A Reactive Force Field Study on the Interaction of Lubricant with Diamond-Like Carbon Structures

Abstract

We studied the degradation of a perfluoropolyether lubricant, i.e., D4OH, in the presence of different components present in hard disks including diamond-like carbon (DLC), using ReaxFF reactive force field based molecular dynamics simulations. To generate the DLC structure, a carbon-diamond structure was melted in an Ar-filled head space box at 7500 K, and was slowly cooled down to 3000 K. Then, a constant pressure (NPT) simulation was performed to adjust the sample volume and reduce the internal stress. Similarly, for growth of functionalized DLC, ethylene molecules were used as the carbon source and they were pyrolyzed in the presence of Ar atoms to make Hfunctionalized DLC (DLC:H). This DLC:H structure was subsequently heated in the presence of nitrogen molecules to make H/N functionalized DLC (DLC:H:N). The results indicate that, by controlling number of Ar atoms and N 2 molecules, it is possible to achieve the experimental H/N/C composition and sp 2 /sp 3 ratio composition targets. Final DLC and DLC:H:N structures have an sp 3 :sp 2 ratio of 27.3, and 31.7%, respectively. Hydrogen and nitrogen contents of 17.9 and 13.7%, respectively, were obtained. After preparing the DLC structures, the spreading mechanism of lubricant droplet containing 9 D4OH strands was investigated over DLC and DLC:H:N structures. It was found that the speed of lubricant spreading over the nonfunctional structure (DLC) is higher than that for the functional structure (DLC:H:N). Moreover, our results indicated that the presence of both DLC and DLC:H:N affects the degradation chemistry of the lubricant.

ReaxFF Reactive Molecular Dynamics Simulations of Mechano-Chemical Decomposition of Perfluoropolyether Lubricants in Heat-Assisted Magnetic Recording

“…Compared with the density functional theory (DFT) and reactive coarse-grained MD, , reactive MD with ReaxFF − force fields is a cost-effective and accurate method providing atomistic-level details of dynamics of systems including chemical reactions, typically more than five orders of magnitude less costly than DFT calculations . Performing ReaxFF MD simulations, Lotfi et al studied the decomposition of polar PFPE D-4OH at elevated temperatures on diamond-like carbon (DLC) surfaces and in the presence of oxygen, water, and nanoparticles of SiO 2 , FeO­(OH), and Fe 2 O 3 . Their simulation results indicated that the PFPE decomposition is accelerated by DLC, water, and the oxide nanoparticles, whereas the effect of oxygen molecules is less significant.…”

“…33 Performing ReaxFF MD simulations, Lotfi et al studied the decomposition of polar PFPE D-4OH at elevated temperatures on diamondlike carbon (DLC) surfaces 15 and in the presence of oxygen, water, and nanoparticles of SiO 2 , FeO(OH), and Fe 2 O 3 . 26 Their simulation results indicated that the PFPE decomposition is accelerated by DLC, water, and the oxide nanoparticles, whereas the effect of oxygen molecules is less significant.…”

“…Molecular dynamics (MD) simulations have also been used to study properties of PFPE films. ,− Particularly, PFPE decomposition has been studied by reactive MD simulations. Compared with the density functional theory (DFT) and reactive coarse-grained MD, , reactive MD with ReaxFF − force fields is a cost-effective and accurate method providing atomistic-level details of dynamics of systems including chemical reactions, typically more than five orders of magnitude less costly than DFT calculations .…”

“…Although high temperature and the shear rate are easily achieved in simulations, the heating time is somewhat long for reactive MD simulations. Even with the ReaxFF method, 1 ns simulations are still computationally expensive because a time step as small as ∼0.1 fs is typically required for the accurate simulation of reaction processes. , As a compromise, temperatures obviously higher than the actual conditions are usually used to accelerate reactive MD simulations.…”

Microtribological Properties of DLC Films Prepared by Different Methods

“…In addition, they also tested the stick-slip phenomenon when the prove tip was slipped over the graphite and mica surface. Since then, numerous studies [16][17][18] have distinguished the characteristics of microtribology from macrofriction. Nonetheless, the microtribology mechanism remains poorly understood.…”