Wear behaviour of NiTi shape memory alloy after oxygen-PIII treatment

Super-elastic Nitinol (NiTi alloys) is predestined for major biomedical applications as its mechanical properties are more closely matched to those of living bones than titanium alloys. Using suitable surface modifications, it is possible to form pure titania (rutile) surface layers, thus prohibiting the outdiffusion of toxic nickel cations. The wear behaviour and the lifetime of such surface layers, produced by oxygen plasma immersion ion implantation (PIII) at 25 kV and 250–550 °C sample temperature, are measured and compared to that of untreated NiTi. Ball-on-disc tests with intermittent impact loading revealed a specific wear volume of the treated samples comparable to that of the base material, slightly lower and higher depending on the process conditions. An increased fatigue lifetime was found for lower temperatures and higher oxygen fluences, indicating that the layer thickness is not the decisive factor. Instead, internal stress relaxation and atomic rearrangement are proposed as the dominant mechanism.