Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases

SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC) _n ) multilayered interphases and a tow of SiC fibers (Hi-Nicalon) have been prepared via pressure-pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH₃SiCl₃/H₂ mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack-spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).     

Microstructure and Mechanical Properties of Three-Dimensional Textile Hi-Nicalon SiC/SiC Composites by Chemical Vapor Infiltration

Three-dimensional textile Hi-Nicalon SiC-fiber-reinforced SiC composites were fabricated using chemical vapor infiltration. The microstructure and mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g·cm⁻³ after the three-dimensional SiC perform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300°C under vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mismatch of thermal expansion coefficients between fiber and matrix. The fracture toughness was 30.2 MPa·m^1/2. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance, and the dynamic fracture toughness of 36.0 kJ·m⁻² was measured using Charpy impact tests.     

SiC/SiC minicomposites with structure-graded BN interphases

BN interphases in SiC/SiC minicomposites were produced by infiltration of fibre tows from BF₃–NH₃–H₂ gaseous system. During interphase one-step processing, the tow travels through a reactor containing a succession of different hot areas. By TEM characterization, the BN interphases were found to be made of a structural gradient: from isotropic to highly anisotropic. The very first coating is poorly organised and allows to protect the fibre from a further chemical attack by the reactant mixture. The minicomposites were tensile tested at room temperature with unloading-reloading cycles. The BN interphases act as mechanical fuses; the fibre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase infiltration. The specimen lifetimes at 700°C under a constant tensile loading were measured in dry and moist air. Compared to a pyrocarbon reference interphase, the BN interphases significantly improve the oxidation resistance of the SiC/SiC minicomposites.     

Thermal Stability of Polycarbosilane-Derived Silicon Carbide Fibers under Reduced Pressures

Three types of polycarbosilane-derived SiC fibers—Nicalon, Hi-Nicalon, and Hi-Nicalon S—were exposed at temperatures of 1573–1773 K under a reduced pressure of 1.3 Pa. The thermal stability of the fibers was investigated through examinations of the gas evolution, grain growth, specific resistivity, fiber morphology, and tensile strength. The thermal decomposition of the silicon oxycarbide phase began at 1523 K; then, active oxidation of the β-SiC crystallites occurred at >1673 K. The active oxidation caused serious damage to the fiber structure, which resulted in significant degradation of the fiber strength. Hi-Nicalon had a tensile strength of ∼0.5 GPa after exposure at 1773 K, although Nicalon and Hi-Nicalon S fibers completely lost their strength, even after exposure at 1673 K. Hi-Nicalon fiber had relatively good thermal stability under reduced pressure.     

Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites: II, Residual Stress in Fibers

Band shifts in Raman spectra were used to assess, on a microscopic scale, the residual strain existing in Hi-Nicalon silicon carbide fibers reinforcing celsian-matrix composites. Uncoated as well as p-BN/SiC-coated and p-B(Si)N/SiC-coated Hi-Nicalon fibers were used as the reinforcements. We unambiguously conclude that the fibers were in a state of compressive residual stress. Quantitative determination of the residual stress was made possible by taking into account the heating induced by laser probing and by using a reference line, of fixed wavenumber. We found fiber compressive residual stress values between 110 and 960 MPa, depending on the fiber/matrix coating in the composite. A stress relaxation-like phenomenon was observed at the surface of p-BN/SiC-coated Hi-Nicalon fibers whereas the uncoated or p-B(Si)N/SiC-coated Hi-Nicalon fibers did not show any stress relaxation in the celsian-matrix composites.     

Fracture Toughness of 2-D Woven SiC/SiC CVI-Composites with Multilayered Interphases

Relations between fracture toughness and fiber/matrix interphases were examined on various SiC/SiC composites made by chemical vapor infiltration (CVI) and reinforced with woven fiber bundles. Strong and weak fiber/matrix bondings were obtained using multilayered interphases consisting of various combinations of carbon and SiC layers of different thickness and using fibers which had been previously treated. Fracture toughness was estimated using the J - integral and using strain energy release rate computed with a model taking into account the presence of a process zone of matrix microcracks. Both approaches evidenced similar trends. It appeared that higher toughness was obtained with those composites possessing strong interphases and subject to dense matrix microcracking.     

Effects of the single layer CVD SiC interphases on the mechanical properties of the SiCf/SiC composites fabricated by PIP process

Owing to the degradation of the mechanical properties of the SiC fiber reinforced SiC matrix (SiCf/SiC) composites with the pyrocarbon (PyC) and BN interphases under oxidation environment and neutron irradiation, single layer SiC interphases prepared by chemical vapor deposition (CVD) process were employed to substitute for them. Effects of the CVD SiC interphases on the mechanical properties and interfacial characteristics of the SiCf/SiC composites fabricated by precursor infiltration and pyrolysis (PIP) process were investigated. Compared with the as-received SiCf/SiC composites, the SiCf/SiC composites with the single layer CVD SiC interphases exhibit an obvious toughened fracture behavior, the flexural strength of which is about 4 times that of the as-received SiCf/SiC composites. From the microstructural analysis, it can be confirmed that the SiC interphases play a key part in protecting the fibers from damage during composite preparation and weakening interfacial bonding, which can provide high in situ fiber strength and appropriate interfacial bonding strength for the SiCf/SiC composites.     

Tensile creep behavior of SiCf/SiC ceramic matrix minicomposites

Single fiber-tow minicomposites represent the major load-bearing element of woven and laminate ceramic matrix composites (CMCs). To understand the effects of fiber type, fiber content, and matrix cracking on tensile creep in SiC_f/SiC CMCs, single-tow SiC_f/SiC minicomposites with different fiber types and contents were investigated. The minicomposites studied contained either Hi-Nicalon™ or Hi-Nicalon™ Type S SiC fibers with a boron nitride (BN) interphase and a chemical-vapor-infiltrated-silicon-carbide (CVI-SiC) matrix. Tensile creep was performed at 1200 °C in air. A bottom-up creep modeling approach was applied where creep parameters of the fibers and matrix were obtained separately at 1200 °C. Next, a theoretical model based on the rule of mixtures was derived to model the fiber and matrix creep-time-dependent stress redistribution. Fiber and matrix creep parameters, load transfer model results, and numerical modeling were used to construct a creep strain model to predict creep damage evolution of minicomposites with different fiber types and contents.     

Mechanical and Dielectric Properties of Mullite Fiber‐Reinforced Mullite Matrix Composites with Single Layer CVD SiC Interphases

Mullite fiber‐reinforced mullite matrix (Mu_f/Mu) composites were fabricated via the solgel process. Prior to the solgel process, SiC coatings were deposited on the fibers by the chemical vapor deposition (CVD) process. Effects of the SiC coatings on the mechanical and dielectric properties of the composites were investigated. The results show that the composites with SiC interphases exhibit evident toughened fracture behavior, and their flexural strength is about 2.37 times that of the as‐received composites. Besides, the complex permittivity of the composites with SiC interphases at X‐band is also increased remarkably due to the existence of carbon in the SiC interphases.     

Influence of surface fibre properties and textural organization of a pyrocarbon interphase on the interfacial shear stress of SiC/SiC minicomposites reinforced with Hi-Nicalon S and Tyranno SA3 fibres

SiC/SiC composites reinforced with 3rd generation SiC fibres (Hi-Nicalon S and Tyranno SA3) are attractive for nuclear applications. The mechanical properties of SiC/SiC minicomposites were studied in relation with the nature of fibres and the textural organization of the pyrocarbon interphase. Two different experimental mechanical procedures were used: the push-out test and the tensile mechanical test. The experimental results demonstrate that the interfacial shear stress is higher for a minicomposite reinforced with Tyranno SA3 fibres. It is also shown that the interfacial shear stress depends on the texture of the carbon interphase. Highly anisotropic pyrocarbon interphase increases the interfacial shear stress. To our knowledge it is the first time that the influence of pyrocarbon texture is observed on SiC/SiC composites using Hi-Nicalon S and Tyranno SA3 fibres. It is also demonstrated that push-out tests are more appropriated than tensile tests to highlight differences of interfacial shear stress measurements.     

Degradation mechanisms of SiC/BN/SiC after low temperature humidity exposure

The environmental degradation of SiC/BN/SiC CMCs under low temperature water exposure is still an unexplored field. This work shows how the effect of low temperature humid environments can be detrimental for turbostratic BN interphases, leading to a drop in mechanical properties. Furthermore, initial low-temperature humid environments can induce a faster degradation during subsequent thermal exposure. In order to understand how low temperature water exposure affects the CMC and how these changes affect the material response to subsequent exposures, intermediate temperature (800 °C) exposures have been studied before and after the low temperature humidity tests. The main challenge of this work consists of understanding how different constituents of the CMC structure (e.g. fibres and interphases) are degrading and consequently affecting the overall bulk mechanical performance and failure modes of the material. For this, linking the change in morphology and chemistry of the interphases with the micromechanical properties each constituent has been crucial.     

Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite

SiC/SiC composites reinforced with near stoichiometric SiC ceramic fibres (Hi-Nicalon S and SA3 Tyranno fibres) are attractive materials to be used in nuclear environment. Netherless, their mechanical properties must be improved and controlled. For example, SA3 Tyranno fibres (TSA3) -reinforced composites exhibit a brittle behaviour whereas composites reinforced by Hi-Nicalon S (HNS) fibres exhibit a conventional damage tolerant response. This difference is related to the nature of the fibre/matrix (F/M) coupling. The aim of this work was to identify the SiC fibres characteristics influencing the F/M coupling and consequently the mechanical properties of the composites. The experimental results point out that the TSA3 fibres exhibit a granular and rough surface leading to an increase of the residual stress and the interfacial shear stress in the SiC/SiC composites. Beside the roughness, the experimental results also point out that the surface chemistry of the SiC fibres significantly influence the F/M bonding.     

Oxidation of SiC/BN/SiC ceramic matrix composites in dry and wet oxygen at intermediate temperatures

The oxidation behavior of SiC/BN/SiC ceramic matrix composites (CMCs) was evaluated from 400° to 800 °C in 100% O₂ and 50% H₂O/50% O₂ gas mixtures. Thermogravimetric analysis (TGA) was utilized to measure weight change during controlled environment exposures at elevated temperatures for 1 and 50 hours. Oxidized CMCs and their oxides were studied post-exposure with scanning electron microscopy and energy dispersive spectroscopy. The oxidation onset and composition transition temperatures were evaluated. Key observations include oxide composition, oxide wetting, oxygen solubility in Hi-Nicalon SiC fibers and BN fiber coating oxidation and volatility behavior as a function of temperature. Degradation in wet environments at 600 °C was most extensive due to the formation of a non-wetting, non-protective surface oxide, allowing oxidant access to the BN fiber coatings followed by oxidation and volatilization. Implications of the CMC oxidation behavior are discussed for CMCs in service.     

电泳沉积时间对SiC_f/SiC复合材料性能的影响

针对电泳沉积结合先驱体浸渍裂解的方法制备SiC_f/SiC复合材料过程,探讨沉积时间对SiC纤维及SiC_f/SiC复合材料性能的影响规律。实验结果得出:随电泳沉积时间的延长,SiC纤维逐渐被腐蚀,致使其单丝强度下降;而在SiC纤维表面覆盖PyC涂层可以有效地保护SiC纤维,由于悬浮液中的通电作用,Py C涂层与SiC纤维的界面结合强度略有降低,纤维单丝强度随电泳时间的延长先增大后减小。5 min的电泳沉积结合9个周期的PIP得到了SiC_f/SiC复合材料最大的弯曲强度为731 MPa,随后其力学性能随着沉积时间的延长先降低后略微回升;SiC_f/SiC复合材料的热导率随沉积时间的延长先增大后减小,10 min电泳沉积得到了常温下SiC_f/SiC复合材料的最大热导率为4.658 W/(m·K)(25℃)。     

Influence of Interface Characteristics on the Mechanical Properties of Hi-Nicalon type-S or Tyranno-SA3 Fiber-Reinforced SiC/SiC Minicomposites

The tensile behavior of CVI SiC/SiC composites with Hi-Nicalon type-S (Hi-NicalonS) or Tyranno-SA3 (SA3) fibers was investigated using minicomposite test specimens. Minicomposites contain a single tow. The mechanical behavior was correlated with microstructural features including tow failure strength and interface characteristics. The Hi-NicalonS fiber-reinforced minicomposites exhibited a conventional damage-tolerant response, comparable to that observed on composites reinforced by untreated Nicalon or Hi-Nicalon fibers and possessing weak fiber/matrix interfaces. The SA3 fiber-reinforced minicomposites exhibited larger interfacial shear stresses and erratic behavior depending on the fiber PyC coating thickness. Differences in the mechanical behavior were related to differences in the fiber surface roughness.     

TEM structure of (PyC/SiC)n multilayered interphases in SiC/SiC composites

Two generations of multilayered interphases, composed of carbon and silicon carbide, have been developed to act as a mechanical fuse in SiC/SiC composites with improved oxidation resistance. Pyrocarbon is an ideal interfacial material, from the mechanical point of view, whereas SiC has a good oxidation resistance. In the multilayered interphase, the carbon mechanical fuse is split into thin sublayers, each being protected against oxidation by the neighbouring SiC-based glass former layers. A first generation of multilayers as synthesised by means of isobaric-CVI with sublayers with micrometric thickness. Then, in order to push forward the concept, pressure pulsed-CVI was involved to deposit nanometric scale sublayers. In this work, transmission electron microscopy was developed to characterise the two generations of materials. The microstructure of the layers and the influence of the fibrous preforms on the structure of the layers were studied. Examinations were then performed on the loaded samples and damaging mode characterised at nanometric scale.     

Tailoring Carbon Fiber/Carbon Nanotubes Interface to Optimize Mechanical Properties of Cf‐CNTs/SiC Composites

C_f/SiC composites were fabricated using fiber coatings including CNTs and matrix infiltration using the polymer impregnation and pyrolysis process. Interface between fiber and CNTs (CF/CNTs) was tailored to optimize mechanical properties of hybrid composites. The tailored interphases, such as Pyrocarbon (PyC) and PyC/SiC, protect fibers from degradation during the growth of CNTs successfully. Hybrid composites with well‐tailored CF/CNTs interface displayed significantly increased mechanical strength (352 ± 21 MPa) compared with that (34 ± 3 MPa) of composites reinforced with CNTs, which grown on carbon fibers directly. The interfacial bonding strength of hybrid composites was improved and optimized by tailoring the CF/CNTs interface. Interfacial failure modes were studied, and a firm interface bonding at the joint where CNTs grown was observed.     

High-temperature flexural strength of I-CVI processed Cf/SiC composites with variable interphases

The effect of single-layer pyrocarbon (PyC) and multilayered (PyC/SiC)_n=4 interphases on the flexural strength of un-coated and SiC seal-coated stitched 2D carbon fiber reinforced silicon carbide (C_f/SiC) composites was investigated. The composites were prepared by I-CVI process. Flexural strength of the composites was measured at 1200 °C in air atmosphere. It was observed that irrespective of the type of interphase, the seal coated samples showed a higher value of flexural strength as compared to the uncoated samples. The flexural strength of 470 ± 12 MPa was observed for the seal coated C_f/SiC composite samples with multilayered interphase. The seal coated samples with single layer PyC interphase showed flexural strength of 370 ± 20 MPa. The fractured surfaces of tested samples were analyzed in detail to study the fracture phenomena. Based on microstructure-property relations, a mechanism has been proposed for the increase of flexural properties of C_f/SiC composites having multilayered interphase.     

先驱体制备SiC纤维的发展历程与研究进展

碳化硅(SiC)纤维具有高强度、高模量、耐高温、抗蠕变、抗氧化等优异性能,是增强耐高温陶瓷基复合材料的关键材料。介绍了先驱体法制备3代SiC纤维的发展历程:从第1代高氧碳含量SiC纤维发展到第2代低氧高碳含量SiC纤维,再到第3代近化学计量比SiC纤维,SiC纤维的微结构从非晶到微晶显著变化,纤维的耐热性能也显著提高。重点比较了第3代近化学计量比SiC纤维(Hi-Nicalon Type S纤维、Tyranno SA和Sylramic纤维等)的性质,结果表明:SiC纤维的热稳定性由近化学计量比SiC微晶的致密度和微结构决定,Sylramic和Tyranno SA纤维的组成和微结构可通过控制Si-C-O纤维的碳热还原反应来实现,烧结助剂的采用及陶瓷烧结工艺的有效应用可提高纤维的致密度。Hi-Nicalon Type S纤维的组成和微结构取决于聚碳硅烷分解过程中特定的气氛和温度。简介了SiC纤维的研究进展并讨论了其发展趋势。     

Single- and Multilayered Interphases in SiC/SiC Composites Exposed to Severe Environmental Conditions: An Overview

Pyrocarbon (PyC), the common interphase for SiC/SiC, is not stable under severe environmental conditions. It could be replaced by boron nitride more resistant to oxidation but poorly compatible with nuclear applications. Other materials, such as ternary carbides seem promising but their use in SiC/SiC has not been demonstrated. The most efficient way to improve the behavior of PyC interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself. Issues related to the design and behavior of layered interphases are reviewed with a view to demonstrate their interest in high-temperature nuclear reactors.