Amorphous carbon (a-C) and several related materials (DLCs) may have ultralow friction coefficients that can be used for saving-energy applications. However, poor chemical bonding of a-C/DLC films on metallic alloys is expected, due to the stability of carbon-carbon bonds. Silicon-based intermediate layers are employed to enhance the adherence of a-C:H films on ferrous alloys, although the role of such buffer layers is not yet fully understood in chemical terms. The chemical bonding of a-C:H thin films on ferrous alloy intermediated by a nanometric SiCx:H buffer layer was analyzed by X-ray photoelectron spectroscopy (XPS). The chemical profile was inspected by glow discharge optical emission spectroscopy (GDOES), and the chemical structure was evaluated by Raman and Fourier transform infrared spectroscopy techniques. The nature of adhesion is discussed by analyzing the chemical bonding at the interfaces of the a-C:H/SiCx:H/ferrous alloy sandwich structure. The adhesion phenomenon is ascribed to specifically chemical bonding character at the buffer layer. Whereas carbon-carbon (C-C) and carbon-silicon (C-Si) bonds are formed at the outermost interface, the innermost interface is constituted mainly by silicon-iron (Si-Fe) bonds. The oxygen presence degrades the adhesion up to totally delaminate the a-C:H thin films. The SiCx:H deposition temperature determines the type of chemical bonding and the amount of oxygen contained in the buffer layer.
Low alloy steel was pack-borided at different processing temperatures (at 850, 950, and 1050 o C) and times (2, 4, and 6 h). The microstructural characterization of boronized steel showed the presence of three zones, namely boronized region containing finer grains and columnar geometry of (Fe, M) 2 B (where M = Cr, Mn, Mo, and Ni), transition zone, and non-boronized core. The concentrations of the alloying elements in (Fe, M) 2 B were increased from the surface to the core of the specimen. The pattern of slope variation of boron concentration-depth profile (obtained using GDOES) was linked with the boride morphology and process temperature. Pack-boriding of steel led to the development of systematic trend in slope variation of overall concentrationdepth profiles of the alloying elements. The composition and morphology of boride affected the trend of slope variation for the boride-forming alloying elements. However, for Al and Si, the trend of slope variation was connected to the boride morphology and the composition of the matrix. Chemistry of the matrix was strongly dependent on the migration kinetics of the alloying elements during the boride growth. The migration kinetics of Cr, Mn, Mo, and C were found almost equivalent to the rate of boride growth. However, Ni, Al, and Si were migrated at a slower rate. Si showed the lowest migration kinetics among the alloying elements. The concentrations of the alloying elements having higher migration kinetics remained constant in the matrix during the boride growth.
Diamond-like carbon (DLC) coatings show strident properties such as high wear resistance and ultra-low friction. However, a widespread use regarding energy efficiency issues is neglected due to the poor adhesion. Silicon adhesion interlayers (SiC x :H) were deposited at different temperatures from 50 to 500°C with hexamethyldisiloxane followed by DLC. The microstructure was analysed by atomic force microscopy, scanning electron microscopy and Raman spectroscopy. The chemical depth profiling and chemical mapping were performed by glow discharge optical emission spectroscopy and energy-dispersive spectroscopy, respectively. Hardness and critical loads were analysed by nanoindentation tests. At higher deposition temperatures the Si-containing interlayers show lower relative content of H, O and Si and higher relative content of C, allowing the formation of more C-C chemical bonds at the outermost DLC/ SiC x :H interface, which is correlated to better adhesion. Finally, an atomistic model is proposed in order to explain the DLC debonding and bonding mechanisms.
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