High-Frequency Induction Brazing of Al2O3 Ceramics With SiO2-Na2O-K2O-CaO Glass Filler Based on Graphite Heat Transfer Medium

High-Frequency Induction Brazing of Al2O3 Ceramics With SiO2-Na2O-K2O-CaO Glass Filler Based on Graphite Heat Transfer Medium | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article High-Frequency Induction Brazing of Al 2 O 3 Ceramics With SiO 2 -Na 2 O-K 2 O-CaO Glass Filler Based on Graphite Heat Transfer Medium Yanyu Song, Guoxiang Sun, Haitao Zhu, Duo Liu, Jiaxin Sheng, Shengpeng Hu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7980247/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract A SiO 2 -Na 2 O-K 2 O-CaO glass filler developed with which the brazing of Al 2 O 3 ceramics was achieved through the high-frequency heating using the graphite as heat conduction in this study. The effects of heating time (110 ~ 140 s) and heating current (130 ~ 170 A) on the microstructure and mechanical properties of the joint were systematically investigated. As thermal input increased, the glass filler exhibited greater fluidity, which initially enhanced shear strength before causing a subsequent decrease. Concurrently, both the amount and size of porosity at the joint were reduced, resulting in a smoother fracture surface. The maximum shear strength of 18.5 MPa, was obtained at a heating time of 130 s and a heating current of 150 A. The excessive heating (140 s and 150 A) caused the glass filler overflow and the interfacial crack formation, reducing strength to a minimum of 5.0 MPa. This study provides significant insights for the development of rapid and efficient bonding of ceramic materials. High-frequency induction brazing Al2O3 ceramic SiO2-Na2O-K2O-CaO glass filler Microstructure Mechanical property Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1 Introduction Alumina ceramics (Al 2 O 3 ) possess high-temperature stability, electrical insulation, mechanical strength, and chemical inertness. These properties are essential for applications in electronic packaging, high-temperature sensors, vacuum technologies, and energy systems [ 1 – 3 ] . The trend toward miniaturization and functional integration in electronic devices has increased the demand for long-term reliability. Forming strong, durable joints between Al 2 O 3 ceramics has become a critical technical barrier, limiting the performance and broader application of advanced ceramic-based technologies [ 4 – 6 ] . Existing ceramic joining techniques mainly include solid-state diffusion bonding, vacuum brazing, and air-reaction brazing [ 7 – 9 ] . Solid-state diffusion bonding demands extremely high surface flatness and cleanliness, suffers from low efficiency, and is unsuitable for large or complex components [ 10 ] . Vacuum brazing is predominantly achieved through the reaction of active elements (e.g., Ti, Cr, Zr) in the filler metal with the base material, leading to the formation of an interfacial compound layer that enables joint formation [ 11 ] . However, significant differences in the coefficients of thermal expansion (CTE) among the reaction layer, the filler metal, and the base material can induce residual stresses, potentially compromising the joint strength [ 12 ] . Air-reaction brazing generally yields low joint strength and offers limited choices of filler materials [ 13 ] . In recent years, high-frequency induction brazing has seen extensive industrial application as a rapid, efficient, and energy-saving process [ 14 ] . It is characterized by highly localized heating, which enables precise temperature control, ensures superior joint quality, and minimizes the heat-affected zone [ 15 ] . Glass fillers are employed as intermediate layers in ceramic bonding, and excellent chemical compatibility with ceramic substrates is demonstrated [ 16 ] . The broad softening temperature range is exhibited, which accommodates rapid high-frequency induction heating. Furthermore, the CTE can be tailored via composition design to match that of the base material [ 17 , 18 ] , thereby enhancing joint strength [ 19 , 20 ] . For instance, Le et al. [ 21 ] fabricated a CaO-Y 2 O 3 -Al 2 O 3 -SiO 2 (CYAS) glass for joining SiC fiber reinforced SiC matrix composites (SiC f /SiC composites), achieving a maximum shear strength of 57.1 MPa after brazing at 1400°C for 30 min. The CTE of the CYAS glass was measured to be 4.3×10 − 6 /K, closely matching that of the SiC f /SiC composite. YiBo et al. [ 22 ] investigated the crystallization behavior of ZnO-Al 2 O 3 -SiO 2 glass, observing the precipitation of ZnAl 2 O 4 . This precipitate led to an increase in the CTE, with the joint exhibiting a maximum flexural strength of 201 MPa. Weiwei et al. [ 23 ] joined MgAl 2 O 4 ceramics using a CaO-Al 2 O 3 -SiO 2 glass filler. Under slow cooling rates of 5 ~ 10°C/min, dissolution of the MgAl 2 O 4 substrate occurred, leading to crystallization of CaAl 2 Si 2 O 8 and Mg 2 Al 4 Si 5 O 18 . The optimal bending strength of 181 ~ 189 MPa was achieved at a bonding temperature of 1300 ~ 1350°C and a cooling rate of 15°C/min. Additionally, the inherent optical transparency of certain glass fillers makes them particularly suitable for joining transparent ceramics [ 24 ] . Glass brazing thus enables high-strength, reliable bonding of precision ceramic components, facilitating the fabrication of complex assemblies for demanding high-performance applications [ 25 , 26 ] . Generally speaking, the elevated SiO 2 content is employed as a network former, enhancing the glass structure and regulating the high-temperature viscosity, which effectively limits excessive flow during brazing [ 27 , 28 ] . Additionally, the incorporation of Na 2 O, K 2 O, and CaO enables precise adjustment of the CTE, allowing it to match the ceramic substrate [ 29 , 30 ] . This adjustment increases the thermo-mechanical stability of the joint. Furthermore, these additives lower the glass transition and softening temperatures, thereby facilitating brazing at temperatures below 700°C. In this study, a SiO 2 -Na 2 O-K 2 O-CaO (SNKC) silicate glass filler was utilized to bond Al 2 O 3 ceramics. The high-frequency induction brazing process was carefully examined, with systematic analysis conducted on the effects of heating time and heating current on the shear strength of the brazed joints. Process parameters were optimized based on the evaluation of fracture morphology and shear testing, while the interfacial reaction mechanisms and joint formation processes were also thoroughly investigated. 2 Experimental Commercially available Al 2 O 3 ceramics were cut into dimensions of 18×8×5 mm 3 and 5×5×5 mm 3 , followed by grinding with SiC sandpaper and polishing with a 1 µm diamond suspension. The as-processed Al 2 O 3 ceramics were subsequently ultrasonically cleaned in ethanol for 20 min. The SNKC glass filler with a composition of 85SiO 2 -8Na 2 O-6K 2 O-1CaO (wt.%) was prepared from raw material powders including quartz sand, sodium carbonate, and limestone. The powders were thoroughly mixed and placed in an alumina crucible, heated at 10°C/min to 950°C, and held at this temperature for 60 min to homogenize the melt. After removing bubbles from the melt, the melt was rapidly injected into room-temperature deionized water to produce glass preforms. After ball milling and drying, the glass filler was obtained, as shown in Fig. 1 (a). Brazing experiments were performed using an SP-25 high-frequency induction brazing system (Ningbo Shuangping Power Technology Co., Ltd.), with graphite employed as the heat transfer medium in the brazed assembly, as shown in Fig. 1 (b). A layer of glass filler was applied to the center of the pretreated Al 2 O 3 substrate (18×8×5 mm 3 ), covering an area of approximately 5×5 mm 2 with a thickness controlled at about 200 µm. A smaller Al 2 O 3 ceramic (5×5×5 mm 3 ) was positioned steadily on top of the filler layer. The assembled specimens were then subjected to the brazing process under controlled heating current and heating time, followed by cooling to room temperature in air, as shown in Fig. 1 (c). The chemical composition and thermal properties of the SNKC glass filler were analyzed using an X-ray diffractometer (XRD, DX-2700) and differential scanning calorimetry (DSC). The microstructure of the SNKC glass filler and the Al 2 O 3 /SNKC/Al 2 O 3 brazed joint were characterized by the field emission scanning electron microscopy (SEM, Merlin Compact, Zeiss) equipped with energy dispersive spectroscopy (EDS). The shear strength of the Al 2 O 3 /SNKC/Al 2 O 3 brazed joint was evaluated using a universal testing machine (Instron 5976) at a crosshead speed of 0.5 mm/min. The load was applied perpendicular to the Al 2 O 3 /SNKC/Al 2 O 3 brazed joint, as illustrated in Fig. 1 (d). A minimum of five specimens was tested for each parameter set. 3 Results and Discussion 3.1 Characterization of SNKC glass and Al 2 O 3 /SNKC/Al 2 O 3 brazed joint The characteristics of the SNKC glass filler is shown in Fig. 2 . SEM image shown in Fig. 2 (a) reveals a morphology distribution at the microscopic scale. The homogeneous spatial distribution of the constituent elements was further confirmed by element distribution (Fig. 2 (b)-(f)), with Si and O being the dominant species that constitute the fundamental glass matrix. The XRD pattern (Fig. 2 (g)) exhibited two distinct broadened diffraction peaks at 2θ = 21.42° and 43.61°, confirming the amorphous structure of the SNKC glass filler matrix. DSC results (Fig. 2 (h)) determine the softening temperature of the SNKC glass filler to be approximately 604°C. Figure 3 shows the microstructure and elements distribution of Al 2 O 3 /SNKC/Al 2 O 3 joint brazed at 150 A for 140 s. As shown in Fig. 3 (a), the glass filler melted and infiltrated the gap of Al 2 O 3 ceramic. No significant defects such as porosity or cracks were identified in the brazed joint, which indicated that the effective bonding of Al 2 O 3 ceramics was achieved under the specified brazing conditions. According to the elements distribution present in Fig. 3 (b)-(f), the brazing seam was mainly consisted of the Si-Na-K-Ca-O phase, with no reaction layer detected at the bonding interface between the Al 2 O 3 ceramic and SNKC glass filler. The mutual diffusion of elements between the Al 2 O 3 ceramic and SNKC glass filler confirmed the integrity and effectiveness of the joint. EDS analysis was performed at various points along the brazing seam, with results shown in the Table 1 . The composition at different locations within the brazing seam consisted entirely of the glass phases with elements of Si, Na, K, Ca, and O contained, indicating the excellent homogeneity. It was attributed to the excellent fluidity of the SNKC glass filler at the high temperatures. Besides, the strong affinity between oxides enabled the SNKC glass melt to effectively penetrate the gap of the Al 2 O 3 ceramic and diffuse into the interior of the Al 2 O 3 ceramic during brazing process, thereby achieving a tight joint. Table 1 EDS analysis results of the points marked in Fig. 3 (a) (at.%). Spot Al Si O K Na Phase A 2.08 48.64 35.47 2.88 10.93 Silicate glass B 1.02 50.58 32.14 2.74 13.52 Silicate glass C 1.86 47.63 33.01 3.01 14.49 Silicate glass D 40.12 0.10 58.18 0.24 1.36 Al 2 O 3 3.2 Effect of heating time on the brazed joints Figure 4 presents the evolution of the interfacial microstructure of the Al 2 O 3 /SNKC/Al 2 O 3 joint as a function of heating time. A progressive reduction in the width of brazing seam from 157 µm at 110 s to 115 µm at 140s was observed with increased heating time. The interface exhibited significant voids and cracks after 140 s due to thermal stress from excessive heating. The narrowing of the joint was attributed to the increased softening of the glass filler under extended thermal exposure, which enhanced its fluidity. As a result, the filler material was more easily expelled from the joint region, leading to the formation of a narrower brazed seam. Further investigation into the effect of heating time on the mechanical properties of the joints was conducted through shear tests on specimens at room temperature, with the results present in Fig. 5 . As heating time increased, the shear strength of Al 2 O 3 /SNKC/Al 2 O 3 joint exhibited a trend of first increasing and then decreasing. At a heating time of 110 s, the poor bonding with the Al 2 O 3 ceramic was attributed to the insufficient softening degree of the SNKC glass filler, with a shear strength of 12.6 MPa. As the heating time increased, the softening degree of the filler increased. When the heating time was 130 s, the shear strength of Al 2 O 3 /SNKC/Al 2 O 3 joint reached a maximum value of 18.5 MPa. Further prolonging the heating time to 140 s caused the shear strength of the Al 2 O 3 /SNKC/Al 2 O 3 joint to suddenly dropped down 5.0 MPa. Excessive heating duration created a high-stress condition in the elevated-temperature environment, leading to interfacial cracking that severely decreasing the mechanical properties of Al 2 O 3 /SNKC/Al 2 O 3 joint. Further investigation into the effects of heating time on fracture morphology was conducted, with the corresponding fracture surfaces of the aforementioned specimens observed under an optical microscope shown in Fig. 6 . When heating time was insufficient, the filler did not soften enough, leading to poor flowability and interfacial pores that compromise connection stability. With extended heating, the filler softened adequately, forming a stronger bond with the base material for enhanced joint integrity. It can be observed that fractures consistently occur primarily at the interface between ceramic and glass. 3.3 Effect of heating current on the brazed joints Under the constant heating time of 130 s, the variation in the interface microstructure of the Al 2 O 3 /SNKC/Al 2 O 3 joint with heating currents of 130 A, 140 A, 150 A, and 160 A is shown in Fig. 7 . As the heating current increased, the width of brazing seam gradually decreased from 158 µm at 130 A to 100 µm at 160 A. This narrowing was attributed to a more thorough softening of the glass filler, which enhanced its fluidity and caused it to be expelled from the joint interface. The gap was observed between the glass filler and the base material at 130 A, preventing proper fusion. This was attributed to the insufficient softening of the filler under the applied processing conditions, which prevented adequate filling of the joint. At 160 A, noticeable cracking occurred within the weld, resulting from excessive heating that induced substantial thermal stresses. To further investigated the effect of heating current on the mechanical properties of the Al 2 O 3 /SNKC/Al 2 O 3 joint, shear tests were conducted on the four above-mentioned groups of specimens at room temperature, as present in Fig. 8 . It was observed that the shear strength initially increased and then decreased with the rise in heating current. At a heating current of 130 A, inadequate softening of the SNKC glass filler caused the poor bonding with the Al 2 O 3 ceramic, resulting in a shear strength of only 9.8 MPa. As the heating current increased, the extent of SNKC glass filler softening was enhanced. The maximum shear strength of 18.5 MPa was achieved at the heating current of 140 A. However, excessive softening of the glass filler was observed with a further increase in heating current, producing substantial filler material outflow. As previously mentioned, this phenomenon adversely affected the joint integrity and caused the reduction in shear strength. Further investigation into the effects of heating current and fracture morphology. The fracture morphologies of the joints of the aforementioned four groups of specimens under the optical microscope are shown in Fig. 9 . Fractures were predominantly observed at the ceramic-glass interface. Similar to the aforementioned mechanism, insufficient softening of the filler under low heating currents resulted in poor flowability, which leaded to pore formation between particles and an unstable connection. As the heating current increased, enhanced softening of the filler promoted bonding with the base material, thereby yielding the greater joint strength. However, when the heating current was further elevated, excessive softening of the filler resulted in heightened fluidity, causing overflow from the joint edges and consequently impairing specimen performance. 3.4 Analysis of the brazing mechanism of Al 2 O 3 /SNKC/Al 2 O 3 joint The induction brazing technology is predominantly employed with electrically conductive metallic materials. Despite its non-metallic nature, graphite is endowed with significant electrical conductivity due to its layered crystalline structure, making it applicable to induction brazing processes [ 31 ] . The electrical resistivity of graphite typically ranges from (8 ~ 13)×10 − 6 Ω·m. When subjected to a high-frequency alternating magnetic field, substantial eddy currents were induced, generating considerable Joule heating through the intrinsic electrical resistance [ 32 ] . In this study, the graphite abutment generated substantial thermal energy while supporting the specimens during brazing operations, efficiently transferring heat to the specimens to achieve efficient and rapid brazing processing. Based on the analysis of the microstructure at the joint interface and the study of the influence patterns of heating time and heating current, it was evident that the formation mechanism of the Al 2 O 3 /SNKC/Al 2 O 3 joint relies on the bonding effect achieved through chemical compatibility between the glass filler and the ceramic substrate during the brazing process. Upon heating to its softening point, the powdered glass filler transforms into a highly viscosity melt, with both the number and size of original powder voids decreasing until they disappear. As the brazing process progressed and temperature rose, the increased fluidity of the filler enabled interfacial reaction with the Al 2 O 3 ceramic through their excellent chemical compatibility, simultaneously filling the brazing seam. However, excessive heat input caused over-softening and overflow of the filler, depleting the effective bonding material and diminishing the mechanical property of the joint. 4 Conclusions In this study, brazing of Al 2 O 3 ceramics was accomplished using silicate glass filler material and high-frequency induction brazing equipment, with heat transfer mediated by high-frequency heating of graphite media. The effect of different heating times and heating currents on the interface microstructure and mechanical properties of the Al 2 O 3 /SNKC/Al 2 O 3 joint was investigated. The width of brazing seam was observed to decrease progressively with increasing heating time and current, while the shear strength initially increased and subsequently declined. Prolonged heating facilitated softening of the filler material, thereby enhancing its fluidity and promoting the filling of inherent pores within the Al 2 O 3 ceramic. These microstructural changes contributed to the formation of smoother fracture surfaces and stronger interfacial bonding. However, excessive heating induced overflow and lateral leakage of the filler, resulting in interfacial cracking and a marked reduction in shear strength. The maximum shear strength of 18.5 MPa was attained under optimized conditions of 130 s heating duration and 150 A current. Declarations Data Availability Statement Data will be made available on request. Authors Contributions Yanyu Song : Writing – original draft, Methodology. Guoxiang Sun : Writing – original draft, Visualization. Haitao Zhu : Writing – review & editing. Duo Liu : Resources, Project administration. Jiaxin Sheng : Investigation, Data curation. Shengpeng Hu : Methodology, Supervision. Xiaoguo Song : Resources, Project administration. Funding This project is supported by National Natural Science Foundation of China (grant No. 52505356), the Taishan Scholar Foundation of Shandong Province (grant No. tsqn202312139), the Shandong Natural Science Foundation (grant Nos. ZR2023JQ021 and ZR2023QE221), the State Key Laboratory of Precision Welding & Joining of Materials and Structures (grant No. MSWJ-24M07), and the Shandong Province University Youth Innovation and Technology Support Program (grant No. 2024KJH013). Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References D. Li, C. He, Y. Wang, et al, High purity Al 2 O 3 ceramic: Metallizing strategy, microstructure and sealing properties, Journal of Asian Ceramic Societies. 11 (2023) 250-259. Y. Li, C. Chen, R. 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Lukowiec, Structure, temperature and frequency dependent electrical conductivity of oxidized and reduced electrochemically exfoliated graphite, Physica E: Low-dimensional Systems and Nanostructures. 99 (2018) 82-90. P. Rolicz, Eddy currents generated in a system of two cylindrical conductors by a transverse alternating magnetic field, Electric Power Systems Research. 79 (2009) 295-300. Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 03 Nov, 2025 Reviewers invited by journal 03 Nov, 2025 Editor invited by journal 01 Nov, 2025 Editor assigned by journal 31 Oct, 2025 First submitted to journal 29 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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9","display":"","copyAsset":false,"role":"figure","size":255587,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of heating current on fracture morphology (t=130s) with (a)130A, (b)140A, (c)150A, (d)160A.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7980247/v1/a264c8093504c3c91312c1ad.png"},{"id":95730478,"identity":"a41856b2-804f-4afd-b79e-67b38caccc45","added_by":"auto","created_at":"2025-11-12 11:36:45","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":61536,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of high-frequency induction brazing.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7980247/v1/0237e5772bffd0e94f85e3d0.png"},{"id":95801348,"identity":"19980fe8-8728-4b3f-ae0a-917d856e13bc","added_by":"auto","created_at":"2025-11-13 08:25:06","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":134875,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the brazing process: (a) low-temperature initial stage, (b) filler melting stage, (c) filler filling stage, (d) filler overflow stage.\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-7980247/v1/f071aebdd886d570d9a45e3f.png"},{"id":96239081,"identity":"49df7778-0607-42b1-bd4c-687207c33dae","added_by":"auto","created_at":"2025-11-19 07:02:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3124573,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7980247/v1/2babddc9-df5b-43b7-b64f-e799a7257f65.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eHigh-Frequency Induction Brazing of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e Ceramics With SiO\u003csub\u003e2\u003c/sub\u003e-Na\u003csub\u003e2\u003c/sub\u003eO-K\u003csub\u003e2\u003c/sub\u003eO-CaO Glass Filler Based on Graphite Heat Transfer Medium\u003c/p\u003e","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eAlumina ceramics (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) possess high-temperature stability, electrical insulation, mechanical strength, and chemical inertness. These properties are essential for applications in electronic packaging, high-temperature sensors, vacuum technologies, and energy systems\u003csup\u003e[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. The trend toward miniaturization and functional integration in electronic devices has increased the demand for long-term reliability. Forming strong, durable joints between Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics has become a critical technical barrier, limiting the performance and broader application of advanced ceramic-based technologies\u003csup\u003e[\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eExisting ceramic joining techniques mainly include solid-state diffusion bonding, vacuum brazing, and air-reaction brazing\u003csup\u003e[\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. Solid-state diffusion bonding demands extremely high surface flatness and cleanliness, suffers from low efficiency, and is unsuitable for large or complex components\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. Vacuum brazing is predominantly achieved through the reaction of active elements (e.g., Ti, Cr, Zr) in the filler metal with the base material, leading to the formation of an interfacial compound layer that enables joint formation\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. However, significant differences in the coefficients of thermal expansion (CTE) among the reaction layer, the filler metal, and the base material can induce residual stresses, potentially compromising the joint strength\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. Air-reaction brazing generally yields low joint strength and offers limited choices of filler materials\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. In recent years, high-frequency induction brazing has seen extensive industrial application as a rapid, efficient, and energy-saving process\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. It is characterized by highly localized heating, which enables precise temperature control, ensures superior joint quality, and minimizes the heat-affected zone\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eGlass fillers are employed as intermediate layers in ceramic bonding, and excellent chemical compatibility with ceramic substrates is demonstrated\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. The broad softening temperature range is exhibited, which accommodates rapid high-frequency induction heating. Furthermore, the CTE can be tailored via composition design to match that of the base material\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, thereby enhancing joint strength\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. For instance, Le et al.\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e fabricated a CaO-Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-SiO\u003csub\u003e2\u003c/sub\u003e (CYAS) glass for joining SiC fiber reinforced SiC matrix composites (SiC\u003csub\u003ef\u003c/sub\u003e/SiC composites), achieving a maximum shear strength of 57.1 MPa after brazing at 1400\u0026deg;C for 30 min. The CTE of the CYAS glass was measured to be 4.3\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e /K, closely matching that of the SiC\u003csub\u003ef\u003c/sub\u003e/SiC composite. YiBo et al.\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e investigated the crystallization behavior of ZnO-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-SiO\u003csub\u003e2\u003c/sub\u003e glass, observing the precipitation of ZnAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. This precipitate led to an increase in the CTE, with the joint exhibiting a maximum flexural strength of 201 MPa. Weiwei et al.\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e joined MgAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e ceramics using a CaO-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-SiO\u003csub\u003e2\u003c/sub\u003e glass filler. Under slow cooling rates of 5\u0026thinsp;~\u0026thinsp;10\u0026deg;C/min, dissolution of the MgAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e substrate occurred, leading to crystallization of CaAl\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e and Mg\u003csub\u003e2\u003c/sub\u003eAl\u003csub\u003e4\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e18\u003c/sub\u003e. The optimal bending strength of 181\u0026thinsp;~\u0026thinsp;189 MPa was achieved at a bonding temperature of 1300\u0026thinsp;~\u0026thinsp;1350\u0026deg;C and a cooling rate of 15\u0026deg;C/min. Additionally, the inherent optical transparency of certain glass fillers makes them particularly suitable for joining transparent ceramics\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e. Glass brazing thus enables high-strength, reliable bonding of precision ceramic components, facilitating the fabrication of complex assemblies for demanding high-performance applications\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. Generally speaking, the elevated SiO\u003csub\u003e2\u003c/sub\u003e content is employed as a network former, enhancing the glass structure and regulating the high-temperature viscosity, which effectively limits excessive flow during brazing\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Additionally, the incorporation of Na\u003csub\u003e2\u003c/sub\u003eO, K\u003csub\u003e2\u003c/sub\u003eO, and CaO enables precise adjustment of the CTE, allowing it to match the ceramic substrate\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. This adjustment increases the thermo-mechanical stability of the joint. Furthermore, these additives lower the glass transition and softening temperatures, thereby facilitating brazing at temperatures below 700\u0026deg;C.\u003c/p\u003e\u003cp\u003eIn this study, a SiO\u003csub\u003e2\u003c/sub\u003e-Na\u003csub\u003e2\u003c/sub\u003eO-K\u003csub\u003e2\u003c/sub\u003eO-CaO (SNKC) silicate glass filler was utilized to bond Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics. The high-frequency induction brazing process was carefully examined, with systematic analysis conducted on the effects of heating time and heating current on the shear strength of the brazed joints. Process parameters were optimized based on the evaluation of fracture morphology and shear testing, while the interfacial reaction mechanisms and joint formation processes were also thoroughly investigated.\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cp\u003eCommercially available Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics were cut into dimensions of 18\u0026times;8\u0026times;5 mm\u003csup\u003e3\u003c/sup\u003e and 5\u0026times;5\u0026times;5 mm\u003csup\u003e3\u003c/sup\u003e, followed by grinding with SiC sandpaper and polishing with a 1 \u0026micro;m diamond suspension. The as-processed Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics were subsequently ultrasonically cleaned in ethanol for 20 min. The SNKC glass filler with a composition of 85SiO\u003csub\u003e2\u003c/sub\u003e-8Na\u003csub\u003e2\u003c/sub\u003eO-6K\u003csub\u003e2\u003c/sub\u003eO-1CaO (wt.%) was prepared from raw material powders including quartz sand, sodium carbonate, and limestone. The powders were thoroughly mixed and placed in an alumina crucible, heated at 10\u0026deg;C/min to 950\u0026deg;C, and held at this temperature for 60 min to homogenize the melt. After removing bubbles from the melt, the melt was rapidly injected into room-temperature deionized water to produce glass preforms. After ball milling and drying, the glass filler was obtained, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a).\u003c/p\u003e\u003cp\u003eBrazing experiments were performed using an SP-25 high-frequency induction brazing system (Ningbo Shuangping Power Technology Co., Ltd.), with graphite employed as the heat transfer medium in the brazed assembly, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). A layer of glass filler was applied to the center of the pretreated Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e substrate (18\u0026times;8\u0026times;5 mm\u003csup\u003e3\u003c/sup\u003e), covering an area of approximately 5\u0026times;5 mm\u003csup\u003e2\u003c/sup\u003e with a thickness controlled at about 200 \u0026micro;m. A smaller Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic (5\u0026times;5\u0026times;5 mm\u003csup\u003e3\u003c/sup\u003e) was positioned steadily on top of the filler layer. The assembled specimens were then subjected to the brazing process under controlled heating current and heating time, followed by cooling to room temperature in air, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe chemical composition and thermal properties of the SNKC glass filler were analyzed using an X-ray diffractometer (XRD, DX-2700) and differential scanning calorimetry (DSC). The microstructure of the SNKC glass filler and the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e brazed joint were characterized by the field emission scanning electron microscopy (SEM, Merlin Compact, Zeiss) equipped with energy dispersive spectroscopy (EDS). The shear strength of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e brazed joint was evaluated using a universal testing machine (Instron 5976) at a crosshead speed of 0.5 mm/min. The load was applied perpendicular to the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e brazed joint, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(d). A minimum of five specimens was tested for each parameter set.\u003c/p\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Characterization of SNKC glass and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e brazed joint\u003c/h2\u003e\u003cp\u003eThe characteristics of the SNKC glass filler is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. SEM image shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) reveals a morphology distribution at the microscopic scale. The homogeneous spatial distribution of the constituent elements was further confirmed by element distribution (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b)-(f)), with Si and O being the dominant species that constitute the fundamental glass matrix. The XRD pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(g)) exhibited two distinct broadened diffraction peaks at 2θ\u0026thinsp;=\u0026thinsp;21.42\u0026deg; and 43.61\u0026deg;, confirming the amorphous structure of the SNKC glass filler matrix. DSC results (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(h)) determine the softening temperature of the SNKC glass filler to be approximately 604\u0026deg;C.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the microstructure and elements distribution of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint brazed at 150 A for 140 s. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), the glass filler melted and infiltrated the gap of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic. No significant defects such as porosity or cracks were identified in the brazed joint, which indicated that the effective bonding of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics was achieved under the specified brazing conditions. According to the elements distribution present in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)-(f), the brazing seam was mainly consisted of the Si-Na-K-Ca-O phase, with no reaction layer detected at the bonding interface between the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic and SNKC glass filler. The mutual diffusion of elements between the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic and SNKC glass filler confirmed the integrity and effectiveness of the joint.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eEDS analysis was performed at various points along the brazing seam, with results shown in the Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The composition at different locations within the brazing seam consisted entirely of the glass phases with elements of Si, Na, K, Ca, and O contained, indicating the excellent homogeneity. It was attributed to the excellent fluidity of the SNKC glass filler at the high temperatures. Besides, the strong affinity between oxides enabled the SNKC glass melt to effectively penetrate the gap of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic and diffuse into the interior of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic during brazing process, thereby achieving a tight joint.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEDS analysis results of the points marked in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) (at.%).\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpot\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAl\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eK\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eNa\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003ePhase\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2.08\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e48.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e35.47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e10.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSilicate glass\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e32.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e13.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSilicate glass\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e47.63\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e33.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e3.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e14.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSilicate glass\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e40.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e58.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Effect of heating time on the brazed joints\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents the evolution of the interfacial microstructure of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint as a function of heating time. A progressive reduction in the width of brazing seam from 157 \u0026micro;m at 110 s to 115 \u0026micro;m at 140s was observed with increased heating time. The interface exhibited significant voids and cracks after 140 s due to thermal stress from excessive heating. The narrowing of the joint was attributed to the increased softening of the glass filler under extended thermal exposure, which enhanced its fluidity. As a result, the filler material was more easily expelled from the joint region, leading to the formation of a narrower brazed seam.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFurther investigation into the effect of heating time on the mechanical properties of the joints was conducted through shear tests on specimens at room temperature, with the results present in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. As heating time increased, the shear strength of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint exhibited a trend of first increasing and then decreasing. At a heating time of 110 s, the poor bonding with the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic was attributed to the insufficient softening degree of the SNKC glass filler, with a shear strength of 12.6 MPa. As the heating time increased, the softening degree of the filler increased. When the heating time was 130 s, the shear strength of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint reached a maximum value of 18.5 MPa. Further prolonging the heating time to 140 s caused the shear strength of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint to suddenly dropped down 5.0 MPa. Excessive heating duration created a high-stress condition in the elevated-temperature environment, leading to interfacial cracking that severely decreasing the mechanical properties of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFurther investigation into the effects of heating time on fracture morphology was conducted, with the corresponding fracture surfaces of the aforementioned specimens observed under an optical microscope shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. When heating time was insufficient, the filler did not soften enough, leading to poor flowability and interfacial pores that compromise connection stability. With extended heating, the filler softened adequately, forming a stronger bond with the base material for enhanced joint integrity. It can be observed that fractures consistently occur primarily at the interface between ceramic and glass.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Effect of heating current on the brazed joints\u003c/h2\u003e\u003cp\u003eUnder the constant heating time of 130 s, the variation in the interface microstructure of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint with heating currents of 130 A, 140 A, 150 A, and 160 A is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. As the heating current increased, the width of brazing seam gradually decreased from 158 \u0026micro;m at 130 A to 100 \u0026micro;m at 160 A. This narrowing was attributed to a more thorough softening of the glass filler, which enhanced its fluidity and caused it to be expelled from the joint interface. The gap was observed between the glass filler and the base material at 130 A, preventing proper fusion. This was attributed to the insufficient softening of the filler under the applied processing conditions, which prevented adequate filling of the joint. At 160 A, noticeable cracking occurred within the weld, resulting from excessive heating that induced substantial thermal stresses.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo further investigated the effect of heating current on the mechanical properties of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint, shear tests were conducted on the four above-mentioned groups of specimens at room temperature, as present in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. It was observed that the shear strength initially increased and then decreased with the rise in heating current. At a heating current of 130 A, inadequate softening of the SNKC glass filler caused the poor bonding with the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic, resulting in a shear strength of only 9.8 MPa. As the heating current increased, the extent of SNKC glass filler softening was enhanced. The maximum shear strength of 18.5 MPa was achieved at the heating current of 140 A. However, excessive softening of the glass filler was observed with a further increase in heating current, producing substantial filler material outflow. As previously mentioned, this phenomenon adversely affected the joint integrity and caused the reduction in shear strength.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFurther investigation into the effects of heating current and fracture morphology. The fracture morphologies of the joints of the aforementioned four groups of specimens under the optical microscope are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Fractures were predominantly observed at the ceramic-glass interface. Similar to the aforementioned mechanism, insufficient softening of the filler under low heating currents resulted in poor flowability, which leaded to pore formation between particles and an unstable connection. As the heating current increased, enhanced softening of the filler promoted bonding with the base material, thereby yielding the greater joint strength. However, when the heating current was further elevated, excessive softening of the filler resulted in heightened fluidity, causing overflow from the joint edges and consequently impairing specimen performance.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Analysis of the brazing mechanism of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint\u003c/h2\u003e\u003cp\u003eThe induction brazing technology is predominantly employed with electrically conductive metallic materials. Despite its non-metallic nature, graphite is endowed with significant electrical conductivity due to its layered crystalline structure, making it applicable to induction brazing processes\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. The electrical resistivity of graphite typically ranges from (8\u0026thinsp;~\u0026thinsp;13)\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e Ω\u0026middot;m. When subjected to a high-frequency alternating magnetic field, substantial eddy currents were induced, generating considerable Joule heating through the intrinsic electrical resistance\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. In this study, the graphite abutment generated substantial thermal energy while supporting the specimens during brazing operations, efficiently transferring heat to the specimens to achieve efficient and rapid brazing processing.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBased on the analysis of the microstructure at the joint interface and the study of the influence patterns of heating time and heating current, it was evident that the formation mechanism of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint relies on the bonding effect achieved through chemical compatibility between the glass filler and the ceramic substrate during the brazing process. Upon heating to its softening point, the powdered glass filler transforms into a highly viscosity melt, with both the number and size of original powder voids decreasing until they disappear. As the brazing process progressed and temperature rose, the increased fluidity of the filler enabled interfacial reaction with the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic through their excellent chemical compatibility, simultaneously filling the brazing seam. However, excessive heat input caused over-softening and overflow of the filler, depleting the effective bonding material and diminishing the mechanical property of the joint.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eIn this study, brazing of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics was accomplished using silicate glass filler material and high-frequency induction brazing equipment, with heat transfer mediated by high-frequency heating of graphite media. The effect of different heating times and heating currents on the interface microstructure and mechanical properties of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SNKC/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e joint was investigated. The width of brazing seam was observed to decrease progressively with increasing heating time and current, while the shear strength initially increased and subsequently declined. Prolonged heating facilitated softening of the filler material, thereby enhancing its fluidity and promoting the filling of inherent pores within the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic. These microstructural changes contributed to the formation of smoother fracture surfaces and stronger interfacial bonding. However, excessive heating induced overflow and lateral leakage of the filler, resulting in interfacial cracking and a marked reduction in shear strength. The maximum shear strength of 18.5 MPa was attained under optimized conditions of 130 s heating duration and 150 A current.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYanyu Song\u003c/strong\u003e: Writing \u0026ndash; original draft, Methodology. \u003cstrong\u003eGuoxiang Sun\u003c/strong\u003e: Writing \u0026ndash; original draft, Visualization. \u003cstrong\u003eHaitao Zhu\u003c/strong\u003e: Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eDuo Liu\u003c/strong\u003e: Resources, Project administration. \u003cstrong\u003eJiaxin Sheng\u003c/strong\u003e: Investigation, Data curation. \u003cstrong\u003eShengpeng Hu\u003c/strong\u003e: Methodology, Supervision. \u003cstrong\u003eXiaoguo Song\u003c/strong\u003e: Resources, Project administration.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project is supported by National Natural Science Foundation of China (grant No. 52505356), the Taishan Scholar Foundation of Shandong Province (grant No. tsqn202312139), the Shandong Natural Science Foundation (grant Nos. ZR2023JQ021 and ZR2023QE221), the State Key Laboratory of Precision Welding \u0026amp; Joining of Materials and Structures (grant No. MSWJ-24M07), and the Shandong Province University Youth Innovation and Technology Support Program (grant No. 2024KJH013).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eD. Li, C. He, Y. Wang, et al, High purity Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic: Metallizing strategy, microstructure and sealing properties, Journal of Asian Ceramic Societies. 11 (2023) 250-259.\u003c/li\u003e\n\u003cli\u003eY. Li, C. Chen, R. 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Rolicz, Eddy currents generated in a system of two cylindrical conductors by a transverse alternating magnetic field, Electric Power Systems Research. 79 (2009) 295-300.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"High-frequency induction brazing, Al2O3 ceramic, SiO2-Na2O-K2O-CaO glass filler, Microstructure, Mechanical property","lastPublishedDoi":"10.21203/rs.3.rs-7980247/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7980247/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA SiO\u003csub\u003e2\u003c/sub\u003e-Na\u003csub\u003e2\u003c/sub\u003eO-K\u003csub\u003e2\u003c/sub\u003eO-CaO glass filler developed with which the brazing of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics was achieved through the high-frequency heating using the graphite as heat conduction in this study. The effects of heating time (110\u0026thinsp;~\u0026thinsp;140 s) and heating current (130\u0026thinsp;~\u0026thinsp;170 A) on the microstructure and mechanical properties of the joint were systematically investigated. As thermal input increased, the glass filler exhibited greater fluidity, which initially enhanced shear strength before causing a subsequent decrease. Concurrently, both the amount and size of porosity at the joint were reduced, resulting in a smoother fracture surface. The maximum shear strength of 18.5 MPa, was obtained at a heating time of 130 s and a heating current of 150 A. The excessive heating (140 s and 150 A) caused the glass filler overflow and the interfacial crack formation, reducing strength to a minimum of 5.0 MPa. This study provides significant insights for the development of rapid and efficient bonding of ceramic materials.\u003c/p\u003e","manuscriptTitle":"High-Frequency Induction Brazing of Al2O3 Ceramics With SiO2-Na2O-K2O-CaO Glass Filler Based on Graphite Heat Transfer Medium","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-12 11:36:40","doi":"10.21203/rs.3.rs-7980247/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-11-03T14:27:58+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-03T06:24:41+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2025-11-01T09:27:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-31T07:44:10+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2025-10-30T03:05:31+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"db6f5b3a-8e27-4fcd-af73-ffe9f9241cbe","owner":[],"postedDate":"November 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-01-26T12:36:48+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-12 11:36:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7980247","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7980247","identity":"rs-7980247","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}