Chemical vapor deposition of a thick film of TiC on a cemented carbide produces an intermediate layer, ∠1 μm thick, of Co6W6C (eta carbide) between the substrate and the 3-μm-thick TiC coating. Although the TiC-coated cemented carbides have a much longer tool life than an uncoated tool, the effect of the intermediate eta phase on this tool life has not been known to date. Normally, the eta phase is avoided in cemented carbides because of its brittleness, but it has been proposed that the eta carbide actually enhances the tool life by forming a diffusion barrier. This hypothesis is being investigated by preparing coated cemented carbides by sputtering. With this method, it is possible to prepare TiC-coated tools with and without the eta-carbide layer and study the effect of the eta carbide on tool life. Tool-life tests with eta-coated carbides show that the eta layer does improve tool life.