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Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons
Surface & Coatings Technology 205 (2010) 498–505
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons F. Cernuschi a,⁎, P. Bison b, S. Marinetti b, E. Campagnoli c a b c
Enea – Ricerca sul Sistema Elettrico S.p.A, Via Rubattino, 54, 20134 Milano, Italy CNR ITC, C.so Stati Uniti, 4, 35100 Padova, Italy Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino
a r t i c l e
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Article history: Received 22 January 2010 Accepted in revised form 8 July 2010 Available online 15 July 2010 Keywords: Thermal barrier coating NDE&T Pulse thermography Thermal diffusivity
a b s t r a c t For thin (b 200 μm) air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) ceramic thermal barrier coatings (TBCs), some non-destructive techniques indicate damage at the bond coat–TBC interface during either ageing or cyclic oxidation tests. However, no technique is available for thick (N 200 μm) APS TBCs. In this work, a semi-quantitative estimation of cracks at the interface of APS TBCs thicker than 300 μm is obtained from thermal diffusivity values measured by using a single side thermographic technique on coupons subjected to thermal cycling. In fact, during thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity. The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface. © 2010 Elsevier B.V. All rights reserved.
1. Introduction TBCs are widely applied for protecting hot path components of gas turbines from high temperature combustion gases [1,2]. Typical TBCs are metal-ceramic multi-layers structures made up of a yttria partially stabilized zirconia (7–8 wt.% Y2O3 + ZrO2) layer deposited either by APS or EB-PVD on a high temperature oxidation/corrosion resistant metallic bond coat (BC) [1,2]. The refractory ceramic porous layer can reduce the temperature of the base metal by 30 °C to 100 °C, depending on the thickness and on specific microstructural properties of the coating. During engine operation, several high temperature cycle-dependent phenomena take place within the TBC system which determine its lifetime. In particular, high temperatures promote the sintering of the TBC and the progressive formation of a thermally grown oxide (TGO) layer at the interface between the BC and the TBC which contributes to the failure by spallation of the top coat. In fact, since the TGO grows at high temperatures in nearly stressfree conditions due to the thermal expansion mismatch among the TGO, TBC and BC, at room temperature relevant residual stresses are induced close to the interface within both the TGO and the TBC. The increase in the TGO thickness promotes the change of the sign of
⁎ Corresponding author. Tel.: +39 02 39924577; fax: +39 02 39925626. E-mail addresses: [email protected], [email protected] (F. Cernuschi). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.024
residual stresses within the TBC (from compressive to tensile, correlating to roughness valleys), allowing micro-cracks previously enucleated within the TBC correlating to roughness peaks to propagate and to coalesce together along the BC–TBC interface. Moreover, crack propagation becomes more favourable because the formation of chemical bonding between splats induced by sintering phenomena allows cracks to propagate through different grains without being deflected, making the TBC fragile [3,4]. Since coating life is strongly influenced by operating conditions, residual life assessment of the operated hot path needs to be performed starting from data furnished by advanced non-destructive techniques. Regarding MCrAlY coatings (M = Ni, Co), some years ago Antonelli et al. developed frequency scanning eddy current equipment that facilitates the estimation of β phase depletion (or the equivalent β phase thickness) caused by both high temperature oxidation and diffusion within the base metal, even if a TBC is present onto the metallic coating [5,6]. Limited to EB-PVD and thin APS TBCs, the Photo Luminescence Piezo Spectroscopy (PLPS) technique allows the measurement of the residual stresses inside the α-alumina TGO layer by the Cr3 + luminescence peak shift [7]. A useful relationship between the value of compressive residual stresses inside the TGO and the TBC integrity has been found on aged samples and components [7–9]. Thus, the early detection of TBC failure at the interface has been facilitated. Impedance Spectroscopy (IS) is reported to be able to detect TGO growth, TBC sintering and crack nucleation close to the BC–TBC
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Table 1 Main micro-structural, mechanical and thermal properties of the two sample sets A and B. Sample Substrate
Bondcoat
A0% A35% A70% A100% B0% B5% B10% B22% B28% B45% B100%
NiCrAlY Amdry 963 APS
Ni base alloy Hastelloy X
Cracked interface [%] High temperature Dwell temperature [°C] TBC Thermal TBC thicknessa [μm] TBC porosity by image analysis [%] dwell time [h] diffusivityb [10− 7 m2/s]
795 ± 3 747 ± 3 854 ± 3 820 ± 4 Ni base alloy IN738 CoNiCrAlYAmdry 419 ± 3 995 HVOF 304 ± 2 314 ± 2 334 ± 2 324 ± 2 313 ± 2 312 ± 2
8±1 8±2 9±1 6±1 11 ± 1 9±1 9±1 11 ± 1 10 ± 1 9±1 9±1
0% 90% 99% n.a. 0% 38% 47% 69% 95% 99% n.a.
n.a 20 20 20 n.a 1.5 1.5 1.5 1.5 1.5 1.5
1100
1050
3.6 ± 0.2 5.3 ± 0.2 5.1 ± 0.3 6.2 ± 0.4 3.9 ± 0.1 4.5 ± 0.1 5.3 ± 0.2 5.4 ± 0.1 4.4 ± 0.2 4.8 ± 0.4 5.3 ± 0.1
a The TBC thickness uncertainty refers to the standard deviation of the average value, being TBC thickness estimated as the average of fifty values taken along the whole specimen section by IA. b The overall thermal diffusivity uncertainty is the sum of two contributions: the uncertainty related to the experimental technique and the uncertainty related to the estimation of the TBC thickness, respectively. When thermal diffusivity is measured both before and after different ageing times on the same coupon, the effect of TBC thickness uncertainty is completely removed from the estimation of the cracked interface.
interface, but both the deposition of a Pt electrode on the top TBC surface and access to the metallic rear face are required to close the electrical circuit. Moreover, no direct quantitative correlation between the idealized model circuit and the TBC micro-structural parameters (TGO thickness, porosity content, percentage of cracked interface, etc.) has been proposed [10–12]. For thin APS TBC coatings, a promising non-destructive technique based on Mid-Infrared reflectance imaging has been proposed by J.I. Eldridge et al. [13]. In particular, in the case of furnace cycled TBC, a correlation between an increase in the reflectance at the infrared wavelength of 4 μm and the delamination progression close to the
interface has been observed, even if the contributions of potential interfering effects are still to be studied. Although several qualitative applications of transient thermography to detect TBC delamination during cycled thermal ageing and after service are reported [14–17], in the case of the APS TBC, nondestructive techniques for the early detection and semi-quantitative quantification of cracks at the interface between the BC and the TBC (independently from the TBC thickness) are not yet available. In this work, a semi-quantitative estimation of cracks at the interface of TBCs is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling.
Fig. 1. Electron back-scattered images of the section of samples (a) A0% - as sprayed, (b) A35% and (c) A70%. Cracks close to the interface are clearly visible of samples A35% and A70%, as pointed out by the arrows.
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Thermal diffusivity of TBC is estimated by inverting TBC surface temperature vs. time using a two layer model which assumes a perfect adhesion between TBC and substrate. During thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity. The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface.
where k, ρ and C are the thermal conductivity, the density and the specific heat, respectively. The TBC thermal diffusivity of a two-layer sample (the TBC and the metallic substrate) is estimated by fitting the spatial average temperature vs. time T(t) of the sample surface after heating by a spot-shaped Dirac pulse (typically a laser pulse). Temperature is measured by an infrared camera at RT, being the maximum TBC surface temperature increase in the range of 15–30 K, depending on TBC features and laser pulse energy.
2. Experimental 2.1. The samples Experiments have been performed on two sets of disk shaped samples A and B, respectively. All nickel base alloy samples, coated with a MCrAlY bond coat and an YPSZ APS top coat, were 25.4 mm in diameter and 3.2 mm in thickness. TBC for set A shows a pseudocolumnar structure with a vertical crack density of roughly 3 mm-1. Set B consists in a standard porous APS TBC. For each set of samples, one sample was in the as–sprayed condition while the others were subjected to high temperature cyclic ageing up to different fractions of their lifetime (this fraction of lifetime was also used to identify each single sample), defined as the number of cycles needed for spalling the TBC from the substrate. For both sets of TBC, three samples have been used to experimentally estimate their average lifetime, before ageing the remaining samples up to a certain percentage of the estimated lifetime. In particular, for set A, the TBC layer completely spalled off from the substrate while for set B, the TBC remained partially constrained to the substrate. Furnace cycle oxidation tests have been performed in the frame of two different projects at two different laboratories, each adopting its own specific cycle. TBC coupons have been made available for testing by the thermographic method independently by the two laboratories. Table 1 summarizes the main microstructural and thermal features of all the samples and the corresponding ageing parameters. In particular, the exposure time at high temperature within each ageing cycle (dwell time) is reported for both TBC sets. 2.2. Microstructural characterization The TBC microstructural characterization was performed using a Zeiss SUPRA 40 scanning electron microscopy (SEM) (Carl Zeiss NTS GmbH, Oberkochen, Germany). TBC specimens were impregnated in vacuum using epoxy resin to better distinguish between true pores and cracks and possible artifacts induced by cross-sectioning and polishing. Regarding the overall porosity evaluation, for each sample, five back-scattered electron images (BEI) taken along each sample cross-section were considered for the image analysis (IA) (Screen Measurement, Laboratory Imaging Ltd., Praha, CZ). For each sample, the percentage of cracked interface between the bond coat and the TBC has been estimated along the whole section, taking into account cracks longer than 25 μm within the first 30 μm from the interface promoted by ageing. 2.3. The thermographic technique Recently, a thermographic technique for non-destructively measuring the thermal diffusivity of TBCs deposited onto gas turbine hot path components such as blades and vanes has been developed [18]; thermal diffusivity α is defined as: α = k = ρC
ð1Þ
Fig. 2. Electron back-scattered images of the section for samples (a) B0% - as sprayed, (b) B5% and (c) B10%. Cracks close to the interface are clearly visible in aged samples.
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Fig. 3. Normalized thermal diffusivity of TBC vs. the percentage of cycles to failure for sets A and B.
The analytical one-dimensional two layer model adopted to fit the experimental data is: TðtÞ =
∞ n2 l2 Q0 n − c pffiffiffiffiffi 1 + 2 ∑ Γ e αc t εC πt n=1
ð2Þ
pffiffiffiffiffiffiffiffiffiffiffi −εs Where Q 0, ε = kρ C , Γ = εεc c + εs and lc are the absorbed energy density, the thermal effusivity, the reflection coefficient at the interface between coating and substrate and the coating thickness, respectively. The subscripts c and s refer to coating and substrate, respectively. The surface area over which the average temperature is computed must completely contain the diffusing heating spot which is typically a roughly circular-shaped spot with a diameter of some millimetres.
As heating source, a 1064 nm wavelength Nd:YAG laser (Theta Industries Inc., Port Washington, NY, USA) generating pulses of about 0.8 ms was used. The energy and the diameter of the heating shots were fixed roughly equal to 0.5 J and 10 mm, respectively. A snapshot focal plane array IR camera sensitive in the range of 7–10 μm (Jade II LW, CEDIP Infrared Systems, Croissy-Beaubourg, France) with a Noise Equivalent Temperature Difference of 30 mK was used to monitor for 3 s after the pulse the temperature transient of the sample surface at frame rate of 150 Hz. With this experiment duration the thermal diffusion length μ=
pffiffiffiffiffiffi αt
ð3Þ
Fig. 4. A sketch of the model used to estimate the percentage of the cracked BC–TBC interface. (a) The sound TBC layer, (b) the cracked TBC modeled as an in-series of two layers and (c) the in-parallel model representing a sample having both a cracked and a sound interface.
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Fig. 5. Normalized thermal diffusivity α/α0 vs. Larson Miller Parameter (LMP) for experimental data. Values of normalized thermal diffusivity for all the samples as estimated by Eq. (6) and literature data are also reported.
results greater than 1 mm, for typical as sprayed thermal diffusivity values of TBC. Owing to the TBC translucency, a thin layer of colloidal graphite was painted on the front sample surface in order to make the TBC opaque to the Nd:YAG laser radiation. Typical experimental uncertainty of this technique is in the range of 5–7%. More details about this technique can be found in the literature [18,19]. 3. Results 3.1. Microstructural characterization Figs. 1 and 2 show the microstructure of each sample. Table 1 summarizes the main microstructural parameters for each set. In particular, for both sets, the overall porosity, the TBC thickness and the percentage of the cracked interface are reported. Aged samples show significant percentages of cracked interface, even at low fractions of spent life (samples B5% and B10%). In particular, only 10% and 1% of the interface remained free of cracks within the first 30 μm of the TBC layer for samples A35% and A70%, respectively. Similarly for set B, only 31%, 5% and 1% of the interface remained free from cracks for samples B22%, B28% and B45%, respectively. Concerning crack length and linking as well as the percentage of the cracked interface, the results of the microstructural characterization are in fairly good agreement with the results reported by Herzog et al. [4]. In fact, at around 50% of the life, an almost complete crack linking with overall crack lengths in the millimetric range is observed.
Considering TBC sintering at high temperatures as the only phenomenon affecting thermal diffusivity variations of TBCs, a monotonic increase of normalized thermal diffusivity for both sets would have been expected. The non-monotonic trend of thermal diffusivity as a function of the spent life can be explained as the effect of the progressive microcrack growth and propagation parallel to the interface between the bond coat and the APS TBC top layer during cyclic oxidation tests. Owing to the relatively fast kinetics of sintering phenomena of TBCs [20,21], at the very beginning of cyclic oxidation ageing, sintering affects thermal diffusivity more significantly than interface damage caused by crack nucleation and growth and explains the observed initial thermal diffusivity increase. At longer times, crack growth becomes increasingly relevant with respect to sintering and promotes a decrease in the measured thermal diffusivity. When TBCs completely spall off from the substrate, the effect of cracks disappears, leaving only sintering phenomena to play a key role, and thus a further increase of
3.2. Thermal diffusivity measurements Table 1 summarizes the measured thermal diffusivity absolute values while Fig. 3 shows the normalized (with respect to the value of the as-sprayed sample) trend of thermal diffusivity as a function of the number of cycles to failure. Notwithstanding that the two sets of samples differed in thickness, both show non-monotonic trends. In fact, for both sets A and B, thermal diffusivity increases for fractions of life smaller than 35% and 28% respectively while at higher fractions (70% for sets A and 45% for set B) a decrease is observed. When the TBC completely spalled off from the substrate, thermal diffusivity reached its highest value (set A) while if TBC was still partially adherent to the substrate, a less significant increase was observed (set B).
Fig. 6. Percentage of the cracked interface as estimated by the proposed method vs. the percentage measured by IA along the section of the samples.
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thermal diffusivity is expected. A similar explanation for aged EB-PVD samples has been given by Azzopardi et al [22]. 4. An approach to a semi-quantitative estimate for the percentage of cracked BC–TBC interface Since thermal diffusivity of TBCs is affected by the progressive cracking inside TBCs close to the BC–TBC interface, some indications about the degree of damage at the interface can be obtained by measuring the thermal diffusivity of TBCs during ageing (real components measurements can be carried out during gas turbine maintenance). In fact, if the experimental variations of thermal diffusivity vs. the number of cycles are compared with the variations caused only by high temperature sintering of TBCs (a rough estimation of thermal diffusivity increase is possible when exposure time and temperature and TBC sintering kinetics are known), the percentage of the cracked interface would be roughly estimated, as described hereinafter. It should be clear that theoretical considerations and modelling have to be performed on the thermal conductivity (the field variable) which is the physical parameter mostly affected by sintering and crack nucleation and propagation. Since the ρC product is supposed to be only slightly affected by sintering and cracking, the ρC value for as sprayed TBC is fixed all along the modelling activity. Under such assumptions, ratios of thermal diffusivities correspond to the ratio of thermal conductivities. In any case, when thermal conductivity is required, it is estimated from the experimental thermal diffusivity, through Eq. (1) using the as sprayed ρC value. The simplest way to model sound and cracked TBCs is as a single sound TBC layer (see Fig. 4(a)) and a one-dimensional in-series twolayer composite where the first and the second layer are the TBC and a thin air gap, respectively (see Fig. 4(b)). In particular, the thermal conductivity of the latter case is
1 = ktwo−layers =
lTBC
. L
kTBC
+
lair
503
Fig. 5 shows the experimental data reported in the literature, the corresponding best fitting straight line (i.e. Eq. (6)), the normalized thermal diffusivity for all the aged samples of sets A and B, as estimated by using Eq. (6) and experimental measurements. In particular, normalized thermal diffusivity values for samples aged at a spent life fraction smaller than 100% were lower than those values estimated by taking into account only sintering phenomena. On the contrary, according to the proposed interpretation, the normalized thermal diffusivity value for sample A100% was very close to the corresponding estimated value; the diffusivity value of sample B100% was lower because it was still partially adherent to the substrate. By applying the proposed procedure, the percentage of the cracked interface has been estimated, as summarized in Fig. 6. Estimations have been performed by fixing the crack thickness lair gap equal to 1 and 2 μm at early and later stages, respectively.
.
kair
L
ð4Þ
where l, k and L are the thickness and the thermal conductivity of each single layer defined by the subscripts and the overall thickness of the two-layer composite, respectively. If the thermal conductivity value of TBC is fixed within the model equal to the value estimated by considering only the effect of sintering, a rough estimation of the percentage of the cracked interface can be obtained by an in-parallel thermal conductivity model sketched in Fig.4 (c) as the relative weight f of the two-layer composite which equates k * defined as k* = f ktwolayers + ð1−f ÞkTBC
ð5Þ
with the corresponding experimentally evaluated thermal conductivity value of the TBC sample. For our purposes, the curve reported in the literature (referring to a set of freestanding APS TBC aged for different times at different temperatures) has been used [21]. In particular, from the knowledge of both the ageing temperature (1100 °C) and the number of cycles, the expected normalized thermal diffusivity increase α / α0 (α and α0 are the thermal diffusivity after a certain time exposure at a certain temperature and in the as- sprayed condition, respectively) caused by sintering has been estimated for aged samples of sets A and B, by using the following equation [21]: α = α0 = 7:85⋅10
−5
⋅LMP−1:2
ð6Þ
The Larson Miller parameter is defined as LMP = T(ln(t + 1) + 20) where temperature T and time t are expressed in Kelvin and hours, respectively.
Fig. 7. High magnification secondary electron images of the section of samples (a) A35%, (b) B10% and (c) B45%, where crack thickness resulted higher than that used in the model.
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5. Discussion In the case of these two sets of furnace cycled TBC it seems that most of the damage at the interface between the bondcoat and the TBC occurs during the first half of the life and then almost delaminated TBC (less affected by the thermal mismatch between TBC and substrate) “survives” for a long time before spalling off. In principle, for a more continuous progression of cracking coverage with time the technique would be sensitive even at higher fraction of spent life. In any case the situation for real components could be completely different. In fact, being components stressed during operation, as soon as a high fraction of crack coverage is reached, TBC would spall off rapidly. For most of the samples, the estimation of the cracked interface by using the proposed method is in fairly good agreement with the quantitative estimation performed along the section by image analysis. These results were obtained by fixing a crack thickness within the model as slightly thinner than the thickness experimentally observed at high magnification by scanning electron microscopy. In fact, as shown in Fig. 7, 3 and 2 μm are the average values of the crack thickness for sets A and set B, respectively. A possible explanation for the good results obtained using an air gap thickness thinner than the thickness observed by IA is probably related to the one-dimensional approximation of the model. In fact, owing to the one-dimensional approximation, the effects of finite dimensions of cracks have not been considered. In the real case, especially at early stages, several small cracks are present at the interface instead of a single crack. As reported in the literature [23–27], the effect on thermal conductivity of many cracks with a higher aspect ratio between their minor and major axes is smaller than the effect of a single crack with a much lower aspect ratio when the overall volumetric fraction is assumed to be equal in both cases; i.e., owing to the one-dimensional approximation, when the true crack thickness is considered, the model underestimates the percentage of the cracked interface especially at early stages where the number of cracks is high, but their size and their aspect ratio are very small. By considering a thinner air gap, a partial compensation of this underestimation effect is obtained. Future work will deal with the development of a 2-D model for data reduction and with the collection of a crack opening database. By this approach it will be possible to fix a suitable crack opening for data reduction and, if a statistically significant number of TBC samples aged at different times will be available for microstructural characterization, even an equation describing crack opening as a function of ageing time could be obtained. These data can be used for inverting experimental data for samples with known thermal history. For samples with unknown thermal history, the
only reasonable approach is to define a threshold value for the normalised thermal diffusivity, without giving a quantitative estimation of crack coverage, as explained also later. Since porosity usually decreases during ageing at high temperatures, when thermal conductivity is estimated from the experimental thermal diffusivity value by Eq. (1), the porosity fraction should be known for each sample. In fact, the density ρ of the porous TBC is linearly related to the porosity fraction. If the porosity cannot be measured, the porosity value should be either assumed to be the same for all the samples or a porosity reduction as a function of ageing time and temperature should be known. Since sintering usually reduces porosity by 1% to 5%, the assumption of a constant porosity fraction, independently from the ageing of the sample, causes the error in the thermal conductivity estimation to be of the same order of magnitude. This error is significantly lower than the thermal conductivity variations caused by sintering and cracking and by the above approximations. In any case, a recently developed photothermal technique seems to be able to simultaneously estimate the thermal diffusivity and density of TBCs and eliminates this error source [28]. As concerns pseudo-columnar TBC samples (set A), the width of vertical cracks appears to increase as a function of the ageing time. It is worth noting that the effect of these vertical cracks on measured through-the-thickness thermal diffusivity is negligible. Since this method could be applied in principle to real components, the TBC thickness has to be estimated by ND techniques. Eddy Currents equipment for coating thickness measurements is available on the market, but the value measured is usually slightly overestimated in respect to the average value measured by IA after sectioning the sample or the component because the probe lift-off is related to the highest roughness peaks. Fig. 8 shows that the relative error in estimating the percentage of the cracked interface caused by the TBC thickness overestimation is not significant as it is lower than 8% (even for an 100 μm overestimation) when a TBC of 400 μm is considered in the simulation. Furthermore, for coupons with an unknown thermal history, the thermal diffusivity measurement can still produce some information if at least the thermal diffusivity of the TBC in the as-sprayed condition is known. In this circumstance, a normalized thermal diffusivity threshold below which the TBC has to be considered close to failure could be properly defined. The application of this NDT technique to real components requires overcoming several difficulties related to the lack of both the thermal history of the blade and the as-sprayed thermal diffusivity all along the surface blade. Furthermore, during service the TBC surface can be
Fig. 8. The relative error in estimating the percentage of cracked interface vs. the TBC thickness overestimation. Simulations have been carried out fixing a true average TBC thickness of 400 μm.
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polluted with deposits that affect TBC thermal diffusivity measurements. Depending on the deposits nature this effect on measured thermal diffusivity can lead to an over- or to an under-estimation of the crack contribution. 6. Conclusions A new analysis of the combined effect of sintering and cracking on apparent TBC thermal diffusivity has been presented. The knowledge of sintering kinetics allows to semi-quantitatively estimate the fraction of cracked interface as a function of the ageing time. Notwithstanding the simplicity of the adopted model, satisfactory indications and a good agreement with image analysis have been obtained, when thermal history and as sprayed thermal diffusivity values of coupons are known. Acknowledgements This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between ERSE and the Ministry of Economic Development - General Directorate for Nuclear Energy, Renewable Energy and Energy Efficiency stipulated on July 29, 2009 in compliance with the Decree of March 19, 2009. The authors acknowledge the technical contribution of Dr. C. Rinaldi, Mr. S. Capelli and Mr. L. Lorenzoni of ERSE. References [1] V.P. Swaminathan, N.S. Cheruvu, in: V.P. Swaminathan, N.S. Cheruvu (Eds.), Advanced Materials and Coatings for Combustion Turbines, ASM International, Materials Park (OH), 1994, p. 1.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons F. Cernuschi a,⁎, P. Bison b, S. Marinetti b, E. Campagnoli c a b c
Enea – Ricerca sul Sistema Elettrico S.p.A, Via Rubattino, 54, 20134 Milano, Italy CNR ITC, C.so Stati Uniti, 4, 35100 Padova, Italy Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino
a r t i c l e
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Article history: Received 22 January 2010 Accepted in revised form 8 July 2010 Available online 15 July 2010 Keywords: Thermal barrier coating NDE&T Pulse thermography Thermal diffusivity
a b s t r a c t For thin (b 200 μm) air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) ceramic thermal barrier coatings (TBCs), some non-destructive techniques indicate damage at the bond coat–TBC interface during either ageing or cyclic oxidation tests. However, no technique is available for thick (N 200 μm) APS TBCs. In this work, a semi-quantitative estimation of cracks at the interface of APS TBCs thicker than 300 μm is obtained from thermal diffusivity values measured by using a single side thermographic technique on coupons subjected to thermal cycling. In fact, during thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity. The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface. © 2010 Elsevier B.V. All rights reserved.
1. Introduction TBCs are widely applied for protecting hot path components of gas turbines from high temperature combustion gases [1,2]. Typical TBCs are metal-ceramic multi-layers structures made up of a yttria partially stabilized zirconia (7–8 wt.% Y2O3 + ZrO2) layer deposited either by APS or EB-PVD on a high temperature oxidation/corrosion resistant metallic bond coat (BC) [1,2]. The refractory ceramic porous layer can reduce the temperature of the base metal by 30 °C to 100 °C, depending on the thickness and on specific microstructural properties of the coating. During engine operation, several high temperature cycle-dependent phenomena take place within the TBC system which determine its lifetime. In particular, high temperatures promote the sintering of the TBC and the progressive formation of a thermally grown oxide (TGO) layer at the interface between the BC and the TBC which contributes to the failure by spallation of the top coat. In fact, since the TGO grows at high temperatures in nearly stressfree conditions due to the thermal expansion mismatch among the TGO, TBC and BC, at room temperature relevant residual stresses are induced close to the interface within both the TGO and the TBC. The increase in the TGO thickness promotes the change of the sign of
⁎ Corresponding author. Tel.: +39 02 39924577; fax: +39 02 39925626. E-mail addresses: [email protected], [email protected] (F. Cernuschi). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.024
residual stresses within the TBC (from compressive to tensile, correlating to roughness valleys), allowing micro-cracks previously enucleated within the TBC correlating to roughness peaks to propagate and to coalesce together along the BC–TBC interface. Moreover, crack propagation becomes more favourable because the formation of chemical bonding between splats induced by sintering phenomena allows cracks to propagate through different grains without being deflected, making the TBC fragile [3,4]. Since coating life is strongly influenced by operating conditions, residual life assessment of the operated hot path needs to be performed starting from data furnished by advanced non-destructive techniques. Regarding MCrAlY coatings (M = Ni, Co), some years ago Antonelli et al. developed frequency scanning eddy current equipment that facilitates the estimation of β phase depletion (or the equivalent β phase thickness) caused by both high temperature oxidation and diffusion within the base metal, even if a TBC is present onto the metallic coating [5,6]. Limited to EB-PVD and thin APS TBCs, the Photo Luminescence Piezo Spectroscopy (PLPS) technique allows the measurement of the residual stresses inside the α-alumina TGO layer by the Cr3 + luminescence peak shift [7]. A useful relationship between the value of compressive residual stresses inside the TGO and the TBC integrity has been found on aged samples and components [7–9]. Thus, the early detection of TBC failure at the interface has been facilitated. Impedance Spectroscopy (IS) is reported to be able to detect TGO growth, TBC sintering and crack nucleation close to the BC–TBC
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Table 1 Main micro-structural, mechanical and thermal properties of the two sample sets A and B. Sample Substrate
Bondcoat
A0% A35% A70% A100% B0% B5% B10% B22% B28% B45% B100%
NiCrAlY Amdry 963 APS
Ni base alloy Hastelloy X
Cracked interface [%] High temperature Dwell temperature [°C] TBC Thermal TBC thicknessa [μm] TBC porosity by image analysis [%] dwell time [h] diffusivityb [10− 7 m2/s]
795 ± 3 747 ± 3 854 ± 3 820 ± 4 Ni base alloy IN738 CoNiCrAlYAmdry 419 ± 3 995 HVOF 304 ± 2 314 ± 2 334 ± 2 324 ± 2 313 ± 2 312 ± 2
8±1 8±2 9±1 6±1 11 ± 1 9±1 9±1 11 ± 1 10 ± 1 9±1 9±1
0% 90% 99% n.a. 0% 38% 47% 69% 95% 99% n.a.
n.a 20 20 20 n.a 1.5 1.5 1.5 1.5 1.5 1.5
1100
1050
3.6 ± 0.2 5.3 ± 0.2 5.1 ± 0.3 6.2 ± 0.4 3.9 ± 0.1 4.5 ± 0.1 5.3 ± 0.2 5.4 ± 0.1 4.4 ± 0.2 4.8 ± 0.4 5.3 ± 0.1
a The TBC thickness uncertainty refers to the standard deviation of the average value, being TBC thickness estimated as the average of fifty values taken along the whole specimen section by IA. b The overall thermal diffusivity uncertainty is the sum of two contributions: the uncertainty related to the experimental technique and the uncertainty related to the estimation of the TBC thickness, respectively. When thermal diffusivity is measured both before and after different ageing times on the same coupon, the effect of TBC thickness uncertainty is completely removed from the estimation of the cracked interface.
interface, but both the deposition of a Pt electrode on the top TBC surface and access to the metallic rear face are required to close the electrical circuit. Moreover, no direct quantitative correlation between the idealized model circuit and the TBC micro-structural parameters (TGO thickness, porosity content, percentage of cracked interface, etc.) has been proposed [10–12]. For thin APS TBC coatings, a promising non-destructive technique based on Mid-Infrared reflectance imaging has been proposed by J.I. Eldridge et al. [13]. In particular, in the case of furnace cycled TBC, a correlation between an increase in the reflectance at the infrared wavelength of 4 μm and the delamination progression close to the
interface has been observed, even if the contributions of potential interfering effects are still to be studied. Although several qualitative applications of transient thermography to detect TBC delamination during cycled thermal ageing and after service are reported [14–17], in the case of the APS TBC, nondestructive techniques for the early detection and semi-quantitative quantification of cracks at the interface between the BC and the TBC (independently from the TBC thickness) are not yet available. In this work, a semi-quantitative estimation of cracks at the interface of TBCs is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling.
Fig. 1. Electron back-scattered images of the section of samples (a) A0% - as sprayed, (b) A35% and (c) A70%. Cracks close to the interface are clearly visible of samples A35% and A70%, as pointed out by the arrows.
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Thermal diffusivity of TBC is estimated by inverting TBC surface temperature vs. time using a two layer model which assumes a perfect adhesion between TBC and substrate. During thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity. The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface.
where k, ρ and C are the thermal conductivity, the density and the specific heat, respectively. The TBC thermal diffusivity of a two-layer sample (the TBC and the metallic substrate) is estimated by fitting the spatial average temperature vs. time T(t) of the sample surface after heating by a spot-shaped Dirac pulse (typically a laser pulse). Temperature is measured by an infrared camera at RT, being the maximum TBC surface temperature increase in the range of 15–30 K, depending on TBC features and laser pulse energy.
2. Experimental 2.1. The samples Experiments have been performed on two sets of disk shaped samples A and B, respectively. All nickel base alloy samples, coated with a MCrAlY bond coat and an YPSZ APS top coat, were 25.4 mm in diameter and 3.2 mm in thickness. TBC for set A shows a pseudocolumnar structure with a vertical crack density of roughly 3 mm-1. Set B consists in a standard porous APS TBC. For each set of samples, one sample was in the as–sprayed condition while the others were subjected to high temperature cyclic ageing up to different fractions of their lifetime (this fraction of lifetime was also used to identify each single sample), defined as the number of cycles needed for spalling the TBC from the substrate. For both sets of TBC, three samples have been used to experimentally estimate their average lifetime, before ageing the remaining samples up to a certain percentage of the estimated lifetime. In particular, for set A, the TBC layer completely spalled off from the substrate while for set B, the TBC remained partially constrained to the substrate. Furnace cycle oxidation tests have been performed in the frame of two different projects at two different laboratories, each adopting its own specific cycle. TBC coupons have been made available for testing by the thermographic method independently by the two laboratories. Table 1 summarizes the main microstructural and thermal features of all the samples and the corresponding ageing parameters. In particular, the exposure time at high temperature within each ageing cycle (dwell time) is reported for both TBC sets. 2.2. Microstructural characterization The TBC microstructural characterization was performed using a Zeiss SUPRA 40 scanning electron microscopy (SEM) (Carl Zeiss NTS GmbH, Oberkochen, Germany). TBC specimens were impregnated in vacuum using epoxy resin to better distinguish between true pores and cracks and possible artifacts induced by cross-sectioning and polishing. Regarding the overall porosity evaluation, for each sample, five back-scattered electron images (BEI) taken along each sample cross-section were considered for the image analysis (IA) (Screen Measurement, Laboratory Imaging Ltd., Praha, CZ). For each sample, the percentage of cracked interface between the bond coat and the TBC has been estimated along the whole section, taking into account cracks longer than 25 μm within the first 30 μm from the interface promoted by ageing. 2.3. The thermographic technique Recently, a thermographic technique for non-destructively measuring the thermal diffusivity of TBCs deposited onto gas turbine hot path components such as blades and vanes has been developed [18]; thermal diffusivity α is defined as: α = k = ρC
ð1Þ
Fig. 2. Electron back-scattered images of the section for samples (a) B0% - as sprayed, (b) B5% and (c) B10%. Cracks close to the interface are clearly visible in aged samples.
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Fig. 3. Normalized thermal diffusivity of TBC vs. the percentage of cycles to failure for sets A and B.
The analytical one-dimensional two layer model adopted to fit the experimental data is: TðtÞ =
∞ n2 l2 Q0 n − c pffiffiffiffiffi 1 + 2 ∑ Γ e αc t εC πt n=1
ð2Þ
pffiffiffiffiffiffiffiffiffiffiffi −εs Where Q 0, ε = kρ C , Γ = εεc c + εs and lc are the absorbed energy density, the thermal effusivity, the reflection coefficient at the interface between coating and substrate and the coating thickness, respectively. The subscripts c and s refer to coating and substrate, respectively. The surface area over which the average temperature is computed must completely contain the diffusing heating spot which is typically a roughly circular-shaped spot with a diameter of some millimetres.
As heating source, a 1064 nm wavelength Nd:YAG laser (Theta Industries Inc., Port Washington, NY, USA) generating pulses of about 0.8 ms was used. The energy and the diameter of the heating shots were fixed roughly equal to 0.5 J and 10 mm, respectively. A snapshot focal plane array IR camera sensitive in the range of 7–10 μm (Jade II LW, CEDIP Infrared Systems, Croissy-Beaubourg, France) with a Noise Equivalent Temperature Difference of 30 mK was used to monitor for 3 s after the pulse the temperature transient of the sample surface at frame rate of 150 Hz. With this experiment duration the thermal diffusion length μ=
pffiffiffiffiffiffi αt
ð3Þ
Fig. 4. A sketch of the model used to estimate the percentage of the cracked BC–TBC interface. (a) The sound TBC layer, (b) the cracked TBC modeled as an in-series of two layers and (c) the in-parallel model representing a sample having both a cracked and a sound interface.
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Fig. 5. Normalized thermal diffusivity α/α0 vs. Larson Miller Parameter (LMP) for experimental data. Values of normalized thermal diffusivity for all the samples as estimated by Eq. (6) and literature data are also reported.
results greater than 1 mm, for typical as sprayed thermal diffusivity values of TBC. Owing to the TBC translucency, a thin layer of colloidal graphite was painted on the front sample surface in order to make the TBC opaque to the Nd:YAG laser radiation. Typical experimental uncertainty of this technique is in the range of 5–7%. More details about this technique can be found in the literature [18,19]. 3. Results 3.1. Microstructural characterization Figs. 1 and 2 show the microstructure of each sample. Table 1 summarizes the main microstructural parameters for each set. In particular, for both sets, the overall porosity, the TBC thickness and the percentage of the cracked interface are reported. Aged samples show significant percentages of cracked interface, even at low fractions of spent life (samples B5% and B10%). In particular, only 10% and 1% of the interface remained free of cracks within the first 30 μm of the TBC layer for samples A35% and A70%, respectively. Similarly for set B, only 31%, 5% and 1% of the interface remained free from cracks for samples B22%, B28% and B45%, respectively. Concerning crack length and linking as well as the percentage of the cracked interface, the results of the microstructural characterization are in fairly good agreement with the results reported by Herzog et al. [4]. In fact, at around 50% of the life, an almost complete crack linking with overall crack lengths in the millimetric range is observed.
Considering TBC sintering at high temperatures as the only phenomenon affecting thermal diffusivity variations of TBCs, a monotonic increase of normalized thermal diffusivity for both sets would have been expected. The non-monotonic trend of thermal diffusivity as a function of the spent life can be explained as the effect of the progressive microcrack growth and propagation parallel to the interface between the bond coat and the APS TBC top layer during cyclic oxidation tests. Owing to the relatively fast kinetics of sintering phenomena of TBCs [20,21], at the very beginning of cyclic oxidation ageing, sintering affects thermal diffusivity more significantly than interface damage caused by crack nucleation and growth and explains the observed initial thermal diffusivity increase. At longer times, crack growth becomes increasingly relevant with respect to sintering and promotes a decrease in the measured thermal diffusivity. When TBCs completely spall off from the substrate, the effect of cracks disappears, leaving only sintering phenomena to play a key role, and thus a further increase of
3.2. Thermal diffusivity measurements Table 1 summarizes the measured thermal diffusivity absolute values while Fig. 3 shows the normalized (with respect to the value of the as-sprayed sample) trend of thermal diffusivity as a function of the number of cycles to failure. Notwithstanding that the two sets of samples differed in thickness, both show non-monotonic trends. In fact, for both sets A and B, thermal diffusivity increases for fractions of life smaller than 35% and 28% respectively while at higher fractions (70% for sets A and 45% for set B) a decrease is observed. When the TBC completely spalled off from the substrate, thermal diffusivity reached its highest value (set A) while if TBC was still partially adherent to the substrate, a less significant increase was observed (set B).
Fig. 6. Percentage of the cracked interface as estimated by the proposed method vs. the percentage measured by IA along the section of the samples.
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thermal diffusivity is expected. A similar explanation for aged EB-PVD samples has been given by Azzopardi et al [22]. 4. An approach to a semi-quantitative estimate for the percentage of cracked BC–TBC interface Since thermal diffusivity of TBCs is affected by the progressive cracking inside TBCs close to the BC–TBC interface, some indications about the degree of damage at the interface can be obtained by measuring the thermal diffusivity of TBCs during ageing (real components measurements can be carried out during gas turbine maintenance). In fact, if the experimental variations of thermal diffusivity vs. the number of cycles are compared with the variations caused only by high temperature sintering of TBCs (a rough estimation of thermal diffusivity increase is possible when exposure time and temperature and TBC sintering kinetics are known), the percentage of the cracked interface would be roughly estimated, as described hereinafter. It should be clear that theoretical considerations and modelling have to be performed on the thermal conductivity (the field variable) which is the physical parameter mostly affected by sintering and crack nucleation and propagation. Since the ρC product is supposed to be only slightly affected by sintering and cracking, the ρC value for as sprayed TBC is fixed all along the modelling activity. Under such assumptions, ratios of thermal diffusivities correspond to the ratio of thermal conductivities. In any case, when thermal conductivity is required, it is estimated from the experimental thermal diffusivity, through Eq. (1) using the as sprayed ρC value. The simplest way to model sound and cracked TBCs is as a single sound TBC layer (see Fig. 4(a)) and a one-dimensional in-series twolayer composite where the first and the second layer are the TBC and a thin air gap, respectively (see Fig. 4(b)). In particular, the thermal conductivity of the latter case is
1 = ktwo−layers =
lTBC
. L
kTBC
+
lair
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Fig. 5 shows the experimental data reported in the literature, the corresponding best fitting straight line (i.e. Eq. (6)), the normalized thermal diffusivity for all the aged samples of sets A and B, as estimated by using Eq. (6) and experimental measurements. In particular, normalized thermal diffusivity values for samples aged at a spent life fraction smaller than 100% were lower than those values estimated by taking into account only sintering phenomena. On the contrary, according to the proposed interpretation, the normalized thermal diffusivity value for sample A100% was very close to the corresponding estimated value; the diffusivity value of sample B100% was lower because it was still partially adherent to the substrate. By applying the proposed procedure, the percentage of the cracked interface has been estimated, as summarized in Fig. 6. Estimations have been performed by fixing the crack thickness lair gap equal to 1 and 2 μm at early and later stages, respectively.
.
kair
L
ð4Þ
where l, k and L are the thickness and the thermal conductivity of each single layer defined by the subscripts and the overall thickness of the two-layer composite, respectively. If the thermal conductivity value of TBC is fixed within the model equal to the value estimated by considering only the effect of sintering, a rough estimation of the percentage of the cracked interface can be obtained by an in-parallel thermal conductivity model sketched in Fig.4 (c) as the relative weight f of the two-layer composite which equates k * defined as k* = f ktwolayers + ð1−f ÞkTBC
ð5Þ
with the corresponding experimentally evaluated thermal conductivity value of the TBC sample. For our purposes, the curve reported in the literature (referring to a set of freestanding APS TBC aged for different times at different temperatures) has been used [21]. In particular, from the knowledge of both the ageing temperature (1100 °C) and the number of cycles, the expected normalized thermal diffusivity increase α / α0 (α and α0 are the thermal diffusivity after a certain time exposure at a certain temperature and in the as- sprayed condition, respectively) caused by sintering has been estimated for aged samples of sets A and B, by using the following equation [21]: α = α0 = 7:85⋅10
−5
⋅LMP−1:2
ð6Þ
The Larson Miller parameter is defined as LMP = T(ln(t + 1) + 20) where temperature T and time t are expressed in Kelvin and hours, respectively.
Fig. 7. High magnification secondary electron images of the section of samples (a) A35%, (b) B10% and (c) B45%, where crack thickness resulted higher than that used in the model.
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5. Discussion In the case of these two sets of furnace cycled TBC it seems that most of the damage at the interface between the bondcoat and the TBC occurs during the first half of the life and then almost delaminated TBC (less affected by the thermal mismatch between TBC and substrate) “survives” for a long time before spalling off. In principle, for a more continuous progression of cracking coverage with time the technique would be sensitive even at higher fraction of spent life. In any case the situation for real components could be completely different. In fact, being components stressed during operation, as soon as a high fraction of crack coverage is reached, TBC would spall off rapidly. For most of the samples, the estimation of the cracked interface by using the proposed method is in fairly good agreement with the quantitative estimation performed along the section by image analysis. These results were obtained by fixing a crack thickness within the model as slightly thinner than the thickness experimentally observed at high magnification by scanning electron microscopy. In fact, as shown in Fig. 7, 3 and 2 μm are the average values of the crack thickness for sets A and set B, respectively. A possible explanation for the good results obtained using an air gap thickness thinner than the thickness observed by IA is probably related to the one-dimensional approximation of the model. In fact, owing to the one-dimensional approximation, the effects of finite dimensions of cracks have not been considered. In the real case, especially at early stages, several small cracks are present at the interface instead of a single crack. As reported in the literature [23–27], the effect on thermal conductivity of many cracks with a higher aspect ratio between their minor and major axes is smaller than the effect of a single crack with a much lower aspect ratio when the overall volumetric fraction is assumed to be equal in both cases; i.e., owing to the one-dimensional approximation, when the true crack thickness is considered, the model underestimates the percentage of the cracked interface especially at early stages where the number of cracks is high, but their size and their aspect ratio are very small. By considering a thinner air gap, a partial compensation of this underestimation effect is obtained. Future work will deal with the development of a 2-D model for data reduction and with the collection of a crack opening database. By this approach it will be possible to fix a suitable crack opening for data reduction and, if a statistically significant number of TBC samples aged at different times will be available for microstructural characterization, even an equation describing crack opening as a function of ageing time could be obtained. These data can be used for inverting experimental data for samples with known thermal history. For samples with unknown thermal history, the
only reasonable approach is to define a threshold value for the normalised thermal diffusivity, without giving a quantitative estimation of crack coverage, as explained also later. Since porosity usually decreases during ageing at high temperatures, when thermal conductivity is estimated from the experimental thermal diffusivity value by Eq. (1), the porosity fraction should be known for each sample. In fact, the density ρ of the porous TBC is linearly related to the porosity fraction. If the porosity cannot be measured, the porosity value should be either assumed to be the same for all the samples or a porosity reduction as a function of ageing time and temperature should be known. Since sintering usually reduces porosity by 1% to 5%, the assumption of a constant porosity fraction, independently from the ageing of the sample, causes the error in the thermal conductivity estimation to be of the same order of magnitude. This error is significantly lower than the thermal conductivity variations caused by sintering and cracking and by the above approximations. In any case, a recently developed photothermal technique seems to be able to simultaneously estimate the thermal diffusivity and density of TBCs and eliminates this error source [28]. As concerns pseudo-columnar TBC samples (set A), the width of vertical cracks appears to increase as a function of the ageing time. It is worth noting that the effect of these vertical cracks on measured through-the-thickness thermal diffusivity is negligible. Since this method could be applied in principle to real components, the TBC thickness has to be estimated by ND techniques. Eddy Currents equipment for coating thickness measurements is available on the market, but the value measured is usually slightly overestimated in respect to the average value measured by IA after sectioning the sample or the component because the probe lift-off is related to the highest roughness peaks. Fig. 8 shows that the relative error in estimating the percentage of the cracked interface caused by the TBC thickness overestimation is not significant as it is lower than 8% (even for an 100 μm overestimation) when a TBC of 400 μm is considered in the simulation. Furthermore, for coupons with an unknown thermal history, the thermal diffusivity measurement can still produce some information if at least the thermal diffusivity of the TBC in the as-sprayed condition is known. In this circumstance, a normalized thermal diffusivity threshold below which the TBC has to be considered close to failure could be properly defined. The application of this NDT technique to real components requires overcoming several difficulties related to the lack of both the thermal history of the blade and the as-sprayed thermal diffusivity all along the surface blade. Furthermore, during service the TBC surface can be
Fig. 8. The relative error in estimating the percentage of cracked interface vs. the TBC thickness overestimation. Simulations have been carried out fixing a true average TBC thickness of 400 μm.
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polluted with deposits that affect TBC thermal diffusivity measurements. Depending on the deposits nature this effect on measured thermal diffusivity can lead to an over- or to an under-estimation of the crack contribution. 6. Conclusions A new analysis of the combined effect of sintering and cracking on apparent TBC thermal diffusivity has been presented. The knowledge of sintering kinetics allows to semi-quantitatively estimate the fraction of cracked interface as a function of the ageing time. Notwithstanding the simplicity of the adopted model, satisfactory indications and a good agreement with image analysis have been obtained, when thermal history and as sprayed thermal diffusivity values of coupons are known. Acknowledgements This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between ERSE and the Ministry of Economic Development - General Directorate for Nuclear Energy, Renewable Energy and Energy Efficiency stipulated on July 29, 2009 in compliance with the Decree of March 19, 2009. The authors acknowledge the technical contribution of Dr. C. Rinaldi, Mr. S. Capelli and Mr. L. Lorenzoni of ERSE. References [1] V.P. Swaminathan, N.S. Cheruvu, in: V.P. Swaminathan, N.S. Cheruvu (Eds.), Advanced Materials and Coatings for Combustion Turbines, ASM International, Materials Park (OH), 1994, p. 1.
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