Partial Evaporation of Strontium Zirconate During Atmospheric Plasma Spraying
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- Zhang, Y., Mack, D.E., Jarligo, M.O. et al. J Therm Spray Tech (2009) 18: 694. doi:10.1007/s11666-009-9360-z
Perovskite-type SrZrO3 has been investigated as a candidate material for thermal barrier coating application. During plasma spraying of SrZrO3, SrO volatilized more than ZrO2 and the coating composition deviates from initial stoichiometry. In this investigation, partial evaporation was investigated by spraying SrZrO3 powders into water. The influences of spraying current, distance and particle size of the powder on the partial evaporation were also investigated in a quantitative way. With optimized spraying parameters, stoichiometric SrZrO3 coating was produced by adding an excess amount of Sr in the precursors before plasma spraying to compensate for the volatilized component.
Keywordspartial evaporationplasma sprayingstrontium zirconatethermal barrier coating
Lattice parameters, nm
Thermal expansion coefficients, K−1
10.9 × 10−6
Melting point, K
Shear modulus, GPa
Debye temperature, K
Vickers hardness, GPa
Thermal conductivity at 1000 °C, W·(m·K)−1
‘‘Steady state’’ sintering rate at 1200 °C, s−1
1.04 × 10−9
Besides electron beam physical vapor deposition, usually TBCs are deposited by atmospheric plasma spraying with typically very high plasma flame temperature. The rapid heating of the particles gives rise to melting of the particles, sublimation, vaporization and evaporation across a liquid-vapor interface. Vaporization is driven by vapor concentration gradients existing between the free stream and the particle surface at temperatures below the boiling point while evaporation happens as the surface temperature reaches the boiling point (Ref 4). Physical vaporization and evaporation of the powders has no component change, which is controlled by vapor diffusion and heat transfer (Ref 5). In addition, plasma spraying can be accompanied by selective evaporation of a component of multi-component powder (Ref 6). As the material is heated in the plasma flame, substantial different evaporation rates of the constituents will lead to stoichiometry changes (Ref 7). For example, partial evaporation of CuO from YBa2Cu3Ox (Ref 6), P2O5 from Ca5(PO4)3OH (Ref 8), La2O3 from La2Zr2O7 (Ref 7) and CeO2 from La2Ce2O7 (Ref 9) have been reported.
During plasma spraying of SrZrO3, SrO was found to volatilize faster than ZrO2 with the result that the coating composition deviated from stoichiometric composition (Ref 2). The existence of excess ZrO2 in the plasma-sprayed SrZrO3 coating might influence the coating properties. To deposit a stoichiometric SrZrO3 coating, an excess amount of Sr must be added in the precursors before plasma spraying to compensate the loss of SrO by partial evaporation.
Although partial evaporation during plasma spraying of multi-oxides is common, detailed investigation of this phenomenon is very limited. Accurate analysis of evaporation mechanisms involved during plasma spraying of multi-oxides is necessary. This paper presents the quantitative investigation of the partial evaporation of SrZrO3 by spraying powders into water.
Commercial SrCO3 (Aldrich, >98%, Munich, Germany) and ZrO2 (Aldrich, 99%, Munich, Germany) powders were mixed by ball-milling with a Sr/Zr atomic ratio of 1.136 and calcined at 1400 °C for 24 h with a heating rate of 5 K/min. This means more than 10% of Sr was added in the precursors. The powders were spray-dried in a spray-dryer (Mobile MinorTM‘2000’ Ex Model H) with N2 as the drying medium, then heat treated at 1300 °C for 5 h to remove the organic binding compounds.
All plasma-sprayed coatings have been produced with a three cathode gun Triplex II in a Multicoat facility produced by Sulzer Metco, Wohlen, Switzerland. The gun was mounted on a 6-axis robot, which allowed well-defined coating sequences. Sand blasted austenitic steel was used as substrate for the coatings. The dimension of the standard steel substrates was a square with a side length of 50 mm and a thickness of 2 mm. Substrate temperatures were measured using the 4 M8 pyrometer system (Land instruments GmbH, Germany). These coatings were sprayed with argon and helium as plasma gases with flow rates of 45 and 6 standard liters per minute (slpm), respectively. The feeding gas was 2 slpm of Ar. The Robert speed was 500 mm/s. Particle speed and temperature monitoring were performed by an Accura spray-g3 diagnostic system (Tecnar, St. Bruno, Canada).
The phase composition of the powders was analyzed by x-ray diffraction (XRD) with a D500 diffractometer (Siemens AG, Germany) equipped with diffracted-beam monochromator for Cu-Kα. Particle size distribution was measured by Fraunhofer diffraction with 633 nm laser light using the Analysette 22 (Fritsch GmbH, Idar Oberstein, Germany). Chemical analysis of samples was performed using optical emission spectroscopy (OES 4.1, TJAIRIS-INTREPID) after evaporation in inductively coupled plasma (ICP). After plasma treatment, the SrZrO3 powders were vacuum impregnated with epoxy. After hardening of the resin, the sample was ground mechanically with SiC abrasive paper and polished on soft disks with diamond suspension. A cross section of the sample was investigated by a scanning electron microscope (SEM) (ModelJXA 840, JEOL, Tokyo, Japan) with an energy dispersive x-ray spectrometer (EDX) (Model Inca, Oxford Instruments, Oxfordshire, UK).
Results and Discussion
Composition of the Synthesized Powders
Powders Plasma-Sprayed into Water
Although the processes of plasma spraying of coatings and plasma treatment of powders are very similar, their final products are totally different. The plasma treatment of powders is usually carried out to improve some of their properties, such as flowability, density, internal or external morphology, etc. (Ref 11). As shown in Fig. 4(c), most of the spherical structures were preserved after plasma treatment. However, some new needle-like structures were formed after plasma treatment. As shown in Fig. 4(d), the surface of the powders became much denser after plasma treatment. Cross section of the powders (Fig. 4e) indicates that most of the spray-dried powders were densified in the plasma flame. Some spheres have a porous center with a dense outer shell. Big holes in the center of powders can be found after being plasma-sprayed into water. As the SrZrO3 powders were prepared by the spray-drying method, particles were initially spherical but very porous. During plasma treatment a large temperature gradient exists between the surface and the core of a porous particle. SrZrO3 has low thermal conductivity and it is possible to have the surface melting, evaporating and the core still remain porous (Ref 12). Therefore, heating, melting and evaporation may proceed at the same time in such a particle (Ref 13).
As shown in Fig. 4(g), after plasma treatment, the composition of the spherical powders is still SrZrO3. However, main composition of the newly formed needle-like structure is Sr and no Zr can be detected in the new structure (Fig. 4h). In spectrum 2 (Fig. 4h), the Ca, Mg, Na, S and Cl are probably from ordinary water used for collecting powders during plasma spraying.
Chemical compositions of the SrZrO3 powders before and after plasma treatment
Sr/Zr, atomic ratio
Before plasma treatment
After plasma treatment
Influence of Plasma Spraying Current, Distance and Particle Size on Sr Evaporation
Influences of plasma spraying current, distance and particle size on Sr evaporation
Particle size, μm
Spraying distance, mm
Sr/Zr decreased, %
The particle size of the powders used for plasma spraying also has an influence on the final Sr/Zr ratio of the coating. The evaporation of SrO can be reduced effectively by increasing the particle size of the powder used for plasma spraying. Comparing with coatings C-D, coatings E-F used larger size powders and showed less Sr evaporation. The rate of heat transfer through the boundary layer is proportional to the surface area of the particle (Ref 14). The evaporation time of a particle is proportional to its surface area and smaller particles have larger evaporation rates (Ref 5, 21). Therefore, the partial evaporation is proportional to the particle diameter. A similar result has also been reported by Cao (Ref 9). During plasma spraying of La2Ce2O7, the evaporation of CeO2 can be reduced effectively by increasing the particle size of the powder for plasma spraying (Ref 9).
For coatings B and C, longer spraying distance leads to a little higher degree of SrO evaporation. With longer spraying distance, the dwell time of the particle in the plume is longer, leading to higher total SrO evaporation.
Stoichiometric SrZrO3 Coating Received
Proposed Forming Mechanisms of Needle-Like Structure and Strontium Carbonate
So far, the detailed forming mechanisms of needle-like structure in powders sprayed into water and SrCO3 in both coatings and powders sprayed into water are still not clear. One mechanism is proposed here.
After spray-drying and before plasma spraying, the powders were annealed at 1300 °C for 5 h to remove organic binder and carbon source. Therefore, the newly formed SrCO3 must have formed during plasma spraying or later. During plasma spraying, molten powders are in contact with the surrounding atmosphere. They may decompose or have chemical reactions with the surrounding atmosphere. For example, oxides might form on the particle surface during plasma spraying of metals. Volatile species might form through thermal decomposition or reaction with the surrounding gas (Ref 5). Like perovskite-structured LaMnO3, LaCoO3 and YBa2Cu3Ox (Ref 6, 22), SrZrO3 also decomposes easily at the melting temperature. The needle-like structure is probably formed from completely molten phase originating from the readily melted ultra-fine Sr-rich SrZrO3 particles in flight. The new structure contained only Sr without Zr. Composition of the needle-like structure may be SrO at the beginning. However, Sr is strong carbonate former like Ba (Ref 23). Formation of SrCO3 from SrO and atmospheric CO2 for the SrZrO3 coatings or plasma-treated powders might be inevitable.
Partial evaporation of SrO happened during plasma spraying of SrZrO3, which is confirmed by plasma spraying of SrZrO3 powders into water. During plasma spraying SrZrO3 into water, some new needle-like structures formed and it contained only Sr. Once the initial composition of the powders is fixed, the final Sr/Zr ratio of the sprayed coating is determined by spraying current, distance and particle size of the powders used. By varying the spraying parameters and adding an excess amount of SrO before plasma spraying, stoichiometric strontium zirconate coatings can be produced. Traces of SrCO3 and metastable tetragonal ZrO2 can be detected in the as-sprayed coating. During sintering SrCO3 reacts with part of ZrO2 to form SrZrO3 again and the remaining metastable tetragonal ZrO2 transformed into ZrO2 (m). After sintering, pure SrZrO3 coating can be received only when the Sr/Zr ratio of the coating is a little larger than 1. In the near future, microstructure optimization and thermal cycling behavior of the received stoichiometric strontium zirconate coating will be investigated.
The authors thank Ms. M. Andreas for spray-drying of the powders, Mr. K.-H. Rauwald for the manufacturing of the plasma-sprayed coatings, Dr. G. Mauer for the particle diagnostic system operation, Mrs. S. Schwartz-Lückge for particle size measurements, Mr. M. Kappertz and Dr. D. Sebold for the sample microstructure characterizations, Dr. W. Fischer and Mr. M. Ziegner for XRD measurements. We would also like to thank Ms. N. Merki at the ZCH, Forschungszentrum Jülich, Germany, who performed the chemical analysis. The experiments described in this paper were carried out in Forschungszentrum Jülich GmbH and the first author was financially supported by NSFC-50825204 and CAS-DAAD program.