Superhard nanocomposite coatings of TiN/Si3N4 prepared by reactive direct current unbalanced magnetron sputtering

Approximately 1.5-μm-thick superhard nanocomposite coatings of TiN/Si3N4 with varying silicon contents were deposited on silicon and stainless steel (SS 304) substrates by reactive direct current (DC) unbalanced magnetron sputtering. An asymmetric bipolar-pulsed DC power supply and a DC power supply were used to sputter Si and Ti targets, respectively in Ar + N2 plasma. Structural characterization of the coatings was done using X-ray diffraction (XRD). The bonding structure of the nanocomposite coatings was characterized by X-ray photoelectron spectroscopy (XPS). The elemental composition of the TiN/Si3N4 nanocomposite coatings was determined using energy dispersive X-ray analysis. The mechanical properties of the coatings were measured using a nanoindentation hardness tester. The surface morphology of the coatings was studied using atomic force microscopy. The nanocomposite coatings exhibited a broad (200) reflection of cubic TiN phase in the XRD data. There was a significant broadening of the (200) reflection with an increase in the silicon content in the nanocomposite coatings, which suggested a decrease in the average crystallite size. Nanoindentation data showed that about 1.5-μm-thick TiN/Si3N4 nanocomposite coatings exhibited a maximum hardness of 5200 kg/mm2 and an elastic modulus of 340 GPa at a silicon concentration of approximately 12 at.%. The hardness and the elastic modulus of the nanocomposite coatings decreased with further increase in the silicon content. Structural changes as a result of heating (400–850 °C) of the nanocomposite coatings in air were characterized using micro-Raman spectroscopy. The Raman data showed that the nanocomposite coatings started to oxidize at 800 °C as compared to TiN, which have been oxidized even at 500 °C. To isolate the oxidation-induced spectral changes as a result of heating of the coatings in air, the samples were also annealed in vacuum at 850 °C under similar conditions. The Raman data of the vacuum-annealed coatings showed no phase transformation even after annealing up to 850 °C. The corrosion behaviors of single-phase TiN and TiN/Si3N4 nanocomposite coatings deposited on stainless steel substrates were investigated using potentiodynamic polarization in 3.5% NaCl solution. The results indicated that the nanocomposite coatings exhibited superior corrosion resistance as compared to the uncoated substrate. The wear data showed that the nanocomposite coatings also exhibited better wear resistance as compared to the uncoated substrate.