Joining Parameters,Microstructure And Mechanical Properties Of Silicon Carbide Ceramic Joints

Continuous silicon carbide(SiC)fiber-reinforced SiC matrix composites(SiC/SiC)is one of the most important high temperature structural material.Although the available manufacturing processes are capable of fabricating nuclear-grade SiC/SiC composites,it is still very difficult to fabricate large or complex SiC/SiC components due to the limitations of technology and condition.To overcome the technological hurdles,it is necessary to develop SiC joining technology and to research its joining mechanism.Since SiC/SiC composites will be deposited a CVD-SiC layer on their surfaces in practical applications,the joining issue of SiC/SiC composites can be treated as similar to that of SiC ceramics.In this study,with the purpose of developing joining technologies that can be used in high temperature environments,the joining parameters,joint microstructure and properties of diffusion bonded and brazed SiC joints were systematically investigated.The main research consequences are drawn below:The high-melt point Ta-5W foils were selected as the interlayer to join SiC ceramic by spark plasma sintering(SPS).The relationship of joint microstructure and strength to joining parameters was investigated.It was found that the diffusion constant of the SiC/Ta-5W interface was about hundred times larger than that obtained by hot-pressing(HP),and the activation energy of the SiC/Ta-5W interface was only one-tenth that of HP.The interfacial reaction phases were closely related to the thickness of the reaction layer,bonding temperature and holding time,and the thickness of the reaction layer and interfacial defects were major factors that determined joint strength.(Ta,W)5Si3 with high coefficient of thermal expansion(CTE)had the highest content in the reaction layer,which was responsible for the degradation of joint strength in the joints with thick reaction layers.With the purpose of decreasing the joining temperature of SiC/Ta-5W/SiC joints,and remaining higher joint strength at the same time.We proposed Ti/Ta-5W/Ti multi interlayers to join SiC.When inserting Ti foils between SiC and Ta-5W,the expected Ti3SiC2 formed at the interface instead of harmful(Ta,W),Si3,thereby reducing the optimal joining temperature of SiC/Ta-5W/SiC joints from 1600? to 1300?simultaneously remaining high joint strength(123.5±25.3 MPa).In addition,it was found that the interlayer with thick thickness or more number required higher joining temperature to achieve a tough bonding.Excellent interface bonding between the tungsten(W)foil and the SiC substrate could be achieved under the bonding condition of 1300? for 10 min by the mean of SPS joining.The reaction layer of W/SiC consisted of W5Si3 and W2C two phases,and the reaction layer had a close modulus to the W foil.The joint strength of W/SiC reached?9.52±2.68 GPa assessing by micro-cantilever bend test.The effects of addition of Y in the 71Si-29Cr(wt.%)eutectic brazing alloy on the microstructure and shear strength of SiC joints were investigated.With the help of appropriate Y addition(0.1 wt.%),it was capable of retarding the grain growth and enhancing the area fraction of eutectic zone,thereby improving the joint strength.The maximum joint strength could be enhanced 20%compared with the joints that using the brazing alloy without the addition of Y.More importantly,brazing of complex shape SiC was achieved by using appropriate Y enhanced 71Si-29Cr(wt.%)eutectic brazing alloy.