Effect of heat treatment on the hardness and wear resistance of electrodeposited Co-B alloy coatings

Cobalt-boride (Co-B) alloy coatings with different boron contents (7.31–15.33 at.% B) were electrodeposited onto an AISI 1018 steel electrode using dimethylamine borane (DMAB) as the boron source and then heat treated at various temperatures ranging from 200 to 500 °C for 60 min under air atmosphere. The composition and morphology of the coatings were analyzed using glow discharge spectroscopy (GDS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The tribological characteristics, such as the hardness, friction coefficient, and wear resistance, were also studied. The results showed that the boron content in the coatings increased as the concentration of DMAB in the electrolytic solution increased. Amorphous Co-B coatings with hardness values ranging from 700 to 820 HV, depending on the boron concentration in the coating, were obtained. When the coatings were heat treated over the temperature range of 200–400 °C, the hardness increased considerably and the wear volume decreased. The XRD patterns of the coatings revealed that the thermal treatment caused a structural change in the Co-B alloys, from an amorphous structure to a crystalline Co metal and a Co3B alloy. The maximum hardness (1280 HV) and the minimum coefficient of friction (0.08) were obtained when the Co-B coatings (15.16 at.% B) were thermally treated at 400 °C.