Low-temperature joining of SiC ceramics by NITE phase using CaO-Al2O3-MgO-SiO2 glass as an additive combined with surface oxidation

Low-temperature joining of SiC ceramics using raw CaO-Al₂O₃-MgO-SiO₂ mixed powders (CAMS-P) and CaO-Al₂O₃-MgO-SiO₂ glass (CAMS-G) as additives for the liquid-phase sintering of SiC nanopowders (NITE phase) in combination with surface oxidation was studied. Using CAMS-P as an additive for SiC joining required 1650 °C and produced a lower shear strength of 77.2 ± 3.5 MPa, compared to untreated SiC (108.2 ± 15.8 MPa). Using CAMS-G as an additive, the compactness of SiC joints obtained at 1550 °C was higher than that of NITE joints obtained at 1650 °C with CAMS-P as an additive. Nonetheless, the low joining temperature resulted in gaps and delaminations at the interface, and the joints strength was only 48.3 ± 5.6 MPa. Incorporating CAMS-G as an additive and surface oxidation achieved reliable NITE phase joining at 1550 °C, with no observed gaps or delaminations at the interface. The resulting joints strength was as high as 100.0 ± 0.8 MPa. The joining mechanism was discussed and compared, involving microstructural analysis.     

Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Microstructure and mechanical properties of hot pressed Al2O3/SiC nanocomposites

Al₂O₃/SiC micro/nano composites containing different volume fractions (5, 10, 15, and 20 vol.%) of SiC were prepared by mixing a sub-micron alumina powder with respective amounts of either micro- or nano-sized silicon carbide powders. The powder mixtures were hot pressed 1 h at 1740 °C and 30 MPa in the atmosphere of Ar. The effect of SiC addition on the microstructure and mechanical properties, i.e. hardness, fracture toughness, and room temperature flexural strength were investigated. The flexural strength increased with increasing volume fraction of silicon carbide particles. The maximum flexural strength (655 ± 90 MPa) was achieved for the composite containing 20 vol.% of coarse-grained SiC, which is more than twice as high as in the Al₂O₃ reference. Hardness and fracture toughness were also moderately improved. The observed improvement of mechanical properties is mainly attributed to alumina matrix grain refinement and grain boundary reinforcement.     

Pressureless joining of SiC ceramics with magnesia-alumina-silica glass ceramics

Liquid phase sintered SiC ceramics were joined using magnesia-alumina-silica (MAS) glass-ceramic fillers without applied pressure. Four different filler compositions with 9.3–25.2 wt.% MgO, 20.7–33.6 wt.% Al₂O₃, and 49.2–68.1 wt.% SiO₂ were studied. The effects of filler composition and joining temperature (1450–1600°C) on the joint strength were investigated. All compositions exhibited an optimum joining temperature at which the maximum joint strength was obtained. A low joining temperature resulted in poor wetting of the SiC substrate due to the high viscosity of the filler. Whereas a high joining temperature caused dewetting and large unfilled sections in the interlayer due to the deleterious interfacial reactions. The joint strength was inversely proportional to the interlayer thickness, which was a function of filler composition and joining temperature. The SiC ceramic joined at 1525°C with MgO-25 wt.% Al₂O₃-60 wt.% SiO₂ filler exhibited a four-point bending strength of 286 ± 40 MPa.     

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.     

High strength and high light transmittance sapphire/sapphire joints bonded using a La2O3-Al2O3-SiO2 glass filler

A novel La₂O₃-Al₂O₃-SiO₂ (LAS) glass was used as filler to join transparent sapphire for obtaining high strength and high light transmittance joints. The results show that the LAS glass filler had compatible coefficient of thermal expansion (CTE) with sapphire and excellent wetting ability on sapphire. During the joining process, no interfacial reaction occurred and the brazing seams were in a completely amorphous state under fast cooling conditions (~50 °C/min). With increased joining temperature, the mutual dissolution and diffusion between sapphire and the LAS filler were enhanced. The flexural strength of joints first increased and then decreased with an increase in the joining temperature from 1400 °C to 1550 °C. The optimal flexural strength of joint reached 325 MPa, which almost was the same as the strength of sapphire substrate. At 500 nm, the in-line transmittance of this joint was 80.5%, which was close to that of sapphire (84.2%).     

Nano-infiltration and transient eutectic (NITE) joining of SiC ceramics applied for the harsh environments

To expand the application of SiC/SiC joints under extreme conditions, Nano-Infiltration and Transient Eutectic (NITE) joining technology with AlN-Y₂O₃ as a sintering additive was successfully developed. The rheological properties of the slurry and the microstructure evolution of the joints were systematically characterized by rheometer, SEM, EDS, EBSD, and TEM, respectively. Both room-temperature and high-temperature flexural strength was measured to evaluate the mechanical properties of the joints. An immersion test with concentrated nitric acids was performed to evaluate the corrosion resistance of the joints. The defect-free joining layer was composed of a dense α-SiC phase, a small amount of YAG(Y₃Al₅O₁₂) distributed in the triangular grain boundary, and a Y-Al-O glass phase from AlN-Y₂O₃. The mechanism of NITE joining could be attributed to the incoherent growth of the newly generated α-SiC in the joining layer along the α-SiC substrate. The maximum room-temperature strength of the joints was 320.5 ± 37.6 MPa. When the test temperatures were 1000 °C, 1200 °C, and 1400 °C, the flexural strength reached 238.7 ± 33.1 MPa, 215.5 ± 52.5 MPa, and 166.9 ± 52.0 MPa, respectively. After immersing the joints in a concentrated HNO₃ for 168 h, the flexural strength was 173.3 ± 12.6 MPa. The joints' excellent mechanical properties and corrosion resistance reveal great application potential under extreme conditions.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al₂O₃–Y₂O₃–SiO₂, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10^?12 m² to 1.26 × 10^?12 m², depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10^?12 m² because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 10⁸ Ω-cm to 1.4 × 10⁹ Ω-cm and from 1.3 × 10⁸ Ω-cm to 1.7 × 10⁹ Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y₂Si₂O₇, YAG, and residual Al₂O₃.     

Fast seamless joining of SiCw/Ti3SiC2 composite using electric field-assisted sintering technique

A pair of Ti₃SiC₂ reinforced with SiC whiskers (SiC_w/Ti₃SiC₂) composites was successfully joined without any joining materials using electric field-assisted sintering technology at a temperature as low as 1090°C (T_i) and a short time of 30 s. The microstructure and mechanical properties of the obtained SiC_w/Ti₃SiC₂ joints were investigated. The solid-state diffusion was the main joining mechanism, which was facilitated by a relatively high current density (~586 A/cm²) at the joining interface. The shear strength of the sample joined at 1090°C was 51.8 ± 2.9 MPa. The sample joined at 1090°C failed in the matrix rather than at the interface, which confirmed that a sound inter-diffusion bonding was obtained. A rapid and high efficient self-joining process may find application in the case of SiC_w/Ti₃SiC₂ sealing cladding tube and end cap.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

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.     

Ultrafast low-temperature near-seamless joining of Cf/SiC using a sacrificial Pr3Si2C2 filler via electric current field-assisted sintering technique

A novel layered structure material, Pr₃Si₂C₂, was synthesized at a low temperature of 850 °C using a molten salt approach for the first time, and subsequently used as the joining filler for carbon fibers reinforced SiC composites (C_f/SiC). A robust near-seamless C_f/SiC joint was successfully obtained at 1509 °C (T_i) for 30 s, while an ultrafast heating rate of 6000 °C/min was applied via electric field-assisted sintering technology. The near-seamless joining process was attributed to the newly precipitated SiC grains, which were densified well with the C_f/SiC matrix by liquid-assisted sintering. The liquid phase was in-situ formed by the eutectic reaction between Pr₃Si₂C₂ and SiC. The shear strength of the near-seamless joint obtained at 1509 °C for 30 s was 17.6 ± 3.0 MPa. The failure occurred in the C_f/SiC matrix. The formation of near-seamless C_f/SiC joints dismisses the issues related to thermal mismatch between C_f/SiC matrices and traditional joining fillers.     

Joining of SiC ceramics by combining NITE-SiC interlayer and its thickness control

Combined with the thickness control of interlayers (∼10 µm and ∼60 µm), SiC nano-powders with Al₂O₃-Y₂O₃-MgO-CaO additive (NITE-SiC) were used as the joining materials to achieve the low-temperature joining of SiC ceramics. Some residual pores were observed in the interlayer with a thickness of ∼60 µm after joining at 1650 ℃, and the shear strength of SiC joints measured was 39.3 ± 5.5 MPa. Observations showed the fracture of joints occurred at the interlayer. When the thickness of the interlayer decreased to ∼10 µm, no cracks or porosity were observed at the interface region after joining at 1650 ℃. The shear strength of SiC joints increased to 69.5 ± 8.9 MPa, and the fracture originated from the matrix. The results demonstrated using NITE-SiC with Al₂O₃-Y₂O₃-MgO-CaO additive as the joining layer and reducing the thickness of the interlayer could lower the joining temperature and significantly improve the mechanical strength of joints.     

Y2O3–Al2O3–SiO2-based glass-ceramic fillers for the laser-supported joining of SiC

Four glass-ceramic filler compositions within the Y₂O₃–Al₂O₃–SiO₂ system were tested for their suitability in laser-supported joining of SiC materials. The compositions differed in terms of their primary crystallization behavior. Joint zone microstructures were investigated and joint tightness was determined using helium leak rate measurements after joining and subsequent annealing at 900 °C and 1050 °C.Yttria- and silica-rich compositions showed a partial crystallization of yttrium silicates during the short laser processing. Subsequent heat treatment effected further crystallization toward equilibrium conditions. Despite the strong change in the degree of crystallization no reduction of the tightness was observed for the best compositions; after 500 h annealing at 1050 °C tightness values of less than 10⁻⁸ mbar l s⁻¹ were measured. These results demonstrate the potential of the investigated filler system for high temperature stable hermetic sealing. At the same time the creation of homogeneously structured joints from glass-ceramic fillers using a laser-supported technology needs the understanding of the crystallization kinetics.     

High-strength SiC joints fabricated at a low-temperature of 1400°C using a novel low activation filler of Praseodymium

High-strength SiC joints were successfully obtained by electric current field-assisted sintering technique at a low temperature of 1400°C using a Pr coating (100 nm) as the initial joining filler. A Pr₃Si₂C₂ transient phase was formed in situ by the interfacial reaction, while the eutectic reaction between Pr₃Si₂C₂ and SiC at ∼1150°C resulted in the formation of a liquid phase. The liquid phase promoted the atomic diffusion at the interface and improved consolidation of the newly precipitated nano-sized SiC with the SiC matrix. This led to the formation of partially seamless joints of SiC. When the thickness of the joining layer decreased from 1 to 100 nm, the content of the residual Pr-O phase at the interface decreased, while the bending strength of the joints increased. A sound SiC joint with a bending strength of 227 ± 12 MPa was obtained at such a low temperature as 1400°C when a 100 nm Pr coating was applied.     

The effects of preparation temperature on the SiCf/SiC 3D4d woven composite

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites have promising applications in aero-engine due to their unique advantages, such as low density, high modulus and strength, outstanding high temperature resistance and oxidation resistance. As SiC fibers are main reinforcements in SiC_f/SiC composites, the crystallization rate and initial damage degree of SiC fibers are seriously influenced by preparation temperatures of SiC_f/SiC composites, namely mechanical properties of SiC fibers and SiC_f/SiC composites are influenced by preparation temperatures. In this paper, KD-II SiC fibers were woven into 3D4d preforms and SiC matrix was fabricated by PIP process at 1100 °C, 1200 °C, 1400 °C and 1600 °C. Digital image correlation (DIC) method was adopted to measure the uniaxial tensile properties of these SiC_f/SiC composites. In addition, finite element method (FEM) based on representative volume element (RVE) was adopted to predict the mechanical properties of SiC_f/SiC composites. The good agreements between numerical results and experimental results of uniaxial tensile tests verified the validity of the RVE. In last, the transverse tensile, transverse shear, uniaxial shear properties were predicted by this method. The predicted results illustrated that axial tensile, transverse tensile and axial shear properties were greatly influenced by the preparation temperatures of SiC_f/SiC composites while transverse shear properties were not significantly various. And the mechanical properties of SiC_f/SiC composites peaked at 1200 °C among these four temperatures while their values reached their lowest points at 1600 °C because of thermal damage and brittle failure of SiC_f/SiC composites.     

Joining of SiC,alumina, and mullite by the Refractory Metal—Wrap pressure-less process

The aim of this work was to discuss the suitability of the joining process called “RM-Wrap” (RM = Refractory Metals, ie, Mo, Nb, Ta, Zr) as a pressure-less and tailorable technique to join several different ceramics such as SiC, alumina, and mullite (3Al₂O₃.2SiO₂). In the RM-Wrap joining technique the refractory metal foil is used as a wrap containing one or more silicon foils. It is performed at 1450°C, under flowing argon, and the resulting joining materials are in situ formed composites made of refractory metal disilicides (MoSi₂, NbSi₂, TaSi₂, or ZrSi₂) embedded in a silicon-rich matrix; their coefficient of thermal expansion has been calculated and the Laser Flash Method was used to measure the thermal diffusivity of one of them (MoSi₂/Si) in 25°C-1000°C range, then to calculate its thermal conductivity. All the obtained joints are uniform, continuous, and crack free. Some preliminary oxidation tests were carried out on all joints at 1100°C, 6 hours in air, giving unchanged morphology of the interface and the joining materials itself; the joint strength of RM-Wrap joined SiC was measured at room temperature using three different mechanical tests: (a) single lap (SL), (b) single lap off-set (SLO) and (c) torsion on hourglass-shaped samples (THG) (on Mo-wrap joined SiC).