Microstructure and Properties of In Situ Synthesized MoB/NiCr Coatings with Different (Mo+B)/(Ni+Cr) Mass Ratios

Abstract:

Three kinds of Mo-B-Ni-Cr ball-milled mixture powders with different (Mo+B)/(Ni+Cr) mass ratios (1:1, 2:1, and 3:1) were deposited by the high velocity oxygen-fuel (HVOF) spraying process to in situ synthesize MoB/NiCr coatings. The microstructure and phase composition of MoB/NiCr coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The effects of different (Mo+B)/(Ni+Cr) mass ratios on the microstructure, microhardness, bonding strength, and corrosion resistance of MoB/NiCr coatings were discussed. The results show that MoB/NiCr coatings with (Mo+B)/(Ni+Cr) mass ratio of 1:1 have the lowest porosity and the largest thickness. Mo₂NiB₂ ternary boride was in situ synthesized in all three kinds of MoB/NiCr coatings. The content of Mo₂NiB₂ ternary boride is increased with increasing the (Mo+B)/(Ni+Cr) mass ratio. The microhardness of MoB/NiCr coatings is increased with increasing the (Mo+B)/(Ni+Cr) mass ratio, while the bonding strength is decreased. After immersion test in molten zinc for 360 h, no zinc or its intermetallic compound can be observed in the surface region of MoB/NiCr coatings according to energy disperse spectrometer (EDS) and XRD analyses. The porosity of the coatings is increased with increasing the (Mo+B)/(Ni+Cr) mass ratio, while the thickness is decreased. Compared with other coatings, the MoB/NiCr coating with (Mo+B)/(Ni+Cr) mass ratio of 1:1 has better corrosion resistance in molten zinc.

Key words:Mo₂NiB₂ ternary boride;high velocity oxygen-fuel;microstructure;microhardness;corrosion resistance