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.
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Figure 1. The IBIS Nanoindentation system from Fischer-Cripps Laboratories.
Testing Unknown Specimens
Once you have confidence in the integrity of the indenter, and that the readings of the instrument are as expected for the standard specimen, you are then ready to test your own specimen. The theoretical basis of the measurement depends on the specimen surface being flat, atomically smooth, and perpendicular to the axis of the indenter shaft.
Surface Roughness
Naturally, all real specimens will have some degree of surface roughness. The rougher the surface, the more variable will be your results. A suitable choice of maximum load will be required to suit the surface roughness of the specimen.
Samples and Sample Preparation
For most applications, the indentation will be performed on the top surface of the specimen. Most specimens are small coupons of about 10 mm size. Larger specimens may be tested if they are properly mounted. The surface of the specimen must be free from grease, finger prints, dust, and preferably free of oxide layers. Ultrasonic cleaning may be required. Any polishing compound residue will have to be completely removed. Even washing with acetone can sometimes leave an oily film on the surface which will affect the results.
Oxide Layers
Oxide layers of a few nm can form on most metal surfaces within a few microseconds and usually cannot be avoided. Care should be taken in the interpretation of results if testing metals.
Mounting Samples
In most nanoindentation instruments, specimens are mounted on removable metal mounts. The attachment of the specimen to the mount is very important. A common mistake for beginners is to use too much glue. A heat softening glue such as “Crystalbond” is suitable for specimen mounting but it is important to ensure that the specimen makes good mechanical contact with the mount, and that the glue only fills in the asperities in the contact surface. If too much glue is used, the compliance of the glued joint will affect the results.
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Figure 2. Specimens mounted on a specimen mount.
Positioning the Specimen
The specimen mount, with specimen attached, then needs to be positioned on to the XY stage of the instrument. The most reliable contact is a magnetic mount, but some instruments use clamps. When using clamps, especially those which clamp the specimen mount from the side, it is important that the clamp does not tilt the mounting up and that good mechanical contact in the Z direction is always achieved.
The Importance of Thermal Equilibrium
It is important that the test not proceed until the sample is at thermal equilibrium with the instrument. Simply handling the specimen for a short time will require at least 10 minutes soak to allow for thermal equilibrium. One of the most common errors in making this type of test is the introduction of finger prints on the surface. The surface has to be absolutely clean for meaningful results to be obtained.
Much more valuable information about nanoindentation can be found in Fischer-Cripps' free downloadable IBIS Handbook of Nanoindentation
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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.