Pitting corrosion behaviour of diamond coated tool steel

Using a hot filament chemical vapour deposition reactor, diamond films of approximately 2-3 μm in thickness were deposited on tool steel specimens with three different interlayer systems, namely CrN, Si and borided steel. The morphology, defect densities and residual stresses of the diamond films were strongly governed by the type of interlayer used. Electrochemical potentiodynamic polarisation studies have been carried out on the diamond deposited tool steel specimens in NaCl solutions of two different concentrations, 0.01 M and 0.50 M, respectively. The type of interlayer used for diamond coating on tool steel has a strong influence on the corrosion potential. The maximum shift in the equilibrium corrosion potential is observed for the silicon interlayer. However, it leads to selective dissolution of the carbide phase upon anodic polarisation. A near total resistance to pitting of the CrN coated tool steel specimens is observed with the introduction of a very thin diamond layer of 2-3 μm. With increasing chloride ion concentration in the electrolyte, reverse trends in the shift of Ecorr are observed for the blank tool steels in comparison to the diamond-coated specimens. Detailed SEM characterisation of the corrosion-tested specimens indicates the possible operating mechanisms.