Structure and wear resistance of high hardness Ni-B coatings as alternative for Cr coatings

• High hardness Ni-B coatings were prepared as alternative for Cr coatings.
• The wear resistance of 300 °C-heat-treated Ni-B coatings is obviously higher than that of hard chromium coatings.
• Coatings with high H/E exhibit good resistance to both abrasion and fatigue.
• Ni-B coatings heat-treated at 300 °C and having the highest H/E exhibit best wear resistance.

Ni-B alloy coatings were electrodeposited using a Ni plating bath containing dimethylamine borane (DMAB) as boron sources. Hard chromium coatings were electrodeposited from conventional Cr (VI) bath for comparison. Ni-B alloy coatings were heat treated at different temperature from 200 °C to 400 °C using vacuum annealing furnace. Crystal structures and surface morphologies of coatings were evaluated by XRD, SEM and TEM, respectively. Their mechanical and tribological properties were tested by microhardness tester, scratch tester and friction wear testing machine, respectively. Wear tracks morphologies were observed through SEM to analyze the wear mechanism of coatings. Nano-hardness and elastic modulus were tested by Nanoindenter. Results show that the hardness of as-plated Ni-B alloy coatings is 700–800 Hv, and it increases to 980 ± 28 Hv after heat treatment at 300 °C for 20 min. The scratch tests show that the Ni-B coatings have a good adhesion to the substrate in both as-plated and heat-treated conditions. Tribological tests show that the 300 °C-heat-treated Ni-B coatings exhibit a lowest wear rate under different oil-lubricated sliding conditions, the wear resistance of which is higher than that of hard chromium coatings. The excellent wear resistance is attributed to its high ratio of hardness to elastic modulus (H/E).