Low-pressure chemical vapour deposition of mullite layers using a cold-wall reactor

Mullite coatings (3Al2O3–2SiO2) were obtained in a cold wall reactor using aluminium trichloride, silicon tetrachloride and nitrous oxide as precursors with nitrogen and hydrogen as carrier gases. The experimental preparation conditions were studied for fixed flow rates of N2O, H2 and N2, substrate temperature T=1200°C, total chamber gas pressure P=20 hPa, as a function of the AlCl3 to SiCl4 input ratio. The chemical composition, crystal structure and morphology of the coatings were determined by means of EDS, XRD and SEM. The aluminium content of the deposits increased, as expected, with the AlCl3 to SiCl4 input ratio. Thin specimens were also characterised by means of TEM. The local composition was determined by EELS, while the crystalline microstructure by electron diffraction. The best experimental parameters to produce mullite layers were then established.