Pyramidal Indenter Research Articles

Correction of squareness measurements of Vickers indenters due to the tilt of the pyramid axis

In Vickers hardness measurements, ISO 6507-2 and 6507-3 Standards require to verify that the quadrilateral of the pyramid indenter base has angles of 90° ± 0.2°. Such measurement is usually performed through optical measuring systems, which, rotating the diamond indenter, allows to evaluate the angles between two consecutive faces with high accuracy. These angles correspond to the angles of the quadrilateral base when the axis of the pyramid is perfectly perpendicular to the seating surface. Nevertheless, when the pyramid axis is tilted, the angles between two consecutive faces are different from the corresponding angles on the quadrilateral base, thus a correction is required. In this work, a method to correct squareness measurements, based on a geometrical model, is presented.

Investigation of articular cartilage structural and biomechanical properties by atomic-force microscopy

Purpose: Although Osteoarthritis (OA) is a painful disease, which leads to a low quality of life for the patient as well as to a high socioeconomic burden, a functional cure remains elusive. Gaining new insights into the onset and progression of the disease is crucial but limited by the lack of biological tools able to display detailed structural and biomechanical properties of healthy and pathological cartilage. However, the cartilaginous ultrastructure and its mechanical properties are key factors for the overall functionality of the joint and should be considered as unique biomarkers of cartilage health status. The direct correlation of OA parameters, analyzed by histology and the OARSI scoring system, with cartilage structural and biomechanical properties assessed by atomic-force microscopy (AFM), will give new insights into the mechanisms underlying OA pathogenesis, into the impact of individual ECM molecules, and into disease progression. Methods: Paraffin embedded hind limbs, collected from euthanized wildtype mice (C57BL/6J), were dewaxed, re-hydrated and sectioned. AFM measurements were carried out using a BioScope Catalyst (Bruker) in combination with an inverse optical microscope (Zeiss LSM 150 Meta). For contact mode imaging and indentation-type AFM silicon nitride cantilevers with a nominal spring constant of 100 mN/m and integrated pyramidal tips with nominal radius of 20 nm were used. Images were recorded in air with a resolution of 512 × 512 pixels at a line rate of 1 Hz. After recording an overview image of the articular cartilage (AC), the scan area was reduced to obtain structural details within the ECM of the superficial layer. After AFM imaging, PBS at pH 7.4 was added to the sample for indentation measurements. Young's modulus was extracted from the approach force-distance curves using a modified Hertz model for a pyramidal indenter. Results: First measurements show that, with the help of AFM, we are able to image cartilage overall structure, collagen fibril orientation and D-band structure, as well as to measure tissue biomechanical properties on deparaffinized tissue sections. Indentation measurements demonstrate differences in the biomechanical properties of the outermost superficial layer to its adjacent middle zone as well as to the pericellular matrix of AC chondrocytes. Conclusions: Although cartilage biomechanical properties will change due to tissue fixation and paraffinization, our measurements on deparaffinized tissue sections are comparable to other AFM studies in the literature, which show similar trends in cartilage biomechanical properties, such as the reduced stiffness of the pericellular matrix of chondrocytes, in cartilage explants or cryo-sections. Our next step is to perform AFM measurements on different murine OA model (e.g. DMM and ageing models) using depraffinized tissue sections. The comparison of cartilage OARSI scoring and other staining methods such as IHC for OA markers, to the ultrastructure and biomechanical properties of cartilage using sections from the same animal and joint, will allow us to directly compare these parameters and gain new insights into the onset and progression of OA.

A flexible model for instrumented indentation of viscoelastic-plastic materials

Abstract

Determination of the best microstructure and titanium alloy powders properties using neural network

Purpose: Create a software product using a probabilistic neural network (PNN) and database based on experimental research of titanium alloys to definition of the best microstructure and properties of aerospace components. Design/methodology/approach: The database creation process for artificial neural network training was preceded by the investigation of the granulometric composition of the titanium powder alloys, study of microstructure, phase composition and evaluation of micromechanical properties of these alloys by the method of material plasticity estimation in the conditions of hard pyramidal indenters application. A granulometric analysis was done using a special complex of materials science for the images analysis ImageJ. Metallographic investigations of the powder structure morphology were carried out on the scanning electron microscope EVO 40XVP. Specimens for micromechanical testing were obtained by overlaying the titanium alloy powders on the substrate made of the material close to chemical composition. Substrates were prepared by pressing the powder mixture under the load of 400 MPa and following sintering at 1300°C for 1 hour. Overlaying was performed by an electron gun ELA-6 (beam current – 16 mA). Findings: According to the modelling results, a threshold of the PNN accuracy was established to be over 80%. A high level of experimental data reproduction allows us a full or partial replacement of a number of experimental investigations by neural network modelling, noticeably decreasing, in this case, the cost of the material creation possessing the preset properties with preserved quality. It is expected that this software can be used for solving other problems in materials science too. Research limitations/implications: The accuracy of the PNN algorithm depends on the number of input parameters obtained experimentally and forms a database for the training of the system. For our case, the amount of input data is limited. Practical implications: Previously trained system based on the PNN algorithm will reduce the number of experiments that significantly reduce costs and time to study. Originality/value: A software product on the basis of the PNN network was developed. The training sample was built based on a series of laboratory studies granulometric composition of the titanium powder alloys, study of microstructure, phase composition and evaluation of micromechanical properties of powder materials. The proposed approach allows us to determine the best properties of the investigated material for the design of aerospace components.

Indentation-based rate-dependent plastic deformation of polycrystalline pure magnesium

Using a Berkovich pyramidal indenter and an instrumented nanoindentation platform, dual stage nanoindentation creep tests, including loading to a pre-set load and holding at the constant load then unloading, were performed on polycrystalline pure magnesium at 300K. Indentation tests were performed at four indentation loading rates of 0.05, 0.5, 5, and 50mN/s at constant load of 45mN and holding time of 400s. These were performed to assess indentation force-displacement response along with average indentation stress, indentation strain rate sensitivity and ambient temperature rate dependent plastic deformation response of the material. To this end, activation volume, a kinetic characteristic of plastic deformation, and density of mobile dislocations in plastically deforming material in the plastic zone around the indenter are discussed in detail. Uncertainties and sources of error, i.e. indentation size effect, surface roughness, and thermal drift, and pile-up/sink-in measuring creep response through nanoindentation are provided in the current paper as well. The microstructure of the material was also studied through optical and scanning electron microscopy to further investigate the microstructure/property correlations in the tested polycrystalline pure magnesium. The results show the dependency of indentation stress, strain rate sensitivity, and activation volume upon depth and loading rate. According to the creep stress exponent measurements, the dominant mechanism of rate dependent plastic deformation for polycrystalline pure Mg at ambient temperature is attributed to obstacle limited dislocation glide. Finally, the contribution of mechanical twins in the plastic zone around the indenter in the studied polycrystalline pure magnesium is briefly discussed.

Application of the nanoindentation method in assessing of properties of cement composites modified with silica-magnetite nanostructures

The results of investigation of the cement composites modified with 5% of silica-magnetite nanostructures of the core-shell type are presented in the paper. The nanoindentation method employing three-sided pyramidal Berkovich indenter was used in the research. The mechanical properties and microstructure of the modified cement composites were evaluated on the basis of the values of hardness and indentation modulus measured inside the cement matrix and in the aggregate-paste interfacial zone. The results were compared with those obtained for the reference composites without nanostructures. The positive influence of the presence of silica-magnetite nanoparticles on the tested properties was found out.

Influence of microstructure on the wear and corrosion behavior of detonation sprayed Cr2O3-Al2O3 and plasma sprayed Cr2O3 coatings

Cr2O3-20wt% Al2O3 cermet composite and Cr2O3 coatings were deposited by the detonation spray coating (DSC) and air plasma spray techniques on Ti-6Al-4V substrates. The microstructure and phase formation in the coatings were analysed. Micro hardness and fracture toughness of coatings were measured using Vickers diamond pyramid indenter. Owing to the well-defined layered structure with individually richer Cr2O3 and Al2O3 phases together with oxide-based solid solution at the splat boundaries, the DSC Cr2O3-Al2O3 composite coating exhibits dense microstructure and better indentation fracture toughness than the well established plasma sprayed Cr2O3 coating. Although the plasma sprayed Cr2O3 coating also exhibits layered structure; the relatively poor indentation fracture toughness is attributed to the significantly higher porosity and weaker inter-splat bonding. Towards understanding the relative tribological behavior, the sliding and three-body abrasive wear tests were conducted. The material removal mechanism and associated degradation behavior of detonation sprayed Cr2O3-Al2O3 coating has been correlated with coating microstructure comparatively against the plasma sprayed Cr2O3 coating. Electro-chemical corrosion tests of coatings in salt solution (artificial sea water) reveals an improved corrosion protection by Cr2O3-Al2O3 composite coating as compared to Cr2O3 coating. Enhanced tribological performance and corrosion resistance was attributed to the dense and crack free structure of Cr2O3-Al2O3 composite coating than its counterpart.

Multiscale Modeling of the Three-Dimensional Meniscus Shape of a Wetting Liquid Film on Micro-/Nanostructured Surfaces.

The design of structured surfaces for increasing the heat flux dissipated during boiling and evaporation processes via enhanced liquid rewetting requires prediction of the liquid meniscus shape on these surfaces. In this study, a general continuum model is developed to predict the three-dimensional meniscus shape of liquid films on micro/nanostructured surfaces based on a minimization of the system free energy that includes solid-liquid van der Waals interaction energy, surface energy, and gravitational potential. The continuum model is validated at the nanoscale against molecular dynamics simulations of water films on gold surfaces with pyramidal indentations, and against experimental measurements of water films on silicon V-groove channels at the microscale. The validated model is used to investigate the effect of film thickness and surface structure depth on the meniscus shape. The meniscus is shown to become more conformal with the surface structure as the film thickness decreases and the structure depth increases. Assuming small interface slope and small variation in film thickness, the continuum model can be linearized to obtain an explicit expression for the meniscus shape. The error of this linearized model is quantitatively assessed and shown to increase with increasing structure depth and decreasing structure pitch. The model developed can be used for accurate prediction of three-dimensional meniscus shape on structured surfaces with micro/nano-scale features, which is necessary for determining the liquid delivery rate and heat flux dissipated during thin-film evaporation. The linearized model is useful for rapid prediction of meniscus shape when the structure depth is smaller than or comparable to the liquid film thickness.

Annealing effect on plastic flow in nanocrystalline CoCrFeMnNi high-entropy alloy: A nanomechanical analysis

The influence of annealing on the constitutive stress-strain response of nanocrystalline (nc) CoCrFeMnNi high-entropy alloy (HEA) was investigated through a series of nanoindentation experiments using five different three-sided pyramidal indenters. The nc HEA, produced by high-pressure torsion (HPT), was subjected to annealing at 450 °C for 1 and 10 h. Microstructural analysis using transmission electron microscopy (TEM) showed that three different nano-scale precipitates (NiMn-, FeCo-, and Co-rich phases) form in the primary single-phase matrix of nc HEA after annealing. The strain-dependent plastic flow response of nc HEA pre- and post-annealing was estimated using the indentation strain and constraint factor, revealing a significant strain softening in nc HEA, which becomes pronounced after annealing. TEM analysis of the deformed material underneath the indenter suggests that the plastic deformation aids in the dissolution of the annealing-induced intermetallic precipitates, which could be the mechanism for the pronounced softening. The dissolution mechanism was rationalized by the destabilization of precipitates during plastic deformation due to the increase in interface energy.

Effects of indenter angle on micro‐scale fracture toughness measurement by pillar splitting

AbstractWe present improvements to a recently developed pillar splitting technique that can be used to characterize the fracture toughness of materials at the micrometer scale. Micro‐pillars with different aspect ratios were milled from bulk Si (100) and TiN and CrN thin films, and pillar splitting tests were carried out using four different triangular pyramidal indenters with centerline‐to‐face angles varying from 35.3° to 65.3°. Cohesive zone finite element modeling (CZ‐FEM) was used to evaluate the effect of different material parameters and indenter geometries on the splitting behavior. Pillar splitting experiments revealed a linear relationship between the splitting load and the indenter angle, while CZ‐FEM simulations provided the dimensionless coefficients needed to estimate the fracture toughness from the splitting load. The results provide novel insights into the fracture toughness of materials at small‐scales using the pillar spitting technique and provide a simple and reliable way to measure fracture toughness over a broad range of material properties.

Multi-cycling instrumented nanoindentation of a Ti-23Nb-0.7Ta-2Zr-1.2O alloy in annealed condition

In this study, depth-dependent mechanical properties (i.e. hardness, elastic modulus, and indentation size effect) as well as strain rate sensitivity of a new class of titanium based alloys, Ti-23Nb-0.7Ta-2Zr-1.2O, are considered. To this end, local deformation micro-mechanisms of this alloy are investigated systematically through a constant-strain rate (CSR) instrumented nanoindentation testing technique. The CSR nanoindentation tests were performed at ambient (room) temperature with nominal indentation depths of 100–2000nm and strain rates of 0.005, 0.05, and 0.5s−1 using a Berkovich pyramidal indenter. Nanoindentation is a particularly reliable tool for studying elastic/plastic behavior of metallic alloys. Large strain gradients around the indenter should reveal and help clarify the thermally activated mechanisms contributing to the plastic deformation processes which can quantitatively be interpreted by examining the rate sensitivity index, m, and activation volume. Indentation results show that the strain rate sensitivity and activation volume, Burger’s vector times activation area, are depth-dependent phenomena revealing different controlling plastic deformation mechanisms.

Application of the Improved Inclusion Core Model of the Indentation Process for the Determination of Mechanical Properties of Materials

The improved Johnson inclusion core model of indentation by conical and pyramidal indenters in which indenter is elastically deformed and a specimen is elastoplastically deformed under von Mises yield condition, was used for determination of mechanical properties of materials with different types of interatomic bond and different crystalline structures. This model enables us to determine approximately the Tabor parameter С = НМ/YS (where НМ is the Meyer hardness and YS is the yield stress of the specimen), size of the elastoplastic zone in the specimen, effective apex angle of the indenter under load, and effective angle of the indent after unloading. It was shown that the Tabor parameter and the size of elastoplastic deformation zone increase monotonically with the increase of the plasticity characteristic indentation test results significantly more informative.

Evaluation of Sinking-In and Cracking Behavior of Soda-Lime Glass under Varying Angle of Trigonal Pyramid Indenter

It is well known that glass undergoes elastic and inelastic deformation under a sharp diamond indenter. Although brittle or less-brittle behavior of glass must be connected with such mechanical responses of glass under the indenter, there has been limited research on in-situ deformation behavior of glass during the loading and unloading indentation cycle. This is because most indentation tests were conducted using a commercial hardness tester for which this information is not available. In this study, the in-situ sinking-in region of glass during indentation test is determined using a custom-designed indentation microscope with trigonal pyramid indenters having different tip angles. It is found that both the shape of contact region and the amount of sinking-in are affected by indenter geometries, and that the projected contact region of the glass sample under Berkovich indenter is not a regular triangle, but a concave triangle with bowed-in edges. This is due to the larger amount of sinking-in under the face than under the ridge of indenter. It is also found that these deformation behaviors of glass are inseparably linked with contact damage or cracking in glass.

Growth and characterizations of L-methioninium picrate single crystal for nonlinear optical applications

An organic nonlinear optical (NLO) single crystal of L-methioninium picrate (LMP) has been grown by slow evaporation solution technique (SEST) at a constant temperature of [Formula: see text]C. Formation of nucleation and growth kinetics of the crystal were observed using an optical microscope for different durations and it was found that the shape of nucleation is similar to the grown large size single crystals of the same material. The characterization techniques such as powder X-ray diffraction (PXRD) analysis, UV–Vis NIR, Vickers microhardness, piezoelectric d[Formula: see text] coefficient, photoconductivity and etching studies have been performed. The crystal system and lattice parameters of the grown crystal were determined by PXRD analysis. The spectroscopic properties of the synthesized compound were confirmed by Fourier transform infrared (FTIR) analysis. The UV–Vis NIR analysis was performed on the grown single crystal to find the optical absorption in the entire visible region. Attempts have been made to investigate the mechanical properties of LMP crystal using Vickers diamond pyramid indenter. In addition piezoelectric, photoconductivity and etching studies were carried out to study the d[Formula: see text] coefficient, photoconducting nature and surface properties respectively.

Удельная энергия упругопластической деформации, необходимая для образования трещины при индентировании упрочняющих покрытий

Current methods of assessing crack resistance of strengthened surfaces using pyramidal indenter are intended only for the case of radial crack formation around the indenter. However, for certain types of hardening coatings, e.g., titanium nitride based coatings, indentation leads to formation of ring rather than radial cracks around the indenter. For this case of crack formation, we propose to use kinetic diagram of indentation to determine the specific energy of elastoplastic deformation which provides formation of the first visible crack. This energy can be used to compare the ability of coatings to withstand any kind of crack formation.

Energy analysis of spherical and Berkovich indentation contact damage in commercial polycrystalline silicon carbide

This research aims at enhancing the understanding of the role of processing and microstructure on the deformation of and fracture of polycrystalline silicon carbide based ceramics when subjected to blunt (spherical) and sharp (pyramidal) indenters. The role of microstructure and material properties on the energy absorption capability of SiC is studied. By examining the behavior of several SiC materials during nanoindentation experiments using spherical and pyramidal indenters, it is possible to make predictions about methods to improve the ductility and fracture toughness of SiC to optimize its energy absorption. The applicability of the area under the irreversible part of the indentation load displacement curve (energy dissipated during loading) to predict the performance of SiC under contact loading is examined. Results from experimental data show that the polycrystalline SiC material properties like hardness and modulus are relatively insensitive to the amount of indentation damage when the indent size is more than the average grain size. It can also be concluded that during pyramidal indentation for the polycrystalline samples analyzed, grain-size plays a much more important role in energy dissipation than the grain boundary phases or any other indentation damage relate effects.

Thin coatings and films hardness evaluation

The existing thin coatings and films hardness evaluation methods based on indentation with pyramidal indenter on various scale levels are expounded. The impact of scale factor on hardness values is performed. The experimental verification of several existing hardness evaluation methods regarding the substrate hardness value and the “coating - substrate” composite hardness value is made.

Improved core model of indentation and its application to measure diamond hardness

A model of the indentation using conical and pyramidal indenters has been proposed, in which not only a sample but the indenter as well are elastoplastically deformed and their materials obey the Mises yield condition. These conditions are characteristic of the measuring of diamond hardness through a diamond indenter. The model that has been proposed generalizes and refines the known simplified Johnson’s model, which uses an elastically deformed indenter. The proposed model makes it possible to determine approximately the sizes of elastoplastic zones in the indenter and sample, the effective apex angle of the loaded indenter and effective angles of the indenter and imprint after unloading. Based on this model a procedure of the determination of the sample and indenter yield strengths (Ys and Yi, respectively) has been developed, in which the relations that use the experimental values of the effective angle of the sample imprint and measured values of the Meyer hardness, HM (mean contact pressure) are added to theoretical relations of the indentation model. The developed computational procedure was applied in indentation experiments on synthetic diamond at the temperature 900°C (at which diamond exhibits a noticeable plastic properties) using natural diamond pyramidal indenters having different apex angles. According to the proposed model, the stress-strain states of samples and indenters have been investigated and their yield strengths and plasticity characteristics were defined. The stress–strain curve of the diamond in the stress-total strain coordinates has been constructed. The strain hardening of diamond was also studied.

Twin pattern evolution in a fine-grained Mg alloy subjected to indentation

A Vickers diamond pyramid indenter was impressed on a fine-grained polycrystalline Mg–3Al–1Zn alloy. Serial polishing in combination with quasi-in-situ electron backscatter diffraction (EBSD) examinations revealed the presence and the 3D spatial distributions of {10–12} extension twins around the indent. Twin chains and completely twinned areas were found in some regions close to the indent. A model of twin pattern evolution around the indent was proposed based on the experimental observations and local strain accommodation analysis.

Mapping Viscoelastic and Plastic Properties of Polymers and Polymer-Nanotube Composites using Instrumented Indentation.

An instrumented indentation method is developed for generating maps of time-dependent viscoelastic and time-independent plastic properties of polymeric materials. The method is based on a pyramidal indentation model consisting of two quadratic viscoelastic Kelvin-like elements and a quadratic plastic element in series. Closed-form solutions for indentation displacement under constant load and constant loading-rate are developed and used to determine and validate material properties. Model parameters are determined by point measurements on common monolithic polymers. Mapping is demonstrated on an epoxy-ceramic interface and on two composite materials consisting of epoxy matrices containing multi-wall carbon nanotubes. A fast viscoelastic deformation process in the epoxy was unaffected by the inclusion of the nanotubes, whereas a slow viscoelastic process was significantly impeded, as was the plastic deformation. Mapping revealed considerable spatial heterogeneity in the slow viscoelastic and plastic responses in the composites, particularly in the material with a greater fraction of nanotubes.