Non-destructive optical characterisation of chromium conversion layers on aluminium

Chromium phosphate conversion coatings were characterised by spectroscopic ellipsometry. A two-layer optical model was used for the simulation and regression analysis procedures. The optical constants of the upper layer of the model are described by a dispersion relation. The second layer of the optical model is an interface layer, located between the aluminium substrate and the top layer. The optical model used both the Forouhi–Bloomer and the Sellmeier dispersion relations. In both cases, the optical constants of the top layer seem to correspond to those of chromium phosphate, provided the coatings are sufficiently thick. When thinner coatings are considered, the presence of other compounds or voids modify the optical constants. Nevertheless, an accurate coating thickness was determined for all conversion times.