Grain Size Dependence of the Fracture Toughness of Silicon Nitride Ceramics

Abstract

Improved K_IC values of monolithic Si₃N₄ ceramics will be expected by growing-in a matrix of microcrystals of β-phase Si₃N₄-an appreciable amount of extensively large, rod-shape crystals of the same phase. It is anticipated that the large grains should play as crack impediments like whiskers mixed in composite ceramics. The test materials prepared by changing firing conditions had such well-grown crystals of different sizes, ranging from 2 to 10 microns in diameter. The fracture toughness (measured by the SEPB method) increases up to 11.3MPa·m^1/2 with the increase in the grain size. Prominent crack bridging along the fracture surface behind the propagating crack tip is found to be the possible toughening mechanism.