Fiber Strength Degradation Research Articles

Microstructure evolution and mechanical properties of SiCf/BN/SiBCN composite after high temperature thermal exposure

The present study investigates the microstructure evolution and subsequent mechanical properties of SiCfiber/BN/SiBCNmatrix composites after high temperature exposure. These composites display a non-brittle failure response under three-point bending retaining 80% of the as-processed strength, even after elevated temperature exposure up to 1350oC for 10h. This is due to crack deflection accompanied by extensive fiber pull-out. In addition, both thermodynamic modelling and phase analysis by XRD show higher matrix degradation in vacuum than in N2 atmosphere due to the lower N2 partial pressure. After thermally exposed at 1500°C, carbothermal reaction in the matrix leads to the formation of a porous composite, and the composites retains a non-brittle failure behaviour. Meanwhile, SiBCN matrix degradation and SiC fiber strength degradation occurs, which results in significant composite strength decrement. Modest increases in the fiber/matrix interfacial shear strength occur upon exposure at temperatures up to 1350°C, and then significantly reduced after expsoure at 1500oC in N2.

Evaluation of heat resistance of SiC-based ceramic fibers via in situ elastic modulus and electrical conductivity measurement at elevated temperatures

SiC-based ceramic fibers have numerous applications as reinforcement materials. Herein, the in situ thermal stability of four types of SiC-based ceramic fibers from three generations was investigated in terms of elastic modulus and electrical conductivity at temperatures up to 1800 °C in a vacuum under low pressure (<10–4 Pa). A specific tensile test device (MacaSiC) for single fibers was used. Mechanical and physical properties were monitored during heating and cooling at 1200, 1500, and 1800 °C for 10 min. Fibers exposed at 1200, 1500, and 1800 °C for 10 min were evaluated by measuring tensile strength retention, and correlating them with fiber crystallinity and microstructural evolution. Third-generation fibers exhibited excellent thermal stability at temperatures up to 1800 °C. The strength degradation of the third-generation SiC fibers was associated with the enlargement of apparent β-SiC crystallites and carbonization caused by the release of Si in the annular region of the fibers.

A novel creep-fatigue life evaluation method for ceramic-composites components

A novel progressive creep-fatigue damage analysis method based on multiscale information was proposed to accurately predict the service life of SiC/SiC composites and their structural components under complex mechanical loads and external environments. First, the strength degradation of SiC fibers involving five damage mechanisms and degradation of the properties of the interface under high-temperature fatigue loading were described quantitatively based on experimental results. Subsequently, a micromechanical model incorporating the global load sharing model, two-parameter Weibull model, shear-lag model, property degradation models of various constituents, and the effect of the braiding angle was utilized to calculate the volume fraction of broken fibers. The fatigue life of 3D 4-directional SiC/SiC composites was predicted and exhibited a trend similar to that of the experimental data. The rupture life of the SiC/SiC composites was calculated considering the stress transfer among microscopic constituents and the influence of creep slow crack growth on the fiber strength. The volume fraction of broken fibers obtained in these micromechanical models was used to perform a gradual strength degradation in the novel progressive creep-fatigue damage analysis method. Microscopic constituents damage and macroscopic property degradation were quantitatively linked in this step. This infuses the micro information into the traditional phenomenological progressive damage method. The gradual stiffness degradation rules were derived from the fitting of the residual elastic modulus obtained from the fatigue and creep experiments. Finally, the damage evolution and service life of SiC/SiC turbine blades were simulated and evaluated as a preliminary validation of the entire process.

Mechanical properties and microstructural evolution of Cansas-III SiC fibers after thermal exposure in different atmospheres

Cansas-III SiC fibers were exposed in argon, air and wet oxygen (12%H2O+8%O2+80%Ar) atmospheres for 1 h at 1000–1500 °C. The pristine fiber consisted of β-SiC, free carbon and SiCxOy phases. After exposure in air and wet oxygen, an amorphous SiO2 layer with embedding α-cristobalite crystals formed, while stacking faults were generated in the SiC core to release the residual stress. With the increasing oxidation temperature, lots of pores formed in the oxide layer, accompanied with the thickening, cracking and spallation of oxide layer. The average tensile strength decreased with the exposure temperature increasing and the exposure atmosphere deteriorating (argon→air→wet oxygen). After exposure at 1400 °C in argon and air, the fiber strength retention rates were 84% and 70%, respectively. However, after exposure at 1300 °C in wet oxygen, the strength retention rate was only 51%, indicating the accelerating oxidation and severe strength degradation of fibers.

Compressive strength degradation of SiC fibers exposed to high temperatures due to impurity-induced internal oxidation

SiC fiber oxidation is a potential factor limiting the operating temperature of SiCf/SiC composites owing to the strength degradation after oxidation. Herein, we fabricated 1-μm-diameter pillars at the core of the fiber cross-sectional surface after SiO2 removal to eliminate surface effects caused by external oxidation. The fiber strength significantly decreased during the first hour of oxidation in dried air at 1400 °C, but this deterioration became less pronounced after 10 h. Simultaneously, the oxidation lowered the Young’s modulus and Weibull modulus. Oxidation considerably increased the porosity and the alterations in the mechanical behavior were primarily caused by the variations in porosity. Oxidation-induced pores were frequently detected at the fiber core and were partially filled with SiO2. Compared with those of the as-received fibers, O impurities in the oxidized fiber core were significantly reduced. Thus, the fiber strength was potentially degraded by the internal oxidation reaction between residual C and O.

Water enrichment/depletion of amorphous carbon coatings probed by temperature-dependent dc electrical conductivity and Raman scattering

Amorphous carbon (a-C) is an appealing disordered multi-phase material widely used as a cost-effective barrier coating (10–100 nm thick) for silica optical fibers to protect them from harsh environments. Those coatings prevent strength degradation of the optical fibers by moisture and, also block hydrogen diffusion into the fiber. In this paper, we experimentally study the in situ temperature-dependent dc electrical conductivity and Raman scattering of dried/wetted a-C coatings under multiple heating/cooling cycles within the range of 40–120 °C. At temperatures of above 80 °C, we observe hysteresis in both the dc electrical resistance and the Raman scattering intensity. One of the mechanisms is related to the escape of intercalated (or spatially confined) water from the a-C coating, causing the resistance decrease due to a closing of the bandgap. The opposite is observed when some water molecules experience dissociative absorption at the edge and basal defects that may be decorated with CH, COOH/COH and COC/CO functional groups. The content of water within the a-C coating is quantified through relative contributions of the water and edge defect related Raman bands. This study expands our knowledge about a-C ultrathin films, originally designed as barrier coatings, towards promising ultracompact humidity sensors and next-generation adsorbents.

Modeling the Effect of Oxidation on the Creep Behavior of SiC/PyC/SiC Mini-composites Under Wet Oxygen Atmosphere

A new model for predicting the creep behavior of SiC/PyC/SiC mini-composites under wet oxygen atmosphere (900 ~ 1200℃, 1% ~ 50%H2O) is developed based on the thermal–mechanical, environmental-micro, fiber strength degradation, and creep-oxidation model. The model firstly takes the effects of the catalysis of water vapor, the oxidation of matrix and interphase, fiber strength degradation due to the grain growing, thermal decomposition, and growth of silica scale, creep of fibers into account comprehensively. The predicted strain–time curves involving three stages presented for the case of a KD-I/PyC/SiC mini-composites are compared to the experimental data at 900℃ under 1%H2O. The model predictions show that the increase of temperature accelerates the consumption of interphase and the growth of silica scale on the fiber, which promotes the failure of fibers by load transfer and fiber degradation. The increase of water vapor pressures promotes the growth of silica scale on the fiber and matrix, but has little influence on the consumption of interphase due to the effect of inhibition of water vapor on the carbon. The effect of creep and oxidation on the matrix crack spacing is checked by the critical matrix strain energy criterion and the results indicate that the creep and oxidation of the mini-composites have no influence on the matrix crack spacing. The application of the method in this work contributes to the analysis of mechanical behavior and failure mechanism of SiCf/SiC structures, such as turbine guide vane, which may serve in the wet oxygen atmosphere for a long period.

Scheelite coatings on SiC fiber: Effect of coating temperature and atmosphere

Scheelite coating was deposited on SiC fiber tows from various liquid-phase precursors followed by heat treatments between 900 °C and 1100 °C in different atmospheres. The tensile strength was fully retained for the coated fibers treated at 900 °C in vacuum. Subsequent heat treatment at 1100 °C in Ar had little effect on the fiber strength, which is explained by the excepted good thermal stability between the scheelite coating and SiC fiber. However, larger strength degradation and poor spool ability of coated fibers prepared in Ar/air were found. Assisted oxidation of SiC fiber by calcium salts is suggested to be responsible for the much larger strength degradation of fibers prepared in Ar/air.

Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure

ZrB2-SiC coatings with varied porous structures were deposited on SiC fiber tows using the sol-gel method and cured at 1400 ℃ in vacuum. Tensile strength of the coated SiC fibers were much lower than that of the uncoated fibers. The bimodal distribution in the Weibull plot of the coated SiC fibers demonstrated that the fracture of the coated fiber can be attributed to two types of defects: the porous structure of the coating and the fiber defects. Detailed morphology and microstructure characterization of the coating and fiber combined with strength calculation were carried out to investigate the individual contribution of the fiber defects and the porous coating layer respectively. The results revealed that apart from the fiber damage during the coating process the porous structure of the fiber coating has a non-negligible effect on the fiber strength, presumably due to a relatively strong bonding between the fiber and coating.

Zero Stress Aging of Glass and Carbon Fibers in Water and Oil—Strength Reduction Explained by Dissolution Kinetics

Understanding the strength degradation of glass and carbon fibers due to exposure to liquids over time is important for structural applications. A model has been developed for glass fibers that links the strength reduction in water to the increase of the Griffith flaw size of the fibers. The speed of the increase is determined by regular chemical dissolution kinetics of glass in water. Crack growth and strength reduction can be predicted for several water temperatures and pH, based on the corresponding dissolution constants. Agreement with experimental results for the case of water at 60 °C with a pH of 5.8 is reasonably good. Carbon fibers in water and toluene and glass fibers in toluene do not chemically react with the liquid. Subsequently no strength degradation is expected and will be confirmed experimentally. All fiber strength measurements are carried out on bundles. The glass fibers are R-glass.

High-temperature microstructural evolution of SiCN fibers derived from polycarbosilane with different C/N ratios

Research into the high-temperature microstructural evolution of SiCN ceramic fibers is important for the aerospace application of advanced ceramic matrix composites in harsh environments. In this work, we studied the microstructural evolution of SiCN fibers with different C/N ratios that derived from polycarbosilane fibers at the annealing temperature range of 1400∼1600 °C. These results showed that the phase separation of SiCxNy phase and the two-dimension grain growth process of free carbon nanoclusters could be processed at the researched temperature range. As the annealing temperature increased to 1600 °C, the crystallization of amorphous SiC and Si3N4 could be detected. SEM and Raman analysis showed that the decomposition and carbothermal reduction of the Si3N4 phase at high temperatures played primary roles in contributing to the fiber strength degradation. Thus, a higher C/N ratio, which is beneficial for inhibiting the decomposition of amorphous Si3N4, helps SiCN fibers retain high tensile strength at high temperatures.

Deformation and rupture behaviors of SiC/SiC under creep, fatigue and dwell-fatigue load at 1300 °C

High temperature creep and fatigue properties are important characteristics of SiC/SiC composites. In this work, dwell-fatigue tests were conducted at 1300 °C for 3D-braided SiC/SiC with and without coatings, and experimental results of dwell-fatigue tests were analyzed together with results of fatigue and creep tests. Then, general quantitative method was adopted to explain the deformation and rupture behaviors of SiC/SiC in creep, fatigue and dwell-fatigue tests. Micromechanical creep model could effectively simulate deformation behavior when the maximum load was under matrix cracking stress. Stress transfer behavior of constituents during creep and fatigue tests provided insights into the difference in macro time/cycle dependent deformation. Next, by incorporating damage evolution model which represents oxidation assisted unbridged crack growth, deformation acceleration in tertiary stage of creep was successfully simulated. Finally, SiC/SiC lifetimes under all fatigue, creep, and dwell-fatigue loads were predicted considering matrix crack propagation and fiber strength degradation. Simulated results correlated well with experimental data, verifying the effectiveness of micromechanical model to predict the creep and fatigue behaviors of SiC/SiC composites at high temperatures.

Processing and mechanical performance of 3D Cf/SiCN composites prepared by polymer impregnation and pyrolysis

Abstract The processing of 3D carbon fiber reinforced SiCN ceramic matrix composites prepared by polymer impregnation and pyrolysis (PIP) route was improved, and factors that determined the mechanical performance of the resulting composites were discussed. 3D Cf/SiCN composites with a relative density of ∼81% and uniform microstructure were obtained after 6 PIP cycles. The optimum bending strength, Young's modulus and fracture toughness of the composites were 75.2 MPa, 66.3 GPa and 1.65 MPa m1/2, respectively. The residual strength retention rate of the as-pyrolyzed composites was 93.3% after thermal shock test at ΔT = 780 °C. It further degraded to 14.6% when the thermal shock temperature difference reached to 1180 °C. The bending strength of the composites was 35.6 MPa after annealing at 1000 °C in static air. The deterioration of the bending strength should be attributed to the strength degradation of carbon fibers and decomposition of interfacial structure.

Time-dependent damage and fracture of fiber-reinforced ceramic-matrix composites at elevated temperatures

ABSTRACTIn this paper, the time-dependent damage and fracture of fiber-reinforced ceramic-matrix composites (CMCs) are investigated considering fiber oxidation and fracture at an elevated temperature. The shear-lag model combined with the fiber/matrix interface oxidation model, the interface debonding criterion, fiber strength degradation model and fiber failure model is adopted to analyze the microstress field in the damaged composite. The relationships between the first steady-state matrix cracking stress, composite tensile strength, fiber/matrix interface debonding and sliding, oxidation temperature and oxidation time are established. The effects of fiber volume fraction, fiber/matrix interface shear stress, fiber strength and oxidation temperature on the evolution of first steady-state matrix cracking stress and composite tensile strength versus the oxidation time are analyzed. The first steady-state matrix cracking stress and composite tensile strength of 2D C/SiC and 2D C/[SiC-B4C] composites after oxidation at 700°C, 1200°C and 1300°C in air atmosphere are predicted for different oxidation time.

Degradation of Nextel™ 610‐based oxide‐oxide ceramic composites by aluminum oxychloride decomposition products

AbstractNextel™ 610 alumina fibers and alumina‐YAG (yttrium‐aluminum garnet) matrices were used to make oxide‐oxide ceramic matrix composites (CMCs) with and without monazite (LaPO4) fiber‐matrix interfaces. Twelve sequential aluminum oxychloride (AlOCl) infiltrations with 1 hour heat treatments at 1100°C and a final 1 hour heat treatment at 1200°C were used for matrix densification. This matrix processing sequence severely degraded CMC mechanical properties. CMC tensile strengths and interlaminar tensile (ILT) strengths were less than 10 MPa and 1 MPa, respectively. Axial fracture of Nextel™ 610 fibers was observed after ILT testing, highlighting the extreme degradation of fiber strength. Extensive characterization was done to attempt to determine the responsible degradation mechanisms. Changes in Nextel™ 610 fiber microstructure after CMC processing were characterized by optical microscopy, SEM, and extensively by TEM. In AlOCl degraded fibers, grain boundaries near the fiber surface were wetted with a glass that contained Y2O3/SiO2 or Y2O3/La2O3/P2O5/SiO2, and near‐surface pores were partially filled with Al2O3. This glass must also contain some Al2O3 and initially some chlorine. AlOCl decomposition products were predicted using the FactSage® Thermochemical code, and were characterized by mass spectrometry. Effects of AlOCl precursors on monazite coated and uncoated Nextel™ 610 fibers tow and filament strength were evaluated. A mechanism for the severe degradation of the oxide‐oxide CMCs and Nextel™ 610 fibers that involves subcritical crack growth promoted by release of chlorine containing species during breakdown of intergranular glasses in an anhydrous environment is proposed.

The fatigue life study of polyphenylene sulfide composites filled with continuous glass fibers

In this study, an effective microscopic model is proposed to investigate the fatigue life of composites containing continuous glass fibers, which is surrounded by polyphenylene sulfide (PPS) matrix materials. The representative volume element is discretized by parametric elements. Moreover, the microscopic model is established by employing the relation between average surface displacements and average surface tractions. Based on the experimental data, the required fatigue failure parameters of the PPS are determined. Two different fiber arrangements are considered for comparisons. Numerical analyses indicated that the square edge packing provides a more accuracy. In addition, microscopic structural parameters (fiber volume fraction, fiber off-axis angle) effect on the fatigue life of Glass/PPS composites is further discussed. It is revealed that fiber strength degradation effects on the fatigue life of continuous fiber-reinforced composites can be ignored.

Control of aluminum phosphate coating on mullite fibers by surface modification with polyethylenimine

Aluminum phosphate (AlPO4) is a promising oxidation-resistant and weak interface for ceramic-matrix composites. In this research, AlPO4 coating was deposited on mullite fibers by an improved liquid-phase method based on electrostatic attraction. A cationic polyelectrolyte, polyethylenimine (PEI), was used for surface modification of mullite fibers. The formation process, phase evolution and microstructure of the coating were studied. The zeta potential of AlPO4 particles, PEI-adsorbed AlPO4 particles, and PEI-adsorbed mullite particles was characterized to find the proper pH value for improving electrostatic attraction. The obtained AlPO4 coating was porous and continuous, whose thickness could be controlled by multiple coating cycles. The relatively low calcination temperature (600 or 1000°C) was a useful heat treatment method to develop bonding between coating and fiber as well as reduce the fiber strength degradation. The phase transformations of AlPO4 have little volume change, and cristobalite AlPO4 is thermal compatible with mullite. Therefore, the coating structure was preserved after calcining at 1200°C. The technique is also applicable for other fibers contained mullite phase to fabricate high-performance AlPO4 coated mullite/mullite composites.

Characterization and high-temperature degradation mechanism of continuous silicon nitride fibers

Amorphous silicon nitride fibers were characterized with respect to their chemical and phase compositions. The high-temperature behavior of the fibers was investigated in N2 and Ar at temperatures ranging from 1350 to 1600 °C through tensile tests, pore volume analysis, elemental analysis, weight loss, X-ray diffraction, and scanning electron microscopy to elucidate the effects of the atmosphere on the fiber degradation mechanism. The fibers demonstrated better high-temperature stability in N2 instead of Ar. In particular, the fibers heat-treated in N2 at 1450 °C retained 43% of their original strength (0.64 GPa), while the fiber strength was completely lost in Ar at the same temperature. The primary reason for the strength degradation of the amorphous fibers was the expanding nanopores before the formation of the large Si3N4 grains. However, a catastrophic decrease in strength was due to the drastic thermal decomposition of SiNxOy and crystallization of Si3N4 grains, influenced by N2 pressure. Thermodynamic considerations were additionally applied to estimate the decomposition and carbothermal decomposition of crystalline Si3N4, thereby explaining the significant discrepancy of crystallization transformation at 1600 °C under different atmospheres.

An Improved Metal-Packaged Strain Sensor Based on A Regenerated Fiber Bragg Grating in Hydrogen-Loaded Boron-Germanium Co-Doped Photosensitive Fiber for High-Temperature Applications.

Local strain measurements are considered as an effective method for structural health monitoring of high-temperature components, which require accurate, reliable and durable sensors. To develop strain sensors that can be used in higher temperature environments, an improved metal-packaged strain sensor based on a regenerated fiber Bragg grating (RFBG) fabricated in hydrogen (H2)-loaded boron–germanium (B–Ge) co-doped photosensitive fiber is developed using the process of combining magnetron sputtering and electroplating, addressing the limitation of mechanical strength degradation of silica optical fibers after annealing at a high temperature for regeneration. The regeneration characteristics of the RFBGs and the strain characteristics of the sensor are evaluated. Numerical simulation of the sensor is conducted using a three-dimensional finite element model. Anomalous decay behavior of two regeneration regimes is observed for the FBGs written in H2-loaded B–Ge co-doped fiber. The strain sensor exhibits good linearity, stability and repeatability when exposed to constant high temperatures of up to 540 °C. A satisfactory agreement is obtained between the experimental and numerical results in strain sensitivity. The results demonstrate that the improved metal-packaged strain sensors based on RFBGs in H2-loaded B–Ge co-doped fiber provide great potential for high-temperature applications by addressing the issues of mechanical integrity and packaging.

Mechanical Behavior of Liquid Route Processed SiCf/Ti Composites Under Longitudinal and Transverse Loadings

Due to the high melting point and strong chemical reactivity of titanium alloys, titanium matrix composites (TMCs) are usually processed through solid-state routes such as the foil-fiber-foil technique. An alternative method consists in the deposition of the matrix on the fibers. However, techniques such as physical vapor deposition lead to a very low deposition rate, contrary to liquid route processing using a levitating liquid alloy sphere held in a cold crucible. In order to investigate the effects of the resulting thermal shock on carbon-coated SiC fibers, and select an appropriate fiber, fibers are subjected to a pure thermal shock using a laser bench facility. These fibers are then tensile tested to failure in order to evaluate the resulting fiber strength degradation and, thus, the maximum acceptable temperature. Mechanical characterization of the liquid route processed TMC is then investigated through longitudinal and transverse tensile and creep tests at temperatures representative of aeronautical applications. The specimens, unbroken after long-duration creep tests, are then subjected to tensile loading to failure: conditions representative of service, i.e., short-time overspeeding of a gas turbine. Finally, interpretation of the mechanical tests through micrographical and microfractographical examinations is focused on the identification of the deformation and failure mechanisms specific to the liquid route processed composite, e.g., nucleation, under either longitudinal or transverse loadings, of internal cracks in the α-phase of the titanium-based matrix, explained through a physical model involving a high shear stress and normal stress combination, leading to cleavage.