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

In recent years (Al,Cr)(2)O-3 coatings manufactured by physical vapor deposition (PVD) gained great interest as alternative to alpha-Al2O3 coatings deposited by chemical vapor deposition (CVD), due to their potential to form corundum-type structure at lower process temperatures, T <= 700 degrees C. In this context, also metastable tetragonal (Al,Cr)(2)O-3 solid solution and semi-crystalline alpha-(Al,Cr)(2)O-3 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 alpha-(Al,Cr)(2)O-3 coatings with high aluminum content, x(Al) >= 70 At.%, is possible at moderate substrate temperature, T-sub = 570 degrees 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 alpha-(Al,Cr)(2)O-3 phases was investigated by annealing tests in vacuum up to T = 1300 degrees C. X-ray diffraction (XRD) measurements after the annealing tests in vacuum revealed a phase stability of semi-crystalline alpha-(Al,Cr)(2)O-3 and metastable tetragonal (Al,Cr)(2)O-3 coating up to T = 600 degrees C and T = 800 degrees 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)(2)O-3 coatings up to a temperature of T = 800 degrees C in vacuum.