Instrumented Micro-indentation Tests Research Articles

Characterization of a copper matrix composite reinforced with nano/submicron-sized boron fabricated via spark plasma sintering

The addition of various reinforcing phases can improve the mechanical properties of copper. This study investigates the enhancement of the mechanical properties of copper by adding boron, focusing on overcoming the challenges associated with the homogeneous distribution of submicron/nanoscale secondary phases in metal matrix composites. Employing a combination of mechanical alloying and spark plasma sintering, a copper-boron composite containing 3 wt% boron was prepared. Scanning electron microscopy equipped with an electron backscatter diffraction detector and energy-dispersive X-ray spectroscopy was utilized to characterize the structure of the sintered samples and mechanically alloyed powder. A two-phase structure containing nano/submicron-sized boron distributed uniformly in the copper matrix was formed in the sintered sample. Instrumented micro-indentation tests were performed to characterize the mechanical behavior of the samples. The sintered composite sample exhibits significantly higher hardness than the sintered copper. The enhanced mechanical performance of the composite is primarily attributed to grain boundary strengthening and microstructural refinement, where nano/submicron-sized boron particles prevent grain growth and refine the microstructure, enhancing hardness and strength. Additionally, dispersion strengthening from hard boron particles and the presence of a high density of twin boundaries within the copper matrix increase resistance to dislocation motion and deformation, and further improving the material's mechanical properties. On the other hand, the composite sample exhibits increased electrical resistivity due to the boron’s role as electron scattering centers. Overall, this study provides a valuable strategy for the design and optimization of advanced copper-based composites with tailored mechanical and electrical properties.

Limits of powder metallurgy to fabricate porous Ti35Nb7Zr5Ta samples for cortical bone replacements

The use of β-Titanium alloys to fabricate metal implants with Young's modulus that resembles bone tissues is presented as an alternative to commercially pure titanium or α-Titanium alloys, although it is still necessary to introduce proper implant porosity to reach the Young's modulus of cortical bones. In this work, porous samples were fabricated by loose sintering (0 MPa) and compared to samples manufactured at 1000 MPa, both sintered under the same conditions. Raw powders and sintered samples of the β-Titanium alloy, Ti35Nb7Zr5Ta, were characterized in detail in terms of both physicochemical and microstructural properties. Moreover, the tribo-mechanical behavior of sintered samples was evaluated by performing ultrasound technique, instrumented micro-indentation (P-h curves), and scratch tests. The bio-functional behavior was studied by impedance spectroscopy and contact angle measurements. The results allowed the evaluation of the limits of conventional powder metallurgy (percentage of porosity, size, and morphology of pores), as well as the influence of the porosity and chemical composition to achieve a better biomechanical and bio-functional behavior that would guarantee bone requirements. The Ti35Nb7Zr5Ta alloy showed relatively high electrical impedance values compared to commercially pure titanium, indicating an improved bio-corrosion behavior. Furthermore, wettability measurements indicated that porous disks fabricated by loose sintering exhibit higher hydrophilicity, often associated with a better antibacterial response.

CONTRIBITION OF MICRO-SILICA AND NANO-MONTMORILLONITE REINFORCEMENTS ON THE MECHANICAL PROPERTIES OF UV-CURABLE THERMOSET RESIN

UV-curable thermoset resins had been utilized in organic coating industry because of their benefits over conventional adhesives like fast curing, less energy consumption and equipment. In this article, the effects of micro and nano-scaled reinforcements on the mechanical properties of a UV-curable thermoset resin were investigated. The reinforcements are chosen to be nano-scaled Montmorillonite (MMT) and micro-scaled Silica (SiO2). The reason for this choice is that the aforementioned particles are non-toxic, low-cost and in the case of MMT; abundant in nature. According to our knowledge, there is no study on the synergistic effects of those two additives in thermoset resins. 
 The instrumented microindentation test results reveal that maximum improvement on hardness (288%) was achieved by single addition of MMT thanks to the well-distributed silicate layers. Conversely, SiO2 addition diminished both strength (-51%) and modulus (-68%) drastically which is attributed to the possible poor dispersion and weak surface attraction. On the other hand, when those additives were utilized together, the property improvements namely; hardness and modulus are observed to be in between of single addition of either additive. It is suggested that SiO2 contribution does not disturb intercalated/exfoliated-MMT structure and similarly by simultaneous MMT reinforcement, quality of SiO2 dispersion is not affected. It is concluded that one benefit of these SiO2-MMT combinations over single MMT reinforcement could be related to plasticity since they result in less plasticity reduction of -22%-27% compared to MMT (-43%) with the further benefit of higher hardness improvement (+66%) than bare SiO2 addition (-51%).

Influence of Femtosecond Laser Modification on Biomechanical and Biofunctional Behavior of Porous Titanium Substrates.

Bone resorption and inadequate osseointegration are considered the main problems of titanium implants. In this investigation, the texture and surface roughness of porous titanium samples obtained by the space holder technique were modified with a femtosecond Yb-doped fiber laser. Different percentages of porosity (30, 40, 50, and 60 vol.%) and particle range size (100–200 and 355–500 μm) were compared with fully-dense samples obtained by conventional powder metallurgy. After femtosecond laser treatment the formation of a rough surface with micro-columns and micro-holes occurred for all the studied substrates. The surface was covered by ripples over the micro-metric structures. This work evaluates both the influence of the macro-pores inherent to the spacer particles, as well as the micro-columns and the texture generated with the laser, on the wettability of the surface, the cell behavior (adhesion and proliferation of osteoblasts), micro-hardness (instrumented micro-indentation test, P–h curves) and scratch resistance. The titanium sample with 30 vol.% and a pore range size of 100–200 μm was the best candidate for the replacement of small damaged cortical bone tissues, based on its better biomechanical (stiffness and yield strength) and biofunctional balance (bone in-growth and in vitro osseointegration).

Bioactive Bilayer Glass Coating on Porous Titanium Substrates with Enhanced Biofunctional and Tribomechanical Behavior

The use of porous titanium samples fabricated by space-holder powder metallurgy with bioactive coatings has already been reported to prevent resorption of the bone surrounding the implant and improve osseointegration, respectively. However, the presence of pores as well as the poor adherence and the brittle behavior inherent to glassy coatings affect the service behavior of implants fabricated from these samples. Therefore, they need to be optimized. In this work, 50 vol.% of porosity titanium substrates were manufactured with different pore range size (100–200 and 355–500 µm) spacer particles and coated with a bilayer of bioactive glasses (45S5/1393). The effect of the pores on the tribomechanical properties and infiltration of the bioactive glass 1393 along with the bioactivity of the bioactive glass 45S5 were evaluated by instrumented micro-indentation and scratch tests and the formation of hydroxyapatite in simulated body fluid. The results obtained were very promising as potential implants for the replacement of small tumors in cortical bone tissues, mainly due to the smaller pores that present an improved biomechanical and biofunctional balance.

Deposition of Cu-based composite coatings on mild steel substrate via electrodeposition method

The mild steel (MS) is a commonly used material in various structural industries due to its low cost and good strength (relatively lower than other processed steel). But, poor corrosion resistance, relatively low hardness, and wear resistance lead to a limit of its application. To enhance these properties, surface engineering with coating are preferred. The present article investigates the synthesis, microstructural and mechanical properties of copper-based electrodeposited coatings on the mild steel. The direct current (DC) power source is used to deposit the pure copper and copper-based composite reinforced with carbonaceous materials, such as graphene (Gr), graphite (Gt), and carbon nanotubes (CNTs). The coatings were deposited using two types of bath containing reinforcement, non-cyanide alkaline glycerol complex and acidic (sulphuric acid) bath. Various deposition parameters, such as bath temperature, time, current density, pH, and stirring of electrolyte were used to get an optimised uniform coating. The wetting agent was added with carbonaceous particles to make it mix and distribute uniform in the electrolyte. The cube-shaped morphology of copper crystals was witnessed in the electrodeposited coatings along with the plate-like structures of graphene and graphite. The morphology and chemical composition of the deposited coatings were analysed with the help of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) respectively. The microhardness and elastic modulus of the electrodeposited coatings were measured using instrumented microindentation test. Among the composite coatings, copper/graphite (Cu/Gt-MS) coating showed the superior hardness of ∼ 5 GPa, due to the uniform distribution of reinforcement.

Localized heat treatment to improve the formability of steel pipes for hydraulic applications: process design and mechanical characterization

In the present paper, authors have demonstrated how a localized induction heat treatment can be advantageously applied, controlled, and mechanically characterized on a specific part—i.e., on steel hose fittings for hydraulic applications. More specifically, the study shows how this specific type of heat treatment facilitates significant localization effects on mechanical properties, and how such a treatment could act as a powerful tool for material optimization in diverse applications. The instrumented micro-indentation test was adopted as the investigation method for mechanical characterization and, due to the reduced amount of material required for the test, has the double advantage of retrieving potential spatial gradients of the mechanical properties without causing permanent damage to the analyzed parts. A measurement of both Vickers hardness and plastic work are required in order to make the indentation necessary to quantify the strength and ductility capability of the parts’ materials. In addition, a customized tensile test, based on a strains measurement obtained through an optical full-field method—i.e., digital image correlation (DIC)—was developed with the aim of identifying and quantifying the correlation between the material properties attainable through a conventional tensile test and those measured by the instrumented micro-indentation test. Finally, it was demonstrated that the proposed customized tensile test, due to the localized heat treatment, is capable of retrieving potential spatial gradients of the material properties.

Tribo-mechanical and cellular behavior of superficially modified porous titanium samples using femtosecond laser

In this work, the surface of porous titanium samples obtained by the space-holder technique was treated with a femtosecond laser to improve their osseointegration. Instrumented micro-indentation and scratch test were implemented to evaluate the tribo-mechanical behavior of the surface of the modified samples. A detailed study of micro-hardness, stiffness, scratch resistance and elastic recovery was performed. Also, in vitro analysis was carried out to evaluate the cellular behavior. Modified samples showed less ALP activity, which could indicate a greater differentiation of the cells. The cell culture was similar in all cases although more differentiated morphology, good cell adherence and biological response were observed on treated samples. Finally, the discs with a pore size between 100–200 μm present being potential candidates for the replacement of small portions of damaged cortical bone tissues.

Characterization of Properties of Poly(vinylidene Fluoride) Using Instrumented Microindentation Test

AbstractThe microhardness evaluation of materials allows a quick prediction of their mechanical properties and the identification of morphological changes, making it an excellent tool for the characterization of parts with microstructural variations over time or that have geometric variations. In this work, instrumented microindentation test is performed on poly(vinylidene fluoride) (PVDF) specimens with different degrees of crystallinity and crystalline structures. The microstructural differences are generated by changing the cooling temperature and the annealing duration. α‐phase and β‐phase are detected by FTIR analysis in all samples, which presented the same spectra. The mathematical relation between the microhardness and the crystallinity degree of PVDF is found, and the theoretical microhardness values of the crystalline and amorphous phases are determined. Microhardness is also related to the melting temperature of the polymer, which varies as a function of crystals perfection. This relationship is expressed through a linear equation.

Splat-boundary reinforcement induced by graphene in plasma-sprayed hydroxyapatite composite coating

Plasma-sprayed HA coatings on metallic implants are widely used for clinical applications. However, typical lamellar structure along with plasma-sprayed coatings usually leads to weak inter-splat adhesion and impair their mechanical properties. In this research, graphene nanosheet (GNS) reinforced HA coatings were fabricated using plasma spray; these GNSs retained their original structure and distributed homogeneously in the as-sprayed coatings. On the basis of instrumented microindentation tests with and without multiple partial unloading, as compared with the monolithic HA coating, the inter-splat friction force increased by ~ 8.7% for the 1.0 wt% GNS/HA coating, and it slightly decreased to ~ 6.5% for the 2.0 wt% GNS/ HA coating due to GNS agglomeration. Meanwhile, the added GNSs contributed greatly to the indentation yield strength of the HA coatings. These results illustrated that these embedded GNSs at splat boundaries are potential in splat-boundary strengthening and resisting splat sliding.

Microstructure and room-temperature mechanical properties of crystalline and amorphous FeAl based melt spun ribbons

The microstructure and mechanical properties of crystalline Fe81%-Al19% and amorphous Fe72%-Al14%-B14% ribbons obtained by the melt spinning technique are studied. A different grain size in each surface of the crystalline ribbon has been found. This is caused by the temperature gradient originated by the manufacturing process. The micro-hardness and the Young's modulus were evaluated by instrumented micro-indentation (P-h curves) and conventional tensile test. The results show different Vicker's hardness on each surface for each of the ribbons, being these values greater in the amorphous ribbon than in the crystalline one. The values of the Young's modulus are 133 ± 11 GPa and 210 ± 20 GPa for the crystalline and amorphous ribbon respectively which are in good agreement with the theoretical values obtained by the mix rule. The surface fracture shows a brittle-like aspect with presence of cleavage in the Fe81%-Al19% ribbon and small localized ductile behavior before reaching the fracture. In the other hand, in the Fe72%-Al14%-B14% ribbon, the surface fracture characteristics reveal a fragile fracture and the presence of chevron marks. There is a concordance between the mechanical behavior and the micro-mechanism responsible for the fracture.

Electrophoretic deposition of chitosan/bioactive glass/silica coatings on stainless steel and WE43 Mg alloy substrates

Two electrophoretic coatings consisting of chitosan/bioactive glass/silica on magnesium alloy (WE43) and on stainless steel (316L) substrates are described. The partial replacement of bioactive glass with silica particles gives a topography favorable for the formation of hydroxyapatite, when the coatings are in contact with simulated body fluid (SBF). Both coatings are compared in terms of composition (thermogravimetric analysis), corrosion resistance (electrochemical impedance spectroscopy and electrochemical polarization curves), scratch resistance (instrumented micro-indentation and scratch test) and bioactivity (immersion in simulated body fluid). Although the addition of silica decreases the adhesion of the coating to the steel substrate, it has no influence on the coating adhesion on the Mg alloy. The results show the advantageous effect of adding silica particles (increasing coating bioactivity and hardness) while keeping the organic (chitosan) concentration constant.

Effect of LED-LCU light irradiance distribution on mechanical properties of resin based materials

The objective of this study is to analyze the light power distribution along the tip end of the light guide of three LED-LCUs (Light Curing Units) and to evaluate its effect on the mechanical properties of a polymer based dental composite. Firstly, the light power distribution over the whole area of LED-LCU light guide surface was analyzed by three methods: visual projection observation, spectral measurement and optical spectral analysis (OSA). The light power distribution and the total irradiance were different for the three LEDs used, but the wavelength was within the camphorquinone absorption spectrum. The use of a blank sheet was quite on hand to make a qualitative analysis of a beam, and it is costless. Secondly, specimens of a hybrid composite with approximately 8mm diameter and 2mm thickness were produced and polymerized by 20s exposition time to each LED-LCU. Thirdly, the elastic modulus (E) and hardness (HV) were measured throughout the irradiated area by instrumented micro-indentation test (IIT), allowing to correlate localized power and mechanical properties. Both E and HV showed to be very sensitive to local power and wavelength dependent, but they followed the beam power profile. It was also shown that the mechanical properties could be directly correlated to the curing process. Very steep differences in mechanical properties over very short distances may impair the material performance, since residual stresses can easily be built over it.

Spherical indentation with multiple partial unloading for assessing the mechanical properties of ZrB2–SiC composites

In this research, spherical instrumented micro-indentation tests with multiple partial unloading were performed to determine the mechanical properties of the ZrB2–SiC composites reinforced with and without the mixture of boron nitride nanotubes (BNNTs) and boron nitride nanoplatelets (BNNPs), the indentation stress–strain curves of these ZrB2–SiC composites were also reconstructed. Results showed that strain-hardening exponents of the sintered ZrB2–SiC composites are in the range of ~0.50 to ~0.52, and the added mixture of BNNTs–BNNPs has little effect on the indentation yield strength of the composites. Synergetic influences of the Orowan-type strengthening effect and the capability of large plastic deformation associated with the added BNNPs might be ascribed to the neglectable changes in the strain-hardening exponent and the indentation yield strength of the (BNNT–BNNP)/(ZrB2–SiC) composites. This research is expected to facilitate a better understanding of the microscopic plastic deformation behavior of ZrB2 and its composites.

Phenomenological description and temperature dependence of serrated flow in Ni–10Pd during high temperature instrumented microindentation

Instrumented microindentation tests were carried out on a Ni–10Pd solid solution from 30°C to 450°C with a loading rate of 125mN/s. When this sample was tested below 350°C, the loading segment of the load–depth curves exhibited a smooth transition. In contrast, for tests conducted above 350°C, the loading segment was initially smooth and then became serrated after reaching a critical load. Increase in testing temperature resulted in an earlier occurrence of serrated flow with a lower load threshold. With the intersections and transitions between the flat “A” segments and steep “B” segments individually defined, the depth and load increment associated with each segment were quantified and analyzed to phenomenologically describe the serrated flow behavior. By accounting for the dislocation reconfigurations and solute–dislocation interactions, a modified cavity expansion model was developed and used to elucidate the underlying physics.

Ageing mechanism and mechanical degradation behaviour of polychloroprene rubber in a marine environment: Comparison of accelerated ageing and long term exposure

Polymers are widely used in marine environments due to their excellent properties and good weathering resistance. Despite this extensive use, their long term behaviour in such an aggressive environment is still not well known. To assess the polymer durability within reasonable durations, it is essential to perform accelerated ageing tests to accelerate the degradation kinetics but without any modification of the degradation process. This study therefore proposes and validates accelerated ageing tests to study marine ageing of a silica-filled chloroprene rubber (CR) used for offshore applications. Several accelerated ageing protocols are investigated for temperatures ranging from 20 to 80 °C in renewed natural seawater. The ageing consequences are characterized using physical measurements (FTIR, solid state NMR) and mechanical testing based on monotonic tension tests. Instrumented micro-indentation tests are also employed, in order to describe accurately the ageing gradients through sample thickness. The measurements obtained on the samples cut from accelerated specimens are compared to those obtained from the topcoat of an offshore flowline aged under service conditions for 23 years. For both kinds of specimens, polychloroprene develops rapid material changes most clearly represented by a considerable increase in stiffness, which allows the accelerated ageing protocols to be validated.

Influence of porosity on the mechanical properties of microporous β-TCP bioceramics by usual and instrumented Vickers microindentation

Beta-tricalcium phosphate [Ca 3(PO 4) 2, β-TCP] is a bioresorbable material showing an excellent biocompatibility. However, sintering of β-TCP is difficult and the material presents poor mechanical strength and a low resistance to crack-growth propagation. In this study, influence of the porosity on the hardness and the elastic modulus is studied by means of usual and instrumented microindentation tests. Nevertheless, indentation diagonals measurement by optical observations is not accurate due to the crack formation around the residual indent. That is why instrumented indentation test which allows deducing the hardness and the bulk modulus from the load–depth curve analysis is used as an alternative method. The corresponding hardness number can be calculated by using the maximum indentation depth (Martens Hardness) or the contact depth determined by Oliver and Pharr's method (Contact Hardness). But in order to give representative values when comparing classical and instrumented hardness measurements, Martens hardness is preferred because its value can be directly related to the value of the Vickers hardness number by simple geometrical considerations. In this work, bioceramics were produced by conventional sintering of β-TCP powders synthesized by aqueous precipitation. Different process conditions were chosen to obtain microporous ceramics with a porosity rate between 0 and 14% in volume. As main results, the elastic modulus is found decreasing between 166 GPa and 108 GPa and the hardness number from 4.4 GPa to 2.2 GPa when increasing the porosity rate. A model connecting mechanical properties to porosity rate and grain arrangement is validated for the elastic modulus whereas deviation is observed for the hardness number. However, we propose an original approach where the relative variation of the two mechanical properties can be expressed with a unique relation as a function of the porosity volume fraction.

Influence of imbibed moisture and organoclay on tensile and indentation behavior of polyamide 66/hectorite nanocomposites

Design with polymeric materials is difficult because moisture absorption during service affects their mechanical properties. Mechanical behavior of polyamide 66/hectorite nanocomposites (PA66CN) with different amounts of imbibed moisture and organoclay was studied using tensile and instrumented micro-indentation tests. PA66CN specimens with imbibed moisture content were obtained by equilibrating in laboratory air and distilled water. The amount of moisture absorbed by the PA66CN samples is decreased with an increase in the amount of organoclay. The imbibed moisture greatly improved the ductility of PA66CN, which was reduced with increase in the amount of organoclay, however with a simultaneous reduction in the tensile modulus and strength. Fractography analysis of failed dry and equilibrated PA66CN specimens showed a clear transition from glassy to rubbery behavior due to absorption of moisture.

Nanoindentation Analysis of Plasticity Evolution during Spherical Microindentation of Bulk Metallic Glass

AbstractUnlike most of crystalline metals, metallic glasses are known to exhibit a fully-plastic behavior or work softening during mechanical deformation. To analyze the characteristics of the deformed region, here a series of instrumented micro- and nano-indentation experiments were performed on a Zr-based bulk metallic glass (BMG) with geometrically self-similar sharp indenter as well as spherical indenters. First, we performed instrumented micro-indentation tests with a spherical indenter on the bonded interfaces of the BMGs. Although adhesive (used for bonding the interfaces) might significantly affect the deformation mode by reducing the constraint, the evolution of subsurface plasticity during spherical indentation was clearly observed. Subsequently, the subsurface plasticity underneath the hardness impressions was systematically examined through nanoindentation. The results are discussed in terms of major change in mechanical responses of BMGs before and after indentation-induced deformation.

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part IThe deformation of three aluminum alloys at 473 K to 833 K

Constant-load indentation tests were performed on wrought-2024, P/M-2024, and wrought-1100 aluminum alloys to assess the capability of the microindentation testing technique for measuring the high-temperature deformation rate controlling parameters of these alloys. The three alloys all display threshold indentation stress σth below which the indentation strain rate eind approaches zero. The nominal inter-obstacle spacing, l*, calculated from σth, increases with temperature in a way that is consistent with the known temperature dependence of the inter-particle spacing and dislocation cell size. The measured activation energy ΔGo of ɛind increases with temperature but remains within the range that is typical of deformation that occurs by dislocation glide limited by weak particles or dislocation/dislocation interactions. The three alloys tested show different trends of ΔGo versus l* and the trends are consistent with the known temperature dependence of the obstacles to dislocation glide.