Contact mechanics in glassy polymers

Abstract

Polymers, primarily semi-crystalline, are widely used in applications where low friction is required; examples are cups in artificial hip joints, bearings and gears. Until now there is no clear indication why some polymers display low friction and others don’t. In this thesis a systematic identification of the role of the intrinsic properties of glassy polymers on singleasperity sliding friction experiments is performed. The problem is analysed using a hybrid numerical/experimental technique. In the numerical part the interaction between indenter and polymer is studied by means of a constitutive model capturing the intrinsic behaviour of glassy polymers, where the interaction between tip and polymer can be influenced by the incorporation of existing friction models. The experimental section concerns the development of reproducible sliding friction experiments, which in a later stage can be compared with simulations before conclusions can be drawn. Starting point is the constitutive model developed in our group over the last decade, which accurately captures the deformation response of glassy polymers, including strain localization phenomena as well as life time predictions. The choice for glassy polymers is, therefore, clearly not motivated by their relevance in low friction applications, but only because they represent a well-characterized class of polymers that allow quantitative predictions. First however some drawbacks of the existing model must be removed. The pre-yield regime itself is highly non-linear and thus correct modelling thereof is important in all simulations where non-homogeneous deformation is applied, like e.g. in indentation and sliding friction. Nevertheless, at present the pre-yield region is modelled as a compressible linear elastic solid and, as a result, details of indentation and unloading are not described quantitatively. The straightforward solution is to extend the existing model to include a spectrum of relaxation times in the pre-yield regime, via use of a multi-mode approach. The thus improved model now indeed also quantitatively predicts the indentation response of polycarbonate for different types of indenter geometries. A second drawback of the existing model is that it cannot deal with multiple relaxation mechanisms, as occur in cases where more than one molecular transition contributes to the stress. This behaviour typically manifests itself when high strain rates are applied, demonstrating a change in slope in the dependence of yield stress on the logarithm of strain rate. Solution of this problem requires a model extension by incorporation of a second, additional, flow process with its own non-linearity, that is, a multiprocess approach. A material which manifests this type of mechanical response is poly(methyl methacrylate); a quantitative prediction of its indentation response is achieved. Generally the friction force is regarded to be an additive composition of a deformationand an adhesion-related component, suggesting that components operate and contribute independently. Although decomposition in independent contributions is impossible to verify in an experimental set-up, it can be conveniently studied by using a numerical approach. Simulations with no adhesive interaction between tip and polymer show almost no influence of sliding velocity on friction force, whereas experiments show a significant influence. In case of an additive decomposition, this would imply a rate-dependence of the adhesive component. By inclusion of the Amontons-Coulomb friction law, which creates an interaction between tip and polymer, the suggested additive decomposition is proved not to be applicable and the large macroscopic deformation response proves to be the result of small changes in local processes. When interaction is taken into account, a bow wave is formed in front of the sliding tip, which leads to an increase in contact area between tip and polymer and results in an increase in friction force. As a consequence the experimentally observed time-dependent behaviour of the friction force can solely be attributed to a polymer’s intrinsic deformation response. Furthermore the influence of a polymer’s intrinsic material properties, such as strain hardening and the thermodynamic state, on the friction force can be studied conveniently.