Demonstration of Long Term Stability in Air at High Temperature for TiAlN Solar Selective Absorber Coatings

High thermal stability and resistance to oxidation at atmospheric conditions need to be critically proven to ensure a long lifetime of the receiver of concentrated solar power plants. In the present work, the titanium aluminum nitride tandem absorbers are showing one of the most promising selective coating solutions for solar receivers operating at high temperature. A series of samples were synthesized by vacuum plasma techniques in an industrial pilot-scale deposition machine. The structure consists in an Inconel 625 substrate with infrared reflective layer, followed by two TiAlN layers (tandem absorber) with refractive index gradation and covered with SiNCH, as an antireflective layer. A programmable electrical tubular furnace was used to apply cumulative aging tests on the samples under static ambient air at temperatures from 500 degrees C to 690 degrees C. One batch of samples was tested cumulatively in air at 500 degrees C, for long-term representative aging, up to a total duration of 732 hours. Other samples were then submitted to cumulative accelerated aging in air at 630 degrees C up to 264 hours, at 660 degrees C up to 366 hours and at 690 degrees C up to 156 hours. Spectral reflectance was systematically measured before and after aging. The results show that the optical properties did not change with aging, demonstrating long-term stability of optical properties at 500 degrees C in air. EDS analysis revealed that the content in oxygen first increases then stabilizes over time (6 at.%), probably indicating the oxidation of the antireflective top layer SiNCH into SiO2 (as was also shown by previous XPS analysis), which then acts both as a barrier for further oxidation and as antireflective material. For higher temperatures, a maximum decrease of solar absorptance of -0.11 was observed. However, emittance also tended to decrease, limiting the decrease of the resulting heliothermal efficiency. In all cases, SEM imaging did not show any physical changes in surface morphology with aging. Overall, this type of selective coating shows both good optical performance and a very good resistance to aging at high temperature in air, rendering it very promising for CSP applications where the solar receiver is at or above 500 degrees C, without the need to maintain it under vacuum.