Optimization of boron concentration in the Ni-B electroless coatings

important place in the defence industry on St-37 mild stell chosen as a substrate material with the autocatalytic chemical reductin method by using NaBH4 as a reduging agent with different NaBH4 concentrations, cross-sections and surface images of the coating samples, their hardness, microstructures, the effects of heat treatment applied at 400 oC and 2 hours and NaBH4 concentration on these properties and in addition, the effect of NaBH4 concentration on the wear behaviour was investigated in the wear test applied to heat treated samples. It was observed that in the coated state, electroless nickel-boron coatings consist of a mixture of amorphous and crystalline regions, uniform, without porosity and typical cauliflower-like appearance, whereas, the cross-sectional images, have a without porosity, uniform coating thickness and columnar and dendritic structure grow together. The concentration of amorphous regions in the microstructure increased with increasing NaBH4 concentration, and in the surface images and cross-sections, the size of grains in the microstructure decreased and a more frequent and homogenous grain distribution was observed. The thicknesses of the coating increased up to 1,4 g/L NaBH4 concentration and showed a tendency to decrease after this concentration. The heat treatment did not show a significant change in the thicknesses of the coatings. Crystallization in coating structure with heat treatment and the incrase in boron content which is included in the coating structure with increasing NaBH4 concentration increased the hardness values. When the wear tests of the electroless nickel-boron coatings were examined, the specific wear rates were decreased due to the decrease in the wear trace width after the wear depending on NaBH4 concentration. When the SEM micrographs in each sample were examined, an adhesion due to the solubility between the nickel atoms in the coating and the iron atoms in the counter materials and furthermore, the formation of thin grooves along the sliding direction observed, whereas in the other samples, no clear adhesion was observed, but thin grooves formed along the sliding direction appeared more clearly.