Use of the Atomic Force Microscope to Study Mechanical Properties of Lubricant Layers
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friction of surfaces in our laboratory. In particular, we will show how the application of modulation methods can be used to study the viscoelastic properties of monolayers of lubricant films. Uses of the Atomic Force Microscope: Modulation Methods The AFM is based on measuring the deflection of a cantilever that supports a sharp tip at one end. The deflection is caused by the interaction forces between the tip and the surface. Two common ways to measure this deflection use light. In the interferometer method, an optical fiber is brought to within a few micrometers of the lever's back side to form an interference cavity. The intensity of the light propagating back into the fiber depends sinusoidally on the cavity gap, which is changed by the deflection of the lever;'4 In the reflection method, light is reflected from the back of the lever into a position-
Introduction Advances in our understanding of the phenomena of adhesion, friction, and lubrication are facilitated by the recent development of new tools that allow the study of contacts in close-to-ideal conditions. These new tools are the surface force spparatus (SFA) and the atomic force microscope (AFM). The first was developed by Israelachvili1 in the 1970s. In this device, contact between two atomically flat surfaces of mica occurs over an area of several micrometers in diameter after the mica sheets, glued onto two perpendicular cylindrical lenses, are compressed. Force, area of contact, and separation distance can be controlled at the atomic scale. The second device, the AFM, was developed by Binnig et al.2 in 1986. The sharp tip of the AFM is a convenient idealization of a single asperity. In addition, the AFM can be used to image the surface in the weak repulsive or in the attractive modes so that minimum perturbation is introduced by the imaging process itself. These two devices have the necessary sensitivity to allow the application of forces weak enough not to dislodge atoms from their sites during contact. The order of magnitude of the force that can lead to the rupture of chemical bonds is a convenient figure to keep in mind in this context. A simple estimate of this force is obtained byo considering a bond-length increase of 1 A as leading to dissociation. For a bond energy of =1 eV, Fb = 1 eV/1 A = 1 X 10""' N. In this article, we will show how the AFM is applied to studies of adhesion and
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Interferometer AFM Optical Fiber
Piezoelectric Tube Scanners Figure la. The atomic force microscope is based on the measurement of forces of interaction between a sharp tip and a surface. These forces bend a cantilever supporting the tip. In (a) the cantilever deflection is measured by the interference of light in the gap between the back of the tip lever and an optical fiber.
MRS BULLETIN/MAY 1993
Use of the Atomic Force Microscope to Study Mechanical Properties of Lubricant Layers
sensitive diode detector. The displacement of the light spot over this detector is converted into electrical signals.5'6 Using a diode composed of four independent parts,
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