Abstract
The aim of this work was to correlate the overall carbon content in NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo starting powders with the resulting microstructure, hardness, and fracture toughness of Ni-bonded NbC cermets. A series of NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo cermets with different carbon content were prepared by conventional liquid phase sintering for 1 h at 1420 °C in vacuum. Microstructural analysis of the fully densified cermets was performed by electron probe microanalysis (EPMA) to assess the effect of carbon and VC or Mo additions on the NbC grain growth and morphology. A decreased carbon content in the starting powder mixtures resulted in increased dissolution of Nb, V, and Mo in the Ni binder and a decreased C/Nb ratio in the NbC based carbide phase. The Vickers hardness (HV30) and Palmqvist indentation toughness were found to decrease significantly with an increasing carbon content in the Mo-free cermets, whereas an antagonistic correlation between hardness and toughness was obtained as a function of the Mo-content in Mo-modified NbC cermets. To obtain optimized mechanical properties, methods to control the total carbon content of NbC-Ni mixtures were proposed and the prepared cermets were investigated in detail.
Highlights
WC–Co cemented carbides are widely used hard materials for cutting tools and wear applications.To improve the sintering behavior, refine the carbide grain size, and fine tune the mechanical properties, transition metal carbides, such as VC, Cr3 C2, NbC, TaC, and Mo2 C, are commonly added in small quantities to WC-Co cemented carbides [1,2,3] and in larger fractions to TiCN–Ni based cermets [4,5,6,7].Compared to WC and Ti(C,N), NbC has a higher melting point (3522 ◦ C) and is, on average, softer [8].The good retention of hot hardness of NbC closes the gap at 800 ◦ C to WC
It is clear that the morphology and particle size of the NbC grains, as well as the wetting between NbC and Ni binder were evolved as a function of the carbon content
The growth of NbC grains in a Co, Ni, or Fe binder was reported to be a result of dissolution and re-precipitation [24]
Summary
WC–Co cemented carbides are widely used hard materials for cutting tools and wear applications.To improve the sintering behavior, refine the carbide grain size, and fine tune the mechanical properties, transition metal carbides, such as VC, Cr3 C2 , NbC, TaC, and Mo2 C, are commonly added in small quantities to WC-Co cemented carbides [1,2,3] and in larger fractions to TiCN–Ni based cermets [4,5,6,7].Compared to WC and Ti(C,N), NbC has a higher melting point (3522 ◦ C) and is, on average, softer [8].The good retention of hot hardness of NbC closes the gap at 800 ◦ C to WC. WC–Co cemented carbides are widely used hard materials for cutting tools and wear applications. To improve the sintering behavior, refine the carbide grain size, and fine tune the mechanical properties, transition metal carbides, such as VC, Cr3 C2 , NbC, TaC, and Mo2 C, are commonly added in small quantities to WC-Co cemented carbides [1,2,3] and in larger fractions to TiCN–Ni based cermets [4,5,6,7]. Despite the promising properties of NbC, the potential of NbC as a major component in hard materials for wear applications was only recently reported by Woydt et al [9,10] and for machining by Uhlmann et al [11]. To control the NbC grain growth and improve the densification behavior, transition metal carbides, such as VC, TiC, Metals 2018, 8, 178; doi:10.3390/met8030178 www.mdpi.com/journal/metals
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