HIGH TEMPERATURE OXIDATION BEHAVIOUR OF HVOF SPRAYED Ni-30 MULLITE COATING ON MILD STEEL

Abstract

The aim is to deposit Ni-Mullite (70:30 wt%) coating on mild steel (MS) by high velocity oxy-fuel (HVOF) thermal spray process. The spraying conditions have been optimized to obtain an adherent coating of ~110 µm thickness.The coated specimens of dimensions (20mmx20mm) had been subjected to atmospheric oxidation at different temperatures of 500 o , 700 o , 900 o and 1000 o C. Oxidation was performed for durations of 5, 18, 30 and 45 hours. The oxidized coatings have been characterized by X-Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDAX). The oxidation rate has been studied in terms of mass gain with respect to time and temperature. It has been seen that till 700ºC, there is negligible change in the mass gain and the oxide layer thickness. Beyond 700 o C the mass gain and thickness of the oxide layer increases considerably up to a temperature of 1000ºC. The growth of the oxidized zone is seen to obey the parabolic law at high temperatures. Field Emission Scanning Electron Microscopy (FESEM) had been used to analyze the surface morphology. The as-coated specimen revealed the formation of porous nickel oxide with mullite particles distributed as fine nodular morphology. However, in oxidized specimens, formation of uniform grains containing aggregates of oxides of iron and that of wurtzite and haematite had been observed. This was also confirmed from EDAX analysis. The formation of iron oxide can be associated with the fact that iron diffuses outward to the surface resulting in oxide formation. This was reflected in the X-ray diffractogram as the highest intensity peaks in the oxidized specimen. This depicts that top surface has facilitated the oxidation of the base metal by the outward diffusion of iron atoms. Thus, to make this coating suitable for use at high temperatures, an interlayer that prevents inter-diffusion of elements between the substrate and coating is required.