Effect of internal oxidation on wear behavior of ultrafine-grained Nb-Zr

The influence of surface modification by internal oxidation on dry sliding wear behavior of ultrafine-grained (UFG) Nb-Zr was investigated using a pin-on-disc type tribometer. The results show that improvement in strength by grain refinement via equal-channel angular pressing/extrusion has no substantial effect on the wear resistance of the non-oxidized UFG samples as compared to the coarse-grained material. This was attributed to the complex wear mechanisms operating in this alloy, such as adhesion leading to smearing, tribo-chemical reactions resulting in strong oxidative wear and also abrasion bringing about scratches and deep grooves. However, internal oxidation by heat-treatment significantly improved the wear resistance of Nb-Zr, especially under low to medium applied pressures due to the hardened diffusion zone with ZrO2 nanoparticles formed in the subsurface layer. Moreover, the improvement was more pronounced in the UFG material, which is attributed to increased diffusion in the UFG microstructure. When the applied pressure was increased above 0.5 MPa, however, the wear rate increased considerably due to the elimination of the hardened subsurface layer. Still, even under high pressures, the oxidized samples demonstrated lower weight loss as compared to non-oxidized samples. Based on the investigations of the worn surfaces, it was determined that internal oxidation mostly eliminates the complex wear mechanisms operational in the non-oxidized samples, especially under low/medium loads. Improvement in wear properties by internal oxidation along with enhanced mechanical properties and previously demonstrated good biocompatibility and superior fatigue performance make internally oxidized UFG NbZr a promising candidate for biomedical applications in the human body.