Tribological coatings for lubrication over multiple thermal cycles

Nanocomposite materials demonstrating multiple temperature-adaptive mechanisms including diffusion, oxidation and/or catalysis mechanisms to yield low friction coefficients of < 0.2 from room temperature to 700 °C were combined with diffusion barrier layers in coatings with different architectures (e.g., layer thicknesses, number of layers, etc.) to examine adaptation of contact surface chemistry and morphology over multiple thermal cycles. Multilayered coatings consisting of ceramic–metal nanocomposite adaptive lubricant layers separated by diffusion barriers allowed adaptation to occur only upon exposure of the lubricant layer by wear, which resulted in prolonged wear life at static and cycled temperatures. It was also observed that a relationship between the number of adaptive lubricant layers and the number of thermal cycles existed, where one thermal cycle consumed two adaptive lubricant layers. The thickness of the adaptive coating layers was also important because diffusion- and oxidation-based adaptation in these particular coatings required a minimum volume of solid lubricant material. The surface roughness of the adaptive coating materials played a significant role in their performance within multilayered coatings, where rough coatings (> 100 nm Ra) failed after relatively few sliding cycles. The utility and application of adaptive coatings materials providing lubrication over multiple thermal cycles is discussed.