Measuring fracture toughness of coatings using focused-ion-beam-machined microbeams
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Measuring the toughness of brittle coatings has always been a difficult task. Coatings are often too thin to easily prepare a freestanding sample of a defined geometry to use standard toughness measuring techniques. Using standard indentation techniques gives results influenced by the effect of the substrate. A new technique for measuring the toughness of coatings is described here. A precracked micro-beam was produced using focused ion beam (FIB) machining, then imaged and loaded to fracture using a nanoindenter.
Determining the mechanical properties of coatings can be very difficult due to their thinness (typically a few microns), effects of the substrate, effects of adhesion, and residual stresses. Using a nanoindenter, it is possible to determine some properties of coatings (hardness and the elastic modulus).1 Some other properties, however, are still very difficult to measure. One of them is the fracture toughness. The classical method for measuring fracture toughness is to fracture a pre-notched sample with welldefined geometry. From the critical load it is then possible to determine the fracture toughness KIc. However, in the case of thin coatings, it is difficult to manufacture a sample of the coating material alone: normal fracture toughness specimens are several tens of millimeters in size or larger. In the case of brittle ceramic materials, toughness is very often measured using indentation techniques. The size of cracks around the indent is measured and then related to fracture toughness using a specific model.2,3 This method is difficult to use for thin coatings because typical indentation crack sizes are several tens of microns to several hundreds of microns; the effect of the substrate will dominate. If the substrate is not brittle it will be difficult to obtain a well-defined fracture geometry. This is also the case for fracture toughness testing using Hertzian indentation.4 Some other methods more specific for coatings have been proposed. The use of a nanoindenter for toughness determination has been described,5,6 and Kim and Nairn7 developed a finite element fracture mechanics model to predict the development of multiple cracks in a coating layer in the presence of a substrate. These methods, however, seem to be usable only in a restricted number of cases, and the values obtained are not always comparable DOI: 10.1557/JMR.2005.0048 J. Mater. Res., Vol. 20, No. 2, Feb 2005
with those obtained with other techniques. Micro-beams and similar micro-mechanical elements have been prepared for many years in silicon, mostly by etching methods.8–11 These methods are not easily applicable to other materials (though silicon-etching has been used to prepare Al and Au beams by Son et al.12). Variants on these micro-beam testing methods have included focused ion beam (FIB) machining of cantilevers of Al-coated Si from material polished to 10 m thickness,13 and use of FIB to put V-notches in Si specimens produced by etching methods.14 The technique used here involves the preparation and testing of pre-cracked
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