Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B2H6 and NH3 in a mixture of H-2 and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous alpha-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp(2)-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films. (c) 2006 Elsevier B.V. All rights reserved.