Depth-sensing Indentation Experiments Research Articles

Adhesive depth-sensing indentation tests: Slopes of the force–displacement curves

Depth-sensing indentation experiments are a very important tool for estimating mechanical properties of modern materials. A review of various aspects of contact problems that used as the theoretical basis for interpretation of depth-sensing nanoindentation experiments is presented. Usually, analytical treatment of the indentation tests is based on analysis of the slopes of the force–displacement curves according to the non-adhesive Hertz contact theory. However, molecular adhesion is crucially important for many physical processes at the micro/nano-scales. Here, depth-sensing indentation techniques are reviewed and analyzed using the recent results obtained for adhesive contact problems. Fundamental relations for adhesive nanoindentation tests are derived for both frictionless and no-slip boundary conditions within the contact region. It is argued that the adhesive effects may be very important for treatment of the nanoindentation tests because the slopes of the force–displacement curves may considerably differ from the slopes derived using the non-adhesive contact theory.

Assessment of mechanical behavior of PLA composites reinforced with Mg micro-particles through depth-sensing indentations analysis

This work deals with the mechanical characterization by depth-sensing indentation (DSI) of PLLA and PLDA composites reinforced with micro-particles of Mg (up to 15wt%), which is a challenging task since the indented volume must provide information of the bulk composite, i.e. contain enough reinforcement particles. The composites were fabricated by combining hot extrusion and compression moulding. Physico-chemical characterization by TGA and DSC indicates that Mg anticipates the thermal degradation of the polymers but does not compromise their stability during processing. Especial emphasis is devoted to determine the effect of strain rate and Mg content on mechanical behavior, thus important information about the visco-elastic behavior and time-dependent response of the composites is obtained. Relevant for the intended application is that Mg addition increases the elastic modulus and hardness of the polymeric matrices and induces a higher resistance to flow. The elastic modulus obtained by DSI experiments shows good agreement with that obtained by uniaxial compression tests. The results indicate that DSI experiments are a reliable method to calculate the modulus of polymeric composites reinforced with micro-particles. Taking into consideration the mechanical properties results, PLA/Mg composite could be used as substitute for biodegradable monolithic polymeric implants already in the market for orthopedics (freeform meshes, mini plates, screws, pins, …), craniomaxillofacial, or spine.

Comparison of nanoindentation techniques for local mechanical quantification of aluminium alloy

Nanoindentation is widely used for the assessment of micromechanical behaviour of multiple phases within the material microstructure. Structural materials typically exhibit large variations in elastic modulus and hardness with increasing indentation depth during depth-sensing indentation experiments because the measured stiffness response is affected by its heterogeneity. In this study, the complicated microstructure of heterogeneous aluminium foam cell walls is described and related micromechanical analyses using various characterization modes performed. The paper compares the results of several nanoindentation approaches for the assessment of hardness and elastic parameters using both the quasi-static indentation and the dynamic modulus mapping method. The results of quasi-static indentation were performed for two different indenter geometries (Berkovich and spherical tips). The material phase properties were analysed with grid indentation method based on statistical evaluation of a large number of indentations (statistical deconvolution technique) as well as with isolated indentations by spherical indenter that revealed overall properties of the foam cell wall. Special attention was paid to the volume affected by the indenter and other factors influencing reliable evaluation of nanoindentation measurements. Intrinsic properties of the individual phases were also verified with modulus mapping technique. Results from the dynamic measurements were consistent with the values obtained in quasi-static measurements using the Berkovich indenter for both main microstructural phases.

Effect of Different Production Methods on the Mechanical and Microstructural Properties of Hypereutectic Al-Si Alloys

In this study, the effects of different production methods like melt spinning, high-energy ball milling, and combined melt spinning and high-energy ball milling on the mechanical and microstructural properties of hypereutectic Al-20Si-5Fe alloys were investigated. While microstructural and spectroscopic analyses were performed using scanning electron microscopy and X-ray diffractometry, mechanical properties were measured using a depth-sensing indentation instrument with a Berkovich tip. Microstructural and spectroscopic analyses demonstrate that high-energy ball milling process applied on the melt-spun Al-20-Si-5Fe alloy for 10 minutes brings about a reduction in the size of silicon particles and intermetallic compounds. However, further increase in milling time does not yield any significant reduction in size. High-energy ball milling for 10 minutes on the starting powders is not enough to form any intermetallic phase. According to the depth-sensing indentation experiments, high-energy milling of melt-spun Al-20Si-5Fe alloys shows an incremental behavior in terms of hardness values. For the Al-20Si-5Fe alloys investigated in this study, the production technique remarkably influences their elastic–plastic response to the indentation process in terms of both magnitude and shape of P-h curves.

Ex-situ depth-sensing indentation measurements of electrochemically produced Si–Li alloy films

We report for the first time ex-situ measurements of the Young's modulus and hardness of Si and Si–Li alloy thin film electrodes at various stages of lithium insertion via depth-sensing indentation experiments. Lithium is electrochemically inserted into the Si electrode, after which its mechanical properties are investigated in an anoxic environment. The Young's modulus was found to decrease from an initial value of 92 GPa for pure Si to 12 GPa at full lithium insertion (Li 15Si 4). The hardness changes from an initial value of 5 GPa for Si to 1.5 GPa for Li 15Si 4.

Comparison of Depth-Sensing Indentation at Ultramicroscopic Contacts by Single- and Multiple-Partial-Unload Cycles

Abstract Thin films typically exhibit variations in modulus and hardness with increasing indentation depth during depth-sensing indentation experiments at ultramicroscopic contacts (“nanoindentation”) because the measured stiffness response is affected by the substrate. There are three techniques commonly used to sample this variation in properties with depth: multiple indentations with increasing peak load, a single indentation with multiple-partial-unload cycles, and continuous stiffness measurement during a single indentation event. Experiments using the first two options were performed on a series of thin film systems and the method of multiple-partial-unloading found to have significant drawbacks when used at the sub-micron length scale that lead to erroneous conclusions.

Characterization of interference thin films grown on stainless steel surface by alternate pulse current in a sulphochromic solution

The aim of this work was to characterize thin interference films grown on the surface of AISI 304 stainless steel for decorative purposes. Films were grown in a sulphochromic solution at room temperature by an alternating pulse current method. The morphology and chemical state of the elements in the films were investigated by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), glow discharge optical emission spectrometry (GDOES), and infrared Fourier transform spectroscopy (FTIR). Depth-sensing indentation (DSI) experiments and wear abrasion tests were employed to assess the mechanical resistance of the films. The coloration process resulted in porous thin films which increased the surface roughness of the substrate. The interference films mainly consisted of hydrated chromium oxide containing iron. Increasing film thickness produced different colors and affected the mechanical properties of the coating-substrate system. Thicker films, such as those producing gold and green colors, were softer but more abrasion resistant.

Mechanics of indentation of plastically graded materials—I: Analysis

The introduction of controlled gradients in plastic properties is known to influence the resistance to damage and cracking at contact surfaces in many tribological applications. In order to assess potentially beneficial effects of plastic property gradients in tribological applications, it is essential first to develop a comprehensive and quantitative understanding of the effects of yield strength and strain hardening exponent on contact deformation under the most fundamental contact condition: normal indentation. To date, however, systematic and quantitative studies of plasticity gradient effects on indentation response have not been completed. A comprehensive parametric study of the mechanics of normal indentation of plastically graded materials was therefore undertaken in this work by recourse to finite element method (FEM) computations. On the basis of a large number of computational simulations, a general methodology for assessing instrumented indentation response of plastically graded materials is formulated so that quantitative interpretations of depth-sensing indentation experiments could be performed. The specific case of linear variation in yield strength with depth below the indented surface is explored in detail. Universal dimensionless functions are extracted from FEM simulations so as to predict the indentation load versus depth of penetration curves for a wide variety of plastically graded engineering metals and alloys for interpretation of, and comparisons with, experimental results. Furthermore, the effect of plasticity gradient on the residual indentation pile-up profile is systematically studied. The computations reveal that pile-up of the graded alloy around the indenter, for indentation with increasing yield strength beneath the surface, is noticeably higher than that for the two homogeneous reference materials that constitute the bounding conditions for the graded material. Pile-up is also found to be an increasing function of yield strength gradient and a decreasing function of frictional coefficient. The stress and plastic strain distributions under the indenter tip with and without plasticity gradient are also examined to rationalize the predicted trends. In Part II of this paper, we compare the predictions of depth-sensing indentation and pile-up response with experiments on a specially made, graded model Ni–W alloy with controlled gradients in nanocrystalline grain size.

Hardness, toughness, and modulus of some common metamorphic minerals

Studies of the hardness and toughness of minerals have historically focused on minerals of the Mohs scale, although, with the exceptions of quartz, orthoclase, and calcite, Mohs phases are not common rock-forming minerals. We report new hardness (H), toughness (resistance to fracture, K IC ), and indentation modulus (E*) data obtained by microhardness and depth-sensing indentation (DSI, or nanoindentation) experiments for common metamorphic minerals: sillimanite, kyanite, andalusite, garnet, quartz, and orthoclase feldspar. Because the experimental techniques involve indentation-induced cracking as well as depth-sensing indentation, the new data set can be used to investigate a range of plastic behavior for minerals in the crust and mantle. The three Al 2 SiO 5 polymorphs have similar H values (~10.12 GPa): kyanite has the largest values and andalusite the smallest. These values are similar to that of quartz (~12 GPa) and greater than that of orthoclase (~7 GPa). Garnet H values vary with composition: for grossular, H ~ 13 GPa, and for almandine-pyrope, H ~ 15 GPa. Although H values for the minerals we analyzed span a range of ~10 GPa, most fracture toughness values are between 1.1.8 MPa·m½. Garnet is much harder than Al 2 SiO 5 , but has a similar to slightly lower K IC (grossular ~1.2 MPa·m½; almandine-pyrope ~1.4 MPa·m½; andalusite 1.8 MPa·m½; sillimanite 1.6 MPa·m½); kyanite K IC is difficult to measure owing to the ease with which kyanite cleaves. Garnet has properties similar to those of cubic zirconia (ZrO 2 ), which we measured as a reference. Another reference mineral, periclase (MgO), has the lowest H (~5 GPa) and the highest K IC (~4 MPa·m½) of minerals we measured. Among the silicates, E* varies significantly from orthoclase (~89 GPa) to quartz (~117 GPa) to garnet (245.260 GPa), and Al 2 SiO 5 has intermediate values: kyanite ~ 186.253 GPa, sillimanite ~ 207 GPa, andalusite ~ 232 GPa.

Microhardness, toughness, and modulus of Mohs scale minerals

We report new results of microhardness and depth-sensing indentation (DSI) experiments for the first nine minerals in the Mohs scale: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, and corundum. The Mohs scale is based on a relative measure of scratch resistance, but because scratching involves both loading and shearing, scratch resistance is not equivalent to hardness as measured by modern loading (indentation) methods; scratch resistance is also related to other material properties (fracture toughness, elastic modulus). To better understand the relationship of hardness to scratch resistance, we systematically determined hardness, fracture toughness, and elastic modulus for Mohs minerals. We measured hardness and toughness using microindentation, and modulus and hardness with DSI (“nanoindentation”) experiments. None of the measured properties increases consistently or linearly with Mohs number for the entire scale.

Mechanical gradient interphase by interdiffusion and antiplasticisation effect—study of an epoxy/thermoplastic system

A stoichiometric amine–epoxy formulation was cured in the presence of a thermoplastic, namely poly(vinylpyrrolidone) (PVP). The epoxy system consisted of the resin diglycidyl ether of bisphenol A (DGEBA) and the aromatic curing agent 4,4′-diaminodiphenylsulfone (DDS). As shown for this system in a former study by Oyama et al. [Oyama HT, Lesko JJ, Wightman JP. J Polym Sci B 1997;35:331–46. [36]], preferential absorption of amine molecules by PVP can occur. In the present study, the focus is on the variations of local elastic properties within the epoxy interphase adjacent to the PVP layer. The curing was performed close to the glass transition temperature, Tα, of the PVP film, namely at 170°C. Variations of the local amine concentration were tracked using energy-dispersive analysis of X-rays (EDX), by taking benefit of the sulfur contained in DDS. Using temperature-dependent dynamic mechanical analysis (DMA), a series of epoxy reference samples of different amine–epoxy concentration ratios, r, was investigated in order to work out the relationship between r and the epoxy storage modulus at room temperature. In the excess-epoxy regime, r<1, the modulus is observed to increase with departure from the stoichiometric ratio, r=1. Considering the respective suppression of the β-transition, the observed characteristic can be explained by an antiplasticisation effect. Depth-sensing indentation (DSI) experiments across the epoxy/PVP interphase provided evidence for strong modulus variations. In consistency with the EDX and the DMA data, in the vicinity of the PVP layer the local epoxy modulus is increased. The total change of the epoxy Young's modulus is ∼1.1GPa. However, the total width of the modulus decay of ∼175μm is ∼2.5 times larger than the one of the DDS concentration gradient. This finding is discussed in terms of additional spatial variations of the DGEBA concentration as well as long-range diffusion currents of DDS induced by the interdiffusion processes and their effect on the final network of crosslinks.

Influence of indentation depth on the determination of the apparent Young's modulus of bi-layer material: Experiments and numerical simulation

Depth sensing indentation experiments have been conducted on stainless steel thin films deposited by reactive magnetron sputtering on AU4G aluminium-alloy substrates to highlight that elastic response of bi-layer materials can be specific of the coating alone only for very low indentation depths. It is shown experimentally that Bückle's rule is at least one order of magnitude too high when applied by default to determine coating-specific elastic response. These results have been confirmed by boundary element numerical simulation underlining that elastic response of a thin film deposited on substrate deviates from the coating’s response above relative indentation depth around 1–2%.

Nano-sized twins induce high rate sensitivity of flow stress in pure copper

We have investigated the rate sensitivity of flow stress and the extent of strengthening in polycrystalline copper containing different volume fractions of nano-sized twins, but having the same average grain size. The specimens were produced by pulsed electrodeposition, wherein the concentration of twins was varied systematically by varying the processing parameters. Depth-sensing instrumented indentation experiments performed at loading rates spanning three orders of magnitude on specimens with the higher density of twins (twin lamellae width ∼20 nm) revealed an up to sevenfold increase in rate-sensitivity of hardness compared to an essentially twin-free pure Cu of the same grain size. A reduction in twin density for the same grain size (with twin lamellae width ∼90 nm) also resulted in a noticeable reduction in rate-sensitivity and hardness. The presence of a high density of nano-scale twins is also seen to impart significant hardness, which is comparable to that achieved in nano-grained Cu. Post-indentation analyses of indented Cu with nano-scale twins in the transmission electron microscope reveal deformation-induced displacement of coherent twin boundaries (CTBs), formation of steps and jogs along CTBs, and blockage of dislocations at CTBs. These processes appear to significantly influence the evolution of thermal activation volume for plastic flow which is some three orders of magnitude smaller than that known for microcrystalline Cu. Transmission electron microscopy also reveals CTBs with a high density of dislocation debris and points to the possibility that displaced CTBs may serve as barriers to dislocation motion and that they may also provide sources for dislocation nucleation, especially near stress concentrations, very much like grain boundaries. Possible consequences of these trends for deformation are explored.

Organosilicate Spin-On Glasses: II. Effect of Physical Modification on Mechanical Properties

Porous copolymer films were synthesized from a methylsilsequioxane:1,2-bis(trimethoxysilyl)ethane (MSSQ:BTMSE) matrix and either an aromatic-core or aliphatic-core porogen at 10, 20, or 30 wt % porogen loading. Films were characterized using scanning electron microscopy (SEM), IR spectroscopy, and ellipsometry. Depth-sensing indentation experiments were performed to measure apparent film modulus, and hardness, H. Indentation load-displacement traces and SEM images were used to determine the threshold load for cracking, The aliphatic-core porogen produced a greater porosity film than the aromatic-core porogen for 10 wt % loading and smaller porosity films for 20 and 30 wt % loadings. IR spectra, normalized for film thickness and density, indicated decreased O-Si-O networking in porous MSSQ:BTMSE films. The combination of increased porosity and decreased O-Si-O networking led to a decreased apparent and H relative to the unmodified MSSQ:BTMSE film. However, low-porosity (approximately 7%), aliphatic-porogen MSSQ:BTMSE films are optimized relative to unmodified MSSQ with smaller dielectric constant and greater and H. © 2004 The Electrochemical Society. All rights reserved.

Stress hysteresis and mechanical properties of plasma-enhanced chemical vapor deposited dielectric films

A comprehensive survey is described of the responses of three plasma-enhanced chemical vapor deposited dielectric film systems to thermal cycling and indentation contact. All three films—silicon oxide, silicon nitride, and silicon oxy-nitride—exhibited significant nonequilibrium permanent changes in film stress on thermal cycling or annealing. The linear relationship between stress and temperature changed after the films were annealed at 300 °C, representing a structural alteration in the film reflecting a change in coefficient of thermal expansion or biaxial modulus. A double-substrate method was used to deduce both thermoelastic properties before and after the anneal of selected films and the results were compared with the modulus deconvoluted from small-scale depth-sensing indentation experiments (nanoindentation). Rutherford backscattering spectrometry and hydrogen forward scattering were used to deduce the composition of the films and it was found that all the films contained significant amounts of hydrogen.

Physical Aging of Polycarbonate: Elastic Modulus, Hardness, Creep, Endothermic Peak, Molecular Weight Distribution, and Infrared Data

For the first time, load and depth sensing indentation (DSI) was used in order to monitor physical aging of bisphenol A polycarbonate for 30 months at room temperature and for 1 month at an elevated temperature. The DSI experiments were combined with differential scanning calorimetry, gel permeation chromatography, and infrared spectroscopy. The endothermic peak of polycarbonate shifted toward higher temperatures upon aging at an elevated temperature and did not change its location upon aging at room temperature. The elastic modulus and hardness of polycarbonate increased in a stepwise fashion during aging at room temperature. The molecular weight distribution broadened slightly, and the trans−trans conformational population increased during annealing. No simple correlation between changes in the mechanical properties and the shift of the endothermic peak during annealing was found. These changes seem to be caused by phenomena of different nature; namely, the changes in the mechanical properties appeared to have a reasonable correlation with free volume relaxation of the polymer, whereas the changes in the endothermic peak may be associated with internal energy changes. The similarities and differences between our results and the results of others are discussed.

On the application of the work-of-indentation approach to depth-sensing indentation experiments in coated systems

The work-of-indentation approach is developed and applied for hardness calculations in soft or hard materials, and also for a range of coated systems. The method appears to be less sensitive than other definitions to the effects of pile-up observed in soft materials. Work-of-indentation hardness is calculated directly from the energy of indentation without the need for estimating penetration depths, areas or volumes, which can be time consuming and inaccurate. In particular, using the plastic component of the work-of-indentation yields consistent results across the range of materials studied. The method breaks down, however, at very low loads and small penetration depths, which is likely to be due to deviations from the ideal tip geometry.

Stress Hysteresis and Mechanical Characterization of Plasma-Enhanced Chemical Vapor Deposited Dielectrics

ABSTRACTTwo plasma-enhanced chemical vapor deposited (PECVD) dielectric films pertinent to microelectronic-based applications were examined for thermo-mechanical stability. Both films–silicon nitride and silicon oxy-nitride–showed significant permanent non-equilibrium changes in film stress on thermal cycling and annealing. The linear relationship between stress and temperature changed after the films were annealed at 300°C, representing a structural change in the film resulting in a change in coefficient of thermal expansion and/or biaxial modulus. A double-substrate method was used to deduce both properties before and after the anneal of selected films and the results compared with the modulus deconvoluted from the load-displacement data from small-scale depth-sensing indentation experiments.

Construction and testing of a nanomachining instrument

This paper presents a nanomachining instrument that was developed for conducting nanocutting, nanoscratching, and nanoindentation experiments. A piezoelectric tube scanner (PZT) is employed to generate three-dimensional machining motions. The sample is moved by the PZT, and the tool is kept stationary during machining. The machining forces are measured by force sensors with a resolution of sub-milliNewtons. The instrument is compact and can be used inside optical microscopes and scanning electron microscopes. In this paper, depth-sensing indentation experiments were performed to test the basic performance of the instrument. The indentation displacement was measured by a capacitance probe situated inside the PZT tube. An experimental system was constructed to locate and image indentations. The system consists of a high magnification microscope to measure coordinates of the indentation relative to a reference corner point on the sample, and an AFM equipped with an on-axis optical imaging system for locating the indentation. A technique was also employed to establish the tool-sample contact to nanometer accuracy. Indentation experiments were carried out on three kinds of materials with different hardness. Experimental results demonstrated the instrument has the ability of performing depth-sensing indentations. The frame compliance was also evaluated from the indentation results.

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens (∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9vqpe0x% c9q8qqaqFn0dXdir-xcvk9pIe9q8qqaq-dir-f0-yqaqVe0xe9Fve9% Fve9qapdbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaie% aacaWFTbaaleqaaaaa!3A30! $$\sqrt m $$ . Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily, due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low fatigue limit of ∼4 pct of ultimate tensile strength.