Hardness and structural correlation for electroless Ni alloy deposits

Electroless nickel (EN) plating has received attention as a hard coating for industrial applications due to its high hardness, uniform thickness as well as excellent corrosion and wear resistance. The electroless Ni-P deposit is a supersaturated alloy in as-deposited state, and can be strengthened by precipitation of nickel phosphide crystallites with suitable heat treatments. However, the hardness of Ni-P films degrades with excessive annealing due to grain coarsening. This is the most severe barrier for electroless Ni-P deposition process from replacing chromium plating in industrial sectors. This problem is addressed in the paper by modifying the conventional electroless Ni-P bath to co-deposit tungsten to increase the hardness of the coating. Structural changes in the coating due to incorporation of tungsten are also highlighted. Deposition is done from an alkaline hypophosphite bath. Deposits with varying tungsten content are synthesized. Chemical analysis shows that tungsten incorporation reduces the phosphorus content in the deposit. Phosphorus content varied from 3 to 7 wt.% depending upon the tungsten incorporation in the deposit which in turn varied between 8 and 18 wt.%. Coatings with high tungsten content possess high hardness when compared to binary Ni-P as well as low tungsten ternary alloy deposits.