Investigation of structured TiAlN- and TiO2-coatings with moth-eye-like surface morphologies

Abstract

This paper is to report on the development and investigation of a specific type of growth mode and surface morphology of PVD-coatings, namely a columnar growth mode where the individual columns are terminated by pyramid-shaped tips. TiAlN was chosen as a representative of the cubic crystal system, TiO2 as a representative of the tetragonal crystal system. The coatings were sputter-deposited on silicon and steel substrates by a reactive RF-magnetron process under variation of total pressure, reactive gas flow and RF-power. Investigations of microstructure and surface morphology were carried out with electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction techniques (XRD) and are described in more detail for TiO2-coatings. The individual columns with diameters between 100 and 500 nm were terminated by pyramid-shaped tips where the geometric forms of the pyramids were determined by the crystal system of the coating material. A power law was found for the dependance of the mean column diameter on coating thickness in qualitative agreement with phenomenological models for thin film growth. Both coating materials were deposited on steel tools and hot embossed into the surfaces of PMMA-clear-view screens. The resulting screen's surface morphologies proved to be comparable to the anti-reflective surfaces of moth-eyes and also exhibited a pronounced decrease in reflectivity.