PERFORMANCE OF PLASMA SPRAYED B4C COATINGS UNDER HIGH HEAT LOADS

In order to investigate the behaviour of B4C coatings under thermal loading, a variety of substrates (fine grain graphite, carbon-carbon composite and the molybdenum alloy TZM) was covered with a coating of B4C by low pressure plasma spray in an inert gas atmosphere. The thickness of the coating was varied between 125 and 250-mu-m. Mechanical tests proved good adhesion of the B4C layer on all substrates. Tiles of the coated materials, 50 x 50 mm2 and 50 x 25 mm2 in size, were then exposed to high thermal fluxes in an ion beam test facility. The power densities applied on the specimens was varied between 3 and 10.5 MW/m2 at pulse lengths up to 10 s (passively cooled specimens only). During thermal loading the surface temperature of the specimens was registered by means of optical pyrometers. Additionally, surface modifications of the specimens were observed with a video camera, and each pulse was recorded on tape. The results showed that already at surface temperatures below 2000-degrees-C crack formation in the B4C coating and delamination occurred, in particular for thick coatings (approximately 250-mu-m in thickness). On graphite substrates no melting of the B4C-layer was observed up to more than 2100-degrees-C (melting temperature of B4C almost-equal-to 2400-degrees-C).