Study On Properties And Structure Of The Dimand-like Carbon Films

This letter studied the doped diamond like carbon(DLC) films prepared on the substrate of domestic high-speed steel W6Mo5Cr4V2 by a Teer hybrid unbalanced magenetron sputter ion-plating and PECVD deposition system. The mechanical property, surface topography and microstructure were observed by metallographic microscope, microhardness instrument, rotating friction tester, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscope (TEM). The thermal stability of DLC films was investigated by thermal analysis and the growth mechanism of films was discussed.The results showed that: the hardness of Mo doped DLC films decreased to 1888 HV because the doped Mo lead to the promotion of sp³ to sp². A transfer layer which had the same structure with graphitic was formed during wear testing. This transfer layer had the function of self-lubricating and decreasing the friction coefficient. As a result, the DLC films had good wear resistance. Toughness of the DLC films was improved because of the existence of ductile metal phase. The thermal stability of Mo doped DLC films was not good. Hydrogen could evolve from DLC films in air at 114℃when graphitization began to appear. The carbon was oxygenized at 262℃.In the DLC films doped with Si, Si could form sp³ bond with C. It advanced the content of sp³ and improved the hardness up to 3538HV. Due to the doping of Si, the formation of graphite transfer layer was hindered; the fiction coefficient increased; the wear resistance decreased. Because Si has the same structure with diamond, it increased the thermal stability of DLC films greatly. Consequently, graphitization of Si doped films occured at 495℃. With the increase of temperature, only a trace amount of of hydrogen precipitated, and also little carbon was oxidized. The antioxidant properties of DLC films was improved.The DLC films doped with Mo and Si simultaneously had the characteristics of the films doped with Mo and Si separately. The hardness was 3024HV which was slightly lower than the Si doped DLC films but higher than the Mo doped ones. And Mo could improve the toughness. Although graphitic transfer film still formed during wear testing, their formation time was delayed because of the doping of a small amount of Si. These film had better thermal stability and higher graphitization temperature of 519℃, which was 257℃or 24℃higher than the Mo or Si doped DLC films. The volume of transformation was significantly reduced. The films had good oxidation resistance.While DLC films were mainly amorphous structure, nanocrystals of Mo, Mo₂C, Mo₂C were formed because of the doped Mo. These particles, which size was about 20～30nm, distributed uniformly in the disordered amorphous carbon network structure. In the Si doped DLC films, owing to the fact that Si didn't form carbide and the films were prepared by UBMS and RF-PECVD system, the growth of columnar crystal was inhibited. Therefore, the films grew as layers with clear interfaces and the thickness of the layers was about 5nm. The DLC films doped with Si and Mo simultaneously had nanocrystals and layered amorphous carbon. And the interfaces between layers were fuzzy because of the lower Si content. Due to the nano-particles formed by the doping of Mo, the growth of layers in large areas was hindered, and the layers had an increase in number but become disordered. These nano-layered structure could inhibit the formation and expansion of dislocations, and significantly improve the strength hardness and wear resistance of the films.Buffer layers used in DLC films enhanced the adhesion between DLC film and the substrate. The compositional gradient in DLC films could eliminate the sudden change of thermal, elastic and plastic mismatch in the interface between different solids, and also could decrease the stress concentration in the films. As a result of the compositional gradient, the interfaces were not clear, which could decrease the stress concentration in the corner, and prevent the generation of cracks, abruption and peeling off of the films.