Effect of Surface Modification on Adhesion of a Metal Bond Coat to PMR Composites

Graphite-reinforced polyimide composites are being considered for use in advanced turbine engines, which require the application of wear- and oxidation-resistant coatings onto the composites. To ensure in-service adhesion of a hard, protective ceramic or cermet coating to these composites, it is first necessary to apply a metal bond coat This study examines the effect that a silica layer has on the adhesion of a proprietary metal bond coat to PMR-15 and PMR-II-50 composite surfaces. Previous work has shown that radio frequency (RF) oxygen plasma etching was the most effective technique for increasing the surface energy of the composites and plasma-enhanced chemical vapor deposition (PECVD) of silica using tetramethoxysilane (TMOS) produced stoichiometric silica films. In addition, UV/ozone chemical vapor deposition (UVCVD) using tetraethylorthosilicate (TEOS) was also used to deposit silica species on the composite surfaces. The surfaces of PMR-15 and PMR-II-50 samples were modified by the following methods: grit blasting; grit blasting, ultra-violet/ozone (UV/ozone) etch, UVCVD of silic; and grit blasting, RF oxygen plasma etch, PECVD of silica. Adhesion pull tests were conducted and the fracture surfaces were examined using both optical and electron microscopy. PMR-15 samples with PECVD silica showed a significant increase in adhesion when compared with both UVCVD silica and grit blasting only. PMR-II-50 showed no significant difference in the adhesion strength for any surface modification method. Electron microprobe analysis of the fracture surfaces showed no evidence that silica was present on surfaces treated by UVCVD. Analyses also showed that the silica bonded more strongly to both the PMR-15 and PMR-II-50 neat resins than to the graphite fibers. The poor surface quality of PMR-II-50 composites is believed to have a major effect on bond coat adhesion strength.