Diffusion and Phase Change During Heat Treatment of Ni-B Coatings on Steel

• Main Author: Shepardson, Kevin W
• Other Authors: Richard D. Sisson, Jr., Advisor
• Format: Others
• Published: Digital WPI 2008
• Subjects: heat treatment; coatings; diffusion; oxidation; ni-b
• Online Access: https://digitalcommons.wpi.edu/etd-theses/416; https://digitalcommons.wpi.edu/cgi/viewcontent.cgi?article=1415&context=etd-theses

Nickel-boron coatings are used to improve friction and wear properties (and, in some cases, corrosion resistance). The nickel-boron coating investigated here is 5-6 wt% boron, and is deposited by electroless plating on a 1018 steel substrate. It is amorphous as-plated, and heat treatment is used to crystallize the coating to improve its hardness. To better understand and predict the effects of heat treatment, samples that had been isothermally annealed at various temperatures from 500ºC to 800ºC for either 2h or 5h were examined by several methods to determine the diffusion effects taking place during annealing. Samples were examined by XRD, both at the coating surface and at multiple depths within the coating. Optical microscopy and SEM were used to view the structure of the coating in cross-section. Cross sections were etched and examined by optical microscopy and SEM, as well as EDS, which was used to develop a Ni-Fe composition profile at the coating-substrate interface. Microhardness measurements were taken and used to develop microhardness profiles. Additional samples were annealed to investigate boron oxidation at the coating surface. Based on the data, there is a reduced amount of Ni3B near the outer surface of the heat-treated coatings, with the thickness of the resultant γ-Ni layer increasing with annealing time and temperature, from 2.4 to just over 13 µm. This low-boron region indicates that boron is diffusing out through the surface of the coating and oxidizing, which the literature indicates should result in the formation of B2O3. Because B2O3 is water-soluble, it is likely that it dissolved during the water quench that concluded most anneals. Diffraction and EDS data also indicate interdiffusion of the nickel in the coating and the iron in the steel substrate. This leads to the formation of a soft interdiffusion layer between the Ni3B coating bulk and steel substrate that appears to be a mix of ferrite and Ni3Fe.