Deposition of hard films on hot-working steel dies for aluminium

In the extrusion process of aluminum alloys at elevated temperature, the dies are used under very severe thermal and mechanical stresses. Furthermore slide friction and adhesive wear occur in the interaction zone between extrusion die and extrusion product. To improve the die performance and the quality of aluminum products, a duplex process combining a plasma nitriding (PN) pretreatment and a plasma-assisted chemical vapour deposition (PACVD) or PVD was applied on the extrusion dies, which are made of the hot-working steel (X38CrMoV5-1). The duplex layers are produced at a temperature of 530 °C. The effects of process parameters during PN, such as duty cycle, nitrogen concentration in process gas and nitriding time, are investigated. The optimised modification of the die surface has been achieved. The duplex layers (TiBN-, CrN- and W–C:H-films on the PN films) have a higher macro-hardness (HV 9.5) and critical load, in comparison with the conventional films without PN treatment. The performance of coated industrial dies are examined by means of direct extrusion.