Preparation and Thermophysical Properties of La2Zr2O7 Coatings by Thermal Spraying of an Amorphous Precursor
- First Online:
- Cite this article as:
- Chen, H., Gao, Y., Luo, H. et al. J Therm Spray Tech (2011) 20: 1201. doi:10.1007/s11666-011-9667-4
- 404 Downloads
Free-standing La2Zr2O7 coatings were obtained by plasma spraying, using an amorphous La-O-Zr precursor as the feedstock. The La-O-Zr precursor powder was prepared by coprecipitation. During thermal spraying, the formation of coatings can be regarded as a joint process of melting-solidification, thermal decomposition, and crystallization. The time required for crystal growth was significantly shortened during spraying. Consequently, the average grain size of coatings was approximately 200 nm, with a narrow distribution (100-500 nm). Coatings prepared by this method show better thermophysical properties than those prepared with crystalline La2Zr2O7 powder as the feedstock. The thermal conductivity of the as-sprayed coating was approximately 0.36-0.47 W/m K and the average coefficient of thermal expansion (CTE) is 11.1 × 10−6/K.
Keywordsamorphous powder feedstock lanthanum zirconate plasma spraying thermal barrier coating thermophysical property
Advanced gas turbines are being developed to operate at high temperatures and thereby increase thermal efficiency. Hot section components in the combustion system are subjected to hot gases, the temperatures of which exceed the melting points of the alloys from which the components are made. Furthermore, they must meet several rigorous requirements, such as long-term high-temperature endurance, thermal shock resistance, and anticorrosion ability, in order to be used in extreme conditions (Ref 1-4). As a result, components must be designed with critically important features including internal cooling and the application of thermal barrier coatings (TBCs) to reduce metal temperatures. Ceramic insulating layers, which are deposited on the metallic components to reduce metal temperatures, are currently used as TBCs. Yttria-stabilized zirconia (YSZ), a state-of-the-art TBC ceramic material, is widely used in industry because of its superior cyclic life. However, it is generally acknowledged that YSZ is to be used to its full potential. At issue are the aging effects on the phase stability of YSZ at temperatures above 1200 °C (Ref 5, 6), as well as on the desirable reductions in the thermal conductivity. Among TBC candidates, lanthanum zirconate (La2Zr2O7, or LZ) is regarded as one of potential alternative materials because of its low thermal conductivity, stable phase structure, and good sintering resistance (Ref 7, 8).
Two main methods, atmospheric plasma spray (APS) and electron-beam-enhanced physical vapor deposition (EB-PVD), are widely used to manufacture these coatings. In general, TBCs made with the APS method have superior thermal insulation properties and lower costs than those made with the EB-PVD process (Ref 9, 10). In addition to the deposition technology, the feedstock also significantly affects the microstructure and the thermophysical and mechanical properties of APS coatings (Ref 11, 12).
Crystalline powder is usually selected as the feedstock. Coarse powders consisting of nanosized crystal aggregates are presently used to produce nanometric and submicrometric deposits (Ref 13). The feedstocks are typically manufactured by vapor condensation and agglomeration (Ref 14), high-energy milling (Ref 15), solution precipitation (Ref 16), or quenching followed by consolidation or agglomeration (Ref 17). The sizes of these agglomerated powders range from 10 to 100 μm, and additional efforts are required to preserve the initial nanostructure of the powders in the sprayed coatings. Generally, the nanosized grains in the powder particles can be subjected to a sintering procedure at high temperatures inside a flame or jet (approximately 104 °C) (Ref 18). Sintering can occur with the formation of a neck among small particles, leading to the melting and growth of nanosized particles and a nonuniform size distribution. The use of finer feedstocks results in smaller lamella in deposited coatings. However, there are some practical difficulties (e.g., feeding and transporting) in the preparation of fine dry powders smaller than 10 μm. The uncontrollable agglomeration of fine particles is a serious problem. It can be caused by the action of electrostatic forces or humidity, and it renders the processing of lightweight dielectric ceramic materials, such as oxides, especially difficult. Another serious problem is the injection of fine, lightweight particles through a viscous jet or flame (Ref 19).
To solve these problems, we propose to use amorphous lanthanum zirconate (La-O-Zr) powders as feedstocks. By taking the advantage of flame heat during plasma spraying, the amorphous lanthanum zirconate particles crystallized during the process of melting-solidification. Thus, fine grains were present in the deposited coating after cooling, and the sintering-induced crystal growth were avoided. This process obviates the need for fine feedstock powders to obtain crystalline nanograins in the deposits. The thermophysical properties of the La2Zr2O7 as-sprayed coatings were also demonstrated to be superior to those of coatings derived from crystalline powders.
The La-O-Zr amorphous powder for spraying was synthesized by a chemical coprecipitation method and agglomerated to provide feedstock with the requisite particle size range for efficient flowability through the APS gun. Briefly, the powder was coprecipitated by adding an aqueous solution of equimolar Zr(NO3)4·8H2O and La(NO3)3·6H2O to a solution of NH4OH (pH 14) with continuous stirring. After reaction, the suspension was filtered, and the precipitate was dried at 110 °C for 10 h. After ball milling (zirconia milling balls with two different sizes, 5 and 10 mm in diameter), the powder was heat treated at 600 °C for 1 h. To obtain crystallized La2Zr2O7 (LZ) powder for comparison, the dried precipitate was sintered at 1200 °C for 1 h. Then the two powders were ground again.
Plasma spraying parameters
Arc current intensity, A
Primary gas (Ar) flow rate, slpm
Secondary gas (H2) flow rate, slpm
Spray distance, mm
Powder carrier gas flow rate, slpm
Injection diameter, mm
Results and Discussion
Basic Properties of the Coatings
EDS results of the precursor after heat treatment at 600 °C
Based on the aforementioned analysis, the grain size of as-sprayed coating can likely be controlled with amorphous feedstock powders with different particle sizes. When the particle size is small (e.g., smaller than that used in this research), the melted outer part will account for a larger proportion than the inner part. Consequently, the inner part may grow into larger crystals after spraying. If the particle size is large, the final grain size in the coating will be small, and amorphous parts may remain.
Density of the as-sprayed coatings
Relative density, %
Future studies should focus on the influences of postheating treatments on coating properties. Resintering, grain growth, and the alteration of thermophysical properties after heat treatment are important topics that are currently being investigated.
By using an amorphous La-O-Zr precursor powder as the feedstock, a nanocrystalline LZ coating, with an average grain size of 250 nm and a uniform microstructure (narrow distribution of grain sizes and pores), was obtained. During thermal spraying, the amorphous powder crystallized, and fine grains were formed. The coating derived from the crystallized feedstock powder had an average grain size of 750 nm and a wider distribution of grain and pore sizes because of sintering.
The thermal conductivity of the as-sprayed coating produced with the amorphous feedstock powder was approximately 0.42 W/m K, which was lower than that of the coating produced with the crystalline feedstock. This was attributed to the increased number of grain boundaries associated with the finer particles and a more uniform microstructure.
The coating produced with the amorphous precursor as the feedstock had an average CTE of 11.1 × 10−6/K, which was similar to that of the metallic bond coating.
The authors would like to thank Inorganic Materials Analysis and Testing Center of Shanghai Institute of Ceramics for the measuring work. This work is supported partly by the Century Program (One-Hundred-Talent Program) and high-tech-oriented projects (YYYJ-0810) of the Chinese Academy of Sciences, NSFC (50972156) and 1052nm02100.