The high intrinsic stresses in cBN films after deposition are reduced significantly employing thermal treatment and ion implantation. In situ stress measurements and ex situ polarized infrared reflection spectroscopy are utilized to study stress relaxation and the phase stability, respectively. Argon ion implantation into cBN films at room temperature and at 340 degreesC reveals that the critical fluence for amorphization increases with increasing temperature. The fraction of cBN transformed into sp(2) bonded BN by Ar ion implantation decreases with increasing substrate temperature for the same amount of stress release.