Investigation On Carbon-based Hard-and-lubricating Multilayered Solid Films

On the basis of the coating concept of multilayer in combination with architecture design of nanosizing, multicomponent, functionally gradient and composite, three series of high-performance carbon-based hard-and-lubricating multilayered solid films were prepared using hybrid deposition techniques, including nanoscale modulation multilayered Cr-C/a-C:Cr composite film, nanoscale tungsten- gradually doped multilayer Wx%-DLC film and self-lubricating diamond/tetrahedral amorphous carbon multilayer composite film. The influences of nanoscale individual layer thickness, elemental concentration gradient and self-lubricating surface layer on the microstructure, constituent, mechanical and tribological properties of the films were investigated. The following results were obtained:(1) The nanoscale modulation multilayered Cr-C/a-C:Cr composite film consisted of a gradient base layer and 10 individual bi-layers of Cr-C and a-C:Cr. The thickness of individual a-C:Cr layer was constant at about 18.5 nm, while that of individual Cr-C layer varied from 11.1 to 55.1 nm. The microstructures, mechanical and tribological properties of the films were highly correlated with the individual Cr-C layer thickness. The scratch test indicated that the adhesion strengths between the film and the substrate were dramatically improved through the nanoscale multilayered structure attached on a graded base layer, acquiring critical loads exceeding 80 N. The highest hardness 13.6 GPa and Young's modulus 164.8 GPa, and lowest friction coefficient 0.21 and wear rate 1.35×10^-6 mm³/Nm were achieved from the coated sample with the individual Cr-C layer thickness of about 40 nm.(2) The nanoscale tungsten-gradually doped multilayer diamond-like carbon film was composed of a functionally graded interlayer, a W-gradually doped DLC sublayer and a W-DLC top layer. The characterized results indicated that the microstructure, mechanical and tribological properties of these films showed a significant dependence on the W concentration gradient. Applying the'compositionally-graded coating'concept, strongly adherent carbon films with critical load exceeding 100 N in scratch test were obtained. The optimum wear performance with friction coefficient of 0.19 and wear rate of 8.36×10^-7 mm³/Nm was achieved for the nanoscale tungsten-gradually doped multilayer DLC film with a graded W concentration ranging from 52.5 to 17.8 at%.(3) The self-lubricating diamond/tetrahedral amorphous carbon multilayer composite film consisted of a thick well-grained diamond base layer with a thickness up to 150μm and a thin ta-C covering layer with a thickness of about 0.3μm, and sp³-C fraction up to 73.93%. Deposition of a smooth ta-C film on coarse polycrystalline diamond films was proved to be an effective way to lower the surface roughness of the polycrystalline diamond film. The wear-resistance of the diamond film was also enhanced by the self-lubricating effect of the ta-C covering film due to graphitic phase transformation. Under dry friction against Si₃N₄ ball, an extremely low friction coefficient of 0.05 was achieved for the PCD/ta-C composite film. The main wear mechanism of the composite films during dry friction was micro-fracturing and polishing.