On the Development of a Protective Oxide System in Rare Earth Oxide Coated Nickel Superalloy under Isothermal Oxidation Conditions

摘要

Isothermal oxidation experiments at 1100℃ in air were carried out to evaluate the protective capability of a new rare earth oxide coating realized by electrodeposition onto a Ni-base single crystal superalloy. A subsequent heat treatment of the RE_xO_y coating already allowed the establishment of a very thin and discontinuous inwardly grown alumina scale. Under isothermal conditions at 1100℃ in air a fully parabolic regime installed from 25h leading to parabolic rate constants of 2.5 10~(-7) mg~ .cm~(-4) .s~(-1) after 200h, similar to those of conventional β-NiAl coatings. The initial, transition and parabolic regimes were ascribed to the major development of NiAl_2O_4/Al_2O_3 mixed oxides by in situ high temperature X-ray diffraction (HT-XRD). No major transient alumina was observed. The a-Al_2O_3 scale intensity increased with increasing oxidation time, in particular with respect the rare earth oxide coating signal. The scanning electron microscopy (SEM) images showed an oxide system consisting on a top NiAl_2O_4 oxide and a bottom α-Al_2O_3 scale underneath the RE_xO_y coating. Alumina grew within the substrate surface. After 500 and 1000h of oxidation, very scarce nodules grew between the alumina and the rare earth oxide deposit. Despite the thermodynamic calculations suggested a REAlOy perovskite at the alumina-RE_xO_y interface, this was not observed experimentally either by XRD or scanning electron microscopy (SEM).