Spectral properties of AlCrNO-based multi-layer solar selective absorbing coating during the initial stage of thermal aging upon exposure to air

A spectrally selective Cr/AlCrN/AlCrNO/AlCrO multi-layer absorber coating has been designed and deposited on stainless steel substrates by the cathodic arc ion plating technique. The spectral properties of the multi-layer coating during the initial stage of thermal aging in air were investigated with the aim of achieving a basic understanding of its relation to microstructure. The composition, structure and surface morphology of the coatings were characterized using XRD, SEM, XPS, TEM and AFM, while the optical properties of the coatings were measured by spectrophotometers. The experimental results indicate that the multi-layer coating with a high solar absorptance of 0.90 and a relatively low emittance of 0.15 are obtained through parameter optimization. The structure analysis suggests that the as-deposited AlCrN and AlCrNO layers, acting as the main absorbers, are consisted of the AlN, Cr2N and AlN, CrN, Cr2N nanocrystallites embedding in amorphous matrix, respectively. While the AlCrO layer, acting as the anti-reflection layer, is consisted of mainly amorphous. After annealing at 500 °C for 2 h in air, a solar selectivity of 0.94/0.12 was achieved while the thermal emittance attained 0.25 at 500 °C. Based on the microstructural analysis, the enhancement in absorptance of the multi-layer AlCrNO-based coating during the initial stage of thermal aging could be attributed to the decomposition of Cr2N nanocrystallites and the recrystallization of the AlCrN and AlCrNO layers. While the enhanced emittance after annealing can be ascribed to the decomposition of the macro-droplets and the decrease of surface roughness. These results could provide new insights into rational use of the muti-layer coatings and spectral properties tuning of solar-selective functional multi-layer coatings and thus promote the development of coatings for photo-thermal conversion at high temperatures.