Mechanism And Tribological Behavior Of Doped Diamond-like Carbon Films

Diamond-like carbon films?DLC?have found a wide range of applications in aerospace,automotive,optical and bio-medical fields in recent years,due to their unique properties such as high hardness,high elastic modulus,low coefficient of friction,exceptional wear resistance,light transmittance,excellent chemical inertness and bio-compatibility,etc.However,there are still some barriers which have to overcome for the use of DLC films to move from an economical niche market to the mass production market.The main drawbacks of DLC films is its very large internal stress that exists irrespective of thin film deposition technique,which leads to the delamination of films from the substrate.In addition,the tribological behavior of DLC films is sensitive to its service environment,which limit their further application in industry.In this research,doping DLC with different elements and compounds to modify their microstructure and properties and still maintaining the amorphous phase of the films have been applied to reduce the compressive stress in the DLC films and diminish the sensitivity of tribological behaviors in practical applications.The W-doped DLC films were successfully prepared on H13 steel and silicon wafer by method combining magnetron sputtering and anode layer ion source combined technology.The gradient transitional layers Cr/CrN/CrNC/CrC/WCrC were deposited between the substrate and the DLC film to improve the interfacial adhesion.The total thickness of the W-DLC was 3.51?m,the adhesion strength of film on H13 steel substrate was HF2 as characterized by VDI 3198,and the hardness of the W-doped DLC film was about 2289 HV.The microstructure and composition of the W-doped DLC film are estimated by TEM,SEM and XRD,respectively.It is observed that nanometer sized tungsten carbide crystals were embedded in the amorphous carbon matrix,dispersedly.The high temperature tribological behaviors of W-doped DLC films were systematically studied ranging from 25? to 500?,it turned out that the incorporation of W atoms into the DLC film can improve the high temperature tribological stability and reduce compressive stress synchronously.The structure and tribological properties were stable under 200?.The graphitization and oxidation were observed at 300?,which deteriorated the wear resistance of as-deposited DLC films.A large number of carbon was lost due to the oxidation of the films,meanwhile,the formation of WO3 was detected on the surface of the samples,so that the wear resistance was almost lost above 400?.Anyway,The coefficient of friction can be as low as 0.15 at 500?,but its wear rate was much higher than that of the samples tested between ambient temperature and 200?.The hydrogenated Si-doped DLC films were prepared on Cr12 MoV steel and Si wafers by using MF-magnetron sputtering and ion beam assisted deposition technology.The structure and properties were strongly influenced by the content of doped Si.The micro hardness of DLC film which contains 3.75 atom percent silicon was 2039 HV and the adhesive strength of the films were more than 30 N.The tribological behaviors of Si-doped DLC film were studied in different relative humidity.The results showed that incorporation of Si in DLC can render its friction against steel counterpart insensitive to moisture.Among which,with content of atomic Si ranging from 3.38 at.% to 3.75 at.%,the friction coefficient is almost independent of the relative humidity,which can be as low as 0.13 in high and low humidity conditions.In order to comply the requirement of multi-environmental adaptation,the WS₂-doped DLC films were designed and prepared on 9310 carburized steel,TC4 alloy and K20 cemented carbide respectively by using MF-magnetron sputtering and ion beam assisted deposition technology.It was observed that the W-S-C composite film was consisted of WC1-x,WS₂ nanometer crystals and amorphous carbon phase.Thus the W-S-C composite films exhibits high hardness and superior abrasion resistance and excellent self-lubricating properties.The WS₂-doped DLC film samples were tested in dry nitrogen and ambient condition respectively.The coefficient of friction can be as low as 0.03 in dry nitrogen and 0.04 in ambient condition with oil lubrication.The wear rate of WS₂-DLC samples was as low as 9.105×10^-8mm3·N^-1·m^-1 in ambient condition without oil lubrication.