Air Plasma Sprayed Thermal Barrier Research Articles

Three-dimensional X-ray micro-computed tomography of cracks in a furnace cycled air plasma sprayed thermal barrier coating

Using X-ray micro-computed tomography, a total of 3 × 3 mm2 area of an air plasma sprayed (APS) thermal barrier coating (TBC) was imaged with a resolution of 0.76 μm at 60% cyclic furnace life. Over 10,000 X-ray micrographs were used to extract the 3-D geometry of the cracks in the top coat. The true shape, center and location of the cracks with respect to bond coat peak–valley roughness features were obtained, which revealed the real nature of cracking characteristics of APS TBCs.

Phase transformations in air plasma-sprayed yttria-stabilized zirconia thermal barrier coatings

Phase transformations in air plasma-sprayed thermal barrier coatings composed of ZrO 2 – 8 wt.% Y 2 O 3 (zirconia - 8 wt.% yttria) are studied using X-Ray diffraction and Rietveld refinement measurements. Samples of TBC deposited onto Inconel 625 substrate were fabricated and heat treated at two different conditions: exposition to 1100oC up to 1000 hours and exposition to temperatures between 700oC and 1100oC during 50 hours. According to Rietveld refinement measurements, the content of the cubic phase in the top coat increases with time and temperature; it starts at 7.3 wt.% and reaches 15.7 wt.% after 1000 hours at 1100oC. The presence of a cubic phase in high amounts is undesirable due its lower mechanical properties compared with the tetragonal phase. After 800 hours of exposure to high temperature, the amount of Y 2 O 3 in the tetragonal phase reduces to 6.6 wt.% and a fraction of this phase transforms to a monoclinic structure during cooling. The monoclinic phase reached 18.0 wt.% after 1000 hours. This phase is also undesirable, not only due to its higher thermal conductivity, but also because the tetragonal-to-monoclinic transformation implies a volume change of circa 5%, which favors crack formation and propagation and compromises the coating integrity.

Explanation of the effect of rapid cycling on oxidation, rumpling, microcracking and lifetime of air plasma sprayed thermal barrier coatings

The effect of cycle duration on thermally grown oxide (TGO) growth kinetics, bond coat rumpling kinetics, microcracking and failure life was examined by cyclic furnace tests at three dwell times and at three temperatures for an air plasma sprayed (APS) thermal barrier coating (TBC) samples with 7wt.% yttria stabilized zirconia (YSZ) top coat and an APS NiCoCrAlY bond coat. Total hot time to failure decreased with increasing cyclic rate, resulting in lower TGO thickness at failure for higher cycling rates. The TBC interface experienced progressive shape changes for the NiCoCrAlY bond coat. The activation energy for TGO growth and the rumpling rate were determined and neither was affected by cyclic rates. Hence the influence of cycling on rumpling and TGO growth does not provide an explanation for the observed effect of cycling on hot time to failure. Data collected from scanning electron microscopy (SEM) images showed that the failure induced in cyclic furnace test was caused by progressive cracking in the top coat. Using over 1000 micrographs, crack length, crack density and crack opening statistics as a function of dwell time and hot time were collected. Average crack length and average crack density were approximately a single valued function of life fraction despite the 33X variation of failure lives in these tests. Crack density decreased and crack length increased after approximately half life, indicating a transition to a crack linking dominated phase. Linear elastic fracture mechanics (LEFM) calculation based on the measured crack opening displacement versus crack length relationship predicted stresses that are impossibly large, indicating that there must be very significant inelasticity present in crack formation and crack growth in TBC.

Thermal shock resistance of air plasma sprayed thermal barrier coatings

Abstract The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.

In situ Raman spectroscopy studies of high-temperature degradation of thermal barrier coatings by molten silicate deposits

Cross-sectional optical micrograph (left) and corresponding Raman ZrO 2 -phase map (right) of CMAS-penetrated 7YSZ air-plasma sprayed thermal barrier coating. In situ Raman spectroscopy is used to understand the high-temperature interactions between air-plasma-sprayed 7 wt.% Y 2 O 3 -stabilized ZrO 2 (7YSZ) thermal barrier coatings (TBCs) and calcium-magnesium-alumino-silicate (CMAS) sand. These interactions result in the depletion of Y 3+ in the YSZ grains. During cooling the Y-depleted YSZ grains transform to the monoclinic phase starting at ∼600 °C. The significant volume expansion associated with this phase transformation could play a role in determining the thermomechanical failure of CMAS-penetrated 7YSZ TBCs during thermal cycling.

Experimental and Finite Element Study of an Air Plasma Sprayed Thermal Barrier Coating under Fixed Cycle Duration at Various Temperatures

Cyclic furnace tests were conducted on Air Plasma Sprayed (APS) Thermal Barrier Coating (TBC) samples which were coated with NiCoCrAlY on a nickel‐based superalloy at three temperatures. From these tests the following were determined: oxidation kinetics and activation energy, bond coat rumpling rate, and cyclic failure lives. All failures occurred in the top coat consistent with engine experience. The measured rumpling rate and oxidation kinetics were used as input for a finite element model that utilized a highly realistic top coat constitutive model that included creep and relaxation behavior as well as tension/compression flow stress asymmetry consistent with available experimental data. The modeling results based on these measured inputs show that the primary sources of stress and strain in descending order are rumpling, oxide growth and finally thermal expansion mismatch between the top coat and underlying metal. Three important implications of these results are; predicted stresses are too low to predict failure based on single crack fracture mechanics, cycling increases accumulated inelastic strain in a fixed time interval providing an additional reason why cycling is damaging and finally to get realistic stresses and strains it is necessary to accurately characterize the geometry changes associated with rumpling.

Experimental study on radiative properties of air plasma sprayed thermal barrier coatings

In this paper the radiative properties of air plasma sprayed (APS) thermal barrier coatings are investigated by measuring the spectral normal-hemispherical reflectance and transmittance of samples with various thickness and porosities in a broad spectral band ranging from 250nm to 15μm. The spectral absorption and scattering coefficients are obtained by a four flux model based on the experimental data. The results show that the porous zirconia ceramics exhibits high scattering coefficient (>15,000m−1) and low absorption coefficient (<500m−1) in the spectral range from 0.5 to 5.5μm. Moreover, the scattering coefficient increases with increasing porosity, while the spectral absorption coefficient is not quite sensitive to the change of porosity. In order to examine the effects of microstructure of porous zirconia coatings, a theoretical model of Mie scattering is established, and a fairly good agreement with the experimental data is achieved. The results indicate that the scattering coefficient heavily depends on the microstructures of coatings, i.e., pore size, porosity and pore distribution, and this will in turn considerably affect the thermal radiation heat transfer inside the coating.

Thermal Fatigue Behavior of Air-Plasma Sprayed Thermal Barrier Coating with Bond Coat Species in Cyclic Thermal Exposure.

The effects of the bond coat species on the delamination or fracture behavior in thermal barrier coatings (TBCs) was investigated using the yclic thermal fatigue and thermal-shock tests. The interface microstructures of each TBC showed a good condition without cracking or delamination after flame thermal fatigue (FTF) for 1429 cycles. The TBC with the bond coat prepared by the air-plasma spray (APS) method showed a good condition at the interface between the top and bond coats after cyclic furnace thermal fatigue (CFTF) for 1429 cycles, whereas the TBCs with the bond coats prepared by the high-velocity oxygen fuel (HVOF) and low-pressure plasma spray (LPPS) methods showed a partial cracking (and/or delamination) and a delamination after 780 cycles, respectively. The TBCs with the bond coats prepared by the APS, HVOF and LPPS methods were fully delaminated (>50%) after 159, 36, and 46 cycles, respectively, during the thermal-shock tests. The TGO thickness in the TBCs was strongly dependent on the both exposure time and temperature difference tested. The hardness values were found to be increased only after the CFTF, and the TBC with the bond coat prepared by the APS showed the highest adhesive strength before and after the FTF.

Optimization of Air Plasma Sprayed Thermal Barrier Coating Parameters in Diesel Engine Applications

In the present paper, an optimization of thermal barrier coating parameters is performed for diesel engine applications. The substrate is A356.0-T7, a cast aluminum alloy which has a vast application in diesel engines, and the alloy is coated by plasma sprayed ZrO2-8 wt.% Y2O3. Parameters including the feed rate of coating powders, the nozzle distance to specimen surfaces, and the coating thickness are optimized by thermal shock fatigue tests and bending tests. Optimum values of the feed rate and the nozzle distance are 30 g/min and 80 mm, respectively, when the objective is considered as maximizing the bending strength. Thermal shock tests demonstrate that lower thickness of coating layers has a better lifetime. By increasing the coating thickness, the thermal fatigue lifetime decreases. The reason is due to higher order of stresses near the interface of the substrate and the bond coat layer, calculated by a finite element simulation. One suggestion to improve the lifetime is to use multiple layers of coatings.

Factors affecting the microstructural stability and durability of thermal barrier coatings fabricated by air plasma spraying

AbstractThe high‐temperature behavior of high‐purity, low‐density (HP‐LD) air plasma sprayed (APS) thermal barrier coatings (TBCs) with NiCoCrAlY bond coats deposited by argon‐shrouded plasma spraying is described. The high purity yttria‐stabilized zirconia resulted in top coats which are highly resistant to sintering and transformation from the metastable tetragonal phase to the equilibrium mixture of monoclinic and cubic phases. The thermal conductivity of the as‐processed TBC is low but increases during high temperature exposure even before densification occurs. The porous topcoat microstructure also resulted in good spallation resistance during thermal cycling. The actual failure mechanisms of the APS coatings were found to depend on topcoat thickness, topcoat density, and the thermal cycle frequency. The failure mechanisms are described and the durability of the HP‐LD coatings is compared with that of state‐of‐the‐art electron beam physical vapor deposition TBCs.

The effect of thermally grown oxide on multiple surface cracking in air plasma sprayed thermal barrier coating system

The objective of this paper is to understand the effect of thermally grown oxide (TGO) on the multiple surface cracking behaviors in an air plasma sprayed (APS) thermal barrier coating system (TBCs). The extended finite element method (XFEM) and periodic boundary conditions are used to investigate the TGO dependences of periodic surface crack driving force and crack propagation path. It is seen that the effect of TGO thickness on the strain energy release rate (SERR) is negligible while ignoring the thickening and elongation strains of the TGO layer. However, the effect of elastic modulus of TGO layer is significant. In particular, for relatively stiff TGO layer the SERR drops to zero as the ratio of surface crack length to coating thickness approximates to one. And vice versa, for a severely damaged TGO layer with relatively compliant modulus, the SERR approaches infinity as surface cracks propagate to the interface. This kind of fracture mechanism is mainly governed by the elastic mismatch across the bimaterial interfaces, which can be used as a guide for the design of good strain-tolerant coating film in APS-TBCs.

Evaluation of microhardness, fracture toughness and residual stress in a thermal barrier coating system: A modified Vickers indentation technique

The evolution of microhardness, fracture toughness and residual stress of an air plasma-sprayed thermal barrier coating system under thermal cycles was investigated by a modified Vickers indentation instrument coupled with three kinds of indentation models. The results show that fracture toughness on the top coating surface after thermal cycles changes from 0.64 to 3.67MPam1/2, and the corresponding residual stress near the indented region varies from −36.8 to −243MPa. For the interface region of coating and bond coat, fracture toughness in the coating close to interface ranges from 0.11 to 0.81MPam1/2, and residual stress varies from −5 to −30MPa, which are consistent with available data. For the lateral region of coating, fracture toughness and residual stress display strong gradient characteristics along the thickness direction due to the special layered structure.

Acoustic emission analysis on tensile failure of air plasma-sprayed thermal barrier coatings

An acoustic emission technique was used to monitor the cracking behavior and fracture process of thermal barrier coatings subjected to tensile loading. Acoustic emission signals were extracted and preformed by fast Fourier transform, and their characteristic frequency spectrums and dominant bands were obtained to reveal fracture modes. Three different characteristic frequency bands were confirmed, corresponding to substrate deformation, surface vertical cracking and interface delamination, with the aid of scanning electronic microscopy observations. A map of the tensile failure mechanism of air plasma-sprayed thermal barrier coatings was established. The fracture strength and interfacial shear strength were estimated as 45.73±3.92MPa and 20.51±1.74MPa, respectively, which are well in agreement with available results.

Suspension and Air Plasma-Sprayed Ceramic Thermal Barrier Coatings with High Infrared Reflectance

Suspension and Air Plasma-Sprayed Ceramic Thermal Barrier Coatings with High Infrared Reflectance

Quality evaluation for air plasma spray thermal barrier coatings with pulsed thermography

AbstractPulsed thermography (PT) as a non-contact non-destructive evaluation (NDE) technique was employed to examine as-sprayed air plasma spray (APS) thermal barrier coatings (TBCs) for quality evaluation. Thickness and microstructural characteristics play a vital role in determining the quality. In this paper, PT logarithmic peak second-derivative method was adopted to measure the thickness of top coat. Time dependent thermal images were used to characterize the microstructure which was confirmed by scanning electron microscope (SEM). The results showed that there was relationship between the temperature distribution of the surface and microstructure change in TBCs. Temperature distribution in thermal images and measurement results of thickness were in fairly good agreement with the microstructure change. It can be concluded that it was possible to employ these NDE methods as quality evaluation for as-sprayed TBCs.

Non-destructive thermographic monitoring of crack evolution of thermal barrier coating coupons during cyclic oxidation aging

The diffused microscopic cracking of the interface between a thick (>300μm) air plasma spray (APS) thermal barrier coating (TBC) and a bond coat cannot be clearly identified by non-destructive evaluation and testing techniques. In this work, a semi-quantitative estimation of cracks at the TBC interface is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling using a single-side thermographic technique. In fact, during thermal cycling, two phenomena occur: sintering, which promotes a significant increase in thermal diffusivity, and cracking, representing an additional thermal resistance which causes an apparent decrease in thermal diffusivity. The results refer to an experimental activity carried out on 28 APS TBC samples cyclic aged at six different fractions of their lifetime. For each sample, the thermal diffusivity was measured at fixed lifetimes, and the evolution of the cracked fraction of the interface was estimated by adopting a two-dimensional inversion model. Furthermore, at each of the six lifetime fractions, some samples were destructively characterized by image analysis, and the results were compared with the estimations given by the inversion model. Good agreement between the non-destructive estimations and image analysis results was obtained. Moreover, a figure of merit incorporating both the cracked fraction and the crack thickness is also proposed for ranking the damage.

The Effect of Vacuum Heat Treatment on the Oxidation Behavior of APS Thermal Barrier Coating

Thermal barrier coating (TBC), which consisted of a NiCoCrAlY bond coat (BC) and a ZrO2-8 wt.%Y2O3 topcoat (TC), was fabricated on the nickel-base superalloy by air plasma spray (APS). The BC and TBC was treated by vacuum heat treatment (VHT). The oxidation of coating with and without VHT has been performed at 1050°C. Oxidation behavior of coatings and thermally grown oxide(TGO) scale were studied by SEM with EDS. As shown in the results, after treating by VHT, a continuous Al2O3 layer formed more rapidly on the VHT coating than that formed on the APS coating, which can act as a diffusion barrier to suppress the formation of other detrimental oxides. The pre-oxidation treatments reduced the growth rate and extend the steady-state growth stage. The TGO in VHT TBC was still a single layerAl2O3 oxide after 120h. However, after same oxidation time, the TGO in APS TBC produced a dual-layer oxide consisting of an inner Al2O3 layer and outer spinel layer. Therefore the VHT improves the oxidation resistance of APS coating.

Determination of mixed-mode interfacial fracture toughness for thermal barrier coatings

In this work, a test method was developed to determine the interfacial fracture toughness of the air plasma sprayed (APS) thermal barrier coatings (TBCs) over a wide range of mode mixities. For this mixed-mode test method, the analytical expressions for the energy release rate and stress intensity factors were derived based on the energy theory and the concept of “equivalence”. The fidelity of these expressions was affirmed by selected finite element analysis. The experimental results showed that the critical energy release rate increased with the increase of the positive mode mixity, which was mainly due to the increase in contact/friction effect and plastic work dissipation with increasing shear mode loading. Furthermore, an elliptical interfacial failure criterion in terms of the stress intensity factors was proposed. The agreement between the experimental results in the literature and those in our work indicated that our test method and the corresponding analytical solutions can well determine the interfacial fracture toughness of the TBCs over a wide range of mode mixities.

Finite element simulation of thermal barrier coating performance under thermal cycling

In this paper, a numerical simulation of stress development within the air plasma-sprayed thermal barrier coating system incorporating nonlinear behavior under compression for the top coat ceramic layer is presented. The nonlinear behavior as well as its evolution with sintering at high temperature is simulated using a microstructure based model. The simulation results indicate that this nonlinearity has a significant role on distribution of the residual stresses in this layer resulting from the thermal cycling. A parametric study is carried out to investigate the effects of the microstructural features of the top coat ceramic layer on residual stress distribution. It is revealed from the simulation results that the variation of porosity has only a negligible effect on the residual stress distribution. In addition, the stresses accountable for the crack growth can be lowered by changing the microcrack densities of the top coat layer within a specified range.

Effects of Thickness on Thermal and Mechanical Properties of Air-Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coating systems (TBCs) prepared by an air-plasma spray (APS) have been used to protect metallic components of gas turbines because of its economic advantage. To enhance the energy efficiency of gas turbine systems, the operating temperature is increased to over 1300 °C, which requires a new material with low thermal conductivity and an increase of TBC thickness. In this study we have focused the microstructure related to the thickness of TBC and their thermal properties, with specific attention to defect species as well as to its morphology with the thermal exposure time. Resintering of TBC happens during thermal exposure in a high temperature, resulting in the less strain tolerance and the higher thermal conductivity. In order to investigate the thermal properties of TBC related to the microstructural evolution, TBCs with different thicknesses of 200 µm, 400 µm, 600 µm, and 2000 µm were deposited on a flat graphite by the APS. The thermal exposure tests were conducted in different dwell time till 800h at 1100 °C. The thermal diffusivity is significantly increased after thermal exposures, depending on the thermal exposure time. Microstructural analysis clearly shows that the variation of thermal diffusivity is ascribed to the coalescence of small cracks and the resintering effect. The hardness values of TBCs are also increased as well. The relationship between mechanical properties and TBC thickness is discussed, including the effect of thickness on thermal properties.