In-situ synthesis, microstructure, and properties of Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] composite coatings by plasma spraying

The high melting point and strong chemical bonding of Nb[B.sub.2] pose a great challenge to the preparation of high-density nanostructured Nb[B.sub.2] composite coating. Herein, we report a novel, simple, and efficient method to fabricate in-situ Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] composite coating by plasma spraying [Nb.sub.2][O.sub.5]-[B.sub.4]C-Al composite powder, aiming at realizing the higher densification and ultra-fine microstructure of Nb[B.sub.2] composite coating. The microstructure and properties of in-situ Nb[B.sub.2]-NbC[Al.sub.2][O.sub.3] composite coating were studied comparatively with ex-situ Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] composite coating (plasma spraying Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] composite powder). The reaction mechanism of [Nb.sub.2][O.sub.5]-[B.sub.4]C-Al composite powder in plasma jet was analyzed in detail. The results showed that the in-situ nanostructured Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] composite coating presented a lower porosity and superior performance including higher microhardness, toughness and wear resistance compared to the plasma sprayed ex-situ Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] coating and other boride composite coatings. Densification of the in-situ Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] coating was attributed to the low melting point of [Nb.sub.2][O.sub.5]-[B.sub.4]C-Al composite powder and the exothermic effect of in-situ reaction. The superior performance was ascribed to the density improvement and the strengthening and toughening effect of the nanosized phases. The in-situ reaction path could be expressed as: [Nb.sub.2][O.sub.5] + Al [right arrow] Nb + [Al.sub.2][O.sub.3], and Nb + [B.sub.4]C [right arrow] Nb[B.sub.2] + NbC . Keywords: Nb[B.sub.2]-NbC-[Al.sub.2][O.sub.3] composite coating; plasma spray; in-situ synthesis; [Nb.sub.2][O.sub.5]-[B.sub.4]C-Al system