*3.3. Mechanical Properties and Fracture Morphology of Titanium*/*Au*/*Al2O3 Joints*

In order to evaluate the effect of brazing temperature and holding time on the mechanical properties of brazed joints, the shear strength of the joints was tested at room temperature, as shown in Figure 8. As shown in Figure 8a, when the joints were brazed at different temperatures varying from 1105 to 1125 ◦C for 1 min, the shear strength of the joints increased first and then decreased. The maximum average shear strength of 20.3 MPa was obtained when the joints were brazed at 1105 ◦C for 1 min.

As shown in Figure 8b, the shear strength of the joints firstly increased and then decreased when the joints were brazed at 1115 ◦C for different holding times prolonged from 1 to 5 min. The maximum value of shear strength reached 39.2 MPa when the holding time was 3 min, which was about twice that of the joints brazed at 1115 ◦C for 1 min.

**Figure 8.** Effect of (**a**) brazing temperature and (**b**) holding time on shear strength of titanium/Au/ Al2O3 joints.

Fracture analysis was conducted using an optical microscope, SEM, and XRD to investigate the fracture location and fracture path of the titanium/Au/Al2O3 joints brazed at 1115 ◦C for 1 min. As shown in Figure 9a,b, Al2O3 ceramic was observed on the fracture surface of the titanium side. Figure 9d shows the crack was initiated at the Al2O3 ceramic and propagated into the brazing seam via the Au/Al2O3 interface during the shear test. The magnified SEM image of Figure 9d is shown in Figure 9e. When the crack propagated into the brazing seam, the joints fractured along the interface of TiAu2 and TiAu4 reaction layers (the interface of zone III and zone IV). The joints brazed at 1115 ◦C for 1 min fractured in the brittle mode. To further investigate the fracture location, reaction phases on the fracture surface of the titanium side were identified using XRD analysis, as shown in Figure 9c. It was evident that the fracture surface of the titanium side consisted of Au, Al2O3, and TiAu2, which corresponded to the fracture path analyses of Figure 9d,e.

**Figure 9.** Fracture analysis of titanium/Au/Al2O3 joints brazed at 1115 ◦C for 1 min after the shear test. (**a**) Fracture surface of Ti alloy side, (**b**) 3D image of (**a**), (**c**) XRD pattern of (**a**), (**d**) fracture path of the Ti/Au/Al2O3 joint, and (**e**) high-magnification image of (**d**).

Figure 10 displays the fracture analyses of the joints brazed at different parameters. It was observed that two types of fracture patterns existed after the shear test. In the first fracture pattern, significantly flat fracture surfaces were clearly observed, and the joints fractured along the Au/Al2O3

interface during the shear tests when brazed at 1105 ◦C for 1 min and 1115 ◦C for 5 min (Figures 9c and 10a). XRD analyses of the fracture surface on the titanium side displayed detectable phases, including Au and Al2O3, which in turn supported the above analysis of the first fracture pattern. Meanwhile, as shown in Figure 10b, a second type of fracture pattern was observed when the joints were brazed at 1115 ◦C for 3 min, identical with that brazed at 1115 ◦C for 1 min. In the second type, the fracture started at the Al2O3 ceramic and propagated along the interface of TiAu2 and TiAu4 reaction layers, which was confirmed by the existence of Au, Al2O3, and TiAu2 in the XRD result.

Variations in shear strength were significant count on the microstructure evolution of the joint. The increase of brazing temperature and holding time can promote the diffusion of active Ti from the titanium substrate and aggregation adjacent to Al2O3 ceramic. When the brazing temperature was lower (or the holding time was shorter), the diffusions of Ti and Au were limited, and the reaction layer of TiOx was extremely thin as the weakest position of the bonding. Therefore, the shear strength of the joints was quite low, and the joint fractured along the Au/Al2O3 interface. With the increase of brazing temperature (or the prolongation of holding time), the TiOx layer thickened, which could improve the metallurgical bonding between brazing alloy and ceramic. Therefore, the shear strength of the joints increased. Fractures occurred at the Au/Al2O3 interface and fragile Ti–Au reaction layers. When the brazing temperature further increased (or holding time was further prolongated), there was a drop in shear strength, which could be attributed to two factors: the over-thickened TiOx layer and the higher stresses resulting from the increased temperature or changed microstructure of Ti–Au IMCs layers in the brazing seam. Based on the above analyses, it can be concluded that a suitable thickness of the TiOx layer adjacent to ceramic had crucial influence on the shear strength of the joints.

**Figure 10.** Fractographs and XRD patterns of titanium/Au/Al2O3 joints brazed at different parameters after the shear test. (**a1**–**a3**) 1105 ◦C for 1 min, (**b1**–**b3**) 1115 ◦C for 3 min, and (**c1**–**c3**) 1115 ◦C for 5 min.

Figure 11 shows nanoindentation test results, which displayed the variation in hardness and elastic modulus of reaction phases for the joint brazed at 1115 ◦C for 3 min. As shown in Figure 11a, the highest hardness (9.9 GPa) and elastic modulus (165.0 GPa) across the joint was found in the Ti3Au

layer, while the Au phase showed the lowest hardness (2.7 GPa) and elastic modulus (115.0 GPa). In order to reveal elastic and plastic behaviors of reaction phases across the joint, typical loads versus depth curves are illustrate in Figure 11b. The deformation process of reaction phases could be divided into elastic-plastic loading and purely elastic unloading. It was apparent that the Au phase possessed the lowest elastic recovery of 14.1%, which recovered 69 nm of the total indentation depth (488 nm). These results showed that the deformation behavior of the Au phase was primarily plastic compared to other phases, which could be beneficial to release residual stress caused by CTE mismatch.

**Figure 11.** (**a**) Hardness and elastic modulus distribution across the joint interface; (**b**) typical load versus depth curves.
