Surface erosion behaviour over NiCrBSi-Al2O3composite coatings

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 degrees, 60 degrees and 90 degrees impingement angles, jet velocity of 20 m s(-1)and erodent size of 200 mu m. 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 HV(0.3)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%Al(2)O(3)exhibited the lowest erosion and all the coatings offered better wear resistance than the uncoated steel.