Al2O3-modified PS-PVD 7YSZ thermal barrier coatings for advanced gas-turbine engines

Plasma spray-physical vapor deposition (PS-PVD), called the third-generation method for thermal barrier coatings (TBCs) fabrication, has great potential for their using in gas-turbine engines. Compared to atmospheric plasma spray (APS), called the first-generation TBCs, and electron beam-PVD (EB-PVD), called the second generation, PS-PVD has many interesting features, including non-line sight deposition, high deposition rate, and microstructural flexibility, among others. Such advantages make them a promising approach to prepare thermal barrier coatings for advanced gas-turbine engines. Using PS-PVD, feather-like columnar TBCs with good strain tolerance and low thermal conductivity can be fabricated. However, prior to their application in gas-turbine engines, some disadvantages, such as CMAS (CaO–MgO–Al2O3–SiO2, etc.) corrosion and oxidation resistance, need to be addressed. In this work, a method to develop Al2O3-modified PS-PVD 7YSZ TBCs was proposed. The experimental results demonstrate that the Al2O3-modification process is an effective approach to address the aforementioned weaknesses of traditional PS-PVD 7YSZ TBCs.


INTRODUCTION
The development of aero-engine has benefited from the progress of thermal barrier coatings (TBCs) [1][2][3][4] . Most research aimed at TBCs was started in the late 1940s and continued through the 1950s to the date [5][6][7] . TBCs were first used in the injectors of X-15 rocket engines and in the combustion chamber of gas-turbine engines in the 1960s 7,8 . Up to now, TBCs for hot components used in gasturbine engines are still a hot research topic, even though the first paper testing turbine blades with TBCs was written in 1976 (refs. 9,10 ). Due to the continuous modification and development of the airplane, the thrust-to-weight ratio continues to increase. Thus, the requirements for TBCs is constantly changing. In a TBCs system, the hot-component materials are constantly being developed, from casting alloys to directionally solidified alloys and single crystal alloys. Additionally, the methods for the preparation of TBCs are continuously updated, from atmospheric plasma spray (APS) to electron beam-physical vapor deposition (EB-PVD) and plasma spray-physical vapor deposition (PS-PVD). Meanwhile, the TBC structural designs are iterated repeatedly. The above-mentioned TBC system progress has been summarized in Fig. 1. Now, the TBCs with different materials and various structure designs have been widely used for turbine blades protection in gas-turbine engines. Laminar-structured rare-earth-doped ZrO 2 (Y 2 O 3 /Ga 2 O 3 /La 2 O 3 /CeO 2 /SmO 2 /Dy 2 O 3 -ZrO 2 ) TBCs prepared by APS have been investigated since 1976 (refs. 3,9,10 ). The thermal conductivity of APS TBCs is in the range of 1.0-2.25 W/mk (25-1000°C) [11][12][13] . A typical TBCs structure is shown in Fig. 2a. In the 1980s, the APS TBCs were unable to meet the necessary service conditions of moving turbine blades in some advanced gas-turbine engines due to high rotation velocities (>20,000 r/min) and frequent takeoffs and landings 14,15 . Thus, EB-PVD TBCs were proposed to compensate the loss of thermal conductivity with good thermal cycle performance, compared to the APS TBCs [16][17][18] . Columnar-structured TBCs (Fig. 2b) with better strain tolerance can be prepared by EB-PVD due to its vapor deposition, in contrast to laminar-structured APS TBCs deposited with liquid/solid mixed states 19 . However, the disadvantage for EB-PVD TBCs is their high thermal conductivity (in the range of 1.6-2.3 W/mk), which needs to be lowered for their practical application 20 . Meanwhile, PS-PVD was presented in the 2000s as a novel approach to achieve such goal 21,22 , by developing feather-like columnar-structured TBCs (Fig. 2c), resulting from multiphase deposition (mainly vapor, some liquid and solid). The PS-PVD was developed based on lowpressure plasma spray by reducing the working pressure (~100 Pa) and improving the plasma power (~180 kW) 23 . With a length and diameter of the plasma flame of up to 2500 and 400 mm, respectively, at low pressures (~150 Pa) 24 , the TBCs materials can be evaporated 25 . Due to the higher concentration of vapor particles compared to EB-PVD (low concentration), feather-like structured 7YSZ coating can be generated by PS-PVD, as shown in Fig. 3a-c.

RESULTS AND DISCUSSION
Performances of 7YSZ TBCs Feather-like TBCs prepared by PS-PVD have attracted much attention worldwide (USA, Germany, Poland, China, etc.) due to its higher thermal cycle performance and lower thermal conductivity 26,27 . Moreover, this technique has the advantages of presenting non-line sight coverage and high deposition rate, which are suitable for the TBCs preparation to protect turbine blades (single, double, multi-blades). However, prior to their application in gas-turbine engines, some challenges in the fabrication of PS-PVD TBCs need to be addressed, such as CMA corrosion and isothermal oxidation [28][29][30][31][32] . During the PS-PVD, the 7YSZ coating is generally deposited not only in the vapor state but also in both liquid and solid states, to a less extent 23,30 . Even though the feedstock powders are injected in the inner plasma gun, part of powders fly along the outer edge of the plasma flame and cannot be effectively evaporated 33,34 . In the 7YSZ coating, the columnar structure results from the vapor deposition and its feather-like structure arises from a shadowing effect 34,35 . Meanwhile, non-evaporated particles, including slightly molten particles, will exist as spherical or flat particles in the columnar coating and influence the vapor atom nucleation, leading to a branch-like structure 30 . Previous experimental results demonstrated that those non-evaporated particles are detrimental to the performance of the TBCs, because the interfaces between spherical grains, flat grains, and columnar grains present weak bond. Thus, such interface is the source of failure for TBCs 26,30 .
The as-sprayed PS-PVD 7YSZ TBCs (DZ125/NiCrAlY/7YSZ) were characterized under typical testing conditions, as shown in Fig. 4. The cross-section and surface microstructure of as-sprayed 7YSZ TBCs are shown in Fig. 4a-c. After a water-quenching thermal cycle, coating spallation occurred at the interface between the flat grain and columnar grain ( Fig. 4d-f). Particle erosion testing (GE standard: E50TF121) was carried out on the TBCs, and it was observed that the same phenomenon of 7YSZ coating spallation occurred at the weak interface between the splat grain and columnar grain (Fig. 4g-i). When particles impact the coating surface, stress and strain will form in the 7YSZ coating simultaneously, accompanied by crack generation. The 7YSZcoated samples were isothermally oxidized in atmospheric furnace at 1000°C. This leads to the formation of a layer of thermally grown oxides (TGOs) on the bond coating. The formation of the TGO layer leads to the generation of strain in the inner 7YSZ coating. Thus, cracks can be generated due to weak bonding between spherical and columnar grains in 7YSZ coating ( Fig. 4j-l).

Performances of Al 2 O 3 -modified 7YSZ TBCs
Based on the deposition mechanism of PS-PVD, it is hard to avoid non-evaporated particles located in the 7YSZ columnar coating. To prevent this, a new approach was proposed, that is, an additional Al film with a thickness of 2-5 μm was deposited onto the PS-PVD 7YSZ TBCs (Fig. 5a, b). Afterwards, the Al film-covered TBCs were annealed in vacuum at 608°C for 1 h, 700°C for 1 h, and finally at 980°C for 2 h (Fig. 5c). During the annealing, the film of Al, due to its low melting point (~667°C), was melt. The molten Al was then penetrated along the vertical gap into the TBCs, driven under capillary force. This capillary force mainly depends on the geometric configuration of pore including radius and wall inclination in 7YSZ coating, as well as wetting property of molten Al on porous coating. After the annealing, the TBCs sample ( Fig.  5d-g) was characterized by X-ray diffraction (XRD), which shows the formation of a-Al 2 O 3 phase (Fig. 5h). For further investigation of the microstructure and morphology of the oxide, the sample in the vicinity of the surface was cross-sectioned by focused ion beam (FIB) (Fig. 5i) for TEM investigation. It clearly indicates the formation of a 600-nm-thick transparent film of an oxide (Fig. 5j). This is assumed to be a-Al 2 O 3 phase, as confirmed by the corresponding high-resolution image (Fig. 5k). Above in situ reaction is feasible based on thermal kinetic analysis. Even though a dense Al 2 O 3 layer was formed on the surface of PS-PVD 7YSZ coating (compare Fig. 5e to Fig. 4b), it will likely not influence its stress tolerance since the gap between columns was not be filled (as seen in Fig. 5f, g).   . d-f Microstructure after water-quenching testing (d cross-section, e surface, f magnified image). g-i Microstructure after particle erosion (g cross-section, h surface, i magnified image). j-l Microstructure after isothermal oxidation (j cross-section, k crack generation in the inner coating, l new surface after spallation).
The Al 2 O 3 -modified 7YSZ TBCs were evaluated as follows. Using the drawing method, the bond strength of Al 2 O 3 -modified TBCs was calculated to be above 50 MPa, based on 10 samples. Additionally, the Al 2 O 3 -modified TBCs were subjected to waterquenching (1100°C for 10 min, followed by atmospheric watercooling for 5 min), using five samples as shown in Fig. 6a (20, 40, 60, and 80 cycles). After 80 cycles, no apparent spallation occurred in the TBC samples, showing good thermal shock resistance and stable performance. Meanwhile, an air-cooling thermal cycle of the Al 2 O 3 -modified TBC samples was performed (1050°C for 5 min, air-cooling for 5 min) with five samples (4000, 9000, 13,000, and 16,000 cylces). After 16,000 cycles, no apparent spallation was observed, as seen in Fig. 6b. During the thermal cycle, TGO grown stress will be generated at the rough interface of 7YSZ coating and bond coating. PS-PVD 7YSZ coating has high porosity and high oxygen diffusion rate. With Al 2 O 3 -modification process, an Al 2 O 3 overlay was in situ synthesized on TBCs surface. This process will lead to porosity decrement. Moreover, the Al 2 O 3 material has low oxygen diffusion rate, which can act as a barrier for oxygen diffusion. Thus, the TGO grown rate can be hindered. Finally, the Al 2 O 3 -modified PS-PVD 7YSZ TBCs show a good thermal shock performance.
Besides good thermal cycle performance, fracture toughness is an important property for TBCs. Three samples (with dimensions 100 × 25 × 2 mm) were characterized with a bend testing at 90°. Analysis of the two images, including front and lateral views, revealed only micro-cracks and no spallation (Fig. 6c). Cylindrical samples of Al 2 O 3 -modified TBCs were tested with isothermal oxidation at 1100°C for 25 and 100 h. The three samples tested showed that the average mass increase rate was 0.057588 g/(m 2 h), based on 100 h, as seen in Fig. 6d. Marine corrosion resistance is another important factor for TBCs, because gas-turbine engines are often in service in marine environments. Five samples were tested with a combustion flame injected with NaCl solution (aviation kerosene and oxygen, at 900°C for 1200 h), seen in Fig. 6e. The experimental results show the mass variation rate is 0.0014, 0.0029, and −0.001, −0.0016, −0.0013 g/(m 2 h), correspondingly.
Traditional PS-PVD 7YSZ TBCs generally present low thermal conductivity due to their high porosity (>15%). In the 7YSZ    Those open gaps and pores will provide paths for molten CMAS infiltration at high temperature. Thus, Al 2 O 3 -modification process were proposed to improve the corrosion resistance of CMAS for PS-PVD 7YSZ coating. Because using above approach, a dense Al 2 O 3 layer was formed on surface of porous 7YSZ coating, which will act as a barrier to hinder the molten CMAS infiltration. A comparison of the CMAS corrosion between the as-sprayed TBCs and Al 2 O 3 -modified TBCs is presented in Fig. 7. The CMAS powders were sprinkled on the surface of TBCs (0.2 g/mm 2 ) for isothermal heating at 1200°C for 24 h (using ZrO 2 disk as a substrate). The experimental results show that a buckling phenomenon was observed in the as-sprayed 7YSZ TBCs after CMAS corrosion (Fig. 7a, b). Because the CMAS react with the 7YSZ coating, leading to spallation (Fig. 7c). However, the Al 2 O 3modified 7YSZ TBCs did not present the wavy crack between the 7YSZ coating and substrate (Fig. 7d, e). It can be hypothesized that a dense Al 2 O 3 overlay was formed on the surface of the Al 2 O 3modified TBCs, which can act as an infiltration barrier, hindering the entrance of the molten CAMS into the porous coating (Fig. 7f).
In China, the APS and EB-PVD 7YSZ TBCs have been widely utilized for the protection of hot components in gas-turbine engines. And the PS-PVD TBCs may soon be used for the protection of turbine blades. A comparison of the performance of APS, EB-PVD, and PS-PVD 7YSZ TBCs is summarized in Table 1, based on 8 years of research in the authors' research group [36][37][38] . Due to their laminar structure, the air-cooling thermal cycle of APS TBCs is lower than 3000. Additionally, a columnar structure can release stress during a thermal cycle. Thus, the thermal cycle of EB-PVD and PS-PVD TBCs is more than 7000 and 8000, respectively. During the thermal cycle, stress will be generated at the interface of 7YSZ coating and bond coating, resulting from thermal mismatch between 7YSZ coating and bond coating, TGO grown stress, etc. [39][40][41] . The rough interface including peak and valley will lead to stress concentration. However, the columnar structure has the ability to release the stress due to different-sized vertical gap among columns, avoiding crack generation. In the Al 2 O 3 -modified PS-PVD TBCs, an Al 2 O 3 overlay was in situ synthesized on the surface, which will decrease the growing rate of TGO 36 . This means that the interfacial stress of the ceramic/bond coating will increase at a low rate, presenting the greatest thermal cycle performance (>16,000). Similarly, the Al 2 O 3 -modified TBCs has the best water-quenching thermal cycle properties (>160), compared to the APS (>60), EB-PVD (>80), and traditional PS-PVD TBCs (>120). In its role as a protective coating for gas-turbine blades, thermal conductivity is an important property. Its laminar structure can hinder heat transmission effectively, thus APS 7YSZ TBCs (>1.0 W/mK) presented the best performance compared to the other TBCs. In addition, other characteristics including bond strength, surface roughness, coating uniformity, and deposition rate were analyzed and summarized in Table 1. Through the above comparison, it can be seen that the Al 2 O 3 -modified PS-PVD 7YSZ TBCs have the best overall performance.
Although the Al 2 O 3 -modified PS-PVD TBCs have good performance in typical simulation testing conditions. However, before practical application of TBCs in gas-turbine engines, the performance of Al 2 O 3 -modified TBCs deposited on turbine blades should be characterized. Thus, a turbine blade with Al 2 O 3modified 7YSZ TBCs was analyzed using a water-quenching testing (1100°C for 10 min, followed by atmospheric water cooling for 5 min). Water quenching is a simple and effective testing to evaluate the performance of TBCs. Figure 8a shows that the turbine blade used for testing consists of an irregular sample, with a high curvature surface and corners, which differs greatly from the regularly shaped samples used previously. The TBCs were deposited on the surface with different states (porosity, density, bond strength, etc.). The turbine blade with Al 2 O 3 -modified TBCs after 20, 60, and 80 cycles are shown in Fig. 8b

Preparation of TBCs
The equipment used for these experiments consists of a PS-PVD multicoat system (Oerlikon Metco

Characterization of TBCs
The thermal cycling properties of PS-PVD 7YSZ TBCs were analyzed. For water-quenching testing, the specimens (sample size: Φ 30 × 5 mm) were heated at an evaluated temperature of 1050°C in a furnace for 10 min followed by direct water-quenching at room temperature for 5 min. For aircooling testing (sample size: Φ 30 × 5 mm), the same conditions were used as before, with a temperature of 1100°C. The isothermal oxidation of the TBCs samples (sample size: Φ 15 × 30 mm) was carried out at 1100°C for 100 h, and the performance was evaluated as a mass increase rate. The simulated marine corrosion of TBCs (sample size: Φ 7 × 60 mm) was conducted by combustion using aviation kerosene and oxygen (900°C for 1200 h). The CMAS (5.6 wt% CaO, 2.7 wt% MgO, 10.9 wt% Al 2 O 3 , 30.2 wt% SiO 2 , etc.) corrosion of TBCs (sample size: Φ 25.4 × 5 mm) was also analyzed. The experiment was designed as follows: CMAS powders mixed with alcohol were prepared as a suspension at a concentration of 5 mg/ml. The TBC samples were dipped into the suspension. After that, the samples were placed in a furnace kept at 1200°C for 24 h, followed by air cooling. The bond strength of the TBCs (sample size: Φ 25.4 × 5 mm) was characterized using the drawing method. The particle erosion performance (sample size: Φ 25.4 × 5 mm) was tested according to GE standard (E50TF121). The surface roughness of the TBCs was tested optical profiler (Contour-GT, Bruker). The phase compositions of as-sprayed and Al 2 O 3modified 7YSZ TBCs were identified by XRD (10-90°; SmartLab, Rigaku, Japan). The microstructure of Al 2 O 3 -modified 7YSZ TBCs was characterized by field emission-scanning electron microscope (Nova-Nono430, FEI) and transmission electron microscopy (TEM; Titan Themis 200, FEI) assisted with FIB (450 S, FEI) milling.

DATA AVAILABILITY
Data that support the findings presented in this manuscript can be provided upon reasonable request by contacting the corresponding author.