Superhard nanocomposite coatings of TiN/a-C prepared by reactive DC magnetron sputtering

Single layer TiN coatings were prepared on silicon (111) substrates using a multi-target reactive DC magnetron sputtering process at various nitrogen flow rates and substrate biases. TiN coatings prepared at a nitrogen flow rate of 0.6 sccm, a power density of 5.4 W/cm2 and a substrate bias of −275 V, showed a nanoindentation hardness of 3300 kg/mm2, whereas amorphous carbon (a-C) coatings prepared under similar deposition conditions exhibited a hardness of 800 kg/mm2. Subsequently, nanocomposite coatings of TiN/a-C were prepared on silicon (111) and M3 tool steel substrates by rotating the substrate back and forth between the titanium and the graphite targets. The nanocomposite coatings were prepared at various carbon concentrations. Structural characterization of the coatings was done by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The compositions of the coatings were determined using energy dispersive X-ray analysis (EDAX). The nanocomposite coatings exhibited a broad (111) reflection of cubic TiN phase in XRD data. Nanoindentation data showed that about 1.5-μm-thick TiN/a-C nanocomposite coatings exhibited a maximum hardness of 4800 kg/mm2 at a carbon concentration of approximately 17 at.%. The TEM micrographs showed that TiN nanocrystals were embedded in a-C matrix and the average crystallite size was 78 Å. The selected area electron diffraction of the nanocomposite coatings showed the presence of both nanocrystalline (TiN) and amorphous (a-C) phases. This was confirmed by high-resolution TEM studies.