Pyramidal Indenter Research Articles

Influence of crystal orientation and Berkovich tip rotation on the mechanical characterization of grain boundaries in molybdenum

In this study, nanoindentation experiments with a continuously measured stiffness are performed in the vicinity of grain boundaries in technically pure molybdenum. A significant change in hardness as a function of indentation depth is put in context with the corresponding deformation patterns in the indented crystals. The difference in hardness increase of 18% compared to 5% of two different grain boundaries with similar misorientation angle could be explained in this way. Five degrees of freedom have to be determined to fully describe a grain boundary. However, the influence of the rotation angle of a pyramidal indenter geometry around the loading axis as yet another loading parameter is assessed by backscattered electron micrographs and orientation deviation maps. Indentation experiments with different rotation angle at the same grain boundary confirm that a hardness increase close to the grain boundary is only apparent if a significant amount of plastic deformation is introduced towards the interface.

Indentation-based characterization of creep and hardness behavior of magnesium carbon nanotube nanocomposites at room temperature

The time-dependent plastic deformation response of magnesium/carbon nanotube (CNT) nanocomposites containing 0.25, 0.5, and 0.75 vol% of carbon nanotubes is investigated through depth nanoindentation tests against monolithic pure magnesium in the present study. The Mg-CNT nanocomposite materials were successfully synthesized via a powder metallurgy technique coupled with microwave sintering followed by hot extrusion to produce 8-mm diameter, long solid bars. All depth-sensing indentation creep tests were conducted at ambient (room) temperature employing a diamond Berkovich pyramidal indenter. These tests are dual-stage, i.e., loading to a prescribed peak load of 50 mN, holding the peak load constant for a dwell period of 500 s, and finally unloading. Various strain rates of 0.01, 0.1, 1, and 10 s−1 were performed to assess the effects of strain rate and dwell time on the ambient temperature creep response of the Mg-CNT nanocomposites. The outcomes of these tests are explained through material hardness, microstructure, the extent of CNT content in each material, and strain rate sensitivity. Upon analyzing the nanoindentation creep tests, the dominant creep mechanism at room temperature was found to be a dislocation creep mechanism. It is also found that CNTs increase the creep resistance of magnesium. Findings of this study can be used as a starting point for a high-temperature creep study on Mg-CNT nanocomposites. This paper is a continued study from our group on time-dependent plastic deformation of Mg nanocomposites (i.e., see Haghshenas et al., Journal of Composite Materials, https://doi.org/10.1177/0021998318808358 ). The short-term goal is to provide a compressive picture of the controlling creep mechanisms and their dependency upon, time, temperature, strain rate, volume fraction of the nanoparticles, and the type of the nanoparticles. Mg, in general, is a notorious material for high-temperature application; therefore, the long-term objective is to propose Mg nanocomposite as reliable replacements for Mg when lightweight and creep resistance are needed. However, to be able to confidently suggest such a replacement detailed understanding on the controlling phenomena, mention as short-term goals, are required.

Measuring biomaterials mechanics with atomic force microscopy. 1. Influence of the loading rate and applied force (pyramidal tips).

Atomic force microscopy (AFM) is today an established tool in imaging and determination of mechanical properties of biomaterials. Due to their complex organization, those materials show intricate properties such as viscoelasticity. Therefore, one has to consider that the loading rate at which the sample is probed will lead to different mechanical response (properties). In this work, we studied the dependence of the mechanical properties of endothelial cells on the loading rate using AFM in force spectroscopy mode. We employed a sharp, four‐sided pyramidal indenter and loading rates ranging from 0.5 to 20 μm/s. In addition, by variation of the load (applied forces from 100 to 10,000 pN), the dependence of the cell properties on indentation depth could be determined. We then showed that the mechanical response of endothelial cells depends nonlinearly on the loading rate and follows a weak power‐law. In addition, regions of different viscous response at varying indentation depth could be determined. Based on the results we obtained, a general route map for AFM users for design of cell mechanics experiments was described.

Characteristics of the deformed zone around Vickers indentations in metals

The stress and plastic strain distributions around pyramidal indenter in steel 10 has been investigated by exploring the Vickers hardness of carefully prepared median plane sections through indentations. Experimental results showed that, the maximum strain is observed at a certain depth below the indentation surface inside the hydrostatic compression zone. The minimum permissible ratio between the coating thickness and the indentation depth, which excludes the influence of the substrate on the indentations results obtained by Vickers indenter, is substantiated. The shape and dimensions of plastic deformation zones around Vickers indentations were determined. As a result, between the indentation volume and the deformation zone volume established directly proportional relationship.

Mechanical and chemical characterisation of bioresorbable polymeric stent over two-year in vitro degradation.

Polymeric stent is a temporary cardiovascular scaffold, made of biodegradable poly (l-lactic) acid, to treat coronary artery stenosis, with expected resorption by the human body over two to three years. In this paper, the mechanical properties of a polymeric stent over two-year in vitro degradation were studied and characterised using atomic force microscopy and nanoindentation techniques, complemented with analyses of weight loss, gel permeation chromatography and differential scanning calorimetry. Atomic force microscopy assessed stent degradation at the surface, whilst nanoindentation was able to investigate the property at a greater depth into the material. No significant changes to the Young’s modulus were observed with the atomic force microscopy due to bulk degradation nature of the polymer. Chemical analyses demonstrated a reduction of molecular weight and an increase of crystallinity, indicating degradation of the stents. Berkovich nanoindentation showed a trend of reduction in modulus over in vitro degradation, which was, however, not continuous due to the variations of measurements associated with the pyramidal indenter tip and the semi-crystalline structure of the polymer.

Indentation modulus at macro-scale level measured from Brinell and Vickers indenters by using the primary hardness standard machine at INRiM

In this paper, the experimental procedure and calculation model for the measurement of the indentation modulus by using the primary hardness standard machine at INRiM in the macro-scale range at room temperature is described. The indentation modulus is calculated based on the Doerner-Nix linear model and from accurate measurements of indentation load, displacement, contact stiffness, and hardness indentation imaging. Measurements are performed with both pyramidal (Vickers test) and spherical indenters (Brinell test). Test force is provided by a dead-weight machine, and the occurring displacement is measured by a laser-interferometric system. The geometrical dimensions of both the Vickers and Brinell indentations are measured by means of a micro-mechanical system and optical microscopy imaging techniques. Applied force and indentation depth are measured simultaneously, at a 16 Hz sampling rate, and the resultant loading-unloading indentation curve is obtained. Preliminary tests are performed on metal and alloy samples. Considerations and comments on the accuracy of the proposed method and analysis are discussed.

Corrections to the stiffness relationship in 3-sided and conical indentation problems

One key relationship in the depth-sensing indentation technique is the proportionality between the contact stiffness and the contact size, as can be proved from the Sneddon's solution of axisymmetric frictionless contact. However, Sneddon's solution is only accurate when the indenter approaches a half-space (e.g., for conical indenter, the half-apex angle approaches 90º) and the interface is frictionless. As Hay et al. (J. Mater. Res., 1999) pointed out, sharp indenters lead to a radial inward displacement on the sample surface, thus leading to extra indentation force needed to push the surface back to conform with the conical indenter. In this paper, we argue that the physical origin arises from the incorrect use of reference and deformed coordinates in the boundary conditions that define Sneddon's problem. This yields two correction factors for both load and depth solutions, which are needed for sharp pyramidal indenters and frictional contact. Approximate solutions are derived which compare favorably well with the finite element simulations. We also find that the stiffness correction factor of three-sided indenter is about 11∼15% times higher than that of conical indenter, and this multiplicative factor is only a weak function of the indenter angle but does not depend on the friction coefficient and Poisson's ratio.

Nanoindentation of Fused Quartz at Loads Near the Cracking Threshold

Nanoindentation with triangular pyramidal indenters with centerline-to-face angles of 35.3°, 45°, 55°, 65.3° and 75° at loads in the range 1–500 mN was conducted on fused quartz to explore how cracks first form and develop in brittle materials during sharp indentation contact. The cracking behavior was documented using high resolution scanning electron microscopy and serial cross sectioning by focused ion beam milling. Results show that cracking is enhanced by sharper indenters and that there is a distinct threshold load below which no cracking is observed. The threshold increases rapidly with indenter angle. A series of indentations made at increasing load with the 45° indenter shows that the first cracks to form are relatively straight radials that emanate outward from the corners of the indentation. However, underneath the indent they do not connect to the hardness impression but rather terminate at what appears to be the elastic-plastic boundary. With increasing load, the radial cracks extend outward and downward, but at an intermediate load they branch to the sides beneath the hardness impression to form lateral cracks. Cracking does not appear to initiate from pre-existing flaws, but is generated from processes associated with plastic deformation beneath the indenter. Results are discussed in terms of prevailing ideas on the origin of the cracking threshold.

Tribological and corrosion performance of DLC coating on sintered Al alloy

DLC formation on sintered Al alloys and its purity is obtained for automotive applications. In this work, initial evaluation of DLC coatings was performed using scratch, micro-hardness, fracture toughness and surface roughness tests on sintered Al. Raman spectra analysis confirms the DLC formation. The DLC coated sintered Al samples exhibits micro-hardness in the range of 2000 to 2300 kg mm−2 when measured using a Vickers diamond pyramid indenter of 0.5 N load applied on it. In some localized regions, even more hardness (>5000 kg mm−2) has been obtained. Scratch test results shows a fairly homogenous coating with strong adhesive nature having almost four times the average critical force compared to electro-statically deposited sample. Fracture toughness and surface roughness are well within the acceptable ranges of DLC coatings. The capability of laser sintering to produce thick DLC coatings with outstanding mechanical and tribological properties and excellent bonding with aluminium offers the possibility to tailor an extreme lightweight, strong and wear-resistant material. Nano indentation result indicates the information of plastic and elastic deformation for industry based applications.

Indentation size effect during measuring the hardness of materials by pyramidal indenter

Abstract The influence of the indentation force on the manifestation of the indentation size effect when measuring Vickers hardness is investigated. It is shown that when introducing the Vickers pyramid, the deformation remains constant, and the main criteria for the indentation size effect are the resistance of the material to the deformation and the specific work of plastic deformation. The influence of the stress state of the material in the deformation zone on the change in the specific work in the process of indentation is established. The indentation conditions are determined, allowing to determine the hardness of the material with the smallest deviation from the true value.

Characterisation of Mechanical Properties of Polymeric Stent using Nanoindentation

Small dimensions of structural elements of stents make the use of traditional testing techniques for their mechanical characterisation cumbersome. Nanoindentation is an alternative approach which overcomes this limitation. In this paper, mechanical properties of a polymeric tube (from which the stent was laser-cut) were characterised by using the nanoindentation technique. Specifically, spherical indentation was carried out to assess the property of the tube, in comparison with the results obtained from pyramidal indentation.

Nanoindentation on the bio-inspired high-performance nature composite by molecular dynamics method

The determination of mechanical properties at the nanoscale is of such importance today that researchers pay special attention to it. Discovering the mechanical properties of biological composite structures in the nanoscale is much interesting today. Top neck mollusk shells are among biomaterial nanocomposites that their layered structures are composed of organic and inorganic materials. Since the nanoindentation process is known as an efficient method to determine mechanical properties like elastic modulus and hardness in small scale, therefore, due to some limitations of considering all peripheral parameters, particular simulations of temperature effect in the atomic scale are considerable. The present article provides a molecular dynamics approach for modeling the nanoindentation mechanism with three types of pyramidal, cubic, and spherical indenters at different temperatures of 173, 273, 300, and 373°K. Based on load-indentation depth diagrams and Oliver–Pharr equations, research findings indicate that the temperature has weakened the power between the biological atoms; this leads to reduced mechanical properties. An increase in temperature causes a reduction in elastic modulus and hardness. There was correspondence between the results obtained from the spherical indenter and experimental data. This study can be regarded as a novel benchmark study for further research studies which tend to consider structural responses of the various bio-inspired nanocomposites.

Measuring Elastic and Plastic Properties of PVK and CBP Thin Films using Triangular Pyramid Indenter

Measuring Elastic and Plastic Properties of PVK and CBP Thin Films using Triangular Pyramid Indenter

Specific Energy of Elastoplastic Deformation Required for Crack Formation at Indentation of Hardening Coatings

Known methods for assessing the crack resistance of very hard or strengthened surface layers of materials by using a pyramid indenter are intended for the cases of the formation of radial cracks around the indent. However, at indentation of a pyramid into some types of hardening coatings, for example, made of titanium nitride, annular rather than radial cracks are formed around the indent. For this case of crack formation, we proposed to use the kinetic diagram of indentation for determination of the specific energy of elastoplastic deformation that is needed for the formation of the first visible crack. This energy can be used to compare the ability of the coating to resist crack formation of any kind.

Estimation of the elastic modulus and the work of adhesion of soft materials using the extended Borodich–Galanov (BG) method and depth sensing indentation

The depth-sensing indentation (DSI) is currently one of the main experimental techniques for studying elastic properties of materials of small volumes. Usually DSI tests are performed using sharp pyramidal indenters and the load-displacement curves obtained are used for estimations of elastic moduli of materials, while the curve analysis for these estimations is based on the assumptions of the Hertz contact theory of non-adhesive contact. The Borodich–Galanov (BG) method provides an alternative methodology for estimations of the elastic moduli along with estimations of the work of adhesion of the contacting pair in a single experiment using the experimental DSI data for spherical indenters. The method assumes fitting the experimental points of the load-displacement curves using a dimensionless expression of an appropriate theory of adhesive contact. Earlier numerical simulations showed that the BG method was robust. Here first the original BG method is modified and then its accuracy in the estimation of the reduced elastic modulus is directly tested by comparison with the results of conventional tensile tests.The method modification is twofold: (i) a two-stage fitting of the theoretical DSI dependency to the experimental data is used and (ii) a new objective functional is introduced which minimizes the squared norm of difference between the theoretical curve and the one used in preliminary data fitting. The direct experimental validation of accuracy and robustness of the BG method has two independent steps. First the material properties of polyvinyl siloxane (PVS) are determined from a DSI data by means of the modified BG method; and then the obtained results for the reduced elastic modulus are compared with the results of tensile tests on dumbbell specimens made of the same charge of PVS.Comparison of the results of the two experiments showed that the absolute minimum in relative difference between individual identified values of the reduced elastic modulus in the two experiments was 3.80%; the absolute maximum of the same quantity was 27.38%; the relative difference in averaged values of the reduced elastic modulus varied in the range 16.20 ... 17.09% depending on particular settings used during preliminary fitting. Hence, the comparison of the results shows that the experimental values of the elastic modulus obtained by the tensile tests are in good agreement with the results of the extended BG method. Our analysis shows that unaccounted factors and phenomena tend to decrease the difference in the results of the two experiments. Thus, the robustness and accuracy of the proposed extension of the BG method has been directly validated.

EVALUATION OF THE MECHANICAL PROPERTIES OF DIFFUSION LAYER IN THE PROCESS OF MICROARC STEEL VANADATION

Traditional processes of thermochemical treatment of steel have a longer duration, so there are proposed the new methods of intensification of diffusion saturation with high-energy impacts on the material surface. In the process of micro-arc alloying the steel product is immersed in a container filled with powder of coal, and is heated by passing electric current. In a powder environment, microdischarges are formed, which are concentrated around the product and create an area of gas discharge with the formation of a carbonaceous gas environment, which enables carburizing of steel. The application of coating containing diffusant allows forming coating of a carbide type due to simultaneous carbon diffusion into alloying elements. The influence of micro-arc surface alloying of steel with vanadium on mechanical properties of diffusion coatings is studied, and the primary mechanism of steel hardening at microarc alloying is revealed. Cylindrical samples of 20 steel were used; the source diffusant was a powder of ferrovanadium. Current density on the sample surface was 0.3 A/cm2, total duration of the process was 3 min. The mechanical properties of coatings were evaluated by means of indentation using pyramidal indenter, at loads of 2.5 mN, 20 mN and 100mN. The diffusion layer with thickness of 170 – 180 μm consists of a base with hardness of 8 – 9 GPa, containing mild etching inclusions of up to 5 μm with microhardness of 21 – 25 GPa. The base of the layer represents an α-solid solution of vanadium in iron, and inclusions are carbides of VC0.863 type. By atomic force microscopy it was established, that the surface relief is defined by single, relatively large carbide particles with a size of up to 3 μm, and by plural nano-sized carbide particles, which act as the strengthening phase, providing high microhardness of the coating. By method of indentation of the hardened layer cross section using different loads hardening effect of the carbide particles is proven. Estimation of possible mechanisms of hardening have shown that the greatest contribution to diffusion layer hardening is made by dispersion component significantly increasing the yield stress of α-solid solution of iron in comparison with the initial state, which is 38 times greater than the contribution of solid-solution hardening.

Depth sensing indentation of magnesium/boron nitride nanocomposites

Magnesium nanocomposites, considered as energy-saving lightweight materials of future, are a fairly new family of composite materials with enhanced specific strength and ductility compared to pure magnesium and/or magnesium alloys. In the present study, time-dependent plastic deformation of novel light-weight magnesium/boron nitride nanocomposites containing 0.5, 1.5 and 2.5 vol% of nano-boron nitride particulates is studied through a depth-sensing indentation approach against monolithic pure magnesium. The synthesis of magnesium–boron nitride nanocomposites was accomplished using powder metallurgy technique coupled with microwave sintering, followed by hot extrusion (end products are 8-mm diameter rods). The depth sensing indentation creep tests were conducted at room temperature (∼0.32Tm of magnesium) using an instrumented indentation platform via a self-similar pyramidal (Berkovich) indenter. To assess the influence of loading rate on the indentation-induced deformation behavior of the materials, a dual stage indentation creep including a constant loading rate followed by a constant load-holding scheme was used; indentation tests were performed on the specimens. The specimens were loaded at rates of 0.05, 0.5, 5, and 50 mN/s to a peak load of 50 mN then force was held constant for 400 s while load/displacement/time data were recorded continuously. The results of the depth sensing indentation tests were correlated and explained using the microstructural characteristics placing special emphasis on the volume fraction of reinforcement and the indentation loading rate. Finally, the controlling creep mechanisms of the magnesium–boron nitride nanocomposites and the base metal (pure magnesium) were discussed in the present paper. The results of this paper can be used as a baseline for high-temperature creep analysis of magnesium–boron nitride nanocomposites which is of engineering significance.

Fracture Toughness Evaluation and Plastic Behavior Law of a Single Crystal Silicon Carbide by Nanoindentation

Nanoindentation-based fracture toughness measurements of ceramic materials like silicon carbide (SiC) with pyramidal indenters are of significant interest in materials research. A majority of currently used fracture toughness models have been developed for Vickers indenters and are limited to specific crack geometries. The validity of the indentation-cracking method for the fracture toughness measurement of single crystal SiC, the elastic-plastic anisotropy and orientation dependence around the c-axis when indented in the <0001> direction is examined using nanoindentation with different pyramidal indenters. The residual impressions are analyzed using scanning electron microscopy to measure the crack lengths and the validity of existing fracture toughness measurement methods and equations is analyzed. A combination of nanoindentation with different pyramidal indenters to produce a wide range of effective strains and finite element simulation is used to extract flow properties of single crystal SiC in the <0001> direction. It is observed that there is no orientation dependence around the c-axis when SiC-6H is indented in the <0001> direction with a Berkovich indenter, i.e., it is transversely isotropic. It is also found that for a Berkovich indenter, the Jang and Pharr model, which is based on the Lawn model for cone/halfpenny cracks, gives approximately constant values at low loads (<1 N), while at higher loads (>1 N), the Laugier model gives constant fracture toughness values. Finite element analysis using equivalent cones is used along with measured hardness values to estimate the yield strength, the work hardening exponents and the stress–strain curve for single crystal SiC-6H in the <0001> direction.

Loop energy: A useful indicator of the hardness of minerals from depth-sensing indentation tests

Depth-sensing indentation tests were performed to obtain the loop energy (equivalent to the energy consumed to produce the indentation) and the residual depth of the indentation using a triangular pyramidal diamond indenter for the minerals in Mohs hardness scale except for diamond, as well as other minerals (apophyllite, forsterite, and tourmaline), at a maximum load ranging from 30 to 100 mN. A new graphic presentation is proposed that shows the hardness of minerals in log(penetration depth)−log(loop energy) space. The data for each mineral under different loads give a straight regression line with a slope of 2.6–2.9 (except for talc, which yields a slope of 2.2), while the data for different minerals under a given load yield a straight regression line with a slope of 1.1–1.2. A theoretical analysis of ideal materials, in terms of log(penetration depth)−log(loop energy) space, shows the existence of two series of parallel regression lines with slopes of 3 (data for each mineral at different loads) and 1 (data for different minerals under a given load). The results show a slight deviation between the measured and theoretical slopes, probably reflecting a progressive change in the mechanical properties of the minerals during the indentation tests.

Comparison between tensile properties and indentation properties measured with various shapes indenters of Copper-Chromium-Zirconium alloy at macroscale level

In this paper the experimental results of tensile properties and indentation properties, as a function of pyramidal and spherical indenters, of Copper-Chromium-Zirconium alloy, in the macro-scale range at room temperature, are presented and compared. Measurements are performed on three Cu-Cr-Zr samples in order to evaluate different heat treatments: two samples are aged for 2 hours in a vacuum furnace at 480 °C, 550 °C and one sample is kept as received. The experimental procedures for the measurement of indentation modulus, by using the primary hardness standard machine at INRiM, and tensile modulus, by means of engineering tensile tests at CIRA, are described.