Summer 2007
In this Issue:
Focus On Metamaterials
MCL Nanoindenter is Ready for Prime Time
Chris Muhlstein, assistant professor of materials science and engineering, oversees the nanoindenter, which is a part of the Materials Characterization Lab user facility. Muhlstein has spent considerable time getting to know the instrument, and in particular, learning how to interpret the data, which is not always a simple task, he says.
"We have spent a long time finding out what the tool can and cannot do. Although nanoindentation instruments have been around for at least a decade, there is no standard method for gathering the data and debate still rages over the reliability of the data. For instance, there is no standard technique for selecting a good sample to work with. Now we've learned on our own how to do that. In fact, we now have more experience with a wider range of samples than any of the manufacturers' labs. They look to us for feedback, which makes working with this tool exciting. It has taken us a long time to feel like we can guarantee results that people expect from a user facility. The major hardware and software upgrades we've put in, including new imaging capabilities, make this a virtually new research tool."
The nanoindenter is designed to measure the mechanical properties of surfaces on a submicroscopic scale. The indenter takes a small diamond, which is shaped as either a pyramid or a sphere, and pushes the diamond tip into the surface of the material being tested. The depth of indentation can range from a couple of hundred nanometers to a maximum of 4 microns. Typically, the radius across the indentation will be 1/2 micron to 5 microns.
By measuring the force required to push the diamond tip into the material compared to the depth of the indentation, the instrument can determine the hardness of the sample. Seeing to what degree the material returns to its previous shape determines its elastic modulus - the stiffness of its atomic bonds. In addition to mechanical properties, the nanoindenter can be used to scratch films and coatings to see how they stand up to wear.
"The tool is unique in that it can probe a very small area at a very small depth," Muhlstein explains. At the same time it can map the entire surface of a material, probing individual grains all across a surface or across the interface of a composite material. Its most common application has been to evaluate thin films on a substrate.
Surface properties of a material often differ from its interior properties due to exposure to the outside world. By investigating these properties with the nanoindenter, researchers can engineer the surface to improve material properties. The tool has proven valuable for studying brittle materials, carbonaceous material coatings, and organic or inorganic coatings on glass. Some organic testing that is time or temperature sensitive may not be suitable for the nanoindenter, but otherwise the instrument is useful across both organic and inorganic platforms, says Muhlstein. "Our next upgrade may be to add some temperature control."
With a couple of years of experience on the instrument, Muhlstein feels confident he can offer researchers the kind of guidance they need to use the tool effectively. "One of the challenges with the nanoindenter is that we cannot measure a single value, because results vary with the depth of the indentation, and even with such variables as atmospheric conditions. We can't just make one measurement; we have to do protracted studies."
To get around that bottleneck, Muhlstein is introducing a new process, a kind of quick feasibility study. He or one of his graduate students will study a specimen for a day to see if the machine can get the data the researcher needs. This can save 4-6 weeks of training time that it would require for a graduate student to learn to get useful data. While the survey experiment will not provide publishable data, the researcher can use the results to make an informed decision about investing in training of students and research associates. "
There are few commercial instruments of this type and only a few places that offer the techniques we do here," Muhlstein remarks. "The tool is too expensive for most single user labs to afford. The University has several hundred thousand dollars invested in this equipment. It's unique to have this particular tool open to the whole university. Other universities I've talked to don't allow general access to the tool. But that is not what MCL is about. There are constraints on how users are trained and what tests can be run, but it is available to a broad audience."
To find out more about the nanoindenter, contact Chris Muhlstein at 865-1523 or Validate to view address - Send Email via form.
More information about this and other instrumentation is available on the Materials Characterization Lab website: http://www.mri.psu.edu/mcl/
MCL is part of the Penn State Materials Research Institute.
