A new method for hardness determination from depth sensing indentation tests
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A new method for hardness determination from depth sensing indentation tests J. Gubicza, A. Juh´asz, and J. Lendvai Department of General Physics, E¨otv¨os University, Budapest, H-1088 M´uzeum krt. 6-8, Budapest, Hungary (Received 18 December 1995; accepted 30 July 1996)
A new semiempirical formula is developed for the hardness determination of the materials from depth sensing indentation tests. The indentation works measured both during loading and unloading periods are used in the evaluation. The values of the Meyer hardness calculated in this way agree well with those obtained by conventional optical observation, where this latter is possible. While the new hardness formula characterizes well the behavior of the conventional hardness number even for the ideally elastic material, the mean contact pressure generally used in hardness determination differs significantly from the conventional hardness number when the ideally elastic limiting case is being approached.
Hardness testing with sharp indenters is generally considered as a simple method for characterizing the mechanical strength of materials. Recently the depth sensing indentation (DSI) test became a widely used method of hardness determination.1–8 In the DSI tests the applied load is registered as a function of indentation depth both during loading and unloading. A schematic load versus penetration depth curve is shown in Fig. 1. The most frequently used DSI method was developed by Oliver and Pharr3 by which the hardness number can be determined without optical observation. In this method the mean contact pressure at the maximum depth is used to characterize the plastic properties of materials. If, however, a considerable part of the deformation during the indentation is elastic, this pressure deviates significantly from the conventional hardness number which is determined by optical observation of the indentation trace. For example, in the limiting case of the ideally elastic material, the conventional hardness number tends to infinity while the mean contact pressure gives finite value since an elastic contact surface is developing between the indenter tip and the ideally elastic material.2 The paper is a continuation of a recently published work9 in which a new semiempirical formula has been developed for the determination of the Meyer hardness10 of materials. With the formula proposed, even the limiting case of the ideally elastic materials can be correctly described. The main results of our recent paper are briefly summarized below to give a basis for the subsequent comparison with another evaluation method.3 Hardness measurements were carried out on different materials by the DSI method using a computercontrolled hydraulic mechanical testing machine with a Vickers indenter. During the loading period the Vickers pyramid penetrates the sample at constant velocity, and 2964
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J. Mater. Res., Vol. 11, No. 12, Dec 1996
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the same velocity is applied in the
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