**2.3 Molybdenum disulphide**

MoS2 has an important place among the mostly used solid lubricants [27], which is obtained from earth's crust in the form of Molybdenite. It is commercialised as inclusions, films, suspensions, or fine particles in the composites after refinement and suitable treatments. The compound's sulphur lamellae are thought of as a laminar solid, and their weak van der Waals bonds make the shearing events that cause layer configurations during sliding easier. Furthermore, the lamellae have the necessary resistance to asperity penetration due to the strong covalent interaction between sulphur and molybdenum [27]. In **Figure 1**, the laminar structure is displayed. At ambient temperature and up to 300°C, the typical COF of unaltered MoS2 is around 0.08. Depending on a number of variables, including the load, the operating circumstances, and the sliding speed, MoS2 can provide enough lubrication in a vacuum up to 1000°C [28]. Oxidation significantly reduces MoS2's efficacy [29, 30]. When MoS2 is oxidised, molybdenum oxide (MoO3) is produced, which increases friction and wear. MoO3has been observed to act abrasively with numerous alloys [27, 31]. Contrarily, MoO3 additions to MoS2 were found to improve tribological behaviour. Powder particle size, air accessibility, inclusion type, and composition are all factors that affect MoS2 oxidation [32]. However, using inclusions with MoS2 to prevent oxidation is demonstrated [27], which eliminates air from the particles. With a constant decrease in friction as temperature increased, NiAl-based composites with a 5 wt%Ti3SiC2–5 wt%MoS2 composition had excellent tribo-behaviour [33]. At 400°C, there was a substantial reduction in the wear rates and COF. In addition to the development of a selflubricating layer, it is believed that the production of TiO2 and SiO2protective oxides will reduce COF and wear. The lowest wear rate and COF were observed at 200°C [34] for ZrO2/Y2O3 composites containing 10 wt% CaF2 and 10 wt% MoS2. MoS2oxidises to MoO3 at higher temperatures, which is a less lubricious [35]. For YSZ coating with Mo, the wear and COF was reduced up to 300°C [36]. However, a coating breakdown happened at temperatures higher than 300°C. Despite this, MoS2 incorporation reduced the COF up to 700°C, however the onset of oxides and MoS2's non-lubricious impact were taken into account. Another tribological investigation [37] on composite

**Figure 1.** *Crystal structure of MoS2.*

### *Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

coatings made of Ni3Si and different amounts of MoS2, BaF2, and CaF2 revealed that the MoS2 broke down into Mo2S3, which enhanced wear and friction. Due to the low quantity of solid lubricant, the composite was also shown to have poor tribological properties at high temperatures. However, as the temperature above 400°C owing to the creation of a glazed layer, an admirable self-lubricating property was seen upon a greater solid lubricant concentration (15 wt% MoS2 and 10 wt% BaF2/CaF2). At 350°C, MoS2 PVD films showed a low and steady COF of 0.15. At around 370°C, MoS2 was observed to degrade to MoO3, albeit [38, 39]. **Figure 2a** shows the relationship between relative wear rates for different MoS2 based solid lubricants [33–37] and **Figure 2b** shows the influence of sliding temperature over COF. The wear rate values at the stated temperature are used to determine the related wear rates after dividing with the wear rates at room temperature.

**Figure 2.** *(a) COF, and (b) wear vs. sliding distance for MoS2***-***based composites and coatings.*

#### **2.4 Graphite**

Graphite has a hexagonal configuration, and is made up of planes of polycyclic carbon atoms, of which the basal planes have weaker bonding. When sliding against graphite, a metallic mating surface exhibits coefficients of friction that range from 0.05 to 0.15 in the surrounding air. But perpendicular to the basal planes has a three times greater coefficient of friction than moving parallel [15]. When tested in a plane that is perpendicular to graphite's basal planes, it is discovered that graphite is soft and lubricates, but when tested in a vacuum or at high altitudes, it is found to be hard and wears out quickly. Dry nitrogen or a vacuum have a coefficient of friction that is typically 10 times greater than that of air. It is necessary to treat graphite with water or another condensable vapour, such as hydrocarbons, in order for it to acquire the lubricious characteristics that it possesses. Because water is absorbed by graphite's hexagonal planes, the bonding energy that holds them together is less when the environment is humid. In reality, due to its high affinity for hydrocarbon lubricants, graphite shows superior lubricity in boundary conditions. Graphite is an effective lubricant in an oxidising environment up to 450°C before failing due to structural deterioration by oxidation. In order to operate over 650°C in military aircraft, and rolling-contact bearings were fitted with powdered graphite lubricants. Impregnated graphite parts are frequently utilised in high-temperature applications like fuel chamber liners, heat shields, and missile nozzle inserts but are not suitable for situations where loading or mechanical shock is rather extreme.

Graphite is frequently utilised in electrically conducting motor and generator brushes and the mechanical seals' rubbing component. Amorphous carbon predominates in situations where a covering with more thermal insulation and less lubricity is needed. Amorphous carbon and graphite can be blended in this situation to fully utilise each material's advantages and disadvantages. Carbon-graphite components are robust, durable, and have little friction. For applications involving harsh chemicals, carbon-graphite bearings that can operate for an extended period of time at temperatures higher than 580°C are perfect.

It can be summarised that the lubricating mechanism of graphite is mechanical in origin and arises from the sliding of one graphite particle over another. Graphite can be used as a dry lubricant or mixed with lubricating oil. For enhanced lubrication, graphite particles may also be included into a grease product. Due of graphite's reputation as a great lubricant, many grades of graphite have been evaluated with a variety of water-based drilling fluids. It was determined that adding dry graphite to a water-based drilling fluid had a negligible effect on friction. Due to the failure of all tests utilising dry graphite, it was determined that the surface of the graphite was hydrophobic or organophilic.

#### **2.5 Graphene**

Graphene, a honeycomb structure containing two-dimensional carbon allotrope, is expected to have superior friction-reducing properties. MoS2 and graphite both employ the same method to lessen slide friction. But unlike graphite, graphene exhibits lubrication in a dry atmosphere. Its usage is well acknowledged in solid or colloidal liquid-based lubricants [40, 41]. Also, it is frequently utilised in the electronics and mechanical sectors because of its remarkable mechanical, electrical (~102 S/m), and thermal qualities. It is significant solid lubricant because to its high strength, simple shearing ability, and chemical inertness. Additionally, it is frequently *Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

employed in nano and micro mechanical systems because of its ultra-thin dimension [42]. Due to the creation of a lubricious tribo-layer up to 400°C, which considerably decreased wear and CoF, graphene nano platelets (1.5 wt%) reinforced NiAl matrix demonstrated outstanding tribological performance [43]. However, when sliding temperature increases up to 500 C, graphene nano platelets (GNPs) lose their protective function owing to oxidation, which causes delamination and extensive adhesive wear. Nevertheless, unstable friction and increased wear were caused by a reduction in lubrication over 600°C as a result of its oxidation. Graphene layers and its oxide were studied for wear behaviour. Graphene layer offered the superior wear protection, and the wear was reduced as compared to steel sliding surfaces by 3–4 orders. However, Graphene oxide showed increased wear rate as compared to graphene layers by 1–2 orders [44]. The majority of research on graphene demonstrates the transitional period of increasing friction and wear over 550–600°C.

#### **2.6 Hexagonal boron nitride (h-BN)**

The "white graphite" known as h-BN is made up of stacked (BN)3 rings. With preferred shear perpendicular to the c-axis, it has anisotropic shear characteristics [10]. The main source of lubrication at high temperatures is lamellar slide along the basal plane. Low strength and poor quality composite materials are caused by h-weak BN's adherence to the majority of metals and ceramics and difficult sintering [1, 10]. Because van der Waals forces are stronger in h-BN layers than in graphite or MoS2, they perform less well in tribological tests. It operates better in high-temperature and humid circumstances due to its improved thermal stability and oxidation resistance, due to which it gets easily sintered. The composites and ceramics achieve better tribological by h-BN. In normal air, h-BN possesses coefficient of friction in the range of 0.2–0.25, and in humid air, less than 0.1 [45–47]. h-BN is added to lubricating oil [48] or water [49] or impregnated into porous surfaces [50–53] as lubricating micro-particles. Although h-BN is more thermally stable than graphite and MoS2, its coefficient of friction is high at ambient temperature; however, it decreases to 0.15 at 600°C. On a ball-on-disk tribometer, pure h-BN and h-BN-10 wt% CaB2O4 were tested against Si3N4 from room temperature to 800°C [46]. Cutting tools made of Si3N4 break down under fatigue loading and high temperatures, especially when hard materials are being machined at high speeds [54]. A Fe2O3, SiO2, and B2O3 tribo-film forms when h-BN is applied to silicon nitride, considerably improving the tribo-pair of austenitic stainless steel. Al2O3, TiB2, and B4C use h-BN as a solid lubricant at high temperatures due to its oxidation resistance and chemical stability [55]. Due to low diffusion coefficient and flaky structure, h-BN is challenging to use as a cutting tool additive [56]. In Al-forming, h-BN substitutes soiled graphite or MoS2 without leaving any stains. Surface quality and tribological performance, such as lubrication-film stability, are determined by the concentration and particle size h-BN powder [57]. A pin-ondisc high-temperature tribometer was used to conduct wear testing from ambient temperature to 400°C [58]. Understanding third body development and velocity accommodation is necessary for controlling wear. Microplastic deformation, brittle fracture, and Grooves are characteristics of BN-based composites [59]. For hightemperature applications, C/C-h-BN-SiC composites that were moulded, carbonised, and infiltrated with liquid silicon shown outstanding self-lubrication, self-healing, and oxidation resistance. At very high braking speeds, adding h-BN to C/C-h-BN-SiC improves friction and wear without clamping stagnation [60]. Titanium alloys were coated with a 15 m thick h-BN coating for high temperature tribological applications,

and they were then rapidly thermally annealed in a furnace using infrared radiation. At 360°C, sliding against paired 15-5PH stainless steel cylinders lowered the friction coefficient from Ti-alloys to Ti h-BN from 0.72 to 0.35 [52, 61]. At room temperature, Ni-P-35 vol.% h-BN autocatalytic composite coating exhibits a 106 mm3 /(Nm) wear rate and a 0.2 friction coefficient when applied to an AISI52100 steel ball [53]. From room temperature to 600°C, nickel-based composites are self-lubricated by silver and h-BN nanopowders [60]. On non-metallic substrates, we also evaluated the mechanical durability and tribological behaviour of h-BN films deposited using an ion beam. Although BN coatings on Si and SiO2 had friction coefficients <0.1, they could shatter when placed on nonmetallic surfaces [48]. **Table 2** represents the tribological behaviour of h-BN at different wear test parameters.

#### **2.7 Transition metal dichalcogenides (TMD)**

TMD solid lubricants are hexagonal layered compounds created by joining transition metals like molybdenum, tungsten, and niobium with chalcogenides like sulphur, selenium, and tellurium [27, 63]. The production of easily-shearable lamellas is caused by the weak adhesion forces (van der Waals) between atoms with sulphur-like characteristics. Strong transition metal and chalcogenide linkages make up the lamellar structures of Mo-, W-, and Nb-disulfides as well as Nb-diselenide. Particularly for vacuum applications, good intrinsic solid lubricants include MoS2 and WS2. Adsorbed substances or additives are not necessary for lubrication. The lubricity rapidly deteriorates as chemicals from the environment, such as H2O, are absorbed, which affects the lamellas' ability to slide. MoS2 films are less susceptible to moisture when Pb, Au, and polytetrafluoroethlene are added (PTFE). In humid air, carbon enhances the performance of burnished and bonded MoS2 [64]. MoS2 films are less susceptible to moisture when Pb, Au, and polytetrafluoroethlene are added (PTFE). In humid air, carbon enhances the performance of burnished and bonded MoS2 [3]. In non-oxidising conditions, MoS2 is thermally stable up to 1100°C, but in air, it oxidises at 350°C. The maximum-use temperature is constrained by the oxidised by product MoO3, which is thought to be abrasive. MoS2 fails early due to a slow oxidative deterioration brought on by water vapour and oxygen in atmosphere. Surface oxidation at 100°C is indicated by a low W6+ content in WO3 peaks. Sulphur virtually vanishes at 500°C, and WS2 transforms into WO3, as shown in **Figure 3** [65]. Closedcage WS2 nanoparticles have been researched for improved lubrication in challenging environments [66].

As lubricants or additives, TMD compounds decrease wear and friction. TMD powders are used in relay and switch contacts, threaded parts, sleeve bearings, and metal-forming dies. Materials containing TMD are employed in bulk composites made of powder metal, PVD thin films, and burnished thick coatings. It does not matter how a material is made; what matters is that it has low friction and low wear over a wide temperature range [2]. For reduced friction in bearings and other sliding/rolling applications, burnished MoS2 or WS2 coatings can be made with the basal planes of the MoS2 or WS2 crystallites parallel to the sliding direction. With proper process control, more uniform coating can be generated using spraying, such as the superior lubrication of SSRMS gears that have last several million cycles without wearing out. MoS2 solid lubrication is used in space for release mechanisms, gears, slip rings, pointing mechanisms, and ball bearings. For upcoming space missions like the ESA's Bepi Colombo mission to Mercury, the rising solid lubrication of MoS2 must function over the wide temperature range with stable and reliable performance [67]. For Hubble and


**Table 2.**

*Tribological behaviour of h-BN.*

*Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

**Figure 3.** *High temperature self-lubricating properties of magnetron-sputtered WS2 coatings [65].*

JWST, MoS2 was selected as the solid lubricant [27]. Resin-bonded MoS2and sputtered coatings are used on satellites and the space shuttle. Depending on humidity and sliding circumstances, resin-bonded spray coatings with heat curing have coefficients of friction over 0.06–0.15 and better wear life. Silicates (such as Na2SiO3) and phosphates (such as AlPO4) are often the inorganic-bonded MoS2 coatings for launch vehicle bearings and gears because they can withstand relatively high temperatures over 650–750°C. Softening or deterioration may result from water or humidity. MoS2 that has been phosphate-bonded lubricates the main differential pivot on the Mars Science Laboratory (MSL) Curiosity Rover. Component friction is decreased via sputterdeposited MoS2 coatings, either with or without soft metals and other compounds. With sputtered coatings, vacuum deposition on big objects is challenging [68].

It can be concluded that the possible alternative of soft metal lubricant is transition metal dichalcogenides (TMDCs), which are semiconductors of the form MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se, or Te). Due of its durability, MoS2 is the most researched material in this family. TMDCs are attractive for fundamental studies and applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing, and personalised medicine due to their unique combination of atomicscale thickness, direct bandgap, strong spin–orbit coupling, and favourable electronic and mechanical properties.

#### **2.8 Binary metallic oxides**

Superior wear resistance is seen in alumina, zirconia, and mullite, although high friction results in debris and cracks in dry air [69]. Some oxides make effective solid lubricants because of their exceptional thermal stability in air, even at high temperatures. They have not been investigated for room-temperature solid lubrication because of their brittleness. They are unable to produce smooth transfer layers on worn surfaces at room temperature because they refuse shear or deform. Debris from

#### *Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

oxide wear is abrasive. In dry air, oxide surfaces are inert and do not produce powerful adhesive bonds like tribological materials. For solid lubrication at high temperatures, soft oxides have been investigated. B2O3, Bi2O3, Ag3VO4, and PbOAg2MoO4 are hightemperature lubricants that are thermally stable and efficient. They cannot lubricate when at normal temperature. Above a particular temperature, the brittle-to-ductile transition results in different friction and wear properties. Above 0.4 to 0.7 Tm, oxides become softer (in K) and to create the connection between ionic potentials and friction coefficient, a crystal-chemical model was proposed [9]. Low melting temperatures and strong ionic potentials are properties of Re2O7, B2O3, and V2O5. Lower binary oxide friction coefficient is caused by oxides with higher ionic potential. This lessens shear strength and explains why vanadium or molybdenum oxides behave lubriously. Contrary to the theory, Bi2O3 and PbO have low friction and low ionic potentials [70]. Oxides are categorised as highly basic, basic, oracidic based on interaction parameter, optical basicity, binding energy, and average ionic polarizability [71]. Increased ionicity and high unshared electron density are associated with low interaction parameters in very basic or basic oxides with low binding energy and high polarizability [70].

It may be possible to understand the complex friction behaviour of binary/mixed oxides based on polarizability by considering how the formation of vacancies and ion hopping at oxide surfaces affect frictional behaviour at high temperatures [71]. To better understand experimental results and generate predictions, computational modelling and simulations are utilised to look at the relationship between oxide crystal structure and frictional behaviour. DFT and MD simulations are used the most often in atomic-scale modelling techniques. DFT calculations are time-consuming and ineffective since they can only be performed on static, nanometre-scale systems. To better understand tribological behaviour on experimentally relevant length scales, molecular dynamics simulations look at the behaviour of moving atoms in a series of system configurations. MenO2n−1, MenO3n−1, and MenO3n−2 are examples of substoichiometric transition metallic oxide compounds that have planar lattice faults that could lead to crystallographic shear planes with less binding forces. At high temperatures, TiOx, VOx, MoOx, and WOx deform due to plastic flow [72]. Good selflubricity is exhibited by vanadium-based Magneli phases in high-temperature hard nitride coatings [73]. Until the V2O5 phase melts at 690°C, these hard coatings reduce friction coefficients from 100 to 700°C [74]. Because lubricious Magneli phases are produced in sliding contact, hard nitrides or carbides, including Wor Mo components, offer higher high-temperature tribological qualities [75].

In order to create compounds or systems with low melting points, which have decreased hardness and shear strength at high temperatures, the difference in ionic potential can be increased [9]. Up to 1000°C, YBa2Cu3Oy exhibits a coefficient of friction over 0.20–0.50, making it a potential high-temperature solid lubricant. From cryogenic to high temperature, the mechanical and tribological characteristics of YBa2Cu3O7-delta/Ag composites were assessed [76]. Strongly lubricious and sticky thin oxide layers can be produced in situ during wear or directly using coating techniques such as reactive magnetron sputtering in an oxygen-containing environment. The modern hard coatings have multiple uses and are highly hard, tough, temperature stable, oxidation resistant, low friction, and wear resistant [77]. Vanadium inclusions lower friction coefficient by self-adapting the hard coating while sliding, and nitride coatings provide excellent hardness and wear resistance [73]. When applied to worn surfaces above 400°C, lubricious V2O5 is produced by the V-containing nitride [78]. Reactive cathodic arc ion-plated (V, Ti) N coatings underwent reciprocating wear tests from room temperature to 700°C. The formation of TiO2 and V2O5 oxides

at 500°C lowered the friction coefficient of (V, Ti) N coatings. To minimise friction coefficient [79], Magneli phase V2O5 oxides were melted over the worn surface while sliding at 700°C. Under high-temperature oxidation conditions, hard nitride coatings produce lubricious metallic oxides. The reduction in friction between 450 and 650°C, slightly below the melting point of V2O5, is significantly associated with V2O5 and related Magneli phases. This conforms to the adaptive lubricating processes described by Voevodin et al. [73, 80]. As a result of in situ lubricious oxide formation, some Ni-Cu-Re, Fe-Re, and Cu-Re alloys exhibit friction coefficients ranging from 0.2 to 0.3.

It can be concluded that adaptive mechanisms have improved the solid lubrication of hard coatings at a range of temperatures. Future research will focus on binary oxides because it is believed that, with the right surface design and composition, they can act as lubricants under certain conditions, despite the fact that the majority of them only maintain low shear strengths over a narrow temperature range, typically at high temperatures. The oxide material is resistant to high temperatures, moist air, and vacuum.

#### **2.9 Ternary metallic oxides**

### *2.9.1 Molybdates*

At high temperatures, lubricious materials include Ag2Mo2O7, Ag2MoO4, CaMoO4, K2MoO4, BaMoO4, CoMoO4, SrMoO4, and ZnMoO4. At room temperature, PbMoO4 and CaMoO4 exhibit low Mohs hardness-3.0 and 3.5, respectively. BaMO4, a scheelitetype tetragonal molybdate, is used in photocatalysts, solid-lubrication, photoluminescence and solid-state lasers. BaMoO4 and SrMoO4 powders are produced using various methods, including electrochemical method, intricate polymerisation method, micro-emulsion route, hydrothermal approach, microwave-assisted synthesis [81]. Tetragonal SrMoO4 has lattice constants of a = b = 0.539 nm and c = 1.202 nm. The layered microstructures of Ag2MoO4 and Ag2Mo2O7, which are similar to WS2, may lessen friction at high temperatures [82, 83]. The lattice parameters of B-Ag2MoO4 are 0.9318 nm, and it features a characteristic AB2O4 cubic spinel structure with exceptional high-temperature stability. Pure molybdenum coatings and plasma-sprayed silver underwent in situ silver molybdate production analysis [84].

When compared to an unaltered Ni-based alloy, plasma-sprayed coatings at 600°C and 800°C reduced friction and wear. Fe-Mo alloys with CaF2 added develop a surface glaze with MoO3, Fe2O3, CaF2, and CaMoO4 after 600°C sliding wear tests. PbMoO4 films made by pulsed laser deposition performed excellent at 700°C, but at ambient temperature, they degraded rapidly [85]. The tribological characteristics of hot-pressed nickel-chromium matrix composites with BaMoO4 were investigated up to 600°C. The NiCr-20 wt% BaMoO4 composite, which displays a lower friction coefficient and almost an order of magnitude lower wear rate at 600°C than unmodified Ni-Cr composite, demonstrates the greatest tribological performance. This is a consequence of the smooth, thick oxide layer that exhibits strong Raman peaks related to BaMoO4 [86]. Because of the cooperative lubrication of Ag and the barium salts BaCrO4 and BaMoO4, Ni3Al matrix composites self-lubricate from ambient temperature to 800°C [87]. Non-lubricious BaAl2O4 must be avoided during manufacturing.

#### *2.9.2 Tungstates*

ZnWO4, CoWO4, CaWO4, BaWO4, and SrWO4 all lubricate well at high temperatures. At 600 to 800°C, CoWO4 exhibits frictional properties between 0.25

#### *Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

and 0.25. Solid electro-optical and lubricant properties are exhibited by AWO4 (A = Ca, Ba, and Sr). BaWO4 is created through solid-state reactions, hydrothermalelectrochemical processes, and high-temperature flux crystallisation. Using a hydrothermal process, BaWO4 powders with whisker-like, flake-like, and olive-like structures have been produced [88]. Solution approaches, employing organic templates and moderate hydrothermal temperatures, polymer, or micro-emulsions, were used to produce dendrite-like, hollow, and organised BaWO4 structures [89]. SrWO4 shows a tetragonal structure. The performance of Ag2WO4 was investigated at elevated temperatures as a tribological material using ab-initio MD simulations [83].

Using a ball-on-disc high-temperature tribometer, powder metallurgy was used to manufacture Ni3Al-based composites with W, BaF2-CaF2, and silver. These materials were then tested up to 800°C for tribological performance. CaWO4 and BaWO4, which provide steady friction and little wear, are formed during high-temperature sliding [90]. Lubrication at high temperatures is provided by thin WS2-ZnO composite coatings. ZnWO4 was produced via the reaction of zinc oxide and tungsten disulfide at high temperatures [91]. A unidirectional ballon-disc wear and friction tester is used to assess the high temperature performance of WS2 and ZnO burnished films [92]. The lubricious ZnWO4 oxide layer formed at 500°C results in reducing the wear and friction. Zinc oxide created a high-temperature lubricant in these composite coatings, whereas WS2 supplied low-temperature lubrication. When subjected to prolonged thermal cycling, however, WS2's low-temperature lubrication will be lost as a result of an irreversible reaction.

#### *2.9.3 Vanadates*

Although they are often ineffectual at generating self-lubricity at ordinary temperature, lubricious ternary oxides ofBi4V2O11, BiVO4, AgVO3 [93], Ag3VO4 [82, 83], and binary V2O5 are effective and thermally stable at high temperatures. Depending on the quantity of oxygen ligands around the vanadium atom, the pentavalent monomer of vanadium oxide can be found in either the meta- or ortho-form. At room temperature, hydrothermal and wet precipitation techniques have been used to create vanadate powders with a variety of particle size distributions and morphologies. When tested with a ball-on-disc friction analyser, VN thin films of varying Ag contents show favourable frictional behaviour up to 1000 \_C, which is attributable to the in-situ synthesis of lubricious Ag3VO4 and Ag-vanadate during sliding [82]. Ag/ VN thin films were pulsed-laser deposited to produce Ag3VO4, AgVO3 and V2O5 [82], which similarly showed enhanced tribological behaviour when slid against an alumina ball over 700–900°C. Due to the development of AgVO3 and Ag3VO4 over 600–800°C [94], laser-clad NiCrAlY–based coatings with V2O5 and Ag2O solid lubricants show enhanced wear resistance. To produce continuous self-lubricity over a wide temperature range, additional inorganic compounds, Ag3VO4, and soft metals were imbedded into textured hard surfaces [95].

#### *2.9.4 Tantalates*

(Cu, Ag)-Ta(Mo, V)-O based ternary metal oxides have minimal friction at high temperatures and are structurally and chemically inert. A layered structure containing-silver tantalate (AgTaO3) with a layered structure can be slid at high temperatures to produce a silver-containing phase, which is a soft metallic phase [96]. Layered AgTaO3 melts and transforms into structural phases at 1172°C [97]. The temperature

dependency of tribological and mechanical sliding processes is related to variations in AgTaO3 structural properties.

A number of processing techniques were looked into for coatings that need lubricious silver tantalate coatings at very high temperatures, including (1) powder films that have been burnished into the substrate (2) monolithic silver tantalate films made by magnetron sputtering (3) coatings made of an adaptive silver nanocomposite / tantalum nitride that, while sliding, forms a lubricious silver tantalate layer on its surface. The friction coefficients of this coating ranged from 0.06 to 0.15 when dry sliding across Si3N4 counter-faces at 750°C [96]. Through friction, heat, and shear stress, a nanocrystalline layer of Ag, Ta2O5, and AgTaO3 was mechanically connected. Silver clusters lessen friction on the sliding surface [98]. Because of the silver's high mobility and quick surface diffusion, AgTaO3 has a low wear resistance. The random movement of silver particle clusters, which occurred at high temperatures and caused system failure in sliding components, was a notable phenomena [98]. Recently, using DFT and MD simulations with newly discovered empirical potential parameters and experimental results, the wear and friction mechanism of three ternary oxides-CuTa2O6, CuTaO3, and AgTaO3 were demonstrated. Experimentally, the composition of the film after sliding is compared with film before sliding, and the growth of Cu or Ag clusters throughout the film development is observed in DFT and MD energy barriers for atomic movement on the surface. Theoretical as well as experimental outcomes results confirmed the effect of metal (or metal oxide) clusters on the sliding surface on wear and friction mechanism [99].

#### *2.9.5 Alkaline earth metallic chromates*

As solid lubricants for self-lubricating metallic or ceramic matrix composites, MCrO4, MCr2O4, and MCrO3 (M = Ba, Sr., and Ca) between chromium sesquioxide and alkaline earth metallic oxides have been explored [11, 100]. M represents +2 alkaline earth metals in MCrO4 oxometallates. Ba2+ cations with a coordination number of 12 and [CrO4]2 tetrahedra make up the constituents of BaCrO4. BaCrO4 has an orthorhombic structure. The crystal structure of BaCrO4 is shown in **Figure 4**. BaCrO4 has a compression coefficient of 0.0357 GPa and a bulk modulus of 28.1 GPa. BaCrO4 is a high-temperature solid lubricant and an oxidising agent, which accelerate vapour-phase oxidation processes by acting as a catalyst. Moreover, it is a model system for researching the morphological regulation of inorganic minerals [11, 101].

Oxometallates with the formula M2+Cr2 3+O4 2− are made up of the oxides of bivalent and trivalent chromium metallic elements. The M2+ to Cr3+ratio radius determines the morphology of their crystals. The majority of MCr2O4 compounds with a spinel structure (M = Co, Ni, Zn, Mg, Cu, Mn, and Fe) are thermally stable. Alkaline earth oxides BaCr2O4, SrCr2O4, and CaCr2O4 are known as chromates [102]. These chromates have multilayer architectures with M2+-separated triangular CrO2 sheets [103]. In the inert (Ar) environment, BaCr2O4 is thermally stable. At 1400°C, it is steady. BaCrO3 has 4, 6, 12, 14, and 27 layers, with c/a ratios of 1.654, 2.433, 4.901, 5.752, and 11.101, respectively. Chemical co-precipitation was used to create BaCrO4 particles with a variety of crystallographic morphologies and sizes for preparing solid lubricants at high-temperature [101]. It was investigated if BaCrO4 particles made using the aqueous solution technique were thermally stable. BaCrO4 has been shown to be thermally stable up to 1400°C by DTA-TG and X-ray diffraction. BaCrO4 breaks down in two phases in vacuum. BaCrO4 breaks down into Ba3(Cr6+Cr5+)2O9**−**x with pentavalent Cr5+ and hexavalent Cr6+ cations, and BaCr2O4 with trivalent Cr3+ cations after vacuum

*Thermal Characteristics and Tribological Performances of Solid Lubricants: A Mini Review DOI: http://dx.doi.org/10.5772/intechopen.109982*

**Figure 4.** *Schematic of BaCrO4 crystal structure [102].*

heat treatments [104]. Due to its tendency to turn green on polished surfaces, BaCrO4 is not thermally stable during vacuum sintering [105].

Thermal stability is one of the most important parameters for lubricants to function in a variety of atmospheres and at high temperatures. With oxygen, BaCrO4 breaks down into Ba3(CrO4)2 and BaCr2O4. BaCrO4 breaks down into BaO Cr2O3 CrO3 and BaCr2O4above 900°C in a non-oxidising environment, as per the Ba-Cr-O phase diagram. By using Cr2O3 and BaCO3powders in stoichiometric ratio in a solid-state reaction, microsized BaCr2O4 particles were created [106]. A prior study found that BaCr2O4 is unstable in air at high temperatures. BaCr2O4oxidises to BaCrO4 and Cr2O3 at 790.2°C. In high-temperature wear testing, an oxidation reaction could aid in selflubrication. The wear track showed spread BaCrO4 at high temperatures and is easily sheared [100]. At 800°C, dry sliding wear was investigated against an Al2O3 ball. BaCr2O4crystallises with [BaO4]-chains and edge-shared CrO6-octahedra, reducing wear and friction. BaCr2O4 ceramics showed low wear and friction over 400–600°C. When BaCr2O4oxidises in air, a self-lubricating layer forms on a worn surface, reducing wear and friction. The relative density of pure BaCr2O4 ceramics decreases with severe oxidation, speeding up wear [100].

Alkaline earth metallic chromates can be used to create self-lubricating materials [11, 100, 105, 106]. By combining BaCrO4 and BaCr2O4 with a metallic or ceramic matrix, electrodeposition [100], low-pressure plasma spraying [11], and powder metallurgy [105] can create self-lubricating composites or coatings. Spark-plasmasintered ZrO2 (Y2O3) matrix composites with BaCrO4 had wear rates of 106 mm3 / Nm and friction coefficients of 0.29–0.32 from room temperature to 800°C [105]. At 800°C, barium chromate softens and forms a self-lubricating coating of fine grains on sliding surfaces subjected to tribo-stress. The in-situ growth of ultrafine nanograin surface glazing brought on at high temperatures by thermo-mechanical recrystallization/deformation is a lubricating process. At high temperatures, plastic smearing and self-lubrication are made possible by grain rotation and grain boundary sliding in the

glaze layer. At high temperatures, the ZrO2-BaCrO4 coating developed using plasmasprayed technique at low-pressure proved lubricious [11].
