Corrosion resistance of nanolayer CrAlN coatings exposed to molten Al-alloy

High pressure die casting (HPDC) is one of the most commonly used technologies for processing of Al-alloys. Die degradation occurs due to different wear mechanisms such as corrosion (soldering), erosion and thermal fatigue. Soldering is a form of corrosion process in which the interaction of cast alloy and tool surface form layers of built-up material that reduce the die performance and castings quality. To mitigate the soldering problems application of nanolayer coatings containing transitional metal nitrides such as CrN, AlN and CrAlN is advised. However, the performance of CrAlN coatings with different chemical composition is still scarce in the literature. Therefore, this study concerned nanolayered CrAlN coating, with three chemical compositions, deposited on plasma nitrided hot-working tool steel. Soldering of the coating to the Al-alloy was evaluated using a detachment test. The contact surfaces were analyzed using a profilometer, focused ion beam (FIB), scanning electron microscopy (SEM) X-ray spectroscopy (XPS). By analyzing the samples' contact surfaces, it was found that soldering and oxidation occurred on the contact surface and through coating defects. These phenomena have manifested as intermetallic compounds, which formed between Al-alloy and the underlying substrate. Samples with higher number of coating defects exhibited a larger amount of soldering. In detachment test, as a result of soldering, a coating layer detached from the substrate together with the casting. This enabled the analysis of the coating back side and the intermetallic compounds that formed through coating defects. These compounds deteriorated the contact surface between the substrate and the coating. This compromised the contact surface of the coating and the substrate material, which lead to coating delamination. It was found that the chemical composition of the coating had a minor influence on soldering than the density of growth defects. The samples with less defects exhibited less soldering. By reducing the number of coating defects, tool life can be extended.