Wear and friction of self-lubricating CuO-TZP composites

In certain applications, including high temperature or vacuum environments, liquid lubricants or greases are not stable. Solid lubricants are potentially suitable candidates for the reduction of friction and wear. Ceramic materials are a suitable candidate for harsh environments such as high temperatures and vacuum. Ceramic components are generally lubricated by thin solid films to achieve low friction and wear. The lifetime of such films is inevitably limited. By incorporating solid lubricant reservoirs into a hard ceramic matrix, friction and wear can be decreased during sliding by realizing a gradual replenishment of the soft solid lubricant to the surface. Therefore, it is of great importance to study the self-lubricating ability of the ceramic contact. In this thesis, a CuO doped zirconia composite has been chosen as a self-lubricating ceramic composite system. The tribological performance of the ceramic composite has been systematically investigated. To understand the self-lubricating ability of the composite, the friction behaviour has been studied at different temperature levels. First, dry sliding tests were conducted at room temperature using different loads and sliding velocities as well as countersurfaces. The wear mechanisms have been investigated using different characterization methods. At room temperature, 5CuO-TZP only shows low friction and wear against an alumina countersurface. Similar dry sliding tests have been carried out at elevated temperatures. A coefficient of friction of 0.35 and a specific wear rate less than 10-6 mm3/Nm were obtained at 600 °C for CuO-TZP sliding against an alumina countersurface. It has been found that a soft copper rich (third body) layer is formed at the interface between the sliding components. The formation of the soft layer as well as the wear mechanism has been explained. A physically-based model has been developed which includes the processes responsible for maintaining the soft third body layer at the interface. The model can predict the thickness of the third body layer under different tribological conditions in the mild wear regime. It can be concluded that the tribological performance of CuO-TZP under dry contact conditions strongly depends on the operational conditions.