Microstructure and mechanical properties of hot-pressed SiC-TiC composites

Hot-pressed SiC-TiC composite ceramics with 0–100 wt% TiC have been investigated to determine the effect of composition (amount of TiC) on the elastic modulus, hardness, flexural strength and fracture toughness,K _IC. The composites exhibited superior mechanical properties compared to monolithic SiC and TiC, especially in fracture toughness,K _IC, value for 30–50 wt% TiC composite. The maximum values ofK _IC and room-temperature flexural strength were 6 MPa m^1/2 for a 50 wt % TiC and 750 MPa for a 30 wt% TiC composite, respectively. The observed toughening could be attributed to the deflection of cracks due to dispersion of the different particles. Although no third phases were detected by both TEM and XRD studies, an EDAX study and resistivity measurements indicated some possibility of solid solutions being present. The composites containing more than 30 wt% TiC, exhibited resistivity lower than 10^–3 cm which is favourable for electro-discharge machining of ceramics.     

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

Sol–gel route to carbon nanotube borosilicate glass composites

Dense borosilicate glass matrix composites containing up to 3 wt% of multiwalled carbon nanotubes were produced by a sol–gel process. The three different silicate precursors employed (tetramethylsilane (TMOS), methyltriethoxysilane (MTES) and methyltrimethoxysilane (MTMS)) yielded transparent xerogels which were subsequently crushed and densified by hot pressing at 800 °C. The dispersion of the carbon nanotubes was aided by using an organic–inorganic binder (3-aminopropyl triethoxysilane) which limited flocculation of the CNTs in the silica sol. After densification, the borosilicate glass composites containing up to 2 wt% CNTs showed significant improvements in hardness and compression strength, as well as thermal conductivity, whilst percolation effects lead to a dramatic increase in electrical conductivity above 1 wt%. This simple approach to disperse CNTs into a technical silicate glass matrix via the sol–gel process focusses specifically on the borosilicate system, but the procedure can be applied to produce other inorganic matrix composites containing CNTs.     

The toughness of epoxy polymers and fibre composites modified with rubber microparticles and silica nanoparticles

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m² by the presence of 20 wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965 J/m² was measured for a ‘hybrid’ epoxy polymer containing 9 wt% and 15 wt% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.     

Properties of composites of 2014 aluminium alloy with Ni-Mo-based metallic glass particles

Composites of 2014 aluminium alloy containing dispersions of metallic glass particles (51.5wt% Ni, 38.0wt% Mo, 8.0wt% Cr and 1.5wt% B) have been prepared by a conventional powder metallurgy route involving powder mixing, compaction, sintering and heat treatment. Physical and mechanical properties of the composites, such as dimensional changes, hardness, electrical resistivity and corrosion behaviour, were studied. Dimensional growth up to a maximum of 6% in a linear direction was observed in all sintered composites. HardnessHv increased from 40 to 55kgmm^–2 with the addition of 4vol% of dispersoid, followed by a gradual decrease with increasing additions of dispersoid. The decrease in hardness above 4vol% of dispersoid was attributed to the presence of increasing amounts of porosity. Electrical resistivity increased from 50nm (for 2014 aluminium alloy) to 180nm at 20vol% dispersoid. The corrosion rate in an artificial sea water environment decreased linearly with the volume fraction of dispersoid. Re-pressing and re-sintering (in an argon atmosphere) of composites containing 4vol% of metallic glass particles resulted in an increase inHv from 55 (argon sintering) to 83kgmm^–2, and a decrease in electrical resistivity from 57 to 52nm due to the increase in density. The corrosion rate in an artificial sea water environment of composites containing 4vol% of metallic glass decreased from 70×10^–3 to 50×10^–3 mgdm^–2 per day due to re-pressing and re-sintering.     

Microstructure and properties of an electroconductive SiC-based composite

In this work, an SiC-based electroconductive composite is obtained through simultaneous addition of MoSi₂ and ZrB₂ particles. The composite material is fully densified by hot pressing at 1860 °C and the microstructure is investigated by SEM-EDS analysis. Microstructural features and mechanical properties are compared to those of a monolithic hot-pressed SiC material. The MoSi₂ and ZrB₂ particles, besides increasing the electrical conductivity of the silicon carbide matrix, also act as reinforcement for the material. Room-temperature strength reaches the value of 850 MPa and the fracture toughness is 4.2 MPa m^0.5. The composite electrical resistivity is of the order of 10⁻³ Ω cm.     

How hybridization with zinc oxide whiskers and carbon fibers affects the thermal diffusivity and mechanical properties of poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid) nanocomposites

The enhanced thermal diffusivity and mechanical properties of poly(l-lactic acid) (PLLA) nanocomposites reported here are based on the percolation network formed when PLLA is hybridized with short carbon fibers (CFs) and functionalized zinc oxide whiskers. The PLLA nanocomposite containing 30 wt% (≈9.5 vol%) ZnO whiskers and 10 wt% (≈8.1 vol%) CFs had a thermal diffusivity almost as high as that of stainless steel and an insulator-level electrical resistivity (>10¹⁰ Ωm). Modifying the surface of the ZnO whiskers by esterifying them using specific alcohols with long linear alkyl chains improved the elastic strength and toughness of the nanocomposites significantly. These results suggest that hybridizing PLLA with short CFs and functionalized ZnO whiskers yields nanocomposites with high thermal diffusivity as well as high electrical resistivity and excellent mechanical properties.     

Fabrication and properties of CNTs/carbon fabric hybrid multiscale composites processed via resin transfer molding technique

Carbon nanotubes (CNTs) are effective fillers/reinforcements regarding improving the properties of polymer. In the present paper, carboxylic acid functionalized CNTs were used to modify epoxy with intent to develop a nanocomposite matrix for hybrid multiscale composites combining benefits of nanoscale reinforcement with well-established fibrous composites. CNTs were dispersed in epoxy by using high energy sonication. At low contents of CNTs, hybrid multiscale composites specimens were manufactured via resin transfer molding (RTM) process. The processibility of CNTs/epoxy systems was explored with respect to their viscosity. The dispersion quality and re-agglomeration behavior of CNTs in epoxy were characterized using optical microscope. A CNTs loading of 0.025 wt% significantly improved the glass transition temperatures (Tg) of the hybrid multiscale composites. Scanning electron microscopy (SEM) was used to examine the fracture surface of the failed specimens. It is demonstrated that the addition of small amount of CNTs (0.025 wt%) to epoxy for the fabrication of multiscale carbon fabric composites via RTM route effectively improves the matrix-dominated properties of polymer based composites. Hybridization efficiency in carbon fiber reinforced composites using CNTs is found to be highly dependent on the changes in the dispersion state of CNTs in epoxy.     

Densification behavior and mechanical properties of spark plasma-sintered ZrC–TiC and ZrC–TiC–CNT composites

Titanium carbide (TiC) and carbon nanotubes (CNTs) were introduced into zirconium carbide (ZrC) ceramics to improve the fracture toughness. ZrC–TiC and ZrC–TiC–CNT composites containing 0–30 vol.% TiC and 0.25–1 mass% CNT were prepared by spark plasma sintering at temperatures of 1750–1850 °C for 300 s under a pressure of 40 MPa. Densification behavior, microstructure, and mechanical properties of the ZrC-based composites were investigated. Fully dense ZrC–TiC and ZrC–TiC–CNT composites with a relative density of more than 98 % were obtained. Vickers hardness of ZrC-based composites increased with increasing TiC content and the highest hardness was achieved with the addition of 20 vol.% TiC. Addition of CNTs up to 0.5 wt% significantly increased the fracture toughness of ZrC-based composites, whereas the addition of TiC did not have this effect.     

Electromechanical and electrothermal behaviours of carbon whisker reinforced elastomer composites

The electromechanical and electrothermal properties of conducting carbon whisker reinforced thermoplastic elastomer (TPE) composites were investigated. The carbon whiskers were derived by a catalytic chemical vapour Deposition (CCVD) process and the TPE was a styrene-ethylene-butylene-styrene (S-EB-S) block copolymer. The electrical resistivity (ϱ) of the composites can be varied either by uniaxial deformation (10¹–10⁸ Ω cm) or by temperature (10¹–10⁵ Ω cm). The temperature-resistivity studies indicated that the resistivity of these composites was influenced by the glass transition temperature (T _g) of the TPE. The ϱ versus 1/T curves exhibited two distinct regimes each with a different negative slope which intersected at the T _g of the elastomer. This was correlated to the T _gof the EB segments in the S-EB-S block copolymer (∼ -50°C) by the dynamic mechanical thermal analysis. Further, uniaxial deformation studies at room temperature (20 °C) demonstrated that the resistivity increased exponentially with the deformation. Processing technique considerations and electron micrographs of the morphology of the composites indicated the formation of polymeric film on the carbon whiskers. Thus, the electrical conduction between carbon whiskers in these highly loaded (33 and 52 vol % fraction) composites occurred through the elastomeric film by electron tunnelling. This is explained on the basis of Mott's electron hopping theory, for conduction through several carbon-polymer-carbon (C-P-C) junctions. Further studies by scanning electron microscopy, dielectric thermal analysis and voltage-current characteristics confirmed this observation. Mechanical and electrical properties of the composites indicated that CCVD carbon whiskers can be used to improve the strength and electrical conductivity of TPEs. The change in resistivity (up to five orders of magnitude) of the composites with respect to the deformation or temperature can find use in electromechanical and electrothermal device applications.     

Effect of MgF<Subscript>2</Subscript> on hot pressed hydroxylapatite and monoclinic zirconia composites

Hydroxylapatite (HA) has been widely used in biomedical applications because of its excellent biocompatibility in the human body. A total of 25 wt% monoclinic (m) zirconia–HA composites (with and without 5 wt% MgF₂) were synthesized to investigate their mechanical properties and phase stability. In HA–m-ZrO₂ composites, HA and m-ZrO₂ reacted to form CaZrO₃ when there was no F⁻ present in the composite and m-ZrO₂ partially transformed to tetragonal ZrO₂. When MgF₂ was added into the system, it improved the thermal stability of the phases, densification, hardness, and fracture toughness of the composites and it caused the m-ZrO₂ to transform completely to t-ZrO₂ by incorporating the Mg²⁺ ions present in MgF₂ in the ZrO₂. Moreover, the stability of HA was improved by incorporating the F⁻ ions from MgF₂ in place of OH⁻ ions in HA. Substitution of OH⁻ by F⁻ ions was verified by the change in HA’s hexagonal lattice parameters. A fracture toughness of 2.0 MPa√m was calculated for the composite containing MgF₂.     

Preparation and properties of the powder SBR composites filled with CNTs by spray drying process

Carbon nanotubes (CNTs) filled powder styrene-butadiene rubber (SBR) composites were prepared by spray drying of the suspension of CNTs in SBR latex. The powder was spherical like and uniform with an average diameter of less than 10 μm. The dispersion of CNTs in the rubber matrix was improved remarkably compared with that in the rubber composites obtained by the conventional mechanical mixing method. Further study about the effect of CNTs on the prepared SBR composites was performed by analyzing the vulcanization process of the SBR powder, thermal and mechanical properties of the vulcanized SBR composites. Differential scanning calorimeter (DSC) analysis indicated that the glass transition temperatures of SBR composites increased with the increasing ratio of CNTs. The vulcanization process showed that CNTs could decelerate the vulcanization of the SBR composites. Dynamic mechanical analysis indicated that the storage modulus of the composites was improved with the CNTs additions, especially when the CNTs addition exceeded 30 phr. Compared with pure SBR composites, the hardness, tensile and tear strengths of the composites filled with 60 phr CNTs enhanced 73.9%, 327.7% and 191.1%, respectively, which should be ascribed to the excellent mechanical properties of CNTs and uniform dispersion of CNTs in the rubber matrix.     

Effects of trace amount of nanometric SiC additives with wire or particle shapes on the mechanical and thermal properties of alumina matrix composites

SiC nanowires (SiCNWs) are suitable candidates used as additives to improve the thermal conductivity of alumina, since they exhibit superior properties such as high chemical and thermal stability. In this study, alumina matrix composites reinforced with very small amount of β-SiC/SiO₂ core–shell nanowires were fabricated by hot-pressing. They were first characterized and compared with alumina matrix specimens containing SiC nanopowder. It was found out that with 0.2 wt% SiC additives, the grain sizes of the alumina specimens were reduced by 20 % of that of the monolithic one, regardless of the shape of the SiC additives. Vickers hardness of specimen containing both SiCNWs and SiC nanopowders slightly increased, while fracture toughness decreased more than that of the monolithic alumina. Thermal conductivity of the specimens increased with increased amount of SiCNWs and was better than those of the specimens containing SiC nanopowders. The alumina composite containing 0.2 wt% SiCNWs had higher thermal conductivity than that of the monolithic alumina by as much as 45 %. From these results, it is clear that only small amount of nanosized SiC as an additive material, particularly SiCNWs, has a significant effect on the properties of alumina matrix composites.     

Fabrication and mechanical properties of WC particulate reinforced Cu<Subscript>47</Subscript>Ti<Subscript>33</Subscript>Zr<Subscript>11</Subscript>Ni<Subscript>6</Subscript>Sn<Subscript>2</Subscript>Si<Subscript>1</Subscript> bulk metallic glass matrix composites

The mechanical behavior of the WC particulate (WC_p) reinforced Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ bulk metallic glass (BMG) matrix composites has been examined. The mechanical properties are improved with increasing WC_p content up to 20 wt%. The ultimate compression strength and plastic strain of the composite containing 20 wt% WC_p are 2.4 GPa and 2.4%, while those of the monolithic BMG are 1.6 GPa and ∼0%, respectively. The multiple shear band formation and crack deflections through WC particles have been identified as the main mechanism for the improved toughness.     

Failure analysis and the optimal toughness design of carbon nanotube-reinforced composites

The combined analysis of the fracture toughness enhancement of carbon nanotube (CNT)-reinforced composites is herein carried out on the basis of atomistic simulation, shear-lag theory and facture mechanics. It is found that neither longer reinforced CNTs nor stronger CNT/matrix interfaces can definitely lead to the better fracture toughness of these composites. In contrast, the optimal interfacial chemical bond density and the optimal CNT length are those making the failure mode just in the transition from CNT pull-out to CNT break. To verify our theory, an atomic/continuum finite element method (FEM) is applied to investigate the fracture behavior of CNT-reinforced composites with different interfacial chemical bond densities. Our analysis shows that the optimal interfacial chemical bond density for (6,6) CNTs is about 5–10% and that increasing the CNT length beyond 100 nm does not further improve fracture toughness, but can easily lead to the self-folding and clustering of the CNTs. The proposed theoretical model is also applicable to short fiber-reinforced composites.     

Microstructure and mechanical properties of titanium carbonitride whisker reinforced β-sialon matrix composites

The microstructure, hardness, fracture toughness and thermal shock resistance were investigated for 15 vol.% TiC_0.3N_0.7 whisker reinforced β-sialon (Si_6−zAl_zO₂N_8−z with z=0.6) composites with additions of three different volume fractions 2, 5 and 20 vol.%, of an yttrium-containing glass oxynitride phase. The composites were prepared by hot pressing at 1750°C for 90 min under a uniaxial pressure of 30 MPa in nitrogen atmosphere. The TiC_0.3N_0.7 whiskers were found to survive without deteriorating in morphology or reacting with the β-sialon matrix and/or the glass phase. The TiC_0.3N_0.7 whiskers had no obvious influence on the matrix microstructure, but their presence improved both the hardness and the fracture toughness of the composites. The highest hardness was obtained for the whisker composite with 2 vol.% glass phase (H_v=18.6 GPa). The fracture toughness and thermal shock resistance improved with increasing glass content. The whisker reinforced composite containing 20 vol.% glass showed the highest fracture toughness (K_1C=6.8 MPa m^1/2). No unstable crack extension occurred during the thermal shock test of the obtained composites in the temperature interval 90-700°C, but above 700°C severe oxidation of the whiskers precludes further evaluation of thermal shock properties by the indentation-quench method applied.     

Titanium diboride copper-matrix composites

Copper-matrix titanium diboride platelet (3–5 μm) composites containing 15–60 vol% TiB2, were fabricated by powder metallurgy, using copper-coated TiB2 (60 vol% TiB2) and various amounts of copper powder. The porosity was ≤0.5% when TiB2 was ≤48 vol%. Above 48 vol% TiB2, the porosity increased abruptly with increasing TiB2 content, reaching 6.7% at 60 vol% TiB2. As a result, the hardness and compressive yield strength dropped precipitously with increasing TiB2 volume fraction beyond 48%. At 48 vol% TiB2, the thermal conductivity was 176 W m-1°C-1, the electrical resistivity was 3.42× 10-6Ωcm, the coefficient of thermal expansion (CTE) was 10.2×10-6°C-1, the compressive yield strength was 659 MPa, and the Brinell hardness was 218. For composites made by conventional powder metallurgy, using a mixture of TiB2 platelets (not coated) and copper powder, the porosity was ≤1.8% when TiB2 was at ≤42 vol%; above 42 vol% TiB2, the porosity increased abruptly and the hardness and compressive yield strength decreased abruptly. The electrical resistivity and thermal conductivity were also affected by the porosity, but less so than the mechanical properties. Composites made using copper-coated TiB2 exhibited lower electrical resistivity, higher thermal conductivity, lower CTE, higher compressive yield strength, greater hardness, greater abrasive wear resistance, greater scratch resistance and lower porosity than the corresponding composites made from uncoated TiB2. This revised version was published online in November 2006 with corrections to the Cover Date.     

碳纳米管-TiB2陶瓷基复合材料的制备与性能研究

研究了用热压烧结(HP)方法制备TiB2-xwt%CNTs-5wt%Ni(x=0.1、0.3、0.5、1、4)复合材料的工艺条件、力学性能和微观结构.用XRD研究了其相组成,用SEM观察了复合材料的断口形貌和裂纹扩展.研究表明碳纳米管的加入使复合材料的硬度、弯曲强度和断裂韧性得到明显的提高,并且在碳纳米管含量为0.5wt%左右时,复合材料的硬度达到20.5GPa,弯曲强度为496MPa,断裂韧性达7.25MPa·m1/2;断口形貌分析表明碳纳米管主要分布于TiB2颗粒的晶界处,复合材料的增韧机制主要是碳纳米管的拔出机制和桥联机制.     

Dependence of the electrical and optical properties of sputter-deposited ZnO:Ga films on the annealing temperature,time, and atmosphere

Effects of annealing process parameters such as annealing temperature, time, and atmosphere on the electrical resistivity and transmittance properties of Ga-doped ZnO (ZnO:Ga) thin films deposited on glass by rf magnetron sputtering were investigated. The electrical resistivity of a ZnO:Ga thin film is effectively decreased with increasing annealing temperature and time in a reducing atmosphere such as N₂ + 5%H₂. This is attributed to passivation of grain boundaries and zinc ions by hydrogen atoms resulting in increases in carrier concentration and mobility. Also the resistivity of 4.9 × 10⁻⁴Ω cm was obtained by annealing at 200°C for 15 h in the same atmosphere, which is not bad for a transparent conductor for solar cell applications. However, annealing at a temperature higher than 400°C is less effective. The lowest resistivity of 2.3 × 10⁻⁴Ω cm was obtained by annealing at 400°C for 1 h in an N₂ + 5%H₂ atmosphere. The optical transmittance of the ZnO:Ga film is improved by annealing regardless of the annealing atmosphere. Annealing in N₂ + 5%H₂ atmosphere widens the optical band gap, while annealing in an O₂ atmosphere makes the band gap narrower, which can be explained as a blue shift phenomenon.     

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.