Air Plasma-Sprayed La₂Zr₂O₇-SrZrO₃ Composite Thermal Barrier Coating Subjected to CaO-MgO-Al₂O₃-SiO₂ (CMAS)

Abstract

La₂Zr₂O₇-SrZrO₃ composite thermal barrier coatings (TBCs) were prepared by air plasma spray (APS). The La₂Zr₂O₇-SrZrO₃ composite TBCs covered with calcium-magnesium-aluminum-silicate (CMAS) powder, as well as the powder mixture of CMAS and spray-dried La₂Zr₂O₇-SrZrO₃ composite powder, were heat-treated at 1250 °C in air for 1, 4, 8, and 12 h. The phase constituents and microstructures of the reaction products were characterized by x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. Experimental results showed that the La₂Zr₂O₇-SrZrO₃ composite TBCs had higher CMAS resistance than 8YSZ coating. A dense new layer developed between CMAS and La₂Zr₂O₇-SrZrO₃ composite TBCs during interaction, and this new layer consisted mostly of apatite (Ca₂La₈(SiO₄)₆O₂) and c-ZrO₂. The newly developed layer effectively protected the La₂Zr₂O₇-SrZrO₃ composite TBCs from further CMAS attack.

Introduction

Thermal barrier coatings (TBCs) with low thermal conductivity, high thermal expansion coefficient (TEC), and acceptable phase stability were applied widely in aero engine turbine blades, thus reducing surface temperature while enhancing engine efficiency (Ref 1). State-of-the-art TBCs are based on 7-8 wt.% Y₂O₃-stabilized ZrO₂ (8YSZ). However, the maximum operation temperature of 8YSZ is limited to 1200 °C for long-term applications. At high temperatures, the phase transitions, accelerated sintering effect, and degradation caused by calcium-magnesium-aluminum-silicate (CMAS) melts result in the premature failure of the 8YSZ TBCs (Ref 2-5). The development of new TBC candidates, which provide high inlet temperatures while increasing engine efficiency and reducing emission, is investigated to satisfy the demands of the next-generation advanced turbine engines.

Among the newly developed TBC candidates, La₂Zr₂O₇ with the pyrochlore structure has low thermal conductivity and acceptable phase stability. However, its fracture toughness and TEC are relatively low (Ref 6, 7). Another TBC candidate material, SrZrO₃ with the perovskite structure, has high melting point (2650 °C) and TEC (10.9 × 10⁻⁶K⁻¹) and also exhibits excellent thermal cycling performance above 1250 °C (Ref 8, 9). However, SrZrO₃ undergoes three phase transitions from room temperature to high temperature, and the transformation from orthorhombic Pnma to pseudo-tetragonal Imma at 700 °C involves a volume change of ~0.14% (Ref 10).

Ceramic composites can effectively overcome the inherent brittleness and inferior mechanical properties of this single-phase ceramic material. La₂Zr₂O₇-La₂Ce₂O₇ composite has better anti-sintering properties and lower thermal conductivity compared with La₂Zr₂O₇ and La₂Ce₂O₇ (Ref 11). The Al₂O₃-YSZ composite coating exhibits better thermal insulation effect and superior thermal shock resistance compared with YSZ coating (Ref 12). The spark plasma-sintered (SPS) La₂Zr₂O₇-SrZrO₃ composite inhibits SrZrO₃ from phase transitions effectively. The fracture toughness (1.95 ± 0.06 MPa m^1/2) and the thermal conductivity (0.91 W m⁻¹ K⁻¹) of La₂Zr₂O₇-SrZrO₃ composite are superior to La₂Zr₂O₇ and SrZrO₃. Therefore, La₂Zr₂O₇-SrZrO₃ composite becomes a promising TBC candidate material (Ref 13).

With the increased surface temperature of TBCs that exceeded the melting temperatures of CMAS (melting temperatures: 1150-1250 °C), the degradation of TBC subjected to CMAS melts has become a crucial issue in the development of next-generation gas turbine engines. In recent years, the interaction of some new TBC candidate materials subjected to CMAS is investigated (Ref 14-16). The pyrochlore TBCs, such as Gd₂Zr₂O₇ and La₂Zr₂O₇, have attracted much attention due to their promising resistance against CMAS attack (Ref 14, 17). They rapidly dissolve into the CMAS melt, and a crystalline apatite is precipitated concurrently, which arrests the penetration of the molten CMAS front.

In this study, the microstructure evolution of the La₂Zr₂O₇-SrZrO₃ composite coating under CMAS attack at 1250 °C for different times was investigated. The interaction mechanism of the coating under CMAS attack was also explored.

Experimental Procedure

In the present study, the La₂Zr₂O₇-SrZrO₃ composite powder was synthesized by solid-state reaction at 1450 °C for 12 h using La₂O₃ (99.99%, Grirem Advanced Materials Co., Ltd., China), ZrO₂ (99.99%, Grirem Advanced Materials Co., Ltd., China), and SrCO₃ (>98%, Shanghai Reagent Co., Ltd., China) as the starting materials.

For plasma spraying, the synthesized La₂Zr₂O₇-SrZrO₃ composite powder was milled with deionized water and subsequently spray-dried. Sieved-size fractions between 45 and 100 μm were used for plasma spraying. The La₂Zr₂O₇-SrZrO₃ composite coatings were plasma-sprayed (Model MC 60, Medicoat AG, Mägenwil, Switzerland) on IN718 superalloy substrates with a diameter of 30 mm and a thickness of 3 mm. The spray parameters for the coatings are listed in Table 1. Before depositing the La₂Zr₂O₇-SrZrO₃ composite coating, the substrate surface was roughened by grit blasting using Al₂O₃ #24 grit, followed by ultrasonic cleaning using ethanol and acetone successively.

Table 1 Spray parameters of the La₂Zr₂O₇-SrZrO₃ coating

The selected CMAS model had a chemical composition of 33CaO-9MgO-13AlO_1.5-45SiO₂ (mol.%) (Ref 5, 14, 18, 19) using CaO, MgO, Al₂O₃, and SiO₂ powders (AR, Grecia Chemical Technology Co., Ltd., China) as the starting materials. The CMAS was prepared by mixing these oxides and milling them in deionized water to form a thick paste, which was subsequently applied to the surface of the TBC specimens with a concentration of 30 mg/cm². After drying, the specimens were heat-treated at 1250 °C for 1, 4, 8, and 12 h. To better understand the phase constituents of the reaction products, the CMAS powder was mixed with the La₂Zr₂O₇-SrZrO₃ composite powder with a weight ratio of 1:1 by ball milling, followed by heat treatment under the same conditions as the TBC specimens.

The phase constituents of the La₂Zr₂O₇-SrZrO₃ composite powder and coating, as well as the mixture of the CMAS powder and the La₂Zr₂O₇-SrZrO₃ composite powder after heat treatment at 1250 °C, were identified by x-ray diffraction (XRD; model D/MAX 2200, Rigaku Co., Ltd., Japan). A scanning electron microscopy (SEM; model JXA 840, JEOL, Japan) equipped with energy-dispersive spectroscopy (EDS) was used for microstructure analyses. All samples for cross-sectional analysis were initially embedded in transparent epoxy resin and then polished with diamond pastes down to 1 μm.

Results

Phase Analyses of the La₂Zr₂O₇-SrZrO₃ Composite Powder and Its Coating

The XRD patterns of the synthesized La₂Zr₂O₇-SrZrO₃ composite powder and its as-sprayed coating are shown in Fig. 1. The synthesized La₂Zr₂O₇-SrZrO₃ composite powder consisted La₂Zr₂O₇ with the pyrochlore structure and SrZrO₃ with the perovskite structure. Both phases crystallized appropriately, and no other impurity phase was observed. For the APS-sprayed La₂Zr₂O₇-SrZrO₃ composite coating, the diffraction peaks were broadened, and the peak intensities decreased for La₂Zr₂O₇ and SrZrO₃, especially for La₂Zr₂O₇. The reasons for these results are as follows. The molten droplets during plasma spraying reach the substrate and solidify to form a coating in an extremely short time, resulting in the development of metastable phase or an inappropriate crystallized phase. In addition, the composite coating may have high activation energy for grain growth of La₂Zr₂O₇ and SrZrO₃. The La₂Zr₂O₇-YSZ nano-composite powder has much higher activation energy for grain growth of La₂Zr₂O₇ and YSZ than its single-phase counterparts (Ref 20).

Fig. 1

XRD patterns of the La₂Zr₂O₇-SrZrO₃ composite powder and its as-sprayed coating

Interaction Behavior Between the La₂Zr₂O₇-SrZrO₃ Composite Coating and the Molten CMAS

Figure 2 shows the cross-sectional microstructures of the La₂Zr₂O₇-SrZrO₃ composite coatings after CMAS attack at 1250 °C for different times. The interaction layer thickness of the composite coating increases gradually to approximately 11, 40, 72, and 102 μm as the interaction time increases after CMAS attack at 1250 °C for 1, 4, 8, and 12 h, respectively. On the contrary, the thickness of the interaction layer of the 8YSZ coating was approximately 250 µm within 12 h heat treatment at 1250 °C (Ref 15), indicating that the La₂Zr₂O₇-SrZrO₃ composite coatings have superior CMAS resistance than the 8YSZ coating. The interaction layer of the La₂Zr₂O₇-SrZrO₃ composite coatings comprised the inner dense layer and outer porous layer. The rod-shaped particles and globular particles have developed clearly in the porous layer after CMAS attack for 4 h as shown in Fig. 2(b).

Fig. 2

Cross-sectional microstructures of the La₂Zr₂O₇-SrZrO₃ composite coatings after CMAS attack at 1250 °C for: (a) 1 h, (b) 4 h, (c) 8 h, (d) 12 h

The high-magnification cross-sectional microstructures of the La₂Zr₂O₇-SrZrO₃ composite coating after CMAS attack at 1250 °C for 1 h are shown in Fig. 3. Evidently, rod-shaped particles and globular particles have formed in the interaction layer. In addition, some globular particles disperse in the CMAS melt. Above the interaction layer, the two distinct phases are marked as “1” (dark gray phase) and “2” (light gray phase), as shown in Fig. 3(a), and the EDS analyses of the two phases are shown in Fig. 3(c). The dark gray phase is mainly composed of Ca, Si, Zr, Mg, and Sr with traces of Al and La, indicating that the formation of the dark gray phase results from a chemical interaction between the CMAS and the La₂Zr₂O₇-SrZrO₃ composite coating. The light gray phase is mainly composed of Ca, Si, and Zr with traces of Sr, Mg, and Al.

Fig. 3

High-magnification cross-sectional microstructures and EDS analyses of the La₂Zr₂O₇-SrZrO₃ composite coating after CMAS attack at 1250 °C for 1 h

Some CMAS remain on the La₂Zr₂O₇-SrZrO₃ composite coating surface after CMAS attack at 1250 °C, resulting in the difficulty to identify the reaction products by XRD. Therefore, to identify the reaction products between the CMAS and the La₂Zr₂O₇-SrZrO₃ composite coating, a mixture of the CMAS powder and the La₂Zr₂O₇-SrZrO₃ composite powder (weight ratio: 1:1) was ball-milled homogeneously and then heat-treated at 1250 °C for different times. The XRD patterns of the heat-treated powder mixture are shown in Fig. 4. Apart from the La₂Zr₂O₇ and SrZrO₃, La-apatite (Ca₂La₈(SiO₄)₆O₂), fluorite (c-ZrO₂), mellilite (Ca₂MgSi₂O₇), and baghdadite (Ca₃ZrSi₂O₉) were developed as the reaction products. The content of La₂Zr₂O₇ decreases as the reaction time increases from 1 to 8 h and then remains almost constant with increasing reaction time, indicating that La₂Zr₂O₇ reacts severely with the CMAS powder. However, minor SrAl₂Si₂O₈ developed after CMAS attack for different times, indicating much less chemical reaction taking place between CMAS and SrZrO₃.

Fig. 4

XRD patterns of the powder mixture of La₂Zr₂O₇-SrZrO₃ and CMAS after heat treatment at 1250 °C: (a) 1 h, (b) 4 h, (c) 8 h, (d) 12 h

Combined with the EDS and XRD analyses, the dark gray phase “1” and light gray phase “2” in Fig. 3(a) are identified as mellilite (Ca₂MgSi₂O₇) and baghdadite (Ca₃ZrSi₂O₉) solid solutions with minor other elements. Sr element has been detected both in dark gray phase “1” and light gray phase “2” (Fig. 3c), indicating the disintegration of SrZrO₃ during CMAS attack. Sr element may dissolve into Ca₂MgSi₂O₇ and Ca₃ZrSi₂O₉ due to the similar ion radius between Sr²⁺ (0.113 nm) and Ca²⁺ (0.118 nm).

The reaction products are summarized in Table 2 based on the XRD results of the powder mixture of La₂Zr₂O₇-SrZrO₃ and CMAS (Fig. 4). Except for the as-sprayed coating compositions of La₂Zr₂O₇ and SrZrO₃, the major reaction products are fluorite (c-ZrO₂) and La-apatite (Ca₄La₆(SiO₄)₆O or/and Ca₂La₈(SiO₄)₆O₂) after CMAS attack for varied times. The Ca₄La₆(SiO₄)₆O transforms into Ca₂La₈(SiO₄)₆O₂ after CMAS attack at 1250 °C for 8 h. Additionally, Mg₂SiO₄, Ca₃ZrSi₂O₉, and SrAl₂Si₂O₈, as minor reaction products, coexist after different interaction times. However, CaAl₂SiO₆ develops after CMAS attack for 1 h, and it transforms into CaAl₂Si₂O₈ gradually with more SiO₂ incorporated as the interaction time increases. CaAl₂SiO₆ and CaAl₂Si₂O₈ are obtained after 4-h interaction, and only CaAl₂Si₂O₈ is found after 8-h interaction. With increasing interaction time, more CaO, MgO, and SiO₂ generate Ca₂MgSi₂O₇. The development of SrAl₂Si₂O₈ is due to the disintegration of SrZrO₃ and resulted from the chemical reaction of SrO with Al₂O₃ and SiO₂.

Table 2 Reaction products of the powders mixture of La₂Zr₂O₇-SrZrO₃ and CMAS

The rod-shaped and globular particles in the interaction layer grow gradually as the interaction time increases, which are clearly shown in the high-magnification cross-sectional microstructures of the La₂Zr₂O₇-SrZrO₃ composite coating after CMAS attack at 1250 °C for 8 h (Fig. 5). The rod-shaped particles comprised Ca, La, and Si by EDS analysis and can be confirmed as La-apatite (Ca₂La₈(SiO₄)₆O₂) with XRD analysis. The globular particles consist primarily of Zr with litter amount of Ca, La, Mg, and Al and can be identified as fluorite (c-ZrO₂) based on the XRD result (Fig. 4). The rod-shaped La-apatite particles and globular fluorite particles are interpenetrated by residual CMAS melt. As shown in Fig. 5(a), the size of the crystalline La-apatite decreases toward the interface between the interaction layer and the La₂Zr₂O₇-SrZrO₃ composite coating, because the crystals closer to the outer surface are likely to have formed earlier and have more time to coarsen. In addition, the relative amount of CMAS decreases toward the interface.

Fig. 5

High-magnification cross-sectional microstructures and EDS analyses of the La₂Zr₂O₇-SrZrO₃ coatings after CMAS attack at 1250 °C for 8 h

Discussion

The experimental results reveal that the microstructures of the La₂Zr₂O₇-SrZrO₃ composite coating vary significantly with increasing interaction time at 1250 °C, whereas the reaction products are relatively insensitive to the interaction time. Most of the CMAS/La₂Zr₂O₇-SrZrO₃ interaction features seem to be in good agreement with the observations in the La₂Zr₂O₇ case (Ref 5, 21). The most important reaction product is the La-apatite. The EDS/XRD results indicate that the apatite produced in this study is a silicate phase, which is either in the form of Ca₄La₆(SiO₄)₆O or Ca₂La₈(SiO₄)₆O₂ depending on the interaction time. After CMAS attack for 8 h, more La³⁺ react with CMAS; thus, Ca₄La₆(SiO₄)₆O transforms into Ca₂La₈(SiO₄)₆O₂ (JCPDS no.29-0337). The general formula of apatite phase is ${\text{A}}_{4}^{\text{I}}$ ${\text{A}}_{6}^{\text{II}}$(SiO₄)₆O_x, where the “A” sites are occupied by alkali metal, alkaline earth metal, or rare earth ions; x is determined by the net valence of the “A” sites. A^I is occupied by larger radius of Ca²⁺ with ninefold coordination, and A^II is occupied by smaller radius with sevenfold coordination relatively (Ref 14, 19). The initial chemical formula of La-apatite is Ca₄La₆(SiO₄)₆O. Some La³⁺ can substitute Ca²⁺ to form Ca₂La₈(SiO₄)₆O₂ because the radius of La³⁺ (0.110 nm) is similar to that of Ca²⁺ (0.118 nm). When more La³⁺ ions react with CMAS, a more stable apatite phase similar to Gd₂Zr₂O₇ (Ref 4, 22) can be produced. After CMAS attack for 12 h, the reaction products are invariant in the mixed powder, and the reaction tends to be stable.

The second reaction product of La₂Zr₂O₇-SrZrO₃, which is subjected to CMAS, is fluorite c-ZrO₂. EDS determined that the fluorite c-ZrO₂ is stabilized predominantly by CaO and La₂O₃, although less amounts of Mg, Al, and Si are involved.

One possible CMAS interaction mechanism of the La₂Zr₂O₇-SrZrO₃ coating is dissolution-reprecipitation mechanism (Ref 14, 19). The CMAS mixture starts to melt at 1250 °C, and La₂Zr₂O₇ grains in the La₂Zr₂O₇-SrZrO₃ coating are wetted by molten CMAS melt and reacted with the melt. The dissolution of the original La₂Zr₂O₇-SrZrO₃ coating occurs with incorporation and migration of La, Zr, and Sr to the melt. The concentration of La in CMAS melt that accumulates to a certain value triggers the crystallization of La-rich melt into La-apatite. In addition, fluorite crystallizes when the Zr content in the melt exceeds the threshold for nucleation. At the same time, mellilite (Ca₂MgSi₂O₇) and baghdadite (Ca₃ZrSi₂O₉) solid solutions with minor other elements in the melt are formed. However, SrZrO₃ in the La₂Zr₂O₇-SrZrO₃ coating reacts much less intensively with CMAS compared to La₂Zr₂O₇. In our study, identifying SrZrO₃ in the interaction layer is difficult, whereas minor amount of Sr is detected in mellilite and baghdadite. The interaction behavior of SrZrO₃ in the La₂Zr₂O₇-SrZrO₃ coating in contact with CMAS melt at high temperatures requires further investigations in the future.

The interaction behavior of the La₂Zr₂O₇-SrZrO₃ coating in contact with CMAS melt is extremely similar to La₂Zr₂O₇. The Poerschke’s results (Ref 19) and our experimental results indicated that only the dissolution of the La₂Zr₂O₇-SrZrO₃ coating in the CMAS melt and reprecipitation of apatite and fluorite are sufficiently fast compared to the CMAS penetration through the coating; the CMAS mitigation goal of the coating can be realized. According to the SEM/EDS results, the dense interaction layer mainly comprised La-apatite and globular fluorite (c-ZrO₂) grains that can effectively protect the coating from CMAS penetration. The reaction between the La₂Zr₂O₇-SrZrO₃ coating and the CMAS melt can occur at high temperature as follows, and the ratio of Ca to La in Ca₂La₈(SiO₄)₆O₂ is calculated according to Fig. 5(c).

$$10{\text{Ca}}_{0.33} {\text{Mg}}_{0.09} {\text{Al}}_{0.13} {\text{Si}}_{0.45} + 4.45{\text{La}}_{0.5} {\text{Zr}}_{0.5} \left( {{\text{La}}_{2} {\text{Zr}}_{2} {\text{O}}_{7} } \right) + 0.2{\text{Sr}}_{0.5} {\text{Zr}}_{0.5} \left( {{\text{SrZrO}}_{3} } \right) \to 2.482{\text{Ca}}_{0.13} {\text{La}}_{0.07} {\text{Zr}}_{0.8} \left( {{\text{c}} - {\text{ZrO}}_{2} } \right) + 4.275{\text{Ca}}_{0.12} {\text{La}}_{0.48} {\text{Si}}_{0.4} \left( {{\text{Ca}}_{2} {\text{La}}_{8} \left( {{\text{SiO}}_{4} } \right)_{6} {\text{O}}_{2} } \right) + 2{\text{Ca}}_{0.5} {\text{Zr}}_{0.17} {\text{Si}}_{0.33} \left( {{\text{Ca}}_{3} {\text{ZrSi}}_{2} {\text{O}}_{9} } \right) + {\text{Ca}}_{0.4} {\text{Mg}}_{0.2} {\text{Si}}_{0.4} \left( {{\text{Ca}}_{2} {\text{MgSi}}_{2} {\text{O}}_{7} } \right) + 3{\text{Ca}}_{0.2} {\text{Al}}_{0.4} {\text{Si}}_{0.4} \left( {{\text{CaAl}}_{2} {\text{Si}}_{2} {\text{O}}_{8} } \right) + {\text{Mg}}_{0.67} {\text{Si}}_{0.33} \left( {{\text{Mg}}_{2} {\text{SiO}}_{4} } \right) + 0.5{\text{Sr}}_{0.2} {\text{Al}}_{0.4} {\text{Si}}_{0.4} \left( {{\text{SrAl}}_{2} {\text{Si}}_{2} {\text{O}}_{8} } \right) + {\text{Ca}}_{0.94} {\text{Mg}}_{0.06} \left( {\text{residue}} \right).$$

(1)

The residue is used to balance the equation.

Conclusions

La₂Zr₂O₇-SrZrO₃ coatings prepared by APS were exposed to CMAS melts at 1250 °C and characterized in terms of microstructure and phase evolution. During CMAS attack, the microstructures of coatings significantly varied with prolonged interaction time, but the reaction products were relatively unaffected. The La₂Zr₂O₇-SrZrO₃ coating rapidly dissolved in molten CMAS, and La-apatite (Ca₂La₈(SiO₄)₆O₂) and fluorite c-ZrO₂ stabilized mainly with Ca and La precipitating concurrently. The acceptable CMAS resistance for the La₂Zr₂O₇-SrZrO₃ coating relative to 8YSZ coating in terms of infiltration depth was due to the formation of a dense interaction layer resulting from the fast chemical reactions between La₂Zr₂O₇ and CMAS. The interaction mechanism of La₂Zr₂O₇-SrZrO₃ coating subjected to CMAS was dissolution-reprecipitation, which was similar to that of La₂Zr₂O₇.