Hot corrosion behaviors of gas tunnel type plasma sprayed La2Zr2O7 thermal barrier coatings

https://doi.org/10.1016/j.jeurceramsoc.2011.10.049Get rights and content

Abstract

Gas tunnel type plasma sprayed free-standing La2Zr2O7 coating specimens with a thickness of 300–400 μm were prepared under optimized operating conditions and were subjected to hot corrosion test in the presence of corrosive impurities such as V2O5, Na2SO4, and Na2SO4 + V2O5 mixtures (60:40 wt%) at two different temperatures for duration of 5 h, i.e. 1000 and 1350 K for V2O5 and Na2SO4 + V2O5 mixtures, 1200 and 1350 K for Na2SO4. For temperatures at 1350 K, the reaction mechanism of V2O5 and the mixture of Na2SO4 + V2O5 are similar and LaVO4 is formed as the corrosive product, which leads to massive phase transformation from pyrochlore to tetragonal and monoclinic phases. Microstructural observations from planar reaction zone (PRZ) and melt infiltrated reaction zone (MIRZ) reveals that the present La2Zr2O7 coating exhibits good hot corrosion resistance in V2O5 environment and moderate for the mixture of Na2SO4 + V2O5, but is worst in Na2SO4 environment.

Introduction

Plasma-sprayed thermal barrier coatings (TBC) are traditionally applied to critical hot sections of the industrial gas turbines, particularly, turbine blades, vanes and combustion chambers in order to protect the hot sections from this high operating temperature and increase the component durability. Simultaneously, the need for enhancing the fuel efficiency has lead to increase in the operating temperatures of gas turbines year after year. Hence, the reliability of the most widely used TBC top coat composition, i.e. yttria stabilized zirconia (YSZ) coatings, has been weakened due to its phase transformation and sintering or densification behavior at higher operating temperatures and corrosive environments, which might result in disintegration of the coating.1, 2, 3, 4

Therefore, development of new materials for TBC in order to address the challenges of the highly demanding operating environments is essential to fulfill the industrial requirements. Recently, it has been found that some very interesting properties are possessed by one composition, i.e. pyrochlore phase lanthanum zirconate (La2Zr2O7). The results of the earlier works have shown that this material has excellent thermal stability, (which is stable up to its melting point: 2573 K), low thermal conductivity (1.56 W m−1 K−1, compared to 2.12 W m−1 K−1 for YSZ4), low sintering rate, and thus becomes a very promising candidate for new TBC material.5, 6, 7 However, relatively low thermal expansion coefficient of about 9 × 10−6 K−1 compared to YSZ with 10–11 × 10−6 K−1 leads to higher thermal stresses due to thermal expansion mismatch between the coating interfaces and causes severe damage in the coating system.8, 9

Vassen et al.10 studied the thermo-physical properties of La2Zr2O7 material and successfully produced a coating through APS technique. Observation from their results clearly underlines that La2Zr2O7 has good potential as a new material for advanced TBCs, even though it has lower Young's modulus and thermal expansion than that of YSZ. Furthermore, their results proved that the thermal conductivity of La2Zr2O7 which is approximately 20% lower than that of YSZ is favorable at elevated temperatures, and also shows excellent thermal stability. Subsequently, they developed YSZ/La2Zr2O7 multilayer layer coating systems in order to enhance the cyclic life time of the coating under high operating temperatures.11, 12 Likewise, Chen et al.13 stated that the graded YSZ/La2Zr2O7 coating offers admirable thermal shock resistance in comparison to that of its duplex and single layer coatings. Actually they have prepared six-layered YSZ/La2Zr2O7 bicomponent graded coatings through plasma spraying. These authors also prepared structured La2Zr2O7 coatings through plasma spraying and studied their thermophysical properties. Astonishingly, the coating exhibited minimum thermal conductivity, i.e. 0.73 W m−1 K−1, which was about 50% lower than previously reported results of conventional microstructure coatings.14

It should be noted that the hot corrosion mechanism of the TBC material against different kinds of corrosive environment is also one of the imperative factors that must be taken into account along with other factors such as thermal conductivity, phase stability, thermal expansion coefficient and mechanical properties of the TBC material while searching for new TBC materials or while validating the existing one as a promising TBC material. Hence the understanding of hot corrosion reaction mechanisms of the La2Zr2O7 against different kinds of corrosive ashes at elevated temperatures is a must for categorizing or widening the application of La2Zr2O7. Since in most of the cases, the TBCs are subjected to face low-quality fuels during operation, the impurities in the highly contaminated fuel can combine to form molten salts, such as sodium sulfate and vanadium compounds and deposit on the coating surface in the combustion environment, giving rise to hot corrosion problems.15

There are only a few earlier reports16, 17 that exemplify the hot corrosion behavior of plasma sprayed La2Zr2O7 coatings against V2O5, Na2SO4 and V2O5, Na2SO4 + V2O5 corrosive environments at elevated temperatures and different time durations, but they are insufficient when compared to the reports of conventional YSZ coating hot corrosion results.18, 19, 20 Hence in this paper, further examinations of the hot corrosion behavior of plasma sprayed La2Zr2O7 coatings against V2O5, Na2SO4, and Na2SO4 + V2O5 mixtures (60:40 wt%) corrosive salts at elevated temperatures was conducted and the results are discussed with regard to corrosive product formation and microstructural changes. For this purpose, free-standing La2Zr2O7 coating specimens with thickness of around 300–400 μm were prepared by using gas tunnel type plasma spray torch under optimized operating conditions. Superiority, unique features of gas tunnel type plasma spraying and the influence of its processing parameters on the ceramic coating formation are well reported in previous publications.21, 22, 23

Section snippets

Experimental procedure

Fig. 1 shows a schematic of the gas tunnel type plasma spraying torch and a photographic image of its high energy density plasma jet, which was used for producing La2Zr2O7 coatings required for this study. For this purpose, La2Zr2O7 powder was commercially procured and its typical surface morphology is shown in Fig. 2. The SEM morphology reveals that the feedstock grains are irregular in nature with average size range of 10–40 μm. Argon gas was used as carrier gas with a flow rate of 5 lpm to

Results and discussion

Fig. 3 shows the XRD patterns of the La2Zr2O7 powder and its plasma sprayed coating. The XRD pattern revealed that the pyrochlore phase of La2Zr2O7 was stable even in the coating obtained at higher plasma current (350 A) under the optimized plasma torch spraying conditions. The main feature of this XRD investigation is that the obtained peaks belonging to the pyrochlore structure shift slightly towards smaller 2θ value and cause larger lattice parameter than that of the initial powder.

Summary and conclusions

Gas tunnel type plasma sprayed free-standing La2Zr2O7 coating specimens with a thickness of about 300–400 μm were prepared under optimized operating conditions and characterized. The coatings retain the pyrochlore phase except for trivial losses of La2O3 and the coatings reflect the unique plasma spraying microstructure, i.e. lamella structures with pores. Furthermore, the coating specimens were subjected to hot corrosion testing in the presence of corrosive impurities such as V2O5, Na2SO4, and

Acknowledgments

The authors would like to express their sincere gratitude to the Japan Society for the Promotion of Science (JSPS) for providing the financial support under the grant No. 21.09299 to conduct this research work. The support provided by Mr. T. Horinouchi during the plasma spraying and sample preparation is also gratefully acknowledged.

References (35)

  • Z. Xu et al.

    Effects of deposition conditions on composition and thermal cycling life of lanthanum zirconate coatings

    Surf Coat Technol

    (2008)
  • J. Pannetier

    Energie electrostatique des reseaux pyrochlore

    J Phys Chem Solids

    (1973)
  • R.L. Jones et al.

    Hot corrosion studies of zirconia ceramics

    Surf Coat Technol

    (1987)
  • Z. Chen et al.

    Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

    J Eur Ceram Soc

    (2009)
  • J. Szekely et al.

    A structural model for gas-solid reactions with a moving boundary-II: the effect of grain size, porosity and temperature on the reaction of porous pellets

    Chem Eng Sci

    (1971)
  • H.Y. Sohn et al.

    A structural model for gas-solid reactions with a moving boundary-III: a general dimensionless representation of the irreversible reaction between a porous solid and a reactant gas

    Chem Eng Sci

    (1972)
  • J.A. Poston et al.

    Thermal decomposition of the rare earth sulfates of Cerium(III), Cerium(IV), Lanthanum(III), and Samarium(III)

    Appl Surf Sci

    (2003)

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