Measurement of Gold Coatings in the Watch Industry
In the medium price segment, “gold” watches are generally made of stainless steel coated with 20 microns of an 18ct Au alloy. The gold plating should serve decorative purposes and at the same time being durable. This expensive coating must be controlled by precise measurements to guarantee performance while limiting cost.
A minimum thickness of 20 microns of 18ct Au alloy is required to ensure the longevity of the gold plating, as the coated parts include the case, the bracelet and the crown – all of which are subject to wear.
Usually, Au coatings are measured using the well established, non-destructive X-ray fluorescence method (XRF). However, the maximum coating thickness determinable with XRF is 10 microns of 18ct Au alloy, necessitating a different non-destructive method: Beta backscattering. The maximum coating thickness measurable with Beta backscattering depends on the radiation source. For watch applications, Strontium is the right choice.
Fig 1: Measurement principle of the beta backscattering method.
Using a Strontium radiation source, beta particles are emitted with high energy (2.27 MeV) to the specimen. The backscattered beta particles from the specimen are then detected by a Geiger-Müller counter. The software of the FISCHERSCOPE® MMS® PC2 calculates the coating thickness from the irradience. A Strontium radiation source allows Au coatings of up to 35 µm to be measured with this method. The size of the measurement spot is determined exactly by the colli-mator used, typically 0.6 x 1.2 mm for this application. Additional sizes are available and collimaters can be changed out easily for other purposes.
Fig 2: Coating thickness measurement of an 18ct Au-plated watch case using FISCHERSCOPE® MMS® PC2 BETASCOPE®
The FISCHERSCOPE® MMS® PC2 multi-measurement system, with its integrated Windows™ CE and network capability, features a large, high-resolution color touchscreen as well as LAN and USB ports. It is perfect for straightforward and nondestructive high-precision coating thickness measurements, as required by the watch industry.
The thickness of 20 µm 18ct Au layers on plated watch parts can be measured non-destructively and easily using the beta backscattering method and the FISCHERSCOPE® MMS® PC2 BETASCOPE®. For further information, your local FISCHER partner will be glad to assist you.