Microstructure And Tribological Properties In Multiple Environments Of Nc-Cu/a-C:H Films

As a kind of novel solid lubricant,amorphous hydrogenated carbon(a-C:H)film,with the characteristics of high hardness,low coefficient of friction,good wear and corrosion resistance and chemically inertness,etc.,is considered as a very promising anti-friction and wear-resistant material and expected to meet the application under a variety of extreme conditions.However,due to the high internal stress and different tribological properties in multiple environments,the practical application of a-C:H film was limited.Doping other elements in an a-C:H film can effectively reduce the internal stress,improve the toughness and tribological adaptability in multiple environments of the film.Copper(Cu),as a weak carbide forming metal,has good ductility and self-lubricating effect,and does not react with carbon in the film.Doping Cu in the a-C:H film may solve the defects of a-C:H film,such as large internal stress,large difference tribological properties in multi environment.In this article,the copper doping a-C:H(nc-Cu/a-C:H)composite films were deposited on single crystal silicon wafer and 304ASS substrate by a mixture gas of argon(Ar)and methane(CH₄)bombarding the copper target using a pulsed magnetron sputtering technique.The doping content of copper was controlled by adjusting the CH₄ flow ratio in the mixture gas.The microstructure and mechanical properties of the films under different CH₄ ratios were investigated.The tribological properties in multiple environments and related mechanism of the films were studied.The main results are as follows:(1)nc-Cu/a-C:H films with a total thickness of 1.25?m and different Cu contents were prepared by adjusting the CH₄ gas ratio.The thin films show a fine grain and compact structure.(2)XPS analysis indicated that as the proportion of CH₄ in the mixed gas increased,the carbon content in the film increased and the Cu content decreased.When the CH₄ ratio reached 65%,the Cu content in the film reduced to 3.35 at.%,and the peak area of sp³C-C decreased significantly,while the peak area of sp²C=C increased.(3)XRD analysis show that no carbon diffraction peak appearing in the films indicated that carbon was existed in the films with a amorphous form.As the proportion of CH₄ increased,the content of Cu in the films gradually decreased,and the intensity of Cu diffraction peak gradually decreased.Cu diffraction peaks were not found in the film at a CH₄ proportion of 65%.Cu mainly existed in a form of nanocrystalline in the films.So nanocrystals/amorphous structure composite films(nc-Cu/a-C:H)were formed.(4)The Raman spectroscopy analysis results showed that as the CH₄proportion increased from 50%to 60%,the G peak position of the films moved slightly towards the higher wave number and I_D/I_G ratio decreased.The film under a CH₄ proportion of 60%had the lowest I_D/I_Gratio and the highest sp³C content.When the CH₄ proportion reached to65%,the G peak position of the film increased significantly to high wave number and the I_D/I_G ratio reached a maximum value.It indicated that the content of sp²C increased significantly.(5)The indentation test indicated that the hardness and elastic modulus of nc-Cu/a-C:H films under CH₄ ratio 50%and 55%were low.However,the films show a good toughness and the best film-substrate adhesion.The hardness of the film under a CH₄ proportion of 60%and with the highest content of sp³C in the film was better than that under CH₄ proportion of 50%and 55%.Although the toughness of the film was slightly poor,there was no continuous observable spalling when the film is under a Rockwell indentation,meaning the film with a better bonding strength.When the proportion of CH₄ was 65%,the film with the lowest Cu content and highest hardness show a worse toughness and film-substrate adhesion.(6)All the nc-Cu/a-C:H films prepared under different CH₄proportions did not fail in the different friction environments(air,3.5%Na Cl solution,SBF solution).The wear mechanism of nc-Cu/a-C:H film in air condition was mainly abrasive wear.The film under the CH₄ proportion of 60%show the lowest friction(0.098)coefficient and the best wear resistance in air(wear rate is 1.77?10^-6mm³·N^-1·m^-1).However,the nc-Cu/a-C:H film under high CH₄ ratio(65%)show a better wear performance in a 3.5%Na Cl aqueous solution and an artificial simulated body fluid(SBF).The film had a strong adaptability to the friction environment.(7)The tribological properties of nc-Cu/a-C:H film under 65%CH₄ratio in the 2-D material(Mo S₂?Mo S₂-C?Mo S₂-V and Mo S₂-Ag)solutions were further investigated.The friction coefficient of the nc-Cu/a-C:H film in in Mo S₂-Ag solution was the lowest(0.042).It may be due to the soft metals Cu and Ag can form a self-lubricating metal-rich transfer film at the friction interface.Colloids formed in the solution with the debris generated by friction with the Al₂O₃ ball reduced the friction coefficient of the film.