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Abstract Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y 2 O 3 composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS. Highlights • Surface diffusion and grain boundary diffusion played different roles in densification. • The density, grain size, and Vickers hardness of tungsten increased as temperature increased. • The densification of the composite accelerated at the initial stage of sintering but decelerated after a certain period. • The densification of the powder at different sintering stages was influenced by different mechanisms. [ABSTRACT FROM AUTHOR]

Biomass-derived carbon has attracted extensive attention because of its unique form, wide range of sources and low cost. However, the construction of efficient biomass-based microwave absorbers with hierarchical structures and multicomponent magnetic/dielectric structures remains a great challenge. Therefore, CoNi-MOF was self-assembled in situ on the plant Juncus effusus by the microwave method, and CoNi/NC/C composites were prepared by in situ pyrolysis in a nitrogen atmosphere. The morphology, electromagnetic parameters, specific surface area and porous structure of the composites were adjusted by controlling the pyrolysis temperature. For a pyrolysis temperature of 600 ℃ and filling of 16 wt%, the composite was found to show an RL min value of − 64.3 dB at 12.88 GHz at a thickness of 2.5 mm, and the effective absorption bandwidth (EAB)reached 7.36 GHz. The excellent microwave absorption performance was attributed to conduction loss, dipole polarization, interface polarization, magnetic loss and balanced impedance matching. • Multimodal 3D structure CoNi / NC / C composites were synthesized by microwave and pyrolysis. • The RLmin of the sample was − 64.3 dB at 2.5 mm, with a broad effective bandwidth of 7.36 GHz. • Metal-O-C and Metal-N bonds were present in the composites. • The performance derives from multiple loss mechanism and tunable impedance match. [ABSTRACT FROM AUTHOR]

Abstract: Short-carbon-fiber-reinforced SiC composites were prepared by precursor pyrolysis–hot pressing with MgO–Al2O3–Y2O3 as sintering additives. The effects of the amount of sintering additives on microstructure and mechanical properties of the composites were investigated. The results showed that the composites could be densified at a relatively low temperature of 1800°C via the liquid-phase sintering mechanism and the composite density and mechanical properties improved with the amount of additives. The amorphous interphase in the composites with more additive content, not only avoided the direct contact of the fibers with matrix, but also improved the fiber–matrix bonding. It proved that the fiber–matrix interphase characteristics played a key role in controlling mechanical properties of the composites. [Copyright &y& Elsevier]

Abstract: The in situ TiB/Ti–4.0Fe–7.3Mo composites were fabricated by plasma spark sintering (SPS) technique using mechanical alloying of Ti, Fe65Mo and B powder. The effects of sintering temperature on densification of the composites and microstructure of the in situ synthesized TiB were investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). A dense composite was obtained after sintering at 1000°C for 5min. Under proper ball milling and SPS conditions in situ TiB reinforced Ti–4.0Fe–7.3Mo composites have been prepared. The in situ TiB showed a typical needle shape grow along [010] direction. The transverse cross-section of TiB grain is a hexagonal shape with the planes of (100), (101) and . The stacking faults in the (100) plane were observed in the TiB needles. [Copyright &y& Elsevier]

2Y-PSZ/steel composites have been sintered by vacuum hot-pressing. The microstructure was studied by TEM after the composites were impacted at the average strain of 1.0×103 S-1 by split Hopkinson method. The results showed that the phase transformation from γ to α′ occurred in the matrix of TRIP steel under dynamic loading. Dynamic mechanical properties of 2Y-PSZ/steel composites were improved because of the transformation of distortion induced Martensite. The morphology of Martensite induced by distortion was mainly twins. The sub-structure of Martensite was mainly twins. But there was no mid-ridge and distortion twins comptetely expanded to the whole Martensite. Moreover, there was dislocation Martensite at the same sample. The inhomogeneous spread of stress wave in 2Y-PSZ/steel composites led to the different dimension of twins Martensite. When dynamic distortion was relatively efficient, Martensitic dimension and twins distance were increased and the second twins would occur. ZrO2 grains are finely distributed in matrix TRIP steel, which improved the dynamic flow stress of 2Y-PSZ/steel composites. Moreover, ZrO2 could undergo tetragonal→monoclinic phase transformation, which would improve the dynamic yield strength of the composites. But when the content of ZrO2 exceeded 20 vol.%, it would restrain the transformation of dynamic distortion induced Martensite in TRIP steel and led to the decrease of the dynamic mechanical properties of 2Y-PSZ/steel composites. [Copyright &y& Elsevier]

LiTaO3/Al2O3 ceramic composites were fabricated by hot pressing and pressureless sintering, respectively, to investigate the effect of piezoelectric secondary phase on the properties of structural ceramics. Phase constitution, microstructure and mechanical properties of the ceramic composites were studied. LiTaO3 could stably coexist with Al2O3 during sintering. LiTaO3 phase was located at the grain boundaries of Al2O3 matrix fabricated by different processing routes. Domain structures were clearly seen in LiTaO3, confirming that LiTaO3 remained hexagonal ferroelectric phase. About 5 vol.%LiTaO3/Al2O3 ceramic composite prepared by pressureless sintering at 1300 °C showed an increase in fracture toughness. Energy dissipation due to domain switching was suggested as a toughening mechanism. With the increasing of LiTaO3 content, the domain switching toughening mechanism was decompensated by the decreased relative density of LiTaO3/Al2O3 ceramic composites. For LiTaO3/Al2O3 ceramic composites prepared by hot pressing at 1500 °C, the bending strength decreased because of the increased grain size of Al2O3 matrix. The toughening effect due to domain switching was also decompensated by the coarsening of Al2O3 matrix grains, the weaker net-like LiTaO3 phase and the weaker bonding strength between LiTaO3 phase and Al2O3 matrix. [Copyright &y& Elsevier]

Fully dense ZrB 2 –TiC composites were produced by reactive pressureless sintering using ZrB 2 and TiC as the starting reactants. The effect of TiC content on microstructure and mechanical properties of ZrB 2 –TiC composites was investigated. The densification and grain refinement were achieved due to the reaction and solid solution coupling effect. Besides, elongated (Ti, Zr)B 2 plate-like grains with a high aspect ratio were obtained for the first time by the reaction of ZrB 2 and TiC. The ZrB 2 -70 mol%TiC composite exhibited good comprehensive mechanical properties of high hardness of 29.0 GPa, flexural strength of 555 MPa, and fracture toughness of 5.4 MPa m1/2 due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects. [ABSTRACT FROM AUTHOR]

Boron nitride-barium aluminosilicate (BN-BAS) composite ceramics were fabricated by the sol-gel method and hot-pressed sintering. The effects of the introduction method of BAS and sintering temperature on the phase assemblage, density, apparent porosity, mechanical properties and microstructures of the h-BN composite ceramics were investigated. The following two introduction methods were adopted: (A) BAS gel powder was prepared by the sol-gel method and then mixed with h-BN for hot pressing sintering, and (B) the BN-BAS composite powder was first prepared by the sol-gel method and then hot-pressed and sintered. The results indicate that the density, bending strength and fracture toughness of the latter are higher than those of the former when other conditions are the same. In the gel step of Group B, BAS has been coated on the surface of BN particles in dots, chemical bonding has existed, the distribution of BAS is more uniform, and it can better serve as a sintering aid. The BN-BAS composites prepared by both methods achieved the maximum mechanical properties at a sintering temperature of 1700 °C, and the density, apparent porosity, bending strength and fracture toughness of Group A were 2.40 g/cm3, 1.82%, 132.8 MPa and 2.3 MPa m1/2, while those of Group B were 2.42 g/cm3, 1.45%, 162.9 MPa and 2.7 MP m1/2. [ABSTRACT FROM AUTHOR]

Abstract: The effects of zinc oxide (ZnO) additive on the crystal structure, densification, and electrical conductivity of the GdSmZr2O7 ceramic have been examined. The GdSmZr2O7 ceramic with and without the addition of 1 wt.% ZnO sintered at temperatures between 1773 and 1973 K exhibits a single cubic pyrochlore-type phase. ZnO is very effective as a sintering aid for the GdSmZr2O7 ceramic, and it reduces the sintering temperature by about 200 K with a positive effect on the total conductivity. The total conductivity of the ZnO-modified GdSmZr2O7, as measured by A.C. impedance spectroscopy under an air atmosphere, is higher than that of the unmodified GdSmZr2O7 at identical temperature levels. At 1173 K, the highest total conductivity of the ZnO-modified GdSmZr2O7 is about fifteen percent higher than that of the unmodified GdSmZr2O7. The 1 wt.% ZnO-modified GdSmZr2O7 is one of good oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10−4 to 1.0 atm at different test temperatures. [Copyright &y& Elsevier]

The surface microstructure of advanced tool materials is critical, as it directly affects subsequent processing or the performance in use as original condition. In this work, the microstructural evolution and formation mechanism of metal capping on the surface of Ti(C, N)-based cermets, during liquid phase sintering were investigated. Surface metal capping on Ti(C, N)-based cermets is formed during the cooling stage of liquid phase sintering, driven by the contraction pressure of ceramic particle skeleton in the middle stage, in which an adequate amount of liquid phase and significant densification are vital prerequisites. The formation of a step-like textured metal capping is attributed to volume contraction during solidification of liquid phase on the sample surface, in which directional solidification caused by carbon gradient is necessary in this process. Transport behavior of molten metal associated with metal capping on Ti(C, N)-based cermets is divided into four stages in order as infiltration, extrusion, aggregation, and evaporation. The energy of crack propagation is greatly dissipated by plastic deformation of metal capping, thereby greatly improving the fracture toughness of original surface of the sample. [Display omitted] • The formation mechanism of surface metal capping and step-like texture on Ti(C, N)-based cermets is revealed. • The transport behavior of molten metal on the surface of Ti(C, N)-based cermets is clarified. • The presence of a metal capping greatly improves the fracture toughness of the original surface of Ti(C, N)-based cermets. [ABSTRACT FROM AUTHOR]

The composition complexity of Ti(C, N)-based cermets poses a great challenge to the controllability of their sintering process. The effects of both refractory TaC addition and original particle size of starting materials on densification behavior and microstructural evolution of Ti(C, N)-based cermets were investigated during liquid phase sintering. The particle rearrangement only plays a very limited role in promoting densification of Ti(C, N)-based cermets at the initial stage of liquid phase sintering, whereas the pore-filling in the presence of liquid phase becomes a dominant densification mechanism in the middle stage or even earlier. Enhanced densification of ultrafine Ti(C, N)-based cermets is realized by introducing refractory TaC additive based on the grain growth-induced liquid phase pore-filling mechanism, which is also verified by an extended study using other refractory carbide additives such as NbC, ZrC and HfC. This work provides a novel idea for the development of fully densified ultrafine Ti(C, N)-based cermets with outstanding mechanical properties, which are highlighted in machining applications of difficult-to-machine materials as well as other wear-resistant parts operating in high-temperature and high-stress environments. [Display omitted] • The effects of both TaC addition and the particle size of starting materials on the densification behavior of Ti(C, N)-based cermets are clarified. • Enhanced densification of ultrafine Ti(C, N)-based cermets is realized by introducing refractory TaC additive. • This work provides a novel idea for the development of fully densified ultrafine Ti(C, N)-based cermets. [ABSTRACT FROM AUTHOR]

Revealing the grain growth behavior during liquid phase sintering is of great significance for scientific application of grain growth inhibition strategies and the regulation of microstructure. In this study, the effects of both TaC addition and the particle size of starting materials on microstructure of Ti(C, N)-based cermets were investigated. Ultrafine Ti(C, N)-based cermets modified with TaC addition exhibit an obvious grain growth of grey phase during liquid phase sintering. The grain growth of grey phase is typically controlled by liquid-phase diffusion, with the grain growth index of 3 and activation energy of 48–66 kJ/mol. In particular, the increase in grain contiguity of grey phase caused by the evaporation of metal liquid phase at high temperatures, faceted transition of grey phase grains, and the loss of carbon and nitrogen are considered to be the main reasons for the decrease in the grain growth rate of grey phase at high temperatures. • The effects of TaC addition and initial particle size of raw materials on microstructure of Ti(C, N)-based cermets were elucidated. • The growth mechanism of grey-phase grains in TaC-modified ultrafine Ti(C, N)-based cermets was revealed. • The reason why the growth rate of grey phase grains in Ti(C, N)-based cermets is reduced at high temperatures was clarified. [ABSTRACT FROM AUTHOR]

Abstract: Self-toughening α-SiAlON/6H-SiC composites containing both α-SiAlON and α-SiC elongated grains in the microstructures were previously fabricated by spark plasma sintering (SPS). In this study, the α-SiAlON/6H-SiC composites were heat treated at 1850°C for 3h. The effects of the heat treatment on the phase compositions, microstructures, and mechanical properties were investigated. The results showed M′ phases formed from crystallization of the intergranular glassy films. No polytype transformation for the SiC phase was observed. Both α-SiAlON and α-SiC grains grow anisotropic in the heat treatment, rather than converted into the brittle equiaxed morphologies. The flexural strength and fracture toughness of the post heat-treated composites were further increased and the 40wt% SiC composition reached 907MPa and 8.5MPam1/2, respectively. [Copyright &y& Elsevier]

Abstract: A series of zirconate compounds with the general formula Sm2– x La x Zr2O7 (0⩽ x ⩽1.0) were prepared by pressureless-sintering method at 1973K for 10h in air. The relative density, structure and electrical conductivity of Sm2– x La x Zr2O7 ceramics were investigated by the Archimedes method, X-ray diffraction and impedance spectroscopy measurements. Sm2– x La x Zr2O7 (0⩽ x ⩽1.0) ceramics exhibit a pyrochlore-type structure. The measured electrical conductivity of Sm2– x La x Zr2O7 ceramics obeys the Arrhenius relation and gradually increases with increasing temperature from 673 to 1173K. Sm2– x La x Zr2O7 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0×10−4 to 1.0atm at all test temperature levels. The electrical conductivity of Sm2– x La x Zr2O7 ceramics decreases with increasing lanthanum content at identical temperature levels. [Copyright &y& Elsevier]

Abstract: Tantalum carbide with a TaC0.9 composition was sintered to 96% relative density by spark plasma sintering at 1400°C for 5min, using commercial micrometric Ta and TaC powders as the starting materials. This achievement was dependent on the delicate rearrangement of the heating process which limits evaporation–condensation and promotes the mass transportation in association with 0.9TaC+Ta=TaC0.9 reaction. The dense TaC0.9 ceramic had a uniform microstructure composed of equiaxed grains, and showed flexure strength up to ∼498MPa. [Copyright &y& Elsevier]

Abstract: Yb- and Y-doped α-SiAlON ceramics were effectively bonded by spark plasma sintering in less than 20min without using any interlayer materials at 1650–1700°C. The microstructures and mechanical properties of the joints were investigated by means of SEM, STEM, EDX, and three-point bending tests. The results showed that α-SiAlON joint formation was accompanied by fundamental rare earth diffusion. The intergranular liquids in the α-SiAlON ceramics at the bonding temperatures provided the diffusion paths for the species. α-SiAlON grain growth across the joining interfaces modified the joint microstructures to secure high bonding strength. [Copyright &y& Elsevier]

Monolithic α-SiC particles and in situ β-Si3N4-reinforced barium aluminosilicate (BAS) glass ceramic matrix composites with good mechanical properties were obtained by spark plasma sintering. However, attempts at the synthesis of ternary phase composites in order to maximize the reinforcing effects by co-adding α-SiC and β-Si3N4 were only partly successful. β-Si3N4 grain growth was remarkably hindered by the coexistent α-SiC, thus the resultant composite suffered a loss of reinforcement efficiency. [Copyright &y& Elsevier]

Abstract: (Nd1−x Yb x )2Zr2O7 (0≤ x ≤1) ceramics were prepared by pressureless-sintering to obtain dense bulk materials. The electrical conductivity of (Nd1−x Yb x )2Zr2O7 was investigated by complex impedance spectroscopy over a frequency range of 20Hz to 2MHz from 723 to 1173K in air. A high-temperature dilatometer was used to analyze thermal expansion coefficient of (Nd1−x Yb x )2Zr2O7 in the temperature range of 373–1523K. The measured electrical conductivity obeys the Arrhenius relation. The grain conductivity of each composition in (Nd1−x Yb x )2Zr2O7 gradually increases with increasing temperature. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x =0.3 to x =0.5. The highest electrical conductivity value obtained in this work is 9.32×10−3 Scm−1 at 1173K for (Nd0.7Yb0.3)2Zr2O7 ceramic. (Nd1−x Yb x )2Zr2O7 ceramics are oxide-ion conductors in the oxygen partial pressure range from 1.0×10−4 to 1.0atm at all test temperature levels. Thermal expansion coefficients of (Nd1−x Yb x )2Zr2O7 gradually decrease with increasing ytterbium content at identical temperature levels. [Copyright &y& Elsevier]

Abstract: (Gd1−x Yb x )2Zr2O7 (0≤ x ≤1) solid solutions were prepared by pressureless-sintering process at 1973K to investigate the substitution effect of Yb for Gd on their electrical properties. Electrical conductivity of (Gd1−x Yb x )2Zr2O7 solid solutions were measured using complex impedance spectroscopy from 673 to 1173K over a frequency range of 2Hz to 2MHz in air. (Gd1−x Yb x )2Zr2O7 are oxide-ion conductor in the oxygen partial pressure range from 1.0×10−4 to 1.0atm at all test temperature levels. The measured values of grain conductivity obey the Arrhenius relation. The grain conductivity of (Gd1−x Yb x )2Zr2O7 increases with increasing temperature. However, the grain conductivity of (Gd1−x Yb x )2Zr2O7 gradually decreases with increasing ytterbium content at identical temperature levels. [Copyright &y& Elsevier]

Abstract: Highly densified Al2O3/LiTaO3 (ALT) ceramic composites were fabricated by hot-pressing in a nitrogen atmosphere. The addition of Al2O3 particles could significantly improve the densification of LiTaO3. Sintering mechanism of the LiTaO3 ceramic incorporated with Al2O3 particles is proposed. Dielectric constant of 5vol.% Al2O3/LiTaO3 (5ALT) composite ceramic was slightly increased in the range from 30kHz to 106 Hz, but the dielectric loss was lowered in the whole range from 103 Hz to 106 Hz. Piezoelectric constant (d 33) of the 5ALT ceramic composite is about 50% of that of LiTaO3 single crystal. [Copyright &y& Elsevier]

Abstract: (Nd1−x Gd x )2(Ce1−x Zr x )2O7 (0≤ x ≤1.0) powders with an average particle size of 100nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd1−x Gd x )2(Ce1−x Zr x )2O7 powders was studied by pressureless sintering at 1600–1700°C for 10h in air. The relative densities of (Nd1−x Gd x )2(Ce1−x Zr x )2O7 solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd1−x Gd x )2(Ce1−x Zr x )2O7 solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd1−x Gd x )2(Ce1−x Zr x )2O7 solid solutions agree well with the Vegard''s rule. [Copyright &y& Elsevier]

Abstract: A 20vol.% ZrCp/W composite is fabricated by hot-pressing, and its mechanical properties can be significantly improved by the addition of ZrC particles. On the other side, the residual pores within ZrC particles clustering regions which are some latent microcrack sources are mainly caused by the relative low sinterability of ZrC particles. In order to eliminate the pores within the particle clusters, the as-sintered composite is annealed at 2300°C for 1h. The microstructure of the annealed composite becomes more uniform and densified. Components of the as-sintered and annealed composites are composed of a dilute solid solution tungsten-matrix, (Zr, W)C solid solution particles and a new W2C phase, but the content of W2C phase in the annealed composite increases. After annealing, the strength properties of the composite decrease, but its plasticity increases. [Copyright &y& Elsevier]

Abstract: To increase operating temperature and improve performance of gas-turbine engines, it is urgently needed to develop new thermal barrier oxides with a lower thermal conductivity than 6–8wt.% yttria-stabilized zirconia. (Yb x Sm1−x )2Zr2O7 (0≤ x ≤1.0) ceramics were synthesized by pressureless-sintered at 1700°C for 10h in air. The relative density, phase structure, morphology and thermal diffusivity coefficients of (Yb x Sm1−x )2Zr2O7 ceramics were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and laser-flash method. Sm2Zr2O7 and (Yb0.1Sm0.9)2Zr2O7 ceramics exhibit a pyrochlore structure, while (Yb x Sm1−x )2Zr2O7 (0.3≤ x ≤1.0) ceramics have a defect fluorite-type structure. The thermal conductivities of (Yb x Sm1−x )2Zr2O7 ceramics first gradually decrease with increasing temperature, and then increase slightly above 800°C due to the increased radiation contribution. YbSmZr2O7 ceramics have the lowest thermal conductivity over the entire temperature range, which is caused by the reduction of cation mean free path in ytterbium–samarium zirconate system. [Copyright &y& Elsevier]

Abstract: A detailed microstructural and mechanical characterization of a PM processed 20vol.% ZrCp/W composite disk indicates that the composite from the edge to the center of the disk has different phase compositions and mechanical properties. The difference is caused by a temperature gradient in the disk during heating and cooling. [Copyright &y& Elsevier]

Abstract: Microstructures of short-carbon-fiber-reinforced silicon carbide composites, prepared by hot-pressing with MgO–Al2O3–Y2O3 as sintering additives, were investigated by means of X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. The results showed that the composites could be densified at a relatively low temperature of 1800 °C via the liquid-phase-sintering mechanism. The amorphous interphase in the composites not only avoided the direct contact of the fibers with the matrix, but also improved the fiber/matrix bonding, so they could improve the densification of the composites and avoid the degeneration of the carbon fiber. The nano silicon carbide derived from polycarbosilane, could play a role of improving the relative density of the composites. [Copyright &y& Elsevier]

Abstract: SiB0.5C1.5N0.5 nanopowders were synthesized by mechanical alloying method, and a predominantly amorphous with a small volume fraction of nanocrystalline phase was formed in the powders. The results showed that the crystallites were SiC, and the polytype of SiC were 3C and 6H. SiB0.5C1.5N0.5 powders were consolidated by spark plasma sintering (SPS) technique at 1800°C under different sintering pressure. The grain boundaries of SiC and BCN phase could not be clearly distinguished for the ceramics sintered under 25MPa. When the pressure increased to 40MPa, SiC phase almost completely crystallized, and its grain size was less than 1μm. BCN phase were mainly distributed at boundary of SiC grain. Some amorphous structures were found in the sintered ceramics. Relative densities of the ceramics increased from 76.6% to 92.4%, and mechanical property values increased two or three times with the increase of sintering pressure. For the ceramics sintered under a pressure of 40MPa, flexural strength, fracture toughness, elastic modulus and Vickers’ hardness attained to 311.5MPa, 3.45MPam1/2, 133.0GPa and 3.68GPa, respectively. The mass of the SiBCN ceramics lose by several percent below 930°C in air, and then keep unchanged above 1100°C. [Copyright &y& Elsevier]

Abstract: α-Sialon ceramics doped with Nd2O3 were prepared by hot-pressing sintering at 1900°C holding for 1h with a heat preservation for 1h at 1500°C. Microscopic observations indicate that elongated α-sialon grains appear and are embedded in the fine equiaxed-grain matrix. A small amount of β-sialon phases and secondary crystallized phases M′ (Nd2Si3−x Al x O3+x N4−x ) also exist in the Nd-α-sialon ceramic. A core/shell structure can be found in the elongated α-sialon and β-sialon grains with a high aspect ratio, in which some misfit dislocations surround the core. The 21R AlN-polytype was observed by transmission electron microscopy (TEM), which could not be detected by X-ray diffraction (XRD) due to its trace amount. Different nucleation and growth modes of sialon grains were also discussed. [Copyright &y& Elsevier]

Abstract: A six-layer functionally gradient material (FGM) of Ti–TiB–TiB2 was rapidly fabricated by spark plasma sintering (SPS) in vacuum. Both B and TiB2 were used and reacted with Ti to synthesize TiB. Adjusting the content ratio of B to TiB2 in individual layers could control the amount of synthesized TiB by two steps during sintering. The composition and microstructure of the layers in FGM were investigated by XRD, SEM and TEM. The synthesized TiB has needle-like, agglomerated and elongated morphologies. The content of agglomerated TiB increased and that of needle shape TiB decreased with increasing the target TiB volume fraction ranging from 20 to 60%. The elongated TiB was observed in the other two layers with higher target TiB volume fraction, which structure is similar to the agglomerated TiB. The FGM with Ti on one side to Ti–TiB–TiB2 on the other were fabricated at relative low sintering temperature and short dwelling time by spark plasma sintering. [Copyright &y& Elsevier]

• The adding vanadium to (TiZrHfVNbTa)C favors eliminating Nb segregation and enhancing densification. • Excess adding vanadium reduces single-phase-forming ability of (TiZrHfVNbTa)C, due to the lattice distortion increment. • Mechanical properties of (TiZrHfVNbTa)C is optimized via adjusting vanadium content and sintering temperature. A series of (TiZrHfVNbTa)C high-entropy ceramics with different vanadium contents have been fabricated by pressureless sintering at 2300 °C–2500 °C for 1 h, utilizing self-synthesized carbide powders obtained by carbothermal reduction. The addition of vanadium is beneficial to promote densification process and refine grain, as well as facilitate the homogeneous distribution of metal elements. The distribution of pores is also modified, almost entirely existing at grain boundary, and the integral mechanical properties achieve optimization. However, excess adding vanadium does not favor forming a single-phase (TiZrHfVNbTa)C high-entropy ceramic. The optimal (TiZrHfVNbTa)C high-entropy ceramic sintered at 2300 °C possesses a high relative density of 97.5 % and homogeneous microstructure with small grain size of 1.2 μm. The flexural strength and Vickers hardness reach 473 MPa and 24.9 GPa, respectively. This work has established a cost-effective and convenient preparation of novel (TiZrHfVNbTa)C high-entropy carbide ceramics. [ABSTRACT FROM AUTHOR]

MXenes have good application prospects due to the attractive mechanical and physicochemical properties. Here, for the first time, silicoboron carbonitride (Si–B–C–N) ceramics with the integration of MXene(Ti 3 C 2 T x) nanosheets were developed, and the influences of MXene additives (1.0–3.0 wt%) on mechanical properties and microstructures were investigated. The ceramics containing 3.0 wt% MXene(Ti 3 C 2 T x) show better mechanical properties including fracture toughness of 7.6 MPa‧m1/2, 36% higher than ceramics without any additives. The toughening mechanisms consist of pull-out, delamination, crack bridging, and crack deflection. This work demonstrates that MXenes are good reinforcements for enhancing the fracture toughness of structural ceramics, and also paves the way for the investigation of high-temperature properties of MXene-containing ceramics. [ABSTRACT FROM AUTHOR]

To improve the passive radiative heat dissipation property of aluminum alloy, a novel-structural SiC/ h BN nanocomposite ceramic coating with high emissivity value (>0.86) was fabricated by one-pot liquid-phase plasma assisted electrophoresis and sintering (LPES) method. The special formation conditions of nanocomposite coating were optimized as: applied voltage was not lower than 600 V, temperature of electrolyte was ranged from 70 to 90 °C and the concentrations of SiC and h BN nanoparticles were 8 g/L and 40 g/L, respectively. The unique parameters contributed to the plasma assisted SiC and hBN nanoparticles synergistic electrophoresis and then sintering together by SiO 2 based glassy phase. The infrared emissivity in the wavelength of 3–20 μm was enhanced by SiC/ h BN nanocomposite coating, especially the value was up to 0.86 in 3–8 μm wavelength. The nanocomposite coating enabled the temperature drop by 10.4 °C for 1 W LED compared with the bare aluminum alloy, which leds to the cooling efficiency of 19.8%, respectively. The SiC/ h BN ceramic nanocomposite coating fabricated by one-pot LPES process shows a novel strategy to design excellent radiative heat dissipation property coatings. [ABSTRACT FROM AUTHOR]

MgO-Al 2 O 3 -SiO 2 (MAS) glass ceramic was used as liquid sintering aid to densify h -BN ceramics, and effects of sintering temperature on the phase composition of MAS, texture behavior of h -BN grains and properties of the composite ceramics were studied. h -BN grains will react with MAS by forming N-M (Al3+ and Si4+) coordinate-covalent bonds and inhibit the crystallization of glass phase. The h -BN grains will rotate parallel to the pressure direction due to the joint action of the liquid sintering aid and the pressure. The sintering temperature will significantly affect the properties of the composite ceramics by effecting the texture behavior of h -BN grains. And higher sintering temperature of 1900 °C will result in the decline of mechanical properties and increase of thermal conductivity due to the decrease of texture degree caused by the grain growth of h -BN. [ABSTRACT FROM AUTHOR]

Abstract Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y 2 O 3 -5Al 2 O 3 (mole ratio of 3:5) and 3Y 2 O 3 -5Al 2 O 3 -4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c -axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y 2 O 3 -5Al 2 O 3 -4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y 2 O 3 -5Al 2 O 3 is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties. [ABSTRACT FROM AUTHOR]

Abstract TiB 2 –40 wt% TiC composites were prepared by pressureless sintering. Prior to the conventional heating at 2000 °C for 1 h for final densification, addition of a low temperature hold at 1500 °C for 4 h was found effective in densification of the composite, refining the microstructure and promoting the anisotropic growth of TiB 2 grains. The TiB 2 -TiC composite prepared by two-step sintering had better mechanical properties with flexural strength of 776 MPa and fracture toughness of 7.43 MPa m1/2, in comparison with the values of 419 MPa and 4.81 MPa m1/2 for composite by conventional one-step sintering. [ABSTRACT FROM AUTHOR]

In this study, amorphous SiBCN powders were mechanical milled and consolidated by low-temperature sintering at different pressure to provide an insight into the densification and mechanical behavior of dense amorphous SiBCN monoliths. Results show that the mechanical properties undergo a gradual variation closely related to the microstructural evolution. The increased degree of densification with few defects and free volume results in significantly improving Young’s modulus and nano hardness in case of high pressure application. The consolidation at relatively low pressure (3 GPa) occurs by heavy deformation with the support of limited short-range diffusion, whereas the densification at higher pressure (5 GPa) is mostly diffusion controlled showing a classic sintering mechanism. At 1000 °C, higher pressure (5 GPa) suppresses crystallization leading to an effective solid-state reaction, while SiBCN monoliths consolidated by lower pressure (3–4 GPa) show a hybrid structure of dominant amorphous phases and few nanocrystals. [ABSTRACT FROM AUTHOR]

Lanthanum-doped titanium-zirconium-molybdenum (La-TZM) alloys have good mechanical properties, including high strength and toughness due to complex secondary phases. Herein, the formation process of the secondary phases in the La-TZM alloy was revealed by thermodynamics calculations. The ΔG values of possible reactions during sintering were obtained. Results show that the formation process of the secondary phases was controlled by different reactions at different stages. At the first stage (0–500 °C), the main reactions are listed as follows: TiH 2 +O 2 (g)→TiO 2 +H 2 ↑(g), ZrH 2 +O 2 (g)→ZrO 2 +H 2 ↑(g), C 6 H 12 O 6 →[C]+ H 2 O↑, [C]+ O 2 (g)→CO 2 (g)↑, [C]+Mo→MoC, La(NO 3 ) 3 ·6H 2 O→ La 2 O 3 +NO 2 ↑+ NO↑+O 2 ↑+H 2 O; [C]+TiH 2 →TiC + H 2 (g)↑, [C]+ZrH 2 →ZrC + H 2 (g)↑. At the secondary stage (680–960 °C), the dominant reactions include: TiH 2 →Ti + H 2 (g)↑, ZrH 2 →Zr + H 2 (g)↑. At the last stage (≥1200 °C), the primary reaction is Ti + Zr+[O]→ZrTiO 4 . The secondary phases in the La-TZM alloy include TiO 2 , ZrO 2 , ZrC, TiC, MoC, ZrTiO 4, La 2 O 3 and the solid Ti and Zr phases. These results can guide the design of new ingredients for Mo alloys. [ABSTRACT FROM AUTHOR]

The 0.8Bi 3.15 Nd 0.85 Ti 3 O 12 (BNdT)-0.2CoFe 2 O 4 (CFO) composite multiferroic ceramics have been fabricated by spark plasma sintering (SPS) at 850 °C. The relative density of as-sintered SPS ceramic reaches 97.4 (±0.3)%. The composites are composed of pure BNdT and CFO phases without any preferred c -orientation. The a -orientation preference is more obvious perpendicular to the pressure direction. The average grain-sizes of BNdT and CFO are 163 and 146 nm, respectively. The BNdT phase has more grains below 100 nm (∼20%). The super energy-dispersive X-ray analyses suggest no serious reaction between BNdT and CFO. The Raman spectrum verifies the nano-structure of the SPS ceramic via the broadening bands and peak shifts. The Curie temperature of the SPS ceramic declines to 560 °C with stabilized dielectric loss. The grain boundary resistance plays a dominant role on impedance above 700 °C. The remanent polarization approaches to 15.2 μC/cm 2 (300 kV/cm) with lower coercive fields (−89/+95 kV/cm). [ABSTRACT FROM AUTHOR]

The oxidation behavior of SiBCN-Zr composites prepared through incorporating sol-gel processing with spark plasma sintering (SPS) was studied in detail. It is indicated from results that a dense amorphous SiO 2 film was formed on the samples superficially in the course of oxidation, which avoided the sample from being further oxidized. The oxidation of ZrB 2 would boost the oxygen to be diffused, which aggravates the oxidation. Amorphous SiO 2 would fill up the pores and avoid ZrB 2 particles from being further oxidized. A preferable anti-oxidation behavior was also manifested by BN(C) arising from the formation of B 2 O 3 phases. [ABSTRACT FROM AUTHOR]

MWCNTs modified Si 2 BC 3 N ceramics were prepared through SPS sintering mechanically alloyed amorphous Si 2 BC 3 N powder and MWCNTs at 1900 °C in N 2 atmosphere. The phase evolutions, microstructures and mechanical properties of the prepared ceramic were characterized by XRD, SEM and TEM (SAED and HRTEM), as well as the properties testing. The results show that the Si 2 BC 3 N ceramics are mainly composed of β-SiC, α-SiC and BN(C) phases. The introduction of MWCNTs can stimulate the formation of β-SiC in comparison with the pure Si 2 BC 3 N ceramics. Meanwhile, the particle sizes of BN(C) and SiC are reduced with the introduction of MWCNTs. Furthermore, the MWCNTs are mainly embedded in the Si 2 BC 3 N matrixes, contributing to enhanced bonding. Judging from the interfacial characteristic, the strengthening and toughening mechanisms of MWCNTs in Si 2 BC 3 N ceramics can be classified as “crack bridging”, “pull-out” and “crack deflection”. Based on these effects, the 1 vol% MWCNTs incorporated Si 2 BC 3 N ceramics have much higher flexural strength and fracture toughness of 462.1 MPa and 5.54 MPa·m 1/2 , respectively. [ABSTRACT FROM AUTHOR]

To meet the ultrahigh temperature requirements of a thermal protection system, an ultrahigh temperature phase of ZrB 2 was introduced into a SiBCN matrix that was fabricated using a reactive spark plasma sintering method. The thermal ablation behavior of SiBCN-Zr composites was investigated using an oxyacetylene flame test. The test results indicated that the ablation behavior of the modified ceramic composites was significantly improved over that of a monolithic SiBCN ceramic. The linear and mass ablation rates of the SiBCN-Zr material were found to be 0.004 mm/s and 4.75×10 −4 g/s, which was indicative of excellent ablation resistance. Analysis of the material after thermal ablation testing showed that ablation products mainly consisted of the ZrSiO 4 , SiO 2 and ZrO 2 phases. A reaction occurred between the SiO 2 and ZrO 2 phases in the central region of the ceramic forming ZrSiO 4 that protected the material from further thermal damage. A loose and porous oxidation layer was found from the matrix based on analysis of a cross-section image. [ABSTRACT FROM AUTHOR]

Porous boron nitride/silicon oxynitride (BN/Si 2 N 2 O) composites were fabricated by pressureless sintering at 1650 °C with Li 2 O as sintering aid. The influence of Li 2 O and hexagonal boron nitride (h-BN) contents on phase, microstructure, mechanical, dielectric and thermal properties of the resulting porous BN/Si 2 N 2 O composites was investigated. Increasing Li 2 O content facilitated densification and decomposition of Si 2 N 2 O into Si 3 N 4 . The apparent porosity of the composites increases with the h-BN content increases and Si 2 N 2 O grain growth was restrained by the dispersed h-BN particles. The dielectric properties and thermal conductivities (TC) were affected mainly by porosity. Porous BN/Si 2 N 2 O ceramic composites with 4 mol% Li 2 O and 25 mol% BN exhibit both low dielectric constant (3.83) and dielectric loss tangent (0.008) with good mechanical and thermal performance, suggesting possible use as high-temperature structural/functional materials. [ABSTRACT FROM AUTHOR]

Sintering densification processes of the composite ceramics (Bi 3.15 Nd 0.85 Ti 3 O 12 (BNdT)/CoFe 2 O 4 (CFO)) have been investigated using dilatometric experiments combining with the TG-DTA, density measurements and microstructure studies. Microstructures analyses and quantitative calculations show that the composite ceramics achieve densification at low temperatures (<1150 °C). The formation of coherent-lattice interfaces between (200)/(020) BNdT and (310) CFO are considered to play an important role on such densification. The intrinsic preferred orientation of BNdT grains is suppressed by CFO phase because of this coherent relationship. Although the sintering activation energies of 0.8BNdT-0.2CFO are about 2.7 times larger than those of pure BNdT due to the pinning effect, the composite ceramic could still be densified, indicating the formation energy of coherent-lattices provided the extra sintering force. The even coercive electric fields of the resulting pure BNdT and 0.8BNdT-0.2CFO ceramic are approximately 89 and 97 kV/cm, respectively, at 250 kV/cm. The polarization of 0.8BNdT-0.2CFO reaches saturation around 430 kV/cm. [ABSTRACT FROM AUTHOR]

Si-B-C-N ceramic monoliths with LaB 6 additive were prepared by mechanical alloying followed by spark plasma sintering. The microstructures and mechanical properties were investigated compared to LaB 6 -free Si-B-C-N counterparts. After sintered at 1900 °C/50 MPa, the La/Si-B-C-N monoliths provide well-developed BN(C) flakes (≤100 nm in thickness) and improved fracture toughness, while the BN(C) ones show no lamellar microstructures in Si-B-C-N systems. The solid-solution of La in BN(C) grains promotes ordered arrangement of BN(C) along the (0002) plane causing in situ growth of BN(C) thus leading to an increase in toughness by 40% (from 5.6 to 7.8 MPa m −1/2 ). [ABSTRACT FROM AUTHOR]

A novel pressure-assisted transient viscous phase sintering process has been used to fabricate dense BN-ZrO 2 -SiC composites at relatively low sintering temperatures. The sintering behavior and densification mechanism of BN-ZrO 2 -SiC composites incorporated with B 2 O 3 as transient viscous phase have been investigated. A rapid densification process occurs at about 1300 °C, which is attributed to the in-situ formation of SiO 2 by chemical reactions among ZrO 2 , SiC and B 2 O 3 . BN-ZrO 2 -SiC composites sintered at 1300 °C under 100 MPa exhibit a relative density of 95.9%, a flexural strength of 242 MPa and a fracture toughness of 4.1 MPa m 1/2 . [ABSTRACT FROM AUTHOR]

Pure h-BN ceramics were processed at different sintering temperatures and hold times using ball milled nanocrystalline or amorphous h-BN powders. Increases in sintering temperature induced greater grain growth but reduced relative densities and mechanical properties. Changes in sintering hold times caused further increases in grain sizes and relative density, but had little impact on mechanical properties. The grain sizes play an important aspect of strength in h-BN ceramics. The relationship of flexural strength and grain sizes in h-BN ceramics conforms to Hall-Petch behavior. When the grain size of h-BN ceramics increased from 0.3 μm to 8 μm, the flexural strength declined from 163 ± 6 to 84 ± 4 MPa. The mechanical properties of h-BN ceramics depend both on relative densities and grain sizes. [ABSTRACT FROM AUTHOR]

Since its invention, alumina ceramics have been extensively investigated for potential various applications. However, their intrinsic brittle nature is still an insurmountable obstacle when they are applied as structural components. This paper provides a simple routs to prepare ductile alumina based composites with the addition of chopped carbon fiber (C sf /Al 2 O 3 -BN). Effects of fiber length and sintering temperature on the microstructure, phase composition, mechanical properties together with fracture behavior were systematically investigated. The results showed that composites with mixed fiber lengths of 12 mm and 1 mm exhibited homogeneous microstructure and striking enhancement in mechanical performances compared with composites with other fiber length. With the increase in sintering temperature from 1500 °C to 1650 °C, interfacial bonding strength increased and interface state converted from mechanical interlocking at 1500 °C into chemical bonding at 1650 °C. Chemical reaction in the composites degraded carbon fiber properties, which resulted in the decrease in mechanical performance of the composites. [ABSTRACT FROM AUTHOR]