The thermal and chemical compatibility of silicon nitride ceramics (Si3N4) to diamond ensures an adequate adhesion for tribological applications, such as mechanical seals, biomaterials or cutting tools for hard machining operations. Most of the recent research efforts on these components focused on nanocrystalline diamond (NCD) due to its small grain size and surface smoothness. The present study reports for the first time NCD coating Of Si3N4 substrates by microwave plasma-assisted chemical vapour deposition (MPACVD) using Ar/H-2/CH4 gas mixture and varying the substrate temperature (650-1050 degrees C) and deposition time. Optimal deposition conditions result in growth rates up to 3.0 mu m h(-1) that allowed the production of continuous NCD films with low roughness (Ra approximate to 22 nm) and grain size of about 10 nm at the lower temperatures and longer deposition times. (c) 2004 Elsevier B.V. All rights reserved.