Development of ceramic matrix composites for application in Ceramic Technology for Advanced Heat Engine program

The objectives of the program are to develop an advanced toughened silicon nitride composite and a process for near net shape part fabrication. This program was initiated in 1985. The initial phase of the program considered particulate and whisker-reinforced composites and examined the effect of TiC and SiC dispersoids on fracture toughness of Si{sub 3}N{sub 4}. The best results were obtained with whisker reinforcements. Silicon carbide whisker-reinforced Si{sub 3}N{sub 4} was selected for further development. A predictive model that relates microstructure-fracture toughness dependence developed and scrutinized in the course of these studies has shown that fracture toughness of polycrystalline ceramics could be affected by changes of grain size and shape as well as strength of intergranular bond. Accordingly, it was shown that a deflection/debonding mechanism could utilize Si{sub 3}N{sub 4} whiskers to toughen Si{sub 3}N{sub 4} body. Si{sub 3}N{sub 4}-SiC composites offer a number of distinct advantages over monolith,'' which in addition to their improved thermal shock and wear resistance (due particularly to improved conductivity and hardness), Si{sub 3}N{sub 4}-SiC composites are tailorable with respect to high-temperature properties. It was considered that in heat engine applications, Si{sub 3}N{sub 4}-SiC whisker composites, due to their higher hardness, thermal conductivity, thermal shock,more » and wear resistance, have a definite advantage and warrant further development. In the current program (Phase 11), 1987--1989, the properties were further improved to achieve nearly two times higher fracture toughness and considerably improved elevated temperature (1400{degrees}C) strength of the composite. These improvements were obtained through optimization of processing and modifications of composite phase assembly, primarily intergranular phase.« less