Synthesis Of Boron Nitride Coatings By Chemical Vapor Deposition Processes

Boron nitride(BN) coatings with(002) planes are parallel to the surface of the substrates, which can form a weak interface in ceramic material composites(CMCs).It has some advantages, such as better oxidation resistance, thermal shock resistance and chemical stability. Therefore, BN has been an optimum choice for interphase materials and is successfully used in various composite systems, especially in SiCf/SiC composites. BN produced by chemical vapor deposition(CVD) is smooth and continuous. Besides the thickness of BN interphase is easily controlled and less damaged to the substrate fibers. Therefore, the CVD process is widely used in the preparation of high quality BN interphase. But the main problem is that how to keep the process stable, which is caused by the complex reactions between BCl3 and NH3 at high temperatures. And little is known about how BN is produced. What’s more, it is still a great challenge to improve the crystallization degree of BN deposited at low temperatures. Our research is focused on the process of BN interphase coatings and the effect of deposition parameters. The deposition mechanism of BN and the effect of post heat-treatment temperatures on the crystallization degree is also studied. The main contents are as follows:1. The deposition rates defined by mass increment as a function of reactor length were measured at different deposition temperatures and residence times using BCl3 and NH3. It found that the deposition rates were variable under different deposition parameters and reached a maximum at a deposition temperature of 650 ℃, a residence time of 0.24 s.2. Based on the study for pyrocarbon deposition the gas-phase reaction and surface reaction during the deposition of BN was analyzed according to the results of deposition rates varied. And a deposition mechanism scheme was built. The results demonstrated that BN was not directly deposited from BCl3 and NH3 but the intermediate products in the gas phase. During the deposition intermediate productswere absorbed on the surface that resulted in the surface reactions products, among which Cl2BNH2 was beneficial to the deposition of BN.3. BN interphase coatings were successfully deposited on Si C fibers by CVD and dip-coating method and its microstructures and phase compositions were investigated.The coatings produced by CVD at low temperatures transformed to h-BN after heat-treatment and its crystallization degree was improved with increasing temperatures. Through the characterization of TGA and filament strength test, we confirmed that CVD-BN coatings can improve the thermal stabilities of SiC fibers and keep the tensile strength of SiC fibers. By means of studying the two methods, we found that CVD process had been the first choice to process uniform and continuous BN interphase coatings.In summary, a method of synthesizing highly crystallized BN coatings by CVD at low temperatures and low pressures is introduced. The deposition mechanism of BN using BCl3 and NH3 is built and a stable deposition parameter for BN deposition has been obtained. The research method in this paper gives a possibility for the future study of BN deposition.