Prediction of nanoindentation hardness profile from a load-displacement curve
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Prediction of nanoindentation hardness profile from a load-displacement curve K. W. Xu, G. L. Hou, B. C. Hendrix, and J. W. He State Key Laboratory for Mechanical Behavior of Materials, Xian Jiaotong University, China
Y. Sun, S. Zheng, A. Bloyce, and T. Bell School of Metallurgy and Materials, The University of Birmingham, Birmingham, United Kingdom (Received 12 July 1996; accepted 10 March 1998)
During the nanoindentation process, the load and depth data are continuously recorded. A single load-displacement curve is thus expected to contain material property information from the whole depth range indented. In the present paper, a new method to obtain the hardness-depth curve has been derived for small depths from the load-displacement curve measured at a large depth, based on the assumption that the elastic properties of the indented material can be obtained from the indentation depth. Using this method, hardness values can be computed for various small depths from a single load-displacement curve. From a series of nanoindentation experiments, it has been proven that the method can be used on both homogeneous and surface-modified materials, such as fused silica, single crystal tungsten, and plasma nitrided steel with and without an iron nitride Fe4 N compound layer. Testings on a series of Ni–P films coated on 15 MnB steel also gave fairly good results.
I. INTRODUCTION
Despite the rapid development of the nanoindentation technique in the past decade, little work has been done to derive hardness values at small depths with the data measured at a relatively large depth. Such mechanical properties as hardness and elastic modulus values of the indented material are normally derived according to the loading and unloading behavior at the maximum load (or depth).1,2 However, the most pronounced characteristic of the nanoindentation technique is its ability to continuously record the loaddisplacement data as the material experiences continuous indentation up to the maximum depth. Accordingly, a single load-unload curve should contain material property information from the whole indented depth range. Obviously, measurements at a relatively large depth are more reliable and can eliminate the detrimental effect of surface roughness and require less control of temperature and vibration during the measurement, which are known to be a serious problem for measurement at extremely low loads. In the study of hardness, its distribution through the depth is of special significance. Surface modification usually introduces a hardness gradient in the near surface region of the material. In many cases it influences the wear, fatigue, and corrosion behavior of the material. On the other hand, it is frequently observed that the apparent hardness obtained by micro- and nanoindentation increases with decreasing load,3 even though there are no J. Mater. Res., Vol. 13, No. 12, Dec 1998
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appreciable changes through the depth in composition,
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