C. Mathew Mate

Molecular conformation and disjoining pressure of polymeric liquid films

Atomic force microscopy, angle resolved x‐ray photoelectron spectroscopy, and ellipsometry are applied to study the conformation of fluorocarbon polymers in molecularly thin liquid films, 5–130 A thick, on solid surfaces. The combination of these techniques shows that the physisorbed polymers at the solid surface have an extended, flat conformation. In addition, the disjoining pressure of these liquid films is determined from atomic force microscopy measurements of the distance needed to break the liquid meniscus that forms between solid surface and force microscope tip. For a monolayer thickness of ∼7 A, the disjoining pressure is ∼5 MPa, indicating strong attractive interaction between the polymer molecules and the solid surface. The disjoining pressure decreases with increasing film thickness in a manner consistent with a strong attractive van der Waals interaction between the liquid molecules and the solid surface.

Atomic force microscopy of polymeric liquid films

We demonstrate the use of the atomic force microscope (AFM) for studying perfluoropolyether polymer liquid films as thin as ∼20 A. With the AFM we are able to measure three distinct properties of the liquid film: (1) its thickness when the thickness of liquid on the AFM tip is taken into account, (2) the meniscus force acting on the AFM tip as a function of depth into the liquid film, and (3) the topography of the liquid/air interface. All three of these measurements can be done with a very high lateral resolution, ∼1000 A, demonstrating the unique capability of AFM for studying liquid films. With AFM we have observed several interesting properties of these polymeric liquid films. First films thinner than ∼300 A are fairly uniformly distributed, while films thicker than ∼300 A slowly dewet the surface. Second, by measuring the meniscus radius of liquid in a micron sized hole on the surface, we can determine the disjoining pressure in a thin liquid film.

Force microscopy with a bidirectional capacitance sensor

A new method for sensing cantilever deflection in the atomic force microscope (AFM), based on capacitance measurement, is described. Parameters governing the design of such an instrument are considered in detail. Two different geometries are compared, wire on plate and an integrated flat plate sensor. The electronic circuitry, providing 6×10−19 F noise in a 0.01–1000 Hz bandwidth, is also described. Implementation of the design ideas into a working AFM in ultrahigh vacuum is demonstrated. This AFM allows simultaneous measurement of cantilever deflection in two orthogonal directions, necessary for our nanotribology studies. The theoretical sensitivity of 5×10−7 F/m is not achieved due to roughness. The bidirectional sensing and imaging capabilities are demonstrated for an Ir tip on cleaved graphite, and a diamond tip on diamond films. The capacitance detection technique is compared and contrasted with other AFM sensors.

Application of disjoining and capillary pressure to liquid lubricant films in magnetic recording

The behavior of liquid lubricant films on the disk surfaces in magnetic recording is analyzed using the concepts of disjoining and capillary pressure. Particular examples analyzed include lubricant distributions on rough and porous disk surfaces, lubricant flow, and the meniscus force acting on the recording slider sitting on the disk surface.

Nanotribology studies of carbon surfaces by force microscopy

Abstract The frictional properties of different types of carbon surfaces, graphite, diamond, hydrogenated amorphous carbon (a-C:H), and C60 buckminsterfullerene, are studied in air at low loads (below 1.5 μN) by friction force microscopy. The different types of carbons have frictional coefficients varying from 0.01 to 0.8, demonstrating the ability of friction force microscopy to distinguish between the different types of carbon. For a-C:H carbon on a magnetic disk surface, the surface topography greatly influences the observed friction, complicating the interpretation of the friction force for this practical surface.

Direct measurement of forces during scanning tunneling microscope imaging of graphite

Abstract The normal force acting on a scanning tunneling microscope tip while imaging a graphite surface in air has been measured directly. Forces in the range of 10 −7 to 10 −6 N are required to achieve tunneling. Further, the force needed to maintain a constant current varies considerably as the tip scans from one part of the graphite unit cell to another. Our results are consistent with a model, originally suggested by Mamin et al., in which the force between the tip and the surface is mediated by a contamination layer, and tunneling occurs at the end of an asperity which pierces this layer. However, we cannot rule out a model where a graphite flake is dragged across the graphite surface to generate an STM image.

Air shear driven flow of thin perfluoropolyether polymer films

We have studied the wind driven movement of thin perfluoropolyether (PFPE) polymer films on silicon wafers and CNx overcoats using the blow-off technique. The ease with which a liquid polymer film moves across a surface when sheared is described by a shear mobility χS, which can be interpreted both in terms of continuum flow and in terms of wind driven diffusion. Generally, we find that the movement of PFPE films can be described as a flow process with an effective viscosity, even when the film thickness is smaller than the polymer’s diameter of gyration. Only in the special case of sparse coverage of a polymer with neutral end groups is the motion better described by a wind driven diffusion process. The addition of alcohol end groups to the PFPE polymer chain results in strong interactions with the substrate, creating a restricted layer having an effective viscosity an order of magnitude larger than the mobile layer that sits on top of the restricted layer.

Coverage and properties of a-SiNx hard disk overcoat

Amorphous silicon nitride (a-SiNx) overcoats are deposited on magnetic disks by rf-reactive sputtering to study their coverage and properties. According to the x-ray photoelectron spectroscopy analysis, a-SiNx has a low coverage-limit of ∼10 A compared with that of the reference a-CNx (∼20 A). The lower coverage-limit of a-SiNx may be attributed to its high density of 3.2 g/cm3, which corresponds to 93% bulk density. By contrast, the density of diamond-like carbon is only 54% that of diamond. This is in agreement with the results of coverage simulation, which reveal that the film coverage thickness decreases by ∼3 A per 10% increase in the relative density. Compared with 45 A a-CNx coated disks, 15 A a-SiNx coated disks have fewer pinhole defects and are more durable in the accelerated flyability test. The superior performance of a-SiNx disk overcoat may be attributed to its dense structure and high hardness (25 GPa).

Nanotribology of lubricated and unlubricated carbon overcoats on magnetic disks studied by friction force microscopy

Abstract Friction force microscopy is used to study the frictional properties of lubricated and unlubricated hydrogenated amorphous carbon overcoats deposited on magnetic disks used in hard disk drives. For carbon overcoats deposited on smooth disks, a strong correlation is found between topography and friction, with the peaks on the surface having low friction and the valleys having high friction. The correlation between friction and topography is attributed to the variation in van der Waals adhesive forces with topography. For carbon overcoats on strongly textured magnetic disks, the correlation between topography and friction is substantially reduced. For both types of disk, the addition of a perflouropolyether lubricant film substantially reduces the magnitude and variation of the frictional forces.

Roughness of molecularly thin perfluoropolyether polymer films

X-ray reflectivity has been used to measure the roughness of perfluoropolyether (PFPE) polymer films on silicon substrates and carbon overcoats. For PFPE on smooth silicon, we find that the rms roughness of the PFPE–air interface increases slowly from about 2 to 4 A as PFPE thickness increases from 5 to 33 A. This increase is consistent with capillary waves roughening the polymer film, but inconsistent with current theories for the dewetting of polymer films. For PFPE on the rougher surface of amorphous hydrogenated carbon, we find that the PFPE polymer smoothes the surface with the rms roughness decreasing from 9 to 4 A. We also discuss the implications of these results on the limits of disk drive technology.