MechAction, Inc.

Nanoindentation, also known as instrumented indentation technique (IIT), is achieved using a high-resolution system that can drive an indenter tip made of diamond into and withdraw it from a sample material. During the process, the loads and displacements of the tip are recorded by positional sensors, from which mechanical properties can be derived. In the past few decades, this technique has been developed sufficiently to satisfy the increasing needs of nano-scale materials and devices characterizations.

The earliest indentation technique traces back to the ancient times when people bit on gold coins to check its authenticity. The ancient gold coins were usually made from pure gold metals, which are ductile and relatively soft. A counterfeit gold coin is usually mixed with other metals like copper which will lesson the coin value. However such process will also harden the coin, which leads to a different result when you bite on it: pure gold will have a dent after biting, while counterfeit gold will grant you a broken tooth! This simple phenomenon dictates the truth of indentation: to measure the hardness of materials (resistance to permanent damage from an outer force).

The human body is probably the most sophisticated indentation machine. You have your teeth as the indenter tip, the jaws and muscles to provide force, while the brain gives you the sensation that the material you bite on is whether soft or hard. The modem indentation systems simply mimic such function by replacing the teeth with a diamond indenter, the jaw with a force actuator, and the brain with force/depth sensors.

One of the very first instrumented hardness testers was the Brinell hardness tester. Introduced in 1900s, the Brinell tester employs a spherical indenter and a fixed load to make indentation impressions on the material surface. By optically measuring the imprint size, the contact pressure (hardness) can be calculated. However, the spherical shape of the indenter limited Brinell's applications on hard materials. In 1919, the first Rockwell Hardness Tester was introduced, which has kept its popularity in the industrial fields through modern days. Compared to Brinell's spherical indenters, the Rockwell employs a cone shape indenter (Rockwell also employs spherical indenters depending on its scales), so it can penetrate deep into materials, which makes it a great tool for hard materials hardness measurements. The Rockwell also introduced the pre-load concepts, which takes away certain influence of the elastic deformation on hardness measurements. Rockwell scales from A through G depending on different loads and tips used. Just about 2 years later, an even "sharper" hardness tool was introduced in 1921: The Vickers Tester employs the so-called Vickers Indenter (a 4-face pyramid shaped tip), which is sharper than a Rockwell cone tip. Unlike Rockwell, the Vickers indenter is a one-tip-fit-for-all situation, as the required calculation is independent of the Vickers tip size. The Vickers Systems are still very popular these days and preferred by hard metals/alloys and ceramic industries.

In the last 2~3 decades, as the world has been more and more inspired by the nano-scale materials and devices, instrumented indentation, inexorably, followed this “expanding” trend. Researchers started to realize the limitations of conventional microhardness testers: 1. The system sensitivity and resolution cannot satisfy the needs of small (submicron) scale measurements; 2. Cannot measure other mechanical properties such as Young's Modulus; 3. Vickers/Knoop tip imperfection. After years of development, a new type of tool called Nanoindenter was introduced to the market in early 90s to satisfy the urgent needs of nano-mechanical testing. The biggest differences of Nanoindenter compared to a microhardness tester are: 1. Change from Static loading to Quasi-Static or Dynamic loading 2. The load/depth data is recorded through both loading and unloading process of indent; 3. Usage of Berkovich indenter (a 3-face pyramid shaped tip). With nanoindentation technique, the measurements of not just hardness, but also Young's modulus, yield strength, fracture toughness, and many other mechanical properties at nanometer scale becomes achievable.