Rk parameters for AFM inspection of Wear
and Surface Finish
When evaluating surface finishing processes and wear phenomena,
we find it valuable to examine AFM (Atomic Force Microscope) images
showing 3-dimensional surface structure, to calculate Ra (average
roughness), Rq (rms roughness) and related roughness parameters
from those images, and to graph height histograms and bearing
ratio curves.
Sometimes the above information does not give a full understanding
of the situation. For example, the Ra values may not correlate
well with qualitative surface differences that are obvious from
the images. We recently developed software to calculate the Rk
parameters from AFM data and to prepare histogram and bearing
data for replotting. We are excited about the value of this additional
information. It has, for example, allowed us to quantify the dramatic
difference between a new and a broken-in wear surface.
The Rk parameters give a numerical summary of information contained
in the bearing ratio curve (Abbott-Firestone Curve), based on
a division of the depth scale into three regions (top, or peak
region; middle, or core region; bottom, or valley region). See
graph below. An example of the interpretation is as follows. When
two surfaces rub together, the peak region is worn away during
the break-in period, the core region bears the load for the life
of the product, and the valley region is available as a lubricant
reservoir. Additional parameters estimate the amount of material
in each region.
Bearing Ratio for Microfabricated Surface
The Rk parameters are not calculated by standard AFM software.
Advanced Surface Microscopy uses its own Rk software when preparing
analytical reports for its clients.
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