3.1 Interfacial microstructure of YSZ/AISI 441 joint
Figure 1a displays the overall SEM image of the YSZ/AISI 441 joint obtained with the pure Ag interlayer at 920 ℃/30 min in air. It is apparent that a tight joint is formed under this condition. Both the YSZ/Ag interface and the AISI 441/Ag interface seem compact and defect-free. The corresponding enlarged SEM images (Fig. 2a1 and 2a2) are obtained for further details about these two interfaces.
A thin and dense oxide layer (~ 2 µm) is formed between Ag and AISI 441 (Fig. 2a2), which has been proved to be composed of (Fe, Mn, Cr)3O4 and Cr2O3 by many studies [25, 40–42]. It is worth noting that the oxide layer formed at the stainless steel interface by the traditional RAB method is typically thicker than 20 µm [25, 32, 33], which is 10 times wider than the above oxide layer. The dramatic reduction of the oxidation layer thickness is due to the absence of CuO and the lower joining temperature (920°C, while the traditional RAB method generally requires temperatures higher than 970°C). Besides, compared with ≥ 20µm thick oxide layer, this thin oxidation layer may help maintain the long-term stability of the joint.
It can be seen from the enlarged view in Fig. 2a1 that the YSZ/Ag interface is still dense without micro-voids or micro-cracks. It is well documented that a tight atomic bonding is essential to fabricate a stable joint [43], while it is hard to judge whether there is atomic bonding between YSZ and Ag only by SEM. Therefore TEM is adopted to further analyze the YSZ/Ag interface. Figure 2b exhibits the bright-field image at the YSZ/Ag interface, and the corresponding elemental distribution (Zr and Ag) is shown in Fig. 2c.
The element distribution indicates that Ag and YSZ are closely joined without apparent gaps. The enlarged view in Fig. 2c1 reveals that a small number of Ag atoms diffuse into the YSZ, forming a diffusion layer with a width of ~ 3 nm. As shown in Fig. 2d, the elemental distributions of Ag, Zr, and O along the yellow line in Fig. 2b also indicate that a diffusion layer (~ 3 nm) exists between Ag and YSZ. This diffusion phenomenon might play a crucial role in forming a hermetic joint. High resolution transmission electron microscope (HRTEM) is used to determine the bonding type between YSZ and Ag. The HRTEM image in Fig. 2e depicts that the crystal lattices of Ag and YSZ are clearly visible. The enlarged view (Fig. 2e1) of region 4 and the corresponding Fast Fourier Transformation (FFT) (Fig. 2e2 and 2e3) note that the [220] zone axis of the ZrO2 is paralleled to the [0—11] axis of the Ag. In addition, a disordered lattice region (~ 3 nm) is observed between Ag and YSZ, which might attribute to the diffusion of Ag into the YSZ. The above TEM analysis demonstrates that atomic bonding is achieved between YSZ and Ag, which is the premise of obtaining an airtight joint with superior mechanical properties.
3.2 Effect of bonding temperature on YSZ/AISI 441 joint
Since the joining temperature is an essential factor affecting the microstructure and properties of the joint, the microstructures of the joints obtained at various joining temperatures (860–970 ℃) for 30 min are observed in this section as displayed in Fig. 3.
It can be seen from Fig. 3a-3d that tight and well-formed joints can be obtained at the temperature of 860–970 ℃. Fine and dense bonding is formed at both the YSZ/Ag interface and AISI 441/Ag interface. At these temperatures, no defects or voids are observed at the YSZ/Ag interface. This may be because Ag is soft enough to fill the gaps at this interface under the pressure of ~ 2 MPa. In addition, since there is no reaction product between Ag and YSZ, and both are stable at 860–950 ℃, no noticeable change of the interfacial microstructure at various temperatures can be observed via the SEM method (the enlarged SEM image of YSZ/Ag interface is not shown).
For the AISI 441/Ag interface (see Fig. 3a1-3d1), the oxidation behavior is intensified with elevating temperatures. At 860°C (Fig. a1), a discontinuous oxide is formed at the AISI 441 interface. The oxide layer is thickened with the increasing temperature, and a continuous and dense oxide layer (2–3 µm in thickness) is observed at 950°C (Fig. d1).
In addition, when the temperature reaches 970 ℃ (higher than the melting point of Ag), the joint quality decreases significantly,which can be seen in the supplementary material (Fig. S2). Most of the Ag flows away from the gap between YSZ and AISI 441, and only a slight residual Ag exists between YSZ and AISI 441. This phenomenon can be attributed to the high contact angle of pure Ag on the YSZ surface and no reaction between YSZ and Ag, resulting in the loss of liquid Ag during joining [34, 44].
3.3 Shear strength of joints at various bonding temperatures
Shear strength is an essential indicator for measuring joint quality, so shear tests are conducted for the joints obtained at different temperatures in this section. Subsequently, the fracture surfaces after shear tests are analyzed by SEM to understand the failure mechanism further.
Figure 4a notes that the shear strength of the joint first increases and then decreases with the increasing temperature (from 860 ℃ to 970 ℃), reaching the maximum (86.1 MPa) at 920 ℃. Figure 4b illustrates the AISI 441-side fracture morphology of the joints obtained at 860°C. The fracture mainly occurs between YSZ and Ag, and no noticeable dimple on the Ag surface is observed. This phenomenon indicates a poor bonding strength between Ag and YSZ at 860°C, owing to insufficient diffusion between Ag and YSZ at lower temperatures. Hence the joints exhibit low shear strength at low temperatures (20.1 MPa at 860°C). When the joining temperature increases to 890°C, the shear strength is significantly increased to 64.6 MPa. As shown in Fig. 4c, part of the fracture begins to occur between Ag and AISI 441. On the one hand, this is due to the increase of the YSZ/Ag interfacial strength as the temperature rises. On the other hand, the thickening of the oxide layer on the AISI 441 surface may reduce the bonding strength at the AISI 441/Ag interface. The maximum shear strength is reached at 920°C. The fracture occurs at both the YSZ/Ag interface and the AISI 441/Ag interface (see Fig. 4d). Besides, many dimples are observed at the Ag layer, indicating a large plastic deformation and high strength at YSZ/Ag interface. A further increase in temperature will reduce the joint strength (55.8 MPa at 950°C), mainly due to the increased oxidation of the AISI 441 surface, as shown in the enlarged view of area 8. In addition, when the joining temperature rises to 970 ℃ (exceeding the melting point of Ag), the shear strength drops sharply to 2.5 MPa. The reason has been explained in detail in Sect. 3.2.
According to the research of Cao et al. [25], the shear strength of the YSZ/AISI 441 joint obtained using traditional Ag-CuO braze at 960 ℃/30 min is ~ 33 MPa. Surprisingly, the joint strength in this study can reach 86.1 MPa, which is 161% higher than that of traditional RAB joints. The significant enhancement in shear strength could be explained by the lower joining temperature (920°C) that reduces the oxidation at the AISI 441 surface. Meanwhile, the absence of CuO further alleviates the oxidation of the metal surface.
3.4 Stability of joints in reducing and oxidizing atmospheres
To assess the long-term stability of the joints in SOFC/SOEC relevant atmospheres, the YSZ/441 joints obtained with pure Ag interlayer at 920 ℃/30 min were aged for 300 h at 800°C in 50% (H2 + Ar)-50% H2O and air atmospheres, respectively. And the aging results are analyzed as follows.
Figure 5 presents the cross-sectional SEM images of the joints after exposure in 50% (H2 + Ar)-50% H2O at 800 ℃ for 300 h. Obviously, aging for 300 h in the reducing atmosphere does not cause any significant change in the overall morphology of the joint. No defects (such as delamination, cracks, or voids) due to reduction are observed. The enlarged views of the two interfaces are subsequently observed for further details. It can be seen from Fig. 5a1-c1 that the YSZ/Ag interface remains compact and well bonded, and the reducing atmosphere has no apparent effect on this interface. The good reduction resistance of the interface may be attributed to the Ag-ZrO2 atomic bonding. The AISI 441/Ag interface also shows excellent stability in reducing atmosphere at 800 ℃, as shown in Fig. 5a2-c2. The oxide layer at the AISI 441 interface remains compact, and its thickness does not seem to change after exposure to reducing atmosphere for 300 h. This may be because the oxide layer is thermodynamically stable under this condition and is difficult to be decomposed by reduction. Notably, the voids caused by the CuO reduction, which is inevitable in the traditional RAB method [4, 31, 34, 37], are completely eliminated.
Figure 6 displays the joint structure after aging in air at 800 ℃ for 300 h. It can be seen from the low-magnification SEM images in Fig. 6a-6c that the joints maintain tight bonding in a wide range without delamination after oxidation for 300 h. Besides, the oxide layer on the AISI 441 surface is thickened with the increasing oxidation time. Also, no apparent defects are observed in cross-sectional SEM images from Fig. 6a1-6c1. The YSZ/Ag interface remains tight and defect-free (the enlarged view at this interface is not shown). The enlarged view at AISI 441 interface (Fig. 6a2-6c2) depicts that the oxide layer thickness only reaches ~ 15 µm after oxidation for 300 h, which is even thinner than that of the joints obtained by the traditional Ag-CuO braze without oxidation test (typically ≥ 20µm) [25, 32, 33]. Furthermore, the oxide layer in this study is much denser, and only a few micro-voids appeared after oxidation for 300 h. Both the thickness and compactness of the oxide layer indicate more superior oxidation resistance and aging performance of the joints fabricated by a pure Ag interlayer.
In brief, the above aging test demonstrates that YSZ/AISI 441 joints obtained with pure Ag interlayer present long-term (≥ 300 h) stability in both wet reducing atmosphere and oxidizing atmosphere. Due to the absence of CuO, the joints are more stable in a reducing atmosphere on the one hand, and the oxide layer formed on the stainless steel surface is much thinner and denser in the oxidizing atmosphere on the other hand. Compared with the traditional RAB method, the method in this paper can greatly improve the service life of the joint, and it is more suitable for fabricating the SOFC/SOEC stacks.