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.     

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.     

Enhanced mechanical properties and thermal cycling stability of Al2O3-4J42 joints brazed using Ag–Cu–Ti/Cu/Ag–Cu composite filler

This study investigates the mechanical properties and microstructure evolution of Al₂O₃-4J42 joints brazed using Ag–Cu–Ti (ACT) and Ag–Cu–Ti/Cu/Ag–Cu (ACTCA) fillers during thermal cycling from 0 °C to 500 °C. The reaction products between Al₂O₃ and brazing filler of these two types of joints are mostly composed of Ti–O compounds, Ti₄Cu₂O and Al-based compounds. Brittle intermetallic compounds (IMCs) are observed in ACT joints, but not found in ACTCA joints. The reaction layer in ACT joints becomes thinner and discontinuous with thermal cycles, while that in the ACTCA joints hardly changes. Besides, the stress-induced cracks occur within the Al₂O₃ ceramic near the Al₂O₃/filler interface in the ACT joints, but no crack is found in the ACTCA joints. The mechanical tests show that the ACTCA joints maintain at least 217%, 154% and 144% higher shear strength than the ACT joints at 0, 10 and 20 thermal cycles, respectively. The Cu interlayer with low yield strength releases stress through plastic deformation, meanwhile acts as a barrier to prevent elements diffusion and the formation of the brittle IMCs, thus improving the mechanical properties and thermal cycling stability of the joints.     

Thermal stability and heat flux investigation of neutron-irradiated nanocrystalline silicon carbide (3C–SiC) using DSC spectroscopy

Nanocrystalline silicon carbide (3C–SiC) particles have been irradiated by neutron flux (2 × 10¹³ n∙cm⁻²s⁻¹) up to 5 h at the TRIGA Mark II type research reactor. At the present work, thermal properties of nanocrystalline 3C–SiC are comparatively investigated before and after neutron irradiation at the 300 K < T < 1300 K ranges. Simultaneously, the DSC (Scanning Calorimetry), TGA (Thermogravimetric Analysis) and DTG (Differential Thermogravimetric Analysis) experiments were conducted from 300 K up to 1300 K. Oxidation mechanism of nanocrystalline 3C–SiC particles have been theoretically and experimentally studied before and after neutron irradiation. The kinetics of mass and heat flux were analysed at the heating and cooling processes using DSC spectroscopy.     

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si₃N₄ powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si₃N₄ composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si₃N₄ particles.     

Thermal properties of silicon carbide composites fabricated with chopped Tyranno® SiAlC fibres

Thermal diffusivity, a, and thermal conductivity, κ, between room temperature and 600 K were investigated for SiC composites containing 0–50 mass% of Tyranno^® SiAlC (SA) fibre (mean length: 394 μm) hot-pressed at 1800 °C for 30 min under a pressure of 31 MPa. The monolithic SiC specimen possessed κ of 32.1 W m⁻¹ K⁻¹ at room temperature; no significant changes were found for the SiC composite containing ≤20 mass% of SA fibre addition. However, further increases in the amount of SA fibre to 50 mass% improved κ to a maximum of 56.3 W m⁻¹ K⁻¹. The value of a for the SiC composite containing 40 mass% of SA fibre was 0.185 cm² s⁻¹ at room temperature and decreased to 0.120 cm² s⁻¹ at 600 K. In addition, SiC composites using 40 mass% of SA fibre with a carbon interface of approximately 100 nm were fabricated. The effect of this interface on a and κ was marginal.     

Electric current–assisted direct joining of silicon carbide

Conventional direct joining technologies are difficult to use with silicon carbide (SiC) materials, especially for fiber composite forms of SiC, because of the harsh conditions required. To reduce the temperature and/or process time required for the direct joining process, an electric current–assisted joining (ECAJ) method was studied. Joining of low–resistivity grade, nitrogen doped β-SiC was demonstrated at a relatively low nominal temperature of 1750 °C with a 10 min hold by enhancing the passage of current through the material. The joining mechanism is discussed in terms of localized overheating and accelerated self-diffusion at the interface. In the case of joining at 2160 °C for 1 min, rapid crystal growth of textured SiC was found at the interface. This study indicates that rapid ECAJ-based direct joining is a practical and appropriate method for joining SiC-based materials.     

Microstructure and mechanical properties of Al2O3/Cu joints brazed with Ag–Cu–Ti+Zn composite fillers

In this study, Al₂O₃ ceramic and Cu bars were brazed with newly designed Ag–Cu–Ti(ABA)+Zn composite fillers. Systematic analysis of the microstructure of the brazed joints indicated that the volatilization of Zn atoms during the brazing process could promote the spreading of liquid brazing fillers on the surface of the Al₂O₃ ceramic, resulting in a uniform dendritic interfacial structure. The typical interfacial structure was an Al₂O₃/TiO/(Cu, Al)₃Ti₃O+Ag(s, s)/Cu interface. Notably, the tensile strength was improved to 20.89 MPa for Al₂O₃/Cu joint brazed with ABA+Zn composite fillers at 900 °C for 20 min, approximately 67.6% higher than the sample brazed without Zn foil. In this case, the fracture model was straight and sharp-angled inside the Al₂O₃ ceramic. In addition, the joint strength decreased with increased brazing temperatures from 900 to 940 °C.     

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.     

Examination of non-shrinkage firing using low-temperature co-fired alumina containing a small quantity of Cu–Ti–Nb–Ag–O additive

In this study, we examined non-shrinkage firing in the planar direction using low-temperature co-fired alumina (LTCA) containing a small quantity of Cu–Ti–Nb–Ag–O additive. A good restraint effect was obtained using AL-43-L alumina as a constraint layer whose particle diameter was several times larger (1 to 2 μm) than that of alumina in the LTCA material TM-5D (0.2 μm). The x-y shrinkage ratio of the non-shrinkage fired sample using AL-43-L as the constraint layer was within 2 %. However, the density of the non-shrinkage fired sample was much lower than that of the normal shrinkage fired sample. By producing a powder using a freeze-dry method, we obtained a well-sintered alumina with a relative density of approximately 95 % at 950°C, which is lower than the melting point of silver. That is, we achieved both suppression of the shrinkage rate in the x-y direction and ensured sinterability below the melting point of silver. The thermal conductivity of the sample was 16.2 W/mK, which is extremely high for low-temperature co-fired ceramics materials.     

Dispersion,slip casting and reaction nitridation of silicon–silicon carbide mixtures

The dispersing behaviour of silicon, silicon carbide and their mixtures in aqueous media were monitored by particle size, sedimentation, viscosity and zeta potential analyses as a function of pH of the slurry. The pH values for optimum dispersion were found to be 4 and 8 for silicon, 10 for SiC and 9 for Si+SiC mixtures. Optimum slips of Si+SiC mixtures were slip cast to obtain green compacts which were nitrided once at 1450°C for 2 or 4 h or successively and cumulatively for 8 (2+6) and 10 (4+6) h in a resistively heated graphite furnace. The binding phases in the nitrided products were found to be fibrous/needle like α-Si₃N₄, flaky grains of β-Si₃N₄ and Si₂ON₂. The products containing 19–47% of silicon nitride as bond/matrix possessed flexural strength (three-point bending) values of 50–85 MPa. ©     

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.     

Characterization of copper–silicon nitride composite electrocoatings

Silicon nitride particles were incorporated to electrolytic copper by co-electrodeposition in acidic sulfate bath, aiming the improvement of its mechanical resistance. Smooth deposits containing well-distributed silicon nitride particles were obtained. The current density did not show significant influence on incorporated particle volume fraction, whereas the variation of particle concentration in the bath had a more pronounced effect. The microhardness of the composite layers was higher than that of pure copper deposits obtained under the same conditions and increased with the increase of incorporated particle volume fraction. The microhardness of composites also increased with the increase of current density due to copper matrix grain refining. The composite coatings were slightly more corrosion resistant than pure copper deposits in 3.5% NaCl solutions.     

Creep behavior of a zirconium diboride–silicon carbide composite

Flexural creep studies of ZrB₂–20 vol% SiC ultra-high temperature ceramic were conducted over the range of 1400–1820 °C in an argon shielded testing apparatus. A two decade increase in creep rate, between 1500 and 1600 °C, suggests a clear transition between two distinct creep mechanisms. Low temperature deformation (1400–1500 °C) is dominated by ZrB₂ grain or ZrB₂–SiC interphase boundary and ZrB₂ lattice diffusion having an activation energy of 364 ± 93 kJ/mol and a stress exponent of unity. At high temperatures (>1600 °C) the rate-controlling processes include ZrB₂–ZrB₂ and/or ZrB₂–SiC boundary sliding with an activation energy of 639 ± 1 kJ/mol and stress exponents of 1.7 < n < 2.2. In addition, cavitation is found in all specimens above 1600 °C where strain-rate contributions agree with a stress exponent of n = 2.2. Microstructure observations show cavitation may partially accommodate grain boundary sliding, but of most significance, we find evidence of approximately 5% contribution to the accumulated creep strain.     

Effect of silicon carbide nanoparticles on the adhesion strength of steel–epoxy composite joints bonded with acrylic adhesives

This paper reports a study on the effect of silicon carbide nanoparticles on the adhesion strength of steel–glass/epoxy composite joints bonded with two-part structural acrylic adhesives. The introduction of nanosilicon carbide in the two-part acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strengths of the adhesive joints increased with adding the filler content up to 1.5?wt%, after which decreased with adding more filler content. Also, addition of nanoparticles caused a reduction in the peel strength of the joints. DSC analysis revealed that Tg values of the adhesives rose with increase in the nanofiller content. The equilibrium water contact angle was decreased for adhesives containing nanoparticles. SEM micrographs revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.     

Effects of Ti thickness on microstructure and mechanical properties of alumina–Kovar joints brazed with Ag–Pd/Ti filler

Alumina, metalized with Ti by magnetron sputtering, has been successfully bonded to a Kovar alloy using Ag–5 wt% Pd filler. The effects of Ti content on the interfacial microstructure and mechanical properties were investigated. The results showed that a reaction layer was formed at the alumina/filler interface. Microanalysis indentified the reaction products to be titanium oxide and Pd–Ti compounds. The thickness of the reaction layer and the residual filler layer played determinable roles on the joint strength. The maximum four-point flexural strength of the brazed joints at room temperature reached as high as 177.3 MPa when the thickness of Ti layer was 3 μm.     

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.     

Effect of carbon and oxygen on the high-temperature properties of silicon carbide–hafnium carbide nanocomposite fiber

The polymer-derived ceramics (PDCs) technique enables relatively low-temperature fabrication of Si-based ceramics, with silicon carbide fiber as a representative product. Polycarbosilane (PCS) has Si-C backbone structures and can be converted to silicon carbide. In the PDCs method, residual or excess carbon is generated from the precursor (C/Si ratio = 2 for polycarbosilane). Because of the non-stoichiometry of SiC, the physicochemical properties of polymer-derived SiC are inferior to those of conventional monolithic SiC. Herein, a silicon carbide-hafnium carbide nanocomposite fiber was optimized by crosslinking oxygen into the PCS fiber by regulating the oxidation curing time. During pyrolysis, carbothermal reduction, and sintering, carbon was removed by reaction with hydrogen and cross-linked oxygen. Non-destructive techniques (X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and high-temperature thermomechanical analysis) were used to investigate the effects of excess carbon. The microstructure of the near-stoichiometric SiC-HfC nanocomposite fiber was more densified, with superior high-temperature properties.     

Influence of Ti3SiC2 content on erosion behavior of Cu–Ti3SiC2 cathode under vacuum arc

In this study, a series of Cu–Ti₃SiC₂ composites with different Ti₃SiC₂ contents were prepared by spark plasma sintering. Their mechanical properties and electrical resistivity were investigated. Through analyzing the morphology and composition of the eroded regions, the effect of Ti₃SiC₂ content on the erosion behavior of Cu–Ti₃SiC₂ cathodes under vacuum arc was studied. Results show that the relative density and bending strength of the Cu–Ti₃SiC₂ composites decrease with the increasing Ti₃SiC₂ content, while the opposite holds for hardness and electrical resistivity. The morphology and phase composition of the erosion zone is dominated by the decomposition process and the amount of Ti₃SiC₂ in the cathode. Cu–Ti₃SiC₂ cathodes containing 10 mass%Ti₃SiC₂ or less displayed relatively flat eroded surface morphology. Cathodes with high Ti₃SiC₂ content suffered more serious erosion with voids, cracks, and severe decomposition of Ti₃SiC₂, all of which contribute to impairing the arc ablation resistance of the composite. Ti₃SiC₂ particles decomposed into TiC and Si vapor; eventually, this TiC also decomposed into Ti vapor and C, leaving a considerable amount of C on the arc affected cathode surface. Excess addition of Ti₃SiC₂ particles not only deteriorates the strength but also the electrical and thermal conductivity of the composite, both of which in turn harms the arc erosion resistance of the material. These results suggest that the optimal Ti₃SiC₂ content is below 10 mass% in the composite.     

Microstructure of friction surface developed on carbon fibre reinforced carbon–silicon carbide (Cf/C–SiC)

We have used TEM to study the microstructure of friction surface of carbon fibre/carbon–silicon carbide composites brake discs after multi braking stop by using organic pads. A friction surface layer was developed consistently on the top of Si regions of the composites, but inconsistently on that of SiC and C. Inside the layer, amorphous silicon/silicon oxides appeared extensively with various non-metallic and metallic crystallites dispersed inside with sizes ranging from a few nanometers to several microns. A coherent interface between the friction layer and the composite surface was established under the braking conditions, whilst its sustainability varied notably in SiC and C regions. Microcracking near the friction surface appeared in SiC and C_f/C regions largely due to the extensive ductile deformation of SiC and weak interfaces between C and C_f. Material joining mechanisms were discussed to enlighten the friction transfer layer development on the surface of the composite discs.