Study On Synthesis,structure And Properties Of Copper/ Chromium Co-doped Diamond-like Carbon Based Films

Due to their high hardness,good wear resistance and wide optical transparency,Diamond like carbon（DLC）films have been widely used in the industrial fields such as aerospace,mechanical engineering.However,the problems arising from high residual stress,poor toughness,hydrophilicity and sensitivity of the environment seriously limit the development of DLC applications.Metal doping has been proved to be an effective method to reduce the residual stress,as well as improve the tribological and hydrophobic properties of films.But mono-metal doping with different metal element is difficult to satisfy the requirement in practical applications,such as high hardness,low stress,strong wear resistance and other comprehensive properties.Based on our theoretical results,herein,we firstly bring forward the idea of Cu/Cr co-doping to improve the properties of DLC films in this thesis.The synthesis,microstructure,and properties of films are focused.The relations among residual stress,mechanical properties,tribological properties and microstructure of films are carried out.Most importantly the mechanism of stress reduction and high hardness maintaining caused by duplex metals is clarified.The application of films in metal substrates like stainless steel and high speed steel was studied.Firstly,the sputtering currents were varied to investigate the effect of metal concentration on the microstructure and properties of Cu/Cr co-doped DLC films.Results showed that the residual stress and hardness firstly decreased and then increased with the metal concentration.In particular,the film with Cu_{11.88 at.%}Cr_{6.57 at.%}exhibited a relatively high water contact angle of 103.6°,lower residual stress of 0.89GPa and high hardness of 13.44 GPa.The combined structural analysis demonstrated that the synergistic Cu/Cr co-doping resulted in the formation of a nanocomposite structure,which consequently caused the reduction in residual stress by grain boundary sliding and diffusion.Meanwhile,the distorted C-C bond length was significantly relaxed by Cu/Cr addition.In addition,the formation of hard Cr carbide nano-particles in amorphous carbon matrix could promote maintaining a high hardness of films,while the tribological performance decreased.Interestingly,noted that the wetting of films variety from hydrophilic to hydrophobic state was attributed to the surface morphology,enhanced graphitization and emergence of copper oxidation phases.And the water contact angle of film could be further increased to129.9°by annealing method at 550℃,which was attributed to the increase of the size and density of the Cu nano-particles on the surface.Meanwhile,the wear resistance of the film was improved in the deionized water lubrication.Secondly,in order to obtain better mechanical and tribological properties of the films,the films were optimized by controlling the deposition temperature and the ratio of Ar/C₂H₂ gas flow.When the sputtering current was fixed at 1.8 A and deposition temperature was 200℃,the results showed that the hardness and wear resistance of films were improved with the decrease of Ar/C₂H₂.A self-organized multilayer structure with a carbon rich layer on the surface and a metal rich layer within the interface was formed when the ratio of Ar/C₂H₂ was 68/12.As a consequencely,high performance of DLC film with residual stress of 0.99 GPa,hardness of 25.9 GPa,friction coefficient of 0.17 and wear rate of 1.12×10^-16 m³/Nm was obtained.Meanwhile,the films exhibited low friction and excellent wear resistance due to the formation of a complete and compact carbon rich transfer film on the contact surface.In addition,it showed good tribological adaptability in oil,deionized water and dry environments.Base on the above results,the mechanism of gaining a low internal stress while maintaining a high hardness with Cu/Cr co-doped was studied.Results showed that there were three areas in the evolution of residual stress and hardness of films with the concentration of Cu/Cr increased,which were amorphous carbon characteristic area,transition area and metal characteristic area.In the amorphous carbon characteristic area and metal characteristic area,the residual stress and hardness restricted each other in the film.When in the transition area,Cu and its oxidation nano-particles,Cr solid solution atoms and Cr-C phase were formed.The restricting effect between stress and hardness got weakened,which play a key role in designing a Cu/Cr-DLC film with low stress and high hardness.Finally,the application of Cu/Cr co-doped DLC films on metal substrates was developed.Based on the structure designing of mono-doped metal DLC films,Cu/Cr doped DLC multilayer films were optimized by soft and hard layer.The DLC film with thickness 1.73μm,substrate-film adherence>45 N,hardness 27.1 GPa,residual stress 1.0 GPa was successfully deposited on the substrates of high-speed steel,stainless steel and silicon wafer,and its applications in screws and holding clamps were explored.