Wear and Nanorheology

Mechanical characterization of worn areas on used obsidian tools

Are worn areas characterized by thin coatings made of transferred debris from the worked material ? Or are they due to the transformation of the obsidian due to repeated friction against the worked material ?


1. Measurement of elastoplastic properties of polishes by nanoindentation

During a nanoindentation test (also called depth-sensing indentation), a pyramidal diamond tip of known geometry is pressed at controlled load onto the sample, and then removed. The hardness and the reduced Young’s modulus of the tested material can be calculated from the continuous measurement of the normal load, the tip displacement and the contact stiffness. When indents are made on worn areas in contact with the wheat during harvesting (experimental material), we obtained similar hardness value for unused and used areas and a Young modulus of 8GPa and 75 GPa. 


2. Nanoscratch tests to investigate ductile/brittle behaviour and adherence of 'polishes'

During nanoscratch tests, the diamond tip scratches the tested material with an increasing normal load. During the test, the normal load, the normal displacement, the tangential force (friction force) and the tangential displacement are recorded. The friction coefficient µ is calculated by dividing the friction force by the applied normal load.

We made this experiments on one tool used for wheat harvesting and one tool for limestone working:

Polishes on obsidian used for wheat harvesting exhibit significantly more brittle behaviour during nanoscratch tests compared to unused obsidian. The difference is not so clear for the obsidian blank used for limestone working as the raw material is more brittle.
• High friction instabilities were observed when the nanoscratch test was performed face first on polish, even at low normal loads (no instability at low load for unused obsidian).
Significantly lower friction instabilities when the nanoscratch is performed edge first, and is higher for polished compared to unused obsidian.

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