Improvement of oxidation resistance of unidirectional Cf/SiO2 composites by the addition of SiCp

Unidirectional carbon fiber reinforced fused silica composites (uni-C_f/SiO₂) with addition of different contents of SiC particle (SiC_p) were prepared by slurry infiltrating and hot-pressing. The model of oxygen infiltrating into the composite was supposed according to the characterization of fiber/matrix interface observed by transmission electronic microscope (TEM). The oxidation process of the composite was analyzed by thermo-gravimetry and differential scanning calorimeter (TG-DSC) method and the oxidation resistance was evaluated by the residual flexural strength and the fracture surface of the composite after heat treatment at elevated temperatures method. The results showed that the oxidation of carbon fiber started at 480 °C and ended at 800 °C and the oxidation of SiC_p started at above 1000 °C in the composite. The addition of 20 wt.% SiC_p had a better oxidation resistance. According to the characterization of fiber/matrix interface observed by TEM, gaps existed at the fiber/matrix interface which resulted from the CTE mismatch of carbon fiber and SiO₂ matrix. While the CTE mismatch between SiC_p and SiO₂ matrix could also result in the pre-existing gaps in the matrix. The oxygen penetrated along the gaps and simultaneously reacted with carbon fiber ends and SiC_p, which filled the gaps at the fiber/matrix interface and the pre-existing gaps in the matrix and subsequently prevented oxygen from infiltrating inward.     

Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.     

Joining of sintered silicon carbide using ternary Ag–Cu–Ti active brazing alloy

Sintered silicon carbide was brazed to itself by Ag–35.25 wt%Cu–1.75 wt%Ti filler alloy at 860 °C, 900 °C and 940 °C for 10 min, 30 min and 60 min. Mechanical properties both at room temperature and high temperature were measured by flexural strength. The interfacial microstructure was investigated by electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results indicate that increased brazing temperature heightens the flexural strength and the maximal four-point flexural strength reaches 342 MPa at room temperature. In addition longer holding times result in thicker reaction layer, which increases mismatch of coefficients of thermal expansion (CTE) between SiC substrate and reaction layer and finally leads to poor mechanical properties due to high residual stresses. High temperature flexural strength decreases with an increase of test temperature due to softening of the filler alloy. A reaction layer composed of TiC and Ti₅Si₃ was observed at the interface of SiC/filler alloy and there is a representative microstructure: SiC/continuous fine TiC layer/discontinuous coarse Ti₅Si₃ layer/filler alloy.     

Ultra high thermal conductivity polymer composites

Epoxy composites based on vapor grown carbon fiber (VGCF) were fabricated and analyzed for room temperature thermophysical properties. An unprecedented high thermal conductivity of 695 W/m K for polymer matrix composites was obtained. The densities of all the composites are lower than 1.5 g/cc. In addition the high value of coefficient of thermal expansion (CTE) of the polymer material was largely reduced by the incorporation of VGCF. Also, unlike metal matrix composite (MMC), the epoxy composite has an electrically insulating surface. Based on the composite thermal conductivities, the room temperature thermal conductivity of VGCF, heat-treated at 2600°C, was estimated to be 1260 W/m K. Furthermore, the longitudinal CTE of the heat-treated VGCF was determined, for the first time, to be −1.5 ppm/K.     

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

Compressive strength of three-dimensionally reinforced carbon/carbon composite

Compressive behavior of three-dimensionally reinforced carbon/carbon composite (3D-C/C) was examined from room temperature to elevated temperatures up to about 3000 K. Three-dimensionally reinforced C/C was found to have an inclination to induce kinks at the ends of specimens due to extremely low shear strength. In order to avoid this type of premature fracture and to conduct high-temperature tests, discussion was made on specimen geometry and testing procedure, and the combination of a dumbbell-shape specimen and test configuration without a supporting jig were found to be suitable for the present study. Using this set-up, the compressive strength of a 3D-C/C was evaluated as a function of temperature up to about 3000 K. The compressive strength of the 3D-C/C monotonically increased with the increase in temperature up to 2300 K, but decreased above this temperature. The strength enhancement was suggested to be caused by improvement in the fiber/matrix interfacial bonding, and the degradation over 2300 K was by softening of the matrix at high temperatures.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300 °C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100 °C, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400 °C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve.     

Preparation and characterization of C/SiC–ZrB2 composites by precursor infiltration and pyrolysis process

Ultra-high temperature ceramic matrix composites (C/SiC–ZrB₂) are prepared by slurry and precursor infiltrations and pyrolysis method. C/SiC–ZrB₂ composites with ZrB₂ volume content from 10% to 24.6%, have balanced performance of fracture toughness (17.7–8.1 MPa m^1/2), flexural strength at room temperature (367–163 MPa) and at high temperature (strength retention 74% at 1800 °C and over 32% at 2000 °C), better oxidation and ablation resistance under oxyacetylene torch environment (recession rate 0.01 mm/s).     

Weak interface dominated high temperature fracture strength of carbon fiber reinforced mullite matrix composites

Weak fiber/matrix interface dominates the toughening properties of ceramic matrix composites. This paper reports a novel sol-gel fabricated carbon fiber reinforced mullite matrix composite, in which the fiber/matrix interface was inherently weak in shear properties (∼25 MPa), measured in-situ by fiber push-in tests. The interface microstructure was chemically sharp, characterized by transmission electron microscopy. The outcome of the weak interface was the full trigger of the toughening mechanisms like crack deflection, etc., leading to significant enhancement of the fracture toughness of the composite (∼12 MPa√m), measured by single edged notch beam method. Finally, due to the weak fiber/matrix interface and large thermal expansion mismatch of the fiber and matrix, the high temperature fracture strength was enhanced in the temperature range from 25 to 1200 °C, which is attributed to the enhancement of the interfacial property at elevated temperatures that favors better load transfers between composite constituents.     

Mechanical properties and microstructures of Cf/SiC-ZrC composites using T700SC carbon fibers as reinforcements

C_f/SiC-ZrC composites were fabricated through mold-pressing and polymer infiltration and pyrolysis (PIP) process using T700SC carbon fibers as reinforcements. The effects of interphases on the mechanical properties and microstructures of composites were studied. Composite showed brittle fracture behavior and low bending stress of 81 ± 24 MPa when no interphase was deposited on the fiber surface. With the deposition of a PyC/SiC interphase, composite showed typical non-brittle fracture behavior and the bending stress increased to 401 ± 64 MPa and a large amount of pulled-out fibers could be observed on the fracture surface. Meanwhile, it could also be concluded from the microstructures of the composites that the existed interphases had a great hindering effect on the infiltration of ZrC into the intra-bundle zones in the slurry infiltration process. The TEM analysis results showed that the carbon fibers were almost not eroded and the brittle fracture behavior may be mainly ascribed to the strong bonding between fibers and matrix.     

Mechanical property and microstructure evolutions of Cf/SiOC composites with increasing annealing temperature in reduced pressure environment

Three-dimensional (3D) carbon fiber reinforced silicon oxycarbide (C_f/SiOC) composites were fabricated through precursor impregnation and pyrolysis process using siloxane resin as precursor. These composites were further annealed at various temperatures to follow their mechanical properties and microstructure evolutions in reduced pressure environment. The decrease in strength of those composites could be attributed mainly to the decomposition of matrixes. When the annealing temperature increased from 1000 to 1250 °C, the mechanical properties of composites did not change too much, despite the matrixes underwent a redistribution of Si-O bonds and Si-C bonds. At temperature above 1300 °C, due to the weight loss and volume shrinkage of matrixes caused by carbothermal reduction, both the flexural strength and elastic modulus of C_f/SiOC composites decreased rapidly.     

Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites

Spark plasma sintering (SPS) of monolithic ZrB₂ ultra-high temperature ceramic and 2–6 vol% graphene nanoplates (GNPs) reinforced ZrB₂ matrix composites is reported. The SPS at 1900 °C with a uni-axial pressure of 70 MPa and soaking time of 15 min resulted in near-full densification in ZrB₂–GNP composites. Systematic investigations on the effect of GNP reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented. Densification mechanisms, initiated by interfacial reactions, are also proposed based on detailed thermodynamic analysis of possible reactions at the sintering temperature and the analysis of in-process punch displacement profiles. The results show that GNPs can be retained in the ZrB₂ matrix composites even with high SPS temperature of 1900 °C and cause toughening of the composites through a range of toughening mechanisms, including GNP pull-out, crack deflection, and crack bridging.     

Joining of Si3N4 to Si3N4 using a AuPd(Co,Ni)–V filler alloy and the interfacial reactions

Au38.0–Pd28.0–Co18.0–Ni7.0–V9.0 (in wt%) alloy was designed as a filler for joining Si₃N₄. The filler alloy showed a contact angle of 77.2° on Si₃N₄ ceramic at 1473 K. The Si₃N₄/Si₃N₄ joint brazed with the rapidly-solidified filler foils at 1443 K for 10 min exhibits an average three-point bend strength of 320.7 MPa at room temperature and the strength values are 217.9 MPa and 102.9 MPa at 1073 K and 1173 K respectively. The interfacial reaction products were composed of V₂N and Pd₂Si, and the elements Co and Ni in the brazing alloy did not participate in the interfacial reactions. The coarse-network-like distribution of refractory Pd₂Si compound within the Au–Pd–Co–Ni alloy matrix throughout the joint contributes to the stable high-temperature joint strengths.     

Preparation and properties of 2D C/SiC-ZrB2-TaC composites

Two-dimensional (2D) C/SiC-ZrB₂-TaC composites were fabricated by chemical vapor infiltration (CVI) combined with slurry paste (SP) method. 2D laminate was prepared by stacking carbon cloth that was pasted with a mixture of polycarbosilane-ZrB₂-TaC slurry. A small amount of carbon fiber tows were introduced into the preform in the vertical direction. After heat-treated at 1800 °C, the 2D laminate was densified with SiC by CVI to obtain 2D C/SiC-ZrB₂-TaC composites. Properties including flexural strength, interlaminar shear strength, and thermal expansion of the composites were investigated. The ablation test was carried out under an oxyacetylene torch flame. The morphologies of the ablated specimens were analyzed. The results indicate that the adding vertical fiber tows and heat-treatment at 1800 °C can greatly improve the mechanical properties of the composites. The co-addition of TaC and ZrB₂ powders into C/SiC composite effectively enhance its ablation resistance.     

Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating

The crystallization behavior of the amorphous phase of the plasma sprayed LaMgAl₁₁O₁₉ (LaMA) coating during thermal aging processes has been investigated. Results indicate that LaMA coating exhibits much similar microstructure and thermal properties such as close coefficient of thermal expansion and specific heat capacity etc. to the sintered LaMA bulk after aging at 1673 K for 20 h. On the other hand, a solid state reaction seems to occur to reform the ideal magnetoplumbite-type LaMA phase coupled with the formations of the La-rich aluminate intermediate phases. When the aging temperature is held between 1273 K and 1473 K, nanosized platelet-like grains as well as sub-grains with high aspect ratios are present. The phase stability has been investigated through the chemical compositions and X-ray diffraction analysis. The recrystallization mechanism of the amorphous LaMA coating has been explored by tracing the microstructure evolutions during thermal aging process.     

High temperature flexural strength,microstructure, phase evolution and anti-oxidation mechanism of Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C

Carbon fiber/phenolic resin composites (CF/Ph) have attracted great interests in the field of thermal protection materials for their characteristics of high specific strength and easy manufacturing. However, CF/Ph are inherently susceptible to oxidation failure at elevated temperatures. In this study, a novel Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB₂ and B₄C was fabricated by an impregnating and compression molding route. Thermal stability, flexural strength, microstructure and phase evolution of the resulting ceramizable composite were studied. The residue yield at 1400 °C and flexural strength after treated at 1400 °C for 15min was 90.4% and 53.1 MPa, respectively, which was increased by 15.9% and 532.1% than that without ceramizable fillers. Surface defects generated by matrix pyrolysis were well healed, and PyC and carbon fibers were covered with dense ceramic layers while the fracture surface was covered with relatively continuous ceramic layers without visible pores. Multiphase ceramics composed of TiB₂, TiO₂, TiC and PyC were identified. Furthermore, oxidation failure and anti-oxidation mechanism was revealed based on the aforementioned characterizations and thermodynamic calculation results. Oxidation resistance got enhanced markedly for synergistic effects of oxygen consuming, carbon fixation, oxygen barrier and endothermic effect, which were derived from ceramization reactions between TiB₂, B₄C, O₂, Al and PyC.     

A hot-pressing reaction technique for SiC coating of carbon/carbon composites

A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating.     

Influence of oxidation on fatigue life of a carbon/silicon carbide composite in water vapor containing environments

The influence of oxidation on the fatigue life of two-dimensional carbon/silicon carbide composites in water vapor containing environments at 1300 °C was investigated. Tension–tension fatigue experiments were conducted at sinusoidal frequency of 3 Hz. Using a stress ratio (σ_min/σ_max) of 0.1, specimens were subjected to peak fatigue stresses of 90, 120 and 150 MPa. The mean residual strength of the specimens after survived 100,000 cycles with a peak stress of 90 MPa was 83.9% of that of the as-received composite. The mean fatigue lives of the specimens subjected to peak fatigue stresses of 120 and 150 MPa were 42,048 and 13,514 cycles, respectively. Oxidation was the dominant damage mechanism, which remarkably decreased the fatigue life. Oxidizing species diffusion within the composite defects was discussed. The higher the applied stresses, the larger the equivalent radius of the defect and the shorter the fatigue life.     

Hydrogen storage properties of Pd nanoparticle/carbon template composites

Theoretical studies predict improved hydrogenation properties for hybrid carbon/metal composites. The hydrogen storage capacity of ordered porous carbon containing Pd clusters was measured. The C/Pd composite was obtained by chemical impregnation of an ordered porous carbon template (CT) with a H₂PdCl₄ solution followed by a reduction treatment. 10 wt.% of palladium clusters were introduced in the carbon porosity; the Pd clusters (2 nm in size) being homogeneously distributed. Thermodynamic hydrogenation properties of both Pd-free CT and the Pd-10 wt.% CT composite have been determined by hydrogen isotherm sorption measurements and thermal desorption spectroscopy (TDS) analysis. The introduction of the palladium into the carbon matrix does not increase the hydrogen storage capacity at 77 K and 1.6 MPa, since here the hydrogen uptake is being attributed to physisorption on the carbon. However, at room temperature and moderate pressure (0.5 MPa), the filling of the CT with 10 wt.% nanocrystalline Pd results in an hydrogen uptake eight times larger than that of the Pd-free CT. After the second cycle, a good reversibility is observed. TDS measurements confirm that the sharp increase of the hydrogen uptake is due to the presence of the Pd clusters in the carbon porosity.     

Mechanical behavior of La0.8Sr0.2Ga0.8Mg0.2O3 perovskites

This paper examines the important mechanical properties of commercially purchased La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ at room temperature and 800 °C. Sr and Mg-doped lanthanum gallates (LSGM) are strong candidates for use as solid electrolytes in lower temperature solid oxide fuel cells operating at or below 800 °C. The material was found to be phase pure with a Young's modulus value of ∼175 GPa. The four point bending strength of the LSGM samples remained almost constant from 121 ± 35 MPa at room temperature to 126 ± 20 MPa at 800 °C. The fracture toughness, as measured by the single edge V notch beam (SEVNB) method, was 1.22 ± 0.06 MPa√m at room temperature, 1.04 ± 0.09 MPa√m at 700 °C followed by a small increase 1.31 ± 0.16 MPa√m at 800 °C. We also report, for the first time, the static subcritical (or slow) crack-growth (SCG) behavior of natural cracks in LSGM performed in four point bending tests at room temperature. The exponent of a power-law representation in the SCG tests was found to be n = 15, a rather low value showing LSGM to be highly susceptible to room temperature SCG.