Infrared Spectroscopy of Ultrablack Films Based on Electrodeposited Ni–P Alloys

Abstract

Introduction. Black coatings with low refl ection coeffi cients for visible and near-IR electromagnetic radiation are widely used to produce optical devices for reducing the scattered light background and in laser-radiation power meters as operating light-absorbing surfaces [1]. It was shown [2] that surfaces formed by chemical etching of chemically deposited Ni–P coatings with P contents of 3–10 at% were some of the blackest. It was established that the low refl ection coeffi cients of such coatings were due to a combination of the surface morphology produced by etching and the formation of a thin light-absorbing surface layer [3, 4]. The black color of the layer was explained by higher nickel oxides and hydroxides in it. However, the role of P in the formation of the black coating was not found. The P must be present in the coating because Ni coatings without P do not form black surfaces during etching in various acids. IR spectroscopy can be used to obtain information about the presence of oxidized P species. The goal of the present work was to study the chemistry of ultrablack fi lms prepared by chemical etching of electrodeposited Ni–P coatings in an oxidizing medium. Experimental. Ni–P coatings of thickness 30 �� m were prepared by electrochemical deposition by the published