The excellent characteristics of diamond such as its high wear resistance and chemical inertness raise the possibility to improve the performance and life time of industrial steel components by the deposition of a protective diamond coating. Therefore, we have investigated the direct deposition of diamond onto tool and stainless steel substrates by means of hot-filament-assisted chemical vapor deposition. Our growth studies show that, without any pretreatment procedure, the formation of interlayers consisting of a mixture of graphite, amorphous carbon and iron carbides prior to diamond film growth is unavoidable. For the tool steel substrates, ballas type diamond with large quantities of graphite is formed. For the stainless steel, the carbon in-diffusion leads to lesser amounts of graphite. However, as a result of the competitive growth of non-diamond phases, the obtained diamond films are still not continuous. The differences in graphitization effects of iron between both types of steel are explained by the differences in generation and decomposition of the metastable cementite (Fe3C) phase. Our results are discussed with the outcomes of recent works on multiple-stage diamond deposition processes and the application of diffusion barrier layers to overcome the above-mentioned problems. (c) 2007 Elsevier B.V All rights reserved.