Optimization of electroless bath process parameter for improving the tribology behavior of Ni-P/CaBr₂ composite coating against the hardened EN-31 steel

Ceramic, polymers and solid lubricants were used as reinforcements in the electroless Ni–P deposit, which is in-soluble in nature. Impingement of in-soluble particles has improved the hardness and wear resistance of the Ni–P deposit. However, the question still remains on the bonding of matrix and reinforcement particle during the wear at high load conditions. Hence, to strengthen the Ni–P deposit, a soluble reinforcement particle (CaBr₂) has been used in this study. A detail study on the role of bath composition (Nickel sulphate and CaBr₂) on the mechanical and tribology behavior of alkaline electroless Ni–P/CaBr₂ composite coatings is reported. Coatings were carried out with various weight combination of CaBr₂ particle (1 g l⁻¹, 2 g l⁻¹, and 3 g l⁻¹) and source of nickel (16 g l⁻¹, 18 g l⁻¹, and 20 g l⁻¹) in the bath. The optimum bath composition to achieve higher hardness, low surface roughness (Ra), low coefficient of friction (COF), and wear rate were analyzed using response surface methodology (RSM). ANOVA was used to identify the influencing factor on the output responses and an empirical model has been formulated. Ni–P/CaBr₂ coating exhibited a high surface roughness of 1.505 μm for the weight combination of 1 g l⁻¹ of CaBr₂ and 20 g l⁻¹ of nickel sulphate. High surface hardness of 793 HV_0.05 and low wear rate of 3.33 × 10⁻⁶ gm.m⁻¹ was achieved for the high weight combination of CaBr₂ particle and nickel sulphate. The obtained experimental results were closer to the predicted results, confirming the significance of the model for optimizing the bath composition for Ni–P/CaBr₂ composite coatings. Various characterization techniques like Scanning Electron Microscope-Electron Dispersive Spectrum (SEM-EDS), x-ray Diffractometer (XRD) and 3D non-contact surface profilometer were used to justify the wear results obtained through the experimentation. (paper)