Rapid vapor-phase densification of refractory composites

The status of vapor-phase routes for the rapid densification of high-temperature composite materials, primarily ceramic-matrix composites, is reviewed. Conventional densification of composites such as carbon-carbon and SiC-SiC is accomplished by isothermal, isobaric chemical vapor infiltration (CVI), either alone or in combination with liquid resin impregnation and thermal annealing. These are multi-step processes which take from several hundred to thousands of hours at high temperature. In this paper we review approaches designed to significantly reduce the processing time and the number of steps required for densification, while producing materials with the desired properties. We describe techniques such as inductively-heated thermal-gradient isobaric CVI, radiantly-heated isothermal and thermal-gradient forced-flow CVI, liquid-immersion thermal-gradient CVI and plasma-enhanced CVI. Different heating methods, such as radiative and inductive, and both hot-wall and cold-wall reactors are compared. Available material properties of composites produced by these techniques are given.