Studies on phase dependent mechanical properties of dc magnetron sputtered TaN thin films: evaluation of super hardness in orthorhombic Ta(4)N phase

The mechanical hardness of the different phases of tantalum nitride (TaN(x), x <= 1) thin films grown by the reactive planar dc magnetron sputtering technique at 300 degrees C have been evaluated. The different phases have been prepared by varying the nitrogen (N(2)) to argon (Ar) gas ratio R in the range 0.04-0.30. The phases present in each sample and their volume fractions are obtained from x-ray diffraction and Ta 4f(7/2) binding energy x-ray photoelectron spectra, respectively. The grown TaN thin films are found to contain mainly cubic-TaN(0.1), orthorhombic-Ta(4)N, orthorhombic-Ta(6)N(2.5), hexagonal-TaN(0.8) and cubic-TaN phases. The hardness corresponding to each phase has been derived from the experimental hardness values using 'rule of mixtures'. A super hardness of similar to 61.9 GPa is observed in orthorhombic Ta(4)N phase. With increase in R, a decrease in the surface oxidation of TaN has been observed. The reduced surface oxidation is attributed to the formation of stoichiometric TaN thin films. The surface morphology is found to change considerably with increase in R.