Study Of The Effect Of Interfacial Structure On The High-Temperature Performance Of CrAlN-base Multilayer Coatings

As protective hard coatings,CrAlN-based multilayered coatings can be effectively used to prolong the material life due to the excellent hardness,friction resistance,superior oxidation resistance.However,the diffusion of elements at high temperatures will lead to the decomposition of structure and corresponding deterioration of high-temperature properties and thermal failure of the coating.Therefore,the study of diffusion mechanism and thermal stability of CrAlN-base multilayers at high temperature is beneficial to the development of coating design.In this paper,CrN,CrAlN and CrAlN-based multilayer coatings with different interfaces were prepared by PVD deposition technique.The influence of the interface types on microstructure,mechanical properties,interfacial diffusion,thermal stability and oxidation behavior of coatings under high temperature was studied.The preferred orientation,residual stress and hardness were significantly affected by the homogeneous interface of CrAlN/CrN multilayer coatings.As the thickness of the CrAlN sublayer increased,the dense oxide layer was formed on the coatings with reduction of the oxygen content at the interface between oxide scale and nitride,which resulted in the improvement of oxidation resistance and reduction of chromium oxide.However,the CrAlN/CrN multilayer structure was unable to improve the thermal stability of coatings.The crystal SiNx sublayer was transformed into an amorphous structure as the layer thickness exceeded a critical thickness of 0.9 nm in CrAlN/SiNx multilayer coatings and the preferred orientation changed from(111)orientation to(200)orientation.Multilayer coatings exhibited higher hardness,compressive residual stress and the lowest wear rate than the monolithic CrAlN,and the coating consisted of 0.9 nm nanocrystalline-SiNx and 5.5 nm nanocrystalline CrAlN sublayer exhibited the highest value of 37.9±2.5 GPa,-5.9±0.2 GPa and 0.7×10-7 mm3/Nm,respectively.The hardness and residual stress decreased with the increase of CrAlN sublayer thickness.Phase transition was not detected in the nanocrystalline multilayer coatings,and the sublayer remained intact after vacuum annealing.However,sublayer interface thermal dissolution in the coatings with amorphous SiNx sublayer resulted in a pitch-off effect at the grain boundary and the formation of w-AlN under the same condition.The influence of interface structure and modulation period of CrAlN/SiNx multilayer coatings on oxidation resistance was investigated at high temperature.The experimental results showed that the multilayers can improve the oxidation resistance compared with the monolithic CrAlN coating.The more stable crystalline interface structure in multilayers was beneficial to the oxidation resistance compared with the crystalline-amorphous interface structure at high temperature.The best oxidation resistance was obtained in multilayers with 0.5 nm crystalline SiNx sublayer.Hence,the oxide types and oxidation resistance of multilayers were influenced by modulation period and temperature.The coatings were slightly oxidized until the temperature raised to the phase transition temperature point.The oxidation resistance of CrAlN/SiNx multilayer coatings was improved due to the reduction of diffusion paths caused by the dispersed nano-amorphous SiO2 in oxide scale at 900℃.However,the multilayer structure with 5.5 nm CrAlN sublayer was destroyed and the 3Al2O3·2SiO2 was formed caused by the dissolved SiO2 which resulted in the incomplete oxide scale after oxidation at 1000℃.Hence,the coating with 5.5 nm CrAlN sublayer was continuous oxidized due to the inner diffusion of oxygen along the grain-boundary and defects.The multilayer coating consisted of 10.7 nm CrAlN and 0.5 nm SiNx exhibited the best oxidation resistance and the oxide scale was only 240 nm thick.The high-density stacking faults caused by oxidation of SiNx were mainly distributed along the grain boundaries and then extend into the multilayer with 5.5 nm CrAlN sublayer,which resulted in destruction of multilayers and corresponding reduction of oxidation resistance.While the low-density stacking faults were diffusely distributed in the columnar grains below the oxide layer in the multilayer coating structure with 10.7 nm CrAlN sublayer which showed slightly impact on oxidation resistance.