Microstructure and Thermal Conductivity

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Sm2Zr2O7 coating. The value of effective thermal conductivity in this case is. 7.75% higher. It is 5.8% higher for 50% wt. 8YSZ - 50% wt., 4.7% higher for 25% wt.
Defect and Diffusion Forum Vol. 336 (2013) pp 91-96 Online available since 2013/Mar/25 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/DDF.336.91

Microstructure and Thermal Conductivity Characteristics of Sm2Zr2O7+8YSZ type TBCs Grzegorz Moskal1, a, Marta Mikuśkiewicz1 1

Silesian University of Technology, Department of Materials Science, 40-019 Katowice ul. Krasińskiego 8, Poland a

[email protected]

Keywords: thermal barrier coatings, microstructure, thermal properties, samarium zirconates

Abstract. The article presents results of microstructural investigations of plasma sprayed ceramic thermal barrier coatings obtained using Sm2Zr2O7+8YSZ and 8YSZ powders in 50/50, 25/75 and 75/25 proportions. The coatings thickness is evaluated and spherical, horizontal and vertical porosity is quantitatively and qualitatively characterized. The results obtained in this investigation were used for adjusted values of thermal conductivity calculations on the basis of results obtained from thermal diffusivity measurements at temperatures between 20 and 1100°C. Introduction The requirements that modern gas turbines have to face are most of all related to increase in the efficiency with simultaneous fulfillment of strict environmental protection policies. One of the ways to meet these requirements is application of thermal barrier coatings (TBC) characterized by ever improving insulation properties. It allows to increase the operating temperature and not influence the durability of metallic components of the turbine, such as combustion chamber. The consequence of these expectations are extensive research investigations seeking for an alternative for yttria stabilized zirconia (8YSZ - ZrO2-8Y2O3) which is the common material applied as the outer insulating layer of TBCs. It is expected that the new type of a ceramic coating will make it possible for the turbine to operate above 1200°C while maintaining phase stability of the ceramic material and thermo-chemical compliance with the substrate material and the bond coat [1-3]. Selection of the new material for the outer insulating layer of TBC is very restricted. Most important restrictions include: (1) high melting temperature, (2) lack of phase transformations in the range between room and operating temperature, (3) low thermal conductivity, (4) chemical inertness in contact with the bond coat and the TGO (thermally grown oxide), (5) possibly low coefficient of thermal expansion mismatch with the substrate material, (6) good adhesion to the substrate material and (7) low tendency for sintering of the porous microstructure [4,5]. No information concerning a material that would meet all these requirements has been published so far. Currently applied 8YSZ oxide is merely a compromise between these requirements. Its main disadvantage is a limited long term operation at 1200°C. At higher temperatures the tetragonal phase transforms to a cubic and monoclinic phase which generates unfavorable stresses and consequently leads to cracking and degradation of the ceramic layer [5,6]. Researches concerned on inventing a new type of ceramic material meeting these requirements (1-7) have been intensively conducted in the last several years. Most promising materials seem to be rare earth zirconates of pyrochlore lattice and general formula A2B2O7. It particularly concerns zirconates of La2Zr2O7, Nd2Zr2O7 i Sm2Zr2O7 type [7-15]. These materials exhibit improved insulating properties in comparison to conventional 8YSZ oxide [16-18], they are characterized by higher phase stability as a function of temperature [19], as well as do not show the tendency to react with 8YSZ oxide [20], which is of importance in the case of DCL layers (double ceramic layers) [21,22] and “composite” coatings of RE2Zr2O7 + 8YSZ type, which are the topic of this paper. Data concerning this type of TBCs cover publications characterizing plasma sprayed “composite” ceramic layers consisting of 8YSZ oxide and Al2O3 [2326], 8YSZ and mullite [27], 8YSZ and Gd2Zr2O7 [28]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 157.158.129.110, Silesian University of Technology, Katowice, Poland-13/08/13,09:34:30)

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The main issue limiting the application of A2B2O7 type compounds in TBCs is their high tendency to react with Al2O3 oxide and to form at least one of the aluminate phases. There is a high tendency of pyrochlore compounds to react with Al2O3 oxide present in the TGO zone which leads to the formation of aluminum phases, for instance REAlO3 type phase of perovskite structure, which rapidly reduces the durability of the TBC [29-37]. This phenomena does not occur in the case of 8YSZ phase because both these oxides do not form transition phases and their mutual solubility is significantly limited [38-40]. Procedure of experiment The study was conducted on three types of TBCs characterized by the following proportions of feedstock powders: 50% wt. 8YSZ - 50% wt. Sm2Zr2O7, 25% wt. 8YSZ - 75% wt. Sm2Zr2O7 and 75% wt. 8YSZ - 25% wt. Sm2Zr2O7. Ceramic layers were obtained using the air plasma spraying (APS) method. The substrate used in this research was nickel superalloy AMS 5599 with a 125 µm thick NiCrAlY bondcoat obtained using the vacuum plasma spraying (VPS) method. The scope of the research covered microstructural evaluation of the ceramic coating using light and electron scanning microscopy, phase composition analysis using XRD from the surface as well as qualitative porosity analysis (spherical, horizontal and vertical). The thermal diffusivity using the laser-flash method was analyzed in the range of 20-1100oC. On the basis of the obtained results and taking into account porosity measurement results an adjusted value of the thermal conductivity was calculated. For this purpose a model (Eq. 1) was used which allows to determine effective thermal conductivity λ [41]:

λ λ

cot

dens

=

1   2 

f

2

 Ph   x 1 − P s 

f [P ] + f 1

s

1

 Ps   x 1 − P h 



f [P ]  2

h

(1),



where: Ph – horizontal pores fraction, Ps – spherical pores fraction, f1 – function expressed by Eq. 2 formula, f2 – function expressed by Eq. 3 formula: 3 f [P ] = 1 − 2 P 1

s

s

(2)

f

2

 2 P  l  [ Ph ] = 1 −  h    π  t 

(3),

where: l/t – average length to thickness of the horizontal pores ratio. The accuracy of the last formula has been experimentally confirmed in investigations concerning modeling of various shaped pores on the value of thermal conductivity of plasma sprayed ceramic layers [41,42]. Results of investigations Exemplary results of microstructural and phase composition investigations of Sm2Zr2O7+8YSZ type TBCs are presented in Fig. 1-3. The morphology of the insulating layer in all cases is typical for APS process with visible pores and microcracks on the lamellar grains. The pores are uniformly distributed on the whole cross-section of the ceramic layer regardless of the phase composition of particular areas (bright areas are Sm2Zr2O7 phase while darker ones are 8YSZ). Distinct boundaries between the areas of these phases presence indicate that no transient phases were formed.

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Fig.1. Microstructure and phase constituents of 50% wt. 8YSZ - 50% wt. Sm2Zr2O7 TBC.

Fig. 2. Microstructure and phase constituents of 25% wt. 8YSZ - 75% wt. Sm2Zr2O7 TBC. Porosity measurements allowed to extract the spherical and horizontal constituents that have the most significant influence on the thermal conductivity. A summary of porosity and TBCs thickness measurements is presented in Tab. 1. The thickness of the insulating layer in all cases are very similar. Some differences can be noticed in the case of porosities though. The highest overall and spherical porosity is exhibited by the coating containing 75% wt. of 8YSZ, it is also the thickest one. The difference in porosity in comparison to the TBC containing 25% wt. of 8YSZ is almost 40%.

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Fig. 3. Microstructure and phases constituent of 75% wt. 8YSZ - 25% wt. Sm2Zr2O7 TBC. Table 1. Results of qualitative and quantitative analysis of porosity in TBC systems. Parametr

8YSZ

50 8YSZ – 50 Sm2Zr2O7

25 8YSZ – 75 Sm2Zr2O7

75 8YSZ 25 Sm2Zr2O7

Total porosity [%]

13,06

9.85

7.88

12.69

Vol. of vert. pores [%]

3,10

2,09

1,34

2.37

Vol. of horiz. pores [%]

4,94

3,40

2,82

4.24

Vol. of speroid. pores [%]

5,12

4,36

3,72

6.08

Av. leng. of horiz. pores [µm]

6,75

6,31

5,75

6.05

Av. thick. of horiz. pores [µm]

5,34

5,35

5,44

5.14

Lengt.to thick. ratio of horiz. p.

1,26

1,18

1.06

1.18

Thick.of ceramic layer [µm]

290

240

230

250

The thermal conductivity investigation results for TBCs are presented in Fig. 4. These results were obtained applying double layer model taking into account thermal resistance (Proteus® by Netzsch software) and were compared with results for 8YSZ. The basic measurement was done for the thermal diffusivity. The value of the thermal conductivity was calculated on the basis of the specific heat, density and coefficient of linear expansion as a function of temperature [17]. It should be noted that all composite coatings exhibit better insulating properties than conventional 8YSZ coating. These results were adjusted using Eq. 1 and on that basis the value of the effective thermal conductivity independent of the porosity for insulating layers Sm2Zr2O7 + 8YSZ was determined. The lowest thermal conductivity in the whole range of test temperatures is exhibited by 75% wt. 8YSZ - 25% wt. Sm2Zr2O7 coating. The value of effective thermal conductivity in this case is 7.75% higher. It is 5.8% higher for 50% wt. 8YSZ - 50% wt., 4.7% higher for 25% wt. 8YSZ - 75% wt. Sm2Zr2O7 and 7.91% higher than in the case of conventional 8YSZ TBC.

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Fig. 4. Thermal conductivity (left) and effective thermal conductivity (right) of TBC`s. Summary The results of this investigation proved that the thermal conductivity of all studied composite coatings is lower than that of 8YSZ and Sm2Zr2O7 type TBCs. From a morphological point of view the structure of the insulating layer is typical for plasma sprayed ceramic coatings and is characterized by the presence of spherical, horizontal and vertical pores. The influence of porosity allows to lower the thermal conductivity of the ceramic layer by about 7.75% in the utmost case. Acknowledgment We gratefully acknowledge the financial support of the Structural Funds in the Operational Program, Innovative Economy (IE OP) financed by the European Regional Development Fund, Project No. POIG.0101.02-00-015/09. References [1] [2] [3]

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Microstructure and Thermal Conductivity Characteristics of Sm2Zr2O7+8YSZ Type TBCs 10.4028/www.scientific.net/DDF.336.91