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

In aluminium die casting, tools are exposed to erosion, corrosion and soldering 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. Hard coatings based on nitrides or carbides of transition metals may protect the steel surface from erosion and soldering of aluminium and improve the resistance against thermal cracking. Thus, they may replace the thick oxide-based die coatings nowadays used in foundries. Within this study, results obtained on the performance of coatings deposited onto die-casting dies by magnetron sputtering and plasma-assisted chemical vapour deposition are presented and discussed. TiN, Ti(C,N), Ti(B,N), and (Ti,Al)(C,N) coatings deposited onto hot-working tool steel have been evaluated in practical die-casting service. Results obtained are compared with those of simulation tests using 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 internal stresses. (C) 2000 Elsevier Science S.A. All rights reserved.