Degradation study of black cobalt coating

Abstract

Black cobalt coatings were electrodeposited on 304 stainless steel substrates. In order to study the thermal stability of the films, they were subjected to several cycles of heating in air at a fixed temperature of 400°C. The phase compositions of fresh samples and of heat treated films were identified using X-ray diffraction. Values of solar absorptance were calculated from the measured reflectance values before and after heat treatment. The studies revealed that the fresh samples of black cobalt were composed of (002) reflection phase (5–727 JCPDS), which suggests that the cobalt grows epitaxially along the [002] direction of the γ₀₀₂ phase of the substrate. The high absorptance of fresh samples and their deep black appearance can be explained in terms of the multiple reflection model, owing to the intrinsic absorption and roughness of the surface. This has a direct bearing on the electrodeposition method, because it is also possible to prepare cobalt with metallic appearance. The thermal oxidation behaviour and morphological changes of the black cobalt were identified as the main causes of the elevated temperature degradation of the coatings.