Elsevier

Thin Solid Films

Volume 368, Issue 1, 1 June 2000, Pages 151-155
Thin Solid Films

Controlling and improving the microtribological properties of Langmuir–Blodgett monolayer films using an external electric field

https://doi.org/10.1016/S0040-6090(00)00733-1Get rights and content

Abstract

The microtribological properties of Langmuir–Blodgett (LB) monolayer films in AC and DC electric fields have been investigated using scanning force microscopy (SFM). Friction and wear characteristics of LB films can be controlled and improved by an external electric field. In a DC field, the friction force of the LB film increases with the external voltage, and its wear-life is significantly shortened. On the other hand, in an AC field, friction force is a function of AC voltage and frequency. With a special combination of voltage and frequency, the friction force almost approaches zero, and the wear-life of the LB film is greatly extended.

Introduction

Micro-miniaturization is a current trend in the development of many electrical devices. Various fabrication techniques and design strategies developed in recent years have led to the production of micro-scale devices, known as micro-electro-mechanical systems (MEMS), for a wide range of applications [1], [2]. Because of the large surface area-to-volume ratio in MEMS, friction forces predominate over inertial and gravitational forces. Friction becomes a more critical problem in MEMS than in general macro-systems [3].

Because of the micro-size of the MEMS components, the traditional lubrication technique of using freely-supported liquid multimolecular layers is not the most desirable. Recently, Langmuir–Blodgett (LB) films have been attracting interest, because the molecular scale materials and technique are compatible with micro/nano scale technologies. Monolayer organic films can even significantly reduce the friction coefficient of an inorganic substrate [4], [5]. However, the weak adhesion of classical LB films to the substrate surface restricts their effective life for continuous sliding. The low-melting temperature and high conformational mobility of such boundary lubricants may cause dynamic instabilities in sliding characteristics [6], [7]. Therefore, it is quite necessary to find a technique that can further reduce the friction force of LB films, as well as extend their wear lives.

It is well known that both friction and wear can influence, or be influenced by surface chemical and physical characteristics, such as chemical activity, roughness, surface potential, etc. There are several research groups already studying the phenomena of friction-induced surface potential [8], [9], [10], [11]. It is reasonable to infer that surface potential may influence the friction and wear behavior of materials. It is also well known that an electric field can influence the surface charge, i.e. the surface-potential properties of materials. Therefore, we can deduce that there may be some relationship between the microtribological properties and an external electric field. There have been only a few investigations focusing on the influence of an external electric field on the microtribology performance of materials, until our recent studies showing that the microtribological properties of mica can be influenced by an external electric field [12].

In our present study, we investigated the influence of an external electric field on the micro-friction and wear properties of the arachidic-acid monolayer.

Section snippets

Experimental details

Monolayer films of arachidic acid (C19H39COOH) were prepared at a pH of 6.0, using the standard vertical-dipping method. Films were deposited onto a Si (100) substrate at 23°C in air, at a transfer pressure of 30 m/Nm with a speed of 1 cm/min.

A commercially-available scanning force microscope (SFM; Nanoscope III, Digital Instrument, Inc.) was used, which detects the deflection and torsion of a cantilever, to identify the surface characteristics of arachidic-acid monolayer films, and to study

Results and discussion

Fig. 2a,bshows the friction forces of two single-line scans without an electric field and in a +50 V DC field, respectively. To determine the friction force, we calculated the average difference between a forward and backward scan, as shown in Fig. 2a. The difference between the forward and backward scans in the lateral-force scan corresponds to twice that of the friction force.

Since the sample roughness value, Ra, was below 0.1 nm over a scan range of 1 μm, the topographical contribution to

Conclusions

The microtribological behavior of LB monolayer films in AC and DC electric fields have been reported in this paper. The friction force and wear-life of LB monolayer films can be controlled and improved by using an external electric field. In a DC field, the friction force increases with the DC voltage, and wear-life is significantly shortened. In an AC field, the friction force can be influenced by the AC voltage and frequency. In a 130 V and 19 kHz AC field, the friction force exhibits nearly

Acknowledgements

A part of this research was supported by the fundamental research on microtribology under extreme/hostile environments project in the special coordination funds for promoting science and technology, under the Science and Technology Agency of Japan. The author, H. Liu, would also like to thank the Alexander von Humboldt Foundation for their AvH fellowship support.

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    At the microscopic scale, the application of external potentials could affect the lubrication properties by altering the structure and orientation of interfacial molecular films at the tribo-interface. Based on AFM friction measurement, it was demonstrated that the lubrication properties of Langmuir–Blodgett (LB) monolayer could be controlled and modified by external potentials, and the friction was almost zero and the wear life could be greatly prolonged under certain ac magnitude and frequency [52]. Further AFM friction studies conducted by Karuppiah et al. suggested that the frictional response to the external potential was more obvious for the low-density molecular monolayer, e.g., ω-mercaptan carboxylic acid monolayer, mainly due to the change in the structural and crystalline order of the film [53].

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