Corrosion tests of PVD coatings with die lubricant used for Al high-pressure die-casting dies

Abstract

Under the extreme working conditions of the Al high-pressure die-casting process, the corrosion of the die surface, provoked by the die lubricant sprayed on the hot die surface for better ejection of the casting, is included. The major failure mechanism of the die is originated by the cyclic temperature change on the die surface, causing thermal fatigue. Although this failure is mainly caused by the temperature gradient between die surface and its bulk volume, a reduction of heat checking occurrence is expected for a die when its surface has good corrosion resistance, which results in a smoother surface. PVD coatings were deposited on hot work steel and corrosion tested with die lubricant. Polarization curves, such as current density potential, were taken for (Ti,Al)N and Cr-based coatings, which were deposited by magnetron sputtering and cathodic arc processes. The coated and uncoated substrates were also tested after an isothermal annealing in air at 720°C, which has shown better results due to the oxide formation on the coatings. SEM analysis was used for a better comparison of the results, and has confirmed the improvements achieved for the coated substrates.