Vapor-Phase-Mediated Phenomena Associated with High Temperature, High Water Content Oxidation of MCrAlX Bond Coats

Oxidation of MCrAlX (M = Ni and Co; X = Y or Re) bond coats was carried out at 1,125 A degrees C in a range of N-2-O-2-H2O environments. A three-step process of (1) oxidation, (2) taper-polishing, and (3) re-oxidation was used to evaluate steady state development of thermally grown oxide (TGO). During initial oxidation, transient (Ni,Co)(Al,Cr)(2)O-4 spinel formed above alpha-Al2O3. Following taper-polishing, no new spinel grew during 1-200 h of re-oxidation in any water vapor environment; spinel growth at the TGO surface by a steady state mechanism-owing to Al-depletion of the bond coat, as predicted elsewhere-was deemed unlikely. Observations of transient spinel volatilizing in wet environments were supported by measurements of nickel volatilizing from pre-fabricated NiAl2O4 spinel pellets as a function of humidity. In some cases, following volatilization, water promoted vapor phase-redeposition of spinel onto adjacent specimen surfaces. Spinel-related conclusions from past humid oxidation experiments for which volatilization phenomena were not considered-and especially for which Al-depletion of the bond coat is cited as the cause for spinel growth-should be reevaluated.