The steel grade SAE 4140 has been nitrided and coated with amorphous diamond like carbon (DLC) from type a-C:H:Si. The nitriding and coating with diamond like carbon was carried out in a plasma assisted nitriding furnace which is extended by plasma assisted chemical vapour deposition for a-C:H:Si coating. The nitriding before coating has been done to increase adhesion and reduce the hardness gradient between the diamond like carbon layer (810 – 1125HV) and the sample material (325HV).

The temperature of the coating process has been varied, from 400 to 550°C. These different samples have been analysed to obtain corrosion resistance data using potentiodynamic polarisation and a salt spray test. The chemical composition of the a-C:H:Si coating has been analysed using glow discharge optical emission spectroscopy (GDOES). Hardness of the coating was identified with nanoindentation. Scanning electron microscopy coupled with energy dispersive x-ray spectroscopy was utilized to analyse the structure and defects on samples after corrosion test.

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