Nanoindentation Results Research Articles

Round Robin into Best Practices for the Determination of Indentation Size Effects.

The paper presents a statistical study of nanoindentation results obtained in seven European laboratories that have joined a round robin exercise to assess methods for the evaluation of indentation size effects. The study focuses on the characterization of ferritic/martensitic steels T91 and Eurofer97, envisaged as structural materials for nuclear fission and fusion applications, respectively. Depth-controlled single cycle measurements at various final indentation depths, force-controlled single cycle and force-controlled progressive multi-cycle measurements using Berkovich indenters at room temperature have been combined to calculate the indentation hardness and the elastic modulus as a function of depth applying the Oliver and Pharr method. Intra- and inter-laboratory variabilities have been evaluated. Elastic modulus corrections have been applied to the hardness data to compensate for materials related systematic errors, like pile-up behaviour, which is not accounted for by the Oliver and Pharr theory, and other sources of instrumental or methodological bias. The correction modifies the statistical hardness profiles and allows determining more reliable indentation size effects.

Preparation of Poly(L-lactic acid)/Hydroxyapatite composite scaffolds by fused deposit modeling 3D printing

Abstract In this work, method of preparation FDM 3D printed PLLA/HAp composite scaffolds of three compositions (12.5, 25, and 50 wt.% of HAp) was proposed. Influence of hydroxyapatite weight fraction on morphological and mechanical properties of PLLA/HAp composite scaffolds was studied. Morphological and mechanical properties of 3D-pinted scaffolds were investigated by scanning electron microscopy and nanoindentation. Results of scanning electron microscopy analysis showed that proposed method of composite mixing allows obtaining highly filled composites with homogeneous distribution of HAp powder in the PLLA-matrix. Results of nanoindentation showed growth of Young’s modulus with an increasing of HAp weight fraction. The maximum value of 8111 ± 714 MPa Young’s modulus was reached for the composite with 50 wt.% of HAp

Micro-scale simulation of nanoindentation characteristics in dual-phase steel

In the present study, nanoindentation technique was employed to derive the flow behavior of ferrite and martensite phases in order to accurately predict the overall tensile stress-strain behavior of dual-phase steel by finite element simulation. A dislocation based formulation was used to estimate the flow characteristics of constituting phases while both the sizes of model and mesh were finalized. Subsequently, finite element analysis was performed to determine the indentation load-depth curves of individual phases. The simulated and experimental load-depth profiles were compared and the values of material constant for ferrite as well as martensite phases were identified. Finally, the predicted flow behavior of constituting phases was incorporated in the real microstructure based RVE model to estimate the overall tensile behaviour. Analyses of nanoindentation results revealed that fine-tunning of the material parameter associated with the dislocation based theory considerably improved the predictability of tensile stress-strain behaviour by finite element simulation for the investigated dual-phase steel.

Abnormal softening of Ti-metallic glasses during nanosecond laser shock peening

Nanosecond laser shock peening (NLSP) has been proved to be an effective way to enhance the mechanical properties of metallic components through modifying the surface microstructures. However, the micromechanism of structure and property fluctuations induced by shock wave propagation is still limited due to the intrinsic defects in crystalline materials. Here, NLSP treatment has been performed into the Ti-metallic glasses. Owing to the absence of dislocations, grain boundary and phase segregation, the structural signals caused by the shock wave could come into sight. According to grid nanoindentation results, the shock-affected region can be divided into three sections: (I) the rapid softening region, (II) the recovery region and (III) the matrix region. The fronted first pop-in event and the reduced hardness found in the subsurface region provide clear evidence of the release wave-induced structural rearrangement. The abnormal change of the average volume of the shear transformation zone (STZ) could be explained by a two-step transformation mechanism, due to the fluctuations of the “weak spot” induced by the release stress wave. By means of these results, this work provides an opportunity to shed light on the common laser-solid interaction.

Effective reinforcements for thermoplastics based on carbon nanotubes of oil fly ash

Carbon nanotubes (CNTs) are widely investigated for preparing polymer nanocomposites, owing to their unique mechanical properties. However, dispersing CNTs uniformly in a polymer matrix and controlling their entanglement/agglomeration are still big technical challenges to be overcome. The costs of their raw materials and production are also still high. In this work, we propose the use of CNTs grown on oil fly ash to solve these issues. The CNTs of oil fly ash were evaluated as reinforcing materials for some common thermoplastics. High-density polyethylene (HDPE) was mainly reinforced with various weight fractions of CNTs. Xylene was used as a solvent to dissolve HDPE and to uniformly disperse the CNTs. Significantly enhanced mechanical properties of HDPE reinforced at a low weight fraction of these CNTs (1–2 wt.%), mainly the tensile strength, Young’s modulus, stiffness, and hardness, were observed. The tensile strength and Young’s modulus were enhanced by ~20 and 38%, respectively. Moreover, the nanoindentation results were found to be in support to these findings. Polycarbonate, polypropylene, and polystyrene were also preliminarily evaluated after reinforcement with 1 wt.% CNTs. The tensile strength and Young’s Modulus were increased after reinforcement with CNTs. These results demonstrate that the CNTs of the solid waste, oil fly ash, might serve as an appropriate reinforcing material for different thermoplastics polymers.

To study the effects of nano-additives and nano-indentation variables on viscoplastic behaviour of a polymeric orthopaedic bone cement

PMMA-based orthopaedic cement is an essential medical grade material in the modern healthcare system which has been widely used for the fixation of prostheses in total joint replacement surgeries. A PMMA bone cement is an intrinsically viscoplastic biomaterial, and a review of its constitutive material models seems to be of great importance to design, functionalize and improve the properties of these biomaterials. In this article, using the simultaneous analyses of experimental nano-indentation results and finite element simulation of a well-known two-layer viscoplasticity model, the effect of adding three types of nano-additives including hydroxyapatite (HA), Alumina (Al2O3), and single-walled carbon nanotubes (SWCNTs) on the viscoplastic behaviour of the cement was examined. Additionally, the effect of the experimental conditions such as loading rate, holding time and unloading rate on the viscoplastic behaviour of the cement was investigated. Making an analogy between the numerical and experimental results, the optimal constitutive material model for each case study was obtained. Moreover, by comparing the results with the condition where no nano-additives were added into the cement matrix, the effectiveness of the nano-supplements on the constitutive model was evaluated.

Nanomechanical characterization of RB-SiC ceramics based on nanoindentation and modelling of the ground surface roughness

Reaction bonded silicon carbide (RB-SiC) ceramics are the primary structure and mold materials for the optical industry and mostly are machined by means of ultra-precision grinding to achieve a satisfactory surface quality. However, it is not easy to attain the theoretical prediction of surface quality, particularly surface roughness, because of different mechanical characterization of Si/SiC phases inside the RB-SiC ceramics. In this work, the nanoindentation tests were performed to investigate the nanomechanical characterization of individual phase inside the RB-SiC ceramics. On the basis of the nanoindentation results of RB-SiC, a theoretical model was established to predict surface roughness in the ultra-precision grinding process, which considered the different removal mechanisms of Si matrix and SiC particles. The comparison of the prediction results of existing and novel models and single-factor experimental results shows that the novel model was well consistent with the experiment.

Influence of metal nanocomposite morphology on Helium implantation response

This work reports on Helium (He) accumulation in novel metal nanocomposites fabricated by phase separation in three-dimensions. These nanocomposites are dense with semi-coherent interfaces that ultimately influence He implantation response. The effect of film morphology on three aspects of He response is investigated: size and distribution of He precipitates, He retention, and He-induced changes in mechanical properties. Transmission electron microscopy analysis showed that He bubbles agglomerated along the vertical phase boundaries in morphologies with lateral and random domains of Cu and Mo. Nuclear Reaction Analysis (NRA) and nanoindentation results show He retention and hardness is morphology dependent.

Beryllium – A Challenge for Preparation and Mechanical Characterization

Abstract Beryllium has an extraordinary combination of material properties such as low density, high melting point, high specific heat capacity, high Young's modulus, high hardness and low atomic number. The conventional investigation of the mechanical properties of Be and Be alloys is only possible under difficult conditions due to the material's toxicity and the resulting restrictions on sample manufacturing. These limitations are avoided, at least partly, when using a depth-sensing hardness test, also called nanoindentation, where the resulting contamination with Be dusts is limited to a controllable extent. For this work, technically pure Be from X-ray exit windows of high-performance X-ray tubes was chosen and its mechanical properties were characterized by means of nanoindentation. This contribution will focus in detail on the preparation of the material as well as the following microstructural characterization by means of light microscopy and scanning electron microscopy. The mechanical results of local nanoindentation will be correlated with the microstructure and compared with known values found in the literature.

Cavitation erosion mechanisms in Co-based coatings exposed to seawater.

The cavitation erosion (CE) of most materials in seawater is more serious than in fresh water due to the onset of corrosion; however, in a previous study we reported results that contradict this widely accepted trend. In this research our objective is to provide fundamental insight into the mechanisms that may be responsible for these earlier results. To accomplish this objective, two types of Co-based coatings, prepared by high velocity oxygen fuel (HVOF) spraying system, were used to further investigate the underlying corrosion-mitigating CE mechanism in seawater. Accordingly, the influence of spraying parameters on microstructure, composition and mechanical properties of the coatings was analyzed on the basis of SEM, XRD, Raman spectroscopy, Vicker's hardness and nano-indentation results. Electrochemical corrosion tests were used to evaluate the corrosion behavior of the Co-based coatings. Their CE performances in seawater and deionized water were comparatively studied by a vibratory apparatus. Results demonstrated that a higher flame temperature facilitated the oxides formation with associated improvements in compactness, hardness and toughness of the coatings. The presence of alumina in combination with the oxides formed in-situ facilitated the formation of an oxidation film on surfaces, and effectively enhanced the charge transfer resistance of the coating, thereby significantly improving the corrosion resistance in seawater. Metallic Co was not only more easily oxidized but also more readily corroded than the alloyed Co. Compactness was identified as an important factor affecting CE resistance of coatings in deionized water, because defects facilitate the nucleation and eventual collapse of bubbles. Moreover, bubble collapse produced a transient high temperature spike in excess of 600 °C that also caused Co and Cr elements to oxidize. Because the CE tests were carried out in seawater, additional Co3O4 and Cr2O3 were generated owing to corrosion that more effectively increased the surface compactness and mechanical properties of the coatings. This behavior was particular notable for coatings with metallic Co and Cr, which should be why seawater corrosion could weaken the CE of Co-based coatings.

A data analysis procedure for phase identification in nanoindentation results of cementitious materials

Measuring accurately phase properties is essential for a realistic mesoscale modeling of materials, and nanoindentation is a popular technique regarding mechanical properties. Given the statistical nature of the grid indentation method, where large arrays of indents are performed blindly, the identification of phases from the distributions of measured properties is an essential step. Many biases can be introduced at that stage when the phases do not have very distinct properties as is often the case for cementitious materials, since many indentation tests may also be in effectively heterogeneous areas. It is proposed in the present work to analyze statistical indentation results on cementitious materials with a hierarchical clustering algorithm making use of enriched information, including the spatial coordinates of the indent. It is shown that it allows to reduce potential biases of the method by eliminating tests in potentially heterogeneous areas and performing model independent identification of the different phases.

Characterization of microscopic mechanical performance for welded joint of 9% Ni steel using nanoindentation

Nanoindentation test, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were introduced to investigate the mechanical properties and microstructure of microscopic regions of the nickel-based welded joint. Five indentation series were performed across the weld zone including base metal (BM), coarse grained heat affected zone (CGHAZ), partially melted zone (PMZ), weld metal near the partially melted zone and weld metal center (WM). The mechanical properties (E, H, and n) were then estimated from the load on sample-displacement into surface curves (P-h) data of nanoindentation and SEM analysis results. The results showed that the average nano-hardness value (H) was obviously increased in the CGHAZ. WM has the highest values of elastic modulus (E). BM has the highest plastic rheology stress exponent (n). At the same time, the relationship between the plastic rheology depth and the plastic rheology was also discussed.

Investigation of the electroplastic effect using nanoindentation

A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. In this work, a novel setup to study the electroplastic effect during nanoindentation on individual phases and well-defined interfaces was developed. Using a eutectic Al-Al2Cu alloy as a model material, electroplastic nanoindentation results were directly compared with macroscopic electroplastic compression tests. The results of the micro- and macroscopic investigations reveal current induced displacement shifts and stress drops, respectively, with the first displacement shift/stress drop being higher than the subsequent ones. A higher current intensity, higher loading rate and larger pulsing interval all cause increased displacement shifts. This observation, in conjunction with the fact that the first displacement shift is highest, strongly indicates that de-pinning of dislocations from obstacles dominates the mechanical response, rather than solely thermal effects.

The investigation on the fabrication and microstructure of NbTi-(NbTi)C Fe-based composite

NbTi-(NbTi)C Fe-based composites are fabricated by in-situ diffusion reaction. The locally reinforced (NbTi)C cermet layer formed by carrying out the reaction of C + β-NbTi → (NbTi)C, which resulted in the formation of the core-annular shell structure with a ductile core (NbTi) and high hardness annular shell (cermet layer). The (NbTi)C cermet layer is composed of two zones, where ZoneⅠ, near the GCI, is composed of micron and sub-micron-sized particles; and ZoneⅡ, near the remnant β-NbTi, is composed of sub-micron-sized particles. The detailed microstructural, elemental and morphological analysis has been carried out to unveil the phase distribution and microstructural changes in the (NbTi)C cermet layer. The results reveal that the cermet layer consists of (NbTi)C particles with different Ti/(Ti + Nb) ratios, ranging from 0.6308 to 0.8330, which have different morphology: the fusiform particles and near spherical or cubic particles. Moreover, the nanoindentation results of (NbTi)C layer indicate that the maximum hardness and elastic modulus is 31.43 GPa and 473.11 GPa, respectively. It is worth mention that the remnant β-NbTi and α-Fe phases, surrounding the (NbTi)C cermet layer, render high plasticity. Therefore, the combination of β-NbTi, α-Fe and (NbTi)C phases can effectively improve the hardness and plasticity of as-prepared composites.

A Combined Experimental and Numerical Study of the Effect of Surface Roughness on Nanoindentation

Gold and copper thin films are widely used in microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) devices. Nanoindentation has been developed in mechanical characterization of thin films in recent years. Several researchers have examined the effect of surface roughness on nanoindentation results. It is proved that the surface roughness has great importance in nanoindentation of thin films. In this paper, the surface topography of thin films is simulated using the extracted data from the atomic force microscopy (AFM) images. Nanoindentation on a rough surface is simulated using a three-dimensional finite-element model. The results are compared with the results of finite-element analysis on a smooth surface and the experimental results. The results revealed that the surface roughness plays a key role in nanoindentation of thin films, especially at low indentation depths. There was good compatibility between the results of finite-element simulation on the rough surface and those of experiments. It is observed that on rough films, at low indentation depths, the geometry of the location where the nanoindentation is performed is of major importance.

Microstructural, Surface Topology and Nanomechanical Characterization of Electrodeposited Ni-P/SiC Nanocomposite Coatings

In the present study, nickel phosphorous alloys (Ni-P) and Ni-P/ silicon carbide (SiC) nanocomposite coatings were deposited by electrodeposition on steel substrates in order for their microstructural properties to be assessed while using SEM, XRD, and three-dimensional (3D) profilometry as well as nanoindentation. The amorphisation of the as-plated coatings was observed in all cases, whereas subsequent heat treatment induced crystallization and Ni3P intermetallic phase precipitation. Examination of the surface topology revealed that the surface roughness follows the deposition characteristics and heat treatment induced microstructural changes. Additionally, substantial improvements in mechanical properties, including hardness, yield stress, and elasticity modulus, were obtained for the Ni-P, Ni-P/SiC nanocomposites when heat treated as seen from the nanoindentation results. A Finite Element Analysis (FEA) was developed to simulate the nanoindentation tests that enable the precise extraction of the Ni-P and Ni-P/SiC nanocomposite coatings’ stress-strain behavior. It is shown that the correlation between the nanoindentation tests and the computational models was satisfactory, while the stress-strain curves revealed higher yield points for the heat-treated samples.

A Surface Transition Layer Model for Size Effect in T2 Copper Micro-Compression

The effects of sample size and grain size on the surface morphology and flow stress of deformed samples were investigated by means of copper micro-cylinder compression experiments at room temperature. The results of SEM showed that when the grain size increased or the sample size decreased, the deformation non-uniformity of samples’ free surfaces increased. Meanwhile, the stress–strain curves showed that during the compression process, the flow stress of the sample also tended to decrease as the grain size increased or the sample size decreased. According to the experimental results of nanoindentation, a surface transition layer model was established on the basis of the surface layer model by considering the mutual constraint of grains and the existence of transition layer grains. The experimental results indicated that the stress–strain curve calculated by the surface transition layer model can more accurately reflect the actual deformation situation of the material compared to the surface layer model.

Investigation on the effect of deposition temperature on structural and nanomechanical properties of electron beam evaporated lanthanum zirconate coatings

In the present study, the effect of deposition temperature in the range 773–973 K on structural and nanomechanical properties of lanthanum zirconate (La2Zr2O7) ceramic coatings deposited on Inconel-718 substrates by EB-PVD process has been investigated. X-ray diffraction and Raman spectroscopy were used to understand the structural details of lanthanum zirconate coatings. Structural studies show that the coatings have defect fluorite structure and are textured at the highest deposition temperature of 973 K. Increase in the surface roughness as a result of increased crystallite size was discussed from SEM and AFM analyses. Scratch test showed higher adhesion strength of the coating with the substrate for the film deposited at higher deposition temperature. Nanoindentation studies indicated that higher deposition temperature leads to enhanced indentation hardness and modulus of the coatings. From the nanoindentation results, resistance to plastic deformation and elastic recovery were also determined to explain the suitability of the lanthanum zirconate as a top coat material for thermal barrier coatings.

Characterization of gradient properties generated by SMAT for a biomedical grade 316L stainless steel

Gradient microstructure generated by surface mechanical attrition treatment (SMAT) has beneficial effects for treated mechanical components, such as improved fatigue behaviour and tensile yield stress. Little effort has been devoted to characterize the local properties of each region in the gradient microstructure generated by SMAT. In this paper, the gradient microstructure was first highlighted by microscopic observation using Electron Back-Scatter Diffraction (EBSD). Nanoindentation was used to characterize the loading-unloading behaviour at different depths beneath the treated surface. All the indentation patterns were observed using Atomic Force Microscopy (AFM) and a pile-up phenomenon was detected. The nanoindentation results were analysed taking into account this pile-up effect on the dimension of the imprints. The results reveal that grain refinement plays a dominant role on the hardness values, while the effect of residual stress is less significant. In addition, correcting the indentation contact area appears to be necessary in the near surface region which is strongly affected by SMAT.

Molecular dynamics simulation of plastic deformation and interfacial delamination of NiTi/Ag bilayer by cyclic-nanoindentation: Effects of crystallographic orientation of substrate

This paper presents a comparative study of plasticity and fracture behavior of the NiTi/Ag bilayer for the different crystallographic orientations of the substrate. Molecular dynamic (MD) simulation was used to determine the deformation mechanism, dislocation density, plastic energy dissipation and delamination of the NiTi/Ag bilayers near the interface, when NiTi aligned at (1 0 0), (1 1 1), (1 1 0), (3 2 1), (2 1 0) and (2 1 1) faces during the cyclic-nanoindentation test. The Griffith energy balance model was used to estimate the energy release associated with the delamination. The results of the simulation are suggested the dependence of deformation mechanism, energy release rate (Gin), interfacial toughness (Kin) and average plastic energy dissipation per unit cycle (Eplastic) of NiTi/Ag bilayer on the crystallographic orientation of the NiTi substrate. Prismatic dislocation loop and the nest-like organization of the twins were responsible for the plastic deformation of the NiTi/Ag bilayers when NiTi aligned at (211,010,111) and (100,210,321) faces, respectively. The values of Gin and Kin of the bilayers were in the range of 64.27–147.71 J/m2 and 77–192 J/m2, respectively. The values of Eplastic is increased in the following order: NiTi (2 1 1) < NiTi (3 2 1) < NiTi (1 0 0) < NiTi (2 1 0) < NiTi (1 1 1) < NiTi (0 1 0). The average dislocation densities produced for the indenter radius of 30 Å were in the range of 1.23 × 1019–1.84 × 1019 m−2. The results show that the NiTi (0 1 0)/Ag case has the larger dislocation density than other bilayers. Both of hardness and the interfacial toughness of the NiTi/Ag at different crystallographic orientations were consistent with the free energy data obtained from Jarzynski’s equality for nanoindentation results, which were in the range of 3.35 × 10−4−7.81 × 10−4 eV/atom. Due to high interfacial toughness and hardness plus lower plastic energy dissipation, the NiTi (2 1 1)/Ag bilayer may show the better results for engineering and medical applications.