Reactivity and microstructure evolution of a CoNiCrAlY/Talc cermet prepared by Spark Plasma Sintering

A mixture of CoNiCrAlY and talc powders is considered as a new candidate composition for abradable seal coating applications. Dense specimen having the composition of 1:20 weight ratio of talc with respect to CoNiCrAlY was prepared using the Spark Plasma Sintering (SPS) technique. The aim of the present article is to investigate the reactivity and microstructure evolution of the β/γ-CoNiCrAlY based cermet. The resulting microstructures were analysed and their compositions determined using standard analytical techniques such as SEM, TEM and X-ray diffraction. After fabrication, the bulk of the material is shown to contain a continuous oxide layer of MgAl2O4 at the periphery of metallic particles, resulting from the reaction between aluminium, which has diffused from the bulk of CoNiCrAlY grains, with magnesium and oxygen delivered during the high temperature decomposition of the talc phase. Thermodynamic calculations results are found to be consistent with the experimental observations. The oxidation behaviour at a high temperature of this cermet was also investigated. It was shown that at its external surface a continuous double layer is formed — one external film at the surface of the sample made of MgAl2O4 and the second one more internal in between the later and the cermet made of α-Al2O3. The oxide scale is protective with low oxidation kinetics typical of alpha alumina growth (kP = 1.8 ⋅ 10 − 7mg2 ⋅ cm − 4 ⋅ s − 1 at 1050 °C in flowing dry air).