Depth-sensing Indentation Research Articles

On the Indentation Modulus of Sintered Materials

When the depth-sensing (nano)indentation is applied to sintered samples, measured properties, which are expected to represent the material of an individual grain, seem to depend on the overall porosity of the macroscopic sample. To understand such a result, it is assumed that while the nanoindenter penetrates into the surface grain and probes the properties of its material, the grain itself serves as another, larger indenter indenting the rest of sample and probing the properties that represent the bulk of material rather than individual grains. Load vs. displacement curve reflects the synergetic response of these two “indenters” and so it contains information about the sample’s mechanical properties at both microscopic and macroscopic scales. Obtained theoretical results agree qualitatively with the experimental data (the dependence of the indentation modulus on the porosity of sample; the indentation size effect).

Mechanical Properties of Metal Matrix Nanocomposites Synthesized by Selective Laser Melting Measured by Depth Sensing Indentation Technique

This work presents an investigation of basic mechanical properties (hardness, Young modulus) of metal matrix composites (MMC) reinforced with non-oxide ceramic nanoparticles by means of nanoindentation measurements. The evaluated materials were manufactured by selective laser melting (SLS/M). As a matrix 316L stainless steel and as a reinforcing phase TiC nanoparticles were used. The influence of nanoscale reinforcement on the mechanical properties of MMC manufacturing with SLS/M process was examined. In this case statistical evaluation of hardness and Young modulus based on nanoindentation data was performed.

Influence of deposition conditions on electrical and mechanical properties of Sm2O3 doped CeO2 thin films prepared by EB-PVD (+IBAD) methods part 2. Indentation hardness and effective elastic modulus

The study of polycrystalline CeO2 + xSm2O3 (x = 0, 10.9–15.9 mol %) thin films deposited by Electron Beam Physical Vapour Deposition (EB-PVD) and Ionic Beam Assisted Deposition (IBAD) techniques on the Si substrate was devoted to the influence of deposition conditions used, namely composition x, deposition temperature T dep and Ar+ ion bombardment, on the (micro)hardness, H pl and elastic modulus, Y with respect to the film structure and microstructure. These mechanical characteristics were investigated by the depth sensing indentation (DSI) technique as the functions of relative indentation depth h rel = h max/t and the values obtained were compared with those obtained by the classical Vickers technique. Results of this study are described and discussed.

On the determination of the film hardness in hard film/substrate composites using depth-sensing indentation

Abstract The main difficulty in the mechanical characterisation of thin films using depth-sensing indentation is the determination of the relative substrate and film contributions to the measured properties of the film/substrate composite. In this study, a three-dimensional numerical simulation of the Vickers hardness test is used to study the influence of the substrate and film mechanical properties on the composite's behaviour under depth-sensing indentation. The particular case of hard films on soft substrates is analysed. In order to understand the behaviour of the composite, a study of the plastic strain distribution under indentation of several composites is performed. A methodology to determine the relative film hardness, i.e. H F / H S ratio, is proposed. The methodology is successfully verified using fictitious and real composite materials.

Mapping the mechanical properties of biomaterials on different length scales: depth-sensing indentation and AFM based nanoindentation.

The micro- and nanomechanical properties of biomaterials are of central importance as surface elasticity, surface elastic response and adhesion play a central role in a large number of biological processes. They influence, e.g. morphogenesis, self-healing abilities and restoration processes, focal adhesion, motility, mechano-transduction, metastasis and are important for drug delivery applications, and others. These processes address fundamental questions in biology, medicine and diagnostics, pharmaceutical research, archaeology, dental as well as cosmetic applications. With nanoindentation as well as AFM force spectroscopic techniques suitable investigation tools are at hand, which can provide mechanical information on different length scales. The development of both techniques resulted in a broad applicability and the number of examples where both techniques have been used to study (soft) biomaterials rapidly increased during the last years.

Towards Quantitative Characterisation of the Small Force Transducer Used in Nanoindentation Instruments

Quantitative characterization of the mechanical properties of materials in micro-/nano-scale using depth-sensing indentation technique demands high performance of nanoindentation instruments in use. In this paper, the efforts to calibrate the capacitive force transducer of a commercial nanoindentation instrument are presented, where the quasi-static characteristic of the force transducer has been calibrated by a precise compensation balance with a resolution of ~1 nN. To investigate the dynamic response of the transducer, an electrostatic MEMS (Micro-Electro-Mechanical System) based on nano-force transfer standard with nano-Newton (10-9 Newton) resolution and a bandwidth up to 6 kHz have been employed. Preliminary experimental results indicate that 1) the force transducer under calibration has a probing force uncertainty less than 300 nN (1σ) in the calibration range of 1 mN; 2) the transient duration at contact points amounts to 10 seconds; 3) the overshoot of engagement is pre-load dependent.

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.

Suspension plasma sprayed bioactive glass coatings: Effects of processing on microstructure, mechanical properties and in-vitro behaviour

Bioactive glass coatings deposited via suspension plasma spraying were studied to improve the adhesion between orthopaedic implants and bone. Fine powders of a bioactive glass, named BG_Ca, having composition (in wt.%): 4.7 Na2O, 42.3 CaO, 6.1 P2O5, and 46.9 SiO2, were produced and dispersed in ethanol to form a suspension used as a feedstock. Various sets of spray parameters were applied in order to define the influence of the deposition process on the final coating properties. Consequently, the coatings were characterized in as-sprayed state and after soaking in a simulated body fluid (SBF) for different periods ranging from 1 to 14days. The microstructural investigations were carried out using environmental scanning electron microscope (ESEM) and X-ray diffraction (XRD). The coatings' adhesion to the substrate was evaluated by means of scratch tests. Finally, hardness and elastic modulus were determined by means of depth-sensing indentation methods.

Development of Innovative Algorithm for Nanomechanics and its Applications to the Characterization of Materials

Understanding major mechanisms affecting material strength such as grain size, grain orientation and dislocation mechanism from atomistic viewpoint can empower scientists and engineers with the capability to produce vastly strengthened materials. Computational studies can offer the possibility of carrying out simulations of material properties at both larger length scales and longer times than direct atomistic calculations. The study has conducted theoretical modeling and experimental testing to investigate nanoscale mechanisms related to material strength and interfacial performance. Various computational algorithms in nanomechanics including energy minimization, molecular dynamics and hybrid approaches that mix atomistic and continuum methods to bridge the length and time scales have been used to thoroughly study the deformation and strengthening mechanisms. Our study has also performed experiments including depth-sensing indentation technique andin-situpico-indentation to characterize the nanomechanisms related to material strength and tribological performance. In this project, we have developed the innovative mutil-scale algorithms in the area of nanomechanics. These approaches were used to studies the defect effect on the mechanical properties of thin film, mechanical properties of nanotubes, and tribological phenomena at nanoscale interfaces.

UV-induced crosslinking of the polybutadiene domains in lamellar polystyrene-block-polybutadiene block copolymer films – An in-depth study

We present an in-depth study of the UV-induced crosslinking of the polybutadiene domains in lamellar polystyrene-block-polybutadiene (PS-b-PB) block copolymer films. The crosslinking process is followed by a combination of depth-sensing indentation, Raman spectroscopy, and differential scanning calorimetry (DSC) investigations. Indentation assesses changes in the mechanical properties of the overall polymer films while Raman experiments directly monitor the consumption of the PB double bonds. Immediately after a short UV exposure, a loose network is formed and the glass transition temperature increases, whereas changes in the mechanical properties of the materials require far longer hardening times. We show that both processes are strongly affected by the amount of crosslinker (Lucirin-TPO®) added (within a range of 5 to 50 wt. % with respect to the PB fraction of the material).

Fabrication of in situ Ni(W)–WC nano composites via mechanical alloying and spark plasma sintering

Microstructural and mechanical properties of spark plasma sintered Ni-30wt% W powders fabricated via mechanical alloying (MA) for 48h were reported in the present study. Due to the intensive WC contamination during MA, the synthesized powders are termed as Ni(W)–WC nanocomposites. The MA’d powders were sintered at temperatures between 800 and 1000°C via spark plasma sintering (SPS) technique and high density (∼97%) Ni(W)–WC composite compacts having high micro hardness values (∼4.30GPa) and high elastic modulus (∼270GPa) were obtained. The effects of sintering temperature, duration and hBN spraying of the graphite die on the phase compositions, i.e. WC content, microstructure and mechanical properties were investigated systematically by using X-ray diffractometer (XRD), Rietveld analyses, scanning electron microscopy (SEM), microhardness and depth-sensing indentation techniques. The crystallite sizes of both the Ni(W) solid solution and the WC phases increased with increasing sintering temperature and durations: An average crystallite size of 39nm for the Ni(W) solid solution phase in the SPS-800 sample increased to 86nm for the SPS-1000-5min sample, likewise, about 14nm crystallite size of WC phase in the SPS-800 sample increased to 78nm for the SPS-1000-5min sample. The SPS-1000-hBN sample had the highest relative density and microhardness values of 97.33% and 4.35GPa, respectively.

Verification of Selected Key Assumptions for the Analysis of Depth-Sensing Indentation Data

Methods for indentation analysis involve assumptions that may be problematic for visco-elastic materials. Three assumptions are discussed: (1) the unloading is predominantly elastic, (2) tip/sample adhesion and friction are negligible, and (3) no cracking occurs around the indent. A series of copolymers is investigated by indentation at room temperature, just below their glass transitions. Closer to the glass transition assumptions (1) and (2) were violated. Erroneous analysis due to extensive creep was identified by comparison of the creep rate at the end of the hold period with the unloading displacement rate and from the power of the fit to the unloading response. Assumption (3) was studied by scanning the indenter tip over the residual indent in polystyrene, which is known to be susceptible to stress localization.

Evaluation of adhesive and elastic properties of materials by depth-sensing indentation of spheres

Work of adhesion is the crucial material parameter for application of theories of adhesive contact. It is usually determined by experimental techniques based on the direct measurements of pull-off force of a sphere. These measurements are unstable due to instability of the load-displacement diagrams at tension, and they can be greatly affected by roughness of contacting solids. We show how the values of work of adhesion and elastic contact modulus of materials may be quantified using a new indirect approach (the Borodich–Galanov (BG) method) 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. Using numerical simulations it is shown that the BG method is simple and robust. The crucial difference between the proposed method and the standard direct experimental techniques is that the BG method may be applied only to compressive parts of the force-displacements curves. Finally, the work of adhesion and the elastic modulus of soft polymer (polyvinylsiloxane) samples are extracted from experimental load-displacement diagrams.

Applications of instrumented indentation testing for the characterization of polypropylene materials

In this study, instrumented indentation testing, or so-called depth-sensing indentation testing, was used to characterize polypropylene (PP) materials. The influence of molecular weight and cooling rate on the hardness was investigated showing that low cooling rates and relatively low molecular weight lead to higher hardness values. In random copolymers of propene with butene and hexene, the correlation between hardness and degree of crystallinity is linear and the conversion to the amorphous state could be detected. Blends of PP with ethylene–propylene elastomer show a strong negative deviation of indentation modulus from linear rule of mixture. This behaviour can be described by Kerner’s model in the case of particulate arrangement and by Davies’ model in the phase inversion region. The addition of high-density polyethylene to PP results in a synergistic behaviour of modulus which can be attributed to nucleation effects. Furthermore, the application of indentation testing to detect physical ageing of a surface of an injection-moulded specimen and heterogeneity of weld lines was shown. Orientations in a biaxially oriented film could be detected using Knoop indenter.

Determination of temperature effect on mechanical properties of templated mesoporous silica aerogels by nanoindentation method

Cross-linking is known as a chemical process to improve the mechanical properties by adding polymer. Therefore, the pore walls of aerogels can be strengthened by coating with polymer. In this study, the mechanical properties of a cross-linked monolithic silica aerogel have been examined by nanoindentation tests at different temperatures. Creep compliance values were analyzed as a function of time by linear viscoelastic equations using the test data of nanoindentation. It is determined that the creep compliance values obtained from nanoindentation are much higher than that of compressive test. Additionally, from the relaxation modulus curves, it can be expressed that the aerogel sample used retains its general long-term viscoelastic behavior at elevated temperatures. Key words: Silica aerogel, nanoindentation, creep behavior, temperature test, linear viscoelasticity, depth-sensing indentation.

Depth-sensing indentation of a transversely isotropic elastic layer: Second-order asymptotic models for canonical indenters

Simple analytical approximations to the frictionless indentation problem for a transversely isotropic layer are obtained for spherical, conical, and pyramidal indenters as well as for axisymmetric indenters of power-low profile and self-similar non-axisymmetric indenters. These approximations are asymptotically exact in the small-contact limit. The results obtained are validated in the case of an isotropic layer for spherical and conical indenters.

Investigation of the effect of relative humidity on polymers by depth sensing indentation

Stereolithography (SL) resins absorb varying amounts of moisture dependent on the relative humidities, which can significantly affect the mechanical properties. In this work, the influence of relative humidity (RH) on the mechanical behaviour of an SL resin is investigated using depth sensing indentation (DSI). The samples were conditioned by two methods. In the first method, samples were pre-conditioned at 33.5, 53.8, 75.3 and 84.5% RH using saturated salt solutions. These preconditioned samples were tested at 33.5% RH, using a humidity control unit (HCU) to control RH in the DSI system. In the second method, samples were conditioned and tested at 33.5, 53.8, 75.3 and 84.5% RH by regulating humidity in the DSI system using the HCU. Temperature was kept constant at 22.5 °C for the conditioning and DSI testing. It was seen that hardness and modulus decreased with increasing RH and conditioning time but recovered significantly when tested after drying. This study demonstrates that RH needs to be taken into account during the DSI testing of polymers.

Mechanical properties of thin films of hydrogenated silicon and their relationship with microstructure

Thin films of hydrogenated silicon were deposited on glass and single-crystalline silicon substrates using a capacitively coupled radio-frequency plasma-enhanced vapor-deposition system with the help of direct-current bias stimulation. Micro-Raman scattering was applied to investigate the microstructure of the thin films obtained. The crystalline volume fraction, X c, was obtained from the Raman spectra. Microscopic mechanical characterization of the thin films was carried out by nanoindentation based on the conventional depth-sensing indentation method. An analytical relation between X c and the elastic modulus was thereby established. The elastic modulus of the film on a glass substrate was found to be lower than that of the film on a monocrystalline silicon substrate with the same X c. The grain size of a phosphorus-doped thin film was smaller than that of the intrinsic one, with greater ordering of the grains and X c was found to be usually above 40%. A film with boron doping was on the opposite side, with X c usually below 40%. In the phosphorus-doped, intrinsic, and boron-doped films, the elastic moduli were lower when the X c values were 45%, 30%, and 15%, respectively.

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.

Determination of the Drucker–Prager parameters of polymers exhibiting pressure-sensitive plastic behaviour by depth-sensing indentation

This paper presents a method to determine the Drucker–Prager parameters of pressure-dependant elastic-perfectly plastic polymeric materials by means of the depth-sensing indentation technique. This is achieved via an inverse analysis of the load–displacement data resulting from two tests performed with Berkovich and spherical tips. The well-posedness and the effective range of application of the proposed method are carefully assessed first. Then the method is tested for two elasto-plastic materials with mild initial strain-hardening (HDPE and PMMA), and the results are compared to those measured using conventional tensile and compression tests. It is found that the pressure-sensitivity indices can be accurately predicted, while the yield stress predictions in tension and compression fall within the non-linear portion of the uniaxial stress–strain curves, i.e., inside the region where plastic deformation begins.