Cristobalite-suppressed Al–SiO₂ ceramic coatings: DFT-guided nanoparticle design for durable oxidation resistance

To address the critical challenge of crack propagation in electrodeposited SiO₂ coatings caused by cristobalite phase transitions, Al nanoparticles were incorporated into SiO₂ coatings for enhanced phase stability and durable temperature oxidation resistance. The incorporation of Al inhibits and optimizes the generation of cristobalite, which suppresses the formation of cracks, thereby reducing the parabolic oxidation rate constant ( ${\mathit{k}}_{\mathrm{p}}$) of the SiO₂ coating by 32.4% after oxidation at 900 °C for 100 h. Density functional theory (DFT) calculations demonstrate that the introduced Al preferentially exists in a substitutional form, effectively stabilizing the K@SiO₂ lattice structure by inhibiting K migration-induced cristobalite precipitation. Thermodynamically, the negative solution energy (−4.367 eV) of K@Si_x−1AlO_2x confirms the spontaneous incorporation of substitutional Al into the K@SiO₂ lattice. Structurally, substitutional Al forms a shorter Al‒K bond (2.91 Å) than does the Si‒K bond (3.39 Å), which can mitigate K-induced channel distortion and hinder the migration of K. Charge distribution analysis reveals that the Mulliken charges of Al (1.040) partially neutralize the Mulliken charges of K (−2.187), reducing electrostatic repulsion and promoting localized bonding. Furthermore, the incorporation of Al can restrain the precipitation of the brittle Z-phase, further contributing to improved oxidation performance.