Mechanical and Thermo-physical Properties of Plasma-Sprayed Thermal Barrier Coatings: A Literature Survey
Atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) have been studied from an extensive review of the dedicated literature. A large number of data have been collected and compared, versus deposition parameters and/or measurement methods, and a comparison was made between two different microstructures: standard APS coatings and segmented coatings. Discussion is focused on the large scattering of results reported in the literature even for a given fabrication procedure. This scattering strongly depends on the methods of measurement as expected, but also—for a given method—on the specific conditions implemented for the considered experimental investigation. Despite the important scattering, general trends for the correlation of properties to microstructure and process parameters can be derived. The failure modes of TBC systems were approached through the evolution of cracking and spalling at various life fractions.
KeywordsAir plasma sprayed (APS) Thermal barrier coatings (TBC) Mechanical properties Cracking
Atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) are widely used in hot sections of gas turbine engines as thermal insulation systems. The current state of the art for manufacturing APS TBCs consists in a bilayer system composed of a 150-µm-thick MCrAlY bond coat and a 250- to 500-µm-thick ceramic top coat of ZrO2 stabilized with 7–8 wt% Y2O3, labeled YSZ. However, following the cumulative cooling related to successive thermo-mechanical cycles imposed to the engine, the thermal expansion misfit between the metallic substrate and the ceramic layer results in thermo-mechanical fatigue leading to a progressive and irreversible damage of the deposit. The failure of plasma-sprayed TBCs upon thermal cycling is usually induced by the spallation of ceramic coating. Consequently, understanding such spalling phenomena and predicting the life of the thermal barriers are major issues for engine makers willing to build and properly implement a relevant model of life to be used for design purpose. This, previously, requires a precise knowledge of the experimental background in the field, and the present work proposes a comprehensive synthesis based on an extensive review of the dedicated literature. A large number of data have been collected and compared, using deposition parameters and/or measurement methods and/or characteristics of cycling as objective criteria of selection. Discussion is focused on the large scattering of results reported in the literature even for a given and fixed fabrication procedure. This scattering strongly depends on the methods of measurement as expected, but also—for a given method—on the specific conditions implemented for the considered experimental investigation.
Process and Coatings Microstructure
Plasma spraying is intended to manufacture thermal barriers with a thickness ranging from 250 μm to 1 mm mainly on static parts as combustion chambers or outer air seals (segments or rings) in aircraft engines and on blades and vanes in land-based engines. It is well known that the thermo-physical and mechanical properties of plasma-sprayed thermal barrier coatings strongly depend on the microstructure, and therefore on process parameters.
On the other hand, our research is focused on dense, vertically segmented or cracked (DVC) microstructure, characteristic of hot spraying and thick coating (1 mm) [3, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. The most important difference with the standard APS coating is the global columnar structure (Fig. 1c, d) . Indeed, vertical cracks run through the coating, parallel to the direction of heat flow, forming segments on cross sections, and columns in 3D. Within the coating, between those macro-cracks, and as a consequence of the specific implemented process parameters, the porosity is smaller than in APS coatings, resulting in a higher thermal conductivity, from 1.2 to 1.6 W/mK, close to values of an electron beam physical vapor deposition (EBPVD) coating. Unlike standard APS deposit, segmented coatings show good mechanical compliance and provide improved tolerance of the ceramic layer to the strain .
Properties of APS TBC
In industry, Young’s modulus is one of the standard, most commonly used parameter entering the global description of the mechanical behavior of TBC in the elasticity domain. It determines the response of the coating to a tensile or compressive loading and is subject to special attention. Recent developments [22, 23, 24, 25] have shown that ceramic coatings exhibit an anisotropy of elasticity between tension and compression, and hysteresis phenomena in the stress–strain curves relative to their mechanical responses. Cracks opening/closing as well as sliding of splats with respect to each other results in a non-linear response and allows a high tolerance for deformation, as stress is easily accommodated. Resulting differences can be observed depending on the type (tension, compression), the magnitude, and the level of solicitation (macro/micro) of the applied load.
The large scattering can be attributed to variations in methods of measurement and—for a given method—to the considered experimental investigation such as the residual stress, the anisotropy of the porosity (normal/parallel to the spray direction), and the initial size of powders.
From a literature survey, general trends regarding the correlation between the engineering properties and both the microstructure and the process parameters can be highlighted.
In addition, in gas turbines, and for long exposure times to high temperatures, changes in stiffness during operation can be really significant and critical. A continuous increase in stiffness was observed over 1000 °C by many researchers [35, 41, 42]. The stiffening underlying mechanisms are related to sintering and the associated microstructural changes upon temperature exposure. For lamellar structure exposed at high temperature (Fig. 3b), sintering occurs and leads to an increase by a factor 5 for modulus values. The sintering speed and rate are functions of the temperature and the initial porosity into the coating.
DVC zirconia has a different behavior: sintering effect is limited (Fig. 3a), due to plasma spraying in hot conditions conferring a nearly non-lamellar microstructure and stability of macro-cracks versus temperature. However, Thompson et al. [35, 36] showed that the behavior of thermal barriers tested in free-standing conditions and those tested as a complete system is different: the presence of the substrate significantly reduces the rate of stiffening of ceramics. It is concluded that, because of the large thermal expansion coefficient of the metal substrate and bond coat, micro-cracks are kept open at elevated temperatures. Thus, it becomes clear that the sintering kinetics is a function not only of the holding temperature and the microstructure of the coating, but also of the thermal stress.
Main thermo-physical and mechanical properties of ZrO2–8% Y2O3
Plasma thermal barrier coating
Vertically cracked plasma thermal barrier coating
Coefficient of thermal expansion (10−6 K−1)
(domain 20–1000 °C)
Thermal conductivity (W/mK)
(domain 20–1000 °C)
Young’s modulus (GPa)
10 in bending test
13 in tension
25 in compression
37 by dynamic measurement
60–150 by indentation test
2–4 in tension
4–8 in compression
15–25 by dynamic measurement
>80 by indentation test
205 by dynamic measurement (Impulse-Excitation-Technique)
13 in tension
33 in bending test
300 in compression
7 in tension
15 in bending test
350 in tension
1000 in bending test
2000 in compression
0.04–0.18 in compression
0.3 attached to substrate
0.034 in compression
0.3 attached to substrate
Fracture toughness (MPa√m)
1–1.2 by SEVNB 1.8–2.2 by indentation test and DCB
0.7 in SEVNBa
3–4 in DCB 4–5 by indentation test
>10 (>1350 HV)
Residual stresses (MPa)
Failure Modes and Lifetime
The failure modes of a thermal barrier coating systems are complex, usually involving various mechanisms, and strongly depend on service conditions. However, for standard APS coatings, ruin of the system occurs mainly by spallation of the ceramic layer during cooling. In this section, only standard APS coatings are concerned, because of the lack of data concerning cracking in DVC TBCs.
To address TBC failure, it is of utmost importance to determine a criterion able to define as precisely and objectively as possible the end of life of the system. However, depending on the authors, the criterion defining the end of life of a sample can greatly vary: a given cumulative crack length, a percentage of spalled area ranging from 10 to 100%, or a subjective assessment defined as “clear deterioration”. The definition of the ruin of the system is particularly critical when data from different publications have to be compared. In order to rationalize our procedure, a failure criterion related to a surface fraction of spall of 100% has been considered and only data from papers using this approach or data derived in such a way are analyzed. This criterion is not at variance with industrial practice, because it is not valid for an actual component, where the curvature of the surface and inhomogeneous temperature distributions may plan a significant role. Nevertheless, this procedure allows to highlight and understand the mechanisms involved in the TBC spallation in laboratory conditions on simple shape samples.
Two microstructures of ZrO2–8% Y2O3 coatings obtained by APS were considered: the standard porous lamellar microstructure and the dense vertically segmented microstructure, both with MCrAlY bond coats. A thorough synthesis of their thermo-physical and mechanical properties has been made, comparing a large amount of data. Despite a wide scattering in the results, and considering the anisotropy of behavior due to the microstructure, general trends have been observed. Due to process parameters, the intrinsic properties of DVC coatings are closer to the dense zirconia than the standard coatings, but the global behavior is weaker. The review of standard APS TBC lifetime has shown that cracking, resulting in the spallation of the ceramic coating, follows similar evolution, regardless of the considered TBC systems. However, no similar data are available for segmented coatings in order to establish a comparison of cracking. Further experiments are being carried out to confirm and complete these results.
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