Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Increased high temperature strength and oxidation resistance of Al4SiC4 ceramics

Al₄SiC₄ bulk ceramics were synthesized by reaction hot-pressing using Al, graphite powders and polycarbosilane (PCS) as starting materials. The present work confirmed that this process was an effective method for the preparation of Al₄SiC₄ ceramics having high relative density and well-developed plate-like grains. The mechanical, thermal properties and oxidation behaviors of the Al₄SiC₄ ceramics were also investigated. The flexural strength, fracture toughness (K_IC) and Vickers hardness at room temperature were 297.1 ± 22 MPa, 3.98 ± 0.05 MPa m^1/2, 10.6 ± 1.8 GPa, respectively. The high-temperature bending strength showed an increasing trend with increasing test temperatures, with the value of 449.7 ± 26 MPa at 1300 °C. The thermal expansion coefficient was 6.2 × 10⁻⁶ °C⁻¹ in the temperature range from 200 °C to 1450 °C. The isothermal oxidation of Al₄SiC₄ ceramics at 1200–1600 °C for 10–20 h revealed that it had excellent oxidation resistance.     

In situ formation of SiC in Al–40Si alloy during high-pressure solidification

The in situ formation of SiC in Al–40Si alloys during the fabrication of SiC/Al–Si composites by high-pressure solidification were investigated. The results demonstrate that the in situ formation of SiC occurs by a gradual conversion of Al₄C₃ and Al₄SiC₄ to SiC. In situ SiC can be formed in an Al–40Si alloy solidified under a pressure of 3 GPa at a temperature of 1373 K. The SiC particles (SiC_p) formed in situ was compatible with the α-Al matrix and the Si phases. The relative density of the SiC/Al-38.6Si composite was 99.9%. The bending strengths of the Al–40Si alloy and the SiC/Al-38.6Si composite obtained by solidification under a pressure of 3 GPa were 200.8 MPa and 322 MPa, respectively, which represents an enhancement of 60.3% as a result of reinforcement by the in situ-formed SiC.     

Microstructure and mechanical properties of hot-pressed SiC nanofiber reinforced SiC composites

This study reports on the preparation and mechanical properties of a novel SiC_nf/SiC composite. The single crystal SiC nanofiber(SiC_nf) reinforced SiC ceramic matrix composites (CMC) were successfully fabricated by hot pressing the mixture of β-SiC powders, SiC_nf and Al–B–C powder. The effects of SiC_nf mass fraction as well as the hot-pressing temperature on the microstructure and mechanical properties of SiC_nf/SiC CMC were systematically investigated. The results demonstrated that the 15 wt% SiC_nf/SiC CMC obtained by hot pressing (HP) at 1850 °C with 30 MPa for 60 min possessed the maximum flexural strength and fracture toughness of 678.2 MPa and 8.33 MPa m^1/2, respectively. The nanofibers pull out, nanofibers bridging and cracks deflection were found by scanning electron microscopy, which are believed can strengthen and toughen the SiC_nf/SiC CMC via consuming plenty of the fracture energy. Besides, although the relative density of the prepared SiC_nf/SiC CMC further increased with the sintering temperature rose to 1900 °C, the further coarsend composites grains results in the deterioration of the mechanical properties for the obtained composites compared to 1850 °C.     

Microstructure and strengthening behavior in high content SiC nanowires reinforced SiC composites

In this study, the high-content SiC_nw reinforced SiC ceramic matrix composites (SiC_nw/SiC CMC) were successfully fabricated by hot pressing β-SiC and sintering additive (Al₂O₃-Y₂O₃) with boron nitride interphase modification SiC_nw. The effects of sintering additive content and mass fraction (5–25 wt%) of SiC_nw on the density, microstructure, and mechanical properties of the composites were investigated. The results showed that with the increase of sintering additives from 10 wt% to 12 wt%, the relative density of the SiC_nw/SiC CMC increased from 97.3% to 98.9%, attributed to the generated Y₃Al₅O₁₂ (YAG) liquid phase from the Al₂O₃-Y₂O₃ that promotes the rearrangement and migration of SiC grains. The comprehensive performance of the obtained composite with 15 wt% SiC_nw possessed the optimal flexural strength and fracture toughness of 524 ± 30.24 MPa and 12.39 ± 0.49 MPa·m^1/2, respectively. Besides, the fracture mode of the composites with 25 wt% SiC_nw content revealed a pseudo-plastic fracture behavior. It concludes that the 25 wt% SiC_nw/SiC CMC was toughened by the fiber pull-outs, debonding, bridging, and crack deflection that can consume plenty of fracture energy. The strategy of SiC nanowires worked as a main bearing phase for the fabrication of SiC/SiC CMC providing critical information for understanding the mechanical behavior of high toughness and high strength SiC nanoceramic matrix composites.     

High-strength SiC joints with a novel in-situ formed SiC/Al4SiC4 joining filler

The in-situ formed SiC/Al₄SiC₄ joining layer was used to join monolithic SiC using an electric field-assisted sintering technique. A multiphase powder of Al₄C₃/SiC/Al₄SiC₄ was used as the initial joining material to obtain the in-situ reaction layer of SiC/Al₄SiC₄ via the appropriate interface reactions. The bending strength as high as 240.5 ± 6.6 MPa was obtained for the sample joined at 1800 °C, which was higher than the strength of the un-joined SiC matrix. Sound joints were obtained when Al₄C₃ was completely transformed to Al₄SiC₄, and a fully dense SiC/Al₄SiC₄ joining layer was consolidated. The integration of the joining layer with the SiC matrix was improved by a high amount of liquid phase formed at the interface. The proposed SiC/Al₄SiC₄ joining layer, with good thermal matching with SiC matrix, shows a great potential to be applied as a joining material for SiC-based ceramic matrix composites.     

Microstructural evolution of amorphous Si2BC3N nanopowders upon heating at high temperatures: High pressures reverse the nucleation order of SiC and BN(C)

Amorphous Si₂BC₃N nanopowders (NPs) were heated at 1000‐1700°C temperatures for 30 minutes in 1 atm N₂. The changes in phases, chemical bonds, and microstructures were investigated by XRD, XPS, NMR, TEM, and first‐principles calculation. Increases in heating temperatures lead to the nucleation and growth of SiC and BN(C) grains, along with partial transformation of SiC₄ units from amorphous to β/α‐SiC, and collapse of B–C–N bonds. SiC nucleates prior to BN(C) at 1 atm, while it goes in the opposite order at high pressures (≥1 GPa). High pressures also shift the initial temperatures of crystallization of amorphous Si₂BC₃N NPs from 1400°C (1 atm) to 1150°C (5 GPa).     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Polymer-derived SiC ceramic aerogels with in-situ growth of SiC nanowires

Herein, the SiC ceramic aerogels with in-situ growth of SiC nanowires (SiC_w/SiC CAs) have been synthesized by polymer‐derived ceramics (PDCs) method. The morphology, microstructure, and phase composition of the as-prepared samples were systematically investigated through SEM, XRD, TEM, Raman spectrum, FT-IR spectrum, and XPS spectrum techniques. The results showed that the as-obtained SiC_w has a diameter of about 80 nm and a length of 1–3.5 μm. In addition, the formation mechanism and evolution process of growth SiC_w were systematically studied using a VLS growth mechanisms. The way in this work could be expanded to synthesize other Si-based porous ceramic aerogel nanostructed with nanowires.     

Synthesis of ZrC–SiC Powders by a Preceramic Solution Route

Homogenous liquid precursor for ZrC–SiC was prepared by blending of Zr(OC₄H₉)₄ and Poly[(methylsilylene)acetylene]. This precursor could be cured at 250°C and converted into binary ZrC–SiC composite ceramics upon heat treatment at 1700°C. The pyrolysis mechanism and optimal molar ratio of the precursor were investigated by XRD. The morphology and elements analyses were conducted by SEM and corresponding energy‐dispersive spectrometer. The evolution of carbon during ceramization was studied by Raman spectroscopy. The results showed that the precursor samples heat treated at 900°C consisted of t‐ZrO₂ (main phase) and m‐ZrO₂ (minor phase). The higher temperature induced phase transformation and t‐ZrO₂ converted into m‐ZrO₂. Further heating led to the formation of ZrC and SiC due to the carbothermal reduction, and the ceramic sample changed from compact to porous due to the generation of carbon oxides. With the increasing molar ratios of C/Zr, the residual oxides in 1700°C ceramic samples converted into ZrC and almost pure ZrC–SiC composite ceramics could be obtained in ZS‐3 sample. The Zr, Si, and C elements were well distributed in the obtained ceramics powders and particles with a distribution of 100 ~ 300 nm consisted of well‐crystallized ZrC and SiC phases.     

Residual thermal stress of SiC/Ti3SiC2/SiC joints calculation and relaxed by postannealing

Residual thermal stresses in SiC/Ti₃SiC₂/SiC joining couples were calculated by Raman spectra and simulated by finite element analysis, and then relaxed successfully by postannealing. The results showed that the thermal residual stress between Ti₃SiC₂ and SiC was about on the order of 1 GPa when cooling from 1300°C to 25°C. The thermal residual stresses can be relaxed by the recovery of structure disorders during postannealing. When the SiC/Ti₃SiC₂/SiC joints postannealed at 900°C, the bending strength reached 156.9 ± 13.5 MPa, which was almost twice of the as‐obtained SiC/Ti₃SiC₂/SiC joints. Furthermore, the failure occurred at the SiC matrix suggested that both the flexural strength of joining layer and interface were higher than the SiC matrix.     

Improved mechanical properties of reaction-bonded SiC through in-situ formation of Ti3SiC2

Reaction-bonded SiC is a ceramic with excellent thermal properties, good corrosion resistance and the characteristic of near-net-shape manufacturing. However, the poor fracture toughness of free Si limits the applications of reaction-bonded SiC. In this study, TiC was added to reaction-bonded SiC and reacted with free Si to form Ti₃SiC₂. The effects of TiC and carbon black on the mechanical properties of reaction-bonded SiC were investigated. The results demonstrated that the in-situ formation of Ti₃SiC₂ and decrease in the content and size of free Si improved the mechanical properties of reaction-bonded SiC ceramics. The mechanical properties of TiC-added reaction-bonded SiC with 17.5 wt% carbon black were superior to those of TiC-added reaction-bonded SiC with 15 wt% carbon black. Moreover, increasing the TiC content of reaction-bonded SiC with 17.5 wt% carbon black from 0 to 7.5 wt% caused an increase in its bending strength from 183.92 to 424.43 MPa and an increase in fracture toughness from 3.7 to 5.24 MPa m^1/2.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Combustion synthesis of SiC-based ceramics reinforced by discrete carbon fibers with in situ grown SiC nanowires

This study proposes a combustion-based ceramic matrix composite processing technique intended on single-step in situ deposition of single-crystal SiC nanowires (SiC_nw) on the surface of carbon fibers (C_f) and formation of SiC_nw–reinforced SiC matrix. This was accomplished by Ta-catalyzed combustion of poly-(C₂F₄)-containing reactive mixtures with pre-mixed chopped C_f. Depending on the combustion conditions, carbon fiber surface is subjected either to formation of diffusion layers, ceramic particle incrustation or growth of continuous arrays of carbon-coated single-crystal SiC_nw with a nearly defect-free lattice, 10–50 nm diameter and 15–20 μm length. Thermodynamics, phase and structure formation mechanisms are explored, and the optimal conditions are outlined for reproducible C_f/in situ SiC_nw dual reinforcement of SiC-based ceramics. Hot pressing at 1500 °C produced C_f/in situ SiC_nw-reinforced ceramic SiC–TaSi₂ specimens with a relative density of 97%, 19 GPa Vickers hardness, 3-point flexural strength σ = 420 ± 70 MPa and fracture toughness K_1C = 12.5 MPa m^1/2.     

Reactive Hot Pressing of ZrC–SiC Ceramics at Low Temperature

Composites of ZrC–SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC_1?x, SiC, and ZrSi. Low‐temperature densification of ZrC?SiC ceramics is attributed to the formed nonstoichiometric ZrC_1?x and Zr–Si liquid phase. Adding 5 wt% Si to ZrC, the three‐point bending strength of formed ZrC_0.8–13.4 vol%SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m^1/2, respectively.     

Superior bending and impact properties of SiC matrix composites infiltrated with an aluminium alloy

A simple process based on melt infiltration was used to modify a silicon carbide (SiC) ceramic and thus improve its mechanical properties. SiC ceramics infiltrated with an Al alloy for 2 h, 4 h, 6 h, and 8 h exhibited outstanding mechanical performance. The three-point bending strength, four-point bending strength, and impact toughness of the SiC ceramics increased by 125–135%, 170–180%, and 140%, respectively, after infiltration with the Al alloy at 900 °C for 4–6 h. The maximum three-point bending strength, four-point bending strength, and impact toughness achieved were 430 MPa, 360 MPa, and 3.5 kJ/m², respectively. Analysis of the processing conditions and microstructure demonstrated that the molten Al alloy effectively infiltrated the gaps between the SiC particles, forming a compact structure with the particles, and some of the Al phases reacted with Si to form Al-Si eutectic phases. Moreover, the results showed that a reaction layer is present on the surface of the SiC sample, which mainly contains the Ti₃SiC₂ phase. Both complete infiltration with the Al alloy and the formation of the Ti₃SiC₂ phase contributed to the improvement of the mechanical properties.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Microscopic and Spectroscopic Characterization of Stacking‐Sequence Disordered SiC

Nanometric silicon carbide (SiC) powder (~5 nm) with a stacking‐sequence disordered structure (SD‐SiC), synthesized from elemental powders of Si and C, was investigated by microscopic and several spectroscopic methods. The structure of SD‐SiC was characterized by transmission electron microscopy (TEM), ¹³C, and ²⁹Si‐NMR, and by infrared (IR), Raman, and X‐ray photoelectron spectroscopy (XPS) methods. TEM characterizations showed relatively large deviations of the lattice parameters in the as‐synthesized SiC, indicative of the presence of stacking‐sequence disorder. IR analysis showed a weaker Si‐C bond in the SD‐SiC than in the 3C‐SiC. XPS determinations showed that C and Si in SD‐SiC are similar to those in 3C‐SiC. Broader peaks of ²⁹Si and ¹³C MAS‐NMR also indicate that the structure of SD‐SiC is different from that of 3C‐SiC. Raman spectroscopy exhibited activities for the crystalline polytypes and the amorphous of SiC but lack of them for the SD‐SiC. The inactivity of Raman spectroscopy for the SD‐SiC along with large deviation of the lattice constant and the extremely broad X‐ray diffraction peaks would indicate that SD‐SiC is a possible intermediate state between conventional polytype SiC and amorphous SiC, that is, a possible new type of SiC.     

Fabrication and oxidation behavior of Al4SiC4 powders

Al₄SiC₄ powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al₄SiC₄ was investigated at 800°C‐1500°C in air by means of thermogravimetry method. It is demonstrated that Al₄SiC₄ powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C‐1100°C, the oxide scales consist of an Al₂O₃ outer layer and a transition layer. Al₄SiC₄ remains the main phase. At 1200°C, some spallation resulting from the increment of Al₂O₃ and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large‐scale Al₂O₃ crystals (outer layer) and mullite with less amount of SiO₂ (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C‐1100°C) and 267.1 kJ/mol (1300°C‐1400°C).     

Microstructural characterization using orientation imaging microscopy of cellular Si/SiC ceramics synthesized by replication of Indian dicotyledonous plants

Woods from three dicotyledonous plants of local origin (mango Mangifera indica), jackfruit (Artocarpus integrifolia) and teak (Tectona grandis) were transformed by pyrolysis into carbonaceous preforms and subsequently converted into cellular Si/SiC ceramics by liquid Si-infiltration and reaction. The pyrolyzed mango, jackfruit and teak were characterized in terms of pyrolysis weight loss, shrinkages, bulk density and microstructures. The Si-infiltrated pyrolyzed woods were found to have densities and porosities in the range of 2.46–2.60 gm cm⁻³ and 1.5–3.6 vol.% respectively. SEM imaging confirmed the preservation of microcellular tissue anatomy of the precursor wood structure in the morphologies of final ceramics. The end ceramics were investigated for the phases present and for crystallographic microtexture. A combination of XPS (X-ray photoelectron spectroscopy), XRD (X-ray diffraction) and OIM (orientation imaging microscopy) was used to establish the presence and the relative locations of silicon (Si), silicon carbide (SiC) and graphite (C). Fine SiC grains did typically surround coarse crystals of Si – the latter had significant presence of Σ3 twin boundaries. Graphite was primarily present in the regions containing SiC and was more textured than the SiC. Distinct orientation relationship could be established between the graphite crystals and the Si grains containing them.