Chapter Technical Meeting
Tuesday March 31st 2009
"The
Geometric Nature of Surface Roughness and Losses with the Skin Effect"
Christopher
A. Brown, PhD, PE
Professor
of Mechanical Engineering
Director,
Surface Metrology Lab
Worcester
Polytechnic Institute
Phone: 508 831-5627
brown@wpi.edu
http://www.me.wpi.edu/People/Brown/
Presentation
ABSTRACT: This
talk is about surface roughness and how to measure and characterize
it in the context of microwave transmission losses. Adhesion issues
will also be considered briefly. The skin effect says that the
amplitude of fields in a conductor decay with depth from the surface.
The amount of decay is indicated by the skin depth. The skin depth
decreases with increasing frequency in good conductors from
millimeters at 1Hz to hundreds of nanometers at 1THz. The
peak-to-valley roughness of ceramic substrates for microstrips can be
micrometers or tenths of micrometers, the same order of magnitude as
the skin depth, and maybe several times greater. This raises several
interesting questions about the interaction of the high frequency
fields with the roughness, and how to best characterize the geometry
of the surface roughness.
Most
of the work on surface roughness uses the arithmetic average
roughness or the root mean square roughness for characterization.
This talk examines the use of scale-sensitive fractal-based
parameters for characterizing surface roughness in a more functional
way. Most engineering surfaces are fractal-like so that the
path-length on profiles increases as the scale of observation
decreases. This leads to the problem of determining the appropriate
scale for characterizing surface roughness, and its possible
dependence on frequency.
Chris
Brown has been on the WPI faculty since 1989. In 1983 he completed
his doctoral work on machining at the University of Vermont. He
spent four years in the Materials Department at the Swiss Federal
Institute of Technology and two years at Atlas Copco's European
research center. Chris has published over a hundred articles on
surfaces and surface metrology and has a patent on a fractal method
for characterizing surface textures. He also develops software for
surface texture analysis. Brown is the vice chair of the US standards
committee for surface texture, and director of WPI’s Surface
Metrology Lab. He teaches courses on Axiomatic Design, Surface
Metrology, Manufacturing, and the Technology of Alpine Skiing.
