Effects of cyclic loading on subsurface microstructural changes of zirconia polycrystals in nanoscale mechanical processing

Yttria stabilized zirconia (YSZ) is an attractive biomaterial with excellent properties, but its deformation mechanism under repetitive loads is still unclear. In this study, the subsurface microstructural changes of YSZ polycrystals under repetitive loading/unloading were investigated by multi-cyclic nanoindentation. Raman spectroscopy analysis revealed that a tetragonal-to-monoclinic (t-m) phase transformation occurred, and multicyclic indentation promoted this transformation significantly. Cross-sectional scanning transmission electron microscopy showed that monoclinic variants were extensively generated beneath the free surfaces outside the indent planes, whereas grain refinement was dominant beneath the indenter tip. In addition, load-displacement curve analysis demonstrated that the t-m phase transformation occurred during unloading rather than loading, and that the phase transformation-induced volume expansion caused an incremental change of residual indent depth. The extension of monoclinic variants was promoted by repetitive shear deformation, resulting in significant pileups, as detected by atomic force microscopy. The effect of sample surface preprocessing on indentation behaviors was also investigated. Microcracking and surface spalling were confirmed around the multi-cyclic indents made on a diamond-turned YSZ sample. Based on the experimental results, a model of subsurface microstructure distribution and evolution was established. The findings from this study help clarify the historical effects in subsurface damaging mechanism of brittle polycrystalline materials caused by cyclic tool-workpiece interactions in nanoscale mechanical processing.