High temperature lubricant is a kind of lubricant that can be used to reduce friction and minimize wear between moving contact surfaces at elevated temperature, including gas, liquid or solid lubricant. High temperature lubricant is an entity functioning as high temperature lubricity. At high temperature, gas lubrication technology has been not mature. Due to decomposition and rapid degradation on exposure to the elevated temperatures, liquid lubricant is limited to less than 350 °C. However, the usable temperature of some solid lubricants can even reach 1000 °C. The use of high temperature solid lubricants is the only viable alternative to reduce friction in many high temperature environments, especially those that include temperatures above 350 °C. High temperature solid lubricant has a wide variety and the complex lubrication mechanism, which mainly includes the following types: (1) layered structure material with weak interlayer force, such as graphite, MoS2; (2) soft metal with multiple slip planes, such as Ag, Au; (3)metal fluorides and oxides with thermal softening, like CaF2, PbO,AgMoO4. The lubricity of solid lubricants depends closely on ambient temperature. Traditional solid lubricants with layered structure like graphite and MoS2 can provide lubrication below 400 °C due to undesirable oxidation at elevated temperatures, while fluoride graphite and WS2 can withstand temperatures up to 500 °C. Moreover, hexagonal boron nitride (hBN) can maintain lubrication at temperatures up to 1000 °C. Some soft metals (e.g., Ag, Au) offer low friction at low and elevated temperatures. Metal fluorides and oxides work quite well as high temperature solid lubricants, but both fail to provide lubrication at room or lower ambient temperatures. High temperature solid lubricant can be used in form of single or composite lubricants for various purposes. Since there is no “universal lubricant” that can operate at a broad temperature range conditions, a synergetic/combining lubricating action, a mixture of two or more solid lubricants, is one of promising strategies to achieve high temperature solid-lubricating materials. The well-known solid lubricant combination is Ag and BaF2/CaF2 eutectic. Soft metal Ag works as a solid lubricant below 500 °C, whereas BaF2/CaF2 eutectic offers the lubricating function above 450 °C. The combination of Ag and BaF2/CaF2 eutectic can provide favorable lubricity from room temperature to 1000 °C. In addition to conventional solution that lubrication is obtained by adding solid lubricant into high temperature matrix during fabrication process, another solution is in situ formation of lubricious compounds by tribochemical reaction during friction process. This design approach has been used in many materials, such as adaptive nitride-based high temperature solid-lubricating coatings and Ni3Al matrix high temperature solid-lubricating composites. Soft high temperature solid lubricant cannot carry heavy load and undergo abrasive wear. Consequently, high temperature materials need be used as matrix to bind solid lubricant and bear load. To achieve high wear resistance and low friction coefficient, high temperature matrix material is required to have suitable compatibility with solid lubricant, such as mechanical properties, physical properties and chemical properties. As for mechanical properties, high temperature matrix material needs to possess favorable hardness, adequate ductility and high strength to confront abrasive wear and fatigue wear. Moreover, high oxidation resistance is vital to forestall oxidative wear at high temperature. Additionally, suitable thermal expansion coefficient is also critical factor to obtain an effectively lubricating film on the worn surface. High temperature matrix material mainly includes metal, intermetallic and ceramic. Furthermore, reinforcing phase is added into high temperature matrix material to enhance the hardness for metal and improve the toughness for ceramic. Sometimes, it is allowed assisting phase to adjust compatibility for tribological system, such as thermal mismatch, corrosive properties, bonding strength, and transfer effect.
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