Oliver-Pharr Method Research Articles

AlGaAs film growth using thermionic vacuum arc (TVA) and determination of its physical properties

In this research, an AlGaAs film was deposited on a microscope slide by means of the thermionic vacuum arc (TVA) technique which is a novel plasma production technique. AlGaAs structures were grown by this deposition technique for the first time and this process occurred in a very short period of time. In order to characterize the produced film, nano-structural, nano-mechanical, optical, and surface properties were determined by field emission scanning electron microscope (FESEM), atomic force microscope (AFM), X-ray diffractometer (XRD) and interferometer. According to the results of the measurements, the mean thickness value of the produced film was obtained as 1.8 μm. The band gap value was determined as 2eV from the Kubelka-Munk plot. The refractive index value was obtained as approximately 3.4. Hardness value was determined as 2 GPa from the Oliver-Pharr method. All these values are consistent with the reported values in the literature for the AlGaAs films produced by different methods. TVA technique appeared as a suitable and promising technique for the production of AlGaAs films.

Deposition of a Mo doped GaN thin film on glass substrate by thermionic vacuum arc (TVA)

Current research presents a new deposition method for GaN thin films that produces in a very short production time for GaN-based solid-state applications. A Mo doped GaN thin film on a glass substrate was produced by thermionic vacuum arc (TVA) technique. The TVA technique is a novel non-reactive plasma technique. The optical properties were determined by Filmetrics F20 interferometer and UV–Vis double beam spectrophotometer. The surface morphology was analyzed using field emission scanning electron microscopy and atomic force microscopy. The mean thickness value was measured as 100 nm by Filmetrics interferometer. The crystalline structure of the produced thin film has a Wurtzite crystal structure (004) as obtained by X-ray diffraction. Hardness value was determined as 14 GPa with the Oliver–Pharr method. The obtained properties are consistent with the values reported in related literature. The findings indicate that the TVA method provides advantages for optical and industrial applications.

Temperature dependent mechanical property of PZT film: an investigation by nanoindentation.

Load-depth curves of an unpoled Lead Zirconate Titanate (PZT) film composite as a function of temperature were measured by nanoindentation technique. Its reduce modulus and hardness were calculated by the typical Oliver-Pharr method. Then the true modulus and hardness of the PZT film were assessed by decoupling the influence of substrate using methods proposed by Zhou et al. and Korsunsky et al., respectively. Results show that the indentation depth and modulus increase, but the hardness decreases at elevated temperature. The increasing of indentation depth and the decreasing of hardness are thought to be caused by the decreasing of the critical stress needed to excite dislocation initiation at high temperature. The increasing of true modulus is attributed to the reducing of recoverable indentation depth induced by back-switched domains. The influence of residual stress on the indentation behavior of PZT film composite was also investigated by measuring its load-depth curves with pre-load strains.

Nanoindentation Characterization of Moisture Damage in Different Phases of Asphalt Concrete

Abstract Traditional microscale testing cannot be performed on asphalt binder, mastic, or aggregate, while they are an integral part of asphalt concrete (AC). Recently, nanoindentation has created an opportunity to characterize mastic and asphalt binder while they reside in an AC sample. In the study, laboratory nanoindentation testing is carried out to characterize moisture-induced damage in different phases of AC. A moisture-induced sensitivity testing (MIST) device is used for moisture conditioning of AC. In the MIST device, an AC sample is fully submerged under water and all-around cyclic pressure is applied through the pores inside an AC sample to cause damage. Damaged AC samples are indented for an extended dwell time of 200 s using an unloading rate of 0.02 mN/s to minimize viscous effects of asphalt on test results. The indentation load-displacement curve is analyzed by the Oliver–Pharr method to obtain elastic modulus and hardness. When comparing wet and dry sample indentation test results, it is observed that the modulus of the wet mastic reduces to 60 % of dry mastic modulus. Overall moisture conditioning reduces the modulus of AC by 70 %. In addition, the creep response of the mastic phase is modeled by a viscoelastic Burger model. The creep compliance value of wet mastic is 42 % higher than that of dry mastic.

Comparison of Four Different Methods to Determine the Hardness of Plasma-sprayed Cr3C2–NiCr Coating by Nano-indentation

Abstract A Cr3C2–NiCr coating with a thickness of 200 μm was deposited on an AISI 1045 steel substrate using a supersonic plasma-spray technique. The hardness of the Cr3C2–NiCr coating was characterized by a nano-indenter equipped with an atomic force microscope (AFM). The AFM images indicated that the nano-indents on the Cr3C2–NiCr coating exhibited significant “pile-up” deformation. Such pile-up behavior needs to be taken into account in hardness determination because the pile-up height accounts for a large proportion of the indentation depth. In this paper, four different methods are compared to determine the hardness of the Cr3C2–NiCr coating: the Oliver–Pharr method, two work-of-indentation methods (total work of indentation and plastic work of indentation), and an AFM analysis method. The results show that the Oliver–Pharr and work-of-indentation methods overestimated the hardness, and the AFM analysis method is considered as a more accurate method for determining the hardness of the Cr3C2–NiCr coating.

Nanomechanical Evaluation of Vapor-Conditioned and Unconditioned Asphalt

Traditionally, moisture damage of asphalt concrete (AC) is evaluated by comparing the macroscale strength and stiffness values of a set of moisture- or vapor-conditioned samples with those of a set of unconditioned AC samples. The moisture damage of a bulk (liquid) asphalt binder is determined by the surface energy components of the binder with a Wilhelmy plate device: the most recent and advanced way to evaluate moisture damage in liquid asphalt. Despite these advancements, moisture damage in AC or asphalt binder is an unsolved issue. Different from previous approaches, this study evaluated the moisture damage of thin films of asphalt deposited on glass substrates, conditioned at 25%, 49%, and 71% vapors, and used a nanoindentation technique for testing. In nanoindentation, a point load indents the asphalt film surface and load–displacement data are recorded. The study analyzed nanoindentation data with the traditional Oliver–Pharr method and a nontraditional mechanical model that captures the viscoelastoplastic behavior of the indented film with spring, dashpot, and rigid (SDR) body elements. The model results showed that the modulus and hardness of asphalt film decreased with an increase in vapor level. The SDR model showed that at higher humidity, at 49% and 71% humidity conditions, the viscosity decreased by approximately 60% in thin film binder.

Mechanical characterization of MgB2 thin films using nanoindentation technique

Mechanical properties of MgB2 thin films deposited by RF magnetron sputtering technique have been studied. MgB2 thin films were deposited approximately 300nm on Si substrate above room temperature, and the thin films were subsequently annealed. Great care has been taken to reduce O2 in the chamber during deposition. Structural and mechanical properties of MgB2 thin films were investigated using the Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and nanoindentation techniques. Unloading segments of nanoindentation curves analyzed using Oliver–Pharr method. Both reduced elastic modulus and nanohardness values show load dependence, i.e., indentation size effect (ISE). Proportional specimen resistance (PSR) model was used to calculate the load independent nanohardness value. Load-independent hardness value was calculated using the proportional specimen resistance (PSR) model. XRD and SEM results show that the MgB2 thin films have fine grain size. Nanoindentation results show that hardness and elastic modulus values were 11.72GPa and 178.13GPa, respectively.

Strain rate effect on semi-crystalline PLLA mechanical properties measured by instrumented indentation tests

Poly-L-lactic acid, PLLA, is a widely studied biomaterial, currently approved for use in a range of medical devices. Its mechanical properties can be tailored giving the material different crystallinity degrees. PLLA presents a complex non-linear behaviour that depends not only on structural parameters such as crystallinity degree but also on external parameters such as strain rate and temperature. Failure of polymeric implants is attributed to their intrinsic time-dependent performance under static loading conditions. This work explores the potential of instrumented indentation tests as a suitable technique to measure the complex non-linear time-dependent mechanical properties of PLLA. The applicability of the Oliver–Pharr method, typically used in ceramic and metallic materials, is discussed through the sensitivity of elastic modulus, hardness and material creep response to different strain rates. Likewise, the strain rate influence depending on the crystallinity degree is considered, providing important information about viscoelastic behaviour of PLLA. Whereas possible, results are compared with those obtained from tensile and Dynamic Mechanical Analysis tests extracted from the literature.

Growth, structural and mechanical analysis of a single crystal ofl-prolinium tartrate: a promising material for nonlinear optical applications

A single crystal of L-prolinium tartrate (LPT), which is an organic non linear optical material, was successfully synthesized and grown using a slow evaporation solution growth technique (SEST). The crystal structure and lattice parameters of the crystal were confirmed by powder X-ray diffraction and it was found that it belongs to the monoclinic crystal system with β = 100.380 and a noncentrosymmetric space group. The presence of strain in the grown ingot was calculated from powder X-ray diffraction measurements. The crystalline perfection was examined by high resolution X-ray diffractometry, which revealed that the crystal contained structural grain boundaries. The optical behavior of the grown specimen was analyzed by photoluminescence (PL) spectroscopy and its time resolved PL decay was calculated. The grown crystal adopted a step wise growth pattern with parallel striations, which was confirmed from the etching technique. Its ferroelectric and piezoelectric properties were also assessed. Its third order non linearity was assessed using an open aperture Z-scan technique. The thermal parameters of the LPT single crystal were calculated using a photopyroelectric technique. The mechanical strength of the single crystal at the micro level was observed by nanoindentation using the Oliver–Pharr method.

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

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 30nm 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.

Accounting for structural compliance in nanoindentation measurements of bioceramic bone scaffolds

Structural properties have been shown to be critical in the osteoconductive capacity and strength of bioactive ceramic bone scaffolds. Given the cellular foam-like structure of bone scaffolds, nanoindentation has been used as a technique to assess the mechanical properties of individual components of the scaffolds. Nevertheless, nanoindents placed on scaffolds may violate the rigid support assumption of the standard Oliver–Pharr method currently used in evaluating the Meyer hardness, H, and elastic modulus, Es, of such structures. Thus, the objective of this research was to use the structural compliance method to assess whether or not specimen-scale flexing may occur during nanoindentation of bioceramic bone scaffolds and to remove the associated artifact on the H and Es if it did occur. Scaffolds were fabricated using tricalcium phosphate and sintered at 950°C and 1150°C, and nanoindents were placed in three different (center, edge, and corner) scaffold locations. Using only the standard Oliver–Pharr analysis it was found that H and Es were significantly affected by both sintering temperature and nanoindents location (p<0.05). However, specimen-scale flexing occurred during nanoindentation in the 1150°C corner location. After removing the effects of the flexing from the measurement using the structural compliance method, it was concluded that H and Es were affected only by the sintering temperature (p<0.05) irrespective of the nanoindent locations. These results show that specimen-scale flexing may occur during nanoindentation of components in porous bioceramic scaffolds or in similar structure biomaterials, and that the structural compliance method must be utilized to accurately assess H and Es of these components.

An improved instrumented indentation technique for single microfibers

Abstract

Room temperature nanoindentation creep of hot-pressed B6O

Nanoindentation has become a widely used and versatile means of characterising the near-surface nanomechanical properties of a wide variety of materials. Yet, the nanomechanical properties of the polycrystalline boron suboxide (B6O) material prepared by uniaxial hot-pressing are sparsely known. We recently reported on the nanoindentation profiles, nanoindentation hardness, and elastic modulus determined by using the Oliver–Pharr method. To complement our earlier reports on the analysis of the load–displacement indentation response of hot-pressed B6O and for the first time, we present and discuss results giving an insight into the temporal evolution of the nanoindentation creep behaviour in B6O ceramics.

Nanoindentation and AFM Studies on Tungsten Carbide Crystals in WC-Co Hardmetal

The room temperature elastic and plastic properties of the WC grains of WC-9%Co hardmetal were investigated by nanoindentation and atomic force microscope (AFM). Two easily distinguishable crystallographic planes (using EBSD analyses) were investigated, namely the basal and prismatic planes, on which nanoindentation tests were performed with different applied loads from 1 mN to 50 mN to determine the hardness and reduced-modulus, respectively. The results deriving from nanoindentation show significantly higher indentation hardness on basal planes (HIT=28.9±0.1 GPa) than on prismatic ones (HIT=21.9±0.1 GPa) over 10 mN load. For loads below that the results were inconsistent. The corresponding indents were checked by AFM and correct values of hardness were found. The discrepancies indicate the inaccuracy of the built-in evaluation procedure (Oliver-Pharr method) in this low load, or more precisely in the low indentation depth range. It is pointed out that below 50 nm contact depth the applied built-in contact area-contact depth function is not appropriate and a new correct contact area-contact depth function is proposed, thus the resulting recalculated hardness values are in good agreement with the AFM measurements.

Numerical Model of Instrumented Indentation by a Rounded Cone Indenter Using Finite Element Method

The paper serves as an introduction to investigation of mechanical properties of functionally graded materials and deals with elastic nanoindentation numerical models. The models were based on the finite element method. Young's moduli were estimated by Oliver-Pharr method. The indenter geometry for which numerical solutions were accomplished was a rounded cone indenter. The effect of tip sharpness was examined by applying an increasing spherical tip radius. The results show that the apparent Young's modulus and the hardness increase linearly with increasing radius of the tip. The effect of approaching interface between two elastic materials on the apparent hardness and indentation modulus was identified in 3D model. The specimen consisted of two materials. First, the interface was linear and parallel to the direction of indentation, so that the Young's modulus changed suddenly. Second, the Young's modulus was continuously changing. The dependence on various boundary conditions of the specimen was also considered.

A Numerical Study of the Effect of the Berkovich Indenter Orientation on the Elastic Response of Anisotropic Material

The effect of the grain orientation of the stainless steel AISI 304 and the effect of the indenter orientation on the indentation moduli was numerically studied by means of the finite element method. The contact areas were evaluated numerically and the indentation moduli was determined according to the Oliver-Pharr method. As a result, the crystallographic orientations in which the indenter orientation plays the most important role were identified. However, the observed indentation moduli variation is within the scatter of the experimental data in practical applications.

The effect of collagen crosslinking on the biphasic poroviscoelastic cartilage properties determined from a semi-automated microindentation protocol for stress relaxation

Given the important role of the collagenous structure in cartilage mechanics, there is considerable interest in the relationship between collagen crosslinking and the mechanical behavior of the cartilage matrix. While crosslink-induced alterations to the elastic modulus of cartilage have been described, changes to time-dependent behavior have not yet been determined. The objective of the study was to quantify changes to cartilage material properties, including viscoelastic coefficients, with crosslinking via indentation. To accomplish this, a semi-autonomous microindentation stress relaxation protocol was first developed, validated and then applied to cartilage specimens before and after crosslinking. The change in mechanical properties with crosslinking was analyzed both in the unloading portions of the test via the Oliver–Pharr method and in the holding portion with an inverse iterative finite element model that represented cartilage as a biphasic poroviscoelastic material. Although both techniques suggested a similar increase in equilibrium modulus in the crosslinked specimens as compared to the controls, distinct differences in the control specimens were apparent, suggesting that the two different techniques may be capturing different aspects of the material behavior. No differences in time-dependent properties were observed between the crosslinked and the control specimens. These results give further insight into the effects of crosslinking in cartilage mechanical behavior. Additionally, the microindentation stress relaxation protocol may enable increased automation for high-throughput testing.

Electric-field-tunable mechanical properties of relaxor ferroelectric single crystal measured by nanoindentation

Electric field dependent mechanical properties of relaxor ferroelectric material Pb(Mn1/3Nb2/3)O3-PbTiO3 are investigated with the nanoindentation technique. Giant electric-field-tunable apparent elastic modulus (up to −39%), hardness (−9% to 20%), and energy dissipation (up to −13%) are reported. Based on experimental data, a characterization method of electromechanical coupled nanoindentation is proposed. In this method, an electric field tunable scaling relationship among elastic modulus, hardness, and indentation work for ferroelectric materials can be determined. In addition, this method can be used to obtain the electric-field-dependent elastic modulus and hardness, and avoid the estimate of contact area in the Oliver-Pharr method. Finally, the different effects on elastic modulus between positive and negative electric fields can be explained by the flexoelectric effect.

Finite element analysis of depth effect on measuring elastic modulus of a core-shell structure for application of instrumented indentation in tooth enamel

Tooth enamel is a complex structure, consisting of numerous enamel rods surrounded by a protein-rich sheath. Considering the possible effect of the protein-rich sheath on the indentation deformation of an enamel rod and the limitation of the Oliver–Pharr method in measuring the elastic modulus of the enamel rod, we used a finite element method to analyze the indentation deformation of an elastic-perfectly plastic cylinder surrounded by an elastic-perfectly plastic film. A concept of the threshold indentation depth was proposed, at which the percentage error of the measured modulus of the cylinder is ±10%. For the indentation depth less than the threshold indentation depth, the elastic modulus measured from the indentation test can be approximated as the intrinsic elastic modulus of the cylinder. The normalized threshold indentation depth strongly depends on the modulus ratio of the film to the cylinder and the ratio of the film thickness to the cylinder radius. The results can be used to guide the use of the Oliver–Pharr method in characterizing the mechanical properties of tooth enamel and bio-composites with core-shell structures.

Numerical and Experimental Study of the Roughness Effects on Mechanical Properties of AISI316L by Nanoindentation

Surface roughness is a commonly used criterion for characterization of surface quality in a machining operation. In the study of micro-scale mechanical properties of machined surface and cutting tool using nanoindentation method, perfect surface finish on the specimen is often required for the reliable indentation result. However, the perfect surface finish is often difficult to obtain from the machining operation due to the dynamic behavior of the machining and the limitation of the cutting tool geometry. In the presented paper, the effect of surface roughness on the nanoindentation measurements is investigated by using finite element method. A 3D finite element model with seven levels of surface roughness is developed to simulate the load-displacement behavior in an indentation process with a Berkovich indenter. The material used in the simulation is AISI 316 L stainless steel, modeled as an elastic-plastic material. The mechanical properties were calculated by combining simulations with the Oliver-Pharr method. The hardness and reduced modulus from the simulation were found to decrease with an increase of roughness. The study showed that the scatter of the load-depth curves and the deviation of the hardness and the reduced modulus are significant affected by the variation of roughness. It was also found that the height of pile-up was little affected by the surface roughness from the simulation. The combined effect of indenter tip radius and surface roughness was also investigated. The study was complemented with experimental tests and the results from these tests support the results from the simulation.