Investigation on tensile behaviors of diamond-like carbon films

https://doi.org/10.1016/j.jnoncrysol.2016.03.025Get rights and content

Highlights

  • The deformation of DLC films can induce their graphitization.

  • Strain-localized regions of DLC films are dominated by their sp2 clusters.

  • The sp2 clusters are responsible for the relaxation of DLC films at small strains and film failures at large strains.

Abstract

The lubrication performance of diamond-like carbon films is significantly influenced by their deformations under loading. However, their deformation mechanisms are unclear so far due to their nanoscale thicknesses and complex microstructures. In this study, these mechanisms are explored by investigating the tensile response of the DLC films via molecular dynamics simulations. The atomic strain localizations are observed, and the regions where they occur are dominated by sp2 clusters. These clusters relax the film at small tensile-strains by releasing its residual energies. The sp3-sp2 transitions are present at large tensile-strains and prefer to occur in the strain-localized regions. This preference significantly improves the graphitization level in these regions and thus promotes the sp2 clusters to propagate. The propagation severely damages the sp3 networks and leads to the failure of the film. This research suggests that reductions of heterogeneities such as existences of large-sized sp2 clusters may be useful to delay the film failure by suppressing the initial strain localizations. It is demonstrated that the propagation of sp2 clusters for the DLC films can be induced by their deformation besides the high friction temperature in their wear tests. This demonstration can help to improve the understanding of their trigological mechanisms.

Introduction

Amorphous solids are materials that lack the long-range order characteristics of crystals and thus exhibit disordered microstructures [1], [2]. The linear defects and dislocations which support plasticity in crystals are inappropriate for interpreting the deformation mechanisms of amorphous solids. As a result, these mechanisms always attract attentions, and many significant discoveries have been made in the past decades [3], [4], [5]. It was found that amorphous solids exhibit localizations of atomic strains and atomic stresses when subjected to external forces. Domains in which the localizations occur are usually ill-packed and have high free-volumes due to low local densities and liquid-like properties [6], [7], [8]. Since the plasticity or localized shear transformations are initiated in these domains, they are commonly regarded as defects [5]. For metallic glasses, the evolutions of these domains can even induce the presence of shear bands which improve ductility of materials [9].

Diamond-like carbon (DLC) films are amorphous solids that combine carbon atoms by hybridized sp3, sp2, and sp bonds [10], [11]. These films exhibit excellent mechanical properties and good wear resistances, and are widely used as solid lubrication films. Lubricities of DLC films are dominated by sp3-sp2 rehybridization transitions (also named graphitization) with the passivation of surface dangling bonds by other atoms or molecules [12], [13], [14], [15]. Recent theoretical works showed that strains can largely induce the sp3-sp2 transitions and strain localizations are observed to play a crucial role in the lubricities of DLC films when the dangling bonds inside them lack efficient passivation [12], [14], [16], [17], [18], [19], [20], [21]. Since both the strain-induced bond transition and the strain localization are closely related to structural evolutions of DLC films, these works indicate that the understanding of the film deformation mechanisms is of significant importance.

So far, few studies have been conducted on these deformation mechanisms, mainly due to the huge experimental difficulties in directly observing the microstructural evolutions of DLC films because of their nanoscale thicknesses [10], [15], [22]. Moreover, previous theoretical works mainly focused on the evolution of atoms at the sliding interface in the wear test [12], [14], [23] instead of the deformation of the whole film.

In view of the many similar properties such as the disorder distributions of atoms and the absence of dislocations shared by most of the amorphous solids, their common theories can provide useful points to investigate the deformation mechanisms of DLC films [3], [4], [5]. For example, the strain localizations may be used to understand the sp3-sp2 transitions [6], and the free-volume theory reminds that sp2 atoms or clusters may act as defects due to their larger atomic volumes as compared with those of sp3 atoms [7], [8].

In the present study, the deformation process of DLC films under tensile loading is explored via molecular dynamics (MD) simulation. The evolutions of microstructures are studied in detail, and the effect of strains on the plasticity and graphitization is investigated. Since the DLC films are comprised of pure C atoms without other doping elements, the results are applicable for non-hydrogenated DLC films especially those with high fraction of sp3 C atoms.

Section snippets

Modelling

The MD simulation is performed by the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [24]. Atomic interactions in DLC films are described by the Tersoff potential which is effective and accurate for carbon systems [25]. Its cutoff distance is set as 2.1 Å.

The initial atomic configurations of DLC films are generated by using the melting and quenching procedure, since it can help to easily obtain their realistic structures [26], [27], [28], [29]. During this generation

Stress-strain curves and potential energy evolution

Fig. 2 shows the obtained stress vs. strain curve. When the tensile-strain initially increases, the stress increases linearly, indicating a linear and reversible elastic deformation process. The slope of the linear curve gives the elastic modulus, as about 539 GPa which agrees well with those reported in the experimental study [36]. As the tensile-strain increases, the stress-strain curve becomes nonlinear and exhibits a decreasing slope. The nonlinear deformation also called inelasticity is

Discussions

The stretched sp3-sp3 and sp2-sp3 bonds are unstable and break easily due to their larger bond lengths than those of the sp2-sp2 bonds. This has also been previously verified by density functional theory (DFT) calculations [21]. Since the bond elongations are induced by atomic strains, one might assume that atoms with the stretched bonds should be highly strained. To evaluate this assumption, distributions of these atoms in terms of their atomic strains are further studied. Surprisingly, the

Conclusions

The deformation mechanisms of DLC films are investigated by conducting a tensile test via MD simulations. At small tensile-strains, the fraction of sp2 atoms Fsp2 and the fraction of sp3 atoms Fsp3 keep constant, but the film structures are relaxed. With the increasing tensile-strain, Fsp3 decreases but Fsp2 increases, indicating the occurrence of sp3-sp2 transitions. When the ultimate tensile strength of the DLC film is reached, it subsequently fails. The atomic strains analysis indicates that

Acknowledgements

This work is financially supported by Ministry of Education (Academic Research Fund TIER 1-RG128/14), Singapore. LB acknowledges the Interdisciplinary Graduate School of Nanyang Technological University, Singapore for providing the Research Student Scholarship.

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