Study of rubber-modified brittle epoxy systems. Part II: Toughening mechanisms under mode-I fracture

The toughening mechanisms in grafted-rubber concentrate (GRC), dispersed acrylic rubber (DAR), and Proteus rubber-modified brittle epoxy (i.e., highly crosslinked) systems are examined using scanning electron microscopy, optical microscopy and transmission electron microscopy techniques. The toughening of the GRC-modified brittle epoxy system is found to be due to cavitation of the GRC rubber particles, followed by formation of limited shear yielding when the crack propagates. Crack bifurcation and crack deflection are also observed in this system. Only crack bifurcation, crack deflection, and possibly crack/particle bridging mechanisms are operative in the DAR-modified brittle epoxy system. In the case of the Proteus rubber-modified system, the rubber appears to be rigid (T_g ≈ 28°C). As a result, the crack/particle bridging mechanism is not observed. Only crack deflection and crack pinning mechanisms are found. These observations are in agreement with the toughness measurement results (see Part I), which indicate that the GRC rubber provides the most effective toughening, followed by the DAR rubber, and then by the Proteus rubber. An approach for toughening brittle epoxies is also discussed.     

Phase-field simulation of microscale crack propagation/deflection in SiCf/SiC composites with weak interphase

To understand the microscale toughening mechanism, the crack propagation, and stress–strain response of unidirectional SiC_f/SiC composites with h-BN interphase under transverse and longitudinal tension are investigated by a promising micromechanical phase field (PF) method along with representative volume element. Of much interest, the calculation results are well consistent with the available experimental results. With a strong dependence on the interphase strength, the toughening mechanisms during crack propagation are well presented, for example, fiber pull-out, crack deflection, and interphase debonding. Furthermore, the longitudinal tensile strength of SiC_f/SiC composites increases with decreasing the interphase strength, where only a weak enough interphase can result in a significant crack deflection by its cracking. In particular, the ratio of the interphase strength along fibers to the matrix strength should be less than 1.254 to ensure crack deflection in the interphase and fiber pull-out. Moreover, the transverse tensile strength of SiC_f/SiC composites reaches a maximum with increasing the interphase thickness into the range of 0.25–0.5 µm.     

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si₃N₄/Si₂N₂O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si₂N₂O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si₂N₂O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si₃N4/Si₂N₂O and fine grained β- Si₃N₄ ceramics.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

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.     

Microstructure and mechanical properties of boron nitride nanosheets-reinforced fused silica composites

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5 wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms.     

Preparation and properties of MoSi2 based composites reinforced by carbon nanotubes

Molybdenum disilicide (MoSi₂) based composites with various contents of carbon nanotubes (CNTs) were made by sintering in vacuum at 1500 °C for 1 h. Mechanical properties of these composites at room temperature revealed the addition of CNTs to have good hardening and toughening effect on the matrix. Especially when adding 6.0% CNTs by volume, the hardness and fracture toughness were improved respectively by about 25.3% and 45.7% compared to pure MoSi₂. Phase identification and microstructure of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HTEM). Multi-walled CNTs were found in the powders synthesized by self-propagating high temperature synthesis (SHS) and SiC phase existed in the sintering samples. Fine grain and the favorable effect of dispersed SiC particles resulted in a high hardness of the CNTs/MoSi₂ composite. The toughening mechanisms for the CNTs/MoSi₂ composites included crack deflection, crack micro-bridging, crack branching, crack bowing and fine-grain pullout.     

环氧树脂增韧研究

综述目前环氧树脂增韧的几种最新机理,如分散相的撕裂和塑性拉伸、钝化基体树脂裂纹、裂纹钉铆、逾渗理论及其他几种增韧机理。探讨了橡胶弹性体、热塑性树脂、柔性链段固化剂、刚性纳米粒子、热致液晶(TLCP)、核-壳结构(CSP)、互穿网络(IPN)等增韧环氧树脂的方法及其相应机理,并指明了今后环氧树脂增韧研究发展方向。     

The toughening mechanism and mechanical properties of graphene-reinforced zirconia ceramics by microwave sintering

ABSTRACT

The graphene/ZrO₂ composites were fabricated by impregnating graphene dispersion into the ZrO₂ ceramic matrix and sintered by microwave, and the microstructure and mechanical properties were investigated. The results showed that the graphene was well dispersed in the ceramic matrix and refined the grain size. The fracture toughness reached 8.62?MPa?m^1/2, confirmed by single-edge notched beam, which was 42% higher than that of the pure ZrO₂. Also, the toughening mechanisms were investigated by micro-hardness testing and showed that a combination of crack deflection, micro-crack and crack bridging increased the fracture toughness.     

Fracture Energy and Strength Behavior of a Sodium Borosilicate Glass-Al2O3 Composite System

Fracture energy and strength were determined for three series within a sodium borosilicate glass-Al₂O₃ dispersed composite system. The average particle sizes of the Al₂O₃ dispersions were
, and
μm. Within each series, composites containing 0.10, 0.25, and 0.40 vol fractions of the Al₂O₃ dispersed phase were vacuum hot-pressed. The fracture energy was determined at 77°K with the double cantilever specimen configuration. Strength was measured by a 4-point flexural test. A significant increase in fracture energy was observed (up to 5 times the fracture energy of the glass without second-phase dispersion). The fracture energy depended on the interparticle spacing and average particle size of the Al₂O₃ dispersion. These results could best be explained by a previously proposed model for the interaction of a crack front with a second-phase dispersion. Surface roughness also contributed to the increased fracture energy. Some composites were strengthened significantly relative to the glass without a dispersion. Calculation of the crack size showed that the Al₂O₃ dispersion increased the crack size of the glass by ∼1 to 3 times the average particle size of the Al₂O₃ dispersion. Thus, the dispersion increased both the fracture energy and the crack size. These opposing parameters ultimately determined the strength behavior of these composites.     

Toughening by Monoclinic Zirconia

The toughening induced by monoclinic ZrO₂ in the absence of microcracking was investigated, using ZnO as the host material. Toughness levels K_c in excess of the host toughness K_c^M were achieved, attaining a peak toughness K_c/K_c^M ∼1.7, at monoclinic ZrO₂ volume concentrations 0.2. This toughening is attributed to crack/particle interactions, associated with the deflection and bowing of the crack by the residual strain field around the monoclinic ZrO₂ particles.     

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.     

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.     

SiC晶须,晶板增韧AlN陶瓷的研究

本文利用现代测试技术对ＳｉＣｗ，ＳｉＣｐ增韧ＡｌＮ材料的力学性能，显微结构进行研究，并分析探讨了材料的增韧机理，结果表明；ＳｉＣｗ可有效改善材料的断裂韧性和断裂强度，其增韧机理主要为裂纹偏转和晶须拔出效应，ＳｉＣｐ加入的断裂韧性起到良好的促进作用，但对材料的断裂强度则有不良的作用，其增韧机理主要为裂纹偏转和分枝效应。     

Preparation of O′-Sialon-based ceramics by two-stage liquid phase sintering and study on the toughening mechanism of ultrafine-grained sintered clusters

Ultrafine-grained O′-Sialon-based ceramics were prepared by two-stage sintering at 1250 °C, with large particle GH4169 superalloy powder and nano Al₂O₃–Y₂O₃ as composite sintering aids. The effects of these aids on the densification, microstructure, and mechanical properties of O′-Sialon-based ceramics during two-stage sintering were also studied. Studies have shown that the densification process of O′-Sialon-based ceramics promoted by composite sintering additives, presents with the characteristics of two-stage liquid-phase sintering. In the first stage, GH4169 formed ultrafine-grained sintered clusters in the sintered material through liquid phase diffusion. In the second stage, the uniformly dispersed nano Al₂O₃–Y₂O₃ realized the uniform sintering of the material. In the fracture process, the ultrafine-grained sintered clusters hindered the crack propagation and promoted multiple deflections of the crack around the edge of the clusters, achieving the effect of crack deflection toughening. This effect, dominated by ultrafine-grained sintered clusters, significantly improved the fracture toughness of O′-Sialon-based ceramics up to 8.52 MPa m^1/2.     

Microstructure and fracture toughness of SiAlCN ceramics toughened by SiCw or GNPs

Mechanical alloying and spark plasma sintering (SPS) were used to prepare dense SiAlCN ceramic and SiAlCN ceramic toughened by SiC whiskers (SiC_w) or graphene nanoplatelets (GNPs). The influences of different reinforcements on the microstructure and fracture toughness were investigated. The SiAlCN ceramic exhibited a fracture toughness of 4.4 MPa m^1/2 and the fracture characteristics of grain bridging, alternative intergranular and transgranular fracture. The fracture toughness of SiC_w/SiAlCN ceramic increased to 5.8 MPa m^1/2 and toughening mechanisms were crack deflection, SiC_w bridging and pull-out. The fracture toughness of GNP/SiAlCN ceramic increased significantly, which was up to 6.6 MPa m^1/2. GNPs played an important role in grain refinement, which resulted in the smallest grain size. Multiple toughening mechanisms, including crack deflection, crack branch, GNP bridging and pull-out could be found. The better toughening effect could be attributed to the larger specific surface area of GNPs and the appropriate interface bonding between GNPs and matrix.     

A review of an innovative concept to increase the toughness of the ceramics by piezoelectric secondary phases

Despite of wide range scope of ceramics for various applications, such as healthcare, space, and energy storage etc., poor fracture toughness restricts their multifunctional performance. The development of various techniques/approaches to improve the fracture toughness of ceramics is in continuum thrust. The present work reviews one of the novel techniques to enhance the toughness of ceramics with the incorporation of piezoelectric secondary phase in the matrix. In addition to the piezoelectricity induced toughening mechanisms such as, energy dissipation due to electro-mechanical phenomenon as well as stress-induced domain switching toughening, other toughening mechanisms such as, transformation toughening, crack bridging, crack deflection and microcrack toughening also contributes to the total observed toughening of piezo-composites. As far as the piezoelectricity induced toughening is concerned, the poling direction and electrical field parameters also affect the toughness of the ceramics.     

Microstructure,mechanical properties,and fracture behaviour of Ti(C,N)-based cermets with a composite structure

Ti(C,N)-based cermets with a composite structure were designed to maintain the balance between strength and toughness. The cermets with the composite structure comprised coarse particles and the matrix, and the coarse particles included fine hard phases compounded in the matrix. A new hard phase grain with a four-layered structure was found. The composite structure of the cermet can contribute to high toughness, and the grain with the four-layered structure in the composite structure imparts high strength and toughness. As the granule size increases, the fracture toughness of the cermets increased, but the hardness and transverse rupture strength (TRS) showed the opposite trend. The toughening mechanisms of the cermet were crack branching, crack bridging, crack deflection, and formation of tear ridges.     

Fracture Behavior of Multilayer Silicon Nitride/Boron Nitride Ceramics

The fracture behavior of multilayer Si₃N₄/BN ceramics in bending has been studied. The materials were prepared by a process of tape casting, coating, laminating, and hot pressing. The Si₃N₄ layers were separated by thin, weak BN interlayers. Crack patterns in bending bars were examined with a scanning electron microscope. The weak layers deflected cracks in bending and thus prevented catastrophic failure. In one well-aligned multilayer ceramic A, a main crack propagated through the specimen although along a zigzag path. A second multilayer ceramic B was made to simulate a wood grain structure. Its failure was dominated by shear cracking along the weak BN layers. Besides crack deflection, interlock bridging between toothlike layers in the wake of the main crack appeared also to contribute to toughening.     

钇补强颗粒弥散陶瓷复合材料增韧机制的微观结构表征

在Al2O3/（W,Ti）C陶瓷复合材料中适量添加稀土元素钇能显著提高其断裂韧性。本文运用SEM与TEM技术,从微观结构的角度探讨了其增韧机制。表明,由于稀土钇的添加,使材料内部形成不同程度的强弱界面,它们与扩展中的裂纹相互作用,使得裂纹桥联、裂纹分支、裂纹偏转以及微裂纹增韧机制得到明显增加和加强,从而以多种增韧机制及其协同作用共同提高稀土补强Al2O3/（W,Ti）C陶瓷复合材料的断裂韧性。