阻燃PA66复合材料翘曲性能研究

对比研究了无玻璃纤维、普通圆形玻璃纤维、扁平玻璃纤维对溴系阻燃聚酰胺66(PA66)复合材料的翘曲性能影响,分别从力学性能、结晶性能、收缩率和横向/纵向收缩率比等因素阐述复合材料翘曲性能.结果表明,相同阻燃剂含量条件下,不加玻璃纤维复合材料的结晶度最高,横向收缩率与纵向收缩率最大,但横向/纵向收缩率比最小,复合材料翘曲...     

Effects of Fiber Volume Fraction on Mechanical Properties of SiC-Fiber/Si3N4-Matrix Composites

The effects of fiber volume fraction on composite mechaniacl properties were examined in SiC-fiber-reinforced Si₃N₄ composites fabricated in our laboratories. Fiber volume fraction was found to have significant effects on important composite properties including failure mode, ultimate strength, matrix-cracking stress, fiber–matrix interfacial shear stress, and work-of-fracture. The composite mechanical properties were improved with increasing fiber volume fraction. However, when the fiber volume fraction was sufficiently large, the composite ultimate strength was degraded. This was related to fiber strength loss as a result of fiber damage from contact with surrounding fibers and abrasive matrix particles during hot pressing.     

Method for Production of α-Alumina Whiskers via Vapor-Liquid-Solid Deposition

α-alumina (α-Al₂O₃, corundum) fibers exhibit high thermal and chemical stability, as well as good mechanical properties, even at high temperatures. Such characteristics make them good candidates for use in composites. Nevertheless, very few methods of producing α-Al₂O₃ fibers are available. In the present work, we describe a method that uses aluminum pieces deposited on SiO₂ powder, in an argon atmosphere, at temperatures in the range 1300°–1600°C. The α-Al₂O₃ fibers are obtained via vapor-liquid-solid deposition. The novel addition of nickel and cobalt (or their oxides) allows the use of temperatures >1500°C, resulting in improved fiber production. We demonstrate that the metals do not contaminate the fibers produced in this way. Finally, we also estimate the tensile strength of the Al₂O₃ fibers produced through this method.     

Fracture Behavior of Directionally Solidified Y3Al5O12/Al2O3 Eutectic Fiber

The strength and fracture of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth technique. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The tensile strength and Weibull's modulus of the eutectic fibers were determined in the as-fabricated state and after extended thermal exposure at 1460°C in air. Fractographic analysis was used to identify and classify the strength-limiting mechanisms. The fracture toughness and crack growth behavior were characterized by an indentation technique. A fracture mechanics analysis was also used to establish the relationships between surface flaw size, tensile strength, and fracture toughness of the fiber.     

Mechanical Properties of Melt-Grown Alumina–Yttrium Aluminum Garnet Eutectics up to 1900 K

Alumina/yttrium aluminum garnet (YAG) eutectic rods of 1 mm in diameter were grown by the laser-heated floating zone method at different rates to obtain microstructures with the same morphology but of very different domain size. The mechanical properties of the rods (hardness, toughness, strength) were measured at ambient temperature in the longitudinal and transverse directions and, in addition, the longitudinal flexure strength was determined up to 1900 K. The fracture resistance and the hardness of the eutectics at ambient temperature were isotropic and independent of the domain size. On the contrary, the longitudinal strength was significantly higher than the transverse one and increased linearly with the growth rate, reaching almost 2 GPa in the rods grown at 750 mm/h, which presented a submicrometer homogeneous microstructure. The critical defect size was equivalent to that of Al₂O₃ and YAG domains in the microstructure, and the strength was proportional to the inverse of the square root of the domain size. In addition, the strength retention of the eutectics was remarkable, and the rods with the finest microstructure withstood 1.53 GPa at 1900 K. The moderate reduction in strength at very high temperature was induced by the homogeneous coarsening of the microstructure.     

Piezoelectric Fibers with Uniform Internal Electrode by Co-Extrusion Process

Piezoelectric fibers with internal electrodes were fabricated by the co-extrusion process. The initial feedrods, which were composed of an outer piezoelectric PZN–PZT layer, a thin conducting PZN–PZT/Ag layer inside, and fugitive carbon black at the center, were co-extruded through a reduction die (1 mm) to form a continuous fiber. After thermal treatment and sintering, the PZN–PZT/Ag layer became the inner electrode, while the carbon black at the center was removed by oxidation to form an empty space. Three different types of fibers were produced: (i) solid fiber filled with an inner electrode, (ii) hollow fiber clad with a uniform inner electrode, and (iii) hollow fiber clad with a partial inner electrode. The piezoelectric properties of the fibers were evaluated in terms of their longitudinal strain (s₃₁) or transverse displacement. When the dimensions of the fiber were 840 μm (outer diameter) × 420 μm (inner diameter) × 40 mm (length), the longitudinal strains of the solid fiber with the inner electrode and hollow fiber clad with the uniform inner electrode were 5.25 × 10⁻⁵ and 8.5 × 10⁻⁵ m/m, respectively, under an applied voltage of 100 V (0.48 kV/mm) at a frequency of 100 Hz. For the hollow fiber clad with a partial inner electrode with the same dimensions, the transverse displacement was 80 μm under the same applied electric field.     

Fabrication and Mechanical Properties of Si3N4/Carbon Fiber Composites with Aligned Microstructure Produced by a Seeding and Extrusion Method

Si₃N₄/carbon fiber composites have been produced with and without seeding by an extrusion and sintering process. In both cases the carbon fibers were aligned along the direction of extrusion, but the Si₃N₄ grains were only aligned in the seeded material. The mechanical properties of the specimens showed anisotropy with respect to the grain alignment, with both strength and toughness being highest in the direction parallel to the extruding direction. In this direction the seeded specimen, where both the Si₃N₄ grains and the carbon fibers were aligned, showed both higher fracture toughness and higher fracture strength than the nonseeded specimen where only the fibers were aligned.     

Mechanical Properties of Hot-Pressed TiB2-ZrO2 Composites

The use of monoclinic ZrO₂ as an additive improves the mechanical properties of TiB₂-based composites without the use of stabilizers. In particular, TiB₂-30% ZrO₂ compacts exhibited a transverse rupture strength of 800 MN/m², few pores, and a K_{I c} of 5 MPa·m^1/2. The high strength and toughness are thought to result mainly from the presence of partially stabilized tetragonal ZrO₂ and from solid solution of (TiZr)B₂ formed in sintering.     

Scanning Force Microscopy Study of Ceramic Fibers for Advanced Composites

The surface roughness and topography of three different Al₂O₃ fibers were evaluated using atomic force microscopy (AFM). The fibers used were as-received, refractory-metal coated, and Y₂O₃/refractory-metal duplex-layer coated. The refractory-metal coating and Y₂O₃ coating on the Al₂O₃ fibers increased the average surface roughness from 13.3 to 17.1 and 18.8 nm, respectively. The topographic image of the fibers evaluated by AFM was compared to that obtained by scanning electron microscopy. Lateral force microscopy (LFM) was used to measure the distribution of the friction force on the refractory-metal-coated Al₂O₃ surfaces, with friction coefficients ranging from 0.2 to 0.8; the average friction coefficient was 0.38. Tailoring the mechanical properties at fiber/matrix interfaces by surface modification of Al₂O₃ fibers to improve the overall mechanical properties of the composites also was proposed.     

Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Strengthening of Porous Alumina by Pulse Electric Current Sintering and Nanocomposite Processing

Porous Al₂O₃ and SiC–dispersed-Al₂O₃ (Al₂O₃/SiC) nanocomposites with improved mechanical properties were fabricated using pulse electric current sintering (PECS). Microstructures with fine grains and enhanced neck growth, as well as high fracture strength, could be achieved via PECS of Al₂O₃. The incorporation of fine SiC particles into an Al₂O₃ matrix significantly increased the fracture strength of porous Al₂O₃. Based on microstructural observations, it was revealed that the refinement of Al₂O₃ grains and neck growth occurred by PECS and nanocomposite processing.     

Carbon-Fiber-Reinforced Composites with Graded Carbon-Silicon Carbide Matrix Composition

Carbon-silicon carbide (C-SiC) graded-matrix composites were prepared via chemical vapor infiltration with carbon and SiC codeposition from methyltrichlorosilane (CH₃SiCl₃), acetylene (C₂H₂), argon, and hydrogen. The graded-matrix composites were prepared by gradually varying the source gases, from C₂H₂ to CH₃SiCl₃, in a semicontinuous process. Oxidation resistance, wear, and mechanical tests were conducted, and the microstructure was observed via optical microscopy, electron probe microanalysis, transmission electron microscopy, and scanning electron microscopy. The results show that this material seems to be macroscopically homogeneous, as an integral part of the entire material; however, microscopic examination shows a gradient variation of the matrix sheaths around each fiber. The oxidation resistance of the material is significantly superior to that of a C/C material, which is a promising thermostructural material, because of its low density, good mechanical properties, and good resistance to oxidation.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

Fiber Effects on Minicomposite Mechanical Properties for Several Silicon Carbide Fiber-Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems

Several different types of SiC fiber tows were coated with BN and composited using chemically vapor-infiltrated SiC to form single-tow minicomposites. The types of SiC fiber included Nicalon^™, Hi-Nicalon^™, and the new Sylramic^™ polycrystalline SiC fiber. The interfacial shear stresses were determined from unload–reload tensile hysteresis-loop tests. The ultimate stress and strain properties also were determined for the minicomposites. The ultimate strengths of the newer Hi-Nicalon and Sylramic fibers were superior to that of Nicalon minicomposites with similar fiber volume fractions. The Sylramic minicomposites had the lowest strain to failure and highest interfacial shear strength, respectively, because of the high modulus of the fiber and the rough surface of this fiber type. The apparent interfacial shear strength increased as the stress increased for the Sylramic minicomposites, which also was attributed to the surface roughness of this fiber.     

Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites and Its Effect on Fiber Strength

Microstructural changes at the interface were analyzed in two Nicalon-fiber ceramic-matrix composites with a dual BN/SiC coating on the fibers after thermal exposure at different temperatures (in the range 800°-1400°C) and in different environments (air and argon). The outer SiC coating acted as a barrier to oxygen, which penetrated into the composite via pipeline diffusion along the BN/fiber interfaces. Oxygen penetration led to the formation of an SiO₂ layer by oxidation of the fiber surfaces. The in situ fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO₂ layer severely degraded the fiber strength. Oxidation was highly dependent on the nature of the BN/fiber interface. The presence of a thin carbon-rich interlayer, which burned out rapidly at high temperature, favored the entry of oxygen and accelerated oxidation of the fibers.     

Optimization of the Mechanical Properties of Silicon Carbide-Fiber-Reinforced Zirconia Titanate Matrix Composites through Controlled Processing

The effects of processing parameters on the microstructure and mechanical behavior of a SiC-fiber-reinforced ZrTiO₄ matrix composite were evaluated through a controlled study. The microstructure was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical behavior was characterized by strength and toughness measurements, which were correlated with the microstructure of the composite. The optimized processing schedule developed included incorporation of CO-heat-treated BN-coated fibers, composite calcination at 530°C, and consolidation via hot-pressing at 1270°C and 17.25 MPa applied pressure in an atmosphere of CO at an overpressure of 111.5 kPa (1.1 atm). Use of this processing schedule improved in situ fiber strength and modified the fiber/matrix interfacial microstructure to ameliorate its sliding and debonding resistance, leading to a composite with average strength over 1 GPa and a toughness of 26 MPa.m^1/2.     

Aramid‐short‐fiber reinforced rubber as a tire tread composite

Mechanical and dynamic‐mechanical properties of a typical tire tread compound reinforced with one part aramid short fibers were investigated in order to predict the effects of fibers on tire tread performances such as rolling resistance and traction. Rubber processing, including mixing and extrusion, was performed in an industrial scale. Fiber orientation as a result of extrusion was evaluated quantitatively and qualitatively using mechanical anisotropy in swelling and scanning electron microscopy, respectively. Unidirectional tensile tests revealed higher modulus, but slightly lower strength and elongation at break for the composites stretched in the longitudinal (orientation) and transverse directions than those for the isotropic reference compound with no fiber. Dynamic mechanical thermal analysis showed that relative values of loss factor for the longitudinal and transverse composites and the reference compound depended on the state of polymer as glassy or rubbery. Therefore, a high loss factor at lower temperatures and a low loss factor at higher temperatures predicted a balanced improvement of tire traction and rolling resistance as a result of fiber addition. Heat build‐up and abrasion experiments showed that addition of fiber did not deteriorate other performances of tire tread. Also, the fibers had negligible effects on processing and vulcanization characteristics of the composite. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Stress Corrosion Cracking of Single-Crystal Tetragonal ZrO2(Er2O3)

The flexure strength of partially-stabilized tetragonal ZrO₂(Er₂O₃) single-crystal monofilaments manufactured by the laser-heated floating zone method was measured as a function of the environment (air versus water) and temperature (from 25° to 800°C) at loading rates spanning three orders of magnitude to ascertain their susceptibility to the environmental conditions. These mechanical tests were completed with parallel tests on fully annealed monofilaments (to relieve the thermal residual stresses induced during growth) and by detailed analysis of the fracture surfaces using scanning electron microscopy and micro-Raman spectroscopy. While environmental susceptibility of ZrO₂(Y₂O₃) in previous investigations was always associated with the destabilization of the tetragonal phase, monoclinic phase was not detected on the fracture surfaces of the ZrO₂(Er₂O₃) monofilaments and it was concluded that slow crack growth in this material at high temperature or immersed in water was due to stress corrosion cracking.     

Combined Effect of Salt Water and High-Temperature Exposure on the Strength Retention of Nextel™720 Fibers and Nextel™720-Aluminosilicate Composites

The relative contribution of fiber strength loss to reported degradation in the mechanical behavior of Nextel^™720-aluminosilicate composites after exposure to salt fog (ASTM B117) was explored. Single filament tension tests were performed on Nextel^™720 (3M, Inc., Minneapolis, MN) fibers after immersion in NaCl solutions followed by high-temperature exposure in air. The results were compared with the behavior of control specimens which received high-temperature exposure but were not immersed in NaCl solution. There was no degradation in fiber strengths for NaCl solutions below 1 wt%. However, significant degradation was observed at 5 wt% NaCl upon exposure to temperatures between 900° and 1150°C, while no degradation was observed upon an exposure to 1200°C. The relative contribution of fiber strength loss to composite degradation was estimated as nearly 50%, indicating that both fibers and matrix/interface degrade from exposure to salt water. X-ray diffraction and transmission electron microscopy of the exposed fibers and composites were conducted to help rationalize the observations. Microstructure of degraded fibers showed presence of Na at grain boundaries near the surface, without any evidence of a crystalline phase, indicating weakening from segregation or formation of an amorphous phase. The degraded composites showed that matrix and fiber/matrix interfaces had Na rich regions/phases.     

Tensile and Stress-Rupture Behavior of SiC/SiC Minicomposite Containing Chemically Vapor Deposited Zirconia Interphase

The tensile and stress-rupture behavior of SiC/SiC minicomposite containing a chemically vapor deposited (CVD) ZrO₂ interphase was evaluated. Fractographic analyses showed that in situ fiber strength and minicomposite failure loads were strongly dependent on the phase contents and microstructure of the ZrO₂ interphase. When the ZrO₂ interphase structure possessed a weakly bonded interface within the dense ZrO₂ interphase coating layer, the interphase sufficiently protected the fiber surface from processing degradation and promoted matrix crack deflection around the fibers. With this weakly bonded interphase, the stress-rupture properties of SiC/SiC minicomposite at 950° and 1200°C appeared to be controlled by fiber rupture properties, and compared favorably to those previously measured for state-of-the-art BN fiber coatings.