Failure of the plasma-sprayed coating of lanthanum hexaluminate

Lanthanum magnesium hexaluminate (LaMgAl11O19, LMA) is an attractive material for thermal barrier coatings (TBCs), and the failure of its coating was studied in this work by thermal cycling, X-ray diffraction, dilatometric measurement and thermal gravimetric-differential thermal analysis. The dilatometric measurement indicates that even though the bulk material of LMA has a higher sintering-resistance than the typical TBC material, i.e. yttria-stabilized zirconia (YSZ), the plasma sprayed coating of LMA has two serious contractions due to the re-crystallization of LMA and phase transitions of alumina. LMA has similar thermal expansion behaviour with alumina, leading to a good thermal expansion match between LMA and the thermally grown oxide layer. On the other hand, the plate-like structure of LMA not only results in a low thermal conductivity, low Youngʹs modulus, but also a high stress tolerance, and these are believed to be the reasons for the long thermal cycling life of LMA coating.