Unveiling the Oxidation Mechanism of Y2O3-Doped Cr2AlC: From Grain Boundary Diffusion Blocking to Interfacial Strengthening
The high-temperature oxidation resistance of Cr2AlC MAX phase ceramics is severely compromised by rapid Al depletion and the formation of a brittle sub-surface Cr7C3 layer. This study elucidates how trace Y2O3 doping (0.25 and 0.5 wt.%) modulates the oxidation behavior of Cr2AlC at 1100 °C. The incorporation of 0.5 wt.% Y2O3 significantly suppresses the parabolic rate constant compared to undoped counterparts. This kinetic suppression is attributed to the Reactive Element Effect (REE), where Y3+ segregation at α-Al2O3 grain boundaries inhibits outward Al3+ diffusion, shifting the scale growth mechanism to inward oxygen diffusion control. Consequently, this retarded Al consumption prevents the decomposition of the Cr2AlC substrate into Cr7C3. While undoped specimens fail due to volume contraction and Kirkendall voiding associated with the Cr7C3 interlayer, specimens with the most effective doping content within the investigated range maintain a stable, atomically sharp α-Al2O3/Cr2AlC interface devoid of voids and decomposition products. The superior adhesion of this interface is attributed to three synergistic factors: the elimination of volumetric mismatch induced by phase transformation, the mechanical interlocking provided by Y-rich oxide pegs, and the intrinsically higher interfacial bonding strength of the α-Al2O3/Cr2AlC system as confirmed by DFT calculations. These findings provide a mechanistic framework for designing oxidation-resistant MAX phase ceramics via reactive element doping.