Sharp Indenter Research Articles

OS1903 微小押込試験の負荷速度増加による荷重増加の要因

The indentation test is a method to extract material plastic and elastic properties by pressing sharp indenter to the specimen. In this test, load and displacement are continuously recorded. In recent our studies, we can transform load-displacement curves into stress-strain curves. However, it is indicated by past research that if strain exceeds more than predefined strain (critical strain), the load-displacement curves are unlikely be affected. In this report, we examined how load-displacement curves are varied while increasing strain rate and relation between strain rate and critical strain by FE analysis. Finite element analysis is used and the analysis data is confirmed valid by comparing experimental result of the Indentation test. As a result, it turned out that higher strain rate become, the more load-displacement curves are affected and high strain area, which is located in direct beneath of the indenter have only little impact on load-displacement curves of the Indentation test.

Effect of Strain Rate on Load-Displacement Relations by Instrumented Sharp Indentation

Effect of Strain Rate on Load-Displacement Relations by Instrumented Sharp Indentation

Nanoscale Thermomechanics of Wear-Resilient Polymeric Bilayer Systems

We explore the effect of an ultrathin elastic coating to optimize the mechanical stability of an underlying polymer film for nanoscale applications. The coating consists of a several nanometer thin plasma-polymerized norbornene layer. Scanning probes are used to characterize the system in terms of shear-force-induced wear and thermally assisted indentation. The layer transforms a weakly performing polystyrene film into a highly wear-resistive system, ideal for high-density and low-power data storage applications. The result can be understood from the indentation characteristics with a hot and sharp indenter tip. The latter gives rise to a deformation mode in the fully plastic regime, enabling a simple interpretation of the results. The softening transition and the yield stress of the system on a microsecond time scale and a nanometer size scale were obtained. We show that the plastic deformation is governed by yielding in the polystyrene sublayer, which renders the overall system soft for plastic deformation. The ultrathin protection layer contributes as an elastic skin, which shields part of the temperature and pressure and enables the high wear resistance against lateral forces. Moreover, the method of probing polymers at microsecond and nanometer size scales opens up new opportunities for studying polymer physics in a largely unexplored regime. Thus, we find softening temperatures of more than 100 °C above the polystyrene glass transition, which implies that for the short interaction time scales the glassy state of the polymer is preserved up to this temperature.

Effects of elastic-plastic properties of materials on residual indentation impressions in nano-indentation using sharp indenter

One of the primary features of nano-indentation technique is that the contact area induced by an indenter is indirectly measured by a relationship between the penetration depth and the known geometry of the indenter. However, this indirect measurement occasionally leads to inaccurate properties of the indented material. The objective of this study is to investigate the effects of E*/σr and the strain hardening exponents n of materials on the behaviors of pile-up and sink-in in nano-indentation and to predict n values of materials from the residual indentation impressions. The relations between the residual indentation profile and n value of the indented material were identified by dimensional analysis. Also, they were numerically formulated using FE analysis of nano-indentation for 140 different combinations of elastic-plastic parameters such as E, σy and n. The parameters of hrp/hm, herp/hm, Rr/hm and HO&P/Hreal were introduced as various dimensionless parameters to represent and quantify the residual indentation profile after indentation. They were subsequently characterized as dimensionless functions using n and E*/σr values. Finally, the validity of these functions was verified through 3D FE analysis of nano-indentation for Al 6061-T6 and AISI 1010 materials.

An inverse problem for adhesive contact and non-direct evaluation of material properties for nanomechanics applications

We show how the values of the effective elastic modulus of contacting solids and the work of adhesion, that are the crucial material parameters for application of theories of adhesive contact to nanomechanics, may be quantified from a single test using a non-direct approach (the Borodich-Galanov (BG) method). Usually these characteristics are not determined from the same test, e.g. often sharp pyramidal indenters are used to determine the elastic modulus from a nanoindentation test, while the work of adhesion is determined from a different test by the direct measurements of pull-off force of a sphere. The latter measurements can be greatly affected by roughness of contacting solids and they are unstable due to instability of the load-displacement diagrams at tension. The BG method is based on an inverse analysis of a stable region of the force-displacements curve obtained from the depth-sensing indentation of a sphere into an elastic sample. Various aspects related to solving the inverse problem for adhesive contact and experimental evaluation of material properties for nanomechanics applications are discussed. It is shown that the BG method is simple and robust. Some theoretical aspects of the method are discussed and the BG method is developed by application of statistical approaches to experimental data. The advantages of the BG method are demonstrated by its application to soft polymer (polyvinylsiloxane) samples.

Characterization of viscoelastic properties of molybdenum disulphide filled polyamide by indentation

In this paper, the creep behavior of molybdenum disulphide (MoS2) filled polyamide 66 composite was investigated through sharp indentation at room temperature. Two types of indentation creep test, the 3-step indentation test, and the 5-step indentation test were considered in order to explore whether the measured creep response is mainly viscoelastic or includes a significant contribution from the plastic deformation developed during the loading phase. The experimental indentation creep data were analyzed within an analytical framework based on the hereditary integral operator for the ramp creep and a viscoelastic–plastic (VEP) model in order to determine the indentation creep compliance function including the short- and long-time modulus. The equivalent shear modulus calculated from the creep compliance function was compared to the indentation plane strain modulus derived from the initial slope of the unloading curve in order to investigate the validity of the Oliver and Pharr method.

Determining Elastoplastic Properties of Thin Film from Loading and Unloading Curve of Sharp Indentation

Determining Elastoplastic Properties of Thin Film from Loading and Unloading Curve of Sharp Indentation

Relationship between subsurface damage and surface roughness of glass BK7 in rotary ultrasonic machining and conventional grinding processes

Subsurface damage (SSD) induced during the abrasive machining process considerably influences the technological application of the optical components. However, to date, there is no rapid and effective method to inspect the depth of SSD. For the purpose of precise and nondestructive evaluation of the SSD depth generated in rotary ultrasonic machining (RUM) and conventional grinding (CG) processes, a theoretical model, ground on indentation fracture mechanics of brittle material, was proposed by analyzing the correlation between the median and lateral crack systems aroused by a sharp indenter. It was found that the SSD depth was nonlinear monotone increasing with square of surface roughness (SR), namely, SSD = χSR2 + l. Utilizing this model, the SSD depth could be quickly and precisely predicted through the SR (p–v value.) of the machined surface, geometrical features of the abrasive, and the material mechanical properties. To validate the feasibility of this method, both RUM and CG tests were conducted on the BK7 glass specimens with a Sauer Ultrasonic 20. Subsequently, the SSD of these specimens was exposed with the polishing–etching technique. The measurement results of SSD depth were consistent with the prediction values of this model, which reflected the feasibility of using this model to rapidly and accurately predict the SSD depth.

Deformation behavior of metallic glass thin films

We report room-temperature deformation behavior of damage-free metallic glass films characterized by nanoindentation and atomic force microscopy. The glass films with thicknesses ranging from 5 μm down to ∼60 nm plastically deform by shear bands when subjected to both spherical and sharp Berkovich indenters. Importantly, we found that gallium contamination from focus ion beam (FIB) milling significantly suppresses shear band formation, indicating that the absence of shear bands in FIB milled samples may be caused by gallium irradiation damage, rather than sample size effect. Finite element simulation reveals that a high stress gradient at the film/substrate interface promotes the plastic deformation of the thin films but does not give rise to significant strain inhomogeneity.

Deformation of plastically compressible hardening-softening-hardening solids

Motivated by a model of the response of vertically aligned carbon nanotube (VACNT) pillars in uniaxial compression, we consider the deformation of a class of compressible elastic-viscoplastic solids with a hardeningsoftening-hardening variation of flow strength with plastic strain. In previous work (Hutchens et al. 2011) a constitutive relation was presented and used to model the response of VACNT pillars in axisymmetric compression. Subsequently, it was found that due to a programming error the constitutive relation presented in the paper (Hutchens et al. 2011) was not the one actually implemented. In particular, the plastic flow rule actually used did not satisfy plastic normality. Here, we present the constitutive formulation actually implemented in the previous work (Hutchens et al. 2011). Dynamic, finite deformation, finite element calculations are carried out for uniaxial compression, uniaxial tension and for indentation of a “half-space” by a conical indenter tip. A sequential buckling-like deformation mode is found in compression when there is plastic non-normality and hardeningsoftening-hardening. The same material characterization gives rise to a Luders band-like deformation mode in tension. When there is a deformation mode with a sharp front along mesh boundaries, the overall stress-strain response contains high frequency oscillations that are a mesh artifact. The responses of non-softening solids are also analyzed and their overall stress-strain behavior and deformation modes are compared with those of hardening-softening-hardening solids. We find that indentation with a sharp indenter tip gives a qualitatively equivalent response for hardening and hardening-softening-hardening solids.

Characterization of stress-strain relationships of elastoplastic materials: An improved method with conical and pyramidal indenters

The load-displacement curve in indentation is widely used to extract elastoplastic properties of materials. It is believed that such a measurement is non-unique and a full stress-strain curve cannot be obtained with a sharp indenter or even plural and spherical indenters. By introducing a ratio of the additional residual area to the area of a profile indenter, we proposed a new set of dimensionless functions. Based on these functions and finite element simulations, analytical expressions were derived between indentation data and elastoplastic properties. It is shown that this method can effectively distinguish highly elastic and plastic solids (Cheng and Cheng, 1999) and mystical materials (Chen et al., 2007), which provides a useful guideline for properly using the indentation technique to measure elastoplastic properties of materials with conical and pyramidal indenters.

Residual stress effect on hardness and yield strength of Ni thin film

Sharp indentation tests with a Berkovich indenter are utilized to investigate the influence of residual stress on the mechanical properties of Ni thin film subjected to plastic deformation. Based on the indention size effect, it is shown that residual stress has almost no influence on hardness in the elastic deformation stage while plastic deformation has a significant impact on the measurement of hardness. The elastic modulus and work hardening exponent are independent of elastic or plastic deformation and the evolution of yield strength follows the stress–strain curve of Ni thin film due to work hardening. Two analytical models are suggested in which the effect of residual strain on hardness and yield strength can be considered.

An Inverse Approach for Extracting Elastic–plastic Properties of Thin Films from Small Scale Sharp Indentation

An inverse method for extracting the elastic-plastic properties of metallic thin films from instrumented sharp indentation has been proposed in terms of dimensional analysis and finite element modeling. A wide range of materials with different elastic modulus, yield strength, and strain-hardening exponent were examined. Similar to the Nix-Gao model for the depth dependence of hardness H, ( H H 0 ) 2 = 1 + h H * h , the relationship between elastic modulus E and indentation depth h can be expressed as ( E E 0 ) 4 = 1 + h E * h . By combining these two formulas, we find that there is a relationship between yield stress σ y 0 and indentation depth h: σ y = σ y 0 ⋅ ( 1 + h H * h ) f ( n ) ⋅ ( 1 + h E * h ) [ 0.25 − 0.54 f ( n ) ] where σ y0 is the yield strength associated with the strain-hardening exponent n, the true hardness H0 and the true elastic modulus E0. f ( n ) = 1 2 ( 1 − n ) is constant, which is only related to n, and h H * and h E * are characteristic lengths for hardness and elastic modulus. The results obtained from inverse analysis show that the elastic-plastic properties of thin films can be uniquely extracted from the solution of this relationship when the indentation size effect has to be taken into account.

Hardness Variation with Indenter Sharpness in an Au Thin-Film

The effects of the indenter shape on hardness were studied from thin-film nanoindentations. Two Berkovich indenters with different operating histories were prepared and their morphologies were measured with an atomic force microscope. The curvature radii of both indenters that were measured through an image analysis were 58.8 nm and 732.2 nm, respectively. The nanoindentations were carried out on a 1.2 microm-thick Au thin-film with a Nanoindenter XP system with both indenters. Various nanoindentation data with indenter exchanges were surveyed, and they showed that the peak indentation loads under the blunter indenter were higher than those of the sharper indenter at the same indentation depths. The indenter sharpness parameter was used to correct the raw nanoindentation curves. The corrected curves overlapped well and the resulting hardness values were consistent regardless of the indenter sharpness. The intrinsic hardness values of the Au thin-film from both indenters agreed with each other, with only a 0.6% difference. This means the indenter sharpness was properly corrected and that the sharpness must be considered when the contact properties are measured at shallow indentations.

The role of probe shape on the initiation of metal plasticity in nanoindentation

The dislocation nucleation stress of crystalline materials is frequently estimated from the maximum shear stress, assuming Hertzian contact up to the first “pop-in” event, which is a sudden displacement burst during load-controlled nanoindentation. However, an irregular indenter tip shape will significantly change the stress distribution, and therefore the maximum shear stress from a Hertzian estimation. Here, we assess possible errors and pitfalls of the Hertzian estimation of initial plastic yield at the nanoscale. The near-apex shapes of two Berkovich indenters, one sharp and one worn, were measured by atomic force microscopy and directly input into finite element analysis (FEA) models for “virtual” nanoindentation experiments on single-crystal tungsten. Experiments were also carried out with those indenters. Excellent agreement is found between experimental and FEA force–displacement relationships, but the discrepancies between Hertzian and FEA estimates of the shear stresses are over 25% for the sharp indenter and over 50% for the blunt indenter. This demonstrates that small irregularities in the shape of indenter tips can cause significant deviations from the Hertzian estimation of dislocation nucleation stress.

The reduced modulus in the analysis of sharp instrumented indentation tests

Abstract

Nanomechanical Testing of the Barrier Thin Film Adhesion to a Flexible Polymer Substrate

Polymers in the form of films are nowadays used in a wide range of applications, from the packaging of several products in food industry to the cutting edge organic optoelectronic devices. Despite their remarkable advantages, the polymer films suffer from poor mechanical and barrier properties. Overcoming of these drawbacks can be achieved by using inorganic, hybrid polymers and/or nanocomposite thin films to coat the flexible polymer substrate. In this way the permeation of the harmful atmospheric gases (H2O and O2) through the polymer can be significantly reduced and, as a result, the product service life is prolonged. In this work, the nanomechanical behavior of 50 μm thick flexible poly(ethylene terephthalate) (PET) substrate coated with AlOx barrier thin film (40 nm and 70 nm thick) is studied. A synergy of state-of- the-art experimental techniques, such as depth-sensing Nanoindentation, Scratch Test and Atomic Force Microscopy (AFM), was used to shed some light on the deformation mechanisms and the adhesion failures of the inorganic thin film on flexible PET substrate. Several nanoindents were made on each sample with a sharp Berkovich-type indenter, in various maximum normal applied load (P)/indentation depths. Analysis of the measured load-displacement (P–h) curves showed that for low loads (P ⩽ 35 μN) the contact between the indenter and the sample was almost elastic. The AFM images provided valuable information about deformation during higher normal applied load (P > 500 μN) nanoindentation, which induces elastic/plastic deformation to the samples. Delamination/cracking of the barrier thin film from the PET flexible substrate was not observed even though the indenter penetrated almost 7 times the thickness of the thin film. On the other hand, cracking, fragmentation and finally delamination of the barrier thin film occurred during the ramping load Scratch Test.

Determination of the mechanical properties of amorphous materials through instrumented nanoindentation

A novel methodology based on instrumented indentation is developed to determine the mechanical properties of amorphous materials which present cohesive-frictional behaviour. The approach is based on the concept of a universal hardness equation, which results from the assumption of a characteristic indentation pressure proportional to the hardness. The actual universal hardness equation is obtained from a detailed finite element analysis of the process of sharp indentation for a very wide range of material properties, and the inverse problem (i.e. how to extract the elastic modulus, the compressive yield strength and the friction angle) from instrumented indentation is solved. The applicability and limitations of the novel approach are highlighted. Finally, the model is validated against experimental data in metallic and ceramic glasses as well as polymers, covering a wide range of amorphous materials in terms of elastic modulus, yield strength and friction angle.

Influence of hydration on nanoindentation induced energy expenditure of dentin

Improved understanding of the effects of hydration and drying in mineralized tissues is highly desirable, particularly for physiologically hydrated biological materials such as dentin. We investigated the influence of hydration on the nanomechanical properties of healthy dentin and hypothesized that drying leads to an increase in indentation induced energy expenditure and hardness. Hydrated and dry dentin were tested with a UMIS set up with a Berkovich indenter at a maximum load of 50mN. Values representative of the energy expenditure behavior were presented as dissipated energy, Ud, recovered energy, Ue, normalized energy expenditure index, ψ, and hardness, H. Energy expenditure index results, which normalize the energy expenditure for each test and describe the relative energy dissipation–recovery behavior of a material, suggested that, for the relatively severe contact strains about a sharp Berkovich indenter, dissipation dominates the mechanical response of both the hydrated and dry dentin. In support of our initial hypothesis, dry dentin presented a significantly higher energy expenditure index than hydrated dentin (p<0.0001). These results were primarily associated with a lower Ue that was found upon drying. Hydration also decreased H significantly (p<0.0001). In summary, this study presents the first direct measurements of the energy expenditure behavior of hydrated and dry dentin using instrumented nanoindentation.

A new method to determine the elastoplastic properties of ductile materials by conical indentation

Based on load-displacement curves, indentation is widely used to extract the elastoplastic properties of materials. It is generally believed that such a measure is non-unique and a full stress-strain curve cannot be obtained using plural sharp and deep spherical indenters. In this paper we show that by introducing an additional dimensionless function of ΔA/A (the ratio of residual area to the area of an indenter profile) in the reverse analysis, the elastoplastic properties of several unknown materials that exhibit visually indistinguishable load-displacement curves can be uniquely determined with a sharp indentation.