High-resolution x-ray tomography workflows to investigate vitreous enameled steel plate full field morphology

Vitreous Enameled Steel plates are a special class of metal-ceramic composite materials obtained by the deposition of vitreous-ceramic layer over the metal surface and the subsequent firing at high temperature. Enamel raw material is made by a mixture of boron-silicate vitreous matrix (frits) and other metal oxides as additives [1-4]. During firing a functionally graded interface is produced [5,6] and it determines the adhesion between coating and substrate. Moreover, during the firing phase several gases [7,8] are release and entrapped into the vitreous matrix in the form of bubbles. The number of bubbles, their dimensions and spatial distribution influence the enamel coating elastic behavior and mechanical strength. Previous finite element models (FEM) [6,9], micrograph analysis based, revealed that bubbles have a great influence on residual stresses distribution and on coating mechanical strength. The idea underpinning the present work is to use High-Resolution X-Ray Tomography (XCT) to obtain the 3D-full field distribution of bubbles in the enamel coating with the purpose of stress analysis. One of the major issue related to the application of XCT to the enameled steel material is due to the great mismatch of x-ray absorption between the metal substrate and the enamel coating. This leads to extremely low contrast and the presence of phase contrast artifacts which compromise the vitreous enamels segmentation process. The aim of the present study is to define a XCT-based protocol in order to properly visualize vitreous enamel layer including bubbles. Three different samples of enameled steels were investigated differing each other for the chemical composition, for the manufacturing process and bubbles. Each specimen was scanned by means a XCT with a voxel size of 1 micron with projections acquired over 180°. The specimens were successfully scanned reducing the artifacts. Each scan was then cropped by using ImageJ and finally segmented and meshed using MeVisLab. These successfully reached meshes will be suitable for future full field finite element modeling investigations of enameled steel materials mechanical properties.