EFFECTS OF PH VALUES IN ELECTROLESS NI PLATING ON MILD-STEEL WITH TIN COATING

TiN films are prepared by reactive r.f. magnetron sputtering on mild steel substrates. An electroless Ni-P plating is introduced as an interlayer in the surface modification of a TiN coating. A hypophosphite-based bath is employed for electroless Ni-P deposits. The electroless Ni-P deposits crystallize during r.f. sputtering, owing to the elevated sputtering temperature, and thus a TiN/Ni3P/Fe coating assembly is formed. The pH value of the bath affects the phosphorus content of the electroless Ni-P deposits, and the phosphorus contents vary from 6.7 wt.% to 11 wt.% as the pH value is changed from 5.5 to 4.0. The as-plated structure is amorphous for the range of 7.6 wt.% to 11 wt.% phosphorus, and is microcrystalline for the case of 6.7 wt.% phosphorus. The employment of an electroless Ni-P deposit results in an increase in surface microhardness and adhesion strength. In addition, an appropriate thickness of electroless Ni-P without a substrate effect is evaluated.