The phase transformation of cubic boron nitride (c-BN) in the near-surface region of a c-BN (I 11) facet induced by argon-ion bombardment has been investigated by reflective electron energy-loss spectroscopy, with the objective of tracking possible growth mechanisms of c-BN in ion-assisted vapor deposition. For example, with an ion energy of 500 eV at a fluence of 5 X 10(16) ions/cm(2) 74% of c-BN were found transformed to h-BN and a-BN within the ion penetration depth of 11 Angstrom. Lowering the argon ion bombardment energy to 200 eV reduced the phase transformation depth to 7 Angstrom. A damage saturation was observed at a fluence of 6 X 10(16) ions/cm(2), at which up to 9% of the cubic phase were transformed. Similar measurements were performed at other ion energies. The present work gives a quantitative explanation of the difficulty in growing pure c-BN films with ion-assisted deposition and the presence of an upper-bound in ion energy above which the formation of phases other than the cubic phase dominate.