Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Mechanical properties of epoxy composites reinforced with a low volume fraction of nanosilica fillers

In this paper we focus on the preparation and mechanical properties of the nanosilica-reinforced, epoxy resin Epikote 828LVEL. Epoxy composites containing two sizes of spherical silica nanoparticles, 130 nm and 30 nm, were prepared at a fixed volume fraction (V_P = 0.5%). To prevent agglomeration, the silica fillers were initially pre-treated with diglycidyl ether of bisphenol A (BADGE). Due to the low content of silica fillers, their inclusion in the matrix was confirmed by the increased roughness of a fracture surface compared to the smooth surface of the neat epoxy. Raman spectroscopy was employed to obtain additional information about the crack-propagation path. The mechanical properties, characterized by a three-point bending test, revealed a 10–20% increase in the composite's modulus of elasticity with 30-nm and 130-nm silica-filler inclusions. Elongation at break, on the other hand, decreased for 5–10% in both composites compared to neat epoxy, suggesting brittle fracture behavior in silica/epoxy composites. The fracture toughness results showed a 25–30% improved toughening for both composites compared to the pure epoxy. The composite's resistance to failure in terms of the impact energy was, however, strongly dependent on the size of the silica: we observed a 30% increase for the 130-nm, and a 60% increase for the 30-nm, silica/epoxy composites, compared to the pure epoxy.     

Hydroxyapatite as a filler for dental composite materials: mechanical properties and in vitro bioactivity of composites

Hydroxyapatite (HAp) powder was treated with -methacryloxypropyltrimethoxy-silane (-MPS) using standard techniques in both non-polar and polar systems. Infrared spectra (DRIFT) and thermogravimetric analysis (TGA) confirmed the presence of -MPS on the surface of the HAp filler particles. Series of experimental composites consisting of bisphenol-a-glycidyl methacrylate (BisGMA) based resin and untreated or treated HAp filler were produced to determine the mechanical properties and in vitro bioactivity. The incorporation of HAp filler into the BisGMA base resin had an enhancing effect on the flexural strength and Young's modulus of the base resin, the latter being increased by a factor of three. The mechanical properties of the filled resin were not affected by the surface treatment of the HAp, but filler loading was found to have a significant effect on Young's modulus. Higher proportions of silane-treated HAp of smaller particle size could be incorporated in the monomer phase giving rise to composites of higher stiffness. Examination of the fracture surfaces showed that the silanized HAp particles maintained better contact with the polymer matrix. In vitro study revealed that the composites incorporating silanized HAp formed a compact and continuous calcium phosphate layer on their surface after 4 weeks immersion in a simulated body fluid (SBF). © 2001 Kluwer Academic Publishers     

改性酚醛树脂处理剂对石英纤维增强芳基乙炔复合材料性能的影响

采用改性酚醛树脂作为石英纤维表面处理剂来提高石英纤维增强芳基乙炔复合材料(SF/PAA)界面性能。通过性能测试,研究处理剂对力学性能和介电性能的影响。通过XPS和SEM分析方法,研究了酚醛树脂表面处理剂对复合材料界面官能团变化和微观形貌的影响。性能测试结果表明改性酚醛树脂处理剂可以显著提高PAA复合材料的力学性能和介电性能。XPS分析结果表明酚醛树脂处理后的石英纤维表面与酚醛树脂发生了化学反应,SEM研究表明酚醛树脂处理后的复合材料界面粘结性能得到显著提高。     

微纳米SiO2/PP复合材料增强增韧的实验研究

为了研究无机刚性颗粒对通用塑料聚丙烯 (PP) 的力学性能的影响, 采用熔融共混方法制备了经硅烷偶联剂A-151处理的SiO₂/PP 复合材料, 并通过其缺口冲击、 拉伸、 弯曲试验和冲击断面的形貌观察, 分析研究了微纳米SiO₂颗粒大小、 填充量、 表面改性以及不同颗粒大小SiO₂混合物对PP复合材料增韧、 增强效果的影响。实验结果表明: 纳米SiO₂的加入可以同时改善其韧性、 刚性和强度; 填充量相同, 颗粒越细, SiO₂/PP复合材料的力学性能越好。SiO₂经改性后填充到PP基体中, 明显改善了颗粒在基体中的分散性及基体与颗粒之间界面结合性能, 使复合材料的综合力学性能得到提高。不同颗粒大小的SiO₂混合后填充到PP基体中, 混合SiO₂的协同效应使复合材料拉伸、 弯曲性能进一步提高, 对PP基体具有更好的增强效果, 但其冲击性能下降。      

Effect of particle size and weight fraction on the flexural strength and failure mode of TiO2 particles reinforced epoxy

Effect of inclusions size and weight fraction on flexural strength and failure mode of composite containing SC-15 epoxy resin and TiO₂ particles has been studied in this investigation. The sizes of particles varied from macro (0.02 mm) to nano (5 nm) scale, and these particles were infused into the part-A of SC-15 through sonic cavitations and then mixed with part-B of SC-15 by using a high speed mechanical agitator. Three-point bending tests were performed on unfilled, 0.5 wt.%, 1.0 wt.% and 1.5 wt.% particles filled SC-15 epoxy to identify the loading effect on mechanical properties of the composites. Results show that 1.0 wt.% nanoparticles reinforced epoxy exhibit the highest mechanical performance. Higher than 1.0%, strength of composite decreased because of poor dispersion. Experimental results also shown that micro-sized particles have little effect on strength of epoxy at such low loading, and strength of composite increased as the size of particles decreased to nano scale. However, degradation in strength was found in 5 nm TiO₂/epoxy system due to agglomeration.     

Preparation and characterization of epoxy nanocomposites by using PEO-grafted silica particles as modifier

EP/SiO₂ nanocomposites, which contained PEO flexible chain, have been prepared via epoxy resin and PEO-grafted silica particles. The PEO-silica particles were obtained by endcapping PEO-1000 with toluene 2,4-diisocyanate (TDI), followed by a reaction with silica sols. The chemical structure of the products was confirmed by IR measurements, and the mechanical properties of composites such as impact strength, flexural strength, dynamic mechanical thermal properties were investigated. The results showed that the addition of the PEO-grafted silica particles to the epoxy/DDS curing system, the impact strength is 2 times higher than that of the neat epoxy. While the storage modulus and the glass transition temperature are a little changed. The morphological structure of impact fracture surface and the surface of the hybrid materials were observed by scanning electron microscope (SEM) and atomic force microscopy (AFM), respectively.     

Effect of thermal cycling on whisker-reinforced dental resin composites

The mechanical properties of dental resin composites need to be improved in order to extend their use to high stress-bearing applications such as crown and bridge restorations. Recent studies used single crystal ceramic whiskers to reinforce dental composites. The aim of this study was to investigate the effects of thermal cycling on whisker-reinforced composites. It was hypothesized that the whisker composites would not show a reduction in mechanical properties or the breakdown of whisker–resin interface after thermal cycling. Silicon carbide whiskers were mixed with silica particles, thermally fused, then silanized and incorporated into resin to make flexural specimens. The filler mass fraction ranged from 0% to 70%. The specimens were thermal cycled in 5 °C and 60 °C water baths, and then fractured in three-point bending to measure strength. Nano-indentation was used to measure modulus and hardness. No significant loss in composite strength, modulus and hardness was found after 10⁵ thermal cycles (family confidence coefficient=0.95; Tukey's multiple comparison test). The strength of whisker composite increased with filler level up to 60%, then plateaued when filler level was further increased to 70%; the modulus and hardness increased monotonically with filler level. The strength and modulus of whisker composite at 70% filler level were significantly higher than the non-whisker controls both before and after thermal cycling. SEM revealed no separation at the whisker–matrix interfaces, and observed resin remnants on the pulled-out whiskers, indicating strong whisker–resin bonding even after 10⁵ thermal cycles. In conclusion, novel dental resin composites containing silica-fused whiskers possessed superior strength and modulus compared to non-whisker composites both before and after thermal cycling. The whisker–resin bonding appeared to be resistant to thermal cycling in water, so that no loss in composite strength or stiffness occurred after prolonged thermal cycling.     

Low shrinkage light curable dental nanocomposites using SiO2 microspheres as fillers

Commercial resin matrixes of dental composites generally utilize diluents such as triethylene glycol dimethacrylate (TEGDMA) to reduce viscosity. However, the diluents exhibited adverse effects such as higher volume shrinkage and diminished mechanical properties of the dental composites. To overcome these adverse effects, developing of both inorganic fillers and resin monomers is necessary to improve the properties of dental composite. In this work, monodispersed silica microspheres with a diameter of 400 nm were synthesized via the Stöber process. The as-prepared particles were silanized with 3-methacryloxypropyltrimethoxysilane (γ-MPS) and used as fillers. Additionally, ethoxylated bisphenol A dimethacrylate (EBPADMA) with lower viscosity and higher molecular mass was introduced as a base resin monomer, which could be used as resin matrixes with a low amount of diluent. Various resin mixtures of EBPADMA, bisphenol A diglycidyl dimethacrylate (Bis-GMA) and TEGDMA were prepared, which had a similar filler content (71 wt.%), and their mechanical properties, volume shrinkage, depth of cure and light transmission were examined. Among them, the resin mixture containing 70% EBPADMA and 30% TEGDMA exhibited the best compression strength (238.1 ± 5.4 MPa), depth of cure (4.02 ± 0.04 mm) and the lowest volume shrinkage (2.27%).     

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Interfacial control of uni-directional SiC_f/SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC_f/SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC_f/SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC_f/SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC_f/SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC_f/SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m². In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC_f/SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC_f/SiC composites and to obtain high-performance SiC_f/SiC composites.     

Development of aligned-hemp yarn-reinforced green composites with soy protein resin: Effect of pH on mechanical and interfacial properties

Unidirectional hemp yarn-reinforced green composites were fabricated with soy protein concentrate (SPC) resin processed at various pH values. To preserve the yarn alignment during the fabrication of green composites, hemp yarn was wound onto a metal frame with slight tension and precured SPC resin was applied to the yarns. Effects of pH values on the tensile properties of the SPC resin and hemp yarn/SPC resin interfacial shear strength (IFSS) were investigated. Increasing pH of the SPC resin from 7 to 12 decreased its fracture stress and Young’s modulus from 13.1 MPa and 357.5 MPa to 8.1 MPa and 156.2 MPa, respectively. At the same time fracture strain and moisture content increased from 31.5% and 15.65% to 53·4% and 19.30%, respectively, indicating resin plasticization. However, hemp yarn/SPC resin IFSS increased from 17.7 MPa at pH 7 up to 28.0 MPa at pH 10, after which it decreased. The fracture toughness of the composites increased up to pH of 10 but further increase in pH reduced the toughness. SEM photomicrographs showed fracture surfaces of hemp yarn-reinforced green composites that indicated better resin/fiber interaction at pH of 10 than 7 or 12.     

Biomicrofibrilar composites of high density polyethylene reinforced with curauá fibers: Mechanical,interfacial and morphological properties

Vegetal fibers are used in polymer composites to improve mechanical properties, substituting inorganic reinforcing agents produced by non renewable resources, like fiberglass. The highest performance formulation in high density polyethylene, HDPE, composites reinforced with curauá fibers were studied, aiming to improve the interphase interaction and optimize the mechanical properties. The fiber content, the type and the concentration of coupling agent were tested. The composites and the pure materials were characterized by Fourier transform infrared spectroscopy and the fiber/matrix phase adhesion was evaluated by scanning electron microscopy. The mechanical properties and the micrographs showed that the best formulation is: 20 wt.% of milled curauá fibers and 2 wt.% poly(ethylene-g-maleic anhydride). The coupled composites are also less hygroscopic than the uncoupled composites. We conclude that the composites reinforced with curauá fibers have mechanical properties comparable to commercially produced composites of HDPE reinforced with fiberglass.     

Hot-pressed A12O3–SiC(–C) composites with polycarbosilane as the precursor

The processing and mechanical behavior of Al₂O₃–xSiC (–C) (x = 1, 2, 5, 10 wt.%, ASx and ASCx) composites prepared by in situ reaction synthesis SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The pyrolysis process of PCS, microstructure, phase structure and mechanical properties of sintered composites were analyzed. Fully dense structure was obtained, and it was found that the fracture toughness and strength were highly improved compared with monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in 1 wt.%SiC composite ASC1. AS1 showed 516 MPa of flexural strength.     

Effect of additive content on transient liquid phase sintering in SiC nanopowder infiltrated SiCf/SiC composites

SiC nanopowder infiltrated SiC_f/SiC composites with a high fiber volume fraction above 50 vol.% were prepared at a relatively low fabrication temperature of 1800 °C by transient liquid phase sintering using Al₂O₃-Y₂O₃-SiO₂ additives. The effects of additive content with 6-18 wt.% were investigated, based on densification, microstructure, mechanical properties and fracture behaviors of the composites. The results showed that the densification and mechanical properties of the composites were greatly improved with the additive content. Microstructural observation indicated that the infiltration of SiC nanopowder inside fiber-bundles were enchanced with the increase of additive content due to the effectively widen space by the reaction between pyrocarbon (PyC) interface and the additives especially with the addition of SiO₂. It proven that the enchanced matrix-PyC interface bonding by the high densification inside fiber-bundles played a key role in the improved mechanical properties and fracture behaviors of the composites.     

Mechanical properties of tri-modal Al matrix composites reinforced by nano- and submicron-sized Al2O3 particulates developed by wet attrition milling and hot extrusion

Among all metal matrix composites, aluminum is the most widely used matrix due to its low density coupled with high stiffness. In this study, aluminum matrix composites reinforced by two sizes of alumina particles (35 nm and 0.3 μm) are prepared by wet attrition milling and hot forward extrusion processes. The effect of the ratio of the nano- to submicron-sized particles (2:8, 3:7, 4:6, 5:5, and 6:4 in weight percent) on mechanical properties of the composites is evaluated by micro-hardness and tensile tests. It is found that by increasing the nanoparticles content, the hardness and strength of the composites first increase and then decrease when the amount of the nanoparticle exceeds 4 wt.%. The tensile fracture surfaces are also observed by scanning electron microscopy.     

Effects of particle size and distribution on the mechanical properties of SiC reinforced Al-Cu alloy composites

This paper studied the combined effects of particle size and distribution on the mechanical properties of the SiC particle reinforced Al-Cu alloy composites. It has been shown that small ratio between matrix/reinforcement particle sizes resulted in more uniform distribution of the SiC particles in the matrix. The SiC particles distributed more uniformly in the matrix with increasing in mixing time. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength and elongation. Yield strength and ultimate tensile strength of the composite reinforced by 4.7 μm sized SiC particles are higher than those of composite reinforced by 77 μm sized SiC particles, while the elongation shows opposite trend with yield strength and ultimate tensile strength. Fracture surface observations showed that the dominant fracture mechanism of the composites with small SiC particle size (4.7 μm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, while the dominant fracture mechanism of the composites with large SiC particle size (77 μm) is ductile fracture of the matrix, accompanied by the SiC particle fracture.     

ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing

ZrB₂–Nb (ZN) composites were prepared through hot-pressing at a temperature of 1800 °C. A contribution of Nb was believed a significant influence on the sinterability, microstructure and mechanical properties of ZN composites. The values of flexural strength of ZN composites rang from 395 to 773 MPa, who are dependent on Nb contents. The highest strength obtained for the ZN composite containing 25 vol.% Nb (773 MPa). A fracture toughness of 7.1 MPa m^1/2 of ZN was revealed, which was much higher than that of monolithic ZrB₂. The improvement in fracture toughness strongly depended on an introduction of Nb–ZrB₂ matrix. Crack deflection and branching were believed to be the toughening mechanism of ZN.     

Fabrication and mechanical properties of porous TiB2 ceramic

Porous material of TiB₂ with improved mechanical properties was fabricated by vacuum and pressureless sintering. The microstructure of the porous ceramic was characterized by the enhanced neck growth between the initially touching particles. This neck growth was ascribed to the selective heating of TiB₂ particles with different dimension. The porous structure prepared by the high-temperature sintering exhibited higher bending strength and fracture toughness in the present experiment. The improved mechanical properties of the sintered composites were attributable to the enhanced neck growth by surface diffusion.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Preparation and properties of unidirectional boron nitride fibre reinforced boron nitride matrix composites via precursor infiltration and pyrolysis route

The unidirectional boron nitride fibre reinforced boron nitride matrix (BN_f/BN) composites were prepared via the precursor infiltration and pyrolysis (PIP) route, and the structure, composition, mechanical and dielectric properties were studied. The composites have a high content and fine crystallinity of BN. The density is 1.60 g cm⁻³ with a low open porosity of 4.66%. The composites display good mechanical properties with the average flexural strength, elastic modulus and fracture toughness being 53.8 MPa, 20.8 GPa and 6.88 MPa m^1/2, respectively. Lots of long fibres pull-out from the fracture surface, suggesting a good fibre/matrix interface. As temperature increases, both of the flexural strength and elastic modulus exhibit a decreasing trend, with the lowest values being 36.2 MPa and 8.6 GPa at 1000 °C, respectively. The desirable residual ratios of the flexural strength and elastic modulus at 1000 °C are 67.3% and 41.3%, respectively. The composites have excellent dielectric properties, with the average dielectric constant and loss tangent being 3.07 and 0.0044 at 2-18 GHz, respectively.