Spherical instrumented indentation of porous nanocrystalline zirconia

Nanoindentation tests with a spherical tip were performed to analyze the stress–strain response on 3-mol%-yttria-doped tetragonal zirconia polycrystals produced by spark plasma sintering (SPS) with porosities in the range 2–21% and nanometric average grain sizes in the range 65–120 nm. Indentation stress–strain curves were obtained by using load (P)–displacement (h) data. Onset of elasto-plastic transition was determined by Hertz fits on P–h curves. Elastic modulus obtained from spherical indentation was similar to the values obtained by Berkovich indentation, with a slight difference with increasing porosity. Microstructural characterisation shows no cracks around and beneath the indentations. Raman spectroscopic analysis of the residual indentation imprints reveals tetragonal to monoclinic phase transformation in the most porous material, whereas no phase transformation was detected in the dense material in spite of its larger grain size. The results are discussed in terms of porosity, grain size and tetragonal-monoclinic transformability.