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

Nitrous oxide, together with aluminium and silicon chlorides, as precursor gases, while hydrogen and nitrogen as carriers, have been fed into a cold-w all reactor, in order to produce crystalline deposited layers of mullite (3 Al(2)O(3).2SiO(2)) on silicon carbide and aluminium nitride substrates. The corresponding deposition kinetics has been studied, for fixed flow rates o f nitrous oxide, hydrogen and nitrogen, a's a function : of the substrate t emperature, from 1100 to 1300 degrees C; of the gas pressure, from 10 to 90 hPa and of the AlCl3, to SiCl4, input ratio. The coating's chemical compos ition, crystal structure and morphology was precisely determined by means o f WDS, XRD and SEM. The aluminium content of the deposits increases first a s a function of pressure. It reaches a maximum for P = 20 hPa, whence it de creases steadily, to finally vanish completely for P = 100 hPa. The best ex perimental conditions to produce mullite coatings were then interpolated, e xperimentally tested, and the side compounds, under such conditions, identi fied.