Rhenium used as an interlayer between carbon-carbon composites and iridium coating: Adhesion and wettability

Iridium (Ir) is one of the most promising materials as a protective coating on carbon/carbon (C/C) composites for very high temperature applications. However, the poor adhesion and thermal stress induced by thermal expansion mismatch between Ir coating and C/C substrate restrict their application. Rhenium (Re) has been selected as the interlayer material between the Ir coating and C/C substrate which can improve the adhesion of Ir coating and relieve the thermal stress. The Re coating was prepared on the C/C substrate by chemical vapor deposition (CVD). The Ir coating was electrodeposited on the C/C and Re/C/C substrates in molten salt. The morphology and microstructure of the Re and Ir coatings were studied by SEM and XRD. The bond strength of the coatings on C/C substrate was measured by coating-pull-off test. The molten salt wettability on the C/C and Re/C/C substrates was determined by drop shape method. The results showed that the CVD Re coating on the C/C substrate had a preferential orientation of < 002 > with many fine regular cracks. The bond strength between the Re coating and C/C substrate was higher than 15 MPa before and after annealing treatment. The contact angles of molten salt on C/C and Re/C/C substrates at 500°C were 128.5° and 43.4°, respectively. The bond strength of as-deposited coatings increased from 3.2 MPa for Ir/C/C to 7.9 MPa for Ir/Re/C/C. After high temperature annealing treatment, however, the bond strength has been reduced by ~ 50%, probably due to the infiltration of molten salt into the C/C substrate.