Indentation strength of silicon nitride ceramics processed by spark plasma sintering technique

We investigated the influence of the microstructure on the true stress-strain curve of silicon nitride based ceramics. The materials were processed by spark plasma sintering technique. Si3N4 with fine, average and coarse microstructures were obtained. Load versus displacement curves (P-h) were obtained by means of instrumented indentation technique using diamond coni-spherical tip. The experimental data were coupled with a minimization method based on the Levenberg-Marquardt algorithm and the non-linear part of the mechanical response was identified. Based on the obtained stress-strain curves, rolling contact simulations were performed. In addition, the nature of Hertzian contact damage was examined in the material with coarse microstructure using diamond indenters of radii 0.2 and 1 mm. The surface damage was observed under optical microscopy while Focused Ion Beam Sectioning technique permitted to image the subsurface damage. An evident size effect was noticed: fracture consisting of classical ring cracks dominated at large scale while distributed microcracks beneath the indent dominated at small scale.