Nanoindentation Evaluation of Brittle Films on Metals Natalia
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NANOINDENTATION EVALUATION OF BRITTLE FILMS ON METALS
Natalia I. Tymiak* , Antanas Daugela **, Trevor F. Page*** , and William W. Gerberich* * Dept. of Chem. Eng. and Mat. Sci. (CEMS),University of Minnesota, Minneapolis, MN 55455 **Hysitron, inc., Minneapolis, MN 55439 *** Materials Division (MMME), The University of Newcastle, Newcastle Upon Tyne, UK ABSTRACT
The present study addresses two cases of brittle ceramic films on metals. With the assistance of AE as a supplementary technique, yield initiation phenomena have been evaluated for W single crystal surfaces under several nm thick native oxide film. An AE sensor coupled to an indenter tip allowed an increasing sensitivity to localized fracture and plasticity events in the vicinity of an indentation contact. A good correlation between AE signals and indentation induced plasticity and fracture has been accomplished for contacts below 100 µN. Second, mechanical behavior of porous nanocrystalline SiC films on Mo substrates was examined. An analysis was based on the P-δ2 approach. With this method, both loading and unloading parts of indentation curves obtained with sharp pyramidal indenters may be represented as P=Kδ2 where P, and δ denote indentation load and displacement respectively. The parameter K includes a material's hardness/modulus ratio and indenter geometry. INTRODUCTION
In most cases, depth-sensing indentation is the only practical means of evaluating the mechanical properties of thin films and coatings. While a number of experimental strategies and models for analyzing data exist for general situations of this type, indentations into brittle ceramic films on metallic substrates raise some added problems. This may be partially attributed to deformation of such films involving a variety of concurrent processes such as plasticity, densification, and fracture. In the limit of very thin brittle films such as e.g. several nm thick native oxides on metals, film deformation and fracture may affect deformation of an underlying substrate dramatically. As this poses quite a challenge for the experimental data interpretation, supplementary techniques such as acoustic emission become increasingly critical and useful. Due to a highly localized nature of indentation induced fracture, an AE sensor placed in the vicinity of a contact is clearly advantageous. For thicker ceramic films, a complex microstructure dependent deformation under indentation contact poses a difficulty for extracting mechanical property of these films from indentation curves. The present study addresses a characteristic example for each of the above two cases. First, plasticity initiation is studied for oxidized W surfaces utilizing AE monitoring with the recently developed sensor coupled with the indenter tip. Second, porous nanocrystalline SiC films on Mo were evaluated. An objective was to deconvolute yield stress and Young’s modulus from a complex indentation response affected by indentation-induced densification and fracture. BACKGROUND
Oxidized W single crystal surfaces. For a variety
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