On the Use of the Nanoindentation Unloading Curve to Measure the Young's Modulus of Polymers on a Nanometer Scale

Summary: The nanoindentation test is a fundamental tool to assess the link between morphology and mechanical properties. The preliminary results of a more exhaustive study about the applicability to polymers of the most used procedure to determine elastic modulus by indentation are reported in this short communication. A departure of the experimental conditions from the theoretical assumptions and results that give rise to the Oliver and Pharr analysis is shown to occur under a wide range of experimental conditions, with applied loads and penetration depths covering several orders of magnitude and using different indenter geometries. Unloading curves with exponents significantly larger than 2 are observed in disagreement with the contact mechanics approach used by Oliver and Pharr.

An AFM image obtained in non contact mode of an indentation induced by a sharp AFM tip with a maximum applied load of ca 1.2 µN on amorphous PET.     

Microcantilever bend testing and finite element simulations of HIP-ed interface-free bulk Al and Al–Al HIP bonded interfaces

Abstract

We report on the strength of Al–Al interfaces and the effects of chemical segregation and interfacial void formation on bond strength using microcantilever bend testing. Interfaces are synthesised via hot isostatic pressing. Microcantilevers of several nominal dimensions were fabricated via focused ion beam and deformed in a nanoindenter. We find increased cantilever strength as a function of decreasing sample size, with a linear dependence of the yield strength on the inverse square root of the length scale characteristic to the cantilever cross-section. The presence of pores and chemical segregation decreases the yield strength of the material by 17% and the accommodated strain energy by 10–15% for strain values in the 6–12% range.     

The mechanical response of core-shell structures for nanoporous metallic materials

Nanoporous gold (NP-Au) exhibits microscale plasticity, but macroscopically fails in a relatively brittle manner. This current study suggests that a core-shell structure can increase both ductility and strength of NP-Au. A core Au foam structure was created using conventional dealloying methods with average ligament size of 60?nm. Nickel was then electroplated on to the NP-Au with layer thicknesses ranging from 2.5?nm to 25?nm. Nanoindentation demonstrated a significant increase in the hardness of the coated Np-Au, to about five times of that of the pure Np-Au, and a decrease in creep by increasing the thickness of the coated Ni layer. Molecular dynamics simulations of Au–Ni ligaments show the same trend of strengthening behavior with increasing Ni thickness suggesting that the strengthening mechanisms of the Np-Au are comparable to those for fcc nano ligaments. The simulations demonstrate two different strengthening mechanisms with the increased activity of the twins in plated Au–Ni ligaments, which leads to more ductile behavior, as opposing to the monolithic Au ligaments where nucleation of dislocations govern the plasticity during loading.     

Nanoscale interfibrillar adhesion in UHMWPE fibers

The goal of this research is to quantify the fibrillar adhesive energy in ultra‐high molecular weight polyethylene fibers, characteristic of nanoscale fibril interactions. Quantification of these energies is vital to the understanding of fibrillar deformation mechanisms that have been shown to play an important role in fiber performance. This is achieved through the development and implementation of a nanosplitting technique developed through the use of AFM‐enabled nanoindentation. This technique allows the quantification of nanoscale adhesive energies through careful monitoring of load and unload curves as well as examination of the residual split through high‐resolution AFM images. Results indicate that the average nanoscale fibril adhesive energy is over 3 times larger than the energy expected from van der Waals interactions alone. This indicates that a significant degree of physical interactions exist between fibrils, beyond van der Waals interactions, in the form of tie‐molecules, fibrillar network junctions, and bridging lamellar crystals. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 391–401     

NANOINDENTATION OF THIN-FILM-SUBSTRATE SYSTEM： DETERMINATION OF FILM HARDNESS AND YOUNG’S MODULUS

In the present paper, the hardness and Young‘s modulus of film-substrate systems are determined by means of nanoindentation experiments and modified models. Aluminum film and two kinds of substrates, i.e. glass and silicon, are studied. Nanoindentation XP Ⅱ and continuous stiffness mode are used during the experiments. In order to avoid the influence of the Oliver and Pharr method used in the experiments, the experiment data are analyzed with the constant Young‘s modulus assumption and the equal hardness assumption. The volume fraction model (CZ model) proposed by Fabes et al. (1992) is used and modified to analyze the measured hardness. The method proposed by Doerner and Nix (DN formula) (1986) is modified to analyze the measured Young‘s modulus. Two kinds of modified empirical formula are used to predict the present experiment results and those in the literature, which include the results of two kinds of systems, i.e., a soft film on a hard substrate and a hard film on a soft substrate. In the modified CZ model, the indentation influence angle, φ, is considered as a relevant physical parameter, which embodies the effects of the indenter tip radius, pile-up or sink-in phenomena and deformation of film and substrate.     

碳化硅薄膜的力学性能测试分析

对利用射频磁控溅射及真空退火方法在(100)硅晶片衬底上制备的纳米晶碳化硅(SiC)薄膜,用纳米压痕仪进行了力学性能测试分析。纳米压痕技术测试给出两块SiC薄膜样品I和II的弹性模量/硬度分别约为106GPa/9.5GPa和175GPa/15.6GPa。纳米划痕技术测试两块SiC薄膜的摩擦系数分别约为0.02~0.15和0.05~0.18,显示出良好的润滑性能;对薄膜的临界附着力等进行测量以评价膜基结合强度,分析了划痕过程中薄膜近表面弹塑性变形和断裂信息。在原子力显微镜下对SiC薄膜样品的初始表面及残余压痕和划痕形貌进行了观察分析,与测试结果相符。综合比较,样品II的整体性能优于样品I。本文中薄膜的弹性模量和硬度值较低可归因于制膜技术的不同和表层碳含量偏高。     

真空磁过滤电弧离子镀法制备DLC膜的机械性能

采用真空磁过滤电弧离子镀法分别在9Cr18和40CrNiMo钢上沉积厚约为0.2μm的类金刚石（DLC）膜。为了检测成膜质量，分别使用纳米压痕和纳米划痕技术表征钢基材和DLC/基材的机械性能，对这种亚微米厚的膜，纳米压痕技术和纳米划痕技术能提供丰富的近表面的弹塑性变形，断裂和摩擦等的信息。同DLC/40CrNiMo相比。DLC/9Cr18的硬度高和固体润滑效果显著，9Cr18是较为理想的基体材料。     

亚微米氮化钛膜的纳米压痕和划痕测定

采用等离子电弧沉积的方法，分别在GT35和40CrNiMo钢上沉积厚约为0．5μm的氮化钛(TiN)膜．为了筛选基材，采用纳米压痕和划痕技术，评价膜基界面结合和固体润滑效果．纳米压痕结果，GT35，40CrNiMo和TiN的纳米硬度／弹性模量的典型值分别约为11．5GPa／330GPa，6．0GPa／210GPa，30GPa／450GPa．纳米划痕结果，GT35有较理想的膜基结合能力；GT35，40CrNiMo，TiN及其有机膜的摩擦系数分别约为0．25，0．45，0．i5，0．i0．同40CrNiMo相比，GT35是较为理想的基体材料．纳米压痕和划痕技术能提供丰富的近表面的弹塑性变形、断裂和摩擦等的信息，是评价亚微米薄膜力学性能的有效手段．     

An experimental/computational approach to identify moduli and residual stress in MEMS radio-frequency switches

In this paper, we identify the Young's modulus and residual stress state of a free-standing thin aluminum membrane, used in MEMS radio-frequency (rf) switches. We have developed a new methodology that combines a membrane deflection experiment (MDE) and three-dimensional numerical simulations. Wafer-level MDE tests were conducted with a commercially available nanoindenter. The accuracy and usefulness of the MDE is confirmed by the repeatability and uniformity of measured load-deflection curves on a number of switches with both wedge and Berkovich tips. It was found that the load-deflection behavior is a function of membrane elastic properties, initial residual stress state and corresponding membrane shape. Furthermore, it was assessed that initial membrane shape has a strong effect on load-deflection curves; hence, its accurate characterization is critical. Through an iterative process and comparison between MDE data and numerical simulations, the Young's modulus and residual stress state, consistent with measured membrane shape, were identified. One important finding from this investigation is that variations in membrane elastic properties and residual stress state affect the load-deflection curve in different regimes. Changes in residual stress state significantly affect the load-deflection slope at small values of deflection. By contrast, variations in Young's modulus result in changes in load-deflection slope at large deflections. These features are helpful to decouple both effects in the identification process.     

Plasma and chromic acid treatments of polycarbonate surface to improve coating–substrate adhesion

The influence of Ar/O₂ plasma activation and chromic acid etching of polycarbonate (PC) surface on the adhesion of coating to substrate was systematically studied by cross‐cut and tape peel methods through temperature‐shock aging tests. The differences between the wettabilities and elemental compositions of plasma‐treated and chromic acid‐treated PC surfaces prior to coating deposition were evaluated by contact angle measurements and X‐ray photoelectron spectroscopy. To elucidate the adhesion failure of the coatings, nanoindentation technique was employed for the quantitative assessment of the nanomechanical changes of coating depositions on PCs after temperature‐shock aging tests. The two surface treatments can significantly improve the hydrophilicity and polarity of the PC surface, resulting in excellent adhesion of the coating on the PC substrate. Temperature‐shock aging tests reveal that the adhesion of coating on plasma‐modified substrates is superior to that of chromic acid‐etched substrates. We propose that the improved adhesion of the coating on the plasma‐modified PC can be attributed to the higher wettability and more cross‐linking of C–O–Si bonds at the coating–substrate interface. Copyright © 2013 John Wiley & Sons, Ltd.