Verlag des Forschungszentrums Jülich

JUEL-4127
Cao, Xueqiang
Development of New Thermal Barrier Coating Materials for Gas Turbines
II, 117 S., 2004



1 . Abstract

Keywords: Thermal Barrier Coatings, Materials, Lanthanum Zirconate, Lanthanum Cerate,
Spray-Drying

New thermal barrier coating (TBC) materials for gas turbines were studied in this work. There are numerous factors to determine the application lives of TBCs, and those factors can be basically grouped into:

1. Properties of the material: melting point, thermal expansion coefficient, thermal conductivity, phase stability between room temperature and the application temperature, sintering ability, chemical stability, purity of the material, etc.
2. Preparation technique of TBCs: there are several techniques to prepare TBCs, such as Electron Beam-Physical Vapor Deposition (EB-PVD), Plasma Spraying (PS) and High Velocity Oxyfuel (HVOF). Due to the special columnar structure of TBCs made by EB-PVD, the as-resulted coatings have much longer thermal cycling lives than those made by other techniques. The preparation procedure of each technique also has great influence on the coating quality.
3. Microstructures of TBCs : distribution of the micropores, direction of the microcracks (vertical or parallel to the coating surface), thickness of the coating, graded distribution of the compositions, etc.
4. Other factors: properties of the bond coat, pre-treatment of the metal substrate, morphology of the powder for the coating, etc.

In this thesis, I concentrated my energy on the improvement of two of the most important properties of the material for TBCs, i.e. thermal expansion coefficient and phase stability, and also the powder preparation by spray-drying. Following are the main points of the work:
1.1. Spray-drying is the process by which a fluid feed material is transformed into a dry powder by spraying the feed into a hot medium. The feed materials are either water-based suspensions with air as the drying gas or ethanol-based suspensions with nitrogen as drying-gas. In general, there are five effects to influence the characteristics of spray-dried powders, i.e. type of atomizer, solid content of the suspension, feeding speed, drying temperature and feeding pressure. Powder characteristics such as size distribution, shape, density and purity have a significant influence on the thermal spraying process and the coating properties .
Spray-dried powders offer an advantage in the application in TBCs where a high porosity (10-20%) is necessary. On the other hand, particles of the spray-dried powders are usually spherical and free-flowing, the powder can be easily fed by the plasma gun. However, not all the spray-dried powders are suitable for thermal spraying. The powder should be dense enough and have a proper size distribution. If the powder is too fine and too porous, the powder can not enter the centre of the plasma flame and will float on the flame surface or be evaporated by superheating before sputtering on the substrate, resulting in a poor deposition efficiency and poor coating-bond strength. In our case, the powder with high density and large particle size is preferred for the plasma spraying of coatings. In this work, we tried to improve the quality of TBCs by using the spray-dried powders and look for new candidates for high temperature TBCs (> 1523 K) . The drying machine used in this work was Mobile Minor ^TM'2ooo' Ex Model H (Niro A/S), pneumatic nozzle and a drying capacity 5 Kg(water)-h^-1 . The inner diameter Ø _inner of the drying chamber was 0.85 m. The two most important properties of spray-dried powders to determine the coating quality are density and particle size. Polyethyleneimine (PEI) acts as both an organic binder and a dispersant giving low viscosity in the suspension . The optimized suspension composition is : ≥ 3 .6 vol% powder + 1 .8 wt% PEI + ethanol, and operational parameters of the drying machine: drying temperature 448 K, feeding rate 55 cm³ -min^{- 1}, feeding pressure 1 .013 x 10⁴ Pa.

1.2. Lanthanum zirconate (La₂Zr₂0₇, LZ) is a newly proposed material for TBCs . The crystal structure consists of the corner-shared Zr0₆ octahedra forming the back-bone of the network and La3+ ions fill the holes which are formed by 6 Zr06 octahedra. It can largely tolerate vacancies at the La³⁺ , Zr⁴+ and 0^2--sites without phase transformation. Both La³⁺ and Zr⁴⁺-sites can be substituted by a lot of other elements with similar ionic radii in case the electrical neutrality is satisfied, giving rise to the possibility of its thermal properties to be tailored. It is one of the few oxides with pyrochlore structure (such as La₂Hf₂0₇, Pr₂Hf₂0₇ , CeZr₂0₇ and Sm₂Ti ₂07) that are phase-stable up to their melting points (2573 K for LZ) and this is a major reason that it is believed to have potential as TBC material. The phase diagram of the La₂0₃-ZrO₂ system indicates that a wide solubility range for the pyrochlore phase with compositions ranging from 0.87(La₂0₃) x 2(ZrO₂) to 1 .15(La₂0₃) .2(ZrO₂) exists. After long-term annealing at 1673 K or thermal cycling, both LZ powder and plasma-sprayed coating still keep the pyrochlore structure . A preferred crystal growth direction in the coating was observed by X-ray diffraction . A considerable amount of La₂0₃ in the powder was evaporated in the plasma flame, resulting in a nonstoichiometric coating . Additionally, compared with the standard TBC material, i.e. zirconia stabilized with 8 wt% yttria (8YSZ), LZ coating has a lower thermal expansion coefficient which leads to higher stress levels in a TBC system. The thermal conductivity and thermal expansion coefficient of LZ are: 1 .56 W.m^-1.K^-1 (2 .1-2 .2 W.m^-1 .K^-1 for YSZ, bulk materials, 1273 K), 9.1-9.7 x 10^-6 K^-1 (10.5-11 .5 x 10-6 K-1 for YSZ, bulk materials and coatings, 298-1273 K) respectively. On the other hand, LZ has much a lower ionic conductivity than 8YSZ (9.2 ± 0.3 x 10^-4 Ω ^-1 .cm^-1 for LZ and 0 .1 Ω ^-1-cm^-1 for 8YSZ, 1273 K, air) due to the existence of stable oxygen Frenkel pairs. Therefore, it is expected that LZ is less oxygen-transparent than 8YSZ and may provide a better bond coat oxidation resistance at high temperatures if their coatings have similar porosities .

1.3. The thermal cycling life of LZ coating is shorter than that of 8YSZ due to its low toughness and lower thermal expansion coefficient. This disadvantage has beenovercome by using layered structure with 8YSZ. A different approach is to increase the thermal expansion coefficient of LZ. Ce0₂ was used to substitute Zr0₂ because materials containing Ce0₂ usually have higher thermal expansion coefficient and lower thermal conductivity than 8YSZ. La₂Ce₂0₇ (LC) is a solid solution of La₂0₃ in Ce0₂ . This solid solution has fluorite structure with 1/8 O-sites as vacancies . Its XRD pattern looks the same as that of Dy₂Hf₂0₇ (defect fluorite) and similar to that of LZ (pyrochlore), but the latter has two weak peaks between 40 and 50 degrees (Cu-K, radiation) that the former does not have . These two peaks help us to distinguish the fluorite and pyrochlore structures.
Mixtures of LZ and LC were heated at 1673 K for 2 x 24 h. The experiment results of X-ray diffraction (XRD) indicate that La₂(Zr_{o .9}Ce_{o .1} )₂0₇ is not a single phase, consisting of LZ and LC, and La₂(Zr_{o ,3}Ce_{o ,7})₂0₇ is almost a single phase with a trace of LZ. The ionic radius of Ce⁴+ (0.97 Å, CN = 8) is much larger than that of Zr⁴⁺ (0.79 Å, CN = 8), therefore it is easy for LZ to be solved into LC but the opposite is difficult.
LC is proposed as a new material for TBCs which has higher thermal expansion coefficient and lower thermal conductivity than 8YSZ. When LC was quenched into cold water (about 301 K) from 1273 K, 873 K and 573 K, it still keeps the fluorite structure, and after long-term annealing at 1673 K, the crystal structure of LC was still stable. LC has an average thermal expansion coefficient of 12.6 x 10^-6 K^-1 between 573 K and 1473 K. The thermal expansion improvement can be attributed to the large size of Ce⁴⁺ and the partial reduction of Ce⁴+ to Ce³⁺ at elevated temperatures, similar to the thermal expansion of La_1-x,Sr_xCo_1-yFe_y0_3-&delta when it is heated. However, it has a sharp decrease (thermal contraction) between 473 K and 573 K with a minimum temperature of 523 K. Furthermore, the higher is the content of oxygen vacancy in the La₂0₃-CeO₂ solid solutions, the more serious is this thermal contraction, implying that the thermal contraction is a result of oxygen vacancy. The reason of the thermal contraction for LC is still not clear. During the plasma spraying process, LC lost some Ce0₂ . Ce0₂ itself has high a melting point (2873 K) but it will be partially reduced to Ce₂0₃ whose melting point is much lower (2193 K) in the reducing atmosphere of the plasma flame. The thermal cycling test of LC coating at 1523 K indicates that this coating has a much better thermal shock resistance than LZ coating and it is comparable to or even better than the coating of the traditional material 8YSZ. The thermal cycling life of the LC coating is strongly dependent on the composition of the coating . The composition of the coating whose thermal cycling life is the longest (3238 cycles) is La₂Ce_{2, o9}0_7.18 , a small deviation from it has an obviously negative influence on the life of the coating.

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