Characterization of interfacial adhesion between carbon-carbon composite and plasma-sprayed aluminum oxide

The integrity of the interface between plasma-sprayed alumina and a carbon-carbon substrate is a critical factor in the design of the thermal protection system for the NASA Solar Probe spacecraft. The carbon-carbon serves as the primary structural material in the thermal shield. Plasma-sprayed alumina is a coating which has been identified for its optical properties. The objective of this research is to provide a methodology to assess the strength of adhesion between carbon-carbon and the alumina coating and determine which factors influence the quality of the bond.;A new methodology, the inverted four-point bend bimaterial fracture test, was developed specifically to measure the critical interfacial energy release rate, Gc, an indicator of interfacial strength, for a bimaterial system containing a layered composite or a brittle coating. For the bimaterial that was selected as the control, Gc was experimentally measured to be 33+/-3 J/m2. This was in close agreement with a finite element model prediction of 37 J/m 2. The model estimated the phase angle of loading to be 89.2°, a state of nearly pure shear.;Tests were conducted on samples with varying material combinations and processing techniques. Results showed that a mild grit blast provided the best carbon-carbon surface preparation in terms of interfacial strength. A comparison of carbon-carbon substrates showed that the control---a specific brand of high density carbon-carbon---yielded the best combination of high interfacial strength and low variability.;It was determined that a thin chemical barrier layer between the carbon-carbon and alumina was necessary during solar orbit. Samples containing two types of bond coat were prepared: TaN and SiSiC. Prior to thermal treatment, the samples with TaN yielded an interfacial strength identical to the control material which contained no bond coat. The samples with SiSiC yielded a higher strength. Two similar sample sets underwent thermal treatment, but neither survived to the extent that mechanical testing could be performed. The SiSiC samples delaminated due to a chemical reaction and the TaN samples incurred considerable contraction and partial debonding of the coating due to alumina phase transformations; however, the TaN served adequately with regard to chemical barrier protection.