| With the development of aerospace,polar,deep-sea and other detection technologies,mechanical systems need to operate under severe conditions such as high load,wide temperature ranges,and strong irradiation.Because the lubricating oil cannot function effectively in this extreme environment and harsh working conditions,the key components are prone to friction loss and failure.Although the traditional hard protective films such as TiN and CrN can improve the life of surface modified parts to a certain extent,their toughness is poor.And due to the thermal expansion coefficient mismatch between the hard films and the alloy matrix,a large residual stress is generated inside the film,which will weaken the bonding strength between the hard film and the matrix.The protective films are very prone to failure when applied in the harsh environment.Therefore,the development of a strong and tough integrated wear-resistant and friction-reducing solid film for the application in harsh environments has become a key problem that needs to be solved urgently.The high entropy alloy film is a new type of material,which has a simple solid solution structure or amorphous structure with a variety of atoms highly soluble in each other.And its unique structure makes it have high entropy effect in thermodynamics,lattice distortion effect in structure,hysteresis diffusion effect in kinetics,and cocktail effect in performance.This makes it exhibit properties that traditional materials can’t have at the same time,such as high strength,wear resistance,corrosion resistance,oxidation resistance,radiation resistance and high temperature stability.It has shown great application value in the field of surface engineering of precision mechanical parts.High entropy alloy films have excellent comprehensive properties,but their hardness and wear resistance are still lower than those of traditional hard films such as TiN and CrN,which cannot meet the requirements of severe wear conditions such as high frequency and high load.Currently,grain refinement strengthening,solid solution strengthening,phase transformation strengthening and other methods are used to improve the strength and toughness of the high entropy alloy films,but their hardness and tribological properties still cannot match those of traditional hard thin films.Therefore,we propose the idea of realizing the strengthening-toughening of high-entropy alloy thin films through the design of nanocomposite thin films.The CuNiTiNbCr high entropy alloy system is designed based on the thermodynamic nanocomposite structure formation principle,and on this basis,the nanocomposite structure（CuNiTiNbCr）Nx,（CuNiTiNbCr）Cx,（CuNiTiNbCr）CxNy high entropy nanocomposite films are produced by high power pulsed magnetron sputtering（HPPMS）.The nanocomposite structure formation mechanism of high entropy nanocomposite films is studied.The strengthening-toughening mechanism and friction-reducing and wear-resisting mechanism of high entropy nanocomposite films are revealed.In this paper,CuNiTiNbCr high-entropy alloy films are deposited at different pressures by HPPMS technology,and the effect of deposition pressure on plasma discharge,film structure and mechanical properties are studied.The research results show that the increase of deposition pressure will increase the plasma discharge and make the substrate temperature rise rapidly.The films prepared at low deposition pressure（0.4 Pa）have an amorphous structure,while at high deposition pressure（1.5 Pa）,the films present a dual-phase structure of amorphous+BCC,which is because with the prolongation of deposition time,the substrate temperature increases rapidly,the diffusion ability of Cu improves,and a large amount of BCC Copper-rich phase precipitation is formed in the film.With the increase of deposition pressure,the structure of the film changes from dense to loose,showing a state of tensile stress,and the hardness and toughness decrease.Based on the research of CuNiTiNbCr thin film,HPPMS is used to prepare（CuNiTiNbCr）Nx high-entropy alloy/nitride nanocomposite films.The effect of nitrogen flow rate on plasma discharge,film composition,microstructure,mechanical properties and tribological properties is studied.The research results showed that with the increase of nitrogen flow rate,the plasma density in front of the target increases,and the nitrogen content in the film increases gradually,reaching a maximum of 27.8 at.%.With the increase of nitrogen content in the film,nitrogen element reacts with Ti,Nb and Cr to form the（TiNbCr）N nano-nitride phase,and then form a composite structure in which the nitride nano-crystals wrapped in the amorphous alloy matrix phase.When the nitrogen content is 17.4 at.%,the（CuNiTiNbCr）Nx film has both high strength and high toughness,the hardness reaches 18 GPa and the toughness reaches 1.17 MPa m1/2,and the protective oxide layer that is well bonded to the matrix is formed during the friction process.Under the combined influence of the above factors,the film exhibits the most excellent tribological properties,with a friction coefficient of 0.48 and a wear rate of 1.36×10-6 mm3/N·m.Based on the preparation of high entropy alloy/nitride nanocomposite（CuNiTiNbCr）Nxfilms,in order to further improve the wear resistance and friction reduction performance of the films,carbon atoms that are easy to form amorphous carbon lubricating phases are introduced into the films instead of nitrogen atoms.High entropy alloy/carbide/amorphous carbon nanocomposite（CuNiTiNbCr）Cx films are prepared by single carbon source and double carbon source methods,and the effect of carbon content on the microstructure,mechanical properties and tribological properties of the films is investigated.The results show that with the increase of carbon content,the carbon atoms in the film first react with Ti,Nb and Cr to form a（TiNbCr）C ceramic phase,and then the excess carbon atoms precipitate in the form of amorphous carbon phase in the film.The structure of the film changes from an amorphous structure to a nanocomposite structure of amorphous（amorphous alloy phase+amorphous carbon phase）-nanocrystalline carbide phase.With the increase of carbon content,the hardness,modulus and compressive stress of（CuNiTiNbCr）Cx films first increase and then decrease.When the carbon content is about 37.2 at.%,the hardness and modulus reach the highest,which are 20 GPa and 243 GPa respectively.At a carbon content of 59.0 at.%,the proportion of amorphous carbon lubricating phase in（CuNiTiNbCr）Cx film reaches the highest,the ratio of amorphous carbon phase to carbide ceramic phase reaches 3.7:1,and an interface lubricating layer mixed with metal oxide and amorphous carbon is formed in the friction process.Under the influence of the above factors,the film presents the best wear properties with a friction coefficient of 0.16 and a wear rate of 2.5×10-6 mm3/N·m.In the above study,the（CuNiTiNbCr）Nx films show better toughness than the（CuNiTiNbCr）Cx films.Therefore,by introducing an amorphous carbon lubricating phase into the（CuNiTiNbCr）Nx film with better toughness,its wear resistance and wear reduction performance can be further improved,and a high-entropy alloy nanocomposite film with both strengthening-toughening and wear resistance can be realized.（CuNiTiNbCr）CxNy high-entropy alloy/carbonitride/amorphous carbon nanocomposite films with different carbon contents are deposited by double-target co-sputtering technology,and the effects of carbon content on the microstructure,mechanical properties and tribological properties of the films are investigated.The carbon content in the film reached a maximum of 37.6 at.%,and the nitrogen content gradually decreases from 26.0 at.%to 15.0 at.%.The introduced carbon atoms will first enter the（TiNbCr）N phase to form a（TiNbCr）CN carbonitride ceramic phase,and then the excess carbon atoms will precipitate to form an amorphous carbon phase.The film forms a nanocomposite structure of amorphous（amorphous alloy phase and amorphous carbon phase）-nanocrystalline carbonitride phase.At a carbon content of 21.0 at.%,the film has the highest hardness of 18 GPa.When the carbon content is 37.6 at.%,the toughness of the（CuNiTiNbCr）CxNy film is the best,reaching 0.96 MPa m1/2,the ratio of amorphous carbon phase to carbonitride ceramic phase is 6.6:1,and a complete surface oxide protective layer is formed during the friction process.Under the combined action of these factors,the film exhibits the best tribological properties,with a friction coefficient of 0.27 and a wear rate of1.0×10-6 mm3/N·m.By introducing N and C atoms into the CuNiTiNbCr high-entropy alloy thin film system,the ceramic phase and amorphous carbon phase are formed in situ,and the preparation of high-entropy nanocomposite thin films with strengthening-toughening,friction reduction and wear resistance is realized.It provides new ideas for the strengthening-toughening structure design and improvement of the anti-wear and friction reduction properties of high entropy nanocomposite films,which makes the thin film system show great application prospects in the field of protection of precision parts in extreme environments. |
PDF Full Text Request