Low-pressure chemical vapour deposition of mullite coatings in a cold-wall reactor

Abstract
Nitrous oxide, together with aluminium and silicon chlorides, as precursor gases, while hydrogen and nitrogen as carriers, have been fed into a cold-wall reactor, in order to produce crystalline deposited layers of mullite (3Al₂O₃.2SiO₂) on silicon carbide and aluminium nitride substrates. The corresponding deposition kinetics has been studied, for fixed flow rates of nitrous oxide, hydrogen and nitrogen, as a function : of the substrate temperature, from 1100 to 1300 °C ; of the gas pressure, from 10 to 90 hPa and of the AlCl₃ to SiCl₄ input ratio. The coating's chemical composition, crystal structure and morphology was precisely determined by means of WDS, XRD and SEM. The aluminium content of the deposits increases first as a function of pressure. It reaches a maximum for P = 20 hPa, whence it decreases steadily, to finally vanish completely for P = 100 hPa. The best experimental conditions to produce mullite coatings were then interpolated, experimentally tested, and the side compounds, under such conditions, identified.