Effect of Nitrogen Partial Pressure on the Electrochemical Evaluation of (Ti-A1)N Coatings Deposited by Reactive Magnetron Sputtering

Abstract

SUMMARYTitanium aluminium nitride films were deposited on stainless steel substrates by reactive magnetron sputtering under various nitrogen partial pressures, using a composite target consisting of alternate arc segments of titanium and aluminium. Electrochemical evaluation of these coatings, carried out by the potentiodynamic measurement technique in deaerated 1N H2S04 solution at room temperature, has shown that initially there is a rapid increase in corrosion resistance of the coatings with increase in partial pressure of nitrogen; a further increase in nitrogen partial pressure leads to a much lower increase in the corrosion rate. The corrosion potential (Ecorr) increased from -339.8 to -268.0 mV with the increase in nitrogen partial pressure from 0.18 to 0.63 mtorr. With further increase in partial pressure of nitrogen to 1.08 mtorr, Ecorr decreased to -303.6 mV. The corrosion current density (Icorr) was found to be least 4.6 μA cm−2 at nitrogen partial pressure of 1.08 mtorr. Coatings were characterized by X-ray diffraction phase analysis, which showed the presence of microcrystalline cubic TiN structure for nitrogen partial pressures of up to 0.88 mtorr. A cubic TiN plus hexagonal AIN structure was present at 0.98 mtorr, while only hexagonal AIN structure was observed at 1.08 mtorr nitrogen partial pressure. Surface hardness measured by microhardness tester using a Knoop indenter showed an increase in surface hardness values with increase in partial pressure of nitrogen. The maximum hardness of 2790 HK25 was observed at a nitrogen partial pressure of 0.98 mtorr. At nitrogen partial pressure of 1.08 mtorr the hardness value decreased drastically to 1746 HK25