Reaction-sequence-regulated Y2O3/t-ZrO2 coherent interfaces enable Y diffusion and YSTZ toughening in ceramic coatings on AZ91D magnesium alloy

As a prototypical structural metallic material, AZ91D magnesium alloy is widely used in aerospace and automotive applications due to its low density and high specific strength, but its poor surface durability remains a critical limitation. To enhance service reliability, ceramic coatings have been developed as protective barriers. In ZrO 2 -doped plasma electrolytic oxidation (PEO) coatings, a key challenge is the inefficient stabilization of yttria-stabilized tetragonal zirconia (YSTZ), caused by severely hindered Y 3+ interfacial transport under non-equilibrium discharge conditions. The rapid and competitive formation of Y 2 O 3 and ZrO 2 generates structurally disordered or weakly coherent interfaces, increasing the diffusion barrier for Y 3+ migration and suppressing t-ZrO 2 stabilization, thereby degrading fracture toughness. To address this issue, a reaction-sequence-regulated PEO strategy combined with machine learning was developed to construct a low-interfacial-energy Y 2 O 3 /t-ZrO 2 coherent interface, converting it from a diffusion barrier into a transport pathway. First-principles calculations reveal a reduced Y 3+ migration barrier of 0.566 eV (9.87% lower than conventional PEO). Optimization shows that interface continuity and strain homogeneity are highly sensitive to Y concentration, where both deficiency and excess suppress diffusion. The optimized coating achieves 74% YSTZ with significantly improved fracture toughness while maintaining high hardness. This work establishes a mechanistic link among reaction control, coherent interfaces, strain-regulated diffusion, and macroscopic performance, providing a general strategy for toughened ceramic coatings.