Properties of cubic boron nitride films with buffer layer control for stress relaxation using ion-beam-assisted deposition

Abstract

Significant ion irradiation during film growth is required for the formation of cubic boron nitride (cBN) films. Meanwhile, a huge level of intrinsic stress possibly induced by the ion bombardment has been frequently reported to result in cracking and/or lack of adhesion of deposited cBN films. The present work has been performed to investigate the interfacial and/or the buffer layer structures with better matching to the cBN film by relaxation of the film stress using ion-beam-assisted deposition (IBAD). Boron nitride films have been synthesized on Si(100) wafer and tungsten carbide (WC) substrates by depositing boron vapor under simultaneous bombardment with nitrogen ions and nitrogen–argon mixture ions in the energy range of 0.5–10 keV. Cubic BN films with enhanced tribological properties have been explored by inserting a BN layer with various B/N compositions as a controlled buffer at the interface. Significant relaxation of the film stress has been observed for the buffer layer with a boron-rich (B-rich) composition (B/N∼10) with hardness maintained at a relatively high value (∼30 GPa). A structural analysis by Fourier transform infra-red spectroscopy (FTIR) and cross-sectional transmission electron microscopy (TEM) confirms that polycrystalline films with a high cBN fraction were synthesized on the B-rich layer. Formation of the cBN films with a B-rich buffer layer enabled the tribological characterizations to be performed, and the tribological properties have been observed to be significantly enhanced with the insertion of the B-rich buffer and the enhancement of atomic intermixing at the initial stage of cBN layer growth. Using the nanoindentation method, the hardness of the cBN films with a high cubic fraction was found to be as high as the values for the bulk cBN.