Nano/macro-hardness and fracture resistance of Si3N4/SiC composites with up to 13 wt.% of SiC nano-particles

Relations between composition and mechanical properties of the Si3N4/SiC micro/nano-composites were studied by combination of nano-indentation and Vickers indentation techniques. The Si3N4/SiC composites were prepared from crystalline Si3N4 powder doped with SiNC amorphous precursor and yttria as the sintering aid. During sintering the SiNC precursor crystallised to yield both SiC and Si3N4. The in situ formed SiC particles were located both inter- and intra-granularly. The presence of SiC nano-particles enhanced the nano- and macro-hardness, and the fracture toughness of the composites. The nano-hardness of Si3N4/SiC composites ranged between 20 and 24 GPa, and depends on the volume fraction of SiC. The nano-hardness of individual Si3N4 grains exhibited large scatter as the consequence of the presence of intra-SiC inclusions, which directly influence the measured values as the harder phase, or by generating large thermal stresses within Si3N4 grains. Consequently the scatter of nano-hardness was much larger than in case of macro-hardness where the measured values are averaged over large area. The nano-indentation of grain boundaries indicates that the boundaries are much softer than the surrounding matrix phase. Apart of indentation size effect (ISE) this is believed to be an additional reason why the measured values of macro-hardness are lower than the nano-hardness. The maximum fracture toughness (5.8 MPa m1/2) was achieved for the composite with the total amount of 8 wt.% SiC, where a percolating network of intergranular SiC particles was formed, as indicated by the measurement of electrical resistivity.