Corrosion behavior and strength degradation of Ti3SiC2 exposed to a eutectic K2CO3 and Li2CO3 mixture

The corrosion of polycrystalline Ti₃SiC₂ was studied in the eutectic Li₂CO₃ (68 at.%) and K₂CO₃ (32 at.%) mixture at 650–850 °C. Ti₃SiC₂ exhibited better corrosion resistance at 650 °C. However, the mass loss was fast when temperature was above 700 °C. It was demonstrated that the surface chemical reaction-controlled shrinking core model could be applied to describe the relationship between the degree of the corrosion and reaction time for the corrosion of Ti₃SiC₂ in the 700–850 °C temperature range. The corresponding apparent activation energy was 206 kJ/mol. Corrosion resulted in roughness of specimen surface. The fracture strength of the corroded samples was evaluated by a three-point bending test. The results showed that the degradation of the fracture strength was about 25% of the original values for the corroded specimens up to 10% weight loss. The mechanism of the strength degradation was discussed based on the analysis of the microstructure and composition of the corroded sample.     

Strength degradation of alumina fiber: Irreversible phase transition after high-temperature treatment

The mechanical properties of alumina AF17-20 fiber after high-temperature treatment have been evaluated through tensile tests on single fiber and bundle. The tensile test on single fibers shows that the temperature has little effect on the elastic modulus of the fibers, which stables around 140 GPa. The test on bundles minimizes the personal errors thus giving a more reliable value of tensile strength. In general, as temperature increases, both the Weibull modulus and the tensile strength decrease gradually. De-sized fibers have the highest tensile strength, but inherent defects like pores still cause slight dispersion of the strength. Further, the strength maintains about 90% after treating below 1200 °C, and this insignificant decline is caused by the decrease of amorphous SiO₂ and the formation of aluminum silicate. In addition, the severe degradation in strength over 1200 °C is mainly attributed to the appearance and growth of mullite grains, which is only about 60% of the initial value.     

Effect of anodic oxidation of PAN-based carbon fibers on the morphological changes of their surfaces

The effects of the electrochemical oxidation of three kinds of carbon fibers obtained from polyacrylonitrile (PAN) are tested. Anodic oxidation at high potentials in 1 mole · dm⁻³ sulfuric acid produces the cracking of the fiber skin perpendicular, parallel or askew to the fiber axis. The orientation of the cracks depends chiefly on the kind of internal defects of the fibers and may be correlated with the as-received fiber's tensile strength. The electrochemical corrosion of the fibers in a 1 mole · dm⁻³ sodium hydroxide solution leads to the generation of surface cavities. These cavities appear chiefly at sites of the surface defects of the carbon fibers.     

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%.     

Effect of high-pressure vapor on the microstructure and mechanical properties of TiO2 continuous fibers

During the preparation of TiO₂ continuous fibers, the organic ligands of the precursor fibers are severely decomposed and generated a large amount of gas, which can reduce the fiber matrix strength. Tt is necessary to choose a suitable treatment strategy to avoid this and obtain high-quality TiO₂ continuous fibers. In this study, flexible continuous TiO₂ fibers with a diameter of about 30 μm were prepared using a high-pressure vapor pretreatment method. The high-pressure vapor pretreatment caused precursor hydrolysis, which promoted the decomposition of the organic ligands in a mild way and prevented fiber fracture caused by the violent oxidative decomposition. The crystallization temperature decreased by 120 °C because of the synergistic effects of vapor and pressure. The hydrolysis of the precursor and the reduction in the crystallization temperature were conducive to the formation of compact fibers with high strength. However, the presence of water vapor caused the fibers to undergo the dissolution-precipitation process simultaneously, forming a large number of defects, which was harmful to its strength. The sample 1501 composed of anatase and rutile showed the highest average tensile strength of 385 MPa because it had fewer defects than the other samples. Although the highest average tensile strength is lower than the reported value of 800 MPa, the method is easy to implement and solves the problem of organics decomposition, which is helpful for industrial preparation.     

Tensile behaviors of ECR-glass and high strength glass fibers after NaOH treatment

ECR-glass and high strength glass (S-glass) fibers were treated in 2 mol/l NaOH solution up to 5 h. The strength maintenance ratio and mass loss ratio of the fibers after treatment were investigated. The surface morphologies were characterized using scanning electron microscopy, and changes of chemical composition were analyzed by energy dispersive X-ray spectroscopy and Fourier transform infrared spectrometry. The alkali resistance and tensile strength of the S-glass fibers are higher compared to those of the ECR-glass fibers as they received less alkaline attack because of the more compact SiO₂ network and the formation of a protective layer on the S-glass fiber surface. The S-glass fibers have a higher mass loss due to the smaller diameter and thinner corrosion layer.     

Electromagnetic characteristics and microstructure stability of Nextel 610 fiber after heat treatment

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.     

The structure and properties of ribbon-shaped carbon fibers with high orientation

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000 °C are about 1.1 μΩ m and above 1100 W/m K at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842 GPa, respectively.     

Fracture Behavior of Woven Silicon Carbide Fibers Exposed to High‐Temperature Nitrogen and Oxygen Plasmas

High‐temperature aero‐thermal heating in a 30 kW inductively coupled plasma torch was used to replicate the effects of harsh oxidizing environments during hypersonic atmospheric entry on fracture behavior and microstructure of two‐dimensional woven SiC fibers. Hi‐Nicalon SiC woven cloths were exposed to surface temperatures over 1400°C with different high‐enthalpy dissociated oxygen and nitrogen plasma flows, and were subsequently deformed in pure tension at room temperature. Changes in fiber microstructure and surface chemistry after thermal exposure were examined by scanning electron microscopy. Pure nitrogen plasmas resulted in a 50% decrease of strength in woven SiC fibers with minimal effects on the fiber structure, except for highly localized surface pitting caused by partial decomposition of silicon oxycarbonitride phase at high temperature. In contrast, exposure to dissociated oxygen and air plasmas led to severe strength reduction and embrittlement over significantly short time scales, corresponding to degradation rates up to 200 times higher than those reported with static heating at equivalent temperatures. The origin of accelerated embrittlement at microscopic scale was found related to complex gas‐surface interactions and high‐temperature oxidizing processes involving the formation of SiO₂ bubbles and microcracks on the surface. These findings are important for the development of outer fabric materials for new flexible thermal protection systems in space applications.     

Tensile strength of directionally solidified chromia-doped sapphire

Tensile fracture properties of directionally solidified chromia-doped c-axis sapphire fibers have been studied in a range of temperature (room temperature up to 1400 °C) and dopant content (0, 300 ppm and 1% of Cr₂O₃). Delayed failure of the fibers was studied by measuring the dependence of the tensile strength on the loading rate and by fractographic studies on the fracture surfaces of the fibers. In all the temperature range, the fibers doped with 300 ppm of Cr₂O₃ are slightly stronger than the pure sapphire fibers. The least strong fibers are those containing 1% of Cr₂O₃. For this badge of material, the beneficial effect of solution hardening is counterweighted by increasing amount of defects caused by a faster fabrication. Slow crack growth appears to be the process controlling delayed failure at higher temperature. Little contribution of slow crack growth to delayed failure is found at the lower temperature.     

Reduction in tensile strength of polyacrylonitrile-based carbon fibers in liquids and its application to defect analysis

It has been observed that the tensile strength of carbon fibers is reduced in liquids, although the exact reason for this phenomenon has not been elucidated yet. In this study, this phenomenon has been measured in various liquids such as glycerin, water, ethyleneglycol, n-heptane, and mixtures of ethyleneglycol and water. This reduction in strength has been attributed to the reduction in free energy at the crack surface due to contact with liquids. Using this phenomenon, fractions of surface and internal defects in the fiber have been estimated and an intrinsic strength (exhibited in the absence of defects) was predicted from the tensile strength of notched fibers. For the polyacrylonitrile-based carbon fiber used in this study, the severe defects tended to be located at the fiber surface, and the intrinsic strength was estimated to be 9.3 GPa, which was almost twice the measured tensile strength of this fiber, i.e., 4.7 GPa.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

Enhancing thermal stability of oxide ceramic matrix composites via matrix doping

To overcome the main limitation of oxide ceramic matrix composites (Ox-CMCs) regarding thermal degradation, the use of matrix doping is analyzed. Minicomposites containing Nextel 610 fibers and alumina matrices with and without MgO doping were produced. The thermal stability of the minicomposites was evaluated considering their microstructure and mechanical behavior before and after thermal exposures to 1300 °C and 1400 °C for 2 h. Before heat treatment, both composite types showed very similar microstructure and tensile strength. After heat treatment, densification, grain growth and strength loss are observed. Furthermore, the MgO dopant from the matrix diffuses into the fibers. As a result, abnormal fiber grain growth is partially suppressed and MgO-doped composites show smaller fiber grains than non-doped composites. This more refined microstructure leads to higher strength retention after the heat treatments. In summary, doping the matrix can increase the overall thermal stability without impairing the room-temperature properties of Ox-CMCs.     

Structural evolution and mechanical properties of Cansas-III SiC fibers after thermal treatment up to 1700 °C

The effects of thermal treatment on the Cansas-Ⅲ SiC fibers were investigated via heating at temperatures from 900 to 1700 ℃ for 1–5 h in argon atmosphere. The composition and morphology of the SiC fibers were characterized and the tensile strength of the SiC fiber bundles was analyzed via two-parameter Weibull distribution analysis. The results showed that the thermal treatment has negligible influence on the microstructure of the SiC fibers at temperatures ≤ 1100 ℃. At temperatures ≥ 1300 ℃, the surface of the fibers became rough with some visible particles. Particularly, at 1700 °C, numbers of holes appeared. With the increasing of heating temperature and holding time, the average tensile strength of the SiC fibers decreased gradually from 1.81 to 1.01 GPa. The decreasing of tensile strength can be attributed to the increase of critical defect sizes, grain growth and phase transformation (β→α) of SiC.     

Manufacture and thermomechanical characterization of wet filament wound C/C-SiC composites

The paper presents manufacture of C/C-SiC composite materials by wet filament winding of C fibers with a water-based phenolic resin with subsequent curing via autoclave as well as pyrolysis and liquid silicon infiltration (LSI). Almost dense C/C-SiC composite materials with different winding angles ranging from ±15° to ±75° could be obtained with porosities lower than 3% and densities in the range of 2 g/cm³. Thermomechanical characterization via tensile testing at room temperature and at 1300°C revealed higher tensile strength at elevated temperature than at room temperature. Thus, C/C-SiC material obtained by wet filament winding and LSI-processing has excellent high-temperature strength for high-temperature applications. Crack patterns during pyrolysis, microstructure after siliconization, and tensile strength strongly depend on the fiber/matrix interface strength and winding angle. Moreover, calculation tools for composites, such as classical laminate and inverse laminate theory, can be applied for structural evaluation and prediction of mechanical performance of C/C-SiC structures.     

Improvement of silicon carbide fibers mechanical properties by Cl2 etching

Silicon carbide fibers combine structural and refractory properties making them good candidates for ceramic matrix composite reinforcement, driving their ultimate failure. The fractographic observation after tensile tests of various silicon carbide fibers, belonging to first, second or third generations, reveals the critical flaw location i.e. internal or surface. An etching treatment under pure chlorine at 500–850?°C can be used to transform the fibers surface on hundreds of nanometers and alter the surface-located flaw population. This way, first-generation SiC-based fibers successfully show an asymptotic improvement of tensile strength (up to 60%) and Weibull modulus (up to twice), when etched on 0.3–1?μm depth range. The lifetime of bundles, under static fatigue conditions at intermediate temperature, is consequently multiplied by a factor ranging from 10 to 40 with a narrower dispersion. Nevertheless, the tensile strength could neither be increased on second-generation Hi-Nicalon nor dramatically dropped on third-generation Hi-Nicalon-S fiber.     

Corrosion-erosion behavior of MgAl2O4 spinel refractory in contact with high MnO slag

The corrosion and erosion behavior of spinel refractory of different compressive strength has been investigated for refractories in contact with a high MnO slag for producing manganese ferroalloys. The finger rotating test (FRT) was adopted to evaluate the degradation behavior at 1550 °C under a rotating condition of 150 rpm. The sample with higher compressive strength (Sample C1) showed higher corrosion resistance than Sample C2. In Sample C1, the refractory progressively corroded from the surface to the inner region by chemical corrosion, while mechanical erosion of the refractory suddenly occurred in Sample C2 as a result of the penetration of the slag within the refractory and the rotating energy.     

Surface defect healing effect of silica coatings on silicon nitride fibers

In this study, silica coatings with different thickness were prepared on silicon nitride fibers by a continuous dip-coating method. The effects of the coatings on the mechanical properties of the silicon nitride fibers were investigated. The SiO₂ coatings with uniform thickness were prepared from a sol solution with a concentration of 0.75 wt% and then heat-treated at 400 °C, and the strength of the fibers was improved by the treated coating. The tensile strength of a coated fiber was approximately 26% higher than that of an uncoated fiber because the thin coating healed the surface defects. Our study also confirmed that the size of sol particles must match that of the flaws on the fiber surface before these flaws could be effectively repaired. Finally, a probable mechanism will be proposed here to explain this effect. The present results demonstrate that the strength of silicon nitride fibers can be enhanced by coating them through the sol–gel process, and the findings are expected to provide guidelines for repairing strength-limiting flaws in other fibers.     

Microstructure,high-temperature mechanical and dielectric properties of novel Si3N4 f/SiNO wave-transparent composites

Continuous Si₃N₄ fiber reinforced SiNO matrix composites (Si₃N_4 f/SiNO composites) were innovatively prepared for long-time high-temperature resistant wave-transparent materials of hypersonic aircraft. The microstructure, high-temperature mechanical and dielectric properties of Si₃N_4 f/SiNO composites were investigated in detail. The as-fabricated Si₃N_4 f/SiNO composites have homogeneous SiNO matrix distribution for the special winding process, which is beneficial for the mechanical strength and wave-transparent properties. The average tensile strength and flexural strength at room temperature is 87.8 MPa and 171.2 MPa respectively, which suggests Si₃N_4 f/SiNO composites have excellent mechanical strength. The tensile strength value decreases to 54.6 MPa after heat-treated at 1000 ℃ for the surface reactions between the SiNO matrix and Si₃N₄ fibers. After heat-treated at 1550 ℃, the composites have the tensile strength value of 24.2 MPa for the high strength retention rate of Si₃N₄ fibers at this temperature. Si₃N_4 f/SiNO composites have excellent room temperature dielectric properties and excellent dielectric stability in different frequency bands (7–18 GHz). The dielectric constant values vary from 3.69 to 3.75 while the dielectric loss attains the order of 10^?3. The dielectric constants and dielectric loss of Si₃N_4 f/SiNO composites are relatively stable from RT to 800 ℃. The as-fabricated Si₃N_4 f/SiNO composites that have excellent high temperature resistance and dielectric properties are the ideal high temperature wave-transparent composites.     

Mechanical and thermal shock properties of Cf/SiBCN composite: Effect of sintering densification and fiber coating

In this work, resin-derived carbon coating was prepared on carbon fibers by polymer impregnation pyrolysis method, then silicoboron carbonitride powder was prepared by mechanical alloying, and finally carbon fiber-reinforced silicoboron carbonitride composites were prepared by hot-pressing process. The effects of sintering densification and fiber coating on microstructure, mechanical properties, thermal shock resistance, and failure mechanisms of the composites were studied. Fiber bridging hinders the sintering densification, causing more defects in fiber-dense area and lower strength. However, higher sintering temperature (1800–2000°C) can improve mechanical properties significantly, including bending strength, vickers hardness, and elastic module, because further sintering densification enhances matrix strength and fiber/matrix bonding strength, while the change of fracture toughness is not obvious (2.24–2.38 MPa·m^1/2) due to counteraction of higher debonding resistance and less pull-out length. However, fiber coating improves fracture toughness greatly via protecting carbon fibers from chemical corrosion and damage of thermal stress and external stress. Due to lower coefficient of thermal expansion, lower fiber loading ratio, less stress concentration at the fiber/matrix interface, and better defect healing effect, lower sintering temperature favors thermal shock resistance of composites, and thermal shock recession mechanisms are the damage of interface.