Encapsulation of Reactive Ti₂AlC and Nb₂AlC Particles via a Boehmite Precipitation Route

Reactive Ti₂AlC and Nb₂AlC particles are encapsulated in a continuous boehmite shell by hydrolysis of ultrafine AlN powder in aqueous environment. The hydrolysis of AlN leads to the heterogeneous precipitation of nanolamellar boehmite on the surface of the M_{n + 1}AX_n phase particles after 24 h reaction time. The boehmite coatings completely cover the MAX phase particles (Nb₂AlC d₅₀ = 7.9 μm; Ti₂AlC d₅₀ = 9.6 μm) with a thickness of 200 nm (Nb₂AlC) and 135 nm (Ti₂AlC). The lamellar structure of the boehmite coating is maintained after calcination at 400 °C in air, resulting in coatings with a maximum surface area of 41 m² g⁻¹ (Nb₂AlC) and 50 m² g⁻¹ (Ti₂AlC). The stability of the coating is assessed by using the coated Nb₂AlC as particle filler in ZrO₂. The coating is stable after the spark plasma sintering process and covers complex features of the particles. The present work shows the feasibility to encapsulate reactive MAX particles with a continuous oxide shell. The coated particles can be used as ductile crack bridging or healing filler in reactive matrixes requiring a spatial separation between filler and matrix. Higher surface charges by Al–O bonds on the surface facilitate the dispersion of the carbide particles in aqueous slurries.