Study Of CVD SiC Coatings Defects Control And Oxidation Behaviors Of C/SiC Composites

Continuous carbon fiber reinforced silicon carbide ceramic matrix composites(CYSiC) are considered as the most promising thermal structural candidate materials for aerospace, energy and nuclear technologies etc. As a key technology for improving high-temperature structural applications of the materials, oxidation protection with anti-oxidation coating system had being paid more and more attentions. And CVD SiC has been wildly used act as oxidation protection coatings materials. But there were little attentions paid for coatings defects control. Ground on the knowledge of the form mechanisms of the coating defects, the research work proposed some thoughts related to preparation/deposition process to control the coating defects. Based on observation of micro-morphologies of the coatings, combined with relative weight change in oxidation, the effects of these modifications on coating defects control and the behaviors of 2D C/SiC at constant temperatures in air condition were investigated. The main contents and conclusions are as follows:1. The behaviors of 2D and 3D C/SiC both protected by a normal multi-layer CVD SiC coatings at constant temperature in air condition were investigated. Both materials showed an identical oxidation kinetics behavior.2. Effects of different coating modification process on coatings defects control and oxidation behaviors of C/SiC were studied. The main results are: grinding is favorable to improve surface smooth degree, while nitrided and slow deposition makes SiC granules fine; The width of coatings gap increased in order of grinding, nitrided and vacuum heat treatment, but gap defects in multilayer coatings could be removed by slow deposition; Temperature of maximum weight loss could be decrease to 600℃ by grinding, vacuum heat treatment or slow deposition, but it will increased to 800℃ after nitrided; Oxidation kinetics curves all varied with the coating modifications.3. The gap defects in multilayer CVD SiC coatings could be effectivelycontrolled by means of slow deposition in low-pressure chemical vapor deposition (LPCVD). 2D C/SiC composites with a multi-layer CVD SiC coating prepared by slow LPCVD showed weight gain at 1300@ in air condition.4. Products on CVD SiC after oxidation were investigated using XRD. Crystallization was observed in silicon dioxide film produced by oxidation of CVD SiC after oxidation in air above 1100@. From XRD pattern, a strong diffraction peak of a-cristobalite was observed.