Tungsten coatings with thickness of 5-500 nm are applied onto plane-faced synthetic diamonds with particle sizes of about 430 and 180 mu m. The composition and structure of the coatings are investigated using scanning electron microscopy, X-ray spectral analysis, X-ray diffraction, and atomic force microscopy. The composition of the coatings varies within the range W-W2C-WC. The average roughness, R (a), of the coatings' surfaces (20-100 nm) increases with the weight-average thickness of the coating. Composites with a thermal conductivity (TC) as high as 900 W m(-1) K-1 are obtained by spontaneous infiltration, without the aid of pressure, using the coated diamond grains as a filler, and copper or silver as a binder. The optimal coating thickness for producing a composite with maximal TC is 100-250 nm. For this thickness the heat conductance of coatings as a filler/matrix interface is calculated as G = (2-10) x 10(7) W m(-2) K-1. The effects of coating composition, thickness and roughness, as well as of impurities, on wettability during the metal impregnation process and on the TC of the composites are considered.