Search Results (7)

The chemical design of new functional materials for solid oxide fuel cells (SOFCs) is of great interest as a means for overcoming the disadvantages of traditional materials. Redox stability, carbon deposition and sulfur poisoning of the anodes are positioned as the main processes that result in the degradation of SOFC performance. In this regard, double perovskite molybdates are possible alternatives to conventional Ni-based cermets. The present review provides the fundamental properties of four members: Sr₂NiMoO_6-δ, Sr₂MgMoO_6-δ, Sr₂FeMoO_6-δ and Sr₂Fe_1.5Mo_0.5O_6-δ. These properties vary greatly depending on the type and concentration of the 3d-element occupying the B-position of A₂BB’O₆. The main emphasis is devoted to: (i) the synthesis features of undoped double molybdates, (ii) their electrical conductivity and thermal behaviors in both oxidizing and reducing atmospheres, as well as (iii) their chemical compatibility with respect to other functional SOFC materials and components of gas atmospheres. The information provided can serve as the basis for the design of efficient fuel electrodes prepared from complex oxides with layered structures. Full article

At present, most Mo₂FeB₂-based cermets are prepared by vacuum sintering. However, vacuum sintering is only suitable for ordinary cylinder and cuboid workpieces, and it is difficult to apply to large curved surface and large size workpieces. Therefore, in order to improve the flexibility of preparing Mo₂FeB₂ cermet, a flux cored wire with 70% filling rate, 304 stainless steel, 60 wt% Mo powder and 40 wt% FeB powder was prepared. Mo₂FeB₂ cermet was prepared by an arc cladding welding metallurgy method with flux cored wire. In this paper, the microstructure, phase evolution, hardness, wear resistance and corrosion resistance of Mo₂FeB₂ cermets prepared by the vacuum sintering (VM-Mo₂FeB₂) and arc cladding welding metallurgy method (WM-Mo₂FeB₂) were systematically studied. The results show that VM-Mo₂FeB₂ is composed of Mo₂FeB₂ and γ-CrFeNi.WM-Mo₂FeB₂ is composed of Mo₂FeB₂, NiCrFe, MoCrFe and Cr₂B₃. The volume fraction of hard phase in WM-Mo₂FeB₂ is lower than that of VM-Mo₂FeB₂, and its hardness and corrosion resistance are also slightly lower than that of VM-Mo₂FeB₂, but there are obvious pores in the microstructure of VM-Mo₂FeB₂, which affects its properties. The results show that WM-Mo₂FeB₂ has good diffusion and metallurgical bonding with the matrix and has no obvious pores. The microstructure is compact and the wear resistance is better than that of VM-Mo₂FeB₂. Full article

The aims of this study were: (1) to produce composite coatings by high velocity oxy fuel (HVOF) spraying with steel matrix reinforced by cermets (a) Cr₃C₂–20%Ni and (b) TiC–20%NiMo, manufactured by mechanically activated synthesis (MAS); (2) to synthesize in situ a carbide reinforcement for iron matrix from a mixture of titanium and carbon during HVOF reactive thermal spraying (RTS); (3) to compare the wear resistance of produced coatings. As a reference, HVOF sprayed coatings from commercial Cr₃C₂–25%NiCr (Amperit 588.074) and AISI 316L were utilized. Study of microstructure revealed the inhomogeneity of the Cr-based MAS coating; the Ti-based MAS coating had typical carbide granular structure, and the Ti-based RTS coating possessed elongated structures of TiC. The X-ray diffraction revealed two main phases in the Cr-based MAS coating: Cr₃C₂ and austenite, and two phases in the Ti-based coatings: TiC and austenite. Among the studied coatings, the Cr-based MAS coating demonstrated the highest low-force hardness (490 HV0.3). During the abrasive rubber wheel test (ASTM G65), the Ti-based MAS coating showed the best wear resistance, followed by Cr₃C₂–25%NiCr and Ti-based RTS coating. In the abrasive–erosive test (GOST 23.201-78), the Ti-based MAS coating was 44% better than Cr₃C₂–25%NiCr coating. The Ti-based RTS coating was 11% more wear resistant than the reference Cr₃C₂–25%NiCr coating. Full article

Boride-based cermet can serve as a good protective coating for low-corrosion and wear-resistant materials, such as carbon steels, due to their mechanical and chemical properties. In this study, M₃B₂ (M: Mo, Ni, Fe, and Cr) boride-based cermet coatings were fabricated on Q235 steel with mixed powders of Mo, B, Ni60, and Cr by laser cladding synthesis, and the effects of laser power on the properties of the cermet layer were investigated. Three laser powers (2200, 2500, and 2800 W) were used at the same scanning speed. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) analysis confirmed that all the coatings were composed of M₃B₂-type borides and {Fe, Ni} alloys. The micro-hardness, corrosion, and frictional experiments showed that the cermet coatings enhanced the corresponding performances of the Q235 steels at the three laser powers. However, the micro-hardness of the coatings decreased as the power increased, and the maximum micro-hardness value was 1166.3 HV (Vickers Hardness). The results of the corrosion and frictional experiments showed that the best performance was obtained at a laser power of 2500 W, followed by 2800 and 2200 W. Full article

The hard and corrosion resistant coatings of Mo₂NiB₂ cermet were prepared by the laser cladding technique. The influences of the Mo:B ratio and the laser scanning speed on the microstructure and property of the Mo₂NiB₂ cermet coatings were investigated. The results showed that the laser scanning speed of 1.5 mm/s and the Mo:B ratio of 1 were more beneficial to the formation of Mo₂NiB₂ cermet than 2.0 mm/s and 0.8, 1.2, respectively. The amount of the Mo₂NiB₂ ceramic phases were decreased from the top layer to the bottom layer of the coating. The changes of microstructure and composition led to the changes of hardness and corrosion resistance of the Mo₂NiB₂ cermet coatings. The coating prepared at the Mo:B ratio of 1 and the scanning speed of 1.5 mm/s possessed the highest hardness, and the hardness gradually decreased from the top layer to the bottom layer of the coating. The formation of Mo₂NiB₂ and {FeM} phases led to the enhanced corrosion resistance of the Mo₂NiB₂ cermet coatings, and the coating prepared at the Mo:B ratio of 0.8 possessed the best corrosion resistance and the minimum corrosion current. Full article

Although Q235 steel materials are widely used in offshore engineering, the service life is severely shortened by its inferior resistance to wear and corrosion in harsh marine working environments. Boride-based cermet composites could be a good surface-protective coating to enhance surface hardness, wear resistance, and corrosion resistance. M₃B₂ (M: Mo, Ni, Fe, Cr) boride-based cermet coatings composed of hard ceramics of M₃B₂-type complex borides and an {Fe, Ni} metal matrix was fabricated on Q235 steels with mixed Mo, Cr, B, and Ni60 powders using a laser cladding synthesis technique. The influences of laser cladding parameters on the microstructure, phase composition, microhardness, and corrosion resistance of the coatings were comprehensively investigated. Results showed that the microstructures of the coatings mainly consisted of three layers, which were, from the top to bottom layer, a metal layer with fewer ceramic phases, a ceramic layer with fewer metal phases, and another metal layer with fewer ceramic phases. The ceramic phases were mainly M₃B₂-type borides, and the metal phases were mainly {Fe, Ni} alloys. The appearance of Fe-enriching metal phases was due to the supply of Fe elements from Q235 substrates. With squash pretreatment and without a remelting aftertreatment, ceramics uniformly dispersed in the cermet coatings, and their sizes decreased. The results of microhardness showed that the microhardness of the coating first increased and then decreased from the top layer to the bottom layer, and maximum microhardness was obtained in the layer of ceramics with less metal phases. An electrochemical corrosion test showed that the cermet coatings (j_corr = 6.35 μA/cm²) could improve the corrosion resistance of Q235 steels (j = 43.76 μA/cm²) by one order of magnitude. Full article

Mo₂NiB₂-Ni cermets have been extensively investigated due to their outstanding properties. However, studies have not systematically examined the effect of the powder milling process on the cermets. In this study, Mo, Ni, and B raw powders were subjected to mechanical ball milling from 1 h to 15 h. XRD patterns of the milled powders confirmed that a new phase was not observed at milling times of 1 h to 15 h. With the increase in the mechanical ball milling time from 1 h to 11 h, raw powders were crushed to small fragments, in addition to a more uniform distribution, and with the increase in the mechanical ball milling time to greater than 11 h, milled powders changed slightly. Mo₂NiB₂-Ni cermets were fabricated by reaction boronizing sintering using the milled powders at different ball milling times. The milling time significantly affected the microstructure and mechanical properties of Mo₂NiB₂-Ni cermets. Moreover, the Mo₂NiB₂ cermet powder subjected to a milling time of 11 h exhibited the finest crystal size and the maximum volume fraction of the Mo₂NiB₂ hard phase. Furthermore, the cermets with a milling time of 11 h exhibited a maximum hardness and bending strength of 87.6 HRA and 1367.3 MPa, respectively. Full article