Hardness-depth profiling on nanometer scale
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I. INTRODUCTION
THE mechanical properties of the near-surface region of a component often determine its service life in tribological applications. This motivates the development and the application of new surface-modification techniques and new surface layers/coatings. For example, multilayers,[1,2] gradient coatings,[3] and surface regions modified by ion implantation[4,5] can be used to tailor the tribological properties of a component. In order to optimize the process treatment to arrive at the desired properties, a fundamental understanding of the relations among processing parameters, microstructure generated, and resulting properties is required. The mechanical properties within those surface layers or modified surface regions can change drastically over depths as small as 10 nm. Hence, if a “real” hardnessdepth profile of the near-surface region is to be determined, a measurement method with a depth resolution on this order, or better, is needed. To characterize the mechanical properties on such a small scale, depth-sensing nanoindentation testing (DSI) can be performed.[6,7] In DSI, a diamond tip is pushed into the material to be probed while the load is increased; subsequently, after having reached a given maximum depth or load, the tip is removed. During this procedure, both the load on and the displacement of the indenter are recorded. This load-vs-displacement behavior represents a fingerprint of the mechanical properties of the material averaged over a certain volume, the size of which depends on both the properties of the material investigated and the shape of the diamond tip used.[8] In any case, this volume and, thereby, the probed depth increases strongly with
M. KUNERT, B. BARETZKY, and E.J. MITTEMEIJER are with the Max-Planck-Institut fu¨r Metallforschung, D-70174 Stuttgart, Germany. S.P. BAKER is with the Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853-1501. Manuscript submitted August 15, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
increasing load. Therefore, to obtain a high depth resolution, a small maximum load should be applied. Hardness-depth profiles are usually assessed by gradually increasing the maximum load. Accordingly, the depth resolution of this load-variation method (LVM) (Figure 1(a)) is reduced severely as the maximum indentation depth is increased. For thin films or modified surfaces, this method measures the combined response of the film/ modified surface and the substrate underneath. This is particularly true at larger indentation depths. A number of models have been developed to extract the “true” film hardness, i.e., the film hardness not influenced by the substrate, from the measured composite hardness obtained using the LVM.[9–15] These simple models do not, however, apply to multilayers, gradient coatings, or ion-implanted surfaces where large hardness gradients can occur.[16,17] The aim of this work was, therefore, to define a measurement method that reveals a more direct picture of the true hardness-depth profile, i.e., a
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