Abstract
Thermal degradation phenomena occurring in nickel-pigmented aluminium oxide coatings were investigated by combining detailed microstructural analysis with modelling of the optical properties. Scanning electron microscopy and Auger electron spectroscopy sputter profiling showed that the initial film consisted of a nickel-pigmented aluminium oxide layer close to the substrate. This layer supported a porous aluminium oxide layer that had a rough outer surface. While the solar absorptance degraded substantially (from 0.94 to 0.71) after heat treatment at 350°C for tens of hours, the aluminium oxide film morphology and thickness remained virtually unchanged and there was apparently no redistribution of nickel within the coating. Instead, the optical quality of the film degraded through oxidation of the nickel particles. These observations were supported by an optical model of the coating which produced the spectral reflectance properties measured both before and after the thermal ageing.
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