Page 44 - CFCM Mar-Apr 2021_Neat

PLATING AND ANODIZING: THICKNESS TESTING

THROUGH THICK

AND THIN

Thickness Testing Methods for Plating Technologies

XRF with Operator.
By Connor Stewart

chieving the correct coating thickness is imperative in the aimed at a sample. The incoming X-rays, or primary X-rays, strike
surface finishing industry. Manufacturers often require the coating surface and excite the elements within the coating. This
Acoatings to meet thickness requirements to ensure the pro- causes the excited elements to emit a unique, secondary X-ray which
duct’s functionality and appearance. Highly regulated industries, is detected by the XRF. Depending on the intensity and energy of the
such as the aerospace and automotive industries, have strict specifi- secondary X-rays, the XRF can determine both the composition and
cations on functional coating thicknesses and a thousandth of an thickness of the coating with impeccable accuracy. Since each
inch can be the difference between sellable product and scrap. element will produce a unique secondary X-ray, an XRF can measure
Even when parts have wide ranges for coating thicknesses like in the thickness of multiple layers and alloyed layers simultaneously.
some powder coating and zinc plating operations, ensuring that the For example, coatings like electroless nickel, which have specific
finished good falls within that range will help shops avoid costly mis- sulphur and nickel compositions, can be determined via XRF. While
takes. While it’s important to achieve the correct coating thickness, these laboratory-grade instruments can give invaluable information
monitoring the coating thickness is equally important. Factors such about coating thicknesses, it still has its limitations. XRFs require that
as how a coating is applied, the geometry of a part and the concentra- the coating and base substrate be discrete, distinguishable layers.
tion of product used will all influence coating thickness. For example, an XRF cannot determine the thickness of brass
Because of the importance and variability of coating thicknesses, over copper since both layers contain copper and will produce indis-
consistently and accurately measuring a coating is an integral part of tinguishable secondary X-rays. Additionally, coatings containing
any surface finishing application. The type of instrument employed lighter elements such as silicon or sulphur are difficult to measure.
to do so depends on the finish and the coating’s application. This These elements are easily excitable by the primary X-rays and pro-
article explores the principles of how these instruments work and duce an excess of secondary X-rays which are difficult to differenti-
when each instrument should be used. ate between. A very powerful, and consequently very expensive
detector is required for those applications.
X-ray Fluorescence While XRFs are expensive and require training and regular main-
Perhaps the most powerful and robust instrument used for coating tenance, handheld XRFs (HXRF) are becoming more available.
thickness testing is the X-ray Fluorescence Spectrometer (XRF). HXRFs operate under the same principle as laboratory XRFs and
This non-destructive and non-contact technique uses X-rays to while being more affordable and easier-to-use, these instruments
probe the thickness of both conductive and nonconductive coatings. often operate with a large margin of error and should not be used for
An XRF functions by producing and emitting X-rays which are thin layer applications.
44 CANADIAN FINISHING & COATINGS MANUFACTURING MARCH/APRIL 2021