**4. Degradation mechanisms**

During service, several kinetic processes occur in parallel. Interdiffusion occurs between the bond coat and the superalloy. Consequently, Al diffuses from the bond coat to form the TGO. Microstructural, chemical, and phase changes occur in all the materials including the ceramic top coat. The rates of these thermally activated processes are expected to increase exponentially with temperature. These processes generally lead to degradation and failure of the coating [5]. During service, failure of the TBC system occurs depending upon the following three factors:

### **4.1. Bond coat degradation**

conductivity and lower processing costs [3]. Recently, various processing techniques have been

The objective of this article is to present an overview of the TBC requirement, application of TBCs, degradation mechanisms for TBCs, different processing techniques used for preparation of TBCs, and their thermal properties. Recent developments in TBC material have been also described. The prospect of innovative materials as bond coat in a TBC system has been

TBCs must have low weight and low thermal conductivity and they should withstand large stress variations due to heating and cooling, as well as thermal shock. They must be chemically compatible with the underlying metal and the thermally grown oxide (TGO) and should operate in an oxidizing environment. TBCs must provide thermal insulation to the underlying superalloy engine parts. They must have strain compliance in order to minimize the thermal expansion mismatch stresses with the superalloy parts. Additionally, they must reflect much of the radiant heat from the hot gas and thereby, preventing it from reaching the superalloy substrate. Further, TBCs must provide thermal protection to the substrate for prolonged

TBCs provide thermal insulation to superalloy engine parts such as the combustor, rotating blades, stationary guide vanes, blade outer air-seals, shrouds in the high-pressure section behind the combustor, and afterburners in the tail section of jet engines. Significant gastemperature increase can be achieved by using TBCs in association with innovative air-cooling approaches than that obtained by earlier materials including single-crystal Ni-based superal‐

During service, several kinetic processes occur in parallel. Interdiffusion occurs between the bond coat and the superalloy. Consequently, Al diffuses from the bond coat to form the TGO. Microstructural, chemical, and phase changes occur in all the materials including the ceramic top coat. The rates of these thermally activated processes are expected to increase exponentially with temperature. These processes generally lead to degradation and failure of the coating [5]. During service, failure of the TBC system occurs depending upon the following three factors:

developed to deposit the ceramic coatings.

**2. Main requirements for TBCs**

service times and thermal cycles without failure [4].

**3. Applications of TBCs**

**4. Degradation mechanisms**

loys [4].

elucidated.

112 Advanced Ceramic Processing

Bond coat plays an important role in promoting the durability of the TBC system. But the role of the bond coat is very complex and poorly understood. In most practical cases, oxidation of the bond coat becomes the predominant coating failure mechanism. During high temperature exposure, NiCrAlY bond coat is oxidized resulting in a TGO layer on the bond coat. After reaching a critical thickness, the TGO becomes prone to spallation, which in turn results to the failure of the TBC system. It is very difficult to establish the exact mechanisms of bond coatinduced TBC failure for various coating types. However, all researchers agree on the signifi‐ cance of spallation or cracking of the TGO for the failure of air plasma-sprayed TBC system. Detailed research is being conducted to find out a solution to the problem regarding bond coat degradation in the TBC system [5]. Nanostructured NiCrAlY bond coat may improve the life expectancy of thermal barrier coatings. Daroonparvar et al. [6] investigated the microstructural evolution of TGO layer on the conventional and nanostructured atmospheric plasma sprayed (APS) NiCrAlY coatings in TBC systems during oxidation. It was observed that the growth of Ni(Cr,Al)2O4 (as spinel) and NiO on the surface of the Al2O3 layer (as pure TGO) in nano TBC systems was much lower compared to that of normal TBC systems during thermal exposure at 1,150°C. These two oxides play a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. This microstructure optimization of the TGO layer is primarily associated with the formation of a continuous, dense, uniform Al2O3 layer over the nanostructured NiCrAlY coating [6]. The interfacial failure mechanism of the TBC system was numerically investigated by Xu et al. [7], considering the role of mixed oxides (MO), which was induced by the discontinuous *α*-Al2O3 at the top coat-bond coat interface. High growth rate of MO will stimulate the initiation and propagation of interface cracks resulting in debonding of the top coat. The high coverage ratio of MO at the interface will accelerate the propagation of an interface crack. Therefore, the durability and performance of TBCs can be improved by suppressing the formation of MO [7]. The prediction for spallation of thermal barrier coatings has proven to be an intricate problem [8]. The spallation usually occurs through buckling that is driven by strain energy release within the ceramic top coat. If the delamination interface is at the bond-coat/TGO interface, then spallation occurs within the underlying TGO layer. Prior to spallation, substantial sub-critical damage must develop at one or both of the TGO interfaces. Evans [8] stated that the strain energy within the TGO produced during cooling contributes significantly to this damage development and not that within the top coat. Critical strain energy within the TGO layer is assumed to be a possible pragmatic method of predicting the spallation. Several factors such as phase changes in the bond coat, mechanical constraint imposed by the top coat on the mechanical stability of the bond coat interface, TGO growth on a non-planar interface on stress development, and localized Al depletion in nucleating fast-growing non-protective TGO influence the TBC failure [8].

#### **4.2. Generation of high residual stress**

Residual stress has a vital effect on the performance of a TBC system. The role of residual stress is very complex and varies with the difference in system configurations. Thermal expansion mismatch between the three layers generates residual stress resulting in degradation of a TBC system. Although extensive research has been initiated to study the effect of residual stress on TBC life, there is still ample scope to carry out this study on novel TBC systems involving novel compositions [9]. High residual stresses are induced in the TBC due to thermal expansion mismatch and bond coat (BC) oxidation leading to failure by spalling and delamination. An analysis of the stress distributions in TBC systems, which is a prerequisite for the understand‐ ing of failure mechanisms, was performed by Sfar et al. [10] using the finite element method (FEM). Cracks in the interface region were considered in the FE models in order to determine the loading conditions for their propagation and thus, the failure criteria of the TBCs as cracking usually occurs at or near the interfaces between BC/TGO and TBC/TGO depending on the processing mode of the TBC. The modified crack closure integral (MCCI) method combined with an FE analysis led to highly accurate energy release rate values. Moreover, this method enables the determination of mode-dependent energy release rates. TBC failure models could be developed and verified using this tool and appropriate crack propagation criteria [10]. Yang et al. [11] investigated the residual stress evolution in air plasma-sprayed yttria-stabilized zirconia (YSZ) TBCs after thermal treatments at 1,150°C. The residual stress in the YSZ layer was measured using Raman spectroscopy and the curvature method. Generally, as-deposited YSZ layer was under compressive stress and subsequently after thermal treatment for 30 h it was under tensile stress partly due to the monoclinic to tetragonal phase transformation in the YSZ layer. Sintering of the YSZ layer occurred with prolonged thermal treatment resulting in the gradual transformation of the residual stress, from tensile to compressive stress. Further, β-NiAl to γ/γ′-Ni3Al phase transformation in the bond coat also plays an important role on the stress development in the top coat [11].
