Contact fatigue behaviour of glassy polymers with improved toughness under fretting wear conditions

Semi-quantitative fatigue wear models have been largely derived in order to account for the wear resistance of polymers substrates sliding against rigid counterfaces which are insufficiently rough for the elastic limit of the polymer to be exceeded during micro-asperity deformation. Direct experimental evidence of such contact fatigue processes, however, are scarce: propagating cracks in polymers are difficult to detect on an asperity scale because of the elastic recovery. As a result, the exact nature of the contact damage micro-mechanisms (crack orientation, crack propagation rates, …) and the way they relate to the bulk material properties remain largely to be established. In this investigation, contact fatigue processes within glassy amorphous polymers contacting smooth glass spheres have been studied under small amplitude oscillating micro-motions, i.e. fretting. Under such contact conditions, it was found possible to induce the early development of a crack network at a macroscopic scale without the complications arising from the extensive generation of wear debris at the contact interface. This approach has been applied to homologous polymer series differing by their toughness properties. Two kinds of systems were considered: (i) epoxy networks modified by antiplasticizing additives; (ii) copolymers of methylmethacrylate with either glutarimide or cyclohexylmaleimide moieties. Crack nucleation and propagation micro-mechanisms within these various polymer materials have been investigated from the resources of in situ contact visualisation during the fretting tests. From a detailed mechanical analysis of the local contact conditions, the relationships between the development of cracks and the toughness properties of the polymers have been established.