Study On Wear And Corrosion Resistance Of Ni-B Coatings As Alternative For Cr Coatings

Ni-B alloy coatings were electrodeposited using a Ni plating bath containing dimethylamine borane(DMAB) as boron sources. The in?uence of different heat-treatment temperature to the coatings on their structure, morphology and properties was investigated. Evaluation of their structural characteristics, hardness, corrosion resistance in 3.5% Na Cl solution and wear resistance in oil- condition. Hard chromium coatings were electrodeposited from conventional Cr(VI) bath for comparison. Ni-B alloy coatings were heat treated at different temperature from 200 ℃ to 400 ℃ 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 electrodeposited Ni-B coatings have nanocrystalline structure. The average thickness of coatings is 45μm, the surface is smoothy and bright. The hardness of as-plated Ni- B alloy coatings is 700 ~ 800 Hv, which increases and then decreases with heat-treated temperature increases. The testing by TEM and electron diffraction proved the nanocrystalline state of the deposits, Electrodeposited Ni-B coatings have nanocrystalline structure and are comprised of a solid solution of boron in f.c.c. Ni lattice.After heat treatment, with temperature increases some peaks corresponding to Ni3 B arises when heat treatment temperature beyond 350℃. After heat treatment, hard Ni3 B particles separate out from the Ni crystal lattice, the grain coarsen. because of Orowan strengthening effects and Hall-Petch negative effect, the hardness increases and then decreases with heat-treated temperature increases. Binding force increase increases a with heat-treated temperature increases.Wear resistance performance of Ni-B alloy coatings has a lot of improvement than that of pure Ni coatings, Ni- Fe alloy coatings under the lubricated sliding condition.The scratch tests show 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℃-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).Ni-B alloy coatings were electrodeposited by constant current method on copper substrates, pure Ni coatings were prepared using similar methods as control samples. These samples’ crystal structure and surface morphology were evaluated by XRD and SEM. The corrosion resistances of electrodeposited coatings both in as-plated and heat treated conditions were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) using modulab electrochemical work station. After salt spray test, surface morphology of corrosion specimens were observed by SEM. Results showed that with electrochemical method and salt spray test in 3.5%Na Cl solution, the coatings showed a good corrosion resistance property; heat treatment influences corrosion resistance of the coatings which improved after heat treatment, the corrosion resistance of 300℃-heat treated Ni-B is the best, 400℃-heat treated Ni-B decreases but better than Ni-B as-plated...