Structure, mechanical characteristics and thermal stability of high speed physical vapor deposition (Al,Cr)2O3 coatings

Abstract

In recent years (Al,Cr)2O3 coatings manufactured by physical vapor deposition (PVD) gained great interest as alternative to α-Al2O3 coatings deposited by chemical vapor deposition (CVD), due to their potential to form corundum-type structure at lower process temperatures, T ≤ 700 °C. In this context, also metastable tetragonal (Al,Cr)2O3 solid solution and semi-crystalline α-(Al,Cr)2O3 coatings were developed by high speed (HS)-PVD technology. The chemical analysis of the coatings in this paper confirmed, that the deposition of semi-crystalline α-(Al,Cr)2O3 coatings with high aluminum content, xAl ≥ 70 At.%, is possible at moderate substrate temperature, Tsub = 570 °C. With regard to the application on cutting and molding tools, the thermal stability of the developed coatings is of great interest. Therefore, the thermal stability of the deposited tetragonal and α-(Al,Cr)2O3 phases was investigated by annealing tests in vacuum up to T = 1300 °C. X-ray diffraction (XRD) measurements after the annealing tests in vacuum revealed a phase stability of semi-crystalline α-(Al,Cr)2O3 and metastable tetragonal (Al,Cr)2O3 coating up to T = 600 °C and T = 800 °C, respectively. Diffusion processes during annealing at high temperatures in vacuum lead to significant structural changes, showing detrimental effects on the mechanical properties of the coatings. Due to thermal induced structural, chemical and mechanical changes, the evaluations confirmed a thermal stability of the (Al,Cr)2O3 coatings up to a temperature of T = 800 °C in vacuum.