A shower head generally has a glossy silver appearance, one might assume solid metal. However, the substrate is very often plastic. The typical layer composition is chrome/nickel/copper on a plastic substrate.
The decorative chrome coatings are in the region of 0.5 μm or even less, the nickel coating is 5 – 10 μm, the copper layer is up to 30 μm or even thicker. The x-ray fluorescence (XRF) method is a non-destructive method of measurement to be used.
Figure 1 shows the measurement setup on a FISCHERSCOPE® X-RAY XDLM®. A measurement application determining the chrome, nickel and copper coatings on a plastic component was used to carry out the measurement.
Figure 1: Shower head positioning in the FISCHERSCOPE® X-RAY XDLM®
The following points must be considered in this measurement:
Alignment of the specimen to the detector (proportional counter tube).
The measurement should always be taken at the highest point. In addition, it is important to know how the detector is aligned in the measuring device. If the shower head is arranged longitudinally to the detector, smaller displacements of the measuring position do not have such great effects on the result.
Figure 2: Measuring points for XRF and coulometric measurements
The specimen must not be tilted, i.e. the point to be measured must be absolutely horizontal. Particular attention must be paid to this requirement as there are often no flat surfaces on the specimen.
Correct video focusing on the measuring point
If the focus is not correct at the measuring point, the analytical software will assume an incorrect measuring distance. This can lead to faulty measurements.
XRF saturation thickness
There are physical limits to XRF measurement. If the coating thickness is too great, there is a risk of saturation. This limit can be estimated on devices with WinFTM® software for an existing measuring application.
In Figure 3, the measuring range limit was determined for the copper coating, on the assumption that the chrome and nickel finishing coatings are 0.2 μm and 7.5 μm thick respectively. The measuring range for the copper coating beneath these is approximately 1 to 25 μm.
Figure 3: Estimate of the measuring range for the copper coating (WinFTM® simulation)
If all points are taken into careful consideration, the shower head can be measured non-destructively by XRF.
Figure 4: Coulometric removal in 3 steps: Cr/Ni/Cu
There is also the alternative option of determining the layer thicknesses of Cr/Ni/Cu coats coulometrically. In this method, the coatings are successively removed at the measuring area and the coating thickness determined in the COULOSCOPE® CMS by way of the time taken for removal.
Table 1 shows the coating thicknesses measured: in the XRF method, four locations were measured around the each of the coulometric measuring area . The coulometric results each consist of a measurement for each coating element. The values measured for the two methods are very close to one another, but a practised observer would note that the methods do not come to exactly the same results.
The reasons for the differences may be found in the positioning of the specimen in XRF measurement; inhomogeneities in the coatings may also play a role. In the case of copper, it is also true that the specimen is close to the XRF saturation thickness and that measuring inaccuracy increases strongly as a consequence
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Table 1: Comparison of measurement results on the shower head
The COULOSCOPE® CMS offers the capability of measuring copper coatings up to approximately 50 μm thick. The coulometric method can thus represent an important complement to the XRF method.