Fracture toughness and reliability in high-temperature structural ceramicsand composites: Prospects and challenges for the 21st Century

The importance of high fracture toughness and reliability in Si3N4, and SiC -based structural ceramics and ceramic matrix composites is reviewed. The p otential of these ceramics and ceramic matrix composites for high temperatu re applications in defence and aerospace applications such as gas turbine e ngines, radomes, and other energy conversion hardware have been well recogn ized. Numerous investigations were pursued to improve fracture toughness an d reliability by incorporating various reinforcements such as particulate-, whisker-, and continuous fibre into Si3N4 and SIC matrices. All toughening mechanisms, e,g, crack deflection, crack branching, crack bridging, etc es sentially redistribute stresses at the crack tip and increase the energy ne eded to propagate a crack through the composite material, thereby resulting in improved fracture toughness and reliability, Because of flaw insensitiv ity, continuous fibre reinforced ceramic composite (CFCC) was Found to have the highest potential for higher operating temperature and longer service conditions. However, the ceramic fibres should display sufficient high temp erature strength and creep resistance at service temperatures above 1000 de greesC. The greatest challenge to date is the development of high quality c eramic fibres with associate coatings able to maintain their high strength in oxidizing environment at high temperature. In the area of processing, cr itical issues are preparation of optimum matrix precursors, precursor infil tration into fibre array, and matrix densification at a temperature, where grain crystallization and fibre degradation do not occur. A broad scope of effort is required for improved processing and properties with a better und erstanding of all candidate composite systems.