Showing papers on "Indentation published in 2014"

Micro/Nanotribology and Its Applications

Abstract: Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of single-crystal silicon, natural diamond, magnetic media (magnetic tapes and disks) and magnetic heads have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurements of atomic-scale friction of a freshly-cleaved highly-oriented pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement has been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macro scales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than the macrofriction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load approaching to the macrofriction at contact stresses higher than the hardness of the softer material. Wear rate for single-crystal silicon is approximately constant for various loads and test durations. However, for magnetic disks with a multilayered thin-film structure, the wear of the diamond like carbon overcoat is catastrophic. Breakdown of thin films can be detected with AFM. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nono scratches. AFM has been modified to obtain load-displacement curves and for nanoindentation hardness measurements with depth of indentation as low as 1 mm. Scratching and indentation on nanoscales are the powerful ways to screen for adhesion and resistance to deformation of ultrathin fdms. Detection of material transfer on a nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thichness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolyers and chemically-bonded lubricant films with a mobile fraction are superior in wear resistance. Finally, AFM has also shown to be useful for nanofabrication/nanomachining. Friction and wear on micro-and nanoscales have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help def'me the regimes for ultra-low friction and near zero wear.

The Hertz-Type and Adhesive Contact Problems for Depth-Sensing Indentation

Abstract: Connections between the Hertz-type contact problems and depth-sensing indentation of materials are studied. Formulations of Hertz-type contact problems with various boundary conditions within the contact area are discussed in detail. The problems under investigations can be subdivided into two large groups: self-similar problems for anisotropic materials with various rheological properties and adhesive contact problems for arbitrary bodies of revolution or for power-law shaped blunt indenters. Specific features of indentation problems are described and the common methods for extracting elastic and adhesive properties of materials are briefly reviewed. The basic formulae are extended to the case of nonslipping boundary conditions between a probe and the material. The main formulae of the JKR theory of adhesion are extended to any material with rotational symmetry of the elastic properties. These materials include not only isotropic or transversely isotropic elastic solids but also homogeneously prestressed isotropic or transversely isotropic nonlinear elastic materials. The BG method introduced for extracting adhesive and elastic properties of isotropic elastic materials from depth-sensing diagrams of spherical indenters, is described and extended to linear or linearized materials with rotational symmetry of the elastic properties.

Spherical indentation method for determining the constitutive parameters of hyperelastic soft materials

Abstract: A comprehensive study on the spherical indentation of hyperelastic soft materials is carried out through combined theoretical, computational, and experimental efforts Four widely used hyperelastic constitutive models are studied, including neo-Hookean, Mooney–Rivlin, Fung, and Arruda–Boyce models Through dimensional analysis and finite element simulations, we establish the explicit relations between the indentation loads at given indentation depths and the constitutive parameters of materials Based on the obtained results, the applicability of Hertzian solution to the measurement of the initial shear modulus of hyperelastic materials is examined Furthermore, from the viewpoint of inverse problems, the possibility to measure some other properties of a hyperelastic material using spherical indentation tests, eg, locking stretch, is addressed by considering the existence, uniqueness, and stability of the solution Experiments have been performed on polydimethylsiloxane to validate the conclusions drawn from our theoretical analysis The results reported in this study should help identify the extent to which the mechanical properties of hyperelastic materials could be measured from spherical indentation tests

Large recovery strain in Fe-Mn-Si-based shape memory steels obtained by engineering annealing twin boundaries

Abstract: It has previously been shown experimentally that nanotwins in cubic boron nitride can lead to significant strengthening. Here, the authors perform simulations to understand the mechanism behind this, reporting a twin-boundary dominated indentation strain stiffening phenomena.

Large indentation strain-stiffening in nanotwinned cubic boron nitride

Abstract: Recent experiments reported a substantial strengthening of cubic boron nitride by nanotwinning. This discovery raises fundamental questions about new atomistic mechanisms governing incipient plasticity in nanostructured strong covalent solids. Here we reveal an unusual twin-boundary dominated indentation strain-stiffening mechanism that produces a large strength enhancement at nanometer-scale twinning size where a strength reduction is normally expected due to the reverse Hall-Petch effect. First-principles calculations unveil significantly enhanced indentation shear strength in nanotwinned cubic boron nitride by bond rearrangement at the twin boundary under indentation compression and shear strains that produces especially strong stress response. This remarkable strain-stiffening mechanism offers fundamental insights for understanding the stress response of nanotwinned covalent solids under indentation.

Atomistic simulation of tantalum nanoindentation: Effects of indenter diameter, penetration velocity, and interatomic potentials on defect mechanisms and evolution

Abstract: Nanoindentation simulations are a helpful complement to experiments. There is a dearth of nanoindentation simulations for bcc metals, partly due to the lack of computationally efficient and reliable interatomic potentials at large strains. We carry out indentation simulations for bcc tantalum using three different interatomic potentials and present the defect mechanisms responsible for the creation and expansion of the plastic deformation zone: twins are initially formed, giving rise to shear loop expansion and the formation of sequential prismatic loops. The calculated elastic constants as function of pressure as well as stacking fault energy surfaces explain the significant differences found in the defect structures generated for the three potentials investigated in this study. The simulations enable the quantification of total dislocation length and twinning fraction. The indenter velocity is varied and, as expected, the penetration depth for the first pop-in (defect emission) event shows a strain rate sensitivity m in the range of 0.037-0.055. The effect of indenter diameter on the first pop-in is discussed. A new intrinsic length-scale model is presented based on the profile of the residual indentation and geometrically necessary dislocation theory.

Failure and deformation mechanisms during indentation in nanostructured Cr/CrN multilayer coatings

Abstract: The aim of the present work was to investigate the microstructure evolution of the nanostructured Cr/CrN multilayer coatings on titanium Ti6Al4V alloy during indentation. Five types of multilayer (Cr/CrN) × 8 coatings were deposited on Ti6Al4V titanium alloy by the PVD vacuum arc method. The coatings had the same total thickness and number of constituent layers, but differed in the thickness ratio of the Cr and CrN constituent layers (QCr/CrN parameter). The indentation measurements were performed with a Berkovich indenter at loads of 100 mN, 500 mN, and 1 N. The SEM and STEM observations were focused on the cracking behaviour in the CrN layers as well as on the microstructure changes in the ductile Cr layers. The investigations demonstrate that the applied surface treatment significantly improves the hardness of the titanium alloy. The hardness of the coatings varied in the range 10 to 35 GPa, depending on the thickness ratio of the layers. Observations performed on cross-sections through the indentation zones revealed small radial cracks, which were observed only in the CrN layers. They initiated at the interface and propagated across a given layer, to be arrested at the next interface. The STEM observations also showed microstructure changes in the Cr layers due to the accommodation of plastic deformation. The two main mechanisms of permanent deformation were observed.

A Raman-spectroscopic study of indentation-induced structural changes in technical alkali-borosilicate glasses with varying silicate network connectivity

Abstract: We report on the deformation behavior of a series of glasses in the ternary alkali-borosilicate system. Vickers indentation was applied to different samples and structural changes were studied by micro-Raman spectroscopy. Selected sodium- and potassium-borosilicate glasses, a soda-lime silicate glass and pure vitreous SiO 2 were chosen as models for technical glasses, as their varying content of non-bridging oxygen atoms affects the connectivity of the silicate and borate sub-networks. Indentation experiments were performed with different loads and two different load times in order to analyze the response of the two sub-networks in the studied glasses. The measured Raman spectra show distinct indentation-induced changes in the glass structure. These changes depend not only on the borate concentration, but also on the silicate sub-network's connectivity and reflect anomalous (densification-driven) and normal (shear-driven) deformation mechanisms. Using vitreous SiO 2 as reference material, indentation-induced structural changes were also correlated to hydrostatic compression experiments from the literature in order to develop a more detailed picture of the apparent evolution of the local compaction in and around the indent.

Adhesion and Wetting of Nanoparticles on Soft Surfaces

Abstract: We study adhesion of spherical and cylindrical nanoparticles on soft (gel-like) substrates using a combination of the molecular dynamics simulations and theoretical calculations. The substrate deformation is obtained as a function of the gel shear modulus, nanoparticle size, surface tension of nanoparticles and substrate, and work of adhesion. It was shown recently that the classical JKR model can only be applied to describe nanoparticle adhesion on relatively stiff substrates. In this so-called adhesion regime the deformation of the substrate is determined by balancing the elastic energy of indentation and the work of adhesion between a nanoparticle and a gel. However, in the case of soft gels when substrates undergo moderate deformations the depth of the indentation produced by a nanoparticle is determined by the surface tension of the gel and the work of adhesion (the wetting regime). We present an analytical model describing crossover between adhesion and wetting regimes. In the framework of this mode...

Indentation of a Rigid Sphere into an Elastic Substrate with Surface Tension and Adhesion

Abstract: The surface tension of compliant materials such as gels provides resistance to deformation in addition to and sometimes surpassing that due to elasticity. This article studies how surface tension changes the contact mechanics of a small hard sphere indenting a soft elastic substrate. Previous studies have examined the special case where the external load is zero, so contact is driven by adhesion alone. Here, we tackle the much more complicated problem where, in addition to adhesion, deformation is driven by an indentation force. We present an exact solution based on small strain theory. The relation between indentation force (displacement) and contact radius is found to depend on a single dimensionless parameter: $\omega=\sigma(\mu R)^{-2/3}(9\pi W_{\textrm{ad}}/4)^{-1/3}$, where $\sigma$ and $\mu$ are the surface tension and shear modulus of the substrate, $R$ is the sphere radius, and $W_{\textrm{ad}}$ is the interfacial work of adhesion. Our theory reduces to the Johnson-Kendall-Roberts theory and Young-Dupre equation in the limits of small and large $\omega$ respectively, and compares well with existing experimental data. Our results show that, although surface tension can significantly affect the indentation force, the magnitude of the pull-off load in the partial wetting liquid-like limit is reduced only by 1/3 compared with the JKR limit, and the pull-off behavior is completely determined by $\omega$.

Mechanical properties of Inconel 625 laser cladded coatings: Depth sensing indentation analysis

Abstract: Ni-base superalloys, like Inconel 625, exhibit a high corrosion and oxidation resistance. For this reason, these alloys are typically used to manufacture engineering components, or coatings for cheaper metallic substrates, which should work in extreme conditions including mechanical loads and an aggressive environment at high temperature. Some of these applications could be found in chemical and petrochemical plants, power generation sector, aircraft engines components, heat exchanger tubing or boilers of waste incinerators. The advantages of using coatings protecting cheaper materials are the significant cost reductions compared to using bulk alloys. In this work, a Ni-base superalloy Inconel 625 was deposited onto a medium alloy steel by laser cladding using a high-power diode laser. A process parameters selection was performed following a Taguchi's array with the aim to reduce the number of performed tests. The mechanical properties (hardness and elastic modulus) of the Inconel 625 coatings were studied by nanoindentation. The mechanical properties were obtained from each indentation as a function of the penetration depth. These results were compared with those obtained previously in two other Inconel 625 coatings processed by cold-spray and laser re-melting cold-spray techniques. A comparison among the mechanical properties and the microstructures of these coatings was carried out to evaluate alternative methods to produce Inconel 625 coatings.

Nanomechanical assessment of human and murine collagen fibrils via atomic force microscopy cantilever-based nanoindentation

Abstract: The nanomechanical assessment of collagen fibrils via atomic force microscopy (AFM) is of increasing interest within the biomedical research community. In contrast to conventional nanoindentation there exists no common standard for conducting experiments and analysis of data. Currently used analysis approaches vary between studies and validation of quantitative results is usually not performed, which makes comparison of data from different studies difficult. Also there are no recommendations with regards to the maximum indentation depth that should not be exceeded to avoid substrate effects. Here we present a methodology and analysis approach for AFM cantilever-based nanoindentation experiments that allows efficient use of captured data and relying on a reference sample for determination of tip shape. Further we show experimental evidence that maximum indentation depth on collagen fibrils should be lower than 10-15% of the height of the fibril to avoid substrate effects and we show comparisons between our and other approaches used in previous works. While our analysis approach yields similar values for indentation modulus compared to the Oliver-Pharr method we found that Hertzian analysis yielded significantly lower values. Applying our approach we successfully and efficiently indented collagen fibrils from human bronchi, which were about 30 nm in size, considerably smaller compared to collagen fibrils obtained from murine tail-tendon. In addition, derived mechanical parameters of collagen fibrils are in agreement with data previously published. To establish a quantitative validation we compared indentation results from conventional and AFM cantilever-based nanoindentation on polymeric samples with known mechanical properties. Importantly we can show that our approach yields similar results when compared to conventional nanoindentation on polymer samples. Introducing an approach that is reliable, efficient and taking into account the AFM tip shape, we anticipate that the present work may act as a guideline for conducting AFM cantilever-based nanoindentation of collagen fibrils. This may aid understanding of collagen-related diseases such as asthma, lung fibrosis or bone disease with potential alterations of collagen fibril mechanics.

Insights into reference point indentation involving human cortical bone: sensitivity to tissue anisotropy and mechanical behavior.

Abstract: Reference point indentation (RPI) is a microindentation technique involving 20 cycles of loading in "force-control" that can directly assess a patient׳s bone tissue properties. Even though preliminary clinical studies indicate a capability for fracture discrimination, little is known about what mechanical behavior the various RPI properties characterize and how these properties relate to traditional mechanical properties of bone. To address this, the present study investigated the sensitivity of RPI properties to anatomical location and tissue organization as well as examined to what extent RPI measurements explain the intrinsic mechanical properties of human cortical bone. Multiple indents with a target force of 10N were done in 2 orthogonal directions (longitudinal and transverse) per quadrant (anterior, medial, posterior, and lateral) of the femoral mid-shaft acquired from 26 donors (25-101 years old). Additional RPI measurements were acquired for 3 orthogonal directions (medial only). Independent of age, most RPI properties did not vary among these locations, but they did exhibit transverse isotropy such that resistance to indentation is greater in the longitudinal (axial) direction than in the transverse direction (radial or circumferential). Next, beam specimens (~2mm×5mm×40mm) were extracted from the medial cortex of femoral mid-shafts, acquired from 34 donors (21-99 years old). After monotonically loading the specimens in three-point bending to failure, RPI properties were acquired from an adjacent region outside the span. Indent direction was orthogonal to the bending axis. A significant inverse relationship was found between resistance to indentation and the apparent-level mechanical properties. Indentation distance increase (IDI) and a linear combination of IDI and the loading slope, averaged over cycles 3 through 20, provided the best explanation of the variance in ultimate stress (r(2)=0.25, p=0.003) and toughness (r(2)=0.35, p=0.004), respectively. With a transverse isotropic behavior akin to tissue hardness and modulus as determined by micro- and nano-indentation and a significant association with toughness, RPI properties are likely influenced by both elastic and plastic behavior of bone tissue.

Raman spectroscopic study of structural changes induced by micro-indentation in low alkali borosilicate glasses

Abstract: We report on the effect of micro-indentation on the local structure of glasses in the ternary borosilicate glass system Na2O–B2O3–SiO2. Indentation experiments were performed with loads ranging from 0.49 to 4.91 N and for loading times of 10–30 s in order to analyze the impact on the silicate and borate sub-networks in glasses ranging from pure SiO2 to binary Na2O–4B2O3, with a ratio of R = Na2O/B2O3 < 0.5. The Raman spectra reveal distinct changes in the glass structure, which can be correlated to different densification mechanisms occurring in the silicate and borate sub-networks, respectively. A clear trend from anomalous (densification-driven) deformation in silica-rich to normal (shear-driven) deformation behavior in alkali borate-rich glasses is observed.

Molecular dynamics simulation of nanoindentation of Fe3C and Fe4C

Abstract: Study of nanomechanical response of iron carbides is important because presence of iron carbides greatly influences the performance and longevity of steel components. This work contributes to the literature by exploring nanoindentation of Fe3C and tetrahedral-Fe4C using molecular dynamics simulation. The chemical interactions of iron and carbon were described through an analytical bond order inter-atomic potential (ABOP) energy function. The indentations were performed at an indentation speed of 50 m/s and a repeat trial was performed at 5 m/s. Load–displacement (P–h) curve for both these carbides showed residual indentation depth and maximum indentation depth (hf/hmax) ratio to be higher than 0.7 i.e. a circumstance where Oliver and Pharr method was not appropriate to be applied to evaluate the material properties. Alternate evaluation revealed Fe3C to be much harder than Fe4C. Gibbs free energy of formation and radial distribution function, coupled with state of the average local temperature and von Mises stresses indicate the formation of a new phase of iron-carbide. Formation of this newer phase was found to be due to deviatoric strain rather than the high temperature induced in the substrate during nanoindentation.

Understanding pop-ins in spherical nanoindentation

Abstract: Pop-ins, or sudden displacement-bursts at constant load in a nanoindentation test, are typically attributed to the difficulty of setting up potent dislocation sources in the very small indentation zones in these experiments. Such displacement (and strain) bursts would intuitively indicate a sharp drop in stress during the pop-in event itself. However, spherical indentation stress-strain curves routinely exhibit a high and stable indentation stress value during the pop-in, and the indentation stresses decrease only after a further finite amount of additional indentation displacement has been applied. In order to understand this discrepancy, we utilize a combination of interrupted spherical indentation tests along with depth profiling of the residual indentation surfaces using in-situ atomic force microscopy (AFM) to study pop-ins. The AFM surface profile maps show that there is an asymmetric profile change over a limited region around the indentation contact area for a single pop-in; the asymmetry disappears upon further loading beyond the pop-in. A plausible sequence of physical processes (related to metal plasticity) occurring underneath the indenter during and immediately after the occurrence of the pop-in is proposed to explain these observations.

Digital Volume Correlation Applied to X-ray Tomography Images from Spherical Indentation Tests on Lightweight Gypsum

Abstract: Cylinders made of lightweight gypsum are extracted from industrial plasterboard and then indented in-situ in an X-ray tomograph. The results from the in-situ experiment show that a compacted zone develops under the indenter, displaying a very sharp boundary with the undamaged material. Tomographic imaging during the mechanical load associated with digital volume correlation enable the displacement fields to be measured during the test. However, because of the inhomogeneous nature of the indentation test, a high spatial resolution for the displacement is called for, and because the range of displacement amplitudes is small, uncertainties on the measured displacement and strain fields are large. In this study, a new methodology is presented to address integrated digital volume correlation based on a library of fields computed from a commercial finite element software. It allows many fluctuations in the estimated displacement fields to be filtered out and the measurement to be much more robust and reliable. This opens new pathways for the identification of mechanical properties.

The elastic mechanical response of supported thin polymer films.

Abstract: Nanoindentation studies of the mechanical properties of sufficiently thin polymer films, supported by stiff substrates, indicate that the mechanical moduli are generally higher than those of the bulk. This enhancement of the effective modulus, in the thickness range of few hundred nanometers, is indicated to be associated with the propagation and impingement of the indentation tip induced stress field with the rigid underlying substrate; this is the so-called “substrate effect”. This behavior has been rationalized completely in terms of the moduli and Poisson’s ratios of the individual components, for the systems investigated thus far. Here we show that for thin supported polymer films, in general, information regarding the local chain stiffness and local vibrational constants of the polymers provides an appropriate rationalization of the overall mechanical response of polymers of differing chemical structures and polymer–substrate interactions. Our study should provide impetus for atomistic simulations t...

The effect of work-hardening and pile-up on nanoindentation measurements

Abstract: Nanoindentation is performed on the cross-section of copper samples subjected to surface mechanical attrition treatment (SMAT) The cross-section of the SMAT samples provides a unique microstructure with varying amounts of work-hardening depending on the distance from the SMAT surface Results show that for a given indentation load the pile-up height decreases and the indentation depth increases as the distance from the SMAT surface increases, both following a power law relationship Based on image analysis of the indented surface this increase in the pile-up height and decrease in indentation depth is attributed to the localization of plastic strain due to the increased resistance to dislocation motion in the work-hardened region For a given amount of work-hardening (in terms of distance from SMAT surface), the indentation depth increased with the indentation load obeying a power law relationship with the exponent ranging from 058 to 068 However, the pile-up height increased linearly with the load, with the rate (slope) increasing with the amount of work-hardening The observed linear increase in pile-up height with indentation load would naturally introduce an indentation size effect (ISE) if the hardness is corrected for the pile-up Interestingly, this ISE associated with pile-up increased with an increase in indentation depth, in contradiction to the ISE associated with strain gradient Deviation of the hardness values corrected for pile-up from the bulk behavior due to surface effect is highlighted and a method to obtain a bulk-equivalent hardness quantity representing the bulk behavior is proposed

Different Cold Spray Deposition Strategies: Single- and Multi-layers to Repair Aluminium Alloy Components

Abstract: Cold spraying is increasingly being used for reconstruction or repair of damaged aluminium alloy components, especially in the aviation industry. Both thin (<0.5 mm) and thick (up to 1 cm) coatings are necessary to achieve dimensional recovery of such components. Thin and above all thick coatings can be deposited in a single pass (single layer) or in several passes (multi-pass), resulting in different thermal and stress effects in the component and the coating itself. The thermal input, the amount and type of residual stresses and the porosity affect various characteristics such as adhesion, crack propagation and mechanical properties of the coating. In this study, two sets (single- and multi-pass) of aluminium alloy (AA6061) coatings with different thicknesses (0.5 mm to 2 mm) were deposited onto AA6061 substrates and compared using metallographic and fractographic analyses, four-point bending testing, residual stress analysis and Vickers microhardness indentation. Finally, the coating adhesion and cohesion were measured using the standard ASTM-C633 adhesion test and tubular coating tensile test. This study demonstrates that the single-layer strategy results in greater adhesion and lower porosity, while multilayer coatings have higher elastic modulus. Independent of the strategy, the compressive residual stress decreases as a function of coating thickness.

Evaluation of the tensile properties of a material through spherical indentation: definition of an average representative strain and a confidence domain

Abstract: In the present article, a new method for the determination of the hardening law using the load displacement curve, F–h, of a spherical indentation test is developed. This method is based on the study of the error between an experimental indentation curve and a number of finite elements simulation curves. For the smaller values of these errors, the error distribution shape is a valley, which is defined with an analytic equation. Except for the fact that the identified hardening law is a Hollomon type, no assumption was made for the proposed identification method. A new representative strain of the spherical indentation, called “average representative strain,” eaR was defined in the proposed article. In the bottom of the valley, all the stress–strain curves that intersect at a point of abscissa eaR lead to very similar indentation curves. Thus, the average representative strain indicates the part of the hardening law that is the better identified from spherical indentation test. The results show that a unique material parameter set (yield stress σy, strain hardening exponent n) is identified when using a single spherical indentation curve. However, for the experimental cases, the experimental imprecision and the material heterogeneity lead to different indentation curves, which makes the uniqueness of solution impossible. Therefore, the identified solution is not a single curve but a domain that is called “solution domain” in the yield stress–work hardening exponent diagram, and “confidence domain” in the stress–strain diagram. The confidence domain gives clear answers to the question of uniqueness of the solution and on the sensitivity of the indentation test to the identified hardening laws parameters.