Abstract: Thermal barrier coatings, as a protective coating composed of a metal bonding layer, a ceramic surface layer and a thermally grown oxide, have been widely used on aero-engine turbine blades. LaZrCeO/YSZ dual ceramic thermal barrier coatings were prepared on Ni-based superalloy substrates by electron beam physical vapor deposition technology. The composition, phase structure and thermal cycle life of the thermal barrier coatings were studied by adjusting the deposition energy of the target. The failure mechanism of the thermal barrier coatings under 1100 ℃ thermal cycling was analyzed. The results show that with the increase of the target deposition energy, the Zr content in the LaZrCeO coating increases continuously, while the La/Ce element ratio is basically consistent with that of the target. At the same time, with the increase of the target deposition energy, the coating phase structure changes from a single fluorite phase to a composite pyrochlore and fluorite phase structure, and then to a single pyrochlore structure. The 1100 ℃ thermal cycling test shows that the LaZrCeO/YSZ dual ceramic thermal barrier coating with a composite pyrochlore and fluorite phase structure has an average thermal cycle life of 1518 times, showing good thermophysical properties. As the thermal cycle progresses, the Al element in the metal bonding layer diffuses outward to form thermally grown oxide (TGO), and the Cr element reacts with LaZrCeO and O to form LaCrO3 and ZrO2. Ni and Co elements diffuse and react with O at high temperatures to form (Ni, Co)(Cr, Al)2)O4 compounds, which causes cracks in the TGO layer or interface layer, reduces the toughness between the metal bonding layer and the ceramic layer, and ultimately leads to the failure of the thermal barrier coating.

Keywords: thermal barrier coating; electron beam physical vapor deposition; phase structure; element content; thermal cycle life; failure mechanism

With the continuous development of advanced aircraft engines, the temperature of the inlet gas at the front of the aircraft engine turbine blades continues to increase, and the thermal barrier coating will be subjected to more harsh cold and hot environments during actual service. The ceramic surface layer of the conventional thermal barrier coating, yttria-stabilized zirconia material (YSZ), will undergo phase change and severe sintering during long-term service at a high temperature of 1200 ℃, resulting in premature failure of the thermal barrier coating. Therefore, the development of new thermal barrier coatings with low thermal conductivity and higher operating temperature has become a key technology for the development of the next generation of high-performance engines.

Among the new thermal barrier coating materials, La2Zr2O7 coating has good thermal stability, low thermal conductivity and high melting point, but compared with YSZ, La2Zr2O7 has a low thermal expansion coefficient, which will increase the thermal stress in the coating during the hot and cold cycle, thereby shortening the thermal cycle life of the coating. Therefore, domestic and foreign research institutions and universities usually use methods such as doping Ce, Pt, Sm and other elements to modify it to increase the thermal expansion coefficient of the material and reduce the thermal conductivity. After Cao et al. doped CeO2 in La2Zr2O7, the thermal expansion coefficient of the material increased and the thermal conductivity decreased. In addition, forming a double ceramic layer with YSZ can also better play the advantages of LaZrCeO. Vassen et al. deposited a double ceramic layer La2Zr2O7/YSZ coating by atmospheric plasma spraying. This double-layer structure coating has a longer thermal cycle life than a single-layer coating. Bobzin et al. compared the thermal cycling performance of single-layer YSZ coating and double-layer La2Zr2O7/YSZ coating deposited by electron beam physical vapor deposition at 1100 °C. The thermal cycling life of the La2Zr2O7/YSZ double-layer coating reached 4140 times.

The microstructure control, mechanical properties and life of thermal barrier coatings are closely related to the deposition process. At present, air plasma spraying (APS) and electron beam-physical vapor deposition (EB-PVD) are the two main methods for depositing ceramic surface layers of thermal barrier coatings in industry. Among them, the coating prepared by EB-PVD process is mainly chemically bonded to the substrate. The special columnar crystal structure improves the strain tolerance of the coating and can significantly reduce the thermal stress caused by the mismatch of thermal expansion with the substrate. Therefore, the EB-PVD coating has very excellent thermal shock resistance. Compared with APS coating, EB-PVD coating has lower surface roughness and does not block the cooling gas channel of the blade, which is beneficial to maintain the dynamic performance of the blade. However, during the EB-PVD coating deposition process, due to the different saturated vapor pressures and melting points of different oxides, the chemical composition of the deposited coating deviates from the stoichiometric ratio of the target material, affecting the mechanical properties and thermal stability of the thermal barrier coating.

Therefore, this work uses EB-PVD technology to prepare four LaZrCeO/YSZ dual ceramic thermal barrier coatings by changing the target evaporation energy during the coating deposition process. The element content (atomic fraction, the same below), phase structure and thermal cycle life of the thermal barrier coating were analyzed to study the failure mechanism of the thermal barrier coating under 1100 ℃ thermal cycling.

1 Experimental materials and methods
1. 1 Experimental materials
This experiment uses the third-generation single crystal high-temperature alloy DD10 as the substrate material for thermal barrier coatings. The nominal composition of the DD10 single crystal high-temperature alloy is shown in Table 1. Wire cutting is used to prepare 30 mm×10 mm×1.5 mm specimens. After the substrate specimens are cut, they are ground with 200#, 600# and 1200# water-abrasive sandpaper in turn until the surface roughness is (1.0±0.2) μm, and then cleaned and dried for later use.

The NiCoCrAlYHf coated metal bonding layer was selected and prepared by vacuum arc plating physical vapor deposition process. The equipment used was A-1000 vacuum arc ion plating (arc ion plating-physical vapor deposition, AIP-PVD) physical vapor deposition equipment, and the deposition time was 200 minutes. The LaZrCeO ceramic surface layer was prepared by EB-PVD process, and the equipment used was UE-207S improved EB-PVD equipment. Table 2 shows the deposition process parameters of LaZrCeO thermal barrier coatings deposited by EB-PVD. Based on the previous research, this experiment further adjusted the target evaporation energy to (1.60~1.70)×104, (2.00~2.10)×104, (2.35~2.45)×104, (2.75~2.85)×104 J/cm2, and deposited four LaZrCeO thermal barrier coatings with different microstructures.

1.2 Experimental method
D8 Advance XRD was used to analyze the coating phase composition, with an operating voltage of 40 kV, an operating current of 40 mA, and a scanning range of 20°~80°. Quanta 600 environmental SEM and JXA-8230 EPMA were used to observe the coating morphology. The composition of metal elements in the coating was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a 725E inductively coupled plasma atomic emission spectrometer. The coating life was tested using a thermal cycler. The isothermal thermal cycle life test method was used, with the heating temperature set at 1100 °C, the furnace kept warm for 55 min, and the air cooled naturally for 5 min as one complete cycle. The sample coating was considered to have failed when the coating peeling area exceeded 10%.

2 Results and Analysis

2. 1 Element Content of LaZrCeO Thermal Barrier Coating

The chemical composition of the target and LaZrCeO coating was analyzed by ICP-AES, and the results are shown in Table 3.

Compared with the target composition, the element content ratio of La/Zr/Ce in the LaZrCeO coating has changed significantly. After analysis, it was found that the main reasons for the element segregation phenomenon in the LaZrCeO coating include the following three aspects: (1) Due to the difference in the melting points of La2O3, CeO2 and ZrO2, the melting points of La2O3, CeO2 and ZrO2 are 2300℃, 2600℃ and 2700℃, respectively. During the evaporation of the target by electron beam heating, the melting points of different oxides are different, resulting in different evaporation orders. La2O3 and CeO2 with low melting points in the target will evaporate first, resulting in a higher content of La and Ce in the coating; (2) Due to the large difference in the saturated vapor pressures of La2O3, CeO2 and ZrO2 [24], at 2200℃, they are 130, 1000 and 0.0375 Pa, respectively. Therefore, when the evaporation equilibrium is reached, the content of Ce and La in the vapor is higher than that of Zr; (3) The lattice energy calculation results of LaZrCeO oxide show that the thermal stability of ZrO2 is better than that of La2O3 and CeO2, which will also lead to higher content of La and Ce in the vapor, while ZrO2 is lower. These reasons together lead to the chemical composition of the coating formed by the solidification of gas phase atoms deviating from the stoichiometric ratio.

Therefore, as the electron beam current value of the evaporation electron gun increases and the evaporation energy increases, the Zr content in the coating will continue to increase. In addition, the La/Ce element ratio is basically consistent with the La/Ce element ratio in the target material, which is also well consistent with the saturated vapor pressure and melting point of La2O3 and CeO2. The evaporation of La and Ce elements is basically synchronous.

2. 2 Morphology and phase structure of LaZrCeO thermal barrier coating
The thermal barrier coating prepared in this experiment is a three-layer structure of a NiCoCrAlYHf bonding layer + a double ceramic layer (YSZ intermediate layer + LaZrCeO surface layer). The NiCoCrAlYHf bonding layer prepared by vacuum arc plating is tightly bonded to the substrate and has a thickness of about 40~50 μm. The thickness of the YSZ intermediate layer and the LaZrCeO surface layer is about 60~70 μm, and no obvious defects are found at the interface between the bonding layer and the YSZ layer, and between the YSZ layer and the LaZrCeO layer. According to previous studies, the change in evaporation energy does not significantly affect the morphology of the coating. The cross-sectional morphology of the four coatings prepared in the experiment is basically the same, as shown in Figure 1.

Figure 2 shows the XRD spectra of four deposited LaZrCeO coatings. The XRD spectra of the LaZrCeO target are basically consistent with the La2Zr2O7 standard card, with only a small angle shift in the main peak position. After comparing the XRD spectra of the four deposited LaZrCeO coatings with the standard card, it was found that when the target evaporation energy was (1.60~1.70)×104J/cm2, the deposited LaZrCeO coating was a single fluorite phase structure. During the deposition process of the coating, the evaporation energy density was low, and the proportion of Zr element in the coating was low. At this time, the La/Zr/Ce atomic ratio of LaZrCeO coating sample 1 was 1∶ 0.175∶0.739. The low content of Zr directly leads to the phase structure of the deposited coating being mainly fluorite phase. At the same time, since the three strong peaks of the pyrochlore phase La2Zr2O7 are at large angles relative to the three strong peaks of the fluorite phase La2Ce2O7, the presence of a small amount of Zr will cause the peak position of the coating fluorite phase to shift to large angles.

When the target evaporation energy is (2.00~2.10)×104J/cm2, the evaporation energy of coating sample 2 is increased compared with that of coating sample 1, and the Zr content in the deposited element composition increases significantly. Therefore, the deposited LaZrCeO coating is a composite pyrochlore and fluorite phase structure. The XRD spectrum
clearly shows the presence of a wider peak, corresponding to the dual-phase composite structure of pyrochlore and fluorite. When the evaporation energy is (2.35~2.45)×104J/cm2, the evaporation energy continues to increase, the Zr content in the deposited element composition continues to increase, and the deposited LaZrCeO coating is a single pyrochlore phase structure. The XRD spectrum clearly shows the presence of sharp diffraction peaks, indicating that the coating has good crystallinity. When the evaporation energy is (2.75~2.85)×104J/cm2, the deposited LaZrCeO coating is a single pyrochlore phase structure. At this time, the evaporation energy continues to increase, and the Zr content in the deposited element composition continues to increase.

Figure 2 (b) shows the result of magnifying the XRD spectrum 25°~35°. It can be found that the composite pyrochlore and fluorite phase structure of LaZrCeO coating sample 2. Coating samples 3, 4 and the target material show a single pyrochlore phase structure, which is shifted to a smaller angle than the standard diffraction peak of La2Zr2O7. LaZrCeO coating sample 1 shows a fluorite phase structure, and the standard diffraction peak of La2Ce2O7 is offset to a larger angle. The reason for the offset of the diffraction peak can be attributed to the large difference between the ionic radius of Zr4+ and the ionic radius of Ce4+, the continuous increase in the Zr element content, the change in the interplanar spacing, and the movement of the diffraction peak to a smaller angle.

2.3 Thermal cycle life of LaZrCeO thermal barrier coating
Figure 3 shows the 1100 ℃ thermal cycle life of LaZrCeO/YSZ thermal barrier coatings prepared by different evaporation electron beam currents. The values ​​are the average thermal cycle life of 5 coating samples under the same conditions. Under the same thermal cycle test conditions, there are certain differences in the thermal cycle life of LaZrCeO/YSZ double-layer thermal barrier coatings prepared by different evaporation electron beam currents. Among them, when the target evaporation energy is (1.60~1.70)×104 J/cm2, the average thermal cycle life of the double-layer thermal barrier coating of the deposited coating sample 1 is 1024 times. As the electron beam current value of the evaporation electron gun increases, that is, the evaporation energy increases, the thermal cycle life of the double-layer thermal barrier coating increases. When the target evaporation energy is (2.00~2.10)×104J/cm2, the average thermal cycle life of the thermal barrier coating of the deposited sample 2 reaches the highest value, which is 1518 times.

As the evaporation energy continues to increase, the average thermal cycle life of the thermal barrier coating begins to show a downward trend. When the evaporation energy is (2.35~2.45)×104J/cm2, the average thermal cycle life of the double-layer thermal barrier coating of the deposited coating sample 3 is 1303 times. When the evaporation energy of the deposited coating is (2.75~2.85)×104J/cm2, the average thermal cycle life of the double-layer thermal barrier coating of the deposited coating sample 4 continues to decrease, and its average thermal cycle life is only 1200 times.

As the evaporation energy increases, the coating changes from a single fluorite phase structure to a composite pyrochlore and fluorite phase structure. Due to the lattice mismatch, this two-way coexistence structure increases the lattice distortion of each phase, thereby enhancing phonon scattering and helping to reduce thermal conductivity. At the same time, the difference in thermal expansion coefficients between the two phases will generate internal stress during the cooling process, which can offset the thermal stress generated by the thermal expansion mismatch. At the same time, the composite pyrochlore and fluorite phase structure has better phase stability, which further improves the thermal cycle life of the coating. When the evaporation electron beam current continues to increase, the coating structure changes to a single pyrochlore phase structure, which has lower thermal conductivity than the single fluorite phase structure, and has better phase change stability and sintering resistance. However, the single phase structure will affect the fracture toughness and strain tolerance of the thermal barrier coating to a certain extent during the thermal cycle process, and is not conducive to the release of thermal stress. Therefore, the thermal cycle life of coating samples 3 and 4 is reduced.

2.4 Failure mechanism of LaZrCeO thermal barrier coating
Under long-term high-temperature service conditions, the metal bonding layer will react with O to generate TGO at the ceramic layer/bonding layer interface, and the formation and evolution of TGO are the key factors leading to the failure of thermal barrier coatings. In the early stage of TGO growth, the uniform and continuous oxide film can prevent the bonding layer from being further oxidized. As the thermal experiment proceeds, the thickness of TGO will increase with a certain rule, and the Al element in the nearby metal bonding layer will be continuously consumed. When the Al content is lower than a certain critical value, it will not be able to meet the requirements of forming a dense Al2O3 layer, and the metal bonding layer may undergo severe internal oxidation, resulting in a decrease in the life of the thermal barrier coating. Therefore, taking the thermal barrier coating of coating sample 2 with the best thermal cycle life as an example, the diffusion of coating elements and the failure mechanism during the thermal cycle process are studied.

2.4.1 Phase structure stability
The XRD phase structure analysis was carried out on the surface of the LaZrCeO/YSZ thermal barrier coating after 1500 thermal cycles at 1100 ℃. As shown in Figure 4. The XRD spectrum of the LaZrCeO coating after 1100 ℃ thermal cycling is basically the same as that of the deposited LaZrCeO coating. Compared with the La2Zr2O7 standard card, the main peak position is also basically the same. The LaZrCeO coating after 1100 ℃ thermal cycling is a two-phase composite structure of pyrochlore (La2Zr2O7) and fluorite (La2Ce2O7). Further compared with the XRD of the deposited LaZrCeO coating, the XRD spectrum of the thermal barrier coating after thermal cycling clearly shows the phenomenon of narrowing of the characteristic peak width, which is caused by the increase in the crystallinity of the LaZrCeO coating after high-temperature thermal cycling. In addition, by analyzing other miscellaneous peaks in the XRD spectrum, it was found that the thermal barrier coating after thermal cycling had 3 characteristic peaks of ZrO2 and 1 LaCrO3. This is because the LaZrCeO coating will partially undergo chemical reactions under 1100 ℃ thermal cycling.

2.4.2 TGO growth and evolution
Related literature shows that Al diffusion and Al2O3 formation can affect the evolution of TGO layer structure. Figure 5 is a cross-sectional EPMA scan of the Al element content and distribution of the LaZrCeO/YSZ double-layer thermal barrier coating after 1100 ℃ thermal cycling. The Al element of the deposited LaZrCeO/YSZ double-layer thermal barrier coating mainly exists in the metal bonding layer, and the Al element content and element distribution are relatively uniform, without obvious segregation. As the 1100 ℃ thermal cycle proceeds, due to the difference in Al element concentration in the ceramic surface layer and the metal bonding layer in the thermal barrier coating system, a gradient of Al element distribution is formed. Under the high temperature condition of 1100 ℃, the Al element in the metal bonding layer will diffuse out of the coating. At the same time, since the thermal barrier coating is a columnar crystal structure with a certain amount of gaps, it is a good conductor for air, and O2 can directly reach the interface between the ceramic layer and the metal bonding layer through the columnar crystal gaps. Because of the concentration difference of the O element in the air and in the thermal barrier coating, the O2 in the atmosphere will tend to diffuse into the coating. When O2 reaches the surface of the metal bonding layer, it will inevitably react chemically with the Al element on the surface under high temperature to generate Al2O3. The reaction equation is shown in formula (1): See formula (1) in the figure.
When the Al element and the O element react chemically, the concentration balance of the Al element at the local interface will be broken. As the chemical reaction proceeds, the Al element will continue to diffuse to the surface. As shown in Figure 5 (b), after 100 thermal cycles at 1100 °C, a layer of TGO (mainly Al2O3) was generated on the surface of the metal bonding layer, and the interface roughness increased with the growth of the TGO layer. As the 1100 °C thermal cycle continued, at 400 thermal cycles, the Al element would continue to diffuse to the surface and react to generate α-Al2O3, resulting in the continuous thickening of the TGO layer and the continuous increase in the interface roughness, as shown in Figure 5 (c).
As the 1100 °C thermal cycle proceeded, the Al element continued to diffuse to the interface and react to generate α-Al2O3 and was consumed. When it reached a certain level, cracks appeared inside the TGO, and with the consumption of the Al element and the evolution of TGO, horizontal cracks and vertical cracks interacted with each other. When it reached 1500 thermal cycles, the thermal barrier coating reached the critical point of failure. At this time, it can be observed that the TGO thickness reaches about 30 μm, and horizontal cracks and vertical cracks are distributed throughout the TGO layer.

2.4.3 Element diffusion in the metal bonding layer
Figure 6 shows the Cr element line scan analysis of the LaZrCeO/YSZ double-layer thermal barrier coating after 1100 °C thermal cycling. From Figure 6 (a), it can be seen that Ni, Co, Cr, Al, Y, and Hf are mainly present in the metal bonding layer. At the same time, from Figure 6 (b), it can be observed that the Cr element has diffused from the metal bonding layer to the TGO layer, and a small part has diffused into the LaZrCeO top coating. This is because in the initial oxidation stage, Cr-containing oxides are formed earlier than α-Al2O3［28］. Under high temperature conditions of 1100 ℃, Cr will react with O to form Cr2O3, and then the LaZrCeO ceramic layer will further react with Cr2O3 and finally generate LaCrO3 and ZrO2, as shown in formula (2), which corresponds to the characteristic peaks of LaCrO3 and ZrO2 appearing in the XRD experimental data after thermal cycling.
Under high temperature conditions of 1100 ℃, as the reaction proceeds, Cr continues to diffuse into the LaZrCeO layer. When reaching 1500 thermal cycles, the thermal barrier coating reaches the critical point of failure. At this time, it can be observed that Cr has diffused into the TGO layer and the LaZrCeO top coating in large quantities. Therefore, the diffusion of Cr and the generation of LaCrO3 are one of the reasons for the failure of the thermal barrier coating.
The Ni and Co elements in the metal bonding layer will also diffuse at high temperatures to form a small amount of NiO and CoO, that is, the brittle phase is generated in the TGO layer, thereby reducing the toughness between the metal bonding layer and the ceramic layer, resulting in the failure of the thermal barrier coating. Figure 7 shows the distribution of Ni and Cr elements in the cross-sectional EMPA surface scan of the LaZrCeO/YSZ double-layer thermal barrier coating after 1100 ℃ thermal cycling. As can be seen from Figure 7, the Co and Ni elements in the metal bonding layer after 1100 ℃ thermal cycling are evenly distributed in the metal bonding layer, but some Co and NiO have diffused outward, and the maximum element diffusion depth is 30 μm. After comparing the diffusion depth of the Al element, it was found that the outward diffusion of Co and Ni elements mainly exists in the TGO layer. According to the literature, the Co element diffuses outward into the TGO layer and reacts with the O element to form Al-Co-O series oxides. The formation of these compounds will reduce the bonding strength and mechanical properties of the TGO layer, and increase the stress concentration area during the cooling and heating process.
Under high temperature conditions, Ni, Co, and Cr elements will further react with Al2O3 and O to form spinel structure (Ni, Co) (Cr, Al)2O4 compounds. The chemical reaction equation is shown in formula (3): See formula (3) in the figure.
The formation of these spinel structure compounds will reduce the bonding strength and mechanical properties of the TGO layer during service, and will further promote the initiation and evolution of TGO cracks during the cooling and heating process.

3 Conclusions
(1) Four double-layer thermal barrier coatings with different microstructures were prepared by EB-PVD method. By changing the evaporation energy, the La/Zr/Ce element content ratio in the prepared LaZrCeO coating changed. The Zr content in the coating increases continuously, while the La/Ce ratio is basically consistent with the La/Ce ratio in the target.
(2) With the increase of evaporation energy, the phase structure of the LaZrCeO layer changes from a single fluorite structure (La2Ce2O7) to a dual-phase structure of fluorite structure (La2Ce2O7) and pyrochlore structure (La2Zr2O7), and then to a single pyrochlore structure (La2Zr2O7). In addition, the increase in evaporation energy causes the Zr4+ content in the coating to increase continuously, causing changes in the inter-crystalline spacing, resulting in a shift in the XRD diffraction peak.
(3) After 1500 thermal cycles at 1100 °C, the phase structure of the LaZrCeO thermal barrier coating still maintains a dual-phase composite structure of pyrochlore (La2Zr2O7) and fluorite (La2Ce2O7), with only a small amount of ZrO2 and LaCrO3. The thermal barrier coating exhibits good thermal stability.
(4) Under high temperature conditions of 1100 °C, the Al element in the metal bonding layer diffuses outward from the coating to form TGO. The Cr element diffuses outward and reacts with O and LaZrCeO to generate a new phase, causing the phase structure of the ceramic surface layer to transform, and then stress concentration is generated during the hot and cold process, resulting in cracks, thereby reducing the bonding between the metal bonding layer and the ceramic layer.
(5) The Ni and Co elements in the metal bonding layer diffuse at high temperatures and are mainly enriched in the TGO layer. They further react with Al2O3 and O to form spinel structured (Ni, Co) (Cr, Al)2O4 compounds, thereby reducing the bonding between the metal bonding layer and the ceramic layer and causing the thermal barrier coating to fail.