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: