Vacuum brazing Nb and BN-SiO2 ceramic using a composite interlayer with network reinforcement architecture

A novel composite interlayer with a reinforced network was designed using a SiC ceramic with a network structure and Ti-Ni-Nb composite filler foils, to which the Nb and BN-SiO₂ ceramic were successfully brazed under vacuum. For a brazing temperature of 1160 °C and holding time of 10 min, the interfacial microstructure of the Nb/BN-SiO₂ ceramic joint was Nb/(βTi,Nb)-TiNi eutectic structure+(βTi,Nb)₂Ni+SiC+TiC/TiN+Ti₂N+TiB+Ti₅Si₃+TiO/BN-SiO₂ ceramic. In addition, the shear strength and nano-hardness were analyzed to evaluate the effect of the composite interlayer with a network reinforcement architecture on the mechanical properties of the joint. During brazing, the Ti-Ni-Nb filler metal infiltrated and reacted with the SiC to form the network reinforcement architecture, resulting in the residual stress being relieved and the mechanical performance of the joint being significantly improved. A maximum shear strength of 102 MPa was achieved, which was 60 MPa (142%) higher than that of the joint brazed without the network reinforcement architecture. A reduction in the residual stress on the BN-SiO₂ ceramic side from 328 MPa to 210 MPa was observed with the network reinforcement architecture, and the fracture path of the joint changed from the surface of the BN-SiO₂ ceramic to the interfacial reaction zone.     

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 Cf/SiC composite to Ti-6Al-4V with (Ti-Zr-Cu-Ni)+Ti filler based on in-situ alloying concept

In this paper, a novel brazing process based on in-situ alloying concept was carried out to join C_f/SiC composite to TC4 alloy at 940 °C for 20 min. Mixed powders of Ti-Zr-Cu-Ni alloy and pure Ti metal were used as interlayer. In the process, Ti-Zr-Cu-Ni alloy melted and then dissolved pure Ti metal via liquid-solid reactions, achieving in-situ alloying of the interlayer. The interfacial microstructure and formation mechanism of the brazed joints were investigated. The effect of Ti powder content on the microstructure and the mechanical properties of joints were analyzed. The results showed that: the maximum lap-shear strength of the in-situ alloying brazed joints was 283±11 MPa when using (Ti-Zr-Cu-Ni)+40 vol% Ti composite filler, and this value was 79% higher than the mechanical strength when using Ti-Zr-Cu-Ni alone. A reaction layer of (Ti,Zr)C+Ti₅Si₃ formed near C_f/SiC composite side, while a diffusion layer of Ti₂Cu+Ti(s,s) formed near Ti-6Al-4V side. In the interlayer, lots of Ti(s,s) were distributed uniformly and few of Ti-Cu compounds were found, contributing to the plasticity of joints. Adding moderate Ti powder was beneficial for improving the interfacial reaction between C_f/SiC composite and filler material, which affected the lap-shear strength of joints.     

Reactive brazing Cf/SiC to itself and to Mo using the NiPdPtAu-Cr filler alloy

The NiPdPtAu-Cr filler alloy was proposed for joining C_f/SiC composites. The wettability on C_f/SiC composite was studied by the sessile drop method at 1200 °C for 30 min. Under the brazing condition of 1200 °C for 10 min, the C_f/SiC-C_f/SiC joint strength was only 51.7 MPa at room temperature. However, when used a Mo layer, the C_f/SiC-Mo-C_f/SiC joint strength was remarkably increased to 133.2 MPa at room temperature and 149.5 MPa at 900 °C, respectively. At the interface between C_f/SiC and Mo, Mo participated in interfacial reactions, with the formation of Cr₃C₂/Mo₂C reaction layers at the C_f/SiC surface. The improvement in the joint strength should be mainly attributed to the formation of MoNiSi. The C_f/SiC-Mo joint strength was 86.9 MPa at room temperature and 73.7 MPa at 900 °C, respectively. After 10 cycles of thermal shock test at 900 °C the C_f/SiC-Mo joint strength of 71.6 MPa was still maintained.     

Reaction joining of SiC ceramics using TiB2-based composites

SiC ceramics were reaction joined in the temperature range of 1450–1800 °C using TiB₂-based composites starting from four types of joining materials, namely Ti–BN, Ti–B₄C, Ti–BN–Al and Ti–B₄C–Si. XRD analysis and microstructure examination were carried out on SiC joints. It is found that the former two joining materials do not yield good bond for SiC ceramics at temperatures up to 1600 °C. However, Ti–BN–Al system results in the connection of SiC substrates at 1450 °C by the formation of TiB₂–AlN composite. Furthermore, nearly dense SiC joints with crack-free interface have been produced from Ti–BN–Al and Ti–B₄C–Si systems at 1800 °C, i.e. joints TBNA80 and TBCS80, whose average bending strengths are measured to be 65 MPa and 142 MPa, respectively. The joining mechanisms involved are also discussed.     

Diffusion bonding of ZrB2–SiC/Nb with in situ synthesized TiB whiskers array

The ZrB₂–20 vol.% SiC (ZS) composites were diffusion bonded to Nb using pure Ti interlayer. Effects of joining temperature on the microstructure and mechanical properties of the joints were investigated. The results show that Ti reacted with Nb and ZS to form a typical three layers in the joint. An in situ synthesized TiB whiskers array which consisted of two types of TiB was produced in the reaction layer. The formation mechanism of TiB was analyzed. Mutual diffusion between Ti and Nb led to a ductile β-(Ti, Nb) layer on Nb side. Joining temperature influenced the thickness of reaction layers and distribution of TiB seriously. The maximum shear strength reached 158 MPa with bonding temperature at 1200 °C for 60 min.     

Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering

The development of reliable joining technology is of great importance for the full use of SiC. Ti₃SiC₂, which is used as a filler material for SiC joining, can meet the demands of neutron environment applications and can alleviate residual stress during the joining process. In this work, SiC was joined using different powders (Ti₃SiC₂ and 3Ti/1.2Si/2C/0.2Al) as filler materials and spark plasma sintering (SPS). The influence of the joining temperature on the flexural strength of the SiC joints at room temperature and at high temperatures was investigated. Based on X-ray diffraction and scanning electron microscopy analyses, SiC joints with 3Ti/1.2Si/2C/0.2Al powder as the filler material possess high flexural strengths of 133 MPa and 119 MPa at room temperature and at 1200 °C, respectively. The superior flexural strength of the SiC joint at 1200 °C is attributed to the phase transformation of TiO₂ from anatase to rutile.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Interfacial microstructure and mechanical properties of zirconia ceramic and niobium joints vacuum brazed with two Ag-based active filler metals

Reliable brazing of a zirconia ceramic and pure niobium was achieved by using two Ag-based active filler metals, Ag-Cu-Ti and Ag-Cu-Ti+Mo. The effects of brazing temperature, holding time, and Mo content on the interfacial microstructure and mechanical properties of ZrO₂/Nb joints were investigated. Double reaction layers of TiO and Ti₃Cu₃O formed adjacent to the ZrO₂ ceramic, whereas TiCu₄+Ti₂Cu₃+TiCu compounds appeared in the brazing interlayer. With increasing brazing temperature and time, the thickness of the Ti₃Cu₃O layer increased with consumption of the TiO layer, and the total thickness of the reaction layers increased slightly. Meanwhile, the blocky Ti-Cu compounds in the brazing interlayer tended to accumulate and grow. This microstructural evolution and its formation mechanism are discussed. The maximum shear strength was 157 MPa when the joints were brazed with Ag-Cu-Ti at 900 °C for 10 min. The microstructure and bonding properties of the brazed joints were significantly improved when Mo particles were added into the Ag-Cu-Ti. The shear strength reached 310 MPa for joints brazed with 8.0 wt% Mo additive, which was 97% higher than that of joints brazed with single Ag-Cu-Ti filler metal.     

Joining of Cf/SiC composite with a Cu–Au–Pd–V brazing filler and interfacial reactions

A new Cu–Au–Pd–V filler alloy was designed for the joining of C_f/SiC composite. Its wettability on the composite was studied with the sessile drop method. After heating at 1473 K for 10 min the filler alloy showed a low contact angle of 5°. The interfacial reactions under the brazing condition of 1443 K/10 min resulted in the formation of VC_0.75 reaction band at the surface of the composite, and the microstructure in the central part of the joint is composed of Cu(Au, Pd) solid solution and Pd₂Si compound. The average three-point bend strength of the C_f/SiC–C_f/SiC joints at room temperature is 135 MPa. The joints also exhibit stable strengths at high temperatures of 873–1073 K. The presence of refractory Pd₂Si compound within the Cu(Au, Pd) solid solution matrix throughout the joint should contribute to the stable high-temperature property.     

Microstructural and mechanical properties of porous Si3N4 carbon coated ceramic brazed with a cobalt-silicon filler

The Co_22.5Si_77.5 (at.%) braze was used to bond porous Si₃N₄ ceramics. The effects of brazing temperature on microstructure and the bonding strength of the joint were studied. The results reveal that no visible reaction layer was observed. The corresponding joint strength was low. In order to improve the joint strength a carbon coated modification of the porous Si₃N₄ substrate was suggested. The impact of this modification on the joint properties was examined. It was established that a SiC reaction layer with a thickness from ∼15 μm to ∼65 μm was formed at the interface and SiC nanowires were observed when the temperature increased from 1280 °C to 1340 °C. The maximum shear strength of the carbon coated and uncoated joints were 115 MPa and 44 MPa, respectively. The significant improvement of the joint strength was attributed to the SiC reaction layer and a strengthening by the presence of SiC nanowires. .     

Interfacial microstructure and mechanical properties of porous-Si3N4 ceramic and TiAl alloy joints vacuum brazed with AgCu filler

Porous Si₃N₄ ceramic was firstly joined to TiAl alloy using an AgCu filler alloy. The effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of porous-Si₃N₄/AgCu/TiAl joints were studied. The typical interfacial microstructure of joints brazed at 880 °C for 15 min was TiAl/AlCu₂Ti/Ag-Cu eutectic/penetration layer (Ti₅Si₃+TiN, Si₃N₄, Ag (s, s), Cu (s, s))/porous-Si₃N₄. The penetration layer was formed firstly in the brazing process. With increasing brazing temperature and time, the thickness of the penetration layer increased. A large amount of element Ti was consumed in the penetration layer which suppressed the formation and growth of other intermetallic compounds. The penetration layer led the fracture to propagate in the porous Si₃N₄ ceramic substrate. The maximum shear strength was ~13.56 MPa.     

Investigation of brazing of Ba(Zn0.33Ta0.67)O3 ceramic with Ti6Al4V alloy

Feasibility of brazing Barium Zinc Tantalate (BZT) ceramics with Ti6Al4V alloy using active and non-active brazing alloy was investigated in the present study for high power RF window application. BZT ceramics were brazed successfully with Ti6Al4V alloy under high vacuum with active brazing alloy (Ticusil® (4.5%Ti) and Cusil-ABA® (1.5%Ti) at 850 °C and non-active brazing alloy (BAg8) at 830 °C. Micrographs at the interface of non-active brazed joints are free of interfacial micro-defects, and exhibit excellent physical contact and good metallurgical bonding with high diffusivity when compared to active-brazed joints. Ceramic-filler interfacial layer thickness is high for samples containing non-active filler (~25 µm) compared to the samples containing active filler materials (~9 µm and ~5 µm). The average shear strength of BZT ceramics brazed with non-active brazing alloy BAg8 is higher (~45.5 MPa) in comparison with active brazing alloys (~33 MPa and ~31 MPa). Fracture surface reveals that the failure is on ceramics side only indicating that the strength of non-active brazed joint is more compared to ceramics.     

Effects of polycarbosilane infiltration processes on the microstructure and mechanical properties of 3D-Cf/SiC composites

Three-dimensional braided carbon fiber-reinforced silicon carbide (3D-C_f/SiC) composites were prepared through eight cycles of infiltration of polycarbosilane (PCS)/divinylbenzene (DVB) and subsequent pyrolysis under an inert atmosphere. The effects of infiltration processes on the microstructure and mechanical properties of the C_f/SiC composites were investigated. The results showed that increasing temperature could reduce the viscosity of the PCS/DVB solution, which was propitious to the infiltration processes. The density and flexural strength of 3D-C_f/SiC composites fabricated with vacuum infiltration were 1.794 g cm⁻³ and 557 MPa, respectively. Compared to vacuum infiltration, heating and pressure infiltration could improve the infiltration efficiency so that the composites exhibited higher density and flexural strength, i.e., 1.944 g cm⁻³ and 662 MPa. When tested at 1650 °C and 1800 °C in vacuum, the flexural strength reached 647 MPa and 602 MPa, respectively.     

Interfacial microstructure and mechanical property of ZrC-SiC ceramic and Ti6Al4V joint brazed with AgCuTi alloy

ZrC-SiC ceramic and TC4 alloy were brazed using AgCuTi alloy. The microstructure and mechanical property of the joints obtained at different brazing parameters were investigated and the reaction mechanism was analyzed. The results indicated that the Ti from the AgCuTi and TC4 reacted with the ZrC in the ceramic to form different shaped TiC crystals adjacent to the ZrC-SiC ceramic. With the increase of brazing temperature or extending of holding time, the dissolution of TC4 became vigorous and much Ti dissolved into the braze alloy. As a result, Ti reacted with the Cu from AgCuTi alloy to form a series of Cu-Ti compounds in the brazing seam due to the strong affinity between Cu and Ti. The Cu-Ti compounds made the hardness and brittleness of brazing seam increase, which deteriorated the property of the brazed joint. The maximum shear strength was 39 MPa obtained at 810 °C for 5 min.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Joining of SiC ceramics via a novel liquid preceramic polymer (V-PMS)

A novel liquid preceramic polymer (V-PMS) was synthsized by modifying polymethylsilane (PMS) with 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane ([CH₃(CH₂CH)SiO]₄, D4Vi), for joining SiC ceramics under ambient pressure. The obtained V-PMS with a viscosity of 125 Pas at room temperature exhibits excellent thermal properties and bonding strength. The ceramic yield of V-PMS treated at 1200 °C under Ar atmosphere is 84.5%, which is 38.3% higher than the original PMS. The shear strengths of the SiC joints joined by V-PMS at 800 °C, 1000 °C and 1200 °C under N₂ atmosphere are 11.9 MPa, 34.5 MPa and 29.9 MPa, respectively. The excellent performances make the obtained V-PMS promising candidates for joining SiC ceramics in high-temperature applications.     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Microstructural evolution during the brazing of Al2O3 ceramic to kovar alloy by sputtering Ti/Mo films on the ceramic surface

Alumina (Al₂O₃) ceramics were metallized by magnetron sputtering Ti/Mo bilayer films on the surface with a subsequent high temperature sintering and were brazed to Kovar alloy (Fe-Ni-Co) using Ag-Cu eutectic alloy. The Ti/Mo metallization film and the brazing seam microstructures were investigated and the correlation between the Al₂O₃/Kovar joining strength and the microstructures of the brazing seam was discussed. The results show that the joining strength is related to the thickness of the Ti/Mo adhension layer which depends on the holding time during brazing. The mutual diffusion of the elements at the interface firstly increases the thickness of the adhension layer as the holding time increases and the Mo film acts as a barrier layer to block the diffusion of Ti atoms into the seam. The optimal brazing joining strength of 72.6±5.0 MPa could be achieved at a brazing temperature of 810 °C for 14 min However, if the holding time is further prolonged, Mo atoms will diffuse into the (Ni, Cu) solid solution, resulting in the diffusion of Ti atoms and the adhension layer becoming indistinguishable. Therefore, the intermetallic Ni₃Ti forms in the seam and the titanium oxide changes from TiO to Ti₂O₃ or Ti₃O₅, which leads to the joint strength decreasing.     

Microstructure and mechanical properties of the C/C composite and Nb joints brazed with an active Ag-based filler metal

An active Ag-based filler metal, containing trace alloy elements of Al, Cu and Ti, was successfully applied to braze C/C composite and Nb. The microstructure and formation mechanism of the C/C composite and Nb brazed joint were investigated in this study. Moreover, the influence of brazing parameters on microstructural evolution and mechanical properties of brazed joints was evaluated. The typical interfacial microstructure of the joint obtained at 950 °C for 600 s was C/C/TiC + AlCu₂Ti/Ag(s, s) + AlCu₂Ti + particle Cu/AlCu₂Ti + AlCuTi + (Ti, Nb)₃Al + Nb(Ti)/Nb. The dispersive AlCu₂Ti phase was uniformly distributed in the Ag matrix, which was a beneficial structure for the brazed joint. The shear strength of the brazed joint was sensitive to the brazing temperature and holding time, which was closely related to the TiC layer bordering the C/C composite. The thickness of the TiC layer first increased as temperature increased to 950 °C, and then decreased when temperature reached 970 °C. The carbon fiber eroded by the filler at 970 °C entered to the brazing seam and reacted with Ti, resulting the reduction of the thickness of TiC, thus damaging the strength of the joint. With extension of holding time from 300 s to 1200 s, the interface reaction became more sufficient. Therefore, the thickness of the TiC layer increased. However, at 1200 s, the over-thick TiC broke the C/C substrate because of the mismatch of coefficient of thermal expansions. The maximum shear strength of the joint (950 °C/600 s) reached 55 MPa at room temperature and 35 MPa tested at 550 °C.