Diamond coatings were successfully deposited on tool steel substrates without using any external diffusion barrier layers. The diamond film deposition was performed in a hot filament chemical vapour deposition (HFCVD) reactor. In the first step, a high substrate temperature and a high methane percentage were used to achieve a faster critical carbon concentration and hence a shorter incubation time for diamond nucleation. Subsequently, the substrates were taken out of the reactor and subjected to ultrasonic scratching in diamond slurry in order to increase the diamond nucleation density. Then the final deposition was performed in the reactor under typical diamond growth conditions. X-ray diffraction analyses showed that Fe3C was the dominant carbide phase formed on the steel surface after the first step. The carbides produced in the first step act as a diffusion barrier for both iron and carbon. Thus, they accelerate the diamond nucleation and help further diamond growth. Scotch tape adhesion tests showed that the diamond film adheres well to the substrate.