Novel Low Thermal Conductivity Ceramic Materials for Thermal

Novel Low Thermal Conductivity Ceramic Materials for Thermal

Barrier Coatings

Wei Pan 1, Qiang Xu 1, Longhao Qi 1, Jingdong Wang 1, Hezhuo Miao 1,

Kazutaka Mori 2 and Taiji Torigoe 2

1 State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and

Engineering, Tsinghua University, Beijing, 100084, China

2 TAKASAGO Research & Development Center, MITSUBISHI Heavy Industries, LTD., HYOGO,

676-8686, Japan Keywords: Ceramics, Thermal barrier coatings, Low thermal conductivity

Abstract. The increase of the efficiency for gas turbines leads to the increasing combustion-chamber temperatures. Rapid degradation of the conventional yttria-stabilized zirconia coatings does not fulfill the requirements at these temperatures for a reliable thermal barrier coatings (TBCs) due to the phase transformation of zirconia and the sintering behaviour. Therefore, it is very important to develop novel ceramic materials for TBCs with low thermal conductivity and long-term stability at high temperatures. In this paper, the developments of potential novel ceramic materials for TBCs with low thermal conductivity are reviewed.

Introduction

With the rapid development of advanced gas-turbines, many parts worked in the combustion system will be suffered from high temperature, high thermal stress, high thermal shock and corrosion. In order to improve the work reliability and lifetime, a thermal insulation layer can be coated on the surface of the parts. The use of the layer can result in large temperature decrease between the hot gas and the metal substrate, thereby ensure that the parts can work at relative lower temperature. These thermal insulation coatings were titled as Thermal Barrier Coatings (TBCs) [1]. Generally, these TBCs are comprised of a top coat, a bond coat and a metal substrate. The top coat with favorably low thermal conductivity, serves as the insulator and the bond coat mediates contact between the top coat and metal alloy substrate [2]. However, there is a growing demand for even higher temperature and higher engine efficiency, providing great motivation for developing alternate TBCs ceramics with lower thermal conductivities that will: (i) extent the life of gas-turbine components, (ii) reduce cooling requirements (thereby decreasing fuel consumption), or (iii) allow for an increase in gas inlet temperatures (thereby increasing thrust) [3].

Thermal barrier coatings of 6-8wt% Yttria Stabilized Zirconia (YSZ) are currently in production to protect metals in the hot sections of military and commercial aircraft gas turbine engines. However, there are two major disadvantages on YSZ materials. The first is that the phase transformation of zirconia will be occurred and results in the failure of TBCs [4]. The second disadvantage is that YSZ begins to sinter at elevated temperature and results in an increase in Young’s modulus and therefore in higher mechanical stresses because of the differential thermal expansion of the coating and base material [5]. This behaviour will increase the thermal conductivity of the coatings. In part, the thermal conductivities of YSZ are higher, which are 2～2.3 W/(m·K) [6]. Therefore, some researchers begin to investigate some novel materials for TBCs with lower thermal conductivity. For this paper, some novel ceramic materials for TBCs are reviewed.

Novel Ceramic Materials for TBCs

YSZ-M x O.

In order to further improve the operating temperature of TBCs, Ibegazene et al reported

some new materials composed of x ％(mol) HfO 2

(x = 0, 25, 50 and 100) + ZrO 2, partially stabi-

lized by 4.53％(mol) Y 2O 3. XRD studies showed

that a metastable, non-transformable, tetragonal

solid solution was the only phase. The results

showed that the phase-transition temperature of

ZrO 2 increased with the addition of HfO 2 [7].

Jones added Sc 2O 3 into YSZ and found that the

phase stability of Sc 2O 3-Y 2O 3-ZrO 2 was better

than that of YSZ at 1400°C [8]. Er 2O 3, NiO, Nd 2O 3, Gd 2O 3 and Yb 2O 3 were added into YSZ to reduce the thermal conductivity of the materials by J.R. Nicholls. The thermal conduc- tivities of the YSZ with the additives were 1.0 W/(m·K), while those of YSZ were between 1.5 and

1.9 W/(m·K) [9].

LaMgAl 11O 19. LaMgAl 11O 19 ceramic with the magnetoplumbite structure has been proposed as a candidate material for TBCs [10-12]. The ceramic consists mainly of alumina, which has been doped by MgO and La 2O 3. In order to obtain a fine grained reactive powder, lanthanum (III) oxide, bayerite (Al(OH)3) and magnesium hydroxycarbonate are used as raw materials. This coating composition ages significantly slower than commercial zirconia based TBCs at a temperature of 1573K (Fig.1)

[11]. LaMgAl 11O 19 ceramic shows an outstanding structural and thermochemical stability up to 1673K and insulating properties comparable to YSZ. The random arrangement of lanthanum hexaaluminate platelets causes a well-balanced microporosity, and lower the thermal conductivity of LaMgAl 11O 19 ceramic [12].

Garnet. Polycrystalline garnets ceramics with compositions of Y 3Al x Fe (5-x)O 12 (x = 0, 0.7, 1.4, and 5.0) are being considered for TBCs [13]. Y 3Al 5O 12 (YAG), in particular, has superior high-temperature mechanical properties and excellent phase/thermal stability up to the melting point (2243K). YAG, with its high-temperature thermal conductivity comparable to that of YSZ but oxygen diffusivity about 10 orders of magnitude lower than in zirconia, implying that YAG can give a better protection to the bond coat.

LaPO 4. Lanthanum phosphate (LaPO 4) ceramic is considered as a potential material for TBCs owing to its high-temperature stability, its low thermal conductivity and high thermal expansion comparable to those properties of dense zirconia [14]. In addtion, LaPO 4 is expected to be relatively inert to molten salts containing sulfate or vanadate ions because the basic lanthanum cation is strongly bonded to the very acidic phosphate group. Stoichiometric LaPO 4 does not react with alumina, which is a positive attribute, but also bonds poorly to it, which is a limitation for the application. LaYbO 3. LaYbO 3 ceramic is a perovskite type material with a high melting point. It is investigated as a potential material for TBCs at temperature above 1573K owing to the low thermal conductivity and the promising low sintering activity. Moreover, long-term annealing tests showed that phase stability of this ceramic from room temperature to temperature above 1573K [15].

Ln 2Zr 2O 7. Ln 2Zr 2O 7 ceramic are a relatively new addition to the menu of TBCs mateirals [16]. They tend to preserve their pore content and architecture to higher temperature, with concomitant benefits to the long-term strain tolerance and insluating efficiency because they exhibit sluggish sintering kinetics.

La 2Zr 2O 7 ceramic has been largely investigated in recent years as a candidate material for TBCs due to the lower elastic modulus, lower thermal conductivity (the thermal conductivity at 973K is

1.6W/m·K), better high-temperature stability and equivalent fracture toughness comparable to YSZ ceramic [17]. Moreover, B. Saruhan investigated 3wt.% Y 2O 3-doped La 2Zr 2O 7 composition with pyrochlore structure by EB-PVD processing [18]. The role of Y 2O 3 can be postulated as that the

Fig.1 Comparison of the aging behavior of lan-

thanum hexaaluminate and Y-PSZ at 1573K [11]

presence of the growth conditions in favor of pertinent pyrochlore structure formation due to the smaller ionic radius of Y2O3 by occupying the vacancies in the lattice. Notably, doping of La2Zr2O7 with 30%Nd, Eu or Gd revealed a systematic variation with ionic mass of the dopant cation and yielded a maximum reduction in thermal conductivity from ~1.55 to ~0.9 W/m·K for Gd at 1073K [19].

In addition, some other Ln2Zr2O7 ceramics with the pyrochlore structure (Ln = Eu, Gd, Sm, Nd) were studied. The results showed that the thermal conductivities of these ceramics were ranging between ~1.5 and ~1.6 W/m·K at 1073K [20-22]. These results are shown in Table 1. It can be seen that the thermal conductivities of Ln2Zr2O7 ceramics are lower than those of YSZ and the thermal conductivities decrease with the atomic number of rare earth element increasing. The main heat carrier in Ln2Zr2O7 ceramics is thought to be phonon. With the increasing of the atomic number of rare earth element, the atomic radius increases gradually and the probability of the phonon umklapp scattering increases. Hence the thermal conductivities of Ln2Zr2O7 ceramics decrease with the atomic number increasing from La to Gd with the same pyrochlore structure.

Table1 Results of thermal conductivities of Ln2Zr2O7 ceramics

Materials Thermal Conductivity at 1073K [W/m·K] Reference

La2Zr2O7~ 1.7 [17]

Nd2Zr2O7~ 1.6 [23]

Sm2Zr2O7~ 1.5 [23]

Gd2Zr2O7~ 1.6 [23]

Outlook

The recent investigations on novel ceramic materials for TBCs reveals vigorous research and development activities combined with growing needs that bode well for the future of this field. According to the recent literatures about TBCs, the critical research and development needs of ceramic materials for TBCs have been described as follows:

Functionally gradient materials (FGMs) are perhaps the most exciting development in the ceramic materials for TBCs. FGMs serve to level off the differences in properties between the ceramics and the substrate so that there is no discernable interface.

The thermal conductivity of the ceramic materials can be reduced further by doping some oxides or nanocrystalline structure. Moreover, the modelling of the low thermal conductivity should be presented. Moreover, the sintering activity of the ceramic materials should be investigated by the examination of the micorstructure and the porosity.

The interfacial reaction between the ceramic materials and the thermally grown oxide (TGO) should be explored in detail. Any high-temperature chemical reaction between the ceramic materials and TGO, and the attendant generation of reaction products and stresses, will adversely affect the durability and performance of TBCs.

Acknowledgments

This work was supported by a grant for National Natural Science Foundation of China, under Contract No. 50232020.

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High-Performance Ceramics III

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Novel Low Thermal Conductivity Ceramic Materials for Thermal Barrier Coatings

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