Study On The Joining Properties Of SiC Ceramics Using Polycarbosilance As The Joining Material

Silicon carbide（Si C）ceramic is a very important and widely used engineering structural material with many excellent properties such as low density,high strength,acid and alkali corrosion resistance,wear resistance and so on.At present,the processing of Si C ceramic has been thoroughly investigated in the past.However,its natures of high brittleness,low toughness,and high melting point make it difficult and expensive to manufacture parts with complex shapes and large sizes.Therefore,the successful development of Si C ceramic joining technology could provide effectively solution to this problem.However,the Si C ceramic joining technologies like nano impregnation and transient eutectic（NITE）phase joining,brazing and MAX（M,early transition metal;A,A-group element;X,C or N）phase have encountered certain problems,such as high joining temperature,complex process,high cost,and the thermophysical properties mismatch between the joining interlayer and the parent material,which limits the application of joining technology.Although the preceramic polymer joining method can successfully join Si C ceramics at relatively low temperature and pressure,the reported problem of low joining strength needs to be solved urgently.In this dissertation,the preceramic polymer of Si C ceramic was used as the joining material,and the Si C ceramics were successfully joined at relatively low temperature and pressure.The thermophysical properties of the joining material,the evolution of the microstructure of the joints,mechanical property,and reinforcement mechanism of the joints were systematically studied.（1）The Si C preceramic polymer of polycarbosilane（PCS）was used as the joining material,and the effect of different coating methods,pre-pressure,joining temperature and heating rate on the phase composition,microstructure,and shear strength of the joint were studied.The application of PCS powders by a screen printing method improves its uniform distribution on the Si C surface,which is conducive to the improvement of the joining strength.Although the amount of ceramic yield from PCS is only 64.78%,the ultra-thin joining interlayer（～2μm）is beneficial to reduce the existence of defects in the joining interlayer.The slow heating rate in the PCS pyrolysis stage can also improve the shear strength of the joints.When the temperature was 1600 ^oC,a higher joining strength was obtained,which was 117.3±15 MPa.（2）The PCS was used as the joining material.The Si C ceramic is quickly joined by spark plasma sintering（SPS）technology,and the average shear strength of the joints is 45.3±11.8 MPa.The oxidation behaviors of Si C joints at different temperatures（600-1200 ^oC）in air were studied.After the joints were heat-treated at different temperatures under ambient air conditions,the shear strength of the joints was improved.The strengthening effect was the best when the heat treatment temperature was 1000 ^oC,and the average shear strength of the joints was 101±6.4 MPa.The evolution in the phase composition,degree of crystallinity,and microscopic morphology of the joining interlayer before and after heat-treatment in air were detected and analyzed.The oxidation of Si C to Si O₂ will produce volume expansion,which can heal the pores and cracks to some extent in defects of the joining interlayer,thus enhancing the shear strength of the joints.（3）The PCS was used as the joining material,and the effects of the amount of B₄C addition and the joining temperature on the phase composition,microscopic morphology,and shear strength of the joints were studied.When B₄C is oxidized to generate B₂O₃,it will produce a volume expansion of about 250%.If B₄C was not added,the maximum average shear strength of the joint was 62.5±9 MPa.When B₄C was added,the maximum average shear strength increased to 104.1±2.1 MPa.The changes in the phase composition as well as microscopic morphology of the joining interlayer were detected and analyzed,and the reinforcement mechanism of the joint by adding B₄C was obtained.