Sliding wear induced subsurface microstructural evolution in nanocrystalline Nb-Ag binary alloys and its impact on tribological performance

Powders of Nb and Ag were first subjected to mechanical alloying to force the formation of supersaturated solid solutions and then compacted to near full density using spark plasma sintering. During the sintering process, Ag precipitated out from the Nb matrix along the Nb grain boundaries, with the size of the precipitates depending on the nominal chemical composition. Pin-on-disk wear tests show that the nanostructured Nb-Ag alloys show good wear resistance. A wear factor of 10⁻⁵ mm³/(N·m) was observed upon sliding against stainless steel 440C. Ag concentration has a significant effect on the wear rate and wear modes of Nb-Ag alloys. For higher Ag concentration with larger initial Ag precipitate size and lower hardness, severe plastic deformation (SPD) during wear induced the spontaneous formation of alternating Nb and Ag nanolayers. These nanolayered structures remain stable up to the sliding surface, leading to a reduction in the wear rate. In contrast, for lower Ag concentration with smaller Ag precipitate size and higher hardness, SPD can dissolve the Ag precipitates and force the formation of supersaturated solid solutions with equiaxed nanograins below the sliding surface. This results in an increase in the wear rate.