Effects of post-sintering annealing on the microstructure and toughness of hot-pressed SiCf/SiC composites with Al2O3-Y2O3 additions

This study examined the effects of post-sintering heat treatment on enhancing the toughness of SiC_f/SiC composites. Commercially available Tyranno^® SiC fabrics with contiguous dual ‘PyC (inner)-SiC (outer)’ coatings deposited on the SiC fibers were infiltrated with a SiC + 10 wt% Al₂O₃-Y₂O₃ slurry by electrophoretic deposition. SiC green tapes were stacked between the slurry-infiltrated fabrics to control the matrix volume fraction. Densification of approximately 94% ρ_theo was achieved by hot pressing at 1750 °C, 20 MPa for 2 h in an Ar atmosphere. Sintered composites were then subjected to isothermal annealing treatment at 1100, 1250, 1350, and 1750 °C for 5 h in Ar. The correlation between the flexural behavior and microstructure was explained in terms of the in situ-toughened matrix, phase evolution in the sintering additive, role of dual interphases and observed fracture mechanisms. Extensive fractography analysis revealed interfacial debonding at the hybrid interfaces and matrix cracking as the key fracture modes, which were responsible for the toughening behavior in the annealed SiC_f/SiC composites.     

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

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.     

Microstructure and mechanical properties of hot-pressed SiC/(W,Ti)C ceramic composites

SiC/(W, Ti)C ceramic composites with different content of (W, Ti)C solid-solution were produced by hot pressing. The effect of (W, Ti)C content on the microstructure and mechanical properties of SiC/(W, Ti)C ceramic composites has been studied. Densification rates of the SiC/(W, Ti)C ceramic composites were found to be affected by addition of (W, Ti)C. Increasing (W, Ti)C content led to increase the densification rates of the composites. The sintering temperature was lowered from 2100 °C for monolithic SiC to 1900 °C for the SiC/(W, Ti)C composites. Results show that additions of (W, Ti)C to SiC matrix resulted in improved mechanical properties compared to pure SiC ceramic. The fracture toughness and flexural strength continuously increased with increasing (W, Ti)C content up to 60 vol.%, while the hardness decreased with increasing (W, Ti)C content.     

The mechanical,thermophysical and electromagnetic properties of UD SiCf/SiC composites in different directions

Unidirectional (UD) silicon carbide (SiC) fiber-reinforced SiC matrix (UD SiC_f/SiC) composites with CVI BN interphase were fabricated by polymer infiltration-pyrolysis (PIP) process. The effects of the anisotropic distribution of SiC fibers on the mechanical properties, thermophysical properties and electromagnetic properties of UD SiC_f/SiC composites in different directions were studied. In the direction parallel to the axial direction of SiC fibers, SiC fibers bear the load and BN interphase ensures the interface debonding, so the flexural strength and the fracture toughness of the UD SiC_f/SiC composites are 813.0 ± 32.4 MPa and 26.1 ± 2.9 MPa·m^1/2, respectively. In the direction perpendicular to the axial direction of SiC fibers, SiC fibers cannot bear the load and the low interfacial bonding strengths between SiC fiber/BN interphase (F/I) and BN interphase/SiC matrix (I/M) both decrease the matrix cracking stress, so the corresponding values are 36.6 ± 6.9 MPa and 0.9 ± 0.5 MPa?m^1/2, respectively. The thermal expansion behaviors of UD SiC_f/SiC composites are similar to those of SiC fibers in the direction parallel to the axial direction of SiC fibers, and are similiar to those of SiC matrix in the direction perpendicular to the axial direction of SiC fibers. The total electromagnetic shielding effectiveness (EM SE_T) of UD SiC_f/SiC composites attains 32 dB and 29 dB when the axial direction of SiC fibers is perpendicular and parallel to the electric field direction, respectively. The difference of conductivity in different directions is the main reason causing the different SE_T. And the dominant electromagnetic interference (EMI) shielding mechanism is absorption for both studied directions.     

Fabrication of TiC coated short carbon fiber reinforced Ti3SiC2 composites: Process,microstructure and mechanical properties

The C_f/Ti₃SiC₂ composites were fabricated through spark plasma sintering (SPS) and hot isostatic pressing (HIP), TiC coated C_f and Ti₃SiC₂ powder were used as starting materials. The improved fracture toughness (K_IC) and Vickers hardness (HV₁) of the TiC coated C_f/Ti₃SiC₂ composite fabricated by SPS were 7.59 MPa·m^1/2 and 7.28 GPa. On this foundation, taking the advantage of better sintering process of HIP, the highest K_IC and HV₁ achieved 8.32 MPa·m^1/2 and 9.24 GPa with fiber content of 10 vol%, which increased by 40% and 65% compared with that of monolithic Ti₃SiC₂. The reasonable control of reactive interface is the main factor for the improved mechanical properties of the composites, the TiC coating effectively protected the fiber structure from interfacial reaction compared with that of the non-coated C_f/Ti₃SiC₂. Meanwhile, the artificially designed and weakly bonded TiC coated C_f can fully exert the toughening mechanisms like fiber pull-out and debonding.     

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.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection     

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 properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm³ and 82 W/(m K), respectively.     

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.     

Effects of random chopped short carbon fibers on microstructure and mechanical properties of joints for Cf/SiC composites by reaction bonding process

Random chopped short carbon fibers (C_sf)/phenol-formaldehyde resin (PF)/SiC powder mixtures are used as filler for the joining of C_f/SiC composites to obtain SiC interlayer at the joining region. The influences of C_sf on the microstructure and mechanical properties have been investigated. Research shows that the introduction of C_sf can improve the microstructure uniformity of the joint and reduce residual silicon content in the interlayer. The joint achieve a high flexural strength of 232?±?33?MPa as the carbon fiber content is 30?wt.%, which is similar to that of the C_f/SiC composites (220?±?21?MPa). The decrease in residual silicon content and the formation of nano-sized SiC particles are the main reasons for high joining strength.     

Effect of Ge on microstructure and mechanical properties of Ti3SiC2/Al2O3 composites

Owing to the good physicochemical compatibility and complementary mechanical properties of Ti₃SiC₂ and Al₂O₃, Ti₃SiC₂/Al₂O₃ composites are considered as ideal structural materials. However, TiC and TiSi₂ typically coexist during the synthesis of Ti₃SiC₂/Al₂O₃ composites through an in-situ reaction, which adversely affects the mechanical properties of the resulting composites. In this study, Ti₃SiC₂/Al₂O₃ composites were prepared via in-situ hot pressing sintering at 1450 °C. Ge, which was used as a sintering aid, improved the purity and mechanical properties of the Ti₃SiC₂/Al₂O₃ composites. This is because Ge replaced some of the Si atoms to compensate the evaporation loss of Si to form Ti₃(Si_1-xGe_x)C₂, which showed a crystal structure similar to that of Ti₃SiC₂. Furthermore, the molten Ge accelerated the diffusion reaction of the raw materials, increasing the overall density of the Ti₃SiC₂/Al₂O₃ composites. The optimum Ge amount for improving the mechanical properties of the composites was found to be 0.3 mol. The flexural strength, fracture toughness, and microhardness of the composite with the optimum Ge amount were 640.2 MPa, 6.57 MPa m^1/2, and 16.21 GPa, respectively. The formation of Ti₃(Si_1-xGe_x)C₂ was confirmed by carrying out X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy analyses. A model crystal structure of Ti₃(Si_1-xGe_x)C₂ doped with 0.3 mol Ge was established by calculating the solid solubility of Ge.     

Effect of holding time on SiC whiskers growth of SiCw/SiC composites based on SLS technology and their mechanical properties

The in-situ SiC whisker and SiC particle composites were prepared by selective laser sintering (SLS) technology, and the longitudinal and transverse growth rates of crystal nuclei at the liquid-solid interface were calculated and analyzed under the traditional vapor-liquid-solid mechanism. A mathematical model of holding time on the number of in-Situ SiC whisker growth was established, and the prediction rate was 95%. The mechanical properties of in-situ SiC whisker and pure SiC samples with similar volume densities were calculated. The results showed that: The longitudinal growth rate of crystal nuclei at the liquid-solid interface was higher than the transverse growth rate. After precursor infiltration pyrolysis (PIP) four-cycle treatment, the fracture toughness per unit volume density of B-1, B-2, and B-3 specimens increased by 198.66%, 225.00%, and 221.05%, respectively, compared with pure SiC specimens, indicating that this method has a vital role in increasing the toughness of SiC ceramics.     

Flexural behaviors and microstructures of SiC/SiC composites fabricated by microwave sintering assisted with heat molding process

To improve densification degree and reduce process time, microwave sintering and heat molding method were combined to prepared SiC matrix reinforced SiC (SiC/SiC) composite via polymer infiltration and pyrolysis process (PIP). The effects of heat molding pressures on the densification process, flexural behaviors and failure modes of the fabricated SiC/SiC were examined via scanning electron microscopy (SEM), computed tomography (CT) technique and mercury intrusion test. Results indicate that heat molding process promoted the densification degrees of SiC/SiC and adjusted the interphase bonding between SiC matrix and SiC fibers on the basis of rapid microwave heating. Owing to the appropriate interphase bonding, SiC/SiC composites fabricated under the heat molding pressure of 3 MPa had preferable flexural properties and failure mode.     

Ti3SiC2-Cf composites by spark plasma sintering: Processing,microstructure and thermo-mechanical properties

MAX phases, and particularly Ti₃SiC₂, are interesting for high temperature applications. The addition of carbon fibers can be used to reduce the density and to modify the properties of the matrix. This work presents the densification and characterization of Ti₃SiC₂ based composites with short carbon fibers using a fast and simple fabrication approach: dry mixing and densification by Spark Plasma Sintering. Good densification level was obtained below 1400 °C even with a high amount of fibers. The reaction of the fibers with the matrix is limited thanks to the fast processing time and depends on the amount of fibers in the composite. Bending strength at room temperature, between 437 and 120 MPa, is in the range of conventional CMCs with short fibers and according to the resistance of the matrix and the presence of residual porosity. Thermo-mechanical properties of the composites up to 1500 °C are also presented.     

Effects of channel modification on microstructure and mechanical properties of C/SiC composites prepared by LA-CVI process

Carbon fiber reinforced SiC matrix composites (C/SiC) with four different deposition channel sizes were fabricated via a novel laser-assisted chemical vapor infiltration (LA-CVI) method. Effects of infiltration channel sizes on microstructure and mechanical properties of C/SiC composites were investigated. The results showed that increasing the size of channels could expand infiltration passages and densification bands, which was consistent with theoretical calculations. Due to the presence of channels, the flexural strength of C/SiC composite increased by 14.47% when the channel diameter was 0.3?mm, compared to C/SiC composites prepared via conventional CVI process. Characteristics of matrix cracking and crack propagation on fracture surface were analyzed by using scanning electron microscopy. LA-CVI C/SiC composites displayed significantly improved damage-tolerant fracture behavior. Thus, findings of this work demonstrate that LA-CVI fabricated C/SiC composites are promising for a wide range of applications, particularly for enclosed-structure and thick-section C/SiC composites.     

Effects of ZrO2 nanoparticles on the microstructure and mechanical properties of ZrO2/AlSi10Mg composites manufactured by laser powder bed fusion

In this work, the nano-ZrO₂ particles were mixed into AlSi10Mg alloy to prepare ZrO₂/AlSi10Mg composites with different x wt.% ZrO₂ (x = 0, 0.15, 0.3, 0.45, 0.6, 0.75). The microstructure, mechanical properties and the anisotropy of the ZrO₂/AlSi10Mg composites fabricated by laser powder bed fusion (LPBF) were studied. The results show that nano-ZrO₂ particles can be uniformly dispersed on the AlSi10Mg powder by the method of pre-dispersion and mechanical mixing. When the mass ratio of ZrO₂ in ZrO₂/AlSi10Mg composites is 0.3 wt%, the values of the tensile strength, yield strength and elongation are 493.64 MPa, 321.30 MPa and 11.74%, respectively. Compared with AlSi10Mg alloy, the tensile strength of ZrO₂/AlSi10Mg composites with 0.3 wt% is increased by 30–55 MPa and the elongation is increased by 3–5%. In addition, the mechanical properties of AlSi10Mg alloy and ZrO₂/AlSi10Mg composites of 0.3 wt% exhibit antistrophic behavior in different direction, which is due to the differences of microstructure, texture and stress distribution between transverse direction (TD) and build direction (BD). Compared with other AlSi10Mg matrix composites, ZrO₂/AlSi10Mg composites of this work show excellent strength and plasticity matching.     

Effects of carbonyl nickel powder addition on the mechanical properties,microstructure, and yttrium segregation of sintered 3YSZ composites

In this study, the effects of the addition of carbonyl nickel powder on the density, microstructure, and mechanical properties of sintered yttria-stabilized zirconia (3YSZ) were investigated. Sintering at 1300 °C resulted in the optimum comprehensive mechanical properties. The addition of 5 vol% carbonyl Ni increased the fracture toughness and flexural strength from 9.51 MPa m^1/2 to 14.5 MPa m^1/2 and from 747 MPa to 873 MPa, respectively. The addition of carbonyl nickel showed greater improvement than did the addition of spherical Ni powder. The dendritic morphology improved the interface bonding between the ceramic and the metal, enabling a bridging mechanism of the ductile phase. However, further Ni addition decreased the mechanical properties. X-ray diffraction results showed that the amounts of the monoclinic phase (M) and cubic phase (C) of 3YSZ increased, whereas the amount of the tetragonal phase (T) decreased. The Y segregation near the Ni particles, which was confirmed by an energy dispersive spectrometer (EDS), caused the phase changes. The segregation of Y occurred during the cooling stage, rather than the holding stage, of sintering. During the cooling stage, the heat mismatch between Ni and ZrO₂ resulted in strong elastic strain energy, which promoted Y segregation.     

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).