Phase decompositions of Gd2Zr2O7+8YSZ TBC systems under the condition of long-term high-temperature oxidation

The article presents the results of research related to the assessment of the phase composition of two-phase TBC systems based on gadolinium zirconate GZO and a conventional zirconium oxide modified by the yttrium oxide 8YSZ. The coatings were made on a nickel superalloy of IN 625 type with a NiCrAlY base bond coat. The technique of atmospheric plasma spray was used for coatings depositions. Three types of composite coatings, made of a mixture of base powders with the proportion of components 25-75, 50-50 and 75-25, were evaluated. The thickness of the ceramic layers was approximately 300 mu m. The TBC systems were then subjected to high temperature oxidation at 1100 degrees C for up to 2000 h in air. Changes in phase composition were evaluated by the XRD method after 48, 175, 500, 1000 and 2000 h of exposure. Detailed microstructural studies with the use of scanning microscopy methods were performed after 2000 h. The GZO + 8YSZ composite coatings were found to be characterized by high susceptibility to changes in phase composition, which is related to the formation of new zirconate phases with a lower content of gadolinium oxide (in the case of zirconates) and phases based on zir-conium oxide with a higher content of gadolinium oxide. The observed changes are diffusive and take place predominantly within the GZO phase. Additional structural effects observed during SEM studies are the phe-nomena of the formation of a network of numerous small voids in the GZO phase, which suggests the course of nonequilibrium diffusion processes. Furthermore, a high concentration of nickel oxide and a slightly lower concentration of chromium oxide were found in the ceramic layer.

Automated summary

The article presents the results of research on the phase composition of two-phase TBC systems using GZO and modified 8YSZ. Composite coatings with different powder proportions were evaluated, and high temperature oxidation was performed to assess changes in phase composition. It was found that GZO + 8YSZ coatings were highly susceptible to phase composition changes, resulting in the formation of new zirconate and zirconium oxide phases. The ceramic layer also exhibited a high concentration of nickel oxide and a slightly lower concentration of chromium oxide.