**2. High temperature self-lubricating alloys**

It is well known that the friction and wear of metal alloys at high temperatures are controlled by their tribochemically generated oxide films [16-20]. Consequently, on a hard metal substrate-formed lubricating soft oxide layer, with low shear strength, results in considerable wear reduction and sometimes a decrease of friction. Hence the idea is reasonable and feasible that realization of self-lubricating property by in situ oxide formation on sliding surface at high temperatures. Peterson M.B. and Li S.Z. applied this concept to develop high temperature self-lubricating alloys, such as Ni-Cu-Re, Co-Cu-Re, and Fe-Re, by lubrication with naturally occurring oxides during the sliding process [19-23].

Meanwhile, the principles of oxide lubrication and to develop alloys based on tribochemically generated oxide films were proposed as to what alloy compositions will produce effective oxide films, what interface temperatures and what operating conditions are necessary and what oxides will be effective[20].

High Temperature Self-Lubricating Materials 111

oxidation and corrosion. The results indicated that Ni3Al may be an excellent matrix for high temperature self-lubricating composite owing to its high temperature strength, good oxidation resistance and corrosion resistance behavior. However, till now, the tribological

Recently, a series of Ni3Al high temperature self-lubricating composites were developed in Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences [15, 48-55]. The selflubricating composites, which consist of Ni3Al matrix with Cr/Mo/W, Ag and BaF2/CaF2 additions, exhibit the low friction coefficient and wear rate at a wide temperature range from room temperature to 1000 °C. Additionally, in order to design and fabricate high temperature self-lubricating composite with excellent tribological property from room temperature to 1000 °C and also explore the friction and wear mechanisms at high temperatures, the effects of solid lubricant and reinforcement on tribological properties of Ni3Al matrix high temperature self-lubricating composites at a wide temperature range from room temperature to 1000 °C were investigated. The tribological behavior was studied from room temperature to 1000 °C on an HT-1000 ball-on-disk high temperature tribometer. The schematic diagram of HT-1000 ball-on-disk high-temperature tribometer is shown in Fig. 1. The rotating disk was made of the sintered sample with a size of 18.5 × 18.5 × 5 mm, and the stationary ball was the commercial Si3N4 or SiC ceramic ball with a diameter of 6 mm. The selected test temperatures were room temperature, 200, 400, 600, 800 and 1000 °C. The tribological tests were carried out at an applied load of 10 or 20 N, sliding speed of 0.2 m/s and testing time of 30 or 60 min. The furnace temperature, which was monitored using a

behavior of Ni3Al matrix composite has not been researched systemically.

thermocouple, was raised at a heating rate of 10-12 °C /min to the set point.

**Figure 1.** The schematic diagram of HT-1000 ball-on-disk high-temperature tribometer

In addition, it is another effective approach to improvement of tribological properties by addition of active elements, such as sulfur and selenium [24,25]. At the interface, heating and sliding produces certain compounds with lubricious properties by tribochemical reaction between the active elements and metal components.
