Journal of Thermal Spray Technology

, Volume 20, Issue 6, pp 1201–1208

Preparation and Thermophysical Properties of La2Zr2O7 Coatings by Thermal Spraying of an Amorphous Precursor

Peer Reviewed

DOI: 10.1007/s11666-011-9667-4

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

Abstract

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.

Keywords

amorphous powder feedstock lanthanum zirconate plasma spraying thermal barrier coating thermophysical property 

Introduction

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.

Experimental

Samples

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.

The flow charts of preparation procedures for amorphous and crystallized LZ powders are shown in Fig. 1. After 5 wt.% polyvinyl alcohol (PVA) was added to the powder, the mixture was ball milled for 30 min, then dried and ball milled again. Finally, agglomerated powders with particle size in the range of 30-45 μm can be obtained for plasma spraying.
Fig. 1

Flow charts for the preparation of amorphous and crystallized LZ powders

Figure 2 shows the scanning electron microscopy (SEM) images of the agglomerated feedstock powders of amorphous (Fig. 2a) and crystallized LZ (Fig. 2b). Thick coatings (about 5 mm) were obtained by plasma spraying (Sulzer-Metco 9 M, Winterhur, Switzerland), using an Ar/H2 mixture for flame generation onto an alloy substrate (aluminum, 60 × 75 mm). Spraying parameters are shown in Table 1. The as-sprayed coating was removed from the alloy by milling off the substrate thoroughly.
Fig. 2

Scanning electron micrographs of the agglomerated feedstock powders. (a) Amorphous LZ. (b) Crystallized LZ

Table 1

Plasma spraying parameters

Parameter

Value

Arc current intensity, A

660

Primary gas (Ar) flow rate, slpm

30

Secondary gas (H2) flow rate, slpm

15

Spray distance, mm

130

Powder carrier gas flow rate, slpm

4

Injection diameter, mm

1.8

Characterization Techniques

The phase compositions of the powder and the coating were determined by x-ray diffraction (XRD; Rigaku RINT2200) using Cu Kα radiation. Surface and cross-section morphology observation was conducted by SEM (JEOL JSM-6700F, Tokyo, Japan). The overview morphology and element compositions of the as-prepared amorphous LZ powder were observed by transmitting electron microscopy (TEM; JEM-2010, Tokyo, Japan), equipped with an energy-dispersive spectrometer (EDS). The thermal conductivity k was determined from the thermal diffusivity α, and the specific heat capacity Cp, using:
$$ k = C_{p} \upalpha \uprho $$
(1)
where ρ is the measured density of the sample, obtained by the Archimedes method with an immersion medium of deionized water. A disk-shaped specimen with 11 mm in diameter and 0.8 mm in thickness was used for thermal diffusivity measurements. The thermal diffusivity of the samples was measured using the laser-flash method, as a function of temperature (in the range of 200-1200 °C) in an argon atmosphere. Both the front and back of the specimen were coated with a thin layer of graphite. These coatings were used to prevent direct transmission of the laser through the specimen. The specific heat capacity was obtained from an investigation of powder specimens using differential scanning calorimetry (DSC), working continuously at a scanning rate of 20 K/min from room temperature to 1200 °C (Model 404, Netzsch, Bayern, Germany) in N2 atmosphere. Coefficient of thermal expansion was obtained by measuring the temperature-dependent length change of the specimen with a size of 3 × 3 × 20 mm using a high-temperature dilatometer (Model 402, Netzsch) from room temperature to 1200 °C in air at a heating rate of 5 K/min. Data were corrected by using a standard material, sapphire as a reference. Simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) of the precursor powders were performed in air with platinum cups as sample holders. The thermal analysis experiments were performed at a heating rate of 10 K/min. The porosity of the coating was calculated from (1 − ρ/ρ0), where ρ0 was the theoretical density. The pore size distributions were determined by the mercury intrusion method, and the grain-size distributions were obtained by randomly measuring the diameters of 400 grains in SEM photos of the coating surfaces.

Results and Discussion

Basic Properties of the Coatings

The XRD pattern of the precursor powders after heat treatment at 600 °C is shown in Fig. 3(a). Only a broad peak can be observed in the profile, indicating the formation of amorphous precursor powders. A heating temperature of 600 °C was chosen based on the following considerations. After precipitation, the metallic ions formed a coordinating polyhedron La-O-Zr (Ref 20). Inside the structure, some H2O molecules and impurity groups, NH4+ and NO3, were attached. According to the TG-DTA curves (Fig. 4), weight loss in the TG curve was minimal at temperatures above 600 °C, indicating that the structural water and impurities could be removed by annealing at 600 °C. Meanwhile, the exothermic peak of crystallization in the DTA curve occurred at approximately 900 °C, indicating that the powder was still amorphous after the impurities had been removed at 600 °C. The impurities must be separated from the powder by heat treatment before spraying because the decomposition of NH4+ and NO3 usually results in the formation of unnecessary pores in coatings. Conversely, by removing the structural water and the remaining OH groups from the powder, heat treatment facilitates the formation of a denser, amorphous structure; the crystallization temperature is approximately 900 °C. Figure 5 shows TEM pictures of the powders after heat treatment at 600 °C. The powders were nonuniform in size. According to an EDS analysis, both La and Zr were detected in one grain and had a molar ratio of approximately 1:1 (Table 2), consistent with the La-O-Zr structure mentioned previously.
Fig. 3

XRD patterns of (a) the feedstock after heat treatment at 600 °C, (b) the as-sprayed coating, produced with the -La-O-Zr precursor, and (c) the as-sprayed coating, produced with the crystalline LZ powder

Fig. 4

TG-DTA curves for the precipitates after drying at room temperature for 5 h

Fig. 5

TEM image of the precursor after heat treatment at 600 °C

Table 2

EDS results of the precursor after heat treatment at 600 °C

Element

Spot 1

Spot 2

Spot 3

O

37.52

35.46

38.50

Zr

32.03

32.41

30.23

La

30.45

32.23

31.26

Molar ratio; measurement spots shown as a cross in Fig. 3

After spraying, a crystalline coating was obtained. As shown in Fig. 3(b), all diffraction peaks in the XRD pattern were assigned to La2Zr2O7 (JCPDS Card No. 73-0444). However, this pattern was slightly different from that of the coating derived from crystallized LZ powders (Fig. 3c). The main diffraction peak in Fig. 3(b) was widened. The factor that grain size of the coating made from amorphous feedstock (coating A) was smaller than that of the coating made from the crystallized feedstock (coating B) accounted for the wider peaks. The average crystallite size of coating A and B, estimated by the Scherrer formula (Ref 21), were 14.5 and 44.3 nm, respectively. Although the observed coating grain sizes (250 nm for coating A and 750 nm for coating B) were substantially different from the estimates, the proportion between the two was almost the same. This difference was attributed to the nonuniform, typically columnar shape of the grains in the coating. In addition, the peak of coating A shifted to the high-angle direction compared with that of coating B. This shift was attributed to residual stress in coating A. Because of the phase transition during spraying, the residual stress was larger in coating A than in coating B, leading to a decrease in interplanar spacing and an increase of diffraction angles. Furthermore, as shown in the SEM pictures, the average grain size was approximately 250 nm for coating A (Fig. 6a) and 750 nm for coating B (Fig. 6b). The grain sizes of coating A were distributed from 100 to 500 nm, and the grain size distribution of coating B ranged from 400 nm to 2 μm.
Fig. 6

Scanning electron micrographs of the coating surface and grain size distribution. (a) Coating A. (b) Coating B

The cause of the difference between the grain sizes of the coating derived from amorphous powders and the coating fabricated with crystallized powders is an interesting point. During spraying, the outer part of the powder normally melts first, leaving the unmelted part inside (Ref 22, 23). For the amorphous feedstock powders, the outer part experienced a crystallization-melting-recrystallization process during spraying. Meanwhile, the inner amorphous part crystallized as the phase transition temperature was reached, and nucleation occurred. Crystalline nuclei appeared in the inner part (Fig. 7a). With heating, nuclei gradually grew at the consumption of the amorphous component. When the particle was jetted on the base material, it quickly cooled and solidified. Because the flight time was short (approximately 3 ms from the inlet to the substrate) (Ref 24), these crystals grew incompletely and had a relatively small size. Moreover, heterogeneous heat conduction from the flame to the feedstock powders could have prevented some amorphous components from crystallizing (Ref 19). This phenomenon can be observed in Fig. 6. The grain boundaries in Fig. 6(a) are not as clear as those in Fig. 6(b), and the surfaces of some grains are coarse. Concerning the spraying process of the crystallized feedstock powders, the initial nanograins (approximately 20 nm) were composed of the aggregated particles (Fig. 7b). As a result of the high flame temperature, the inner part of the tiny crystals sintered, and the necks were formed among adjacent crystals (Fig. 6b, indicated by arrows). Finally, large submicrometric crystals formed due to Ostwald ripening. As shown in Fig. 7(b), crystals with different sizes were observed in the substrate.
Fig. 7

Thermal spraying process. (a) Amorphous powder as feedstock. (b) Crystallized LZ powder as feedstock

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.

Thermophysical Properties

The thermal conductivities of the two as-sprayed coatings derived from different feedstock powders are shown in Fig. 8. Both curves decrease before 900 °C and increase after 900 °C. The thermal conductivity increase in the high-temperature region above 900 °C was attributed to the effects of thermal radiation (Ref 25). Coating A had an average thermal conductivity of 0.42 W/m K, with a minimum value of 0.36 W/m K. The average thermal conductivity of coating B was 0.68 W/m K with a minimum value of 0.64 W/m K. The fine crystal size of coating A resulted in its low thermal conductivity. As mentioned previously, coating A had an average grain size of approximately 250 nm, which was smaller than the average size of 750 nm for coating B. Both experiments and theoretical calculations show that smaller TBC grain sizes are favorable to promote thermal insulation (Ref 26-29). When the grain size is smaller than 400 nm (Ref 29), the thermal conductivity of the coating can be dramatically decreased. The thermal resistance effect among grain boundaries, also known as Kapitza thermal resistance, leads to discontinuous temperature changes at each grain boundary (Ref 26). Therefore, a large number of grain boundaries are beneficial for the reduction of thermal conductivity.
Fig. 8

Thermal conductivities of as-sprayed coatings produced with (a) the amorphous precursor and (b) the crystallized LZ powder as feedstocks

The existence of pores in the coating can also effectively reduce the thermal conductivity because air has a low thermal conductivity. Table 3 shows the relative densities and porosities of coatings A and B. Coating A had a density of 5.53 g/cm3 and a porosity of 8.60%, whereas coating B had a density of 5.34 g/cm3 and a porosity of 11.74%. Figure 9 also shows that the porosity of coating B was higher than that of coating A, implying that, theoretically, coating B may have a lower thermal conductivity. However, the pore geometry also plays a significant role. Generally, most coatings prepared by plasma spraying have round or oval pores, but the size and distribution of these pores change with fabrication parameters. As shown in Fig. 10, coating A had a relatively narrow pore distribution concentrated around 550 nm. The pore distribution curve of coating B showed peaks at 40, 150, and 550 nm, and 1.5 μm. These results are consistent with the sizes and distributions of pores observed in the cross-section morphologies of the two coatings (Fig. 9). According to a previous report, the thermal conductivity of porous ceramics with a uniform microstructure (i.e., with narrower pore and grain size distributions) is lower than that of ceramics with nonuniform microstructures (Ref 30). Therefore, coating A, with a narrow pore distribution, was more likely to have a lower thermal conductivity.
Table 3

Density of the as-sprayed coatings

Samples

Density, g/cm3

Relative density, %

Porosity, %

Coating A

5.53

91.40

8.60

Coating B

5.34

88.26

11.74

Theoretical density: 6.05

Fig. 9

Cross-sectional morphologies of (a) coating A and (b) coating B

Fig. 10

Pore size distributions of coating A and coating B

Figure 11 shows the coefficients of thermal expansion (CTEs) of coatings A and B. The average CTE was approximately 11.1 × 10−6/K for coating A and 9.6 × 10−6/K for coating B. The latter value is 17.7% higher than that of the former. The higher CTE value of coating A could have been a result of the small grain size and the remaining amorphous La-O-Zr component, or it could have been caused by the porosity, pore size, and distribution. Generally, pores in the shape of holes or microcracks heal during heating. Thus, a coating with a higher porosity and wider pore size distribution may have a lower CTE. Conversely, the CTE of a coating with a lower porosity may not decrease due to resintering at high temperatures. The observed CTE was similar to that of the state-of-the-art 7YSZ (approximately 11.2 × 10−6/K) (Ref 31) and the metallic bond coating, implying a good thermal match between coating A and the base material.
Fig. 11

Thermal expansion coefficients of coating A and coating B

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.

Conclusions

  • 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.

Acknowledgments

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.

Copyright information

© ASM International 2011

Authors and Affiliations

  1. 1.State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics (SIC)Chinese Academy of Sciences (CAS)ShanghaiChina
  2. 2.Graduate University of Chinese Academy of SciencesBeijingChina
  3. 3.The Key Laboratory of Inorganic Coating Materials, Shanghai Institute of CeramicsChinese Academy of SciencesShanghaiChina

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