In-Situ Observation of Scratch/Indentation True Contact Area

In contact mechanics analysis, converting the stiffness calculated from load versus depth (in the case of indentation) or load contact versus contact radius (in the case of scratching) behaviors into a stress strain relationship is a major problem. Generally, when directly recording the load from the load cell, the true contact radius and the true depth depend on a model which depends on the kind of behavior (elastic, elastic-plastic, plastic).This is not a real problem for materials such as steel and other materials that quickly yield during contact. However, no models are available to consider the viscoelastic and/or viscoplastic behavior of the material or an elastic behavior at large strain, such as polymeric materials.

New apparatus was developed to study this problem. It controls the tip velocity over a large range, at a wide range of temperatures and is equipped with an “optical microscope” to perform in-situ control and measurement of the contact area and of the groove left on the surface. The prototype of this apparatus built under the name “Micro Visio Scratch”¹ at the Charles Sadron Institute is now available as an option for all types of Anton Paar Indentation and Scratch Testers.

Figure 1. Anton Paar Nanoscratch Nanoindentation tester including the in-situ vision set-up

Figure 1 shows the prototype Mirau interferometer integrated with an Anton Paar Open Platform, which is mounted in a vacuum chamber. This optical device may be shifted under the Nanoscratch Module or under the Nanoindentation Tester Module. This article discusses the recent results acquired with this setup. The indenter tip used has a spherical geometry for two reasons: the stress is uniformly distributed around such an indenter and the in-situ vision allows controlling the mean contact strain proportional to the ratio a/R (where a is the contact radius and R the tip radius).

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.