Micro X‐ray diffraction and fluorescence tomography for the study of multilayered automotive paints

Abstract

Combined microscopic X-ray fluorescence/microscopic X-ray diffraction (µ-XRF/µ-XRD) tomography is a recently developed method that allows the visualization of the distribution of chemical elements and the associated crystalline phases inside complex, heterogeneous materials of extended thickness (millimeter range) in a nondestructive fashion. In this paper, the accuracy and resolution with which the individual layers in a multilayer stack of automotive paints can be distinguished is evaluated, and some of their properties measured. A paint layer system of eight layers was investigated, in which eight different crystalline substances were identified, each layer consisting of an organic, synthetic resin doped with finely milled inorganic compounds that serve as pigments or to strengthen the layer. In the XRD tomograms, all paint layers could be straightforwardly distinguished and their average thickness calculated. In case the filtered back projection method was used for tomogram reconstruction, a spatial resolution comparable to the microbeam size was obtained indicating no significant reconstruction blurring. When a more robust reconstruction method, such as the maximum-likelihood expectation maximization method, was employed, tomograms showing fewer artifacts were obtained, but with a spatial resolution that was two times worse. In the corresponding XRF tomograms, significant self-absorption distorted the element-specific tomograms corresponding to the low-energy ( Keywords: X-ray diffraction tomography; X-ray powder diffraction; automotive paint; pigment identification IntroductionFor the three-dimensional characterization of meso-to millimetersized objects in terms of their chemical and structural properties, several X-ray-based imaging techniques have been developed. The major advantage of these methods is that the three-dimensional structure and other properties of such materials can be determined in a noninvasive and nondestructive manner, i.e. without physically sectioning the materials under investigation. Many situations exist in which it is undesirable or not allowed to destroy material during an analysis: for example, forensic materials must be retained as judicial evidence; precious materials should retain their value and not show traces of sampling; unique cultural/paleontological artifacts should remain available for other investigations, etc.Many of these methods involve tomographic scanning of the sample materials via irradiation with penetrative types of radiation. In this paper, we devote our attention to the possibilities offered by combined X-ray fluorescence/X-ray diffraction microbeam (µ-XRF/µ-XRD) tomography for nondestructive characterization of paint multilayer systems. Such materials are characterized by a relatively large number of 5-50 µm thick layers of X-raytransparent materials (alkyd or acryl resins), doped with a variety of fine-grained inorganic components and therefore represent, on one hand, materials with a well-defined two-dimensional structure while, on the other, contain a ...

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