Microstructures of cold-sprayed coatingsinvestigated by X-ray Microtomography

The cold spray coating build-up results in a complex conformation of steadily embedded and highly deformed particles. The knowledge of the resulting threedimensional architecture of joined and interlocked particles is a key issue to be addressed in order to better predict mechanical and functional properties in relation with compacity and porosity of cold spray coatings. This is also notably true for the assessement of the characteristics of cold spray composite coatings. In this work, X-ray Microtomography (XMT) experiments and analyses are described in case of cold spray coatings. Such non-destructive technique was very helpful to enable a “true” visualization of the microstructure with a reliable assessement of the porosity, i.e. without artefacts such as smearing effects. In this paper, investigations have been achieved on a laboratory microtomograph and also on a high resolution large scale facility at the European Synchrotron Radiation Facility (beam line 19). The aim was to carry out the phase imaging of pores and cold spray matrix of various cold spray coatings involving pure metals of different atomic numbers. Four different Ag cold spray coatings obtained with four different powders has been imaged by XMT through attenuated contrast imaging. Porosity was measured in a very narrow range between 0.004% to 0.66% and both measurements made with laboratory and ESRF facilities involving 850 µm × 1160 µm × 2500 µm and 280 µm × 280 µm × 573 µm respectively were in good agreement. Further comparison has been achieved in case of Ag/Cu composites involving mixed powders. The resulting images of large scanned volumes were analyzed to discriminate and quantify each constituent distribution and content. Moreover, in addition to porosity assessement, the three-dimensional morphology of series of numerous individual embedded particles were also initiated through the development of advanced 3D segmentation of XMT images stacks of a Al cold spray coating sample. Therefore, the work presented here enabled the volumetric description of the actual morphology of numerous deformed cold spray particles in order to move from a two-dimensional assessement to a sound three-dimensional inspection and quantification of dense or slightly porous cold spray coatings, including composites.