Response to thermal cycling of duplex-coated hot work tool steels at elevated temperatures

Abstract

Hot work tool steel samples were plasma nitrided and coated with a thin hard AlTiN film using the Cathodic Arc Physical Vapour Deposition process in the present work. The duplex-coated samples thus obtained were submitted to thermal cycling under conditions that mimic the steel thixoforming process. Compressive stresses generated at the surface due to a thermal expansion mismatch with the substrate lead to blistering of the AlTiN coating. These blisters produce wrinkles and develop into surface cracks with increasing number of cycles. AlTiN coating starts to spall off at those locations encircled by these cracks, leaving the substrate exposed to very hostile conditions. The nitrided substrate is very prone to oxidation and scaling is inevitable. Likewise, the nitrided surface fails to provide the required substrate support for the AlTiN coating as the hardening introduced at the surface by plasma nitriding is almost completely erased after only 350 thermal cycles. Having lasted for a maximum of 750 thermal cycles, the duplex-coated hot work tool steel fails to offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures.