Discussion of the indentation hardness of a glass-ceramic with participate microstructure

On the basis of a theory previously developed by the authors for the indentation hardness of glass matrix, particulate composites, an attempt was made to interpret published hardness data for a ZnO-Al₂O₃-SiO₂ glass-ceramic in which gahnite (ZnAl₂O₄) crystal particles are dispersed in a glass matrix as a major crystalline phase. The elastic moduli for gahnite were estimated using both the bulk modulus-molar volume relationship and the density-Poisson's ratio relationship, established for oxide crystals. After determining the variation of the matrix Young's modulus with heat-treatment, the variation of the overall hardness with volume fraction of crystal phase as well as the crystal-size effect were discussed. The hardness behaviour of the present glass-ceramic could be interpreted well in terms of the properties and amounts of the constituent phases and the microstructural effects.     

基于纳米压痕法的埃洛石纳米管/环氧复合材料力学性能表征

采用球磨法在环氧树脂中分散了不同质量分数(0、5wt%和10wt%)的埃洛石纳米管(HNTs),通过哌啶固化剂固化,制备了HNTs/环氧树脂复合材料,并利用纳米压痕法测试了HNTs/环氧树脂复合材料的弹性模量、硬度和蠕变性能。SEM和TEM观测表明:HNTs在环氧树脂中分散情况较好。纳米压痕实验结果表明:在不牺牲HNTs/环氧树脂复合材料弹性模量、硬度以及玻璃化转变温度的基础上,HNTs明显提高了环氧树脂基复合材料的抗蠕变性能,这主要是由于HNTs和环氧基分子链形成了新的交联结构,增加了材料的交联密度,刚性纳米粒子限制了环氧基分子链的活动性。     

Conditions of applying Oliver–Pharr method to the nanoindentation of particles in composites

The indentation test is a popular experimental method to measure a material’s mechanical properties such as elastic modulus and hardness, and the Oliver–Pharr method is commonly used in commercial indentation instruments to obtain these two quantities. To apply the Oliver–Pharr method correctly in all of these cases, it is essential to know the limitations of this method. The present study focuses on the applicability of the Oliver–Pharr method to measure the mechanical properties of particles in composites. The finite element method is used to undertake virtual indentation tests on a particle embedded in a matrix. In our numerical studies, the indentation “pile-up” phenomenon is generally observed in our numerical case studies, which indicates that the contact area used for predicting the elastic modulus should be measured directly, not be estimated from the indentation curve. The Oliver–Pharr method based on the real contact area is applied to estimate the elastic modulus of the particles by using the indentation curve from the numerical simulation, with the estimated elastic modulus being compared with the input value. Applying the real contact area value (not the one predicted from the indentation curve) we show that the Oliver–Pharr method can still be applied to measure the elastic modulus of the particle with sufficient accuracy if the indentation depth is smaller than the particle-dominated depth, a value defined in this work. The influences of the matrix and particle properties on the particle-dominated depth are studied using a dimensional analysis and parametric study. Our results provide guidelines to allow the practical application of the Oliver–Pharr method to measure the elastic modulus of particles in composites. This could be particularly important where particles are formed in situ in a matrix (as opposed to being preformed and subsequently incorporated in a matrix), or when the modulus of individual performed particles is required such as for subsequent modelling, but the modulus of individual material particles (or its material) cannot readily be determined.     

Some issues on nanoindentation method to measure the elastic modulus of particles in composites

The application of the indentation method to measure the elastic modulus of particles embedded in a composite is theoretically investigated in this paper by finite element simulation. The Oliver–Pharr method, which is widely used in commercial nanoindentation instruments, is used to probe the elastic modulus of the particle from the simulated indentation curve. The predicted elastic modulus is then compared with the inputted value. Two cases are studied, that of a stiff particle embedded in a soft matrix and a soft particle embedded in a stiff matrix. In both of these cases, there exists a particle-dominated depth. If the indentation depth lies within this particle-dominated depth, the Oliver–Pharr method is able to be applied to measure the particle’s elastic modulus with sufficient accuracy if the real contact area is used. This could lead to an experimentally-convenient method of determining the primary properties of individual particle, providing they can be well dispersed in the polymeric matrix.     

Analysis of effective elastic modulus for multiphased hybrid composites

Short fiber reinforced composites inherently have fiber length distribution (FLD) and fiber orientation distribution (FOD), which are important factors in determining mechanical properties of the composites. Since the internal structure has a direct effect on the mechanical properties of the composites, a Micro-CT was used to observe the three dimensional structure of fibers in the composites and to acquire FLD and FOD. It was successful to investigate FLD, FOD, and fiber orientation states and to predict the elastic modulus of the hybrid system. Since hybrid composites used in this study consist of three phases of particles, glass fibers, and matrix, theoretical hybrid modeling is required to consider reinforcing effects of both particles and glass fibers. Interaction between the particles and matrix was considered by using a perturbed stress–strain theory, the Tandon–Weng model. In addition, the laminating analogy approach (LAA) was used to predict the overall elastic modulus of the composite. Theoretical prediction of hybrid moduli indicated that there was a possibility of poor adhesion between glass fibers and matrix. The poor interfacial adhesion was confirmed by morphological experiments. This theoretical and experimental platform is expected to provide more insightful understanding on any kinds of multiphased hybrid composites.     

Nanoindentation behaviour of aluminium based hybrid composites with graphite nanofiber/alumina short fiber

Nanoindentation was performed on Al based hybrid composites containing graphite nanofiber/alumina short fiber. Measurement of hardness and elastic modulus were carried out using the continuous stiffness method (CSM) with an indentation depth of about 2000 nm. To find out the hardness and moduli of the composites at various local regions, nanoindentation tests were carried out in different locations of the sample. In all cases, the measured values for hardness and modulus are a mixture of the effects of the presence of the graphite nanofiber (GNF) and alumina short fiber (Al₂O_3sf) and of the precipitation phases in the Al matrix. Moreover, the large number of dislocations in the plastic zone formed in the indented region of the GNF contributes to an increase in values of both modulus and hardness. The enhancement of indentation properties of the Al matrix is attributed to the possible indentation on the Al₄C₃ phase or MgAl₂O₄ precipitate. The hardness values and material pile-up are also shown to depend significantly on the mismatch between the moduli of the Al matrix and reinforcements.     

Mechanical properties of a lithium glass–ceramic matrix (LZSA) reinforced with TiC or (W,Ti)C particles: A preliminary study

Despite their generally low strength and hardness values, glass–ceramics show good potential to be used in structural applications at room temperature instead of other costlier ceramic materials. This work investigates the effect of dispersed hard carbide particles on the sintering behaviour and the mechanical properties of a lithium glass–ceramic. The glass was mixed with 30 wt.% TiC or (W,Ti)C and hot-pressed at 650 °C (30 MPa, 30 min, Ar). The results obtained compare the properties of the composites with those of the parent glass and demonstrate that the addition of hard particles significantly improves the mechanical strength of the glass–ceramic matrix.     

Mechanical and fracture properties of ternary PE/PA6/GF composites

This article studies the mechanical properties of short fiber reinforced polymer blends comprised of a soft thermoplastic matrix (polyethylene, PE), a rigid dispersed thermoplastic phase (polyamide-6, PA6) and glass fiber reinforcement. These ternary composites are designed as a model system to investigate the impact of the mutual interactions of the three phases on the composite mechanical properties. For this purpose two types of fibers are used, dispersed-phase and matrix-phase compatible fibers, respectively.     

Theoretical approach to the fracture of two-phase glass-crystal composites

This paper proposes a fracture theory for two-phase glass-crystal composites. It is hypothesized that the fracture mechanisms of such solids consist of the processes of crack nucleation and of crack propagation round the dispersed particles. At lower volume fractions of dispersed phase, macroscopic fracture will occur as a result of the growth of the micro-cracks originating in the vicinity of the pre-existing structural imperfections through a heterogeneous nucleation process; in this case, strength decreases with the proportion of the dispersed phase. At higher volume fractions where further crack propagation is prohibited by the hard crystalline particles, the process of crack propagation round the dispersed particles may be responsible for the macroscopic fracture of the composite; in this case, strength is an increasing function of the volume fraction. Expressions are formulated for mechanical strength of the glass-crystal composites, based upon the nucleation theory and Griffith's criterion. The published data on the strength of glass-alumina composites are used for the verification of the theory. The proposed theory explains well the strength behaviour of glass-alumina composites, and in particular, the dependence of the strength reduction on particle size at lower volume fractions.     

Thermite Synthesis of TiC/FeNiCr Cermet with Double-Layer Structure

TiC/FeNiCr cermet with TiC particles as hard phases and FeNiCr alloy as binder phase was in situ synthesized by thermite reactions under high gravity. A double-layer structure was obtained, including an upper layer enriched with TiC particles and an under layer with few TiC particles. Between the two layers, no interfacial line, pores, or defaults existed. A large amount of needle-like Cr₇C₃ phases were homogeneously dispersed in the FeNiCr binder phase as multiple reinforcements. A braiding structure was formed between the precipitated NiAl phases and the matrix, where the two phases kept a coherent or semi-coherent relationship. The hardness and wear resistance were evaluated, and the upper layer possessed high hardness and excellent wear resistance.     

砌浆层状复合材料珍珠层的应变率效应

为了分析具有砌浆结构的层状复合材料的应变率效应,以珍珠层为研究对象,采用纳米压入法测试珍珠层力学性能,利用连续刚度测量法得到不同加载速率下材料的硬度值和弹性模量。利用扫描电子显微镜观察珍珠层不同方向的砌浆微结构形貌,并结合微观结构对比分析不同压入深度和不同应变率两种工况下,珍珠层表层与横断面方向的力学性能。结果表明:在相同加载条件下,珍珠层表层方向的弹性模量小于其横断面方向的弹性模量,而表层方向硬度值则大于横断面方向的硬度值;当应变率恒定时,珍珠层弹性模量与硬度随压入深度增加而增加,当压入深度达到750nm后,弹性模量不再随压入深度变化而变化;当压入深度恒定时,硬度值、弹性模量和弹性回复率均随着应变率的增加而变大。     

Wedge indentation of a thin film on a substrate based on micromorphic plasticity

In this article, we use the small strain micromorphic plasticity (MP) to study the wedge indentation of a thin film on a substrate and find qualitative agreement with experiments. A two-dimensional plane strain finite element formulation of the entire MP theory framework is outlined. The generalization of the radial return method for modeling the elasto-plastic deformation is presented. The numerical results show that the MP theory is capable of describing the initial fall in hardness at small depth of indentation and then the rise at larger depth for a soft film on a hard substrate. The indentation force and hardness increase with decreasing film thickness for a given depth. It is also shown that the hardness falls monotonically as the indentation depth increases and never approaches a constant for a hard film on a soft substrate. Contrary to the soft film/hard substrate system, the force and hardness diminish with decreasing film thickness for a given depth. Besides, the influences of internal length scale and hardening modulus of the film on hardness predictions are investigated.     

Trans-interface diffusion-controlled coarsening

Accurate theoretical predictions of the volume-fraction dependence during diffusion-controlled coarsening of a polydisperse assembly of particles have proved difficult. Here, a new model of coarsening is presented, involving diffusive transport through the coherent interface between ordered and disordered phases, which atomistic calculations show has a ragged structure. The interface is a diffusion bottleneck when the ordered phase is dispersed. It is predicted that the square of the average radius grows linearly with time, that the depletion of solute decreases as the inverse square-root of time, and that there is no effect of volume fraction on kinetics and the scaled particle-size distributions. These differ dramatically from predictions of modern theories of diffusion-controlled coarsening. Data on coarsening in Ni-Al alloys is examined. We show that no other theory is consistent with the experimentally observed absence of an effect of volume fraction on coarsening of ordered gamma' (Ni3Al) precipitates in a disordered Ni-Al (gamma) matrix, and the strong volume-fraction dependence of coarsening of gamma precipitates in an ordered gamma' matrix.     

Characterization of carbon nanotube/nanofiber-reinforced polymer composites using an instrumented indentation technique

An instrumented indentation technique was tested on three types of carbon nanotube/nanofiber-reinforced composites to investigate its applicability for measuring mechanical properties (elastic modulus and hardness). There was good agreement in the measured elastic modulus between the instrumented indentation and uniaxial tension tests for the case of a nanocomposite with a harder epoxy matrix material. In contrast, there was a considerable difference in elastic modulus between the two tests for the case of a nanocomposite with a softer polystyrene matrix material. A modified area function was then developed for the nanocomposite with the softer polystyrene matrix material, and this eliminated the difference in elastic modulus between the two test techniques. Thus, the instrumented indentation technique can be used for evaluating the mechanical properties of polymer matrix nanocomposites with an added advantage that a small sample size can be used. The instrumented indentation test was also utilized in the case of a patterned nanotube array-reinforced epoxy matrix composite. This clearly showed the modulus of the array nanocomposite improved considerably compared to that of the neat epoxy resin.     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

Effective elastic moduli of three-phase composites with randomly located and interacting spherical particles of distinct properties

A micromechanical analytical framework is presented to predict effective elastic moduli of three-phase composites containing many randomly dispersed and pairwisely interacting spherical particles. Specifically, the two inhomogeneity phases feature distinct elastic properties. A higher-order structure is proposed based on the probabilistic spatial distribution of spherical particles, the pairwise particle interactions, and the ensemble-volume homogenization method. Two non-equivalent formulations are considered in detail to derive effective elastic moduli with heterogeneous inclusions. As a special case, the effective shear modulus for an incompressible matrix containing randomly dispersed and identical rigid spheres is derived. It is demonstrated that a significant improvement in the singular problem and accuracy is achieved by employing the proposed methodology. Comparisons among our theoretical predictions, available experimental data, and other analytical predictions are rendered. Moreover, numerical examples are implemented to illustrate the potential of the present method.     

Stress in particulate reinforcements and overall stress response on aluminum alloy matrix composites during straining by analytical and numerical modeling

SiC and Al₂O₃ (10–20v%) particle-reinforced Al-2618 matrix composites subjected to tensile loading were selected to simulate stress–strain curves and average stress in particles, and to examine mechanical properties experimentally in comparison. A particle-compounded mechanical model was established based on Eshelby equivalent inclusion approach to simulate stress–strain curves by introducing secant modulus and tangent modulus techniques, and to calculate stress in particles and in matrices. The same modeling work was carried out by FEM analysis based on the unit cell model using a commercial ANSYS code. The modeling and experiment were also applied to compare the mechanical behaviors between hard matrix and soft matrix, which were produced under different heat treatments. Through the comparison of the results between simulations and experiment, it is shown that Eshelby particle-compounded mechanical model can predict the stress–strain curve of the composites with both hard matrix and soft matrix, while the FEM model can match the experimental data with only hard matrix. The modeling was also carried out to study the influence of different volume fractions and aspect ratios on elastic modulus and yield strength of the composites with different reinforcing particles to get a better understanding of strengthening mechanisms of the composites.     

聚氨酯(PU)/凹凸棒土(AT)原位复合材料制备及表征

凹凸棒土(AT)经过提纯,采用原位聚合方法制备聚氨酯(PU)/凹凸棒土(AT)复合材料.通过SEM、FT-IR、DMA等测试方法对PU/AT复合材料的结构进行了表征.结果表明:复合材料的热稳定性和玻璃化温度较PU都有明显提高,拉伸强度也有较大提高.并分析了AT对复合材料结构的影响和作用机理.     

Grid indentation analysis of composite microstructure and mechanics: Principles and validation

Several composites comprise material phases that cannot be recapitulated ex situ, including calcium silicate hydrates in cementitous materials, hydroxyapatite in bone, and clay agglomerates in geomaterials. This requirement for in situ synthesis and characterization of chemically complex phases obviates conventional mechanical testing of large specimens representative of these material components. Current advances in experimental micro and nanomechanics have afforded new opportunities to explore and understand the effect of thermochemical environments on the microstructural and mechanical characteristics of naturally occurring material composites. Here, we propose a straightforward application of instrumented indentation to extract the in situ elastic properties of individual components and to image the connectivity among these phases in composites. This approach relies on a large array of nano to microscale contact experiments and the statistical analysis of the resulting data. Provided that the maximum indentation depth is chosen carefully, this method has the potential of extracting elastic properties of the indented phase which are minimally affected by the surrounding medium. An estimate of the limiting indentation depth is provided by asssuming a layered, thin film geometry. The proposed methodology is tested on a “model” composite material, a titanium-titanium monoboride (Ti–TiB) of various volumetric proportions. The elastic properties, volume fractions, and morphological arrangement of the two phases are recovered. These results demonstrate the information required for any micromechanical model that would predict composition-based mechanical performance of a given composite material.