Development of thermophysical properties of Gd2Zr2O7 ceramics for thermal barrier coating applications

Thermal Barrier Coatings (TBCs) are widely utilized in gas turbines to protect components exposed to high-temperature gas flows, enhance system durability, and improve fuel efficiency. These coating systems typically consist of a metallic bond coat applied onto a superalloy substrate, topped with a ceramic top coat. Today, the most common and effective TBC material is zirconia (ZrO2) stabilized with 8 wt.% yttria (Y2O3), known as 8YSZ. These ceramic coatings, which are highly resistant to elevated temperatures, can function effectively up to approximately 1200 °C and provide critical protection in engine and turbine systems thanks to their low thermal conductivity and high thermal shock resistance. Zirconia exists in a monoclinic crystal structure at room temperature; as the temperature rises, it transforms to a tetragonal phase at 1170 oC and to a cubic phase above 2370 oC. These phase transitions involve a volumetric change of about 3–5%, which compromises the integrity of the coating and significantly reduces its service life. Therefore, the development of next-generation materials with superior thermal, mechanical, and chemical properties is essential to replace current coatings. Such materials can extend the lifespan of turbine components, reduce maintenance intervals, and improve overall system efficiency while ensuring safer operation under extreme thermal conditions. In line with this goal, Gd2Zr2O7 (GZO) ceramics have emerged as promising candidates due to their higher thermal stability, lower thermal conductivity, and desirable thermal expansion properties. These ceramics are considered a potential alternative to YSZ in thermal barrier coating systems The use of Gd2Zr2O7 (GZO)-based ceramics in thermal barrier coatings (TBCs) has been increasingly widespread due to their advantageous properties. However, limited fracture toughness, dependence on rare-earth elements, and high production costs pose certain challenges to the sustainability of these materials. In this research, eight samples of GZO ceramics were produced by doping with various ratios of non-rare-earth elements such as Fe, Mo, and Ti, along with Yb, and the effects of these dopants on the mechanical and thermophysical properties were investigated. The undoped GZO ceramic exhibited a defective fluorite (F) phase, while the doped GZO ceramics formed in the pyrochlore (P) phase. At 900 oC, the thermal conductivity of undoped GZO was measured as 2,21 W.m-1.K-1, whereas in the doped ceramics, it decreased to as low as 0,89 W.m-1.K-1. At 1150 oC, the thermal expansion coefficient of undoped GZO was determined to be 11,48 × 10-6.K-1, while in the doped GZO ceramics, these values ranged from 10,39 × 10-6.K-1 to 11,88 × 10-6.K-1. In terms of mechanical properties, the hardness of undoped GZO was measured as 10,88 GPa, while the doped ceramics exhibited hardness values ranging from 8,64 GPa to 10,99 GPa. The elastic modulus of the undoped GZO was 307 GPa, whereas the doped samples showed values between 302 GPa and 343 GPa. The fracture toughness of the undoped GZO was calculated as 1,36 MPa.m1/2, while for the doped ceramics, it ranged from 0,92 MPa.m1/2 to 1,64 MPa.m1/2. Based on these findings, it can be concluded that the doped GZO ceramics possess potential for TBC applications. Furthermore, reducing the reliance on rare-earth elements in these ceramics may contribute to the development of more sustainable and environmentally friendly coating materials.