Deposition and process parameter optimization of electroless Ni-B coating from a stabilizer free bath to achieve enhanced microhardness, scratch-hardness and adhesion using taguchi’s methodology

Abstract

Electroless nickel boron coatings have wide industrial usage. However, they are generally obtained from a lead-stabilized bath. The present work investigates and optimizes the scratch-hardness and microhardness obtained from stabilizer-free electroless nickel boron bath in a quest to eliminate lead nitrate/heavy metals, which are potentially toxic. The bath temperature, heat treatment temperature, and duration were varied at three levels. Enhanced scratch-hardness (12.581 GPa) was obtained at 85 °C bath temperature and heat treatment at 350 °C for 1 h. At the same time, the highest microhardness (886.17 HV100) was obtained at a parametric combination of 95 °C bath temperature and heat treatment at 450 °C for 1 h. Multi-objective optimization was carried out using grey relational analysis. The parametric combination predicted in multi-objective optimization was 85 °C bath temperature and heat treatment at 350 °C for 1 h where the microhardness was 846.34 HV100. Furthermore, an analysis of variance was also carried out to investigate the importance of the factors in controlling scratch-hardness and microhardness. The highest contribution was observed from heat treatment duration. Further investigation of the optimized coating was done by the progressive scratch test, which recorded that the first critical load of failure improved compared to non-heat treated electroless Ni-B coatings. The coatings were also characterized using field emission scanning electron microscope, energy dispersive x-ray spectroscopy, and x-ray diffraction. The coatings in optimized condition showed no transverse or chevron cracks within 5–24 N.