Abstract: Carbon-carbon composites were densified by SiC from mixing gases of "2 and MTS using chemical vapor infiltration technique. A parametric matrix of various kinetic conditions of temperature, pressure and reagent gas concentration were studied. Temperature is the primary kinetic parameter of the CVI process. The structure of coating is strongly affected by pressure and concentration of reagent gases. In this work, we discover that 950°C is the turning temperature of CVI process.
Introduction Chemical vapor infiltration (CVI) is a densification process used to form the matrix and to fill the voids of a composite by depositing specific materials from a gaseous phase heated in a reaction atmosphere. In CVl densification process, the time required as well as the uniformity for infiltration is strongly dependent on the key process variables such as temperature, pressure, reagent concentration, and flow rate (Lackey, 1990). Therefore, in order to reduce the time required for infiltration and to improve the uniformity of infiltration from part-to-part, we have to understand the influence of these major variables on the process In this work we try to find the optimized key process variables of CVI. With these variables, the specimens can reach the best bulk density, structure and mechanical properties. Experimental Setup Experiments of SiC infiltration were performed on random carbon fiber performs 2" x 1 "x 1/8" in size, with density around I. 25 g/cm and porosity of 39 percent Mixed gases of methytrichlorosilane (MTS) and hydrogen were used as the reagent. These experiments were conducted using a parametric matrix of various kinetic conditions of temperature and pressure and different ratios of gas chemical composition. The mechanical property testing included only the flexural bending test It was tested and compared by static measurement of natural frequency of vibration and by dynamic 3-point bending.