Thermo-mechanical/chemical properties and contact behaviors of molecularly thin lubricants

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2021-05

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Abstract

Inside a modern day hard-disk drive (HDDs), the gap between the head and the magnetic media has been drastically shrunk down to several nanometers. Therefore, understanding the interaction between the head and the media surface at the head-disk interface (HDI) plays a vital role to ensure a safe design and error-free data reading and writing. As the gap dimension has reduced significantly, the intermittent contact between the head and media surface during HDD operation has become more frequent. Therefore, to protect both surfaces from wearing out each other is one of the most important aspects of proper HDI design. Generally, diamond-like carbon (DLC) is used on the head and disk surfaces as an ultra-thin protective coating due to its high mechanical strength, wear resistance, thermal stability, chemical inertness, and low friction. However, during the HDD operation, it is found that the intermittent contact between the head and disk surfaces may result in carbon overcoat surface damage and eventually read/write errors of the HDD. So, perfluoropolyether (PFPE) lubricant is applied on the disk DLC surface to reduce friction and protect the disk surface from wear and oxidation. To understand the interaction between the DLC surface and the lubricants, it is essential to know the surface properties of the DLC which can eventually help tuning its manufacturing process parameters to achieve the desired surface quality. We performed molecular dynamics (MD) simulations to measure the surface energy of amorphous carbon (a-C) in relation to its hybridization state. To control the sp2/sp3 ratio of a-C, two different approaches were attempted, i.e., changing the quenching rate and the density of carbon atoms. The results showed that the quenching rate did not yield any appreciable change in sp2/sp3 ratio, whereas the number of sp3 bonding was proportional to the density. The measured surface energy showed a non-monotonous correlation with the hybridization state. The trend of surface energy with carbon hybridization state can be interpreted by the combined effects of non-polar interaction (or dispersive component) induced by sp3 bonds and polar interactions by dangling bonds. As mentioned above that, at high-speed surface contact situation during HDD operation at elevated temperature, DLC can go through significant degradation of its mechanical and chemical properties. In our next work we have investigated the thermomechanical contact behavior and its effect on the atomic structure of carbon atoms at the head disk interface by molecular dynamics approach. To improve the surface wear property, PFPE lubricants of different molecular weights and structures have been introduced on the DLC surface and their performance in respect to lubricity has been investigated. Results showed that lubricants have a positive effect on reducing the friction force, wear, and degradation of the carbon atoms. Moreover, we also quantified the amount of lubricant transfer from the disk to the head slider during the scratching which leads us to a better understanding of the lubricity of PFPE lubricants at the atomic level as well as suggests a guideline for lube design for this specific purpose. Now, when these lubricants are exposed to organic and inorganic airborne contaminants at the HDI, the contaminant molecules are attracted onto or adsorbed into the lubricants, which accordingly changes the thermo-chemical/mechanical properties and tribological performance of the lubricants. We investigated the effects of SiO2 contaminants on the airshear properties, friction force, and material transfer of PFPE lubricants. MD simulations were run with two different temperatures, i.e., 300 K and 700 K and it was observed that higher the temperature, the more is SiO2 adsorption into the lubricants. The adsorbed SiO2 particles increased the stiffness of PFPE lubricants leading to less airshear displacement. In addition, the adsorbed SiO2 particles caused to increase the magnitude of friction and the amount of lubricant pick-up during the sliding contact with a nanosized DLC tip.

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Keywords

HDI Contact, Contaminant

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