Surface pre-oxidation improves the wear performance of Si3N4 against UHMWPE

The present research was aimed at understanding the tribological properties of silicon nitride (Si3N4) ceramic, as a candidate material for artificial femoral heads, before and after surface pre-oxidation under dry and wet wear testing conditions. Si3N4 ceramic heads were systematically tested against ultra-high molecular weight polyethylene (UHMWPE) spherical pins under controlled conditions in order to substantiate if and how their surface pre-oxidation could improve the wear behavior of the couple in the long term through slowing down the degradation processes occurring in the softer UHMWPE counterpart during wear loading. The results of this research suggest that a pre-oxidative cycle conducted on Si3N4 heads not only improves the tribological properties of Si3N4/UHMWPE couples, but also reduces oxidation of the UHMWPE counterpart under wet conditions. The reasons for this peculiar behavior were attributed to the presence of a silica layer that fills residual roughness on the surface and leads to the formation of an orthosilicate layer in a lubricated tribolayer. This, in turn, decreases and stabilizes the friction coefficient, and leads to an overall improved performances of Si3N4/UHMWPE couples.