A solution to the challenges of particle erosion and thermal cycle for PS-PVD 7YSZ thermal barrier coatings


 Advanced aero-engine is a key technique that is used all over the world, where many high-temperature components such as turbine blades and combustor, are made of Ni/Co/Fe based superalloys. However, they need high-temperature protection to avoid fast performance degradation. Generally, the superalloy high-temperature components are protected by thermal barrier coatings (TBCs) obtained via an atmospheric plasma spray (APS) and an electron beam-physical vapor deposition (EB-PVD). Here, a novel 3rd generation TBCs process using plasma spray-physical vapor deposition (PS-PVD) is presented, showing a more promising use than the traditional APS and EB-PVD. The PS-PVD feature uses evaporating ceramic powder, which results in the deposition of a feather-like columnar coating. This special microstructure showed good strain tolerance and non-line-of-sight (NLOS) deposition, giving great potential for application. In a working aero-engine, the high-temperature components face a serious environment, where foreign particle erosion is a great challenge and is the first barrier to the application of PS-PVD TBCs. As a solution, an Al-modification approach was proposed in this investigation. The results demonstrate that this approach can improve particle erosion resistance. Also, the thermal cycle performance had an apparent optimization.


Introduction
The aero-engine is known as the crown jewel in the industry. Highly safe, high-performance aero-engines are important for the realization of human air travel. As an advanced technology, the aero-engine has attracted much attention all over the world since 1903. Highly e cient power and a high thrust-weight ratio have always been the aim. To realize these targets, increasing the turbine inlet temperature (TIT) is a direct approach. However, those high-temperature superalloy components in the aero-engine cannot meet the requirements of an advanced engine, due to the relatively low melting point (~ 1150°C) [1][2][3][4] . Generally, the surface temperature of the turbine blades in a service environment (≥ 1200°C) is much higher than its melting point, even with the high-e ciency gas-lm cooling technology [5][6] . Considering this limitation, thermal barrier coatings (TBCs) were proposed to reduce the working temperature of the high-temperature components. After more than 60 years of research and development, the TBCs prepared by atmospheric plasma spray (APS) and electron beam-physical vapor deposition (EB-PVD) have been applied extensively in aeroengines [7][8] . The APS process has many advantages, such as low cost and high e ciency. [9][10] The EB-PVD TBCs also have many merits, such as high strain tolerance and high bond strength. [11][12] However, to prolong the TBCs lifetime, there are many challenges to overcome, such as foreign particle erosion, and frequent thermal cycle performance [13][14][15] .
The constant progress of the aero-engine TIT process as well as increasing pollution restrictions determine the development of heat-resistant coatings. The new deposition technique of plasma sprayphysical vapor deposition (PS-PVD) has a different deposition mechanism as compared with the traditional processes. The APS TBCs have a laminar structure obtained through splat deposition, which causes the challenges of low strain tolerance and low bond strength. [16][17] . The EB-PVD TBCs have a columnar structure obtained via vapor-deposited atoms, and they face the challenges of high cost and high thermal conductivity [18][19] . The PS-PVD process can avoid these limitations to some extent. In this investigation, the key feature of the PS-PVD process is the option of evaporating ceramic powders, which enables the deposition of a feather-like columnar ceramic coating [20][21][22] . The PS-PVD TBCs have many advantages, such as the uniform coating on irregularly shaped components, a high deposition e ciency, and a low thermal conductivity [23][24] . However, the characteristic high porosity of PS-PVD TBCs hurts particle erosion performance [25][26] , which is the great challenge before application.
Facing this challenge, an Al-modi cation method was proposed for PS-PVD 7YSZ TBCs in this investigation. Through magnetron sputtering, appropriate thickness of Al lm was deposited on TBCs surface. Subsequently, the Al-deposited TBCs were carried out with vacuum heat treatment. Then an a-Al 2 O 3 dense overlay will be in-situ synthesized through reaction between Al and ZrO 2 . This dense layer will provide a positive effect for TBCs. The main difference of Al-modi cation from traditional approaches is a-Al 2 O 3 phase resulting from in-situ synthesis. Thus, this overlay has good bond with original 7YSZ TBCs. Possibility, Al-modi cation is an important approach to improve the performance of PS-PVD 7YSZ TBCs 2. Experimental Section
To further improve their performance, the 7YSZ TBCs were deposited with Al lm on their surface by magnetron sputtering. And then, the Al-deposited 7YSZ TBCs samples with different Al lm thicknesses of 5, 10, and 20 µm were carried out with vacuum heat treatment, forming Al-modi ed 7YSZ TBCs. During deposition, Al target (99.99%) was used and the direct current, voltage, and pressure were set as 3 A, 150 V, and 5×10 − 3 Pa, respectively. And the Al-deposited TBCs samples were treated with a certain parameter (608°C for 1 h, 700°C for 1 h, and 980°C for 2 h). Above experiment process can be seen in the following supplementary video.

Plasma ow simulation
The plasma ow of the PS-PVD was equivalent to a quasi-equilibrium continuous medium, and its ow process was described by an N-S equation. Therefore, it is accurate to establish the plasma ow model by a CFD (Computational Fluid Dynamics) simulation. The internal and external ow eld model of the spray gun was established by ANSYS FLUENT (Ansys 2020 R2, URL: //www.ansys.com/), which included consideration for the coupling effect of the internal and external ow eld. To show the turbulent characteristics of the plasma ow, the SST-KW two-equation turbulence model was used during the process of the CFD calculations.

TBCs characterization
The microstructures of the as-sprayed and Al-modi ed 7YSZ TBCs were characterized by eld emissionscanning electron microscope (FE-SEM, Nova-Nono430, FEI) and transmission electron microscopy (TEM, Titan Themis 200, FEI) assisted by focused ion beam (FIB, 450S, FEI) milling. The phase compositions of the as-sprayed and Al-modi ed TBCs were identi ed by X-ray diffraction (10-90°, SmartLab, Rigaku, Japan). Also, the particle erosion resistances of the TBCs were evaluated by a scratch tester (HH-3000, CAS, China). This is a dead-loaded machine where a separate scratch is made for each applied load. The load was continuously increased from 0 to 80 N with a rate of 100 N/m and a scratch length of 4 mm.
The tester was tted with friction coe cient monitoring equipment which was used as an on-line failure monitor. The particle erosion performance of both types of TBCs (sample size: Φ25.4 × 5 mm) was carried out based on the GE standard E50TF121 (impingement angle 20 °, distance 101.6 mm, erosion media 240 grit) at room temperature. The thermal cycling properties of the as-sprayed and Al-modi ed PS-PVD 7YSZ TBCs were analyzed by water-quenching testing. The specimens (sample size: Φ25.4 × 5 mm) were heated at an evaluated temperature of 1100 ℃ in a furnace for 10 min followed by direct water-quenching at room temperature for 5 min.

Challenges of PS-PVD 7YSZ TBCs
A typical failure in TBCs can be seen in Fig. 1a (Turbo Fan Engine CFM56, Boeing 737). With fast development of aero-engine, the TIT will increase continually, which lead to requirements of high performance TBCs. And the progress of superalloy and TBCs had been summarized in Fig. 1b. For TBCs, APS, EB-PVD and PS-PVD are important preparation method. The APS TBCs have disadvantages of low strain tolerance and low bond strength. (Fig. 1c) [16][17] . The EB-PVD TBCs have disadvantages of high cost and high thermal conductivity (Fig. 1c) [18][19] . The PS-PVD has a different deposition mechanism as compared with the traditional processes, which can avoid these weaknesses to some extent. The core feature of PS-PVD process is the option of evaporating ceramic powders, which will obtain a feather-like columnar ceramic coating (Fig. 1c) [23][24] . However, the characteristic high porosity of PS-PVD TBCs is the great challenge before application.
Inherent weakness of high porosity for PS-PVD TBCs results from its high velocity and high concentration vapor deposition (Fig. 2a), which is different from EB-PVD 27 . The high velocity of the plasma ow and the high concentration of vapor particles in the PS-PVD process creates a non-line-of-sight (NLOS) deposition, which forms a feather-like columnar coating (Fig. 2b) 23 . However, a high concentration leads to a shadow effect created during the nucleation and growth of the vapor particles on the substrate, which is the biggest difference when compared with the EB-PVD process. Additionally, the size of the plasma ame is large, resulting in a large deposition area on the substrate. Figure 2b shows that the feather-like columnar 7YSZ coating has a different sized gap from the bottom to the top area, where the columns are made of nano-sized grains. A dense coating at the bottom is also a columnar structure, but the gap is very small.
Previous investigations indicate that the promising feather-like columnar microstructure depends on a temperature gradient and the velocity gradient of a plasma ame [27][28][29] . During the PS-PVD spray, the plasma temperature (Fig. 2c) and velocity (Fig. 2d) distribution were modeled through a computational uid dynamics (CFD) simulation. The gas ow rate at the inlet of the spray gun was 95 L/min, the boundary pressure of the out ow eld was 150 Pa, and a superalloy substrate was set at 1000 mm from the outlet of the spray gun. Figure 2c shows that when the plasma gases left the spray gun outlet, the temperature was still as high as 20,000 ℃. With an increase in the distance from the gun outlet, the plasma temperature dropped rapidly. The plasma temperature at the position of the substrate decreased to about 1400 ℃. As seen in Fig. 2d, when the plasma left the outlet of the plasma gun, it had a velocity of 7000 m/s. With the increase of the distance from the gun outlet, the plasma velocity dropped rapidly, to 300 m/s when it reached the substrate (1000 mm).

Solution for particle erosion resistance
It is known that the service environment of TBCs (Fig. 3a) includes high temperatures, particle erosion, and CMAS corrosion [1][2][3][4] . Due to the high vapor particle concentration, the feather-like columnar 7YSZ coating had a high porosity. This feature led to a low thermal conductivity and high-stress tolerance 30 .
However, this microstructure resulted in a negative effect on particle erosion resistance 31 . As a solution to the issue, an Al-modi ed approach was proposed (Fig. 3b) where an Al lm was deposited on the featherlike columnar 7YSZ coating. After a vacuum heat treatment, the Al lm in ltrated into the columnar coating and reacted with the 7YSZ coating, forming a dense α-Al 2 O 3 overlay. To optimize the process, Al lms with different thicknesses (5, 10, 20 µm) were deposited onto the TBCs surface. Figure 3c shows a cross-sectional microstructure evolution with the Al lm before and after vacuum heat treatment. The 5 µm Al-modi ed TBCs were characterized by TEM (Fig. 3d), which shows the cross-sectional elemental analysis including HAADF (High Angle Annular Dark Field) image and Al, O and Zr mappings. The experimental results demonstrated that the microstructure of the deposited and Al-modi ed TBCs are in accordance with the schematic diagram in Fig. 3b.
The Al-modi cation process aimed to improve particle erosion performance. Firstly, to characterize the erosion resistance, the as-sprayed and Al-modi ed TBCs were compared using scratch testing. Scratch micrographics of different TBCs with different-sized micro-cracks are shown in Figs. 4a-d, where the various positions P1, P2, and P3 corresponding to the initial position, middle position, and terminal position of the scratch path are presented. Using a scratch comparison method of the 3D surface pro le, the as-sprayed TBCs had a depth of 72 µm, which was deeper than the 5, 10, and 20 µm Al-modi ed TBCs (71, 57, and 56 µm). Moreover, the friction forces and friction coe cients of the as-sprayed TBCs had the highest value among the PS-PVD TBCs. Thus, the above phenomenon indicates that the scratch depth was getting shallower with the increasing Al thickness.
Therefore, it can be concluded that the various scratch depth depends on the hardness of the TBCs. The Al-modi cation process contributed to increasing hardness and it was also expected to improve the particle erosion resistance of PS-PVD TBCs.
The above assumptions shown in Fig. 5a have been demonstrated by particle erosion testing, which was based on the testing standard GEAE-E50TF121. The erosion test specimen holder was in accordance with the GE drawing 4013240-525 (Fig. 5b). The weight losses of the as-sprayed and 20 µm Al-modi ed TBCs were compared in Fig. 5c and the corresponding surface images after the particle erosion failure was seen in Fig. 5d. When the erodent exposure mass of grit reached 60 g, the color of the bond coating was seen (Fig. 5). However, the 20 µm Al-modi ed TBCs lost 90 g of grit before the appearance of the colored bond coating (Fig. 5). The above results showed that the Al-modi ed TBCs had a better erosion resistance than the as-sprayed PS-PVD 7YSZ TBCs.

Comparison of thermal cycle performance
Apart from the particle erosion resistance, the thermal cycle performance is an important parameter for the characterization of the TBCs representing frequent take-off and landing resistance 30,32 . The images of the as-sprayed and Al-modi ed PS-PVD with different water-quenching cycles were shown in Fig. 6. The 20 µm Al-modi ed TBCs showed an optimal thermal cycle performance (Fig. 6a) compared to the other APS and EB-PVD TBCs [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] . In this work, the as-sprayed and Al-modi ed TBCs were compared, shown in Figs. 6b-i. The rst spallation of the as-sprayed TBCs occurred in 162 cycles and after 198 cycles and the TBCs had been completely stripped from the surface of the substrate. With the 5 µm Almodi ed TBCs, the rst spallation occurred in 100 cycles, and after 115 cycles, the TBCs separated from the substrate. Correspondingly, the 10 µm Al-modi ed TBCs had rst and nal spallation occur in 131 and 162 cycles, respectively.
The above results show that the Al modi cation process had no positive effect on the thermal cycle performance for the PS-PVD 7YSZ TBCs. However, when the Al thickness increased to 20 µm, the Almodi ed TBCs had rst spallations occur in 198 cycles. As the thermal cycle increased to 350 cycles, the TBCs were still not completely stripped from the substrate, they only showed an increase in the spallation area. Thus, the 20 µm Al-modi cation was the best process for improving the thermal cycle performance of PS-PVD 7YSZ TBCs.
The results showed that the 20 µm Al-modi cation was the optimal process for improving the thermal cycle performance of the PS-PVD 7YSZ TBCs. The surface microstructures of the 20 µm Al-modi ed TBCs are shown in Fig. 7a. The gap between the cauli ower tops still can be seen and the magni ed images indicated that many nanowires had been formed on the surface. Some cauli ower tops were connected by nanowires. The cross-sectional microstructure was shown in Fig. 7b, where the positions P1, P2, and P3 represent different areas. After the 20 µm Al-modi cation, a dense overlay was formed on the top columnar coating. In the inner coating, the porous microstructure was still preserved. However, after 350 water-quenching cycles at 1100 ℃, many cauli ower tops had been stripped from the surface and the grain size including nanowires had increased (Fig. 7c). The cross-sectional microstructures at different positions (P1, P2, and P3) were seen in Fig. 7d, and were becoming denser than the crosssection in Fig. 7b. The evolutionary process of the phase composition in the PS-PVD 7YSZ coating from the as-sprayed to the Al-modi cation to the water quenching state was shown in Fig. 7e. It indicated that there were three phases, M-ZrO 2 , T'-ZrO 2 , and C-ZrO 2 , in the as-sprayed 7YSZ TBCs. After the 20 µm Almodi cation, a new phase, a-Al 2 O 3, was observed due to the in-situ synthesis between the Al and ZrO 2 in the heated vacuum treatment 29,49 . After 350 water quenching cycles, the peaks of the M-ZrO 2 , T'-ZrO 2 , and C-ZrO 2 phases became shorter and wider because the small grain size had grown (Fig. 7f). Based on the analysis of the T'-ZrO 2 patterns, the grain sizes of the as-sprayed TBCs, the 20 µm Al-modi ed TBCs, and the 20 µm Al-modi ed TBCs after 350 thermal cycles were 21, 35, and 247 nm, respectively. However, the a-Al 2 O 3 peaks increased, which means the degree order of the grain structure had increased.
Additionally, based on the analysis of the T'-ZrO 2 patterns, the lattice stress of the as-sprayed TBCs, the 20 µm Al-modi ed TBCs, and the 20 µm Al-modi ed TBCs after 350 thermal cycles were 0.4, 0.2 and 0.41 GPa, respectively (Fig. 7g). This means that the Al-modi cation process did not add lattice stress to the PS-PVD 7YSZ TBCs.

Conclusions
Considering the challenges of creating PS-PVD 7YSZ TBCs, this study sheds light on a new Almodi cation solution for particle erosion and thermal cycle. It was found that the Al-modi cation not only improved the particle erosion resistance but also increased the thermal cycle performance. Moreover, the aspiration is that the PS-PVD 7YSZ TBCs can be rapidly implemented to protect the high-temperature components in an aero-engine. Thus, the hope is that the current study may inspire further attempts to clear the application barriers. The Al-modi cation is a normal approach. And this technique can be extended to others structural coating such as environmental barrier coating, densi ed coating, etc.