Facilitating low-temperature diffusion bonding between oxygen-free Al2O3 ceramic and pure Cu through inclusion of 0.8 La (wt.%) to Ti pre-metallized interlayer: Microstructural evolution, metallurgical reactions, and mechanical properties
Introduction
Aluminum oxide (Al2O3) ceramic and Al2O3-based ceramic composites are widely used as structural materials in nuclear reactors and aerospace, due to their excellent mechanical properties at high temperatures, satisfactory resistance to radiation, and good oxidation and corrosion resistance [1]. However, the high hardness and low fracture toughness of Al2O3 ceramics makes it difficult to machine Al2O3 components and join them to other metallic materials. Heterogeneous bonding (i.e., active metal bonding or diffusion bonding) has attracted considerable attention in industry for the attachment of Al2O3 ceramic to itself or to metals [2,3]. Note that ceramic surfaces require pre-metallization [4], [5], [6], [7], which generally involves the highly-complex and expensive process of Ni plating [8,9]. Numerous bonding methods have been developed, including solid state diffusion bonding, partial transient liquid phase bonding, and active metal brazing [10], [11], [12], [13], [14], [15], [16]. Note that the mechanical properties of the ceramic components depend heavily on the constituents of the active filler material.
The use of active filler materials is particularly well-suited to industrial applications, due to its simplicity and applicability to joints of various shapes and sizes. The soundness of the joints depends primarily on the chemical reactions between the ceramic surface and active elements in the filler material. The most common elements are Ti, Zr, and Hf, which have proven particularly effective in adjusting the wetting and adhesion characteristics of Al2O3 ceramic surfaces following in-situ chemical reactions [17], [18], [19], [20], [21]. Note that these interfacial reactions (particularly those involving Ti) are highly controllable [22]. Researchers have also been working to improve the properties of joints through the addition of a soft interlayer or the in-situ formation of particles or whiskers with a low coefficient of thermal expansion (CTE) [23], [24], [25]. Unfortunately, ceramic joints based on active metallic fillers are unable to withstand high-temperature fabrication techniques, due largely to the fragility of the reaction layers of Ti-Cu-O compounds and Ti oxides on the surface of the Al2O3 ceramic [26], [27], [28], [29]. This has severely limited the applicability of Al2O3 ceramics and necessitated the development of low-temperature active filler metals to facilitate the bonding of ceramics to metals. Ti, Zr, and Hf-based active filler materials have been shown to suppress oxide formation, whereas cerium (Ce) [30,31] and lanthanum (La) [32,33] have been shown to enhance atomic substitution at interfaces. Overall, the high oxygen affinity of such elements reduces the formation of oxides and intermetallic compounds to improve bonding strength.
In the current study, we employed low-temperature (250, 300 and 350°C) hot press diffusion to create bonds between oxygen-free Al2O3 ceramic substrates and pure Cu substrates using a thin layer of Ti-0.8La (wt.%) as a pre-metallization layer. The proposed scheme includes two simple steps: 1) active metallization of the Al2O3 ceramic surface using Ti-0.8La (wt.%) as a composite filler and 2) diffusion brazing of a metallized oxygen-free Al2O3 ceramic substrate and pure Cu substrates at 250, 300, and 350°C. A wide range of experiments were performed to assess the feasibility of the proposed low-temperature diffusion bonding method. The primary function of the experiments was to compare the metallurgical reactions, microstructural evolution, and properties of joints produced under a range of bonding temperatures. The overall objective in this study was to determine whether the inclusion of La in the solder could be used to reduce the temperature of diffusion bonding between Al2O3 and Cu substrates.
Section snippets
Materials, diffusion bonding process, and joint characterization
Success in the diffusion bonding of joints depends on a combination of factors, the most important of which is the preparation of material surfaces, as this determines the initial contact area between diffusion couples. Thus, the diffusion bonding experiments in this study began with the preparation of materials to be joined: 1) Oxygen-free Al2O3 ceramic substrates and pure Cu substrates (10 × 10 × 0.5 mm) were sequentially washed in acetone, isopropanol, and ethanol to remove contaminants, and
Microstructure at interfacial of Al2O3 ceramic and Cu substrates
Fig. 2(a)~(c) present backscattered electron images (BEI) of Al2O3 / Cu diffusion-bonded joints respectively processed at temperatures of 250, 300, and 350°C. Fig. 3(a)~(c) present corresponding line scans of the constituent elements in each joint. The line scan results confirm that the Ti-0.8La (wt.%) interlayer facilitated the solid-state diffusion bonding of the Al2O3 ceramic substrate to the Cu substrate via and the formation of a 150~250 nm diffusion bonding zone (i.e., reaction zone). As
Conclusions
This study investigated the low-temperature diffusion bonding of metal and ceramic substrates using a Ti-0.8La (wt.%) pre-metallization layer. Experiment results support the following conclusions:
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Bonding at temperatures of 250, 300, and 350°C was shown to facilitate the formation of strong diffusion bonds between Al2O3 and the Cu substrate. The strong diffusion of La and Ti toward the Al2O3 substrate can be attributed to the strong affinity of La for oxygen and the ease with which Al is
Authors' contribution
Dr. Chun-Ming Lin conceived the experimental design, analyzed the results, and reviewed and edited the manuscript. In addition, Miss Tseng-Pei-Hsins conducted the experiments, prepared the draft manuscript, including both the text and the figures and tables.
Declaration of Competing Interest
This manuscript has not been published or presented elsewhere in part or in entirety, and is not under consideration by another journal. All authors have approved the manuscript and agreed with submission to your esteemed journal. There are no conflicts of interest to declare.
Acknowledgements
The authors would like to thank the Ministry of Science and Technology of Taiwan for the financial support of this study under Contract no. MOST 105-2218-E-027 -011-MY3. Finally, the authors are grateful to Miss Tseng-Pei-Hsins for his assistance in publishing this work.
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