Elsevier

Thin Solid Films

Volume 736, 31 October 2021, 138906
Thin Solid Films

Nano-indentation and nano-scratching of pure nickel and NiTi shape memory alloy thin films: an atomic-scale simulation

https://doi.org/10.1016/j.tsf.2021.138906Get rights and content

Highlights

Abstract

Issues with contact and friction in thin-film devices are inevitable in the field of micro-engineering, especially in micro-electro-mechanical systems (MEMS). In this study, the nano-indentation and nano-scratching behaviours of pure nickel and NiTi shape memory alloy (SMA) nano-films, two common materials used in MEMS, were comparatively investigated using large-scale molecular dynamics simulations. For the nickel nano-film structure, the load–displacement (P–h) curve demonstrated distinct elasto-plastic characteristics, and visible permanent deformation remained after unloading. In contrast, for the NiTi SMA, the Ph curve formed a hysteresis loop, and no evident deformation remained on the surface after unloading. It is noteworthy that during scratching, despite a lower applied normal pressure on the nickel nano-film than on the NiTi SMA, the friction and friction coefficient of the former were much higher. These differences in performance are closely related to the super-elasticity induced by the martensitic transformation within the NiTi SMA.

Introduction

Friction and wear are two important issues for metal contacts in engineering operations and technological applications, especially for thin-film micro-devices [1]. For instance, in micro-electro-mechanical systems (MEMS), contact occurs on an extremely small scale, and the friction performance will not only determine the service characteristics but also affect the working life. Thus, a full understanding of the friction behaviour at the micro- and nano-scale is essential for the design of small-scale devices.

In recent years, nickel–titanium (NiTi) shape memory alloy (SMA) thin films, with unique properties arising from the shape memory effect such as super-elasticity (pseudo-elasticity), high power-to-weight ratio, etc., have attracted tremendous attention for their potential application in the MEMS field [2]. Furthermore, it has been reported that NiTi SMAs have better wear resistance than conventional materials due to their unconventional deformation mechanisms [3], [4], [5], [6], [7], [8]. Therefore, much experimental and theoretical research has been carried out on the tribological performance of SMAs [9], [10], [11], [12], [13]. However, the micro mechanism of wear resistance of NiTi SMAs remain largely unknown due to the complex effect of plastic deformation and martensitic transformation induced by stress and temperature [6,14].

Molecular dynamics (MD) simulations are a highly effective approach to studying micro-scale localised contact and friction using three-dimensional nano-indentation and nano-scratching [15]. Compared with experiments, MD simulations can elucidate dynamic variations in atomic structures and stress levels beneath the indenter towards establishing the deformation mechanism. MD nano-indentation has been widely used to determine the nano-hardness and contact pressure of materials and is ideal for measuring the mechanical properties of thin films. For example, Fu et al. investigated the nano-indentation behaviour of Cu/Ni nano-twinned multilayer films [16] and vanadium nitride thin films [17,18] using MD simulations, and Ayoub et al. [19] employed MD nano-indentation to identify the underlying deformation mechanism which controls the plastic behaviour of metal/ceramic multilayers. Similarly, Huang et al. studied the anisotropy effects in diamond [20] and the nucleation and propagation of dislocations in nano-twinned diamond films [21] using a spherical indenter. MD nano-indentation has also been applied to metals, such as BCC Fe [22], FCC Cu [23], HCP Ti [24], NiTi alloy [25], metallic glasses [26], thin graphdiyne films [27], etc. The nano-scratching technique can be used to study the friction and wear of micro- and nanostructures. In recent years, MD nano-scratching has been employed to investigate the surface mechanical behaviours of Fe [28], pure Al [29], monocrystalline/nanocrystalline Cu [30,31], Ni films [32], multi-layer graphene [33], 3C SiC [34], etc. A recent detailed review of nano-indentation and scratching using atomistic-based modelling is available in the literature [15].

In order to study the wear mechanism of NiTi SMA at atomic scale, in present work, large-scale MD simulations of nano-indentation and nano-scratching were conducted on a near-equiatomic NiTi alloy by using of a spherical diamond indenter. For comparison, a pure nickel thin film, a material which is commonly used in MEMS, was also simulated under the same conditions. As a typical phase-change material, the NiTi SMA has excellent friction and wear properties. Compared with traditional elasto-plastic material such as pure Ni, there are marked differences in the wear resistance mechanism of the SME. A comparative study would help to reveal the effect of the martensitic transformation on the friction and wear properties.

Section snippets

Interatomic potentials

In the MD simulations for NiTi SMA, the second nearest neighbour modified embedded-atom method (2NN-MEAM) potential [35], which has been used to study phase transformations [36], compression [37,38], cyclic loading [39], and nano-indentation [25], was selected to describe the interaction between Ni and Ti atoms. It should be noted that the 2NN-MEAM potential function used in present work is still insufficient in the plastic deformation of NiTi SMA, the simulation results most likely

Nano-indentation

To determine the fundamental deformation mechanisms of the pure Ni and NiTi SMA thin films, MD simulations of nano-indentation were first performed for different maximum depths. Fig. 2(a) shows the load–depth (P–h) curves of pure Ni under indentation depths of 15, 20, and 25 Å. When the indentation depth is 15 Å, the residual depth after unloading is small. With increasing indentation depth, the slope of the unloading curve and remaining depth after unloading increase, as indicated in Fig. 2(a)

Conclusions

In summary, MD simulations were performed to investigate the fundamental deformation behaviours of pure Ni and NiTi SMA films under nano-indentation and nano-scratching using a spherical indenter. It was found that the P–h curves of NiTi under nano-indentation form hysteresis loops with different depths and the indentation mark disappears after unloading. On the other hand, for pure Ni, V-shaped dislocations and prismatic partial dislocation loops form during nano-indentation, which cause most

CRediT authorship contribution statement

Zenglu Song: Writing – original draft, Project administration. Xiao Tang: Data curation, Formal analysis. Xiang Chen: Methodology, Writing – review & editing. Tao Fu: Resources. Huanping Zheng: Software. Sheng Lu: Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We want to acknowledge Prof. Xianghe Peng, Dr. Cheng Huang, and Dr. Yinbo Zhao for their discussions and helpful support. The numerical calculations in this paper were performed on TianHe-1(A).

Funding

The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China [grant numbers 11932004,11802047, 11802045, 11602042], the open fund of the State Key Laboratory [grant numbers SKLMT-KFKT-201711, SV2018-KF-28], Chongqing Research Program of Basic Research and Frontier Technology [grant numbers cstc2016jcyjA0594, cstc2016jcyjA0443].

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