FAILURE MODES OF PVD COATINGS IN MOLTEN Al-ALLOY CONTACT

Abstract

This paper deals with a study of the failure mode of thin PVD coatings in alternated contact with
molten aluminum alloy. CrN and ZrN monolayer coatings deposited through cathodic arc
evaporation were used. The coatings morphology was assessed by SEM and their mechanical
properties evaluated by nanohardness test performed at room temperature. An experimental test
rig which cyclically immerses coated steel samples in molten Al-alloy and in a cooling bath was
applied. The thermal gradient from the coating to the steel core was exalted by internal cooling
channels placed in the internal cavity of samples. Periodical SEM inspections were performed to
assess the damaging levels introduced by the test and to study the related decrease of substrate
protection capability. Descriptions and interpretations of the damages evolutions were derived.
The main conclusions achieved are that both coatings suffered by the formation of corrosion
pits, which were due to a corrosion attack of the steel substrate localized at coating defects sites.
In particular, at pores locations the corrosion was fast, whereas at droplets sites it required a
certain incubation time. Once corrosion pits were formed they exhibited an initial tendency to
expand laterally, but they did rapidly stabilize in terms of lateral dimensions. Later on two
different failure modes acted in ZrN and in CrN. Extended delamination due to a marked
mismatch of mechanical properties between the coating and the steel substrate developed in
ZrN. On the contrary, thermal cracking due to lower hardness levels developed in CrN, but with
limited delamination. Accordingly the steel substrate protection capability was evaluated to be
higher in CrN than in ZrN.
Keywords: PVD