**4. Conclusion**

Many industrial applications, such as advanced propulsion systems in aerospace and aviation, metal processing, automotive, nuclear power engines, electric railways, etc., depend on an understanding of the tribological (wear and friction) behaviour of self-lubricating composites and solid lubricants in harsh environments. Research has been done on solid lubricant compounds that are environmentally friendly. A few examples of solid lubricants are laminar solids, polymers, chemically stable fluorides, soft metals, binary or ternary oxides, chromates, sulphates, and combinations. (1) Noble metals (Ag and Au) with increased ductility and plastic deformation, provide decent lubricity. Polymer composites made with polyimides or PTFE can lubricate up to 350°C. (2) Graphite and DLC transfer films lubricate in wet air, while MoS2/WS2 transfer films are good lubricants in dry N2 and vacuum. At specific temperatures, layer-lattice solid lubricants (Graphite fluoride, MoS2, etc.) can oxidise or dissociate, as can the complex chalcogenides ZnMoOS3, Cs2WOS3, and Cs2MoOS3. In oxidising circumstances, the non-layered inorganic compounds CaF2 and BaF2/CaF2 eutectic are chemically stable. They provide high lubricity from 500 to 900°C and have low shear strength and facile film-forming ability. Over a wide temperature range, the superior thermal stability and lubricity is exhibited by alkaline earth chromates (BaCrO4 and BaCr2O4), sulphates (SrSO4 and BaSO4), and their solid solutions. (3) For extremely high temperatures, oxide lubrication will be investigated. To lessen brittleness at low temperatures, lower the oxide particle size. Rather than dislocation activity in ultrafine grains, sliding or rotation of grain boundaries in this instance causes plastic deformation. Powder metallurgy, thermal spraying, electroplating, laser cladding, physical/chemical vapour deposition, and additive manufacturing have all been used to create self-lubricating composites and coatings. (4) Solid lubricants' synergistic effects related to tribology for vacuum, humidity, temperature, or load are heavily studied. Adaptive processes are designed to achieve low-meltingpoint and easy-to-shear binary and ternary compounds considering some parameters such as, melting of soft metals or temperature-activated diffusion, interfacial tribo-reaction or environmental-assisted oxidation. (5) The tribological surfaces for reversible temperature, humidity, load, or vacuum are necessary for wide-range solid

lubrication through a number of heat cycles. Micro-laminate structures are utilised to create temperature-adaptive composites or coatings with lubrication that limits tribo-reaction, melting, oxidation, or diffusion across a number of heat cycles. (6) A breakthrough that might extend the life and performance of air and spacecraft is adaptive solid lubrication, which can operate from ambient temperature to 1000°C or more. The applications are the seal parts and contact bearings (sliding and rolling) for high-speed dry machining, side dams for thin-strip steel casting, high-tech propulsion systems, pantograph contact strips for electric trains. For better lubrication systems at low/high temperature, and high vacuum, pressure and chemical reactivity, the microstructurally engineered solid lubricants are crucial.
