In-depth quantitative analysis of conducting coatings by radiofrequency glow discharge optical emission spectrometry: influence of the source operation methodology

Abstract

Two operation modes using a laboratory-built radiofrequency (rf) glow discharge (GD) are studied for in-depth analysis of conducting, Zn-type coatings based on the emission yield concept: 'method 1' which uses constant pressure and constant delivered power with variable dc-bias and 'method 2', using constant pressure and constant dc bias but variable rf power. For steels with a coating of electroplated ZnNi, selected as a model, good qualitative profiles were observed in both operation modes. Good correlation factors for emission yields were obtained with a simple calibration for emission intensities of the element spectral lines versus the product 'sputtering rates × elemental concentrations' at the operating conditions with standard samples of different matrices. Therefore, the conversion of the measured emission intensities of the qualitative profiles into elemental concentrations and the erosion time into depth reached was also pursued. Both methods proved to give appropriate results in terms of quantitative depth profiles for electroplated ZnNi coatings. Quantitative profiles for galvanneal and hot dipped zinc were also obtained with both methods. Method 1 provided slightly better results in all cases. The behaviour of emission yields of the several elements (Zn, Fe, Si, Ni, Al and Pb) measured in different matrices (certified reference materials) has been studied versus delivered dc bias (method 1) and discharge power (method 2). Results showed that Zn and Fe emission yields were not influenced by dc-bias or power (at the operating conditions used). Conversely, emission yields calculated for Si, Ni, Al and Pb showed a decreasing trend with increasing dc-bias and also with decreasing delivered power to the discharge.