Tribological behavior of electroless Ni-P coatings in alkaline environment and optimization of coating parameters using Taguchi based grey relational analysis

Electroless nickel coating is an autocatalytic coating whose behavior is very much dependent on the composition of electroless bath. The present paper studies the synthesis of electroless Ni-P (nickel-phosphorous) coatings on mild steel substrate and optimization of the coating process parameters for minimum friction and wear in alkaline (NaOH) solution using Taguchi based grey relation analysis. The study is carried out using different combinations of four coating process parameters, namely, concentration of nickel source (A), concentration of reducing agent (B), deposition temperature (C) and annealing temperature (D) with three level each (1, 2 and 3). The wear and friction tests are conducted with a pin-on-disk tribometer. Taguchi based grey relational analysis is employed for the optimization of this multiple response problem using an L-27 orthogonal array. The optimal combination of parameters is found as A3B3C1D2. ANOVA is performed to find out the significance of the coating parameters and their interactions. Annealing temperature and deposition temperature are most influence in controlling friction and wear behavior. The surface morphology and composition of coatings are studied with the help of scanning electron microscopy (SEM), energy dispersed X-ray (EDX) analysis and X-ray diffraction (XRD) analysis. It is found that Ni-P coating is amorphous in as-deposited condition but gradually turns crystalline with heat treatment. The worn surface morphology reveals abrasive wear mechanism.