SMOOTH ULTRAHARD COATINGS ON METALS

Implant Sciences Corporation (ISC) proposes non-conventional, nanocrystalline, ultrahard SiC for coating Cu arc heater segments. Its high electrical resistivity (10xE11 ohm-cm), high thermal conductivity (270 W/mK), and excellent oxidation resistance makes SiC the most promising candidate material for this application. Diamond, CBN, BeO and AlN were considered but eliminated because they failed to meet the selection criteria. A novel, low temperature ion beam induced chemical vapor deposition (IICVD) process will be used to deposit smooth, high purity, nanocrystalline SiC coatings at ambient temperatures. The low process temperature reduces residual stress, grain growth and contamination - problems experienced with conventional, high temperature CVD. To promote adhesion, a functionallly graded Cu-SiC layer will be interposed between coating and substrate by co-evaporation of Cu during initial stages of SiC deposition. The SiC coating will be characterized in terms of its thickness, smoothness, hardness, impurity content, crystal structure, microstructure, continuity, adhesion, electrical resistivity, thermal conductivity, oxidation resistance and structural integrity under high heat flux loads. The compositional gradient and thickness of the blended layer also will be determined. Based on these measurements, optimum processing conditions and properties will be defined. At the end of Phase I, ISC will deliver a coated disk to AEDC for evaluation.