Preparation And Tribological Properties Of NiCrW-based Self-lubricating Composites

Wear is the main form of failure in metallurgical equipment parts after they have been used for a long time under harsh conditions,such as high temperature,heavy load and vibration.This means that the material for the equipment parts should not only have certain mechanical properties,but also good tribological properties in a wide temperature range.In this paper,nickel-plated graphite was prepared by electroless plating method,and WS₂ nanosheets was synthesized by solid phase reaction method.A nickel alloy-based self-lubricating composite material possessing high mechanical properties and good self-lubricating performance in a wide temperature range was prepared by powder metallurgy with nickel-plated graphite and WS₂nanosheets used as lubricants and NiCrW alloy powder used as a matrix material.The tribological properties of the composite materials were tested by HT1000 high-temperature friction and wear tester,the microstructure and composition of lubricants and wear scars were analyzed by scanning electron microscope,energy dispersion spectrometer and X-ray diffractometer.The self-lubricating mechanisms of composites in a wide temperature range were studied.The details are as follows:1?The nickel-plated graphite with uniform thickness and compactness was prepared by electroless plating.NiCrW/graphite composites were prepared via powder metallurgy using pure graphite and nickel-plated graphite as lubricants,and the NiCrW alloy powder as the matrix.The microstructure,mechanical properties and tribological properties of composites added with nickel-plated graphite were better than those of composites added with pure graphite due to good dispersion and combination of nickel-plated graphite.With the increase of either graphite content or temperature,friction coefficient and wear rate of NiCrW/graphite composites decreased first and then increased.The tribological properties of NiCrW/graphite composites were better when the content of nickel-plated graphite was 3wt.%,and its friction coefficient was 0.27-0.42 from room temperature to 500?.It exhibited best tribological performance at 300? with 0.27 of friction coefficient and 2.7×10^-55 mm³·N^-1m^-11 of wear rate.NiCrW/graphite composites possessed certain self-lubricating properties from room temperature to 500? due to the lubricating layer composing of graphite and NiCr₂O₄ formed on the surface had antifriction properties,and the carbides Fe₃C and Cr₃C₂ in the material enhanced its wear resistance.2?The highly pure hexagonal layered WS₂ nanosheets were prepared by solid-phase reaction method.The NiCrW/WS₂ composites were prepared by powder metallurgy with Cu powder added,WS₂ nanosheets as lubricant and NiCrW alloy powder as matrix.With the increase of WS₂ content,the density and hardness of the composites first increased and then decreased,the friction coefficient decreased gradually and the wear rate decreased first and then increased.When the content of WS₂ was 10wt.%,the composites had best tribological properties and higher density and hardness.With temperature increasing,the friction coefficient of the material gradually decreased,and wear rate increased.The friction coefficient of the composite with 10wt.%WS₂ from room temperature to 500? was 0.23-0.37,which had better self-lubricating properties.WS₂ acted as a lubricant at room temperature,meanwhile,CrS generated by the reaction between WS₂ and Cr had high-temperature lubrication,leading to the excellent self-lubrication performance in a wide temperature range.And increased wear rate at high temperature was due to the"self-consumption"phenomenon of metal sulfides.3?NiCrW/graphite/WS₂ composites were prepared by powder metallurgy with Cu powder added,graphite and WS₂ nanosheets as lubricants and NiCrW alloy powder as matrix.WS₂respectively reacted with graphite and Cr to form the WC hard phase and the CrS,Cr₂S₃lubricating phases during sintering,and the composite containing 3wt.%graphite and 10wt.%WS₂ had higher density and hardness.As lubricants content or temperature increased,the friction coefficient of the composite decreased gradually,the wear rate decreased first and then increased,composites containing 3wt.%graphite and 10wt.%WS₂ had lower wear rate from room temperature to 500? which ranged?2.7-3.1?×10^-55 mm³·N^-1m^-11 and friction coefficient ranged0.13-0.35,indicating the existence of synergistic lubrication of graphite and WS₂ endowing the material better self-lubricating properties in a wide temperature range.Graphite played a major role in lubricating at room temperature,when the temperature rised,the metal oxides formed on the surface of the substrate began to have lubricating properties.CrS and Cr₂S₃ played a major role in lubrication at 500?,and increased wear rate was due to the"self-consumption"phenomenon of CrS and Cr₂S₃.