Nanoindentation is a method of measurement of the mechanical properties of small volumes of materials using an instrumented indentation technique. Elastic modulus, hardness, fracture toughness, creep and dynamic properties such as storage and loss moduli can be measured. In this and subsequent articles, we will look at some of the issues facing the user of a nanoindentation instrument. Our purpose is to educate and inform the prospective user of this type of equipment as to what can be measured and what factors influence the results obtained.
Figure 1. The IBIS Nanoindentation system from Fischer-Cripps Laboratories.
One of the most important laboratory issues to be considered when undertaking nm scale indentations is thermal drift. Thermal drift is a variation in the force or depth signal arising from thermal expansion or contraction of the specimen. To some extent, thermal drift may also be caused by the electronics unit having not reached a stable temperature. So, before any test is begun, the following must be observed:
Allow the instrument to arrive at its working temperature. Usually a minimum of one hour warm up time should be considered from a cold start. Note that the signals involved are in the microvolt region and any deviation in gain, resistance and capacitance in the circuitry will have an observable effect.
Make sure the specimen is at thermal equilibrium with the instrument and the laboratory. If a nanoindentation test takes say, 1 minute, then an expansion of the specimen by say 10 nm over that minute can be a serious error. Even instruments that claim to negate thermal drift of this kind by a surface reference will be affected, but to a lesser extent (at the expense of other problems). The best way to ensure that thermal stability has been achieved is to make contact with the specimen in a manual, or diagnostic mode, of the instrument, zero the depth sensor, and then plot the depth sensor output as a function of time. Naturally this test will only be of significance if the specimen does not creep under indentation loading. If the depth sensor output is stable, such as illustrated below, then testing can proceed.
The Importance of Obtaining a Series of Indentations
Usually a series of indentations is done in any one test. It is often best to arrange matters so that the maximum load is increased by a small increment at each indentation. This way you can obtain results of mechanical properties as a function of depth of penetration, and in doing so, make a decision about what is the best load to apply. When a series of indentations has been performed, the resulting load displacement curves should be nicely overlaid.
Figure 2. Load displacement curves at different maximum loads on the same specimen.
Spacing between Indentations
The spacing of the indentations should be such that the impression from one does not influence the readings taken for the next indentation. Generally speaking, for a Berkovich indenter, the aspect ratio is about 7 to 1, so that for indentations of about 1 ìm should be spaced about 8 to 10 ìm apart as a minimum.
Optimal Testing Conditions
The tests are best performed in unattended mode, and preferably overnight. This is to minimise influence from activity in the laboratory. If performing a series of tests, a helpful hint is to arrange things so that the higher loads are run first. In this way, the more sensitive lower loads are run last where the chances of interference by external matters is minimised, and the chances of the instrument being fully operational in terms of temperature are maximised.
Much more valuable information about nanoindentation can be found in Fischer-Cripps' free downloadable IBIS Handbook of Nanoindentation
This information has been sourced, reviewed and adapted from materials provided by Fischer-Cripps Laboratories.
For more information on this source, please visit Fischer-Cripps Laboratories.