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Accurately testing exact hardness of a material, in depth

July 30, 2013 — In today's precision manufacturing environment, designers of products as diverse as car airbag sensors, computer microchips, drill bits and paint often need to know the mechanical properties of their materials' down to the nanometer scale. Scientists have now built a machine that sets a new standard of accuracy for testing one of those properties: a material's hardness, which is a measure of its resistance to bumps and scratches.

The new machine is called the Precision Nanoindentation Platform, or PNP. It was created in response to the need to test tiny novel devices, components and coatings in diverse industrial settings, said Douglas Smith, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, who was part of the design team.

"In the material science community there are more and more components and materials that just don't exist on the macro scale," Smith said. His team tested the new instrument's performance on a synthetic polymer known as poly (methyl methacrylate), or PMMA, which is a lightweight plastic used as a thin film during fabrication processes in the semiconductor industry and employed as thick panels in large aquarium tanks or the spectator protectors that ring hockey rinks.

The work is published in the journal Review of Scientific Instruments, which is produced by AIP Publishing.

How the New Instrument Works

The existing generation of nanoindentation instruments work by bringing a shaft with a tiny, extremely hard tip into contact with a sample and measuring how the sample surface deforms in response to a known applied force. In the past, these instruments typically have been designed to measure the deformation via the displacement of the tip and shaft relative to their mount, but this can lead to measurement error, because the instrument frame can deform under stress or drift due to random thermal gradients in the environment.

To avoid these effects, Smith and his team designed the PNP to measure hardness via the actual penetration depth of the indenter tip into the specimen. They did this by placing two tiny tuning forks on either side of the indenter tip that resonate at 32 kilohertz, well above the limit of human hearing. When the tips of the tuning forks approach the surface of the specimen being measured, they feel a slight attraction that subtly shifts their resonant frequency without causing any detectable deformation of the specimen surface. By sensing this shift, the machine continuously monitors the actual position of the tip relative to the specimen surface -- a process known as "top referencing" or "surface referencing."

The improvements built into the PNP allow it to test properties beyond the reach of previous nanoindentation devices, said Smith. For example, the machine can measure whether a material responds to pressure by deforming slowly over long periods of time, a process known as viscoelastic creep. "I don't want to say it is the best instrument out there, but it has certain advantages that we really like," said Smith.

While the PNP is state-of-the-art, don't expect to see it available for purchase any time soon. "We love the PNP," said Smith, but he added that it would be expensive and cantankerous to operate in an industrial setting. Instead, NIST scientists plan to use the machine to create standard reference materials and reference data for industry. Commercial instrument owners can then use these materials to calibrate the machines they use to characterize nano-scale components or ultra-thin coatings.

And for the rest of us? We can look forward to a new generation of ever more precisely built consumer products.

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