Structure and wear of electroless nickel coatings

Abstract

The structure and properties of electroless nickel are highly sensitive to heat treatment owing to its pronounced metastability. The heat treatment process induces crystallization of amorphous nickel to nickel phosphides and fcc nickel by a mechanism similar to that of the solidification of liquid alloys. The process produces a twofold increase in hardness, the elimination of severe adhesive wear, and a deterioration in the abrasive scratch resistance. The susceptibility of the as deposited coating to severe adhesive wear during unlubricated sliding against plain carbon steel is due to the low interfacial and high surface free energies of the two surfaces, resulting in a high compatibility. Heat treatment removes severe wear by changing the structure from amorphous nickel to Ni3P, which presents a surface incompatible to steel. The abrasive scratch resistance of the as-deposited coating is a result of its relatively high fracture toughness, which is attributable to the absence of heterogeneities in the amorphous structure. The relevance of the data to engineering applications is discussed.MST/539