ThinLayerAnalysis
Ball cratering
-
A large steel ball-bearing (typically 20mm - 40mm in
diameter)
is rotated against the surface of interest together with a fine diamond
slurry. This forms a shallow spherical depression
in the
test piece.
Where the depression cuts through coating layers these appear as
concentric rings that can be measured using a calibrated microscope.
From this, the coating thickness may be calculated.
However, unlike a bevel of constant angle, the magnification
of
the ball crater is greatest in the centre and reduces towards the
perimeter. A major advantage is that special test samples
are
generally not required and investigations can be made on finished
components. Tests typically take from a few minutes to an
hour
depending on the depth to be measured and the magnification required.
Examination
of the crater shape reveals not only the layer thickness,
but also the morphology of the buried interfaces. By making
the
crater at specific sites, defects in the coating can be investigated.
This can often reveal the reason for lack of adhesion
between
coatings or subsurface corrosion. As the ball crater
provides
its own depth calibration, abrasion
resistance of the material can be investigated by monitoring the
erosion rate.
Ball cratering is useful for layers of 200nm upwards. However,
the thinnest layer we have measured is a 90 nanometre thick TiN film
deposited on a silicon semiconductor wafer.
The above photograph shows the working part a simple ball crater tool.
The ball shown is of 40mm diameter and is resting against a
square aluminium pillar where the test sample would normally be
positioned. The bed of this machine permits a range
of vices and
holders to be rigidly fixed and the shaft on which the ball sits can
rotate at speeds from less than 60 rpm to over 1200 rpm, giving it
a very flexible perfomance.
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