iTSSe TSS ADVANCED MATERIALS & PROCESSES | MARCH 2026 47 iTSSe TSS case of Batch 2, the formation of individual agglomerations could be observed (see Fig. 3b). As presented by Racek, such artifacts can be attributed to substrate asperities, also known as “speckle formation[7].” The hypothesis formulated hereby is based on the impact of splats that have not undergone complete melting. These splashed particles are distributed radially and adhere to a projected area in the form of roughness peaks. After this initial stage, the speckles undergo an increase in size with each subsequent deposition cycle. In the specific microstructure from Batch 2, the speckles begin to form approximately between cycles 4 and 5. Therefore, it is assumed that the agglomerated solids from the initially degraded suspension replace the roughness peaks as seeding point. Once a sufficient quantity of material has accumulated inducing an adequate amount of elastic energy, vertical cracks begin to form even within the speckle. These cracks may also follow the radially oriented band line between the individual deposition layers. In addition, the formed speckles are estimated to mask the surrounding coating surface, thus reducing the adhesion quality (as indicated by the dark area around the speckle). The speckle formation also affects the surface topography. While the large splat artifacts are prominent for both batches, the speckles produce a rougher surface for Batch 2 (see Fig. 3c)[4]. This may also affect the coating thickness and DE. The larger coating thickness, as determined via measurement screw, exhibited greater variability due to the uneven surface texture. Conversely, the DE reflects the effect of the speckles through its reduced coating weight. In combination with the formed speckles, the larger particles resulting from the degraded suspension were expected to yield an elevated coating porosity. However, the data in Table 2 does not support this assumption and may also be an artifact of the increased particle temperature for improved particle deformability. CONCLUSIONS The present study focuses on the investigation of the impact of degradation on suspension for spray techniques. The findings can be summarized as follows: • An increase in the size of the solid fraction was observable over time in terms of suspension degradation. This phenomenon was attributed to the agglomeration of fine particles with disadvantageous surface-to-volume ratio. • It has been demonstrated that this suspension degradation can result in the development of what is colloquially referred to as speckles within the coating. The agglomerated solid content within the suspension functions as a seed. FEATURE Fig. 3 — Microstructures (a) for Batch 1 (left) and Batch 2 (right) with a magnified insert of a speckle (b). Vertical cracks are indicated by red arrows. The surface topographies (c) are shown for Batch 1 (top) and Batch 2 (bottom). The figures are partwise adapted from Schmitt et al.[4], published under CC BY 4.0. TABLE 2 — DEPOSITION EFFICIENCY (DE) COATING THICKNESS (tc), ARITHMETIC MEAN ROUGHNESS (Ra) AND POROSITY FOR BATCH 1 AND BATCH 2 DE, % tc, µm Ra, µm Porosity, % Batch 1 Batch 2 Batch 1 Batch 2 Batch 1 Batch 2 Batch 1 Batch 2 65.8 60.1 136.0 ± 4.0 158.0 ± 10.0 1.9 ± 0.6 2.7 ± 0.3 4.2 ± 0.2 4.1 ± 0.2 (a) (b) (c)
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