Measurement of hardness and elastic modulus by depth sensing indentation: Improvements to the technique based on continuous stiffness measurement

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Effects of lithiation on the fracture toughness and mechanical properties of LiMn2O4 cathode battery materials

The micro-pillar splitting method has been used to assess the influence of lithiation on the fracture toughness of LixMn2O4 micro-particles used as cathode materials in lithium ion battery composites. The materials under investigation consisted of hard LiMn2O4 particles embedded in a soft and compliant epoxy matrix to form the composite electrode. Five different samples were extracted from commercial battery cells at different states of charge (SoC% = 0-20-50-75-100%). These correspond to different lithium concentrations in the particles, as measured by inductively coupled plasma optical emission spectrometry (ICP-OES). Experimental results from the pillar splitting experiments show a significant effect of the SoC%, and therefore the lithiation level, on the fracture toughness and failure mechanisms of the LixMn2O4 particles. Specifically, the toughness of the fully charged electrodes (de-lithiated material) is much lower that the fully discharged electrodes. SEM observation of split pillars (see figure) confirms a significant change in toughness of the materials as a function of the lithium concentration in the particles. The results compare well with recent investigations where a loss in ductility of electrode materials has been observed after de-lithiation. This suggests that a knowledge of the changes in toughness of the materials may be extremely important for prediction of in-service damage of the electrodes due to diffusion-induced stress during charge/discharge cycles. An analysis of pillar splitting for a hard film on a compliant substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. This ensures a correct estimation of the critical splitting load in the case of the composite materials studied in this investigatio

Effects of adhesion on the measurement of thin film mechanical properties by nanoindentation

Experiments have been performed on soft aluminum films deposited on hard ceramic substrates to explore the influences of interfacial adhesion on mechanical property measurement by nanoindentation. The substrate materials included soda-lime silicate glass, aluminum oxynitride (ALON), and (100) sapphire. Thin films of high purity aluminum were sputtered onto each substrate to a thickness of 500 nm. Because the films were deposited simultaneously, the only major difference in the specimens was the nature of the substrate, which exerts an important influence on film adhesion through interfacial chemistry. Of the substrates examined, aluminum adheres strongly to glass and sapphire, but poorly to ALON. In addition, two different types of aluminum films on sapphire were examined - one with and the other without a 10 nm interlayer of amorphous carbon which significantly reduces film adhesion. Testing revealed important differences in the hardness of the specimens when measured by standard nanoindentation methods. Characterization of the residual hardness impressions by high resolution scanning electron microscopy showed that the hardness differences arise from an influence of interfacial debonding and film delamination on pile-up in the film. Furthermore, when the pile-up is accounted for in contact area determinations, the film hardness is actually independent of the substrate, thus indicating that the hardness differences observed in nanoindentation testing are an artifact of the testing analysis procedure. Results of the experiments are documented and discussed. 8 refs., 6 figs., 1 tab

Nanoindentation hardness of soft films on hard substrates: Effects of the substrate

The ability to accurately measure the mechanical properties of thin metallic films is important in the semiconductor industry as it relates to device reliability issues. One popular technique for measuring thin film mechanical properties is nanoindentation. This technique has the advantage of being able to measure properties such as hardness and elastic modulus without removing a film from its substrate. However, according to a widely-held rule of thumb, intrinsic film properties can be measured in a manner which is not influenced by the substrate only if the indentation depth is kept to less than 10% of the film thickness, which is often not practical. In this work, a method for making substrate independent hardness measurements of soft metallic films on hard substrates is proposed. The primary issue to be addressed is the substrate-induced enhancement of indentation pile-up and the ways in which this pile-up influences the contact area determined from analyses of nanoindentation load- displacement data. Based on experimental observations of soft aluminum films on silicon, glass, and sapphire substrates, a simple empirical relationship is derived which relates the amount of pile-up to the contact depth. From this relationship, a simple method is developed which allows the intrinsic hardness of the film to be measured by nanoindentation methods even when the indenter penetrates through the film into the substrate

A multi-country level analysis of the environmental attitudes and behaviours among young consumers

Environmental issues have become more prominent internationally and are increasingly featured in discussion by governments, business and academics. This paper presents the results of a study which examines the concerns for environmental issues and purchase behaviours of a sample of 1173 young consumers in England, Germany, Portugal and Spain; countries which represent different realities in terms of economic development, social context and cultural issues. An analysis of the differences between the respondents from the four countries regarding concepts such as man-nature orientation, generativity, environmental concern, consumer perceived effectiveness, conservation behaviour and environmentally-friendly buying behaviour is presented. The results obtained confirm the existence of significant differences between countries for almost all variables. © 2013 Copyright Taylor and Francis Group, LLC

Inaccuracies in Sneddon`s solution for elastic indentation by a rigid cone and their implications for nanoindentation data analysis

Methods currently used for analyzing nanoindentation load-displacement data to determine a material`s hardness and elastic modulus are based on Sneddon`s solution for the indentation of an elastic half-space by a rigid axisymmetric indenter. Although this solution is widely used, no attempts have been made to determine how well it works for conditions of finite deformation, as is the case in most nanoindentation experiments with sharp indenters. Analytical and finite element results are presented which show that corrections to Sneddon`s solution are needed if it is to be accurately applied to the case of deformation by a rigid cone. Failure to make the corrections results in an underestimation of the load and contact stiffness and an overestimation of the elastic modulus, with the magnitude of the errors depending on the angle of the indenter and Poisson`s ratio of the half-space. For a rigid conical indenter with a half-included tip angle of 70.3{degrees}, i.e., the angle giving the same area-to-depth ratio as the Berkovich indenter used commonly in nanoindentation experiments, the underestimation of the load and contact stiffness and overestimation of the elastic modulus may be as large as 13%. It is shown that a simple first order correction can be achieved by redefining the effective angle of the indenter in terms of the elastic constants. Implications for the interpretation of nanoindentation data are discussed

An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments

International audienceThe indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%

Critical issues in measuring the mechanical properties of hard films on soft substrates by nanoindentation techniques

This study explores the difficulties encountered when using conventional nanoindentation techniques to measure the Young`s modulus and hardness of hard films on soft substrates. In general, the indentation measurement of film/substrate systems is affected by four material properties: the Young`s modulus and hardness of the film, and the Young`s modulus and hardness of the substrate. For the particular case of a hard film on a soft substrate, there is a tendency for the material around the hardness impression to sink-in which results from the large difference in yielding of the two materials. In this study, a model system consisting of NiP on annealed Cu was used to explore the behavior. This system is interesting because the film and substrate have similar Young`s moduli, minimizing the elastic behavior as a variable. In contrast, the hardness of NiP is approximately 7--8 GPa, and that of the annealed copper is less than 1 GPa, providing a factor of 10 difference in the plastic flow characteristics. Experimental results indicate that standard analytical methods for determining the contact depth, hardness and Young`s modulus do not work well for the case of a hard film on a soft substrate. At shallow contact depths, the measured indentation modulus is close to that of the film, but at larger depths sink-in phenomena result in an overestimation of the contact area, and an indentation modulus which is less than the Young`s modulus of both the film and substrate. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide critical details of the physical processes involved, and illustrate how the standard data analyses overestimate the true contact area

Nanoindentation of soft films on hard substrates: The importance of pile-up

Nanoindentation is used for measuring mechanical properties of thin films. This paper addresses potential measurement errors caused by pile-up when soft films deposited on hard substrates are tested this way. Pile-up is exacerbated in soft film/hard substrate systems because of the constraint the substrate exerts on plastic deformation of the film. To examine pile-up effects, Al films 240 and 1700 nm thick were deposited on hard glass and tested by standard nanoindentation. In Al/glass, the film and substrate have similar elastic moduli; thus, any unusual behavior in nanoindentation results may be attributed to differences in plastic flow alone. SEM examination of nanoindentation hardness impressions in the film revealed that common methods for analyzing nanoindentation data underestimate the true contact areas by as much as 80%, which results in overestimations of the hardness and modulus by as much as 80 and 35%, respectively. Sources of these errors and their effect on measurement of hardness and elastic modulus are discussed, and a simple model for the composite hardness of the film/substrate system is developed. This model could prove useful when it is not possible to make indentations shallow enough to avoid substrate effects

The correlation of indentation size effect experiments with pyramidal and spherical indenters

Experiments were conducted in annealed iridium using pyramidal and spherical indenters over a wide range of load. For a Berkovich pyramidal indenter, the hardness increased with decreasing depth of penetration. However, for spherical indenters, hardness increased with decreasing sphere radius. Based on the number of geometrically necessary dislocations generated during indentation, a theory that takes into account the work hardening differences between pyramidal and spherical indenters is developed to correlate the indentation size effects measured with the two indenters. The experimental results verify the theoretical correlation