Study On The Preparation And High-temperature Properties Of TiAlCrTaWn Multicomponent Hard Coatings

The surface coatings have good hardness and wear resistance,which can significantly improve the service life and processing efficiency of cutting tools.Under thermal load,TiAlN coatings will undergo spinodal decomposition accompanied by hardness enhancement,and have been widely used in industrial applications.However,the development of advanced cutting technologies and the application of difficult-to-machine materials have put forward higher requirements to the thermal stability and oxidation resistance of coated tools.TiAlN coatings will be completely oxidized at 900°C,in addition,the formation of the hexagonal Al N stable phase after spinodal decomposition stage will cause a sharp decline in mechanical properties.The coated tools will be subjected to a certain impact during the machining process,leading to coating crack and peel off.Moreover,the coatings can also creep under the combined action of high temperature and high stress,resulting in the degradation of cutting performances and premature failure.In this thesis,the alloying method was employed to develop TiAlCrTaWN multicomponent coatings and investigated with respect to structural,mechanical,thermal,and tribological properties.The main research findings are summarized as follows:1)TiAlN,TiAlCrN,TiAlTaN,TiAlWN and TiAlCrTaWN multicomponent coatings exhibit face-centered cubic structure,in which Cr,Ta,and W are dissolved in TiAlN to form solid solutions.The as-deposited hardness of each coating is not much different,all exceeding32 GPa,and TiAlCrTaWN coatings exhibit more excellent scratch and indentation toughness.The two multicomponent TiAlCrTaWN and TiAlTaN coatings exhibit better creep resistance at room temperature.The creep mechanism of TiAlN and TiAlWN coatings is grain boundary sliding,the TiAlCrN coating is diffusion creep mechanisms,and the creep behavior of TiAlTaN and TiAlCrTaWN coatings are related to dislocation movement.2)The TiAlCrTaWN multicomponent coatings exhibit better thermal stability than TiAlN.The co-alloying of Cr,Ta,and W significantly broadens the temperature range of spinodal decomposition of TiAlN-based coatings,and effectively inhibits the subsequent formation of hexagonal Al N.The formation of hexagonal Al N phase was observed after annealing at1100°C for the Ti_0.21Al_0.28Cr_0.20Ta_0.16W_0.15N coating with higher mixing entropy.Then,its hardness remained at 33.5±0.7 GPa,much higher than the 26.8±0.7 GPa of TiAlN coating.3)The oxidation resistance of TiAlCrTaWN coatings is significantly better than TiAlN and singl alloyed TiAlCrN,TiAlTaN,TiAlWN.During the oxidation process,TiAlCrTaWN coatings also form a double-layer oxide structure similar to TiAlN,the outer layer is rich in Al and the inner layer is rich in Ti.The rapid diffusion of Cr in the initial stage of oxidation to form Cr₂O₃ is conducive to the subsequent growth of Al₂O₃;In contrast,the slow diffusion rate of Ta and W elements tends to dissolve in the inner Ti O₂,which promotes the direct formation of rutile Ti O₂,on the other hand,effectively reduces the oxygen vacancy concentration in the inner layer and inhibits the inward diffusion of oxygen.4)The TiAlCrTaWN multicomponent coating exhibits excellent wear resistance at high temperatures.TiAlN and TiAlCrTaWN coatings mainly show abrasive wear at room temperature,while the wear mechanism becomes adhesive and oxidation wear at high temperature.The reason TiAlCrTaWN coating has better wear resistance at high temperature is mainly due to its better oxidation resistance.