Heat treatment effects on microstructure and properties of CVI SiC/SiC composites with Sylramic™-iBN SiC fibers

Chemical-vapor-infiltrated (CVI) SiC/SiC composites with Sylramic?-iBN SiC fibers and CVI carbon, BN, and a combination of BN/C interface coating were heat treated in 0.1-MPa argon or 6.9-MPa N₂ at temperatures to 1800 °C for exposure times up to 100 hr. The effects of thermal treatment on constituent microstructures, in-plane tensile properties, in-plane and through-the-thickness thermal conductivities, and creep behavior of the composites were investigated. Results indicate that heat treatment affected stoichiometry of the CVI SiC matrix and interface coating microstructure, depending on the interface coating composition and heat treatment conditions. Heat treatment of the composites with CVI BN interface in argon caused some degradation of in-plane properties due to the decrease in interface shear strength, but it improved creep resistance significantly. In-plane tensile property loss in the composites can be avoided by modifying the interface composition and heat treatment conditions.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

Microstructural,strength, and creep characterization of Sylramic™, Sylramic™-iBN and super Sylramic™-iBN SiC fibers

The chemical composition, microstructure, strength, and thermal stability of polymer-derived Sylramic? SiC fibers fabricated by Dow Corning and COI Ceramics, Inc., and nitrogen-treated Sylramic? SiC fibers, referred to as Sylramic?-iBN and Super Sylramic?-iBN SiC fibers, were investigated and compared. The baseline Sylramic? SiC fibers fabricated by both vendors as well as the nitrogen-treated Sylramic? SiC fibers are composed mostly of β-SiC (～97 wt%) with small amounts of TiB₂ (～2 wt%), amorphous carbon (～1 wt%) and trace amounts of B₄C. Most of the amorphous carbon is segregated at the core/interior of the fibers. Both baseline and nitrogen-treated Sylramic? SiC fibers have similar grain size and pore size distribution, except for a thin layer of in-situ grown crystalline BN (30–70 nm) on the surface of Sylramic?-iBN and Super Sylramic?-iBN fibers. Wide variation in strength within a batch as well as between batches is observed in both baseline and nitrogen-treated Sylramic? SiC fibers but both types of fibers are microstructurally stable at temperatures to 1800 °C in argon and nitrogen environments compared to Nicalon?-S and Tyranno®-SA SiC fibers. Under the same creep condition, Super Sylramic?-iBN fibers show better creep resistance compared to Sylramic?, Sylramic?-iBN, Hi-Nicalon?-S, and Tyranno®-SA fibers. Possible reasons for strength variability and the mechanism of in-situ BN formation on Sylramic? SiC fibers are discussed.     

Relationships between the accumulative damage and dielectric properties of woven BN-coated Hi-Nicalon™ SiC fibre-reinforced SiC matrix composites

The accumulative damage behaviour of BN-coated Hi-Nicalon™ SiC fibre-reinforced SiC matrix composite was examined under tensile cyclic loading at room and elevated temperatures. The accumulative damage occurring during the cyclic loading was quantitatively characterised using the damage parameter obtained by the hysteresis loop curves. The damage parameter increased with increasing applied stress beyond the matrix cracking stress, and it subsequently retained a nearly constant value until just before fracture. Moreover, the dielectric constant, dielectric loss and loss tangent of the composite were measured before and after the fracture in the frequency range 1–1000 MHz. The dielectric properties had similar frequency dependency before and after the fracture. However, the dielectric constant, dielectric loss and loss tangent were lower in the post-fractured specimens than in the pristine ones. The reduction of the dielectric properties was associated with the accumulative damage stored in the specimens. In addition, the relationships between the dielectric properties and the damage parameter were described in detail.     

Heat treatment effects on microstructure and mechanical properties of CVI SiCf/PyC/SiC composites with Cansas-Ⅲ SiC fibers

The microstructure and mechanical properties of CVI-Cansas-III/PyC/SiC composites were systematically investigated after heat treatment under high temperature argon atmosphere, ranging from 1000 °C to 1500 °C, for different time durations. The results showed that the Cansas-III fibres degraded with increasing heat treatment temperature, resulting in degradation of the fibre properties due to pyrolysis of the SiOC phase inside the fibres. The bending strength of the composites remained nearly constant upon heat treatment at 1000 °C and 1250 °C, while a decline in bending strength was observed upon increasing the heat treatment temperature and time, specifically at 1350 °C and above. Moreover, the composites maintained their pseudo-plastic fracture behaviour below 1450 °C, while displaying brittle fracture of the ceramic after 100 h of heat treatment at 1500 °C, due to the complete crystallisation of the fibres.     

Microstructure and properties of ablative C/ZrC–SiC composites prepared by reactive melt infiltration of zirconium and vapour silicon infiltration

C/ZrC-SiC composites with a density of 3.09 g/cm³ and a porosity of 4.8% were prepared by reactive melt infiltration and vapour silicon infiltration. The flexural strength and modulus were 235 MPa and 18.3 GPa, respectively, and the fracture toughness was 7.0 MPa m^1/2. The formation of SiC and ZrSi₂ during vapour silicon infiltration, at the residual cracks and pores in the C/ZrC, enhanced the interface strength and its mechanical properties. The high flexural strength (223 MPa, c. 95% of the original value) after oxidation at 1600 °C for 10 min indicated the excellent oxidation resistance of the composites after vapour silicon infiltration. The mass loss and linear recession rate of the composites were 0.0071 g/s and 0.0047 mm/s, respectively and a fine ablation morphology was obtained.     

Microstructure and ablation behavior of carbon/carbon composites infiltrated with Zr–Ti

Carbon/carbon(C/C) composites infiltrated with Zr–Ti were prepared by chemical vapor infiltration and reactive melt infiltration. Their microstructure and ablation behavior at different temperatures and time were investigated. The results show that C/C composites infiltrated with Zr–Ti have good interface cohesion between carbon fibers, pyrocarbon and carbide. Compared with C/C composites and C/C–ZrC composites, the synthesized sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 exhibits better ablation resistance at 2500 °C due to the newly formed protective layer composed of ZrTiO₂ pinned by ZrO₂ grains after ablation. The ablation resistance of the sample with Zr_0.57Ti_0.43C_1.01 increased gradually with the decrease of temperature from 3000 °C to 2000 °C, whereas the ablation resistance of the sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 first increased obviously and then decreased slightly. In addition, the work indicates that surplus particles or liquid phases of oxides cannot protect the matrix, and that the liquid oxides may even cause severe ablation. Furthermore, a protective layer of oxides tends to be formed with the increase of ablation time.     

Friction and wear properties of C/C–SiC braking composites

Two series of C/C–SiC composites were fabricated via precursor infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) using porous C/C composites with different original densities as preforms, respectively. The tribological characteristics of C/C–SiC braking composites were investigated by means of MM-1000 type of friction testing machine. The friction and wear behaviors of the two series of composites were compared and the factors that influence the friction and wear properties of C/C–SiC composites were discussed. Results show that the friction and wear properties relate close-knit to the content of SiC and porosity. As the original preform density increasing, the content of SiC and porosity decrease, and then the friction coefficient increases obviously, the braking time and the wear rate both decrease. Preparation techniques play an important role in the tribological properties of C/C–SiC composites. Compared with PIP process, the samples from CVI have a little higher friction coefficient, shorter braking time and higher wear rate.     

Preparation and ablation properties of SiC nanowire-reinforced ZrC–SiC coating-matrix integrated C/C composites

A modification of the precursor infiltration pyrolysis (PIP) method was explored to prepare the integrated doped ceramic matrix and coating by the added SiC nanowires layer and shape-stabilization process. The epitaxial layer of SiC nanowires provided surficial attachments for the precursor. And the shape-stabilization process aggregated loose ceramic particles into a coating. Then the SiC nanowire-reinforced ZrC–SiC coating-matrix integrated C/C (S/SZ-CZ/C) composite was simply prepared by the modified PIP method. The bonding strength between the coating and matrix of the S/SZ-CZ/C composite was improved. Through the ablation test, the mass and linear ablation rate of the S/SZ-CZ/C composite were 0.46 mg/s and 0.67 μm/s, which were 60.34 % and 74.91 % lower than those of the SiC nanowire-reinforced C/C–ZrC (S/CZ/C) composite, respectively. The integration of the coating and matrix enabled the formation of a continuous oxide layer of molten SiO₂ and ZrO₂ in the ablation process, which helped to block the oxygen and heat during the ablation test. Thus the ablation resistance of the materials was systematically and effectively improved.     

Controllable fabrication and mechanical properties of C/C–SiC composites based on an electromagnetic induction heating reactive melt infiltration

To regulate the microstructures of carbide ceramic-doped C/C (C/C-ceramic) composites using reactive melting infiltration (RMI) in a controlled manner, an electromagnetic induction heating RMI (ERMI) was proposed and used to fabricate typical C/C–SiC composites herein. Because the tedious heating and cooling regions could be bypassed using ERMI, excessive graphitization and ceramic overreactions of the ERMI-C/C-SiC composites were effectively avoided, which made the interfacial bonding strength (τ) of the ERMI-C/C–SiC composites (～25.7 MPa) much lower than that of the CRMI-C/C-SiC composites (～36.1 MPa) (fabricated using conventional RMI (CRMI)). A weaker τ value triggered strengthening/toughening mechanisms such as crack deflection, and crack arrest, which ultimately led to higher flexural strength and displacement of the ERMI-C/C–SiC composites than the CRMI-C/C–SiC composites. The proposed ERMI exhibited relatively good controlling capability to regulate the microstructures of C/C-ceramic composites.     

Creep behavior and failure mechanisms of CVI and PIP SiC/SiC composites at temperatures to 1650 °C in air

Creep properties of 2D woven CVI and PIP SiC/SiC composites with Sylramic™-iBN SiC fibers were measured at temperatures to 1650 °C in air and the data was compared with the literature. Batch-to-batch variations in the tensile and creep properties, and thermal treatment effects on creep, creep parameters, damage mechanisms, and failure modes for these composites were studied. Under the test conditions, the CVI SiC/SiC composites exhibited both matrix and fiber-dominated creep depending on stress, whereas the PIP SiC/SiC composites displayed only fiber-dominated creep. Creep durability in both composite systems is controlled by the most creep resistant phase as well as oxidation of the fibers via cracking matrix. Specimen-to- specimen variations in porosity and stress raisers caused significant differences in creep behavior and durability. The Larson-Miller parameter and Monkman-Grant relationship were used wherever applicable for analyzing and predicting creep durability.     

Influence of annealing on the mechanical properties of SiC–Si composites with sub-micron SiC microstructures

The influence of annealing temperature (1000, 1100 and 1200°C) on the mechanical properties of SiC–Si composites has been evaluated. Three SiC powders with particle sizes in the range of 0.24 to 0.7 μm were used to produce the composites. Before application the SiC powders were treated with hydrofluoric acid to remove the extent of SiO₂. With this treatment a successful infiltration of green-bodies especially produced of SiC powder with a mean particle size of 0.24 μm was possible. The bending strength decreased with decreasing SiC starting particle size as well as with increasing annealing temperature. However, the fracture toughness was independent on SiC starting particle size and annealing temperature. XRD diffraction analysis showed that internal stress, expressed by broadening of XRD peaks, is low and had no effects on the mechanical properties of the composites.     

Effect of heat treatment on the mechanical properties of PIP–SiC/SiC composites fabricated with a consolidation process

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been considered as candidates for heat resistant and nuclear materials. Three-dimensional (3D) SiC/SiC composites were fabricated by the polymer impregnation and pyrolysis (PIP) method with a consolidation process, mechanical properties of the composites were found to be significantly improved by the consolidation process. The SiC/SiC composites were then heat treated at 1400 °C, 1600 °C and 1800 °C in an inert atmosphere for 1 h, respectively. The effect of heat treatment temperature on the mechanical properties of the composites was investigated, the mechanical properties of the SiC/SiC composites were improved after heat treatment at 1400 °C, and conversely decreased with increased heat treatment temperature. Furthermore, the effect of heat treatment duration on the properties of the SiC/SiC composites was studied, the composites exhibited excellent thermal stability after heat treatment at 1400 °C within 3 h.     

Study of the interactions between HfB2 and Hi-Nicalon™ fiber

This work aims at studying the interactions between Hi-Nicalon SiC fiber and HfB₂ in presence of various sintering additive, such as Si₃N₄ or ZrSi₂. Spark plasma sintering was used as suitable technique to interrupt the sintering at intermediate temperatures and thus analyze the microstructure evolution at the various densification stages. SEM investigations enabled to disclose the interaction mechanisms not only among fiber and matrix, but also revealed the curious mechanisms of formation of core–shell structures in HfB₂ grains when ZrSi₂ was employed.     

Preparation and formation mechanism of C/C–SiC composites using polymer-Si slurry reactive melt infiltration

This study proposes a polymer-metal slurry reactive melt infiltration (RMI) method to overcome the limitations of conventional RMI in modifying irregular geometric carbon–carbon (C/C) preforms. Herein, polycarbosilane (PCS), polysiloxane, phenol-formaldehyde, and epoxy resin, which were introduced to prepare slurries with Si powder, and subsequently used to modify cylindrical C/C preforms into C/C–SiC composites. Results show that the PCS–Si slurry has the best RMI capability, by which, a cylindrical C/C preform (1.35 g·cm⁻³) was modified successfully to into a dense C/C–SiC composite (1.92 g·cm⁻³). PCS plays a vital role in fixing the coating to prevent it from falling off the surface of the C/C preform in PCS–Si slurry RMI. Both of the degree of densification and flexural strength of the C/C–SiC composites increase with an increase in the thickness of the PCS–Si slurry coating. The overreaction of the PCS–Si slurry RMI was effectively suppressed because the content of Si powder is reasonably controlled in the PCS–Si slurry coating. Moreover, nozzle-shaped C/C composites were successfully modified into a C/C–SiC composite for the first time using PCS–Si slurry RMI.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrC composites

C/SiC–ZrC composites were prepared by a combining slurry process with precursor infiltration and pyrolysis, and then annealed from 1200 °C to 1800 °C. With rising annealing temperature, their mass loss rate increased, and the flexural strength and modulus decreased from 227.9 MPa to 41.3 MPa and from 35.3 GPa to 22.7 GPa, respectively. High-temperature annealing, which elevated thermal stress and strengthened interface bonding, was harmful to the flexural properties. However, it improved the ablation properties by increasing the crystallization degree of SiC matrix. The mass loss rate and linear recession rate decreased with increasing annealing temperature and those of the samples annealed at 1800 °C were 0.0074 g/s and 0.0011 mm/s respectively. Taking mechanical and ablation properties into consideration simultaneously, the optimum annealing temperature was 1600 °C.     

Oxidation resistance and thermal cycling properties of the SiCnws–SiC coating on the C/SiC composites

SiC coatings reinforced with SiC nanowires were prepared on carbon/silicon carbide (C/SiC) composites through chemical vapor reaction route and chemical vapor deposition (CVD). The SiC nanowires were introduced to mainly improve the interface bonding properties of the coating and C/SiC composites. The microstructure, phase composition, thermal cycling, and bonding strength of the SiCnws–SiC coating were investigated. After nine thermal cycles, the weight loss of the SiCnws–SiC-coated C/SiC composites was only 4.6 wt.%. Tensile test results show that the tensile strength of the SiCnws–SiC-coated C/SiC composites was more than 4.5–4.6 MPa. The introduction of SiC nanowires effectively improved interface bonding strength, thus enhancing the thermal cycling and mechanical properties of the coating.     

Processing and properties of Nextel™ 720 fiber reinforced alumina-zirconia ceramic matrix composites

The continuous Nextel? 720 fiber-reinforced zirconia/alumina ceramic matrix composites (CMCs) were prepared by slurry infiltration process and precursor infiltration pyrolysis (PIP) process. The introduction of submicron zirconia powders into the aqueous slurry was optimized to offer comprehensively good sintering activity, high thermal resistance and good mechanical properties for the CMCs. Meanwhile, the zirconia and alumina preceramic polymers were used to strengthen the porous ceramic matrix through the PIP process. The final CMC sample achieved a high flexural strength of 200 MPa after one infiltration cycle of alumina preceramic polymer and thermal treatment at 1150 °C for 2 h. The flexural strength retention of the improved CMC sample was 104% and 89% respectively after thermal exposure at 1100 °C and 1200 °C for 24 h.     

Development of SiC–Si composites with fine-grained SiC microstructures

This work summarises the influence of the original particle-size of the SiC powder on the mechanical properties of silicon infiltrated SiC (SiC-Si) composite. These composites are based on a defined SiC particle-size structure. Using α-SiC powders with a mean particle-size of 12·8, 6·4, 4·5 and 3 μm, a clear linear enhancement of the bending strength with decrease of SiC-particle-size was observed. However, a further decrease of the SiC particle-size (from 3 to 0·5 μm) brought no increase of the strength and toughness, respectively. ©     

Fabrication and microstructure evolution of Csf/ZrB2–SiC composites via direct ink writing and reactive melt infiltration

Fiber damage and uniform interphase preparation are the main challenges in conventional short fiber reinforced ceramic matrix composites. In this work, we devel...