In recent years, there has been wide interest among tribologists around the world in the new advanced ceramics, which are characterized by high strength, hardness, improved toughness, corrosion resistance and good insulating properties. Since these properties are often retained at high temperatures, the advanced ceramics are excellent candidate materials for high-performance engineering applications such as low heat rejection engine, bearings, cutting tools, dies, etc. Many of the potential applications involve sliding or rolling contacts, and therefore there is a need for advanced ceramics with reliable tribological characteristics.
For the past several years, the study of wear and frictional behaviour of transformation toughened zirconia ceramics has been conducted in our Tribology Laboratory. Preliminary results revealed that, contrary to the predictions, self-mated zirconia ceramics suffer from high friction and considerable wear in unlubricated contacts. Although traditional liquid lubricants, as subsequent tests have demonstrated, can successfully lubricate zirconia ceramics, they are not suitable for high temperatures and/or vacuum environment. The challenge, therefore, is to find some form of solid lubrication which would reduce wear and friction of zirconia ceramics down to the levels acceptable for their use as machine components.
Studies conducted in our laboratory include the investigation of metal-zirconia and polymer-zirconia tribological systems. The tests have shown that the ceramic is usually protected from damage by a metal or polymer film transferred onto the ceramic surface. In addition, in cases when the transfer film is characterized by low shear strength, it acts as solid lubricant and the reduction of friction in the system is observed. The main drawback, however, of solid lubricant films is their short lifetime and the necessity of their frequent replenishment. When a hard ceramic slides over a solid lubricant film, the film is usually rapidly worn through, especially when higher loads and speeds are applied.
SEM micrograph of the wear track on the YTZP ceramic after unlubricated sliding in air.