Studies on High Temperature Wear and Erosion Behaviour of Partially Oxidized Plasma Spray Coatings

Abstract

The effect of partially oxidized coatings in improving wear and erosion resistance was investigated in this study. The challenge in the current situation is to process a new system of powders containing metallic and oxide phases. Partially oxidized powders containing metallic and oxide phases were processed by flame spraying the alloy powders into distilled water and allowing the oxide layer to form while keeping the tougher alloy in the middle of the particle. Partially oxidized powders were prepared with a flame spray process by spraying NiCr, NiCrBSiFe alloy powders and pure Al powder into distilled water. Oxide layer and heat-affected layer at the cross-section of partially oxidized powders were analyzed using metallurgical phase analyzer software. Partially oxidized NiCr, NiCrBSiFe feedstock powders are processed from alloy powders, whereas NiCr+Al feedstock powder is the mixture of 30 weight percent of partially oxidized Al and 70 weight percent of NiCr alloy powder. These feedstock powders were deposited on MDN321 steel substrate using air plasma spray process. The effect of partial oxidization on microstructure, microhardness, density, bond strength, and porosity of the coatings were analyzed using SEM/EDS, Vickers hardness tester, water immersion method, pullout method and image analyzer software respectively. The friction and wear behaviour of partially oxidized coatings were assessed using a pin-on-disc tribometer by varying loads (10, 20 and 30 N), sliding velocities (1, 2 m/s) and temperatures (Room Temperature, 200, 400 and 600 °C) for 3000 m sliding distance. Worn surfaces of NiCrBSiFe and NiCr coatings consist of oxide phases of SiO2, NiO, Cr2O3 and NiCr2O4 at elevated temperatures. These phases contributed to reducing the wear rate by five folds in coated steels compared to uncoated steels at 600 °C. The wear rate in coating decreases with an increase in temperature. The coefficient of friction was reduced gradually with the temperature in coatings and substrate. The wear rate coefficient of NiCr coating was 1.7 times higher than the NiCrBSiFe coating. ii The test results of NiCr+Al coating indicated that at room temperature, frictional heat generated due to applied load produce three-body abrasion at the interface caused to increase the wear and friction in the coating. The oxide film formed at high temperature avoids surface degradation at the interface and reduce the wear and friction. The worn surfaces at 600 °C consist phases of α-Al2O3, NiO, and Cr3O. These phases are contributing to improving the wear resistance of NiCr+Al coating more than 4-times compared to uncoated steels under varying load and sliding velocities. The coefficient of friction reduced with increase in temperature due to generated oxides act as lubricants at the interface. The solid particle erosion behaviour of partially oxidized coatings was analyzed using air jet erosion tester by varying temperatures (RT, 200, 400, 600, and 800 °C) and impact angles (30, 45, 60, 75, and 90°). An air jet erosion test was carried out using Al2O3 erodent of grit size 50 μm. The effect of temperature and impact angle on volumetric erosion loss was studied using SEM, EDS, and XRD analysis. Partially oxidized NiCrBSiFe coating exhibited better erosion resistance compare with partially oxidized NiCr, NiCr+Al coatings and MDN321 steel substrate. NiCr coating demonstrates maximum volumetric erosion loss at 45° impact angle, whereas NiCrBSiFe at 60° impact angle under all tested temperatures. Non-contact three- dimensional optical profilometer was used to quantify the volumetric erosion loss.