Thomas E. Karis

Spreading Profiles of Molecularly Thin Perfluoropolyether Films

Perfluoropolyethers (PFPEs) are widely used as lubricants on magnetic recording media. The mobility of the PFPE on the protective carbon overcoat of the media is widely accepted to be intimately coupled to the resulting tribological performance. The flow properties of molecularly thin films of nonpolar PFPEZ and polar PFPE Zdol fractions on solid surfaces were investigated by measuring the spreading profiles. The spreading of Zdol exhibits terraced profiles with the formation of a molecular foot, a shoulder and a vertical step. To describe these features of Zdol spreading, we measured the Zdol thickness dependence of the surface energy, which is then used to calculate the thickness dependence of the disjoining pressure. The polar component of the Zdol surface energy exhibits oscillations as a function of PFPE thickness. The resulting oscillations in the disjoining pressure can be used to qualitatively describe the origins of terraced spreading. The characteristic Zdol spreading profile and surface energy ...

Perfluoropolyether characterization by nuclear magnetic resonance spectroscopy and gel permeation chromatography

Abstract Perfluoropolyethers (PFPEs) are vital lubricants in the aerospace and magnetic recording industry, and they have potential applications in micro-electromechanical and optoelectronic devices. Precise control of their physical properties requires analytical tools, such as nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC) to characterize, chain composition end groups, and molecular weight distribution. The 19 F chemical shifts for Z-, D- and K-type perfluoropolyethers, and six different end groups are compiled into tables within. Application of NMR spectra to derive the number average molecular weight and chain composition is illustrated. A GPC system was developed to measure the perfluoropolyether Zdol molecular weight distributions. Significant variation in the Zdol refractive index with molecular weight was observed in the differential refractometer during GPC. The index variation compared favorably with that calculated from the molar refraction of subunits and end groups. Oligomer peaks found in the low molecular weight tail of the molecular weight distribution are well described by a statistical model. Both the NMR spectroscopy and GPC were simultaneously employed to follow the evolution of the molecular weight distribution during Zdol evaporation.

Surface diffusion of thin perfluoropolyalkylether films

The spreading characteristics of thin polymeric liquid films of perfluoropolyalkylethers (PFPEs) Fomblin Z15 and Fomblin Zdol (hydroxyl terminated PFPE) on silica surfaces have been measured by scanning microellipsometry (SME). We estimated the surface diffusion coefficients and propose a modified diffusion equation to interpret the spreading phenomenon from film thickness profiles measured with SME. We investigated the spreading of Z15 as a function of binary blend ratio of monodisperse Z15 fractions and found that the surface diffusion coefficients of the blends do not obey a simple linear mixing rule. A summary of thin PFPE film spreading characteristics as a function of molecular weight, film thickness, chain‐end functionality, temperature, and humidity is presented.

Surface diffusion and flow activation energies of perfluoropolyalkylether

Surface diffusion of perfluoropolyalkylether (PFPE) Fomblin Z15 and Fomblin Zdol (hydroxyl terminated PFPE) on silicon wafers was investigated over the temperature range of 25 to 50°C using scanning microellipsometry. Zdol exhibits a much lower mobility and a distinctly different thickness profile as compared to Z15. The activation energy for surface diffusion of Zdol is higher than that of Z15, reflecting the stronger affinity of its hydroxyl end groups for the substrate. The viscosity flow activation energy Eη*is compared with that of surface diffusion Ed*yielding Ed*≈ Eη*for Z15, and Ed*≈ 1.5Eη*for Z

Lubricant spin-off from magnetic recording disks

As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10 000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 Å of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 Å thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface.

The relationship between PFPE molecular rheology and tribology

The tribology of several perfluoropolyether (PFPE) lubricants was studied using a pin on disk (POD) test. During the POD test, PFPE is incrementally removed from the track with each sliding cycle. The number cycles to failure, NF, is detected as a sudden increase in the friction coefficient. Molecular theory for polymer melt rheology was employed to develop a universal scaling rule. The PFPE removal rate coefficient is proportional to a parameter containing the bulk viscosity, degree of polymerization, and temperature and structure scaling coefficients. The parameter is a measure of the frictional resistance to segmental sliding along the surface in the contact zone. The temperature scaling coefficient corrects for the absence of free volume in the molecularly‐thin lubricant film. The structure scaling coefficient accounts for differences in the energy barriers to internal rotation. This is the first description of a relationship for the tribological properties of PFPEs that takes into account their viscosity, molecular structure, degree of polymerization, and temperature.

Lubricant Bonding Effects on Thin Film Disk Tribology

The magnetic recording industry predominantly uses Zdol to lubricate the carbon overcoat of magnetic recording disks. Zdol comprises a perfluoropolyether chain terminated with hydroxyl end groups that are capable of reversibly bonding to the carbon overcoat. Contact start/stop (CSS) tests were done to investigate the effects of Zdol lubricant bonding, thickness, and relative humidity on durability. The durability improved with increasing thickness of fully bonded or mobile Zdol. The durability decreased with increasing initial bonded fraction and with decreasing relative humidity. The bonded fraction increased with time during the tests at elevated temperature and low relative humidity. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1–4, 2000

An overview of rheology in the computer industry

The rheological characterization of a wide variety of materials used in computer hardware is described. The materials and applications presented include liquid crystal polymers for components, solder paste for surface mounting of integrated circuits, screen printable polyimide for dielectric patterns, new photosensitive polyimide to reduce processing steps, hot-melt adhesive for temporary bonding, pressure-sensitive adhesive for permanent bonding of optical recording disks, bearing grease for magnetic recording disk drive spindles, fluoropolymer lubricants for magnetic recording disks, magnetic particle suspensions for magnetic tape and disks, toner for laser printing, thermoplastic polymer for rapid prototyping, and cathode paste for rechargeable lithium batteries. Rheological tests appropriate for each of the materials were designed to provide key information about its performance in the intended application. This overview provides insight into the relation between interpretation of rheological test data and materials performance in engineering applications as well as for process control. Rheology is essential to the development of computer hardware and peripheral devices.

Pin-on-disk tribology of thin-film magnetic recording disks

Tribological studies of friction and wear were performed on carbon overcoated thin‐film magnetic recording media with pin‐on‐disk tests. Scanning microellipsometry was employed to measure the wear of carbon overcoats on rigid magnetic media. Severe wear produced after the carbon film wore through was measured by mechanical profilometry. The wear rate of the carbon is three orders of magnitude lower than that of the underlying metallic layers and is nearly proportional to slider load. Micrographs of the wear tracks and the magnitude of the wear coefficient from the Archard wear equation indicate three‐body abrasive wear.

Oxidation Chemistry of a Pentaerythritol Tetraester Oil

Synthetic oils have come into widespread use due to their inherent stability, consistent physical properties, and reproducible composition. As ever increasing demands are being placed on oil performance in magnetic recording disk drives by increasing rotation rates up to 10,000 rpm, it is important to know the ultimate limitations of these functional fluids due to oxidation; In this study, the authors focus on a pentaerythritol ester oil. Accelerated aging tests were carried out on the oil at elevated temperature. Tests were also done with 50 ppm of dissolved iron in the oil. The progress of oxidation was followed by analytical techniques including UV/visible, infrared, and proton nuclear magnetic resonance spectroscopy, and gel permeation chromatography, thin layer chromatography, viscometry, and differential scanning calorimetry. A new technique of laser desorption mass spectrometry with jet cooling, which provides the parent ion mass spectrum, provided the mass distribution of intermediate oxidation pr...