Al2O3 coatings deposited by filtered vacuum arc -: characterization of high temperature properties

Thin Al2O3 coatings can provide oxidation protection on metallic high temperature components. For this purpose Ni-base alloys for gas turbine applications are plasma spray coated with MeCrAlY alloy. At high temperatures this alloy forms a native Al2O3 scale for oxidation protection. In order to increase the efficiency of this oxidation protection a thin artificial Al2O3 layer can be deposited on the metal surface. Such a coating has to be dense with a very low void fraction and with good adhesion to the substrate. A reactive plasma are process was selected to deposit Al2O3 coatings of 0.5 mu m thickness onto IN713LC and onto Si (100). In order to reduce the contamination of the coatings with Al macroparticles two different magnetic filtering methods were investigated. One magnetic particle filter consisted of a 90 degrees torus segment with a series of solenoid coils and internal baffles as particle traps. The other filter was a magnetic island which guided the plasma around a central barrier as particle filter. Results from deposition experiments with both filtering methods were compared with each other. The effect of the substrate temperature during deposition was investigated in the range of 200-800 degrees C. Films deposited below 300 degrees C were amorphous; at 400-750 degrees C gamma-Al2O3 was dominant; alpha-Al2O3 was deposited at 800 degrees C with an applied bias voltage of -300 V. In addition, the stability of the coatings under vacuum heating to 1000 degrees C and under thermal shock conditions by laser healing to 1500 degrees C surface temperature was investigated. It was found, that coatings deposited at substrate temperatures of 600-800 degrees C are stable under heating to 1000 degrees C, but form cracks under laser heating to 1500 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.