Elsevier

Acta Materialia

Volume 46, Issue 11, 1 July 1998, Pages 3793-3804
Acta Materialia

Micro/nanoscale friction and wear mechanisms of thin films using atomic force and friction force microscopy

https://doi.org/10.1016/S1359-6454(98)00062-7Get rights and content

Abstract

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales ranging from atomic and molecular to microscales. At most solid–solid interfaces of technological relevance, contact occurs at numerous asperities; a sharp AFM/FFM tip sliding on a surface simulates just one such contact. However, asperities come in all shapes and sizes. To study the effect of tip radius, experiments are conducted using an AFM tip with radii ranging from about 50 nm to 14.5 μm. The effect of the radius on adhesive forces and coefficient of friction at different relative humidities is measured. It is found that adhesive forces at low humidities do not change with tip radius whereas these increase with tip radius at high humidities. Coefficient of friction increases with the tip radius at all humidities. Samples coated with perfluoropolyether lubricant, which are hydrophobic in nature, are less sensitive to the environment and the tip radius. It appears that hydrophobicity of liquid films can be studied using large radii tips. Another objective of this study is to understand material removal mechanisms on microscale. Silicon surfaces are micromachined using a sharp diamond tip in an AFM. It is found that wear rate as well as the coefficient of friction is negligible up to a certain load and these increase rapidly above this load. The critical load corresponds to the local hardness of the silicon implying that the wear rate is negligible and the coefficient of friction is very low if the deformation is primarily elastic. The range of scanning velocity used in this study has little effect on the wear rate. SEM studies show that at light loads used in AFM, material is removed by ploughing and fine particulate debris is observed. At higher loads, cutting type including ribbon-like debris is observed. SEM and TEM studies of the wear region suggest that the material is removed in a brittle manner or chipping without major dislocation activity and crack formation. Evolution of wear of thin films also has been studied. Wear is found to be initiated at nano-scratches. These fundamental studies have provided insight to molecular origins of friction and wear mechanisms.

Introduction

In magnetic rigid disk drives, the magnetic head slider flies on an air bearing on the order of 50 nm above a disk surface[1]. Physical contact between the slider and disk occurs during starts and stops. To protect against frequent contact occurring between the slider and disk surfaces, the disk is coated with about 10–15 nm thick hard protective coating, mostly amorphous hard carbon, normally referred to as diamond-like carbon (DLC), which is then coated with a molecularly thin film of perfluoropolyether lubricant. Many sliders are also coated with 5–10 nm thick amorphous hard carbon. To achieve ever increasing recording densities, the slider flying heights in future drives may be less than 25 nm and may even be in continuous contact with the disk surface. Furthermore, the thickness of overcoat and lubricant films may have to be reduced to a few molecular layers. With such ultrathin films, a fundamental understanding of the molecular origins of tribological phenomena at the slider–disk interface is essential.

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales2, 3, 4. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, boundary lubrication and for nanofabrication/nanomachining purposes.

At most solid–solid interfaces of technological relevance, contact occurs at numerous asperities. A sharp AFM/FFM tip sliding on a surface simulates just one such contact. However, asperities come in all shapes and sizes. A question then arises on what is the effect of radius of a single asperity (tip) on the friction performance. A technique has been developed to produce AFM tips with different radii. Experiments are conducted using AFM tips with radii ranging from about 50 nm to 14.5 μm. The effect of radius on the adhesive forces and coefficient of friction of uncoated and lubricated silicon samples is measured. Another objective of this study is to understand material removal mechanisms during wear on microscale. Uncoated and DLC-coated silicon surfaces are micromachined using a sharp diamond tip in an AFM. The results of these studies are the subject of this paper.

Section snippets

Test apparatus

Micro/nanoscale friction and wear studies were performed using a commercial AFM/FFM. Surface roughness and friction measurements at two scan sizes of 1 μm×1 μm and 10 μm×10 μm were made using the method described in Ref.[2]. The tip was scanned along the cantilever axis to obtain coefficient of friction and it was scanned orthogonal to the cantilever axis to obtain friction force maps. Coefficient of friction was obtained from the slope of friction force data measured as a function of normal loads

Unlubricated surfaces

Adhesive force and friction measurements were made at 0, 15, 45 (ambient) and 65% RH for all tips. Further measurements were taken with the Si3N4 tip at 85% RH. The microtips gave extremely high adhesive forces beyond 65% RH and no data could be taken beyond this humidity setting. Coefficient of friction was measured for scan sizes of 10 μm×10 μm and 1 μm×1 μm. Representative surface height and friction force maps obtained from three of the tips (0.05, 3.8 and 14.5 μm radius) are shown in Fig. 3 for

Conclusions

Friction and wear studies are crucial in developing fundamental understanding on micro- to nanoscales. Asperity (tip) radius and relative humidity have been shown to affect coefficient of friction and adhesive force. Trends and mechanisms have been identified and discussed. Hydrophobicity of lubricants can be studied with large radii tips. For a sliding interface requiring near-zero friction and wear, contact stresses should be below the hardness of the softer material to minimize plastic

Acknowledgements

Microtips were obtained by V. N. Koinkar and sample lubrication was done by Z. Zhao. Financial support for this research was provided by the Office of Naval Research, Department of the Navy (Contract No. N00014-96-1-10292). The information herein does not necessarily reflect the position or policy of the government, and no official endorsement should be inferred.

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