Strength Evaluation of Sintered Silicon Nitride Ceramics by means of a New Test System at Elevated Temperatures

A test system for precisely evaluating the tensile strength of ceramics at elevated temperatures was newly developed. Four-point bending and tensile tests were performed on sintered silicon nitride ceramics in the temperature range from room temperature to 1300°C. The track of a ductile fracture was observed on the fractured surfaces of the tensile specimens tested at temperatures above 11OO°C, suggesting the occurrence of plastic deformation. The fracture stress of bending specimens was about 1.18-1.43 times as large as that of tensile specimens. The test temperature dependency, however, was very similar in both the fracture stresses throughout the entire test temperature range. The fracture toughnesses from tensile and bending specimens were almost the same in the test temperature range from R.T. to about 800°C. The fracture toughness obtained from the chevron-notch-type specimen was almost coincident with that from the Knoop-indented specimen in the temperature range from R.T. to 900°C. There was, however, some difference at temperatures above this, suggesting the occurrence of adhesion at the crack tip in the Knoop-indented specimen.