Strength Of SiC Fibers Research Articles

Ultrahigh room and high − temperature mechanical properties of SiCf/SiC composites prepared by hybrid CVI and PIP methods: Effects of PIP temperature

Unidirectional (UD) SiCf/SiC composites were prepared using chemical vapor infiltration (CVI)-polymer infiltration and pyrolysis (PIP) hybrid procedure at different PIP temperatures of 1100 °C (1100PIP), 1300 °C (1300PIP), and 1500 °C (1500PIP). The effect of PIP temperature on the microstructure of each component was studied. Results showed that SiC fiber strength and interfacial shear strength (IFSS) were the main factors affecting the mechanical properties of the composite. At 1100 °C, the fiber was thermally stable and IFSS was high, due to which 1100PIP achieved ultrahigh mechanical performance with tensile strength of 901.0 ± 87.7 MPa, flexural strength of 2186.5 ± 192.5 MPa, and toughness of 80.6 ± 12.0 MPa·m1/2. At 1300 °C, IFSS decreased slightly, due to the crystallization of BN interphase. Hence, the mechanical performance of 1300PIP decreased slightly to 789.8 ± 42.9 MPa, 1935.9 ± 163.2 MPa, and 58.2 ± 4.0 MPa·m1/2, respectively. At 1500 °C, severe fiber ceramization and decrease in IFSS caused severe decline in mechanical performance to about half of that of 1100PIP. The crack could be deflected not only at the fiber/BN (F/B) interface, but also at the CVI SiC/PIP SiC (C/P) interface, due to the existence of free carbon layers at the C/P interface, which played an important role in improving the strength and toughness of the composite. 1300PIP also showed excellent strength at high − temperature. At 1350 °C and 1500 °C, its flexural strengths were as high as 1529.0 ± 73.0 MPa and 1223.1 ± 81.1 MPa, respectively. The thermal conductivity and thermal expansion coefficient were also tested. Their values were mainly affected by the grain size and thermal stabilities of the SiC fiber and PIP matrix.

Microstructure evolution and mechanical properties of SiCf/BN/SiBCN composite after high temperature thermal exposure

The present study investigates the microstructure evolution and subsequent mechanical properties of SiCfiber/BN/SiBCNmatrix composites after high temperature exposure. These composites display a non-brittle failure response under three-point bending retaining 80% of the as-processed strength, even after elevated temperature exposure up to 1350oC for 10h. This is due to crack deflection accompanied by extensive fiber pull-out. In addition, both thermodynamic modelling and phase analysis by XRD show higher matrix degradation in vacuum than in N2 atmosphere due to the lower N2 partial pressure. After thermally exposed at 1500°C, carbothermal reaction in the matrix leads to the formation of a porous composite, and the composites retains a non-brittle failure behaviour. Meanwhile, SiBCN matrix degradation and SiC fiber strength degradation occurs, which results in significant composite strength decrement. Modest increases in the fiber/matrix interfacial shear strength occur upon exposure at temperatures up to 1350°C, and then significantly reduced after expsoure at 1500oC in N2.

An analytical model for the high temperature fracture strength of SiC fiber reinforced ceramic matrix composites considering oxidation and residual thermal stresses

An analytical model for the high temperature fracture strength of SiC fiber reinforced ceramic matrix composites considering oxidation and residual thermal stresses

SiC fibers with different diameters exhibiting excellent high-temperature resistance and oxidation resistance

SiC fibers have important application in aerospace and other high-tech equipment due to their excellent mechanical properties and thermal resistance. In this work, SiC fibers with diameters of 9, 14, 60 μm were prepared in order to investigate the influence of different diameters on the precursor-derived SiC fibers. The results showed that the polycrystalline structure and near stoichiometric composition of the SiC-9 and SiC-14 fibers were achieved by the “one-step” approach. However, the sintering for SiC-60 fiber was insufficient due to the largest diameter. Moreover, the tensile strength of SiC fibers decreased from 1.88 GPa to only 0.28 GPa with increment of the fiber diameter. The fibers exhibited excellent high temperature resistance with tensile strength retention reaching over 94% after being kept at 1900 °C in argon for 1 h. Furthermore, the SiC fibers with sintered and densified microstructure showed good oxidation resistance. This work could help to further optimize the structure of the polycrystalline near stoichiometric SiC fibers for higher mechanical and thermal performance.

Role of H2 and Ar as the diluent gas in continuous hot-wire CVD synthesis of SiC fiber

W-core SiC fiber was synthesized with CH3SiCl3 as the precursor by continuous hot-wire CVD approach. Using H2 and Ar as the diluent gas, the temperature profile and the production of atomic hydrogen (H) in high-speed deposition process were studied by computational flow dynamics (CFD) simulation, and the growth kinetics and microstructure of the deposit were systematically investigated. H2 significantly increases the H production, which reduces the formation of amorphous phase and structural defects (e.g. stacking faults and twins) during SiC growth. Ar could establish a steep thermal gradient in reactor, which enhances the deposit’s surface quality. The mean tensile strength of SiC fiber reaches its peak value (3.15 GPa) with H2 at 1200 sccm, and a high Weibull modulus (14.7) is achieved with Ar at 1600 sccm. These findings indicate H2 is crucial to promote the mechanical performance of SiC fiber, while Ar is beneficial to improving its uniformity.

Study of SiC fiber axial compressive behavior using tensile recoil measurement

AbstractSiC fibers have been widely investigated as reinforcements for advanced ceramic matrix composites owing to their excellent high‐temperature properties. However, the axial compressive strength of SiC fibers has not been thoroughly studied. In this study, the compressive behavior of two SiC fiber types containing different compositions and thermal degradation were characterized by tensile recoil measurements. Results illustrated that the SiC fiber compressive strength was 30%–50% of its tensile strength, after heat treatment at 1200℃–1800℃ for 0.5 h in argon. The fiber compressive failure mechanism was studied, and a “shear‐bending‐cleavage” model was proposed for the recoil compression fracture of pristine SiC fibers. The average compressive and tensile strengths of the pristine SiC‐II fiber were 1.37 and 3.08 GPa, respectively. After treatment at 1800℃ for 0.5 h in argon, the SiC‐II fiber compressive strength decreased to 0.42 GPa, whereas the tensile strength reduced to 1.47 GPa. The mechanical properties of the fibers degraded after high‐temperature treatment. This could be attributed to SiC grain coarsening and SiCxOy phase decomposition.

Mechanical Properties of Interface Layers in SiC/TiAl Composite

Although metal matrix composites (MMC) for the high temperature structural material have been investigated extensively for many years, applications of MMC have been still limited. Among many combinations between the ceramic fibers and the matrix materials, combination of SiC fiber and TiAl based intermetallic compounds has been expected to be one of the best combination, since both SiC fiber and TiAl have demonstrated the capabilities of the low density heat resistant materials. SiC fiber reinforced TiAl composites have been successfully fabricated using hot press method. Optimum temperature and pressure have been determined. SiC/TiAl composite having relatively low fiber volume fraction shows nearly an ideal elastic property applying the law of mixture. Effects of interface layers on the mechanical properties of composites have been studied in detail. Micro-indentation on a single fiber was carried out to examine the pull out strength of SiC fiber quantitatively. Estimated shear stress on the interface was 145-195MPa, those values are quite reasonable since the tensile strength of TiAl matrix was 420MPa and the maximum shear stress would be the half of tensile strength according to Schmid law. Three-point bending tests have been carried out to evaluate the mechanical properties of composites. Fiber volume fraction 8.9% specimen shows ideal bending stiffness compare with the calculated values based on the low of mixture. Reaction layers and the interface between SiC fiber and TiAl have been analyzed by SEM-EDS and XRD. At least two or more reaction layers have been identified. These reaction layers can be explained based on the Si-Ti-C ternary equilibrium phase diagram at 1373K. Optimum conditions of interface structure will be discussed

Effects of temperature and atmosphere on microstructural evolution and mechanical properties of KD-II SiC fibers

KD-II SiC fibers were heat treated from 800 °C to 1500 °C in argon and air atmospheres to comparatively analyze the effect of temperature and applied atmosphere on the microstructural evolution and mechanical behaviors of the fibers. The results show that sizes of the β-SiC grains, free carbon clusters, and micropores increase in both atmospheres as the temperature rises. Consequently, the strength of SiC fibers, in general, decreases with the increase in heat treatment temperature. However, SiC fibers responded differently during heat treatment at various temperatures in air compared to argon. The passive oxidation in air could cause more defects in the fibers. At 1200 °C, a lot of oxidation products were formed on the surface of the fibers; at 1400 °C, cracks appeared on the oxidation (silica) scale; and at 1500 °C, spallation of the oxide occurred. The cracking and spallation of the surface oxide may stem from the internal stress produced at the interface between the oxide scale and the unoxidized SiC core, which caused an earlier fiber failure in the air environment. As a result, the fiber strength treated in the air was lower than that treated in argon when the temperature was above 1200 °C.

Effect of Thermal Exposure on the Interface Microstructure and Interfacial Shear Strength of the SiC Fiber Reinforced AlFe5Si2 Matrix Composite

The effect of high temperature on the interface microstructure and interfacial shear strength of the continuous SiC fiber reinforced AlFe5Si2 matrix (45 vol.% SiCf/AlFe5Si2) composite was investigated. The composite was prepared by hot isostatic pressing (HIP), and then thermally exposed at 260/300/350/400/450 °C for 20/50/100 h, respectively. The interfacial shear strength of the composite with and without thermal exposure was measured by push-out test. The interface microstructure and phase compositions of the composite with and without thermal exposure were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Results showed that the interfacial reaction took place at the interface between SiC fiber and aluminum matrix and formed Al4C3 phase during the preparation process. The activation energy of Al4C3 growth at the C/Al interface was about 163 kJ/mol. With increasing the thermal exposure temperature and time, the interfacial shear strength of the composite declined, especially at temperatures above 400 °C. The brittle Al3.21SiO0.47 compound formed at temperatures above 400 °C mainly resulted in the severe strength deterioration of the interfacial reaction zone.

Revealing the formation mechanism of the skin-core structure in nearly stoichiometric polycrystalline SiC fibers

The polymer-derived SiC fibers have broad application prospects in the fields of aerospace, nuclear industry and high-tech weapon. Oxygen plays an essential role in adjusting the composition, structure and tensile strength of SiC fibers. Our studies have found that introducing too much oxygen during air curing process will form the skin-core structure in nearly stoichiometric polycrystalline SiC fibers. In order to reveal the formation mechanism of the skin-core structure, gradient oxygen was introduced into the fibers. The morphologies, phase distributions and defects of the fibers were well characterized. By strictly controlling the introduction of oxygen, the polycrystalline product fiber exhibits intragranular fracture behavior and excellent high-temperature resistance. The retention rate of its tensile strength can reach up to 91% and 61% after exposure at 1800 °C for 1 h and 10 h, respectively. The present results give valuable insights into the structural optimization of the nearly stoichiometric polycrystalline SiC fibers.

Reduced Pressure Curing on Polycarbosilane Precursor for Synthesis of Silicon Carbide Fiber

Low pressure curing method with iodine vapor was used on low softening temperature polycarbosilane (PCS) precursor for fabrication of continuous SiC fiber at relatively low temperature. The low curing temperature can provide with a wide range of softening temperature PCS precursors, especially with low softening PCSs, which have a good spinnability, but many difficulties with conventional oxidation curing method. The low pressure curing method having the presence of iodine vapor have shown the more positive effect on pyrolysis with early stage crystallization of β-SiC at 1300 °C. Crystal size of β-SiC, cured at 0.008 kPa is around 2–3 nm larger than cured at 101 kPa. In addition, the higher tensile strength of SiC fiber at elevated temperature can be obtained at 0.008 kPa with a value of 2.1 GPa, compare to 1.3 GPa at 101 kPa of curing pressure condition.

Model for SiC fiber strength after oxidation in dry and wet air

AbstractThe strengths of oxidized SiC fibers were modeled from the effects of SiO2 scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO2‐SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal‐Grove oxidation kinetics for glass and crystalline scales. Kolmogorov‐Johnson‐Mehl‐Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed.

Failure Modeling of SiC/SiC Mini-Composites in Air Oxidizing Environments

An iterative method was presented for simulation of the failure process of SiC/SiC mini-composites with pyrolytic carbon interphase exposed to air oxidizing environments under a constant load at 900 °C. This method was based on the possibility fracture strength of SiC fibers caused by random defects and the fiber stress distribution in mini-composites. The fiber strength probability model and Monte Carlo simulation were combined to generate the fracture strength along SiC fibers at 900 °C. The influence of fiber arrangement on fiber stress distribution was assessed to simplify the geometry model which was used to calculate the fiber stress distribution in the mini-composites. The failure process of the mini-composites was simulated, and the calculated oxidation life of the mini-composites matches the experimental data well with an error of −9.40%.

SiC fiber strength after low pO2 oxidation

AbstractHi‐Nicalon™‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO2 of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO2, 1400°C with 100 ppm pO2 or higher, and 1300°C with 50 ppm pO2 or higher. When active oxidation did not occur, a glass SiO2 scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO2. Fiber strengths were lowest after heat treatment at 1300°C and a pO2 of 3.7 ppm, 1400°C and a pO2 of 20 ppm, and 1500°C and a pO2 of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.

Effect of heat treatment on the microstructure and properties of CVD SiC fiber

In this study, the effect of heat treatment on the room temperature strength of W-core SiC fiber produced by chemical vapor deposition (CVD) was investigated. Thermal exposure in the temperature range of 900–1000°C decreases the strength of the SiC fiber. Fracture morphology analysis indicates that failure initiations predominantly take place at the W-core/SiC interface. A reaction layer that formed at the W-core/SiC interface during thermal exposure degraded the fiber strength and an empirical linear relationship of strength vs thickness of the reaction layer can be obtained. The kinetics of the growth of the W-core/SiC reaction layer were determined.

Microstructure and mechanical properties of SiC fibres after exposure at elevated temperature

To understand the service behaviour of SiC fibres, the effects of ambient environment and temperature on the microstructure, mechanical property and oxidation behaviour of these fibres were investigated. The result shows that, the surface of SiC fibres becomes rough after exposure in air from 973 to 1573 K due to the formation of small SiO2 particles, and a smooth SiO2 film will be formed on the SiC fibre at 1773 K. In Ar atmosphere, SiC fibres will change into clusters of large SiC crystals after heat treatment for 2 h at 2373 K. The tensile strength of SiC fibres decreased by 66 and 95% when the fibres were exposed at 1773 K for 5 min in air and 2373 K for 2 h in Ar, respectively. This degradation is associated with the evaporation of CO and SiO from the fibres as well as with SiC grain growth in the fibres.

Effect of microwave sintering time on the flexural properties of the SiC/SiC composites

In order to investigate the effect of microwave sintering time on the mechanical properties of SiC/SiC composites, four groups of SiC/SiC composites were fabricated via polymer impregnation and pyrolysis (PIP) process using microwave heating with different sintering time‐ 0.5h, 1.0h, 1.5h and 2.0h (at 1100°C) at each PIP cycle. The flexural properties, the interfacial debonding strength between SiC fiber and matrix of the fabricated SiC/SiC composites and the SiC fiber filament strength after heat treatment were investigated. The results indicated that after heat treatment the residual strength of SiC fibers as reinforcement decreased and the interfacial debonding strength increased with the increase of the microwave sintering time at each PIP cycle, which together resulted in the changing tendency that the flexural strength and toughness of the as-fabricated SiC/SiC composites increased firstly and then decreased.

Growth stress in SiO2 during oxidation of SiC fibers

A method to calculate growth stress in SiO2 scales formed during SiC fiber oxidation was developed. Calculations were done for Hi-NicalonTM-S SiC fibers using previously measured Deal-Grove oxidation kinetics parameters. Initial compressive stresses in SiO2 of ∼25 GPa from the 2.2× oxidation volume expansion are rapidly relaxed to lower levels by flow of silica with a shear-stress dependent viscosity. At >1200°, viscous flow of silica relaxes stress to negligible levels. At 700° – 900 °C, compressive axial and hoop stress at the GPa level persist, but radial stresses are much smaller. The decrease in growth stress with increase in temperature is a consequence of larger activation energy for silica viscosity than for oxidation kinetics. Radial expansion of the outer scale eventually causes hoop stress to become tensile in the outer scale, and axial stress becomes tensile from the Poisson effect. Tensile hoop stresses can be >1 GPa for thick scales formed at <1000 °C. Approximate analytical expressions for growth stress for some limiting cases are discussed. Growth stresses were also calculated for crystallized SiO2 scales; these were qualitatively consistent with microstructural evidence of stress. Assumptions and limitations of the calculation method are discussed, along with the possible effects of growth stress on SiC oxidation kinetics and on SiC fiber strength.

Mechanical and microwave dielectric properties of SiCf/SiC composites with BN interphase prepared by dip-coating process

Abstract The BN interphase of SiC fiber-reinforced SiC matrix (SiCf/SiC) composites was fabricated by dip-coating process with boric acid and urea as precursor. The results show that the tensile strength of SiC fiber decreases about 30% after BN coating treatment, but the BN coating has little influence on the electrical resistivity of SiC fiber. Compared with the as-received SiCf/SiC composites, the SiCf/SiC composites with BN interphase exhibit a toughened fracture behavior, and the flexural strength is about 2 times that of the as-received SiCf/SiC composites. From the microstructural analysis, it can be confirmed that the BN interphase plays a key part in weakening interfacial bonding, which can improve the mechanical properties of SiCf/SiC composites remarkably. Owing to the close dielectric properties between SiC and BN, the complex permittivity of SiCf/SiC composites with and without the BN interphase is similar.

Effect of Interface Reaction on Interface Shear Strength of SiC Fiber Reinforced Titanium Matrix Composites

Abstract The finite element model, which considered the interfacial reaction layer was developed to evaluate interfacial shear strength of titanium matrix composites (TMCs) and analyze the effect of interface reaction on interfacial mechanical properties. The results show that the interfacial shear strength of SiC/Timetal-834 evaluated is about 500 MPa, and that the interfacial shear strength increases as the interfacial reaction enhances. Moreover, the relationship between the thickness of the interfacial reaction layer and the interfacial shear strength is built.