3.1. 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-PVD27. The high velocity of the plasma flow 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 flame 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 flame27–29. During the PS-PVD spray, the plasma temperature (Fig. 2c) and velocity (Fig. 2d) distribution were modeled through a computational fluid dynamics (CFD) simulation. The gas flow rate at the inlet of the spray gun was 95 L/min, the boundary pressure of the outflow field 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).
3.2. Solution for particle erosion resistance
It is known that the service environment of TBCs (Fig. 3a) includes high temperatures, particle erosion, and CMAS corrosion1–4. 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 tolerance30. However, this microstructure resulted in a negative effect on particle erosion resistance31. As a solution to the issue, an Al-modified approach was proposed (Fig. 3b) where an Al film was deposited on the feather-like columnar 7YSZ coating. After a vacuum heat treatment, the Al film infiltrated into the columnar coating and reacted with the 7YSZ coating, forming a dense α-Al2O3 overlay. To optimize the process, Al films with different thicknesses (5, 10, 20 µm) were deposited onto the TBCs surface. Figure 3c shows a cross-sectional microstructure evolution with the Al film before and after vacuum heat treatment. The 5 µm Al-modified 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-modified TBCs are in accordance with the schematic diagram in Fig. 3b.
The Al-modification process aimed to improve particle erosion performance. Firstly, to characterize the erosion resistance, the as-sprayed and Al-modified 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 profile, the as-sprayed TBCs had a depth of 72 µm, which was deeper than the 5, 10, and 20 µm Al-modified TBCs (71, 57, and 56 µm). Moreover, the friction forces and friction coefficients 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-modification 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-modified 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-modified TBCs lost 90 g of grit before the appearance of the colored bond coating (Fig. 5). The above results showed that the Al-modified TBCs had a better erosion resistance than the as-sprayed PS-PVD 7YSZ TBCs.
3.3. 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 resistance30, 32. The images of the as-sprayed and Al-modified PS-PVD with different water-quenching cycles were shown in Fig. 6. The 20 µm Al-modified TBCs showed an optimal thermal cycle performance (Fig. 6a) compared to the other APS and EB-PVD TBCs33–48. In this work, the as-sprayed and Al-modified TBCs were compared, shown in Figs. 6b-i. The first 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 Al-modified TBCs, the first spallation occurred in 100 cycles, and after 115 cycles, the TBCs separated from the substrate. Correspondingly, the 10 µm Al-modified TBCs had first and final spallation occur in 131 and 162 cycles, respectively.
The above results show that the Al modification 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 Al-modified TBCs had first 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-modification was the best process for improving the thermal cycle performance of PS-PVD 7YSZ TBCs.
The results showed that the 20 µm Al-modification was the optimal process for improving the thermal cycle performance of the PS-PVD 7YSZ TBCs. The surface microstructures of the 20 µm Al-modified TBCs are shown in Fig. 7a. The gap between the cauliflower tops still can be seen and the magnified images indicated that many nanowires had been formed on the surface. Some cauliflower 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-modification, 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 cauliflower 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 cross-section in Fig. 7b. The evolutionary process of the phase composition in the PS-PVD 7YSZ coating from the as-sprayed to the Al-modification to the water quenching state was shown in Fig. 7e. It indicated that there were three phases, M- ZrO2, T’-ZrO2, and C-ZrO2, in the as-sprayed 7YSZ TBCs. After the 20 µm Al-modification, a new phase, a-Al2O3, was observed due to the in-situ synthesis between the Al and ZrO2 in the heated vacuum treatment29,49. After 350 water quenching cycles, the peaks of the M-ZrO2, T’-ZrO2, and C-ZrO2 phases became shorter and wider because the small grain size had grown (Fig. 7f). Based on the analysis of the T’-ZrO2 patterns, the grain sizes of the as-sprayed TBCs, the 20 µm Al-modified TBCs, and the 20 µm Al-modified TBCs after 350 thermal cycles were 21, 35, and 247 nm, respectively. However, the a-Al2O3 peaks increased, which means the degree order of the grain structure had increased. Additionally, based on the analysis of the T’-ZrO2 patterns, the lattice stress of the as-sprayed TBCs, the 20 µm Al-modified TBCs, and the 20 µm Al-modified TBCs after 350 thermal cycles were 0.4, 0.2 and 0.41 GPa, respectively (Fig. 7g). This means that the Al-modification process did not add lattice stress to the PS-PVD 7YSZ TBCs.