Oxidation protective barrier coatings for high-temperature polymer matrix composites
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James K. Sutter NASA Lewis Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135
Maria A. Schuerman Xavier University, Cincinnati, Ohio 45207
Elizabeth A. Crane John Carroll University, Cleveland, Ohio 44118 (Received 13 December 1993; accepted 17 February 1994)
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 (c-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.
I. INTRODUCTION Application of the polymer matrix composite (PMC) materials in aircraft frames and engines is limited by the high-temperature performance of the resin.1 The primary benefit in replacing currently used metals with PMC's would be the weight savings afforded by using lower density, higher specific strengths PMC's. In addition, processing and fabrication costs would be lower. However, a major limitation of using PMC's at elevated temperatures is the oxidation of the polymer resin, which degrades the mechanical properties. Numerous synthetic approaches are underway to improve the longterm thermo-oxidative stability to a target temperature of 425 °C (800 °F) by eliminating the oxidative weak point at the polymer chain ends2 while retaining the desirable rheological properties of the resin. PMC systems using the PMR-II-50 resin demonstrate the optimum blend of high mechanical strength, good processability (viscoelasticity), and high-temperature stability required for aerospace applications. This study investigates whether a protective coating for PMC's can extend the useful lifetime, or maximum use temperature, of the PMC by retarding the oxidation of the resin: an approach analogous to coatings for carbon-carbon composites, but with a considerably lower temperature goal. Implicitly, the coating must be an adherent, dense, oxidation-resistant diffusion barrier that J. Mater. Res., Vol. 9, No. 6, Jun 1994 http://journals.cambridge.org
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can limit oxygen penetration to the resin surface. In addition, it must be applied to completely cover the large, complex shapes of actual fabricated components. Finally, it must be unaffected by the thermal gradients and resulting stresses experienced by a coated PMC under operational conditions. A variety of coating materials and deposition techniques are discussed relative to the unique requirements of PMC's. II. COATING MATERIAL A
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