Mechanical properties of hot isostatically pressed Si₃N₄ and Si₃N₄/SiC composites

The mechanical properties of hot isostatically pressed monolithic Si₃N₄ and Si₃N₄−20 vol. % SiC composites have been studied by microindentation at temperatures up to 1400 °C. Indentation crack patterns and microstructures have been examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. It is shown that dense Si₃N₄ base materials can be synthesized by HIPing without densification aids. Both the monolithic Si₃N₄ and the Si₃N₄/SiC composites exhibit high hardness values which gradually decrease with increasing temperature. Both types of material show low fracture toughness values apparently because of strong interfacial bonding. On the other hand, the fracture toughness of the composite is about 40% higher than that of the monolithic material, due to the presence of the 20 vol. % SiC whiskers. A crack deflection/debonding mechanism is likely to be responsible for the higher toughness observed in the composite. High resolution electron microscopy shows that the grain boundaries in both samples contain a thin SiO₂ layer.