Thermal stability of γ-Al 2O 3 coatings for challenging cutting operations

Abstract

Crystalline PVD Al 2O 3 coatings offer great potential for their use in cutting operations. They promise high hot hardness and high oxidation resistance at elevated temperatures. Alumina exists in different crystallographic phases. α-Al 2O 3 appears to be the only thermodynamically stable phase at all common temperatures and pressures. Today there are many efforts to generate α-Al 2O 3 by means of physical vapour deposition. In this regard one problem is the high deposition temperature, which does not allow the deposition on temperature-sensitive materials. Another promising candidate is γ-Al 2O 3 which is more fine-grained than α-Al 2O 3 and can be deposited at lower temperatures. At high temperatures γ-Al 2O 3 might be transformed into α-Al 2O 3, which could limit the application temperature. But until now it is not clearly proved, up to which temperatures γ-Al 2O 3 thin films are stable and which mechanisms influence the stability. In the present work different (Ti,Al)N/γ-Al 2O 3 coatings are deposited on cemented carbides by means of Magnetron Sputter Ion Plating (MSIP). The (Ti,Al)N bond coat was employed to improve adhesion of γ-Al 2O 3 on the substrate. It could be shown that the γ-phase is stable in vacuum up to 1200 °C. In the atmosphere the formation of α-Al 2O 3 begins at 900 °C and it is influenced by the choice of transition zone between the (Ti,Al)N interlayer and γ-Al 2O 3. The results show that the thermal stability of the γ-phase and therefore the application temperature of the coating can be enhanced by the choice of interlayer.