**5. Ceramic matrix high temperature self-lubricating composites**

Advanced structural ceramics are expected to be suitable for tribo-systems because of their high hardness and corrosion resistance at high temperature [84,85]. A major challenge in advanced structural ceramics is to develop long-lifetime and reproducible ceramic sliding components for use in mechanical systems that involve high loads, velocities and temperatures. As the friction of unlubricated ceramic surfaces at elevated temperatures is usually high and unacceptable, it is necessary to find ways of effectively lubricating ceramics. Ceramic matrix composites in which solid lubricant is dispersed throughout the structure are advantageous when long lubrication life is required, compared to selflubricating coatings. In recent years, ceramic matrix high temperature self-lubricating composites have attracted the attention of many researchers.

#### **5.1. Zirconia matrix high temperature self-lubricating composites**

Tetragonal zirconia polycrystals stabilized by yttria present a good combination of fracture toughness and bending strength, which is related to the stress-induced phase transformation of tetragonal ZrO2 (Y2O3) into monoclinic symmetry. Therefore, zirconia ceramics are potential candidates for a host of engineering applications, especially at high temperatures. However, the friction coefficient of zirconia ceramics in dry sliding is enough high not to acceptable for engineering applications. Consequently, it is quite necessary to research and develop ZrO2 (Y2O3) matrix high temperature self-lubricating composites.

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

118 Tribology in Engineering

and CaMoO4.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

**Friction coefficient**

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

Friction coefficient Wear rate

**Temperature/o**

**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)

**5. Ceramic matrix high temperature self-lubricating composites** 

composites have attracted the attention of many researchers.

**5.1. Zirconia matrix high temperature self-lubricating composites** 

20 200 400 600 800 1000

Advanced structural ceramics are expected to be suitable for tribo-systems because of their high hardness and corrosion resistance at high temperature [84,85]. A major challenge in advanced structural ceramics is to develop long-lifetime and reproducible ceramic sliding components for use in mechanical systems that involve high loads, velocities and temperatures. As the friction of unlubricated ceramic surfaces at elevated temperatures is usually high and unacceptable, it is necessary to find ways of effectively lubricating ceramics. Ceramic matrix composites in which solid lubricant is dispersed throughout the structure are advantageous when long lubrication life is required, compared to selflubricating coatings. In recent years, ceramic matrix high temperature self-lubricating

Tetragonal zirconia polycrystals stabilized by yttria present a good combination of fracture toughness and bending strength, which is related to the stress-induced phase transformation of tetragonal ZrO2 (Y2O3) into monoclinic symmetry. Therefore, zirconia ceramics are potential candidates for a host of engineering applications, especially at high temperatures. However, the friction coefficient of zirconia ceramics in dry sliding is enough high not to acceptable for engineering applications. Consequently, it is quite necessary to research and develop ZrO2 (Y2O3) matrix high temperature self-lubricating composites.

**C**

**Wear rate(10-4mm**

**N**

**m**

**)**

**3 -1**

**-1**

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 temperatures (seen in Fig. 10).

**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)

**Figure 9.** Variations of wear rates of the ZrO2-MoS2-CaF2 composite at different temperatures (tested at an applied load of 10 N and sliding speed of 0.2m/s against SiC ceramic ball)

High Temperature Self-Lubricating Materials 121

exhibited a distinct improvement in wear resistance and frictional characteristics at elevated temperatures. The self-lubricating behavior was dominated by a synergistic effect. The lubricating film as a mixture of Ag and CaF2 on friction surfaces was responsible for the

Si3N4-based ceramics are potential substitutes for more traditional materials for these specific applications due to their high hardness, excellent chemical and mechanical stability under a broad range of temperatures, low density, low thermal expansion and high specific stiffness [93]. The incorporation of solid lubricants is a goal to further enhance the tribological performance of Si3N4 [94-98]. The published papers indicated that Cscompounds are exceptional promises as high temperature lubricants for Si3N4 ceramic. Cscompound provided favorable lubrication on Si3N4 from room temperature to 750 °C, especially with an average value of 0.03 at 600 °C. The synergistic chemical reactions occurred between the cesium compounds, Na2SiO3, and the Si3N4 surface to provide the

The class of refractory oxygen-free compounds possesses a layered structure and a unique combination of metal and ceramic properties, which are generally described by the formula Mn+1AXn, where M is the transition metal, A is the preferentially subgroup IIIA or IVA element of the periodic table, and X is carbon or nitrogen [99-102]. They are characterized by a low density; high thermal conductivity, electrical conductivity, and strength; excellent corrosion resistance in aggressive external media; resistance to high-temperature oxidation; and tolerance to thermal shocks. Additionally, due to their layered structure and by analogy with hexagonal boron nitride and graphite, it is proposed that they are self-lubricating and possesses low friction coefficient. However, in the previous literature on friction and wear, Mn+1AXn phases did not exhibit the expected tribological properties at high temperatures. Although there exists the debate on their intrinsically self-lubricating behavior, they could be appropriate candidate for high temperature self-lubricating matrix due to the combination of metals and ceramics properties [103-107]. In order to lubricate Mn+1AXn phases, many efforts have been made in recent years. Among them, Mn+1AXn matrix composites employed Ag as solid lubricant are the promising materials for high

High temperature self-lubricating composites with good high temperature anti-oxidation ability have been developed to reduce friction and wear from room temperature to high operating temperatures in many tribological systems. Since it is difficult or impossible for a bulk monolithic material to possess all the above mentioned surface properties [112], much attention has been paid to metallic matrix composite coatings which contain solid lubricants

**5.3. Silicon nitride matrix high temperature self-lubricating composites** 

**5.4. Mn+1AXn matrix high temperature self-lubricating composites** 

reduction of friction and wear at elevated temperature.

remarkable performance.

temperature tribological applications [107-111].

**6. High temperature self-lubricating coatings** 

**Figure 10.** XRD patterns of the ZrO2-MoS2-CaF2 composite (a) and its worn surfaces at different temperatures: 200 °C (b), 400 °C (c), 600 °C (d), 800 °C (e) and 1000 °C (f)

#### **5.2. Alumina matrix high temperature self-lubricating composites**

Alumina is a promising material at high temperature because of its excellent chemical stability and low price. However, tribological experiments of alumina sliding against itself at high temperature show high friction coefficient and wear rate. Solid lubrication becomes necessary to overcome this problem. In order to lubricate alumina ceramics, many efforts have been made in recent years. Among them, alumina matrix composite employed Ag and fluoride as solid lubricants is a successful example [90-92]. The Al2O3-Ag-CaF2 composite exhibited a distinct improvement in wear resistance and frictional characteristics at elevated temperatures. The self-lubricating behavior was dominated by a synergistic effect. The lubricating film as a mixture of Ag and CaF2 on friction surfaces was responsible for the reduction of friction and wear at elevated temperature.

### **5.3. Silicon nitride matrix high temperature self-lubricating composites**

120 Tribology in Engineering

1E-6

1E-5

1E-4

Wear rate(mm3/Nm)

1E-3

0 200 400 600 800 1000

C

Temperature/o

**Figure 9.** Variations of wear rates of the ZrO2-MoS2-CaF2 composite at different temperatures (tested at

**Figure 10.** XRD patterns of the ZrO2-MoS2-CaF2 composite (a) and its worn surfaces at different

Alumina is a promising material at high temperature because of its excellent chemical stability and low price. However, tribological experiments of alumina sliding against itself at high temperature show high friction coefficient and wear rate. Solid lubrication becomes necessary to overcome this problem. In order to lubricate alumina ceramics, many efforts have been made in recent years. Among them, alumina matrix composite employed Ag and fluoride as solid lubricants is a successful example [90-92]. The Al2O3-Ag-CaF2 composite

**5.2. Alumina matrix high temperature self-lubricating composites** 

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

temperatures: 200 °C (b), 400 °C (c), 600 °C (d), 800 °C (e) and 1000 °C (f)

Si3N4-based ceramics are potential substitutes for more traditional materials for these specific applications due to their high hardness, excellent chemical and mechanical stability under a broad range of temperatures, low density, low thermal expansion and high specific stiffness [93]. The incorporation of solid lubricants is a goal to further enhance the tribological performance of Si3N4 [94-98]. The published papers indicated that Cscompounds are exceptional promises as high temperature lubricants for Si3N4 ceramic. Cscompound provided favorable lubrication on Si3N4 from room temperature to 750 °C, especially with an average value of 0.03 at 600 °C. The synergistic chemical reactions occurred between the cesium compounds, Na2SiO3, and the Si3N4 surface to provide the remarkable performance.

#### **5.4. Mn+1AXn matrix high temperature self-lubricating composites**

The class of refractory oxygen-free compounds possesses a layered structure and a unique combination of metal and ceramic properties, which are generally described by the formula Mn+1AXn, where M is the transition metal, A is the preferentially subgroup IIIA or IVA element of the periodic table, and X is carbon or nitrogen [99-102]. They are characterized by a low density; high thermal conductivity, electrical conductivity, and strength; excellent corrosion resistance in aggressive external media; resistance to high-temperature oxidation; and tolerance to thermal shocks. Additionally, due to their layered structure and by analogy with hexagonal boron nitride and graphite, it is proposed that they are self-lubricating and possesses low friction coefficient. However, in the previous literature on friction and wear, Mn+1AXn phases did not exhibit the expected tribological properties at high temperatures. Although there exists the debate on their intrinsically self-lubricating behavior, they could be appropriate candidate for high temperature self-lubricating matrix due to the combination of metals and ceramics properties [103-107]. In order to lubricate Mn+1AXn phases, many efforts have been made in recent years. Among them, Mn+1AXn matrix composites employed Ag as solid lubricant are the promising materials for high temperature tribological applications [107-111].

#### **6. High temperature self-lubricating coatings**

High temperature self-lubricating composites with good high temperature anti-oxidation ability have been developed to reduce friction and wear from room temperature to high operating temperatures in many tribological systems. Since it is difficult or impossible for a bulk monolithic material to possess all the above mentioned surface properties [112], much attention has been paid to metallic matrix composite coatings which contain solid lubricants prepared by various processes, such as PS coatings by plasma spray [11,113], Ni/hBN composite coating by laser cladding [114], adaptive nitride-based coating by unbalanced magnetron sputtering [115], and Ni3Al matrix composite coating by powder metallurgy [116].

High Temperature Self-Lubricating Materials 123

sinterability as well [114]. The resulting Ni-coated hBN particulates are then used to prepare a self-lubricating wear-resistant composite coating on the stainless steel substrate with the assistance of laser cladding. Laser cladding Ni/hBN composite coating on the stainless steel substrate was composed of metallic Ni and hBN, and a small amount of B-matrix interphases, and it had high hardness and uniformly distributed constituent phases. The friction and wear behavior of the laser cladding Ni/hBN coating was strongly dependent on test temperature. The coating had good friction-reducing and anti-wear abilities as it slid against the ceramic counterpart at elevated temperatures up to 800 °C, which could be owing to the good lubricating performance of the hBN particles as a kind of hightemperature solid lubricant. The wear rate of the coating increased to some extent as the test temperature rose from 600 °C up to 800 °C, which could be attributed to the decrease in the

strength of the coating at excessively high-temperature.

atomic structure.

microstructure.

**6.3. Adaptive nitride-based high temperature self-lubricating coatings** 

Adaptive tribological coatings have been recently developed as a new class of smart materials that are designed to adjust their surface chemical composition and structure as a function of changes in the working environment to minimize friction coefficient and wear between contact surfaces [12, 122-124]. At a wide temperature range, VN/Ag adaptive tribological coatings produced using unbalanced magnetron sputtering exhibited excellent self-lubricating properties [115]. The friction coefficient was found to vary from 0.35 at room temperature to about 0.15-0.20 in the 700-1000 °C range. After tribotesting, Raman spectroscopy and X-ray diffraction measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000 °C. Upon cooling, silver and vanadium oxide were found. Silver reduced the friction coefficient at low temperatures, while the Ag3VO4 phase provided low friction at high temperatures due to its layered

**6.4. Intermetallics matrix high temperature self-lubricating composite coatings** 

Powder metallurgy is a convenient method to prepare bulk components with fine and dense microstructures. In contrast to the plasma spraying technique, where some components may be lost during the deposition process, the final composition is the same as that of the starting powders. Hence powder metallurgy is applied to prepare the coatings with a fine and dense

One of the most attractive engineering properties of Ni3Al alloys is their increasing yield strength with increasing temperature up to about 650-750 °C. This type of strength behavior suggests that the Ni3Al-based intermetallic alloys may have good wear properties in the peak-strength temperature range. Consequently, investigations of Ni3Al intermetallics for

tribological coating matrix at high temperatures were initiated.

### **6.1. PS high temperature self-lubricating coatings**

In the past 40 years, the PS100, PS200, PS300 and PS400 families of plasma sprayed coatings with self-lubricating behavior were developed at NASA Lewis Research Center (shown in Table 2) [11,113,117-121]. The PS100 family of nickel-glass-solid lubricant-containing coatings pioneered the concept of combining the functions of individual constituents to produce a composite solid lubricant coating. PS200 coatings developed the composite concept, which consisted of a hard nickel-cobalt-bonded chrome carbide matrix and solid lubricants of Ag and BaF2/CaF2 eutectic. The PS300 coating system replaced the harder chrome carbide of PS200 coatings with chrome oxide, eliminating the necessity of costly diamond grinding and providing improved resistance to oxidative changes in hightemperature air. This coating was not very hard but had desirable tribological performance, such as good wear resistance and low friction coefficient, especially at elevated temperature up to 650 °C. NASA has recently developed a new solid lubricant coating, PS400, due to several drawbacks of PS300, namely the need to undergo a heat treatment for dimensional stabilization and poor initial surface finish. These four distinct families of coatings were engineered over the last four decades to address specific tribological challenges encountered in various aerospace systems.



## **6.2. Ni/hBN high temperature self-lubricating composite coating**

The non-wettability and poor sinter ability of hBN restrict its applications as a solid lubricant, though it has a graphite-like lamellar structure. Fortunately, the hBN powders electroplated with Ni can improve the wettability with 1Cr18Ni9Ti stainless steel and sinterability as well [114]. The resulting Ni-coated hBN particulates are then used to prepare a self-lubricating wear-resistant composite coating on the stainless steel substrate with the assistance of laser cladding. Laser cladding Ni/hBN composite coating on the stainless steel substrate was composed of metallic Ni and hBN, and a small amount of B-matrix interphases, and it had high hardness and uniformly distributed constituent phases. The friction and wear behavior of the laser cladding Ni/hBN coating was strongly dependent on test temperature. The coating had good friction-reducing and anti-wear abilities as it slid against the ceramic counterpart at elevated temperatures up to 800 °C, which could be owing to the good lubricating performance of the hBN particles as a kind of hightemperature solid lubricant. The wear rate of the coating increased to some extent as the test temperature rose from 600 °C up to 800 °C, which could be attributed to the decrease in the strength of the coating at excessively high-temperature.

#### **6.3. Adaptive nitride-based high temperature self-lubricating coatings**

122 Tribology in Engineering

in various aerospace systems.

Binder matrix Harder Solid

**Table 2.** Comparision of the NASA plasma spray coating

PS100 NiCr Glass Ag+Fluorides Soft-high wear

lubricants

**6.2. Ni/hBN high temperature self-lubricating composite coating** 

PS200 Ni-Co Cr3C2 Ag+Fluorides Hard-low wear, (abrasive to counter face

PS300 NiCr Cr2O3 Ag+Fluorides Moderate hardness, mildly abrasive to

PS400 NiMoAl Cr2O3 Ag+Fluorides Excellent dimensional stability and surface

The non-wettability and poor sinter ability of hBN restrict its applications as a solid lubricant, though it has a graphite-like lamellar structure. Fortunately, the hBN powders electroplated with Ni can improve the wettability with 1Cr18Ni9Ti stainless steel and

General attributes

dimensionally stable)

requires heat treatment

counter face, poor dimensional stability-

finish, poor initial low temperature tribology

Coating designation

[116].

prepared by various processes, such as PS coatings by plasma spray [11,113], Ni/hBN composite coating by laser cladding [114], adaptive nitride-based coating by unbalanced magnetron sputtering [115], and Ni3Al matrix composite coating by powder metallurgy

In the past 40 years, the PS100, PS200, PS300 and PS400 families of plasma sprayed coatings with self-lubricating behavior were developed at NASA Lewis Research Center (shown in Table 2) [11,113,117-121]. The PS100 family of nickel-glass-solid lubricant-containing coatings pioneered the concept of combining the functions of individual constituents to produce a composite solid lubricant coating. PS200 coatings developed the composite concept, which consisted of a hard nickel-cobalt-bonded chrome carbide matrix and solid lubricants of Ag and BaF2/CaF2 eutectic. The PS300 coating system replaced the harder chrome carbide of PS200 coatings with chrome oxide, eliminating the necessity of costly diamond grinding and providing improved resistance to oxidative changes in hightemperature air. This coating was not very hard but had desirable tribological performance, such as good wear resistance and low friction coefficient, especially at elevated temperature up to 650 °C. NASA has recently developed a new solid lubricant coating, PS400, due to several drawbacks of PS300, namely the need to undergo a heat treatment for dimensional stabilization and poor initial surface finish. These four distinct families of coatings were engineered over the last four decades to address specific tribological challenges encountered

**6.1. PS high temperature self-lubricating coatings** 

Adaptive tribological coatings have been recently developed as a new class of smart materials that are designed to adjust their surface chemical composition and structure as a function of changes in the working environment to minimize friction coefficient and wear between contact surfaces [12, 122-124]. At a wide temperature range, VN/Ag adaptive tribological coatings produced using unbalanced magnetron sputtering exhibited excellent self-lubricating properties [115]. The friction coefficient was found to vary from 0.35 at room temperature to about 0.15-0.20 in the 700-1000 °C range. After tribotesting, Raman spectroscopy and X-ray diffraction measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000 °C. Upon cooling, silver and vanadium oxide were found. Silver reduced the friction coefficient at low temperatures, while the Ag3VO4 phase provided low friction at high temperatures due to its layered atomic structure.

#### **6.4. Intermetallics matrix high temperature self-lubricating composite coatings**

Powder metallurgy is a convenient method to prepare bulk components with fine and dense microstructures. In contrast to the plasma spraying technique, where some components may be lost during the deposition process, the final composition is the same as that of the starting powders. Hence powder metallurgy is applied to prepare the coatings with a fine and dense microstructure.

One of the most attractive engineering properties of Ni3Al alloys is their increasing yield strength with increasing temperature up to about 650-750 °C. This type of strength behavior suggests that the Ni3Al-based intermetallic alloys may have good wear properties in the peak-strength temperature range. Consequently, investigations of Ni3Al intermetallics for tribological coating matrix at high temperatures were initiated.

Recently, a Ni3Al matrix coating containing Ag, Mo and BaF2/CaF2 was fabricated by the vacuum hot-pressed sintered technology [116]. Fig. 11 presented the morphology of the interface of the composite coating. A small amount of nickel powders were spread on the surface of the substrate to improve the wettability between the coating and AISI 1045 carbon steel. The Ni3Al layer on the layer of nickel powder reduced the stress concentration between the substrate and the coating as well as to improve the bonding strength. The mixed composite powders Ni3Al-Mo-Ag-fluorides were spread on the layer. After sintering, it could be seen that there were no pores and cracks near the interface region and the composite coating layer was well adhered to the substrate (Fig. 11a). The morphology of the interface of the coating (after thermal shock) was shown in Fig. 11b. It showed that the coating did not peel off and even no cracks after the thermal shock, indicating that the coating possesses excellent bonding strength.

High Temperature Self-Lubricating Materials 125

The microhardness depth profile in the transverse cross-section of the composite coating was shown in Fig. 12. It was found that the coating was in three layers according to hardness. The Vickers hardness (HV) of the nickel interlayer was 1.40±0.50 GPa, and that of the Ni3Al layer is 3.80±0.50 GPa, and at the top surface, the HV is 3.60±0.50 GPa. The result showed that the top layer was not the hardest part of the coating, and the hard Ni3Al layer provided efficient support in the coating. The multilayer structure of the coating could

The average friction coefficients of the coating were presented in Fig. 13. The friction coefficients of the coating were approximately 0.35 from 25 to 800 °C, however, when the temperature reached 1000 °C, the friction coefficients fell to 0.24. In comparison, the friction coefficient of the AISI 321 stainless steel was much higher than that of the coating. Fig. 14 showed the wear rates of the coating at various temperatures in air. Although the wear rates of the coating were higher than that of the AISI 321 stainless steel at the temperature between 200 and 400 °C, but they were lower at room temperature and high temperature

0.7 Coating

AISI 321 Steel

0 200 400 600 800 1000

Temperature/C

**Figure 13.** Average friction coefficients of the Ni3Al-based composite coating at temperatures ranging from room temperature to 1000 °C in air (tested at an applied load of 20 N and sliding speed of 0.2 m/s

These results proved that the coating offered good self-lubricating property at a wide temperature range from room temperature to 1000 °C. The low friction coefficient of the coating was mainly attributed to Ag and fluorides eutectic at the temperature below 800 °C; at high temperatures, the molybdates, which formed in the tribochemical reaction, acted as

reduce the stress concentration, and improve the bonding strength.

(above 600 °C) and remained a stable level.

0.2

effective lubricants (seen in Figs. 15 and 16).

against Si3N4 ceramic ball)

0.3

0.4

0.5

Friction coefficient

0.6

**Figure 11.** SEM micrographs of transverse cross-sections of the Ni3Al-based composite coating: (a) before thermal shock; (b) after thermal shock.

**Figure 12.** Microhardness profile of the Ni3Al-based composite coating

The microhardness depth profile in the transverse cross-section of the composite coating was shown in Fig. 12. It was found that the coating was in three layers according to hardness. The Vickers hardness (HV) of the nickel interlayer was 1.40±0.50 GPa, and that of the Ni3Al layer is 3.80±0.50 GPa, and at the top surface, the HV is 3.60±0.50 GPa. The result showed that the top layer was not the hardest part of the coating, and the hard Ni3Al layer provided efficient support in the coating. The multilayer structure of the coating could reduce the stress concentration, and improve the bonding strength.

124 Tribology in Engineering

coating possesses excellent bonding strength.

a b

before thermal shock; (b) after thermal shock.

0

1

substrate

**Figure 12.** Microhardness profile of the Ni3Al-based composite coating

Ni layer

2

3

Microhardness (GPa)

4

5

6

Recently, a Ni3Al matrix coating containing Ag, Mo and BaF2/CaF2 was fabricated by the vacuum hot-pressed sintered technology [116]. Fig. 11 presented the morphology of the interface of the composite coating. A small amount of nickel powders were spread on the surface of the substrate to improve the wettability between the coating and AISI 1045 carbon steel. The Ni3Al layer on the layer of nickel powder reduced the stress concentration between the substrate and the coating as well as to improve the bonding strength. The mixed composite powders Ni3Al-Mo-Ag-fluorides were spread on the layer. After sintering, it could be seen that there were no pores and cracks near the interface region and the composite coating layer was well adhered to the substrate (Fig. 11a). The morphology of the interface of the coating (after thermal shock) was shown in Fig. 11b. It showed that the coating did not peel off and even no cracks after the thermal shock, indicating that the

**Figure 11.** SEM micrographs of transverse cross-sections of the Ni3Al-based composite coating: (a)

Ni3Al layer

Ni layer

Substrate


Ni3 Al layer composite coating

Distance from the interface (m)

The average friction coefficients of the coating were presented in Fig. 13. The friction coefficients of the coating were approximately 0.35 from 25 to 800 °C, however, when the temperature reached 1000 °C, the friction coefficients fell to 0.24. In comparison, the friction coefficient of the AISI 321 stainless steel was much higher than that of the coating. Fig. 14 showed the wear rates of the coating at various temperatures in air. Although the wear rates of the coating were higher than that of the AISI 321 stainless steel at the temperature between 200 and 400 °C, but they were lower at room temperature and high temperature (above 600 °C) and remained a stable level.

**Figure 13.** Average friction coefficients of the Ni3Al-based composite coating at temperatures ranging from room temperature to 1000 °C in air (tested at an applied load of 20 N and sliding speed of 0.2 m/s against Si3N4 ceramic ball)

These results proved that the coating offered good self-lubricating property at a wide temperature range from room temperature to 1000 °C. The low friction coefficient of the coating was mainly attributed to Ag and fluorides eutectic at the temperature below 800 °C; at high temperatures, the molybdates, which formed in the tribochemical reaction, acted as effective lubricants (seen in Figs. 15 and 16).

High Temperature Self-Lubricating Materials 127

1000 °C

800 °C

600 °C

20 40 60 80 100

2(°)

**Figure 16.** XRD pattern worn surfaces of the Ni3Al-based composite coating at different temperatures

There is no doubt that search for newer and better high temperature self-lubricating materials will continue in coming years, since the application conditions of future mechanical systems will undoubtedly be much more demanding than the current ones.

To fulfill engineering application, a general design appraisal of high temperature selflubricating material can be proposed as follows: low friction (friction coefficient < 0.2); high wear resistance (wear rate < 10-6 mm3/Nm); and wide temperature range (from room

To meet these requirements, matrix and solid lubricant selected are essential for novel high

Intermetallic alloys own the comprehensive mechanical properties for industrial applications by micro- or macro-alloying process. They are now serious candidates for structural applications requiring reduced density, excellent fatigue strength, corrosion/oxidation resistance, and service at temperatures up to 1000 °C. Intermetallic alloys have attracted the attention of many researchers as promising tribomaterials for

It should be realized by designers and engineers that there is no "universal lubricant" that can operate at a broad temperature range conditions. A synergetic lubricating action, a mixture of two or more solid lubricants, is one of promising approaches to fabricate high temperature self-lubricating materials. Furthermore, research on novel solid lubricant is also

mechanical components operating under severe or hostile environments.

 

temperature to high temperature of above 1000 °C).

temperature self-lubricating material.

a response to the requirements.

**7. Conclusions** 

Ni3

NiMoO4

NiAl2 O4 Ag2

Al Ag Mo BaF2 CaF2

MoO4

BaMoO4 CaMoO4 Ni NiO

**Figure 14.** Wear rates of the Ni3Al-based composite coating at temperatures ranging from room temperature to 1000 °C in air (tested at an applied load of 20 N and sliding speed of 0.2 m/s against Si3N4 ceramic ball)

**Figure 15.** XRD patterns of the sintered Ni3Al-based self-lubricating composite coating

**Figure 16.** XRD pattern worn surfaces of the Ni3Al-based composite coating at different temperatures
