Laser interferometric measurement of the surface tension of thin foils
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I. INTRODUCTION Many techniques have been utilized to measure the surface tension (surface free energy) of metals. Unfortunately, the reliability of the data generated with many of these techniques is questionable at best. One of the more reliable techniques for measuring the surface tension of metals is the zero creep technique.1'2 The zero creep technique relies on the phenomenon of high temperature creep. Creep occurs when a material at constant elevated temperature is subjected to a constant load, causing the material either to contract or to elongate. Creep occurs appreciably only at temperatures greater than the Tammann temperature3 {~V2 Tm) of the material. However, most surface tension measurements are done within 85-95% of the melting point of the metal. The reason for this is unclear; however, it is likely due to the fact that the level of strains which occurs as the temperature is reduced (approaching the Tammann temperature) is not easily measured with techniques used by previous investigators. The zero creep technique has been used by many researchers to measure the surface tension of metal wires1'4"8 or foils2'910 in a vacuum or in an inert atmosphere. In all of the investigations where wires were used as samples, it was necessary to remove the sample from the furnace, and, in some cases from the gaseous environment in order to measure sample strain. A microscope stage or a traveling microscope was used to measure sample strains with reported accuracies of 2-5 /xm. In the investigations involving foils, the sample length measurements were made in situ by periodically taking x-ray shadowgraphs of the sample. The reported error associated with this method of length measurement was ±10-20 nm. An approach has been developed which allows the surface tension of foils to be determined at lower tem744 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 4, Apr 1991 Downloaded: 17 Mar 2015
peratures without removal from the experimental environment and with improved accuracy. A modified Michelson interferometer was used to measure sample length changes in situ and in real time. The interferometer allows length measurements approaching a fraction of the wavelength, A, of the light source which is a red HeNe laser (A = 0.6328 ju,m). II. METHODOLOGY The technique which has been developed to measure the surface tension of foils couples two well-known technologies: the zero creep technique and the technique of laser interferometry. The theory behind each of these technologies has been described in detail elsewhere,11 so only a brief description will be given here. A. The zero creep technique The zero creep technique was developed by Udin, Shaler, and Wolf1 for wires and was later extended for use with thin foils by Hondros.2 Hondros suggested that foils were more sensitive than wires because the wire gauge was limited due to the loads needed to stress the sample. Also, very thin foils, approximating a surface, are readily available. When shaped into a cylinder, the sample will tolerate large loads without a
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