C(B) materials as interphases in SiC/SiC model microcomposites

A specific test procedure has been developed to compare the high temperature lifetimes of SiC/SiC microcomposites with various interphases in air and under mechanical loading. The interphases, namely pure pyrocarbon (PyC) or C(B) materials with uniform or variable boron contents in the thickness, were prepared by chemical vapour deposition (CVD). Uniform addition of boron in PyC interphases improved their oxidation resistance and consequently the lifetimes of the microcomposites. However, room temperature tensile tests have shown that this improvement occurs to the detriment of the mechanical properties even when a non-brittle behaviour is maintained. In the case of variable boron contents, compositional gradient interphases (CGI) in which boron content increases from the fibre interface to the matrix interface allow the mechanical fuse properties of PyC to be combined with the oxidation resistance of a C(B) material. This revised version was published online in November 2006 with corrections to the Cover Date.     

SiC/SiC minicomposites with (PyC/TiC)n interphases processed by pressure-pulsed reactive CVI

(Pyrocarbon/titanium carbide) _n multilayered interphases were prepared within SiC/SiC minicomposites by a new method: pressure-pulsed reactive chemical vapour infiltration (P-RCVI). This method combines P-CVI with reactive chemical vapour deposition (RCVD). Minicomposite tensile tests with unload-reload cycles have shown that the interfacial shear stress depends on the number of TiCl₄ gas pulses used for the processing of TiC sub-layers. TEM observations have shown, that with a few gas pulses, the carbide nucleates as isolated grain islands which disturbs the structural anisotropy of the pyrocarbon. This structure results in a good mechanical fibre/matrix load transfer. By increasing the number of gas pulses, the TiC sub-layers become continuous and it is possible to partially consume the highly ordered pyrocarbon sub-layers, but, in that case, the load transfer is poor. The specimen behaviour in air at 700°C under a constant tensile loading was assessed. Compared with a pure pyrocarbon reference interphase, the interphases containing TiC significantly improve the lifetime of the SiC/SiC minicomposites.     

BN/SiC复合界面层对SiC纤维和PIP-Mini复合材料力学性能的影响

采用化学气相渗透(CVI)工艺, 在SiC纤维表面沉积BN和BN/SiC复合界面层, 对沉积界面层前后纤维的力学性能进行了评价。采用聚合物浸渍裂解(PIP)工艺进行致密化, 制得以原纤维、BN界面层和BN/SiC界面层纤维增强的三种Mini-SiC_f/SiC复合材料, 研究其微观结构和拉伸性能。结果表明: 采用CVI工艺制得的界面层厚度均匀、结构致密, 其中BN界面层中存在六方相, 晶体尺寸为1.76 nm; SiC界面层结晶性较好, 晶粒尺寸为18.73 nm; 沉积界面层后SiC纤维的弹性模量基本保持不变, 拉伸强度降低。与SiC_f/SiC相比, PIP工艺制备的SiC_f/BN/SiC和SiC_f/(BN/SiC)/SiC-Mini复合材料所能承受的最大拉伸载荷和断裂应变明显提升, BN界面层起主要作用。由断面形貌分析可以看出, SiC_f/BN/SiC和SiC_f/(BN/SiC)/SiC复合材料的纤维拔出明显, 说明在断裂时消耗的能量增加, 可承受的最大载荷增大。     

Interface characterization by TEM,AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase

The fibre-matrix interfacial zone formed during the isothermal/isobaric chemical vapour infiltration processing of SiC fibres (ex-polycarbosilane)/boron nitride/SiC matrix composites has been analysed by TEM/electron energy loss spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy. In the composites, the boron nitride interphase (deposited from BF₃-NH₃) is made of turbostratic boron nitride, almost stoichiometric but containing some oxygen (less than 5 at %). The boron nitride layer stacks are randomly orientated except in the very vicinity of the fibre surface where they lie almost parallel to the substrate. The long chemical vapour infiltration treatment at 1000 °C used to infiltrate the SiC matrix acts as an annealing treatment for the metastable ex-polycarbosilane fibres which gives rise to the growth of an SiO₂/carbon amorphous double layer at the boron nitride/fibre interface. Deflection of microcracks arising from the failure of the brittle SiC-matrix occurs at the boron nitride/SiO₂ interface considered to be the weaker link in the matrix/boron nitride interphase/SiO₂/carbon/fibre sequence. It is suggested that the combination of the boron nitride layered interphase and SiO₂/carbon fibre decomposition products might play an important role in determining the propagation path of microcracks in the fibre/matrix interfacial zones and could be responsible, at least to some extent, for the non-brittle behaviour of such composites.     

Brazing of SiC and SiCf/SiC composites performed with 84Si-16Ti eutectic alloy: microstructure and strength

The microstructure and strength of brazed joints for monolithic SiC and SiC_f/SiC composites are presented and discussed; the brazing technique is based on the use of the 84Si-16Ti (at%) eutectic alloy. The rather low melting point of the used alloy allows to avoid a degradation of the fibre/matrix-interfaces in the composite materials. All the joints did not show any discontinuities and defects at the interface and revealed a fine eutectic structure. Moreover, in the case of composites, the joint layer appeared well adherent both to the matrix and the fibre interphase, and the brazing alloy infiltration looked sufficiently controlled. High resolving electron microscopic investigations of the microstructure and of the nanochemistry (HREM, EELS, esp. ELNES) revealed atomically sharp interfaces without interdiffusion or phase formation at the interlayer leading to the conclusion that direct chemical bonds are responsible for the adhesion. The joints of SiC_f/SiC composites showed 71 ± 10 MPa shear strength at RT and nearly the same values at 600°C.     

The interphase and interface microstructure and chemistry of isothermal/isobaric chemical vapour infiltration SiC/BN/SiC composites: TEM and electron energy loss studies

The boron nitride interphase and its interfaces in two-dimensional-SiC/BN/SiC composites have been analysed by transmission electron microscopy and electron energy loss spectroscopy. BN was deposited by isothermal/isobaric chemical vapour infiltration from BCl₃–NH₃–H₂ mixture at moderate temperature. BN and the fibre/BN interface exhibit different features depending on the nature of the Nicalon^TM fibre surface, raw or treated prior to the BN deposition. When untreated fibres are used, a carbon-rich layer and silica clusters are formed during the manufacturing of the composite. In that case, the interphase is poorly organized and presents a porous microstructure and a large carbon content. With the treated Nicalon^TM fibre, no formation of a new interlayer is observed at the fibre–BN interface and the interphase exhibits a better organized turbostratic microstructure with no voids. Additionally, in both types of composites, a carbon-rich layer is formed at the BN–matrix interface during the SiC infiltration step at about 1000 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fibre and interfacial properties measured in situ for a 3D woven SiC/SiC-based composite with glass sealant

In situ fibre strength Weibull parameters and fibre pullout length distributions were investigated for a 3D woven SiC/SiC-based composite with glass sealant oxidation protection system after tensile testing at 1000°C and 1200°C in air. Results were similar to those previously observed for unsealed specimens tested in vacuum (1200–1380°C), but significantly different from the case of unsealed specimens tested in air (1100–1200°C). Overall, it was concluded that the glass sealant provided excellent oxidation protection for the composite at the test conditions employed, i.e. short-term exposure up to 1200°C in air.     

Micro/minicomposites: a useful approach to the design and development of non-oxide CMCs

Micro (one single filament) and mini (one single fiber tow) non-oxide composites (C/C; C/SiC and SiC/SiC) with simple (PyC or BN) or complex interphases [C (B) or (PyC-SiC)_n multilayers] are fabricated in a short time by CVD/CVI. The fiber/matrix interfacial zone is characterized by AES and TEM. Tensile tests are used to assess the mechanical properties and the Weibull statistical parameters of both the fiber and matrix, as well as the fiber–matrix interfacial parameters (τ_i; l_d; G_ic). The tensile stress–strain behavior has been modelled. The tensile curves exhibit the same features as those previously reported for real nD-composites. Lifetime at high temperatures in air is characterized through static/cyclic fatigue tests and modelled. It is improved by replacing conventional pyrocarbon by highly engineered interphases. The micro/mini composite approach is used in the optimization of processing conditions and to derive parameters necessary for the modelling of the thermomechanical and chemical behavior of composites with more complex fiber architectures.     

Effect of the filament nature on fatigue crack growth in titanium based composites reinforced by boron,B(B4C) and SiC filaments

Crack propagation testing has been applied to synthetic metal matrix composites (MMC) in order to compare failure mechanisms in Ti-6Al-4V alloy reinforced by uncoated boron, B(B₄C) and chemical vapour deposition (CVD) SiC filaments. The impeding effect of the fibres leads to low crack growth rates, compared to those reported for the unreinforced Ti-6Al-4V alloy and to higher toughness despite the presence of the reinforcing brittle phases. After long isothermal exposures at 850° C, the MMC crack growth resistance is reduced mainly due to fibre degradation, fibre-matrix debonding and an increase in matrix brittleness. However, for short-time isothermal exposures (up to about 10 h for B/Ti-6Al-4V, 30 h for B (B₄C)/Ti-6Al-4V and 60 h for SiC/Ti-6Al-4V) the crack growth resistance is significantly increased. This improvement is related to the build up of an energy-dissipating mechanism by fibre microcracking in the vicinity of the crack tip. This damaging mechanism allowing matrix plastic deformation is already effective for boron and B(B₄C) in the as-fabricated state, but occurs only after 10 h of thermal exposure at 850° C in the case of SiC/Ti-6Al-4V composites.     

Raman study of SiC fibres made from polycarbosilane

We have examined the evolution of Raman spectra of SiC fibres through structural and compositional transformations caused by heat treatment. The SiC fibre was made from polycarbosilane. Raman spectra of the SiC fibre indicate that it consists of (i) amorphous or microcrystalline SiC, (ii) carbon microcrystals, and (iii) silicon oxide. The amount of microcrystalline carbon in the fibre increases with heat treatment temperature up to 1400° C, and it decreases abruptly in those fibres heat treated above 1500° C. The tensile strength of the fibre drops virtually to zero after the heat treatment at 1500° C. Carbon microcrystals are precipitated from the Si-C random network with excess carbon, and they are distributed uniformly in the fibre. These carbon particles suppress the growth of SiC crystals. It is shown that the carbon microcrystals play an important role in maintaining the high mechanical strength of the SiC fibre.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

Design,Fabrication, Characterization and Simulation of PIP-SiC/SiC Composites

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature and fusion applications. Polymer impregnation and pyrolysis (PIP) is a conventional technique for fabricating SiC/SiC composites. In this research, KD-1 SiC fibers were employed as reinforcements, a series of coatings such as pyrocarbon (PyC), SiC and carbon nanotubes (CNTs) were synthesized as interphases, PCS and LPVCS were used as precursors and SiC/SiC composites were prepared via the PIP method. The mechanical properties of the SiC/SiC composites were characterized. Relationship between the interphase shear strength and the fracture toughness of the composites was established. X-ray tomographic scans of the SiC/SiC composites were performed and the closed porosities of the composites were calculated. The compatibility of the SiC/SiC composites with liquid LiPb at 800 °C and 1000 °C was investigated. High-resolution synchrotron X-ray tomography was applied to the SiC/SiC composite and digital volume correlation was employed for Hertzian indentation testing of the SiC/SiC composite. A Cellular Automata integrated with Finite Elements (CAFE) method was developed to account for the effect of microstructure on the fracture behavior of the SiC/SiC composite.     

Synthesis of SiC fibre with low oxygen content and high tensile strength using a polyblend precursor

SiC fibre with low oxygen content and high tensile strength was first synthesized in our laboratory. The SiC fibre was obtained by using a polyblend of polycarbosilane (PC) and hydroxy-terminated-polybutadiene (HTPB) as a precursor. It was found that PC could react with HTPB to form cross-linked polymers at temperatures around 260 ° C, so the HTPB can be used as a curing agent. Consequently, the need for oxygen to be introduced in the air-curing process is reduced and SiC fibre with low oxygen content and higher tensile strength can be made. The chemical compositions, the oxidation resistance and chemical stability of the SiC fibre were also studied here.     

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Non-destructive testing of satellite nozzles made of carbon fibre ceramic matrix composite,C/SiC

Carbon fibre ceramic matrix composite materials, C/SiC, are excellent candidates as lightweight structural materials for high performance hot structures such as in aerospace applications. Satellite nozzles are manufactured from C/SiC, using, for instance, the Liquid Polymer Infiltration (LPI) process.In this article the applicability of different non-destructive analysis methods for the characterisation of C/SiC components will be discussed. By using synchrotron and neutron tomography it is possible to characterise the C/SiC material in each desired location or orientation. Synchrotron radiation using tomography on small samples with a resolution of 1.4 μm, i.e. the fibre scale, was used to characterise three dimensionally fibre orientation and integrity, matrix homogeneity and dimensions and distributions of micro pores. Neutron radiation tomography with a resolution of about 300 μm was used to analyse the over-all C/SiC satellite nozzle component with respect to the fibre content. The special solder connection of a C/SiC satellite nozzle to a metallic ring was also successfully analysed by neutron tomography. In addition, the residual stress state of a temperature tested satellite nozzle was analysed non-destructively in depth by neutron diffraction. The results revealed almost zero stress for the principal directions, radial, axial and tangential, which can be considered to be the principal directions.     

Processing, microstructure and mechanical properties of hot-pressed SiC continuous fibre/SiC composites

SiC (SCS-6TM) continuous fibre/SiC composites were fabricated by hot-pressing at 1700°C in vacuum using an Al sintering additive. Analytical transmission electron microscopy was used to investigate the microstructure of the composites. The room-temperature mechanical and high-temperature creep properties of the composites were investigated by four-point bending. The SiC powders used were sintered at a relatively low sintering temperature to high density (97% of theoretical density) with the addition of the Al sintering additive. It is believed that the Al additive is very efficient for the densification of SiC. The SiC fibres maintained their original form and microstructure during fabrication. The SiC matrix reacted with the outermost carbon sublayer in the fibre, forming a thin (1.8–4.8m) interfacial layer, which was composed of Al4C3, Si–Al–C, and Si–Al–O phases. The incorporation of SiC fibre into a dense SiC matrix significantly increased the room-temperature failure strain and improved the high-temperature creep properties. In addition, the incorporation of SiC fibre into a porous SiC matrix increased the room-temperature failure strain, but did not contribute to the high-temperature creep properties.     

SiC纤维/LAS复合材料的TG-DTA-MS研究

本文首次用TG-DTA-MS联用技术对SiC纤维/LAS微晶玻璃复合材料的热分解过程、机制及其与晶化的关系进行了研究。提出了该复合材料界面形成碳层的热力学和质谱分析判别依据,并对晶化前后的复合材料的热分解行为作了实验对比和理论分析。      

Effect of matrix liquid phase on interphase formation in SiC fibre-reinforced Si2N2O-Al2O3-CaO composites

Matrix compositions based on Si₂N₂O, with Al₂O₃ and CaO additions, were used to hot press Nicalon SiC fibre-reinforced composites at 1600 °C. With both CaO and Al₂O₃ additions, eutectic melting formed an appreciable volume of liquid phase during hot pressing, which remained as a stable glassy phase in the cooled composites. This liquid phase fostered formation of 240 nm thick carbon-rich interphases between the fibres and the matrix. These interphases showed relatively low interfacial shear strength and resulted in composites which showed non-catastrophic, notch-independent fracture. Matrices using either Al₂O₃ or CaO did not form adequate liquid phase to form coarse interphases, and fracture was catastrophic in nature. Post-heat treatment of the composites at 1000 °C showed peripheral oxidation (removal of the carbon content of the interphase) indicating limited protection afforded when glassy phase was present in the matrix. Controlled cooling in the hot press did not cause the liquid regions to devitrify.     

Health monitoring in continuous glass fibre reinforced thermoplastics: Tailored sensitivity and cyclic loading of CNT-based interphase sensors

In this study, we report on different approaches for tailoring the resistance as well as the sensitivity of interphase sensors based on carbon nanotubes (CNTs). The two main aspects in affecting their initial resistance as well as the sensitivity of the systems during mechanical loading are the yarn coating content and the CNT-weight fraction of the coating. Varying those factors, the conducted tensile tests show that the initial resistance as well as the sensitivity of the interphase sensors can be adjusted within a certain range. Additionally, it is shown that glass fibre (GF)-yarns with low coating contents allow identifying critical loads for the interphase, which are found to be below the ones for GF failure. Performing cyclic tensile loading above and below this critical stress value has a significant effect on the interphase life-time. In order to assess the interphase damage quantitatively, new parameters based on the resistance change are introduced. Those parameters allow for direct comparison and characterisation of different GF modifications, i.e. interphases, during mechanical testing by cyclic loading of the interphase sensors.     

Influence of a multilayered matrix on the lifetime of SiC/BN/SiC minicomposites

The mechanical behaviour at room temperature and the lifetime in air at 700°C under static loading of SiC/BN/SiC minicomposites have been investigated. The minicomposites consisted of a single tow of Hi-Nicalon fibres coated with a BN interphase and a CVI-SiC matrix. For few of them, a BN layer was introduced within the matrix. All the minicomposites were heat treated at high temperature to improve the BN crystallinity. In some cases, the BN interphase was submitted to a specific treatment before the infiltration of the SiC matrix, to further improve its crystalline state. The differences in interfacial zone, as assessed by TEM, were correlated with those in mechanical properties. A significant improvement of the mechanical behaviour at room temperature and the lifetime of the minicomposites with a multilayered matrix was observed. The multilayered matrix is efficient when a silica layer on both sides of the BN layer within the matrix is present.