High Temperature Self-Lubricating Materials

Since the strong internal order and mixed (metallic and covalent/ionic) bonding, intermetallic compounds often offer a compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough to sacrifice some toughness and ease of processing [36-38]. Since Aoki and Izumi reported the remarkable achievements of ductility in Ni3Al alloys by B doping in , structural intermetallics and related materials have been actively investigated. Intermetallics have given rise to various novel high temperature self-lubricating materials developments.

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4.1. Ni3Al matrix high temperature self-lubricating composites

Ni3Al is the intermetallic compound that has been most intensively studied from both fundamental and practical points of view [39-47]. In the past years, a great deal of work has been addressed to the study of the effect of alloying elements, mechanical properties, 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 behavior of Ni3Al matrix composite has not been researched systemically.

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 self-lubricating 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 °C. Additionally, in order to design and fabricate high temperature self-lubricating composite with excellent tribological property from room temperature to °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 °C were investigated. The tribological behavior was studied from room temperature to °C on an HT- ball-on-disk high temperature tribometer. The schematic diagram of HT- 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 °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 thermocouple, was raised at a heating rate of 10-12 °C /min to the set point.

4.1.1. Effect of solid lubricant on the tribological behavior

To obtain high temperature self-lubricating materials with well tribological and mechanical properties, suitable solid lubricant selected is very important. Since no single material can provide adequate lubricating properties over a wide temperature range from room temperature to high temperatures (800 or even °C), many efforts are made to a synergetic lubricating action of the composite lubricants, namely, the combination of low temperature lubricant and high temperature lubricant [56].

The conventional solid lubricants, such as MoS2 and graphite, cannot meet the demand on tribological and mechanical properties due to their inadequate oxidation resistance in air above 500 °C. Hexagonal boron nitride (hBN) has been considered an effective solid lubricant for high temperature applications since it has a graphite-like lamellar structure. However, the non-wettability and poor sinterability of hBN would restrict its applications. Except for the above layered lubricants, soft noble metal Ag and Au should be as a promising lubricant for Ni3Al at low temperatures (below 450 °C) due to the low shear strength and stable thermochemistry.

It was found that Ag added into the Ni3Al matrix composite exhibited no reactants between Ag and other additives detected after the hot-sintering process. Moreover, the composite with Ag had higher strength than those with graphite or MoS2. Furthermore, during frictional process, Ag kept favorable thermal stability at low temperatures, whereas oxidation reaction could happen between Ag and other additives in the composite at high temperatures. It is noteworthy that the oxidation products like AgMoO4 are beneficial to improvement of lubricity.

In a search for even higher temperature solid lubricants for Ni3Al, many efforts have been performed on inorganic salts and fluorides of alkali metals [51-53].

Fluorides have shown promise as high-temperature solid lubricants to provide low friction coefficient and wear according to the previous references [57,58]. Ni3Al-Cr-Ag-BaF2/CaF2 composites were synthesized by powder metallurgy technique [15,51,59]. XRD results indicated that components in the sintered Ni3Al matrix composites did not react on each other and no any new compound formed during the fabrication process. XRD patterns of worn surfaces after frictional tests presented that at 600 °C, BaCO3 in the form of weak peak appears, and at 800 °C, no BaF2 peaks present but BaCrO4 peaks were found. Fluorides served as high temperature lubricants and exhibited a good reduce-friction performance at 400 and 600 °C. However, at 800 °C, BaCrO4 formed on the worn surface due to the tribo-chemical reaction at high temperatures provided an excellent lubricating property.

Inorganic salts are obvious candidates for consideration owing to low shear strength and high ductility at elevated temperatures. The high temperature lubricious behavior of some sulfates, chromates, molybdates and tungstates has been extensively studied [60-66]. Important early work on high-temperature solid lubricant reported that molybdates appeared to be the promising high-temperature solid lubricants [56]. As a high-temperature solid lubricant, and similar to CaWO4 and CaMoO4, BaMoO4 has scheelite structure and adequate thermophysial properties [67, 68]. However, till now, the lubricious behavior of BaMoO4 has not been explored in detail. Recently, BaCrO4 has attracted much attention due to its lubricating property at a wide temperature range [62]. BaCrO4 has an orthorhombic structure, and its thermal data shows that the BaCrO4 phase is thermally stable to 850 °C [69,70]. Therefore, they could be expected as promising high-temperature solid lubricants for Ni3Al.

It can be noted that no BaMoO4 peaks presented but Ni, Mo and BaAl2O4 peaks were found in XRD results of the sintered Ni3Al matrix composites, and the peaks of Ni, Mo and BaAl2O4 get stronger with the increase of BaMoO4. This means that the formation of Ni, Mo and BaAl2O4 results from high-temperature solid state reaction between Ni3Al and BaMoO4 during the fabrication process. However, during the sliding process at high temperatures, BaMoO4 re-formed on the worn surfaces. The occurrence of BaMoO4 is possible when considering the higher temperature rise at the instantaneous contacting surface in the rubbing process at high temperatures. It could come from the oxidation of Mo and then a reaction with BaAl2O4. The frictional results showed that Ni3Al matrix composites with addition of BaMoO4 offered better friction behavior than the monolithic Ni3Al above 600 °C. The addition of BaMoO4 could improve the tribological property, but lead to a decrease in hardness. Below 400 °C, Ni3Al matrix composites with addition of BaMoO4 wre non-lubricating, unless at 600°C, re-formed BaMoO4 provided a well lubricity.

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The same as BaMoO4, Ni3Al composites with addition of BaCrO4 showed the absence of BaCrO4 but the formation of BaAl2O4 during the fabrication process. At high temperatures, it was found the re-formation of BaCrO4 on the worn surface. Since BaMoO4 and BaCrO4 as solid lubricants for Ni3Al intermetallics only have low friction coefficient at narrow temperature range, they should not be used solely.

4.1.2. Effect of reinforcement on the tribological behavior

From the point of view of the principle of tribology, the ideal composition of a high temperature solid lubricant material should be composed of high strength matrix, reinforcement and solid lubricant. Reinforcement plays a significantly role in mechanical properties and tribological behavior. Generally, the reinforcement can be classified into two categories: one is the hard ceramic phase, and the other is the soft metal phase. In order to promote the tribological performance of Ni3Al matrix composites, the different kinds of reinforcements were added.

Titanium carbide is selected as reinforcement because it is a ceramic with high melting point, extreme hardness, low density, moderate fracture toughness, and high resistance to oxidation and corrosion and a very good wettability with Ni3Al [71-75]. Observations on TiC reinforced Ni3Al matrix composite showed that the mechanical properties were improved, although the friction and wear performance were not promoted [59].

Chromium additions to Ni3Al, as a solution, have been reported the effectiveness of alloying about 8 at % Cr for suppressing the oxygen embrittlement of Ni3Al alloys at intermediate temperatures [39-42]. Additionally, Cr particles, as reinforcement, can improve the strength of Ni3Al-Cr composite at low temperatures, whose strength is determined by the strength of the Cr particles and the good bonding between the matrix and Cr reinforcement [76]. The results presented that Cr added to Ni3Al matrix composite not only enhanced mechanical strength but also ameliorated tribological performance [15]. Further study on the Ni3Al-Cr-Ag-BaF2/CaF2 self-lubricating composite was carried out by tailoring the composition of the additives [48,51,59]. It was found that Ni3Al-20 % Cr-12.5 % Ag-10 % BaF2/CaF2 (in weight) composite offered the low friction coefficient 0.24-0.37 and wear rate 0.52-2.32 × 10-4 mm3/Nm at a wide temperature range from room temperature to °C (shown in Fig. 2). Especially at 800 °C, the excellent self-lubricating performance was obtained among the composites.

XRD results of the sintered sample and worn surfaces of Ni3Al-20 % Cr-12.5 % Ag-10 % BaF2/CaF2 composite after tests at different temperatures were represented in Fig. 3. There were no reactants among the Ni3Al, fluorides, Ag and Cr detected after the hot-sintering process in XRD result of the sintered sample. However, peaks of BaCO3 and NiO appeared on worn surface at 600 °C, and as did little BaCrO4. Moreover, peaks of chromates get stronger with increase in temperature from 800 to °C, indicating that large amounts of chromates formed on worn surfaces owing to the complex reaction including high temperature reaction and tribo-chemical reaction. Also, XPS results in Fig. 4 demonstrated the formation of chromates on worn surfaces at high temperatures. The favorable self-lubricating property of Ni3Al-BaF2-CaF2-Ag-Cr composite at a broad temperature range was attributed to the synergistic effects of Ag, fluorides and chromates formed at high temperatures.

Moreover, another self-lubricating composite Ni3Al-Mo-Ag-BaF2/CaF2 offers acceptable mechanical strength and excellent tribological properties over a wide temperatures ranging from room temperature to °C, as shown in Table 1 and Figs. 5 and 6 [49,54,55].

Table 1.

Compressive strength of the Ni3Al-Mo-Ag-BaF2/CaF2 composite at different temperatures

In addition, tungsten as reinforcement for Ni3Al-Ag-BaF2/CaF2 composite is selected based on the premise that fluoride and tungsten are expected to react with oxygen at high temperatures and create tungstate lubricants on the worn surface. As expected, barium and calcium tungstates with lubricious properties contributed to low friction coefficient at elevated temperatures [50].

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