Studies on ultra high solar absorber black electroless nickel coatings on aluminum alloys for space application

A simple process of blackening of electroless nickel is investigated to produce ultra high solar absorber coating on the aluminum alloys. Process optimization was carried out by investigating the influence of various operating conditions, viz., pH of electroless nickel solution, thickness of electroless nickel deposit, formulation, concentration, temperature and processing time of etching solutions on the physico-optical properties of the black coating. The nature of the coating was evaluated by the thickness measurement, micro hardness evaluation and EDX studies. The microstructure of the deposits was examined under scanning electron and optical microscope. The space worthiness of the coating was evaluated by the environmental tests, viz., humidity, corrosion resistance, thermal cycling, thermo-vacuum performance and thermal stability tests. Optical properties of the coating were measured before and after each environmental test to ascertain its stability. The blackened electroless nickel provides high solar absorber in the order of 0.995. The results of these studies clearly indicate that the coating process described herein is extremely suitable as a solar absorber coating for space and allied application.