Interaction of ceria and ytterbia in air within temperature range 1500 – 600 °C

Phase equilibria and structure transformations in the CeO₂–Yb₂O₃ system have been studied in air within the temperature range 1500 - 600 °C in the full concentration range using X-ray diffraction analysis (XRD) and petrography methods. It was established that the system is characterized by the formation of solid solutions on the basis of cubic modification of Yb₂O₃ (C- type) and fluorite CeO₂ (F- type) separated by two-phase (F + C) region. The systematic study that covered whole composition range excluded formation of new phases. Solubility limits and concentration dependences of lattice parameters were determined for the phases forming in the system.     

Microstructure and mechanical performance evolution of C/C–ZrC composites exposed to oxyacetylene flame

C/C–ZrC composites were prepared by isothermal chemical vapor infiltration (ICVI) combined with reactive melt infiltration (RMI). The ablation behavior of the C/C–ZrC was investigated using an oxyacetylene flame. The effect of ablation time on the microstructure and mechanical property evolution of the composite was studied. The results showed that as the ablation time prolonged, the linear and mass ablation rates of the composite increased firstly and then stabilized. After 15 s ablation, the flexural strength and modulus of the C/C–ZrC were interestingly increased by 141.8% and 40.9%, which reached 138.42 MPa and 6.45 GPa, respectively. During ablation, the preferential oxidation effect of ZrC could mitigate the oxidation of pyrolytic carbon (PyC) and carbon fibers, and the volume change induced by the ZrC →ZrO₂ phase transformation could weaken its bonding with PyC, which was beneficial for releasing the internal residual stresses of the C/C–ZrC and then contributed to the mechanical performance improvement.     

Oxidation and microstructure of SiCf/SiC composites in moist air up to 1600°C by X-ray tomographic characterization

SiC_f/SiC composites that possess PyC or BN interface layers were fabricated and then oxidized in moist air at 1000, 1200, 1400, and 1600°C. High-resolution CT was used for capturing 3D images and quantifying the SiC phase, mesophase, and voids. The oxidation behavior and microstructural evolution of SiC_f/SiC with PyC or BN interface are discussed in this study. The microstructure of the SiC_f/SiC with a PyC layer was seriously damaged in moist air at high temperature, whereas the BN interface layer enhanced the oxidation resistance of the SiC_f/SiC. These results are also confirmed by using XRD, oxidation mass gain, tensile testing, and SEM measurements. The results of the oxidation behavior and microstructural evolution for SiC_f/SiC oxidized in dry air are also compared with the results of this study. Comparing the SiC_f/SiC with a PyC interface layer, the composite with a BN interface layer oxidized in moist air exhibits a high void growth rate and a low SiO₂ grain growth rate from 1000 to 1600°C. This work will provide guidance for predicting the service life of SiC_f/SiC for multiscale damage rate models of materials at a local scale and will also provide guidance on the life service design of SiC_f/SiC materials.     

Creep behavior and failure mechanisms of CVI and PIP SiC/SiC composites at temperatures to 1650 °C in air

Creep properties of 2D woven CVI and PIP SiC/SiC composites with Sylramic™-iBN SiC fibers were measured at temperatures to 1650 °C in air and the data was compared with the literature. Batch-to-batch variations in the tensile and creep properties, and thermal treatment effects on creep, creep parameters, damage mechanisms, and failure modes for these composites were studied. Under the test conditions, the CVI SiC/SiC composites exhibited both matrix and fiber-dominated creep depending on stress, whereas the PIP SiC/SiC composites displayed only fiber-dominated creep. Creep durability in both composite systems is controlled by the most creep resistant phase as well as oxidation of the fibers via cracking matrix. Specimen-to- specimen variations in porosity and stress raisers caused significant differences in creep behavior and durability. The Larson-Miller parameter and Monkman-Grant relationship were used wherever applicable for analyzing and predicting creep durability.     

Influence of SiC content on the oxidation of carbon fibre reinforced ZrB2/SiC composites at 1500 and 1650 °C in air

The microstructure and the oxidation resistance in air of continuous carbon fibre reinforced ZrB₂–SiC ceramic composites were investigated. SiC content was varied between 5–20?vol.%, while maintaining fibre content at ～40?vol.%. Short term oxidation tests in air were carried out at 1500 and 1650?°C in a bottom-up loading furnace. The thickness, composition and microstructure of the resulting oxide scale were analysed by SEM-EDS and X-Ray diffraction. The results show that contents above 15?vol.% SiC ensure the formation of a homogeneous protective borosilicate glass that covers the entire sample and minimizes fibre burnout. The scale thickness is ～90?μm for the sample containing 5?vol.% SiC and decreases with increasing SiC content.     

Mechanical behavior of zirconium diboride–silicon carbide ceramics at elevated temperature in air

The mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics were characterized from room temperature up to 1600 °C in air. ZrB₂ containing nominally 30 vol% SiC was hot pressed to full density at 1950 °C using B₄C as a sintering aid. After hot pressing, the composition was determined to be 68.5 vol% ZrB₂, 29.5 vol% SiC, and 2.0 vol% B₄C using image analysis. The average ZrB₂ grain size was 1.9 μm. The average SiC particles size was 1.2 μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680 MPa and strength increased to 750 MPa at 800 °C. Strength decreased to ～360 MPa at 1500 °C and 1600 °C. The elastic modulus at room temperature was 510 GPa. Modulus decreased nearly linearly with temperature to 210 GPa at 1500 °C, with a more rapid decrease to 110 GPa at 1600 °C. The fracture toughness was 3.6 MPa·m^½ at room temperature, increased to 4.8 MPa·m^½ at 800 °C, and then decreased linearly to 3.3 MPa·m^½ at 1600 °C. The strength was controlled by the SiC cluster size up to 1000 °C, and oxidation damage above 1200 °C.     

Interaction of ceria and erbia in air within temperature range 1500–600 °C

Phase equilibriums and structure transformations in the CeO₂–Er₂O₃ system have been studied within 1500–600 °C in the full concentration range using XRD and petrography methods. It was established that the system is characterized by the formation of solid solutions on the basis of cubic modification of Er₂O₃ (C-type) and fluorite CeO₂ (F-type). Solubility limits and concentration dependences of lattice parameters were determined for the phases forming in the system.     

Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics

Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m^1/2, which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness.     

Microstructure and ablation mechanism of C/C–SiC composites

C/C–SiC composites were prepared by molten infiltration of silicon powders, using porous C/C composites as frameworks. The porosities of the C/C–SiC composites were about 0.89–2.8 vol%, which is denser than traditional C/C composites. The ablation properties were tested using an oxyacetylene torch. Three annular regions were present on the ablation surface. With increasing pyrocarbon fraction, a white ceramic oxide layer formed from the boundary to the center of the surface. The ablation experimental results also showed that the linear and mass ablation rates of the composites decreased with increasing carbon fraction. Linear SiO₂ whiskers of diameter 800 nm and length approximately 3 μm were formed near the boundaries of the ablation surfaces of the C/C–SiC composites produced with low-porosity C/C frameworks. The ablation mechanism of the C/C–SiC composites is discussed, based on a heterogeneous ablation reaction model and a supersaturation assumption.     

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.     

Microstructure and mechanical properties at room and elevated temperatures of reactively hot pressed TiB2–TiC–SiC composite ceramic tool materials

TiB₂-based composite ceramic tool materials with different amounts of TiC and SiC were fabricated via a reactive hot pressing process. The mechanical properties at room temperature and flexural strength at 800–1300 °C were tested in ambient air. The composition and microstructure before and after the high-temperature strength tests were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectrometer (EDS). The flexural strength increment/degradation mechanisms at elevated temperatures were investigated. In-situ synthesized TiC improved the sinterability and mechanical properties of the materials at both room and elevated temperatures. Comparing with TTS (TiB₂–15.9 wt%TiC–10.6 wt%SiC) and TS (TiB₂–22.4 wt%SiC), TTS3 (TiB₂–8.1 wt%TiC–16.4 wt%SiC) had the optimum room temperature mechanical properties, i.e., flexural strength of 862 MPa, fracture toughness of 6.4 MPa m^1/2, hardness of 22.8 GPa, and relative density of 99.3%. The improved mechanical properties were ascribed to the fine grain size. The flexural strength of the TTS composite at 800 °C was higher than that at room temperature. The improvement of the flexural strength was attributed to the healing of preexisting flaws and the relief of residual stress. Substantial strength degradation took place when the temperature exceeded 1000 °C, due to softening of the grain boundaries, surface oxidation and elastic modulus degradation.     

Strength and deformation of concrete with variable content of moisture at elevated temperature up to 90°C

It is known, that the change of mechanical properties of concrete due to elevated temperature is also influenced by the moisture content. This change was primarily studied for prestressed concrete reactor vessels (PCRV). Because a PCRV is a mass concrete structure, the results of this research cannot be transfered to slender members. To simulate drying conditions of the latter, specimens of differing initial moisture content were subject to elevated temperatures and defined climates. The results of tests reveal the differing influence of moisture on strength and modulus of elasticity. The compressive strength is partly increased, partly decreased as the moisture content grows. Tensile strength and modulus of elasticity are weakened by decreasing moisture. Also the thermal strain as function of type of aggregate and moisture content was studied. The changes are caused by the alteration of structure of cement stone and by microcracks due to incompatibility.     

Microstructure evolution and mechanical behavior of a mullite fiber heat-treated from 1100 to 1300°C

In this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ-Al₂O₃ and δ-Al₂O₃ with amorphous SiO₂ at 1150°C. The stable α-Al₂O₃ formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.     

Oxidation resistance of SiCf/SiC composites with three-layer environmental barrier coatings up to 1360 °C in air atmosphere

Atmospheric plasma spraying (APS) was used to prepare three-layer environmental barrier coatings (EBCs) Si/Yb₂SiO₅/LaMgAl₁₁O₁₉ (LMA) on a SiC_f/SiC substrate. Isothermal aging test of the specimens were performed between 1000 and 1360 °C for 500 h. The flexural strength of the specimens after isothermal aging was investigated. Microcracks and holes were observed in the as-sprayed EBCs because of the shock cooling during the APS process, but reduced after isothermal aging, and the EBCs became denser. At least 80.07% of the flexural strength of the SiC_f/SiC substrate with EBCs was maintained after isothermal aging, but only 35.91% strength was maintained without EBCs. In particular, the retention ratio of flexural strength was 90.72% after isothermal aging at 1360 °C, despite a reaction between the layers of the EBCs. All the specimens with EBCs showed “pseudo-plastic” fracture, compared with the brittle fracture of specimens without EBCs.     

Tension-tension fatigue behaviour of 3D braided SiCf/SiC composite with film cooling holes at 1350 °C in air

The tension-tension fatigue behaviour of SiC_f/SiC composites with film cooling holes (FCHs) was investigated at 1350 °C in air. The number of drilled holes were 1, 5, 10 (rectangular arrangement) and 11 (triangular arrangement), and the average diameter was 0.5 mm. The fatigue stresses were 80 MPa and 120 MPa at a frequency of 3.0 Hz, and the R ratio of minimum stress to maximum stress was 0.1. The stress-strain hysteresis curve, strain accumulation and modulus decrease were investigated, and characterization of the fracture morphology and microstructure was performed to reveal the initiation and evolution of damage during fatigue cycles. The results indicated that the FCHs exhibit little effect on the ultimate tensile strength of the composites. The fatigue life decreases with an increasing number of FCHs, and the effect of the FCHs on the fatigue life gradually decreases as stress levels increase. Specimens with triangular arrangement 11-H exhibit the worst fatigue life, and the damage mechanism is significant relative to large-area of fibre oxidation on the fracture surface. The weakening effect of FCHs on the fatigue performance of the composites is mainly due to the oxidation and embrittlement of fibres around the hole, and it is significant relative to initial crack nucleation and propagation in the coating and matrix.     

Static fatigue of Hi-Nicalon™-S fiber at elevated temperature in air,steam, and silicic acid-saturated steam

A facility for testing SiC fiber tows in static fatigue and creep at elevated temperatures in air and steam was developed. Static fatigue of Hi-Nicalon™-S fibers was investigated at 800°C-1100°C at applied stresses between 115 and 1250 MPa in air, in Si(OH)₄(g)-saturated steam, and in unsaturated steam. Fibers tested in Si(OH)₄(g)-saturated steam and in air had silica scales throughout the test sections, but those tested in unsaturated steam did not develop scales near the steam injection point. Fiber lifetimes in static fatigue were shortest in unsaturated steam, intermediate in Si(OH)₄(g)-saturated steam, and longest in air. Failure strains did not exceed 0.3%. Steady-state strain rates and static fatigue lifetimes are modelled empirically by the Monkman-Grant relationship. Failure mechanisms are discussed.     

Semiconductor-conductor transition of pristine polymer-derived ceramics SiC pyrolyzed at temperature range from 1200°C to 1800°C

This paper studies the effect of pyrolysis temperature on the semiconductor-conductor transition of pristine polymer-derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C-1800°C is presented. At relatively lower pyrolysis temperatures (1200°C-1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C-1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal-like conductive response and their resistivity increases monotonically with the increasing measuring temperature.     

Ablation of C/SiC,C/SiC–ZrO2 and C/SiC–ZrB2 composites in dry air and air mixed with water vapor

C/SiC composites with different additives (ZrO₂ and ZrB₂) were fabricated by CVI and CVD and their oxidation and ablation properties at 1700–1800 °C were investigated. Two different ablation test conditions, dry air and air mixed with water vapor, are compared. The ablation test results are reviewed, the weight loss rates are presented and the corresponding micro-structures are investigated in detail. The results show that in dry air, the weight loss rate of C/SiC composites is greater than those with ZrO₂ and ZrB₂ additives. However, in air mixed with water vapor (5 wt%) to simulate the hygrothermal condition, the weight loss rates of these three composites all become relatively smaller. A model is proposed to predict the weight loss of C/SiC composites and it agrees well with the experimental data.     

The effect of a graphite addition on oxidation of ZrB2–SiC in air at 1500 °C

Dense ZrB₂ containing 15 vol.% SiC and 15 vol.% graphite was exposed to flowing air at 1500 °C. A layered scale structure developed that consisted of (1) a uniform SiO₂-rich layer on the surface, (2) a layer of ZrO₂ and SiO₂, (3) a layer of ZrO₂ (4) a partially oxidized layer composed of porous ZrB₂, ZrO₂, and graphite, and (5) unaffected ZrB₂–SiC–C. A thermodynamic model based on volatility diagrams and consistent with the experimental observations was constructed to explain the development of the layered structure. Oxidation behavior was consistent with passive oxidation and formation of a protective surface layer. Analysis indicated that it may not be possible to form a protective surface layer without actively oxidizing SiC and producing a porous partially oxidized layer between the outer protective layer and the underlying unoxidized material.     

High temperature thermal physical performance of SiC/UO2 composites up to 1600 °C

SiC was introduced to UO₂ matrix by spark plasma sintering (SPS) to improve the thermal conductivity. The microstructure evolution and thermal physical properties up to 1600?°C were firstly reported. The grain growth and the formation of equiaxed grain structure were inhibited by the addition of SiC. The critical SPS sintering temperature, above which SiC was positive on improving thermal conductivity, was discovered to be 1300?°C. Two equations were proposed to calculate the thermal diffusivity and thermal conductivity of SiC/UO₂ sintering at 1500?°C. Each percent of SiC fraction brought about 3% increment in thermal conductivity. The coefficient of thermal expansion (CTE) was decreased after SiC addition. Such improvement in thermal conductivity and decrease in CTE were beneficial to the fuel safety in accident condition.