Effect of grain boundary chemistry and grain morphology on the mechanical behaviour of silicon carbide

Abstract

Silicon carbide based materials are being increasingly used as a room temperature structural material. Unfortunately, there have been very few studies of the mechanical behaviour of SiC at room temperature especially studies incorporating slow crack growth. In order to address this gap, this thesis will focus on room temperatures only. SiC materials are produced with a wide range of grain boundary chemistries and grain morphologies with the aid of sintering additives. Therefore, in this thesis two different chemistries were produced using solid state (SS-SiC) sintering with carbon and boron or liquid phase sintering (LP-SiC) using alumina and yttria, materials with fine and coarse grains were produced for each chemistry. The fracture toughness at room temperature is improved by a factor of two using LP- sintering and by introducing elongated grains. The toughening mechanisms responsible for this improvement are discussed. The magnitude of the short range closing forces becomes larger as grain size increases irrespective of chemistry. This was expected for materials with intergranular failure, but not for materials that exhibit transgranular failure. FIB technique was used to investigate the toughening mechanisms of short range cracks and new findings are proposed. Slow crack growth (SCG) investigations in air and in water at room temperature show that the presence of glass pockets and oxide complexions makes LP-SiC materials prone to stress corrosion, while materials such as SS-SiC with no glass or oxides in their microstructure are immune to SCG. Possible suggestions are discussed to quantify the effect of grain size on SCG. For all the studied materials no apparent cyclic fatigue in air was observed. This was expected for brittle materials such as SS-SiC, but not for LP-SiC. In order to clarify the significance of the measured mechanical properties, a lifetime analysis in the presence of artificial and natural defects is presented.