The Conductivity of Thermal Barrier Coating in Ceramics

Thermal barrier coating or TBC is made of low thermal conductivity ceramics have seen increasing application in gas turbine engines to provide thermal insulation to metallic components from hot gas in the engines used for aircraft propulsion, power generation, and marine propulsion [1–5]. The use of TBCs, along with internal cooling of the underlying superalloy components, provides a temperature reduction of up to 300 K in the surface of the superalloy component. This enables engines to operate at temperatures above the melting temperature of the superalloy, thereby improving the energy efficiency and performance of engines. On the other hand, TBCs contribute to reducing metal temperature, thus improving the duration capability of components.

The structure of TBCs

Usually, a thermal barrier coating has a four-layered structure: the ceramic thermal barrier layer, the metallic bond coat layer, a thermally grown oxide (TGO) layer between the topcoat and bond coat, and substrate. Each layer has its own specific physical and chemical properties, which provide the required functions in TBC.

The ceramic thermal barrier layer provides thermal protection to the underlying materials. Also, this layer works as a shield to protect the underlying metallic parts from erosion and corrosion. The metallic bond coat is to protect the underlying superalloy substrate from oxidation, balance thermal mismatch between the topcoat and substrate, and prevent interdiffusion of elements in the substrate and bond coat.

The operating temperature for the bond coat often exceeds 1273 K. Due to oxidation of bond coat, a TGO layer is inevitably formed. The phase constituents of the TGO depend on the operating temperature, the thermal exposure time and the composition of the bond coat. In the initial stage, accompanying outward diffusion of elements from the bond coat and inward diffusion of oxygen from hot gas, the formation of TGO is dominated by a combined mechanism with external oxidation and internal oxidation.

The phases in TGO

The TGO mainly consists of spinel phases, such as NiAlO2 and NiCrO2, which are, however, thermodynamically unstable and detrimental to oxidation resistance [6,7]. Subsequently, the growth of TGO is mainly governed by internal oxidation. As a result of selective oxidation, an α-Al2O3 is formed, replacing those spinel phases. In some cases, new spinel phases appear again after long-term thermal exposure when a continuous α-Al2O3 layer could not be maintained due to Al depletion in the bond coat. A continuous, defect-free TGO comprising α-Al2O3 is desirable to TBC, because α-Al2O3 has a very low oxygen ionic diffusivity and provides an excellent diffusion barrier, retarding further oxidation of the bond coat [8]. Additionally, α-Al2O3 is chemically stable, contributing to improved bonding between ceramic topcoat and bond coat.

The substrate materials in a thermalbarrier coating system are usually Ni- or Co-based superalloys, which include traditional polycrystalline, directionally solidified and single-crystal (SC) superalloys. At high operating temperatures, interdiffusion between substrate and bond coat occurs. The interdiffusion has a profound effect on the mechanical properties of the substrate and thermal cycling lifetime of TBCs [9]. For new generation SC alloys, additions of high concentrations of refractory elements can ensure superior high temperature capability; however, they leave the superalloys prone to microstructure instability. A well-known topologically close-packed (TCP) phase and a so-called secondary reaction zone (SRZ) are formed due to the interdiffusion, and have been found to degrade the mechanical properties of superalloys [10,11]. It then makes sense to consider the substrate and the TBC as an integrated system when designing a TBC for a specific superalloy substrate.