This invention relates to a method of depositing a high-emissivity carbon coating on a substrate by reactive RF sputtering. More specifically, the invention relates to a method of depositing a carbide-carbon coating on a substrate by reactive RF sputtering having a composition gradient such that the coating is composed primarily of a carbide compound near the substrate and primarily of carbon near the exterior surface.
Thermoelectric generators are frequently used as an energy source in aerospace applications. Temperature regulation in these generators is a critical problem for both orbital satellites and deep space probes. A high-emissivity coating on the metallic thermoelectric generator casings would improve the performance of satellite thermoelectric systems by radiating large quantities of heat while operating at low temperatures. The high-emissivity coating must be capable of withstanding the stresses imposed during launch and the elevated temperatures and vacuum conditions encountered during operation.
Carbon has many of these properties and would serve well as a high-emissivity coating. However, carbon does not adhere well to the metals often used in thermoelectric generator casings, namely iridium and aluminum. Attempts have been made to blend carbon with an epoxy and paint the mixture onto the casings, but this process adds undesirable weight to the generator. A more promising method is to sputter deposit a coating with a composition gradient such that the surface of the coating near the substrate is rich in material that adheres well to the substrate while the exterior surface of the coating is rich in carbon. Previous methods of deposition have involved rotating a substrate between two targets of different materials and gradually changing the deposition parameters on the two targets to obtain the desired composition gradient. This method is too complicated to be practical for the massive parts of the generator which must be coated.