Hydrothermal silica coatings on aluminum powders: interfacial chemistry, grain refinement, and mechanical response upon sintering

A surfactant-free hydrothermal process was designed and explored for the conformal coating of SiO₂ on aluminum powders, with a focus on understanding the coating mechanism and its effect on the microstructure and mechanical properties of the sintered materials. The coating process was found to involve a series of hydrolysis reactions of the silicon precursor, followed by the adsorption of silanol groups on the hydroxylated aluminum surface, and subsequent condensation reactions to form stable Al-O-Si bonds. Microstructural analysis confirmed the presence of a multilayer core-shell structure consisting of an alumina-rich inner shell, an Al-Si-O interfacial transition region, and an amorphous silica outer shell. After pressing and sintering at 650°C, the SiO₂-coated powders exhibited pronounced microstructural refinement. After pressing and sintering, the average grain size of the unmodified aluminum was 46.9 ± 10.5 μm, whereas the SiO₂-modified sample exhibited a significantly smaller grain size of 28.5 ± 8.7 μm. The study shows a clear improvement in aluminum's properties, with strength and ductility increasing simultaneously. The ultimate tensile strength increases from 64.9 ± 0.7 MPa to 95 ± 1.7 MPa, with the elongation at break rising from 2.83 ± 0.12% to 7.61 ± 0.08%. This proves that hydrothermal oxide coatings can stabilize grain boundaries while achieving strong interfacial bonding, thereby enabling enhanced mechanical properties in aluminum alloys. This technique provides a green, cost-effective, and sustainable solution for developing advanced powder feedstocks for powder metallurgy, which can be used for the automotive and aerospace industries.