Optimization of Ni-B coating bath and effect of DMAB concentration on hardness and wear

Abstract

In this study, reducing agent amount, stabilizer amount, coating bath temperature and stirring speed parameters were examined for optimization of electroless Ni-B coatings, and the effect of reducing agent amount on hardness and wear of the coating was investigated. The pH of the electroless Ni-B coatings made for 1 h was set to 6, nickel chloride (NiCl2.6H2O) as the nickel source, sodium acetate (C2H3NaO2) as the complexing agent, thiourea as the stabilizer (CH4N2S) and DMAB ((CH3)2NBH2) as the reducing agent was used in the coatings. The bath parameters were varied as; the amount of reducing agent (2 g/L, 3 g/L and 4 g/L), coating bath temperature (65°C, 70°C and 75°C), stabilizer amount (0.5 mg/L, 1 mg/L, and 2 mg/L) and stirring speed (250, 300 and 350 rpm). After determining the optimum conditions, the effects of the amount of reducing agent on the coating were examined. Scanning Electron Microscopy (SEM), X-ray diffractometer (XRD), Vickers nano-hardness test and tribometer were used for characterization of coatings. In the optimization studies, it was observed that the coating morphology was in the form of cauliflower and columnar structures were formed in the cross-section images. According to optimization results on the morphologies, the samples which compact and free-crack have been investigated phase structure, hardness and tribology properties. In XRD analysis, coatings produced with different amounts of reducing (DMAB) are amorphous as-deposited, after annealed at 400°C for 2 h, nickel borides (Ni2B and Ni3B) phases are formed and increased the preferential orientation (111) of the nickel crystal plane. The effect of DMAB concentration on coating was examined in as-deposited condition and post annealing. The formation of nickel borides after annealing increases the hardness. The effect of DMAB concentration on the coating after annealed was examined and it was observed that the wear resistance of the dense and homogeneously dispersed sample in the coating matrix was more perfect than other samples. Consequently, hardness (850 HV), coefficient of friction (0.37) and wear rate (2.92 × 10−7 mm3/Nm) of B2 sample which free-crack and compact was obtained. These results indicate a high strengthen B2 sample produced with 3 g/L DMAB concentration and 70°C bath temperature could utilize in a harsh medium.