DEVELOPMENT ON HIGH TEMPERATURE INSULATION MATERIALS. PART I PYROLYTIC DEPOSITION OF ALUMINUM AND SILICON NITRIDES.

Aluminum nitride has been deposited on molybdenum, tungsten, and graphite by passing vapors of the AlCl3 NH3 complex over the hot substrates. Microstructure and density measurements indicate that the deposited material has a porosity of 20%. This and the presence of residual amounts of chlorine probably account for the high temperature electrical properties of the coating being poorer than expected of high purity aluminum nitride. Molybdenum substrates, when completely covered with these coatings are protected for oxidation by air at least up to 700 C. Pyrolytic silicon nitride has been prepared using SiF4 2NH3, SiCl4+NH3 and SiH4+NH3 as source materials. The gaseous mixture of silane and ammonia has been the most thoroughly studied and appears to be the most promising. Very adherent, relatively soft, amorphous films have been deposited on substrates heated in the 750 to 800 C range. Very hard crystalline coatings of Si3N4 were obtained in the 1100-1200 C range. These films have high spectral emissivity and provide oxidation protection for molybdenum substrates.