Characterization of the Ti-doped diamond-like carbon coatings on a type 304 stainless steel

The microstructure and chemistry of a functionally gradient Ti-doped diamond-like carbon (DLC) coating prepared by a closed field unbalanced magnetron sputtering system (CFUBMS) on a type 304 stainless steel was analyzed by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and scanning electron microscopy (SEM). Three distinct regions including the steel substrate, Ti interlayer, and the functionally gradient coating are observed from the cross-section TEM micrograph. Further analysis of the graded coating using selected area diffraction reveals that the early deposited film is mainly composed of TiN located next to the Ti interlayer; subsequently it changes to a mixture of TiCo0.7N0.3 and TIG, and finally a very complicated mixture containing Ti, TiC, graphite, possibly diamond and amorphous carbon near the coating surface is observed. The graded Ti-doped DLC coating exhibits columnar structure and preferred orientation illustrated by the arced electron diffraction patterns. Chemical analysis carried out by EELS of the surface coating layer using the plan-view TEM specimen indicates that the bonding state of carbon in the Ti-doped DLC coating depends on the content of Ti. For a Ti-rich region, the carbon K edge favors the formation of a TI-C bond, whereas graphitic and/or amorphous carbon dominates in the Ti-deficient regions. In addition, the coated specimens were immersed in a 60% H3PO4 (85 + %) and 40% H2SO4 (95 + %) solution to investigate their corrosion resistance and then examined by SEM and energy-dispersive x-ray spectroscopy (EDS). It is found that the surface coating layer, in which microcracks were observed in the plan-view TEM micrograph, has lower packing density than the subsurface coating. By analysis of the secondary and backscattered electron images, and x-ray mapping, it is believed that the submicron-sized particles that survived the corrosion test in the Ti-doped DLC coating are diamond microcrystallites. (C) 1999 American Vacuum Society. [S0734-2101(99)01203-8].