The tribological property profile of hard metals and metal matrix composites based on niobium carbide

The term “tribology” relates to the friction and wear behavior of materials, as well as their lubrication, and is essentially a system property involving the material itself, the counterbody material, operating condition stresses and also environmental conditions. Hard metals have been used extensively under typical tribological conditions encountered in the mining and processing industry with the intention of increasing lifetime and performance of components. The typical and well-established hard metal is WC-Co and its modifications, which have been developed over the last several decades and are used in a vast range of applications. In contrast surprisingly little work has been done on niobium carbide (NbC) based hard metal, despite repeated indications that this material should have very favorable tribological properties. Therefore, an extensive research program has been initiated firstly to identify the intrinsic properties of NbC and their influencing factors and secondly to develop hard metal, as well s metal matrix composite (MMC) materials, based on NbC. This work is focused on the tribological profile of NbC-based materials, as well as their potential applications and environmental implications. Microhardness and elastic properties depend on the C/Nb ratio. At room temperature hard metals of stoichiometric niobium carbide (NbC1.0) have an elastic modulus of around 440 GPa. Those of under-stoichiometric niobium carbide (NbC0.88) have an elastic modulus of around 405 GPa while melt grown NbC0.70 shows an elastic modulus of approximately 365 GPa. The microhardness, however, increases with decreasing C/Nb ratio. Stoichiometric and sub-stoichiometric NbC possess a pronounced intrinsic wear resistance, either as hard metal or as a hard phase in metal matrix composites. Some of the recent activities have focused on the tribological behavior of Fe3Al-NbC metal matrix composites prepared by pyro-metallurgical synthesis. It is compared to different Spark Plasma Sintered (SPS) NbC-based hard metals bonded by cobalt or Fe3Al under dry sliding conditions. The wear resistance under dry sliding conditions of the present Fe3Al-NbC, containing about 60 volume percent NbC, is shown to be close to that of NbC-based hard metals. No particle extraction or fragmentation of the NbC particles was seen in the wear tracks of the Fe3Al-NbC composite, as a metallurgical interface was formed between the matrix and NbC grains. The physical properties of melt-grown NbC match those observed in sintered material originating from NbC powder converted from Nb2O5 by carbo-thermal reduction. (AU) Copyright © 2018 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved