From Splat Spreading to Coating Densification: Substrate Thermal Effects in Plasma-Sprayed NiCrAl-TiB2 Coatings

Controlling molten droplet deposition and coating densification is crucial for improving the surface properties of lightweight aluminum alloys; however, the relationship between substrate temperature, splat spreading–solidification behavior, and the resulting microstructure of atmospheric plasma-sprayed (APS) composite coatings remains insufficiently understood. The key novelty of this work is to establish a multiscale relationship between the temperature-dependent behavior of individual splats and the subsequent densification and mechanical properties of the bulk coating. Establishing how individual deposition units evolve into a dense coating structure is essential for the rational control of coating microstructural and mechanical integrity. This study investigated the droplet deposition behavior, microstructural evolution, and hardness of NiCrAl-10 wt.% TiB2 coatings fabricated on 6061-T6 aluminum alloy substrates at substrate temperatures of 230, 330, and 430 °C. Increasing substrate temperature promoted splat spreading and morphological continuity by delaying solidification and extending the liquid-state lifetime. On polished substrates, the average splat diameter increased from 47.82 ± 5.4 to 57.54 ± 3.1 μm, while coating porosity decreased from 1.48 ± 0.42% to 0.55 ± 0.26%. Meanwhile, microhardness increased from 317.7 ± 3.9 to 336.3 ± 3.2 HV0.1. The dominant γ-Ni(Cr,Al) phase remained unchanged. These results establish a direct mechanistic link between substrate temperature, individual splat evolution, and bulk coating properties, providing a mechanistic basis for optimizing APS fabricated NiCrAl-10 wt.% TiB2 coatings.