Application of hard coatings in aluminium die casting - soldering, erosionand thermal fatigue behaviour

In aluminium die casting, tools are exposed to erosion, corrosion and solde ring due to the frequent contact of the tool surface to the casting alloy, to heat checking and gross cracking due to thermal fatigue and to oxidation due to high pouring temperatures. The gradual destruction of die surfaces during service decreases casting piece quality and limits die lifetime. Har d coatings based on nitrides or carbides of transition metals may protect t he steel surface from erosion and soldering of aluminium and improve the re sistance against thermal cracking. Thus, they may replace the thick oxide-b ased die coatings nowadays used in foundries. Within this study, results ob tained on the performance of coatings deposited onto die-casting dies by ma gnetron sputtering and plasma-assisted chemical vapour deposition are prese nted and discussed. TiN, Ti(C,N), Ti(B,N), and (Ti,Al)(C,N) coatings deposi ted onto hot-working tool steel have been evaluated in practical die-castin g service. Results obtained are compared with those of simulation tests usi ng thermal cycling immersion tests in liquid aluminium, thermo-gravimetric analysis, and stress measurements during thermal cycling. Maximum lifetime of the coated casting die is achieved for careful optimization of hardness, adhesion, oxidation resistance and high temperature stability, and interna l stresses. (C) 2000 Elsevier Science S.A. All rights reserved.