Microstructure, composition and formation mechanism of ultra-black surfaces on the electrodeposited nickel-phosphorous coatings

Light absorbing materials are of practical importance in the production of optical devices in aerospace technology. A new method of light absorbing coatings preparation on the basis of electrodeposited nickel-phosphorus coatings has been proposed. Modification of the electrolyte for nickel-phosphorus plating by the addition of saccharine allowed preparing an ultrablack surface with a reflectance coefficient less than 0.5%. Chemical composition of both surfaces has been studied using X-ray photoelectron spectroscopy. The mechanism of their formation has been proposed. It has been found that etching in nitric acid leads to the partial oxidation of both surfaces with the formation of needle like microstructure.It also results in surface enrichment with phosphorus and formation of nickel phosphides particles of the variable composition. Products of the coating oxidation represent the matrix of amorphous nickel polyphosphates and polyphosphoric acids where fine Ni-P particles are distributed. Such composite structure is a well light absorbing medium. Formation of hypophosphite protective layer on the Ni-P coating obtained from saccharin free electrolyte reduces the thickness of light absorbing layer and changes its composition. Thus light absorbance ability of the coatings can be caused by both distribution of fine nickel-phosphorus particles in dielectric matrix and specific microstructure of their surfaces.

Automated summary

A new method of preparing light absorbing coatings based on electrodeposited nickel-phosphorus coatings has been proposed. By modifying the electrolyte, an ultrablack surface with a low reflectance coefficient can be achieved. The chemical composition and formation mechanism of both surfaces have been studied. It has been found that etching in nitric acid leads to oxidation and the formation of a needle-like microstructure. The light absorbance ability of the coatings is caused by the distribution of fine nickel-phosphorus particles in a dielectric matrix and the specific microstructure of their surfaces.