Surface erosion behaviour over NiCrBSi-Al2O3 composite coatings

Abstract

For hydro turbine components High Velocity Oxy Fuel (HVOF) thermal spraying process was adopted for three different types of coatings i.e NiCrBSi-5%Al2O3 (N5A), NiCrBSi-10%Al2O3 (N10A) and NiCrBSi-15%Al2O3 (N15A) on Grade 420 Stainless steel, the coatings being characterized by Scanning Electron Microscope (SEM), x-ray Diffraction (XRD) and optical microscope. With measurements of surface roughness, thickness, micro hardness and porosity of the coatings, the erosion tests were conducted in a slurry-jet erosion tester at 30°, 60° and 90 ° impingement angles, jet velocity of 20 m s−1 and erodent size of 200 μm. The optical microscopy images of the coated samples exhibited lamella structure of the coatings. The N15A coating exhibits highest micro hardness value of 715 ± 31 HV0.3 followed by N10A (557 ± 22 HV0.3) and N5A (407 ± 15 HV0.3). The grade 420ss shows the lowest micro hardness value (272 ± 5 HV0.3). The surface morphology of the eroded samples was studied using SEM. The mechanisms involved in erosion of all the coated surfaces are removal of splats, lip formation, ploughing action and micro cutting whereas of bare steel, ploughing action, micro cutting and lip formation along with appearance of pores and craters. It was found that amongst the three coatings NiCrBSi-15%Al2O3 exhibited the lowest erosion and all the coatings offered better wear resistance than the uncoated steel.