3.4.4 ElectroDeposited Coating Thickness SRM's

Stefan Leigh
Statistical Engineering Division, ITLCarlos Beauchamp
Metallurgy Division, MSEL There are numerous industrial applications of measurements of the thickness of coatings: assessment of wear or build-up, compliance verification for anticorrosive coatings, precheck for successful assembly of coated parts, determination of optimal thicknesses for conductivity, insulation, or solderability. The earliest prototypes of the now most commonly used instruments for quick nondestructive evaluation of coating thicknesses for magnetic coatings on nonmagnetic substrates or nonmagnetic coatings on magnetic substrates were in fact developed by the Electrochemical Processing Group of the National Bureau of Standards in the 1940's. While implementations vary, all such instruments make use of the attenuation or distortion of a magnetic probe's field by the nonferric coating or substrate. And since that time, NIST has produced and maintained primary coating standards and disseminated secondary standards in the form of Standard Reference Materials (SRM's) for coating thickness assay.

Five years ago a major redesign and reissue effort for this family of SRM's was initiated. The new SRM's consist of sets of chromium-copper-plated steel coupons certified for coating thicknesses in the 2.5$\mu$m-2000$\mu$m range. Members of SED participated in all initial and ongoing design, analysis, and Quality Assurance aspects of the project. Work over the past year has concentrated mainly on the analysis of the calibration curve relating very homogeneous coupons chosen as primary standards, with thicknesses measured by atomic absorption, to subsets of the numerous secondary - SRM issue - standards measured by magnetic induction.

After clean-up of outliers and transcription errors, and comparison of portions of the global calibration curve to segments generated using other methods (optical cross-section and x-ray fluorescence), suitable descriptions - models - of the calibration curve were sought. Global and segmented parametric and nonparametric versions of calibration models were considered. A global smoothing spline derivative plotted on a logarithmic scale gives strong indication of both how to segment and how to parametrically model the data. With appropriate segment points chosen, appropriate parametric models become EXP[cubic], EXP[cubic], RECIP[quadratic]. For certification purposes, uncertainties are quoted as confidence bounds on the respective model-predicted mean curves. For information purposes, simple statistics on the actual fluctuation of coating depth over the surface of the coupon are also given.

Figure 27: Derivative of smoothing spline fit to global calibration curve plotted on a logarithmic scale.

       

Date created: 7/20/2001
Last updated: 7/20/2001
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