Title: Rapid, Automated Measurement of Layer Thicknesses on Steel Coin Blanks Using LIBS Depth-Profiling

We report application of a near-real time method to determine layer thickness on electroplated coin blanks. The method was developed on a simple Laser-Induced-Breakdown Spectroscopy (LIBS) arrangement by monitoring relative emission line intensities from key probe elements via successive laser ablation shots. This is a unique LIBS application where no other current spectroscopic method (ICP or XRF) can be applied effectively. Method development is discussed, and results with pre-calibrated coins are presented. Copyright George Asimellis, Aggelos Giannoudakos, and Michael Kompitsas OCIS codes: 120.0120, 120.6200 , 140.3440, 300.6360, 350.3390 OSA Published by Introduction The Royal Canadian Mint (RCM) plating facility in Winnipeg, Canada, has identified a problem with performing Quality Control of their electroplated coin blanks. Typically, coins, of a ferritic steel core, are electro-plated with three (3) layers, two (2) layers of approximately 4-8 μm-thick nickel, separated with a middle layer of 7-10 μm thick copper. A rapid and automated technique was sought to perform near real-time quality assurance on production coin blanks to replace current methods of coin sectioning and subsequent optical metallographic measurements, because these mechanical methods are slow and labor intensive. Laser-Induced Breakdown Spectroscopy (LIBS) enables multi-elemental identification and quantitative analysis requiring little or no sample preparation. A schematic of LIBS arrangement is shown in Figure 1. A high-power laser pulse is focused just above or below the sample surface, creating a localized area of material removal (less than 1 mm -wide and sub-μm deep per shot). Particles from the ablated material are subsequently ionized and thus a hot plasma is created. Upon plasma cooling ions recombine, and consequent excited atoms relax, radiating atomic emission lines, characteristic of the ablated sample constituent elements. By spectrally examining the optical emission it is possible to identify the ablated area’s atomic composition. This is achieved by a highresolution spectrograph, on which the optical plasma radiation is directed, by means of fused-silica optics and steering mirrors. The spectrum is recorded on a gated, intensified CCD detector. Thus, specific quantitative element analysis can be accomplished in near real time . This work is focusing on the applicability of LIBS for rapid identification of plated layer thickness. The elemental composition of the ablated layer can be analyzed for every successive shot as the laser penetrates deeper into the material at a repeatable rate. Thus the number of shots required to penetrate a specific layer can be determined by monitoring the appearance of new probe elemental OSA Published by lines or the sharp decline/increase in the line intensities (photon count) of existing elemental lines. Thus LIBS can provide detailed knowledge of the layered depth profile on a very localized sample area, less than 1 mm-wide . It is this characteristic depth-profiling capability of LIBS that provided motivation for this work. An analytical technique for trace element analysis that has been applied to coin analysis is Energy-Dispersive X-Ray Fluorescence (EDXRF). Compared to this technique, LIBS is advantageous in the aspect of being able to detect all elements, independent of their atomic number, and, more important, can provide a detailed information of elemental constitution of each (sub-μm) ablation layer. In addition, because the area of the ablation crater is in the vicinity of 1 mm, detailed depthprofiling analysis is possible. For this development a simple laboratory LIBS setup was used (figure 1) using a fundamental Nd:YAG laser (YG 981, Quantel, France) with pulse energy 30 mJ and pulse duration of 7 ns, conventional laser optics delivery to the target and plasma optical emission collection, a CzernyTurner (HRP, Jobin-Yvon, Edison, NJ, USA) spectrometer fitted with 2,400 lines/mm holographic grating. A gated intensified CCD camera (GEN II, Andor Technologies, Belfast, UK) with 15% quantum efficiency (average) was used for optical signal detection. Coin samples were placed on a servo-controlled positioning stage. The programmable stage can position the sample on the desired location relative to the incident laser beam with μm precision, allowing the desired number of shots at each spot and then moves to the next spot or next coin.

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