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Although significant increase in mechanical properties has been achieved in α-SiAlON ceramics by introducing self-toughening bimodal microstructure, the bimodal microstructure may not always result in better cutting performance of α-SiAlON tool inserts. Herein Dy-α-SiAlON ceramics were prepared by hot-pressing at 1900°C under 30MPa. Diameter and aspect ratio of the large elongated α-SiAlON grains pivotal to formation of bimodal microstructures were regulated by using 0-5wt.% Dy2O3 as a sintering additive. Mechanical properties and cutting performance of α-SiAlON tool inserts were evaluated. The results showed good Vickers hardness (19.0-19.7GPa) and fracture toughness (3.61-4.18MPa•m1/2) increasing with crack propagation length. However, transverse section of the large elongated grains was revealed as critical crack to cause premature fracture of the α-SiAlON ceramics. Fracture rather than attrition wear was recognized as dominant mechanism for tool failure, indicating that for better cutting performance of the α-SiAlON tool inserts a selftoughening microstructure consisting of thinner grains with large aspect ratio is required.

Three multi-phase tantalum carbide composites (TaC z -Ta 5 Si 3) were fabricated by Spark plasma Sintering (SPS) using Ta, TaC and SiC powders. SiC was added to control the C:Ta ratio (z = 0.6, 0.74, 1.0), regulating the phase composition and grain morphology while also serving as a deoxidizer. Composite TaC 0.6 -Ta 5 Si 3 had Ta 2 C as the major phase and contained Ta 4 C 3−x , TaC and Ta 5 Si 3. It exhibited an excellent combination of high flexure strength (714 ± 21 MPa) and toughness (13.9 ± 0.2 MPa·m1/2), despite a low Ta 4 C 3−x fraction, a phase known to have very high toughness. The outstanding performance was attributed to the elongated Ta 2 C grain morphology, highly laminated intra-granular Ta 4 C 3−x laths, and the refined Ta 2 C grain size. Composite TaC 0.74 -Ta 5 Si 3 , characterised by equiaxed grain morphology, exhibited good flexure strength (571 ± 36 Mpa) and toughness (7.6 ± 1.1 MPa·m1/2) due to the presence of Ta 4 C 3−x laths in the c-TaC. Lastly, composite TaC 1.0 -Ta 5 Si 3 , composed of c-TaC and Ta 5 Si 3 , exhibited moderate strength (545 ± 25 MPa) and toughness (5.3 ± 0.1 MPa·m1/2). • Ta 2 C-dominant and TaC-dominant composites (TaC z -Ta 5 Si 3) are fabricated using Ta, TaC and SiC. • SiC additives control the C:Ta and effectively promoted the sintering through oxide removal. • TaC 0.6 -Ta 5 Si 3 has the best toughness due to the elongated Ta 2 C grains and intra-granular laths. • TaC 0.74 -Ta 5 Si 3 with the equiaxed TaC has good toughness due to the intra-granular laths. • TaC 1.0 -Ta 5 Si 3 with the equiaxed TaC is free of intra-granular laths and has the lowest toughness. [ABSTRACT FROM AUTHOR]

A new route based on HfO2 + B4C + C → HfB2 + HfC + CO reaction was developed to synthesize HfB2‐HfC hybrid powders. X‐ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and transmission electron microscopy were used to evaluate the powder products. The effects of excess additions of B4C and C on the phase constituents of the powder products were investigated. Thermodynamics of relevant reactions for the synthesis of the hybrid powders were calculated to guide the selection of compositions and processing parameters. The results indicated that HfB2‐HfC hybrid powders were obtained by reaction at 1750°C for 1 h. HfB2:HfC phase content ratios showed a dependence on B4C and C additions. The obtained hybrid powders exhibited good oxidation resistance with an oxidation activation energy of 303 kJ/mol. Good sinterability of the powder products was demonstrated by spark plasma sintering at 2100°C. The consolidated ceramics based on them measured good Vickers' hardness values (>26.0 ±.5 GPa) and fracture toughness (>3.78 ±.27 MPa·m1/2). [ABSTRACT FROM AUTHOR]

Abstract TaC ceramics with 0.03–0.60 wt% of boron additions were prepared by hot pressing at 2100 °C for 1 h under a pressure of 40 MPa. Effects of boron content on densification, phase composition, microstructure, mechanical properties and oxidation resistance of the TaC ceramics were investigated. When the boron content was 0.12 wt% and above, full density was obtained due to reactions between boron and oxygen impurity at presence of TaC. Minor phases of TaB 2 and C were formed in the 0.24 and 0.60 wt% B compositions after gas-out of the oxygen impurity. Microstructure of the TaC ceramics was refined with increasing in boron content. The TaC ceramic with 0.24 wt% of boron showed the best mechanical properties with a Vickers hardness, flexural strength and fracture toughness of 17.7 GPa, 534 MPa and 4.6 MPa m1/2, respectively. When more boron was added, interfacial bonding of the TaC grains was strengthened causing a decrease in fracture toughness. Oxidation resistance of the TaC ceramics increased with boron content. Particularly, the 0.60 wt% B composition showed a weight gain of 0.0018 g/cm2 after oxidization at 800 °C in air for 3 h. [ABSTRACT FROM AUTHOR]

Abstract Tantalum carbide ceramics with an overall composition of TaC 0.6 were prepared by hot pressing at 1700 °C for 1 h under 30 MPa pressure with fcc TaC and bcc Ta powder as starting materials. Effects of adding 0–30 mol% Cu on densification, phase formation, microstructural evolution, mechanical properties and electrical conductivity of the obtained composite ceramics were investigated. Full density could be reached at 20 mol% Cu addition and above. Reactions between TaC and Ta resulted in the final phases of hexagonal Ta 2 C and rhombohedral ζ-Ta 4 C 3-x. Plate-shaped ζ laminate grew in both TaC and Ta 2 C and consequently had their sizes confined by the parental grains. Addition of 20–30 mol% Cu could significantly grow the intermediate Ta 2 C grains, hence controlling length of the ζ laminates precipitated in them to form bimodal microstructures consisting of large grains. The 20 mol% Cu composition had a better flexural strength (706 ± 46 MPa) and fracture toughness (9.2 ± 1.7 MPa m1/2). Electrical resistivity at room temperature of the composites decreased with Cu content from 1.03 μΩ m at 0 mol% addition to 0.12 μΩ m at 30 mol% Cu. Highlights • TaC 0.6 ceramics with 0–30 mol% Cu addition were hot pressed at 1700 °C for 1h. • The 10–30 mol% Cu Compositions reached relative densities >97%. • Microstructure consisting of large Ta 2 C and small TaC y grains was formed. • ξ-Ta 4 C 3-x precipitated in both TaC y and Ta 2 C grains. • The 20 mol% Cu composition had good flexural strength and fracture toughness. [ABSTRACT FROM AUTHOR]

Abstract TaC-Al 2 O 3 composites were prepared by hot pressing. Influence of Al 2 O 3 content ranging from 10 to 40 vol. % on densification, phase composition, microstructure, mechanical properties and oxidation behavior of the TaC-Al 2 O 3 composites was investigated. With 30 and 40 vol. % Al 2 O 3 addition a closed porosity was achieved. The Al 2 O 3 particles were uniformly distributed among TaC grains retarding grain growth and resulting in refined microstructures with grains below 2 μm in size. The most densified composite with 40 vol. % Al 2 O 3 addition exhibited good mechanical properties with a Vickers' hardness of 17.8 GPa, a flexural strength of 485 MPa and a fracture toughness of 5.4 MPa·m1/2. After holding at 700°C for 3 h in air, the dense 30 and 40 vol. % Al 2 O 3 compositions showed hardly noticeable and mainly surface oxidation, whereas less densified TaC-Al 2 O 3 composites with 10 and 20 vol. % Al 2 O 3 content and with open porosity were disintegrated to powders. Highlights • TaC-Al 2 O 3 composites with 10-40 vol% Al 2 O 3 were prepared by hot pressing. • Satisfying densification could be reached at 30 vol% Al 2 O 3 addition and above. • The dense TaC ceramics with 30-40 vol% Al 2 O 3 had good mechanical properties. • Oxidation resistance was significantly improved by adding 30-40 vol% Al 2 O 3. [ABSTRACT FROM AUTHOR]

Abstract: Zirconium diboride (ZrB2) ceramics were prepared by reactive hot pressing of ZrB+B powder mixture. Formation of a transient liquid due to eutectic reaction of ZrB2+Zr→Leu(ZrB2+Zr) at 1661°C following peritectic decomposition of 2ZrB=ZrB2+Zr at 1250°C during heating up of the ZrB+B mixture facilitated densification. The liquid phase was subsequently eliminated via reaction of B with Zr in the eutectic liquid Leu(ZrB2+Zr) to result in a dense ZrB2 ceramic. Full density was reached after reactive hot pressing at 1900°C under 30 MPa for 1 h. The ZrB2 ceramic had a refined microstructure consisting of grains of <1.5 μm in size and relatively good Vickers hardness (21 ± 2 GPa) and flexural strength (595 ± 63 MPa). [ABSTRACT FROM AUTHOR]

Abstract Tantalum carbide ceramics containing 0.25–3.40 wt% Al as densifying aid were prepared by hot pressing at 1900 °C under 30 MPa for 60 min. Effects of Al addition on densification, phase composition, microstructure and mechanical properties of the TaC-Al ceramics were investigated. Al was an effective oxygen getter which reacted with volatile oxygen impurities such as Ta 2 O 5 on surfaces of TaC particles to form non-volatile minor phase of Al 2 O 3 , thus slowed TaC grain growth via inhibiting the non-densifying matter transport by evaporation-condensation. When Al content was larger than the value that was needed for complete impurity reaction of 10Al + 3Ta 2 O 5 = 6Ta + 5Al 2 O 3 , presence of Al liquid at high temperatures improved densification of the TaC-Al ceramics because Al liquid spread along the grain boundaries and wet the TaC grains. The TaC ceramic with 0.75, 1.50 and 3.40 wt% Al addition reached a relative density value of 91.6, 97.5 and 100%, respectively, with a similar average grain size of 3.5–4.8 μm. Fe impurity dissolved in Al and formed Al 6 Fe; slight reaction of Al with TaC formed a thin layer of Ta-Al-C compound around TaC grains. The TaC ceramic with 1.50–3.40 wt% Al additions showed good Vickers hardness, Young's modulus and flexural strength in the range of 12.4–13.2 GPa, 413–479 GPa and 532–673 MPa, respectively. Increase in oxidation resistance of TaC ceramics was observed at 600 °C in air with a few percentages of Al additions. Highlights • TaC-Al ceramics with 0.25–3.40 wt% Al were prepared by hot pressing at 1900 °C. • Al reacted with volatile oxygen pieces in TaC powder to form non-volatile Al 2 O 3. • Substitution of Al 2 O 3 for volatile Ta-O inhibited TaC grain growth. • The 1.50 wt% Al material was fully densified and had good mechanical properties. • Formation of minor phases in TaC-Al composite ceramics was discussed. [ABSTRACT FROM AUTHOR]

Tantalum carbide (TaC)-based composites containing 0.1–30 mol% lanthanum hexaboride (LaB 6 ) were prepared by hot pressing at 1900 °C for 30 min under 30 MPa pressure. Effects of LaB 6 content on densification, microstructure and mechanical properties of the consolidated TaC-LaB 6 composites were investigated. Sinter-forge was performed to test creep resistance of the TaC-LaB 6 composites at temperatures up to 1950 °C. Addition of LaB 6 increased densification of the TaC-LaB 6 composites and the compositions with ≥1 mol% LaB 6 almost reached full densities. LaB 6 reacted with TaC and the Ta 2 O 3 impurity to form minor phases of TaB 2 , B 2 O 3 and C. Average size of the TaC grains in the consolidated composites showed a decrease trend with increase in the LaB 6 content. The TaC-LaB 6 composites with 1–2 mol% LaB 6 addition had relatively good flexural strength, Vickers hardness and fracture toughness with a value of 530 MPa, 15.6 GPa and 5.29 MPa m 1/2 , respectively. Addition of LaB 6 modified interfacial bonding strength of the TaC grains, hence increase in creep resistance under sinter-forge up to 1950 °C. [ABSTRACT FROM AUTHOR]

Metallic Si as sintering aid was effective in densifying tantalum carbide ceramic (TaC) by spark plasma sintering ( SPS) at 1700°C. Full density was reached at 5.0 mol% Si addition (equivalent to 1.088% Si in weight) and above. Enhanced densification of TaC ceramic with Si was associated with decrease in oxygen content from ~0.24 wt% in TaC powder to ~0.03 wt% in consolidated specimen. Rest of the oxygen species was collected at multigrain conjunctions to form SiO2-based liquid at high temperatures. Upon cooling, Ta, Si, O, and C dissolving in the liquid precipitated minor phases of TaSi x and SiC of low concentrations. Microstructure of TaC ceramics was refined by the Si addition, with average grain size decreasing from 11±8 μm at 1.0 mol% Si to 3±2 μm at 7.5 mol% Si addition. Ta solute in SiC and Si solute in TaC were evidenced. TaC ceramic containing 7.5 mol% Si had a relatively good flexural strength and fracture toughness of 646±51 MPa and 5.0 MPa·m1/2, respectively. [ABSTRACT FROM AUTHOR]

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si 3 N 4 as sintering aid. Effects of the Si 3 N 4 addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si 3 N 4 reacted with TaC and tantalum oxides such as Ta 2 O 5 to form a small concentration of tantalum silicides, SiC and SiO 2 , with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si 3 N 4 addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si 3 N 4 to 4 µm at 2.50 mol% Si 3 N 4 addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si 3 N 4 addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples. [ABSTRACT FROM AUTHOR]

TaC ceramics were prepared by hot pressing at 1900 °C for 30 min under 30 MPa mechanical pressure. Effects of the addition of 20 mol% ZrC or 5–20 mol% ZrC plus 5 mol% Cu as sintering aids on densification, phase composition, microstructure and mechanical properties of the consolidated materials were investigated. The results showed the pure TaC had a relative density of 86%of the theoretical value. Densities >97% theoretical were reached in the materials when the sintering aids were used. ZrC and TaC reacted to form solid solutions. The grain average size decreased from 18 µm in the pure TaC to 6 µm at 20 mol% addition. Addition of 5 mol% Cu further refined the microstructures. The 20 mol% ZrC+5 mol% Cu and the 10 mol% ZrC+5 mol% Cu composition had good flexural strength of 608 MPa and fracture toughness of 5.4 MPa m 1/2 , respectively. [ABSTRACT FROM AUTHOR]

The effects of the addition of 5mol.% aluminum, copper, silver or gold on the densification, microstructures, mechanical properties and oxidation resistance in thin air of TaC ceramics were investigated. Full density and relatively high flexural strength (up to 738MPa) were observed in the case of the Au addition after spark plasma sintering at 1700°C for 5min under 30MPa pressure. Due to wetting of the TaC grains by a thin metallic film, the consolidated materials showed enhanced oxidation resistance against thin air. [ABSTRACT FROM AUTHOR]

Abstract: TaC/SiC composites with 20vol.% SiC addition were densified by spark plasma sintering at 1600–1900°C for 5min under 40MPa. Effects of sintering temperatures on the densification, microstructures and mechanical properties of composites were investigated. The results showed the materials achieved >98% of theoretical density at a temperature as low as 1600°C. While the TaC grains grew slightly with the sintering temperature increasing, the SiC particles in materials decreased in size. Equiaxed to elongated grain morphology transformation was observed in the SiC phase in the 1900°C material to obtain a higher flexural strength and fracture toughness of 715MPa and 6.7MPam1/2, respectively. Lattice enlargement of the TaC phase in the 1900°C material suggested possible Si diffusion into TaC grains. Ta was also detected in SiC grains by energy dispersive spectroscopy. Glassy pockets present at multi-grain junctions explained the enhanced densification. [Copyright &y& Elsevier]

Abstract: TaC/SiC composites with 5–40vol.% 6H–SiC were fabricated by spark plasma sintering at 1800°C for 5min. Effects of SiC concentrations on the densification, microstructure, and mechanical properties of the materials were investigated. The results showed the densification process of the TaC/SiC composites was composed of six sequent stages. SiC addition was effective in eliminating the closed porosities in the final stage to yield fully dense TaC/SiC composites at 20vol.% SiC concentrations and above. TaC grain sizes decreased with SiC concentration increase. The 40vol.% SiC composition had the highest flexure strength and fracture toughness up to 703MPa and 6.8MPam1/2 due to the reinforcement of the SiC agent. The TaC/SiC composites with 20 and 40vol.% SiC showed good flexural strength at 1400°C. [Copyright &y& Elsevier]

Abstract: A TaC composite with ∼11wt% in situ TaB2 was fabricated by spark plasma sintering at 1600–1900°C. The densification process was studied by analysis of the densifying shrinkage of the powder compacts. Three distinct stages for densification were determined. The starting powder mixture of TaC, B4C and Ta completed reaction to form the desired TaC and TaB2 at temperature <1583°C. At around ∼1750°C, the TaC/TaB2 material significantly improved relative density to ∼95% with rapid grain growth. The final densification took place very rapidly at 1900°C by releasing a high pressure vapor. [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: The thermal shock behavior of 30wt%BAS/Si3N4 composite was evaluated by the water-quenching technique. The results show that the investigated composite possesses excellent thermal shock resistance. The critical temperature difference (ΔT c) of the composite is over 1100°C and the retained strength at a quenching-temperature difference of 1100°C is hardly affected by the quenching cycles. It is attributed to its high strength and low Young''s modulus. The in situ growth of elongated β-Si3N4 grains are also beneficial to the improvement of thermal shock resistance due to the increased crack propagation resistance by crack deflection and grain pull-out mechanisms. In addition, the formation of an oxide layer on the sample surface before quenching may act as an additional thermal barrier and leads to a reduction in the intensity of thermal shock. [Copyright &y& Elsevier]

Yb-, Y- and Dy-doped mixed α/β-SiAlON ceramics were fabricated by hot-pressing using 10wt.% BaAl2Si2O8 (BAS) glass ceramic as an additive. The results showed that BAS could effectively facilitate the anisotropic growth of both α- and β-SiAlON grains. The α-SiAlON grains as well as the β-SiAlON grains were highly elongated. The obtained mixed α/β-SiAlONs possess much better mechanical properties than those of conventional mixed α/β-SiAlONs containing equiaxed α-SiAlON grains in their microstructure. [Copyright &y& Elsevier]

Abstract: Refractory Y-α-SiAlONs with elongated microstructure were obtained by hot pressing and post-heat treatment using BaAl2Si2O8 (BAS) as an additive. The results showed that BAS not only served as a liquid phase sintering aid to promote densification, but also facilitated the development of elongated α-sialon grains, resulting in excellent room and high-temperature mechanical properties. Increasing the amount of BAS addition and post-heat treatment could further effectively promote the anisotropic growth of α-SiAlON grains, the room-temperature flexural strength and fracture toughness of 10wt% BAS/Y-α-SiAlON could reach 634MPa and 7.2MPam1/2, respectively. The resultant materials could maintain the room strength up to 1400°C due to the extensive crystallization of BAS. [Copyright &y& Elsevier]

Barium aluminosilicate (BAS) glass–ceramic composites reinforced with multi-wall carbon nanotubes (MWNTs) were fabricated by hot-pressing. The results showed that BAS glass–ceramic served as an effective liquid phase sintering aid to promote the densification of the composites. The incorporation of MWNTs could significantly improve the mechanical properties of the BAS matrix. [Copyright &y& Elsevier]

Abstract: Dense 40wt%BAS/Si3N4 self-reinforced composite was synthesized by a two-step pressureless sintering process. The results showed that the BAS glass–ceramic not only served as an effective liquid phase sintering aid for attaining full densification and completing the α- to β-Si3N4 phase transformation, but also remained as a structural matrix. A bimodal microstructure could be obtained via this two-step process without the addition of special β-Si3N4 seeds. It is due that the first step could suppress the α- to β-Si3N4 phase transformation while allowing densification, and then at the higher temperature second step, the continued transformation could be used to facilitate the growth of the β-nuclei formed in the first step. The obtained composite exhibits excellent mechanical properties compared to unreinforced BAS matrix. The flexural strength and fracture toughness could reach 565MPa and 7.4MPam1/2, respectively. The crack deflection, grain bridging and pullout are considered as major toughening mechanisms in this composite. [Copyright &y& Elsevier]

TaC x (x = 0.5, 0.55, 0.6 and 0.7) ceramics with 20 mol% Al addition were prepared by hot pressing TaC, Ta and Al powder mixtures. Effects of the overall C:Ta ratios and the Al addition on constituent phases, microstructures and mechanical properties of the consolidated materials were examined. Results showed that O impurity contained in the starting powders was collected in form of Al 2 O 3 due to strong affinity of O with Al. The TaC x -Al composites were easy to densify due to presence of molten Al at its melting point (T m = 660 °C) and a transient Al–Ta eutectic liquid at higher temperatures. After densification, the metallic liquids were eliminated ultimately by formation of refractory Ta 4 AlC 3 via reaction of Al, Ta and TaC. Reactions between the remnant Ta and TaC after Ta 4 AlC 3 formation resulted in ζ-Ta 4 C 3-z or h-Ta 2 C depending on the initial C:Ta ratio. The Ta 4 AlC 3 compound was compatible with the tantalum carbides. Microstructures of the TaC x -Al (x = 0.5–0.7) composites were consisted of ζ-Ta 4 C 3-z and Ta 4 AlC 3 fine lamellar grains to benefit the mechanical properties of the composites. The TaC 0.7 –20mol% Al composition had satisfying mechanical properties (flexural strength 613 ± 28 MPa and fracture toughness 11.2 ± 0.8 MPa m1/2). TG-DSC evidenced an increase in oxidation resistance of the TaC x ceramics by the addition of 20 mol% Al. TaC x -20mol% Al (x = 0.5,0.55,0.6,0.7) composites prepared by hot pressing powder mixtures of TaC, Ta and Al are featured by easy densification, coexistence of Ta 4 AlC 3 with ζ-Ta 4 C 3-z /h-Ta 2 C and lamellar growth of the constituent grains in the microstructures as shown here in the SEM micrograph to result in good flexural strength and fracture toughness (613 ± 28 MPa and 11.2 ± 0.8 MPa m1/2, respectively, at x = 0.7). Addition of 20 mol% Al improved oxidation resistance of the tantalum carbide ceramics by increasing the oxidation initiation temperature from about 550 °C for pure TaC ceramics to 750 °C for the TaC x -Al composites. [Display omitted] • TaC x (x = 0.5–0.7) ceramics with addition of 20mol%Al were prepared. • The composites were easy to densify due to presence of a transient Al–Ta liquid. • The Al–Ta liquid was replaced by formation of the ultimate refractory Ta 4 AlC 3 and ζ-Ta 4 C 3-z /h-Ta 2 C. • Lamellar growth of the constituent phases imparted good fracture toughness to the composites. [ABSTRACT FROM AUTHOR]

TaC 0.6 -Cu composites were fabricated by hot pressing of TaC, Ta and Cu powder mixtures. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, scanning-transmission microscopy, X-ray energy-dispersive spectroscopy, and selected area electron diffraction were used to investigate formation process and growth behavior of the ζ-Ta 4 C 3-x lamellae. Cu addition did not change the reaction sequence of TaC + Ta → Ta 2 C + TaC y → ζ-Ta 4 C 3-x to form the targeted ζ-Ta 4 C 3-x phase, but enhanced growth of ζ-Ta 4 C 3-x lamellae by introducing a dissolution-reprecipitation mechanism due to presence of Cu liquid at high temperatures. ζ-Ta 4 C 3-x could directly nucleate and grow on h-Ta 2 C grain surface, meanwhile C immediately reacted with Ta solutes in Cu liquid after the C atoms diffusing along the {111} TaCy stacking faults out of parental γ-TaC y grains to initiate extension of ζ-Ta 4 C 3-x lamella outside the γ-TaC y grains, thus grew large ζ-Ta 4 C 3-x lamellae with a uniform thickness in the microstructure of the TaC 0.6 -Cu composites. The TaC 0.6 -Cu composites with large ζ-Ta 4 C 3-x lamellae in the microstructures showed increase in fracture toughness. • TaC 0.6 -Cu composites were fabricated by hot pressing. • Large ζ-Ta 4 C 3-x lamellae with a uniform thickness brought good fracture toughness. • A mechanism was proposed to explain the extremely large ζ-Ta 4 C 3 lamellae growth. [ABSTRACT FROM AUTHOR]

Porous α-SiAlON ceramics were fabricated using the camphene-based unidirectional freeze casting method, in which a gradient porous structure was formed as a result of the decreased solidification velocity in the freezing direction. Microstructure, porosity and pore size distribution of different parts of as-prepared samples were examined and compared, and correlated with their mechanical properties. For a given solid loading, the overall pore size and porosity of the top part were greater than those of the bottom part. Interestingly, despite its higher porosity, the flexural strength and fracture toughness of the top part were both higher than those of the bottom part, suggesting that apart from porosity, pore morphology and size affected mechanical properties of as-prepared porous α-SiAlON ceramics. [ABSTRACT FROM AUTHOR]

TaB 2 –SiC composites with 0–20 wt% SiC contents were prepared by reaction hot pressing using Ta, B and SiC powders as starting materials. Densification, constituent phases, microstructure and mechanical properties, namely Vickers hardness, fracture toughness and flexure strength, were evaluated by means of inspection of sintering shrinkage, density measurement, XRD, SEM, TEM, HRTEM, EDS, SAED, indentation and three-point bending tests. Phase formation, including dominant (TaB 2 , SiC) and minor (TaC, glassy SiO 2) phases, was checked by thermodynamic assessment relevant with Ta 2 O 5 and B 2 O 3 impurities introduced into the reaction system by the starting powders. The results showed that mixtures of Ta, B and SiC powders would initiate self-propagating combustion synthesis (SPCS) at about 900 °C; a transient hold at 800 °C for 1 h could suppress this reaction. Volatilization of B 2 O 3 impurity in association with local high temperature due to SPCS helped reduce the TaC and glassy SiO 2 minor phase formation. TaB 2 –SiC composites could reach 99 % relative densities with 3 wt% SiC addition and above. Compositions with <7.5 wt% SiC additions showed uniform dispersion of SiC particles at multi-TaB 2 grain junctions while compositions with higher SiC content revealed SiC segregation. Bending strength of 503 MPa and fracture toughness of 6.69 MPa⋅m1/2 could be reached with 7.5–20 wt% SiC addition. • TaB 2 –SiC composites were prepared by reaction hot pressing of Ta, B and SiC powder mixtures. • Formation of dominant phase TaB 2 , minor TaC and SiO 2 impurity was discussed. • Effects of SiC content on microstructure and mechanical properties were investigated. [ABSTRACT FROM AUTHOR]

Abstract: Gradient porous β-SiAlON ceramics were prepared by unidirectional freezing of camphene-based suspensions on a cold substrate at 0°C and subsequent sintering at 1800°C for 2h. Effects of solid loading content in the suspensions on porosities, pore structures and strengths of the porous ceramics were investigated. The results showed that a gradient microstructure consisting of aligned channels, dendritic and equiaxed pores from the bottom to the top of the sample was created by the freezing process. Both the microstructure observation and the pore size distribution indicated that with the increase of the solid content from 10vol% to 30vol%, the porosity decreased from 71.3% to 36.1% and the pore size decreased from 20μm to 5μm. As a result, the compressive strength increased significantly from 49MPa to 313MPa. The sample fabricated from the slurry with 10vol% solid loading content showed a large strain for failure up to ∼9%. [Copyright &y& Elsevier]

Abstract: Porous Si3N4 ceramics were prepared by freeze casting using liquid N2 as refrigerant. The pore structure, porosity, α→β-Si3N4 transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α→β-Si3N4 phase transformation, but was beneficial to the mechanical properties of the obtained porous Si3N4 due to its unique pore structure. [Copyright &y& Elsevier]

Abstract: Dense short silicon carbide (SiCsf) and carbon fibers (Csf) reinforced BaAl2Si2O8 (BAS) glass-ceramic composites with silicon nitride were fabricated by hot-pressing technique. The phase characterization, microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and three-point bending tests. The results showed that short silicon carbide and carbon fibers disperse homogeneously in BAS matrix, and had good chemical compatibility with the glass-ceramic matrix. The addition of Si3N4 could successfully eliminate the microcracks in the BAS matrix induced by the thermal mismatch between the fiber and matrix. Both the added short fibers could effectively reinforce the BAS glass-ceramic by the associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects. [Copyright &y& Elsevier]

Abstract: The effects of the excess Lu2O3 addition and post heat treatment on the phase composition, morphology of α-SiAlON grains and mechanical properties of Lu-α-SiAlON were investigated. The results show that incorporation of excess Lu2O3 facilitates the formation of elongated α-SiAlON grains. Post heat treatment not only further promotes the growth of α-SiAlON grains with a larger aspect ratio, but also results in the adjustment of phase compositions and eliminates the residual glass phases between α-SiAlON grains. [Copyright &y& Elsevier]

TaC x (x = 0.5, 0.55, 0.6, 0.7) ceramics with 20 mol% Fe, Co or Ni addition were prepared by hot pressing TaC and Ta powder mixtures at 1800 °C. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, X-ray energy dispersive spectroscopy and selected area electron diffraction were used to investigate the constituent phases and microstructures of the composites. Vickers hardness, flexural strength and fracture toughness were measured. Intermetallic compounds Fe 7 Ta 3 , Co 2 Ta and Ni 3 Ta were formed due to reactions of Fe, Co and Ni with Ta. The remaining TaC and Ta reacted to form hexagonal Ta 2 C, rhombohedral ζ-Ta 4 C 3-z or cubic TaC y depending on the overall C:Ta ratios. The microstructure of the TaC 0.5 –20 mol% Fe, Co, Ni compositions (where C:Ta = 0.5) were composed of intermetallics and fine anisotropic Ta 2 C and ζ-Ta 4 C 3-z grains. As the C:Ta ratio increased to 0.55–0.7, the ζ-Ta 4 C 3-z and TaC y grains were enlarged in size and equiaxed in the TaC 0.55 -, TaC 0.6 -, and TaC 0.7 –20 mol% Fe, Co, Ni compositions. Addition of Ni instead of Fe and Co benefitted to refine microstructures of the composites. The TaC 0.5 –20 mol% Fe, Co, Ni compositions had good flexural strength (580-650 MPa) and fracture toughness (9.3–9.5 MPa·m1/2) due to growth of fine lamellar grains, despite the high Ta 2 C content. • TaC x (x = 0.5, 0.55, 0.6, 0.7) ceramics with 20 mol% Fe, Co or Ni addition. • Ta 2 C, ζ-Ta 4 C 3-z or cubic TaC y formed depending on overall C:Ta ratio. • Addition of Ni instead of Fe and Co benefitted to refine microstructures. • The TaC 0.5 compositions were composed of fine anisotropic Ta 2 C and ζ-Ta 4 C 3-z. • Good strength and toughness of TaC 0.5 compositions despite high Ta 2 C content. [ABSTRACT FROM AUTHOR]

Novel in situ toughened ScLu–α-SiAlON ceramics with high optical transmittance were prepared by hot-pressing. The obtained material achieves the maximum transmittance of ∼60% in the region of 2500–5000nm for the sample of 1.0mm in thickness. The development of elongated microstructure does not result in an obviously adverse effect on optical transmittance. This indicates that it is possible to produce α-SiAlON ceramics with good mechanical and optical properties. [Copyright &y& Elsevier]

Abstract: Dense silicon carbide (SiC)-based ceramics were obtained by hot-pressing using BaAl2Si2O8 (BAS) as an additive. The results showed that BAS could serve as an effective liquid sintering aid to promote densification and facilitate the growth of SiC grains. The relative densities, flexural strength and fracture toughness of the composites increase with increasing the BAS content. The flexural strength and fracture toughness of 40wt%BAS/SiC composite could reach 510MPa and 7.0MPam1/2, respectively, The main toughening mechanisms are crack deflection, crack bridging and SiC grain pullout. The complete crystallization of BAS is beneficial to the high temperature mechanical properties and the obtained 40wt%BAS/SiC composite could maintain the room strength up to 1200°C. [Copyright &y& Elsevier]

Abstract: Highly porous Si3N4 with macroscopically aligned pores were prepared by the unidirectional freeze casting process using liquid N2 as refrigerant. The increased sintering temperature had no obvious influence on the porosity, but significantly promoted α→β-Si3N4 phase transformation and the growth of fibrous β-Si3N4 grains, and changed the pore structure. The flexural strength and fracture toughness of the sample sintered at 1900°C could reach 73MPa and 1.1MPam1/2, respectively. [ABSTRACT FROM AUTHOR]

Abstract: The thermal shock properties of 5wt% BAS/α-SiAlON ceramics fabricated by spark plasma sintering (SPS) technique have been valuated with an indentation-quench method based on Vickers cracks. The obtained materials possessed excellent thermal shock resistance due to the formation of self-reinforced microstructure. The co-doped α-SiAlONs exhibited better thermal shock resistance, undergoing a more stable crack growth than the single-doped ones. It is attributed to the higher fracture toughness. [ABSTRACT FROM AUTHOR]

Abstract: Porous barium aluminosilicate (BAS)/Si3N4 composites were prepared by freeze casting process using liquid N2 as refrigerant. The pore morphologies, α → β-Si3N4 phase transformation, and mechanical properties of the resultant porous ceramics were strongly affected by the addition of polyacrylamide (PAM) into the slurries. The incorporation of 2wt.%PAM changed the ice crystals structure from lamellar with dendrites into featureless columnar ones, the feature of which was duplicated by the pores in the sintered ceramics. The lamellar pores with dendrites could favor the α → β-Si3N4 phase transformation, but were detrimental to the mechanical properties. The sample with 1wt.%PAM exhibited the highest strength and toughness in the resultant porous composites due to its columnar pore structure with less dendritic pores and higher aspect ratio of β-Si3N4 grains. A relative dense layer was formed at the bottom of the samples with PAM and its thickness increased with increasing PAM content. [ABSTRACT FROM AUTHOR]

Abstract: cBN/β-SiAlON composites with different cBN contents were fabricated by spark plasma sintering (SPS) process. The densification, thermal stability of cBN, microstructure and mechanical properties of the resultant composites were investigated. The addition of cBN particles to β-SiAlON could increase the Vickers hardness, flexural strength and fracture toughness of β-SiAlON. It is attributed to the high density, no obvious cBN to hBN transformation and good interface bonding between cBN grains and β-SiAlON matrix. [Copyright &y& Elsevier]

Abstract: Y-α-SiAlON (Y1/3Si10Al2ON15) ceramics with 5wt.%BaAl2Si2O8 (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600°C without holding and using a heating rate of 100°Cmin−1, but the transformation from α-Si3N4 to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800°C for 5min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2GPa and fracture toughness of 6.8MPam1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300°C. [Copyright &y& Elsevier]