| AVS 54th International Symposium | |
| Tribology | Friday Sessions |
| Session TR4+SE-FrM |
| Session: | Friction and Wear of Engineered Surfaces Macro- to Nanoscale Approaches |
| Presenter: | C. Muratore, Air Force Research Laboratory |
| Authors: | C. Muratore, Air Force Research Laboratory J.J. Hu, Air Force Research Laboratory A.A. Voevodin, Air Force Research Laboratory |
| Correspondent: | Click to Email |
Plasma processing allows precise control of the composition, microstructure and architecture of nanocomposite coating materials, enabling the design of materials that automatically adapt to variable aerospace environments, including humid and dry ambient air, space, and high temperature oxidizing conditions. Self-adaptive materials are of particular interest in tribological applications because most solid lubricants are effective in only a narrow range of ambient conditions. Thoughtful selection of multiple solid lubricant phases and the development of novel lubricant delivery mechanisms such as controlled diffusion, catalysis and chemical reactions with the surrounding atmosphere enhanced by contact between moving surfaces have recently been realized to achieve adaptation in diverse environments. For example, nanocomposite yttria-stabilized zirconia (YSZ) coatings containing Ag, Mo and other nanosized inclusions depend on temperature-activated adaptations to yield low friction (<0.2) from 25-700 °C in air. For YSZ-Ag-Mo nanocomposites, lubrication below 500 °C results from diffusion and coalescence of silver at the surface. Above 500 °C, the silver is pushed out of the wear contact, exposing a limited quantity of molybdenum in the coating to air, and initiating the controlled formation of MoO3 and other compounds exhibiting low shear strength and thus easy sliding at high temperatures. While these adaptations at the surface reduce friction, irreversible compositional and structural changes resulting from the migration of film components can compromise the mechanical properties of the coating, thus reducing its utility over long periods at high temperature or through multiple thermal cycles. Moreover, some lubrication mechanisms occur over the entire coating surface in addition to the area experiencing wear, thus wasting the limited quantity of lubricious material stored in the coating. Coating architectures that control adaptation rates or selectively inhibit adaptation on coating surfaces unaffected by wear have been developed to increase the lifetime of adaptive tribological materials. Additionally, smart coatings with wear sensing capability have been developed to facilitate coating development and to improve reliability in critical applications.