Juai Ruan

Atomic-Scale Friction Measurements Using Friction Force Microscopy: Part 1--General Principles and New Measurement Techniques

Abstract : Friction force measurements using modified atomic force microscopy, called here Friction Force Microscopy (FFM), are becoming increasingly important in the understanding of fundamental mechanisms of friction, wear, and lubrication, and to study interfacial phenomena in micro- and nanostructures used in magnetic storage systems and Microelectromechanical Systems (MEMS). FFMs can be used to study engineering surfaces in dry or wet conditions. A review of existing designs of FFMs and methods of friction force measurements is presented. In terms of friction force measurements, there are important issues related to the basic operation and calibration of these instruments which have not been fully studied. A new method of measuring friction fore using a commercial FFM and a calibration Procedure for conversion of measured data to normal and friction forces are presented. Microscale friction data of selected materials are presented and discussed in light of macrofriction measurements.

Atomic‐scale and microscale friction studies of graphite and diamond using friction force microscopy

Friction between graphite and diamond surfaces against a sharp silicon nitride tip was measured using a friction force microscope (FFM). Atomic‐scale friction images of a freshly cleaved highly oriented pyrolytic graphite exhibited the same periodicity as that of the graphite surface; however, the peaks in friction profiles and those in corresponding topography profiles were displaced relative to each other. Using the Fourier expansion of the interaction potential, the conservative interatomic forces between the FFM tip and the graphite surface have been calculated. It is shown that the variations in atomic‐scale friction and the observed displacement between the peaks in the frictional (or lateral) force and those in the corresponding topography can be explained by the variations in interatomic forces in the normal and lateral directions. Thus, the observed variation in friction force may not necessarily occur as a result of the commonly believed atomic‐scale stick‐slip process, but can be due to variati...

Frictional behavior of highly oriented pyrolytic graphite

Friction of a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface and a roughened graphite surface against a silicon nitride tip is measured using a friction force microscope. The cleaved graphite surface is an atomically smooth surface of (0001) plane with a small fraction of line‐shaped regions. It is observed that the coefficient of friction is extremely small (<0.006) for a freshly cleaved HOPG surface of (0001) plane. However, the coefficient of friction is more than an order of magnitude larger in some line‐shaped regions on the surface compared with that of smooth regions of (0001) planes. Transmission electron microscopy analysis indicates that the line‐shaped regions consist of graphite planes of different orientations [other than (0001)] as well as amorphous carbon. This result suggests that (0001) graphite planes have the lowest coefficient of friction in comparison with other surface orientations or amorphous carbon. The coefficient of friction of a roughened graphite surface is ...

Tribological properties of polished diamond films

Despite the rapid progress being made in the synthesis of diamond films and recent interest in polishing of diamond films, no systematic measurements of friction and wear on polished diamond films have been reported. In the present study, chemomechanical and laser polishing techniques are used, and friction and wear data on the chemomechanically polished diamond films are presented. With the chemomechanical polishing technique used in this study, the rms roughness of hot filament chemical vapor deposited diamond films can be reduced from about 657 to about 170 nm with rounding off of sharp asperities with no change in the diamond structure. The polished films exhibit coefficient of friction (∼0.1) and wear rates much lower than that of unpolished films. Friction and wear properties of the polished films are comparable to that of single‐crystal natural diamond. Based on this study, it is concluded that polished films are potential candidates for tribological applications.

Atomic-Scale Friction Measurements Using Friction Force Microscopy: Part II—Application to Magnetic Media

Abstract : Atomic Force/Friction Force Microscopes (AFM/FFM) were used to study tribological properties of metal-particle tapes with two roughnesses, Co-gamma Fe2O3 tapes (unwiped and wiped), and unlubricated and lubricated thin-film magnetic rigid disks (as-polished and standard textured). Nanoindentation studies showed that the hardness of the tapes through the magnetic coating is not uniform. These results are consistent with the fact that the tape surface is a composite and is not homogeneous. Nanoscratch experiments performed on magnetic tapes using silicon nitride tips revealed that deformation and displacement of tape surface material occurred after one pass under light loads (approx. 100 nN). A comparison between friction force profiles and the corresponding surface roughness profiles of all samples tested shows a poor correlation between localized values of friction and surface roughness. Detailed studies of friction and surface profiles demonstrate an excellent correlation between localized variation of the slope of the surface roughness along the sliding direction and the localized variation of friction. Atomic-scale friction in magnetic media and natural diamond appears to be due to adhesive and ratchet (roughness) mechanisms. Directionality in the local variation of atomic-scale friction data was observed as the samples were scanned in either direction, resulting from the scanning direction and the anisotropy in the surface topography. Atomic-scale coefficient of friction is generally found to be smaller than the macro coefficient of friction as there may be less ploughing contribution in atomic-scale measurements.

Nanoindentation studies of sublimed fullerene films using atomic force microscopy

Nanoindentation studies of sublimed fullerene films have been conducted using an atomic force microscope (AFM). Transfer of fullerene molecules from the as-deposited films to the AFM tip was observed during the indentation of AFM tip into some of the samples, whereas such a transfer was not observed for ion-bombarded films. The fullerene molecules transferred to the AFM tip were subsequently transported to a diamond surface when the diamond sample was scanned with the contaminated tip. This demonstrates the capability of material manipulation on a molecular scale using AFM. Atomic-scale friction of the fullerene films was measured to be low. Ability of fullerene films to form transfer film on the mating AFM tip surface may be partly responsible for low friction.

Tribological performance of thin film amorphous carbon overcoats for magnetic recording rigid disks in various environments

Abstract An ultrahigh vacuum tribotester equipped with Auger electron analyzer and mass spectrometer is used to study the friction and wear behavior of amorphous carbon coated magnetic thin film rigid disks. The tests include continuous sliding of Al 2 O 3 -TiC sliders against lubricated and unlubricated polished and textured disks. Operating environment is found to play an important role in the tribological performance of the carbon overcoat. Wear lives were longest in dry nitrogen and argon environments as compared with ambient environment (with oxygen and water vapor), in agreement with some of the previously reported results. The wear lives of the carbon coatings in vacuum were inferior to those in other environments. In comparison, surface roughness and lubrication play a less significant role in friction and wear for the disks tested. Based on this study, we conclude that poor wear life in vacuum results from intimate slider-to-disk contact. The long life in dry Ar and nitrogen as compared with Ar+O 2 and ambient environment, results from the absence of tribochemical oxidation prevalent in the oxidizing environment.

A scanning tunnelling microscopy study of fullerene films

Atomic-scale and micro-scale images of C60 (fullerene) films using scanning tunnelling microscopy (STM) are presented. The atomic configuration of a C60 molecule has been clearly observed. Solvated fullerene molecules appear to pack in a hexagonal array on gold-coated freshly cleaved mica substrate, but they do not follow any long range order. STM images of ion-bombarded C60 films show that the bombarded films ar smoother and possibly denser than the as-deposited films. Tribological tests on as-deposited and ion-bombarded films show that these films exhibit low friction and wear that are comparable to those of best known solid lubricants. STM images of a worn film (which has been rubbed against a steel ball) show that nearly spherical wear particles are formed which may roll in a sliding contact resulting in low friction and wear.