A new technique to measure the true contact area using nanoindentation testing

Nanoindentation technique requires the determination of projected contact area under load for calculation of modulus and hardness of materials. This projected contact area is usually calculated by models which take into account the pile-up or sink-in phenomena around the tip. The most commonly used model was developed by Oliver and Pharr [1] which can precisely model the sink-in around the tip, but cannot account for pile-up. Another model developed by Loubet et al can be used [2]. It can take into account the pile-up and the sink-in phenomena and can precisely measure the projected contact area for a large range of materials, except for materials with high strain hardening exponent. Other techniques, like post mortem measurements, can be used. However these measurements do not take into account the elastic recovery during unloading. A new technique to estimate the true projected contact area will be presented. It consists of combining two models (The Dao et al. model and the Kermouche et al. model) that are used normally to calculate the representative stress and the representative strain in indentation. Consequently, the projected contact area calculation does not depend on any contact area model. Moreover, it can account for the pile-up or sink-in phenomenon and the strain hardening of the material, which is not possible with the actual models used. This new technique requires measuring indentations parameters like the maximum load, the contact stiffness and the loading curvature. It requires also the use of two tetrahedral indenters: a Berkovich tip and a tetrahedral tip where the included semi-angle is 50°. The method was tested on three different samples: glass, PMMA and 100C6 steel. For indentations on glass and PMMA samples, the projected contact area was precisely measured. For indentations on 100C6 steel sample, the method was adapted to take into account the Indentation Size Effect observed at small indentation depths. The projected contact area values measured with this new technique will be presented and compared to the values calculated with classical literature models. Also, the limits of the technique will be discusse

Is the second harmonic method applicable for thin films mechanical properties characterization by nanoindentation? Is the second harmonic method applicable for thin films mechanical properties characterization by nanoindentation?

The second harmonic method is a dynamic indentation technique independent of the direct indentation depth measurement. It can be used to determine near-surface mechanical properties of bulk materials more precisely than classical dynamic nano-indentation. In this paper, the second harmonic method is extended to the measurement of the mechanical properties of thin PMMA layers deposited onto silicon wafers. It is shown that this new technique gives precise results at small depths (less than 100nm), even for films with a thickness lower than 500nm, which was not possible to achieve with the classical CSM method. However, experimental and numerical results obtained both with classical nanoindentation and second harmonic methods differ at high indentation depth. Using FE simulations and AFM measurements, it is shown that the contact depth calculation with classical models can explain this difference

The role of β-titanium ligaments in the deformation of dual phase titanium alloys

Multiphase titanium alloys are critical materials in high value engineering components, for instance in aero engines. Microstructural complexity is exploited through interface engineering during mechanical processing to realise significant improvements in fatigue and fracture resistance and strength. In this work, we explore the role of select interfaces using in-situ micromechanical testing with concurrent observations from high angular resolution electron backscatter diffraction (HR-EBSD). Our results are supported with post mortem transmission electron microscopy (TEM). Using micro-pillar compression, we performed in-depth analysis of the role of select {\beta}-titanium (body centred cubic) ligaments which separate neighbouring {\alpha}-titanium (hexagonal close packed) regions and inhibit the dislocation motion and impact strength during mechanical deformation. These results shed light on the strengthening mechanisms and those that can lead to strain localisation during fatigue and failure

Some recent advances in nanomechanical testing: High strain rates, variable temperatures, fatigue and stress relaxation, combinatorial experimentation

In the first part of the talk, I will present two recently developed platforms for high temperature nanomechanical testing. The first platform allows for variable temperature and variable strain rate testing of micropillars in situ in the scanning electron microscope. By utilizing an intrinsically displacement-controlled micro-compression setup, which applies displacement using a miniaturized piezo-actuator, we’ve recently extended the attainable range of strain rates to up to~ 103 s−1, and enabled cyclic loading up to 107 cycles and load relaxation tests. Stable, variable temperature indentation/micro-compression in the range of -45°C to 600°C is achieved through independent heating and temperature monitoring of both the indenter tip and sample and by cooling the instrument frame. A second system allows for measurements at lower loads ex-situ in a dedicated vacuum chamber in the range of -150 °C to 700 °C. The cryo temperature is achieved by means of a liquid nitrogen line, while the high temperature is generated by three independent heat sources for the sample and the two tips of the differential displacement measurement system, establishing an infrared bath in the measurement area. In the second part several case studies will be presented. Using these new capabilities, we examine the plasticity of electrodeposited nanocrystalline Nickel, of combinatorial thin film libraries, of hard nanocrystalline ceramic thin films. Activation parameters such as activation volume and activation energy were determined and discussed in view of the most probable deformation mechanism. High strain rates and cyclic fatigue tests were performed on nanocrystalline Ni. The strain rate sensitivity seems to increase for strain rates higher than 10 s-1 suggesting a change in deformation mechanism with increasing strain rate. Cyclic fatigue tests up to 1 million cycles were performed on nanocrystalline Ni microbeams and compared with existing data from literature. Combinatorial libraries of bulk metallic glasses were synthesized by a combination of gradient sputtering and evaporation. Hardness and Young’s modulus was mapped as a function of temperate, strain rate and composition. The results are discussed in the light of shear band kinetics. Finally, a wide range of chromium nitride-based hard coatings was investigated using in situ micro-cantilever bending and compression testing. This allowed the first direct measurement of the high temperature compressive strength and fracture toughness

A new dynamic module for in-situ nanomechanical testing at high strain rate

In-situ nanomechanical testing is commonly used to probe surface mechanical properties of bulk materials or thin films, like hardness, Young’s modulus, Yield stress…Actually most of the instruments can measure these properties only statically, i.e. a low frequency, leading to property measurement only at low strain rate (usually 10-1s-1 by nanoindentation). This is mainly caused by the low resonance frequency of the system, preventing making tests at higher speed. Performing high dynamic measurements could bring new information on materials properties like deformation mechanism at high strain rate, or high dynamic fatigue properties. A new high dynamic module usable for in-situ mechanical testing has been developed. It is composed of a small piezotube attached directly behind the tip. Because of the small dimensions of the module, his resonance frequency is very high (higher than 50kHz) in comparison to classical nanomechanical testers, permitting to perform and measure precisely the signals at very high frequency. Moreover, it can be used as a sensor and as an actuator, in x, y and z directions which gives to this module a very large range of measurements. Firstly, the characteristics, the performances and the limits of the new high dynamic module will be presented. Secondly some indentations experiments performed at high strain rate on nanocrystalline nickel with the in-situ nanomechanical tester (Alemnis Gmbh) equipped with the high dynamic will be presented and discussed (Fig. 1). Finally, some micropillar compression at high strain rate on the same material will be described and discussed

About the plastic response of silicate glasses at the micronscale

Despite their brittleness, silicate glasses undergo plastic deformation at the micron scale. Mechanical contact and indentation are the most common situations of interest. The plasticity of glasses is characterized not only by shear flow but also by a permanent densification process. We present novel observations of the deformation and fracture of amorphous silica micropillars of various sizes using In Situ SEM Micro-Compression (Fig 1), that can help better understand the mechanisms occurring prior to its fracture [1]. Exhibiting one of the highest ratios of shear stress on shear modulus, fused silica thus further distinguishes itself from other amorphous materials. Moreover, nanocompression allows successful observations of crack initiation and growth. In parallel to this experimental investigation, atomistic simulations [2] aiming to investigate the theoretical plastic response of silicate glasses under coupled shear-pressure stress state was run. The results were interpreted in terms of volumetric and shear hardening. A buckling-like behaviour is clearly evidenced at low density (large free-volume) whereas a BMG-like is observed for samples densified until saturation. Thanks to this rich set of data, it seems now possible to define a constitutive model taking into account both nanomechanical results, i.e. nanopillars, nanoindentation, diamond anvil cell, and molecular dynamics simulation Despite their brittleness, silicate glasses undergo plastic deformation at the micron scale. Mechanical contact and indentation are the most common situations of interest. The plasticity of glasses is characterized not only by shear flow but also by a permanent densification process. We present novel observations of the deformation and fracture of amorphous silica micropillars of various sizes using In Situ SEM Micro-Compression (Fig 1), that can help better understand the mechanisms occurring prior to its fracture [1]. Exhibiting one of the highest ratios of shear stress on shear modulus, fused silica thus further distinguishes itself from other amorphous materials. Moreover, nanocompression allows successful observations of crack initiation and growth. In parallel to this experimental investigation, atomistic simulations [2] aiming to investigate the theoretical plastic response of silicate glasses under coupled shear-pressure stress state was run. The results were interpreted in terms of volumetric and shear hardening. A buckling-like behaviour is clearly evidenced at low density (large free-volume) whereas a BMG-like is observed for samples densified until saturation. Thanks to this rich set of data, it seems now possible to define a constitutive model taking into account both nanomechanical results, i.e. nanopillars, nanoindentation, diamond anvil cell, and molecular dynamics simulation

MFGE8 does not influence chorio-retinal homeostasis or choroidal neovascularization in vivo

Purpose: Milk fat globule-epidermal growth factor-factor VIII (MFGE8) is necessary for diurnal outer segment phagocytosis and promotes VEGF-dependent neovascularization. The prevalence of two single nucleotide polymorphisms (SNP) in MFGE8 was studied in two exsudative or “wet” Age-related Macular Degeneration (AMD) groups and two corresponding control groups. We studied the effect of MFGE8 deficiency on retinal homeostasis with age and on choroidal neovascularization (CNV) in mice. Methods: The distribution of the SNP (rs4945 and rs1878326) of MFGE8 was analyzed in two groups of patients with “wet” AMD and their age-matched controls from Germany and France. MFGE8-expressing cells were identified in Mfge8+/− mice expressing ß-galactosidase. Aged Mfge8+/− and Mfge8−/− mice were studied by funduscopy, histology, electron microscopy, scanning electron microscopy of vascular corrosion casts of the choroid, and after laser-induced CNV. Results: rs1878326 was associated with AMD in the French and German group. The Mfge8 promoter is highly active in photoreceptors but not in retinal pigment epithelium cells. Mfge8−/− mice did not differ from controls in terms of fundus appearance, photoreceptor cell layers, choroidal architecture or laser-induced CNV. In contrast, the Bruch's membrane (BM) was slightly but significantly thicker in Mfge8−/− mice as compared to controls. Conclusions: Despite a reproducible minor increase of rs1878326 in AMD patients and a very modest increase in BM in Mfge8−/− mice, our data suggests that MFGE8 dysfunction does not play a critical role in the pathogenesis of AMD

Iron, Ferritin, Transferrin, and Transferrin Receptor in the Adult Rat Retina

PURPOSE. The retina and other tissues need iron to survive. However, the normal iron metabolism in rodent retinas had not been characterized. This study was intended to investigate iron and iron homeostasis protein (ferritin, transferrin [Tf] and transferrin receptor [Tf-R]) distribution in 20-to 55-day-old rat retinas. METHODS. Iron was revealed on retinal sections directly by proton-induced x-ray emission (PIXE) and indirectly by electron microscopy (EM). Ferritin, Tf, and Tf-R proteins were localized by immunohistochemistry. Transferrin expression was localized by in situ hybridization (ISH). Transferrin and ferritin proteins and mRNA were analyzed by Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR), respectively. RESULTS. Iron is widely and unevenly distributed throughout the adult rat retina. The highest concentration was observed by PIXE in the choroid and the retinal pigmented epithelial cell (RPE) layer, and in inner segments of photoreceptors (IS). Outer segments of photoreceptors (OS) also contain iron. EM studies suggested the presence of iron inclusions inside the photoreceptor discs. Choroid, RPE, and IS showed a strong immunoreactivity for ferritin. Transferrin accumulated mainly in the IS and OS areas and in RPE cells but can also be detected slightly in retinal capillaries. Western blot analysis for Tf and ferritin confirmed their presence in the adult neural retina. By RT-PCR, H-and L-chains of ferritin and Tf mRNAs were expressed in neural retina, but the main sites of Tf synthesis observed by ISH were the RPE and choroid cell layers. Tf-R immunoreactivity was detected in the ganglion cell layer, inner nuclear layer, outer plexiform layer, IS, RPE, and choroid. These results were similar for all stages studied. CONCLUSIONS. For the first time, the present study characterized both iron and iron homeostasis proteins in rodent retinas. In the outer retina, iron and ferritin shared the same distribution patterns. In contrast, Tf, mainly synthesized by RPE cells and detected in OS and IS areas, probably helps to transport iron to photoreceptors through their Tf-R. This is a likely pathway for filling iron needs in the outer retina. (Invest Ophthalmol Vis Sci. 2000;41:2343-235

Protective efficacy of cross-reactive CD8⁺ T cells recognising mutant viral epitopes depends on peptide-MHC-I structural interactions and T cell activation threshold

Emergence of a new influenza strain leads to a rapid global spread of the virus due to minimal antibody immunity. Pre- existing CD8+ T-cell immunity directed towards conserved internal viral regions can greatly ameliorate the disease. However, mutational escape within the T cell epitopes is a substantial issue for virus control and vaccine design. Although mutations can result in a loss of T cell recognition, some variants generate cross-reactive T cell responses. In this study, we used reverse genetics to modify the influenza NP336-374 peptide at a partially-solvent exposed residue (N->A, NPN3A mutation) to assess the availability, effectiveness and mechanism underlying influenza-specific cross-reactive T cell responses. The engineered virus induced a diminished CD8+ T cell response and selected a narrowed T cell receptor (TCR) repertoire within two Vb regions (Vβ8.3 and Vβ9). This can be partially explained by the H-2DbNPN3A structure that showed a loss of several contacts between the NPN3A peptide and H-2Db, including a contact with His155, a position known to play an important role in mediating TCR-pMHC-I interactions. Despite these differences, common cross-reactive TCRs were detected in both the naïve and immune NPN3A-specific TCR repertoires. However, while the NPN3A epitope primes memory T-cells that give an equivalent recall response to the mutant or wild-type (wt) virus, both are markedly lower than wt->wt challenge. Such decreased CD8+ responses elicited after heterologous challenge resulted in delayed viral clearance from the infected lung. Furthermore, mice first exposed to the wt virus give a poor, low avidity response following secondary infection with the mutant. Thus, the protective efficacy of cross-reactive CD8+ T cells recognising mutant viral epitopes depend on peptide- MHC-I structural interactions and functional avidity. Our study does not support vaccine strategies that include immunization against commonly selected cross-reactive variants with mutations at partially-solvent exposed residues that have characteristics comparable to NPN3A. © 2010 Valkenburg et al.Link_to_subscribed_fulltex