Scratch resistance of CrN coatings on nitrided steel

Duplex coatings consisting of a nitrided steel covered by a hard CrN coating have been produced in an industrial Hauzer HC 750 PVD machine. Hot work tool steel was used as substrate and was plasma nitrided up to a depth of 65 μm. The nitriding parameters were chosen such that nitriding was achieved without formation of iron nitrides at the surface. Hard CrN0.94 coatings with thickness up to 10 μm were deposited on all nitrided specimens in the same PVD machine. A set of 42 specimens was obtained with independent variation of nitriding depth and coating thicknesses.The samples were mechanically characterized by scratch testing and nanoindentation. The hardness of the steel substrates measured by nanoindentation increased from 10 to 16 GPa due to nitriding. Increasing the nitriding depth of the duplex coatings results in a significant increase of the scratch hardness (up to two to three times). However, increasing the coating thickness up to 10 μm hardly influences the scratch hardness.The relative importance of coating thickness and nitriding depth for scratch resistance and load carrying capacity of the substrate is discussed.