Time-Dependent Deformation in Room-Temperature Indentation Experiments using a Nanoindenter
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TIME-DEPENDENT DEFORMATION IN ROOM-TEMPERATURE INDENTATION EXPERIMENTS USING A NANOINDENTER SHEFFORD P. BAKER*, TROY W. BARBEE, Jr.** AND WILLIAM D. NIX* *Stanford University, Department of Materials Science and Engineering, Stanford, CA 94305 "**LawrenceLivermore National Laboratory, Livermore, CA 94550 ABSTRACT Time-dependent deformation in room-temperature indentation is a well known phenomenon which is frequently overlooked in depth-sensing indentation experiments. In this study, the timedependent displacements which occur during depth-sensing indentation experiments using a Nanoindenter were investigated. Time-dependent displacements were found to result both from plastic deformation of the sample and experimental drift. A simple correction for experimental drift was applied in experiments on sputtered copper thin films and reasonable strain rate sensitivities were obtained. The sources of experimental drift were identified and measured. It appears to be possible to measure the rate-dependence of plastic deformation in a variety of materials using this instrument. INTRODUCTION The depth-sensing indentation tester is a powerful tool for investigating the mechanical properties of thin films. These devices compile a record of load and displacement throughout the indentation process which can then be interpreted to obtain a measure of the hardness and stiffness of the test material [1]. The basic principles of one such machine, a Nanoindenter, are illustrated in Figure 1. Force is applied through a shaft to the indenting tip by means of a coil and permanent magnet. Displacements are measured by a capacitance gage. The indenter shaft, including the tip, the center plate of the capacitance gage and the load coil, is suspended on fine leaf springs. We have tested a wide variety of materials using this instrument and have observed time-dependent displacements in permanent magnet virtually every experiment. Although some load coil investigators have designed depth-sensing indentation ,-indenter shaft experiments to measure time-dependent deformation [2], in general we, and other groups who use similar suspending spring techniques, do not fully account for the timedisplacement gage dependence of such experiments. More typically we adopt a procedure similar to that of traditional hardness suspending spring tests in which the time scale of various segments tip holder (loading, holding at fixed load) of the experiment are 4diamond tip fixed and the results obtained under that scheme are recorded. Figure 1. Schematic of Indenter Shaft in the Nanoindenter. Three types of displacement are measured by a depth-sensing indentation tester: elastic displacements of the sample and testing machine, plastic displacements in the sample, and experimental drift. In order to interpret the data, we must be able to accurately separate these three components of displacement. This paper is a preliminary report on our efforts to separate and understand the time-dependent displacements, both plastic and thermal drift, that occur during indenta
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