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.     

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.     

Mechanical properties of ZrB2–SiC ceramics prepared by polymeric precursor route

ZrB₂–SiC composite ceramics with varying compositions (6.4, 22.3, and 61.5 vol% ZrB₂–SiC) were synthesized and spark plasma sintered (SPS) for 30 min under argon atmosphere. Ceramics showed relatively uniformly distributed phases with small spherical crystallized grains. Vickers hardness and fracture toughness of ceramics were measured, and scratch and tribological behaviors of sintered ceramic specimens were also investigated. According to experimental results, materials having different inter- and trans-granular fractures showed different wear loss, friction efficient, and tribofilm morphology. Ceramics chemically reacted with moisture while being tribotested, leading to the formation of a tribofilm on the bottom of wear track. Characteristics of silica/hydride silica revealed the formation of tribofilms with different morphologies, thereby implying that several key factors are involved in determining the efficiency of this process.     

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.     

Ablation of C/SiC,C/SiC–ZrO2 and C/SiC–ZrB2 composites in dry air and air mixed with water vapor

C/SiC composites with different additives (ZrO₂ and ZrB₂) were fabricated by CVI and CVD and their oxidation and ablation properties at 1700–1800 °C were investigated. Two different ablation test conditions, dry air and air mixed with water vapor, are compared. The ablation test results are reviewed, the weight loss rates are presented and the corresponding micro-structures are investigated in detail. The results show that in dry air, the weight loss rate of C/SiC composites is greater than those with ZrO₂ and ZrB₂ additives. However, in air mixed with water vapor (5 wt%) to simulate the hygrothermal condition, the weight loss rates of these three composites all become relatively smaller. A model is proposed to predict the weight loss of C/SiC composites and it agrees well with the experimental data.     

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.     

Synthesis,densification and characterization of TaB2–SiC composites

The TaB₂–27.9 vol% SiC composite was synthesized by self-propagating high-temperature synthesis starting from mechanically activated Ta, B₄C and Si reactants. The obtained powders were spark plasma sintered at 1800 °C and 20 MPa for 30 min total time, thus obtaining a 96% dense product. The latter one was characterized in terms of microstructure, hardness, fracture toughness, and oxidation resistance. The obtained results, particularly the fracture toughness, are promising when compared to those related to analogous materials reported in the literature and fabricated with similar and different processing routes.     

Effect of SiC content,additives and process parameters on densification and structure–property relations of pressureless sintered ZrB2–SiC composites

The effect of SiC content, additives, and process parameters on densification and structure–property relations of pressureless sintered ZrB₂–(10–40 vol%) SiC particulate composites have been studied. The ZrB₂–SiC composite powders mixed by ball-milling with 1.2 wt% C (added as phenolic resin) and 3 wt% B₄C have been uniaxially cold-compacted and sintered in argon environment at 1950–2050 °C for 2 h, or at 2000 °C for durations between 1/2 and 3 h. The amount of densification is found to increase with sintering duration, and by prior holding at 1250 and 1600 °C for reduction of oxide impurities (ZrO₂, B₂O₃ and SiO₂) on powder particle surfaces by the aforementioned additives. Presence of SiC with average size smaller than that of ZrB₂ appears to aid in densification by enhancing green density, increasing WC content by erosion of milling media, and inhibiting matrix grain growth. Both SiC and WC appear to aid in reduction of oxide impurities. Furthermore, the impurities enriched in W, Fe and Co obtained from milling media are found to be segregated at ZrB₂ grain boundaries, and appear to assist in densification by forming liquid phase, which completely wets the ZrB₂ grains. Hardness increases with SiC content or with sintering duration till 1 h, but decreases for periods ≥2 h due to grain growth. The experimentally measured elastic moduli approaches corresponding theoretically predicted values with increasing SiC content due to reduction in porosity.     

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.     

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.     

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.     

TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite

The interfacial behavior of spark plasma sintered ZrB₂–SiC nanocomposite doped with graphene nano-platelets was investigated by transmission electron microscopy (TEM). A powder mixture including ZrB₂ matrix, 20?vol% SiC and 10?vol% graphene was used as the starting material. X-ray diffraction analysis did not exhibit any in situ phase formation in the prepared nanocomposite. TEM observations verified the diffusion-controlled sintering. This study clarifies that graphene nano-platelets additive in the prepared nanocomposite did not engage in reactive sintering process, unlike many previous research studies addressing reactive sintering role for carbon additives.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.     

Effects of strain rate and temperature on compressive strength and fragment size of ZrB2–SiC–graphite composites

The quasi-static (strain rate of 10⁻⁴ s⁻¹) and dynamic compression experiments (strain rate of 200–1500 s⁻¹) of ZrB₂–SiC–graphite composites are conducted at 293 K and 1073 K. The initial compressive strength and Weibull modulus are calculated to handle the discrete quasi-static experimental data. Considering effects of strain rate and temperature, the compressions of ZrB₂–SiC–graphite composites are investigated. The results show that both compressive strength and fragment size are higher at 1073 K than those at room temperature. The compressive strengths increase with increasing strain rate at room temperature and 1073 K, whereas fragment sizes decrease. Moreover, a micromechanical model is utilized to characterize the effect of strain rate on the compressive strength. The predictions of this micromechanical model are good agreement with the experimental results. Meanwhile, the fragment sizes of dynamic compressive specimens are analyzed through analytical approaches.     

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.     

ZrB2–SiC gradient oxidation protective coating for carbon/carbon composites

To improve the oxidation protective ability of carbon/carbon composites, ZrB₂–SiC gradient coating was prepared on the surface of C/C composites by an in-situ reaction method. The ZrB₂–SiC gradient coating consisted of an inner ZrB₂–SiC layer and an outer ZrB₂–SiC–Si coating. The phase composition and microstructures of the multiphase coating were characterized by XRD, EDS and SEM. Results showed that the inner coating is mainly composed of ZrB₂ and SiC, while the outer multiphase coating is composed of ZrB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The oxidation behavior of the coated C/C composites at 1773 K in air was investigated. Results show that the gradient ZrB₂–SiC oxidation protective coating could protect C/C from oxidation for 207 h with only (4.56±1.2)×10⁻³ g/cm² weight loss, owing to the compound silicate glass layer with the existence of thermally stable phase ZrSiO₄.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

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.     

Preparation and characterization of TiO2, ZnO,and TiO2/ZnO nanofilms via sol–gel process

The preparation of the TiO₂, ZnO, and TiO₂/ZnO nanofilms was conducted on glass via sol–gel process. The prepared film was detailedly characterized by means of OM, SEM, XRD, and EDS. The results showed that the obtained pure TiO₂ was composed of nanoparticles. For pure ZnO it consisted of nanoparticles and large agglomerates. Both the microstructural morphology and the crystallization of the prepared TiO₂/ZnO composite film were strongly related to the Ti/Zn ratio in the film. With a Ti/Zn ratio less than 1/1, the composite film was absence of cracks. Poor crystallization was definitely observed for the composite film with Ti/Zn ratio of 3/1 and 1/1. The EDS analysis revealed homogeneous distribution of Ti and Zn elements in the film.