Crack propagation analysis of SiCf/SiC composites by gas permeability measurement

Permeability of helium gas through the NITE-SiC_f/SiC composites after applying tensile stress was measured experimentally in a vacuum apparatus. Tensile stress equal to 1.1-1.2 times the proportional limit stress (PLS) was applied parallel to the direction of the reinforced fiber. Results of the permeability experiments revealed that the permeability rapidly increased when threshold stress was applied on the specimens. The permeability of helium gas was governed by the narrowest diameter of the permeation pathway. In the case of NITE composites, the diameter of the pathway was calculated to be below 0.65 μm. The NITE composites exhibited superior performance even when the applied stress was greater than 1.2 times the PLS. Fiber bundles considerably magnified the permeability of helium gas because of the relatively large pore size of the intra-fiber bundles. Transverse cracks propagated with increasing stress and they connected fiber bundles when the applied stress was 1.10-1.15 times the PLS.     

Effect of SiC ceramics on thermoelectric properties of SiC/SnSe composites for solid-state thermoelectric applications

Tin selenide (SnSe) based thermoelectric materials with varying amounts of embedded silicon carbide (SiC) particles were fabricated, and their thermoelectric properties were investigated. The SiC particles were evenly distributed in the SnSe matrix, thereby leading to the formation of the SiC/SnSe composite samples. The introduction of SiC into the SnSe matrix improved the power factors, owing mainly to an increase in the Seebeck coefficient, and a decrease in the thermal conductivity arising from the formation of phonon-scattering centers. Consequently, a ZT of 0.125 (at 300 K) was obtained for the SiC/SnSe composite with a SiC content of 1 wt%; this value was larger than that of the pristine SnSe. The results of this study indicate that the introduction of SiC particles into the SnSe matrix constitutes an efficient strategy for achieving thermoelectric enhancement for solid-state applications.     

Joining of SiCf/SiC using a layered Ti3SiC2-SiCw and TiC gradient filler

A layered filler consisting of Ti₃SiC₂-SiC whiskers and TiC transition layer was used to join SiC_f/SiC. The effects of SiC_w reinforcement in Ti₃SiC₂ filler were examined after joining at 1400 or 1500 °C in terms of the microstructural evolution, joining strength, and oxidation/chemical resistances. The TiC transition layer formed by an in-situ reaction of Ti coating resulted in a decrease in thermal expansion mismatch between SiC_f/SiC and Ti₃SiC₂, revealing a sound joint without cracks formation. However, SiC_f/SiC joint without TiC layer showed formation of cracks and low joining strength. The incorporation of SiC_w in Ti₃SiC₂ filler showed an increase in joining strength, oxidation, and chemical etching resistance due to the strengthening effect. The Ti₃SiC₂ filler containing 10 wt.% SiC_w along with the formation of TiC was the optimal condition for joining of SiC_f/SiC at 1400 °C, showing the highest joining strength of 198 MPa as well as improved oxidation and chemical resistance.     

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.     

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.     

Synthesis of C/SiC core-shell fibers through siliconization of carbon fibers with SiO gas in semi-closed reactor

Novel C/SiC core-shell fibers have been synthesized through incomplete conversion of carbon fibers by their siliconization with SiO gas. The synthesis was performed in the laboratory-made semi-closed batch-type reactor at 1380 °C for 3 h using a 9:1 M ratio mixture of Si and SiO₂ powders as a solid source of SiO gas. The conversion rate of carbon into SiC was 34.0%. All synthesized fibers had a distinct C/SiC core-shell composite structure. The fiber product was of fairly good uniformity in respect of the shell thickness which varied approximately from 0.6 μm to 0.8 μm depending on the location of fibers inside the reactor. It was revealed that the formation of the shell was the result of inward growth of the SiC product layer. The effectiveness of the proposed semi-closed reactor for the synthesis of C/SiC core-shell fibers has been demonstrated.     

Preparing SiC/diamond coatings via chemical vapor deposition of SiC on diamond-coated graphite and their frictional properties

SiC/diamond coatings with excellent frictional properties were successfully prepared using graphite as substrate. Diamond particles with size of 25–38 μm were firstly bonded on graphite substrate through PVA glue, followed by chemical vapor deposition (CVD) of SiC with varied MTS flow on the diamond-coated graphite substrate to enhance the adhesion of diamond particles. The influence of the MTS flow on the SiC coatings was investigated. The results showed that polycrystalline SiC coating with good crystallinity has been obtained. With MTS flow increasing, the SiC grains feature increased surface roughness and greater sizes of the SiC crystallite resulting from the co-deposition of SiC and carbon with increased carbon containing species. Reciprocating sliding wear tests were conducted to investigate the coefficient of friction. With increasing applied load, while the low-flow specimens showed a remarkable increase in the friction coefficient resulting from degradation of the SiC coatings, the high-flow specimens maintained a relatively low friction coefficient during wear tests indicating strong holding force to diamond particles of the SiC coatings. The reason for low friction coefficient of the high-flow specimens was that GCr15 steel ball was wearing by the SiC/diamond coatings with good affinity to the substrate resulting in a flat–flat contact on the contact area.     

Thermal conductivity studies on Si/SiC ceramic composites

Ceramic heat exchangers are increasingly used in many nuclear power plants. Silicon carbide has been treated as a promising material for heat exchanger application since it has good thermal conductivity and corrosion resistance. In this work, four different types of Si/SiC ceramic composites were prepared by liquid silicon infiltration technique. Thermal conductivities of these ceramic composites at different temperatures are measured by the laser flash thermal conductivity method. Results show that the presence of free carbon and voids are notably affecting the thermal conductivity of these materials.     

Appropriate thickness of pyrolytic carbon coating on SiC fiber reinforcement to secure reasonable quasi-ductility on NITE SiC/SiC composites

Pyrolytic carbon (PyC) coating of silicon carbide (SiC) fibers is an important technology that creates quasi-ductility to SiC/SiC composites. Nano-infiltration and transient eutectic-phase (NITE) process is appealing for the fabrication of SiC/SiC composites for use in high temperature system structures. However, the appropriate conditions for the PyC coating of the composites have not been sufficiently tested. In this research, SiC fibers, with several thick PyC coatings prepared using a chemical vapor infiltration continuous furnace, were used in the fabrication of NITE SiC/SiC composites. Three point bending tests of the composites revealed that the thickness of the PyC coating affected the quasi-ductility of the composites. The composites reinforced by 300?nm thick coated SiC fibers showed a brittle fracture behavior; the composites reinforced 500 and 1200?nm thick PyC coated SiC fibers exhibited a better quasi-ductility. Transmission electron microscope research revealed that the surface of the as-coated PyC coating on a SiC fiber was almost smooth, but the interface between the PyC coating and SiC matrix in a NITE SiC/SiC composite was very rough. The thickness of the PyC coating was considered to be reduced maximum 400?nm during the composite fabrication procedure. The interface was possibly damaged during the composite fabrication procedure, and therefore, the thickness of the PyC coating on the SiC fibers should be thicker than 500?nm to ensure quasi-ductility of the NITE SiC/SiC composites.     

Crack healing behaviour and high-temperature strength of mullite/SiC composite ceramics

Mechanical properties of sintered mullite/SiC ceramics related to its significant crack healing behavior are discussed in this paper. This investigation was made on four kinds of specimens such as as-received smooth, heat treated smooth, pre-cracked and pre-crack healed. Pre-crack sizes were 100 and 200 μm and they were semi-elliptical in shape. The main conclusions were obtained as follows: (a) mullite/SiC composite ceramic has ability to heal crack, (b) the best healing condition was found to be 1300 °C in air for 1 h, (c) maximum crack size able to be healed is semi-elliptical crack of 200 μm in diameter, (d) crack-healed zone has enough strength up to 1200 °C and most specimens failed outside the crack-healed zone.     

SiC nanoparticles incorporation in electroless NiP-Graphene oxide nanocomposite coatings

The presence of SiC nanoparticles within the Graphene oxide (GO) incorporated electroless deposited NiP layers (NiP-GO) on carbon steel substrate was studied in this work. The effect of co-deposition of GO nanoplatelets and/or SiC nanoparticles on the morphology and structure of the heat-treated NiP coatings were investigated by scanning electron microscope and X-ray diffraction, respectively. The results revealed that the heat-treated NiP and NiP–SiC coatings consisted of Ni and Ni₃P phases, whereas the NiP-GO also contains the intermediated Ni₂P and Ni₁₂P₅ metastable phases due to the incomplete precipitation of Ni₃P. Such metastable phases are significantly decreased by the incorporation of SiC nanoparticles in NiP-GO coatings. The mechanical properties of the coatings were characterized by microhardness measurement and “pin on disk” wear test. The corrosion tests were conducted in aqueous 3.5 %wt NaCl using electrochemical measurement for Ni–P, NiP-GO, NiP–SiC, and NiP-GO-xSiC coatings. By co-deposition of SiC nanoparticles, the hardness of NiP-GO is significantly increased and the wear loss is reduced, especially at a high sliding distance during the wear test. The corrosion behavior of the NiP-GO coatings containing different amounts of SiC nanoparticles has been investigated.     

The effects of phenolic resin-derived PyC interlayers on microstructure and mechanical properties of Cf/SiC composites

A novel, easy and cost-effective way, infiltration and pyrolysis of phenolic resin solution, was exploited to prepare pyrolytic carbon (PyC) interlayers for carbon fiber/silicon carbide (Cf/SiC) mini-composites. X-ray photoelectron spectroscopy, dynamic contact angle measurement and scanning electron microscope were carried out to characterize chemical structure of carbon fibers (CFs), wetting properties between CFs and phenolic resin solution and microstructure of CFs and their composites, respectively. Remarkably, SEM results showed regulation of uniformity and thicknesses of PyC interlayer could be achieved through controlling the concentration of phenolic resin solution and oxidation condition of CFs. When CFs were treated by 10?min' oxidation with 40?mg/L ozone followed by dip-coating with 4?wt% phenolic solution, uniform PyC interlayer with approximately 120?nm were prepared on CFs. The corresponding Cf/SiC specimens had the largest increase in tensile strength and work of fracture with the improvement of 26.2% and 71.6% from the PyC-free case.     

Microstructure and ablation mechanism of graphite/SiC composites under oxy-acetylene flame

Graphite/SiC composites were prepared by reactive metal infiltration (RMI). The microstructure and the ablation mechanism under oxy-acetylene flame were investigated. The ablation surface was composed of a central zone, an intermediate zone and an outer zone. The surface of the intermediate zone was covered by a SiO₂ layer due to the oxidation of Si and SiC. A particle layer, which consisted of SiC particles and graphite particles, was found beneath the SiO₂ layer due to the large inner stress. In the central zone, an extra SiO layer was detected owing to the high temperature and the few oxygen in the particle layer. Due to the good wettability with graphite, the SiO₂ layer exhibited excellent ablation resistance by inhibiting oxygen diffusion and lowering the mechanical erosion of the flame. Besides, the evolution of the gases formed inside the composite helped to improve the ablation resistance.     

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.     

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.     

SiC coating toughened by SiC nanowires to protect C/C composites against oxidation

A dense SiC coating toughened by SiC nanowires was prepared on carbon/carbon (C/C) composites using a two-step technique of chemical vapor deposition (CVD) to protect them against oxidation. The morphologies and crystalline structures of the coatings were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. SiC nanowires played a role in decreasing the size of the cracks and improving the thermal shock resistance of the coating. The result of thermal shock between 1773 K and room temperature for 21 times indicates that, compared with the SiC coating without SiC nanowires, the average size of the cracks in the SiC coating toughened with SiC nanowires reduced from 5 ± 0.5 to 3 ± 0.5 μm. The weight loss of the SiC coated C/C composites decreased from 9.32 to 4.45% by the introduction of SiC nanowires.     

High temperature oxidation of SiC under helium with low-pressure oxygen. Part 3: β-SiC-SiC/PyC/SiC

In the frame of generation IV gas-cooled fast reactor (GFR), the cladding materials currently considered is a SiC/SiC-based composite with a pyrocarbon interphase and a β-SiC coating on the surface to close the porosity (noted β-SiC-SiC/PyC/SiC). These elements are subjected to temperatures going from 1300 to 1500 K in nominal operating conditions to 1900-2300 K in accidental conditions. The coolant gas considered is helium pressurized at 7 MPa.After a thermodynamic study carried out on the oxidation of β-SiC under helium and low oxygen partial pressures, an experimental approach was made on β-SiC-SiC/PyC/SiC composites under active oxidation conditions (1400 ≤ T ≤ 2300 K; 0.2 ≤ pO₂ ≤ 2 Pa). This study follows two preceding studies carried out on two polytypes of SiC: α (Part 1) and β (Part 2) under the same conditions. In these studies, the influence of the crystalline structure on the transition temperature between passive and active oxidation and on the mass loss rate was discussed.The experimental study allows to determine the oxidation rates in incidental and accidental conditions under pO₂ = 0.2 and 2 Pa. The variation of the mass loss rates according to the temperature for β-SiC-SiC/PyC/SiC oxidized under pO₂ = 0.2 and 2 Pa shows the existence of three domains in the zone of active oxidation. These tests also show the weak impact of the oxygen partial pressure on the mass loss rate of the material in this range of pressure for temperatures lower than 2070 K. On the other hand, beyond 2070 K, an increase of the mass loss rate leading to important damage of the material has been observed, at lower temperature under pO₂ = 0.2 Pa than under pO₂ = 2 Pa. This variation was associated to the effect of the oxygen partial pressure on the sublimation temperature of SiC. Similar experiments were performed on pre-oxidized samples and on the face without CVD β-SiC coating and both the results are close to the ones obtained for the face with the CVD β-SiC layer.     

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.     

Nucleation and growth of SiC at the interface between molten Si and graphite

In the present work, the evolution of the SiC layer formed at the interface between liquid silicon and solid carbon is studied using a diffusion couple configuration. Reaction conditions were isothermal, with a temperature of 1450 °C maintained from 2 min to 4 h. The rapid heating and cooling of the Si–C diffusion couple specimens were achieved using a Pulse-Electric Current Sintering system. Crystallographic, compositional, and phase distribution data obtained after different reaction times were used to develop a two-stage model for SiC growth at the interface between molten Si and C. Initially, the formation of SiC at the interface is governed by diffusion of C into the molten Si and dissolution/reprecipitation of formed SiC nuclei. These nuclei further grow into larger SiC grains at the Si–C interface and this initial stage is successfully modeled using the Johnson-Mehl-Avrami-Kolmogorov model. Once a continuous SiC layer forms at the Si–C interface, the growth of SiC is controlled by the diffusion of C through the SiC layer, which can be modeled using a power rate law. However, the nature of this diffusion is difficult to determine with certainty since the rate laws for both grain boundary and bulk diffusion fit experimental data equally well.     

Microstructure and properties of (SiC, TiB2)/B4C composites by reaction hot pressing

(SiC, TiB₂)/B₄C composites were fabricated by reactive hot-pressing B₄C, Si₃N₄, -SiC and TiC powders, with (Al₂O₃ + Y₂O₃) as sintering additives. According to the thermodynamics principles, the possible reaction equations and the reaction products for the system were determined. By means of XRD, SEM of surface thermally etched and TEM the phase composition was determined. It was shown that the phase composition of sintered body was B₄C, -SiC, BN and TiB₂, and the matrix was B₄C and -SiC. The typical values of hardness, bending strength, fracture toughness and the relative density of the composites can reach HRA 88.6, 554 MPa, 5.6 MPa m^1/2 and 95.6%, respectively. Furthermore, the microstructures of the composites were analyzed by TEM, SEM and energy spectrum methods. The results show the presence of laminated structure and a clubbed frame dispersion phase and bunchy dispersion phase among the matrix. Some intragranular structures were also found in the B₄C grains. Microstructural analysis indicates that the new formed phase, uniform and fine grains, and the layered and clubbed structure play an important role in improving the properties of the composites. Fractography and crack propagation suggest that crack deflection and crack bridging are the possible toughening mechanisms.