**3. Ni matrix high temperature self-lubricating composites**

There is a great need in current technology for solid lubricated systems that will perform satisfactorily over a wide range of temperatures. Ni matrix high temperature self-lubricating composites play a significant role in attaining this goal and achieve applications. In the past years, a series of self-lubricating composites based on nickel alloys have been developed by powder metallurgy methods [26-35]. Of these materials, PM212, which was developed by NASA Glenn (previous NASA Lewis) Research Center, shows promise for use over a wide range of temperatures (ranging from room temperature to 900 °C) [26,27]. This material is comprised of metal Ni-Co binder, ceramic Cr3C2 matrix and solid lubricants CaF2/BaF2 and Ag. These three components play roles in providing binding, wear resistance and selflubrication, respectively. Another NASA PM304 (NiCr-Cr2O3-Ag-BaF2/CaF2) possesses well tribological behavior from room temperature to 650 °C [28], however, above 800 °C, the decline in mechanical property degrades its wear resistance. Based on the design view of NASA, many efforts are made to explore new self-lubricating composites based on nickel alloys, such as Nickel alloy-graphite-Ag, Nickel alloy-WC/SiC-PbO, Nickel alloy-Ag-CeF3, Nickel alloy-graphite-CeF3, Nickel alloy-MoS2-graphite, etc. [29-35]. Although the Ni-based high temperature self-lubricating composites attract much attention, their friction and wear properties over a wide temperature range are inferior to those of PM212.
