In-situ synthesis, microstructure, and properties of NbB2-NbC-Al2O3 composite coatings by plasma spraying

Abstract

AbstractThe high melting point and strong chemical bonding of NbB2 pose a great challenge to the preparation of high-density nanostructured NbB2 composite coating. Herein, we report a novel, simple, and efficient method to fabricate in-situ NbB2-NbC-Al2O3 composite coating by plasma spraying Nb2O5-B4C-Al composite powder, aiming at realizing the higher densification and ultra-fine microstructure of NbB2 composite coating. The microstructure and properties of in-situ NbB2-NbC-Al2O3 composite coating were studied comparatively with ex-situ NbB2-NbC-Al2O3 composite coating (plasma spraying NbB2-NbC-Al2O3 composite powder). The reaction mechanism of Nb2O5-B4C-Al composite powder in plasma jet was analyzed in detail. The results showed that the in-situ nanostructured NbB2-NbC-Al2O3 composite coating presented a lower porosity and superior performance including higher microhardness, toughness and wear resistance compared to the plasma sprayed ex-situ NbB2-NbC-Al2O3 coating and other boride composite coatings. Densification of the in-situ NbB2-NbC-Al2O3 coating was attributed to the low melting point of Nb2O5-B4C-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: Nb2O5 + Al → Nb + Al2O3, and Nb + B4C → NbB2 + NbC.