Corrosion phenomena of plasma nitrided steel, duplex CrN and TiAlN coatings subjected to molten Al-alloy

High pressure die casting (HPDC) is a progressively developing technology used for mass production of complex, near-net shape and thin-walled components of light alloys. Its production efficiency greatly depends on a die quality and its endurance. Nowadays this is usually improved by application of ceramic coatings produced by physical vapor deposition (PVD) on its surfaces. However, future development of these coatings still requires fundamental knowledge about oxidation and corrosion mechanisms acting in these specific cases. Therefore, we investigated the performance of plasma nitrided steel, duplex CrN and TiAlN PVD coatings prepared to different degrees of surface roughness. Corrosion behaviour in Al-Si-Cu cast alloy was evaluated by ejection test, performed with conventional (CS) and delayed cast alloy solidification (DS) for 5 and 20 min. Beside corrosion in a casting process this test simulates the ejection process of a die core from a casting. The force required for ejection is a measure of cast alloy soldering (corrosion) tendency toward pin material. Different microscopy and analytical techniques were employed for the analysis of samples surfaces after the ejection tests. On all samples subjected to CS tests cast alloy built-up layer formed due to galling and mechanical soldering effects. In conditions of DS tests, plasma nitrided sample was attacked by aluminium, a corroded layer formed which easily sheared under lower force during the ejection process. On the other side, lower ejection force recorded in DS tests for smoother coated samples is attributed to the thickening of the casting oxide scale that occurred due to the consumption of oxygen from coatings oxide layers. For both coatings, in DS tests, a nondetrimental corrosion of underlying nitrided substrate occurred through the coating growth defects. Results of focused ion beam analysis of different corrosion sites revealed the morphology and chemistry of corrosion products which suggested possible mechanisms that act in such processes.

Mots clés

high pressure die casting; aluminium alloy; PVD; plasma nitriding; CrN; TiAlN; surface roughness; oxidation; corrosion; growth defects