Magnetron Sputtered TiN Coatings Modified by Chromium, Nickel and Electroless Nickel (EN) Interlayers on Mild Steel

摘要

TiN films were deposited by reactive d.c. magnetron sputtering on mild steel substrate. Because of presence of inherent porosity and internal stresses, TiN coatings cannot be relied to give corrosion protection; also TiN coatings need a good load support. Electroplated chromium and nickel and electroless nickel (EN) were deposited by plating as interlayers below the TiN coatings to study comparatively the effect of these interlayers. Chromium was electrodeposited from conventional sulfate catalysed chromic acid bath and nickel using Watt's bath while EN by electroless deposition process using acidic hypophosphite bath. Three different thicknesses viz. 2, 4 and 10 mum of interlayers were employed to study their influence on TiN coatings. The corrosion behaviour of TiN coated mild steel samples with and without the interlayers was studied using potentiodynamic measurement technique in deaerated 1N H_2SO_4 electrolyte. It was observed that TiN coatings on mild steel without any interlayer almost mimic the behaviour of the substrate, while the presence of any of the three interlayers showed a considerable improvement in the corrosion resistance.Microhardness measurements have shown that the surface hardness values of TiN coatings on mild steel increased to 1907 HK_(25) from about 1000 HK_(25) when employed with chromium interlayer, and to 1484 HK_(25) with EN interlayer, while electroplated nickel as an interlayer increased the hardness to a lesser extent (1289 HK_(25)).X-ray diffraction analysis has shown that the phases present in the TiN coatings were mainly TiN (200) and TiN (111). Chromium coated samples were found to show the broadening of Fe (110) peak, which also corresponds to Cr (110) peak. Interlayer nickel showed the presence of mainly Ni (200) and Ni(111) phases while EN was amorphous in structure. The crystalline phases Ni_3P and Ni were evident when TiN top-coat was given by d.c. magnetron sputtering on EN plated mild steel samples.