Preparation of an oxidation protection coating for c/c composites by low pressure chemical vapor deposition

Preparation defects in a SiC coating on C/C substrates are considered to be the controlling factor which decreases the oxidation protection life at the temperature of preparing. To decrease the number of such defects, a low-pressure chemical vapor deposition (LPCVD) method, instead of general CVD was used, and the substrates were suspended, rather than supported. Multi-deposition was employed to seal the preparation defects at the edges of the substrates produced by the suspension. The SiC coating, which was of a high quality, had a smaller crack width and a better interfacial binding. Every layer in the multi-layer coating was uniform and smooth, and the layer thickness could be easily controlled. The oxidation tests indicated that the preparation defects in the coating could not be sealed by multi-deposition as the gaps between the multi-layers acted as channels for oxygen diffusion towards the defects in the inner layer. The weight loss decreased as the multi-deposition times were increased. An analysis showed that no matter how many layers the coating consisted of, failure always began from the preparation defects in the inner layer which were located at the edges of substrates, and a cavity would be formed beneath the inner layer by direct oxidation of the C/C. The oxidation process in C/C substrates with a multi-layer coating was controlled by the rate of oxygen diffusion along the interlayer gaps which were thinner and longer than the defects. The more the multi-deposition times, the larger the length of the gaps, and the smaller the oxygen transport rate.