X-ray microtomography of heavy microstructures: the case of Plasma-Sprayed Tungsten and Tungsten/Steel MMC

In this paper X-ray microtomography on Plasma Sprayed Tungsten (PS-W) deposited on ferritic-martensitic steel is presented and discussed. PS-W is a challenging material for microtomography since it exhibits a random porous network at different length-scales and it is hardly penetrable by X-rays. Results are presented by application of monochromatic synchrotron light at beamline ID-19 of the ESRF. The key challenges were, firstly, to optimize the beam parameters considering the inherent trade-off between photon energy (penetration depth) and spatial resolution and, secondly, to develop effective signal filtering algorithms. Despite the low signal-to-noise ratio detected, representative volumes of PS-W could be reconstructed with good image quality and micrometric resolution (voxel size = 1.4 μm). As an important by-result, we report excellent image quality and higher penetration depth by applying the same setup on a ferrous microstructure, namely a 6%W/Steel MMC used as interlayer between PS-W and a Steel substrate. The paper reports a detailed 3D morphological analysis of all inclusions in PS-W and W/Steel disclosing a complex connected pore network in both media. The analysis is presented in terms of, multiphase volume fraction, ratio of percolation and 3D shape descriptors. 3D percolation patterns are analyzed in detail and sensitivity to segmentation threshold for the noise-affected PS-W region is discussed. As a significant result, percolation of the porous phase was found throughout the entire coating thickness in PS-W. In W/Steel percolation was found in the perpendicular direction and interpreted as delamination onset caused by thermomechanical stress.