Nanoindentation of Ni-Ti Thin Films

Ni-Ti thin films of various compositions were sputtered-deposited on silicon substrates. Their mechanical properties (hardness and Young's modulus) were then determined using a nanoindenter equipped with a Berkovich tip. This paper examines the effects of composition on the mechanical properties (hardness and Young's modulus) of the sputter deposited Ni-Ti thin films. This is of particular interest since the actuation properties of these shape memory alloy films are compositionally sensitive. The surface-induced deformation is revealed via Atomic Force Microscopy (AFM) images of the indented surfaces. Which show evidence of material pile-up that increases with increasing load. The measured Young's moduli are also shown to provide qualitative measures of the extent of stress-induced phase transformation in small volumes of Ni-Ti films.     

Inkjet printing of conductive Ag lines and their electrical and mechanical characterization

This paper aims to investigate the effects of the substrate, the printed line thickness and the sintering temperature on the electrical resistivity, Young's modulus and hardness of inkjet-printed Ag thin films. Electrical resistivity was determined from the four-point method and Young's modulus and hardness were evaluated from nanoindentation test. Several models for evaluating Young's modulus and hardness were used and compared to account for the influence of substrates. It is noted that Ag lines on glass have higher resistance and resistivity than those on polyimide (PI) since Ag lines on glass and PI have tensile and compressive residual thermal stresses, respectively, due to the difference of coefficient of thermal expansion between Ag lines and substrates. Young's modulus of Ag films on glass can be predicted by the modified King and Bec models considering the substrate effect, but these models offer unstable results for Ag films on PI. Young's modulus and hardness of Ag films increase with the sintering temperature, and they are little affected by the film thickness when fully sintered.     

Mechanical properties of porous silicon by depth-sensing nanoindentation techniques

Porous silicon (PS) was prepared using the electrochemical corrosion method. Thermal oxidation of the as-prepared PS samples was performed at different temperatures for tuning their mechanical properties. The mechanical properties of as-prepared and oxidized PS were thoroughly investigated by depth-sensing nanoindentation techniques with the continuous stiffness measurements option. The morphology of as-prepared and oxidized PS was characterized by field emission scanning electron microscope and the effect of observed microstructure changes on the mechanical properties was discussed. It is shown that the hardness and Young's elastic modulus of as-prepared PS exhibit a strong dependence on the preparing conditions and decrease with increasing current density. In particular, the mechanical properties of oxidized PS are improved greatly compared with that of as-prepared ones and increase with increasing thermal oxidation temperature. The mechanism responsible for the mechanical property enhancement is possibly the formation of SiO₂ cladding layers encapsulating on the inner surface of the incompact sponge PS to decrease the porosity and strengthen the interconnected microstructure.     

Determination of mechanical properties of thermally grown oxide on Fecralloy by nano-indentation

Nano-indentation was used to measure the mechanical properties of the thermally grown oxide (TGO) on a Fecralloy substrate. Due to the influence both of the substrate and the indenter size effect (ISE), the measured hardness and Young's modulus of the TGO system decreased with increasing indentation depth. Models were proposed to determine the mechanical properties of the TGO with consideration of both the substrate effect and the ISE. In addition, the ratio of hardness to Young's modulus (H/E) can be related to the ratio of irreversible work to total work (W_ir/W_t) during the indentation process.     

Study of the mechanical properties of CeO2 layers with the nanoindentation technique

The mechanical properties of CeO₂ layers that are undoped or doped with other elements (e.g. Zr and Ta) are a topic of special interest specially in the manufacturing of superconductor buffer layers by pulsed electron deposition. Nowadays, the trend is to produce small devices (i.e. coated conductors), and the correct mechanical characterization is critical. In this sense, nanoindentation is a powerful technique widely employed to determine the mechanical properties of small volumes. In this study, the nanoindentation technique allow us determine the hardness (H) and Young's modulus (E) by sharp indentation of different buffer layers to explore the deposition process of CeO₂ that is undoped or doped with Zr and Ta, and deposited on Ni–5%W at room temperature. This study was carried out on various samples at different ranges of applied loads (from 0.5 to 500 mN). Scanning electron microscopy images show no cracking for CeO₂ doped with Zr, as the doping agent increases the toughness fracture of the CeO₂ layer. This system, presents better mechanical stability than the other studied systems. Thus, the H for Zr–CeO₂ is around 2.75 · 10⁶ Pa, and the elastic modulus calculated using the Bec et al. and Rar et al. models equals 249 · 10⁶ Pa and 235 · 10⁶ Pa respectively.     

Influence of sintering temperatures on hardness and Young''s modulus of tricalcium phosphate bioceramic by nanoindentation technique

Nanoindentation experiments on tricalcium phosphate (TCP) bioceramic sintered at different temperatures were performed with a Berkovich indenter for determining hardness and elastic modulus from load and displacement data. The hardness and Young's modulus increased with the increase of sintering temperature up to 1300 °C, but the Young's modulus decreased with the further increase of sintering temperatures at 1400 and 1500 °C. X-ray diffraction (XRD) results showed that the transformation β→-TCP happened when the sintering temperature reached around 1400 °C, which contributed to the decreases of modulus at 1400 and 1500 °C. Scanning electron microscopy (SEM) results showed that the sintering effect was improved with the increase in sintering temperature.     

The influence of cellulose nanocrystals on the microstructure of cement paste

This paper reports the influence of raw and sonicated cellulose nanocrystals (CNCs) on the microstructure of cement paste. A novel centrifugation method is designed to measure the concentrations of the adsorbed CNCs (aCNCs) on the cement surface, and the free CNCs (fCNCs) which are mobile in water. It is found that, the majority of the CNCs (>94%) are aCNCs. More importantly, sonication does not significantly reduce the amount of aCNCs (reduction of less than 2%). We surmise that, after sonication, the aCNCs are primarily dispersed over the cement surface, instead of becoming fCNCs via sonication. Isothermal calorimetry and energy-dispersive X-ray spectroscopy (EDX) results support this theory. The water desorption tests show that the total porosities of cement pastes with raw and sonicated CNCs are 14.8% and 14.4%, which showed a reduction from 16% for the plain cement paste. The porosity reduction is a result of an increase in the degree of hydration. The advantage of sonicated CNCs is they are dispersed, avoiding therefore agglomerates that can lead to pores, voids, and air entrapment. The nanoindentation results show that the reduced indentation modulus on the interfacial regions between cement particles and the low density CSH is increased when CNCs are used.     

Phase evolution and mechanical properties of novel nanocrystalline Y2(TiZrHfMoV)2O7 high entropy pyrochlore

High entropy pyrochlores(HEP)are potential candidates as dispersoids in the oxide dispersed strength-ened steels or alloys,which can be used in nuclear reactors and supercritical boilers.For the first time,HEP oxides Y2(TiZrHfMoV)2O7 were synthesized with Y2Ti2O7 as a base structure with the B site(Ti)substituted with five cations through reverse co-precipitation technique in the nanocrystalline form at lowest synthesis temperature.The synthesis parameters for Y2(TiZrHfMoV)2O7(5C)and other derived compositions(five compositions of four cationic systems with each cation eliminated at B site from 5C)are optimised to obtain lower crystallite and particle sizes.5C has a smaller crystallite size(27 nm)than other single-phase compositions.The cation's influence,oxidation state,and oxygen vacancy in the phase formation were analysed through XPS.The single-phase HEPs are consolidated through spark plasma sin-tering.Y2(TiZrHfMo)2O7(4C-V)shows the highest hardness among the compositions reported so far due to its finer grain size,and Y2(TiHfMoV)2O7(4C-Zr)has a higher Young's modulus compared to other single-phase composition due to its higher degree of order in the structure.     

Mechanical properties of single crystal tungsten microwhiskers characterized by nanoindentation

The nanoindentation results in this work showed that the one-dimensional single crystal tungsten microwhiskers fabricated by vapor deposition possess unique yielding behavior. The average hardness of the microwhiskers measured on their (1 1 1) surfaces was 8.44 GPa, significantly higher than that of the bulk tungsten ranged from 3.4 to 5.8 GPa. The hardness increase was attributed to the lacking of dislocation avalanche in the 1D single crystal tungsten that was often observed in the nanoindentation of the bulk tungsten. However, the values of elastic modulus of the microwhiskers measured on the (1 1 1) surfaces were considerably scattered, whose average value is much lower than the reported value of 410 GPa for the bulk tungsten.     

Tensile strength of hydrated cement paste phases assessed by micro-bending tests and nanoindentation

The paper is focused on the experimental investigation of cement paste's tensile strength and related mechanical properties at the micrometer level. Small scale specimens with micrometer dimensions in the form of cantilever beams having a triangular cross section and ≈20 μm in length are fabricated by means of a focused ion beam and tested in bending with the aid of a nanoindenter. Elastic properties are evaluated from both bending and nanoindentation tests for all the phases with very close agreement. The phase separation is performed with SEM-based image analysis and the deconvolution of grid nanoindentation results. The load-deflection curves of bent beams are monitored up to the failure for distinct microlevel phases, namely for inner and outer products and Portlandite. The tensile strength of the phases is directly derived from the load-deflection curves in the range of 264 MPa (for the outer product) to 700 MPa (for the inner product and Portlandite). Moreover, the load-deflection curves are used for the supremum estimates of fracture energies for individual hydrated cement phases. Low values of the energies in the range of 4.4 − 20 J/m² were found. The values obtained experimentally in this paper correspond well with those published in recent multiscale or molecular dynamics models.     

Constitutive modeling of the aging viscoelastic properties of portland cement paste

Analytical approaches for modeling aging viscoelastic behavior of concrete include the time–shift approach (analogous to time-temperature superposition), the solidification theory, and the dissolution–precipitation approach. The aging viscoelastic properties of concrete are generally attributed solely to the cement paste phase since the aggregates are typically linear elastic. In this study, the aging viscoelastic behavior of four different cement pastes has been measured and modeled according to both the time–shift approach and the solidification theory. The inability of each individual model to fully characterize the aging viscoelastic response of the materials provides insight into the mechanisms for aging of the viscoelastic properties of cement paste and concrete. A model that considers aging due to solidification in combination with inherent aging of the cement paste gel (modeled using the time–shift approach) more accurately predicted the aging viscoelastic behavior of portland cement paste than either the solidification or time–shift approaches independently. The results provide evidence that solidification and other intrinsic gel aging mechanisms are concurrently active in the aging process of cementitious materials.     

Creep Behavior and Its Influence on the Mechanics of Electrodeposited Nickel Films

In order to improve the accuracy and comparability of hardness and elastic modulus measurements in nanoin-dentation, an evaluation of the creep behavior and its influence on the mechanical properties of the electrode-posited nickel film has been conducted. The influence of loading time and hold period on the hardness and elastic modulus results at maximum load 5000 μN has also been examined. It is found that with increasing the loading time, the creep value is decreased. However, the creep value is increased when the hold period is increased. The elastic modulus results are more reliable if the hold period is longer. If the hold period is long enough, the loading time has no remarkable effect on the hardness and elastic modulus measured.     

Effect of EVA on the fresh properties of cement paste

The improved workability effect of latex in dry mortars has not been fully clarified. The purpose of this research was to investigate the influence of the EVA copolymer on the cement hydration and on the rheological properties of cement pastes. The results pointed to a minor influence of EVA on cement hydration and to a ball-bearing effect. In fact, the shear thinning behavior of reference paste at 15 min after mixing changed to shear thickening owing to the EVA addition. This behavior could be explained by the decrease in the interparticle separation distance as consequence of the solid content increase in case of shearing detachment of weakly adhered EVA particles from the cement particles surfaces. The expected EVA plasticizing effect was observed at 60 min. Such behavior points to the stabilization of EVA on the cement particles surfaces, thus resulting in a steric barrier effect.     

Nanoindentation and microstructure analysis of resistance spot welded dual phase steel

A nanoindentation hardness study has been conducted on the tempered region and the base metal in a dual phase steel subjected to rapid thermal cycles of resistance spot welding. Nanohardness results revealed “softening” at nano-scale for tempered martensite when compared to martensite in the base metal. At the tempered region, the ferritic matrix presented a slight reduction in hardness while the tempered martensite seemed to have a major contribution to the measured softening at micro-scale.     

Characterization of the mechanical damage of a chemically degraded cement paste

Cement-based materials as concrete interact with their environment. In the presence of aqueous mediums, dissolutions and precipitations of solid species composing the cement paste induce a chemical damage, which may affect the structure integrity. This paper presents a mechanical characterization of the Young modulus evolution of chemically damaged cement paste, based on a microstructural approach coupled to a reactive-transport model. The method has been successfully applied to cement pastes leached by pure water. It is concluded that the capillary porosity and its spatial distribution within the material are the parameters, which mainly rule the stiffness evolution.     

微纤丝取向对木材细胞壁力学性能的影响研究

以马尾松为研究对象,运用原位成像纳米压痕技术测试了两种不同微纤丝取向（微纤丝角）下马尾松细胞壁纵向弹性模量和硬度之间的差异,由此探索微纤丝角对其细胞壁力学性能的影响,结果表明,上述两种力学性能指标与微纤丝角之间存在负相关关系,微纤丝角从13.5°增加到21.7°,马尾松管胞细胞壁纵向弹性模量降低了18.77%,硬度减少了18.37%。纳米压针从细胞壁的纵向压入时,细胞壁变形机制以塑性变形为主,微纤丝角越大,最大压入深度也越大,残余变形量也越大,但残余变形占总变形的比例几乎相同,约为73%。     

纳米压痕仪和激光超声技术检测薄膜弹性模量

在薄膜材料的力学性能测定中,弹性模量是衡量材料软硬程度的重要指标。为了测定弹性模量,本文选取不同种类和厚度的金属薄膜材料,采用纳米压痕技术(nanoindentation)和激光超声技术(laser-acoustics)两种测试方法相互比较,以确保测试的准确性。两种方法在膜厚较大的试样测试中得到了大致相符的试验结果,相对误差最小达到2%。     

Rheology of cement paste under high pressure

The objective of this study was to investigate whether cementitious materials undergo changes during pumping processes due to pressure variation. The influence of pressure on the rheological properties of cement pastes, which assumably represented the lubricating layer that forms along the profile of concrete during pumping, was evaluated using a rotational rheometer with a high-pressure cell. Cement pastes with water-to-cement ratios ranging from 0.35 to 0.6 were tested according to a protocol designed to simulate the conditions of an actual pumping process based on field tests. The shear rates, shearing durations, and pressure levels from 0 to 30 MPa were experimentally simulated. The test results indicated that below a certain water-to-cement ratio (0.40) elevated pressures lead to changes in the rheological properties, while changes were negligible when the ratio was above this threshold. Further, at low water-to-cement ratios the thixotropy of the cement paste can reverse into rheopexy after pressurization.