Comparison of in-situ oxide formation and post-deposition high temperature oxidation of Ni-aluminides synthesized by cathodic arc evaporation

Abstract

Thin coatings were synthesized by cathodic arc evaporation of powder metallurgically fabricated Al-Ni targets with the chemical compositions of Al75Ni25, Al67Ni33 and Al52Ni48 atomic percent. The coatings were produced both either from pure metallic vapour or in reactive oxygen atmosphere. Phase transformations and chemical composition at the target surface were investigated by X-ray diffraction (XRD) and by energy-dispersive X-ray spectroscopy (EDS) techniques and compared with the phases composition obtained in the coating. The deposition in non-reactive mode produces intermetallic coatings containing different Ni aluminides. The formation of the aluminides can be controlled by the target composition and is by trend predictable from the Al-Ni phase diagram. The coatings produced in reactive mode are composed of both Al3Ni and Al3Ni2 with the additional formation of AlNi and Al2NiO4 in all the coatings. The coatings composed of intermetallics show high indentation hardness in the range of 10 GPA while the coatings composed of both oxides and intermetallics demonstrate exceptional high hardness of about 30GPa. All coatings have been annealed in ambient air up to 1200°C and were investigated by in-situ XRD analysis in order to follow in detail their oxidation process. For all coatings, the formation of α-Al2O3 and Al2NiO4 could be observed after annealing forming a thin high temperature stable protective layer for remaining AlNi. The coatings deposited in reactive atmosphere contain additional NiO, independently of their original composition.