High temperature oxidation of SiC-coated Fe-Cr-Al-Mo alloys in muffle and concentrated solar furnaces

The maximal temperature of a solar receiver is limited by its resistance to oxidation, thermal cycling and me-chanical stresses as it is a key component of solar power plant with a central tower. Currently used Ni-based alloys cannot heat the air coolant at temperatures beyond 1000 K due to the severe oxidation that may affect the surfaces exposed to air at high temperatures, affecting the solar absorptivity of the receiver. A SiC coating should protect a high temperature metallic alloy from oxidation and improve its solar absorptivity, only if the accommodation between the metallic substrate and the ceramic coating is optimized to prevent the coating from cracking and peeling due to the thermal expansion of the assemblage. To reduce the interfacial stresses, SiC was deposited using High Temperature Chemical Vapor Deposition on a pre-oxidized Fe-Cr-Al-Mo alloy, so that a 2 mu m-thick alumina layer could work as an accommodation layer. The oxidations performed in muffle and solar furnaces (associated to ex situ characterizations using scanning electron microscopy and energy-dispersive X-ray spectroscopy mapping, X-ray diffraction, and room-temperature normal spectral emissivity measurements) have demonstrated that this multilayer structure could support long-term oxidation and thermal stresses up to1400 K without significant changes regarding the spectral emissivity, but the coating peels if the sample is treated at 1600 K. The multilayer structure can therefore work beyond the current working point of solar receivers, nevertheless the accommodation still has to be improved in case of accidental exposures to higher solar fluxes.