DEPENDENCE OF HIGH-TEMPERATURE TENSILE-STRENGTH ON DISPLACEMENT RATE FOR HOT-PRESSED SILICON-NITRIDE

Tensile tests of hot-pressed silicon nitride were conducted in a wide range of displacement rates from 0.0005 to 50.0 mm min-1 at 1260 and 1380 ° C, and the dependence of strength on displacement rate was discussed in conjunction with the change of fracture surface features. As displacement rate was lowered, strength was degraded and stress-displacement diagrams altered from linearity to non-linearity, accompanying appearances of yielding phenomena. Strength was largely dependent on displacement rate in the high-rate region and rather independent in the low-rate region. The fracture surface features were drastically changed from brittle fracture to subcritical crack-growth fracture with decreasing displacement rate. A further increase in stress following the lower yielding point in the low-rate region was attributed to increments of fracture resistance due to the pull-out contribution of elongated grains. © 1990 Chapman and Hall Ltd.