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 d(50) = 7.9 mu m; Ti2AlC d(50) = 9.6 mu m) with a thickness of 200 nm (Nb2AlC) and 135 nm (Ti2AlC). The lamellar structure of the boehmite coating is maintained after calcination at 400 degrees C in air, resulting in coatings with a maximum surface area of 41 m(2) g(-1) (Nb2AlC) and 50 m(2) 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.