- Nitriding was carried out in low pressure plasma excited by single- or dual-frequency discharge modes, at a substrate temperature of 523 K, followed by the deposition of 3 μm thick TiCN or TiN/TiCN coatings at a PH15-5 substrate temperature of 723 K. The nitrided layer was comprised of two distinct sublayers, namely a compound layer and a diffusion layer, with a total thickness of ∼ 60 μm. The compound layer was γ′-Fe 4 N and the diffusion layer was a solid solution of nitrogen in iron. The thickness of the compound layer fabricated by a single mode plasma is ∼ 5 μm, while that fabricated by dual-frequency mode plasma is ∼ 35 μm. It was found, using a ball-on-disk test, that the plasma nitrided layer fabricated by dual-frequency mode improved wear resistance by nearly one order of magnitude and improved the erosion resistance by a factor of two, compared with untreated steel. This improvement was common to the two nitriding treatments and both types of hard coatings. In particular, a thicker compound layer did not impair the wear resistance or the erosion resistance of the duplex treatment. The erosion resistance shows a linear dependence on the hardness of the uppermost nitrided or deposited layer.