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In the present work it was shown that the addition of VC or a combination of VC + Ti(C0.7N0.3) was efficient in reducing the NbC grain growth during liquid phase sintering, when compared to NbC-Ni equivalents. A finest NbC grain size of 1.48 µm was obtained in the NbC-15 Ti(C0.7N0.3)-10 Ni-7.5 VC cermet, exhibiting a hardness of 1486 +/- 7 kg/mm2 and a fracture toughness of 8.7 +/- 0.1 MPa m 1/2.
Research and development of WC grades is 90 years ahead of niobium carbide-based hard materials, which showed already as light-weight and unexplored material a very high potential for many technical applications, especially for wear protection and machining. NbC evoluted recently from lab scale to pilot scale, especially in areas where established WC-based materials are causing economic, environmental or technical concerns. Benchmark trials of NbC-based hard metals indicated already a favorable performance in machining of steel components.
The current study reports on the influence of the Addition of 5–15 vol% VC or/and Mo2C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo2C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents.
The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV30) of 1077 ± 22 kg/mm2 and an indentation toughness of 9.1 ± 0.5 MPa·m1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm2, whereas the toughness increased to 11.3 ± 0.1 MPa·m1/2. Addition of 5 and 10 vol% Mo2C into a NbC-12 vol% Ni mixtures generated the same values in HV30 and KIC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC+4 vol% Mo2C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.
NbC has a relatively high hardness (19.6 GPa) and melting temperature (3600 °C) and is usually applied as a grain growth inhibitor in WC–Co hardmetals. The current study reports on the influence of the sintering technology and overall chemical composition on the microstructure and mechanical properties of NbC-based hardmetals, using Co as a binder. The partial replacement of NbC by 5 wt.% WC, Mo2C, VC or TiC allows one to significantly improve the hardness and toughness of the NbC–Co materials. The influence of different binders, i.e., Co, 316L stainless steel and Fe3Al on the microstructure and mechanical properties of NbC-based hardmetals was also investigated. The powder mixtures were sintered in the solid state by PECS and liquid state by conventional vacuum sintering. Detailed microstructural analysis was conducted by EPMA elemental mapping and WDS point analysis. Mechanical properties, including Vickers hardness and indentation toughness were assessed.
The origin of the intrinsic wear resistance of NbC-based materials is investigated through an assessment of the tribological performance of fully dense, crack-free spark plasma sintered Nb2O5 (here as a reduced polymorph: monoclinic Nb12O29 or NbO2.416). The most likely wear mechanism on NbC is the tribo-oxidation to Nb2O5. The unlubricated (dry) friction and wear behavior of alumina (99.7%) mated against rotating disks of crack-free niobium(V)oxide (Nb2O5) under unidirectional sliding (0.03–10m/s; 22°C and 400°C) and oscillation (f=20 Hz, dx=200 mm, 2/50/98% rel. humidity, n=105/106 cycles) will be presented. The microstructure and mechanical properties of the crack-free Nb2O5 are assessed. The tribological data obtained are benchmarked with different NbC grades, ceramics, cermets and thermally sprayed coatings.
Niobium carbide for wear protection - tailoring its properties by processing and stoichiometry
(2016)
Niobium carbide is a hardly explored carbide but its functional profile shows a high potential for wear protection and tribological applications, which are currently dominated by tungsten carbide. Surprisingly little information is available on niobium carbide (NbC). Niobium carbide can be either synthesized by carbothermal conversion of Nb2O5 or be metallurgically grown and leached out. Furthermore, NbC hardmetal grades can be bonded by all known metallic binders and processed and sintered in exactly the same way as WC-based hardmetals. Niobium is today largely available. NbC can be efficiently produced, provides comparably low friction in many relevant tribo-contacts and displays low wear. NbC and Nb2O5 have so far no REACH classification related to human toxicology and are not listed as substances of very high concern contrary to WO3 and Co3O4. This contribution demonstrates the key characteristics of NbC and discusses its sustainability and reliable value chain.