Measuring and modelling the instrumented indentation (nanoindentation) response of coated systems

Abstract

We have been determining the ways in which the mechanical properties of coated systems, including those with very thin (<100 nm) coatings, can best be characterized, at the appropriate small spatial scales, using instrumented indentation techniques (IIT). Our approaches have been to critically assess the differing types of sample information available from load–displacement (P–δ) curves, P–δ ² analyses and P–S ² data (where S is the contact stiffness). Parallel insights into the deformation mechanics controlling the contact response have been sought using transmission electron microscopy (TEM), high resolution scanning electron microscopy (HRSEM) and scanning probe microscopy (SPM) to characterize both the detailed appearance of resultant indentations and their underlying deformation structures. Increasingly, many coatings are multilayered and thus there is a need for a modelling approach that can predict the mechanical properties of any proposed multilayer design. Since testing all possible stack sequences and combinations is impossible, we have successfully developed an energy-dissipated-per-unit-volume-of-plastic-zone model to predict the effective hardness, elastic modulus and IIT response in such cases. Such studies are furthering our understanding of the fundamental origins of the mechanical behaviour of coated systems, with particular emphasis on the scale sensitivity of responses and with the purpose of informing systems’ design. New insights have occurred such as the differing scale-sensitivities of system hardness (H _sys) and elastic modulus (E _sys) resulting in there being a critical range of contact scale range over which a maximized elastic contact response can be expected.

Acknowledgements

Work at Newcastle has been supported by EPSRC, EU TMR and Pilkington Technology Centre, Ormskirk, UK. TFP thanks the University of Tennessee (Knoxville (UTK)) for a Visiting Research Professorship during the summer of 1997 when the P/S ² work was initiated and, similarly, Oak Ridge National Laboratory (ORNL) for making facilities and support available as a Guest Scientist for the same period. EPSRC (UK) is thanked for the provision and support for the Newcastle Nano Indenter and AFM facilities. Eric Herbert took part in this study as part of his first and higher degree programs in the Faculty of Engineering at UTK. Dr Sarah Hainsworth, Dr Martin McGurk and Dr Eva Berastegui and Dr Isobel Arce-Garcia are thanked for useful discussions and the citation of their work. Dr Natalia Tymiak (University of Minnesota) is thanked for obtaining the data in figure 6 during a student exchange with Newcastle. Lynne Toase provided valued help with the preparation of the manuscript.