High strain rate compressive response of the Cf/SiC composite

Carbon fiber reinforced ceramic owns the properties of lightweight, high fracture toughness, excellent shock resistance, and thus overcomes ceramic's brittleness. The researches on the advanced structure of astronautics, marine have exclusively evaluated the quasi-static mechanical response of carbon fiber reinforced ceramics, while few investigations are available in the open literature regarding elastodynamics. This paper reports the dynamic compressive responses of a carbon fiber reinforced silicon carbide (C_f/SiC) composite (CFCMC) tested by the material test system 801 machine (MTS) and the split Hopkinson pressure bar (SHPB). These tests were to determine the rate dependent compression response and high strain rate failure mechanism of the C_f/SiC composite in in-plane and out-plane directions. The in-plane compressive strain rates are from 0.001 to 2200?s^?1, and that of the out-plane direction are from 0.001 to 2400?s^?1. The compressive stress-strain curves show the C_f/SiC composite has a property of strain rate sensitivity in both directions while under high strain rate loadings. Its compressive stiffness, compressive stress, and corresponding strain are also strain rate sensitive. The compressive damage morphologies after high strain rate impacting show different failure modes for each loading direction. This study provides knowledge about elastodynamics of fiber-reinforced ceramics and extends their design criterion with a reliable evaluation while applying in the scenario of loading high strain rate.     

Dynamic response of ceramic shell for titanium investment casting under high strain-rate SHPB compression load

The mechanical performances of ceramic mold are crucial for the quality of casts in investment casting. However, most of the previous researches were focused on the quasi-static performance which is not sufficient for the accurate failure analysis of shell mold under complex stress state. In this investigation, dynamic mechanical behaviors of Al₂O₃-SiO₂ ceramic shell for investment casting have been studied using split Hopkinson pressure bar (SHPB) at high strain rates. Sand pack samples and pure slurry samples were considered for the testing in order to further understand the mechanism of fracture. Weibull approach was then applied to describe the strength distribution of ceramic shells. The dynamic increase factor (DIF) of compressive strength increased from 1.23 (863?s^?1) to 2.03 (1959?s^?1) indicating the high dependency of mechanical property to strain-rate. The cross-section and fracture surface were analyzed through scanning electron microscopy (SEM). The microstructural investigations showed that the crack propagation in the ceramic shell is mainly through the weak interface between sand particles and slurry region under quasi-static load. At high strain-rate, the crack propagation path is different which extends through the well sintered slurry region and even runs through the sand particles. The mechanism of crack propagation path is analyzed based on Griffith criterion. In addition, the feature of stress-strain curves indicates the layered structure which plays an important role in the process of fracture.     

Microstructural evolution in infiltration‐growth processed MgB2 bulk superconductors

The study reports phase and microstructural evolution in MgB₂ bulk superconductors fabricated by an infiltration and growth (IG) process. Three distinct stages, (1) intermediate boride formation, (2) bulk liquid Mg infiltration, and (3) MgB₂ layer formation, were identified in IG process after detailed examination of series of samples prepared with varied heating conditions. The intermediate phase Mg₂B₂₅, isomorphous to β‐boron, was detected prior to MgB₂ phase formation in stage (1). Due to volume expansion involved in stage 1, cracks formed in the β‐boron particles and propagated radially inwards during stage 3. The growing MgB₂ particles sintered simultaneously with formation of grain boundaries during the process, as evidenced by the measured hardness and critical current density in these samples. From our observations, we estimate the total time needed for complete transformation to MgB₂.     

Dynamic compressive response and fracture mechanisms of ZrB2–SiC ceramic based on discrete element method

To investigate the dynamic compression behaviors of fracturing and damage evolution of ZrB₂–SiC ceramic, this paper proposes a discrete element method to carry out the dynamic compressive behavior of ZrB₂–SiC ceramic. This study is based on three-dimensional discrete element-finite difference coupling modeling to realize the reproduction of the splitting Hopkinson pressure bar experiment process. Micro-parameters of the linear parallel bond model are obtained by calibrating dynamic compression strengths, stress–strain curves, and fracture characteristics of ZrB₂–SiC ceramic. The dynamic compressive stress–strain curve can be divided into four stages according to the microcrack evolution and acoustic emission: stage I, linear elastic stage; stage II, microcrack initiation and then stable development stage; stage III, increment stage of microcracks before peak strength; stage IV, increment stage of microcracks after peak strength. The dynamic damage evolution with strain shows a Weibull distribution. The shape and scale parameters change with strain rate. In addition, under the dynamic compression, crack initiation stress, fracture pattern, and fragment size distribution of the ZrB₂–SiC ceramic composite exhibited a significant strain-rate dependence.     

Dynamic compression behavior of reactive spark plasma sintered ultrafine grained (Hf,Zr)B2–SiC composites

This paper reports the dynamic compression behavior of ultrafine grained (Hf, Zr)B₂–SiC composites, sintered using reactive spark plasma sintering at 1600 °C for 10 min. Dynamic strength of~2.3 GPa has been measured using Split Hopkinson Pressure Bar (SHPB) tests in a reproducible manner at strain rates of 800–1300 s⁻¹. A comparison with competing boride based armor ceramics, in reference to the spectrum of properties evaluated, establishes the potential of (Hf, Zr)B₂–SiC composites for armor applications.     

Investigation of microstructure and properties in short carbon fiber reinforced silica-based ceramic cores via atmosphere sintering

Short carbon fiber (C_sf) reinforced silica-based ceramic cores for investment casting were prepared by an injection molding approach and sintered in air and N₂ atmospheres, respectively. SEM and XRD results present that there are some in-situ formed silicon carbides (SiC) in sintered samples. Moreover, as for the ceramic cores sintered in N₂ atmosphere, the peaks in XRD patterns related to the cristobalite increase with an increment in C_sf content, which may be attributed to the adhesion interface provided by the C_sf and the decreased crystallization free energy. Interestingly, the sample sintered in N₂ exhibits a higher flexural strength about 16.2 MPa, which is 155 % times than that of the samples sintered in air. This is originated from an obvious composite coating consisting of fused silica, SiC and cristobalite on the C_sf. In addition, the sintering necks can further enhance the interfacial bonding strength between the fibers and ceramic cores matrix.     

Effects of SiC,SiO2 and CNTs nanoadditives on the properties of porous alumina-zirconia ceramics produced by a hybrid freeze casting-space holder method

Highly porous alumina-zirconia ceramics were produced by adding space-holder materials during freeze casting. To increase the strength of porous ceramics, different amounts of nanoadditives (silicon carbide-SiC, silica-SiO₂, and multi-wall carbon nanotubes-CNTs) were added. Space-holder materials were removed by preheating, and solid samples were produced by sintering. Up to 68% porosity was achieved when 40% space-holder was added to the solid load of slurry. Wall thicknesses between pores were more uniform and thinner when nanoadditives were added. Compressive tests revealed that SiC nanoparticles increased the strength more than other nanoadditives, and this was attributed to formation of an alumina-SiC phase and a uniform distribution of SiC nanoparticles. Results indicated that by including 20% space-holder materials and 15% SiC nanoparticles, the density decreases by 33.8% while maintaining a compressive strength of 132.5 MPa and porosity of 43.4%. Relatively low thermal conductivities, less than 3.5 W/K-m, were measured for samples with SiC nanoparticles.     

Dynamic compressive strength of alumina ceramics

Dynamic (220–510 s^?1) and quasi-static (0.001 s^?1) compression experiments are conducted on alumina ceramics implemented with two types of tungsten carbide inserts, cylindrical and step-shaped. Split Hopkinson pressure bar (SHPB) tests with in-situ, high-speed optical imaging are adopted to capture the damage and failure of ceramic samples under dynamic compression. The compressive strength of alumina ceramic samples with step-shaped inserts is 15%–30% higher than that with cylindrical inserts commonly used in previous studies, under both dynamic and quasi-static loading. Damage occurs first at the two ends of ceramic samples with the cylindrical inserts, followed by edge fracture and splitting cracks penetrating the sample. However, damage is initiated in the sample region away from the sample ends for the step-shaped inserts, and oblique and secondary transverse cracks dominate the failure process. The different damage modes in the case of step-shaped inserts result in the delayed damage initiation and sample failure, and consequently high compressive strengths. Finite element modelling (FEM) of the SHPB tests provides strength and damage evolution features consistent with the experiment using the Johnson–Holmquist (JH-2) model. FEM reveals equivalent, tensile and shear stress concentrations at the two ends of samples with cylindrical inserts. The stress concentrations are responsible for the damage initiation and growth at the sample ends and the following splitting cracks, consistent with the high-speed images. In contrast, homogeneous stress distributions are achieved in the sample with the step-shaped inserts, ensuring simultaneous damage development across the sample. Overall, the step-shaped inserts in conjunction with cylindrical samples can yield reliable strength measurements for ceramics and ceramic-like materials.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

In situ formation of mullite strengthened SiC porous ceramics via gelcasting with high strength and good alkali resistance

Mullite-bonded porous SiC ceramics sintered in air by gelcasting are still challenges due to the high porosity induced severe oxidation of SiC, which results in the formation of large amount of detrimental cristobalite phase. Here in this work, small amounts of Y₂O₃ and CaF₂ were added in SiC and Al(OH)₃ raw materials as sintering additives for the in situ growth of mullite reinforcement. This additive system promoted the reaction between oxidation-derived SiO₂ from SiC and Al₂O₃ decomposed from Al(OH)₃ to mullite phase. Almost no cristobalite phase was detected when sintered at 1450℃/2 h with CaF₂ addition of more than 2.0 wt%. Mullite whisker reinforcement was in situ formed due to the gas reaction mechanism caused by CaF₂ addition. Thus obtained porous SiC ceramics exhibited a flexural strength of 67.6 MPa at porosity of 41.3%, which maintained exceeding 36 MPa after 8 h corrosion in 10 wt% NaOH 80℃ solution, being the best performance up to now. This high performance of porous SiC was attributed to the additive induces proper phase control and in situ formation of whisker-like mullite reinforcement.     

Preparation and properties of porous SiC–Al2O3 ceramics using coal ash

In this paper, we first reported that porous SiC–Al₂O₃ ceramics were prepared from solid waste coal ash, activated carbon, and commercial SiC powder by a carbothermal reduction reaction (CRR) method under Ar atmosphere. The effects of addition amounts of SiC (0, 10, 15, and 20 wt%) on the postsintering properties of as-prepared porous SiC–Al₂O₃ ceramics, such as phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal shock resistance, and thermal diffusivity have been investigated. It was found that the final products are β-SiC and α-Al₂O₃. Meanwhile, the SEM shows the pores distribute uniformly and the body gradually contacts closely in the porous SiC–Al₂O₃ ceramics. The properties of as-prepared porous SiC–Al₂O₃ ceramics were found to be remarkably improved by adding proper amounts of SiC (10, 15, and 20 wt%). However, further increasing the amount of SiC leads to a decrease in thermal shock resistance and mechanical properties. Porous SiC–Al₂O₃ ceramics doped with 10 wt% SiC and sintered at 1600°C for 5 hours with the median pore diameter of 4.24 μm, room-temperature compressive strength of 21.70 MPa, apparent porosity of 48%, and thermal diffusivity of 0.0194 cm²/s were successfully obtained.     

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.     

Effect of sintering atmosphere on the grain growth and hardness of SiC/polysilazane ceramic composites

Silicon carbide (SiC) monoliths were synthesised using nano-size SiC powder mixed with/without polysilazane by hot pressing at 1750°C for 1?h under an applied pressure of 20?MPa in N₂ or Ar atmosphere. The effects of polysilazane and sintering atmosphere on the microstructure and hardness of SiC were examined. The grain sizes of the SiC ceramics sintered in N₂ atmosphere with and without the polysilazane were 161 and 605?nm, while the density for those samples were 96.5 and 98.1%, respectively. It was shown that Si₂N₂O was formed for the SiC/polysilazane composite and sintered in N₂. In addition, the sample mixed with polysilazane followed by sintering in N₂ atmosphere revealed a quite high hardness in spite of its relatively low density. It was suggested that Si₂N₂O phase played an important role for the inhibition of grain and subsequent high hardness.     

Microstructure and mechanical properties of carbon-precursor-added B4C and B4C−SiC ceramics subjected to pressureless sintering

Organic-carbon-precursor-added B₄C and B₄C–SiC ceramics were subjected to pressureless sintering at various temperatures. The carbon precursor increased the densification of the B₄C and B₄C–SiC ceramics sintered at 2200 °C to 95.6 % and 99.1 % theoretical density (T.D.), respectively. The pyrolytic carbon content of the B₄C–SiC composite decreased with increasing SiC content. The graphitization degree of pyrolytic carbon decreased slightly with the amount of carbon precursor and content of SiC. The 95 wt. % B₄C–5 wt. % SiC composite added with 7.5 wt. % carbon precursor and sintered at 2200 °C outperformed the other B₄C–SiC composites, and its sintered density, flexural strength, Young’s modulus, and microhardness were 98.6 % T.D., 879 MPa, 415 GPa, and 28.5 GPa, respectively. These values were higher than those of composites prepared via pressureless sintering and comparable to those of composites fabricated via hot pressing and/or using metal or oxide additives.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

Electrochemical corrosion of solid and liquid phase sintered silicon carbide in acidic and alkaline environments

Solid and liquid phase sintered silicon carbide (SiC) ceramics are used in aggressive environments, e.g. as seals and linings in chemical plant equipments. There exist data concerning corrosion of solid phase sintered SiC (SSiC), but there are only few data concerning their electrochemical corrosion behaviour. The corrosion of liquid phase sintered SiC ceramics (LPS SiC) containing yttria aluminium oxide grain boundary phases has been investigated by standard methods that have shown the decisive influence of the oxide grain boundary on the corrosion stability of these materials. But no electrochemical investigations are known. In this study therefore, potentiodynamic polarisation measurements have been used to determine the corrosion mechanisms of SSiC and LPS SiC ceramics at room temperature in acidic and alkaline environments. The investigation has shown a pronounced electrochemical corrosion in acids and alkaline solutions for both types of materials. In HCl and HNO₃ pseudo-passivity features due to the formation of a thin layer of SiO₂ on the surface were observed, whereas in NaOH soluble silicate ions were observed resulting in more pronounced corrosion. Microstructural observations of initial and corroded samples revealed that the residual carbon found in the microstructure of SSiC did not dissolve preferentially. The corrosion current densities of the LPS SiC materials were caused by the dissolution of SiC and not by the corrosion of the oxide grain boundary phase. The corrosion current densities of the LPS SiC materials investigated were lower than those of the SSiC materials.     

Low temperature pressureless sintering of α-SiC with Al2O3 and CeO2 as additives

The combination of Al₂O₃ and CeO₂ was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al₂O₃ and Y₂O₃ as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al₂O₃ and Y₂O₃ as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al₂O₃ and CeO₂ as liquid phase sintering additive for SiC.     

On Compressive Brittle Fragmentation

Dynamic brittle fragmentation is typically described using analytical and computational approaches for tensile stress‐states. However, most fragmentation applications (e.g., impact, blast) involve very large initial compressive stresses and deformations. In this study, the compressive fragmentation of brittle materials is investigated experimentally across a range of materials: silicon carbide, boron carbide, spinel, basalt and a stony meteorite. Analysis of our experimental results suggests that there exists two different regimes in the fragment size distributions, based on two brittle fragmentation mechanisms. The first is a mechanism that produces larger fragments and is associated with the structural failure of the sample being tested. This mechanism is influenced by the loading conditions (rate, stress state) and sample geometry. The second fragmentation mechanism produces comparatively smaller fragments and arises from the coalescence of fractures initiating and coalescence between defects in regions of large stresses and contact forces (e.g., between two fractured surfaces from the larger fragments). A framework is developed for comparing experimental compressive fragmentation results with tensile fragmentation theories. The compressive experimental results are shown to be adequately described by the theories using the new framework.     

Synthesis of SiC/SiO2 core–shell powder by rotary chemical vapor deposition and its consolidation by spark plasma sintering

SiC (core) and SiO₂ (shell) powders were synthesized via rotary chemical vapor deposition (RCVD). The SiC particles (3C, <1 μm in diameter) were coated with a layer of SiO₂ (10–15 nm in thickness). Using spark plasma sintering, the SiC/SiO₂ nanopowders were then synthesized into SiC/SiO₂ composite bodies. Although a phase transformation from 3C to 6H was observed at above 2123 K in the sintered monolithic SiC bodies, sintered SiC/SiO₂ bodies did not display such phase transformation. In addition, SiC/SiO₂ bodies did not exhibited grain growth until the sintering temperature reached 2223 K. The density and Vickers hardness of the sintered SiC/SiO₂ bodies increased with increasing sintering temperature. The highest density and hardness of SiC/SiO₂ composite bodies were 98.1% and 24.4 GPa at 2223 K, respectively, which were higher than the corresponding values of 90% and 14 GPa for monolithic SiC bodies.