Pyramidal Indenter Research Articles

Evaluation of wear suppression for phospholipid polymer-grafted ultra-high molecular weight polyethylene at concentrated contact

The wear resistance of cross-linked polyethylene grafted with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC-grafted CLPE) and unmodified CLPE loaded by an alumina roller were evaluated in a simulated synovial fluid (0.5wt% hyaluronate solution containing 30vol% fetal bovine serum). To evaluate the wear depth distribution within the wear scar, we compared the projected area of wear and non-wear for indents on polyethylene surfaces produced by a triangular pyramidal indenter and estimated the local wear depths on the basis of the changes in the projected area. The worn depth of PMPC-grafted CLPE was almost a third lower than that on CLPE.

Mapping strain fields induced in Zr-based bulk metallic glasses during in-situ nanoindentation by X-ray nanodiffraction

A pioneer in-situ synchrotron X-ray nanodiffraction approach for characterization and visualization of strain fields induced by nanoindentation in amorphous materials is introduced. In-situ nanoindentation experiments were performed in transmission mode using a monochromatic and highly focused sub-micron X-ray beam on 40 μm thick Zr-based bulk metallic glass under two loading conditions. Spatially resolved X-ray diffraction scans in the deformed volume of Zr-based bulk metallic glass covering an area of 40 × 40 μm2 beneath the pyramidal indenter revealed two-dimensional map of elastic strains. The largest value of compressive elastic strain calculated from diffraction data at 1 N load was −0.65%. The region of high elastic compressive strains (<−0.3%) is located beneath the indenter tip and has radius of 7 μm.

Nanomechanical characterization of porous anodic aluminum oxide films by nanoindentation

Nanostructured thin films have gained great interests in recent years due to their significantly enhanced properties and great potential for various applications. Nanoindentation techniques are commonly used to measure nanomechanical properties of thin films and the surface layers of bulk materials. In this article, nanoindentation tests coupled with computer modeling were proposed to characterize nanostructured porous anodic aluminum oxide (AAO) films grown on glass. A three-dimensional nanoindentation model was developed. The Young's modulus and hardness were measured by nanoindentation using a Berkowich pyramidal sharp tip indenter. The effects of the substrate and porous structure of the film on the coating measurements were investigated. The dilemma for extracting the mechanical properties of a porous structural film in nanoindentation has been pointed out, and an alternative approach of combined modeling/experimental provided a justification by considering the porous structure of the film and minimizing the influence of the substrate. Computer modeling results were validated by experimental data.

Modeling and analysis of the grit level interaction in diamond wire sawing of silicon

This paper analyzes the interaction between diamond grits and crystalline silicon (c-Si) in the ductile regime of diamond wire sawing through theoretical calculations and finite element modeling (FEM). Cutting depth per grit is calculated based on the sawing parameters and is found to be in the range of 41 to 143 nm for a typical diamond wire sawing operation. These depths are smaller than the critical depth of cut for brittle-to-ductile cutting mode transition in single point diamond turning/scribing of c-Si as well as the value derived from indentation fracture mechanics. The shape of the cutting grit plays an important role in determining the normal force and the critical depth of cut. The cutting force calculated by the FEM is approximately 12 mN for a spherical grit/indenter shape and 30 mN for a faceted pyramidal (Vickers) indenter at a 70-nm scribing depth. It is shown ductile mode cutting performance is sensitive to grit shape, and grits with sharp edges can easily lead to brittle fracture. The theoretical cutting depth per grit indicates the likely dominance of ductile mode cutting in diamond wire sawing, albeit, in reality, it is usually a mixture of ductile and brittle cutting. This discrepancy is more likely related to the grit size and shape variation in an actual diamond wire as well as to the dynamics of the operation.

Effect of temperature and strain rate on the mechanisms of indentation deformation of magnesium

Abstract

Indentation property and corrosion resistance of electroless nickel–phosphorus coatings deposited on austenitic high-Mn TWIP steel

A multilayer coating using electroless nickel-phosphorus (Ni–P) was applied on a twinning-induced plasticity (TWIP) steel containing nominally 25wt.% Mn and 3wt.% Al to improve the indentation hardness and corrosion properties. Microindentation tests with two different indenters, namely, a three-sided pyramidal Berkovich indenter and a ball indenter were performed to study the mechanical response, the indentation hardness and elastic modulus of the coatings in conditions: as-plated, and post treated (PT) at 350°C and 700°C for 1h. The deformation morphology underneath the indenters was examined using a scanning laser microscope. The results showed that Ni–P coatings could significantly enhance the surface hardness of the TWIP steel. Significant improvement in the corrosion resistance could be observed in a sulfuric acid solution for the Ni–P coated steel compared to the uncoated substrate TWIP steel.

Prediction of residual stress and deformation of enameled steel

The firing process to bond an enamel frit onto a steel substrate is conducted under a high-temperature condition of approximately 830°ýC for 3min, which leads to the thermal deformation of the enameled steel product. This thermal deformation of the enameled steel is one of the main problems with the firing process. The thermal deformation is also affected by the residual stress of the enamel coating, which is generated by the mismatch between the thermal expansion coefficient of the enamel and that of the steel substrate. The objective of this study was to predict the deformation and residual stress of enameled steel using an analytical model and finite element analysis. To verify the results from the prediction methods, firing and cooling experiments were performed using the same conditions as such methods, and the experimental results were compared to those of the prediction methods. The residual stress was measured using a crack indentation technique based on a pyramidal diamond indenter. The magnitude of the residual stress could be determined by comparing the crack lengths in the unstressed and stressed specimens. The results of the prediction methods were in good agreement with the experimental results with respect to the deformation and residual stress.

Mechanical properties of β-HMX.

BackgroundFor a full understanding of the mechanical properties of a material, it is essential to understand the defect structures and associated properties and microhardness indentation is a technique that can aid this understanding.ResultsThe Vickers hardness on (010), {011} and {110} faces lay in the range of 304–363 MPa. The Knoop Hardnesses on the same faces lay in the range 314–482 MPa. From etching of three indented surfaces, the preferred slip planes have been identified as (001) and (101). For a dislocation glide, the most likely configuration for dislocation movement on the (001) planes is (001) [100] (|b| = 0.65 nm) and for the (101) plane as (101) left[10overline{1}right] (|b| = 1.084 nm) although (101) [010] (|b| = 1.105 nm) is possible. Tensile testing showed that at a stress value of 2.3 MPa primary twinning occurred and grew with increasing stress. When the stress was relaxed, the twins decreased in size, but did not disappear. The twinning shear strain was calculated to be 0.353 for the (101) twin plane.ConclusionsHMX is considered to be brittle, compared to other secondary explosives. Comparing HMX with a range of organic solids, the values for hardness numbers are similar to those of other brittle systems. Under the conditions developed beneath a pyramidal indenter, dislocation slip plays a major part in accommodating the local deformation stresses.Graphical abstractHMX undergoing tensile testing.Electronic supplementary materialThe online version of this article (doi:10.1186/s13065-015-0091-6) contains supplementary material, which is available to authorized users.

Composition and cooling-rate dependence of plastic deformation, densification, and cracking in sodium borosilicate glasses during pyramidal indentation

Plastic deformation, densification, and cracking of sodium borosilicate (NBS) glasses were examined during indentation with a three-sided pyramidal indenter. Compositions of 74.0SiO2–10.0B2O3–16.0Na2O (NBS1), and 74.0SiO2–20.7B2O3–4.3Na2O–1.0Al2O3 (NBS2) (mol%) were investigated. The effect of thermal history was additionally considered for the NBS2 composition, which lies near the boron anomaly line. Hardness, elastic modulus, and fracture toughness were estimated with instrumented indentation techniques. Atomic force microscopy (AFM), Raman micro-spectroscopy, and post-indent annealing experiments were used to analyze surface topographies, densification, and recovery of deformed material. The results indicate that NBS1 exhibits a greater hardness and elastic modulus, and undergoes less densification than the NBS2 glasses. Different casting conditions influenced the plastic deformation and onset of crack initiation in NBS2. Interpretation of fracture toughness measured by indentation is complicated by residual stresses, densification during contact, and model assumptions. However, distinct differences in elastic modulus, plastic deformation and cracking between the glasses were noticeable. Such results and observations are discussed in terms of structural changes in the glass.

A dual triangular pyramidal indentation technique for material property evaluation

Abstract

OBSERVATION OF PHASE TRANSITIONS FEATURES IN SILICON AT HIGH LOCAL PRES-SURE UNDER INDENTATION

Instrumented indentation is a very promising technique for studying structural phase transitions in crystalline materials such as silicon.In the present work silicon samples with different crystallographic orientations and doping rates were investigated using indentation with a pyramid indentor of Berkovich type. For studying the electrical properties the indentor was made of semiconductor boron−doped single crystalline diamond. For verification of structural transitions in silicon Raman spectroscopy technique was applied.Electrical current through the contact area under the indentor was measured simultaneously with recording the mechanical response of the material during the indentation process. It is shown that variation in current value gives additional information about the conditions of contact between the indentor and the sample surface. The influence of variations in resistivity and contact area on the measured electrical current value is discussed.

Extracting uniaxial responses of single crystals from sharp and spherical hardness measurements

This investigation provides a mechanistic background to mechanical property extractions performed from single-crystal microhardness measurements. The analysis concerns both spherical and sharp (pyramidal) indentations. We show that the uniaxial stress–strain curves inferred from hardness measurements in single crystalline units of material are coincidental with those attained under true uniaxial loadings along specific crystalline orientations. Landmark hardness relations that were originally developed for power-law strain hardening polycrystalline aggregates are extended to single-crystal indentations. Mechanical property extractions in crystals violating power-law hardening because of a marked critical resolved shear stress and/or extreme strain hardening saturation are subsequently addressed. The analysis is pertinent to the assessment of multiple-glide deformation stage-II and strain hardening saturation stage-III of cubic single crystals from indentation experiments.

Characterization of Biomechanical Properties of Primary Endothelial Cells Exposed to Shear Stress

Mechanotransduction, the process where cells convert biophysical signals into chemical signals, directs the behavior and function of endothelial cells (ECs) because of their unique position on the luminal surface of blood vessels. ECs are in direct contact with blood and can sense changes in shear stress caused by alterations in flow rate. To investigate the biophysical implications of this response we recapitulated physiological levels of shear stress in vitro using two systems, imaged and measured the consequent cytoskeletal changes with immunofluorescence, and quantified cortical stiffness distributions. We developed a sampling method for Atomic Force Microscopy (AFM) stiffness measurements by which force-indentation data is collected in a uniform grid (force map) over multiple cells within a monolayer. In this way, the sampling bias resulting from the selection of force-indentation measurements was reduced. Hertzian contact modeling for AFM was used to specifically isolate the cortical stiffness of the cell using a pyramidal indenter and small indentation depths (100 nm). Nonparametric statistics were used to compare the distributions of moduli from a given experiment, as the data distribution was not Gaussian. Actin filament alignment was quantified using Hough Transforms on immunofluorescence images and increased when cells were exposed to shear stress. The medians of stiffness distributions within sheared EC monolayers increased in magnitude when compared to static monolayers, yielding a method-dependent increase in cortical stiffness of up to 60%. This demonstrates that physiological levels of shear stress dramatically affect ECs morphology in a way that cannot be simulated under static conditions. We concluded that the biophysical milieu of a cell's physiological microenvironment must be recapitulated in order to represent characteristic cellular mechanical properties in vitro.

Evaluation of indentation fracture toughness for brittle materials based on the cohesive zone finite element method

We propose indentation fracture toughness evaluation methods based on finite element analysis adapting the cohesive zone model. Establishing an appropriate finite element model, we examine the effects of material properties on the size of the cracks formed by Vickers indentation, and then suggest a regression formula for the estimation of the fracture toughness. The formula is extended to other types of pyramidal indenters including variations of the indenter angle and the number of indenter edges. The suggested methods are examined by comparison of the experimental data, and the fracture toughness can be accurately estimated from pyramidal indentation data.

Annealing-induced indentation recovery in thin film metallic glasses: Effects of indenter tip geometry, film thickness and film composition

Annealing-induced indentation recovery behaviours of sputtered Zr50Cu28Al14Ni8, Cu48Zr42Al6Ti4, Hf46Cu45Al6Ti3, and Fe65Ti13Co8Ni7B6Nb1 (in at. %) thin film metallic glasses (TFMGs) on silicon substrate are examined. Influences of indenter tip geometry, film thickness and film composition on indentation recovery of TFMGs have been studied. Nanoindentations are performed at room temperature with two three-sided pyramidal indenters (Berkovich and cube-corner), followed by annealing performed within the supercooled liquid temperatures. The results reveal that the extent of recovery of indentation by Berkovich indenter is higher than that of cube-corner owing to higher compressive stress underneath the Berkovich tip and larger radius of curvature of the indent left behind after Berkovich indentation. The numerical modelling of depth profile of Zr-based TFMG shows the blunter the indenter tip is the higher the recovery. The thicker film also shows larger recovery. In addition, because of the difference in bonding force among constituent elements, Zr-based TFMG exhibits larger recovery than Cu-, Hf- and Fe-based TFMGs.

Characteristics of indentation cracking using cohesive zone finite element techniques for pyramidal indenters

During indentation of brittle materials, cracks may be generated around the impression, depending on load conditions, material and indenter geometry. We investigate the effect of indenter geometry (centerline-to-face angle and number of edges) on crack characteristics by indentation cracking test and finite element analysis (FEA). Considering conditions for crack initiation and propagation, an FE model is employed featuring cohesive interfaces in zones of potential crack formation. After verification of the FE model through comparison with experimental results for Vickers and three-sided pyramidal indentation, we study the crack morphology for diverse indenter geometries and establish a relation between the crack length c and the number of indenter edges nc. Together with a relation between indenter angle ψ and crack length c, we can predict the length of the crack induced by other types of indenter from the crack size obtained with a reference indenter.

Experimental and theoretical confirmation of the scaling exponent 2 in pyramidal load displacement data for depth sensing indentation.

A series of articles by Kaupp et al. have recently been published in "Scanning," containing erroneous claims about the curvature of pyramidal nanoindentation loading curves. The present paper recalls the theoretical reasons why, for self-similar indenter shapes like pyramidal or conical indentations, the load scales with the indentation depth squared. Furthermore, experimental evidence for that behavior is provided for a wide variety of materials, ranging from ceramics to metals and polymers.

Dry sliding wear characteristics of carbon nanotube/alumina nanocomposites under a sharp pyramidal indenter

Hot-pressed multiwalled carbon nanotube (MWCNT)/alumina (Al2O3) nanocomposites offered improved flexural strength (σ4–FS), hardness (HV) and fracture toughness (KIC–SENB) over pure Al2O3. While HV and σ4-FS of nanocomposites improved by 8–10%, KIC-SENB of 0.13wt% CNT/Al2O3 nanocomposite (~7.45MPam0.5) was ~1.3 and ~2.1 times higher than hot-pressed and pressureless sintered Al2O3, respectively. Unlubricated sliding wear experiments at 1–20N normal load (FN) under a sharp pyramidal indenter revealed enhanced resistance to catastrophic failure and wear of nanocomposites over pure Al2O3. At FN=20N, the lowest wear rate (WR≈0.02mm3/Nm) of 0.07wt% CNT/Al2O3 nanocomposite was found to be ~50% lower than that of monolithic Al2O3. Scratch tests on ground and polished samples revealed notable changes in WR and wear mode as a function of surface roughness (Ra). Wear mode maps indicated that while in ground specimens the primary mechanism was excessive flake formation, severity of flake formation reduced significantly in polished samples and turned toward powder formation and/or ploughing due to effective indenter tip/specimen surface interaction in absence of stochastic surface irregularities present in ground samples. Present study indicated that for characteristic wear analysis of CNT/Al2O3 nanocomposites, Ra should be ≤100nm.

Method for Determining the Plastic Properties of Metallic Materials by Instrumented Indentation with Dual Pyramidal Indenters

Method for determining the plastic properties of metallic materials was proposed based on the functional relationships between representative stress, representative strain and nominal hardness which were established with the aid of dimensional analysis and finite element simulation. The errors of 0.2% yield strength and strain hardening exponent of five engineering metals were from-17.1% to 15.4% and from -0.125 to 0.11, respectively,which satisfied the need of engineering application and verified the effectiveness of the method.

Deformation and fracture of LLM-105 molecular crystals studied by nanoindentation

Mechanical deformation of crystalline high explosives plays an important role in both the fabrication of polymer-bonded explosives and controlling their sensitivity to mechanically-induced decomposition. Here, we study the deformation behavior of (010)-oriented LLM-105 and β-HMX molecular crystals by nanoindentation with pyramidal (Berkovich) and spherical (19 μm-diameter) indenters. Results reveal indentation elastic moduli of 21 and 18 GPa and Berkovich hardness of 0.73 and 0.65 GPa for LLM-105 and HMX, respectively. For LLM-105 (but not for HMX), indentation stress remains essentially unchanged for spherical indentation strains of ∼10–25%, suggesting that inelastic deformation above a certain strain proceeds via flow at constant stress. Both materials exhibit highly anisotropic surface fracture patterns after Berkovich and spherical indentation, consistent with fracture along (011) cleavage planes. No deformation-induced material decomposition is observed in either material for the indentation conditions used.