δ-A15 and bcc phases coexist in sputtered chromium coatings with moderate oxygen contents

Abstract

Thin films of nanocrystalline Cr with grain sizes of around 20 nm were grown on glass substrates by D.C. magnetron sputtering in an argon atmosphere containing oxygen. The thickness of the films ranged between 30 and 135 nm, and the oxygen content between 11% and 13%. We studied the chemical composition and crystalline structure by means of X-ray spectroscopy, X-ray reflectivity and X-ray diffraction in both Bragg-Brentano and grazing incidence geometries. Our results show the coexistence of metastable δ-A15 and bcc phases in all the Cr films. The combination of the two X-ray diffraction modes allowed us to observe that growth of the δ-A15 phase in the [200] direction is favored when a certain critical film thickness is exceeded for equivalent values of oxygen content. Our results show the importance of the coexistence of both crystalline Cr phases and being aware of their textures, in order to avoid possible misinterpretations of phase transformations associated with critical stress, thickness or oxygen content.