An investigation on oxidation behaviour of high velocity oxy-fuel sprayed Inconel718-Al2O3 composite coatings

The grey cast iron (GCI) components such as exhaust manifolds, turbocharger housings, and furnace parts are subjected to high temperature oxidation. GCI alone cannot retain its mechanical strength at elevated temperatures due to this high temperature oxidation. The composite coatings can effectively be used to counter the high temperature oxidation induced degradation of the GCI components. In the present work, Inconel718-Al2O3 (INAL) based composite coatings were deposited on GCI substrate by using high velocity oxy-fuel (HVOF) spray process. The effects of varying Al2O3 contents (10, 20, and 30 wt%) on the microstructure, porosity, microhardness, and oxidation resistance of Inconel718 (IN718) coating were studied. High temperature cyclic oxidation testing was conducted on these coatings and bare GCI substrate in a laboratory tube furnace at 900 degrees C in air. The kinetics of oxidation were determined by using weight change measurements of the bare GCI and coated specimens at regular intervals for a duration of 50 h. The important phases present in the feedstock powders, deposited coatings, and the oxidised specimens were identified by using X-ray diffraction (XRD). The surface morphology and chemical compositions of the coated and tested specimens were determined by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The microhardness of IN718 coating was found to be improved with an increase in alumina contents and a maximum average hardness of about 801 +/- 40 HV0.2 was observed in case of IN718-30 wt% Al2O3 composite coating. The increase in alumina contents resulted in an increase in porosity and proportion of un-melted feedstock powder particles in the IN718 coating. The coating with 30 wt% alumina contents showed the maximum resistance to oxidation with a minimum weight gain of 1.29 mg/cm(2) as compared to the other coatings. The improved oxidation of composite coatings can be attributed to the formation of protective phases such as Al2O3, NiCr2O4 and Cr2O3 on its exposed surface during the hot corrosion condition.