Microform Metrology

Technical Contact:	Jun-Feng (John) Song
	301-975-3799

What is microform?

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With developments in computer science and technology, microelectronics, precision engineering, and micromechanics, many engineering parts with small sizes and tight tolerances must be measured with high accuracy to ensure their engineering functional requirements. This includes one-, two-, and, three-dimensional measurements ranging in size from approximately 1 µm to 1 mm, combined with the measurements of curves, angles, geometric form, and position errors, as well as surface roughness and waviness.

How do we measure microform?

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By profiling methods such as contacting stylus techniques or by area profiling methods such as phase-shifting interferometric microscopy, confocal microscopy, or a rastered stylus technique, combined with different surface sampling procedures, profile extraction techniques and algorithms.

NIST microform calibrations for Rockwell diamond indenters

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Figure 1. Rockwell indenter.

Figure 2. A stylus instrument and x-y rotary stage.

The Rockwell indenter is a diamond cone with a 120-degree apex angle blended in a tangential manner with a spherical tip of 200 µm radius (see Figure 1). The calibration grade indenters specified in the ISO 6508-2:2005 and ASTM E18-07 standards require a mean angle of (120 ± 0.1) degrees and mean radius of (200 ± 5) µm, which must be measured over an x-y-z range approximately (900 x 900 x 300) µm.

A stylus instrument and an x-y-rotary stage (see Figure 2), combined with NIST designed calibration and check standards, calibration and uncertainty procedures, algorithms, and software, are used for this purpose. The expanded measurement uncertainties are ± 0.4 µm for the 200 µm tip radius, and ± 0.01 degrees for the 120-degree cone angle calibrations. Meanwhile, profile deviations from the least-squares radius, cone flank straightness, holder axis alignment error, and surface roughness can also be measured.

How the NIST microform metrology facility supports U.S. industry

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The NIST microform calibration system has shown the highest accuracy for calibrations of Rockwell diamond indenters in the world. Indenters calibrated here are used in the NIST Rockwell Hardness calibration facility in the Materials Science and Engineering Laboratory. Under a new CIPM Key Comparison project, NIST and several other national laboratories are teaming up to establish a worldwide unified Rockwell hardness scale based on precisely calibrated standard grade Rockwell diamond indenters.

Reference

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Song, Jun-Feng (John), Low III, Samuel R., Pitchure, David J., Germak, A., Desogus, S., Polzin, T., Yang, H., Ishida, H., Barbato, G., “Establishing a Worldwide Unified Rockwell Hardness Scale With Metrology Traceability,” Metrologia 34 (4), pp. 331-342, BIPM, Paris, France (1997).