Thermodynamic study and preparation of Si-B-N ceramic coating by LPCVD from SiCl 4 -NH 3 -BCl 3 -H 2 -Ar system
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tion 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 1 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 a/b-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.
I. INTRODUCTION
Combining the advantages of Si3N4 and BN ceramics, Si–B–N ceramics exhibit low density, high thermal stability, high mechanical performance, good oxidation, and a low dielectric constant.1,2 Si–B–N ceramics are promising electromagnetic wave transparent materials for electromagnetic windows or radomes on high-speed aircrafts and supersonic near space vehicles which endure high temperatures, thermal shock, rain erosion and particle impingement, and which also require a stable wave transparent capability at the same time. Si–B–N ceramics can make ceramic fibers,3–8 films,9–14 interphase coatings,15–20 and ceramic matrices.21–24 Up to now, main fabrication methods of Si–B–N ceramics are polymer-derived ceramic (PDC) routes5,21–28 and chemical vapor deposition (CVD).17–19 PDC routes have been widely used to make fine Si–B–N fibers and Si–B–N ceramic matrices for the designable precursors. On the other hand, low-pressure chemical vapor deposition or infiltration process (LPCVD/CVI), which yields smooth, continuous, and dense ceramics at a lower temperature with a high purity and well-controlled composition and microstructure, has been widely used in the fabrication of Si–B–N films, interphase coatings, and matrices in ceramic fiberreinforced ceramic composites.29–32 The microstructure and phase composition of Si–B–N ceramics fabricated by CVD vary with vapor precursors Contributing Editor: Yanchun Zhou a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.323
and deposition parameters. Hirai et al.33 illustrated that the Si–B–N film prepared by LPCVD from SiCl4– NH3–H2–B2H6 gas sources at deposition temperatures ranging from 1100 to 1300 °C was composed of amorphous Si3N4 and turbostratic BN. Besmann34 performed a thermodynamic analysis of the CVD system of H2SiCI2–BCl3–NH3 and indicated that the solid phase assemblage Si3N4 1 BN was stable over a large region. Essafti11,14 fabricated Si–B–N films by LPCVD using SiH4–B2H6–NH3 and focused on the effects of temperature and gas
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