Morphological studies relating to the fracture stress and fracture toughness of silicon nitride

Microstructure and mechanical properties of HP (Hot Pressed), HP/GP (Gas Pressed), and HP/HIP (Hot Isostatic Pressed)-Si3N4 are studied using scanning electron microscopy; bending tests and the indentation fracture method. The grain diameter distribution is analyzed to clarify the relationship between microstructure and mechanical properties; and also the bending strength and fracture toughness. It is shown that bending strength increases with decreasing grain diameter. The results also show that a Hall-Petch type of relationship is obtained between grain diameter and fracture strength. The fracture toughness shows a linear relationship with sigma(F) root beta(a), where sigma(F) = bending strength, beta = a proportionality factor and d(a) = average grain diameter, and is closely related to the aspect ratio of Si3N4 grains. It is concluded, from the morphological analysis, that a microstructure composed of Si3N4 grains, with both a small grain diameter and a large aspect ratio, is effective in improving both the fracture strength and fracture toughness.