Other than ordinary lubricating ways to add oil-based liquid or semi-solid grease at the friction interface, self-lubricating composite materials can form a continuous lubricating film during friction processes to achieve self-lubricating properties with solid lubricant phase in the structure and are more adaptable to modern engineering applications such as in vacuum, high temperature, high pressure, radiation environments, etc. Due to the strong adhesion and good film forming properties, WS2 and MoS2 are among the most widely used lubricating particles for preparation of self-lubricating composite coatings which can be added through a low temperature wet chemical method (electrodeposition, electroless plating) or a high temperature physical method (laser cladding, thermal spraying). The anti-friction properties of self-lubricating composite coa-tings are closely related to the content and distribution of WS2/MoS2 in the coatings. When the content of the solid lubricant is too low, it is not enough to ensure the formation and stability of the lubricating film, so the purpose of reducing friction is not achieved. When the content is too high, the accumulation of lubricant molecules is prone to viscoelastic frictional resistance, and the coating even peels off, leading to wear mechanism of malignant furrows. Studies have shown that the agglomeration of solid lubricant particles results in poor surface flatness of the coating and loose coating. However, the dispersion of WS2/MoS2 solid lubricant particles in the coating under the existing process conditions cannot reach a monodispersed state. Low-temperature wet chemical method shows good throwing ability, chemical stability, and flexibility. The limit of which is the poor dispersion of particles in the plating solution and the precise control of the particle content in the coatings. At present, the powder may be modified by adding surfactant or metal coating treatment to enhance the wettability between the solid lubricant particles and the substrate. For both electrodeposition and electroless plating, the deposition rate of metal and the content of WS2/MoS2 particles in the coating are influenced by pH value and WS2/MoS2 solid content in the plating solution. The coating prepared by high temperature physical method has the characteristics of good adhesion to the substrate, high density and high hardness. The biggest problem currently exists is that WS2/MoS2 will decompose over 400 ℃, which not only causes the lubrication phase to decrease, but also reduces the friction reducing characteristics. Moreover, the generated gas causes pores to reduce the cohesive bonding force in the coatings, if the surface forms a hard phase, it will fall off and cause abrasive wear, which impairs the performance of the coating. At present, the decomposition of WS2/MoS2 particles can be reduced by powder surface metallization, but cannot be completely avoided. The WS2/MoS2 decomposition can otherwise be used to form other lubricating phases in the molten pool to improve the self-lubricating performance of the coatings. This paper reviews the progress in the preparation of self-lubricating composite coatings containing WS2/MoS2 solid lubricants, evaluating the advantages and limitation of the related process factors, the key influencing factors for the content and distribution of solid lubricants in coatings and the possible solution are proposed as well. © 2019, Materials Review Magazine. All right reserved.
