Effect of thermal spray on the microstructure and adhesive strength of high-velocity oxy-fuel-sprayed Ni-Cr coatings on
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. INTRODUCTION
IN the recent past, thermal-spray technology has been extensively used in the area of thermal-barrier coatings for aerospace and gas turbine components.[1,2,3] Another important area for applying the thermal-spray technology is in ultrasupercritical (USC) boilers components. New coal-fired power plants with improved thermal efficiency make a significant contribution to the reduction in the emission gas. Increasing the steam operating temperature and pressure will proportionally increase the plant efficiency, with a reduction in the emission gas.[4] Material used in the power plant should withstand creep and steam oxidation, while increasing the steam operating temperatures. The ultrasteel project at our institute aims to develop a material that can withstand the steam operating temperature of 650 °C.[5] The featured material should possess both creep and steam oxidation resistance along with ease of fabrication. The existing modified 9Cr-1Mo steel (ASME T91) suffers severe oxidation and creep at this temperature. This necessitated improved mechanical and oxidation properties of the material that can withstand the increased operating temperatures. To overcome the creep problem, new alloy developments are under progress, and there are some promising results already reported.[4,5,6] The addition of W, Si, B, N, and V in the 9Cr-type steels showed a marked improvement in their creep resistance. A recent study from our group on nanosized carbonitride-strengthened ferritic steel showed a remarkable improvement in the creep properties.[6] However, these newly developed alloys showed only a marginal improvement in their
steam oxidation resistance.[4,5,7] The addition of 3 wt pct Palladium in to the CrW NbV steel developed at our institute showed an excellent steam oxidation resistance. But, the cost of this alloy is too high due to the Palladium addition.[4] The thermal-spray process can be considered to overcome the steam oxidation of USC boiler components, since it can offer a thick film and flexibility to deal with on-site modifications. In our previous study,[8–11] attempts have been made with 80Ni-20Cr and 50Ni-50Cr coatings on a 9Cr-1Mo steel substrate using the high-velocity oxy-fuel (HVOF) process. The HVOF spray of a 50Ni-50Cr coating yielded good protection until 750 °C. In the case of 80Ni-20Cr coatings, although the coating did not show any scale initiation at the coating/substrate interface, the diffusion of iron from the substrate to the coating was noticed. On the other hand, iron and nickel diffusion in the 50Ni-50Cr coatings was less significant for the tested duration of 1000 hours. In addition to oxidation resistance, it is important to evaluate the interfacial adhesive strength of the coating, since the coating should withstand the high temperature for several years in the targeted application. In the present investigation, the adhesive strength of these two Ni-Cr coatings has been evaluated in the as-coated and aged conditions. The changes in the adhesive strength with aging durations we
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