Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

Hi‐Nicalon?‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO₂ scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO₂ scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon?‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.     

Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

The crystallization kinetics for SiO₂ formed by oxidation of Hi-Nicalon^™-S SiC fiber between 800 and 1600°C in Si(OH)₄(g) saturated steam were determined. Glass SiO₂ scale always formed first. Glass scale eventually crystallized to cristobalite, and during further oxidation the scale formed directly as cristobalite. Growth stress relaxed by viscous flow in SiO₂ that formed as glass. Cristobalite formed by crystallization of this glass was relatively undeformed. In SiO₂ that formed directly as cristobalite, growth stress relaxed by intense plastic deformation accompanied by dynamic recrystallization. There were therefore two layers in cristobalite scale: a heavily deformed inner layer and an undeformed outer layer. These layers were distinguished by TEM. SiO₂ crystallization times were determined from the thicknesses of undeformed cristobalite and the SiC oxidation kinetics for glass scale formation. SiO₂ crystallization kinetics were determined from the crystallization time distributions at different SiC oxidation temperatures in steam. For all temperatures the crystallization time growth exponent (n) was 1. There was a large decrease in crystallization rate between 1000 and 1100°C. Between 800 and 1000°C the activation energy (Q) for crystallization was 65 kJ/mol, between 1100 and 1500°C it was 110 kJ/mol, and at 1600°C it was ~500 kJ/mol. Analysis methods and results are discussed.     

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 SiC_xO_y 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 SiC_f/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.     

In situ Y2Si2O7 coatings on Hi-Nicalon-S SiC fibers: Phase formation and fiber strength

Y₂Si₂O₇ coatings were formed on Hi-Nicalon-S SiC fibers by reaction of solution-derived YPO₄ coatings with glass SiO₂ scales formed by fiber oxidation. Two oxidation methods were used: pre-oxidation, where fibers were oxidized prior to YPO₄ coating, or post-oxidation, where fibers were first coated with YPO₄ and then oxidized. Fibers with YPO₄/SiO₂ films were heat-treated in argon at 1200°C for 20 hours to react YPO₄ and SiO₂ to Y₂Si₂O₇. The effects of SiO₂ to YPO₄ film thicknesses on fiber strength and on the Y₂Si₂O₇formation kinetics were investigated. An optimized process to obtain single-phase continuous Y₂Si₂O₇ coatings on Hi-Nicalon-S fibers with low loss in fiber strength is suggested.     

Evolution of microstructure and tensile strength of Cansas-II SiC fibers under air oxidizing atmosphere

The influence of oxidation on the microstructure and tensile strength of Cansas-II SiC fibers at 900–1500 ℃ in the air was investigated in depth. The growth of β-SiC grains ordering as well as the increase of the size of free carbon in the SiC core occurred due to the thermal exposure. The thickness of the amorphous SiO₂ layer increases with the temperature, starting to transform to cristobalite at 1200 ℃. The activation energy in the ambient air is determined as 148KJ/mol, similar to that of Hi-Niaclon fibers (107∼151 KJ/mol). With the growth of the SiO₂ layer, lots of bubbles appeared in the SiO₂ layer due to the release of excess CO gas. Moreover, many cracks occurred on the fiber surface caused by the residual stress. The mean tensile strength decreased from initial 2.7 GPa to 0.3 GPa after the treatment at 1500 ℃, which could be mainly attributed to the SiO₂ layer.     

Silicon carbide fiber oxidation behavior in the presence of boron nitride

The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O₂ at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO₂ due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.     

Modeling Environmental Degradation of SiC‐Based Fibers

Experimental data on grain growth and oxidation kinetics of SiC‐based fibers, as well as the accompanying strength degradation, in argon, air, and moist air are interpreted using a mechanistic model. The grain growth from thermal history is modeled using conventional models, and its influence on strength is modeled assuming that the flaw size scales with grain size. The model for fiber oxidation uses available relevant thermodynamic and kinetic data for reactions, vapor pressures, oxygen permeation, and boundary layer effects to capture scale thickness data reported by several prior works, in static or flowing air, moist air, and steam. The effect of the oxide scale on strength was modeled assuming that the flaw size scaled with scale thickness. The resulting model is compared with experimental data and is shown to capture most of the data in the literature on degradation of HiNicalon^? and HiNicalon^? type S fibers.     

Degradation mechanism of near stoichiometric SiC fibers after air and Ar–H2O–O2 corrosion at 1000–1500°C

The near stoichiometric SiC fiber has been reported to play significant roles in the application of aeroengine field. An in-depth understanding on the degradation mechanism of the fiber during its corrosion in air and under a simulated aeroengine environment (P_H2O:P_O2:P_Ar = 14:8:78 kPa) will shine a light on the performance evaluations of the near stoichiometric SiC fiber-based materials as well as the development of their potential applications. In this study, X-ray diffraction, scanning electron microscope, and FIB-TEM were utilized to analyze the mechanical properties and microstructure of the fiber. After oxidation in dry air and Ar–H₂O–O₂ for 1 h, respectively, the fiber strength retention rate has been found to decrease with the increased oxidation temperature. The raise in oxidation temperature also led to the increase of the thickness and the crystallization rate of the oxide scale. The most different oxidation behaviors of SiC being treated under the simulated environment than in air are the lower oxidation activation energy and the higher crystallization activation energy for cristobalite. Water vapor can promote the oxidation reaction and inhibit the crystallization of cristobalite in the oxide scale. Few significant differences have been observed otherwise in the oxidation process and oxidation chromatography crystallization mechanism of fibers being treated under different conditions. The increase of oxide layer thickness and the formation of cristobalite impair the structural integrity and compactness of the oxide scale and thus lead to the deterioration of the mechanical properties of SiC fibers. Therefore, it is proposed that oxidation resistance of SiC fiber can be improved by insulating the reaction between the oxidizing agents and the SiC fiber or by increasing the crystallization temperature of cristobalite in the oxidation process and reducing the crystallization rate.     

Surface oxidation behavior in air and O2-H2O-Ar atmospheres of continuous freestanding SiC films derived from polycarbosilane

In this contribution, thermodynamic computational calculations firstly carried out on Ar-Si-C-O/Ar-Si-C-O-H database demonstrate that passive oxidation is main reaction of continuous freestanding SiC films in both air and 14%H₂O/8%O₂/78%Ar atmospheres. SiC films were subsequently annealed at 1300?°C, 1400?°C and 1500?°C for 1?h in air and O₂-H₂O-Ar atmospheres. Results suggest that modulus, hardness and resistivity decrease whereas crystallite size of β-SiC and α-cristobalite increase with elevated annealing temperature. In particular, hardness of wet oxidized samples is lower than that of air oxidized ones. Additionally, their oxidation kinetics models were also established and verified by annealing at 1200?°C in air and wet oxygen for different time from 1?h to 100?h. Oxidation of continuous freestanding SiC films is identified to follow parabolic oxidation kinetics, and water could effectively enhance the oxidation rates. It is revealed that SiO₂ layer can protect SiC films from further oxidation, and their thickness increases with prolonged annealing time. In this study, a dense and uniform SiO₂ layer with a thickness of 1.1–1.6?µm was produced for sacrificial and passivation layer based on suitable thermal oxidation process (annealing at 1000?°C for 5?h in O₂-H₂O-Ar environment). Interestingly, fast diffusion paths in this oxide layer could effectively accelerate oxidation process of SiC films. These obtained achievements would promote further applications of SiC films on microelectromechanical systems (MEMS) devices in harsh environments.     

Effect of Ti combined with Si and C on mechanical performance and oxidation resistance of SiC castables for plasma gasifier

Gaseous products released during the oxidation of SiC at 1700?°C lead to serious degradation of SiC castables. Ti combined with Si and carbon black are added to improve the mechanical behavior and oxidation resistance of SiC castables in this study. The mechanical behavior, isothermal oxidation, microstructure, and thermodynamic analysis are used to evaluate the properties of SiC castables. The result shows that SiC castables with more Ti exhibit better degradation resistance at high temperature oxidation atmosphere. The preferential oxidation of metal Ti to TiO₂ reduces the oxidizing gases and increases the content of SiO (g) in the matrix, which is beneficial for the generation of SiC fibers; in turn, this reinforces the mechanical behavior. In addition, a certain amount of TiO₂ dissolves into SiO₂ glass following the decrease in viscosity. TiO₂ is not only more difficult to volatilize than SiO₂, but also can decrease the viscosity of SiO₂ glass to improve the mobility of the liquid, which is good for healing the pores on the surface and protecting the inner SiC from being oxidized; this improves the mechanical properties and oxidation resistance.     

Passive oxidation kinetics for glass and cristobalite formation on Hi‐Nicalon™‐S SiC fibers in steam

Parabolic rate constants for SiO₂ glass ( B _G ) and cristobalite ( B _C ) scale formation during passive oxidation of SiC in steam were determined. Cristobalite scale that originally formed as glass and as scale that formed afterward, directly as cristobalite, was distinguished by TEM. A method to determine B _G and B _C from many thickness measurements of the 2 different scale layers was developed. The method was applied to Hi‐Nicalon?‐S SiC fiber oxidation in Si(OH)₄ saturated steam between 500 and 1600°C. At 1500°C and lower temperatures, glass scale formed more rapidly than cristobalite scale. B _G and B _C had activation energies of 80 ± 5 kJ/mol and 95 ± 5 kJ/mol, respectively. At 1600°C, cristobalite scales formed much faster than glass scales. Many scales spheroidized after oxidation at temperatures beneath 1000°C, and continuous scales that did form had wide variation in thickness. This made kinetics analysis after low temperature steam oxidation problematic.     

SiC fiber strength after low pO2 oxidation

Hi‐Nicalon?‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO₂ of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO₂, 1400°C with 100 ppm pO₂ or higher, and 1300°C with 50 ppm pO₂ or higher. When active oxidation did not occur, a glass SiO₂ scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO₂. Fiber strengths were lowest after heat treatment at 1300°C and a pO₂ of 3.7 ppm, 1400°C and a pO₂ of 20 ppm, and 1500°C and a pO₂ of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.     

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.     

Tensile creep properties and damage mechanisms of 2D-woven C/HfC–SiC composites in high temperature

2D-C/HfC–SiC composites were prepared by a combination of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). Creep tests were performed at 1100°C in air under different stress conditions. Unlike most, C/SiC and SiC/SiC ceramic matrix composites only underwent primary and secondary creep stages, and the C/HfC–SiC composites underwent tertiary creep stage in the creep process. The reason was that the mechanical properties of C/HfC–SiC materials prepared by PIP + CVI methods were different from those prepared by traditional methods. The microscopic morphological analysis of the sample fracture showed that the oxidation products SiO₂ and Hf–Si–O glass phases of the HfC–SiC matrix played a crack filling role in the sample during creep. In turn, it provided effective protection to the internal fibers of the sample. The creep failure of C/HfC–SiC composites in a high-temperature oxidizing atmosphere was caused by the oxidation of the fibers. The total creep process was dominated by the oxidation of carbon fibers. It is noteworthy that there was the generation of Hf_xSi_yO_z nanowires in the samples after high-temperature creep. The analysis of the experimental data showed that the creep stress had a linear negative correlation with the creep life.     

Monazite Coatings on SiC Fibers I: Fiber Strength and Thermal Stability

Commercially available SiC fibers were coated with monazite (LaPO₄) using a continuous vertical coater at 1100°C. Coated fibers were heat treated in dry air, argon, and laboratory air at 1200°C for 1–20 h. The tensile strengths of uncoated and coated fibers were measured and evaluated before and after heat treatment. Fiber coating did not degrade SiC fiber strength, but heat treatment afterwards caused significant degradation that correlated with silica scale thickness. Possible strength degradation mechanisms for the coated fibers are discussed. Coating morphology, microstructure, and SiC oxidation were observed with scanning electron microscopy and transmission electron microscopy. Monazite reacted with SiC to form lanthanum silicate (La₂Si₂O₇) in argon, but was stable with SiC in air. Despite the large coefficient of thermal expansion difference between monazite and SiC, micron thick monazite coatings did not debond from most types of SiC fibers. Possible explanations for the thermomechanical stability of the monazite fiber coatings are discussed.     

Model of Oxidation‐Induced Fiber Fracture in SiC/SiC Composites

Stress rupture of SiC/SiC composites at intermediate temperatures in oxidizing environments is the result of a series of internal chemical and thermomechanical processes that lead to premature, localized fiber fracture. This article presents analytical models for two potentially critical steps in this process. The first involves the generation of tensile stresses in the fibers due to SiO₂ scale formation (following removal of fiber coatings) and the associated reduction in the applied stress required for fiber fracture. The second occurs once the gaps produced by coating removal are filled with oxide and subsequent oxidation occurs subject to the constraints imposed by the matrix crack faces. In this domain, the failure model is couched in terms of the stress intensification within the fibers caused by constrained oxidation. The models incorporate the combined kinetic effects of oxide growth and viscous flow. The competing effects of increased oxidation rate and accelerated stress relaxation with increasing temperature on fiber stress feature prominently in the results. The results suggest that, in dry air environments, the highest risk of fiber fracture occurs at temperatures in the range 840°C–940°C. In this range, the oxide scales grow at appreciable rates yet the resulting growth stresses cannot be mitigated sufficiently rapidly by viscous flow.     

Oxidation behavior and ablation properties of MDF-based biomorphic SiC composites

Biomorphic SiC composites were fabricated by infiltration of liquid Si into a preform fabricated from medium-density fiberboard (MDF). The phase compositions, microstructures, oxidation behaviors, and ablation properties of the composites were investigated. The composites were oxidized at elevated temperatures (up to 1450 °C) in air to study their oxidation behavior. Pores and cracks initially formed from the oxidation of residual carbon, followed by melting of residual Si. The ablation resistance of a composite was gauged using an oxy-acetylene torch. The formation of a SiO₂ layer by the oxy-acetylene flame improved the ablation resistance because molten SiO₂ spread over the ablated surface and partially sealed the pores, thus acting as an effective barrier against the inward diffusion of oxygen.     

Oxidation behavior and microstructure evolution of SiC-ZrB2-ZrC coating for C/C composites at 1673 K

To protect carbon/carbon (C/C) composites against oxidation, a SiC-ZrB₂-ZrC coating was prepared by the in-situ reaction between ZrC, B₄C and Si. The thermogravimetric and isothermal oxidation results indicated the as-synthesized coating to show superior oxidation resistance at elevated temperatures, so it could effectively protect C/C composites for more than 221 h at 1673 K in air. The crystalline structure and morphology evolution of the multiphase SiC-ZrB₂-ZrC coating were investigated. With the increase of oxidation time, the SiO₂ oxide layer transformed from amorphous to crystalline. Flower-like and flake-like SiO₂ structures were generated on the glass film during the oxidation process of SiC-ZrB₂-ZrC coating, which might be ascribed to the varying concentration of SiO. The oxide scale presented a two-layered structure ~130 µm thick after oxidation, consisting of a SiO₂-rich glass layer containing ZrO₂/ZrSiO₄ particles and a Si-O-Zr layer. The multiphase SiC-ZrB₂-ZrC ceramic coating exhibited much better oxidation resistance than monophase SiC, ZrB₂ or ZrC ceramic due to the synergistic effect among the different components.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.