra A. Lanjewar. Application - “PhD Scholarship in Polymer Tribology” Page 6

Friction – Effect of Test Parameters

In highly elastic and smooth surfaces, the basic mechanism of friction is adhesion, while transition into

other friction mechanism occurs when polymer transforms into glassy state [9]. In glassy state, more

mechanical energy is expended in plastic deformation of surface layers on polymer. These mechanisms were

observed for both amorphous as well as crystalline polymers [37]. Effect of testing conditions on friction

during wear is different for different polymers which is as detailed below.

(i) Load - Under certain contact conditions, some polymers exhibit exception to the first law of friction

which tells that friction force is proportional to the normal applied load [9]. As a function of load,

friction coefficient in polymers passes through a minimum owing to a transition from elastic contact

to a plastic contact [30], [38], [39], [35], [40].

(ii) Sliding velocity - Relationship between sliding velocity and frictional force is complicated due to the

change in contact temperatures and interface behaviour [9], [41]. Within a limited range of velocities

(0.01-1.0 cm/s), an independent relationship is observed [42], [43]. In low range of velocities, viscous

flow increases the friction [44], [45], [46] whereas at higher velocities elastic behaviour of polymers

either affects only slightly or decreases the friction force [47], [48].

(iii) Polymers as viscoelastic materials are very sensitive to frictional heating [9]. Friction dissipates up to

almost 90-95% of total mechanical energy as heat. Other mechanisms of mechanical energy

dissipation include plastic deformation of material, hysteresis, dispersion and viscous flow. Increase in

test temperature increases specific wear rates and lowers friction coefficients [1]. Effect of

temperature on friction is believed to be dependent on the changes in the mechanical properties of

polymers with temperature. A relationship of coefficient of friction was established with hardness and

shear strength for some polymers [49], [50] [51]. However effect of temperature on adhesion is not

considered in these correlations [37].