Microstructure characterization and compressive performance of 3D needle-punched C/C–SiC composites fabricated by gaseous silicon infiltration

3D needle-punched C/C-SiC composites were fabricated from carbon fiber reinforced carbon (C/C) preforms, with densities of 1.05?g/cm³ and 1.28?g/cm³, by the gaseous silicon infiltration (GSI) method at fabrication temperatures from 1500?°C to 1800?°C. The compressive strengths and elastic moduli in transverse direction are larger than those measured under longitudinal compression except that samples fabricated from 1.28?g/cm³ density exhibit lower elastic moduli in transverse direction than in longitudinal direction. The compressive strength and modulus increase with fabrication temperature at 1500?°C and 1600?°C, and then decrease with higher fabrication temperature. Samples fabricated from the lower density C/C preforms have greater compressive strength and modulus. X-ray tomography was applied before and after the mechanical tests to characterize the microstructure and damage patterns, and the results indicated that for C/C-SiC composites fabricated at 1700?°C from 1.28?g/cm³ density C/C preform the matrix has a volume fraction (vol%) of 36.9%, and the initial intra-bundle cracks (0.6?vol%) display a space crossing structure while the inter-bundle pores (6.0?vol%) are special irregularly distributed.     

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.     

C/C–ZrC composite prepared by chemical vapor infiltration combined with alloyed reactive melt infiltration

A high performance and low cost C/C–ZrC composite was prepared by chemical vapor infiltration combined with zirconium–silicon (Zr: 91.2 at.%; Si: 8.8 at.%) alloyed reactive melt infiltration. The density of the as-received composite is 2.46 g/cm³ and the open porosity is 5%. Due to the reaction between the pyrolytic carbon and Zr–Si alloy in the composite, ZrC and Zr₂Si phases were formed, the formation and distribution of which were investigated by thermodynamics and phase diagram. The as-received C/C–ZrC composite, with the flexural strength of 239.5 MPa, displayed a pseudo-ductile fracture behavior. Ablation properties of the C/C–ZrC composite were tested by a pulse laser. The linear ablation rate was 0.028 mm/s. A ZrO₂ barrier layer was formed on the ablation surface and the composite presented excellent ablation resistance.     

Microstructure and ablation behavior of SiC coated C/C–SiC–ZrC composites prepared by a hybrid infiltration process

In this study, C/C–SiC–ZrC composites coated with SiC were prepared by precursor infiltration pyrolysis combined with reactive melt infiltration. The pyrolysis behavior of the hybrid precursor was investigated using thermal gravimetric analysis-differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy techniques. The microstructure and ablation behavior of the composites were also investigated. The results indicate that the composites exhibit an interesting structure, wherein a ceramic coating composed of SiC and a small quantity of ZrC covers the exterior of the composites, and the SiC–ZrC hybrid ceramics are partially embedded in the matrix pores and distributed around the carbon fibers as well. The composites exhibit good ablation resistance with a surface temperature of over 2300 °C during ablation. After ablation for 120 s, the mass and linear ablation rates of the composites are 0.0026 g/s and 0.0037 mm/s, respectively. The great ablation resistance of the composites is attributed to the formation of a continuous phase of molten SiO₂ containing SiC and ZrO₂, which seals the pores of the composites during ablation.     

Microstructure,mechanical and anti-ablation properties of SiCnw/PyC core-shell networks reinforced C/C–ZrC–SiC composites fabricated by a multistep method of chemical liquid-vapor deposition

C/C–ZrC–SiC composites reinforced by SiC nanowire (SiCnw)/pyrocarbon (PyC) core-shell networks were prepared by a multistep method of chemical liquid-vapor deposition (CLVD). The microstructure, mechanical property and ablation resistance were researched. The investigations presented that the PyC was deposited on the SiC nanowires, and the micro-scale core-shell structures were produced. Moreover, these micro-scale structures not only connected with the fibers and matrices, but also filled the pores in the composites. In contrast with C/C–ZrC–SiC composites, the flexural modulus and strength of SiCnw/PyC-C/C–ZrC–SiC composites increased by 36.91% and 44.53%, and the fracture mode was changed from the brittle to pseudo-plastic fracture. After the oxyacetylene torch ablation at two temperatures for 90s, the composites strengthened by SiCnw/PyC core-shell possessed a better resistant ablation. At ablation temperature of 2300 °C, the mass loss rate and linear reduction rate of the composites with core-shell networks decreased by 66.18% and 57.55% in contrast with the non-reinforced composites, and declined by 56.46% and 57.48% at ablation temperature of 3000 °C. The obvious decrease of ablation rates was ascribed to the dense microstructure, the small coefficient of thermal expansion (CTE), the good thermal conductivity, and the resistant ablation roles of SiCnw/PyC core-shell systems.     

Microstructure and mechanical properties of WB2–B4C composites fabricated by boro/carbothermal reduction and SPS

The phase composition, microstructure, and mechanical properties of the WB₂–B₄C composites fabricated by a combination of boro/carbothermal reduction and spark plasma sintering (SPS) method with WO₃, B₄C, and graphite as raw materials were investigated in this study. The experimental results showed that the relative density of the as-sintered WB₂–B₄C composites was ∼93.1% and ∼99.5%, respectively, after being SPS sintered at 1600°C under the applied load of 30 MPa for 10 min. Scanning electron microscope analysis showed that a network structure with WB₂ grains surrounded by B₄C grains was observed after sintering. Analyses of high-resolution TEM showed semi-coherent interface and lattice distortion transition region between WB₂ and B₄C grains. The Vickers hardness of WB₂–B₄C composite increased to 22.3 ± 0.9 GPa at 9.8 N owing to the fully dense, solid solution of C, and three-dimensional network structure. Moreover, the fracture toughness and flexural strength of WB₂–B₄C composite reach 6.04 ± 0.81 MPa m^1/2 and 750 ± 80 MPa, respectively, which could be attributed to the semi-coherent interface between WB₂ and B₄C grains.     

Microstructure and ablation mechanism of C/C–SiC composites

C/C–SiC composites were prepared by molten infiltration of silicon powders, using porous C/C composites as frameworks. The porosities of the C/C–SiC composites were about 0.89–2.8 vol%, which is denser than traditional C/C composites. The ablation properties were tested using an oxyacetylene torch. Three annular regions were present on the ablation surface. With increasing pyrocarbon fraction, a white ceramic oxide layer formed from the boundary to the center of the surface. The ablation experimental results also showed that the linear and mass ablation rates of the composites decreased with increasing carbon fraction. Linear SiO₂ whiskers of diameter 800 nm and length approximately 3 μm were formed near the boundaries of the ablation surfaces of the C/C–SiC composites produced with low-porosity C/C frameworks. The ablation mechanism of the C/C–SiC composites is discussed, based on a heterogeneous ablation reaction model and a supersaturation assumption.     

Microstructure evolution and performance of B4C–NdB6 composites fabricated by in situ hot pressing

B₄C–NdB₆ composites were fabricated by in situ hot pressing at different temperatures (1950–2150°C) with B₄C and Nd₂O₃ (2–4 wt%) as raw materials. The microstructure evolution of the composites with sintering temperature and Nd₂O₃ content was studied in detail, and the influence of pressure on the sintering of B₄C with different contents of Nd₂O₃ was also investigated. The performance of the fabricated composites was researched and the toughening mechanism was discussed. The results indicate that Nd₂O₃ can react with B₄C to form the thin-sheet intermediate products (Nd(BO₂)₃, Nd₂CO₅) first, which then transform to band-shaped NdB₆. Pressure can reduce the distance of B₄C and Nd₂O₃, accelerating the mass transfer and contributing to the formation of NdB₆. NdB₆ and intermediate products are first in agglomerate structure at 1950°C, and then the agglomerates are broken to form dispersive micron and submicron NdB₆ at 2000°C by the synergistic function of pressure, diffusion at high temperature, and liquid phase sintering. NdB₆ can enhance the densification owing to the bonding function. Excessive Nd₂O₃ content leads to residual pores, and excessive temperature (2150°C) results in the coarsening of phases. The coexistence of transgranular and intergranular fracture of NdB₆ promote the fracture toughness.     

Microstructure and tribological properties of PIP-SiC modified C/C–SiC brake materials

A combination method of precursor infiltration and pyrolysis (PIP), chemical vapor infiltration (CVI) and liquid silicon infiltration (LSI) was proposed to prepare PIP-SiC modified C/C–SiC brake materials. The SiC ceramic matrix pyrolyzed by polymethysilane (PMS) homogeneously dispersed in the fiber bundles region, which improved the plough resistance of local C/C region and the wear resistance of C/C–SiC brake materials. When the braking speed rises to 28 m/s, the fluctuation range of friction coefficient was limited to 0.026. The linear wear rate of the as-prepared composites was could be ~50% less than that of C/C–SiC, when the braking speed was above 15 m/s (for instance, the wear rate of 1.02 μm/(side·cycle) at 28 m/s less than 2.02 μm/(side·cycle) of traditional C/C– SiC). The fading ratio D of CoF under wet conditions was ~11%. The results showed that introducing PIP-SiC could stabilize the braking process and effectively prolong the service life of C/C–SiC brake materials.     

Microstructure and tribological properties of microwave-sintered Ti0.8Ni–0.3Mo/TiB composites

In this study, the Ti–0.8Ni–0.3Mo/XTiB (X = 5, 10, 15, and 20 wt%) composites were prepared using the microwave-sintering assisted powder metallurgy technique, and tribological properties were investigated. X-ray diffraction and scanning electron microscopy (SEM), with the microscope capable of energy dispersive spectroscopy (EDS), were used to characterize the mixed powder. The density and microhardness of the Ti–0.8Ni–0.3Mo/TiB composites were examined. The Ti–0.8Ni–0.3Mo/TiB composites exhibited a hardness of 260 HV, which is a 20% improvement over Ti–0.8Ni–0.3Mo. Tribological properties were studied by conducting experiments using a pin-on-disc wear tester at varying loads, sliding distances, and speeds. The Ti–0.8Ni–0.3Mo/TiB composites exhibited a reduction in wear loss and coefficient of friction values owing to TiB hardness and good bonding with the matrix. The tribological properties of the Ti–0.8Ni–0.3Mo/TiB composites were enhanced by the addition of TiB particles, which resist wear and friction.     

Fabrication and properties of 2D C/C–ZrB2–ZrC–SiC composites by hybrid precursor infiltration and pyrolysis

Abstract

Two dimensional C/C–ZrB₂–ZrC–SiC composites were fabricated through precursor infiltration and pyrolysis process using a mixture of polycarbosilane and ZrB₂ precursor and ZrC precursor as the impregnant. The microstructures, mechanical properties and ablation properties of the composites were investigated. The results showed that the homogeneity of the composite improved on using novel precursors that can dissolve with polycarbosilane through the formation of nanocomposite matrix. The flexural strength and fracture toughness first increased and then decreased on increasing the pyrocarbon content in the composite. Compared with the C/C–SiC composite, the ablation resistance of C/C–ZrB₂–ZrC–SiC composite was greatly enhanced. The mass loss rate and linear recession rate exposed to the plasma torch were 1?7 mg/s and 1?8 μm/s, respectively. The formation of a ZrO₂–SiO₂ glassy layer on the surface significantly contributed to the excellent ablative property of the composite.     

Floating catalyst chemical vapor infiltration of nanofilamentous carbon reinforced carbon/carbon composites – Densification behavior and matrix microstructure

Nanofilamentous carbon (NFC) reinforced carbon/carbon composites were prepared by floating catalyst film boiling chemical vapor infiltration from xylene pyrolysis at 1000–1100 °C using ferrocene as a catalyst. The influence of the catalyst content on the densification behavior and matrix microstructure of the composites was studied. Results showed that the deposition rate of pyrocarbon (PyC) was enhanced remarkably by the catalyst. The density of the composites deposited at a catalyst content of 0–2.0 wt% decreased along both the axial and the negative radial directions. Rough laminar (RL) PyC matrix was formed at 0–0.8 wt% catalyst content by heterogeneous nucleation and growth. A hybrid matrix consisting of RL and isotropic (ISO) PyCs appeared at a catalyst content of 1.2–2.0 wt%. The reasons for this ISO PyC formation were attributed to the deposition of carbon encapsulated iron particles and homogeneous nucleation. A reinforcing network composed of NFCs and vapor grown carbon fibers was formed on the fiber/matrix interface and within the matrix in this floating catalyst process. The structure of NFC transformed from nanotube to nanofiber when the catalyst content was over 0.5 wt%, around which composites of a high density of 1.75 g/cm³ and uniform RL PyC matrix were produced rapidly.     

Preparation and characterization of C/SiC–ZrB2 composites by precursor infiltration and pyrolysis process

Ultra-high temperature ceramic matrix composites (C/SiC–ZrB₂) are prepared by slurry and precursor infiltrations and pyrolysis method. C/SiC–ZrB₂ composites with ZrB₂ volume content from 10% to 24.6%, have balanced performance of fracture toughness (17.7–8.1 MPa m^1/2), flexural strength at room temperature (367–163 MPa) and at high temperature (strength retention 74% at 1800 °C and over 32% at 2000 °C), better oxidation and ablation resistance under oxyacetylene torch environment (recession rate 0.01 mm/s).     

Effects of 1600 °C annealing atmosphere on the microstructures and mechanical properties of C/SiC composites fabricated by precursor infiltration and pyrolysis

Effects of 1600 °C annealing atmosphere on microstructures and mechanical properties of the C/SiC composites fabricated by PIP route were remarkable. Due to carbothermic reductions, the ratios of weight loss of the C/SiC composites were all above 7 wt% in 1 h. Consequently, the mechanical properties all had a significant drop during the first hour of annealing because of the bonding between the fibers and matrix remarkably weaken by cracks and pores. And then the flexural strengths gradually decreased with the annealing time increasing, when the flexural moduli slightly changed within the range of 44.2–49.7 GPa. However, the fracture behaviors of the C/SiC composites annealed under Ar faster became brittle than the C/SiC composites annealed under vacuum. The C/SiC composites annealed under Ar for 5 h and under vacuum for 10 h both became brittle mainly due to the sensitive to annealing of the weak carbon interphase, while the C/SiC composites annealed under Ar for 7 h became brittle mainly due to the chemical bonding between the fibers and matrix. And these phenomena were confirmed by the post densification and the stress-releasing annealing.     

Microstructure and mechanical properties of TiB2–B4C ceramics fabricated by hot-pressing

In this study, TiB₂–B₄C composite ceramics were prepared using Y₂O₃ and Al₂O₃ as the sintering aids. Different contents of B₄C were added to seek promoted comprehensive mechanical properties of the composites. The mixed powders were sintered at 1850 °C under a uniaxial loading of 30 MPa for 2 h via hot-pressing. Through the measurement of XRD, SEM and related mechanical properties, the influence of B₄C content on the microstructure and mechanical properties of TiB₂–B₄C composites ceramics was discussed. The experimental results show that TiB₂–B₄C composite ceramics exhibit excellent mechanical properties, which can be attributed to the dense microstructure and fine grain size. In addition, TiB₂–B₄C composite ceramic shows a relatively high comprehensive properties when the addition amount of B₄C is 20 wt%. The relative density, Vickers hardness, fracture toughness and flexural strength are measured to be 99.61%, 27.63 ± 1.73 GPa, 4.77 ± 0.06 MPa m^1/2, 612.5 ± 28.78 MPa, respectively.     

Pore geometry and permeation characteristic of 3D–Cf/SiC composites fabricated via precursor infiltration and pyrolysis

To investigate the correlation of pore geometry and permeation characteristic, this paper evaluated the three-dimensional braided and/or woven carbon fabrics reinforced silicon carbide (3D–C_f/SiC) composites by mercury intrusion porosimetry, scanning electron microscopy and bubble point measurement. The flowrate–pressure curves of N₂ through C_f/SiC panels were measured by pressure apparatus at room temperature, then the flow modes conversion were analyzed, and permeability K was calculated. The pore geometry of 3D–C_f/SiC is supposed to be a three dimensional network composed of multi-sized interconnecting chambers, channels and cracks with sizes from microns to nanometers. The permeability prediction by porosity proves that the contents and sizes of the full open inter-bundle channels are the determinant factors for the intrinsic through-flow capability of the composite. The capillary bundle model displays feasibility to predict K when the actual full-open pore size distribution is obtained by appropriate means, such as bubble point method.     

C/SiC–ZrB2–ZrC composites fabricated by reactive melt infiltration with ZrSi2 alloy

C/SiC–ZrB₂–ZrC composites were prepared by reactive melt infiltration (RMI) combined with vacuum pressure impregnation (VPI) method. B₄C–C was first introduced into C/SiC composites with a porosity of about 30% by impregnating the mixture of B₄C and phenol formaldehyde resin, followed by pyrolysis at 900 °C. The molten ZrSi₂ alloy was then infiltrated into the porous C/SiC–B₄C–C to obtain C/SiC–ZrB₂–ZrC composites. The flexural strength was tested. The ablation behavior was investigated under an oxyacetylene torch flame. It has been found that the C/SiC–ZrB₂–ZrC showed a high flexural strength and an excellent ablation resistance. The reactions between ZrSi₂ alloy and B₄C–C were studied, and a model based on these reactions was built up to describe the formation mechanism of the matrix.     

Microstructure,mechanical and tribological properties of Ti–B–C–N films prepared by reactive magnetron sputtering

Quaternary Ti–B–C–N thin films are deposited on high-speed steel substrates by the reactive magnetron sputtering (RMS) technique. The microstructure, mechanical and tribological properties of Ti–B–C–N films with different carbon contents (from 28.9 at.% to 54.2 at.%) are explored systematically. The microstructure of Ti–B–C–N films deposited by RMS is consisted mainly of Ti(C, N) nano-crystals embedded into an amorphous matrix of a-C/a-CN/a-BN/a-BC. As the carbon content increases, the crystalline size of the films diminishes, but the hardness linearly increases from 14 GPa to 26 GPa. The friction coefficient of the films sliding against steel GCr15 balls in air decreases with the increase of carbon content, which shows that Ti–B–C–N films with both higher hardness and lower friction coefficient can be obtained by means of increasing the carbon concentration in the films.     

Microstructure and mechanical performance evolution of C/C–ZrC composites exposed to oxyacetylene flame

C/C–ZrC composites were prepared by isothermal chemical vapor infiltration (ICVI) combined with reactive melt infiltration (RMI). The ablation behavior of the C/C–ZrC was investigated using an oxyacetylene flame. The effect of ablation time on the microstructure and mechanical property evolution of the composite was studied. The results showed that as the ablation time prolonged, the linear and mass ablation rates of the composite increased firstly and then stabilized. After 15 s ablation, the flexural strength and modulus of the C/C–ZrC were interestingly increased by 141.8% and 40.9%, which reached 138.42 MPa and 6.45 GPa, respectively. During ablation, the preferential oxidation effect of ZrC could mitigate the oxidation of pyrolytic carbon (PyC) and carbon fibers, and the volume change induced by the ZrC →ZrO₂ phase transformation could weaken its bonding with PyC, which was beneficial for releasing the internal residual stresses of the C/C–ZrC and then contributed to the mechanical performance improvement.     

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.