Hot-filament chemical vapour deposition of diamond onto steel
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X, 164 p.
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Applied Molecular Physics
The main goal of this project was to establish the feasibility of depositing well adhering polycrystalline diamond coatings on steel substrates. It is well known that the growth and adhesion of diamond layers directly onto steels is complicated by the high carbon solubility and the high thermal expansion coefficient of the steel substrate material. Due to the catalytic effect of iron on the formation of graphitic soot, the deposition of adhering and high quality diamond films is hampered as well. In order to solve these problems, interlayer systems, which act as diffusion barriers for both atomic iron and carbon, are required. Using CrN interlayers, continuous and adherent diamond films are obtained on the high-speed steel, whereas only individual crystallites are observed on the stainless steel due to partial delamination of the initially grown films. Ultrathin Si interlayers are efficient diffusion barriers for Fe but not for C during diamond growth onto steel. The third type of interlayer system is produced by means of pack boriding. The resulting boride layers show a very high surface hardness, which gradually decreases on approaching the bulk. The presence of FeB at the surface results in very high thermal stresses and delamination of the diamond films on both types of steel. If only a Fe2B phase is present, continuous diamond films with low residual stresses are grown on both borided stainless and tool steel. A direct comparison made with the strongly adhering diamond layers on the molybdenum substrates indicates only a slightly inferior adhesion for the diamond coated stainless steel with the CrN interlayer. Using electrochemical polarization studies, the corrosion resistance of blank tool steel in sodium chloride solutions is found to be very low, but is significantly increased by the deposition of protective diamond layers. However, the effect of the applied interlayer systems on the shift of the corrosion potential is very strong
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