Optical Performance, Thermal Stability, and Failure Analysis of the WNx-Si3N4 Multilayer Solar Selective Absorbing Coatings

The development of high-temperature-tolerance solar selective absorbing coatings (SSACs) is beneficial to improve the conversion efficiency of concentrated solar power. In the present work, a TiN/Mo/WNx-Si3N4 bilayer/SiO2 cermet-based SSAC was developed, which was deposited on different substrates by magnetron sputtering. The designed WNx-Si3N4 SSAC exhibits a high solar absorptivity of 0.952, accompanied by a relatively low emissivity of 0.059 at 25 °C and 0.134 deduced at 600 °C. Moreover, the WNx-Si3N4 SSAC deposited on a stainless-steel (SS) substrate shows great thermal stability at 650 °C in vacuum. The solar absorptivity shows a slight decrease from 0.952 to 0.939 and the emissivity at 25 °C decreases from 0.059 to 0.045 after being annealed at 650 °C in vacuum. Furthermore, when being annealed at 700 °C in vacuum, the WNx-Si3N4 SSAC deposited on the quartz glass substrate still keeps stable, but the coating on the SS substrate is dramatically destroyed. A comprehensive failure analysis was conducted utilizing FT-IR, SEM, Raman, and XPS characterizations. The degradation of the WNx-Si3N4 SSAC deposited on the SS substrate at 700 °C is caused by the severe elemental diffusion between the absorbing layers and the antireflection layer. Moreover, the cracks induced by the thermal stress provide rapid channels for elemental diffusion.