An interface material is deposited on the fibers. This interphase acts as a deflection layer for the matrix cracks. It consists essentially of PyC, boron nitride, or a multilayer ((PyC/SiC)n or (BN/SiC)n sequences). PyC-based interphases have been the subject of extensive studies and have been shown to be the most appropriate with respect to controlling crack deflection and mechanical properties. With the CVI process, the gas precursor is CH4 for carbon, and BCl3 and NH3 for boron nitride. Multilayered inter­phases may be deposited via pulsed CVI.

2.12.3.2 Infiltration of the SiC Matrix:

The CVI Process

The basic chemistry of making a coating and a matrix by CVI is the same as that of depositing a ceramic on a substrate by CVD.13-15 The reactions consist of crack­ing a hydrocarbon for deposition of carbon and crack­ing of methylchlorosilane for deposition of SiC. In the I-CVI process (isobaric isothermal CVI) the preform is kept in a uniformly heated chamber. Temperature and pressure are relatively low (<1200 °C, <0.5 atm).

A few alternative CVI techniques have been pro­posed to increase the infiltration rate.15,28,29 These techniques require more complicated CVI chambers and are not appropriate to the production of large or complex shapes or a large number of pieces.

The forced CVI (F-CVI) technique was proposed in the mid-1980s.29 The precursor gas is forced through the bottom surface of the preform under a pressure P1, and the exhaust gases are pumped from the opposite face under a pressure P2 < P1. The fibrous preform is heated from the top surface and sides, and cooled from the bottom (cold) surface. The densification times are significantly shorter when compared to I-CVI (10-24 h for a SiC matrix, a few hours for carbon), and the conversion efficiency of the precursor is relatively high. However, the technique is not appropriate for complex shapes. Only one preform per run can be processed, and complex graphite fixtures are required to generate the temperature and pressure gradients.

In order to overcome the aforementioned limita­tions of the F-CVI technique, alternative techniques using thermal gradients or pressure gradients have been examined for many years.15 In the thermal gradient process, the core of the fibrous preform is heated in a cold-wall reactor. The heat loss by radia­tion is favorable to get a lower temperature in the external surface. The densification front advances progressively from the internal hot zone toward the cold side of the preform. In the P-CVI process, the source gases are introduced during short pulses.15 The P-CVI process is appropriate for the deposition of thin films or multilayers.

2.12.3.3 Infiltration of the SiC Matrix: