Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development

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Abstract

Lanthanum hexaaluminate (LHA) with a magnetoplumbite structure is a promising competitor to yttria partially stabilized zirconia (Y-PSZ) as a thermal barrier coating (TBC), since most zirconia coatings age significantly, including undesired densification at temperatures exceeding 1100 °C. The microstructure of calcined lanthanum hexaaluminate powders and thermally sprayed coatings show a platelet structure. The magnetoplumbite structure is characterized by the highly charged La3+ cation located in an oxygen position in the hexagonal close-packed structure of oxygen ions. Ion diffusion is strongly suppressed vertical to the crystallographic c-axis, thus hindering sintering densification. In contrast to the oxygen ion conducting zirconia, lanthanum hexaaluminate permits operating temperatures above 1300 °C because of its thermal stability and electrically insulating properties. This study describes the optimization of powder preparation for thermal spraying by spray drying and the development of parameters for atmospheric plasma spraying (APS) in order to produce homogeneous crystalline coatings with controlled micro-porosity and residual stresses. The phases were characterized by X-ray diffraction (XRD).

Introduction

TBCs are used to protect gas turbine components in power plants and aircraft jet engines, which are subjected to excessive temperatures and hot corrosion [1]. Besides the low thermal conductivity required, other important properties are high thermal-shock resistance, hot corrosion resistance and long-term stability at high temperatures.

The tendency to reach higher efficiency for gas turbines leads to increasing combustion-chamber temperatures. At temperatures above 1100 °C, rapid degradation of the zirconia coating starts and conventional Y-PSZ coatings do not fulfill the requirements for a reliable TBC [2], [3]. Microcracks and micropores begin to heal, which leads to densification of ceramic coatings. Tolerance against the internal stresses caused by the different thermal expansion behavior of the metal substrate and ceramic coating decreases distinctively, and flaws begin to form. As a consequence, the thermal conductivity of the coating increases, as well as thermal stresses on the substrate. The oxygen ion conductivity at temperatures above 700 °C accelerates this process and causes oxidation of the interlayer between the substrate and coating [4], forming thermally grown oxides (TGO). The swelling of the TGO layer is finally followed by chipping off of the TBC and failure of the predamaged component.

Atmospheric plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD) are the two most common deposition techniques for thermal barrier coatings. In some cases, low-pressure plasma spraying (LPPS), high-velocity oxygen fuel spraying (HVOF) and chemical vapor deposition (CVD) are also used [5]. TBCs deposited by APS have lower resistance to thermal cycling than those deposited by EB-PVD, but due to the comparatively cost-effective deposition conditions, high deposition rate and deposition efficiency, APS is a well-established, competitive deposition technique.

The ceramic coating is applied on top of a metallurgical interlayer, usually a MCrAlY (M=Co, Ni) alloy, which acts as a diffusion barrier and bonding agent and equalizes the different coefficients of thermal expansion of the ceramic coating and the substrate. The MCrAlY interlayer can be affected by oxidation at high temperatures.

In spite of similar thermophysical properties, LHA shows superior structural and thermochemical stability compared to Y-PSZ [6]. LHA crystallizes in the magnetoplumbite structure, a superlattice of the spinel structure (Fig. 1) [7].

According to the model of closest packed spheres, there are two general possibilities for stacking of oxide ions. In hexagonal structures, the sequence is AB in cubic ABC. In the magnetoplumbite structure of lanthanum magnesium hexaaluminate, one LaO3 layer (Fig. 2) is followed by four spinel layers (Fig. 3). The consequence is a mixture of hexagonal and cubic layers, leading to hexagonal symmetry in general and therefore to anisotropic physical properties.

The LaO3 layer represents a crystallographic mirror plane. The highly charged La3+ cation in the magnetoplumbite structure is located in an oxygen position and effectively suppresses the diffusion of oxide ions because of the minimized amount of vacancies in the structure, especially in the composition LaMgAl11O19. LHA crystallizes in the habit of platelets (Fig. 4). The random arrangement of these platelets causes a well-balanced microporosity, and therefore lower thermal conductivity.

The thermophysical properties are important for prediction of the performance of coating materials. Fig. 5 shows selected thermophysical properties of an APS-sprayed LHA coating [7]. The thermal diffusivity (α) was determined by the laser flash method, the heat capacity (cp) by thermoanalysis and the density (ρ=3.85 g cm−3) by the principle of Archimedes (theoretical density DX of LaMgAl11O19 is 4.285 g cm−3). The heat conductivity (λ) was calculated from these values (λ=ρ·cp·α).

Two important thermophysical properties influencing the lifetime of TBC materials are thermocycling and the thermal shock resistance. These are mainly influenced by the microstructure, coefficient of thermal expansion (CTE) and aging behavior of the TBC. A comparison of the aging behavior (50 h at 1300 °C) of LHA and Y-PSZ is shown in Fig. 6.

The shrinkage of LHA at the end of the experiment is approximately eight-fold less than that of Y-PSZ. During the first thermal loading up to a temperature of 1300 °C, the plasma-sprayed LHA showed a shrinkage of 1.7% between 800 and 900 °C and of 0.3% between 1100 and 1200 °C. The shrinkage can probably be correlated to initial crystallization, since the volume change is irreversible and only appears after the first thermal load. The crystalline phase is stable up to at least 1300 °C and shows no phase transformation once the LHA crystalline phase is formed.

The thermal expansion of the LHA between 100 and 1300 °C ranges from 7.7 to 9.3×10−6 K−1 and from 10 to 11.1×10−6 K−1 for Y-PSZ coatings [7]. The difference between the CTE of LHA and Y-PSZ decreases at higher temperatures because of the temperature dependency of the former. The thermomechanical and chemical properties of LHA have already been described in [7].

Section snippets

Powder processing

The initial stage of the powder processing is the fabrication of a fine-grained ‘raw powder’ with the desired stoichiometry (LaMgAl11O19) [6] that provides the advantageous thermophysical properties and outstanding thermal stability. In order to obtain a fine-grained reactive powder, lanthanum (III) oxide, bayerite [Al(OH)3] and magnesium hydroxycarbonate were used. Bayerite was utilized as the alumina precursor material. It provides an intermediate cubic stage (γ-Al2O3) after calcination that

Coating deposition and characterization

Recent work has shown that the APS technique is suitable for the deposition of LHA coatings [7], [10]. Due to the high thermal and kinetic energy of this technique, the LHA powder is fused in the plasma and propelled onto the substrate, where it forms the coating layer. The deposition via thermal spraying induces quenching of the coating material. Under these conditions (rapid heating and cooling), various materials tend to form partially non-crystalline phases or stay amorphous. At high

Results and discussion

Spray-dried granules with subsequent calcination were produced in an appropriate grain size distribution for thermal spraying of TBC. Fig. 8 shows a SEM image of the presintered granules. The granules are almost perfectly spherical; a closer look at the surface shows that they consist of randomly arranged LHA platelets. The size and distribution of the granules are specific for the spray drying equipment and the manufacturing conditions employed.

X-ray diffraction shows that the main component

Conclusions

The processing of spray-dried and calcined granules of LHA by atmospheric plasma spraying is a promising method for the development of advanced thermal barrier coatings. The grain size and size distribution of the spray powders were optimized. Spray drying conditions can be adapted from the pilot plant data to larger economic-scale drying systems by modifying parameters such as the drying temperature and air/fluid ratio of the slurry at the nozzle.

Sintering to thermochemically and structurally

Acknowledgments

We would like to thank Dipl.-Ing. Christian Friedrich, MTU Aero Engines GmbH, Munich, for his co-operation in APS spraying.

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