Study On Preparation And Tribological Properties Of Nickel-alloy Matrix Self-lubricating Coatings At Elevated Temperatures

Nickel-alloys matrix self-lubricating materials have become the first choice for air foil bearings and other aerospace high-temperature friction parts due to their high mechanical strength,wide temperature range,low friction coefficient,and low wear rate.In this paper,nickel-based compositepowders（Ni Cr Mo Al-Ag-Ba F₂/Ca F₂）werepreparedby high-energyballmillingcombinedwithspraydryinggranulation technology.Nickel-alloys matrix self-lubricating coatings were prepared by plasma spraying at different spraying distances.The spraying distance was used to study the coating morphology.The mechanism of the bonding strength was used to obtain the optimal coating distance;the friction and wear behavior of the coating in the wide temperature range（25-800°C）was studied;The effects of different friction rates and different working temperatures on the tribological properties of the coatings were also investigated.The main findings were as follows:The spray-drying equipment was used to improve the flowability of the nickel-alloys matrix composite powder,and the spraying distance of 50-200mm were also tested by using agglomerated powder.The results show that when the spraying distance was too close,it will lead incomplete melting of powder,combined with the substrate is not strong,more powder were floating on the surface of coating;At the same time,this will make the plasma jet flow to the substrate material,and the quality of the coating will be affected;When the spraying distance was too far,the powder takes a long time to flight,the kinetic energy is insufficient,the powder burns too much,and the substrate can not form a complete occlusion.The sprayed coating was smooth and uniform and the thickness was precisely,showing a better spraying deposition when coating were sprayed in the range of 60-140 mm.By analyzing the microcosmic morphology,powder melting state and tensile strength of the coating under this range,it is concluded that when the spraying distance is 100 mm,the coating surface has fewer voids and micro cracks,the coating is dense,and the bonding strength is high.Therefore,it was determined that the optimal spraying distance for nickel-alloys matrix self-lubricating coating is 100 mm.The nickel-alloys matrix self-lubricating coating was prepared using the optimal spraying distance parameters.The coating structure is dense and theoriginalphaseremainswell.Scanningelectronmicroscopy,microhardness tester,and universal tensile tester were used to analyze the effects of coating morphology and mechanical properties.Friction test were carried out by sliding the nickel-alloys matrix coaing disks against Inconel718 pin,the testing temperatures range was from 25-800°C.The results showed that nickel-alloys matrix self-lubricating coatings composed of Ni-Cr,Ni₃Al,Mo,Ag and Ca F₂/Ba F₂ phases were obtained by plasma spraying process.The coating has excellent self-lubricating properties in the wide temperature range of 25 to 800°C,and the average friction coefficient is lower than 0.25.The wear rate is on the order of10^-5mm³N^-1m^-1.The low friction coefficient and wear rate of the coating at wide temperatures range were benefited from the"precipitation effect"of the lubricants Ag and Ba F₂/Ca F₂,and synergistic effect of Ag,Ba F₂/Ca F₂and Ag₂Mo O₄,Ni Cr₂O₄ and Ba Mo O₄ oxides at high temperature.A pin-on-disk HT-1000 friction-meter was used to test the friction and wear properties at different rubbing speeds（0.2¹.0 m/s）and under different testing temperatures（25⁸00°C）.The microstructures,phase compositions and wear mechanism of the coating were analyzed by SEM,EDS and Raman.The results indicated that at a wide temperature range from25°C to 800°C,the friction coefficient of the coating firstly increases and then decreases with the temperature increased.The friction coefficient reaches the maximum value of 0.37 at 400°C;and then reduces to a minimum value of 0.17 at 800°C.The friction coefficient of the coating showed a trend of decreasing firstly and then increasing with increasing sliding speed also.The lowest friction coefficient of the coating is about0.29⁰.17 at 0.8 m/s,and then increases to 0.27⁰.2 at 1.0 m/s.The favorable lubricating properties of the coating could be attributed to the low shear of the soft metal Ag,and especially the synergistic lubrication effects of Ba F₂/Ca F₂,Ag and high-temperature products such as molybdate and chromate contribute the coating with excellent lubricity properties under temperature range of 600⁸00°C.