Properties of plasma sprayed La2Zr2O7 coating fabricated from powder synthesized by a single-step solution combustion method
Highlights
► Synthesis of plasma sprayable La2Zr2O7 powder without agglomeration step ► Rietveld analysis of the plasma sprayed La2Zr2O7 coating ► Plasma sprayed coating showed the presence of zirconia along with La2Zr2O7. ► La2Zr2O7 coating exhibited a thermal conductivity value of 1.08 Wm− 1 K− 1 at 900 °C.
Introduction
Ceramic thermal barrier coatings (TBCs) have been used for decades to extend the life of combustors and high turbine stationary and rotating components. By using TBCs, one can increase the operating temperature and in turn the performance of gas turbines or diesel engines can be increased [1]. At present, thermal barrier coatings (TBCs) of Y2O3 partially stabilized ZrO2 (YPSZ) films [2], [3], [4] are widely used to protect hot section parts of aircraft and land-based turbines for reducing the temperature of metal substrates. Currently, state-of-the art TBCs are based on 7–8 wt.% YSZ which corresponds to 4–4.6 mol% YSZ. However, YPSZ has a limited temperature capability due to accelerated sintering and phase transformations at high temperatures. As a result, a worldwide effort has been undertaken to identify new candidates for TBC application [5], [6], [7], [8]. In recent studies, rare earth zirconates with the type of Ln2Zr2O7 have been investigated and the results show that these materials are significant for the top-coat ceramic materials for future TBCs.
Pyrochlore oxides with the general formula A2B2X6Y, where A, B are cations and X, Y are anions, are very important ceramics and have received considerable attention because of their many technological applications [9], [10], [11]. Pyrochlore R2Zr2O7 (R = rare earth metal) compounds have been studied for their optical, electrical and catalytic properties because these materials have potential applications as hosts of fluorescence centers [12] and oxidation catalysts [13]. The structure of pyrochlore can accommodate a diverse chemistry, including over 500 synthetic compositions. The A-site (16d) coordination polyhedron is a distorted cube that generally contains larger cations; the B site (16c) is a distorted octahedron; the Y anion site (8b) may be empty [14]. It is well known that the substitutional solid solution is formed by the substitution of Zr4 + cation by trivalent rare-earth cation when a trivalent rare-earth oxide is doped into ZrO2 [14]. The substitution of two Zr4 + cations with two La3 + or Y3 + cations is accompanied by the incorporation of one oxygen vacancy, to maintain the electroneutrality of the lattice. The defect chemistry due to codoping can be represented by using the Kröger–Vink notation by the following equation:where, LnZr′ represents an Ln3 + cation that occupies a Zr4 + cation site (single negative charge), Vo″ is a doubly charged (positive) oxygen vacancy, and Oox is an O2 − anion on an oxygen site (neutral charge). The electric charges are defined with respect to the pure ZrO2 lattice. The Eq. (1) shows that the higher the content of Ln2O3 is, the more oxygen vacancies are created. The content of La2O3 is 33 mol% in La2Zr2O7, while that of Y2O3 is only 4.02 mol% in 8YSZ. Clearly, the concentration of oxygen vacancies in La2Zr2O7 is significantly higher than in 8YSZ. Therefore, the thermal conductivity of La2Zr2O7 is much lower than that of 8YSZ due to the scattering of the phonons by the oxygen vacancies. In addition to the phonon scattering by the oxygen vacancies, another reason of low thermal conductivity of La2Zr2O7 is the scattering of phonons by the substitutional cation. Low thermal conductivity is one of the most critical requirements for TBCs and hence La2Zr2O7 is a potential candidate for TBC applications.
The performance of TBCs depends largely on the synthesis procedures and conditions. There are a large number of reports on the synthesis of La2Zr2O7 powders. Dhas and Patil have reported the preparation of fine particle rare earth metal zirconates Ln2Zr2O7 by using carbohydrazide and urea as fuels [15]. La2Zr2O7 preparation has been reported by solid-state reaction, nitric acid dissolution route, sol–gel technique and hydrothermal method [16], [17], [18], [19], [20]. The formation and sintering of La2Zr2O7 by the hydrazine method have been reported by Matsumura et al. [21]. Pyrochlore La2Zr2O7 nanocrystals with cubic structure were prepared by stearic acid combustion method using zirconium nitrate and lanthanum nitrate as raw materials, stearic acid as solvent and dispersant [22]. Wang et al. have reported the preparation and characterization of nanostructured La2Zr2O7 feed stock by spray drying process for plasma spraying [23]. Yugeswaran et al. prepared plasma sprayed free-standing La2Zr2O7 coating specimens by using gas tunnel type plasma from commercial powders and studied the hot corrosion behavior of thermal barrier coatings [24]. Chen et al. have reported the thermophysical properties of La2Zr2O7 coatings by thermal spraying an amorphous precursor [25]. Cao et al. demonstrated the successful manufacture of La2Zr2O7 TBCs by plasma spraying (PS) using spray dried powder [26].
There are no reports on the preparation of free flowing La2Zr2O7 powders suitable for plasma spraying in a single-step. In this communication, we report the combustion synthesis and characterization of plasma sprayable La2Zr2O7 powder and the obtained coating.
One can expect the formation of metastable phases like that observed in the alumina system or other phases during plasma spraying of La2Zr2O7 due to the rapid quenching taking place during solidification, or due to the evaporation of La2O3 in the plasma flame. It is also important to know the phase fractions of the different phases formed during plasma spraying from a technological point of view. In the past decade, the Rietveld method for the analysis of X-ray and neutron diffraction patterns has been widely used to obtain refined crystal structure parameters as well as quantitative phase compositions. It is a good technique to extract information about the crystallographic unit cell, quantitative phase amounts, crystallite size, micro strain, etc., of the measured sample [27]. There have been reports on the Rietveld refinement of the plasma sprayed alumina, hydroxyapatite, TiO2, etc. [28], [29], [30] and nevertheless on the plasma sprayed La2Zr2O7 coating. In the present study, the Rietveld refinement was carried out for the plasma sprayed La2Zr2O7 coating.
Section snippets
Powder preparation and characterization
In a typical experiment, the plasma sprayable La2Zr2O7 powder was prepared by solution combustion method as follows: 25 g of La2O3 (99.9% purity, Loba Chemie) and 49.40 g ZrOCl2·8H2O (98.9% purity, Loba Chemie) were dissolved separately in dilute nitric acid (1:1) and slightly heated to get a clear aqueous solution containing lanthanum nitrate and zirconyl nitrate. To this clear solution, 26.83 g urea (99.5% Merck) (oxidizer to fuel ratio was unity (O/F = 1)) was added and the solution was
Powder characteristics
We assume that La(NO3)3 and ZrO(NO3)2 formed by dissolving lanthanum oxide and zirconium oxychloride in dilute nitric acid, reacts with urea yielding respective oxides which in turn combine to form the required product in oxygen atmosphere. Based on this assumption, the theoretical equations for the formation of La2Zr2O7 may be written as follows:
The morphology of as-prepared La2Zr2O7 particles is shown in
Conclusions
The solution combustion method was successfully used for the preparation of the plasma sprayable La2Zr2O7 powder. This is the first report on the application of combustion synthesis for the preparation of the plasma sprayable La2Zr2O7 powders. This process appears to be simple and fast for preparing plasma sprayable powders without any agglomeration step. The as-formed powders show good flowability and contain traces of La(OH)3 and La2O3. The SEM images show blocky angular shapes of particles
Acknowledgments
The authors acknowledge the financial assistance received from the CSIR Network project on Nanostructured Advanced Materials (NWP-00-51-01). The authors thank the Director, NAL for his constant encouragement. Help received from Mr. Siju and Mrs. S. Latha in FESEM and particle size analysis is acknowledged. The authors thank Dr. V. Shubha and Arul Paligan for the thermal conductivity measurements. The help received from Anand and Jyothi in plasma spraying and density measurements is
References (48)
- et al.
Surf. Coat. Technol.
(2001) - et al.
Surf. Coat. Technol.
(2002) - et al.
Surf. Coat. Technol.
(1991) - et al.
Acta Mater.
(2003) - et al.
Mater. Chem. Phys.
(2008) - et al.
Mater. Res. Bull
(1998) - et al.
Solid State Commun.
(1997) - et al.
Mater. Lett.
(2008) - et al.
Powder Technol.
(2011) - et al.
J. Eur. Ceram. Soc.
(2012)
Surf. Coat. Technol.
Biomater.
Thin Solid Films
Mater. Chem. Phys.
Mater. Chem. Phys.
J. Solid State Chem.
Surf. Coat. Technol.
Ceram. Int.
J. Alloy Compd.
J. Am. Ceram. Soc.
Mater. Today
J. Am. Ceram. Soc.
Ceram. Eng. Sci. Proc.
Surf. Coat. Technol.
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