Microstructure And Properties Of The Joints Based On Si-CaO/Al₂O₃-Si Laminated Layer For Diffusive Bonding Of C/C Composites

Carbon/carbon(C/C for short)composites have the advantages of low density,high strength,low expansion coefficient,good friction performance,good thermal impact resistance and high dimensional stability.It is one of the few candidate materials used above1600?,and is considered to be one of the most promising high-temperature materials in mechanical and structural materials.For example,as a high-speed braking material,aero-engine high-temperature structural components(turbine blade,fire shield,throat lining,wing leading edge,etc).However,it is difficult to produce the parts with complicated structure because of the high cost,long period,complicated working procedure and the influence of the equipment,which has great limitation for the application of C/C composites.Therefore,in order to solve this problem,the Si-CaO/Al₂O₃-Si laminated structure is used as the intermediate layer and the hot-press sintering is used to diffusely connect the C/C composites.CaO/Al₂O₃is a kind of glass ceramics,which is a kind of composite material combining crystal phase and glass formed by high temperature melting,forming and heat treatment.This material has excellent properties such as high mechanical strength,thermal shock resistance,chemical resistance and low dielectric loss.The microstructure of the obtained joint is analyzed by X-ray diffraction and scanning electron microscope.A shear strength test on a universal testing machine to test its mechanical strength.This connection process is low-cost,fast and efficient.The main work and research results are as follows:(1)SEM and EDS characterization of the cross section of the joint found that the interface structure is:C/C matrix/Si and Si C layer/Ca-Al-O-Si compound/Si and Si C layer/C/C matrix.Through the previous experimental work and analysis,it is found that the heating temperature and holding time have a significant impact on the mechanical properties of the joint.Therefore,within the scope of the subsequent test process involved,with the extension of the holding time and the connection temperature,the shear strength of the joint at room temperature basically shows a trend of first increasing and then decreasing.Finally,the process parameters are:the joint made under the heating temperature of 1500°C and the holding time of 90 minutes has a maximum shear strength of 38.17 MPa.(2)The fracture obtained after the mechanical test of the obtained joint is analyzed,and four fracture crack propagation models are summarized.Model 1 is that the crack propagates along the interface,the intermediate layer and the matrix and finally forms a fracture.In this work,the section of the joint that we made after the shear test is based on model 1.Model 2 is the fracture along the interface between the connecting layer and the C/C matrix.Because of the junction of two different materials at the interface,this is also the most prone to failure area.Model 3 shows that the cracks propagate along the intermediate layer,and the strength of the connecting layer in these areas is lower than the interlayer shear strength of the interface layer and the C/C matrix.Model 4 is that the fracture occurs on the C/C matrix close to the connecting layer.This failure mode is the most satisfactory result:the shear strength of the connecting layer and the interface layer are both higher than the interlaminar shear strength of the C/C matrix.(3)The connection layer material is glass ceramic material,which is characterized by high brittleness and low toughness.The carbon fiber in the C/C composites will have a certain toughening effect,and a certain pseudoplastic characteristic will be reflected in the load-displacement curve of the C/C composites.Therefore,in the load-displacement curve of the connection joint,the curve shows different types due to the different failure position.When fracture occurs in model 1,the matrix,intermediate layer and interface layer are all loaded,and the load-displacement curve presents a stepped fracture mode;When a fracture occurs in model 2,it indicates that the strength at the interface between the connecting layer and the C/C matrix is not high,which is lower than the strength of the matrix and the connecting layer.When subjected to shear stress,it will peel off immediately,and the load-displacement curve will also show brittleness fracture mode;When the fracture of model3 occurs,it is mainly carried by the intermediate layer,because the intermediate layer is made of glass material and has typical brittle characteristics,so it presents a brittle fracture mode.When the model 4 fractures,it is mainly loaded by the matrix.When the C/C composites is subjected to shear stress,some of the carbon fiber is pulled out,which plays a role of toughening,so the load-displacement curve presents a pseudoplastic fracture characteristic.(4)The interdiffusion and chemical reaction take place in the middle layer at high temperature.With the increase of heating temperature and holding time,the phase changes and some stable compounds are formed.These compounds penetrate into the C/C Matrix to form a mechanical interlock,thus improving the mechanical properties of the joint as a whole.