Discrepancies in the hardness data and the role of grinding-induced surface effects for a porous zirconate ceramic

The conventionally sintered La2Zr2O7 (LZ) ceramic surfaces illustrated notable structural and microstructural changes post grinding and polishing operations. X-ray diffraction studies confirmed presence of in-plane compressive strain on polished surfaces; no phase segregation/separation was noted at surface or bulk level. The scanning electron micrographs revealed that the LZ ceramic grains on the polished top surfaces accommodated this strain by changing their size distribution from unimodal (observed for fractured and unpolished LZ surfaces with average size of 1.2 +/- 0.5 m) to a bimodal (with two modes occurring at 270 nm and 840 nm, respectively) distribution. Possibly, this grain size refinement led the pathway for obtaining surprising high hardness when probed via nanoindentation (13.8 GPa at 4 mN load). This hardness data made this 85% dense LZ sample comparable to its high density (>98%) counterpart (13.8 GPa). The hardness values were noted to have a strong dependence on the penetration depth (h(max)). For h(max) values beyond 500 nm (and loads30 mN), the hardness values depicted a significant decline (by one order of magnitude) and lowered down to 1.8 GPa for 30 mN load. The strain-induced microstructural changes and the strong dependence of the hardness data (with h(max)) paves the path for an in-depth understanding of the mechanism by which the grinding-induced strain is modifying the grain structures; especially for the smaller (mode I) grains on the polished LZ surface.