The wear behaviour of ion implanted biomaterials

Abstract

The tribological performance of biomaterials used for artificial joints is of much importance, and require low coefficients of friction, resistance to wear and the ability to withstand many millions of cycles under a multitude of loading regimes. Currently used material combinations include Ti6A14V, 316L stainless steel and Co-Cr-Mo articulating against UHMWPE. Although typical wear rates are low (60 mm(^3)/10(^6) cycles), the UHMWPE wear debris produced during articulation has been linked to osteolysis, leading to loosening of prostheses and necessitating revision surgery. This study aimed to characterise the surfaces and quantitatively assess the tribological performance of such biomaterials when surface modified by N(^+) ion implantation. Beyond this, investigation of the physical effects of the N(^+) ion implantations were carried out with a view to determination of an optimum ion implantation protocol. The tribological performance of the materials, were quantitatively assessed using multidirectional pin-on-plate wear testing. Surface characterisation of the materials, were studied using a combination of optical microscopy, AFM, non-contacting interferometry, SEM, and XPS. A significant increase in the surface microhardness of the modified materials was measured post ion implantation. This was attributed to the formation of ion implantation induced lattice disorder and hard phase nitride precipitates on the metallic surfaces, and cross-linking incorporating new formed chemical bonds on the polymeric surfaces. N(^+) ion implantation with 5 x 10(^15) N(^+)ions/cm(^2) significantly enhanced the wear resistance of UHMWPE by ≈ 55 % when articulated against 2 x lO(^17) N(^+) ions/cm(^2) implanted Ti6A14V; by ≈ 48 % when articulated against 2 x lO(^17) N(^+) ions/cm(^2) implanted stainless steel; and by ≈ 48 % when articulated against 2 x 10(^17) N(^+) ions/cm^ implanted Co-Cr-Mo. The technique of ion implantation offers potential as a modification method, to improve wear resistance of these biomaterials for articulating applications such as in total joint replacement.