**4. Intermetallics matrix high temperature self-lubricating composites**

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 1979, structural intermetallics and related materials have been actively investigated. Intermetallics have given rise to various novel high temperature self-lubricating materials developments.

#### **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.

110 Tribology in Engineering

are necessary and what oxides will be effective[20].

reaction between the active elements and metal components.

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

properties over a wide temperature range are inferior to those of PM212.

novel high temperature self-lubricating materials developments.

**4.1. Ni3Al matrix high temperature self-lubricating composites** 

**4. Intermetallics matrix high temperature self-lubricating composites** 

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 1979, structural intermetallics and related materials have been actively investigated. Intermetallics have given rise to various

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,

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

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

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 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 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

## *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 1000 °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].

High Temperature Self-Lubricating Materials 113

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

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

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

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

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,

temperature range, they should not be used solely.

reinforcements were added.

*4.1.2. Effect of reinforcement on the tribological behavior* 

although the friction and wear performance were not promoted [59].

for Ni3Al.

lubricity.

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 tribochemical 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.

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*

112 Tribology in Engineering

*4.1.1. Effect of solid lubricant on the tribological behavior* 

temperature lubricant and high temperature lubricant [56].

strength and stable thermochemistry.

improvement of lubricity.

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 1000 °C), many efforts are made to a synergetic lubricating action of the composite lubricants, namely, the combination of low

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

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

In a search for even higher temperature solid lubricants for Ni3Al, many efforts have been

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

performed on inorganic salts and fluorides of alkali metals [51-53].

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].

High Temperature Self-Lubricating Materials 115

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 1000 °C (shown in Fig. 2). Especially at

800 °C, the excellent self-lubricating performance was obtained among the composites.

**Figure 4.** XPS results of worn surfaces of Ni3Al-20%Cr-12.5%Ag-10%BaF2/CaF2 composite after tests at

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 hotsintering 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 1000 °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 selflubricating 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

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 1000 °C, as shown in Table 1 and Figs. 5 and 6 [49,54,55].

different temperatures: (a) Ca2p 3/2 photoelectron peak; (b) Ba3d 5/2 photoelectron peak

temperatures.

**Figure 2.** Variations of friction coefficients and wear rates of Ni3Al-20%Cr-12.5%Ag-10%BaF2/CaF2 composite at different temperatures (tested at an applied load of 20 N and sliding speed of 0.2 m/s against Si3N4 ceramic ball)

**Figure 3.** XRD results of the sintered sample: (a) and worn surfaces of Ni3Al-20%Cr-12.5%Ag-10%BaF2/CaF2 composite after tests at different temperatures: 600 °C (b), 800 °C (c) and 1000 °C (d)

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 1000 °C (shown in Fig. 2). Especially at 800 °C, the excellent self-lubricating performance was obtained among the composites.

114 Tribology in Engineering

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

against Si3N4 ceramic ball)

20

**Friction coefficient**

200 400 600 800 1000

Friction coefficient Wear rate

**C**

0

1

2

3

**Wear rate(10-4mm**

**N**

**m**

**)**

**3 -1**

**-1**

4

5

**Temperature/o**

**Figure 2.** Variations of friction coefficients and wear rates of Ni3Al-20%Cr-12.5%Ag-10%BaF2/CaF2 composite at different temperatures (tested at an applied load of 20 N and sliding speed of 0.2 m/s

**Figure 3.** XRD results of the sintered sample: (a) and worn surfaces of Ni3Al-20%Cr-12.5%Ag-10%BaF2/CaF2 composite after tests at different temperatures: 600 °C (b), 800 °C (c) and 1000 °C (d)

**Figure 4.** XPS results of worn surfaces of Ni3Al-20%Cr-12.5%Ag-10%BaF2/CaF2 composite after tests at different temperatures: (a) Ca2p 3/2 photoelectron peak; (b) Ba3d 5/2 photoelectron peak

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 hotsintering 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 1000 °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 selflubricating 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 1000 °C, as shown in Table 1 and Figs. 5 and 6 [49,54,55].


High Temperature Self-Lubricating Materials 117

**4.2. NiAl matrix high temperature self-lubricating composites** 

properties at elevated temperatures.

Among the intermetallic family, NiAl has been selected for elevated temperature structural applications due to its low density, high oxidation resistance, high melting pointing and high conductivity [77-81]. However, NiAl is not widely used in structural applications due to its poor ductility at ambient temperatures and low strength and creep resistance at elevated temperatures. Alloying is one of effective approach that has been used successfully to improve the room temperature fracture toughness, yield strength and ductility of brittle intermetallics. NiAl-28Cr-6Mo eutectic alloys are regarded as the most logical choice of the multielement system examined to date because of their relatively high melting point, good thermal conductivity and high elevated temperature creep resistance as well as higher fracture toughness [79, 80]. Thus NiAl-28Cr-6Mo alloy may be an excellent matrix for high temperature self-lubricating composite. Recently, NiAl matrix high temperature selflubricating composites also have been explored [82, 83]. NiAl matrix composite with various high temperature solid lubricants, such as oxide and fluoride, provide excellent lubricating

It is well known that the addition of soft oxide is one of effective approach to reduce friction and wear at high temperatures because the softening oxide could offer low shear strength and high ductility and the formation of a glaze film would protect the sliding surface from heavy wear. NiAl, NiAl-Cr-Mo alloy and NiAl matrix composites with addition of oxides (ZnO/CuO) were fabricated by powder metallurgy route [82]. It was found that some new phases (such as NiZn3, Cu0.81Ni0.19 and Al2O3) formed during the fabrication process due to a high-temperature solid state reaction. The results indicated that the monolithic NiAl had high friction coefficient and wear rate at elevated temperatures due to poor mechanical properties. The incorporation of Cr(Mo) not only enhanced mechanical properties evidently but also improved high temperature tribological properties greatly. NiAl matrix composite with addition of ZnO showed superior wear resistance at 1000 °C among the sintered materials, which was due to the formation of the ZnO layer on the worn surface. NiAl matrix composite with addition of CuO exhibited self-lubricating performance at 800 °C, which was attributed to the presence of the glaze layer containing CuO and MoO3. Meanwhile, it had the best tribological properties among the sintered materials at 800 °C.

In addition, CaF2 added into NiAl matrix composite exhibited favorable friction coefficient about 0.2 and excellent wear resistance about 1 × 10-5 mm3/Nm at high temperatures (800 and 1000 °C) [83]. The excellent self-lubricating performance was attributed to the formation of the glaze film on the worn surface, which was mainly composed of CaCrO4 and CaMoO4 as high temperature solid lubricants. However, the composite had poor tribological performance at low temperatures. Addition of Ag evidently reduced friction coefficient and enhanced wear resistance at low temperatures. It indicated that Ag functioned as a favorable solid lubricant for NiAl intermetallic at low temperatures. However, it was adverse to friction and wear at elevated temperatures because of the decrease in the strength of material. On the whole, NiAl-Cr-Mo-CaF2-Ag composite provided self-lubricating properties at a broad temperature range between room temperature and 1000 °C (shown in

**Table 1.** Compressive strength of the Ni3Al-Mo-Ag-BaF2/CaF2 composite at different temperatures

**Figure 5.** Variations of friction coefficients and wear rates of the Ni3Al-Mo-Ag-BaF2/CaF2 composite at different temperatures (tested at an applied load of 20 N and sliding speed of 0.2 m/s against Si3N4 ceramic ball)

**Figure 6.** Evolution of friction coefficient of the Ni3Al-Mo-Ag-BaF2/CaF2 composite with sliding time from room temperature to 1000 °C (tested at an applied load of 10 N and sliding speed of 0.2 m/s against Si3N4 ceramic ball)

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].

#### **4.2. NiAl matrix high temperature self-lubricating composites**

116 Tribology in Engineering

ceramic ball)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

against Si3N4 ceramic ball)

elevated temperatures [50].

**Friction coefficient**

**Friction coefficient**

Temperature/°C 20 800 900 1000 Compressive strength/MPa 1200 230 100 43

**Wear rate(10-4mm**

**N**

**m**

**)**

0

0

200

400

600

**Temperature/**

**C**

**o**

800

1000

1

2

3

4

5

**3 -1**

**-1**

**Table 1.** Compressive strength of the Ni3Al-Mo-Ag-BaF2/CaF2 composite at different temperatures

Friction coefficient Wear rate

**Temperature/o**

**Figure 5.** Variations of friction coefficients and wear rates of the Ni3Al-Mo-Ag-BaF2/CaF2 composite at different temperatures (tested at an applied load of 20 N and sliding speed of 0.2 m/s against Si3N4

> Friction coefficient Temperature

0 20 40 60 80 100 120

**Time/min**

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

**Figure 6.** Evolution of friction coefficient of the Ni3Al-Mo-Ag-BaF2/CaF2 composite with sliding time from room temperature to 1000 °C (tested at an applied load of 10 N and sliding speed of 0.2 m/s

20 200 400 600 800 1000

**C**

Among the intermetallic family, NiAl has been selected for elevated temperature structural applications due to its low density, high oxidation resistance, high melting pointing and high conductivity [77-81]. However, NiAl is not widely used in structural applications due to its poor ductility at ambient temperatures and low strength and creep resistance at elevated temperatures. Alloying is one of effective approach that has been used successfully to improve the room temperature fracture toughness, yield strength and ductility of brittle intermetallics. NiAl-28Cr-6Mo eutectic alloys are regarded as the most logical choice of the multielement system examined to date because of their relatively high melting point, good thermal conductivity and high elevated temperature creep resistance as well as higher fracture toughness [79, 80]. Thus NiAl-28Cr-6Mo alloy may be an excellent matrix for high temperature self-lubricating composite. Recently, NiAl matrix high temperature selflubricating composites also have been explored [82, 83]. NiAl matrix composite with various high temperature solid lubricants, such as oxide and fluoride, provide excellent lubricating properties at elevated temperatures.

It is well known that the addition of soft oxide is one of effective approach to reduce friction and wear at high temperatures because the softening oxide could offer low shear strength and high ductility and the formation of a glaze film would protect the sliding surface from heavy wear. NiAl, NiAl-Cr-Mo alloy and NiAl matrix composites with addition of oxides (ZnO/CuO) were fabricated by powder metallurgy route [82]. It was found that some new phases (such as NiZn3, Cu0.81Ni0.19 and Al2O3) formed during the fabrication process due to a high-temperature solid state reaction. The results indicated that the monolithic NiAl had high friction coefficient and wear rate at elevated temperatures due to poor mechanical properties. The incorporation of Cr(Mo) not only enhanced mechanical properties evidently but also improved high temperature tribological properties greatly. NiAl matrix composite with addition of ZnO showed superior wear resistance at 1000 °C among the sintered materials, which was due to the formation of the ZnO layer on the worn surface. NiAl matrix composite with addition of CuO exhibited self-lubricating performance at 800 °C, which was attributed to the presence of the glaze layer containing CuO and MoO3. Meanwhile, it had the best tribological properties among the sintered materials at 800 °C.

In addition, CaF2 added into NiAl matrix composite exhibited favorable friction coefficient about 0.2 and excellent wear resistance about 1 × 10-5 mm3/Nm at high temperatures (800 and 1000 °C) [83]. The excellent self-lubricating performance was attributed to the formation of the glaze film on the worn surface, which was mainly composed of CaCrO4 and CaMoO4 as high temperature solid lubricants. However, the composite had poor tribological performance at low temperatures. Addition of Ag evidently reduced friction coefficient and enhanced wear resistance at low temperatures. It indicated that Ag functioned as a favorable solid lubricant for NiAl intermetallic at low temperatures. However, it was adverse to friction and wear at elevated temperatures because of the decrease in the strength of material. On the whole, NiAl-Cr-Mo-CaF2-Ag composite provided self-lubricating properties at a broad temperature range between room temperature and 1000 °C (shown in Fig. 7) [59]. Especially at 800 °C, the composite offered excellent friction reduction about 0.2 and wear resistance about 7 × 10-5 mm3/Nm at high temperatures. The low friction coefficient at a wide temperature range could be attributed to the synergistic effect of Ag, CaF2, CaCrO4 and CaMoO4.

High Temperature Self-Lubricating Materials 119

It was reported that the additives of graphite, MoS2, BaF2, CaF2, Ag, Ag2O, Cu2O, BaCrO4, BaSO4, SrSO4 and CaSiO3 were incorporated into zirconia ceramics, respectively, to evaluate their potentials as effective solid lubricants over a wide operating temperature range [13,86- 88]. It was found that the ZrO2 (Y2O3) composites incorporated with SrSO4 exhibited low steady-state friction coefficients of less than 0.2 and small wear rates in the order of 10−<sup>6</sup> mm3/Nm at low sliding speed from room temperature to 800 °C. The formation, plastic deformation and effective spreading of SrSO4 lubricating film were the most important

Recently, a ZrO2 matrix high temperature self-lubricating composite with addition of MoS2 and CaF2 as lubricants prepared using hot pressing method was investigated from room temperature to 1000 °C [14,89]. The ZrO2-MoS2-CaF2 composites had favorable microhardness (HV 824±90) and fracture toughness (6.5±1.4 MPa m1/2), and against SiC ceramic exhibited excellent self-lubricating and anti-wear properties at a wide temperature range. At 1000 °C, the ZrO2 matrix composite had a very low coefficient of friction of about 0.27 and wear rate of 1.54×10-5 mm3/Nm, as shown in Figs. 8 and 9. The low friction and wear were attributed to a new lubricant CaMoO4 which formed on the worn surfaces at high

0 5 10 15 20 25 30

Time/min

**Figure 8.** Evolution of friction coefficient of the ZrO2-MoS2-CaF2 composite with sliding time at 1000 °C

(tested at an applied load of 10 N and sliding speed of 0.2m/s against SiC ceramic ball)

factor to reduce friction and wear rate over a wide temperature range.

temperatures (seen in Fig. 10).

Friction coefficient

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

**Figure 7.** Variations of friction coefficients and wear rates of NiAl-Cr-Mo-CaF2-Ag at different temperatures (tested at an applied load of 10 N and sliding speed of 0.2 m/s against Si3N4 ceramic ball)
