Journal
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
Volume 528, Issue 27, Pages 7901-7908Publisher
ELSEVIER SCIENCE SA
DOI: 10.1016/j.msea.2011.07.001
Keywords
Residual stress; Ring-core; Focused ion beam (FIB); Digital image correlation (DIC); Thin films; Hole drilling method
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Funding
- German Science Foundation (DFG) [SFB 499 N01]
- U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division
- Alexander von Humboldt Foundation
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A new methodology for the measurement of depth sensitive residual stress profiles of thin coatings with sub-micrometer resolution is presented. The two step method consists of incremental focused ion beam (FIB) ring-core milling, combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained by comparison of the experimentally measured surface strain with finite element modeling using Schajer's integral method. In this work, a chromium nitride (CrN) CAE-PVD 3.0 mu m coating on steel substrate, and a gold MS-PVD 1.5 mu m on silicon were selected for the experimental implementation. Incremental FIB milling was conducted using an optimized milling strategy that produces minimum re-deposition over the sample surface. Results showed an average residual stress of sigma = -5.15 GPa in the CrN coating and sigma = +194 MPa in the Au coating. These values are in reasonable agreement with estimates obtained by other conventional techniques. The depth profiles revealed an increasing residual stress from surface to the coating/surface interface for both coatings. This observation is likely related to stress relaxation during grain growth, which was observed in microstructural cross sections, as predicted by existing models for structure-stress evolution in PVD coatings. A correlation between the observed stress gradients and the in-service mechanical behavior of the coatings is proposed. Finally, critical aspects of the technique and the influence of microstructure and elastic anisotropy on stress analysis are analyzed and discussed. (C) 2011 Elsevier B.V. All rights reserved.
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