Diffusion of silver to surfaces plays a major part in high-temperature lubrication.

An experimental study has been performed to learn about the physical and chemical mechanisms of self-lubrication of coatings that comprise nanostructured composites of yttria-stabilized zirconia (YSZ), silver, and molybdenum. These and other YSZ-based nanocomposite coatings have received increasing attention in recent years because they offer a combination of hardness, toughness, resistance to wear, and low-friction C properties that make them attractive for reducing wear and friction and increasing the lifetimes of hot, sliding components of mechanical systems. In addition to the excellent mechanical and thermal stability of the basic YSZ ceramic material, the nanocomposite structures of these coatings, consisting of combinations of amorphous and crystalline phases, provide a “chameleon” surface adaptation, in which different phases turn into lubricants in response to different test environments, contact loads, sliding speeds, and temperatures. Moreover, proper sizing of nanocrystalline grains can restrict crack sizes and create large volumes of grain boundaries, thereby increasing the toughness and contact-load-bearing capabilities of these coatings.

The YSZ-Ag-Mo Coating Is Covered with a TiN surface layer containing a random array of pinholes. This layer retards the diffusion of Ag to the surface, thereby reducing the rate of depletion of lubricant in order to prolong the high-temperature wear lifetime.

In the study, the focus was on the evolution of microstructure, diffusion of silver, and contact surface oxidation processes in sliding contact at high temperature in air. The microstructures of the coatings were determined by x-ray diffraction and transmission electron microscopy. Coefficients of friction were measured by use of a high-temperature ball-on-disk tribometer. The wear scar surfaces and coating cross-sections were studied by use of scanning electron, transmission electron, scanning transmission electron, and focused ion beam microscopes, which provided the information on spatial distributions of chemical compositions, including distributions of silver and molybdenum along with microstructural features.