A novel method for assessing the coating uniformity of hot-melt coated particles using micro-computed tomography

Three-Dimensional Characterization of Dry Particle Coating Structures Originating from the Mechano-fusion Process

Dry particle coating processes are of key importance for creating functionalized materials. By a change in surface structure, initiated during coating, a surface property change and thus functionalization can be achieved. This study introduces an innovative approach employing 3D X-ray micro-computed tomography (micro-CT) to characterize coated particles, consisting of spherical alumina particles (d50 = 45.64 μm), called hosts, surrounded by spherical polystyrene particles (d50 = 3.5 μm), called guests. The formed structures, hetero-aggregates, are generated by dry particle coating using mechano-fusion (MF). A deeper understanding of the influence of MF process parameters on the coating structures is a crucial step toward tailoring of coating structure, resulting surface property and functionalization. Therefore, the influence of rotational speed, process time, and total mechanical energy input during MF is explored. Leveraging micro-CT data, acquired of coated particles, enables non-stereologically biased and quantitative coating structure analysis. The guest's coating thickness is analyzed using the maximum inscribed sphere and ray method, two different local thickness measurement approaches. Particle-discrete information of the coating structure are available after a proper image processing workflow is implemented. Coating efficiency and guest's neighboring relations (nearest neighbor distance and number of neighbors inside search radius) are evaluated.

Accuracy of a reference-free Monte Carlo approach for SEM-EDS thickness assessment of TiN coatings onto diverse substrates

Energy dispersive spectroscopy (EDS) experiments were carried out in the scanning electron microscope (SEM) on top of either stainless steel (SS), silicon or glass substrates covered with TiN nitride coatings. The nominal coating thicknesses were 0.43, 1.1, 2.1 and 3.0 μm. From the EDS spectra obtained, the ratio between the strongest peak coming from the substrate and the Ti_k peak, from the coating, was assessed at an increasing microscope accelerating voltage (V₀) between 10 and 30 kV. This ratio is afterward called Intensity-ratio (I_R). A methodology was developed for assessing the coating thickness from the experimental I_R and the V₀ by using Monte Carlo simulations of the dispersion of electrons inside the covered samples and the concomitant emission of characteristic X-ray photons. Three freeware Monte Carlo graphical user interfaces were used, and the predicted thicknesses were compared between them, namely Casino, MC-Xray and DTSA. At fixed V₀, numerical simulations predict a mono-valued and smooth relationship between I_R and coating thickness. Monte Carlo predictions on the coating thickness were compared with reference thicknesses independently assessed by cross-sectional micrographs obtained in the SEM. Absolute value of percentage discrepancy between the predicted and the reference thicknesses was lower than 30 %, wherein the lower the V₀, the less accurate the predicted thickness. In general, none of the three pieces of software compared outperformed the others, although predicted thicknesses significantly varied between them. For V₀ higher than 20 kV, 90 % of predicted thicknesses deviated less than 15 % from the reference value. Predicted thicknesses for TiN coatings onto Si substrates presented the largest discrepancy regarding reference values.