Study On Design And Tribological Properties Of High-Temperature Self-lubricating CoCr-MoS₂ Coatings

CoCr matrix self-lubricating coatings have excellent high-temperature mechanical properties,corrosion resistance and tribological properties at high temperature conditions,so the coatings have broad application prospects in the aerospace field.At present,there are few theoretical studies on high-temperature self-lubricating anti-wear coatings in China.There is no systematic research on the lubrication mechanism of the coatings at high-temperature environments.Traditional CoCr matrix self-lubricating coatings have a narrow applicable temperature range,a complicated preparation process and high precious metal content and other defects.In this paper,the CoCr matrix high-temperature self-lubricating coatings are prepared on the surface of Inconel718 alloy by using a powder metallurgy technology（P/M）,with MoS₂ as the lubricating phase and Ni,W as the alloying elements.The tribological properties of the coatings under different loads,speeds and temperatures are systematically studied,and the wear mechanism of the coating is analyzed.The bonding strength of the coating is tested by thermal shock experiment,and the stress distribution at the interface between coating and substrate is analyzed using ANSYS software.MoS₂ as a solid lubricant can significantly improve the friction and wear performance of the coating at 20～600°C.After adding MoS₂ to the CoCr matrix coatings,it formed solid lubricating phases Mo₂S₃ and Cr_xS_y with other metal elements.At low temperature,the friction coefficient of the coatings gradually decreased with the increase of MoS₂.At high temperature,the coatings have an excellent high-temperature tribological properties due to the formation of a solid lubricating film which consisted of chromate,oxide and sulfide on the worn surface.Ni element can stabilize theγ（fcc）phase in the coating,making the CoCr matrix coating have good toughness and plasticity at low temperature.The CoCr-MoS₂ matrix coating with Ni as the alloying element has good tribological properties at different speeds,loads and temperatures,the wear rate of the coatings is below 5.5×10-5 mm3N-1m-1 over a wide temperature range,and the wear resistance of the coatings is 4～17 times higher than that of the substrate from 20to 600°C.W element can stabilize theε（hcp）phase in the CoCr matrix coating.The CoCr-MoS₂ matrix coating with W as the alloying element has a hardness increase of 32%compared with the coating with Ni as the alloying element.The abrasive debris generated by the coating during the friction process is hard,which is not conducive to the formation of the friction layer on the worn surface at low temperatures.400°C is close to the phase transition temperature of the CoCr matrix coating（417°C）,the hardness of the coating is reduced at this time,and the wear rate is increased.At 800°C,W element stabilizes theε（hcp）phase in the CoCr matrix coating,improves the coating’s resistance to adhesive wear,the coating has better tribological properties at high temperature.ANSYS finite element software is used to analyze the residual stress distribution inside the CoCr matrix coating.The results show that the samples coated with the CoCr matrix coating are deformed upward in the center and downward in the coating edge after cooling;The deformation of the two coating systems gradually increase from the center to the edge and is distributed in a ring shape;The deformation at the edge of the coating is the largest;The residual stress in each part of the CoCr matrix coating with W as the alloying element is significantly bigger than the coating with Ni as the alloying element.The residual stress distribution near the center of the two coatings is uniform,and the stress gradient is large at the edge of the coating joint surface,and cracks are most likely to occur.The thermal shock test is used to test the bonding strength of the coating and the substrate.After the experiment,the cross section of the coating is observed with a scanning electron microscope,and it is found that the bonding surface of the coating is well combined,and no obvious pores and cracks are generated.The bonding strength is high.