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.     

Microstructure and mechanical strength of transient liquid phase bonded Ti3SiC2 joints using Al interlayer

Based on the structure characteristic of Ti₃SiC₂ and the easy formation of Ti₃Si_1−xAl_xC₂ solid solution, a transient liquid phase (TLP) bonding method was used for bonding layered ternary Ti₃SiC₂ ceramic via Al interlayer. Joining was performed at 1100–1500 °C for 120 min under a 5 MPa load in Ar atmosphere. SEM and XRD analyses revealed that Ti₃Si(Al)C₂ solid solution rather than intermetallic compounds formed at the interface. The mechanism of bonding is attributed to aluminum diffusing into the Ti₃SiC₂. The strength of joints was evaluated by three point bending test. The maximum flexural strength reaches a value of 263 ± 16 MPa, which is about 65% of that of Ti₃SiC₂; for the sample prepared under the joining condition of 1500 °C for 120 min under 5 MPa. This flexural strength of the joint is sustained up to 1000 °C.     

Thickness-dependent phase evolution and bonding strength of SiC ceramics joints with active Ti interlayer

A robust solid state diffusion joining technique for SiC ceramics was designed with a thickness-controlled Ti interlayer formed by physical vapor deposition and joined by electric field-assisted sintering technology. The interface reaction and phase revolution process were investigated in terms of the equilibrium phase diagram and the concentration-dependent potential diagram of the Ti-Si-C ternary system. Interestingly, under the same joining conditions (fixed temperature and annealing duration), the thickness of the Ti interlayer determined the concentration and distribution of the Si and C reactants in the resulting joint layer, and the respective diffusion distance of Si and C into the Ti interlayer differentiated dramatically during the short joining process (only 5 min). In the case of a 100 nm Ti coating as an interlayer, the C concentration in the joint layer was saturated quickly, which benefited the formation of a TiC phase and subsequent Ti₃SiC₂ phase. The SiC ceramics were successfully joined at a low temperature of 1000 °C with a flexural strength of 168.2 MPa, which satisfies applications in corrosive environments. When the Ti thickness was increased to 1 μm, Si atoms diffused easily through the diluted Ti-C alloy (a dense TiC phase was not formed), and the Ti₅Si₃ brittle phase formed preferentially. These findings highlight the importance of the diffusion kinetics of the reactants on the final composition in the solid state reaction, particularly in the joining technique for covalent SiC ceramics.     

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.     

Residual thermal stress of SiC/Ti3SiC2/SiC joints calculation and relaxed by postannealing

Residual thermal stresses in SiC/Ti₃SiC₂/SiC joining couples were calculated by Raman spectra and simulated by finite element analysis, and then relaxed successfully by postannealing. The results showed that the thermal residual stress between Ti₃SiC₂ and SiC was about on the order of 1 GPa when cooling from 1300°C to 25°C. The thermal residual stresses can be relaxed by the recovery of structure disorders during postannealing. When the SiC/Ti₃SiC₂/SiC joints postannealed at 900°C, the bending strength reached 156.9 ± 13.5 MPa, which was almost twice of the as‐obtained SiC/Ti₃SiC₂/SiC joints. Furthermore, the failure occurred at the SiC matrix suggested that both the flexural strength of joining layer and interface were higher than the SiC matrix.     

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.     

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.     

Dependence of the microstructure and properties of TiC/Ti3SiC2 composites on extra C addition

TiC/Ti₃SiC₂ composites were synthesized with Ti/Si/C and Al (in which extra C addition ranges from 0 to 25 wt.%) as starting powders by hot-pressed sintering method at 1400 °C under 30 MPa. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the phase composition and the fracture surface. The results reveal that with the increase of extra C addition, the content of Ti₃SiC₂ phase decreases while the content of TiC phase increases. Graphite phase is detected in the samples with extra C addition of 20 wt.% and 25 wt.%. The bending strength decreases from 554.81 MPa to 57.44 MPa due to the decrease of the densification and Ti₃SiC₂ phase content. The electrical conductivity falls from 42,474.52 s/cm to 1524.95 s/cm, resulting from lower Ti₃SiC₂ phase content and higher contact resistance.     

Facile synthesis of Ti3SiC2 powder by high energy ball-milling and vacuum pressureless heat-treating process from Ti–TiC–SiC–Al powder mixtures

In this article, Ti/TiC/SiC/Al powder mixtures with molar ratios of 4:1:2:0.2 were high energy ball-milled, compacted, and heated in vacuum with various schedules, in order to reveal the effects of temperature, soaking time, thickness of the compacts, and carbon content on the purity of the sintered compacts. X-ray diffraction and scanning electron microscopy were employed to investigate the phase purity, particle size and morphology of the synthesized samples. It was found that the Ti₃SiC₂ content nearly reached 100 wt.% on the surface layer of the sintered compacts prepared in the temperature range from 1350 °C to 1400 °C for 1 h. Powder containing 91 wt.% Ti₃SiC₂ was successfully synthesized by heating 6 mm green compacts of 4Ti/1TiC/2SiC/0.2Al at 1380 °C for 1 h in vacuum. The excessive carbon content failed to improve the purity of Ti₃SiC₂ powder. TiC phase was the main impurity in the formation process of Ti₃SiC₂.     

Revealing the strengthening mechanism in Si3N4 ceramic joint by atomic force microscopy coupled with nanoindentation techniques

The composite filler has been widely introduced for joining ceramic. However, the underlying formation and strengthening mechanisms in the joint remain uncertain. In this study, a commercial Ag–Cu–Ti brazing alloy with Mo particles reinforcement has been introduced for joining Si₃N₄ ceramic and the effect of Mo particles on the microstructure and flexural strength of the joint was investigated. Nanoindentation was employed to characterize the mechanical properties for reaction phases in the joints. The modulus and hardness values for Cu–Ti intermetallics and brazing alloy in the joint were first reported, providing a strong evidence to elucidate the strengthening mechanism. In addition, the strength was increased from 200 MPa, with Ag–Cu–Ti alone, to a maximum of 429 MPa while using Ag–Cu–Ti + 5 vol.% Mo composite filler. We are convinced that, for a well-bonded joint, the thermal expansion mismatch between the joined materials and the plastic deformation in the brazing alloy determined the joint strength.     

High temperature properties of the monolithic CVD β-SiC materials joined with a pre-sintered MAX phase Ti3SiC2 interlayer via solid-state diffusion bonding

Monolithic high purity CVD β-SiC materials were successfully joined with a pre-sintered Ti₃SiC₂ foil via solid-state diffusion bonding. The initial bending strength of the joints (∼ 220 MPa) did not deteriorate at 1000 °C in vacuum, and the joints retained ∼ 68 % of their initial strength at 1200 °C. Damage accumulation in the interlayer and some plastic deformation of the large Ti₃SiC₂ grains were found after testing. The activation energy of the creep deformation in the temperature range of 1000 – 1200 °C in vacuum was ∼ 521 kJmol⁻¹. During the creep, the linkage of a significant number of microcracks to form a major crack was observed in the interlayer. The Ti₃SiC₂ interlayer did not decompose up to 1300 °C in vacuum. A mild and well-localized decomposition of Ti₃SiC₂ to TiC_x was found on the top surface of the interlayer after the bending test at 1400 °C in vacuum, while the inner part remained intact.     

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.     

Brazing of silicon nitride ceramic composite to steel using SiC-particle-reinforced active brazing alloy

Silicon carbide particles have been introduced as reinforcements in a commercially available active metal braze filler alloy (Incusil ABA, Wesgo Metals) used for the joining of ceramic-to-metal. The effect of particle reinforcement of the braze filler on the flexural strength of ceramic to metal joints has been investigated at room temperature and at elevated temperatures. An average four point flexural strength of nearly 400 MPa is achieved at room temperature when using Incusil ABA + 30 vol.% SiC (sandwich foil system) compared to 330 MPa with Incusil ABA alone. At a test temperature of 250 °C relaxation of residual stresses in the joints results in an average flexural strength of approximately 520 MPa when using Incusil ABA + 10 vol.% SiC. These values compare with an average room temperature flexural strength of nearly 800 MPa for the ceramic composite. The reaction products of the braze alloy at the joint interface were identified by SEM.     

Fabrication of Ti3SiC2 and Ti4SiC3 MAX phase ceramics through reduction of TiO2 with SiC

Ti₃SiC₂ and Ti₄SiC₃ MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO₂ using SiC as a reductant, followed by hot pressing of the products under 25 MPa of pressure at 1600 °C. It was found that both Ti₃SiC₂ and Ti₄SiC₃ may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti₃SiC₂ and Ti₄SiC₃ under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti₄SiC₃ compared to Ti₃SiC₂. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.     

Microstructure and microindentation of Ti3SiC2 – Titanium filler brazed joints by tungsten inert gas (TIG) process

Herein we study the joining of Ti₃SiC₂ - a MAX phase - with a Ti filler (Ti₃SiC₂/Ti-filler) using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₃SiC₂ comes into contact with the molten Ti - filler during the TIG-brazing operation, it starts decomposing into TiC_x and a Si-rich liquid. Simultaneously, the molten Ti infiltrates into the Ti₃SiC₂ resulting in a 200 µm thick duplex region, comprised of TiC_x and a Ti-rich phase with some dissolved Si. Both Si and C are found in the solidified Ti; the Si source is from the Si-rich liquid, while the presence of C indicates that some of the C diffused into the Ti. Upon cooling, C- containing Ti- rich lamellae form the solidified Ti. Microindentation results of the decomposed Ti₃SiC₂ layer show an increase in hardness and a decrease in elastic modulus relative to T₃SiC₂. Notably, no cracks were observed.     

Improved mechanical properties of Al2O3 ceramic by in-suit generated Ti3SiC2 and TiC via hot pressing sintering

Al₂O₃ multi-phase composites with different volume fractions of SiC varying from 0 vol% to 30.0 vol% were fabricated by vacuum hot pressing sintering at 1600 °C under the pressure of 30 MPa for 2.0 h. The aim of this work was to investigate the effect of SiC content on the morphology and mechanical properties of the Al₂O₃ multi-phase composite. The results show that the addition of SiC and Ti can produce new strengthening and reinforcing phases include Ti₃SiC₂, TiC, Ti₅Si₃, which would hamper the migration of grain boundaries and promote sintering. The mechanical performances could reach the comprehensive optimal values for 20.0 vol% SiC, delamination and transgranular fracture being the major crack propagation energy dissipation mechanisms.     

Effect of excess silicon content on the formation of nano-layered Ti3SiC2 ceramic via infiltration of TiC preforms

The aim of this work was to investigate the effect of silicon content on the formation and morphology of Ti₃SiC₂ based composite via infiltration of porous TiC preforms. The gelcasting process was used for fabrication of preforms. It was found that the infiltrated sample at 1500 °C for 90 min from a mixture of 3TiC/1.5Si containing 92 wt.% Ti₃SiC₂. With the increasing of TiC and SiC impurity phases, Vickers hardness was increased to the maximum value of 12.9 GPa in Ti₃SiC₂–39 wt.%TiC composite. Microscopic observations showed that the Ti₃SiC₂ matrix was composed of columnar, platelike and equiaxial grains with respect to silicon content.     

Effect of brazing parameters on microstructure and mechanical properties of Cf/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy

Reliable brazing of carbon fiber reinforced SiC (C_f/SiC) composite to Nb-1Zr alloy was achieved by adopting a novel Ti45Co45Nb10 (at.%) filler alloy. The effects of brazing temperature (1270–1320 °C) and holding time (5–30 min) on the microstructure and mechanical properties of the joints were investigated. The results show that a continuous reaction layer (Ti,Nb)C was formed at the C_f/SiC/braze interface. A TiCo and Nb(s,s) eutectic structure was observed in the brazing seam, in which some CoNb₄Si phases were distributed. By increasing the brazing temperature or extending the holding time, the reaction layer became thicker and the amount of the CoNb₄Si increased. The optimized average shear strength of 242 MPa was obtained when the joints were brazed at 1280 °C for 10 min. The high temperature shear strength of the joints reached 202 MPa and 135 MPa at 800 °C and 1000 °C, respectively.     

Brazing SiCf/SiC and TZM alloy with AuPdTiCrCu dual active element filler metal

For the joining of SiC_f/SiC and TZM alloy, an AuPdTiCrCu filler metal with dual active element of Ti and Cr was synthesized. The brazing process was carried out under the condition of 1200°C for 10 min. It was shown that fine Cr-based solid solutions were dispersed in the filler metal. When brazing, the element Cr and Ti moved toward the base metal and reacted with them to form into Mo-Cr-Si and Mo-Cr-Ti-Si compounds. These two active elements could promote the metallurgical reaction of the joint. Meanwhile, it was found that some Pd-Si compounds, fragments of (Au, Cu)_ss and soft Au-Ti phases formed in the middle regions of the joint. (Au, Cu)_ss and Au-Ti phases distributed around the Pd-Si compounds. In addition, the joining mechanism was discussed and the mechanical properties of the joints were tested. The results showed that the average four point flexural strength of the joints reached 88.5 MPa at room temperature and 46.8 MPa at 500°C. The fracture characters of the joints were discussed in the last.     

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.