Ceramics for High-Performance Applications. 18. The Strength of Silicon Nitride after Exposure to Different Environments.

摘要

The room temperature fracture strength of HS-130 hot-pressed silicon nitride was determined by testing 25 specimens in four-point bending. Large statistical scatter, typical of such strength measurements, prompted the development of a test method that would allow strength determination using only a few specimens. It was found that a crack could be reproducibly formed by making a Knoop hardness indentation on the tensile surface of a bend specimen. Scatter in fracture strength was reduced to under 3 percent, and measurements of the crack dimensions were used to calculate fracture surface energy by fracture-mechanics techniques. Fracture strength was found to increase after exposure at temperatures above 1173K, relatively independent of the exposure environment. Elevated-temperature exposure to 0.00001 torr-vacuum, 10 to the minus 15 and 150-torr oxygen, and 760-torr nitrogen atmospheres did not cause any measurable change in the crack dimensions. The strength increase is assumed to result from a decrease in residual stress in the vicinity of the crack tip due to localized plastic deformation. The highest values of fracture energy were measured on specimens annealed above 1500 K, and these values were close to the 39 J/m superscript 2 obtained by other investigators. (Author)