UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

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.     

Ti3Si(Al)C2-based ceramics fabricated by reactive melt infiltration with Al70Si30 alloy

Dense Ti₃Si(Al)C₂-based ceramics were synthesized using reactive melt infiltration (RMI) of Al₇₀Si₃₀ alloy into the porous TiC preforms. The effects of the infiltration temperature on the microstructure and mechanical properties of the synthesized composites were investigated. All the composites infiltrated at different temperatures were composed of Ti₃Si(Al)C₂, TiC, SiC, Ti(Al, Si)₃ and Al. With the increase of infiltration temperature from 1050 °C to 1500 °C, the Ti₃Si(Al)C₂ content increased to 52 vol.% and the TiC content decreased to 15 vol.%, and the Vickers hardness, flexural strength and fracture toughness of Ti₃Si(Al)C₂-based composite reached to 9.95 GPa, 328 MPa and 4.8 MPa m^1/2, respectively.     

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi₂-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi₂-SiC-Si coating with dense structure consisting of Si, MoSi₂ and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600 °C and TGA test from room temperature to 1500 °C were used to evaluate the oxidation resistance ability of the MoSi₂-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600 °C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500 °C due to the formed SiO₂ layer on the surface of coating. After oxidation for 220 h at 1600 °C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO₂ layer and MoSi₂-SiC-Si layer.     

Effects of Fe addition on the mechanical and thermo-mechanical properties of SiC/FeSi2/Si composites produced via reactive infiltration

Remaining silicon in SiC-based materials produced via reactive infiltration limits their use in high-temperature applications due to the poor mechanical properties of silicon: low fracture toughness, extreme fragility and creep phenomena above 1000 °C. In this paper SiC–FeSi₂ composites are fabricated by reactive infiltration of Si–Fe alloys into porous Cf/C preforms. The resulting materials are SiC/FeSi2 composites, in which remaining silicon is reduced by formation of FeSi₂. For the richest Fe alloys (35 wt% Fe) a nominal residual silicon content below 1% has been observed. However this, the relatively poor mechanical properties (bending strength) measured for those resulting materials can be explained by the thermal mismatch of FeSi2 and SiC, which weakens the interface and does even generate new porosity, associated with a debonding phenomenon between the two phases.     

Processing of biomorphic porous TiO2 ceramics by chemical vapor infiltration and reaction (CVI-R) technique

Porous biomorphic TiO₂ ceramics were manufactured from paper preforms by chemical vapor infiltration and reaction (CVI-R) in a three-steps process. First, the cellulose fibers of the paper were converted into carbon (C_b) by pyrolysis in an inert atmosphere. Then, C_b-template was infiltrated with a precursor system consisting of TiCl₄, CH₄ and H₂ to produce porous TiC ceramics, which were oxidized in a final step with air at temperatures in the range of 400–1200 °C. Depending on the conversion degree, TiC/TiO₂ or TiO₂ ceramics were obtained. The kinetics of the oxidation process was studied by thermal gravimetric analysis (TGA) and activation energies of 63 and 174 kJ mol⁻¹ were estimated for the lower (400–800 °C) and higher (950–1200 °C) temperature regions, respectively. The TiO₂ ceramics were characterized by Raman spectroscopy (anatase/rutile ratio), SEM/EDX (morphology, composition) and nitrogen gas adsorption (pore structure). It was shown, that the anatase/rutile ratio as well as the pore structure of the resulting TiO₂ ceramics could be controlled varying the oxidation temperature. The TiO₂ samples obtained by oxidation of TiC biomorphic porous ceramics are lightweight but nevertheless have very good mechanical performances. Their bending strength varies between 30 and 40 MPa at a porosity of 65–70%. These structures have many potential applications, e.g. light structured materials, implants because of their bio-compatibility, catalyst support or catalyst for photo catalytic applications.     

Thermal conductivity of porous SiC composite ceramics derived from paper precursor

Biomorphic porous SiC composite ceramics were produced by chemical vapor infiltration and reaction (CVI-R) technique using paper precursor as template. The thermal conductivity of four samples with different composition and microstructure was investigated: (a) C-template, (b) C-SiC, (c) C-SiC–Si₃N₄ and (d) SiC coated with a thin layer of TiO₂. The SiC–Si₃N₄ composite ceramic showed enhanced oxidation resistance compared to single phase SiC. However, a key property for the application of these materials at high temperatures is their thermal conductivity. The later was determined experimentally at defined temperatures in the range 293–373 K with a laser flash apparatus. It was found that the thermal conductivity of the porous ceramic composites increases in the following order: C-template < C-SiC < C-SiC–Si₃N₄ < SiC–TiO₂. The results were interpreted in regard to the porosity and the microstructure of the ceramics.     

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.     

A new route to fabricate SiB4 modified C/SiC composites

In order to improve the oxidation and thermal shock resistance of 2D C/SiC composites, dense SiB₄–SiC matrix was in situ formed in 2D C/SiC composites by a joint process of slurry infiltration and liquid silicon infiltration. The synthesis mechanism of SiB₄ was investigated by analyzing the reaction products of B₄C–Si system. Compared with the porous C/SiC composites, the density of C/SiC–SiB₄ composites increased from 1.63 to 2.23 g/cm³ and the flexural strength increased from 135 to 330 MPa. The thermal shock behaviors of C/SiC and C/SiC–SiB₄ composites protected with SiC coating were studied using the method of air quenching. C/SiC–SiB₄ composites displayed good resistance to thermal shock, and retained 95% of the original strength after being quenched in air from 1300 °C to room temperature for 60 cycles, which showed less weight loss than C/SiC composite.     

Densification of ZrB2-based composites and their mechanical and physical properties: A review

This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB₂ ceramics and ZrB₂-based composites. Hot-pressing is the most commonly used densification method for the ZrB₂-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB₂-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB₂ ceramics and ZrB₂-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB₂-based composites is in the range of 2–6 MPa m^1/2. Fine-grained ZrB₂ ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi₂. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB₂-based ceramics. In addition, the ZrB₂-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB₂-based composites attractive candidates for high-temperature thermomechanical structural applications.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

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.     

Corrosion behavior of porous silicon nitride ceramics in different atmospheres

The corrosion behavior of silicon nitride (Si₃N₄) ceramics with a porosity of 46% at 1200–1500 °C under different conditions including dry O₂, O₂ containing 20 vol% H₂O and Ar containing 20 vol% H₂O is compared. The results show that porous Si₃N₄ ceramics exhibit good oxidation resistance up to 1200 °C. Their corrosion behavior varies depending on the temperature and atmosphere. Water vapor can obviously affect the morphology of the reaction product and thus accelerate the corrosion rate due to its specific inward diffusion mechanism and devitrified effect at high temperature. In view of the reaction kinetics, it proceeds in a diffusion-controlled manner in dry O₂ while follows the parabolic-linear law at water-containing atmosphere. Furthermore, a new model considering both oxidation and volatilization reactions is established. These provide a baseline for expanding the application fields of non-oxide porous ceramics such as Si₃N₄ and silicon carbide (SiC) etc.     

Effect of SNNWS content on the microstructure and properties of SNNWS/Si-C-N ceramic composites via PIP

Si-C-N ceramic composites containing well distributed silicon nitride nanowires (SNNWs) were fabricated by die-pressing and precursor infiltration and pyrolysis process at a low temperature. The structure, composition, mechanical and thermophysical properties of the composites were investigated. The results show that the composites consisted of amorphous SiCN, α-Si₃N₄ and α-cristobaslite. The composites with different contents of SNNWs possessed a density of 2.02–2.07 g cm^?3 and open porosity of 7.9–9.9%. SNNWs can effectively restrain the contraction of matrix with a decrease by 25% in linear shrinkage. The composites with 3 wt% SNNWs owned the highest flexural strength (83.7 MPa) and elastic modulus (54.0 GPa) at room temperature, which increase by 13.2% and 62.3% respectively, compared with pure SiCN ceramics. The SNNWs displayed good reinforcement function at high temperature due to the fact that the composites with 7 wt% SNNWs had a 96.8% retention rate of bending strength at 1200 °C. The composites had relatively low coefficient of thermal expansion and thermal diffusivity which were less than 2.2 × 10^?6 K^?1 and 0.62 mm² s^?1, respectively.     

Effect of oxidation treatment on KD–II SiC fiber–reinforced SiC composites

Continuous silicon carbide fiber–reinforced silicon carbide matrix (SiC_f/SiC) composites have developed into a promising candidate for structural materials for high–temperature applications in aerospace engine systems. This is due to their advantageous properties, such as low density, high hardness and strength, and excellent high temperature and oxidation resistance. In this study, SiC_f/SiC composites were fabricated via polymer infiltration and pyrolysis (PIP) with the lower–oxygen–content KD–II SiC fiber as the reinforcement; a mixture of 2,4,6,8–tetravinyl–2,4,6,8–tetramethylcyclotetrasiloxane (V4) and liquid polycarbosilane (LPCS), known as LPVCS, was used as the precursor; while pyrolytic carbon (PyC) was used as the interface. The effects of oxidation treatment at different temperatures on morphology, structure, composition, and mechanical properties of the KD–II SiC fibers, SiC matrix from LPVCS precursor conversion, and SiC_f/SiC composites were comprehensively investigated. The results revealed that the oxidation treatment greatly impacted the mechanical properties of the SiC fiber, thereby significantly influencing the mechanical properties of the SiC_f/SiC composite. After oxidation at 1300 °C for 1 h, the strength retention rates of the fiber and composite were 41% and 49%, respectively. In terms of the phase structure, oxidation treatment had little effect on the SiC fiber, while greatly influencing the SiC matrix. A weak peak corresponding to silica (SiO₂) appeared after high–temperature treatment of the fiber; however, oxidation treatment of the matrix led to the appearance of a very strong diffraction peak that corresponds to SiO₂. The analysis of the morphology and composition indicated cracking of the fiber surface after oxidation treatment, which was increasingly obvious with the increase in the oxidation treatment temperature. The elemental composition of the fiber surface changed significantly, with drastically decreased carbon element content and sharply increased oxygen element content.     

Near net-shape fabrication of alumina glass composites

The purpose of the present study is to fabricate alumina glass composites by melt infiltration with better dimensional control through reducing both the presintering and infiltration temperature. Main efforts were put to develop glasses that are chemically compatible with alumina. After extensive investigations, a glass of 21SiO₂–24B₂O₃–35Al₂O₃–15Li₂O–5CaO wt.% was successfully developed. The glass shows good chemical compatibility with alumina at elevated temperatures and low viscosity above 900 °C. Dense alumina glass composites can be fabricated by the melt infiltration process at 950 °C, which is 150 °C lower than the current state-of-art. Investigations showed improved net-shape capability for the newly developed composites, where the total linear shrinkage for the sintering and infiltration at 950 °C is less than 0.1%, as compared with the shrinkage of 0.5% induced by the presintering and infiltration at 1100 °C. Preliminary mechanical tests showed that the fracture strength and toughness of the composites are 303 MPa and 3.4 MPa m^1/2, respectively. The lower processing temperature and the better dimensional control are the major advantages for the newly developed alumina glass composites.     

Oxidation behaviour of Si3N4–TiN ceramics under dry and humid air at high temperature

The oxidation in air of Si₃N₄-based ceramics containing 35 vol.% of TiN secondary phase and different amounts of sintering additives has been studied at different temperatures up to 1400 °C in dry or humid environment. The oxidation starts by crystal growth of TiO₂ at the surface, then a multilayered scale develops under the rutile layer from 1000 °C. This subscale is composed of silicon nitride in which TiN particles are oxidized to agglomerates of rutile, glass and pores. The oxidation process is controlled by the matter transports, which take place in the intergranular phase. These transport phenomena are affected by the changes in distribution and composition of the glassy phase and by humidity which modifies the glass network structure and thus the in-diffusion rate. From 1200 °C, Si₃N₄ grains are also oxidized, the additional glass formed closes the residual porosities yielding scales more compact and developing an autoprotective behavior. At 1400 °C, glass phase crystallizes into cristobalite and the rutile top layer becomes discontinuous. Only composites with low amounts of sinter additives keep an autoprotective oxidation mode.     

Glass infiltration of gelcast zirconia-toughened alumina ceramics for dental restoration

This study investigated the effects of process parameters and material characteristics in glass infiltration of gelcast zirconia-toughened alumina (ZTA) ceramics for dental applications. Nine types of lanthanum-based silicate glasses with different concentrations of La₂O₃, B₂O₃, and Li₂O were prepared and characterized on the basis of their melting temperature, crystal structure, and thermal characteristics. These glasses were infiltrated into a ZTA matrix that was semi-sintered at 1200 °C to obtain a ceramic material with low shrinkage and high strength. The performance of the glass-infiltrated ceramic samples obtained with various glasses for various infiltration durations and temperatures was evaluated on the basis of the linear shrinkage rate, three-point bending strength, fracture morphology, and elemental composition. From the results, the infiltration temperature was identified as 1150 °C, and the optimum glass composition was 30%La₂O₃–15%Al₂O₃–15%B₂O₃–15%SiO₂–5%Li₂O by mass. The glass-infiltrated gelcast ZTA ceramic has a strength of 291.24 ± 27.94 MPa with a shrinkage of 1.8548 ± 0.2663%. These findings could provide a basis for further development of gelcast dental ceramics, which will make all-ceramic oral restoration less reliant on special equipment and materials.     

Effects of polycarbosilane infiltration processes on the microstructure and mechanical properties of 3D-Cf/SiC composites

Three-dimensional braided carbon fiber-reinforced silicon carbide (3D-C_f/SiC) composites were prepared through eight cycles of infiltration of polycarbosilane (PCS)/divinylbenzene (DVB) and subsequent pyrolysis under an inert atmosphere. The effects of infiltration processes on the microstructure and mechanical properties of the C_f/SiC composites were investigated. The results showed that increasing temperature could reduce the viscosity of the PCS/DVB solution, which was propitious to the infiltration processes. The density and flexural strength of 3D-C_f/SiC composites fabricated with vacuum infiltration were 1.794 g cm⁻³ and 557 MPa, respectively. Compared to vacuum infiltration, heating and pressure infiltration could improve the infiltration efficiency so that the composites exhibited higher density and flexural strength, i.e., 1.944 g cm⁻³ and 662 MPa. When tested at 1650 °C and 1800 °C in vacuum, the flexural strength reached 647 MPa and 602 MPa, respectively.     

Application of the chemical vapor infiltration and reaction (CVI-R) technique for the preparation of highly porous biomorphic SiC ceramics derived from paper

Chemical vapor infiltration and reaction (CVI-R) is used to produce biomorphic high porous SiC ceramics based on biological structures such as paper. The paper fibers are first converted into a biocarbon (C_b) template by a carbonization step. In a second step methyltrichlorosilane (MTS) in excess of hydrogen is infiltrated into the C_b-template by CVI technique, depositing a Si/SiC layer around each fiber. The reaction (R) between biocarbon and excess silicon to form additional silicon carbide occurs during a subsequent thermal treatment as a third step of the ceramization process. Due to the mild infiltration conditions (850–900 °C) the initial micro- and macrostructure of the carbon preform is retained in the final ceramics. The applied characterization methods after every step of the ceramization process are X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, Thermal Gravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM). The bending strength of the resulting porous ceramics is measured by the double ring bending test. It is found that a slight excess of free Si in relation to the amount of carbon from the C_b-template must be deposited in the Si/SiC layer to achieve a nearly complete conversion of the C_b-templates into SiC ceramic. The weight gain after infiltration has to be at least 400 wt.%. Varying the infiltration conditions such as temperature, MTS concentration and infiltration time, ceramics in a wide range of porosity (55–80%) and mechanical properties (5–40 MPa) can be produced. A thermal treatment temperature of 1400 °C is found to be optimal for the reaction between the deposited Si and the biocarbon.