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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]

Sr‐containing ceramic products produced from geopolymer, (Na, Sr)‐GP, have been reported as excellent candidate materials for the immobilization of radionuclide 90Sr, despite sintering temperature at 1200°C. In this work, the product possessing the best immobilization performance (leaching rate of 1.29% for Sr2+) can be produced at 1000°C in the method of liquid‐phase sintering via the addition of fluxing agent B2O3 at 2.0 wt% in the geopolymer precursor. The mechanical properties of the geopolymer precursor (Na, Sr)‐GP were also enhanced via the promotion role of B2O3 to geopolymerization. It was revealed that the overall anti‐leaching ability of the Sr‐containing ceramic product not only depends on the microstructure of the loading substrate but also can be affected by the chemical state of radioactive strontium retention in the loading matrix. [ABSTRACT FROM AUTHOR]

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]

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]

A type of boron nitride–magnesium aluminum silicate (BN‐MAS) composite ceramics was fabricated by hot‐press sintering at different sintering temperatures. The relationship between the sintering temperature and microstructure was investigated by analyzing the interaction between hexagonal boron nitride (h‐BN) and the MAS phase. The main MAS phase in the composite ceramics is the α‐cordierite phase at a sintering temperature of 1300°C. At temperatures above 1400°C, the inhibitory effect of h‐BN on the crystallization of the MAS system is significant, and MAS mainly exists in the form of an amorphous phase. The composite sintered at 1700°C exhibited the highest bending strength of 218MPa. h‐BN and MAS were co‐enhanced. MAS can be used as an effective liquid‐phase sintering aid to assist in the sintering of h‐BN, whereas h‐BN can absorb the fracture energy of the composite ceramics through the pull‐out and bridging effect of the particles. [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]

• 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]

Heat dissipation material with programmable anisotropic property is very challenging, yet can realize the controllable thermal diffusion for heating device. In this work, anisotropic Si3N4 ceramics with oriented grains are prepared to adjust and improve the mechanical and thermal properties under the applied stress field by rolling film forming technology. Through the design of the sintering aids in the process of liquid‐phase sintering, the orientation degree of the Si3N4 grains is programmable as well as the mechanical property and the thermal property of the Si3N4 ceramics. As a consequence, the obtained Si3N4 ceramics show significant anisotropy in mechanical properties and thermal conductivity. The typical fracture toughness and thermal conductivity along the grain orientation direction are 10.6 MPa⋅m1/2 and 45.45 W/(m⋅K) while they are 4.5 MPa⋅m1/2 and 66.42 W/(m⋅K) in the direction perpendicular to the oriented grain, respectively. This grain orientation method paves the way for the thermal performance design and the production of programmable heat dissipation material. [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]

A type of hexagonal boron nitride-magnesium aluminosilicate composite ceramics was fabricated by hot-pressed sintering under different pressures. The relationships between sintering pressure, microstructure, and mechanical properties were investigated. The results indicate that sintering pressure has a great influence not only on the mechanical properties of composite ceramics, but also on crystallization of magnesium aluminosilicate (MAS) and structural order of hexagonal boron nitride. Under the given sintering pressure of 30 MPa, the fracture toughness of composites was greatly improved 66 % (from 2.49 to 4.12 MPa m, with similar densities). It also exhibits the highest bend strength of 245 ± 23 MPa. The fracture behavior of composites and the mechanism of sintering pressure which affect the crystallization of mullite in MAS were also discussed. [ABSTRACT FROM AUTHOR]

The fabrication of dense amorphous Si-B-C-N monoliths is a processing challenge given that it is hard to avoid crystallization at the sintering temperatures needed to attain full density up to 1900°C for conventional hot pressing and SPS methods. We report here successful densification of amorphous Si2BC3N monoliths achieved by heating at 1100°C and 5 GPa. The relationships between microstructure, types of chemical bonding, and mechanical properties were investigated. The strong amorphous 3-D networks of Si-C, C-B, C-N (sp³), N-B (sp³), and C-B-N bonds provide high densities at high applied pressure and thus amorphous Si2BC3N monoliths show high hardness of 29.4 GPa and elastic modulus of 291 GPa. The amorphous structure is lost with crystallization of β-SiC and BN(C) reducing contributions from Si-C, C-N (sp³), and C-B-N bond networks thereby decreasing mechanical properties. [ABSTRACT FROM AUTHOR]

β – SiC powder mixed with 10 wt.% Al 2 O 3 and Y 2 O 3 powder in a molar ratio of 2: 3 was consolidated by spark plasma sintering (SPS) at 1700–1900 °C. Three SPS routes were designed to contrast the effects of passing a strong direct current (DC) on the densification behavior of the SiC materials. The wetting of the Y – Al – Si – O liquid to the SiC particles was increased by the DC, hence better densification. The resultant SiC ceramic had a finer uniform microstructure and high flexural strength. [ABSTRACT FROM AUTHOR]

In order to improve the welding performance of WC–Co to steel, a multilayer functionally graded (WC–Co)–Ni composite (MFGWC) was successfully fabricated by sintering the multilayer green compact, and a Ni-rich layer was covered on the compact surface. The microstructure, composition and micro-hardness of the MFGWC were investigated in this paper, results showed that the MFGWC was composed of WC–Co substrate, multi-transitional layers and pure Ni layer. The key part, multi-transitional layers formed between Ni layer and WC–Co substrate, was obtained successfully. The micro-hardness of the composite gradually decreased from 1300 HV to 66 HV along the direction of WC–Co substrate to pure Ni layer, this was due to a decrease in the amount of WC particles and the corresponding increase of Ni phases. The multi-transitional layer played a role in reducing the residual stresses that were induced by the difference of thermal expansion between WC–Co substrate and steel. [ABSTRACT FROM AUTHOR]

The effects of a single additive (SnO 2 , Ta 2 O 5 , or WO 3 ) on the crystal structure, densification, and electrical conductivity of the GdSmZr 2 O 7 ceramic have been examined. The GdSmZr 2 O 7 ceramic with or without a single additive of 1 wt.% SnO 2 , Ta 2 O 5 , or WO 3 sintered at temperatures between 1773 and 1973 K exhibits a single cubic pyrochlore-type phase. The relative densities of different compositions increase with increasing sintering temperature. The total conductivity of different samples obeys the Arrhenius relation. The total conductivity of the GdSmZr 2 O 7 ceramic is not improved by the addition of 1 wt.% SnO 2 , Ta 2 O 5 , or WO 3 . A single additive, SnO 2 , Ta 2 O 5 , or WO 3 , is not an effective sintering aid for the GdSmZr 2 O 7 ceramic. [ABSTRACT FROM AUTHOR]

In the present paper, aluminum matrix composites (AMCs) reinforced by nanodiamond (ND) were fabricated by hot-press sintering. The morphology and microstructure of ND/Al composite powders for different milling hours were observed. The effect of ND content on microstructure, mechanical properties and tribological behavior of AMCs was investigated. The relative density, hardness, compressive strength and wear resistance of AMCs with different ND contents were compared. The flake-shape aluminum particles with uniformly distributed ND particles were formed by ball milling for 15 h. A laminated structure of AMCs was formed by hot-press sintering at 550 °C for 5 min. ND was distributed at the grain boundary of Al matrix. At the interface between Al and ND, a layer of Al with high dislocation density was formed, which was closely connected with a layer of amorphous carbon. With increasing of ND content, more and larger pores were presented in AMCs, which caused a decrease in relative density. The hardness, compressive strength and wear resistance of AMCs were significantly improved in comparison with sintered aluminum. The AMC with 2.5% ND obtained the highest hardness and compressive strength and the lowest coefficient of friction and wear rate. [Display omitted] • A laminated structure is formed in the sintered ND-Al composite material. • At the interface between Al and ND a layer of Al with high dislocation density is formed. • The composite with 2.5% ND exhibits the highest hardness and compressive strength and the lowest COF and wear rate. [ABSTRACT FROM AUTHOR]

A series of TiB 2 –TiC composites were successfully fabricated through one-step and two-step pressureless sintering using powders obtained by high-energy ball-milling. Effects of ball-milling time on crystal size and morphology of powders as well as microstructural evolution and mechanical properties of TiB 2 –TiC composites were investigated. The crystal size of TiB 2 –TiC powders decreases to 14.7 nm after ball milling for 30 h. Two-step pressureless sintering is beneficial to the densification process without obvious grain coarsening. The core-rim structure of carbides in TiB 2 –TiC composites is formed by nonuniform diffusion of W atoms from the outside to the interior of TiC grains. The phenomenon of selective solution of WC into TiC grains was carried out through experiments and verification of first-principles DFT calculations. An in-situ TiB 2 plate toughened microstructure is developed for TiB 2 -40 wt %TiC composites two-step sintered at 1500 °C for 4 h and 2000 °C for 1 h with excellent comprehensive performances. [ABSTRACT FROM AUTHOR]

In the present study, Fe matrix composites reinforced with nano-diamond (ND) particulates were fabricated by hot-press sintering. The influence of the concentration of ND on the mechanical and tribological properties of as-sintered Fe/ND composites was investigated. The microstructures and properties of the Fe/ND composites were examined using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. A pearlite microstructure is formed due to the reaction between Fe and ND in the sintering. The presence of ND provides higher hardness, compressive strength, and flexural strength. The composite with 1 wt% ND exhibits 51.5% higher hardness, 37.4% higher compressive strength, and 76.2% higher flexural strength than sintered Fe. In addition, the wear resistance will increase as the ND concentration increases. Fe/2 wt% ND composite exhibits much lower friction coefficient (COF) and higher wear resistance than a Fe/1 wt% ND composite and sintered Fe. • A microstructure of pearlite was formed in the sintered Fe/ND composites. • Fe/ND composites exhibit higher hardness, compressive strength, and flexural strength than sintered Fe. • Fe/ND composite with higher content of ND produce lower friction coefficient (COF) and higher wear resistance. [ABSTRACT FROM AUTHOR]

h-BN composite ceramics were prepared by hot-pressing using La 2 O 3 and Al 2 O 3 as additives to form liquid phases during sintering. The effect of the amorphization/nanocrystallization of raw h-BN powders by high-energy ball-milling on the microstructure evolution of h-BN composite ceramics was investigated. The amorphization/nanocrystallization of raw h-BN powders significantly promotes grain growth during liquid-phase sintering. The specimen prepared using ball-milled h-BN powders possesses a higher crystallization degree, larger grain size, higher texture degree and more significant anisotropic thermal conductivity than the specimen prepared using raw h-BN powders. Image 1 [ABSTRACT FROM AUTHOR]

Two-step pressureless sintering was investigated for TiB 2 –40wt%TiN system. The calculation for phase equilibria was performed, which indicates that a significant nitrogen loss can occur at 1700 °C while ZrO 2 readily dissolves into TiN lattice from 1600 °C. The XRD analysis revealed that nitrogen loss was greater during first step at low temperature but smaller for second step at high temperature. As a result, the high-temperature processing of the second step provided better densification and mechanical properties than those of one-step sintering. • TThermodynamic calculation for phase equilibria was done to understand the nitrogen behavior as a function of temperature. • Considering a significant nitrogen loss during high temperature processing, two-step sintering was studied to improve the densification and mechanical properties of TiB 2 –40wt%TiN. • Two-step sintered TiB 2 -TiN composites would be an idea to improve the densification and toughening of boride-nitride ceramic composites. [ABSTRACT FROM AUTHOR]