Effect of nitrogen partial pressure on the electrochemical evaluation of (Ti-Al)N coatings deposited by reactive magnetron sputtering

Titanium aluminium nitride films were deposited on stainless steel substrat es by reactive magnetron sputtering under various nitrogen partial pressure s, using a composite target consisting of alternate arc segments of titaniu m and aluminium. Electrochemical evaluation of these coatings, carried out by the potentiodynamic measurement technique in deaerated 1N H2SO4 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 l ower increase in the corrosion rate. The corrosion potential (E-corr) incre ased from -339.8 to -268.0 mV with the increase in nitrogen partial pressur e from 0.18 to 0.63 mtorr With further increase in partial pressure of nitr ogen to 1.08 mtorr, E-corr decreased to -303.6 mV The corrosion current den sity (I-corr) was found to be least 4.6 muA cm(-2) at nitrogen partial pres sure 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 TW plus hexago nal AlN structure was present at 0.98 mtorr, while only hexagonal AlN struc ture 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 pressu re of 0.98 mtorr. At nitrogen partial pressure of 1.08 mtorr the hardness v alue decreased drastically to 1746 HK25.