The Nanoscale World

Donald Cohen, Ph.D., has been using white light interferometers since 1994 to help industry solve problems related to squeaks, leaks, friction, wear, appearance, adhesion and other surface texture related issues. His company, Michigan Metrology, LLC, has worked extensively with the autfomotive and biomedical device communities, as well as many other industries, to leverage the various measurement and analysis capabilities of surface profilometers in optimizing component design, eliminating failures, and creating more efficient and more robust components. Often, companies wish to improve quality or performance but are unsure what measure can be used to control their processes. Comprehensive surface metrology can quantify texture, form, critical dimensions, and defects to give these companies data that can be compared against theoretical models or known performance issues. Those parameters that strongly correlate to improved part function or quality can then be incorporated in development and quality control processes.

The useful applications for this technology have been widely spread across the auto component industry. Cohen has utilized Veeco’s Wyko(R) surface profilers extensively to gain insight into the function of critical automotive engine parts, such as cylinder bores, crank shafts, pistons, and various transmission and braking elements. In many instances, he found that the surface texture was related directly to the friction at the component interfaces and their propensity for wear. For example, three-dimensional optical profilometry has helped brake manufacturers improve the frictional interface and reduce the noise and vibration resulting from the interaction between the brake rotor and friction pad.

Similarly, automotive transmissions have a number of critical interfaces, such as clutch and gear contacts, in which optical profilometry has proven useful for functional optimization. As the auto industry moves toward more “drive-by-wire” applications, the three-dimensional texture/geometry measurement of electronic sensors, electronic controls and other MEMS devices has been essential in design optimization. Similarly, careful attention is required in the design and manufacture of fluid handling components for flammable liquids such as gasoline. Three-dimensional optical profilometry has contributed greatly to the understanding of the surface texture of these pump mechanisms and sealing interfaces. As quality demands have increased, the industry has continued to drive dimensional tolerances to less than 1 micron and surface texture average roughness requirements to less than 25 nanometers. The success of Michigan Metrology’s measurement service is a testament to how beneficial Cohen’s three-dimensional characterization of surface wear has been in understanding issues underlying expensive warranty returns.

As the technology behind optical profilometry evolves, new measurement capabilities are enabled, and Cohen’s consulting work is often on the lookout for new techniques and surface parameters. One key addition has been the development of an extra-long working distance objective lens that allows the measurement of hard-to-reach surfaces. Additionally, with the use of special fixtures and external mirrors, component internal surfaces can be measured without requiring sample sectioning. Thus, an inner surface of a component can be measured before and after use to study surface wear.

3D surface micro-texture measurement of a connecting rod pin bore ID using Veeco's NT8000, an extra-long working distance lens, and Cohen’s mirror arrangement.

The biomedical device field likewise has offered additional metrology challenges for Cohen’s firm, such as measurements requiring non-contact, high-speed, three-dimensional texture analysis of geometrically complicated structures. Typically, Cohen utilizes his Veeco profiler to investigate the small complex geometries found in biomedical components, rather than using a more conventional stylus device. The ability to measure features such as balls, sockets, sliding interfaces and fine wires non-destructively has allowed for the 100% inspection of components prior to patient implantation. Additionally, with the high speed measurement modes, custom display creation and database features, high-volume inspection of components are now possible to assure device quality as products move from development to mass production.

3D surface micro-texture measurement of an orthopedic spinal implant using a Veeco NT8000 Optical Profiler with an extra-long-working distance objective.

To handle these difficult and often widely variable measurement requirements, Dr. Cohen has made full use of the great flexibility in speed, Z height, and field of view offered by optical profiling technology. These systems are also easily customized, and Dr. Cohen has developed a number of unique modifications to allow measurement at unusual angles and within less than ideal environments. The measurement of various orthopedic implants (e.g., knee, hip, spine, and dental) and fine structures (such as stents for cardiovascular applications) have been addressed with Cohen’s hardware/software adjustments and custom fixtures.

With many of the newer technology features, such as thick film measurement, high-speed large field stitching, and the latest ISO 25178-2 analysis parameters, optical profiling experts like Cohen are poised to enable further advancements in the automotive and biomedical fields.

References

1. D. K. Cohen, 3D Surface Micro-Texture/Wear Measurement and Analysis (Invited). Material Science and Technology, Sept 16-20, 2007, Detroit Michigan.
2. Ziv Mazor and Donald Cohen, “Preliminary 3-dimensional Surface Texture Measurement and Early Loading Results with a Microtextured Implant Surface,” The International Journal of Oral & Maxillofacial Implants, Vol. 18, No 5, 2003.
3. Mark Shuster, Dana Combs, Jim Pillar, Doug Burke and Donald Cohen, “Development of the Methodology for 3-D Characterization of Oil Shaft Seal Surfaces,” SAE 2002-01-0661, March 2002.
4. Hazem Barmada and Donald Cohen, “Surface Analysis of Bileaflet Prosthetic Heart Valve Leaflet Leading Edge,” ASAIO Journal , 1998.