Effect of Crack Healing on the Strength of a Si_3N_4-Material Used for Applications in the Metallurgical Industry

摘要

Among engineering ceramics, those based on dense silicon nitride have outstanding mechanical and thermal properties, making them suitable for the metal handling industry. However, they have some limitations in terms of strength under conditions of severe mechanical or thermal loading, where in general tensile stresses are highest at surfaces and especially at free edges. In part such limitations can be alleviated by improvements in strength through material structural refinement to remove significant internal defects, and through greater care in machining processes which introduce both surface cracks and surface residual stresses. However, there are severe economic barriers to achieving these goals. Previous work [1-3] has shown that subjecting dense silicon nitrides to an oxidizing heat treatment after machining can introduce a measure of surface crack healing. In the present work, using a commercial silicon nitride in an as-ground condition the effects of such a heat treatment have been examined in terms of strength changes observed in flexural strength test specimens. Some significant improvements in strength have been achieved. Microstructural investigations reveal the formation of a thin glassy coating with healing of cracks and in-filling of surface-connected pores that can act as fracture origins. Optimized heat treatment conditions have then been applied to complete rings and to ring segments cut from large tubular components, again demonstrating room temperature strength improvements. This work has demonstrated the clear potential for improvements in mechanical reliability in industrially produced components.