Protection of graphite from oxidation at 2100 deg C.

Complete oxidation protection of graphite has been achieved at temperatures up to 2100°C through the use of iridium coatings. Oxygen permeability measurements show that iridium is virtually impervious to oxygen up to 2200°C, and carbon diffusion through iridium also was found to be slow. Coating methods that were investigated include slurry-dipping and sintering, foil cladding, vapor plating, and electroplating. Best results were obtained by a combination of several methods. The iridium coated graphite was subjected to plasma torch tests at both atmospheric and subatmospheric pressures. In all tests, complete protection of the graphite substrate was achieved. At atmospheric pressure and 2050°C, the recession rate of the coating was 0.0054 in./hr. At stagnation pressures of about 45 torr and 2050°C, the recession rate was too small to be determined. It is suggested that iridium coatings also might protect refractory metals from oxidation.