This article presents experimental results for morphological assessment of sub-50-nm thick hydrogenated amorphous carbon (a-C:H) overcoats. The films were grown by rf plasma enhanced chemical vapor deposition using Ar/C2H2 gas mixture onto heterogeneous (Al2O3-TiC) ceramic substrates. They were analyzed using complementary information from scanning electron microscopy, energy dispersive x-ray analysis and atomic force microscopy. We found that the bias deposition voltage and the nature of the substrate (Al2O3 or TiC regions) adversely affect both growth rate and microstructure. Argon inclusion in the C2H2 plasma during the first stage of growth is responsible for the observed effects. These results suggest that the amorphous carbon layer is denser on the TiC than on the Al2O3 region. (C) 1999 American Institute of Physics. [S0021-8979(99)00823-3].