Study On Tribological Properties And Failure Mechanism Of V₂AlC MAX Phase Coating In Wide Temperature Range

With the rapid development of modern industry,the working conditions of the core components and technologies of friction system are becoming more and more demanding.Flakes,for example:air bearings,aircraft brakes,rolling element bearing,gas machine blade,a variety of key parts such as satellite components need to experience fast start in low temperature,high temperature stability for a long time running process,or low to high temperature cycles in the friction and wear problems in the process of service becomes the key affecting the service life of the whole system security and reliability.Traditional lubricating materials are not suitable for the current harsh conditions,and most of the lubricating materials studied at present are aimed at a specific temperature range,and do not have a wide temperature range lubrication effect.Therefore,it is necessary to develop a kind of solid lubrication coating which can be used in a wide range and has good lubrication effect at room temperature to high temperature.In this paper,V₂Al C,a typical representative of MAX phase system,was used to study its tribological properties from room temperature to 700°C by taking advantage of its significant thermal stability,oxidation resistance,machinability and lubricity.And explore its service life at 600°C,and finally reveal its failure mechanism.In this paper,arc composite magnetron sputtering technology combined with subsequent heat treatment was used to successfully prepare high-purity V₂Al C MAX phase coating.The coating structure is compact and defectless,and there are many al-rich"tumor"particles on the surface,which is caused by the low melting point（660°C）of Al target and high sputtering power（3.2 k W）.The friction coefficient of V₂Al C coating is 0.85 at 300°C.The friction coefficient and wear rate of the coating at 500°C are 0.8 and 3.2×10-4mm3/（Nm）,respectively.At 500°C,the surface is rough and the mechanical properties decrease,resulting in high friction coefficient and wear rate.The average friction coefficient and wear rate are the lowest at 600°C,which are 0.42 and1.1×10-5mm3/（Nm）,respectively.V₂Al C coating is oxidized and phase transition occurs at 600°C,and V₂O₅ is generated to lubricate.The V₂O₅ film can be spread over the worn surface and acts as a continuous lubricant.At the same time,the V₂Al C main phase retained in the friction process plays an appropriate bearing role,thus reducing the wear rate of the coating.When the temperature rises to 700°C,the oxidation degree increases,the formation rate of soft V-O phase accelerates,the structure becomes more loose and finally collapses.Although the friction coefficient is low,the wear rate is the largest.V₂Al C coating always maintains a low friction coefficient of 0.4 during the long friction process of 120 m,and there is no obvious fluctuation.The wear rate increases linearly with the friction distance,but it is relatively low,which is due to the wear surface can provide appropriate V₂O₅ lubrication film to maintain lubrication.The formation of appropriate V₂O₅ lubrication phase ensures the retention of V₂Al C main phase,effectively provides support,and reduces wear.As the oxidation time increases,the hardness of the coating gradually decreases,but the V₂Al C coating with the oxidation time of 3 h still has excellent performance under the external force of 100 g N,showing high hardness,which benefits from the excellent high-temperature stability of the coating,V₂Al C main phase retention provides effective support,the hardness of the coating does not decrease significantly.It can be said that the coating can be stable at600°C for a long time.This achievement extends the suitable temperature of V-based coating to 600°C,and proposes a method of preparing MAX phase protective coating suitable for high temperature friction environment.The formation of VO₂,V₂O₅ and V₆O₁₃ v-series oxides can be observed at 650°C for 5 min.The peak intensity of V₂Al C decreases significantly,and the presence of V₂Al C is only detected at 41.2°.The peak intensity of VO₂ and V₂O₅ increases at 30min.V₂AlC peak intensity decreased slightly.After oxidation at 700°C for 5 min,the oxidation phases are VO₂,V₂O₅ and Al₂O₃,and V₂Al C phase completely disappears,indicating that the coating has been completely oxidized.The oxide generated after oxidation for 15 min and 30 min is similar to that generated for 5 min,but the peak intensity is different.At 700°C,V and Al elements in the coating rapidly diffuse to the surface and form oxides such as V₂O₅.Under saturated vapor pressure,V₂O₅ volatilizes seriously.When the generation rate does not reach the volatilization rate,a cavity structure will be generated on the surface,which will accelerate the internal diffusion of oxygen,thus accelerating the oxidation rate of the coating.The formation of many holes will lead to poor bonding between the coating and the transition layer,which will peel off under the action of temperature.The dual effect of volatilization and spalling leads to rapid failure of the coating at temperatures above 650°C.