The influence of substrate material on thermal stability of selective solar absorbing black chrome

The role of the substrate in optical degradation of solar absorbing black chrome films has been investigated. Past work in establishing the optical properties, microstructure, and optical degradation characteristics of these electroplated coatings has used nickel as the substrate. For temperatures less than 450[degrees]C the primary mode of degradation was believed to be changes in the film. It was later found that the thermally activated mechanism of substrate diffusion and oxidation existed for temperatures as low as 400[degrees]C. These results led to a consideration of other materials in the hope of finding a more thermally stable substrate. The metals to be considered here as substrates are copper, gold, nickel, chromium and stainless steel #304. The experimental techniques incorporate hemispherical reflectivity, normal emissivity, scanning electron microscopy, and Auger electron spectroscopy. Black chrome coating on each substrate candidate undergoes equivalent analyses (1) in the "as-prepared" case, (2) after air heating to 300[degrees]C for 24 hours, and (3) air heating to 450[degrees]C for 24 hours. In this work, diffusion at 450[degrees]C was shown to exist for all of the substrate materials considered. Stainless steel #304 however, demonstrated a selective diffusion and oxidation mechanism which significantly reduced optical degradation of the black chrome for temperatures up to 450[degrees]C. Of the candidates considered, this alloy substrate was found to be the most suitable for enhancement of thermal stability of black chrome.