Effect of Geometry,Loading and Elastic Moduli on Critical Parameters in a Nanoindentation Test in Polymeric Matrix Composites with a Nonhomogeneous Interphase

Fiber nanoindentation models are developed for polymeric matrix composites with nonhomogeneous interphases. Using design of experiments, the effects of geometry, loading and material parameters on the critical parameters of the indentation test such as the load–displacement curve, the maximum interfacial shear and normal stresses are studied. The sensitivity analysis shows that the initial load–displacement curve is dependent only on the indenter type, and not on parameters such as fiber volume fraction, interphase type, thickness of interphase, and boundary conditions. The interfacial tensile radial stresses are not sensitive to indenter type, or to type and thickness of interphase, while the interfacial compressive radial stresses are sensitive mainly to boundary conditions and thickness of interphase; however, the influence of these factors on the interfacial radial stresses can be large. In contrast, the interfacial shear stress is sensitive to all factors, but the influence of the factors is relatively small.     

Development of an improved energy-based method for residual stress assessment

In this study, an improved energy-based method is proposed to measure the equi-biaxial residual stress of bulk materials using spherical indentation. Through analysis of the load–depth curve and residual indentation profile, two calculation methods of the energy of residual stress are proposed, one of which is determined from the load–depth curve and the other based on the Hertizan theory. Finite element analysis was used to establish their relationship, which is the function of the residual stress and material parameters. According to reverse analysis, the proposed method can be used to quickly and effectively determine residual stress. To verify the model, indentation experiments were performed using a ZHU0.2/Z2.5 testing machine. A comparison with conventional X-ray diffraction results showed that the improved method is suitable for the quantitative assessment of residual stress in industrial facilities.     

Evaluation of the substrate effect on indentation behavior of film/substrate system

A method to evaluate the substrate effect quantitatively in film indentation is proposed. For the thin film deposited on the substrate, the power function relationship is used to describe the loading curve of the film indentation behavior. The loading curve exponent of the power function which is the fitting parameter can reflect the substrate effect quantitatively. The finite element method is used to simulate the nanoindentation process of the film/substrate system. The loading curve exponent can be obtained from the simulation results. A substrate effect factor based on the loading curve exponent is defined to characterize the effect of the substrate on film indentation. Meanwhile, the dimensionless function of the loading curve exponent related with the material properties and indentation depth is obtained. The results can be helpful to the measurement of the mechanical properties of thin films by means of nanoindentation.     

Effect of sample tilt on nanoindentation behaviour of materials

Analysis of nanoindentation is based on the elastic solution of a rigid indenter perpendicularly penetrating a flat contact surface. In reality, nanoindentation is often performed on a tilt sample surface due to sample tilt mounting or the existing roughness of a polished or raw surface. In this study, finite element simulations as well as nanoindentation experiments on a fused-quartz sample with different tilt angles were carried out to investigate the influence of sample tilt on nanoindentation behaviour of materials. It was found that sample tilt results in increases in the indentation load, contact area and contact stiffness at the same penetration depth. The contact area increase caused by sample tilt cannot be accounted for by Sneddon's equation, commonly used in nanoindentation analysis. This results in a significant underestimation of indentation projected contact area, which in turn leads to an overestimation of the mechanical properties measured by nanoindentation.     

Finite-element analysis of the mechanical behavior of Au/Cu and Cu/Au multilayers on silicon substrate under nanoindentation

Finite-element analysis of the nanoindentation into Au/Cu and Cu/Au multilayers was performed to deduce their mechanical characteristics from nanoindentation response. Different bilayer thicknesses, numbers, and sequences were studied using the load–displacement curve, hardness, indentation, and the residual surface profile as well as the von Mises equivalent stress. The characteristics of the multilayers were found to be dispersed between the Au and Cu. Nevertheless, if the indentation depth is smaller than the uppermost individual layer thickness of the multilayers, the intrinsic properties can be obtained. Using the von Mises equivalent stress as a failure criterion, the results showed that thinner multilayers would induce a greater potential of shear banding deformation. PACS 61.43.Bn; 62.20.-x; 68.03.Hj; 68.05.Cf; 68.08.De     

On the uniqueness and sensitivity issues in determining the elastic and plastic properties of power-law hardening materials through sharp and spherical indentation

A systematic study of the uniqueness, reversibility and sensitivity issues associated with seven indentation-based methods of property extraction demonstrates that: (i) The indentation algorithms generally identify the elastic and plastic properties of materials uniquely for most materials. (ii) The indentation forward algorithms (wherein the indention responses are determined from the elastic and plastic properties of the indented materials) and the reverse algorithms (wherein the elastic and the plastic properties of materials are extracted from the indentation responses) are distinct for each indentation method and are internally consistent in that the differences in the elastic and plastic properties determined through the reverse analysis and the ‘true’ material properties are generally small for a large number of materials, for each of the seven methods. (iii) While the differences in the indentation response parameters predicted by each of the seven indentation methods (for a particular material) could be small, there could be considerable dispersion in the elastic and plastic properties predicted by the reverse algorithms of the seven methods (for a particular set of indentation response parameters). (iv) In the forward analysis, small uncertainties in the elasto-plastic properties lead to small uncertainties in the predictions of the indentation response of materials. The sensitivity distribution is generally heterogeneous and symmetric across positive and negative variations in the material elasto-plastic properties. (v) In the reverse analysis, the elastic modulus exhibits low sensitivity, while the yield strength and the strain-hardening exponent generally exhibit high sensitivity to uncertainties in the indentation response parameters. The sensitivity distribution is heterogeneous and asymmetric across positive and negative variations in the indentation response parameters. (vi) The representative stresses are fairly robust to uncertainties in the indentation response parameters. Consequently, dual sharp and spherical indentation methods, which identify multiple representative stresses, exhibit reduced sensitivity in the determination of the plastic properties.     

Probing mechanical properties of thin film and ceramic materials in micro- and nano-scale using indentation techniques

In this study, we report on the mechanical properties, failure and fracture modes in two cases of engineering materials; that is transparent silicon oxide thin films onto poly(ethylene terephthalate) (PET) membranes and glass-ceramic materials. The first system was studied by the quazi-static indentation technique at the nano-scale and the second by the static indentation technique at the micro-scale. Nanocomposite laminates of silicon oxide thin films onto PET were found to sustain higher scratch induced stresses and were effective as protective coating material for PET membranes. Glass-ceramic materials with separated crystallites of different morphologies sustained a mixed crack propagation pattern in brittle fracture mode.     

Analysis of the substrate effects of strain-hardening thin films on silicon under nanoindentation

Mechanical properties of thin films on substrates can be evaluated directly through nanoindentation. For a comprehensive study, thin films should be characterized via Young’s modulus, yield stress and strain-hardening exponent at constant temperature. In this paper, we evaluate these effects of thin films on silicon substrate through finite element analysis. Thin films, from soft to hard relative to the silicon substrate, are investigated in three categories: soft films on hard substrates, soft to hard films on no elastic mismatch substrates, and hard films on soft substrates. In addition to examining the load-displacement curve, the normalized hardness versus normalized indentation depth is checked as well to characterize its substrate effect. We found that the intrinsic film hardness can be acquired with indentation depths of less than 12% and 20% of their film thickness for soft films on hard substrates and for soft to hard films on no elastic mismatch substrates, respectively. Nevertheless, nanoindentation of hard films on soft substrates cannot determine the intrinsic film hardness due to the fact that a soft substrate cannot support a hard film. By examining the von Mises stresses, we discovered a significant bending phenomenon in the hard film on the soft substrate. PACS 61.43.Bn; 62.20.-x; 68.03.Hj; 68.05.Cf; 68.08.De     

应力预释放对单晶硅片的压痕位错滑移的影响

单晶硅片的压痕位错在一定温度下的滑移距离反映了硅片的机械强度. 压痕位错的滑移是受压痕的残余应力驱动的, 因此研究残余应力与位错滑移之间的关系具有重要的意义. 本文首先采用共聚焦显微拉曼术研究了单晶硅片压痕的残余应力经过300或500 ℃ 热处理后的预释放, 然后研究了上述应力预释放对压痕位错在后续较高温度(700–900 ℃)热处理过程中滑移的影响. 在未经应力预释放的情况下, 压痕位错在700–900 ℃热处理2 h后即可滑移至最大距离. 当经过上述预应力释放后, 位错在900 ℃热处理2 h后仍能达到上述最大距离, 但位错滑移速度明显降低; 而在700和800 ℃时热处理2 h后的滑移距离变小, 其减小幅度在预热处理温度为500 ℃时更为显著. 然而, 进一步的研究表明: 即使经过预应力释放, 只要足够地延长700和800 ℃ 的热处理时间, 位错滑移的最大距离几乎与未经预应力释放情形时的一样. 根据以上结果, 可以认为在压痕的残余应力大于位错在某一温度滑移所需临界应力的前提下, 压痕位错在某一温度滑移的最大距离与应力大小无关, 不过达到最大距离所需的时间随应力的减小而显著增长.     

Mechanical and tribological properties of Ni/Al multilayers—A molecular dynamics study

Mechanical and tribological properties of multilayers with nanometer thickness are strongly affected by interfaces formed due to mismatch of lattice parameters. In this study, molecular dynamics (MD) simulations of nanoindentation and following nanoscratching processes are performed to investigate the mechanical and tribological properties of Ni/Al multilayers with semi-coherent interface. The results show that the indentation hardness of Ni/Al multilayers is larger than pure Ni thin film, and the significant strength of Ni/Al multilayers is caused by the semi-coherent interface which acts as a barrier to glide of dislocations during nanoindentation process. The confinement of plastic deformation by the interface during nanoscratching on Ni/Al multilayers leads to smaller friction coefficient than pure Ni thin film. Dislocation evolution, interaction between gliding dislocations and interface, variations of indentation hardness and friction coefficient are studied.     

TiN薄膜在纳米压痕和纳米划痕下的断裂行为

利用磁控溅射方法在Si(111)衬底上制备了具有(111)和(222)择优取向的TiN薄膜. 用纳米压痕和纳米划痕方法研究了该薄膜的变形和断裂行为. 用扫描电子显微镜、纳米压痕原位原子力显微镜及原位光学显微镜并结合加-卸载 曲线及划痕曲线获得了薄膜发生变形和断裂的微观信息. 在压痕试验中, TiN薄膜在压入深度为200 nm时表现为塑性变形及压痕周围的局部断裂, 随着压入深度的增大, 塑性变形和局部断裂变得越显著, 当最大压入深度达到临界值1000 nm时, 薄膜和衬底间发生了界面断裂. 在划痕实验中, 100 mN及200 mN的最大载荷均可以引起界面断裂. 最大为200 mN的载荷使得薄膜发生界面断裂的位置比用100 mN载荷时的位置提前, 但其临界断裂载荷和100 mN时及压痕实验时的临界界面断裂载荷基本相同. 关键词： TiN薄膜 纳米压痕 纳米划痕 界面断裂     

Effect of solid solution strengthening by iridium on the nucleation of dislocations in a molybdenum single crystal during nanoindentation

The nucleation of dislocations in single crystals of molybdenum and a Mo-1.5 at % Ir solid solution has been investigated by nanoindentation. In the curve of indentation of a Berkovich indenter into the single crystals, an abrupt transition from elastic to plastic deformation has been observed at a depth of 20–40 nm due to the nucleation of dislocations in the initially dislocation-free region under the contact. Alloying of molybdenum by iridium results in a twofold increase in shear stresses under which dislocations nucleated in the contact. Therefore, the solid solution impurity of iridium in molybdenum leads to an increase not only in the plastic strain resistance (to an increase in the hardness) but also in the elastic shear stresses under which dislocations are generated (homogeneously or heterogeneously) in the contact. The latter effect cannot be explained only by an increase in the elastic moduli because of its smallness; however, it is determined to a large extent by a higher degree of perfection of the solid solution crystal as compared to unalloyed molybdenum.     

Study on mechanical properties after laser forming

Laser forming is a means of processing materials in a novel manner. The mechanical properties of specimens after laser forming are investigated. By tension tests, the tension properties are analyzed to establish Ramberg–Osgood constitutive equations under different laser processing parameters. Experimental data show that the yield strength and tensile strength are improved after laser forming, while the elongation percentage is reduced. Based on the distribution of residual stresses as well as residual strains after the laser forming process, the fatigue life under different laser processing parameters is studied using low-cycle fatigue tests. The residual compressive plastic strain is the most important reason for improving the fatigue life of low carbon steel after laser forming. The fatigue fracture mechanism is shown through the analysis of macro-fracture and micro-fracture using the scanning electronic microscope.     

Effect of vacancies on incipient plasticity during contact loading

Atomistic simulations and experimental nanoindentation tests were used to examine the effect of vacancies on the inception of plastic deformation in Ni. Molecular dynamics have shown the effect of vacancy position on the yield load and demonstrate a variety of mechanisms which are responsible for the inception of plastic deformation during indentation. In cases where the vacancy position is close to regions of high shear stresses the nucleation of dislocations is related to the location of a vacancy; however, homogeneous nucleation of dislocations is also observed for vacancy-containing crystals. Complementary experiments have been used to demonstrate the effect of indenter size on the onset of yielding in the presence of vacancies. Both the simulations and experiments show that larger indenter tips increase the chance of weakening the material in the presence of vacancies.     

Analysis by speckle interferometry of the dependency of yield stress on residual stress

We used electronic speckle pattern interferometry (ESPI) to measure in situ displacement fields nondestructively and with high resolution (10⁻² μm) by using the interferometry principle and the phase-shift technique. We measured the depth profile of the residual stress in steel pipe manufactured by thermomechanically controlled processing using a quantitative model, which explains the relationship between residual stress and displacement measured by ESPI in chemical etching. We analyzed the variation of yield stresses measured by the indentation technique and the residual stresses at various depths. The relationship between the residual stresses and the yield stresses was consistent with simulated results and can be used for indirect evaluation of the residual stresses from the yield stresses.     

SiC/Al2O3纳米复合材料压痕残余应力压谱法测量研究

利用压力荧光光谱法研究了5％ SiC／Al2O3纳米复合材料压痕周围残余应力的大小及其分布规律。结果发现在压痕区及压痕周围10μm的应力区范围中,荧光R-线明显增宽,而且压应力与离压痕中心距显著相关。通过获得的R双峰(R1和R2荧光光谱)频率变化,计算了压痕区及其周围复杂应力场中的等静压力。结果表明：沿着压痕区主对角线方向和边界垂直平分线方向,其应力分布存在十分相似的变化规律。同时发现在压痕周围存在着对称残余应力区。     

Atomistic simulations of the effect of a void on nanoindentation response of nickel

Three-dimensional molecular dynamics simulations have been performed to investigate the effect of a void on the nanoindentation of nickel thin film.The radius and depth of the void are varied to explore how they influence the nanoindentation.The simulation results reveal that the presence of a void softens the material and allows for a larger indentation depth at a given load compared to the no void case.The radius and depth of the void have a major effect on the indentation behaviors of the thin film.It is also observed that the void will collapse during the nanoindentation of crystal with void.And when the indentation depth is sufficiently large,the void will disappear.It is found that the indentation depth needed to make the void disappear depends on the void size and location.     

Nanoindentation characterization of GaN epilayers on A-plane sapphire substrates

Gallium nitride (GaN) epilayers was deposited on a-axis sapphire substrate by means of metal-organic chemical vapor deposition (MOCVD) method. The GaN epilayers has been investigated in their repetition pressure-induced impairment events from nanoindentation technique and, the relative deformation effect was observed from atomic force microscopy (AFM). From the morphological studies, it is revealed that none of crack and particle was found even after the indentation beyond the critical depth on the residual indentation impression. The ‘pop-in’ event was explained by the interaction of the deformed region, produced by the indenter tip, with the inner threading dislocations in the GaN films. Pop-in events indicate the generation and motion of individual dislocation, which is measured under critical depth and, no residual deformation of the GaN films is observed.     

Interface Separation in Residually-Stressed Thin-Film Structures

Plasticity is a significant contributor to the interfacial fracture resistance of multilayer thin-film structures containing ductile layers. Salient parameters affecting plasticity contributions to interfacial fracture energy including the ductile layer thickness, yield strength, and the maximum cohesive stress governing interface separation, have been reported. However, the effects of residual stresses in the thin-film layers on such plasticity contributions have not been considered. We explore the effect of residual stresses on debonding with particular attention to the relationship between the stress state in both ductile and elastic layers and the resulting macroscopic debond energy. Using multiscale simulations it is shown that residual thin-film stresses can alter plasticity in the ductile layer and significantly influence the macroscopic fracture energy. A simple yield-based model to account for this behavior is proposed.     

Volumetric and timescale analysis of phase transformation in single-crystal silicon during nanoindentation

Clarifying the phase transformation process and mechanism of single-crystal silicon induced by high pressure is essential for preparation of new silicon phases. Although many previous researches have focused in this area, the volume of high-pressure phases and the duration of phase transformation are still unclear. In this paper, the volume change and the duration of phase transformation from Si-II phase into Si-XII/Si-III phases were investigated quantitatively by introducing a holding process in the unloading stage of a nanoindentation test. Experimental results indicate that the high-pressure phase volume is dependent strongly on the maximum indentation load and independent of the loading/unloading rate and the holding time at the maximum indentation load, while phase transformation duration is independent of the aforementioned experimental parameters. By analyzing the results, a critical volume of Si-XII/Si-III phases was identified which determines the occurrence of sudden phase transformation, and a modified nucleation and growth mechanism of high-pressure phases was proposed. These results provide new insights into high-pressure phase transformations in single-crystal silicon.