Investigation on tensile behaviors of diamond-like carbon films
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.
References (61)
- et al.
Atomistic simulation and modeling of localized shear deformation in metallic glasses
Prog. Mater. Sci.
(2011) - et al.
An atomistic study of deformation of amorphous metals
Acta Metall.
(1983) Atomic level stresses
Prog. Mater. Sci.
(2011)- et al.
Effect of different ion beam energy on properties of amorphous carbon film fabricated by ion beam sputtering deposition (IBSD)
Nucl. Instrum. Methods Phys. Res., Sect. B
(2011) - et al.
An investigation of the relationship between graphitization and frictional behavior of DLC coatings
Surf. Coat. Technol.
(1996) - et al.
The tribological performance of selected solid lubricant films in sand-dust environments
Wear
(2011) Fast parallel algorithms for short-range molecular dynamics
J. Comput. Phys.
(1995)- et al.
A modified Tersoff potential for pure and hydrogenated diamond-like carbon
Comput. Mater. Sci.
(2013) - et al.
VMD: visual molecular dynamics
J. Mol. Graph.
(1996) - et al.
Shear-induced disappearances of energy minima and plastic deformation in polymer glasses
Comput. Theor. Polym. Sci.
(1999)
Structural properties and growth evolution of diamond-like carbon films with different incident energies: a molecular dynamics study
Appl. Surf. Sci.
Formation of chemical short range order and its influences on the dynamic/mechanical heterogeneity in amorphous Zr–Cu–Ag alloys: a molecular dynamics study
Intermetallics
The mechanical properties of a nanoglass/metallic glass/nanoglass sandwich structure
Scr. Mater.
Atomistic simulations of interfacial sliding in amorphous carbon nanocomposites
Compos. Sci. Technol.
Structural models of aC and aC: H
Diam. Relat. Mater.
Formation and evolution of sp 2 clusters in amorphous carbon networks as predicted by molecular dynamics annealing simulations
Diam. Relat. Mater.
Atomic-level structure and structure–property relationship in metallic glasses
Prog. Mater. Sci.
Nature of disorder and localization in amorphous carbon
J. Non-Cryst. Solids
Plastic deformation in metallic glasses
Acta Metall.
A study of the wear mechanism of diamond-like carbon films
Surf. Coat. Technol.
The correlation between shear elastic modulus and glass transition temperature of bulk metallic glasses
Appl. Phys. Lett.
Correlation between average melting temperature and glass transition temperature in metallic glasses
Appl. Phys. Lett.
On the plasticity event in metallic glass
Philos. Mag. Lett.
Local topology vs. atomic-level stresses as a measure of disorder: correlating structural indicators for metallic glasses
Mater. Res. Lett.
Indicators of internal structural states for metallic glasses: local order, free volume, and configurational potential energy
Appl. Phys. Lett.
Radial distribution function and structural relaxation in amorphous solids
Phys. Rev. B
Tribological mechanism of hydrogenated amorphous carbon film against pairs: a physical description
J. Appl. Phys.
A shear localization mechanism for lubricity of amorphous carbon materials
Sci. Rep.
Wear, plasticity, and rehybridization in tetrahedral amorphous carbon
Tribol. Lett.
Evidence of graphitization of diamond-like carbon films during sliding wear
J. Mater. Sci.
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