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.     

SIMS,EDX, EELS,AES, XPS study of interphases in nicalon fibre-LAS glass matrix composites

Secondary ion mass spectrometry (SIMS) and energy dispersive X-ray spectroscopy (EDX) were used to analyse interphases which develop at the fibre—matrix interface during fabrication of SiC nicalon fibre—LAS (Li₂O, Al₂O₃, SiO₂) or LAS+Nb₂O₅ glass matrix composites. SIMS was performed on fibres extracted from composites by dissolution of the matrix in hydrofluoric acid (HF). The composition of the interphases which remain on the fibre periphery was studied using sputter-depth profiling. EDX analysis of the interphases was performed on nicalon-LAS+Nb₂O₅ composite cross-sections. These investigations show that in both composites the reaction products at the fibre—matrix interface consist of a carbon layer and a silicate phase, rich in oxygen, located between the carbon layer and the fibre core. The analyses also demonstrate the efficiency of SIMS for analysing compositional changes in the near surface of fibres of small diameter.     

SIMS,EDX, EELS,AES, XPS study of interphases in nicalon fibre-LAS glass matrix composites

Auger electron (AES), electron energy loss (EELS) and X-ray photoelectron spectroscopy (XPS) were used to identify the reaction products at the fibre-matrix interface in SiC nicalon fibre-LAS (Li₂O, Al₂O₃, SiO₂) or LAS + Nb₂O₅ glass matrix composites. Chemical bonding of the different elements was investigated by AES using sputter-depth profiling on fibres extracted from two matrices by etching in hydrofluoric acid. The chemistry of the silicium was studied by EELS in nicalon-LAS + Nb₂O₅ composite cross-sections. XPS was performed on fibres extracted from the nicalon-LAS + Nb₂O₅ composite to confirm EELS and AES results. These investigations show that in both composites the reaction scale at the fibre-matrix interface consists of a carbon layer next to the matrix and of a silicate phase rich in oxygen which contains carbon, probably in the form of a silicon oxycarbide, and which is located between the carbon layer and the fibre core.     

Assessment of the mechanical behaviour of glass fibre composites with a tough polydicyclopentadiene (PDCPD) matrix

The use of a tough thermoset polydicyclopentadiene (PDCPD) as a matrix material for composites was explored. A PDCPD–glass fibre composite and an equivalent epoxy composite were compared. Fibre–matrix adhesion quality was assessed by transverse bending tests. The materials were subjected to compression tests, impact tests, static tensile tests and tensile fatigue tests. The results indicate that the tough behaviour of the PDCPD matrix markedly influences the composite damage resistance. The size of the impact damage in the PDCPD composite was half of that in the epoxy composite. The tensile tests indicated no significant difference in tensile strength, but the damage before failure was found to be much more severe in the epoxy samples. The fatigue results showed a much lower variation in fatigue life for the PDCPD material than for the epoxy material, as well as clear differences in damage development for the two materials.     

Thermal shock behaviour of unidirectional, 0°/90°, and 2-D woven fibre-reinforced CVI SiC matrix composites

The thermal shock behaviour of NicalonTM fibre-reinforced chemical vapour infiltrated SiC matrix composites with three different types of fibre architecture, unidirectional, 0°/90°, and 2-D woven, has been studied using the water quench technique. Thermal shock induced damage was characterized by the destructive four-point flexure technique and the nondestructive technique of Young's modulus measurement by the dynamic resonance method. It was shown that the unidirectional and 0°/90° composites did not possess satisfactory mechanical properties or resistance to thermal shock because these fibre architectures prevented the composites from attaining high density during infiltration. Excess carbon coating was also found in the unidirectional and 0°/90° composites. Oxidation of this carbon coating contributed to the property degradation at high quench temperature difference. By contrast, the composite with 2-D woven fibre architecture created using the 0°/30°/60° cloth lay-up showed superior mechanical properties and thermal shock resistance. The nondestructive technique of Young's modulus measurement by the dynamic resonance method was successfully used in detecting the thermal shock damage.     

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.     

Mechanical behaviour of the interphase between matrix and reinforcement of Al 2014 matrix composites reinforced with (Ni3Al)p

The 2XXX series aluminium alloys reinforced with intermetallics present a special behaviour due to the reaction between matrix and reinforcement. This reaction forms an interphase that influences the mechanical and chemical behaviour of the composite, reducing the capability of the material to improve its properties after heat treatment. In this work, an approach is made to the study of this interphase, using particulate intermetallics with the same chemical and stoichiometric composition but obtained in different ways: mechanical alloying and gas atomising. A microstructural study was carried out by SEM, including qualitative analysis, showing the chemical gradient formed at the interphase both as-extruded and after T6 treatment. The mechanical behaviour of the interphase is studied through nanoindentation that allows the determination of hardness and Young modulus. Finally, all these properties are correlated with a fractrographic study of the fracture mechanisms. A harder interphase is formed for the mechanically alloyed system promoting a transgranular cleavage fracture micromechanisms, while intergranular cleavage fracture is found in atomised intermetallic containing composites.     

Evolution of Si (and SiC) nanocrystal precipitation in SiC matrix

Si_1−xC_x films with varying ratio of carbon to silicon (C/Si) were fabricated by magnetron co-sputtering from a combined C and Si target. The composition in films was changed by adjusting the ratio of sputtered target's area between C and Si. Analysis of X-ray photoelectron spectroscopy for as-deposited films shows that C/Si atomic ratios of our films have ranges of 0.33-1.02. Thermal annealing of as-deposited films was carried out at various temperatures from 800 to 1100 °C in a conventional furnace. Fourier transform infrared spectra show a shift of Si-C stretching peak towards higher wavenumbers from ∼ 737 cm^− 1 to ∼ 800 cm^− 1 with increasing annealing temperature. From the results of Raman spectroscopy, X-ray diffraction and transmission electron microscopy, it was found that the dominant type of nanocrystals changes from Si to SiC in the films annealed at 1100 °C when the C/Si atomic ratio increases from 0.33 to 1.02.     

The effects of SiC whiskers and an SiC film coating deposited by chemical vapor infiltration (CVI) on a porous cordierite substrate

This study was performed with the goal of reducing environmental pollution caused by nano-particle materials. SiC whiskers were grown on a porous cordierite substrate to enhance its filtering efficiency, performance, and durability by controlling pore morphology. We investigated two different methods: SiC whisker growth (A) and SiC film coating after the SiC whisker growth (B). These experiments were performed using the chemical vapor infiltration (CVI) process to grow the whiskers in the inner pore without closing it. After a 1 h deposition at 1200 °C, the compressive strength of the whiskered porous cordierite body increased from 24 MPa to 60 MPa (250%) in experiment (A) and to 82 MPa (342%) in experiment (B). The mean pore size was reduced after whisker growth (A) as well as after additional film coating (B). The adhesion strength between the whiskers and the cordierite substrate increased with the additional film coating (B). Consequently, the separation of whiskers from the substrate was minimized. N₂ gas was injected and a permeability change was observed which explains the separation of the whiskers from the substrate. This method makes the filtration of nano-sized particles feasible.     

Synthesis of SiC/BN nanocomposite powders by carbothermal reduction and nitridation of borosilicate glass, and the properties of their sintered composites

A new synthetic method for the fabrication of SiC/BN nanocomposites was devised to attain strong machinable ceramics. SiC/BN nanocomposites that contained 10, 20, and 30?vol% hexagonal BN (h-BN) were successfully fabricated by sintering SiC-BN nanocomposite powders by carbothermal reduction and nitridation of borosilicate glass powders. Homogeneous mixtures of silica (SiO(2)), boric acid (H(3)BO(3)), and carbon powder were heated in a nitrogen atmosphere to synthesize SiC-BN nanocomposite powders. Borosilicate glass was obtained by reacting SiO(2) and B(2)O(3) above 800?°C, and SiC and turbostratic BN (t-BN) were obtained by reacting borosilicate glass with carbon powder and nitrogen gas at 1500?°C. Carbothermal reduction followed by nitridation yielded SiC-BN nanocomposite powder composed of nanosized SiC and t-BN. By hot-pressing nanocomposite SiC-BN powders containing 7?wt% Al(2)O(3) and 2?wt% Y(2)O(3), machinable SiC/BN nanocomposites were obtained without a significant decrease in their fracture strength.     

Interaction of Al-Si,Al-Ge,and Zn-Al eutectic alloys with SiC/Al discontinuously reinforced metal matrix composites

Interactions between Al-Si, Al-Ge, and Zn-Al eutectic alloys with SiC whisker-reinforced aluminium metal matrix composites were studied as a function of temperature above the eutectic melting temperature. Penetration extended several millimetres into the composite for the Al-Si and Al-Ge alloys but was restricted to a thin surface layer (50 m) for the Zn-Al alloy. The extent of the penetration zone for the aluminium alloys containing silicon and germanium was also affected by the thermal-mechanical treatment of the composite: limited penetration was observed for hot-pressed material whereas extensive penetration was observed for mechanically worked material. Mechanisms for the observed phenomena are discussed in terms of the wettability of the SiC whiskers by the eutectic alloys, the formation of channels during mechanical working as well as the fine grain size of the composite.     

Interface structure and fractography of a magnesium-alloy,metal-matrix composite reinforced with SiC particles

The interfacial structure in SiC-particle-reinforced, as-cast and heat-treated magnesium-alloy-matrix composites was investigated using analytical electron microscopy. No extensive chemical reactions were observed between the magnesium and the SiC particles or the SiC and the eutectic phase. However, most of the eutectic phase appeared to nucleate at the surface of the SiC particles. In addition to the lamellar eutectic, a fine eutectic and Mg₂Si particles have been identified at the SiC surface using nanoprobe micro-analysis. As with the aluminium base composite, precipitation was observed to take place on dislocations, and dense precipitation was found to occur in the stress fields around the SiC particles. Examination of the fracture surface indicated that the bonding between the SiC/eutectic is stronger than between the SiC/magnesium matrix. Intergranular cracks have been observed both in the fracture surface and also in a polished and etched section. The fracture surface tends to exhibit a more brittle morphology in the composite than is observed in the alloy.     

High hardness BaCb-(BxOy/BN) composites with 3D mesh-like fine grain-boundary structure by reactive spark plasma sintering

Boron carbide B4C powders were subject to reactive spark plasma sintering (also known as field assisted sintering, pulsed current sintering or plasma assisted sintering) under nitrogen atmosphere. For an optimum hexagonal BN (h-BN) content estimated from X-ray diffraction measurements at approximately 0.4 wt%, the as-prepared BaCb-(BxOy/BN) ceramic shows values of Berkovich and Vickers hardness of 56.7 +/- 3.1 GPa and 39.3 +/- 7.6 GPa, respectively. These values are higher than for the vacuum SPS processed B4C pristine sample and the h-BN -mechanically-added samples. XRD and electronic microscopy data suggest that in the samples produced by reactive SPS in N2 atmosphere, and containing an estimated amount of 0.3-1.5% h-BN, the crystallite size of the boron carbide grains is decreasing with the increasing amount of N2, while for the newly formed lamellar h-BN the crystallite size is almost constant (approximately 30-50 nm). BN is located at the grain boundaries between the boron carbide grains and it is wrapped and intercalated by a thin layer of boron oxide. BxOy/BN forms a fine and continuous 3D mesh-like structure that is a possible reason for good mechanical properties.     

Microwave absorbent: preparation,mechanical properties and r.f.-microwave conductivity of SiC (and/or mullite) fibre reinforced Nasicon matrix composites

The electrical and mechanical properties of composites made with Nicalon® NLM202 SiC (and Nextel® 440) woven fibres in a sol-gel prepared Nasicon matrix are reported. Local microhardness, interfacial shear stress and three point flexural strength have been determined and correlated to the conclusion of an X-ray microanalysis of the fibre-matrix interphases. The radio frequency (r.f.) to microwave conductivity of the composites has been measured and compared with that of their constituents (Nasicon matrix, SiC fibre) and with the direct current (d.c.) conductivity of extracted fibres.     

Synthesis and characterization of dual particle (MWCT+B4C) reinforced sintered hybrid aluminum matrix composites

ABSTRACT

Metal matrix nanocomposites (MMNCs) consist of a metal matrix reinforced with nanoparticles, featuring physical and mechanical properties very different from those of the matrix. Especially carbon nanotubes (CNTs) can improve the matrix material in terms of wear resistance, damping properties, and mechanical strength. The present investigation deals with the synthesis and characterization of aluminum matrix reinforced with micro-B₄C particles, and multiwall carbon nanotubes (MWCNTs) which have been prepared by powder metallurgy route. Powder mixture containing fixed weight (%) of B₄C and different wt% of MWCNT as reinforcement constituents that are uniaxial cold pressed and later green compacts are sintered in continues electric furnace. Microstructure and Mechanical properties such as microhardness and density are examined. Microstructure of samples has been investigated using scanning electron microscope (SEM). X-ray diffraction(XRD), energy dispersive x-ray (EDAX), atomic force microscope (AFM), and transmission electron microscope (TEM). TEM microstructure of the nanocomposite shows the homogeneous dispersion of MWCNT in the aluminum matrix. The results indicated that the increase in wt % of MWCNT improves the bonding and mechanical properties.     

具有优异纺丝性的陶瓷先驱体含铝聚碳硅烷的合成及表征

选择适当分子量的低分子量聚碳硅烷与Al(AcAc)3在自制常压高温合成装置中合成了含铝碳化硅纤维的先驱体--聚铝碳硅烷(polyaluminocarbosilane,PACS).并对PACS进行了软化点测试、傅立叶红外光谱(FT-IR)分析、凝胶渗透色谱(GPC)测试、元素分析以及可纺性研究.由中等分子量聚碳硅烷为原料合成出的软化点为194.8～220.1,Si-H键含量为0.857,数均分子量为2353,分子量分布呈"双峰"分布的PACS,经熔融纺丝得到了直径为5μm、表面光滑、直径均匀的原丝,表现出了优异的纺丝性能.     

基于峰值力纳米力学模量成像技术的碳纤维/树脂复合材料界面尺寸与性能的原位表征

为准确测定复合材料界面结构的尺寸与性能,提出一种原位峰值力纳米力学模量成像(PF-QNM)技术,对其测试原理、校准方法和适用性进行分析,并采用PF-QNM技术对碳纤维/聚醚醚酮(T300/PEEK)、碳纤维/聚醚砜(T300/PES)和碳纤维/环氧树脂(T700/TR1219B)三种复合材料的界面尺寸和各组分原位模量进行测试。结果标明:该方法的横向分辨率可以达到纳米尺度,测得树脂、界面、纤维区域的弹性模量数值呈梯度上升趋势,区分度明显,T300/PEEK、T300/PES和T700/TR1219B复合材料界面厚度分别为(69.3±7.9)nm、(101.3±10.2)nm和(48.4±5.4)nm。实验范围内,热固性复合材料界面厚度小于热塑性复合材料。对模量成像图统计分析可得,T300/PEEK、T300/PES和T700/TR1219B复合材料的树脂区平均弹性模量分别为4.36 GPa、4.96 GPa和3.59GPa,与其宏观弹性模量数值较为接近。     

Interactions between tungsten carbide (WC) particulates and metal matrix in WC-reinforced composites

A variety of experimental techniques have been used to investigate the interactions between tungsten carbide (WC–Co 88/12) particulates and the matrix in some new wear resistant cobalt-based superalloy and steel matrix composites produced by hot isostatic pressing. The results show that the chemical composition of the matrix has a strong influence on the interface reaction between WC and matrix and the structural stability of the WC particulates in the composite. Some characteristics of the interaction between matrix and reinforcement are explained by the calculation of diffusion kinetics. The three-body abrasion wear resistance of the composites has been examined based on the ASTM G65-91 standard procedure. The wear behavior of the best composites of this study shows great potential for wear protection applications.