Modeling and simulation of depletion kinetics and mechanisms of nanostructured polymeric lubricants

• Main Author: Li, Bei
• Other Authors: Wong Chee How
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics
• Online Access: http://hdl.handle.net/10356/63644
• Institution: Nanyang Technological University

Nanostructured perfluoropolyether (PFPE) films have drawn great interest in micro- and nano-electro-mechanical systems. In particular, PFPEs are commonly used as lubricant layers to reduce friction and wear between flying heads and rotating disks in hard disk drives (HDDs). However, the need for ultrahigh areal data densities of HDDs requires a decrease in flying height and the magnetic grain size, which brings about many critical stability issues related to the use of PFPEs at the head disk interface (HDI). Therefore, the main research work of the PhD project is to investigate the tribological and thermal depletion behaviors of ultrathin PFPEs using molecular dynamics (MD) simulations. Under infinitesimally low flying height (< 5 nm), the effect of intermolecular interaction between PFPE and the slider/disk can no longer be ignored and its effect on the lubricant redistribution at near-contact HDI was studied. It was shown that the lubricant film will lose its stability to form menisci between the slider and disk when the distance between these two surfaces is lower than a critical value dc. The Hamaker theory application to van der Waals force was also introduced to validate the MD results. In both methods, the critical separation dc increases approximately linearly with the lubricant thickness for slider surface with or without roughness. Moreover, the MD simulations revealed that the lubricant meniscus formation depends largely on the lubricant thickness and the slider-to-disk separation, but less so on the slider roughness. Understanding the performance of PFPEs under femtosecond laser irradiation is of great importance in enhancing the stability and reliability of heat-assisted magnetic recording HDDs. In the simulations, thermal-induced lubricant depletion under static laser heating was first investigated. It revealed that the lubricant undergoes severe depletion with an increase in laser heating duration, giving rise to aggravated lubricant evaporation and raised ridges. During cooling process, the strong polar interactions between end groups and disk hinder the recovery and redistribution of depleted lubricant chains, resulting in an undersaturated lubricant film. Moreover, the effects of laser scanning velocity, laser power, laser spot size, and lubricant film thickness on the lubricant depletion under moving laser irradiation were explored. It was shown that the lubricant depletion can be significantly reduced at large scanning velocities. For a given spot size, the lubricant evaporates with an increasing rate as the laser power rises. However, the total lubricant depletion initially increases with the laser power, and then remains almost constant as the laser power approaches a threshold value before continuing its exponential increasing trend. For a given laser power, a critical laser spot size was observed, at which maximum depletion and thermodiffusion occur at the film, while the lubricant evaporation steadily decreases with increasing spot size. Additionally, the lubricant with a bilayer structure has a much larger depletion depth and a rather smaller width when compared to the lubricant film with thickness of around one monolayer. Although evaporation is significantly enhanced at large laser powers or small laser spot sizes, thermodiffusion is the primary mode of lubricant depletion under moving laser heating. This thesis also details the investigation of the surface depletion kinetics of PFPEs under heat treatment. During rapid heating, the lubricant decomposes at an increasing rate with temperature T, while a maximum rate was observed during desorption due to the influence of lubricant-to-disk interaction. During the isothermal stage, the rate constants for lubricant desorption and decomposition were calculated based on a first-order, coverage independent kinetics-controlled reaction model. The kinetics of lubricant depletion showed that lubricant desorption is favored over decomposition at high temperatures and is the major cause of lubricant degradation and failure on a solid surface. Additionally, it was found that the end group functionality of PFPEs can greatly influence lubricant desorption, but show little effect on lubricant decomposition.