Investigations of the effect of Si addition on graphite elimination and the oxidation behavior of SiC joint using Inconel 625 powder filler

Graphite, whose presence is harmful to the mechanical property, is one of the main reaction products between Ni-based alloy and SiC. This paper reports a method of eliminating graphite based on thermodynamic calculations. Different amounts of Si powders are added into the Inconel 625 powder filler to adjust the interfacial reactions during the brazing process. When the Si content in the liquid reaches the theoretically calculated value, the reactions between the Inconel 625 filler and SiC substrate are suppressed, and no graphite forms on the interface. The graphite-free SiC joints show good oxidation resistance. The joint's strength maintains at 25.7 MPa while that with graphite drastically drops to 5.5 MPa after 185 h oxidation in air. The reaction products and oxidation process of the SiC joints are also carefully analyzed in this paper.     

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.     

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.     

界面粘接对填充复合材料力学性能的影响

利用原位聚合的方法将聚甲基丙烯酸甲酯（PMMA）包覆在滑石粉的表面,制得了含PMMA粘接层的滑石粉／PVC复合材料。聚合物粘接层类似于粘接材料的粘接剂的作用,它很好地改善了复合材料的界面粘附性,提高了复合材料的机械强度,由于聚合物粘和基体聚合物的相互扩散以及界面内应力的存在,和昨合材料中存在一个最佳的聚合物粘接层。     

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.     

Effect of SiC powder on the properties of SiC slurry for stereolithography

Silicon carbide (SiC) is a kind of structural ceramics with excellent properties and it is widely used in industrial fields. Stereolithography (SL) is a potential additive manufacturing technique to fabricate fine complex SiC components, the resin-based SiC slurry with superior rheological and photo-polymerization properties is important for SL. In this paper, we investigated the influence of SiC powder on the properties of the SiC slurries for SL. The physical characteristics of SiC powder such as particle size, size distribution and appearance were tested and observed, and their influence on the dispersion, sedimentation and photo-polymerization property of the SiC slurry were investigated and discussed in detail based on their correlative theory, we finally prepared SiC slurry with superior rheological and photo-polymerization properties, and fabricated the fine complex SiC green body with low defects, high accuracy and high bending strength successfully. The SiC slurry with the solid content of 40 vol% was fabricated by the SiC powder with the median diameter D₅₀ ≈ 10.0 μm and a narrow particle size distribution, it is Bingham fluid with good fluidity and the viscosity of it is 464.40 mPa s under the shear rate of 51.08 s^?1, the cured SiC parts with Z – axis dimension change of 0.75% was finally fabricated, the three points bending strength of it is 50.18 MPa. Our research work provides some fundamental understanding of the SL technique for fabricating fine complex SiC components, explored a suitable way to fabricate high quality SiC green parts through SL, and offers some valuable references for preparing SiC slurry with superior rheological and photo-polymerization properties.     

Microstructure evolution and mechanical property of ZrC-SiC/Ti6Al4V joints brazed using Ti-15Cu-15Ni filler

ZrC-SiC ceramic and TC4 alloy were successfully brazed using a self-prepared Ti-15Cu-15Ni filler. The microstructure and mechanical property of the joints obtained at different brazing temperatures were investigated. The results indicated that Ti from the Ti-15Cu-15Ni and the TC4 reacted with the ZrC-SiC to form TiC phase adjacent to the ZrC-SiC ceramic. In the brazing seam, Ti₂(Ni, Cu) intermetallic compounds zone (IMCs Zone), Hypoeutectic Zone and Hypereutectoid Zone formed. The brazing temperature affected the dissolution of TC4 into the braze filler significantly, and then determined the microstructure of the joint. The formation of α-Ti in the brazing seam could decrease the hardness and the brittleness of the brazing seam, which was beneficial to the property of the brazed joint. The joint strength reached a maximum value of 43 MPa when the joint was brazed at 970 °C and cracks propagated in the ZrC-SiC substrate near the brazing seam.     

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.     

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiC_f)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiC_f using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiC_f/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiC_f/PyC/SiCNWs. Three mini-composites, SiC_f/PyC/SiC, SiC_f/PyC/SiCNWs/SiC, and SiC_f/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiC_f/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiC_f/SiC mini-composites were investigated.     

Reactive wetting and interfacial reaction mechanism of ZrC-SiC ceramic and Ag-Zr filler

In this study, the isotherm wetting and spreading behaviors of molten Ag-Zr filler on ZrC-SiC ceramic surface was investigated using a sessile drop method in vacuum. The effect of Zr content in Ag-Zr filler on the wetting behavior was studied, and the wetting mechanism was revealed in details. The results showed that the contact angle of Ag-Zr filler on the ZrC-SiC surface decreased from 140 ° to 20 ° with the Zr content varied from 0 to 8 wt.%, and the Zr was the key factor to the wetting process. The wetting dynamic analysis illustrated that the interfacial reaction controlled the wetting of Ag-Zr filler on the ZrC-SiC surface. Due to the Zr activity difference at Ag-Zr/ZrC interface, the ZrC released C into Ag-Zr filler, and reacted with Zr to form new-born ZrC with higher Zr content. The formation of the new-born ZrC promoted wetting and spreading of Ag-Zr filler on ZrC-SiC surface.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

Interfacial microstructure and mechanical properties of SiC joints achieved by reactive air brazing using Ag-V2O5 filler

SiC ceramics are successfully brazed via reactive air brazing using Ag-V₂O₅ fillers. The wettability of SiC ceramics by Ag-V₂O₅ fillers is investigated. Interfacial microstructure of SiC joints is analyzed by scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy. Effect of the brazing filler composition on the microstructure and mechanical properties of SiC joints is studied in detail. The V₂O₅ from the brazing fillers is found to react intensively with SiC, and the SiO₂ reaction layer with the thickness of ?7 μm is formed on the SiC surface which ensures a good wetting of the brazing filler on SiC ceramics. The brazing seam is composed of Ag and VO₂ with small amount of remaining V₂O₅. The maximum shear strength (?58 MPa) is achieved when using the optimized brazing process (Ag-8V₂O₅, 1050 ℃/30 min, the loading pressure is ?20 kPa and the cooling rate is 2 ℃/min).     

Effect of adding of SiC particulate on the microstructure and shear strength of SiC ceramic joint brazed with Si-24Ti alloy

In order to refine the grain size of TiSi₂ silicide and reduce the formation of micro-defects in the joint, and thereby increasing the joint strength of SiC ceramic brazed with Si-24Ti (wt.%), a small amount of SiC particulates were added in the brazing alloy. The microstructure and mechanical strength of SiC joints was investigated by using field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction spectrometer, and shear strength test. The results indicated that SiC particulates enhanced the nucleation and grain refinement of the TiSi₂ and Si phase. The adding of appropriate content of SiC (<1 wt.%) could effectively refine the size of TiSi₂ phase and increase the fraction of Si-TiSi₂ eutectic zone. However, excess addition of 1.5 wt.% SiC caused the coarsening of TiSi₂ phase due to the clustering of added fine SiC particulates. With the increasing of SiC particulate content, the shear strength of the joints increased at first and then decreased. The maximum shear strength of 106.3 MPa of SiC joint was obtained for the joint brazed with 1 wt.% SiC addition, which was ~19% higher than that of the joint brazed without SiC particulates.     

Synthesis of SiC powder by RF plasma technique

Continuous synthesis of SiC nanoparticles by RF thermal plasma method has been studied. Precursor mixtures comprised commercial silica powder and various types of carbon source including graphite, char, carbon black as well as the carbonaceous residue of tire pyrolyses. The obtained SiC consisted of nanosized particles that were crystallized mainly in β phase with traces of α. The conversion rate of the silica precursor to SiC varied between 60% and 73% depending on the type of carbonaceous material and on the carbon excess. The main obstacle to achieve higher conversion is the rapid cooling of reactive species that can also be attributed to formation of nanosized particles.     

Joining of SiC using CoFeCrNiCuTi high entropy alloy filler by electric current field assisted sintering

SiC ceramics were brazed by electric field-assisted sintering technology using CoFeCrNiCuTi high-entropy alloy as joining filler. The effect on the interface microstructure and bend strength of brazed joints at different brazing temperature was systemically studied. The interfacial reaction was controlled by adjusting the brazing temperature. The main components in the brazing seam are high-entropy alloys FCC (HEAF), C, TiC, CrC, and Cr₂₃C₆ phase. Furthermore, the maximum bending strength of 54 MPa was found when brazed at a lower temperature of 1125℃. In addition, due to the electric field-assisted sintering technology and the high-entropy effect of the CoFeCrNiCuTi filler, the diffusion of elements and the formation of solid solution were accelerated. This suggests that the current field was beneficial to improve the inter-diffusion between the CoFeCrNiCuTi filler and SiC ceramics. Consequently, the low-temperature rapid brazing of SiC ceramics was realized, and this technology provides a new filler system for ceramic brazing.     

Brazing mechanisms of the Ti2AlC joints using a pure Al filler metal

Brazing of Ti₂AlC ceramics has been successfully performed using a pure Al filler metal, in the temperature range 1023 K–1173 K and with holding time ranging from 0 to 30 min. The microstructure of the Ti₂AlC joints was studied, and the mechanical properties of the joints were evaluated by shear strength test. It is observed that the Al filler has weak effect on the stability of Ti₂AlC substrate, and only a small amount of decomposition products including the TiC_x and TiAl₃ compounds can be observed in the joints. In addition, the formation of an inter-diffusion layer in the Ti₂AlC substrate is considered as the major brazing mechanism. The maximum shear strength of the Ti₂AlC joints using the pure Al filler metal is 95 MPa, with an electrical conductivity of 2.21×10⁶ S m⁻¹, after holding the sample at 1123 K for 10 min.     

Microstructural evolution and mechanical property of a SiCf/SiC composite/Ni-based superalloy joint brazed with an Au-Cu-Ti filler

A new Au-Cu-Ti filler featuring superior mechanical properties was developed to enable the brazing of a SiC_f/SiC composite (CMCs) to itself and a Ni-based superalloy (GH536). The progression of the interfacial reactions was studied using a combination of thermodynamic calculations and experimental observations. It was found that the interfacial reaction was Ti-dominant at the early brazing stage and then gradually transformed to Ni-dominant with the continuous dissolution of the GH536 substrate. Thus, the typical microstructure of GH536/Ti-Ni-Cr-Fe+(Au, Cu)ss + MoNiSi/Ni-Cr-Fe-Si-C (Ni₂Si + Fe₂Si + Cr₃C₂)+(Au, Cu)ss/Ni₂Si + TiC+(Au, Cu)ss/ Cr₃C₂+Ni₂Si + TiC + Fe₂Si/CMCs could be described by the following three stages: a Ti-dominated stage, full interdiffusion stage, and Ni-dominated stage. A maximum shear strength of 36 MPa was obtained for joints brazed at 1050℃ for 10 min, at which a failure occurred at the CMCs/brazing seam interface. The control of the interfacial reactions and the stress relaxation of (Au, Cu)ss contributed to the superior mechanical performance of the composite.     

Effect of pyrocarbon texture on the mechanical and oxidative erosion property of SiC coating for protecting carbon/carbon composites

SiC coating was conducted on C/C composites with rough laminar (RL) or smooth laminar (SL) pyrocarbon matrix separately. The residual stress, elastic modulus and microhardness of RL-SiC (RLS) and SL-SiC (SLS) coatings were investigated. The results showed that compared with SLS, RLS coating possessed smaller residual stress and higher hardness and elastic modulus, which was beneficial for its resistance to cracking and then contributed to the anti-oxidation performance improvement. At temperatures of 300–1400 °C, its mass loss was only 2.41%. At 1500 °C, it showed good self-sealing ability and could provide C/C composites against oxidation at least 120 h.     

Effect of SiC interphase on the mechanical,high-temperature dielectric and high-temperature microwave absorption properties of the SiCf/SiC/Mu composites

In this study, SiC interphase was prepared via a precursor infiltration-pyrolysis process, and effects of dipping concentrations on the mechanical, high-temperature dielectric and microwave absorption properties of the SiC_f/SiC/Mu composites had been investigated. Results indicated that different dipping concentrations influenced ultimate interfacial morphology. The SiC interphase prepared with 5 wt% PCS/xylene solution was smooth and homogeneous, and no bridging between the fiber monofilament could be observed. At the same time, SiC interphase prepared with 5 wt% PCS/xylene solution had significantly improved mechanical properties of the composite. In particular, the flexural strength of the composite prepared with 5 wt% PCS/xylene solution reached 281 MPa. Both ε′ and ε′′ of the SiC_f/SiC/Mu composites were enhanced after preparing SiC interphase at room temperature. The SiC_f/SiC/Mu composite prepared with 5 wt% PCS/xylene solution showed the maximum dielectric loss value of 0.38 at 10 GHz. Under the dual action of polarization mechanism and conductance loss, both ε′ and ε′′ of the SiC_f/SiC/Mu composites enhanced as the temperature increased. At 700 °C, the corresponding bandwidth (RL ≤ ?5 dB) of SiC_f/SiC/Mu composites prepared with 5 wt% PCS/xylene solution can reach 3.3 GHz at 2.6 mm. The SiC_f/SiC/Mu composite with SiC interphase prepared with 5 wt% PCS/xylene solution is expected to be an excellent structural-functional material.