Brazing SiC ceramic using novel B4C reinforced Ag–Cu–Ti composite filler

The development of new composite fillers is crucial for joining ceramics or ceramics to metals because the composite fillers exhibit more advantages than traditional brazing filler metal. In this research, novel B₄C reinforced Ag–Cu–Ti composite filler was developed to braze SiC ceramics. The interfacial microstructure of the joints was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of B₄C addition and brazing temperature on the microstructure evolution and mechanical properties of the joints was analyzed. The results revealed that TiB whisker and TiC particles were simultaneously synthesized in the Ag-based solid solution and Cu-based solid solution due to the addition of B₄C particles. As the brazing temperature increased, the thickness of Ti₃SiC₂+Ti₅Si₃ layers adjacent to SiC ceramic increased. Desirable microstructure similar to the metal matrix reinforced by TiB whisker and TiC particles could be obtained at brazing temperature of 950 °C. The maximum bending strength of 140 MPa was reached when the joints brazed at 950 °C for 10 min, which was 48 MPa (~52%) higher than that of the joints brazed using Ag–Cu–Ti filler.     

Joining of SiC ceramics using the Ni-Mo filler alloy for heat exchanger applications

In order to meet the sealing demands of SiC heat exchanger, the Ni-Mo filler alloy was designed, prepared and employed to braze SiC ceramics. Wetting behavior of the Ni-Mo filler alloy on SiC ceramics and interfacial microstructure of the brazed joints were systematically characterized using optical observation furnace and XRD, SEM, EDS, TEM, respectively. Flexural strengths of the brazed joints at room temperature and high temperature were measured with four-point flexural strength method. HCl immersion test was performed to evaluate the corrosion resistance of the joints. The Ni-Mo filler alloy exhibited excellent wettability on SiC ceramics. During the process of brazing, SiC reacted with element Ni of the Ni-Mo filler alloy, resulting in the formation of Ni₂Si + graphite reaction layer adjacent to the SiC substrate. Ni₃Mo₃C and Ni₂Si compounds were precipitated at the center of brazing seam. When the brazing temperature increased from 1250 ℃ to 1400 ℃, the thickness of Ni₂Si + graphite layer increased gradually. The maximum room-temperature flexural strength of 174 ± 33 MPa was obtained when brazed at 1300 ℃ for 40 min. The joints also exhibited stable high-temperature strength and acid corrosion resistance. When the test temperature was 700 ℃, 800 ℃, 900 ℃, the joints gave the strength retention rate of 92.5 %, 79.8 %, 67.2 %, respectively. It was believed that the formation of high melting point phases played an important role. Residual strength of the joints after HCl corrosion exceeded 130 MPa, revealing a good potential for applications in corrosion environment.     

Joining of SiC using CoFeCrNiCuTi high entropy alloy filler by electric current field assisted sintering

SiC ceramics were brazed by electric field-assisted sintering technology using CoFeCrNiCuTi high-entropy alloy as joining filler. The effect on the interface microstructure and bend strength of brazed joints at different brazing temperature was systemically studied. The interfacial reaction was controlled by adjusting the brazing temperature. The main components in the brazing seam are high-entropy alloys FCC (HEAF), C, TiC, CrC, and Cr₂₃C₆ phase. Furthermore, the maximum bending strength of 54 MPa was found when brazed at a lower temperature of 1125℃. In addition, due to the electric field-assisted sintering technology and the high-entropy effect of the CoFeCrNiCuTi filler, the diffusion of elements and the formation of solid solution were accelerated. This suggests that the current field was beneficial to improve the inter-diffusion between the CoFeCrNiCuTi filler and SiC ceramics. Consequently, the low-temperature rapid brazing of SiC ceramics was realized, and this technology provides a new filler system for ceramic brazing.     

Reactive air brazing of 3YSZ ceramic to aluminized Crofer22H stainless steel using Ag–CuO fillers

In this work, an effective protected Al₂O₃ layer was fabricated first on the surface of Crofer22H by reactive air aluminization to suppress cathodic poisoning in solid oxide fuel cells. Then, the reactive air brazing of aluminized Crofer22H to 3YSZ ceramics was carried out by using Ag–CuO fillers. The effects of CuO content and brazing temperature on the interfacial microstructure and mechanical properties of the joints were studied in detail. The results indicated that maintaining a continuous Al₂O₃ layer was the prerequisite for achieving a reliable joint. And the interfacial reaction gradually became intense with the increase of CuO content, and the continuity of the Al₂O₃ layer was disrupted as it was increased to 6 mol%. The well-connected joints could be achieved when brazing at 970–1100°C with Ag–2CuO filler. The interfacial microstructure did not obviously change with increasing temperature, and the joint strength was stable at ∼35 MPa. While the interfacial reaction gradually weakened with increasing temperature as brazing with Ag–4CuO filler, the well-connected joints were obtained when brazing above 1000°C, and the maximum shear strength of ∼44.3 MPa was obtained when brazing at 1050°C for 30 min.     

Brazing ZrO2 ceramic and TC4 alloy by novel WB reinforced Ag-Cu composite filler: Microstructure and properties

Residual stresses in ceramic-metal joints is the important factor for their reliable implementation in cutting-edge industries. Composite fillers is reported to be a promising approach to reduce the residual stresses. Until now, few experimental researches on the brazing of ZrO₂ ceramic and TC4 alloy using composite fillers have been reported. In this study, to release the residual stresses and improve the joints strength, novel WB reinforced Ag-Cu composite filler was fabricated to braze ZrO₂ ceramic and TC4 alloy. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) were applied for the analysis of microstructure and phases structure in the joints. The TiB whiskers and W particles were in situ synthesized via the reaction between active Ti and WB particles, and randomly distributed in the brazing seam. The effect of brazing temperature and WB content on interfacial microstructure and mechanical strength in the brazed joints were investigated. When brazed at 870 °C for 10 min, favorable microstructure reinforced by TiB whiskers and W particles in the brazing seam was achieved with 7.5 wt% WB addition in composite filler. The maximum average shear strength of the joints was 83.2 MPa, which about 59.4% increase over the joints without WB addition.     

Brazing YSZ ceramics by a novel SiO2 nanoparticles modified Ag filler

Yttria-stabilized zirconia (YSZ) ceramics are soundly brazed in air using a novel amorphous SiO₂ nanoparticles (NPs) modified Ag filler. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction are employed to characterize the interfacial microstructure of YSZ joints. A uniform amorphous SiO₂ layer is observed on the YSZ substrate surface. Micro-sized SiO₂ clusters are dispersed in the Ag braze matrix, which forms the brazing seam. The content of SiO₂ NPs and the brazing temperature exert significant effects on the formation of YSZ joints. The maximum shear strength of YSZ joints reaches 47 MPa under the optimized brazing conditions (Ag–2SiO₂, 1050 °C/30 min, 2 MPa load pressure).     

A novel high entropy CoFeCrNiCu alloy filler to braze SiC ceramics

In order to reduce intermetallic compound formations in brazed joints, a CoFeCrNiCu high entropy alloy was invented and employed to braze SiC ceramics. Results show that SiC ceramics were tightly and strongly brazed with the CoFeCrNiCu filler. Microstructure, phase and shear strength were systematically studied for joints brazed at different temperature. Main compositions were identified as high-entropy FCC, Cu(s, s), Si(s, s), and Cr₂₃C₆ phases, regardless the brazing temperature differences. After being brazed at 1453 K, the joint reached a maximum shear strength of 60 MPa, much higher than those brazed with conventional AgCuTi filler. Thanks to high entropy effect of CoFeCrNiCu filler, random solid solution turned out in the seam and benefitted joint quality. The successful use of CoFeCrNiCu high entropy alloy as fillers can expand the application range of high entropy alloys and provide a new filler system to braze ceramics.     

Study of ion bombardment of SiC ceramics: Surface and interfacial reaction modification

Ar ion bombardment was conducted to modify the SiC surface microstructures, which had a vital effect on the interfacial microstructure and shear property of brazing joints. The amorphous layer with thickness of ∼120 nm was formed on the bombarded surface, accompanied with plenty of dislocations and twins beneath the amorphous layer. Reliable SiC/AgCu-Ti/SiC joints were brazed in vacuum at 900 °C for 10 min, and the interfacial microstructure was investigated by SEM, EDS and TEM in detail. When the ion bombarded SiC was used as substrates, the microstructure of brazing beam was optimized as SiC / Ti₅Si₃ + TiC mixed layer / Ag(s,s) + Cu(s,s) containing TiCu / Ti₅Si₃ + TiC mixed layer / SiC, in which the interfacial stratification was eliminated compared to the conventional SiC brazing. The shear strength was improved to 30.9 MPa with ion bombardment, which was ∼72.6 % higher than that of the original SiC joints without ion bombardment. The proposed Ar ion bombardment method provides a novel way to modify the brazability of ceramics.     

Microstructure evolution and bonding strength of the Al2O3/Al2O3 interface brazed via Ni-Ti intermetallic phases

In this study, Al₂O₃ workpieces were vacuum brazed by using Ni-45Ti binary alloy. The interfacial microstructure evolution of the joints obtained at different brazing temperatures was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The bonding strength of the joints was measured by shear testing. The results indicated that Ni₂Ti₄O and AlNi₂Ti were the main reaction products in the joint areas. Moreover, the Ti₂Ni intermetallic compound formed in the brazing seam. The typical layer structure of the brazed joints was Al₂O₃/AlNi₂Ti/Ni₂Ti₄O/Ti₂Ni + NiTi/Ni₂Ti₄O/AlNi₂Ti/Al₂O₃. With the brazing temperature increasing, the thickness of the Ni₂Ti₄O reaction layer adjacent to the Al₂O₃ substrate increased significantly, while the AlNi₂Ti phase had a tendency to dissolve with the brazing temperature increasing. The mechanism for the microstructure evolution was also discussed. The maximum shear strength of 125.63±4.87 MPa of the joints was obtained when brazed at 1350 °C for 30min. The fracture occurred hardly in the interface between Al₂O₃ and Ni-45Ti filler alloy.     

Improvement for Ti3SiC2/Cu joint brazed using composite fillers with abnormal expansion ceramic particulates

Reducing the residual stresses and improving the mechanical strength of large-scale ceramic/metal brazing joints is an important problem that must be solved for its practical engineering application. Using composite filler with solid-state phase transformation ceramic particulates, it is theoretically feasible to relieve the residual stress and improve the mechanical properties of ceramic/metal brazed joints. In this study, Cu mesh, Ag–28Cu–2Ti (wt.%), and yttria-stabilized zirconia (0.6 mol.% YSZ solid-state phase transformation ceramic particulates) composite power fillers were used in the brazing of Ti₃SiC₂ ceramic and pure copper. The microstructure of joints and YSZ particulates in the interface was investigated and confirmed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM), and transmission electron microscopy (TEM). In addition, the effect of YSZ particulates content on the mechanical properties of joints was investigated and evaluated by the shear strength. The results show that the interfacial phases were mainly Ti₅Si₃, TiC, Ti_xCu, Ag (s, s), Cu (s, s), and YSZ particulates. Moreover, most of YSZ particulates undergo the solid-state phase transformation from tetragonal zirconia (t-ZrO₂) to monoclinic zirconia (m-ZrO₂) during the cooling process of brazing. The abnormal volume expansion of the solid-state phase transformation reduced the thermal mismatch between Ti₃SiC₂ ceramic and filler, thereby reducing the residual stress in the interface of joint. When using composite filler with 6 wt.% YSZ particulates, the shear strength of Ti₃SiC₂/Cu joint reached the maximum. The maximum average shear strength of the joints was 80.2 MPa, which was about 103.6% more than the joint without YSZ particulates.     

Wetting behavior and brazing of titanium-coated SiC ceramics using Sn0.3Ag0.7Cu filler

By coating active titanium, Sn0.3Ag0.7Cu (SAC) filler wetted SiC effectively, as the contact angle decreased significantly from ~145° to ~10°. Ti₃SiC₂ and TiO_x (x ≤ 1) reaction layers were formed at the droplet/SiC interface, leading to the reduction of contact angle. Reliable brazing of SiC was achieved using titanium deposition at 900°C for 10 minutes, and the typical interfacial microstructure of Ti-coated SiC/SAC was SiC/TiO_x + Ti₃SiC₂/Sn(s,s). Comparing to direct brazing, Ti–Sn compounds in the brazing seam were effectively reduced and the mechanical property of joints was dramatically improved by titanium coating. The optimal average shear strength of SiC joints reached 25.3 MPa using titanium coating- assisted brazing, which was ∼62% higher than that of SiC brazed joints using SAC-Ti filler directly.     

Brazing SiC ceramics and Zr with CoCrFeNiCuSn high entropy alloy

CoCrFeNiCuSn high-entropy alloy and Cu foam composite interlayer was used as a filler for the brazing of SiC ceramics and Zr. The microstructure and mechanical properties of the brazed joint at room temperature and high temperatures as well as the brazing mechanism were systematically investigated. The microstructure is adjusted by controlling the brazing temperature. The main phases in the joint were identified at different brazing temperatures to be a high-entropy alloy phase, α-Zr (s, s), Zr₂Cu, (Zr, Sn) and Zr(Cr, Fe)₂ Laves phase. The joint brazed at 1040 °C for 20 min exhibited a maximum shear strength of 221 MPa at room temperature and an average shear strength of 207 MPa at 600 °C. The room temperature and high temperature-strength obtained here are much higher than those obtained for joints brazed using a conventional filler. Owing to the high-entropy effect, the joint matrix is mainly composed of solid solution phase, which improves the strength and thermal stability of the joint. The existence of the hard Zr(Fe, Cr)₂ Laves phase and the soft α-Zr (s, s) phase in the joint significantly improves its strength and plasticity.     

Joining of sintered silicon carbide using ternary Ag–Cu–Ti active brazing alloy

Sintered silicon carbide was brazed to itself by Ag–35.25 wt%Cu–1.75 wt%Ti filler alloy at 860 °C, 900 °C and 940 °C for 10 min, 30 min and 60 min. Mechanical properties both at room temperature and high temperature were measured by flexural strength. The interfacial microstructure was investigated by electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results indicate that increased brazing temperature heightens the flexural strength and the maximal four-point flexural strength reaches 342 MPa at room temperature. In addition longer holding times result in thicker reaction layer, which increases mismatch of coefficients of thermal expansion (CTE) between SiC substrate and reaction layer and finally leads to poor mechanical properties due to high residual stresses. High temperature flexural strength decreases with an increase of test temperature due to softening of the filler alloy. A reaction layer composed of TiC and Ti₅Si₃ was observed at the interface of SiC/filler alloy and there is a representative microstructure: SiC/continuous fine TiC layer/discontinuous coarse Ti₅Si₃ layer/filler alloy.     

The preparation of SiCP/Ag–Cu–Ti hybrid tapes and application in the joining of silicon carbide ceramics

Abstract

A novel method of tape casting to fabricate ceramic-particulate-reinforced composite filler alloy tapes with low organics (no more than 6?wt.%) was developed, with which SiC_P/Ag–Cu–Ti hybrid tapes were successfully prepared and used in joining of sintered silicon carbide ceramics. The stress rheometer, scanning electron microscopy and energy dispersive spectrometer were used to characterize the rheological properties of slurry and microstructure of green tapes and joints. The slurry for tape casting consists of dispersant, binder, solvent, a mixture of ceramics particulates and metal powders, and no plasticizer was added. Castor oil phosphate was proven the suitable dispersant for the slurry, and the content of dispersant, binder, solid loading was optimized as 3?wt.%, 2.6?wt.% and 26?vol.%, respectively. The fabricated hybrid tapes possess good ductility and uniform thickness. The SiC particulates were homogeneously distributed in the metal powders matrix in both sides of the green tapes, and the distribution was retained in the SiC/SiC joints.     

Microstructural evolution and mechanical properties of TiB2-TiC-SiC ceramics joint brazed using Ti-Ni composite foils

A novel TiB₂-based ultra-high-temperature ceramic containing 60 vol.% TiB₂, 20 vol.% TiC, and 20 vol.% SiC was fabricated by hot pressing and subsequently joined using the brazing technique. Ti-based filler was used as the brazing alloy by taking advantage of the reaction between Ti and TiB₂-TiC-SiC. The effects of the brazing temperature on the microstructure and mechanical properties of the brazed joint were investigated. The results showed that Ti in the filler reacted with the TiB₂-TiC-SiC ceramics and formed a reaction layer I that comprised TiB and TiC. The brazing seam was composed of TiB, TiC, Ti₅Si₃, Ti₂Ni, and TiNi. When the brazing temperature was increased, the reaction between TiB₂-TiC-SiC ceramics and the filler was observed to become vigorous; this led to an increase in the growth of the reaction layer I. Meanwhile, the continuous Ti₂Ni layer in the brazing seam gradually disappeared; it was replaced by TiB and Ti₅Si₃. The room temperature shear strength reached a maximum value of 168 MPa when the joint was brazed at 1040 °C for 30 min; while it was 104 and 81 MPa at test temperature of 600 °C and 800 °C, respectively. In addition, the effects of TiB whiskers on the coefficient of thermal expansion of the brazing seam and fracture of the brazed joint were discussed.     

Joining of SiC ceramics using CaO-Al2O3-SiO2 (CAS) glass ceramics

CaO-Al₂O₃-SiO₂(CAS) glass ceramics were designed and prepared using a melt-quench approach. The coefficient of the thermal expansion (CTE) of the synthesized CAS (4.12 × 10⁻⁶ K⁻¹) matched perfectly with that of the SiC ceramic (4.01 × 10⁻⁶ K⁻¹). Thermal analysis of the CAS was conducted. Then the joining of the SiC ceramics by the CAS glass ceramics under various process parameters were conducted. The bonding temperature affects the fluidity of the CAS glass and the oxidation of the SiC substrate. The holding duration decides the infiltration of the CAS glass into the SiC substrate. The optimal bonding parameter is 1400 ℃/10 min and the corresponding highest shear strength of the SiC/CAS/SiC bonded joints in average was 56 MPa. Fracture observation was also conducted to help analyze the relationship between the interfacial microstructure and the joint strength. Finally, the formation mechanism of the SiC/CAS/SiC bonded joints was proposed.     

Brazing of Al2O3 to Ti-6Al-4V alloy with in situ synthesized TiB-whisker-reinforced active brazing alloy

65.9Cu-24.4Ti-9.7TiB₂ (wt.%) composite filler was used to join Al₂O₃ and Ti-6Al-4V alloy. 30 vol.% TiB whiskers were in situ synthesized as reinforcements in joints. Brazing temperature was 890 °C, 910 °C, 930 °C, 950 °C and 970 °C, and the holding time was 0, 5, 10, 20, and 30 min. The microstructure and mechanical properties of brazed joints were analyzed by scanning electron microscope equipped with energy dispersive spectrometer, shear test and nano-indentation test. Results show that reaction layer Ti₄(Cu,Al)₂O forms at Al₂O₃/brazing alloy interface. The reaction between TiB₂ powders and Ti atoms in brazing alloy brings on in situ synthesizing TiB whiskers in (Ti,Al)₂Cu and AlCu₂Ti intermetallics. Formation of TiB whiskers minimizes the mismatch of thermal expansion coefficient between Al₂O₃ and brazing alloy, and makes the ductile-rigid-ductile multiple layer present in joints, which reduces residual stress of joints. The maximum shear strength of joints can reach 143.3 MPa when the brazing temperature is 930 °C, and holding time is 10 min.     

Interfacial reactions and joining characteristics of a Cu–Pd–V system filler alloy with Cf/SiC composite

A novel composition of Cu–Pd–V filler alloy was designed for the joining of C_f/SiC composite. The filler alloy was fabricated into brazing foils with a thickness of 0.15 mm by a rolling process. The alloy′s wettability on the C_f/SiC composite was studied with the sessile drop method. After heating at 1473 K for 10 min the Cu–Pd–V filler alloy showed a low contact angle of 6° on the composite. A VC_0.75 reaction band was formed at the surface of the C_f/SiC composite under the brazing condition of 1443 K /10 min, and the microstructure in the central part of the joint was composed of (Cu, Pd) solid solution and eutectic-like phase of Pd₂Si+Cu₃Pd. The interfacial reaction mechanism is discussed. The room-temperature three-point bend strength of C_f/SiC–C_f/SiC joints brazed with Cu–Pd–V filler alloy at 1443 K for 10 min is 128 MPa, and the joint strengths at temperatures of 873–1073 K are even higher than the room-temperature strength. The presence of refractory Pd₂Si compounds within the joints should contribute to the stable high-temperature joint strengths.     

Microstructure and joining strength evaluation of SiC/SiC joints brazed with SiCp/Ag–Cu–Ti hybrid tapes

SiC particulates were mixed with Ag–Cu–Ti powders to fabricate SiC_P/Ag–Cu–Ti (SICACT) sheets by tape casting process, which were used to braze the sintered SiC ceramics with the structure of SiC/Ag–Cu–Ti foil/SICACT sheet/Ag–Cu–Ti foil/SiC. Microstructure and joining strength both at room temperature and at high temperature were characterized by electron probe X-ray microanalyzer, electron dispersive spectroscopy, transmission electron microscopy, and flexural strength test. The SiC particulates from the SICACT sheets were randomly distributed in the filler alloy matrix and reacted with Ti from the filler alloy. Reaction products TiC and Ti₅Si₃ were found in the interfacial reaction layer. With the increase in SiC particulates volume fraction, the joining strength at room temperature first increased, and then decreased, which was affected by both CTE mismatch and the thickness of the reaction layer. In addition, the joining strength of joints brazed using SICACT sheets at 600?°C can reach 197 MPa, which was obviously higher than that brazed using Ag–Cu–Ti filler alloy.     

Solid-state joining of SiC using a thin Ti3AlC2, TiC,or Ti filler

SiC monoliths containing 5 wt.% Al₂O₃-Y₂O₃ additive were joined using a thin Ti₃AlC_2, TiC, or Ti filler. After joining at 1900 °C for 5 h under 3.5 MPa, the joint properties were compared in terms of the microstructure, phase evolution, joining strength, and possible elimination of the joining layer. Although all samples showed a sound joint, the microstructure differed according to the filler. SiC joined with Ti₃AlC₂ filler showed an indistinguishable joining interface due to the filler decomposition followed by solid-state diffusion into the SiC base, whereas TiC filler remained at the interface without showing decomposition or diffusion. In contrast, the Ti filler showed a possible elimination of the joining layer because of the diffusion of Ti and the formation of TiC. The mean joining strengths for the Ti₃AlC₂, TiC, and Ti fillers were 300, 234, and 248 MPa, respectively, which were comparable to that of the base SiC material (250 MPa).