FISCHER FISCHERSCOPE X-RAY 5000 Series, Operator'S Manual, Page: 88 / 188

Mass per unit area and geometric coating thickness Correlation between mass per unit area and coating thickness
88 FISCHERSCOPE
®
X-RAY
Essentially, the measurement of the x-ray fluorescence is an
indirect count of the excited atoms under a surface with a precisely
defined size, the so-called “measurement spot”.
13.1 Correlation between mass per unit area and
coating thickness
If a coating is comprised of several elements, the instrument initially
measures the mass per unit area of each individual element.
Then the respective masses per unit area of all elements are added
taking into account their weight proportions, and a mean value is
computed.
The division of the mean value of the mass per unit area through
the weighted mean density provides the sought coating thickness
Th.
Th = Fm
ges
/

mean
NOTE: Important note for practical applications:
The computation assumes a homogeneous composi-
tion of the sample. When preparing the sample, en-
sure that a representative density is stated. The
analysis may provide a wrong result if samples are
not homogenous and only a small portion of the sam-
ple is used for the analysis.
!
Important:
The thickness measurement using X-ray fluorescence assumes a
well-defined and constant density

.
The two schematic drawings in Figure 2 show that the X-ray
fluorescence method is actually blind to the (geometric) thickness:
Two coatings with the same geometric thickness are presented
here (prerequisite: same material, i.e., coating element).
The presentation of process 1 shows the atoms of the coating being
stacked “loosely”. However, this cannot be measured as the
spectrum does not show a measurable line. The reason may be that
an additional “matrix element” is present in the coating.
The presentation of process 2 shows that the atoms are
significantly closer to one another.
Correspondingly, the density is greater than the one shown in
Figure 1. The XRF method can provide the correct (geometric)

mean
mean specific density 1/

tot
= 1/

1

c
1
+ 1/

2

c
2
+ 1/

3

c
3
+ ...
1, 2, ... Continuous number of the elements in the analyzed coating
c Element concentration
Th Coating thickness