Toughening alumina with nickel aluminide inclusions

Both ceramics and intermetallics are potential materials for high temperature applications. Recent studies indicated that the toughness of ceramics can be improved by adding intermetallics. Among the intermetallics studied, NiAl is an attracting material for its low density and high melting point. In the present study, the toughening behaviour of Al₂O₃–NiAl composites is investigated. In order to determine the contribution from the matrix and reinforcement, the composites containing 0–100 vol% NiAl are prepared by hot-pressing. The toughness of the Al₂O₃–NiAl composites is higher than the values predicted by the rule of mixtures, i.e. the contribution from only the matrix and reinforcement. The toughness enhancement is contributed by the combination of crack deflection and the plastic deformation of NiAl grains. The toughening mechanism for the Al₂O₃-rich composites is mainly crack deflection. The contribution of the plastic deformation of NiAl to toughening effect increases with increasing NiAl content. It is due to the fact that more NiAl grains are interconnected with each other as NiAl content is increased.     

Whisker Toughening: A Comparison Between Aluminum Oxide and Silicon Nitride Toughened with Silicon Carbide

Two whisker-toughened materials have been studied, with the objective of identifying the mechanisms that provide the major contribution to toughness. It is concluded that, for composites with randomly oriented whiskers, bending failure of the whiskers obviates pullout, whereupon the major toughening mechanisms are the fracture energy consumed in creating the debonded interface and the stored strain energy in the whiskers, at failure, which is dissipated as acoustic waves. The toughening potential is thus limited. High toughness requires extensive pullout and, hence, aligned whiskers with low fracture energy interfaces.     

Improved fracture resistance and toughening mechanisms of GNPs reinforced ceramic composites

The R-curve behavior and toughening mechanisms of graphene nano-platelets (GNPs) reinforced ceramic composites are investigated. A toughening model is developed with the consideration of interface debonding, crack bridging and pull-out of GNPs, which can be used to quantify the contribution of different mechanisms to the improved toughness of ceramic composites. The theoretical results agree well with the experimental data when GNPs homogeneously dispersed in ceramic matrix. All prepared GNPs/ceramic composites exhibit a raising R-curve behavior owing to the toughening mechanisms induced by GNPs, and the curve becomes steeper with increasing GNPs content, indicating that the fracture resistance and flaw tolerance are improved. The dominant toughening mechanism is GNPs pull-out, which is followed by crack bridging and interface debonding. Furthermore, the analytical model suggests that improving GNPs properties, interfacial sheer strength and reducing GNPs thickness can improve the fracture toughness of ceramic composites.     

Toughening Behavior Involving Multiple Mechanisms: Whisker Reinforcement and Zirconia Toughening

Since the contribution of transformation toughening increases with the loca crack resistance (which is proportional to the toughness of the matrix), ZrO₂ particles were added to a toughened, whisker-reinforced ceramic matrix. Analysis revealed that the combination of these multiple toughening agents should result in ceramic composites tougher than (1) that achieved by either mechanism by itself or (2) the sum of the two processes. The toughness of mullite could be increased 1.8-and 2.4-fold with a 20 vol% addition of ZrO₂ particles or SiC whiskers, respectively. However, when 20 vol% of both ZrO₂ particles and SiC whiskers were added, the toughness was increased at least 3-fold with monoclinic m -ZrO₂ and by >5-fold with tetragonal t -ZrO₂. The differences in the toughening achieved when t -ZrO₂ vs m -ZrO₂ is present in the SiC-whisker-reinforced mullite are attributed to differences in their interdependencies upon the whisker reinforcement.     

Toughening of Glass by a Piezoelectric Secondary Phase

A toughening concept for glass, based on exploiting the ferroelastic effect of piezoelectric particles embedded in a glass matrix, is described. It is hypothesized that the domains within a piezoelectric phase will align themselves in the direction of the stress field around an advancing crack, thus absorbing energy and contributing to toughening. A powder technology route was optimized to fabricate lead-containing glass-matrix composites with up to 30 wt% of a lead zirconate titanate (PZT) particulate phase. An increase in fracture toughness of >50% was achieved via the addition of 30 wt% of PZT particles. Although other toughening mechanisms could be excluded in the present composites, the actual contribution of piezoelectric toughening remains under investigation.     

Toughening alumina with silver and zirconia inclusions

Both silver and zirconia inclusions are added into an alumina matrix, the strength and toughness of the composites are determined. The toughening agents prohibit the grain growth of the matrix, the strength of alumina is, therefore, enhanced. The addition of two toughening agents also enhances the toughness of alumina. The presence of Ag inclusions raises the transformation ability of ZrO₂; however, the toughness increase of the Al₂O₃–ZrO₂–Ag composites is slightly lower than the sum of the toughness increase of Al₂O₃–ZrO₂ and of Al₂O₃–Ag composites. The present study demonstrates that the toughening effects contributed by a transformation toughening agent and a ductile toughening agent can interact with each other; nevertheless, such interaction depends strongly on the microstructure of the composites.     

TiC颗粒弥散Al2O3复合材料的阻力曲线行为

研究了ＴｉＣ颗粒弥散ＡＩ２Ｏ３复合材料的阻力曲线行为,发现ＴｉＣ颗粒对ＡＩ２Ｏ３基体的增韧效果在很大程度上取决于ＴｉＣ颗粒的尺寸,在ＴｉＣ颗凿尺寸较小的情况下,复３事材料的断裂韧性与ＡＩ２Ｏ３着ＴｉＣ颗粒尺寸的增大而增大,本工作 地裂纹扩展途径的观察,简要讨论了ＴｉＣ颗粒弥散ＡＩ２Ｏ３复合材料中的增韧机制。     

Transformation Toughening and Fracture Behavior of Molybdenum Disilicide Composites Reinforced with Partially Stabilized Zirconia

The effects of yttria stabilization (0–6 mol%) on the fracture toughness of molybdenum disilicide composites reinforced with 20-vol%-yttria-stabilized zirconia particles are elucidated. Fracture toughness tests were conducted under three-point bend loading using single-edge-notched specimens. The stress-assisted martensitic phase transformation of zirconia associated with the fracture process was then studied using optical interference microscopy and laser Raman spectroscopy. Stress-induced martensitic transformation from tetragonal to monoclinic phase was observed only in the plastic wake of the material stabilized with 2 mol % yttria. The degree of toughening in this composite was also predicted using micromechanical models that assess the combined effects of transformation toughening and crack deflection. However, the fracture toughness of the 2-mol%-yttria-stabilized composite was in the same range as those of the other composites that did not exhibit evidence of transformation toughening. The toughening in the other composites is explained by considering the effects of crack deflection, residual stresses, and microcracking induced by residual stresses that occur as a result of the thermal expansion mismatch between the matrix and the reinforcements.     

纳米CaCO3和POE增韧废旧PP复合材料的研究

采用了纳米CaCO3和乙烯-辛烯共聚物（POE）对废旧聚丙烯（PP）进行增韧改性，借助于力学性能测试、SEM和偏光显微镜等观察手段对这一共混体系的增韧机理进行了研究。结果表明，纳米CaCO3和POE对废旧PP具有良好的增韧作用，两者有协同增韧效果；POE对废旧PP的增韧符合剪切屈服理论，纳米CaCO3的增韧机理是诱导PP产生大量的裂纹，形成空穴群，吸收冲击能；废旧PP／POE／纳米CaCO3复合材料的球晶尺寸细化，球晶边界模糊，非晶区域增大，材料的韧性明显提高。     

聚合物纳米复合材料韧性和破坏行为

在总结高分子材料增韧机理、高分子纳米复合材料冲击破坏行为的基础上,探讨了高分子纳米复合材料的增韧机理。纳米无机粒子起应力集中的作用导致界面脱黏、空化与基体屈服是其增韧高分子材料的主要原因,而碳纳米管则起桥联裂纹、偏转裂纹方向、传递界面应力使聚合物基体屈服而增韧高分子材料。对聚合物／层状填料纳米复合材料而言,分散在聚合物基体中的插层或剥离的无机纳米片层对复合材料银纹的形成有抑制作用,其二维几何结构不利于片层周围基体的屈服与界面脱黏、空化,因而不能增韧高分子材料,反而还导致了聚合物／层状填料纳米复合材料抗冲击强度的降低。高分子材料的纳米复合目前很难达到橡胶增韧的效果,若同时采用纳米复合技术与官能化弹性体增韧技术,可望设计、制备出一系列高强度、高韧性的高分子纳米复合材料。     

Toughness Anisotropy in Textured Ceramic Composites

In this investigation, a model predicting toughness anisotropy in textured ceramics containing elongated grains and in composites reinforced with rod-shaped particles is presented. The model predictions are based on the assumption that crack deflection is the only toughening mechanism. In the model, toughness anisotropy is calculated as a function of texture degree. For composite materials, the volume fraction of the reinforcement phase is also an input parameter. Correspondence between model and experiment was established by comparing measured toughness anisotropies in β-Si₃N₄ and Al₂O₃/SiC whisker composites to model predictions. In these model predictions, measured orientation distributions from hot-pressed and hot-forged specimens were employed. The potential for relating other toughening mechanisms in a similar format is also addressed, since the model and experimental measurements give different results. The crack deflection model simultaneously overpredicts the toughening enhancement and underpredicts the toughening anisotropy observed in the experiments.     

Effect of carbon particle and carbon fiber on the microstructure and mechanical properties of short fiber reinforced reaction bonded silicon carbide composite

Variation of microstructure and mechanical behavior was investigated with the content increase of carbon particles and carbon fiber in the reaction bonded silicon carbide composites. The composites were prepared by slip casting and liquid silicon infiltration. The bulk density is raised with the increase of carbon black due to the formation of fine β-SiC particles. The flexural strength increases for the reduction of residual Si and the formation of SiC framework; whereas a very high carbon content reduces the flexural strength. The fracture toughness is controlled by the contents of carbon particle and carbon fiber. Thus, fiber debonding, fiber pullout and crack deflection are considered as the main toughening mechanisms. Annealing treatment can effectively improve both the flexural strength and fracture toughness. An increase by 49% of fracture toughness is obtained. A series of structural models are proposed to illustrate the structure changes of carbon fiber.     

Hard,tough and strong ZrO2–WC composites from nanosized powders

Yttria-stabilised zirconia (Y-TZP) based composites with a tungsten carbide (WC) content up to 50 vol.% were prepared from nanopowders by means of conventional hot pressing. The mechanical properties were investigated as a function of the WC content. The hardness increased from 12.3 GPa for pure Y-TZP up to 16.4 GPa for the composite with 50 vol.% WC, whereas the bending strength reached a maximum of 1551 MPa for the 20 vol.% WC composite. The toughness of the composites could be optimised by judicious adjustment of the overall yttria content by mixing monoclinic and 3 mol% Y₂O₃ co-precipitated ZrO₂ starting powders. An optimum fracture toughness of 9 MPa m^1/2 was obtained for a 40% WC composite with an overall yttria content of 2 mol%. The hardness, strength as well as fracture toughness of the ultrafine grained composites with a nanosized WC source was significantly higher than with micron-sized WC. The experimentally measured contribution of the different observed toughening mechanisms was evaluated as a function of the WC content. Transformation toughening was found to be the major toughening mechanism in ZrO₂–WC composites with up to 30 vol.% WC, whereas the contribution of crack deflection and bridging is significant at a secondary phase content above 30 vol.%.     

Fracture and toughening behavior of aramid fiber/epoxy composites

The effect of short Aramid fibers on the fracture and toughening behavior of epoxy with high glass transition temperature has been studied. Fine dispersion of the fibers throughout the matrix is evidenced by optical microscopy. Compared with neat epoxy resin, the fracture toughness (K_IC) of the composites steadily increases with increasing fiber loading, indicating that addition of Aramid fibers has an effective toughening effect to the intrinsically brittle epoxy matrix. Scanning electron microscopy (SEM) indicates that formation of numerous step structures for fiber‐filled epoxy systems is responsible for the significant toughness improvement. SEM and transmitted optical microscopy show that fiber pullout and fiber breakage are the main toughening mechanisms for the Aramid fiber/epoxy composites. POLYM. COMPOS. 26:333–342, 2005. © 2005 Society of Plastics Engineers.     

Transformation Toughening in Sol–Gel-Derived Alumina-Zirconia Composites

The microstructures of α-Al₂O₃ seeded sol–gel-derived alumina-zirconia composites containing 20 wt% unstabilized zirconia (processed from zirconium n-propoxide) were very fine, with submicrometer alumina grains and small, mainly intergranular zirconia particles, the latter having a critical size for the tetragonal-to-monoclinic phase transformation of 0.45 μm. The corresponding ratios of the toughening contribution to the matrix toughness are relatively low (δK_c/K₀<1). This finding is confirmed by an analysis of the tetragonal zirconia particle size dependence of the stress-induced transformation toughening.     

Al2O3增强ZrO2陶瓷的制备及性能研究

本文采用热分解法制备Al2O3微粉、化学共沉淀法制备(Y,Ce)—ZrO2超细粉,通过适当工艺制备出ZrO2／Al2O3复合陶瓷。经研究发现,添加Al2O3,可抑制ZrO2晶粒的长大,提高基体的强度和韧性。当Al2O3含量达到30%(质量分数)时,复合陶瓷的抗弯强度为986MPa,断裂韧性为13.7MPa*m1／2。材料性能的提高可归结为Al2O3颗粒的弥散增韧和ZrO2陶瓷的相变增韧叠加作用的结果。     

Microstructure and Mechanical Properties of Mullite–Silicon Carbide Composites

Mullite-SiC-whisker composites were prepared by powder processing using two commercial SiC whiskers. These composites were prepared by sintering rather than hot-pressing. A mulliteSlC-powder composite and a base line mallite material were also prepared for comparison with the two whisker composite materials. Fracture toughness measurements showed significant enhancement in only one of the whisker composite materials. The microstructure of the four materials was examined by scanning electron microscopy and transmission electron microscopy to assist in the explanation of the mechanical behavior of these composites. The examinations suggested that most of the toughening results from second-phase particles, with only limited toughening from effects associated with whiskers per se. In one case, higher toughness was partially associated with the formation of sialon phase by reaction with the whiskers and the furnace environment.     

Comprehensive Study on Using VTBN Reactive Oligomer for Rubber Toughening of Epoxy Resin and Composite

While vinyl-terminated butadiene acrylonitrile is frequently used for the toughening of vinylester and polyester, very limited research has been conducted on modification of epoxy with this oligomer. Herein, the effect of vinyl-terminated butadiene acrylonitrile addition to epoxy in bulk and glass reinforced composite is systematically investigated. Thermo-physical behavior and mechanical characteristics of the samples are determined. To interpret the test results, the void content of reinforced samples is measured and fracture surface of the specimens is investigated. It is found that vinyl-terminated butadiene acrylonitrile improves the toughness with slight negligible effects on other characteristics. Incorporation of 15 phr of vinyl-terminated butadiene acrylonitrile increases the K_IC of epoxy from 0.6 MPa to 2.3 MPam^0.5. Similarly, addition of 15 phr vinyl-terminated butadiene acrylonitrile leads to 67% enhancement in the interlaminar fracture toughness of composites. The toughening mechanisms and toughness transfer from bulk to composite are discussed.     

Enhancing mechanical properties of fused silica composites by introducing well-dispersed boron nitride nanosheets

Well-dispersed boron nitride nanosheets (BNNSs) reinforced fused silica composites were successfully fabricated by surface modification assisted flocculation method. Surface modification can enhance the performance of flocculation process. BNNSs were homogeneously mixed with fused silica through the electrostatic interaction between hydroxylated BNNSs with negative charge and amino-modified fused silica with positive charge. The BNNSs can act as excellent nanofillers for enhancing the mechanical properties of fused silica composites. Approximately 74% and 48% increases in flexure strength and fracture toughness can be achieved for the 1.5 wt% BNNSs/fused silica composite, respectively. The toughening mechanisms were analyzed by microstructural characterization, especially for pull-out mechanism.     

Silicon Carbide Platelet/Alumina Composites: III, Toughening Mechanisms

This is the last part of a series of papers on the processing and fracture behavior of SiC-platelet/Al₂O₃ composites. The objective of this paper was to identify the mechanisms involved in the toughening process. A hot-pressed composite with a SiC volume fraction of 0.3 was chosen as the model system for study. Based on microstructural observations, crack deflection and grain bridging were both identified as possible toughening mechanisms and were further investigated. A Modified two-dimensional crack deflection model is presented to account for the anisotropic microstructure in hot-pressed platelet composites, in which preferred platelet orientation was present. Relative toughness values were predicted for two crack propagation directions assuming crack deflection is the toughening mechanism. Fracture toughness measurements for specific crack directions were made using a bridge indentation technique. The correlation of experimental results with theoretical predictions is discussed. To distinguish the effect of grain bridging from crack deflection, an in situ observation of crack growth was conducted. The results showed no distinct rising T -curve behavior for cracks in the size range 80 to 500 μ m. Measurement of fracture surface roughness was also made and the implications are discussed. The results indicate that crack deflection is the dominant toughening mechanism in the SiC-platelet/ Al₂O₃ composites studied herein.