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.     

Effects of SiC nanowires and whiskers on high-entropy carbide-based composites sintered at low and high temperatures

This study aimed to investigate the toughening effects of SiC nanowires (SiC_nw) and SiC whiskers (SiC_w) on high-entropy carbide based composites prepared at different temperatures (1600°C and 2000°C). At low temperature (1600°C), SiC_nw and SiC_w maintain their original morphology and properties, and exhibit the good toughening effects. The SiC_nw with larger aspect ratio and more curly wires exhibit a much stronger toughening effect on the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites reinforced with 15 vol.% SiC_nw, which shows the highest value of fracture toughness about 6.7 MPa∙m^1/2. However, at high sintering temperature (2000°C), SiC_nw and SiC_w are prone to thermal-induced damages, which significantly reduces their mechanical properties, and thus, toughening effects on (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites. The addition of SiC_w, which have better thermal stability at 2000°C, results in the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8–15 vol.% SiC_w composite exhibiting relatively better fracture toughness, about 3.7 MPa∙m^1/2. Based on the results of the current study, the critical influence of SiC_nw and SiC_w on the toughening of (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites is highly dependent on their high-temperature thermal stability.     

Oxidation protection and mechanism of the HfB2-SiC-Si/SiC coatings modified by in-situ strengthening of SiC whiskers for C/C composites

To improve the oxidation resistance and alleviate the thermal stress of the HfB₂-SiC-Si/SiC coatings for C/C composites, in-situ formed SiC whiskers (SiC_w) were introduced into the HfB₂-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiC_w on isothermal oxidation and thermal shock resistance for the HfB₂-SiC-Si/SiC coatings were investigated. Results showed that the SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent oxidation resistance for C/C composites with only 0.88% weight loss after oxidation for 468?h at 1500?°C, which was markedly superior to 4.86% weight loss for coatings without SiC_w. Meanwhile, after 50 times thermal cycling, the weight loss of the SiC_w-HfB₂-SiC-Si/SiC coated samples was 4.48%, which showed an obvious decrease compared with that of the HfB₂-SiC-Si/SiC coated samples. The SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent adhesion to the C/C substrate and had no penetrating cracks after oxidation. The improved performance of the SiC_w-HfB₂-SiC-Si/SiC coatings could be ascribed to the SiC_w, which effectively relieved CTE mismatch and remarkably suppressed the cracks through toughening mechanisms including whiskers pull-out and bridging strengthening. The above results were confirmed by thermal analysis based on the finite element method, which demonstrated that SiC_w could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiC_w-HfB₂-SiC-Si/SiC coating can provide a promising fail-safe mechanism during the high temperature oxidation by the formation of HfSiO₄ and SiO₂, which can deflect cracks and heal imperfections.     

Residual thermal stress of SiC/Ti3SiC2/SiC joints calculation and relaxed by postannealing

Residual thermal stresses in SiC/Ti₃SiC₂/SiC joining couples were calculated by Raman spectra and simulated by finite element analysis, and then relaxed successfully by postannealing. The results showed that the thermal residual stress between Ti₃SiC₂ and SiC was about on the order of 1 GPa when cooling from 1300°C to 25°C. The thermal residual stresses can be relaxed by the recovery of structure disorders during postannealing. When the SiC/Ti₃SiC₂/SiC joints postannealed at 900°C, the bending strength reached 156.9 ± 13.5 MPa, which was almost twice of the as‐obtained SiC/Ti₃SiC₂/SiC joints. Furthermore, the failure occurred at the SiC matrix suggested that both the flexural strength of joining layer and interface were higher than the SiC matrix.     

Fast seamless joining of SiCw/Ti3SiC2 composite using electric field-assisted sintering technique

A pair of Ti₃SiC₂ reinforced with SiC whiskers (SiC_w/Ti₃SiC₂) composites was successfully joined without any joining materials using electric field-assisted sintering technology at a temperature as low as 1090°C (T_i) and a short time of 30 s. The microstructure and mechanical properties of the obtained SiC_w/Ti₃SiC₂ joints were investigated. The solid-state diffusion was the main joining mechanism, which was facilitated by a relatively high current density (~586 A/cm²) at the joining interface. The shear strength of the sample joined at 1090°C was 51.8 ± 2.9 MPa. The sample joined at 1090°C failed in the matrix rather than at the interface, which confirmed that a sound inter-diffusion bonding was obtained. A rapid and high efficient self-joining process may find application in the case of SiC_w/Ti₃SiC₂ sealing cladding tube and end cap.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Bonding strength and thermal shock resistance of a novel bilayer (c-AlPO4-SiCw-mullite)/SiC coated carbon fiber reinforced CMCs

Mullite coating, SiC whiskers toughened mullite coating (SiC_w-mullite), and cristobalite aluminum phosphate (c-AlPO₄) particle modified SiC_w-mullite coating (c-AlPO₄-SiC_w-mullite) were prepared on SiC coated C/SiC composites using a novel sol-gel method combined with air spraying. Results show that molten SiO₂ formed by the oxidation of SiC whiskers and molten c-AlPO₄ improved the bonding strength between mullite outer coating and SiC–C/SiC composites due to their high-temperature bonding properties. The bonding strength between mullite, SiC_w-mullite, c-AlPO₄-SiC_w-mullite outer coatings and SiC–C/SiC composites were 2.41, 4.31, and 7.38 MPa, respectively. After 48 thermal cycles between 1773 K and room temperature, the weight loss of mullite/SiC coating coated C/SiC composites was up to 11.61%, while the weight losses of SiC_w-mullite/SiC and c-AlPO₄-SiC_w-mullite/SiC coatings coated C/SiC composites were reduced to 7.40% and 5.12%, respectively. The addition of appropriate SiC whiskers can considerably improve the thermal shock resistance of mullite coating owing to their excellent mechanical properties at high temperature. In addition, c-AlPO₄ particles can further improve the thermal shock resistance of SiC_w-mullite coating due to their high-temperature bonding and sealing properties. No obvious micro-pores and cracks were observed on the surface of c-AlPO₄-SiC_w-mullite coating after 48 thermal cycles due to timely healing effect by formation of secondary mullite.     

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.     

Ti3SiC2 interphase coating in SiCf/SiC composites: Effect of the coating fabrication atmosphere and temperature

The SiC fibers were coated with Ti₃SiC₂ interphase by dip-coating. The Ti₃SiC₂ coated fibers were heat-treated from 900 °C to 1100 °C in vacuum and argon atmospheres to comparatively analyze the effect of temperature and atmosphere on the microstructural evolution and mechanical strength of the fibers. The results show that the surface morphology of Ti₃SiC₂ coating is rough in vacuum and Ti₃SiC₂ is decomposed at 1100 °C. However, in argon atmosphere, the surface morphology is smooth and Ti₃SiC₂ is oxidized at 1000 °C and 1100 °C. At 1100 °C, Ti₃SiC₂ oxidized to form a thin layer of amorphous SiO₂ embedded with TiO₂ grains. Meanwhile, defects and pores appeared in the interphase scale. As a result, the fiber strength treated in the argon was lower than that treated in vacuum. The porous Ti₃SiC₂ interphase fabricated under vacuum was then employed to prepare the SiC_f/SiC mini composite by chemical vapor infiltration (CVI) combined with precursor infiltration pyrolysis (PIP), and can effectively improve the toughness of SiC_f/SiC mini composite. The propagating cracks can be deflected within the porous interphase layer, which promotes fiber pull-outs under the tensile strength.     

Synthesis of Ti3SiC2 coatings onto SiC monoliths from molten salts

Coatings with composition close to Ti₃SiC₂ were obtained on SiC substrates from Ti and Si powders with the molten NaCl method. In this work, the growth of coatings by reaction in the salt between monolithic SiC substrates and titanium powder is obtained between 1000 and 1200 °C. At 1000 °C, a coating of 8 µm thickness is formed in 10 h whereas a thin coating of 0.5 µm has been grown in 2 h. A lack in silicon was first found in the coatings prepared at 1100 and 1200 °C. For these temperatures, the addition of silicon powder in the melt had a favorable effect on the final composition, which is found very close to the composition of Ti₃SiC₂. The reaction mechanism implies the formation of TiC_x layers in direct contact with the SiC substrate and the presence of more or less important quantities of Ti₃SiC₂ and Ti₅Si₃C_x in the upper layers.     

Fabrication of Ti3SiC2 and Ti4SiC3 MAX phase ceramics through reduction of TiO2 with SiC

Ti₃SiC₂ and Ti₄SiC₃ MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO₂ using SiC as a reductant, followed by hot pressing of the products under 25 MPa of pressure at 1600 °C. It was found that both Ti₃SiC₂ and Ti₄SiC₃ may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti₃SiC₂ and Ti₄SiC₃ under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti₄SiC₃ compared to Ti₃SiC₂. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.     

Pore and microstructure change induced by SiC whiskers and particles in porous TiB2–TiC–Ti3SiC2 composites

TiB₂–TiC–Ti₃SiC₂ porous composites were prepared through a plasma heating reaction using powder mixtures of Ti, B₄C SiC whiskers (SiC_w) and SiC particles (SiC_p). The effects of the SiC_w and SiC_p content on pore structures, phase constituents, microstructure, and crystal morphology of TiC were studied. The results show that TiC, TiB, Ti₃B₄ phases are formed within the 5Ti+B₄C system. With the addition of SiC_w and SiC_p, the TiB and Ti₃B₄ phases are reduced, sometimes even disappeared. Interestingly, the content of TiB₂ and TiC increased, resulting in Ti₃SiC₂ and TiSi₂ being formed. The porosity of composites increases notably with the addition of SiC_w. However, with the increase of SiC_p, the porosity of the composites first decreases, followed by an increase. After adding the specified amount of SiC_w/SiC_p, the compressive strength of composites are improved significantly. Additionally, the pore size of the composites are decreased significantly with the addition of SiC_w/SiC_p. During the plasma heating process, some Si atoms will diffuse into the TiC lattice, which in turn made the cubic TiC grains into hexagonal lamellar TiC or Ti₃SiC₂ grains.     

Fabrication of Ti3SiC2-Ti4SiC3-SiC ceramic composites through carbosilicothermic reduction of TiO2

Dense Ti₃SiC₂-SiC, Ti₄SiC₃-SiC, and Ti₃SiC₂-Ti₄SiC₃-SiC ceramic composites were fabricated through carbosilicothermic reduction of TiO₂ under vacuum, followed by hot pressing of the as-synthesized products under 25 MPa at 1600°C. In the reduction step, SiC either alone or in combination with elemental Si was used as a reductant. A one-third excess of SiC was added in the reaction mixtures in order to ensure the presence of approximately 30 vol.% SiC in the products of synthesis. During the hot pressing step, the samples that contained Ti₃SiC₂ showed better densification compared to those containing Ti₄SiC₃. The obtained composites exhibited the strength properties typical of coarse-grained MAX-phase ceramics. The flexural strength values of 424 and 321 MPa were achieved in Ti₃SiC₂-SiC, and Ti₃SiC₂-Ti₄SiC₃-SiC composites, respectively. The fracture toughness values were 5.7 MPa·m^1/2.     

Theoretical design and preparation of SiC whiskers catalyzed by Fe-oxides on carbon fibers

This work reported an in-situ vapor-liquid-solid (VLS) preparation method of SiC whiskers (SiC_w) catalyzed by Fe-oxides on carbon fibers, which could provide a method for preparing SiC_w/carbon fiber composites. The mechanism of the SiC_w was theoretically designed and then experimentally validated using XRD, SEM, and TEM. Fe₂O₃ was chosen as a Fe-oxide catalyst and directly loaded on carbon fibers by the impregnation process. The results showed that SiC_w were successfully prepared on carbon fibers at 1600 °C under the protection of flowing nitrogen, utilizing quartz and graphite as gas-phase generation sources. The prepared SiC_w were β-SiC and grew along the (111) crystal plane, with spherical droplets on top formed by Fe₂O₃ catalysts. SiC_w were microstructurally observed to have widths of 500–1000 nm and lengths of more than 15 μm, respectively.     

Compressive creep of SiC whisker/Ti3SiC2 composites at high temperature in air

The compressive creep of a SiC whisker (SiC_w) reinforced Ti₃SiC₂ MAX phase-based ceramic matrix composites (CMCs) was studied in the temperature range 1100-1300°C in air for a stress range 20-120 MPa. Ti₃SiC₂ containing 0, 10, and 20 vol% of SiC_w was sintered by spark plasma sintering (SPS) for subsequent creep tests. The creep rate of Ti₃SiC₂ decreased by around two orders of magnitude with every additional 10 vol% of SiC_w. The main creep mechanisms of monolithic Ti₃SiC₂ and the 10% CMCs appeared to be the same, whereas for the 20% material, a different mechanism is indicated by changes in stress exponents. The creep rates of 20% composites tend to converge to that of 10% at higher stress. Viscoplastic and viscoelastic creep is believed to be the deformation mechanism for the CMCs, whereas monolithic Ti₃SiC₂ might have undergone only dislocation-based deformation. The rate controlling creep is believed to be dislocation based for all the materials which is also supported by similar activation energies in the range 650-700 kJ/mol.     

Processing and mechanical properties of B4C-SiCw ceramics densified by spark plasma sintering

Homogenous distribution of whiskers in the ceramic matrix is difficult to be achieved. To solve this problem, B₄C-SiC_w powder mixtures were freeze dried from a slurry dispersed by cellulose nanofibrils (CellNF) in this work. Dense B₄C ceramics reinforced with various amounts of SiC_w up to 12 wt% were consolidated by spark plasma sintering (SPS) at 1800 °C for 10 min under 50 MPa. During this process, CellNF was converted into carbon nanostructures. As iron impurities exist in the starting B₄C and SiC_w powders, both thermodynamic calculations and microstructure observations suggest the dissolution and precipitation of SiC_w in the liquids composed of Fe-Si-B-C occurred during sintering. Although not all the SiC_w grains were kept in the final ceramics, B₄C-9 wt% SiC_w ceramics sintered at 1800 °C still exhibit excellent Vickers hardness (35.5 ± 0.8 GPa), flexural strength (560 ± 9 MPa) and fracture toughness (5.1 ± 0.2 MPa·m^1/2), possibly contributed by the high-density stacking faults and twins in their SiC grains, no matter in whisker or particulate forms.     

Influence of small amounts of Fe and V on the synthesis and stability of Ti3SiC2

Polycrystalline bulk samples of (Ti_1-yMe_y)₃SiC₂, where Me=Fe or V and y=0.01 to 0.1, were fabricated by reactive hot isostatic pressing of a mixture of Ti, C (graphite), SiC and Fe or V at 1450°C for 4 h under a pressure of 60 MPa. X-ray diffraction and scanning electron microscopy of the fully dense samples have shown that small amounts of Fe and V interfere with the reaction between Ti, C and SiC leading to the presence of SiC, TiC_x, as well as different Fe and V-containing phases in the final microstructures. The presence of these impurity phases also reduces the temperature at which Ti₃SiC₂ decomposes. The decomposition is manifested by the formation of a network of pores when the samples are annealed at 1600°C, a temperature at which pure Ti₃SiC₂ is thermally stable. The concentration threshold for this decomposition is as low as 1 at%.     

Effect of SiC whiskers on the anti-oxidation properties of Yb2Si2O7/mullite/SiC tri-layer environmental barrier coating for CMCs

The mullite and ytterbium disilicate (β-Yb₂Si₂O₇) powders as starting materials for the Yb₂Si₂O₇/mullite/SiC tri-layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti-oxidation properties of Yb₂Si₂O₇/mullite/SiC tri-layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb₂SiO₅, α-Yb₂Si₂O₇, and β-Yb₂Si₂O₇ were gradually formed between 800 and 1000°C, and Yb₂SiO₅ and α-Yb₂Si₂O₇ were completely transformed into β-Yb₂Si₂O₇ at a temperature above 1200°C. The weight loss of Yb₂Si₂O₇/(SiC_w–mullite)/SiC tri-layer coating coated specimens was 0.15 × 10⁻³ g cm⁻² after 200 h oxidation at 1400°C, which is lower than that of Yb₂Si₂O₇/mullite/SiC tri-layer coating (2.84 × 10⁻³ g cm⁻²). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β-Yb₂Si₂O₇ outer coating, but also improve the self-healing performance of the mullite middle coating owing to the self-healing aluminosilicate glass phase formed by the reaction between SiO₂ (oxidation of SiC whiskers) and mullite particles.     

Wetting behavior and brazing of titanium-coated SiC ceramics using Sn0.3Ag0.7Cu filler

By coating active titanium, Sn0.3Ag0.7Cu (SAC) filler wetted SiC effectively, as the contact angle decreased significantly from ~145° to ~10°. Ti₃SiC₂ and TiO_x (x ≤ 1) reaction layers were formed at the droplet/SiC interface, leading to the reduction of contact angle. Reliable brazing of SiC was achieved using titanium deposition at 900°C for 10 minutes, and the typical interfacial microstructure of Ti-coated SiC/SAC was SiC/TiO_x + Ti₃SiC₂/Sn(s,s). Comparing to direct brazing, Ti–Sn compounds in the brazing seam were effectively reduced and the mechanical property of joints was dramatically improved by titanium coating. The optimal average shear strength of SiC joints reached 25.3 MPa using titanium coating- assisted brazing, which was ∼62% higher than that of SiC brazed joints using SAC-Ti filler directly.     

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.