J. Mater. Sci. Technol., Vol.23 No.1, 2007

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• Review Application of Rare Earths in Thermal Barrier Coating Materials Xueqiang CAO Key Lab of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Changchun 130022, China [Manuscript received November 2, 2006]

Rare earths are a series of minerals with special properties that make them essential for applications including miniaturized electronics, computer hard disks, display panels, missile guidance, pollution controlling catalysts, H2 -storage and other advanced materials. The use of thermal barrier coatings (TBCs) has the potential to extend the working temperature and the life of a gas turbine by providing a layer of thermal insulation between the metallic substrate and the hot gas. Yttria (Y2 O3 ), as one of the most important rare earth oxides, has already been used in the typical TBC material YSZ (yttria stabilized zirconia). In the development of the TBC materials, especially in the latest ten years, rare earths have been found to be more and more important. All the new candidates of TBC materials contain a large quantity of rare earths, such as R2 Zr2 O7 (R=La, Ce, Nd, Gd), CeO2 -YSZ, RM eAl11 O19 (R=La, Nd; M e=Mg, Ca, Sr) and LaPO4 . The concept of double-ceramiclayer coatings based on the rare earth materials and YSZ is effective for the improvement of the thermal shock life of TBCs at high temperature. KEY WORDS: Rare earth; Thermal barrier coatings; Gas turbine

1. Introduction Ceramic coatings were first considered in the late 1940s[1] . The drive for the increased aircraft engine thrust and fuel efficiency has resulted in continual increases in hot section temperatures. Figure 1 shows the inner-structure of a gas turbine, of which the TBCs are clearly shown on the combustion room and turbine blades. Turbine blade materials are usually nickel-based superalloys whose incipient melting temperatures have been increased to 1589 K which has slowly approached the melting point of the nickel alloys, i.e. about 1672 K[2] . Several generations of superalloys have been developed over the past 30 years to make the increase of turbine inlet temperature possible. However, the limits of stress rupture, surface protection, and the melting point make this increasingly difficult. In addition, the amount of air that can be used for cooling in high-performance engines is limited. The use of TBCs has the potential to extend these advances in aircraft engine development by providing a layer of thermal insulation between the airfoil and the hot gas. With the turbine blade cooling technology available today, a 250 µm thick TBC can reduce the average metal temperature by 111 K to 167 K[3] . TBCs have been used successfully since the mid-1970s for life extension of combustor and afterburner components. The first application of TBCs for aerospace was developed by NASA. The TBC of CaO-ZrO2 /NiCr on the exhaust nozzle of the X-15 rocket plane is believed to be the first use of TBCs in the manned flight, and the use of 12Y2 O3 ZrO2 /NiCrAlY coated turbine J-75 blades marked the beginning of the modern era of TBCs[4] . Properties of some ceramics that can be used in heat engines are summarized in Fig.2 and marked with down-arrows. Prof., Ph.D., to whom correspondence should be addressed, E-mail: [email protected].

The selection of TBC materials is restricted by some basic requirements: (1) high melting point (>2173 K), (2) no phase transformation between room and the operation temperatures, (3) low thermal conductivity (10×10−6 K−1 ), (6) good adherence to the metallic substrate and (7) low sintering rate of the porous microstructure. Among these properties, thermal expansion coefficient and thermal conductivity seem to be the most important. The number of materials that can be used as TBCs is very limited and so far only a few materials have been found to basically satisfy these requirements. The development of new TBC systems has been described by Schaefer[6] and Cao[7] . In this review, the number before YSZ is for the molar percent of Y2 O3 in ZrO2 . Except the typical TBC material 3.94-4.52YSZ (zirconia stabilized by 3.94-4.52 mol fraction Y2 O3 , in weight percent 7-8 wt pct), several other ceramic coatings such as Al2 O3 , TiO2 , mullite, CaO/MgO-ZrO2 , YSZ, CeO2 YSZ, zircon and La2 Zr2 O7 , etc. have been evaluated as TBC materials. Rare earth oxides have very high melting points, low thermal conductivities and large thermal expansion coefficients, and the addition of any rare earth oxide into a ceramic material usually results in the reduction of thermal conductivity and the increase of thermal expansion coefficient which are two very important properties for a TBC material. The properties and application of rare earths in TBC materials are summarized in this review. China0 s paramount leader, Xiaoping Deng, indicated in 1992 when he inspected the south of China: “The Middle East has oil, and China has rare earths accounting for 80% known reserves of the world. Compared with the Middle East0 s oil, the status has very important strategic significance.”

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Fig.1 Inner-structure of a gas turbine whose combustion room and turbine blades are coated with TBCs (a) and the theory of TBCs (b)[5]

Fig.2 Linear thermal expansion coefficients (TECs) and thermal conductivities of heat engine materials. Numbers above bars are thermal-shock resistance parameters[8] , PSZ and FSZ are for partially and fully stabilized zirconia, respectively, and In 600 is a type of superalloy

2. Rare Earths Rare earth is a unitive term of 17 chemical elements including La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc. It also has the names of “Vitamin of modern industry” and magical “treasury of new materials” due to its special functions. It got the name of rare earth at the end of 18th century when it was discovered, by its undissolvable property which behaves like earth and the idea of that it was very rare and expensive. Actually, rare earth0 s content in the lithosphere is higher than those of normal metals such as Cu, Pb, Zn and Ag. The fancy name “rare earth” has been used up today. Rare earths are the basic elements of many functional materials with special properties such as luminescence, magnetism, electrical conductivity and H2 -storage. They are widely used in the information technology, energy system, metallurgy, environmental protection, spaceflight, defense technology and also agriculture. As shown in Table 1, rare earth elements have similar electronic structures and ionic radii, and therefore they have similar chemical properties. Rare earth elements are strong oxygen-binding, and oxides are the

most stable ones in their related compounds. Rare earth oxides absorb moisture and CO2 readily in air. Except for CeO2 , the other rare earth oxides are dissolvable in HCl and HNO3 . The melting points of rare earth oxides are above 2500 K and tend to increase with the atomic number, because the reduced ionic radius results in the increase of R-O bond strength. In Table 2 are listed the melting and boiling points of rare earth oxides. Except for the valence-changeable elements such as Ce, Pr, Eu and Tb, the other rare earth oxides have similar chemical, physical and mechanical properties. With Y2 O3 as an example, the stability, thermal and mechanical properties of rare earth oxides are briefly introduced. In the structural materials, the most popularly used rare earth element is Y.Normally, Y2 O3 is white, but it looks olive-green if it contains a large amount of oxygen vacancies. Y2 O3 has a cubic structure at high temperature (see Fig.3), which could be regarded as the derivation of the fluorite structure, in other words, half oxygen is lost in each semi-oxide of YO1.5 compared with ZrO2 . In its structure, only 6 corners are occupied by O atoms, and each crystal cell has 32YO1.5 , resulting in a large cell parameter of

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Table 1 Comparison of electronic structures and ionic radii of rare earth elements[9] Atomic No.

Element

39 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Y La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

R

Electronic structure R+2 R+3 R+4

1

2

4d 5s 5d1 6s2 4f1 5d1 6s2 4f3 6s2 4f4 6s2 4f5 6s2 4f6 6s2 4f7 6s2 4f7 5d1 6s2 4f9 6s2 4f10 6s2 4f11 6s2 4f12 6s2 4f13 6s2 4f14 6s2 4f14 5d1 6s2

0

4f2 4f4 4f6 4f7

4f13 4f14

4d 4f0 4f1 4f2 4f3 4f4 4f5 4f6 4f7 4f8 4f9 4f10 4f11 4f12 4f13 4f14

4f0 4f1 4f2

7

4f 4f8

R 0.1801 0.1879 0.1825 0.1828 0.1821 0.1811 0.1804 0.2042 0.1801 0.1783 0.1774 0.1766 0.1757 0.1746 0.1939 0.1735

Radius/nm R+2 R+3

0.1110 0.1090

0.0940 0.0930

0.0893 0.1061 0.1034 0.1013 0.0995 0.0979 0.0964 0.0950 0.0938 0.0923 0.0908 0.0894 0.0881 0.0869 0.0858 0.0848

R+4

0.0920 0.0900

0.0840 0.0840

Table 2 Melting (Tm ) and boiling points (Tb ) of rare earth oxides[9] R 2 O3

Tm /K

Y2 O3 La2 O3 Ce2 O3 Pr2 O3 Nd2 O3 Pm2 O3 Sm2 O3 Eu2 O3

2708 2593 2513 2568 2598 2593 2618 2598

Tb /K 3620 3730 3760 3760 3780 3790

R2 O3

Tm /K

Tb /K

Gd2 O3 Tb2 O3 Dy2 O3 Ho2 O3 Er2 O3 Tm2 O3 Yb2 O3 Lu2 O3

2713 2658 2683 2693 2698 2703 2708 2763

3900 3900 3900 3920 3950 4070 3980

Fig.4 Crystal structure of the rare earth oxide as functions of temperature and ionic radius[10] . A=hexagonal structure whose cation is coordinated with 7 oxygen atoms, B=monoclinic structure, C=cubic structure, H=normal hexagonal structure

for Y2 O3 is: ∆G0f (Y2 O3 , s) = −1.923 × 106 + 3.125 × 102 T (1) In the temperature range of 2500 K-2700 K, the free energy of formation for YO is: ∆G0f (YO, g) = −4.598 × 104 + 5.392 × 101 T (2) The free energy of thermal decomposition for Y2 O3 can be expressed with the following equations: Fig.3 Crystal structure of Y2 O3

1.0604 nm. The theoretical density of Y2 O3 is 5.03 g·cm−3 and the melting point is about 2708 K. The cubic structure transforms into the hexagonal at 2543 K and then into the monoclinic structure at 673 K-1173 K. The crystal structure as functions of ionic radius and temperature is shown in Fig.4. At elevated temperatures, Y2 O3 has a high thermal stability. In the temperature range of 1800 K2000 K, the free energy of formation ∆G0f (J·mol−1 )

Y2 O3 (s) = 2YO(g) + O

(3)

∆G0f = −1.831 × 106 + 2.047 × 102 T

(4)

The experimental value of free energy of thermal decomposition for Y2 O3 is 0.704×106 kJ·mol−1 . The loss of some oxygen at elevated temperature and adsorption of oxygen when it is cooled are reversible, and this process does not have influence on the crystal structure of Y2 O3 . Young0 s modulus of the fully-densified Y2 O3 is E0 =174.4 GPa and it has a relationship with the

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porosity (p): E/E0 =1-2.18p. Between 298 K and 1273 K, the averaged thermal expansion coefficient and heat capacity of Y2 O3 are 8.3×10−6 K−1 and 0.543 J·g−1 ·K−1 , respectively. In this temperature range, there is no phase change. Y2 O3 is very sintering-resistant and has a low sintering activation energy of 96.14 kJ·mol−1 , the sintering process obeys the equation of ∆l/l0 =ktn (t is time, k is a constant and n≈0.007). With the help of high pressure (1223 K, 5 MPa), Y2 O3 could reach a relative density above 99%. Under vacuum or in H2 atmosphere, the sintering temperature for a full densification is above 2173 K. Among all the rare earth elements, three of them (Ce, Pr, Tb) are thermally unstable and valencechangeable. The oxides of Ce have many forms, including CeO2 , Ce2 O3 , CeO1.818 , CeO1.778 and CeO1.714 , and CeO2 is the most stable. At elevated temperatures in air, all these oxides would be oxidized to CeO2 . At 1573 K, CeO2 could be reduced into Ce2 O3 by H2 or C. The oxides of Pr are also complicated, including Pr2 O3 , PrO2 , Pr6 O11 and Pr7 O12 . Pr6 O11 is the most stable among them, and it can be expressed with the formula Pr2 O3 ·4PrO2 which is dark at room temperature (Pr2 O3 looks apple-green). In the temperature range of 473 K-623 K in air, Pr2 O3 will be oxidized into PrO2 . The oxides of Tb mainly include Tb2 O3 , TbO2 and Tb4 O7 . Between 873 K and 1073 K, Tb4 O7 would be reduced to Tb2 O3 by H2 . 3. Application of Rare Earths in TBC Materials 3.1 Zirconia stabilized by rare earths 3.1.1 Stabilization of zirconia Zirconia with the formula of ZrO2 has three phases, including the cubic (C-ZrO2 ), the tetragonal (T-ZrO2 ) and the monoclinic (M-ZrO2 ) phases. Under the atmospheric pressure, the pure ZrO2 exists in the C, T or M-phase. Above 3.5 GPa, there are still several other tetragonal phases which are unstable and have no observable influence on the mechanical properties of the material. C-ZrO2 has the fluorite structure with the space group of Fm3m and is stable only above 2643 K up to its melting point of 2983 K. Between 1443 K and 2643 K, C-ZrO2 transforms into T-ZrO2 (space group P42 /nmc) whose structure is very similar to the former. Compared with the structure of C-ZrO2 , T-ZrO2 is a little bit elongated and the O atoms shift along the c-direction[11,12] . M-ZrO2 is stable below 1443 K, it has the space group of P21 /c whose cell parameters are a=0.51454 nm, b=0.52075 nm, c=0.53107 nm and β=99.23◦[13] . The phase change of T→M is the most complicated and interesting among all the phase changes. In this process, the crystal cell volume is expanded for about 3.5%, corresponding to the decrease of theoretical density from 6.1 g·cm−3 to 5.8 g·cm−3 and may lead to the formation of microcracks. On the other hand, this is also believed to be the reason for the toughening of zirconia composite materials[14] . The phase transformation processes of both the pure ZrO2 and 4.52YSZ are simply expressed in Fig.5. Due to the structural instability, pure ZrO2 is useless in structural materials. With the addition of elements of valence +2 (MgO, CaO), +3 (rare earth

oxides) or +4 (HfO2 , CeO2 ), zirconia could exist in the C or T-phase even down to room temperature. Among all the structural stabilizers of zirconia, Y2 O3 functions the best. For the TBC material, the addition of Y2 O3 is usually between 3.94 mol fraction and 4.52 mol fraction (partially stabilized zirconia), and may increase to 12.0 mol fraction (fully stabilized zirconia) for other applications such as the solid electrolyte of SOFC (solid oxide fuel cell) and the oxygen sensor. One of the stabilization mechanisms of zirconia by foreign elements is the formation of oxygen vacancy (VO•• ) which could be expressed with the following expressions (Kr¨oger Vink notation):

VO•• 2−

0.05Y2 O3 + 0.9ZrO2 = Zr0.9 Y0.1 O1.95

(5)

•• × 0 2YY + 2Zr× Zr → 2YZr + V O + 1/2O2

(6)

has two positive charges, leading to the shift of towards VO•• and far away from Zr4+[11,15] . The phase transformation of 4.52YSZ is some different from that of pure ZrO2 and is dependent on the heat treatment (see Fig.5). The C-phase of 4.52YSZ (C-4.52YSZ) transforms into another Cphase with higher Y2 O3 -content and the T-phase with lower Y2 O3 -content during annealing between 2573 K and 873 K (Process (1) in Fig.5). The Cphase with higher Y2 O3 -content transforms into the T-phase which would transform into the M-phase during annealing below 873 K. However, if C-4.52YSZ is quenched such as the coating processes of plasma spraying (PS, cooling speed about 106 K·s−1 ) and physical vapour deposition, the Y3+ -diffusion would be suppressed and the C-4.52YSZ would transform into a tetragonal phase whose Y2 O3 -content is the same as that of C-4.52YSZ (Process (2) in Fig.5). This phase is named the nontransformable tetragonal phase (T0 -phase), therefore in the 4.52YSZ coating made by PS or electron beam-physical vapour deposition (EB-PVD), the main phase is T0 (>80%). During long-term annealing at high temperatures, the T0 phase would also transform into the C and T-phases and later into the M-phase due to the Y3+ -diffusion. In the process of PS, if the melting condition of the TBC material is not perfect (for example, the powder is too fine or too coarse, the duration of the powder in the plasma flame is short), the coating would contain a lot of M-phase, resulting in a short life of the coating. Figure 6 shows the phase diagrams of the Y2 O3 ZrO2 system. The earliest report about YSZ was in the beginning of 1950s[16] , and such research was popular in 1960s[17,18] . The stabilization of zirconia by Pr2 O3 has been studied by Corradi[20] . The function of Pr2 O3 is similar to that of Y2 O3 , but the stabilization mechanism of the former seems to be more complicated than that of the latter. The addition of Pr2 O3 into ZrO2 at high temperature not only leads to the formation of oxygen vacancies in the crystal cell of ZrO2 , but Pr2 O3 also decomposes as described by the equation Pr3+ →Pr4+ . 1 mol fraction PrO1.5 can only partially stabilize the T-phase and the product is a mixture of the T and Mphases, and the T-phase transforms into the M-phase above 1273 K. 10 mol fraction PrO1.5 can fully stabilize the T-phase, and the main product of the heat treatment at 1273 K for 2 h is still the T-phase. O

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Fig.5 Phase transformation processes of pure ZrO2 and 4.52YSZ

Fig.6 Phase diagrams of the Y2 O3 -ZrO2 system in the whole (a) and low (b) Y2 O3 -content range, respectively[19]

Both CeO2 and ZrO2 have the same crystal structure, i.e, cubic fluorite. Even though the ionic radius of Ce4+ is larger than that of Zr4+ , in the solid solution of CeO2 -ZrO2 , it was found that Ce4+ selectively diffused into the crystal lattice of ZrO2 but the opposite process was not observed[21] . For instance, if the mixture of YSC (Y2 O3 -CeO2 ) and YSZ (Y2 O3 -ZrO2 ) is heated above 1573 K, Ce4+ would selectively diffuse into the crystal lattice of ZrO2 , leading to the formation of the solid solution of Y2 O3 -CeO2 -ZrO2 . 3.1.2 YSZ coatings 3.94-4.52YSZ is the most widely studied and used TBC material because it provides the best performance in high-temperature applications such as diesel engines and gas turbines[24–32] . 10.7-12YSZ coatings have also been studied[33,34] . The YSZ coatings made by EB-PVD often perform better than that made by PS due to its columnar microstructure. This structural characteristic imparts excellent strain tolerance to the coating and the coating has an excellent adhesion to the substrate. The microstructures of 4.52YSZ coatings made by PS and EB-PVD are compared in Fig.7. The content of the T0 -phase in the coating is critical to the life of the coating, in other words, it should be as high as possible. It is clearly indicated in Fig.8 that the content of the T0 -phase has a sudden decrease

above 1473 K, and this is why the coating can not be used for long-term application above 1473 K. YSZ coating has been proved to be more resistant against the corrosion of Na2 SO4 and V2 O5 than the zirconia coating stabilised by CaO or MgO[28] . A major disadvantage of YSZ is the limited operation temperature (