Delineating the role of surface characteristics on the solar selectivity of colored chromium oxide coating on 304 stainless steel substrate

Solar selective colored chromium oxide coatings were prepared on the SS304 substrate through the electrochemical deposition route to provide more options and flexibility in aesthetically design solar absorbers with acceptable energy performance. The variation of colors in the coatings was achieved by varying the coating thickness as a function of deposition time. Color changes in the coatings were also evident in the samples that were annealed under vacuum. Electron microscopic images show that nanometer sized pores are present on the coating surfaces. The pore-density, pore-size distribution and surface roughness exhibit significant changes with variations in deposition time and annealing temperature. Initially, the selectivity factor of 6.08 was recorded in the as prepared sample coated for 30 min, which improved to 7.36 on annealing the samples under vacuum at 700 °C, however, it drops to 4.22 when annealed at 900 °C. A similar trend was also observed for samples that are coated for 60 min. The results indicate that the pore-density, pore-size distribution and surface roughness play a dominant role in governing the solar selectivity of the coatings. Nanoindentation tests were performed on the coated samples and the results show that mechanical properties of the solar absorber coatings degrades with the increase in coating thickness but improve significantly with increase in annealing temperature.