Thermal barrier coatings (TBCs) are multi-material, multi-layer systems used to provide thermal insulation and oxidation resistance to underlying structural components.1
Image credit: KLA Instruments
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Typically, TBCs consist of (1) an intermetallic bonding layer; usually an MCrAlY alloy, where M = Ni, M = Co or both; (2) a top layer of porous ceramics such as yttria-stabilized zirconia (YSZ); and (3) a thermally grown oxide (TGO) layer that during high-temperature operation forms at the interface between the top layer and the bond layer.
To add to this complexity, different microstructural features such as porosity, interlayer interface, cracks, and crack boundaries can also coexist for thermal spray coatings.
In addition, material degradation such as depletion of aluminum in the bond layer, growth of TGO at the interface of the top layer, and densification of the top layer occur with time and temperature because these coatings often withstand extreme environments.2
These degradation mechanisms can operate simultaneously while in operation and can instigate changes in its composition and/or microstructure.3 Figure 1 represents a sectional view of an aircraft engine, revealing the pressure and temperature variation across of the air flow path.
Figure 1. Sectional view of a GE9X4 commercial aircraft engine showing temperature and pressure variation across the airflow path. Image credit: GE Aviation
There is a strong case for applying nanomechanical analysis techniques to the study of TBC, given the complexities inherent in TBC systems and the advantages of emerging materials characterization techniques such as high-speed mechanical property mapping . This, therefore, facilitates the study of the temporal and spatial properties with respect to the different layers of TBC.5
Figure 2. (a) Schematic illustration of the multi-layered and multifunctional nature of the thermal barrier coating system and (b) cross-sectional scanning electron microscopy image of a TBC. Schematic reprinted from MRS Bulletin October 2012. Image credit: KLA Instruments
Experimental method
TBCs in this method were tested using a KLA InstrumentsTM Nano Indenter® that uses NanoBlitz 3D, a high-speed mapping technique where each indentation typically takes less than a second. The time required for approaching the surface, detecting surfaces, loading, unloading and positioning the sample for the next indentation are included in the measurement time.
For the interface regions of the bond layer, top layer, and top bond layer of coated and thermally cycled samples, maps of different sizes (typically containing more than 10,000 indents) were generated.
The hardness and elastic modulus for each indentation were calculated using the standard Oliver-Pharr method after performing an area function calibration on fused silica samples and correcting for load frame compliance.6
Results and discussion
One of the most vital regions of the TBC is the top binding layer interface. The main microstructural variations and their consequent mechanical property variations occur in this region during operation, which ultimately affects the thermal cycling life of the TBC.
The formation of a thermally grown oxide (TGO) layer is the most significant event at the interface of the upper junction layer.7 As a result of a reaction between the incoming oxygen (through the layer upper) with the interdiffused aluminum (from the β-NiAl of the bonding layer) at high temperatures, TGO is formed.
It is now known that having a dense contiguous layer of TGO with a parabolic growth rate prevents further oxidation of both the bonding layer and the underlying substrate. However, this layer causes severe stress incompatibilities and mismatch stresses beyond a certain critical thickness, which causes the TBCs to be progressively damaged and ultimately delaminated8,9.
Therefore, it is extremely important to determine the local elastic modulus in the TGO. However, since the optimal thickness of TGO is typically on the order of several microns, there has been no reliable measurement of local mechanical properties when sandwiched between the bond layer and the top layer.
In the case of the bonding layer, high-speed mapping at the micron length scale demonstrates that it is now possible to measure the mechanical properties of TGO.
Figure 3 shows the microstructure (top row), hardness map (middle row), and elastic modulus map (bottom row) at the top bond layer interface for both the coated and after 5, 10 and 100 thermals. cycles
Figure 3. Cross-sectional SEM micrographs and corresponding maps of hardness and elastic modulus at the bond top layer interface show the growth of TGO for (a, first column) the coated state; (b, second column) after 5 thermal cycles; (c, third column) after 10 thermal cycles; and (d, fourth column) after 100 thermal cycles. Image credit: KLA Instruments
Typical corrugations of plasma spray coatings are shown at the interface. The TGO (dark region at the interface) can be observed in five thermal cycles, and the thickness of the TGO increases with the thermal cycle, as shown by a comparison of the microstructure of the coated and thermally cycled samples.
A similar trend is shown by the corresponding maps of hardness and elastic modulus, in which the TGO is identifiable by the regions of highest hardness and elastic modulus at the interface. An interesting phenomenon to highlight is that a parabolic behavior is shown by the thickness of the TGO measured from the maps, which in the past has been observed in microstructural studies.10,11.
The depletion of the β-NiAl phase, in addition to the growth of TGO with an increasing number of thermal cycles, acts as a source of aluminum on the bonding layer side and can be observed from the microstructure and maps of hardness
One of the main driving forces for the development of misfit stresses is the difference in elastic modulus due to the growth of the TGO. As shown in Figure 3, these stresses can lead to the production of microcracks in the top layer just above the interface, micrograph (d), for the case of the sample subjected to 100 thermal cycles.
The maps also capture the reduction in hardness and elastic modulus in the cracked regions.
In summary, the NanoBlitz 3D high-speed mapping method can be used to measure the local mechanical properties at the interface of different layers of a coating and for the specific case of a top layer interface of a TBC. This technique facilitates the measurement of local elastic properties, which is ideal for use in finite element analysis (FEA), specifically for the simulation of TBC delamination subjected to thermal cycling.
The high-speed mapping data must be deconvoluted to determine the phase-level information of the mechanical properties of the various layers of the TBC. The spatial information about the phases after deconvolution is retained by the clustering algorithm used in this work, which facilitates the reconstruction of a phase map from the mechanical properties map.
Also, the clustering algorithm does not require an expected output range, which is often required for curve fitting procedures.
Figure 4 shows the deconvoluted map obtained from the hardness map and the microstructure for the case of a sample subjected to five thermal cycles.
Figure 4. (a) Microstructure, (b) hardness map, and (c) deconvolved hardness map of the bonding layer after five thermal cycles. Image credit: KLA Instruments
From the deconvoluted hardness map in Figure 4c it can be seen that the property map in Figure 4b has been separated into three different groups based on the hardness data, which in this case are (1) β-NiAl, ( 2) γ/γ ′-Ni and (3) oxides due to internal oxidation.
To represent the mean and standard deviation of the corresponding phases, the mean and standard deviation of the data points in each cluster can be assumed. See reference 5 for more detailed information on data deconvolution.
Conclusions and summary
In conjunction with the NanoBlitz 3D high-speed mapping technique, a Nano Indenter from KLA Instruments was used to investigate the thermal barrier coatings, and specifically the top bond layer interface.
Even at the interface between the various layers of TBC and the porous top layer, an excellent correlation was found between the local mechanical properties at the micrometer length scale and the microstructure.
When it comes to microstructure-based finite element analysis, the phase-level properties obtained from this analysis can be easily used. At the same time, the development of data-driven models to predict the residual life of TBCs is facilitated by the large datasets obtained through extensive mapping.
References and further reading
- DR Clarke, M. Oechsner, NP Padture, “Thermal barrier coatings for more efficient gas turbine engines”, MRS Bulletin 37 (2012) 891–898,
- V. Kumar, K. Balasubramanian, “Progress update on failure mechanisms of advanced thermal barrier coatings: a review,” Progress in Organic Coatings, v. 90 (2016) 54-82,
- BG Mendis, B. Tryon, TM Pollock, KJ Hemker, “Microstructural observations of as prepared and thermal cycled NiCoCrAlY bond coats”, Surface Coating Technology 201 (2006) 3918–3925,
- B. Vignesh, WC Oliver, G. Siva Kumar, P. Sudharshan Phani, “Critical evaluation of high-speed nanoindentation mapping technique and data deconvolution on thermal barrier coatings,” Materials & Design, vol. 181…