Measurement of fracture toughness by nanoindentation methods: recent advances and future challenges

  • Date: Dec 14, 2015
  • Time: 02:00 PM - 03:00 PM (Local Time Germany)
  • Speaker: Dr. Marco Sebastiani
  • Roma TRE University, Engineering Department, Rome (Italy)
  • Location: Max-Planck-Institut für Eisenforschung GmbH
  • Room: Seminar Room 1
  • Host: Prof. Gerhard Dehm / Dr. Christoph Kirchlechner
The analysis of deformation and failure mechanisms in small-scale devices and thin films is a critical issue, not yet solved. In this presentation, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever, beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials. For pillar splitting, cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius, and fracture toughness for a given material. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. Micro-pillars are then produced by Focused Ion Beam (FIB) ring milling, being the pillar diameter approximately equal to its length; this ensures full relaxation of pre-existing residual stress in the upper portion of the specimen. Nanoindentation splitting tests are performed in-situ and the deformation mechanisms corresponding to each class of materials have been investigated. Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ~25% is found between the pillar splitting technique and double-cantilever testing. The limitations of the method are finally discussed. In particular, a minimum pillar’s diameter for the nucleation and growth of a crack during indentation is identified and quantified for a wide range of materials properties. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred.
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