Joining of Ti-coated monolithic SiC using a SiCw/Ti3SiC2 filler by electric field-assisted sintering

Monolithic SiC, for the first time, was successfully joined using a SiC whisker-reinforced Ti₃SiC₂ composite (SiC_w/Ti₃SiC₂) filler via electric field-assisted sintering technique. A thin Ti coating layer was formed on the SiC surface to minimize the residual stress at the joint interface by transforming it into a TiC gradient layer. After optimizing process parameters, a joint strength higher than 250 MPa was obtained, which is higher than the other values reported in the literature. Failure occurred at the SiC base rather than the joining interface because of the improved joint strength by the incorporation of SiC_w. The addition up to 15 wt. % SiC_w in the filler layer improved the joint strength by various strengthening mechanisms. On the other hand, the joint strength was lower with 20 wt. % SiC_w addition, indicating the importance of thermal expansion mismatch between SiC_w and Ti₃SiC₂ to obtain a sound SiC joint.     

Solid-state joining of SiC using a thin Ti3AlC2, TiC,or Ti filler

SiC monoliths containing 5 wt.% Al₂O₃-Y₂O₃ additive were joined using a thin Ti₃AlC_2, TiC, or Ti filler. After joining at 1900 °C for 5 h under 3.5 MPa, the joint properties were compared in terms of the microstructure, phase evolution, joining strength, and possible elimination of the joining layer. Although all samples showed a sound joint, the microstructure differed according to the filler. SiC joined with Ti₃AlC₂ filler showed an indistinguishable joining interface due to the filler decomposition followed by solid-state diffusion into the SiC base, whereas TiC filler remained at the interface without showing decomposition or diffusion. In contrast, the Ti filler showed a possible elimination of the joining layer because of the diffusion of Ti and the formation of TiC. The mean joining strengths for the Ti₃AlC₂, TiC, and Ti fillers were 300, 234, and 248 MPa, respectively, which were comparable to that of the base SiC material (250 MPa).     

Joining of sintered silicon carbide using ternary Ag–Cu–Ti active brazing alloy

Sintered silicon carbide was brazed to itself by Ag–35.25 wt%Cu–1.75 wt%Ti filler alloy at 860 °C, 900 °C and 940 °C for 10 min, 30 min and 60 min. Mechanical properties both at room temperature and high temperature were measured by flexural strength. The interfacial microstructure was investigated by electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results indicate that increased brazing temperature heightens the flexural strength and the maximal four-point flexural strength reaches 342 MPa at room temperature. In addition longer holding times result in thicker reaction layer, which increases mismatch of coefficients of thermal expansion (CTE) between SiC substrate and reaction layer and finally leads to poor mechanical properties due to high residual stresses. High temperature flexural strength decreases with an increase of test temperature due to softening of the filler alloy. A reaction layer composed of TiC and Ti₅Si₃ was observed at the interface of SiC/filler alloy and there is a representative microstructure: SiC/continuous fine TiC layer/discontinuous coarse Ti₅Si₃ layer/filler alloy.     

Preparation and mechanical properties of Ti3SiC2/SiC functionally graded materials

Ti₃SiC₂/SiC functionally graded materials (FGMs) were prepared via hot-pressing sintering followed by positioning impregnation. Positioning impregnation is a novel technique for local impregnation targeted at graded layers that exhibit poor sintering behaviour. The positioning impregnation process significantly densified layers with SiC volume fractions of more than 70% while only slightly affecting the densities of the other layers and preserving sufficiently weak interfaces between layers. FGMs that were hot pressed at 1600 and 1700 °C and then subjected to impregnation showed not only high flexural strengths but also zigzag load-displacement behaviour. The flexural strengths of these FGMs were 436 and 485 MPa, respectively; in comparison, the values for the FGMs without impregnation that were hot pressed at 1600, 1700 and 1800 °C were 235, 268 and 328 MPa, respectively. Moreover, the fracture toughnesses of these FGMs were 8.23 and 7.15 MPa m^1/2, respectively; in comparison, the values for the FGMs without impregnation that were hot pressed at 1600, 1700 and 1800 °C were 6.77, 7.05 and 4.65 MPa m^1/2, respectively.     

Joining of SiC monoliths using a thin MAX phase tape and the elimination of joining layer by solid-state diffusion

This paper reports the joining of SiC monoliths using a thin MAX phase tape filler, such as Ti₃AlC₂ and Ti₃SiC₂, and the subsequent phenomena leading to the elimination of the joining layer via solid-state diffusion of the MAX phase into the SiC base material, particularly with the decomposition of the Ti₃AlC₂ filler. The base SiC monolith, showing?≥?99% density, was fabricated by hot pressing SiC powder after adding 5?wt. % Al₂O₃-Y₂O₃ sintering additive. A butt-joint configuration was prepared and joined by hot pressing under a pressure of 3.5?MPa. The effects of the experimental parameters, including the type and thickness of the joining filler, temperature as well as the holding time, were examined carefully in terms of the microstructure, phase evolution and joining strength. The joining interface could be eliminated from the SiC base when the SiC monoliths were joined at 1900?°C using a thin Ti₃AlC₂ tape, showing a high joining strength ～300?MPa. Moreover, fracture during the mechanical test occurred mainly at the base material rather than the joining interface, indicating excellent joining properties. These findings highlight the elimination of the joining interlayer, which might be ideal for practical applications because the absence of a joining filler helped preserve the excellent SiC mechanical properties of the joint.     

Joining of SiCf/SiC using polycarbosilane and polysilazane preceramic mixtures

Polycarbosilane (PCS)/polysilazane (PSZ) preceramic mixtures with weight ratios of 100/0, 75/25, 50/50, 25/75, and 0/100 were used as a filler for the joining of SiC_f/SiC to obtain high purity SiC at the joining region. SiC_f/SiC was fabricated by the electrophoretic infiltration of a SiC-based matrix phase into Tyranno SA3 SiC fabrics followed by hot-pressing at 1750?°C under 20?MPa for 2?h in an Ar atmosphere. Microstructural analysis confirmed a sound join without cracks after joining at 1750?°C for 2?h under a pressure of 10?MPa. SiC was the only phase remaining at the joint when PCS was used, while a small amount of Si₃N₄ along with the main SiC were observed in the join using PSZ. The flexural strengths of the butt-joint SiC_f/SiC were 120 and 137?MPa for the samples joined using a pure PCS and PSZ at 1750?°C, respectively, whereas those joined using the mixture fillers showed relatively lower strength.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Mechanical properties of the SiCf/SiC composites reinforced with KD-I and KD-II fibers fabricated assisted by a microwave heating method

SiC_f/SiC composites using KD-I and KD-II SiC fibers braided preforms as the reinforcements were fabricated by applying the polymer impregnation and pyrolysis (PIP) technique with a microwave heating assistance. The microwave heating temperature was 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively. Microstructures, flexure properties, and fracture behaviors of the composites were investigated. The KDIISiC_f/SiC composites exhibited higher flexure properties and improved non-brittle fracture characteristics than those of the KD-ISiC_f/SiC composites. The differences in the flexural properties, fracture behaviors and microstructures between the KD-I and KDIISiC_f/SiC composites were discussed based on the tensile properties of the SiC filaments and the interfacial bonding statues in the composites.     

Fabrication of the tube-shaped SiCf/SiC by hot pressing

A manufacturing technique for fabricating a dense tubular SiC long fiber-reinforced SiC composite (SiC_f/SiC) by hot pressing was developed. After infiltrating a SiC-based matrix phase, containing a 12 wt% of Al₂O₃–Y₂O₃ sintering additive, into the fine voids of a Tyranno^TM-SA3 SiC fabric preform by electrophoretic deposition combined with the application of ultrasonic pulses, hot pressing was performed using 2 types of specially designed molds filled with graphite powder to transfer the vertical hot press force efficiently to the sidewalls of the tubular SiC_f/SiC. Compared to the low density (~60%) of SiC_f/SiC hot-pressed using a conventional mold, a density >95% could be acquired using a special mold filled with graphite powder as a pressure delivering medium. This method is suitable for fabricating a dense tubular SiC_f/SiC, which cannot be obtained using a conventional extrusion method.     

Synthesis and characterization of Si/SiC ceramics prepared using cotton fabric

A Si/SiC ceramic was prepared from cotton fabric by the reactive infiltration of liquid silicon into the carbon template. A large density difference between the samples has been observed. This may be due to the variation in the pore size and its distribution within the sample. Scanning electron microscopy with energy dispersive spectroscopy shows the presence of three distinct phases, i.e., SiC, free Si and free carbon. X-ray diffraction pattern also confirms the presence of SiC and Si phases. However, there is no peak corresponding to carbon. So, it is inferred that the carbon exists in amorphous form. Micro-hardness, fracture toughness and bending strength of the ceramics were also studied. The values are lower than commercially available SiC ceramics. This may be due to the highly porous nature of cotton fabric-based SiC, as compared to commercially available SiC.     

Microstructure and mechanical properties of SiC platelet reinforced BaOAl2O32SiO2 (BAS) composites

BaOAl₂O₃2SiO₂ (BAS) glass–ceramic powders were prepared by sol–gel technique. SiC platelet reinforced BAS glass–ceramic matrix composites with high density and uniform microstructure were fabricated by hot-pressing. The effect of additional crystalline seeds on hexagonal to monoclinic phase transformation of Barium aluminosilicate was studied. The effects of SiC platelet content on the microstructure and mechanical properties of the composites were also investigated. The results showed that the flexural strength and fracture toughness of the BAS glass–ceramic matrix composites can be effectively improved by the addition of silicon carbide platelets. The main toughening mechanism was crack deflection, platelets' pull-out and bridging. The increased value of flexural strength is contributed to the load transition from the matrix to SiC platelets.     

Joining of SiC ceramics using the Ni-Mo filler alloy for heat exchanger applications

In order to meet the sealing demands of SiC heat exchanger, the Ni-Mo filler alloy was designed, prepared and employed to braze SiC ceramics. Wetting behavior of the Ni-Mo filler alloy on SiC ceramics and interfacial microstructure of the brazed joints were systematically characterized using optical observation furnace and XRD, SEM, EDS, TEM, respectively. Flexural strengths of the brazed joints at room temperature and high temperature were measured with four-point flexural strength method. HCl immersion test was performed to evaluate the corrosion resistance of the joints. The Ni-Mo filler alloy exhibited excellent wettability on SiC ceramics. During the process of brazing, SiC reacted with element Ni of the Ni-Mo filler alloy, resulting in the formation of Ni₂Si + graphite reaction layer adjacent to the SiC substrate. Ni₃Mo₃C and Ni₂Si compounds were precipitated at the center of brazing seam. When the brazing temperature increased from 1250 ℃ to 1400 ℃, the thickness of Ni₂Si + graphite layer increased gradually. The maximum room-temperature flexural strength of 174 ± 33 MPa was obtained when brazed at 1300 ℃ for 40 min. The joints also exhibited stable high-temperature strength and acid corrosion resistance. When the test temperature was 700 ℃, 800 ℃, 900 ℃, the joints gave the strength retention rate of 92.5 %, 79.8 %, 67.2 %, respectively. It was believed that the formation of high melting point phases played an important role. Residual strength of the joints after HCl corrosion exceeded 130 MPa, revealing a good potential for applications in corrosion environment.     

Fabrication of tubular SiCf/SiC using different preform architectures by electrophoretic deposition and hot pressing

This paper reports the processing feasibility of electrophoretic deposition combined with hot pressing in the fabrication of dense tubular SiC_f/SiC composites using a cylindrical mold. A simulation of pressure distribution using ANSYS software was performed by varying the angular inclinations in a cylindrical mold with an ‘out → in’ configuration so as to ensure a maximum and uniform conversion of vertical hot press force to the lateral side of a centrally-located preform through graphite powder. The simulation revealed an inhomogeneous pressure distribution along the height of the preform, which could be minimized by mold optimization to achieve a more uniform tube density. To verify this, two different preform architectures such as 0/90° woven 2-D fabric rolled in a jelly state and filament winding with two plies having an inter-ply angle of 55° were hot-pressed using a mold fabricated based on the simulation after infiltrating the matrix phase by electrophoretic deposition. The density of the tube could be increased with more uniform microstructures. Although the tube using a filament winding preform exhibited a lower flexural strength (105 MPa) and relative density (90%) than those with the preform rolled in a jelly state (221 MPa, 95%), the results revealed a high degree of fiber pull-out due to the PyC coating on the SiC fiber.     

Fabrication and Properties of Ti3SiC2/SiC Composites

Ti3SiC2/SiC composites were fabricated by reactive hot pressing method. Effects of hot pressing temperature, the content and particle size of SiC on phase composition, densification, mechanical properties and behavior of stress-strain of the composites were investigated. The results showed that ： （ 1 ） Hot-pressing temperature influenced the phase composition of Ti3SiC2/SiC composites. The flexural strength and fracture toughness of composites increased with hot pressing temperature. （2） It became more difficult for the composites to densify when the content of SiC in composites increased. It need be sintered at higher temperature to get denser composite. The flexural strength and fracture toughness of composites increased when the content of SiC added in composites increased. However, when the content of SiC reached 50 wt%, the flexural strength and fracture toughness of composites decreased due to high content of pore in composites. （3） When the content of SiC was same, Ti3SiC2/SiC composites were denser while the particle size of SiC added in composites is 12. 8 μm compared with the composites that the particle size of SiC added is 3 μm. The flexural strength and fracture toughness of composites increased with the increase of particle size of SiC added in composites. （4） Ti3SiC2/SiC composites were non-brittle fracture at room temperature.     

Joining of SiC ceramics via a novel liquid preceramic polymer (V-PMS)

A novel liquid preceramic polymer (V-PMS) was synthsized by modifying polymethylsilane (PMS) with 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane ([CH₃(CH₂CH)SiO]₄, D4Vi), for joining SiC ceramics under ambient pressure. The obtained V-PMS with a viscosity of 125 Pas at room temperature exhibits excellent thermal properties and bonding strength. The ceramic yield of V-PMS treated at 1200 °C under Ar atmosphere is 84.5%, which is 38.3% higher than the original PMS. The shear strengths of the SiC joints joined by V-PMS at 800 °C, 1000 °C and 1200 °C under N₂ atmosphere are 11.9 MPa, 34.5 MPa and 29.9 MPa, respectively. The excellent performances make the obtained V-PMS promising candidates for joining SiC ceramics in high-temperature applications.     

Fatigue behavior and damage evolution of SiC/SiC composites under high-temperature anaerobic cyclic loading

In the present study, the fatigue behavior and damage evolution of SiC/SiC minicomposites at elevated temperatures in oxygen-free environment are investigated which are important for their application and are still unclear. The high-temperature fatigue test platform is developed and the fatigue stress-life curve and the stress-strain response are obtained. The test result shows that the life of the material at elevated temperature is shorter than that at room temperature under the same stress level. Moreover, the hysteresis loop width and the residual strain increase with the increasing of the cycles while the hysteresis modulus decreases during the fatigue cycling. The evolution process of matrix cracks is observed using the real-time remote detection system. It is found that matrix cracking is insensitive to the cyclic loading which is similar to room temperature and is due to that the degeneration of the interfacial shear stress reduces the area of high stress in matrix. The fiber/matrix interfacial shear stress under different cycles is determined based on the fatigue modulus of each hysteresis loop. The result shows a fatigue enhancement phenomenon for the interface which is not observed at room temperature.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

Joining of SiC by Al infiltrated TiC tape: Effect of joining parameters on the microstructure and mechanical properties

SiC ceramics were successfully joined by Al infiltrated TiC tapes at 900-1100 °C for 0.5-2 h in vacuum. Phase constituents, microstructure and mechanical strength of the prepared SiC joints were characterized. The prepared SiC joints display dense interlayer and crack-free interface. The interlayer primarily consists of TiC and Al phases, together with small amount of TiAl₃ and trace of Al₄C₃. With increasing the joining temperature or time, the interface layer either thickens or grows to multiple layers. The bending strengths of the SiC joints are higher than 190 MPa as bonded at present conditions, and are closely related with the property of interface and interlayer.     

High-strength SiC joints with a novel in-situ formed SiC/Al4SiC4 joining filler

The in-situ formed SiC/Al₄SiC₄ joining layer was used to join monolithic SiC using an electric field-assisted sintering technique. A multiphase powder of Al₄C₃/SiC/Al₄SiC₄ was used as the initial joining material to obtain the in-situ reaction layer of SiC/Al₄SiC₄ via the appropriate interface reactions. The bending strength as high as 240.5 ± 6.6 MPa was obtained for the sample joined at 1800 °C, which was higher than the strength of the un-joined SiC matrix. Sound joints were obtained when Al₄C₃ was completely transformed to Al₄SiC₄, and a fully dense SiC/Al₄SiC₄ joining layer was consolidated. The integration of the joining layer with the SiC matrix was improved by a high amount of liquid phase formed at the interface. The proposed SiC/Al₄SiC₄ joining layer, with good thermal matching with SiC matrix, shows a great potential to be applied as a joining material for SiC-based ceramic matrix composites.     

Microstructural evolution and mechanical performances of SiC/SiC composites by polymer impregnation/microwave pyrolysis (PIMP) process

SiC/SiC composites were prepared by polymer impregnation/microwave pyrolysis (PIMP) process, and their microstructural evolution and the mechanical performances were characterized. Using non-coated Tyranno SA fiber preforms as reinforcement and impregnation with only allylperhydropolycarbosilane (AHPCS) into the preforms, Tyranno SA/SiC composite (TSA/SiC) with higher density was obtained. While using carbon-coated Tyranno SA fiber preforms, Tyranno SA/C/SiC composite (TSA/C/SiC) with lower density were also fabricated. In this composite, SiC particulate was loaded with polymer precursor (AHPCS) in the first cycle impregnation. Microstructural observation revealed that pore and crack formation was affected by processing conditions. Bending strength was also dependent on the microstructural evolution of the samples. In TSA/SiC composite, relatively strong interfaces contribute to effective load transfer so that higher bending strength could be reached. In the TSA/C/SiC composite, weak interfaces provide a relatively lower strength. Meanwhile, different microstructural evolution and interfacial properties of the composites lead to the variation of the fracture behaviors.