Abstract
The aim of this paper is to investigate the WC-10Co-4Cr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (Y2O3/ZrO2; YSZ) nanoparticles deposited on the CA6NM turbine steel by using the high-velocity oxy-fuel (HVOF) thermal spraying technique. In the HVOF technique, the hot jet of the semi-solid particles strikes against the workpiece and creates a layer of coating of varying thickness on the substrate material. The coatings fabricated with HVOF were analyzed by scanning electron microscope (SEM) / energy-dispersive x-ray spectroscopy (EDS). The phase identification of a crystalline material was made with the x-ray diffraction (XRD) technique. The mechanical properties in terms of porosity, surface roughness and microhardness of the nanocomposite coatings were also evaluated. The SEM/EDS analysis showed that dense and homogeneous coatings were developed by the reinforcement of YSZ nanoparticles. The peaks of XRD graphs of WC-10Co-4Cr coating reinforced with 5 and 10 % of YSZ nanoparticles revealed that the WC was present as a major phase and W2C, Co3W3C, Co, Co6W6C, Co6W and Y2O3/ZrO2 nanoparticles were observed as a minor phase. The porosity level decreased up to 42 and 56 % by the addition 5 and 10 % of YSZ nanoparticles as compared with conventional WC-10Co-4Cr coating. The surface roughness values for WC-10Co-4Cr conventional coating, 95 % (WC-10Co-4Cr) + 5 % YSZ and 90 % (WC-10Co-4Cr) + 10 % YSZ nanocomposite coated samples were found to be 5.03, 4.89 and 4.28 respectively. The nanocomposite coatings reinforced with 10 % YSZ nanoparticles exhibited the highest microhardness value (1278 HV). The WC-10Co-4Cr coatings reinforced with 10 % of YSZ nanoparticles resulted in low porosity, low surface roughness and high microhardness. During the coating process, the nanoparticles of YSZ flow into the pores and are dispersed in the gaps between the micrometric WC particles and provide a better shield to the substrate material. The WC-10Co-4Cr with 10 % of YSZ nanoparticles showed better results in terms of mechanical and microstructural properties during the investigation.
Highlights
Hydroelectric power plants have a significant contribution to power generation
The oxide formation was reduced as the nanoparticles of YSZ filled the porous gaps. It was analyzed by scanning electron microscope (SEM)/energy-dispersive x-ray spectroscopy (EDS) that the nanoparticles of YSZ are well dispersed in WC-10Co-4Cr coating powders, which are responsible for decreasing the porosity and surface roughness of the nanocomposite coatings
The X-ray diffraction (XRD) peaks corresponding to Y2O3/ZrO2 indicate that the YSZ nanoparticles are dispersed in the gaps between the micrometric WC particles or partially agglomerated in the bonding phase
Summary
Half of the hydroelectric power resources lie only in the Asian countries [1]. India lies at the seventh position in hydroelectric power production in the world. Due to the increase of sediment content in the water, the problem of erosion becomes more critical during the rainy season and sometimes may cause the major shutdown of these hydroelectric power stations, which causes huge financial loss to the hydropower industry [2]. Major components of the hydro turbines, which are badly eroded, are draft tube, facing plate, runner inlet and outlet, shaft seal, guide vanes, nozzle, spear and spiral casing. Traditional steels used in the manufacturing of hydro turbine components are not able to overcome the problems that occurred due to erosion in the hydroelectric power stations. Different types of surface coatings can be deposited to enhance the life of the materials used in the fabrication of hydro turbine components [3]
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