3.1 Surface characterizations of C/C after Cr metallization process
Figure 2 shows the SEM micrograph of C/C surface with different metallization processes. It can be seen that, as a result of the heat treatments, a metallic layer is obviously obtained on C/C surface which indicates that carbides layer might be formed. During the heat treatment, solid-state reaction between the C/C and Cr would take place. XRD analyses shows that the carbides layer consisted of Cr7C3 and Cr3C2 when the C/C was metallized at 1100℃ for 2h, as shown in Fig. 3. when the C/C was metallized at 1300℃ for 1h, Cr23C6 was present and the intensity of the diffractive peaks of Cr23C6 is obviously higher than that of the Cr7C3 and Cr3C2, indicating that Cr23C6 was formed at external surface of the metallic layer. Besides, the morphology of Cr23C6 was quite similar to that of Cr-treated SiC surface [27], where the product Cr23C6 was also located at outer layer in the coating. It is noted that all the C/C samples had cracks normal to the surface. The cracks must have appeared during the cooling process due to the thermal residual stresses caused by the CTE mismatch between carbides and the C/C substrate. The CTE (α) of Cr23C6, Cr7C3, Cr3C2 is 10.3×10− 6K− 1, 9.4×10− 6K− 1, 10.1×10− 6K− 1[27], respectively, which is much higher than that of the C/C (0–2×10− 6K− 1 ). Such a mismatch in the CTE inevitably results in high thermal residual stresses at the interface, leading to cracks in the carbides layer. However, it should be mentioned that the metallic layer did not spall off during the fast-cooling process. As seen from Fig. 2c, the metallic layer was tightly bonded to C/C substrate.
3.2 wetting behavior
A simple wetting experiment was conducted by direct putting an Ag15Pd alloy ball (~ 5mg) on the modified C/C with a flat surface. As seen in Fig. 4, the braze exhibits good wettability and the contact angle is approximately 25°. The penetration of the brazing material into the C/C matrix was also clearly observed. Such a good wettability was also obtained in the Cr modified C/C-Cu system [24]. Pure Ag cannot wet graphite; the contact angle is as high as 120°. According to Singh’s study, C/C also cannot be wetted by Pd alloys, such as PdCo and PdNi [23]. In our case, the excellent wetting behavior of AgPd alloy was contributed to the pre-coated carbides layer on the C/C surface. Generally, Cr containing brazes can always well wet the graphite due to the reaction between Cr and C to form carbides during the brazing process. However, the wetting behavior of braze in our case is non-reactive wetting, which is distinctive to that of Cr containing brazes on graphite [14], naming typical reactive wetting.
3.3 Microstructures of the brazed joint
The cross-sectional backscattered electron (BSE) images of the C/C- Al2O3 joints are shown in Fig. 5. The EDS compositional analysis results at the marked point are listed in Table 3. As shown in Fig. 2a, all the laser-machined holes in C/C composites have been completely filled with braze, as a result of the excellent wettability. It is clearly to see that a thin and continuous reaction layer was formed at C/C/braze interface. The joint displayed well-bonded interface and was free of defects such as cracks and voids. Besides, penetration of the braze into the C/C matrix through the intrinsic open pores and cracks can be clearly observed. It is noted that some residual Cr powders (dark phase) still remained in the laser-machined holes. In addition, PdCr compound (gray phase) was also observed. The present of residual Cr and PdCr may due to the excess Cr powders brushed into the laser-machined holes. As we known, pure Cr has abnormally high vapor pressure (e.g., at 1200℃ the vapor pressure of Cr is approximately 1.5Pa [27]). During the heat treatment, some Cr volatized and reacted with C/C substate to form carbides while the residual Cr remained as solid state. Thus, the PdCr compound might form due to the reaction between Pd and the residual Cr during the brazing process.
On the Al2O3 block side, a reaction layer (marked B) with composition of 49.3Ni-27.3Pd-17.7Mo-6.5Cr (at. %) was formed at braze/Al2O3 interface. The enrichment of Cr in layer B must come from the dissolving and diffusing of residual Cr on C/C side. The microstructure of the joint region is composed of homogeneous solution with composition of Ag-12.6Pd (at. %).
(b) C/C/braze interface
Figure 6 shows the BSE image of the Al2O3-superalloy joint which also reveals intimately contacted interface and is free of cracks and voids. A reaction layer (marked C) was formed at Al2O3/braze interface and the joint appears to be microstructurally sound. The EDS examination shows the reaction layer C is enriched with Ni, Pd, Mo and Cr. The segregation of Cr in layer C indicates that Cr in the superalloy diffused across the molten braze had dissolved in the NiPd solution. On the superalloy side, an obvious diffusion zone with width of about 20µm is observed, as shown in Fig. 6b. This diffusion zone is enriched in Ni and Pd alloying with some other alloying elements such as W, Fe, Al. It should be mentioned that these alloying elements were not detected in the joint zone. The joint zone is comprised of homogeneous solid solution microstructure with composition of Ag-13.8Pd (wt. %).
Figure 7 shows BSE image of C/C-Superalloy joint without an Al2O3 interlayer. As seen in Fig. 7a, the laser-machined hole was also completely filled with braze and the interface between C/C and braze in the holes was well-bonded. Unfortunately, crack was found in C/C matrix adjacent to the interface in the region between the two braze spikes, as shown in Fig. 7b. The appearance of cracks indicates that even if the brazed seam with good plasticity cannot alleviate residual stress.
3.4 Mechanical strength of joint
The mechanical strength of joints is shown in Table 1. It can be seen that all the joints using Al2O3 interlayer and a zig-zag interfacial structure exhibited quite high shear strength up to 82 MPa and shear strength up to 54 MPa, respectively. However, the joint without a zig-zag interfacial structure has a rather low strength of only 10 MPa. Thus, it can be clearly concluded that the zig-zag interfacial structure exhibits a dramatic increase in the joint strength comparing to the flat joining interface. Moreover, it is also observed that the strength of joint with C/C metallized at 1100℃ is higher than that of joint with 1300℃. With the increase of annealing temperature, a lower joint strength is obtained. The influence of the Cr metallization process on the joint strength is contributed to the morphology of reaction layer formed during Cr metallization process. Figure 8 shows BSE image of interface using C/C with Cr metallization process at 1300℃×1h. It can be seen that in case of higher annealing temperatures, the reaction layer is relatively loose, and these pores are filled with infiltrating braze. In addition, cracks have also appeared in the reaction layer. Therefore, the non-dense reaction layer and cracks lead to a decrease of interfacial bonding strength.
Table 1
Mechanical properties of C/C-superalloy joint using Al2O3 interlayer
Metallization process | Shear strength (MPa) | Flexural strength (MPa) |
1100℃×2h | 54 ± 6 | 82 ± 3 |
1200℃×2h | 44.5 ± 9 | / |
1300℃×1h | 26.8 ± 6 | 72 ± 1 |
Images of the fracture surface of joint after tests and bending strength tests is shown in Fig. 9 and Fig. 9, respectively. It can be seen that the fracture morphology of the joint exhibits completely different fracture modes. For the joint using C/C without laser machining, a relatively flat fracture is present and joint mainly cracked at or near the boundary between C/C substate and reaction layer. In case of joint with laser machined C/C, the joint revealed a failure through the transition region between C/C and braze. Some amount of C/C matrix especially fibers with orientation parallel to the joining interface are attached on the fracture surface on metal side. Some other typical fractures, such as pulled out braze spikes and broken spikes are also found, as shown in Fig. 10. The complex fracture morphology indicates that the load-bearing capacity of the joint has significantly improved during loading process. It can be summarized that the using the laser machine process on C/C surface can substantially improve the strength and reliability of the joint.