Strength properties of structural ceramics at elevated temperatures. Relationship between fracture toughness and fracture strength of smooth specimen.

Flexural strength tests and fracture toughness tests on two kinds of ceramics (silicon nitride and silicon carbide) were carried out by four-point bending at room temperature and elevated temperatures. The fracture toughness was measured by using the chevron-notched specimens and by using the controlled surface flaw specimens which have a semi-circular surface crack induced by a knoop indenter.Furthermore, the fracture toughness of smooth specimen was evaluated by using √area, which is the square root of the area of the defect projected to the direction of the principal stress. These results were discussed in comparison with the fracture toughness values obtained by the chevron-notched specimens and the controlled surface flaw specimens.The results obtained are as follows:(1) The fracture toughness values depended on the method of measurement. The values of silicon nitride and silicon carbide obtained from the chevron-notched specimens were higher than those obtained from the controlled surface flaw specimens at elevated temperatures.(2) The fracture toughness values of silicon nitride obtained from the controlled surface flaw specimens decreased rapidly when the temperature exceeded 1000°C. It was clear that its rapid decrease was caused by slow crack growth.(3) The fracture toughness values of silicon carbide obtained from the controlled surface flaw specimens were influenced by the healing of crack at about 1200°C.(4) The temperature dependency of the fracture toughness value of silicon nitride obtained from the smooth specimens coincided with that of the fracture toughness obtained from the controlled surface flaw specimens.