**1. Introduction**

Gas-turbine engine, which is known as the heart of aircraft, covers many leading-edge technologies in advanced manufacturing industry, and represents the level of a nation's science and technology. Higher thrust-weight ratio, longer service lifetime and better reliability are regarded as the development trend for the gasturbine engine. Increasing the turbine inlet temperature is an important method for increasing the thrust-weight ratio. Key technologies for increasing the turbine inlet temperature include single crystal superalloy, air cooling and thermal barrier coatings (TBCs) technology. Specially, TBCs can lower the operating temperature of superalloy blade. The temperature reduction caused by TBCs is equal to the sum of temperature rise induced by superalloy over the last 30 years. Besides, TBCs can protect the metallic blade from oxidizing, and improve the service safety and lifetime of the blade [1–3].

Among traditional TBCs, porous ceramics with low thermal conductivity are prepared on superalloy substrate to achieve heat insulation effect. But the mismatch of thermo-mechanical properties between porous ceramics and superalloy leads to the difficulty in reliable bonding. A bond coat is necessary [2]. Thus, TBCs is a typical multilayer, multi-material system, as shown in **Figure 1**. TBCs mainly include (1) the ceramic top coat, (2) the thermally grown oxide (TGO), (3) the bond coat, and (4) superalloy substrate. The ceramic top coat usually adopts porous ceramics to achieve heat insulation effect. The common ceramic composition is the zirconia

**Figure 1.** *The schematic of typical TBCs.*

'partially stabilized' with about 6–8 wt.% yttria (7YSZ), which has the good heatinsulating property and long thermal cycle life. TGO, whose composition is α-Al2O3, is formed by the reaction between aluminum diffusing from the bond coat and exterior oxygen. TGO can provide good bonding of TBC to bond coat. The bond coat contains the source of elements to create TGO in oxidizing environment and provides oxidation protection, primarily of NiCoCrAlY- or NiAlPt-based compositions. The superalloy substrate, which has high strength at high temperatures, can experience complicated mechanical loads during service [1, 2, 4].

Recently, many methods have been developed to prepare the ceramic top coat, such as air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) process [5, 6]. In APS process, the ceramic feedstock are injected into the high temperature plasma plume, heated to the molten or high-plasticity condition, impinge onto the surface of specimen with a certain momentum, and rapidly solidify. Then the lamellar microstructures, which consist of a large number of overlapped splats, are formed, as shown in **Figure 2(a)**. In EB-PVD process, ingots of a ceramic composition are vaporized in a vacuum chamber using a focused electron beam. The ceramic vapor gradually deposit on the specimen and form columnar microstructure, which are perpendicular to the surface of specimen, as shown in **Figure 2(b)**.

TBCs prepared by APS process, which has lower cost, has been widely used in the larger stationary components. However, the failure behavior such as premature spallation of TBCs during preparation and operation process is still an overriding concern. The premature spallation of TBCs may expose the superalloy substrate to hot gases, resulting in the oxidization of superalloy. This may influence the service performance, lifetime and safety of aircraft, even lead to catastrophic damage [7]. The premature spallation of TBCs mainly results from intralayer and interlayer fractures [8]. The fracture behavior of materials is closely related to stress state. When the stress in a material is tensile, and is larger than strength, the fracture behavior may happen. The change of stress state is considered as the driving force

*Stress among the APS-Prepared TBCs: Testing and Analysis DOI: http://dx.doi.org/10.5772/intechopen.85789*

**Figure 2.** *The microstructures of TBCs prepared by: (a) APS and (b) EB-PVD.*

for the initiation, propagation of cracks [9]. Besides, the stress state may influence the macroscopic properties of TBCs [10]. Specially, the microstructures of TBCS prepared by APS process are typical lamellar microstructures. The lamellar microstructures may not totally melt and bond each other. There are a large number of defects such as pores and microcracks between lamellar microstructures [11]. As the defects are sensitive to stress state, the material near the defects tends to fracture under stress, leading to the spallation of coatings. Therefore, the analysis and characterization of stress in TBCs prepared by APS process is necessary for the study of failure mechanism and service reliability.

This chapter reviews the research progress on the analysis and characterization of stress among the APS-prepared TBCs in recent years. The origin, category, measurement technology and characterization method for stress are focused. It is believed this will be helpful for the investigation on failure mechanism, and promote the application and development of TBCs.
