Tribology of Electroless Ni-P Coating Under Lubricated Condition: An RSM and GA Approach

1. Introduction

Surface coatings play a very important role from tribological point of view in reduction of friction and wear between mating surfaces thereby increasing their lifetime. Several surface coating deposition techniques are available out of which electroless deposition technique has gained immense popularity and has evolved over the years to become a mature subject of research. Electroless means without the use of electricity. It is an autocatalytic reduction process wherein the metal to be deposited is reduced onto a catalytically active substrate due to the action of a reducing agent present in the coating bath. This eliminates the use of electricity and a wide variety of substrates can be coated with ease. Deposit uniformity is another key feature of this method i.e. a sharp edge and a blunt hole receives the same amount of deposition (Sahoo & Das, 2011).

Several metals can be deposited by this method such as nickel, copper, gold, silver, cobalt and palladium out of which electroless nickel coating is the most popular variant (Sudagar et al., 2013). Electroless nickel coatings are classified as pure nickel, nickel alloy and nickel composite coatings. Hypophosphite reduced binary Ni-P coating has excellent physical, mechanical, electrical, wear and corrosion resistance properties and low coefficient of friction (Sahoo, 2009; Sahoo & Pal, 2007). Electroless Ni-P coating can be further classified as low phosphorus (3 – 5% P), medium phosphorus (6 – 9% P) and high phosphorus (10 – 14% P) coatings (Sudagar et al., 2013). The phosphorus content determines—to a large extent—the morphology and crystal structure of the deposits which in turns determines the tribological properties of the coatings (Sahoo & Das, 2011). High phosphorus Ni-P coatings are amorphous in nature and exhibit high resistance to corrosion but the wear resistance and microhardness decreases. When the percentage of phosphorus in the as-deposited coatings increases, the microstructure of the deposits transforms from crystalline to a mixture of nanocrystalline and amorphous phases and finally to full amorphous phase (Yan et al., 2008). In the same study, it is reported that a record high value of microhardness and wear resistance of Ni-P coating can be obtained by controlling the phosphorus content. Sahoo (2008, 2009) has obtained the optimal condition of coating bath parameters for minimum friction and wear of the coatings.