Plastic deformation and cracking resistance of SiC ceramics measured by indentation

Hardness is deemed as a measurement of resistance to plastic deformation of materials, but cracking, accompanying the deformation of ceramics, has recently been reasonably interpreted as one of the essential contributors to the widely known size effect of hardness, also called the indentation size effect. On the basis of the physical activities behind the indentation size effect, we intend to have the indentation size effect measurement adapted as a means to quantitatively measure the plastic deformation and the cracking damage of ceramics. We studied three types silicon carbide ceramics that were manufactured from same chemical formulation and processing route. Two of the three were given a post-sintering annealing at different dwell times, giving a distinctive microstructure from homogeneous equiaxial grains to highly heterogeneous elongated grains. By fitting the indentation size effect data with proportional specimen resistance model, the a1 and a2 parameters are extracted with good confidence. The a2 value is interpreted as the “true hardness”, the determined values of which were found to be similar for all samples. However, the a1, relatable to the indentation size effect, increased dramatically together with the indentation-induced cracking around Vickers indents observed under SEM. Considering that the cracking resistances of all three samples showed limited variance, we discuss, using a simple illustrative model, the possible microstructural factors which may contribute to the cracking damage exhibited under a defined loading condition

Contact damage of silicon carbide ceramics with different grain structures measured by Hertzian and Vickers indentation

We have used Hertzian and Vickers indentation to investigate contact damage in sintered SiC ceramics, one consisting of uniform, fine-grains and the other a coarse, elongated grain structure. Cracking-resistance measured by Hertzian indentation, showed no discernible difference, nor did the Vickers hardness. However, numerical analysis of the Vickers indentation size effect, performed using the proportional specimen resistance model, indicates 77.3% greater surface energy, mostly realised through cracking, is experienced by heterogeneous SiC per unit area of indentation impression. This is typified by an observable increase in the number of radial cracks generated around Vickers impressions, which has been found to artificially increase the K IC determined by Vickers indentation fracture. Quantitative measurements of pre-existing flaws by Hertzian indentation show that heterogeneous SiC retains a higher density of larger flaws. Relationships between the differences in cracking around Vickers indents and the pre-existing flaw populations of these two SiC ceramics are discussed