| With the development of modern aviation,aerospace and other high-tech fields,the service temperature for high-temperature moving parts of hightemperature machinery is up to 1000℃.Moving parts of high-temperature machinery have an urgent demand for lubricating materials in a wide temperature range during the engine start-stop process.The self-lubricating coating formed by mixing soft metal and transition metal into the adhesive phase can achieve continuous lubrication during a wide temperature range.However,the rapid dissipation of soft metal reduces the service life of self-lubricating coating over a wide temperature range.Hence,the lubrication problem under multiple thermal cycles service conditions has become a key factor affecting the reliability and service life of mechanical systems.This thesis investigated the dissipation mechanism of soft metals in wide temperature range self-lubricating coatings and the life extension methods of wide temperature range self-lubricating coatings.In this thesis,the investigation was based on the theory of the applied surface science and thermodynamics of material,combined with the coating design principle of temperature adaptation.Diffusion and dissipation mechanisms of soft metal in Ni-based composite coating prepared by atmospheric plasma spraying(APS)technology,and the effect on the mechanical properties,tribological properties,and multi-cycle lubrication life of the composite coating with the addition of different core-shell structure particles were studied systematically.The lubrication and wear mechanisms of the composite coating over a wide temperature range were analyzed.The evolution law of composition,micro structure and tribological properties over a wide temperature range of the composite coating was described.And the mechanism of the core-shell structure prolonging the life of the Ni-based composite coating was investigated.The main contents and conclusions are as follows:(1)NiCrAlY-Cu and NiCrAlY-Ag composite coatings were prepared by APS technology,and the diffusion and dissipation of soft metal lubricant in Ni-based composite coatings were studied.Results showed that under the influence of heat,the soft metal lubricant shows vertical diffusion from the inside of the composite coating to the surface of the composite coating,and the diffusion intensifies with the increase of temperature.Under the influence of load-heat coupling,the soft metal lubricant occurs lubrication dissipation,and the soft metal lubricant occurs parallel diffusion from the stress-affected area to the non-stress-affected area inside the composite coating.After diffusion and dissipation,the contents of soft metal lubricant involve in lubrication decreased sharply,resulting in deteriorating the lubricating performance of the composite coating.(2)Ag@Ni particles were prepared by electroless plating and NiCrAlY-MoAg@Ni composite coatings were prepared by APS technology.The effect of electroless plating parameters on the coating thickness of Ag@Ni particles,and the effect on the mechanical properties,tribological properties,and multi-thermal cycle lubrication life of composite coatings with adding Ag@Ni core-shell structure particles were studied.Results showed that the mass concentration of NaH2PO2 has the most obvious effect on the coating thickness of the Ag@Ni during the process of electroless plating.The existence of Ni plating layer for the Ag@Ni core-shell structure enhances the interfacial bonding strength between Ag and NiCrAlY.Compared to the NiCrAlY-Mo-Ag composite coating,NiCrAlYMo-Ag@Ni composite coating has better mechanical properties and tribological properties.Ag@Ni core-shell structure greatly slow-release the diffusion of Ag in the composite coating,resulting in the continuous supply of Ag on the composite coating surface enabling the continuous formation of the lubricating film composed of Ag2MoO4 on the composite coating surface,which greatly improves the multi-thermal cycle lubrication life of the composite coating.After 5 thermal cycles,the friction coefficient and wear rate of NiCrAlY-Mo-Ag@Ni composite coating are as low as 0.25 and 1.5×10-5 mm3/(N·m).(3)Mo@Ag@Ni particles were prepared by electroless plating and NiCrAlY-Mo@Ag@Ni composite coatings were prepared by APS technology.The effects of multilayer core-shell structure on the mechanical properties,tribological properties,and multi-thermal cycle lubrication life of the composite coating were further investigated.Results showed that the mechanical properties of the NiCrAlY-Mo@Ag@Ni composite coating are further improved,which is attributed to Mo@Ag@Ni enhances the interfacial bonding strength between Ag,Mo,and NiCrAlY.During the high-temperature friction process,the core-shell structure of Mo@Ag@Ni promotes the occurrences of high-temperature tribochemical reaction on the composite coating surface,accelerates the formation of Silver molybdate,and endows the composite coating with excellent tribological properties.Mo@Ag@Ni particles also can slow-release the diffusion of Ag and improve the multi-thermal cycle lubrication life of the composite coating.The mechanism of the prolonged lubrication life of the soft metal core-shell structure on the composite coating is that the core-shell structure inhibits the diffusion speedy of Ag in the composite coating,resulting in Ag being provided continuously for the worn track interface to produce sufficient Ag2O.When the Ag2MoO4 of the worn track surface is consumed,the adequate Ag2O from the worn track interface reacts with MoO3 to form a continuous Ag2MoO4 lubrication layer on the surface.The continuous formation of Ag2MoO4 lubricating film on the worn track surface endows the composite coating with a longer lubrication service life.In summary,the solid lubricant core-shell structure material can well prolong the service life of the composite coating,which will provide a theoretical basis and key technical support for the development of a new type of long-life selflubricating coating over a wide temperature range. |
PDF Full Text Request