Microstructure, Deposition Characteristics, Crystallization And Oxidation Behavior Of CVD B_xC Coatings

When the continuous fiber reinforced silicon carbide matrix composites (CMC-SiC) are exposed to oxidizing environment over 450℃,the life of them can be shortened due to the oxidation of the fibers and the PyC interlayers between fibers and matrixes.The B_xC coatings/matrixes prepared by CVD/CVI technique can be oxidized into liquid B₂O₃ at the temperature range of 600-1000℃in oxidizing atmosphere,which can seal cracks to prevent these ceramic materials from oxygen attacking.In present thesis,a systemic study was carried out on the CVD B_xC coatings. Through the microstructure analysis of these coatings deposited under various conditions,the relationship between deposition parameters and microstructure and composition was established,and the deposition mechanism was explored.Finally, the crystallization and oxidation behavior of the B_xC coatings was investigated.The main studied contents and results are as follows:1.The microstructure and phase composition of CVD B_xC was studied.Based on the microanalysis results,two kinds of CVD B_xC coatings were found:the one showed a brick-like morphology,with a high content of B and without free carbon, and possessed a certain degree of crystallinity;the other presented a cauliflower-like morphology,with a low content of B and a certain content of free carbon.They were amorphous state.2.The relationship between deposition parameters and microstructure/ composition was researched,and the deposition mechanism was explored.The results obtained are showed as follows:Deposition temperature and the flow ratio of BCl₃/CH₄ had effect on the microstructure or phase composition of the B_xC coatings, but the flow ratio of H₂/CH₄ had little effect on them;The B_xC coatings were deposited through two kinds of mechanisms.3.The crystallization and oxidation behavior of the CVD B_xC coatings was studied.The B_xC coatings were subjected to following experimental conditions: 1600-2000℃,Ar atmosphere;400℃,700℃,1000℃,1200℃,dry oxygen atmosphere;700℃,1000℃,1200℃,wet oxygen atmosphere;Simulated service environment.After these treatments,the microstructure and phase composition was investigated,and the following conclusions were obtained:In inertial atmosphere,the B4C grains and graphite carbon would formed in the coatings at high temperature,and the grain size and graphitization degree increased with the temperature;In dry or wet oxygen atmosphere,this kind of coatings could protect the matrix materials from oxygen attacking when the temperature was below 1000℃,especially,at 700℃(moderate temperature),the coatings showed good self-healing property;At simulated service environment,the microstress could be produced in the coatings,which would resulted in the formation of microcracks.