Nanoindentation on ceramics

Project manager: Prof. Dr.-rer. nat. Gerold A. Schneider
Projekt worker: Dr. rer. nat. Torben Scholz

Prof. Dr. M. V. Swain
Department of Biomaterials, Faculty of Dentistry,
University of Sydney, Australia

Dr. J. Muñoz-Saldaña, Dr. J. Espinoza Beltrán
Centro de Investigación y de Estudios Avanzados del IPN,
Unidad Querétaro, México

Prof. Dr. W. J. Clegg
Department of Materials Science,
University of Cambridge, Great Britain

Recently, it is has become possible to perform indentation tests at dimensions of tens to hundreds of nanometers using this nano- and microindentation methods. Nanoindentation is widely used for measuring the elastic modulus, E, and the hardness, H, of small volumes of material and thin films. A more critical appraisal of the force-displacement response of the samples during nanoindentation experiments can however provide far more significant insights into the mode and onset of plastic deformation or fracture of a material. We analyse the force-displacement curves in brittle materials indented with cube corner indenters. They show typically display two pop-in events (Figure 1).

Figure 1: Schematic drawing of the three different stages during indentation of brittle materials.

Initially the force-displacement response is elastic and is associated with the Hertzian like contact of the indenter, which enables us to calculate the elastic modulus of the material very precisely. Furthermore it was possible to image twofold symmetric arrangements of domains (Figure 2) around indents in barium titanate single crystals using the so called “piezo response force microscopy”. The residual domain structure was unambiguously correlated to the known orientation of the crystals.

Figure 2: Topography image with contour plot (8 nm height spacing/contour line) (a) with corresponding x/ y-component of the polarization vector of a 90°-aa-domain area after indentation (b), (c).

The second pop-in event is associated with development of median/radial cracks about the indentation impression. The crack length (in the order of 100 nm) can be used to estimate the fracture toughness of the material at this extremely small scale. Furthermore at these small indentation depths classic plasticity theory predicts constant hardness using a geometrically self-similar indenter on a homogenous material. Nevertheless a strong size dependent indentation hardness result is well known for metallic materials. This so called “indentation size effect” (ISE) characterized by an increasing hardness (up to a multiple of the macroscopic hardness) as the indentation depth is reduced to the order of microns or sub-microns. We have examined this effect for a very low load region using ceramic materials and various indenter shapes.
Moreover transmission electron microscopy cross-sections of the indents – prepared by focus ion beam – showed paired parallel slip lines with 45° to the original surface (Figure 3). Therefore the initiation of dislocations by indentations in bulk BaTiO3 has been verified.

Figure 3: (a) shows a bright field image of an 11 mN indent in a {001} orientated BaTiO3 crystal. Moreover the orientation of the TEM foil was determined to be {001} by the diffraction pattern in (b). Clearly visible are the shear bands beneath the indent in the bright field image. The angle between the sample surface and the shear bands is approximately 45° (c).

You can read the following articles for further information:
Fracture toughness from submicron derived indentation cracks
Domain rearrangement during nanoindentation in single-crystalline barium titanate measured by atomic force microscopy and piezoresponse force microscopy
Indentation size effect in barium titanate with sperical tipped nanoindenters
“Nanoindentation initiated dislocations in barium titanate (BaTiO3)”
The articles appeared in Applied Physics Letters. Copyright 2004-2007 American Institute of Physics. They may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physic: