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MoCoB–Co cermets with varying Mo/B atomic ratios (AR Mo/B) and Co contents were fabricated, and their microstructure, mechanical properties, and corrosion behavior were investigated. The MoCoB hard phase exhibited the maximum grain size and anisotropy when the AR Mo/B was 0.84, which degraded the mechanical properties. For AR Mo/B ≥ 1.08, a third phase, Co 7 Mo 6 , and dense three-dimensional networks composed of stacking faults and ε-martensite plates were observed. These three-dimensional networks limited the dislocation slip in face-centered cubic γ-Co and ε-martensite, improving the hardness of the cermet and decreasing the transverse fracture strength (TRS). The corrosion resistance of the cermets was influenced by the MoCoB particle size, the Mo content of the binder phase, and the ε-martensite content. With an increase in the AR Mo/B , the corrosion resistance of cermets initially decreased and then increased. Additionally, as the Co content increased, the hardness gradually increased, whereas the TRS and fracture toughness steadily decreased. For AR Mo/B = 0.92 and a Co content of 45 at.%, the cermet displayed the best overall performance, with hardness, TRS, and fracture toughness values of HRA 89, 1720 MPa, and 13.2 MPa m1/2, respectively. [ABSTRACT FROM AUTHOR]

To find high-performance ternary borides as the hard phase of the cermets, first-principles calculation was applied to systematically investigate the structure, mechanical, electronic and thermal properties of 26 ternary borides MⅠ x MⅡB x (x = 1 or 2). The phase stability and mechanical properties were evaluated by formation enthalpy and elastic constants. Debye temperatures were calculated to study the thermal conductivity, and the hardnesses of the candidates were predicted via the overlap populations calculation and hardness model. The results show that all the candidates possess thermodynamic and mechanical stability. Most of the MⅠ x MⅡB x (x = 1 or 2) exhibit a better ductility than WC, except Ta 2 FeB 2 , Mo 2 MnB 2 , Nb 2 CrB 2 , Nb 2 FeB 2 and Ti 2 ReB 2. Moreover, sorted from high to low, Nb 2 CrB 2 , Mo 2 CoB 2 , W 2 MnB 2 , Mo 2 FeB 2 , Ti 2 ReB 2 , NbFeB, MoCoB, Mo 2 NiB 2 , Nb 2 FeB 2 , and Mo 2 NbB 2 have a larger Debye temperature than WC. All the candidate ternary borides exhibit a lower hardness than WC, with the maximum hardness of 26.3 GPa for MoCoB. The analysis of electronic properties indicates that all the MⅠ x MⅡB x (x = 1 or 2) exhibit metallic, ionic and covalent hybrid properties. The calculated results are expected to provide guideline for developing ternary boride-based cermets or coating materials that may replace WC-Co cermet. • Ternary boride-based cermets may replace WC-Co. • The mechanical and thermal properties of 26 MⅠ x MⅡB x (x = 1,2) were investigated. • A novel high hardness and thermal conductivity MoCoB-Co cermet was proposed. • A scientific foundation for accelerated design of tungsten substitutable cermets. [ABSTRACT FROM AUTHOR]

The influence of N 2 atmosphere on the microstructure of gradient zone in Ti(C,N)-Mo 2 C-Ni cermet was systematically investigated by the coupling analysis of experimental characterization and thermodynamic calculation. Under the guidance of calculated carbon window, the composition of Ti(C,N)-Mo 2 C-Ni cermet was designed, and the cermet was produced via liquid-phase sintering at 1450 °C for 2 h under N 2 pressure of 20, 200, 400 and 600 mbar. The microstructure and element distribution of cermet were analyzed by using Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray spectroscopy (EDX). A homogeneous microstructure was obtained for cermet sintered in 20-mbar nitrogen atmosphere, whereas the thickness of gradient layer increased with nitrogen pressure. EDX mapping demonstrate that Mo and Ti are enriched in gradient zone, while Ni is lacking and partially segregated near the surface. The diffusion of elements in cermet is caused by the different nitrogen activity between surface and interior. The carbonitride grains show typical black core and gray rim structure in the bulk of cermets, while it present light-gray core and gray rim in the surface gradient layer. In addition, the Vickers microhardness measurement was performed for the gradient zone of cermets, and the hardness increased for cermets sintered in higher nitrogen pressures, which exhibit slower grain growth phenomena. [ABSTRACT FROM AUTHOR]

MoCoB–Co cermets with different Mn contents were synthesized using an in-situ sintering reaction, and the effect of Mn addition on the microstructure, interfaces, and mechanical properties was investigated. The addition of Mn inhibited the transition of the binder phase from fcc to hcp. Adding an appropriate Mn can increase the density of the material and refine the hard phase grain. With increasing Mn content, the hardness decreased marginally and then gradually increased, and the transverse rupture strength (TRS) and fracture toughness (K IC) gradually decreased. The cermet with 6 wt% Mn addition had the smallest grain size and lower porosity. The hardness of the cermet increased from HRA 89.0 to HRA 91.7. The interface bonding strength of MoCoB/hcp-Co is higher than that of MoCoB/fcc-Co. After isothermal aging treatment, the content of hcp-Co in the binder phase increased markedly; thus, the K IC and TRS increased from 11.3 MPa m1/2 and 1200 MPa–13.3 MPa m1/2 and 1750 MPa, respectively. [ABSTRACT FROM AUTHOR]

Among the ternary borides, the Mo–Co–B system is of great interest because of its excellent hardness, toughness, and stability performance. Eight samples with 60 at.% Co were designed to investigate the isothermal section of Mo–Co–B system at 1073 K in the Co-rich portion. Scanning electron microscopy, energy-dispersion spectroscopy, electron probe microanalysis, differential scanning calorimetry, and X-ray diffraction were used to investigate the phase equilibria of the samples. The formation enthalpies of the ternary borides were obtained by first-principles calculations to serve as key information for thermodynamic assessment. By coupling the reviewed experimental data from the literature, the presently determined phase equilibria, and the calculated formation enthalpies of the compounds, the thermodynamic parameters for the Mo–Co–B ternary system were optimized and used to calculate the isothermal sections, vertical section, and liquidus projection of the system. Comprehensive comparisons showed that the calculated results are in reasonable agreement with the reported phase diagram and thermodynamic data. [ABSTRACT FROM AUTHOR]