Indentation plastic displacement field: Part II. The case of hard films on soft substrates
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Indentation plastic displacement field: Part II. The case of hard films on soft substrates T. Y. Tsuia) Advanced Micro Devices, One AMD Place, Sunnyvale, California 94088
Joost Vlassak and William D. Nix Department of Materials Science and Engineering, Stanford University, Stanford, California 94305 (Received 3 November 1998; accepted 10 March 1999)
The plastic displacements around Knoop indentations made in hard titanium/aluminum multilayered films on soft aluminum alloy substrates have been studied. Indentations were cross-sectioned and imaged using focused-ion-beam (FIB) milling and high-resolution scanning electron microscopy (SEM), respectively. The FIB milling method has the advantage of removing material in a localized region without producing mechanical damage to the specimen. The micrographs of the cross-sectioned indentations indicate that most of the plastic deformation around the indentation is dominated by the soft aluminum substrate. There is a very small change in the multilayered film thickness around the indentation—less than 10%. The plastic deformation of the thin film resembles a membrane being deflected by a localized pressure gradient across the membrane. Stress-induced voids are also observed in the multilayered film, especially in the area around the indentation apex. The density and the size of the voids increase with indentation depth. Indentation sink-in effects are observed in all of the indentations inspected. Based on the experimental results, the amount of sink-in of the hard film–soft substrate composite is larger than the bulk substrate and film alone. This is confirmed by the finite element analyses conducted in this work. I. INTRODUCTION
Hard protective coatings, such as titanium nitride and diamondlike carbon thin films, are commonly used in tribological applications. The hardness (H) values of these thin films are in the range of 10–40 GPa,1–7 significantly harder than the substrate materials (H , 1–5 GPa) on which they were deposited. For example, the aluminum substrate used in the magnetic hard disk industry has a hardness of approximately 1 GPa, which is considerably softer than the diamondlike carbon coating, H , 14 GPa.7 In order to develop better thin film protective coatings, it is important to understand the thin film mechanical properties and the film/substrate deformation mechanisms during the tribological contact. One of the common experimental procedures to measure such properties is the instrumented depth-sensing indentation technique. During the indentation experiment, the applied load and indenter penetration depth are constantly monitored. The hardness and elastic modulus can be calculated from the load-displacement data using the analysis method of Oliver and Pharr.8 Since the substrate and the thin film have different elastic and plastic properties, the measured indentation results vary with indentation depth. When the indentation depth is small, the measa)
Address all correspondence to this author. e-mail: ttsui
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