A development of new displacement measuring system for evaluating the strength of fine ceramics at elevated temperatures

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It is needed to measure precisely the displacement produced on a specimen by loading at elevated temperatures for the reliable evaluation of the strength of engineering ceramics. The present study aims to develop the high temperature strength testing system of engineering ceramics which is equipped with the precise displacement measuring system newly devised : The testing system was constituted by the laser beam type displacement measuring system, the specially designed electric furnace, the jigs made of Silicon-Nitride ceramic and Ni-base super alloy, and the closed loop type material testing system. The main results obtained in the four-point bending test of the Silicon-Nitride ceramic by using this testing system are summarized as follows.1. The displacement measuring system developed in the present study makes it possible to detect the displacement of 200-300 um yielded on the Silicon-Nitride ceramic with the precision of about 2% of the displacement measured at test temperatures over 1,000゜C.2. The silicon-nitride ceramic shows the linear relutionship between load P and displacement at test temperatures below 1,100゜C . The P vs. relation, however,deviates from the above linear relationship in the positive direction of displacement axis. This suggests that the Silicon-Nitride ceramic produces plastic deformation at these temperatures.3. The flexural strength remains unchanged in test temperature range from room temperature to 1,000゜C. This reaches the maximun value around 1,100゜C, and then decreases drastically at test temperatures over 1,200 ゜C. The fracture toughness measured by the chevron notch type specimed is nearly constant over test temperatures from room temperature to 1,100 ゜C, and then this reaches the minimun value around 1,100 ゜C. Thus, the Silicon-Nitride ceramic exhibits the minimum fracture toughness around test temperature where the flexural strength reaches the maximum.