The development of transfer layers and their role in polymer tribology

doi:10.1016/S0043-1648(00)00469-5

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Volume 245, Issues 1–2, October 2000, Pages 92-99

https://doi.org/10.1016/S0043-1648(00)00469-5 Get rights and content

Abstract

When polymers slide against metal counterfaces, transfer films are formed. This is also the case when sliding occurs between a polymer and another polymer. In the latter case, the transfer of material has been documented by infrared studies which show that material transfer occurs from a polymer of low cohesive energy density to one of higher cohesive energy density. The transfer film formed on a non-polymer counterface is governed by the counterface material and roughness, and of course the sliding conditions. The growth of transfer film with the number of passes is presented and the effect of counterface roughness is examined. The mechanism of wear is discussed with respect to the transfer film.

It is shown that when polymers are modified, such as by the addition of fillers, the transfer film affects the tribological behavior. Some fillers affect the development of transfer film and enhance its adhesion to the counterface. Such fillers reduce the wear rate of polymer, often drastically. On the other hand, there are many fillers which have no such effect on the transfer film and wear in these cases is increased. The results of friction and wear behavior for both types of fillers are discussed in view of the transfer film characteristics. The analyses of transfer films as performed by XPS for a few cases are also presented.

Introduction

The phenomenon of material transfer during sliding between metal and metal, polymer and metal, and polymer and polymer is important from both the scientific and practical considerations. This is so because material transfer is the genesis of film development. The transfer film affects the friction and wear behavior of sliding pairs. The role of transfer film in polymer–metal sliding contacts has long been realized as being responsible for the gradual transition from a transient wear behavior to steady state wear behavior. However, the role of sliding variables such as velocity, load and temperature on the transient wear behavior as well as their effect on transfer film is not clear. The development of transfer film in this state by abrasion of the softer polymeric material by the harder metal asperities is oversimplified and needs to be studied as a function of the variables involved.

In most practical systems, the contact during sliding occurs between a metal and a polymer. This is a preferred arrangement because of the incompatibility of the mating materials as well as the good heat transfer characteristics of the metallic material. In some instances, rarely though, there may be contact between two different polymers as well. In a polymer–metal system, transfer invariably occurs from polymer to metal but the direction of transfer in polymer–polymer systems is not that obvious.

As for the role of transfer films, it is widely believed that transfer films provide shielding of the soft polymer surface from the hard metal asperities. As most polymers are self-lubricating materials, the transfer film of polymer can act as a lubricant so that the coefficient of friction is much lower as compared to that between metal and metal, and it can further be lowered by proper lubrication. It is, however, not clear why transfer films are selectively formed and what governs the stability and thickness of these films during repetitive sliding.

There are three things that contribute to material transfer: the deformation of the surface under load, the fracture of material in the substrate and the transfer of this material to the other surface. The deformation and fracture are governed by the stress state in the contact zone which is affected by normal load, contact geometry, and the coefficient of friction. The effect of the factors such as the cohesion between transfer film layers and the adhesion between transfer film and counterface have also not been studied.

Most of the techniques used to study transfer films are qualitative. The lack of quantitative methods hinders understanding of the effect of various factors on the growth and steady state condition of the transfer film. The variables expected to affect the transfer film are sliding conditions such as velocity, load, atmosphere, and temperature; structure of the polymer in terms of backbone flexibility, side chains, pendant groups, and crystallinity; and mechanical topography, surface roughness, and chemical reactivity of the counterface. The presence of so many factors and the interaction among them makes the understanding of the role of these factors on transfer film very difficult. For example, the temperature rise at the interface changes the flexibility of molecules and also affects the reactivity of the counterface.

The objective of this paper is to discuss the role of polymer transfer film in tribological contacts and examine how it can be modified.

Section snippets

Transfer between polymer and polymer

Jain and Bahadur [1] investigated the transfer of material in sliding between polyethylene films and disks of polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polypropylene, and poly(methyl methacrylate) and between polyethylene terephthalate (mylar) films and the disks of polyethylene and PVC by infrared spectroscopy and differential thermal analysis. The estimate of the amount of material transferred was made in terms of the absorbance A which is defined as the log of the ratio of

Transfer between polymer and metal

The transfer of PTFE to the counterface has been reported even in static contact. Brainard and Buckley [2] detected the transfer of PTFE to metal surfaces using Auger emission spectroscopy (AES) and field ion microscopy. Yang [3] studied the transfer of PTFE on to 316 stainless steel and silicon wafers using infrared spectrophotometry and found that it was strongly time and temperature dependent and reached a steady state after a certain period of contact. It is, thus, not surprising that

Concluding remarks

1.
Transfer films are formed in sliding between polymer and polymer as well as polymer and metal. In the former case, material transfer occurs from a polymer of low cohesive energy density to one of higher cohesive energy density. In the latter case, the transfer invariably occurs from polymer to metal.
2.
Transfer film develops because of adhesion and interlocking of the fragments of material into metal asperities.
3.
Inorganic particulate materials used as the fillers in polymers may either increase or

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The development of transfer layers and their role in polymer tribology

Abstract

Introduction

Section snippets

Transfer between polymer and polymer

Transfer between polymer and metal

Concluding remarks

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Tribol. Int.

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