Application of thick film and bulk coating technology to the Subterrene program

The Subterrene is a rock-melting earth-penetration system representing a significant advancement in excavation technology. Temperatures of 1700–2000 K are involved, and heated refractory metal or alloy probes are used. Corrosion problems are severe on the exterior surfaces of the refractory metals used; bulk coatings having the required properties would be extremely desirable, provided they could be deposited upon a substrate as a tenacious coating. Within the Subterrene, pyrographite radiant heaters are operating at temperatures to 2450 K. Graphite receptors added to improve the radiant heat transfer react with the molybdenum and tungsten penetrator bodies to form the respective carbides. A 30-μ thick CVD film of TaC may be applied to inhibit this reaction. The techniques of applying these films, their nature, and the results of their application, are discussed. The environment of an operating penetrator is described, and the requirements and properties of exterior coating are outlined.