Abstract
This paper deals with a study of the failure mode of thin PVD coatings in alternated contact withmolten aluminum alloy. CrN and ZrN monolayer coatings deposited through cathodic arcevaporation were used. The coatings morphology was assessed by SEM and their mechanicalproperties evaluated by nanohardness test performed at room temperature. An experimental testrig which cyclically immerses coated steel samples in molten Al-alloy and in a cooling bath wasapplied. The thermal gradient from the coating to the steel core was exalted by internal coolingchannels placed in the internal cavity of samples. Periodical SEM inspections were performed toassess the damaging levels introduced by the test and to study the related decrease of substrateprotection capability. Descriptions and interpretations of the damages evolutions were derived.The main conclusions achieved are that both coatings suffered by the formation of corrosionpits, 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 acertain incubation time. Once corrosion pits were formed they exhibited an initial tendency toexpand laterally, but they did rapidly stabilize in terms of lateral dimensions. Later on twodifferent failure modes acted in ZrN and in CrN. Extended delamination due to a markedmismatch of mechanical properties between the coating and the steel substrate developed inZrN. On the contrary, thermal cracking due to lower hardness levels developed in CrN, but withlimited delamination. Accordingly the steel substrate protection capability was evaluated to behigher in CrN than in ZrN.Keywords: PVD
Highlights
The continuously developing research on vapour deposition technologies allowed developing new concepts for the design of complex thin coatings for tribological applications [1,2,3] and fabrication of mono, multi-layer and superlattice architectures [4,5,6]
This paper reports the results of a study on failure modes of CrN and ZrN monolayer coatings using cyclic immersion tests in molten aluminum alloy, using both die lubricant additions and internal cooling circuit
In ZrN coating thermal stresses provided by cyclic immersion in molten aluminum alloy are mainly released by cracks nucleating around the corrosion pits and propagating through the coating/substrate interface interface, with progressive decohesion resulting in extended delamination; in some cases de-cohesion of coating is provided giving partial coating delamination
Summary
The continuously developing research on vapour deposition technologies allowed developing new concepts for the design of complex thin coatings for tribological applications [1,2,3] and fabrication of mono-, multi-layer and superlattice architectures [4,5,6]. Such alloys are aggressive in the liquid state [25, 26] due to a complex mix of degradation mechanisms, i.e. thermal fatigue, corrosion/soldering and erosion by hard particles and high velocity liquid fluxes Such harsh operating environment results in frequent requests of dies re-working, with the consequent downtime of the fabrication lines. This paper reports the results of a study on failure modes of CrN and ZrN monolayer coatings using cyclic immersion tests in molten aluminum alloy, using both die lubricant additions and internal cooling circuit Such coated systems studied are commercially proposed for the protection of dies in the aluminum alloy die casting technology. Quality assessments of coated systems were performed through nanohardness tests at different loads and SEM in the as deposited state
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