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.     

Development of SiC–SiC joint by reaction bonding method using SiC/C tapes as the interlayer

SiC/C tapes with different compositions and thicknesses were used to join pressureless sintered silicon carbide ceramics by reaction bonding method. The microstructure of the joints and the influences of joint thickness and residual silicon content in joint layer on the 4-point flexural strength of as joined SiC ceramics have been investigated. Specimens with high flexural strength can be achieved through the control of the composition and the thickness of the joint layer. The highest flexural strength of the joined specimens with the joint thickness of 13 μm can reach 346 ± 35 MPa and 439 ± 31 MPa at room temperature and 1250 °C, respectively. The microstructure development and the reaction bonding mechanism were also studied.     

Microstructure evolution of SiC/SiC joints during ultrasonic-assisted air bonding using a Sn–Zn–Al alloy

Direct soldering of SiC ceramic in air at 230 °C was achieved using a Sn–9Zn–2Al alloy assisted by ultrasonic wave within seconds. Experimental results indicated that a sound metallurgical bond was formed between the SiC ceramic and Sn–9Zn–2Al alloys. The dependence of interfacial microstructure evolution on ultrasonic action duration time was investigated. Two types of interfacial structures at the interface were observed as the ultrasonic action duration time increased. An amorphous SiO₂ layer was identified at the interface for ultrasonic exposures of 1 s, which was the oxide layer formed on the SiC ceramic surface during heating. A layer of amorphous alumina with a thickness of ~ 6.8 nm formed at the interface under ultrasonic action for over 4 s. The shear strength of joints could reach up to 44 MPa. The formation of the alumina layer at the interface was attributed to the redox reaction of Al from the filler metal and SiO₂ on the SiC ceramic surface under the action of ultrasonic waves. The rapid interfacial reaction was principally induced by the acoustic cavitation and streaming effects at the liquid/solid interface.     

Effects of Al addition on the phase transformation and interfacial evolution in multilayer Ti-B4C composite

The fully-dense multilayer Ti-B₄C composite doped with 6 wt% Al was fabricated via tape-casting and hot-pressing sintering at 1800 °C and under a uniaxial pressure of 30 MPa for 60 min. The effects of Al addition on the phase composition, interfacial microstructure and fracture toughness of the laminate composite were investigated. Based on the results of WDS and EDS, Al addition was proved to be effective on accelerating atom diffusion between Ti and B₄C due to the melting pool around interface where liquid Al enriched, besides, it helps to transform the interfacial bonding method of physical to metallurgical. Finally, the improvement on toughness of Al doped composite can be attributed to the strong metallurgical bonding and hybrid fracture mode of interface. Our study may provide a potential method for producing high strength and toughness multilayer metal/ceramic composites.     

Rapid diffusion bonding of WC-Co cemented carbide to 40Cr steel with Ni interlayer: Effect of surface roughness and interlayer thickness

WC-Co cemented carbides were rapidly diffusion bonded to 40Cr steels with pure Ni as interlayers by utilizing plasma activated sintering (PAS). The bonding was carried out at 750 °C for 13 min under a pressure of 40 MPa. It was found that the roughness of the initial surfaces still plays an important effect on the microstructure and mechanical behavior of the joints diffusion bonded by PAS irrespective of the electric current applied during bonding. The adoption of smoother original surfaces was significantly favorable to eliminate the interfacial interstices and microvoids. Correspondingly, the shear strength of the diffusion bonded joints increased with decreasing surface roughness. Additionally, the effect of interlayer thickness on the shear strength of the joints was also evaluated, and the results showed that the strength decreased sharply when thicker interlayer was employed. A maximum value of shear strength, 293.07 MPa, was obtained when the original surfaces was ground with P1200 grit SiC paper and at the same time 50 µm thick interlayer was used. In this case, the fracture initiated and run predominantly along the bonding interfaces instead of in the WC-Co substrate.     

Characterization of carbon/carbon composite/Ti6Al4V joints brazed with graphene nanosheets strengthened AgCuTi filler

Carbon/carbon (C/C) composite and Ti6Al4V alloy (wt%) were successfully brazed with graphene nanosheets strengthened AgCuTi filler (AgCuTiG). Graphene nanosheets (GNSs) with low CTE and high strength were dispersed into AgCuTi filler by ball milling. The interfacial microstructure was systematically characterized by varieties of analytical means including transmission electron microscopy (TEM). Results show that typical interfacial microstructure of the joint brazed at 880 °C for 10 min is a layer structure consisting of (Ti6Al4V/diffusion layer/Ti2Cu + TiCu + Ti3Cu4 + TiCu4/GNSs + TiCu + TiC + Ag(s,s) + Cu(s,s)/TiC/C/C composite). The interfacial microstructure and mechanical properties of brazed joints changed significantly as temperature increased. High temperature promoted the growth of TiCu and TiC phases, which were attached to GNSs. Meanwhile, the diffusion layer and primary reaction layers thickened as temperature increased, while the thickness of brazing seam decreased. The maximum shear strength of 30.2 MPa was obtained for the joint brazed at 900 °C for 10 min. GNSs decreased the thickness of brittle reaction layers and promoted the formation of TiCu and TiC phases in brazing seam, which caused the strengthening effect and decreased the CTE mismatch of brazed joints. The fracture modes are also discussed in this paper.     

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. .     

Effect of SiC nanowires on the thermal shock resistance of joint between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics

In order to improve the bonding property of joint between SiC modified carbon/carbon (C/C) composites and Li₂O–Al₂O₃–SiO₂ (LAS) glass ceramics, SiC nanowires were attempted as the reinforcement materials in the interface region of SiC transition layer and Li₂O–MgO–Al₂O₃–SiO₂ (LMAS) gradient joining interlayer. The C/C–LAS joint with SiC nanowire-reinforced interface layer was prepared by a three-step technique of pack cementation, in situ reaction and hot-pressing. The microstructure and thermal shock resistance of the as-prepared joints were examined. The average shear strength of the joined samples with SiC nanowires increased from 24.9 MPa to 31.6 MPa after 40 thermal cycles between 1000 °C and room temperature, while that of the joined samples without SiC nanowires dropped from 21.4 MPa to 8.3 MPa. The increase of thermal shock resistance of the C/C–LAS joints was mainly attributed to the toughening mechanism of SiC nanowires by pullout, bridging and crack deflection.     

Microstructural evolution and characterization of interfacial phases in diffusion-bonded SiC/Ta–5W/SiC joints

The evolution of reaction phases formed in diffusion-bonded SiC/Ta–5W/SiC joints was investigated at a joining temperature of 1500–1700 °C for 10–90 min. The effects of bonding temperature and holding time on the phase evolution were found to be directly correlated with the thickness of the interfacial reaction layer when a 100-μm-thick Ta–5W interlayer was used for joining. In the case of a ~7-μm-thick reaction layer, the interfacial phase constitution consisted of a layered SiC/(Ta,W)C/(Ta,W)₅Si₃/(Ta,W)₂Si/(Ta,W)_xSi_y/Ta–5W structure. In the reaction layer with a thickness of ~11–26 μm, the interfacial structure evolved into SiC/(Ta,W)C/(Ta,W)₅Si₃/(Ta,W)C/(Ta,W)_xSi_y/(Ta,W)₂Si/(Ta,W)_xSi_y/Ta–5W, in which an additional (Ta,W)C/(Ta,W)_xSi_y layer was inserted between (Ta,W)₅Si₃ and (Ta,W)_xSi_y owing to the precipitation of carbon from the (Ta,W)₅Si₃ layer. When the Ta–5W interlayer was fully consumed to form a stable reaction product, namely the equilibrium state, (Ta,W)_xSi_y and (Ta,W)₂Si were eventually transformed into (Ta,W)₅Si₃, and the final interface structure that was obtained was SiC/(Ta,W)C/(Ta,W)₅Si₃/(Ta,W)C/(Ta,W)₅Si₃/(Ta,W)C. This achievement will benefit the design, control, and characterization of SiC/metal interfaces.     

Microstructure and mechanical property of SiC whisker coating modified C/C composite/Ti2AlNb alloy dissimilar joints prepared by transient liquid phase diffusion bonding

SiC whisker coating was prepared on the surface of C/C composite successfully by CVD, and transient liquid phase (TLP) diffusion bonding was employed to realize the joining of SiC whisker coating modified C/C composite and Ti₂AlNb alloy using Ti–Ni–Nb foils as interlayer. The microstructure, shear strength and fracture behavior were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD) and universal testing machine. The results show that SiC has good compatibility with C/C composite, and gradient interface formed between SiC-modified C/C composite and Ti₂AlNb alloy. When the bonding experiment was carried out under bonding temperature of 1040 °C and holding time of 30min with 5 MPa pressure in vacuum, the joints formed well and no obvious defects can be observed. The typical microstructure of joints is C/C composite/SiC + TiC/Ti–Ni compounds + Ti–Ni–Nb solid solutions/residual Nb/diffusion reaction layer/Ti₂AlNb alloy. With the increasing of bonding temperature, the thickness of joining area increased due to sufficient element diffusion. However, when bonding temperature is elevated to 1060 °C, some defects such as cracks and slag inclusions exist in the interface layer between interlayer and Ti₂AlNb. The joints with maximum average shear strength of 32.06 MPa are bonded at 1040 °C for 30min. C, SiC and TiC can be found on the fracture surface of joints bonded at 1040 °C which indicated that fracture occurred at the interface layer adjacent SiC layer.     

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

Fabrication and characterization of cordierite-bonded porous SiC ceramics

A reaction bonding technique was used for the preparation of cordierite-bonded porous SiC ceramics in air from α-SiC, α-Al₂O₃ and MgO, using graphite as the pore-forming agent. Graphite was burned out to produce pores and the surface of SiC was oxidized to SiO₂ at high temperature. With further increasing the temperature, SiO₂ reacted with α-Al₂O₃ and MgO to form cordierite. SiC particles were bonded by the cordierite and oxidation-derived SiO₂. The reaction bonding characteristics, phase composition, open porosity, pore size distribution and mechanical strength as well as microstructure of porous SiC ceramics were investigated. The pore size and porosity were strongly dependent, respectively, on graphite particle size and volume fraction. The porous SiC ceramics sintered at 1350 °C for 2 h exhibited excellent combination properties, the flexural strength of 26.0 MPa was achieved at an open porosity of 44.51%.     

Microstructure and mechanical properties of the spark plasma sintered TaC/SiC composites: Effects of sintering temperatures

TaC/SiC composites with 20 vol.% SiC addition were densified by spark plasma sintering at 1600–1900 °C for 5 min under 40 MPa. Effects of sintering temperatures on the densification, microstructures and mechanical properties of composites were investigated. The results showed the materials achieved >98% of theoretical density at a temperature as low as 1600 °C. While the TaC grains grew slightly with the sintering temperature increasing, the SiC particles in materials decreased in size. Equiaxed to elongated grain morphology transformation was observed in the SiC phase in the 1900 °C material to obtain a higher flexural strength and fracture toughness of 715 MPa and 6.7 MPa m^1/2, respectively. Lattice enlargement of the TaC phase in the 1900 °C material suggested possible Si diffusion into TaC grains. Ta was also detected in SiC grains by energy dispersive spectroscopy. Glassy pockets present at multi-grain junctions explained the enhanced densification.     

Direct thermal diffusion bonding of Nd:YAG/YAG composite transparent ceramics by vacuum sintering

Nd:YAG/YAG transparent ceramics were prepared by vacuum sintering at 1780 °C for 40 h and annealed at 1450 °C for 20 h in air. Two separately polished Nd:YAG/YAG samples were bonded into monolithic and uniform composite-material followed by vacuum sintering at 1790 °C for 50 h under uniaxial pressure of 60 MPa, and then annealed at 1450 °C for 100 h in air. The fracture strength of bonded samples at the bonding interface is higher than that of as-prepared Nd:YAG/YAG samples. Meanwhile, the extinction coefficient of bonded samples is 0.0305 cm⁻¹ which is an improvement over as-prepared samples. The microstructure of the contact interface reveals the disappearance of the contact layer at the bond due to the grain growth and coalescence mainly based on grain boundary diffusion at higher temperatures and longer heat-treated time, which indicates that the bonding technology is beneficial to the fabrication of the thick composite materials.     

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.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Processing,properties and microstructure of SiC foam derived from epoxy-modified polycarbosilane

SiC foams having controlled porosity were fabricated using epoxy modified polycarbosilane (EMPCS). The EMPCS was synthesized by refluxing adequate amount of epoxy and polycarbosilane (PCS) in THF solution at 150 °C. The EMPCS having epoxy content of 0%, 10% and 20% by weight were termed as PCS, 10EMPCS and 20EMPCS respectively. Thermal foaming of the EMPCS was carried out at 1000 °C under inert atmosphere followed by ceramization at 1200, 1400 and 1600 °C under vacuum. The cell size of the ceramized SiC foam was found to be varying between 100 and 700 µm. The ceramized SiC foams were characterized for their density, porosity and compressive strength. Total porosity was found to be 81.8 ± 3.9, 87 ± 4.1 and 90.6 ± 4.6% for the PCS, 10EMPCS and 20EMPCS based SiC foams while their bulk densities were found to be 0.6 ± 0.03, 0.4 ± 0.02 and 0.3 ± 0.01 g/cc respectively. Compressive strength was found to be the highest for the SiC foams ceramized at 1600 °C for all the types of EMPCS. The compressive strength of the 10EMPCS is found to be 2.2 ± 0.2 MPa, 2.5 ± 0.2 MPa and 3.8 ± 0.3 MPa for the foams pyrolyzed at 1200 °C, 1400 °C and 1600 °C respectively while the strength was 1.9 ± 0.1 MPa, 2.1 ± 0.2 MPa and 2.9 ± 0.2 MPa for the 20EMPCS based SiC. The 20EMPCS based SiC foam of thickness 10 mm was exposed to oxy-acetylene flame for 120 s, back face temperature was found to be around 300 °C. Microstructure and phase analysis was carried out to understand the effect of epoxy content and ceramization temperature on physical, mechanical and thermal properties of different types of the SiC foams.     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

A heat-resistant preceramic polymer with broad working temperature range for silicon carbide joining

A room temperature curable heat-resistant adhesive with broad working temperature range was prepared through organic and inorganic modification. The preceramic polymethylsiloxane showed low bonding strength for silicon carbide from 400 °C to 600 °C because of the decomposition of polymer network. So the modification with epoxy resin was used to generate strong blending and copolymerization network which decomposed at higher temperature over 500 °C. The ceramization of active fillers and preceramic polymer compensated the bonding strength with rising temperature, thus eliminating the weak stage from 400 °C to 600 °C. The modification with fillers greatly improved its bonding strength at high temperature over 1000 °C. Consequently, the modified adhesive exhibited outstanding bonding strength tested at room temperature between 9.29 ± 0.56 MPa and 37.28 ± 1.33 MPa after heat-treatment from 25 °C to 1500 °C and the bonding strength directly tested at the temperature from 25 °C to 800 °C over 8.21 ± 0.40 MPa. The adhesive shows the potential to extend the application for engineering ceramic joining.     

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.