Surface & Coatings Technology 285 (2016) 120–127

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Surface & Coatings Technology

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A novel low-thermal-conductivity plasma-sprayed thermal barriercoating controlled by large pores

Masayuki Arai a,⁎, Hiroya Ochiai b, Tatsuo Suidzu c

a Department of Mechanical Engineering, Faculty of Engineering Division I, Tokyo University of Science, 6-3-1, Niijuku, Katsushika-ku, Tokyo 125-8585, Japanb Graduate School of Engineering, Department of Engineering, Tokyo University of Science, 6-3-1, Niijuku, Katsushika-ku, Tokyo 125-8585, Japanc Thermal Spraying Technology R&D Laboratory, TOCALO Co., Ltd., 14-3, Minamifutami, Futami-Cho, Akashi, Hyogo 674-0093, Japan

⁎ Corresponding author.E-mail address: marai@rs.tus.ac.jp (M. Arai).

http://dx.doi.org/10.1016/j.surfcoat.2015.11.0220257-8972/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 September 2015Revised 11 November 2015Accepted in revised form 14 November 2015Available online 15 November 2015

In order to reduce the thermal conductivity of thermal barrier coatings (TBCs) for land-based gas turbineapplications, porous TBC (P-TBC) with a microstructure consisting of a distribution of large open pores was de-veloped. A powder mixture of zirconia and polyester was used to form such open pores within the microstruc-ture. The results showed that the thermal conductivity of the P-TBC monotonically decreased as the porosityincreased. In addition, compared with conventional TBC with a thermal conductivity value of 1.2 W/(mK), thethermal conductivity of P-TBC with a polyester content 15 wt.% achieved a value as low as 0.3W/(mK). This im-plies that the presence of large open pores leads to a dramatic reduction in the thermal conductivity, and thethermal conductivity can be freely controlled by adjusting the polyester content of the powder. A SEM-image-based finite element model was constructed. The FE analysis revealed that the presence of pores disturbed theheat flow. Thus, compared with the relatively straight lines that occurred for the TBC, which possessed fewpores, it was considered that the presence of open pores within the P-TBC increased the length of the heatflow lines. This provides an explanation for the reduction in the apparent thermal conductivity of the P-TBC.

© 2015 Elsevier B.V. All rights reserved.

Keywords:Gas turbineTBCLow thermal conductivityPorosityFEMSEM-image-based model

1. Introduction

The thermal efficiency of a land-based gas turbine can be effectivelyimproved by increasing the turbine inlet temperature (TIT). However,such improvement increases the temperature of metallic parts, such asthe blade, stator, and combustor within the gas turbine. In order to pro-tect these parts from the combustion gas environment, thermal barriercoatings (TBCs) are usually deposited onto the outer or inner surfaces ofthe parts. TBCs typically consist of two layers, namely, bond coat and topcoat. The bond coat improves the adhesive strength between the topcoat and substrate, andprevents high-temperature corrosion of the sub-strate; the top coat prevents heat transfer from the combustion gas tothe substrate. Thus, materials with low thermal conductivity, such aspartially-stabilized zirconia, are generally chosen. Recently, a top coatmaterial with lower thermal conductivity has been developed.

Top coatmaterialswith a very low thermal conductivity are typicallydeveloped through the addition of dopants and controlling the micro-structure of the materials. Several studies have been focused on theaddition of dopants to the material. Shen et al. [1] focused on the co-doping of zirconia with either Y3+ and Ta5+, or Yb3+ and Ta5+ to benon-transformable under a high-temperature environment. TheYbTaO4-doped and YTaO4-doped zirconia showed thermal conductivity

values of 1.5 W/(mK) and 2.0 W/(mK), respectively, compared withthat of 3 W/(mK) for a well-known dense 8YSZ material. On additionof the dopants, it was considered that there was a reduction in the ther-mal conductivity because of a photon scattering due to the formation ofoxygen vacancies. For identical reasons to the aforementioned study,Zhao et al. [2] examined the influence of Ti addition on the thermal con-ductivity. The results showed that thermal conductivity of Ti-doped YSZdecreased as the Ti content increased. In particular, the 8TiYSZ materialdemonstrated the lowest thermal conductivity of 1.98 W/(mK) at1273 K. The study also showed that the Ti-dope YSZ material had im-proved phase stability and fracture toughness, and thus Ti-doped YSZcould potentially be used as a new TBC material. Rahaman et al. [3] fo-cused on Gd2O3 rather than Y2O3, as a phase stabilizer in ZrO2. They re-ported that the sintering temperature increased with Gd2O3-doping,and the associated thermal conductivity of the plasma-sprayed Gd2O3-ZrO2 coating was 2.3 W/(mK). Marocsan et al. [4] reported that thethermal conductivity of dysprosia-stabilized ZrO2 (DyPSZ), depositedby plasma spraying, was 0.76 W/(mK). Thermal conductivity was alsosignificantly influenced by the plasma-spraying conditions; the coatingdepositedwith a lower plasmapower had a lower thermal conductivity.

Furthermore, studies have also been conducted on the thermal con-ductivity of microstructure-controlled coatings. Kulkarni et al. [5] re-ported on the thermal conductivity of TBCs deposited by atmosphericplasma spraying (APS) using four types of powers, namely, fused andcrushed (F&C), sol–gel (SG), agglomerated and sintered (A&S), and

Table 1Spraying parameters for the bond coat.

Spray gun Sulzer-Metco UniCoat + F4-MB

Powder AMDRY 9952Blasting condition White alumina (#54), 0.6 MPaPlasma gas Ar + H2 gasSpray distance 120 mmTorch traverse speed 750 mm/sPitch 5 mmCoating thickness 0.1 mm

Table 2Spraying parameters for the top coat

(a) Common parameters

Spray gun Sulzer-Metco UniCoat + F4-MB

Powder gas flow 3.0 nlpmPlasma gas Ar + H2 gasTorch traverse speed 750 mm/sPitch 5 mm

(b) TBC

Powder 204NS-G

Spray distance 120 mmParticle speed 152 m/sParticle temperature 2579 KSubstrate temperature 410 KCoating thickness 1.0 mm

(c) P-TBC

Powder 204NS-G/polyester

Spray distance 120 mmParticle speed 71 m/sParticle temperature 2507 KSubstrate temperature 424 KCoating thickness 1.0 mm

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plasma densified (HOSP) powders. The results showed that the porositydecreased in the following order: A&S N SG N HOSP N F&C, and the ther-mal conductivity was decreased in the following order: HOSP N SG N

A&S N F&C. These results indicate that it is difficult to determine a rela-tionship between the porosity and thermal conductivity. In particular,the HOSP coating achieved a thermal conductivity value as low as 0.7W/(mK). Kulkarni et al. demonstrated that the inter-lamellar pores,micro cracks and globular pores within the microstructure strongly af-fected themacroscopic thermal conductivity. Jordan et al. [6] developedthe TBC with layered porosity, referred to as Inter-Pass Boundaries, bythe solution plasma spray process (SPPS), and associated thermal con-ductivity reached 0.63 W/mK. Thus, they demonstrated that the cyclicfurnace durability and the erosion resistance of the developed coatingsare comparable to those of APS TBC used in current gas turbine blade.The thermal conductivity of plasma-sprayed TBC is known to be lowerthan that of electron beam physical vapor deposited (EB-PVD) TBC.However, the EB-PVD technique significantly enhances the interfacialstrength of the TBC, and thus it can be used to manufacture airplanejet engine blades, especially. Hass et al. [7] and Gu et al. [8] developeda unique microstructure with a zig-zag shaped column, which was fab-ricated by rotating the sample within the chamber during the electronbeam deposition process. This approach aims to produce a heat flowpath along the zig-zag structure for the physical reason that the heatflowpath of the zig-zag structure is longer than that of a straight columnstructure. They showed that the associated thermal conductivity de-creases as the wavelength of the zig-zag decreases. By introducingsuch a complex microstructure, the thermal conductivity of EB-PVDTBC achieved a value similar to that of the plasma-sprayed TBC, approx-imately 1.0 W/(mK).

Currently, to develop advanced TBCs with low-thermal conductivityfor applications such as gas turbines for electric power generation, boththe cost of the deposition, and the enhanced mechanical properties ofthe coatings are important. In consideration of this, the plasma sprayingtechnique is still attractive.

In this study, in order to reduce the thermal conductivity of TBCs de-posited by a typical APS technique, excellent thermal insulation can po-tentially be achieved by the development of the microstructures with adistribution of large open pores that contain air. The formation of suchopen pores within the microstructure of the TBC can be achieved by uti-lizing a powder mixture of zirconia and polyester. The melting tempera-ture of polyester is very low; therefore, polyester sites can be easilyevaporated by performing a post heat treatment, which consequentlyleads to the formation of the open pores. Herein, scanning electron mi-croscopy (SEM) was used to determine the size of open pores and mea-sure the microstructural porosity of the top coats deposited withpowders of various zirconia/polyester weight ratio. The results of thecharacterization were compared with the apparent thermal conductivitymeasured by a laser flash technique. Finally, the thermal conductivity ofthe porous-TBC, which is simply denoted as P-TBC in this paper, will beexamined by conducting finite element analysis on the SEM image-based model.

2. Specimen preparation

In this study, austenitic stainless steel type 304 was prepared as thesubstrate, with a geometry of 50 × 30 × 4.9mm. Co-based alloy powder(CoNiCrAlY; AMDRY 9952) was deposited as the bond coat by APS; thethickness of the bond coatwas 0.1mm,which is typical of coating on anactual gas turbine blade.

Table 1 shows the bond coat spraying conditions. The top coat wassubsequently deposited with the following powder mixtures: zirconia(204NS-G)/polyester = (100 wt.%/0 wt.%), (95 wt.%/5 wt.%), (90 wt.%/10 wt.%), and (85 wt.%/15 wt.%). Following the deposition of the topcoat, a post heat treatment was conducted at 1073 K for 2 h using anatmospheric electric furnace to evaporate the polyester within the topcoat. Table 2 shows the top coat spraying conditions. In this study,

P-TBC is denoted as P-TBC (-), and the final digit (-) corresponds totheweight content of the polyester. For instance, the top coat consistingof zirconia/polyester = (90 wt.%/10 wt.%) is denoted as P-TBC (10).However, the top coat consisting of zirconia/polyester = (100 wt.%/0wt.%) is still denoted as TBC. Fig. 1 shows a cross-sectional SEM imagesof the TBC and P-TBC microstructure. As shown in Fig. 1(a), the TBC mi-crostructure is highly dense, and does not possess any large defects. Fur-thermore, open pores, linked with neighboring pores, were uniformlydistributed within the P-TBC microstructure, especially in P-TBC (15).

3. Measurement procedure

A laser flash measurement system (LFA-502; Kyoto Denshi KogyoCo., LTD., Japan) was utilized to measure the apparent thermal conduc-tivity of a freestanding top coating extracted chemically from the TBCand P-TBC. Fig. 2 shows a diagram of the measurement system appara-tus used in this study. The thermal conductivity was assessed by themethod discussed in the following section. A temperature-timeelevation curvewas obtained by radiation thermometer when the sam-ple surface was heated by laser irradiation. Subsequently, the thermaldiffusivity, α, was evaluated by applying a half-time method to thetemperature–time elevation curve. Using the same equipment, theheat capacity, cp, was also measured by differential scanning calorime-try. Consequently, the thermal conductivity could be evaluated usingthe following simple formula:

λ¼ρcpα ð1Þ

where ρ is the density of the sample.

Fig. 1. Cross-sectional SEM images of P-TBC. (a) TBC, (b) P-TBC (5), (c) P-TBC (15).

Fig. 2. Diagram of the laser flash measurement system used to measure thermal conduc-tivity of freestanding coating.

Fig. 3. Image of the open pores extracted by image binary processing for the P-TBC (10)sample.

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A sample of the freestanding top coat that was chemically extractedfrom the TBC and P-TBC coated plate was prepared for thermal conduc-tivity measurements. Theweight and size of the sampleweremeasuredto determine the density, ρ, and the surface of the sample was paintedblack prior to the laser flash measurement. Following the preparation,the sample was set in the equipment.

In order to characterize themicrostructure of the TBC and P-TBC, theporosity was evaluated from the SEM image by the following proce-dure: to extract the areas of the open pores, the SEM image shown inFig. 1 was initially transformed into a black and white image by image

binary processing as shown in Fig. 3. The porosity was subsequentlyassessed by using the open pore area to total picture area ratio.

4. Measurement results and discussion

Fig. 4 shows the relationship between the porosity and polyestercontent of the P-TBC. The porosity linearly increases with the polyestercontent, except in the case of 0 wt.% polyester. The TBC (correspondingwith polyester 0 wt.%) did not conform to this linear relationshipbecause of the different pore structures of thematerials; the TBCmicro-structure is characterized by small cracks and globular pores, while theP-TBCmicrostructure is characterized by large open pores. The differentmicrostructures can easily be compared in Fig. 1. Consequently, it wasconfirmed that this linear relationship is owing to the pore density, be-cause the average size was almost identical for all different polyestercontents.

Fig. 5 shows the relationship between the porosity and thermalconductivity at near-room temperature. Here, the average porosity isplotted against the polyester content. The thermal conductivity of theP-TBC decreases as the porosity increases. The thermal conductivity of

Fig. 4. Relationship between porosity and polyester content of TBC and P-TBC.Fig. 6. Temperature dependence of thermal conductivity.

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the P-TBC (15) achieved a value as low as 0.3 W/(mK), compared with1.2 W/(mK) for the TBC; this implies that the presence of large openpores leads to a dramatic reduction in the thermal conductivity. Inaddition, it is recognized that the thermal conductivity can be freelycontrolled by adjusting the polyester content of the powder by follow-ing the monotonic curve of Fig. 5.

Fig. 6 shows a comparison of the thermal conductivity at varioustemperatures under different polyester contents for the P-TBC. Thethermal conductivity of the TBC (1.2 W/(mK)) at 300[K] is reduced to1.0 W/(mK) at 570 K. This could be attributed to photon scatteringwhich commonly occurs in zirconia ceramics [9,10]; however, thereduction in temperature is very small. In contrast, for all polyestercontents, there is relatively little change in the thermal conductivity ofthe P-TBCs with the increased temperature.

A number of theoreticalmodels have been developed to estimate theapparent thermal conductivity, λ, of an idealized microstructure withregularly distributed spherical pores with identical radii. In this study,we have focused on the Voigt and Reuss model (V-R model), andHashin–Shtrikman model (H-S model) [11].

Fig. 5. Relationship between porosity and thermal conductivity measured at near-roomtemperature.

The V-R model is expressed by:

λ¼ 1−ϕð ÞλYSZ þ ϕλPORE upper limitð Þλ ¼ λYSZλPOREð Þ

1−ϕð ÞλPORE þ ϕλYSZf g lower limitð Þ ð2Þ

and the H-S model is expressed by:

λ¼ 1−ϕð ÞλYSZ þ ϕλPORE−1−ϕð Þϕ λYSZ−λPOREð Þ2

3λYSZ− 1−ϕð Þ λYSZ−λPOREð Þupper limitð Þ

λ¼ 1−ϕð ÞλYSZ þ ϕλPORE−1−ϕð Þϕ λYSZ−λPOREð Þ2

3λPORE þ ϕ λYSZ−λPOREð Þlower limitð Þ

ð3Þ

where, λYSZ is the thermal conductivity of the zirconia matrix, λPOREis the thermal conductivity of the air in the pores and ϕ(0bϕb1) isthe porosity. To evaluate these expressions, it was assumed thatλYSZ=1.29 W/(mK) and λPORE=0.026 W/(mK).

Fig. 7 shows a comparison of thesemodels and our experimental re-sults. The experimental data correlates the upper and lower lines of the

Fig. 7. Comparison of the thermal conductivity determined by theoretical models and ex-perimental results.

Table 3FE calculation parameters.

Ambient temperature T 298 K

λYSZ 1.29 W/(mK)λPORE 0.0257 W/(mK)h 100 W/(m2K)Model thickness t 0.00098 m

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theoretical models. The theoretical models can roughly estimate thethermal conductivity of the P-TBC. However, these models, whichinclude some assumptions to simplify the calculation, cannot preciselydetermine the thermal conductivity as a function of porosity.

In the following chapter, a strict analysis will be conducted using aSEM-image-based model with the aid of finite element procedure.

5. SEM-image-based finite element analysis

Here, to evaluate the heat flow within the complicated microstruc-ture and the changes in the associated thermal conductivity with thevarious porosity levels, a SEM-image-based finite element model wasconstructed.

The SEM-image-based numerical model was prepared by the fol-lowing procedure: first, a cross-sectional SEM image of a P-TBC coatedsample was stored as bmp image, and it was subsequently transferredinto a dxf digital format by binary image processing. This digital datawas directly recorded by the MENTAT pre-processor of the MARC com-mercial finite element solutionMARC. An outline of this image data wasextracted along the boundary using the curve function. The areaenclosed by this contour line was divided by an auto-mesh tool. Fig. 8shows the finite element meshes obtained.

As boundary conditions, a fixed temperature condition was pre-scribed for the top surface T2 and the following convection heat transferin the lower boundary was taken into account.

q¼−h T−T1ð Þ ð4Þ

Fig. 8. SEM-image-based FE model. (a) TBC, (b) P-TBC (10).

where T is the ambient temperature, T1 is the lower surface temper-ature and h is the heat transfer coefficient. The apparent thermalconductivity, λ, of the entire model, which incorporates open pores,can be assessed from a combination of Fourier's law and Eq. (4). Here,λYSZ was identified by adjusting its value to be identical to that of theTBC. The ambient temperature, T, was fixed at 298 K under the top

Fig. 9. Temperature contour plot of P-TBC analyzed under the assumption of insulationconditions at pore wall. (a) TBC, (b) P-TBC (5), (c) P-TBC (15).

Fig. 10. Comparison of thermal conductivity values obtained from experimental data andthose estimated by FE analysis under the assumption of insulation conditions at the porewall.

Fig. 12. Idealized shape and arrangement of poremodel. (a) Pore shapes, (b) distance, (c)tilt angle, (d) aspect ratio.

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surface temperature change as analysis parameter. Regarding the othercalculation parameters, the heat transfer between the pore wall and airwithin the pores, and the insulation conditions at the pore wall wereboth considered. The calculation parameters are listed in Table 3.

Fig. 9 shows the temperature contour plots for the TBC and P-TBC. Inthis case, insulation conditions were assumed. As the color changesfrom yellow → red → blue, the temperature decreases. It is obviousthat the heat flow is disturbed by the existence of pores. In particular,as the size of the pores increases, a heat flux accumulates around thepore wall. Thus, in comparison to the relatively straight line that oc-curred for the TBC, which possessed few pores, the presence of openpores increased the length of the heat flow line length for the P-TBC.This provides an explanation for the reduced apparent thermal conduc-tivity of the P-TBC.

Fig. 10 shows the comparison of our experimental results and thethermal conductivity at near-room temperature obtained from the FEresults calculated under the assumption of insulation conditions at theporewall. It is recognized that our experimental results generally corre-late with the FE results. In contrast with this result, the FE results, whichwere calculated under the assumption of heat transfer between thepore wall and the air within the pores, revealed that the influence ofheat transfer at the pore wall is very small.

Fig. 11. Comparison of thermal conductivity at 570 K.

Fig. 11 shows the comparison of the FEMdata and experimental dataat 570 K. Under high temperature conditions, the thermal conductivityvalues estimated by the FEM do not correlate with the experimentaldata. However, the variation in the thermal conductivitywith the poros-ity level showed a similar trend. This disparity can be attributed to thedifferences in the porosity levels between the actual SEM image andthe associated finite element model. Specifically, on construction ofthe SEM image-based model, the globular pores and micro crackswere eliminated from the actual SEM image. The presence of thesemicro defects may affect the estimation of the thermal conductivity inthe finite element calculation. However, the thermal conductivity ofP-TBC (15), which was estimated by the finite element model underthe assumption of heat conduction within the pores, is very similar tothe experimental data. Thus, the heat conduction mechanism withinthe pores in addition to the heat transfer based upon the lattice wavesin a ceramic solid are of significant importance rather than the presenceof a small defect. Furthermore if the thermal conductivity is estimated

Fig. 13. Influence of pore shape on apparent thermal conductivity.

Fig. 15. Variation in thermal conductivity with aspect ratio.

126 M. Arai et al. / Surface & Coatings Technology 285 (2016) 120–127

by a numerical calculation, the radiation transfer between pore wallswill become significant for temperatures greater than 1200 K.

6. Discussion

The thermal conductivity of the TBC (1.0W/(mK)) was dramaticallyreduced to 0.3 W/(mK) by the introduction of large open pores. Withregard to coating deposition process technology, it is clear that the ther-mal conductivity of ceramic TBC can be controlled by alternating thepolyester content of the powder mixture. In this section, we considermethods of future reducing the thermal conductivity under a fixed po-rosity level; therefore the influence of the pore geometry on the appar-ent thermal conductivity is of interest. To achieve this, finite elementanalysiswas performedusing an idealizedmodel; several arrangementsof various kinds of pore shapes were used.

For this trial, we focused on the influence of the idealized shape andarrangement of pores on the apparent thermal conductivity. Under afixed porosity level, the model parameters, namely, the pore shape,distance between pores, and the tilt angle and aspect ratio of the ellipseassumed as the idealized pore shape were varied as shown in Fig. 12.

Fig. 13 demonstrates the influence of the pore shape on the apparentthermal conductivity. In this case, a square, circle, rhomboid, triangle,and inverted triangle were sequentially assumed as the idealizedshape. The application of the various pore shapes had little influenceon the thermal conductivity. Specifically, the heat flow disturbancethat occurred due to the various angles of the shapes was not an impor-tant factor.

Fig. 14 demonstrates the influence of the angle of the ellipse on thethermal conductivity. The thermal conductivity increased as the tiltangle was increased. Fig. 15 demonstrates the change in the thermalconductivity with respect to the aspect ratio of the major and minoraxes of the elliptical pore. It shows that the thermal conductivity de-creases as the aspect ratio decreased, which implies that a samplewith needle-shaped poreswould achieve the lowest thermal conductiv-ity. Here, we consider the projection area of the pores normal to theglobal heat flow direction. A decrease in the tilt angle and aspect ratioleads to a reduction in the projection area of the pore, which indicatesa reduction in the area of the heat-flow path. Thus, to reduce the ther-mal conductivity effectively, it can be recommended that the poresshould be arranged to reduce the projection area normal to the heat-flow direction.

In order to achieve a structure consisting of large needle-shapedopen pores, the shape of the powder and spraying conditions will bestudied in future work. This should result in the development of an ad-vanced TBC with lower thermal conductivity.

Fig. 14. Variation in thermal conductivity with regard to the tilt angle of the elliptical pore.

7. Conclusion

In this study, P-TBC consisting of amicrostructurewith a distributionof large open pores containing air, which functioned as excellentthermal insulation was developed to reduce the thermal conductivityof TBCs for land-based gas turbine applications. A powder mixture ofzirconia and polyester was used to develop such open pores withinthe microstructure. The results showed that the thermal conductivityof P-TBC monotonically decreases as the porosity increases. In particu-lar, the thermal conductivity of the P-TBC with a polyester content15 wt.% achieved a value as low as 0.3 W/(mK), compared with that of1.2 W/(mK) for conventional TBC. This implies that the presence oflarge open pores dramatically leads to a reduction in the thermal con-ductivity, and the thermal conductivity can be freely controlled byadjusting the polyester content of the powder.

A SEM-image-based finite element model was constructed to inves-tigate both the heat flows within the complicated microstructure, andthe influence of the porosity on the associated thermal conductivity.The results showed that the heat flow was disturbed by the presenceof pores within the microstructure. Thus, compared with the relativelystraight line that occurred for the TBC, which possessed few pores, itwas considered that the presence of open pores within the P-TBC in-creased the length of the heat flow lines. This provides an explanationfor the reduction in the apparent thermal conductivity of the P-TBC.

Finally, based on the finite element analysis, the influence of an ide-alized shape and arrangement of pores on the apparent thermal con-ductivity was discussed. From the results, it was determined that thethermal conductivity could be effectively reduced if the pores were ar-ranged to reduce the projection area normal to the heat flow direction.

In order to check commercial acceptance of the P-TBC developedhere, cyclic electric furnace test and erosion resistance test will beperformed. Now we have developed the surface modification of P-TBCsystem by introducing a high-power laser irradiation technique.

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