Friction and wear behavior of electroless Ni–P and Ni–W–P alloy coatings

Abstract

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. An alternative to Ni–P coatings is the ternary alloy coating with the addition of a third element to the binary Ni–P system. The ternary Ni–M–P alloy, where M is typically a transition metal such as W, Co, Mn, Re and Mo has superior properties compared to the binary Ni–P alloy, and therefore, could extend its range of applications. The present paper discusses the synthesis and tribological behavior of Ni–P and Ni–W–P deposits. Deposition is done from an acidic electroless bath. Deposits with varying tungsten content were synthesized. Chemical analysis shows that tungsten incorporation reduces the phosphorus content in the deposit. Phosphorus content varied from 5 to 12 wt.% depending upon the tungsten incorporation in the deposit which in turn varied between 2 and 14 wt.%. Coatings with high tungsten content exhibit very good wear resistance compared to binary Ni–P as well as low tungsten ternary alloy deposits.