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In this paper, the effect of water on the friction and wear of a carbon fiber reinforced epoxy tested under reciprocating sliding against a stainless steel counter body is reported. The tribological behavior of unidirectional carbon fiber reinforced epoxy composite was investigated in ambient air and in demineralized water, and the role of water on the (sub-) surface degradation is discussed. The effect of sliding direction relative to the fiber orientation has been studied. The correlation between the debonding of carbon fibers at the fiber-epoxy interface, and the wear behavior of the carbon fiber composite are discussed based on an indepth analysis of the worn surfaces done by environmental scanning electron microscopy, white light interferometry, atomic force microscopy, and focused ion beam. We demonstrate that the carbon fiber reinforcement greatly improves the tribological properties of epoxy under sliding in both dry environment and demineralized water. A reciprocating sliding performed along an anti-parallel direction to the fiber orientation under dry conditions results in a large degradation by debonding and breaking of the carbon fibers compared to sliding in 2 parallel and perpendicular directions. Immersion in water has a harmful effect on the wear resistance of the carbon fiber reinforced epoxy composite. The competition between crack growth and the wear rate of epoxy matrix and/or carbon fibers in the sliding track determines the level of material loss of the composite in both test environments.

The impact of processing-induced chain orientation on the sliding wear behaviour of ultrahigh molecular weight polyethylene (UHMWPE) was investigated. The orientation of the molecular network of UHMWPE was done by means of uniaxial tension up to different residual strains. We found that high residual strain levels (higher than 0.45) enabled the sliding dissipated energy of UHMWPE to be decreased in dry conditions. In particular, oriented UHMWPE with a residual strain of 0.85 exhibited, at 500 000 sliding cycles in dry conditions, a decrease in volumetric wear loss by a factor of 3.3 and 19.4 compared with the reference UHMWPE tested in directions parallel and perpendicular to the chain direction, respectively. It is argued that oriented UHMWPE exhibits less adhesion during interfacial wear than the reference material, and hence orientation of UHMWPE bulk may be an alternative treatment to crosslinking for dry sliding conditions. In the case of sliding testing conducted in Ringer's solution, the benefit of the initial chain orientation was quite weak due to a lubrication effect of the solution that markedly limited the effect of chain orientation on the sliding behaviour.