Depth-sensing Indentation Research Articles

Improvement of viscoelastic, elastic and plastic properties of Poly(L-lactide)/Graphene Oxide-Graft-Poly(L-lactide) nanocomposites by modulation of grafted chain length

The full potential of poly(L-lactide) (PLLA) reinforced with graphene oxide, GO, nanocomposites tends to be hindered by aggregation of the nanofillers. In order to promote a good dispersion of GO in the PLLA matrix, polymer grafting techniques have been explored to anchor PLLA onto functionalized GO surface to produce GO-PLLA hybrids. For this purpose, PLLA with a terminal triple bond was synthesized by ring-opening polymerization. By controlling the concentration of monomer to initiator, PLLA samples with three different chain lengths have been prepared and later coupled to azide-functionalized GO using click chemistry. These hybrids were then mixed with commercial PLLA and cast films have been prepared. The ensuing nanocomposite films were studied using Depth Sensing Indentation and the results have shown that PLLA matrix-filler interaction can be modulated by controlling the chain length of PLLA graft. This is a critical point because the enhancement of these interactions provides a stronger matrix-filler interphase which improves the stress transfer between both phases and increases the contribution of the interphase to the stiffening of the nanocomposites. Therefore, it has been proven that judicious functionalization of GO is an effective procedure for improving the strength, the stiffness and the creep resistance of the PLLA composites.

Development of an Approach to Determining the Representative Volume Element of the Al/SiC Metal Matrix Composite Material Fabricated by Squeeze Casting

This work is aimed at solving the challenging problem of developing an approach to determining the representative volume of an Al/SiC metal matrix composite material (MMC) with 55 vol % silicon carbide synthesized by a unique squeeze casting technology. This volume is referred to as representative volume element (RVE). Initially, the minimum unit cell size was found by a metallographic method. The RVE of the MMC, where the average filler content is equal to the average filler content in the composite material, begins with a 40 × 40-μm region. The RVE is determined by the finite element method. To perform numerical RVE calculations, we solved the two-dimensional problem of tension an element in the cross section of the MMC consisting of hard SiC particles and a pure aluminum matrix. A stress–strain curve recorded during uniaxial tension is used as the response of MMC to an external action. The problems with regions of 40 × 40, 50 × 50, 70 × 70, 100 × 100, 200 × 200 and 400 × 400 μm are solved. The results of the numerical calculations demonstrate that the minimum RVE of the Al/SiC MMC with 55 vol % silicon carbide is 200 × 200 μm. In addition, depth-sensing indentation is recommended as an experimental method for estimating RVE. When a pyramidal indenter is pressed into the surface of an MMC sample, the indentation diagram reflects the influence of both the composite matrix and silicon carbide particles. The indentation force (correspondingly, surface area) used in experiments is such that the influence of the site of indentation on the indentation diagram is minimal. Instrumented indentation used to determine RVE demonstrates that it is 200 × 200 × 200 μm.

Polyamide Surface Layer Nano-Indentation and Thermal Properties Modified by Irradiation.

This study describes the effect of electron radiation on the nano-mechanical properties of surface layers of selected polyamide (PA) types. Electron radiation initiates the cross-linking of macromolecules in the polyamide structure, leading to the creation of a 3D network which fundamentally changes the properties of the tested polymers. Selected types of polyamide (PA 6, PA 66 and PA 9T) were exposed to various intensities of electron radiation (33 kGy, 66 kGy, 99 kGy, 132 kGy, 165 kGy and 198 kGy). The cross-linked polyamides’ surface properties were measured by means of the modern nano-indentation technique (Depth Sensing Indentation; DSI), which operates on the principle of the immediate detection of indenter penetration depth in dependence on the applied load. The evaluation was preformed using the Oliver–Pharr method. The effect of electron radiation on the tested polyamides manifested itself in the creation of a 3D network, which led to an increase of surface layer properties, such as indentation hardness, elastic modulus, creep and temperature resistance, by up to 93%. The increase of temperature and mechanical properties substantially broadens the field of application of these materials in technical practice, especially when higher temperature resistance is required. The positive changes to the nano-mechanical properties as well as mechanical and temperature capabilities instigated by the cross-linking process were confirmed by the gel volume test. These measurements lay the foundation for a detailed study of this topic, as well as for a more effective means of modifying chosen properties of technical polyamide products by radiation.

Creep behaviour of elastomeric nanocomposites by flat punch indentation: Influence of graphene modification and content

Creep can be understood as the resistance of a material to a continuous stress and represents an important property for engineering design. The creep behaviour of a series of nanocomposites with electromechanical applications based on a thermoplastic elastomer, poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS), and small amounts of graphene has been explored using cylindrical flat punch depth-sensing indentation. Special attention is paid to the influence of the nature of the polymer brushes (polyethylene or polystyrene) attached to the graphene surface. Common methodologies for indentation creep analysis are critically examined and brought together to provide a comprehensive picture of creep deformation. It is found that graphene significantly reduces the creep rate and this is most significant under viscoplastic deformation than in the linear viscoelastic regime. Most interesting, polyethylene branches reduce visco-elastic-plastic flow in a more effective manner and this is attributed to the selective interaction of the filler with the matrix chains.

LOCAL NANO-MECHANICAL PROPERTIES OF CROSS-LINKED POLYBUTYLENE

Cross-linking is a process in which polymer chains are associated through chemical bonds. The cross-linking level can be adjusted by the irradiation dosage and often by means of a cross-linking booster. The polymer additional cross-linking influences the surface nano and micro layers in the way comparable to metals during the thermal and chemical-thermal treatments. Polybutylene terephthalate (PBT) can be found in a group of structural polymers, which are often used in industry, especially in automotive. Applying the technology of electron radiation induces a creation of 3D network structure, which improves the local mechanical properties. These were later measured by a depth sensing indentation (DSI) test. This state of the art method is based on immediate detection of indentation depth in relation to applied force. The creation of 3D network caused an increase in nano-mechanical properties values, such as indentation hardness and indentation modulus, in comparison to the virgin material. The indentation hardness rose by 80%, while the indentation modulus elevated by 62%. The selected structural materials, e.g. PBT, were modified by the electron irradiation in a positive way and as such could be moved to a group of high performance materials.

Influence of Injection Moulding Process Parameters on High-Density Polyethylene Surface Hardness

Commodity polymers are a common part of everyday life. They consist mainly of polyolefins such as polyethylene, polypropylene. They are primarily used for ease of processing, cost and especially chemical resistance. The disadvantages of these polymers are low mechanical properties as well as temperature resistance. Any improvement in the mechanical properties can extend the application possibilities of the commodity polymers to the areas reserved for the construction polymers. This paper deals with changing two injection moulding process parameters - melt and mould temperature to high-density polyethylene (HDPE) surface hardness. HDPE hardness was measured using the method of Depth-Sensing Indentation (DSI) on three different instruments (ultranano-, nanoand micro-hardness tester). It has been found that as the melt and mould temperature increases, the hardness slightly increases.

PALS and nanoindentation as complementary methods for investigating cellulose derivatives (esters and carbamates)

New cellulose derivatives were synthesized differing in the nature of the substituent bonded with the main chain. The substituent contains a halogen atom and a methyl group at different positions. The new cellulose derivatives were studied by 1H NMR, Positron Annihilation Lifetime Spectroscopy (PALS), and Depth-Sensing Indentation (DSI) (nanoindentation). PALS is a powerful material characterization technique that can give information about the dimensions of the free volume holes (fvh) and their relative number density in polymer materials. The fvh are believed to be related to the molecular and segmental mobility of the molecular chains and, therefore, to the mechanical properties of a polymer. On the other hand, the nanoindentation characteristics such as universal, Martens, and indentation hardness, indentation modulus, creep, and a number of others give information about the resistance of a material against elastic and plastic deformations. This indirectly leads the interpretation of the nanoindentation results to the corresponding structural elements and their changes due to the indentation deformation. It was established that in the cases of cellulose derivatives the nanoindentation characteristics were influenced non-uniformly by the dimension and architecture of the substituent and the degree of substitution. Usually, the indentation modulus and all hardness characteristics are more sensitive to structural parameters than the creep and the elasticity. The obtained NMR, PALS, and DSI results were discussed in order to shed light on to what extent the free volume holes affect the mechanical properties of the studied groups of cellulose derivatives.

Low Temperature Nanoindentation: Development and Applications.

Nanoindentation technique at low temperatures have developed from initial micro-hardness driving method at a single temperature to modern depth-sensing indentation (DSI) method with variable temperatures over the last three decades. The technique and implementation of representative cooling systems adopted on the indentation apparatuses are discussed in detail here, with particular emphasis on pros and cons of combination with indentation technique. To obtain accurate nanoindentation curves and calculated results of material properties, several influence factors have been carefully considered and eliminated, including thermal drift and temperature induced influence on indenter and specimen. Finally, we further show some applications on typical materials and discuss the perspectives related to low temperature nanoindentation technique.

On the propagation of methodological uncertainties in Depth Sensing Indentation data analysis: A brief and critical review

A systematic and critical review about the propagation of uncertainties related to the Depth Sensing Indentation data analysis standard procedures and its effect on the estimation of Young´s modulus of materials is reported. Uncertainties related to experimental issues like calibration of indenter area profile and determination of effective applied force and their propagation in estimating the indenter-sample reduced elastic modulus were discussed. On the other hand, theoretical uncertainties related to the contact mechanics fundamentals used for calculating the Young´s modulus of materials by using experimentally determined reduced elastic modulus in indentation experiments were also discussed. Theoretical uncertainties associated to assume fixed values for the mechanical properties of the indenter (Poisson´s ratio and Young´s modulus) and sample (Poisson´s ratio) to estimate the sample Young´s modulus were studied. In general, all the results suggest the experimental uncertainties are the main source of error in estimating the sample Young´s modulus, however the uncertainties associated to theoretical procedures can be comparable with them in some cases.

Outcomes of ultra-fast (3 s) photo-cure in a RAFT-modified resin-composite

ObjectiveThis study evaluates critical material properties resulting from ultra-fast (3 s) photo-polymerization at high radiant emittance of a pre-production, novel bulk-fill resin-based composite (RBC) modified for reversible addition-fragmentation chain transfer (RAFT) polymerization. MethodsThe output characteristics of the associated light curing unit (LCU) were measured on a laboratory-grade spectrometer. Real-time Fourier Transform Infrared Spectroscopy (FTIR) and mechanical investigations (depth-sensing indentation with a linear and spatial distribution of the measured properties, and three-point bend tests) were performed using, as reference material, an established bulk-fill RBC of comparable chemical composition. Micro-mechanical properties were mapped to quantify material tolerance to sub-optimal curing conditions (exposure distance of 5 mm and an angulation of the LCU of 20° and 30°) vs. ideal curing conditions (exposure distance of 0 mm and no angulation), with 3 s polymerization. Weibull statistics, one- and multiple-way analysis of variance (ANOVA) and the Tukey honestly significant difference (HSD) post hoc-test (α = 0.05) were used for data comparison. ResultsThe change in cure mechanism to RAFT polymerisation gave slightly faster initial polymerisation kinetics, but DC measured 300 s post irradiation was similar, irrespective of material, curing depth or polymerisation condition. Slightly better polymerisation, in layers thicker than 4-mm, was identified in the RAFT polymerised RBC. However, slightly lower flexural modulus and hardness, up to 1.5-mm subsurface, were related to the ca. one wt.% lower inorganic filler content. SignificanceRAFT polymerisation induced comparable properties to a RBC cured via free radical polymerisation of comparable chemical composition. The RAFT polymerised RBC with high irradiance for 3 s was equivalent to 10 s of moderate irradiance. However, the clinical tolerance for 3 s irradiance should be limited to an exposure distance of 5-mm and angulation of the LCU should be avoided. If this is not possible, an additional 3 s polymerisation is recommended.

Nano-Mechanical Properties of Surface Layers of Polyethylene Modified by Irradiation.

This study’s goal was to describe the influence of a wide range of ionizing beta radiation upon the changes in surface layer mechanical properties and structural modifications of selected types of polymer. Radiation crosslinking is a process whereby the impingement of high-energy electrons adjusts test sample structures, thus enhancing the useful properties of the material, e.g., hardness, wear-resistance, and creep, in order that they may function properly during their technical use. The selected polymers tested were polyolefin polymers like polyethylene (Low-density polyethylene LDPE, High-density polyethylene HDPE). These samples underwent exposure to electron radiation of differing dosages (33, 66, 99, 132, 165, and 198 kGy). After the crosslinking process, the samples underwent testing of the nano-mechanical properties of their surface layers. This was done by means of a state-of-the-art indentation technique, i.e., depth-sensing indentation (DSI), which detects the immediate change in the indentation depth associated with the applied force. Indeed, the results indicated that the optimal radiation dosage increased the mechanical properties by up to 57%; however, the beneficial levels of radiation varied with each material. Furthermore, these modifications faced examination from the structural perspective. For this purpose, a gel test, Raman spectroscopy, and crystalline portion determination by X-ray all confirmed the assumed trends.

Copper-Graphene Composite Materials: Manufacturing Technology and Properties

The article presents the results of preliminary research into the feasibility of copper-graphene composite application in the production of shaped charge liners in HEAT ammunition from graphene-coated copper grain powders by powder metallurgy techniques. Copper powder grains were coated with graphene with a machine and process developed at the Institute of Precision Mechanics in Warsaw (Poland). The characteristics of the applied powdered materials (including particle size distribution) were determined in this work. The paper discloses the result of graphene identification by Raman spectroscopy. The presence of graphene was confirmed in the processed copper powder. The paper discusses the preparation of copper powder by grinding (refinement) and reduction for consolidation. Powder mixtures of pure copper and graphene powder were applied at different component ratios. P/M compacts and sinters (agglomerates) for the test specimens were made from the proposed mixtures by die pressing and sintering in dissociated ammonia gas. Examples of microstructures and selected test results of material properties are shown for the produced sinters. The paper shows a selection of test results for the copper-graphene composites produced by PPS (Pulse Plasma Sintering) from 100% graphene-coated copper powder. The properties of the produced composite materials were determined, including their density, porosity, and a selection of mechanical properties identified by DSI (Depth Sensing Indentation). It was found that the copper-graphene composite met the primary design criteria applied to shaped charged liners for HEAT munitions. In “traditional” powder metallurgy processes, high-density products can be produced if the composite material features a low content of graphene-coated copper powder; PPS, however, makes the production viable with 100% graphene-coated copper powder.

Depth-Sensing Indentation as a Micro- and Nanomechanical Approach to Characterisation of Mechanical Properties of Soft, Biological, and Biomimetic Materials.

Classical methods of material testing become extremely complicated or impossible at micro-/nanoscale. At the same time, depth-sensing indentation (DSI) can be applied without much change at various length scales. However, interpretation of the DSI data needs to be done carefully, as length-scale dependent effects, such as adhesion, should be taken into account. This review paper is focused on different DSI approaches and factors that can lead to erroneous results, if conventional DSI methods are used for micro-/nanomechanical testing, or testing soft materials. We also review our recent advances in the development of a method that intrinsically takes adhesion effects in DSI into account: the Borodich–Galanov (BG) method, and its extended variant (eBG). The BG/eBG methods can be considered a framework made of the experimental part (DSI by means of spherical indenters), and the data processing part (data fitting based on the mathematical model of the experiment), with such distinctive features as intrinsic model-based account of adhesion, the ability to simultaneously estimate elastic and adhesive properties of materials, and non-destructive nature.

Atmospheric Pressure Plasma Polymerized Oxazoline-Based Thin Films-Antibacterial Properties and Cytocompatibility Performance.

Polyoxazolines are a new promising class of polymers for biomedical applications. Antibiofouling polyoxazoline coatings can suppress bacterial colonization of medical devices, which can cause infections to patients. However, the creation of oxazoline-based films using conventional methods is difficult. This study presents a new way to produce plasma polymerized oxazoline-based films with antibiofouling properties and good biocompatibility. The films were created via plasma deposition from 2-methyl-2-oxazoline vapors in nitrogen atmospheric pressure dielectric barrier discharge. Diverse film properties were achieved by increasing the substrate temperature at the deposition. The physical and chemical properties of plasma polymerized polyoxazoline films were studied by SEM, EDX, FTIR, AFM, depth-sensing indentation technique, and surface energy measurement. After tuning of the deposition parameters, films with a capacity to resist bacterial biofilm formation were achieved. Deposited films also promote cell viability.

Evaluation of elastic and adhesive properties of solids by depth-sensing indentation

ABSTRACT To describe properly interactions between contacting solids at micro/nanometre scales, one needs to know both adhesive and mechanical properties of the solids. Borodich and Galanov have introduced an effective method (the BG method) for identifying both characteristics from a single experiment on depth-sensing indentation by a spherical indenter using optimal fitting of the experimental data. Unlike traditional indentation techniques involving sharp indenters, the Borodich-Galanov methodology intrinsically takes adhesion into account. It is essentially a non-destructive approach. These features extend the scope of the method to important applications beyond the capabilities of conventional indentation. The scope of the original BG method was limited to the classic JKR and DMT theories. Recently, this restriction has been overcome by introducing the extended BG (eBG) method, where a new objective functional based on the concept of orthogonal distance curve fitting has been introduced. In the present work, questions related to theoretical development of the eBG method are discussed. Using the data for elastic bulk samples, it is shown that the eBG method is at least as good as the original BG method. It is shown that the eBG can be applied to adhesive indentation of coated, multilayered, functionally graded media.

Elastic constants determination of anisotropic materials by depth-sensing indentation

Depth-sensing indentation is a useful and powerful tool for the mechanical characterization of materials at the micro and nano scale. This technique allows the determination of the Young modulus from the analysis of the load-penetration depth curve according to specific theoretical models. One of the most used models is that one proposed by Oliver and Pharr. However, when a material with anisotropic mechanical properties is tested, Oliver and Pharr’s theory is no longer suitable to describe the contact mechanics between the indenter tip and the tested material. This paper provides an overview of the theoretical models developed for the evaluation of the elastic constants of anisotropic materials through depth-sensing indentation. Specifically, the cases of generally anisotropic and orthotropic materials are described in order to cover the entire range of anisotropy. Examples on how these models can be applied for the mechanical characterization of generally anisotropic topological insulators and transversely isotropic pyrolytic carbon are also reported. This topical overview represents a useful tutorial for the evaluation of the elastic constants of anisotropic materials by depth-sensing indentation by shading light on the contact mechanics at the micro and nano scale.

Structure and mechanical properties of nanocrystalline Ni/Cu multilayer coatings produced by the electrocrystallization method

The paper presents the results of studies on the structure and mechanical properties of nanocrystalline multilayer Ni/Cu coatings. and for comparative purposes, also of nanocrystalline Ni and Cu coatings deposited by the electrocrystallization method. The structure of the produced coatings was characterized by the X-ray diffraction method and scanning electron microscopy. The tests of mechanical properties were carried out using the Depth Sensing Indentation (DSI) method. The curves of dependence of the loading force as a function of the penetration depth of the indenter of the tested coatings were determined. as well as the parameters characterizing the tested materials: indentation hardness (HIT), Martens hardness (HM), elastic modulus (EIT), elastic strain index (KH) and creep (CIT). The scratch test evaluated the adhesion of the material of coatings to the steel substrate as well as between the individual layers in multilayer coatings. Multilayer Ni/Cu coatings are characterized by, nanocrystalline structure, compact construction and uniform thickness over the entire covered surface. The materials of the tested coatings differ significantly in the course of plastic deformation and elastic deformation. The thickness of the layers in multilayer Ni/Cu coatings affects their mechanical properties.

Probing of the mechanical properties and monitoring of the drying process of azopolymer thin films for optical recording

Azopolymers prepared in the form of thin films are promising candidates as media for optical recording due to their photo-anisotropic properties. The thermo-physical and mechanical properties of the prepared films, as well as the uniformity of their thickness and smoothness of the formed layers are important characteristics for application of these materials. In the present study, thin films of two amorphous and one liquid crystalline azopolymers were characterized and, in the experiment, a commercially available amorphous azopolymer was included as a base for comparison. Atomic force microscopy was applied for surface topography determination for thin film samples of all four azopolymers and roughness parameters as the average roughness and root mean square values are estimated. These values show that amorphous azopolymers are characterized by a high smoothness and a flatter surface while the thin film from the liquid crystalline polymer shows a structure with pronounced topographic features and thus exhibits much higher surface roughness. The conducted depth-sensing indentation of non-exposed thin films prepared by spin-coating on glass substrates allowed to determine mechanical properties such as universal and indentation hardness, elastic modulus and elastic part of indentation work. The creep behavior of the tested materials was also analyzed and the elastic-plastic behavior for all azopolymers was established. The most pronounced plastic behavior was observed for the liquid crystalline one. We obtained different temporal scales of the drying process of solutions of the tested materials in a polymer casting experiment by a quantitative dynamic speckle analysis. Analysis involved statistical processing of correlated sequences of laser speckle patterns, reflected the properties of the respective materials and was supported by the reported mechanical characteristics.

Linking air-void system and mechanical properties to salt-scaling resistance of concrete containing slag cement

This paper is focused on correlating salt-scaling resistance to material performance of concrete made with various water-to-cement ratios (w/cm), ternary blends of slag cement and fly ash, and air contents. The fly ash incorporation was kept constant at 20% cement content replacement for all mixes. The material performance of the mixtures was evaluated using air-void system, sorptivity, abrasion, compressive strength, and depth-sensing indentation (DSI) tests.Test results show that effect of concrete properties, i.e., sorptivity, surface hardness, and air content on salt scaling resistance of concrete is relative to mixture proportion. For a w/cm of 0.55 the average total salt scaling after 45 F-T cycles was found about 80% higher than that of mixes made with w/cm of 0.35. The results associated with mixtures made with slag cement were found misleading, since they exhibited improved mechanical properties, while lowered salt-scaling resistance. In addition, 3–6% of air entrainment appeared beneficial for concrete mixtures made with w/cm of 0.55, when subjected to high potential salt scaling environments.

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.