CORROSION TESTS OF PVD COATINGS WITH DIE LUBRICANT USED FOR AL HIGH-PRESSURE DIE-CASTING DIES

Under the extreme working conditions of the Al high-pressure die-casti ng process, the corrosion of the die surface, provoked by the die lubr icant sprayed on the hot die surface for better ejection of the castin g, is included. The major failure mechanism of the die is originated b y the cyclic temperature change on the die surface, causing thermal fa tigue. Although this failure is mainly caused by the temperature gradi ent between die surface and its bulk volume, a reduction of heat check ing occurrence is expected for a die when its surface has good corrosi on resistance, which results in a smoother surface. PVD coatings were deposited on hot work steel and corrosion tested with die lubricant. P olarization curves, such as current density potential, were taken for (Ti,Al)N and Cr-based coatings, which were deposited by magnetron sput tering and cathodic are processes. The coated and uncoated substrates were also tested after an isothermal annealing in air at 720 degrees C , which has shown better results due to the oxide formation on the coa tings. SEM analysis was used for a better comparison of the results, a nd has confirmed the improvements achieved for the coated substrates. (C) 1998 Elsevier Science S.A. All rights reserved.