Study On Preparation And Properties Of NiCr-Cr₃C₂-BaF₂·CaF₂-Ag Wide Temperature Range Self-lubricating Composite Coating

High-temperature self-lubricating coatings are of special significance in improving the lifetime and performance of the mechanical hot parts owing to their excellent mechanical properties,wear resistance,oxidation resistance and low friction NiCr-Cr₃C₂-BaF₂·CaF₂-Ag coating is a type of traditional self-lubricating coating,which has been widely used as bearing or seal applications in nuclear industry and thermal gas turbine engines.The excellent resistance of the coating to wear benefits from the particulate reinforcement of bonded chromium carbide and the high temperature lubricity of embedded BaF₂·CaF₂ eutectic.However,the lubrication effect of the fluoride eutectic is formed only after a brittle-to-ductile transition takes place at temperatures close to 400?.The operating temperature of many aerospace components continuously range from room temperature to high temperature(650? and above).The poor tribological behavior of NiCr-Cr₃C₂-BaF₂·CaF₂-Ag coating at low to moderate temperatures often leads to quick failure of components resulting from excessive wear of coating or the couple partIn this paper,silver is added as low-temperature solid lubricant to improve the friction and wear performance of NiCr-Cr₃C₂-BaF₂·CaF₂ coating at low and medium temperature.NiCr-Cr₃C₂-BaF₂·CaF₂-Ag composite powder was prepared by drum-type mechanical mixing method,then NiCr-Cr₃C₂-BaF₂·CaF₂-Ag composite coating was prepared by high velocity oxy-fuel spraying technology.In addition,NiCr-Cr₃C₂-BaF₂·CaF₂-Ag coating and NiCr-Cr₃C₂ coating were prepared as comparisons.The microstructure and phase composition of the coating were observed and determined by X-ray diffraction,scanning electron microscope,energy spectrometer,transmission electron microscope and Raman spectroscopy.The friction and wear behavior of the coatings under dry sliding against Si3N4 balls from 25? to 800? was evaluated with a ball-on-disk high temperature tribometer.The tribological mechanism of NiCr-Cr₃C₂-BaF₂·CaF₂-Ag composite coating was discussed with the help of surface morphology and composition of the wear scars.The main results are shown as follows(1)The NiCr-Cr₃C₂-BaF₂·CaF₂-Ag composite powder prepared by the drum-type mechanical mixing method greatly retains the original morphology of the agglomerated powder NiCr-Cr₃C₂-BaF₂·CaF₂-Ag and the ion atomized powder Ag,ensuring the fluidity of the to-be-sprayed powder.(2)As-sprayed BCF-Ag coatings exhibit dense and continuous splats-like microstructure.Silver particles are fully melted and well spread on the substrate,and feature as pancake-lamellas in the Ag-BCF coatings.The fluorides in the coatings are in the forms of both amorphous and crystal structure.(3)Introducing appropriate amount of silver can effectively improve friction and wear performance of BCF-Ag composite coatings in a wide temperature range.Due to the lubrication effect of metallic silver in the temperature range of 25-350?,BCF-Ag coatings basically possess superior friction coefficients and wear rates than BCF coating at 25?,200? and 350?.The compact glazed layers formed at 500? and 650? on the worn surface of 10Ag-BCF coatings,mainly containing AgCrO2,makes a great contribution to the improvement of its frictional behavior and wear resistance at 500?and 650?.As temperature increases to 800?,the formation of BaCrO4 on the worn surfaces of BCF and 10Ag-BCF coatings plays an important role in reducing abrasion and friction.