Page 64 - Mechanics of Asphalt Microstructure and Micromechanics

Mechanical Proper ties of Constituents 57

2.5.3 Nanoindenter
The nanoindenter is a popular nanomechanical test instrument for measuring mechan-
ical properties such as hardness, elastic modulus, fracture toughness, wear resistance,
coefficient of friction, and viscoelastic properties of thin films, coatings, and particles
with nanometer spatial resolution using controlled indentation of surfaces (Oliver and
Pharr, 1992; Vinci and Vlassak, 1996; Fischer, 2004; Kelsall, et al., 2005). Nanoindenter
dimensions are very small, less than 50 μm in some cases. They are made with precise
angular geometry in order to achieve the highly accurate readings required for nanoin-
dentation. Nanoindentation, also called depth sensing indentation or instrumented inden-
tation, is using instruments that can continuously record small loads and displacements
with high accuracy and precision. Using a sample stage controlled by piezoelectric
drivers, a nanoindenter can be positioned with submicron accuracy within the micro-
structure of a sample. Useful data such as Young’s modulus and hardness can be ob-
tained from the loading-unloading versus displacement curve after testing.
Many types of nanoindenters are currently in use based on their tip geometry. Some
available geometries are three- and four-sided pyramids, wedges, flat, cones, cylinders,
filaments, and spheres. Nanoindenters such as Berkovich, cube corner, Vickers, and
Knoop have been used extensively for hardness testing. To meet the high precision re-
quired, nanoindenters must be inspected and measured with equipment and standards
provided by the National Institute of Standards and Technology (NIST). Nanoindenters
are made mostly of diamond and sapphire, although other hard materials can be used
such as quartz, silicone, tungsten, steel, tungsten carbide, and almost any other hard
metal or ceramic material. Diamond is the most commonly used material for nanoinden-
tation due to its properties of hardness, thermal conductivity, and chemical inertness.

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