Haigang Chen

Effects of Molecular Structure on the Conformation and Dynamics of Perfluoropolyether Nanofilms

With the increase of areal density of magnetic recording, several new perfluoropolyether (PFPE) lubricants, including A20H, DDPA-S, and Ztetraol multidentate, have been recently reported to enhance the performance and reliability of hard disk drive with ultra-low head-media spacing. At the molecular level, the static conformation and detailed dynamic structure of these PFPE films are still not well-determined. In this paper, molecular dynamics simulations with bead-spring models were employed to investigate the conformation and dynamics of new types of PFPE films. The detailed static structures were revealed by functional end bead density profiles and visualization. The distribution of anisotropic radius of gyration was examined to investigate the conformations of PFPE monolayers. And the self-diffusion coefficient quantifying the mobility of films was compared for these PFPE monolayers. It was found that the molecular structure played a critical role in the conformation and dynamics of PFPE thin films

Molecular rheology of perfluoropolyether lubricant via nonequilibrium molecular dynamics simulation

Rheological properties of perfluoropolyether (PFPE) systems can be particularly important in designing effective lubricants that control the friction and wear in their tribological applications. Using equilibrium/nonequilibrium molecular dynamics (MD) simulation, we examined the rheological properties of PFPE lubricant, such as viscosity and viscoelastic properties, by observing the time-dependent system in shear motion. Strong dependence of PFPE bulk viscosity on PFPE molecular structure (e.g., end group functionality) and external conditions (e.g., temperature) was observed. Via a nonlinear regression method, we further extracted the relaxation time spectrum based on our simulated dynamic moduli data, where the importance of end group couplings was emphasized. In addition, we adopted two solid surfaces to mimic the head and disk, which will provide a further understanding in lubricant nanorheology and nanotribology

Surface energy and adhesion of perfluoropolyether nanofilms on carbon overcoat: The end group and backbone chain effect

In this paper, we have investigated the surface energy and adhesion of one functional PFPE (Zdol) and two series of nonfunctional PFPEs (Z and D) on carbon-overcoated disk surfaces. The effects of end group functionality, backbone chain flexibility, molecular weight, and film thickness were systematically examined. Our results indicated that nonfunctional PFPEs have weak attraction with carbon overcoat. However, due to backbone chain effect, Z has slightly stronger attraction than D. Based on the surface energy analyses and bonded thickness results, schematic bonding models were proposed, which indicate strong hydrogen bonding∕ordered packing structure∕low mobility for functional PFPE films and weak attraction∕less-ordered packing structure∕high mobility for nonfunctional PFPE films.

A wavelet/moment analysis for the surface morphology of lubricant films

Molecular dynamics simulations using a coarse-grained, bead-spring model were performed to generate the morphology of submonolayer and monolayer lubricant films. Different morphological states, such as isolated islands, connected perfluoropolyether bridge network, and monolayers with defects were found for various surface coverage. We introduced the Kohlrausch–Williams–Watts function and the wavelet transform as well as calculated several parameters such as the correlation length, roughness exponent, and wavelet entropy to analyze the surface morphologies. A strong correlation was found between the surface morphologies and transport properties via correlating the parallel component of the self-diffusion coefficient with the wavelet entropy per molecule. Two distinct correlations at low and high surface coverage regions were found where the transition between these two regions depends on the number of beads per molecule and endgroup functionality.

Relationship between surface coverage and end group functionality of molecularly thin perfluoropolyether films

The relationship between surface coverage and film thickness for perfluoropolyether (PFPE) films with different functional end groups was investigated by measuring the dispersive surface energy. It was found that the strength of end group functionality played an important role in the surface coverage; i.e., PFPE with the stronger end group functionality has the smaller surface coverage. Molecular dynamics simulations with a bead-spring model were employed to investigate the conformation of PFPE films, where the anisotropic radii of gyration were analyzed as a function of the strength of end group functionality. It was found that PFPE became flatter if the strength of end group functionality decreased, which, in turn, increased the surface coverage.

Viscoelastic liquid bearing modeling via lattice Boltzmann method

As the need for higher areal recording density in hard disk drives (HDDs) requires continuous reduction of the fly height in head-disk interface, the tribological issues become critical during operation. In viscoelastic liquid bearing (VLB) technology, thin liquid films instead of air bearing are used for lubrication. In this paper, we developed a computational tool based on lattice Boltzmann method (LBM), which is accurate, fast, robust, and highly parallelizable, for the accurate modeling of a non-Newtonian ultrathin liquid film undergoing extremely high shear rates. We adopted truncated power-law and the Bird-Carreau models to take into account the shear rate dependent viscosity via the relaxation time in our LBM scheme. After the feasibility study of the pressure driven microchannel, the pressure and flow fields under the model slider were calculated for different constitutive relationships. Together with the order of magnitude analyses, our LBM simulations, using a physically realistic viscoelastic c...

Molecular dynamics simulation of binary mixture lubricant films

Due to the requirements for ultra-small head-media spacing hard disk drives (HDDs), the mixture of lubricants may become feasible and promising alternative for future HDD. In this paper, molecular dynamics simulations with a bead-spring model were employed to examine the detailed structure, conformation, and dynamics of binary mixture lubricant films by analyzing the anisotropic radius of gyration and the self-diffusion coefficient as a function of volume fraction. Our simulation results indicate that the binary mixture monolayer can be more suitable as a disk lubricant in comparison with the single component. The conformation and mobility change by tuning the volume fraction was discussed in conjunction with the optimal lubricant selection.

Ultraviolet Bonding of PFPE With Carbon Overcoat: Surface Energy and Spreading

Ultraviolet (UV) irradiation effects on the functional perfluoropolyether (PFPE), i.e., Ztetraol films, were investigated by studying the bonded film thickness as well as the surface energy of PFPE lubricated media using Lifshitz-van der Waals and Lewis acid/base (LW/AB) theory. It was found that short-time UV irradiation enhanced the bonding of Ztetraol film with ion beam deposited carbon overcoat, which is attributed to the direct photodissociation of PFPE molecules. Long-time UV irradiation degraded PFPE chain and caused the loss of lubricant by evaporation. L-t plot was further employed to analyze the spreading behaviors of Ztetraol films, where L represents the distance of the leading edge of the lubricant film travels in the time t. With UV irradiation, the spreading of Ztetraol films was retarded possibly due to the enhancement of bonding of Ztetraol with overcoat and cross-linking between Ztetraol molecules.

Lifshitz-van der Waals and Lewis Acid-Base Approach for Analyzing Surface Energy of Molecularly Thin Lubricant Films

The dispersive and polar components of surface energy, which are typically obtained from the two-liquid geometric method through contact angle measurement, have been extensively used to study the interfacial properties of perfluoropolyether (PFPE) lubricated media used in hard disk drive industry. In this paper, an alternative approach, Lifshitz-van der Waals and Lewis acid-base (LW/AB) theory, which provides additional information on the nature of lubricant/overcoat interaction, has been applied to the study of the surface energy of PFPE-coated media. Here, the surface energy is decomposed into Lifshitz-van der Waals (LW), Lewis acid, and Lewis base components. The results of Lewis acid and Lewis base components of surface energy suggest that the bonding of functional PFPEs with a carbon overcoat is a Lewis acid-base interaction; the overcoat is an electron donor and functional PFPEs are electron acceptors. It was also found that the LW surface energy has a sequence of Ztetraol<Zdol<AM2001<A20H<Z03 for films with the same thickness, which is in the reverse sequence as found in the bonded thickness of these PFPEs. A molecular-level bonding mechanism is proposed to explain the experimental results

A novel simulation of air∕liquid bearings based on lattice Boltzmann method

A novel three dimensional (3D) simulation tool based on lattice Boltzmann method (LBM), which is more efficient than the conventional computational fluid dynamics approach, was developed to analyze the head∕disk interface (HDI) dynamics. For handling general geometrical structures, Taylor series expansion and least squares based LBM was employed to simulate the flow conditions under a model slider. Truncated power-law and Bird–Carreau models were incorporated to capture the non-Newtonian fluid flow behavior of lubricant liquid film undergoing extremely high shear rates by modifying the relaxation time as a function of shear rate. After the benchmark study of two dimensional∕3D Poiseuille flows, pressure and shear profiles under the model slider were calculated for different constitutive relationships. The results indicate that our LBM techniques could be an attractive computational tool for next generation HDI design.