Nanocomposite and nanostructured tribological materials for space applications

Satellites and space-born systems have a number of moveable mechanical parts, whose reliability is severely limited by degradation of lubricants and excessive wear. Many systems may remain in space for 10-30 years and, therefore, exposed to atomic oxygen, solar radiation, energetic particles, and temperature cycling from cryogenic to 400 °C. Furthermore, these systems are frequently tested on the ground and stored for many years under controlled environments before launching. Also, reusable launch vehicles are so planned that they will operate with space-terrestrial cycling and with temperature spikes in excess of 800 °C during re-entry. A “chameleon” tribological coating concept was developed to address this challenge. This approach relies on the coating to change its surface (both chemistry and structure) to self-adjust to the environment and thus achieve long durability. The first “chameleon” coatings were made of WC, WS2, and diamond-like carbon (DLC); they provided superior mechanical toughness and performance in space/terrestrial environmental cycling. In order to address the temperature variation, the second generation of “chameleon” coatings were made of yttria stabilized zirconia (YSZ) in a gold matrix with encapsulated nanosized reservoirs of MoS2 and DLC. Encapsulation of MoS2 nanoparticles into Al2O3 matrix and high temperature lubrication with low melting point glassy ceramic phases were also explored. “Chameleon” coatings with various chemistries are discussed along with their characterization by various analytical, mechanical, and tribological methods. Coating toughness was remarkably enhanced by a grain boundary sliding mechanism. Unique friction and wear performance was demonstrated by testing in controlled humidity air, dry nitrogen, vacuum, 500-600 °C air, and in environmental cycling.