Description

One of the core technologies in the design and manufacture of the next-generation hard disk drives is the head-disk interface (HDI). The design of HDI must provide sufficient stability and durability for tens of thousands of hard drive start/stop cycles. However, the intermittent contacts between the head and disk are often unavoidable. To avoid and minimize disk damage, the surface of hard drive disks is often protected by a diamond-like carbon (DLC) coating, which is in turn covered by a thin layer of ZDOL lubricants. When the head and disk are at a close proximity, e.g., 3 nm, intermittent contacts during the operation can create large impact forces and generate heat, which in turn can cause serious lubricant damage and depletion. Hence, a comprehensive understanding of the structural and mechanical properties of ZDOL lubricants on a DLC overcoat is essential to ensure a stable and reliable HDI. We report our recent work on all-atom MD simulations on a ZDOL polymer film/diamond-like carbon substrate system. We present our computational model, which includes the construction of the DLC substrate and ZDOL lubricant layer, the atomic potentials, the simulation procedures, and the boundary conditions. Using this model, we first analyze the structural and thermo-mechanical properties of the ZDOL polymer film, focusing on the formation of layered structure, the anisotropic radius of gyration, the anisotropy of lubricant diffusion coefficient, and the activation energies for lubricant diffusion. Nanoindentation has been widely used to characterize the mechanical properties of multicomponent polymer materials. We perform molecular dynamics simulations to investigate the mechanical response of ZDOL film on DLC by nanoindentation. It is found that significant atomic reshufflings occur at the surface layer of the polymer in the stage from approaching to small contact force. These atomic motions, which arise from different van der Waals interaction profiles between different atomic species, not only change surface adhesion strength, but also generate atomic level plasticity. We also perform molecular dynamics simulations to investigate the nanoscale frictional behavior of a ZDOL film sandwiched between two DLC coatings. We show that the ZDOL film behaves like a solid and can perform either a motion-station movement or a continuous motion with a fluctuating velocity. The friction traction follows the modified Amonton’s law but fluctuates violently at low contact pressures. The magnitude of atomic level friction forces is found to be highly nonuniform, and their directions can even be opposite. This study reveals interesting insights into the structures and mechanical and frictional behavior of ZDOL film/DLC substrate system and provides useful guidelines for the design of nanoscale lubricant systems.

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Molecular dynamics simulations of the structures and mechanical properties of ZDOL polymer films on diamond-like carbon

One of the core technologies in the design and manufacture of the next-generation hard disk drives is the head-disk interface (HDI). The design of HDI must provide sufficient stability and durability for tens of thousands of hard drive start/stop cycles. However, the intermittent contacts between the head and disk are often unavoidable. To avoid and minimize disk damage, the surface of hard drive disks is often protected by a diamond-like carbon (DLC) coating, which is in turn covered by a thin layer of ZDOL lubricants. When the head and disk are at a close proximity, e.g., 3 nm, intermittent contacts during the operation can create large impact forces and generate heat, which in turn can cause serious lubricant damage and depletion. Hence, a comprehensive understanding of the structural and mechanical properties of ZDOL lubricants on a DLC overcoat is essential to ensure a stable and reliable HDI. We report our recent work on all-atom MD simulations on a ZDOL polymer film/diamond-like carbon substrate system. We present our computational model, which includes the construction of the DLC substrate and ZDOL lubricant layer, the atomic potentials, the simulation procedures, and the boundary conditions. Using this model, we first analyze the structural and thermo-mechanical properties of the ZDOL polymer film, focusing on the formation of layered structure, the anisotropic radius of gyration, the anisotropy of lubricant diffusion coefficient, and the activation energies for lubricant diffusion. Nanoindentation has been widely used to characterize the mechanical properties of multicomponent polymer materials. We perform molecular dynamics simulations to investigate the mechanical response of ZDOL film on DLC by nanoindentation. It is found that significant atomic reshufflings occur at the surface layer of the polymer in the stage from approaching to small contact force. These atomic motions, which arise from different van der Waals interaction profiles between different atomic species, not only change surface adhesion strength, but also generate atomic level plasticity. We also perform molecular dynamics simulations to investigate the nanoscale frictional behavior of a ZDOL film sandwiched between two DLC coatings. We show that the ZDOL film behaves like a solid and can perform either a motion-station movement or a continuous motion with a fluctuating velocity. The friction traction follows the modified Amonton’s law but fluctuates violently at low contact pressures. The magnitude of atomic level friction forces is found to be highly nonuniform, and their directions can even be opposite. This study reveals interesting insights into the structures and mechanical and frictional behavior of ZDOL film/DLC substrate system and provides useful guidelines for the design of nanoscale lubricant systems.