Design and Selection of Chemically Deposited Ni-P-W Coatings for Optimum Tribological Behavior

Introduction

The performance and life of engineering components can be enhanced by applying hard coatings over the surface of the components. The thickness of coatings range between 10 and 500 μm and their rates of deposition can provide the required product quality at relatively low capital and operating costs. The coating processes are broadly classified into two groups: electroplating and electroless or chemical coating. Since the study of electroless/chemical coating process by Brenner and Riddell (1946), a series of research studies have been performed and the process is accepted by various industries like electrical, aerospace, automotive, chemical, electronics, etc (Mallory & Hadju, 1991; Riedel, 1991). In chemically deposited coating process, the substrate is submerged in to a chemical solution called bath which consists of solution of metal ions, reducing agent, complexing agents, stabilizers, buffer solution etc. Few metal coatings applied by electroplating can match the thickness uniformity of an electroless coating finish. Because these protective coatings are chemically applied, they create deposits of highly consistent depth across all surfaces, including edges and complex interior geometries. Electroless coating has several advantages over conventional electroplating technique, except the life of the bath. The advantages are that the quality of the deposit, namely the physical, tribological and mechanical properties improve. Moreover the non-conductive substrate can be coated by this electroless method. In this process, a sharp edge receives the same thickness of deposit as a blind hole does and it offers extremely bright deposits, which are comparable with electroplated bright nickel. The desirable properties can be varied by choosing different pH, temperature and composition of the bath. Electroless nickel coatings has assumed the greatest commercial importance among the electroless coating.