Study On High Temperature Thermal Stability Of Al₃BC₃ In Vacuum Or Nitrogen Atmosphere

Al-B-C system includes various compounds such as Al₃BC₃,Al₈B₄C₇,AlB₂₄C₄,etc.Among these compounds,Al3BC3 is one of the most promising high-temperature lightweight structural ceramics with low density,high hardness and toughness,and high chemical stability.Material damage may occur in different environments when used in high temperature environments,the thermal stability of materials is important for its applications.The stability of Al₃BC₃ under oxidizing atmosphere and Ar atmosphere has been studied,which is related to temperature and atmosphere.Nitrogen is also an inert gas commonly used in laboratories and industries as well as argon.However,the thermal stability of Al₃BC₃ in nitrogen has never been reported.Direct nitriding of Al₃BC₃ without adding a sintering aid can provide a feasible method for the preparation of AlN-BN by pressureless sintering.Based on the stability of ternary carbide Al₃BC₃ in different atmospheres（vacuum and N₂）at high temperature,the thermal stability and decomposition products of ternary carbide Al3BC3 under vacuum and nitrogen were studied.Porous AlN-BN composites were prepared by using Al3BC3 as raw material,and the effects of heat treatment temperature and holding time on the phase composition and microstructure of the product were investigated.（1）In vacuum,Al（g）in Al₃BC₃ first escapes from the crystal lattice and is rapidly extracted from the system.The product after complete consumption of Al is a mixture of C and B₄C.Since Al（g）escapes from the Al₃BC₃ lattice,it is a process from the surface to the inside,and the surface of the Al₃BC₃ block may have completely decomposed at1600℃.The decomposition of the Al₃BC₃ powder has occurred at 1400℃and completely decomposed at 1600℃.（2）The thermal stability study of Al₃BC₃ in N₂ shows that Al（g）first escapes from the Al3BC3 lattice and then reacts with N2（g）to form AlN.The final products of Al3BC3under N₂ atmosphere are AlN,BN and C.The thermal stability study of Al₃BC₃ powder in N₂ atmosphere shows that the formation of nitriding product will hinder the further progress of nitriding reaction;the impurity atmosphere CO（generated by the reaction of O₂ impurity and graphite felt in N₂）can promote Al₃BC₃ and N₂.Reaction;the difference in thermal stability of Al₃BC₃ raw materials of different particle sizes in N₂ is caused by the difference in contact area between Al₃BC₃ and N₂.（3）Based on the study of the stability of ternary carbide Al₃BC₃ under high temperature in different atmospheres（vacuum and N₂）,the porous Al₃BC₃ body was prepared by direct foaming method,and then the blank was placed in a high temperature tubular furnace under a flowing N2 atmosphere.Heat treatment provides a feasible method for preparing porous AlN-BN composites.