Oxidation and Hot Corrosion Behaviour of Dual Phase 8YSZ + La2Zr2O7 and 8YSZ + Gd2Zr2O7 TBC Systems

Abstract

The article presents research results on thermal barrier coatings based on dual-phase systems of the type 8YSZ + Ln₂Zr₂O₇. Previous studies have shown that composite materials of this type are characterized by an increased tolerance to deformation and a significantly lower thermal conductivity coefficient value than single-phase coatings. Therefore, two-phase composite coatings are expected to demonstrate the ability to operate for a more extended time without compromising their integrity and without ceramic layer spallation. The scope of the presented analysis included assessing the phase constituent stability under oxidation conditions at 1100 °C for 2000 hours and hot corrosion in Na₂SO₄ at 950 °C for 300 hours. The XRD analyses showed that in the 8YSZ + La₂Zr₂O₇ system, there is no significant interaction between components of the TBC under analyzed conditions. In the case of the system containing Gd₂Zr₂O₇, an intensive tendency to mutual interactions was demonstrated, expressed by the formation of non-stoichiometric Gd_xZr_1−xO_2−x/2 with a fluorite-type lattice. The intensity of this process was much higher with the assistance of Na₂SO₄. The XRD test results were confirmed by SEM/EDS analyses, which showed the presence of areas significantly different in chemical composition and morphology in the case of the 8YSZ + Gd₂Zr₂O7 coatings and relatively small morphological changes in the case of the 8YSZ + La₂Zr₂O₇ system. These analyses were verified during tests on model powder mixtures. The model tests covered the temperature range from 800 to 1400 °C (24 h at each temperature). The research conducted confirmed the previously obtained results. The research suggests that two-phase coatings 8YSZ + Ln₂Zr₂O₇ may be a very effective solution for applications in gas turbines. Despite intense phase changes, they showed high durability (at least 2000 hours), practically without the effects of the ceramic layer’s disintegration. However, the desirable or undesirable nature of the observed phenomena (order-disorder transition, formation of chromites) requires further analysis.