Abradable thermal barrier coatings (TBCs) play a crucial role as thermal insulators and dynamic seals in turbine engines. However, the conventional choice, yttria-stabilised zirconia (YSZ), exhibits vulnerabilities, such as sintering induced stiffening, phase transformations, and susceptibility to molten siliceous debris. This study investigates the potential of magnesia-alumina spinel (MAS) as an alternative top coat material for TBCs due to its superior resistance to sintering and remarkable high temperature stability.

The study comprises an extensive examination of the thermal evolution in MAS, encompassing stiffness, density, microstructure, and phase stability, in benign environments. Comparative analysis with YSZ showed that MAS demonstrates enhanced resistance against stiffening. This stiffening phenomenon primarily resulted from microcrack closure during initial thermal exposure, with subsequent dominance of density driven stiffening. The gradual formation of alumina phase was observed within MAS, but had minimal impact on mechanical properties due to its gradual development.

In situ characterisation of MAS during initial thermal exposure revealed an early initiation of stiffening at low temperatures, leading to a twofold increase in Young’s modulus at 1200°C. An intermediate temperature effect (520-900°C) was observed, characterised by a sudden increase in thermal expansion and the release of energy, accompanied by shifting oxygen positions indicative of structural changes.

Furthermore, the interaction between YSZ, MAS, and gadolinium zirconate (GZO) with molten debris, specifically calcia-magnesia-alumina-silica (CMAS) mixtures, was explored. YSZ exhibited significant deterioration in the presence of CMAS, with solution-reprecipitation leading to the occurrence of the tetragonal-to-monoclinic phase transformation and a rise in stiffness. In contrast, MAS and GZO displayed excellent chemical resistance to CMAS. In MAS this was attributed to the formation of a protective anorthite layer and the chemical similarities between CMAS and MAS. The CMAS-GZO interaction results in an apatite-fluorite reaction layer, limiting CMAS infiltration.

Despite their merits, both MAS and GZO experience substantial stiffening during thermal exposures. In MAS, CMAS induced stiffening occurred due to the formation of a stiff CMAS layer, limiting compliance. Conversely, GZO exhibited limited CMAS induced stiffening but suffered from intrinsic stiffening.

In conclusion, MAS demonstrates improved performance over YSZ in benign and CMAS rich environments. However, addressing intrinsic stiffening in the MAS system and further optimising CMAS resistance are essential to mitigate further against in service spallation.