Oxidation protective barrier coatings for high-temperature polymer matrix composites

Three coating techniques (metal-organic chemical vapor deposition, magnetron sputtering, and plasma-enhanced chemical vapor deposition) were employed to deposit different coating materials (alumina, a superalloy, and silicon nitride) on graphite-fiber-reinforced polyimide composites to protect against oxidation at elevated temperatures. Adhesion and integrity of the coatings were evaluated by isothermal aging (371 °C for 500 h) and thermal cycling (25 to 232 °C for 1000 cycles and −18 to 232 °C for 300 cycles). Best results were achieved with a plasma-deposited, amorphous silicon nitride (a-SiN: H) coating, which withstood stresses from 0.18 to −1.6 GPa. The major factors affecting the suitability of a-SiN: H as an oxidation protective coating are the surface finish of the polymer composite and the presence of a sizable hydrogen content in the coating.