Structure of mullite coatings grown by chemical vapor deposition

Dense mullite coatings have been deposited on SiC and Si3N4 substrates by chemical vapor deposition (CVD). The coatings were compositionally graded with the Al/Si ratio increasing from 1.5 at the substrate/coating interface, to a value of 8 at the coating surface. This gradation of composition is discussed in light of changes in the concentration of oxygen vacancies, ordering of the vacancies, formation and size of domains and the observed changes in lattice parameters.