Improving Atmospheric Plasma Spraying of Zirconate Thermal Barrier Coatings Based on Particle Diagnostics
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Lanthanum zirconate (La2Zr2O7) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of La2Zr2O7 a considerable amount of La2O3 can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La2O3-ZrO2 system, in the composition range of pyrochlore structure, the stoichiometric La2Zr2O7 has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro™-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (Gd2Zr2O7) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance.
Keywordsatmospheric plasma spraying gadolinium zirconate lanthanum zirconate particle diagnostics thermal barrier coating
Thermal barrier coatings (TBCs) are key elements in the design of advanced gas turbines for both aircraft engines and land-based power generation. They make it possible to achieve the increased gas inlet temperatures required to meet growing demands on improved fuel efficiency, lower emissions, and higher thrust and power (Ref 1). TBC systems consist typically of two layers: a bond coat layer and an insulative ceramic topcoat. The bond coat is often a MCrAlY alloy (M = Co, Ni) and has two major functions. It improves the bonding between the substrate and the topcoat, and it protects the substrate from corrosion and oxidation. The ceramic topcoat provides heat insulation due to its low thermal conductivity resulting from its bulk material characteristics as well as from microstructural features such as pores and voids (Ref 2).
At the end of the 1970s, zirconia stabilized by 6-8 wt.% yttria (YSZ) was established as an almost ideal ceramic topcoat material (Ref 3) because of its high fracture toughness and thermal expansion coefficient. It is deposited by either electron beam physical vapor deposition (EB-PVD) or by atmospheric plasma spraying (APS). YSZ does not form the equilibrium phase (monoclinic and cubic zirconia) during either of the deposition processes but rather a metastable t′-phase due to rapid cooling. It does not significantly transform to the equilibrium phases even at longer operating times and temperatures up to 1200 °C. However, above this temperature limit the material undergoes undesirable diffusion-induced decomposition into high yttria and low yttria phases (Ref 4). While the first of these transforms on cooling to the cubic phase with low toughness, the latter converts to the monoclinic phase which is accompanied by a considerable volume change leading to strains and cracks. Furthermore, above 1200 °C significant sintering occurs, which causes an increase of the Young’s modulus affecting the strain tolerance and hence reducing the thermal cycling lifetime (Ref 5, 6).
These shortcomings regarding such elevated turbine gas inlet temperatures have initiated many research activities seeking even better ceramics than YSZ (Ref 7-9). Apart from perovskites, also hexaaluminates, doped zirconia, and ceramics with pyrochlore structure offer very attractive characteristics. In particular, several zirconate pyrochlores show low thermal conductivities as well as high thermal stabilities. Among them, lanthanum zirconate La2Zr2O7 (LZ) is phase stable to its melting point as it can largely tolerate vacancies at the La3+, Zr4+, and O2− sites. At the same time, it displays a lower thermal conductivity (1.56 W m−1 K−1, bulk material) and a lower sintering tendency when compared to YSZ. However, the thermal expansion coefficient of 9.1 × 10−6 K−1 (30-1000 °C) (Ref 7) is low in relation to bond coats and Ni-base alloy substrates (~15 × 10−6 K−1) and the toughness is poor (Ref 10). The thermal expansion coefficient may be raised by replacing lanthanum by other trivalent rare earth elements having similar ionic radii. This can also lower the thermal conductivity (Ref 11) or even offer the option of temperature sensing by luminescence (Ref 12). However, the problem of low fracture toughness remains crucial. For this reason, pyrochlores are applied in combination with YSZ in double-layer TBC systems (Ref 13, 14). Graded coating systems were also investigated (Ref 15). YSZ is applied as the first ceramic layer since TBC failure is often initiated by cracks occurring close to the bond coat. This YSZ interlayer can, furthermore, prevent possible reactions between the pyrochlore and the alumina-based scale (thermally grown oxide, TGO) formed on the bond coat under thermal load (Ref 16). Here, LaAlO3 was formed above 1200 °C. In contrast, no significant chemical reaction was observed at YSZ LZ interfaces after thermal cycling.
The La2O3·ZrO2 phase diagram shows a stable pyrochlore region up to the melting temperature of 2280 °C (Ref 17). In more recent work a melting point of 2295 ± 10 °C is given (Ref 18). According to the same reference, the stability region ranges from approximately 33 to 35 mol% La2O3 at 1500 °C (La/Zr atomic ratio 0.99-1.08). Below the melting temperature, no fluorite phase occurs which is less ordered than the pyrochlore phase. Due to the high fusion enthalpy of ~350 kJ mol−1, solidification starts directly in the form of pyrochlore. However, considerable amounts of metastable fluorite phase are formed in the case of rapid solidification, which is typical of plasma spray conditions. Such fluorite phase is transformed to pyrochlore at temperatures above 1000 °C (Ref 19, 20). This is not critical as such transformation is not associated with significant volume change.
The processing of LZ by APS is challenging because La2O3 is prone to evaporate in the plasma plume resulting in non-stoichiometric coatings (Ref 16). This might be detrimental for TBC performance as thermophysical properties are affected (Ref 21). Similar problems are known from LZ EB-PVD coatings (Ref 22). La2O3 shows considerably higher vapor pressure when compared to ZrO2 (Ref 23). Although reference data from the literature are not in good agreement, it is evident that the vapor pressure of lanthania is at least one magnitude higher than that of zirconia. Also evaporation rates of La2O3 were found to be considerably higher in the temperature range between 2250 and 2500 °C (Ref 24). Evaporation products are LaO, O and O2 (Ref 25).
Cao et al. (Ref 16) attempted to prevent such non-stoichiometries by adjusting specific excess of La2O3 in the powder. It was concluded that the highest La content in the coating is obtained by applying a feedstock with a La/Zr atomic ratio between 1.0 and 1.05. Both at smaller and also larger ratios, less La was detected in the coatings. The reason for this was assumed to be that the evaporation temperature decreases at La2O3 contents below and above the compositional existence range of the pyrochlore phase in the same manner as the liquidus line decreases when it approaches the eutectic compositions immediately adjacent on the left and right of this pyrochlore peak. However, no reliable reference data is available on the way in which the evaporation temperature is dependent on the La2O3 content.
In the present work, APS experiments were performed with a TriplexPro™-200 plasma torch at different power levels to adjust different degrees of evaporation and thus stoichiometry. The results were verified by chemical, phase, and crystallographic analyses. Although LZ was the focus of this work, some investigations with Gd2Zr2O7 (GZ) were also performed for comparison since GZ does not show such a strong tendency to evaporate the rare earth constituent.
Particle temperature distributions were measured in flight by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected indicating evaporation and solidification processes. Based on these, as well as on chemical and crystallographic analyses, process parameters can be defined to provide stoichiometric coatings displaying good thermal cycling performance.
The samples used in this study were coated by APS in a Multicoat facility (Sulzer Metco, Wohlen, Switzerland) with a TriplexPro™ gun mounted on a six-axis robot. The 9 mm diameter nozzle was used with a 1.8 mm diameter feedstock injector. The plasma gas composition was 46 standard liters per minute (slpm) Ar and 4 slpm He. The current was varied between 200 and 525 A yielding plasma torch input powers between 15 and 51 kW. Particle in-flight diagnostics were performed by the DPV-2000 system (TECNAR Automation Ltd., St. Bruno, QC, Canada), which enables particle velocities, temperatures, and diameters to be measured in flight. The operating principles are described elsewhere (Ref 26).
The feedstock was a commercially available LZ powder (Praxair LAO-109, d10 = 7 μm, d50 = 44 μm, d90 = 80 μm) exhibiting a spherical morphology. The powder feed rate was 3.5 g min−1 and the carrier gas flow 2.0 slpm Ar. The spray distance was 90 mm and the robot speed 500 mm s−1. The same parameters were applied to process two GZ powders. One was commercially available (Praxair GDO-103, d10 = 25 μm, d50 = 42 μm, d90 = 72 μm) with a spherical morphology; the other one was a fused and crushed (f + c) experimental powder (d10 = 11 μm, d50 = 18 μm, d90 = 33 μm).
Coatings were sprayed on graphite substrates and subsequently stripped by grinding. Parts of the freestanding coatings were used for chemical analysis by optical emission spectroscopy with inductively coupled plasma (ICP-OES).
The remaining samples were investigated by x-ray diffraction analysis (XRD) and then embedded into epoxy resin to prepare metallographic polished cross sections for microscopic investigation and elemental analysis. XRD was performed on a D4 ENDEAVOR diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) and scanning electron microscope investigation (SEM) on an Ultra55 model (Carl Zeiss NTS GmbH, Oberkochen, Germany) combined with an energy-dispersive x-ray INCAEnergy355 spectrometer (EDS, Oxford Instruments Ltd., Abingdon, Oxfordshire, UK). For SEM examination, the samples were coated with approximately 2 nm platinum. EDS point analysis was performed on LZ samples with an acceleration voltage of 15 kV. Instead of the reference patterns of the EDS system internal database, a polished cross section of LZ feedstock particles was analyzed and defined as standard. The composition of this powder was confirmed by chemical analysis to be almost stoichiometric.
Results and Discussion
Deposition Rate and Chemical Composition
Chemical analysis yields an almost stoichiometric composition of the LZ powder feedstock (33.3 mol% La2O3, the La/Zr atomic ratio is nearly 1.0). Moreover, impurities of 0.7 mol% HfO2, 0.1 mol% CaO, and 0.1 mol% SiO2 were determined. The GZ powders show moderate gadolinium excess (Gd/Zr ratio is 1.07 and 1.11, respectively).
The La evaporation is in good agreement with the figure of 26% found by Chen et al. in their plasma-sprayed LZ coatings (Ref 21). Based on their reported process conditions, a torch input power of 50 kW can be estimated. Cao et al. (Ref 16) reported a maximum La loss of 13% relative to the content in the feedstock, but at a lower torch input power of 21 kW.
The GZ coatings show solely defective fluorite phase. According to the phase diagram, solidification starts as fluorite phase. With further cooling, the formation of pyrochlore below approximately 1530 °C is suppressed due to the high cooling rate at deposition on the substrate. Probably, such low temperatures are not reached by any particles still in flight so that pyrochlore phase cannot be formed then.
Homogeneity of Chemical and Phase Composition
Monitoring Melting, Evaporation and Solidification During Spray Process
Two further peaks appear at variable temperatures for both LZ and GZ. Regarding LZ, one of these peaks occurs from the beginning at 15.0 kW and vanishes above 32.6 kW, and the other one exists at 19.6 kW and above. Furthermore, above 32.6 kW the distributions are increasingly scattered. A similar development is observed using the spherical GZ powder (not shown in this paper), but at a higher power level and with less scattering.
In contrast, the slope of the GZ liquidus line in the phase diagram is small. Thus, the solidification temperature is hardly dependent on the gadolinia content so that only one peak at almost constant temperature is observed in the particle temperature distributions. The mean temperatures of this peak are in good agreement with the theoretical GZ melting temperature of approximately 2600 °C.
The two other distributions at variable temperatures in both the LZ and GZ cases are associated with molten particles. The reason for the appearance of two distributions is assumed to be the change of the composition at the radiating particle periphery due to the rare earth oxide evaporation. The remaining zirconia is assumed to have different emissivities compared to the zirconates. Thus, the results of two-color pyrometry are affected. Furthermore, if evaporation is intense, the particle radiation is scattered by vapor clouds so that the particle distributions are spread and scattered. This is evident in particular for LZ at 35.6 kW and above.
Lanthanum zirconate tends to experience the evaporation of lanthania during plasma spraying. Above a specific power level, more lanthania is evaporated at the particle surfaces during flight so that lanthanum-depleted zones occur in the deposited coatings at the splat peripheries. As a consequence, unstabilized zirconia is formed and these non-stoichiometries are expected to be detrimental for applications in thermal barrier coatings. Thus, lanthania evaporation during spraying must be prevented as much as possible.
Based on these results, plasma spray parameters can be identified for the manufacture of stoichiometric lanthanum zirconate thermal barrier coatings so that the advantages of this material may be exploited. Particle diagnostics were found to be an effective tool to detect and monitor lanthania evaporation during the spray process. The composition of the deposited coatings can be analyzed on the basis of lattice parameters obtained by XRD and Rietveld refinement.
The authors would like to gratefully acknowledge helpful discussions with Dr. Maria Ophelia Jarligo as well as the work of Mirko Ziegner (XRD analysis) and of Mark Kappertz (metallographic preparation), all Forschungszentrum Jülich, IEK. They also thank Sabrina Kohnen and Hannelore Lippert, Forschungszentrum Jülich, ZCH, for the chemical analyses.