On the compositional and thermal stability of sputter deposited Inconel based multilayer solar absorber coating

Abstract

A multilayer concept has been used to coat Inconel-718 in the presence of Ar+O₂ atmosphere. The coating structure of metal oxide/metal/metal oxide was deposited on stainless steel SS304 substrate with the bottom most oxide layer as non-stoichiometric and the top oxide layer as stoichiometric in nature. This led to the solar absorptance of 0.887 and emissivity of 0.19. The absorption of this multilayer stack was increased by depositing an additional layer of SiO₂ which improves the absorptance in the range of 0.940–0.951 without affecting the emittance (0.17 – 0.19). Field Emission Scanning Electron Microscopy analysis was carried out for studying the morphological properties of the coating. The reflectance properties of the coating were analyzed using UV–Vis-NIR spectroscopy and the X-ray diffraction analysis was used for the structural properties. The findings of these studies highlight Inconel's suitability as a solar selective absorber coating. Contrary to high temperature stability of bulk Inconel, thermal stability investigations of the coating reveal its stability only up to 300 °C in air for long durations. A detailed metallographic analysis conducted on both the bulk and the metal layer, to compare the intermetallic phases present, revealed the absence of the intermetallic phases in the metallic layer. This absence indeed caused the deterioration in thermal stability of the absorber layer. Further, energy dispersive X-ray analysis revealed that, unlike the bulk material, the sputtered Inconel layer exhibited absence of Nb composition. This absence of Nb is significant, as it contributes to the formation of intermetallic phases, thereby, influencing the observed differences in thermal behavior.