ThinLayerAnalysis
Investigation of the copper plating on a decorative brass plated hinge
Decorative
materials, such as brass, are generally not plated directly
onto the base metal, but onto an intermediate copper layer. This
investigation was aimed at determining the copper
layer thickness, and also to see if this layer is used to
make a
smoother surface to improve the appearance of the final layer.
Figure
1
shows the hinge, together with a one penny coin for scale.
The test craters can be seen on the right hand
leaf of the hinge, two are visible, one to breach the decorative layer
and expose the copper and other through the entire stack.
Figure 1
Figure 2 ball crater in the
plated hinge
The
ball craters were made using a 30mm ball. This size was chosen to give
adequate magnification of both layers, without taking excessive
time.
The
ball crater shown in figure 2 is the deeper of the two that were made
and shows all three layers. From this single crater the
layer thickness of both the copper (5.0 µm) and the
decorative coating (2.4µm) can be calculated.
However, other useful information can also be
gained from the image.
The
direction of the final surface finish is the same as that of the
original rolled steel base. This can be seen from the crater
base which is squarer than would be expected, indicating that the base
/
copper interface is rough. At the upper side of the crater
base a copper track can be seen crossing through the steel and, in the
lower right corner of the steel, a copper island can be seen.
This is where the copper has filled low points in
the steel
surface.
From this image the extent and form of the surface finish of
the original rolled steel sheet can be determined.
Although
this looks very dramatic it must be remembered that the magnification
of the vertical scale is greatest at the crater centre, and, in this
case, the middle of the crater is less than one micron below the steel
/ copper interface!
Inspection of the copper - brass interface does not
show
the deep grooves visible in the steel base, so it does appear that
the copper layer provides an improvement in surface finish
for the decorative brass coating.