Sdr Analysis of Surface Topography

Ra characterizes the average roughness calculated from
a surface profile.

The Sdr (developed interfacial area ratio)
is a three-dimensional surface parameter that quantifies,
as a percentage, the increase in area due to the measured
relief compared to the projected area.
It thus provides complementary information to
roughness parameters calculated from a profile.

Two surfaces can therefore have a comparable Ra
while having very different reliefs.

Ra → profile reading.

Sdr → reading of the surface
development in 3D.

Sdr is most meaningful when interpreted
with roughness parameters and 3D observation
of the surface.

This information is complementary and allows
for a more complete understanding of the surface condition.

Two surfaces can exhibit a similar Ra value
while the shape, distribution, and organization
of their reliefs differ.

This is particularly relevant when comparing surfaces
obtained by different processes.

Mechanical polishing acts by abrasion on the surface.

Electropolishing acts by controlled electrochemical
dissolution.

With comparable Ra, these two processes can therefore lead
to different microtopographies.

3D observation and Sdr enable an objective assessment of
this difference beyond a single average
roughness value.

On pharmaceutical or critical equipment,
removing a piece of metal to analyze a scratch,
impact, or anomaly can be impossible
or disproportionate.

When suitable, taking a replica allows
for local reproduction of the microtopography
without cutting the equipment or removing metallic material.

Preparation and removal conditions
and reproduction fidelity are taken into account.

This approach provides objective elements
to determine if an anomaly requires monitoring,
further investigation, or intervention.

Sdr complements roughness parameters and
three-dimensional observation by quantifying
the additional area related to a surface’s relief.

Comparing measurements before and after treatment
highlights changes in
microtopography and provides an objective assessment of the resulting
finish quality, particularly after electropolishing.

Combined with a controlled measurement protocol,
Sdr thus constitutes a relevant indicator
for documenting results, comparing different
finishes, and verifying the achievement of topographic
objectives defined with the client.

Interpretation takes into account the material,
initial state, measurement conditions,
and desired function.
Other functional performances are evaluated,
when necessary, by appropriate complementary controls.