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Raising the configurational entropy in a solid solution ceramic is regarded as a promising strategy to improve the mechanical properties of ceramics, especially when five or more elements are mixed to form so‐called high‐entropy ceramics. However, in this study, we report that the binary (TiZr)C solid solution ceramics can demonstrate high hardness comparable or even superior to high‐entropy ceramics. Followed by a carbothermal reduction synthesis of carbide powders, the bulk ceramics were synthesized by hot pressing. Via increasing the hot pressing temperature to 2200°C, a full solid solution of equimolar (TiZr)C was obtained in contrast to phase separation at lower sintering temperatures, for example, 2000 and 2100°C. The dislocation networks are observed in the single‐phase (TiZr)C ceramic and should be the product of competition between enthalpy and entropy in a binary full solid solution. These defects finally contribute to the high nano‐hardness of 41.9 ± 1.4 GPa (H) and the Vickers hardness of 22.0 ± 0.6 GPa (HV at 49 N). [ABSTRACT FROM AUTHOR]

We report for the first time the synthesis of a core‐shell structured composite powder with a core of Zr(Ti,Ta)C and a shell of Ti,Ta(Zr)C at 1700°C and investigate the formation mechanism for the core‐shell structure. The medium‐entropy (Ti,Zr,Ta)C ceramics with fine grains (1.1 ± 0.4 μm) and relative density of 94.8% was prepared by hot‐pressing at 2100°C. The flexural strength of (Ti,Zr,Ta)C at 1000°C (493 ± 21 MPa) was close to the room temperature (511 ± 52 MPa). As the temperature increased from 1600°C to 1800°C, the flexural strength was increased significantly, with an ultra‐high flexural strength of 725 ± 32 MPa at 1800°C. The existence of the core‐shell structure in the powder suppressed the grain growth due to the sluggish diffusion effect. The ultra‐high strength of (Ti,Zr,Ta)C ceramics was attributed to its fine microstructures, high fracture toughness, and the reinforced the grain boundary strength. [ABSTRACT FROM AUTHOR]

Medium‐entropy (Ti,Zr,Hf)C ceramics were prepared by hot pressing a dual‐phase medium‐entropy carbide powder with low oxygen content (0.45 wt%). The results demonstrate that the medium‐entropy (Ti,Zr,Hf)C ceramics sintered at 2100°C had a relative density of 99.2% and an average grain size of 1.9 ± 0.6 μm. The flexural strength of (Ti,Zr,Hf)C carbide ceramics at room temperature was 579 ± 62 MPa. With an increase in temperature to 1600°C, the flexural strength showed an increase up to 619 ± 57 MPa, and had no significant degradation even up to 1800°C. The high‐temperature flexural strengths of (Ti,Zr,Hf)C were obviously higher than those of the monocarbide ceramics (TiC, ZrC, and HfC). The primary strengthening mechanism in (Ti,Zr,Hf)C could be attributed to the high lattice parameter mismatch effects between TiC and ZrC, which not only inhibited the fast grain coarsening of (Ti,Zr,Hf)C ceramics, but also increased the grain‐boundary strength of the obtained ceramics. [ABSTRACT FROM AUTHOR]

A series of ZrB 2 -ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B 4 C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB 2 : 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB 2 -ZrC-SiC composites with various SiC content from 0 to 20 vol%. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fine-grained Mo–12Si–8.5B alloys were prepared by hot pressing sintering. [•] The microstructure control is achieved by mechanical alloying process. [•] The yield stress and especial the flexure strength of alloys are superior. [ABSTRACT FROM AUTHOR]

ZrB2–20vol% SiC (ZS) ceramics based on ZrB2 starting powders obtained by different boro/carbo-thermal reductions involving ZrO2 +B4C, ZrO2 +B4C+C, and ZrO2 +B, were fully densified by hot pressing at 1900–2000°C. The flexural strength of these ZS ceramics was measured from room temperature up to 1600°C. At 1600°C, the flexural strength of the ceramics is 460±31, 471±32 and 345±11MPa, respectively. The evolution of the strength as function of temperature is explained in terms of the differences in oxygen content, nature of fracture, grain sizes, grain boundary phases and microstructural defects. [ABSTRACT FROM AUTHOR]

Abstract: ZrB2–SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB2 and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also improved. [Copyright &y& Elsevier]

• Glass containing Na 2 O has a lower crystallization temperature relative to that containing K 2 O. • Glass ceramics obtained by powder-sintering show a low degree of densification. • Fully dense glass ceramics containing Na 2 O are obtained by hot pressing sintering. • Glass ceramics containing K 2 O are not fully densified after hot pressing. The influences of different alkali metal oxides, sintering methods, process conditions on the crystallization behavior and densification of CaO-Al 2 O 3 -SiO 2 -Fe 3 O 4 -R 2 O (R=Na, K) glass ceramics were investigated. It can be concluded that the initial crystallization temperature and crystallization peak temperature of glass containing K 2 O were higher than those of glass containing Na 2 O, respectively. The increase of sintering temperature promoted the precipitation of crystalline phase in glass ceramics, meanwhile, the crystalline phases transformed from single magnetite to magnetite and CaFeSi 2 O 6. The glass ceramics obtained by powder-sintering showed a low degree of densification. The fully dense glass ceramics containing Na 2 O can be obtained by hot pressing sintering above 680 °C for 2 h. However, due to the high viscosity of the glass containing K 2 O, the degree of densification of K 2 O containing glass ceramics was relatively low even after the hot pressing sintering. Magnetite based glass ceramics can be used as candidates for magnetic materials. [ABSTRACT FROM AUTHOR]

Fully dense and magnetically controllable glass ceramics was successfully synthesized by method of hot pressing using CaO-Al 2 O 3 -SiO 2 -Na 2 O glass powder and Fe 3 O 4 powder as raw materials. The influences of sintering temperature and time, content and particle size of Fe 3 O 4 , as well as particle size of glass powder on the densification and magnetic properties of samples were investigated. It was found that the saturation magnetization gradually increased with increasing magnetite content. In addition, the samples containing smaller size magnetite particles had a higher coercivity. However, for samples using smaller size glass powder, magnetite particles could partially dissolve into the glass matrix, which led to the decrease of saturation magnetization and the increase of coercivity. It was also concluded that the precipitation of crystalline phase from smaller size glass powder caused the decrease of degree of densification, and after decreasing the sintering temperature, the degree of densification of product was enhanced. [ABSTRACT FROM AUTHOR]

SiC and HfC were used to tailor microstructures and improve properties of hot pressed HfB 2 -based ceramics. SiC and HfC particle, as the second phase, could hinder grain growth of HfB 2 . Compared with HfC, SiC grains grew slower, which was more effective to hinder grain growth of HfB 2 . The ternary ceramics HfB 2 –20vol%HfC–20vol%SiC had the finest microstructure, indicating the effect of SiC and HfC on grain growth. Several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, which are harmful to mechanical properties of boride-SiC ceramics. The predominant fracture mode of HfB 2 –20vol%SiC was transgranular, however, in HfB 2 –10vol%HfC–10vol%SiC and HfB 2 –20vol%HfC–20vol%SiC ceramics, the dominating fracture mode shift from transgranular to intergranular. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m 1/2 and 5.05 MPa m 1/2 , which were 17.3% and 26.7% higher than HS20 (3.72 MPa m 1/2 ), respectively. The flexural strengths of HfB 2 –HfC–SiC ceramics were improved to ~750 MPa, which were about 1.3 times higher than that of HfB 2 –20SiC. [ABSTRACT FROM AUTHOR]

Dense TiC-based ceramics were obtained at 1500 °C by hot pressing, benefiting from Ti and Si additives in the TiC matrix. The in-situ formed Ti 3 SiC 2 platelet grains refined TiC grains and reinforced the obtained ceramics. The TiC–20 vol.% Ti 3 SiC 2 ceramic exhibits a much higher flexural strength of 1003 MPa, compared with 450 MPa of the pure TiC ceramic. Owing to the formation of continuous heat-conducting paths by the bridged Ti 3 SiC 2 platelet, a high thermal conductivity over 33 W/m · k was measured in TiC–40 vol.% Ti 3 SiC 2 at 500 °C, which is nearly 1.5 times than that in the pure TiC ceramics with a similar grain size. [ABSTRACT FROM AUTHOR]

Toughened ZrB 2 –SiC composites have been prepared by an introduction of amorphous carbon spheres. After hot pressing, microstructure observation showed that the carbon phases were present in the form of thin layer at grain boundaries, or in the form of agglomerates at the multigrain junctions. The carbon spheres led to significant improvement of fracture toughness from 4.96 ± 0.46 MPa m 1/2 to 7.52 ± 0.19 MPa m 1/2 . The toughening mechanism was attributed to the weak interfaces, which occurred due to the presence of carbon layer formed at grain boundaries after sintering. [ABSTRACT FROM AUTHOR]

Abstract: HfB2 and HfB2–10vol% HfC fine powders were synthesized by carbo/boronthermal reduction of HfO2, which showed high sinterability. Using the as-synthesized powders and commercially available SiC as starting powders, nearly full dense HfB2–20vol% SiC (HS) and HfB2–8vol% HfC–20vol% SiC (HHS) ceramics were obtained by hot pressing at 2000°C/30MPa. With the incorporation of HfC, the grain size of HHS was much finer than HS. As well, the fracture toughness and bending strength of HHS (5.09MPam1/2, 863MPa) increased significantly compared with HS (3.95MPam1/2, 654MPa). Therefore, it could be concluded that the incorporation of HfC refined the microstructure and improved the mechanical properties of HfB2–SiC ceramics. [Copyright &y& Elsevier]

Abstract: This work compared phase composition, microstructures and mechanical properties of ZrB2—20vol.% SiC—5vol.%Yb2O3 prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength. [Copyright &y& Elsevier]

A high-entropy M 2 AlC-MC (M=Ti, Zr, Hf, Nb, Ta) composite was produced by reactive hot pressing using Ti, Zr, Hf, Nb, Ta, Al, and graphite as raw materials. Successful solution of the five transition metals in both hexagonal M 2 AlC and cubic MC was confirmed by XRD, SEM, atom-resolved STEM and EDS elemental analyses. The high-entropy M 2 AlC shows larger delocalization of atoms and higher lattice strain than the high-entropy MC along with considerable amount of Al vacancies. The formation mechanism of the high-entropy composite has been discussed. The Vickers hardness of the composite is measured to be 11.1 ± 1.1 GPa. Image, graphical abstract [ABSTRACT FROM AUTHOR]

B 4 C–ZrB 2 ceramics were reactively hot pressed by three-step process using B and C with Zr (C–B–Zr mixtures) or ZrO 2 (C–B–ZrO 2 mixtures) as raw materials. Based on thermodynamic analysis and experimental results, the C–B–Zr or C–B–ZrO 2 mixtures were firstly held at 1000 °C, then held at 1650 °C, and finally held at 2000 °C under 30 MPa. B 4 C–ZrB 2 ceramics were obtained by using the three-step process and controlling the reaction sequence. The ZrB 2 grains in B 4 C–ZrB 2 ceramics from C–B–Zr mixtures had a bimodal grain size distribution including fine grains with size of 1–2 μm and large agglomeration-like grains with size of 10–30 μm, while that in the ceramics from C–B–ZrO 2 mixtures showed homogeneous distribution with small grain size of 1–2 μm. The B 4 C–ZrB 2 ceramics from C–B–ZrO 2 mixtures showed higher relative density, more homogeneous and fine-grained microstructure, higher hardness (33.0 ± 1.3 GPa), and comparable toughness (4.1 ± 0.2 MPam 1/2 ), compared with the ceramics from C–B–Zr mixtures. [ABSTRACT FROM AUTHOR]

Chemical reactivity of hot-pressed Si 3 N 4 –ZrB 2 ceramics at 1500–1700 °C under N 2 and Ar atmosphere is investigated by X-ray diffraction, thermodynamic calculation, and phase diagram. Results show that there are chemical reactions in Si 3 N 4 –ZrB 2 ceramics. The chemical reactions accelerate the decomposition and phase transformation of Si 3 N 4 . The reaction products include ZrN, BN, ZrSi 2 , SiC or Si, which strongly depend on the atmosphere or temperature. The source of N 2 for ZrN and BN products is both atmosphere and decomposition of Si 3 N 4 at 1600 °C or above, whereas that is only atmosphere at 1500 °C. There is no reaction in Si 3 N 4 –ZrB 2 ceramics at 1500 °C under Ar. The MgO–Lu 2 O 3 sintering adds lead to the significant increase of relative density of Si 3 N 4 –ZrB 2 from 48% to 94% at 1500 °C in Ar. Therefore, it is possible that dense Si 3 N 4 –ZrB 2 ceramics with no reaction could be achieved at 1500 °C or below in Ar. [ABSTRACT FROM AUTHOR]

Abstract: Using nano-sized starting powders, TiN ceramics were hot pressed under different sintering atmospheres, which were nitrogen, argon and vacuum (5Pa). The effects of sintering atmosphere on the densification process and microstructures of TiN ceramics were investigated. At 1500°C, ceramics densified under vacuum demonstrated higher density compared with nitrogen and argon, owing to the high nitrogen vacancy concentration which came from the dissociation of TiN under vacuum circumstance. At 1600°C, however, the densification process under vacuum will be hindered because of the decomposition of TiN. Combining a lower sintering temperature, smaller grain size and higher grain boundary concentration, the argon atmosphere is more suitable for fabrication of TiN ceramics as nuclear materials. [Copyright &y& Elsevier]

Abstract: ZrC ceramics with additions of MC (M=V, Nb, and Ta) were prepared by hot pressing, and the effect of MC additions on densification was analyzed in terms of the solubility limit and kinetics of formation of MC solid solutions with ZrC. VC additions of 2.5–10vol% (3.5–13.5mol%), which is higher than its solid solubility limit of 1.3vol% (1.8mol%), effectively promoted the densification process and nearly fully dense ZrC ceramics were obtained by hot-pressing at 1900°C. In contrast, both NbC and TaC additions, which can form unlimited solid solutions with ZrC, have no obvious contribution to ZrC densification. This is because formation of the solid solution of NbC and TaC with ZrC matrix requires higher temperature and longer time due to their stronger bonding energy, compared to VC. SEM observation demonstrated that the VC addition resulted in larger grains, compared to ZrC ceramics with NbC and TaC additions. [Copyright &y& Elsevier]