Thermodynamic study and preparation of Si-B-N ceramic coating by LPCVD from SiCl₄-NH₃-BCl₃-H₂-Ar system

The codeposition characteristics of Si-B-N ceramics from the SiCl₄-NH₃-BCl₃-H₂-Ar system at lower temperatures and phase transformation of as-prepared Si-B-N ceramics at temperatures from 1200 to 1800 °C were investigated. Thermodynamic analysis results indicated that the BN + Si₃N₄ dual phase region existed from 800 to 1200 °C and that 800 °C was an optimum deposition temperature to deposit Si-B-N ceramic coating. Deposition efficiencies at equilibrium for Si₃N₄ and BN were high, particularly at temperatures below 1000 °C. Pressure and dilution ratio of H₂ had little influence on deposition efficiencies of BN and Si₃N₄ at 800 °C. The amorphous Si-B-N ceramic coatings were successfully deposited at 800 °C from the same precursor system and contained N-B and N-Si bonds by XPS analysis. It kept amorphous below 1600 °C in N2 and partly transformed to α/β-Si₃N₄ when heat treated at 1600 °C in N₂ for 2 h. These results demonstrated that the composite Si-B-N ceramics could be fabricated at 800 °C and used below 1600 °C.