Thermal Fatigue Testing of Duplex-Coated Hot Work Tool Steels at Elevated Temperatures

Thermal fatigue is one of the principal failure mechanisms in thixoforming dies. PVD coatings provide adequate protection against oxidation of hot work tool steels but fail to improve thermal fatigue resistance. The extensive softening of the tool steel substrate and the thermal expansion mismatch between the hot work tool steel and the PVD coatings are held responsible. Hot work tool steel samples were plasma nitrided before coating with CrN/AlTiN using the Cathodic Arc Physical Vapour Deposition process. The duplex-coated samples thus obtained were submitted to thermal cycling under conditions which approximate thixoforming of steels. Plasma nitriding before PVD provides the much needed substrate support and has a very favourable impact on the thermal fatigue performance of the tool steel. The results from the thermal fatigue tests of the duplex-coated tool steel samples will be discussed and compared with tool steel samples coated via PVD alone.