Nondestructive optical characterization of chemical conversion coatings on aluminum

Chromium phosphate conversion coatings on aluminum have been characterized with nondestructive optical techniques. Complementary vibrational spectroscopy techniques, i.e., Fourier transform infrared spectroscopy and confocal micro-Raman spectroscopy, prove the presence of chromium phosphate as principal component in the coating. Additionally, aluminum oxide and indications for the presence of chromium oxide and aluminum fluoride are found. Reflection/absorption infrared spectroscopy (RAIRS) allows analysis of coatings as thin as 40 nm, while confocal micro-Raman spectroscopy is limited to thicknesses above about 150 nm. Compared to RAIRS spectra, the interpretation of Raman spectra is easier due to the absence of peak shifts which occur in the RAIRS spectra. Spectroscopic ellipsometry allows determination of the morphological characteristics of the conversion coatings, e.g., the coating thickness, using a simulation and regression procedure based on a two-layer optical model. The optical constants of the upper layer, which in a first approximation can be attributed to the chromium phosphate part of the conversion coating, can explain the greenish appearance of the thickest conversion coatings. A linear relationship exists between the coating thickness and the conversion time. An analogous linear relation exists between the conversion time and the peak areas of most of the absorption peaks in the RAIRS spectra.