A study on the tribological behaviour of ultra-high molecular weight polyethylene (UHMWPE) coated with a Ni-P layer

The tribological behaviour of ultra-high molecular weight polyethylene (UHMWPE) has been investigated using friction and wear tests at room temperature (∼25°C) and also at a low temperature (∼-20°C) in air, vacuum or CO₂ saturated vapour for UHMWPE rubbing against itself and also against a steel counterpart. A sticking phenomenon took place in saturated CO₂ vapour at low temperature (∼-20°C), which was produced by severe adhesion between the UHMWPE polymer rubbing pair. The sticking phenomenon was prevented by a Ni-P coating deposited on the surface of the UHMWPE by means of high-speed electro-plating. Non-continuous transfer films of UHMWPE on the Ni-P surface layer and the steel surface were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The studied coating has a solid lubrication character, so that its sliding friction coefficient μ and relative wear weight loss of the pin w_t decreased; the wear mechanism changed from adhesive wear to surface fatigue wear. It is concluded that the tribological behaviour of the UHMWPE polymer could be improved with a metallic coating, such as the Ni-P coating used in this study.