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.
Selecting the Right Load for Nanoindentation Measurement
Selecting the right load for the specimen to be tested is a necessary step in nanoindentation measurement. The maximum load chosen determines the depth of penetration of the indenter into the surface. If the load is too high, then the indenter may penetrate too far – a serious problem when testing thin films because the results will be then influenced by the properties of the substrate. On the other hand, if the load is too low, then the results will be affected by the roughness of the specimen surface. Hand-in-hand with load selection is the indenter geometry. For a sharp-tipped indenter, at low load, the sharpest possible tip should be used so that a fully developed plastic zone occurs at the shallowest possible depth. For sphero-conical indenters, a very high load may be required for large radius indenters.
Selecting Maximum Load and obtaining Reliable Results
The choice of maximum load for nanoindentation testing depends upon the nature of the specimen (whether it is a thin film, a thick polymer coating, bulk steel, a ceramic etc). If a low load is required, say from about 50 mN up to 1 mN, then certain aspects of the response of the instrument, and the specimen must be considered in order to obtain reliable results.
The Importance of Tip Sharpness
If the objective of the test is to measure elastic modulus, then all that is required is a “clean” load displacement curve and an accurate area function. The actual tip sharpness is immaterial although a sharper tip provides more “depth” and hence a better signal to noise ratio. If the objective of the test is to measure hardness, then at low load, the sharpest possible tip is required so as to obtain a fully developed plastic zone. It should be borne in mind that the instrument really measures the mean contact pressure, which can only be equated to hardness when the plastic zone is fully developed and the mean contact pressure has become constant. In both cases, measurements of E and H at low load require the best possible surface – the equations used to calculate these quantities assume an atomically flat surface and any surface roughness that is on the scale of the penetration depth will cause scatter in the results.
Figure 2. Differences in hardness H as a function of penetration depth on a hard thin film for a blunt indenter and a sharp indenter.
Taking Measurements at Low Loads
To obtain data at low load, the laboratory conditions must be as stable as possible with respect to both mechanical/acoustic noise and temperature stability. The test parameters must also be arranged so that as much data as possible is collected in the shortest possible time so that the effect of any thermal expansions or contractions at the contact are kept to a minimum.
Factors that Affect Hardness Values
Be wary of results of any nanoindentation instrument for depths of penetration less than 50 nm. The sharpness of the tip, the surface condition, and the nature of the specimen material (particularly the ratio E/H) have a very significant effect on the reported value of H.
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.