Mechanical properties and microstructures of 2D Cf/ZrC–SiC composites using ZrC precursor and polycarbosilane

Two-dimensional C_f/ZrC–SiC composites were fabricated through mold-pressing and polymer infiltration and pyrolysis (PIP) using T700SC plain weave fiber fabrics as reinforcements with ZrC precursor and polycarbosilane. The mechanical properties and microstructures of the composites with 34, 45, and 56% fiber fraction were investigated. All composites showed a typical non-brittle fracture behavior and a large amount of pulled-out fibers were observed on the fracture surface. The bending strength and elastic modulus of the composite with 56 vol% fiber fraction increased up to 582 ± 80 MPa and 167 ± 25 GPa, with increasing fiber fraction. The mass loss and linear recession rate of the composites during the oxy-propane torch test were 0.008 g/s and ?0.003 mm/s, respectively. The formation of a ZrSiO₄ melt on the surface of the composite significantly contributed to the excellent ablative property of the 2D C_f/ZrC–SiC composites.     

Mechanical behaviour of carbon fibre reinforced TaC/SiC and ZrC/SiC composites up to 2100°C

The microstructure and elevated temperature mechanical properties of continuous carbon fibre reinforced ZrC and TaC composites were investigated. Silicon carbide was added to both compositions to aid sintering during hot pressing. Fibres were homogeneously distributed and no fibre degradation was observed at the interface with the ceramic matrix even after testing at 2100 °C. The flexural strength increased from 260 to 300 MPa at room temperature to ∼450 MPa at 1500 °C, which was attributed to stress relaxation. At 1800 °C, the strength decreased to ∼410 MPa for both samples. At 2100 °C plastic deformation resulted in lower strength at the proportional limit (210–320 MPa), but relatively high ultimate strength (370–440 MPa). The sample containing ZrC had a lower ultimate strength, but higher failure strain at 2100 °C due to the weak fibre/matrix interface that resulted in fibre-dominated composite behaviour.     

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.     

Densification,microstructure, and mechanical properties of ZrC–SiC ceramics

ZrC–SiC ceramics were fabricated by high-energy ball milling and reactive hot pressing of ZrH₂, carbon black, and varying amounts of SiC. The ceramics were composed of nominally pure ZrC containing 0 to 30 vol% SiC particles. The relative density increased as SiC content increased, from 96.8% for nominally pure ZrC to 99.3% for ZrC-30 vol% SiC. As SiC content increased from 0 to 30 vol%, Young's modulus increased from 404 ± 11 to 420 ± 9 GPa and Vickers hardness increased from 18.5 ± 0.7 to 23.0 ± 0.5 GPa due to a combination of the higher relative density of ceramics with higher SiC content and the higher Young's modulus and hardness of SiC compared to ZrC. Flexure strength was 308 ± 11 MPa for pure ZrC, but increased to 576 ± 49 MPa for a SiC content of 30 vol%. Fracture toughness was 2.3 ± 0.2 MPa·m^1/2 for pure ZrC and increased to about 3.0 ± 0.1 MPa·m^1/2 for compositions containing SiC additions. The combination of high-energy ball milling and reactive hot pressing was able to produce ZrC–SiC ceramics with sub-micron grain sizes and high relative densities with higher strengths than previously reported for similar materials.     

Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites

Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB₂–SiC–ZrC–C_f ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB₂:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB₂–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (C_f: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:C_f components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% C_f had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.     

Fabrication of Cf/ZrC–SiC composites using Zr–8.8Si alloy by melt infiltration

Cf/ZrC–SiC composites were fabricated by melt infiltration at 1800 °C using Zr–8.8Si alloy and carbon felt preforms. Microstructural analysis showed the formation of both ZrC and SiC phases in the matrix, in which ZrC acted as a main composition of the resulting composites. The results showed that carbon matrix reacted preferentially with Si of Zr–8.8Si alloy, which caused the formation of SiC first and then ZrC. The designed carbon coating by pyrolysis prevented the severe reaction between fibers and the melt. The composites could be more dense and uniform with the bending strength of 53.3 MPa, when preforms had a high open porosity (47.2%) with small size pores (10–40 μm).     

Mechanical property improvement and microstructure observation of SiCw–AlN composites

SiCw–AlN composites were produced with different sintering additives. Eight wt% Y₂O₃ was found a suitable sintering additive for mechanical property improvement of SiCw-AlN composites. With the sintering additive, SiCw–AlN composites with whisker content ranging from 10–30 wt% were hot-pressed to high density, and the flexural strength and fracture toughness were significantly improved with the increase of SiC whisker content. SEM and TEM observation indicated that several kinds of toughening mechanism contributed to the increasing toughness, including crack deflection, crack branching and whisker bridging process. The interfacial boundaries of the composites were also discussed in detail. ©     

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.     

Tensile fatigue behavior of plain-weave reinforced Cf/C–SiC composites

The fatigue behavior of plain-weave C_f/C–SiC composites prepared by liquid silicon infiltration (LSI) was studied under cyclic tensile stress at room temperature. The specimens were loaded with stress levels of 83% and 90% of the mean static tensile strength for 10⁵ cycles. The cross-sections and fracture surfaces of the fatigued specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. The results show that the specimens can withstand 10⁵ fatigue cycles with a stress level of 90% of the static tensile strength. The retained strengths after fatigue for 10⁵ cycles with stress levels of 83% and 90% are about 19% and 11% higher than the static tensile strength. Due to the observation of the microstructures a relief of the thermal residual stress (TRS) caused by stress-induced cracking is probably responsible for the enhancement. Furthermore, the fracture surfaces indicate that the fatigue stress results in interfacial debonding between the carbon fiber and matrix. Additionally, more single-fiber pull out was observed within the bundle segments of fatigued specimens.     

Long-term oxidation and ablation behavior of Cf/ZrB2–SiC composites at 1500, 2000, and 2200°C

Although C_f/ZrB₂–SiC composites prepared via direct ink writing combined with low-temperature hot-pressing were shown to exhibit high relative density, high preparation efficiency, and excellent flexural strength and fracture toughness in our previous work, their oxidation and ablation resistance at high and ultrahigh temperatures had not been investigated. In this work, the oxidation and ablation resistance of C_f/ZrB₂–SiC composites were evaluated via static oxidation at high temperature (1500°C) and oxyacetylene ablation at ultrahigh temperatures (2080 and 2270°C), respectively. The thickness of the oxide layer of the C_f/ZrB₂–SiC composites is <40 μm after oxidizing at 1500°C for 1 h. The C_f/ZrB₂–SiC composites exhibit non-ablative properties after oxyacetylene ablation at 2080 and 2270°C for >600 s, with mass ablation rates of 3.77 × 10⁻³ and 5.53 × 10⁻³ mg/(cm² s), and linear ablation rates of −4.5 × 10⁻⁴ and −5.8 × 10⁻⁴ mm/s, respectively. Upon an increase in the ablation temperature from 2080 to 2270°C, the thickness of the total oxide layer increases from 360 to 570 μm, and the carbon fibers remain intact in the unaffected region. Moreover, the oxidation and ablation process of C_f/ZrB₂–SiC at various temperatures was analyzed and discussed.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating

A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB₂ powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 °C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB₂ powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB₂ and a compositionally similar ZrB₂–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB₂ reference sample was slow to densify until 1800 °C and was not fully dense even at 2000 °C, while the sol–gel modified ZrB₂ powder completed densification by 1800 °C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB₂ powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth.     

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.     

Effect of SiC particles on rehological and sintering behaviour of alumina–zircon composites

The effect of additions of SiC particulates on rheological and sintering behaviour of slip-cast alumina–zircon composites has been investigated. Finely divided alumina, zircon and silicon carbide powders were first processed into slips, using polyacrylite dispersant (0.5 wt.%) to create highly concentrated, stable aqueous suspensions at 40 vol.% loadings, from which test specimens which were then slip cast and dried. They were subsequently sintered in air for 2 h at 1650 °C. Rheological properties of the prepared slips were evaluated and related to the amount of added SiC. After sintering, the resultant porosities, fractional densities, crystallographic phases present, and microstructures were determined.     

Architectural engineering inspired method of preparing Cf/ZrC–SiC with graceful mechanical responses

Inspired by grouting technique in architectural engineering, an innovative method of slurry injection and vacuum impregnation was put forward to introduce nanosized ZrC–SiC ceramics into PyC modified 3-D needle-punched carbon fiber preform homogeneously and continuously, followed by spark plasma sintering to prepare C_f/ZrC–SiC with graceful mechanical responses. The composite possessed improved fracture toughness and work of fracture at 5.37 ± 0.25 MPa∙m^1/2 and 951 ± 12 J/m², 50% and nearly one order of magnitude higher than those of ZrC–SiC composite, respectively, with rivaling flexural strength at 177 ± 8 MPa synchronously. A graceful fracture mode was embodied in an obviously extended yield plateau with increased failure displacement by 300%. This enhancement was attributed to the uniform and continuous combination of ZrC–SiC with carbon fiber preform as well as protection and interface tailoring from PyC coating. The study offered an easy and effective method of preparing 3-D fiber reinforced ceramic matrix composites.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Influence of sol–gel derived ZrO2 and ZrC additions on microstructure and properties of ZrB2 composites

ZrB₂ powder was coated with 5% ZrOC sol–gel precursor and sintered by SPS. Relative densities >98% were achieved at 1800 °C with minimal grain growth and an intergranular phase of ZrC. Carbon content in the precursor determined the type of reinforcing phase and porosity of the sintered composites. XRD, SEM and EDS studies indicated that carbon deficiency resulted in ZrO₂ retention, improving ZrB₂ densification with oxide particle reinforcement. Excess carbon resulted in ZrC formation as the reinforcing phase, but could yield porosity and residual carbon at grain boundaries. These two types of ZrB₂ composites displayed different densification and microstructural evolution that explain their contrasting properties. In the extreme oxidative environment of oxyacetylene ablation, the composites with ZrC-C maintained superior leading edge geometry; whereas for mechanical strength, a bias towards the residual ZrO₂ content was beneficial. This highlighted the sensitivity of processing carbon-precursors in the initial sol–gel process and the carbon content in ZrB₂-based composite systems.     

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.     

Effects of density and heat treatment of C/C preforms on microstructure and mechanical properties of C/C–SiC composites

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C–SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C–SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C–SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C–SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon–carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C–SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.     

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.