Paper detail

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

In this paper synchrotron microtomography on Plasma Sprayed Tungsten (PS-W) is presented and discussed. PS-W is a challenging material for microtomography since it exhibits a random porous network at different length-scales (from nanometers to micrometers) and is hardly penetrable by X-rays. Furthermore, inner porosity causes strong internal scattering. 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 limited signal-to-noise ratio detected, large volumes of PS-W could be reconstructed with good image quality and micrometric resolution (voxel size = 1.4 μm). As an important result, we report excellent image quality and higher penetration depth by applying the same setup on a ferrous microstructure, namely a 10%W/Steel MMC used as interlayer between PS-W and a ferritic/martensitic steel substrate. The paper reports a detailed 3D morphological analysis of all inclusion types in PS-W and W/Steel, which led to disclosure of a complex connected porous 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 towards segmentation threshold for the noise-affected PS-W region is discussed. Remarkably, percolation of the porous phase was found throughout the entire coating thickness of PS-W. In W-Steel MMC percolation was found in the perpendicular plane and interpreted as onset of delamination caused by thermomechanical stress.