**2.10 Alkaline earth metallic sulfates**

High-temperature solid lubricants, papermaking, cosmetics, electronics, pigments, and ceramics all use alkaline earth metallic sulphates like anhydrite, celestite, and baryte [107]. Baryte and celestite's exceptional lubricating properties are closely correlated with their morphologies and structural makeup. At normal temperature, the lubricating processes involve sliding along the (001) basic plane, and at high temperatures, in situ production of ultrafine nanograin surface glaze. Similar to BaCrO4, SrSO4 is made up of Sr2+ cations and [SO4]2 tetrahedra. Seven [SO4]2 tetrahedra form the coordinates for each Sr2+ cation. SrSO4 crystal planes (002) and (210) are seen in **Figure 5**. Controlled nucleation and growth were used to create alkaline earth sulphate particles with distinct and well-defined crystallographic morphologies [108].

With lattice values of 0.8359, 0.5352, and 0.6866 nm for SrSO4; and 0.8881, 0.5454, and 0.7157 nm for BaSO4, respectively, these substances have orthorhombic structures. With just an orthorhombic to a monoclinic phase change (i.e., structural change) at 1100°C and almost minimal weight loss up to 1300°C, baryte, celestite, and their sulphate solid solutions are thermally resistant. Surface energy, supersaturation, and reaction diffusion are three factors that have an influence on crystal formation and make it challenging to form sulphate hierarchical structures [107, 108]. Without the use of surfactants or templates, SrSO4 nanocrystals with a range of features, from a needle-like to a tablet-like shape, were produced using a simple aqueous solution method [107]. The (020) and (210) planes affect the crystalline morphology of SrSO4. Monodispersed peanut-type SrSO4 particles with average length of 1.7 m and aspect ratio of 1.4 were created at room temperature. The mean pore size and BET surface area of these peanut-shaped SrSO4 particles were 34.3 nm, and 20.9 m2 /g, respectively. The chemically precipitated BaxSr1**−**xSO4 solid solution nanocrystals are

**Figure 5.** *Schematic of atomic arrangements at planes of (a) (002), and (b) (210) in SrSO4 crystal [102].*

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

characterised by orthorhombic structure and ellipsoidal shape. BaxSr1**−**xSO4 solid solutions are indexed as a single orthorhombic phase with the space group Pbnm (62) and changing composition parameters [109].

Moreover, baryte-like structure containing-alkaline earth sulphates exhibit exceptional self-lubricity, thermochemistry stability, and innocuity. At high temperatures, these sulphates lubricate. BaSO4-impregnated hard surfaces with a texture can self-lubricate [95]. Only temperature shows a considerable effect on the tribological properties of baryte, and material is widely accessible and inexpensive. A lubricant for brake pads is baryte. Brake pad baryte is increased to minimise friction and wear. Variations in sliding velocity and rubbing pressure have minimal impact on the brake pads' friction that contain baryte. BaSO4 is added in greater amounts to friction materials to achieve excellent coefficient stability and fade resistance. Baryte can withstand high temperatures and does not alter much at 300°C [110]. To create self-lubricating, sulfate-containing composites, several techniques such as spark plasma sintering, hot pressing, electrodeposition, physical vapour deposition, plasma spraying, etc. were developed.

By using burnishing [111], electrodeposition [112], pulsed laser deposition [113], and powder metallurgy [69, 106],(Ba, Sr) SO4, SrSO4, and, BaSO4may be injected or generated to create self-lubricating composites or coatings. At high temperatures, ZrO2 (Y2O3)-Al2O3-50BaSO4 composites show superior friction and wear characteristics than unmodified ZrO2(Y2O3)-Al2O3 ceramics. The frictional behaviour of composite against an alumina ball as a function of wear cycle and temperature is shown in **Figure 6** and worn surface at 800°C is shown in **Figure 7**. At 800°C, BaSO4 composite has a 0.33 friction coefficient and a 4.72106 mm3 /(Nm) wear rate. High temperatures cause a layer of self-lubricating fine-grained BaSO4 to be form on sliding surfaces, avoiding direct balls to oxide ceramic tribo-contact. At high temperatures, lubricating operations result in a surface glaze with ultrafine nanograins because of thermo-mechanical recrystallization/deformation. Plastic smearing

**Figure 6.** *Friction coefficients of ZrO2(Y2O3)-Al2O3-50BaSO4 composite at different temperatures [102].*

**Figure 7.** *Worn surface view of ZrO2(Y2O3)-Al2O3-50BaSO4 composite at (a) 10 μm, and (b) 1 μm after wear test performed at 800°C [69].*

and self-lubricity are caused by the BaSO4 nanograins' grain boundary sliding and rotating in the glaze layer [69].

Coatings that include alkaline earth sulphate boost tribology. For SrSO4-Ag or SrSO4 coatings on silicon nitride or ZrO2(Y2O3)-Al2O3ceramics throughout a broad temperature range, chemical precipitation verifies low wear as well as friction coefficient [114]. When covered with silver, CaSO4 films produced using a pulsed laser are flexible and readily malleable, lubricating more effectively than CaF2 [113]. Using a high-frequency reciprocating ball-on-block tribometer with induction heating, the wear and friction properties of Al2O3 and SUS316 stainless steel coated with powder films are studied. Up to 800°C in air, Al2O3 coated with chemically precipitated SrSO4 and BaSO4 powder exhibit low friction coefficients. Flake-shaped BaSO4 powder coatings over alumina have lower friction coefficients than lump-shaped films. Between ambient temperature and 800°C in air, coatings of BaSO4-10mass% Ag on SUS316 show typical friction coefficients of 0.2–0.4 [111].

#### **2.11 Silicates**

Each O2 ion is linked with two Si4+ ions to form the (SiO4) 4− units that make up silicates. SiO4 tetrahedra may be combined to develop rings/chains. These can be converted to sheets or double chains. Silicates have the ability to interchange cations that are not SiO4 tetrahedra, such as Si4+ and Al3+, without compromising oxygen coordination. Layered micas may easily cleave in a plane while being hard. The layer-to-layer Van der Waals coupling is weak. Commercial ceramics' machinability is improved by the use of micas and other minerals like serpentine and attapulgite. Tetrahedral SiO4 group condensation occurs repeatedly and produces chains, cyclic, and larger polymeric structures [115]. Metal rubbing surfaces react with sodium or potassium silicate to produce a lubricating layer. Aluminium-magnesium silicate and other silicate-based materials like Al4 [Si4O10](OH)4 shield engine surfaces and reduce friction and wear. Silicate sliding contacts can fix themselves [116]. The tribofilm's diverse mineral composition demonstrates that the mechanical damage can be self-repaired at sliding surfaces using these additives. A sticky melt layer in silicates causes them to lubricate rubbing contacts at high temperatures. Low-temperature silicates behave like hard solids, where friction is mostly unaffected by deformation strain rate. Tool performance and durability are enhanced by lubrication. At 920°C,

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

inorganic sodium metasilicate lowers wear rate and friction coefficient (by half) [117]. A covering made of a Sb2O3nanocomposite, MoS2, and magnesium silicate hydroxide was created by Wang et al. This 150–250 nm thick composite coating is created by burnishing powders of antimony trioxide, molybdenum disulfite, and lamellate magnesium silicate hydroxide onto a copper substrate. At 400°C, a superlubricity condition is achieved by the composite coating, in which the friction coefficient lowers to less than 0.01 within 100 revolutions. This is because the magnesium silicate hydroxide, molybdenum disulfide, and antimony trioxidephase all act as lubricants, allowing for straightforward shearing. In the case of magnesium silicate hydroxide, the sliding motion releases O-H-O, OH-, O-Si-O, OH-Mg-OH, and Si-O-Si groups from its layered structure [118]. The nickel superalloy substrate is subjected to coated by burnishing magnesium silicate hydroxide-C-Sb2O3 and demonstrates high-temperature super-lubricity as a result of the formation of a silicate-containing carbon layer with an easily shearable composition [119].

#### **2.12 Caesium oxythiomolybdate**

The high-temperature solid lubricants (Cs2MoOS3, ZnMoOS3, and Cs2WOS3) have been studied for ceramic bearings in single-use engines. The US Air Force Research Laboratory developed the complex chalcogenide Cs2MoOS3 in 1987 at Wright-Patterson Air Force Base (WPAFB). The purpose was to lubricate silicon nitride bearings in air at temperatures, speeds, and loads as high as 760°C, 1.2 million DN, and 890 N, respectively for 5–6 hours. To avoid failure and ensure a long wear life, burnished caesium oxytrithiomolybdate-based lubricants must be replenished. By reproducing the target chemistry, pulsed laser deposition makes Cs2MoOS3 films that are very adherent. These films were developed for silicon nitride bearings in order to interact with the environment and produce new lubricious phases at high temperatures [120, 121]. At 600°C, this adaptable lubricant exhibits 0.03 as a friction coefficient. Additionally, 700°C and room temperature are OK; however, 300°C and 800°C are not. As the test temperature increases, components interact with O2 and one another to generate a high-temperature lubricious phase. At 650°C, the friction coefficient of caesium oxytrithiomolybdate (Cs2MoOS3) covered with sodium silicate is less than 0.2 [121]. Over 200°C, Cs2MoOS3 becomes an unstable lubricant. Powdered Cs2MoOS3 is oxidised at temperatures between 600 and 800°C to produce Cs2MoO4, caesium oxides, Cs2SO4, etc. At 300 to 600°C, the oxidation of Cs2MoOS3 produces lubricious oxides such as MoO3 and Cs2MoO4on ceramics. The friction coefficients of Cs2MoOS3 coatings on alumina and zirconia substrates are low. With a friction coefficient of less than 0.1, Cs2MoOS3films are effective lubricants on silicon nitride and silicon carbide in the temperature range of 600–750, and 500–600°C, respectively. Production of caesium silicate glass and the softening of oxides are necessary for lubrication [120]. For Si3N4bearings at high temperatures, Cs2WOS3 and ZnMoOS3 are the thermodynamically stable lubricants [6, 120]. Rosado et al. [122] proposed that silicon nitride bearings with caesium tungsten (Cs2WOS3) bonded coatings might be lubricated with low shear strength glass. At 650°C, lubricious Cs-based compounds' tribological properties and high-temperature rolling contact persistence were examined on Si3N4 balls [6]. The best outcomes came when an in-situ produced caesium silicate reaction layer was paired with a hydrated caesium silicate-bonded covering. Combining these factors resulted to rolling friction coefficients below 103 and low wear coefficients, resulting in very extended endurance lives despite high contact loads. The low wear coefficients allowed for this to happen. Adding alkali

ions to a combination of silicate glass can make the glass more fluid. These glasses have been used as steel lubricants for the over 70 years. Hot extruded steel surfaces can be lubricated by glass lubricants at 600°C by reducing area and increasing length significantly [123]. For one-time use and brief periods of time, thin PLD films work well. It is necessary to assess the tribological performance and dependability of Si3N4 piston pins, high-temperature seals, intake and exhaust valves, roller followers, cam lobes, camshafts, etc.
