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

The high melting point and strong chemical bonding of NbB 2 pose a great challenge to the preparation of high-density nanostructured NbB 2 composite coating. Herein, we report a novel, simple, and efficient method to fabricate in-situ NbB 2 -NbC-Al 2 O 3 composite coating by plasma spraying Nb 2 O 5 -B 4 C-Al composite powder, aiming at realizing the higher densification and ultra-fine microstructure of NbB 2 composite coating. The microstructure and properties of in-situ NbB 2 -NbC-Al 2 O 3 composite coating were studied comparatively with ex-situ NbB 2 -NbC-Al 2 O 3 composite coating (plasma spraying NbB 2 -NbC-Al 2 O 3 composite powder). The reaction mechanism of Nb 2 O 5 -B 4 C-Al composite powder in plasma jet was analyzed in detail. The results showed that the in-situ nanostructured NbB 2 -NbC-Al 2 O 3 composite coating presented a lower porosity and superior performance including higher microhardness, toughness and wear resistance compared to the plasma sprayed ex-situ NbB 2 -NbC-Al 2 O 3 coating and other boride composite coatings. Densification of the in-situ NbB 2 -NbC-Al 2 O 3 coating was attributed to the low melting point of Nb 2 O 5 -B 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 2 O 5 + Al → Nb + Al 2 O 3 , and Nb + B 4 C → NbB 2 + NbC.