A tough SIAION ceramic based on alpha-Si3N4 with a whisker-like microstructure

Silicon nitride (Si3N4) is a light, hard and strong engineering ceramic(1,2). It can withstand harsh environments and support heavy loads at temperatures beyond those at which metals and polymers fail. It can also be manufactured reliably at a reasonable cost and in large quantities. There are two forms of silicon nitride(3): alpha-Si3N4 and beta-Si3N4. The former is harder, but only the latter is currently used in engineering applications, because only this form can be given a microstructure resembling a whisker-reinforced composite(1,2,4), which gives it the necessary toughness and strength. Here we report the synthesis of a tough alpha-Si3N4 solid solution with this kind of microstructure. This material is 40% harder than beta-Si3N4 and is equally strong and tough. Its hardness (22 GPa) is exceeded only by boron carbide and diamond (which are both brittle). These properties mean that this form of alpha-Si3N4 Should be preferred over beta-Si3N4 for all engineering applications, and it should open up new potential areas in which the ceramic can be applied.