Thermal stability test and ageing mechanisms study of different solar selective absorbing coatings

Long term thermal stability and temperature induced ageing mechanisms of different solar absorber coatings are investigated in this study. These coatings are chromium, chromium nitride and chromium oxynitride based coatings on aluminium substrate. The coatings were annealed at 278°C for 600h during the first round test. The second round test was performed at 308°C for 342h. An additional test at 400°C for 200h is designed for the sake of obtaining more degradation information. Thermal stability, microstructure, composition distribution, element diffusion and oxidation were studied using UV/VIS/NIR spectrophotometer, FTIR spectrometer, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Auger electron spectroscopy (AES) respectively. The result shows that the main ageing mechanism is diffusion and oxidation of Cr, Al, N and O. And the microstructure defects, such as enlarged grain boundaries, micro cracks and holes, act as a facilitating role during the ageing process because they provide fast diffusion paths for short-circuit diffusion.