To overcome the poor sinterability and low fracture toughness (KIC) of zirconium carbide (ZrC) ceramics, a novel multiscale microstructure design was proposed via a two-step in situ reactive spark plasma sintering (SPS) process using ZrC, TiSi2, and B4C powders. During sintering, TiSi2 preferentially reacted with B4C to form TiB2 and primary silicon carbide (SiC), while the released Si further reacted with ZrC to yield ZrSi2 and secondary SiC. The two-step SPS (1600 °C/3 min + 1800 °C/10 min) promoted complete in situ reactions, liquid-phase sintering, and interdiffusion of Zr/Ti, leading to the formation of (Zr,Ti)C and (Ti,Zr)B2 solid solutions. With the addition of 30 mol% TiSi2 and 15 mol% B4C, the multiphase ceramics exhibited a refined submicrostructure (grain size < 500 nm), achieving a high flexural strength of 824±46 MPa and KIC of 7.5±0.5 MPa·m1/2. The synergistic enhancement in strength and toughness is attributed to a multiscale strengthening/toughening mechanism: solid-solution strengthening at the atomic scale, effective grain boundary pinning by nanosized primary and secondary SiC particles at the nanoscale, and toughening through crack deflection and bridging by TiB2–SiC agglomerates and the higher-toughness ZrSi2 phase at the microscale. This work provides a viable and innovative approach for designing high-performance ultrahigh-temperature ceramics through tailored in situ reactions and microstructural control.
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The mechanism underlying the influence of carbon vacancies on the comprehensive properties of multi-component carbide ceramics has been thoroughly investigated. A series of (TiZrNbMo)Cx ceramics with varying carbon content were fabricated using spark plasma sintering (SPS). Detailed examinations were conducted on the phase composition, microstructure evolution, as well as mechanical and thermal properties, in response to carbon content variation. The variations in bonding states and charge distribution were calculated to elucidate the mechanism through the influence of carbon vacancies. The observed nano hardness peak of (33.3 ± 0.4) GPa in the C0.75-22 sample is attributed to the enhanced strength of the M−C covalent bond induced by the presence of carbon vacancies. Moreover, the exceptional lattice stability and resistance to compression were further validated through theoretical simulations of compression deformation performed via ab initio molecular dynamics (AIMD). Additionally, the presence of carbon vacancies was found to enhance the phonon and electron scattering, and thus led to reduce the thermal conductivities.
Ultra-high temperature ceramics include a series of high melting point materials, such as transition metal carbides, nitrides and borides, especially transition metal carbides with excellent high-temperature mechanical properties, stable physico-chemical properties, corrosion resistance and radiation resistance, showing broad prospects for application in the hypersonic aircraft, rocket engines, fourth-generation nuclear reactor, and other extreme environments. However, the traditional single-component transition metal carbide ceramics have been unable to meet the emerging requirements under extreme environments, there is an urgent need to develop a new high-performance material under ultra-high temperature. In recent years, the concept of multi-component “high entropy” has greatly expanded the scope of material composition design and property optimization. Compared with single-component carbides, multi-component ceramics perform better in terms of overall properties, including hardness, creep resistance, oxidation resistance and radiation resistance. These improvements mainly result from their complex component composition, electronic structure and lattice distortion. At present, the influence of elemental species on the microstructure evolution and mechanical properties of carbide high-entropy ceramics is not reported. In this paper, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are prepared by hot press sintering, and the effects of Me elemental species on the physical phase, microstructure evolution, and mechanical properties of (TiZrNbTaMe) high-entropy ceramics are investigated.
In this work, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics with equimolar ratio were prepared by carbothermal reduction-assisted hot pressing using transition metal oxides and carbon black as raw materials. The oxides and carbon black were mixed using a planetary ball mill (Fritsch, model P4, Germany). Carbide powders were obtained by carbothermal reduction under vacuum using a pressureless sintering furnace (WS0404, Ningxia Sincere Co. Ltd., China) with a process of 1500 ℃ /1 h. The synthesized carbide powders were loaded into graphite molds, and the ceramic samples were prepared by a two-step hot pressing method (AVS, model 1540, USA). The samples were held at 1850 ℃ for 1 h, and then at 2100 ℃ for 0.5 h under pressure of 30 MPa or 10 Pa. Phase analysis was carried out by X-ray diffractometry (XRD; D/max-B, Rigaku, Japan) using Cu-Kα rays. Scanning electron microscopy (SEM; Quanta 200FEG, USA) was used to analyze the microstructure and elemental content and distribution. Transmission electron microscopy (TEM; Talos F200X, USA) was used to analyze the microstructure, elemental content and distribution, and grain boundary characteristics of the materials. The relative densities of the samples were measured using image analysis software (Photoshop, Adobe, USA) based on the pores in the SEM photographs. Vickers hardness was measured using a Vickers hardness tester (HVS-30) at a load of 9.8 N with a holding time of 15 s. Fracture toughness was also measured using the indentation method. The interaction parameters between the metal elements were also calculated using DFT calculation.
The high-entropy ceramics are all characterized by an FCC crystal structure. Except for the (TiZrNbTaW)C sample, the porosity of the other three high entropy ceramics is low, and their density exceeds 98% and the element distribution is relatively uniform. Compared with the (TiZrNbTaMo)C sample, the grain sizes of the (TiZrNbTaV)C and (TiZrNbTaCr)C samples are significantly reduced, which are 2.69 μm and 5.39 μm, respectively. This shows that the addition of V and Cr elements helps to improve the sintering properties of the (TiZrNbTaMe)C system and inhibit grain growth. The element content analysis results of (TiZrNbTaCr)C show that the Cr element segregates significantly at the grain boundaries, while the other four metal elements are distributed more evenly. The segregation of Cr at the grain boundaries may be related to chromium carbides with low melting points. For example, the melting point of Cr3C2 is about 1810 ℃, while the sintering temperature is as high as 2100 ℃, suggesting that chromium carbides may form a liquid phase and aggregate at the grain boundaries during sintering. In addition, the complex composition of high-entropy ceramics may also affect the solid solubility of Cr. The interaction coefficients between Cr and each metal element in transition metal carbides show that the interaction parameter values of Cr and other metal elements are high. It is difficult for Cr to form a solid solution with other metal elements, thus tending to be enriched at the grain boundaries. Therefore, (TixZr0.4–xNb0.2Ta0.2Cr0.2)C ceramics with different Ti and Zr contents (x=0, 0.2, 0.3 and 0.4) were designed and prepared. The effect of Ti content on the solid solubility of Cr in the (TiZrNbTaCr)C sample was investigated. As the x value increases from 0 to 0.4, the solid solubility of Cr in the grains increases from 3.18% to 8.68%. It shows that the increase of Ti content is beneficial to improve the solid solubility of Cr. In addition, for the (TiZrNbTaCr)C sample, the high lattice distortion leads to solid solution strengthening in the grains and the high density of the system increases its hardness, which has the best mechanical properties. Its Vickers hardness and fracture toughness reach 29.8 GPa and 3.71 MPa·m1/2, respectively.
The four (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are all face-centered cubic structures, and the elements are uniformly distributed in the high-entropy ceramic systems except for the (TiZrNbTaCr)C sample. The (TiZrNbTaCr)C sample is also characterized by the presence of a significant Cr segregation at grain boundaries, resulting in a low Cr content inside the grains. The (TiZrNbTaCr)C sample has the best overall mechanical properties, with the Vickers hardness and fracture toughness reaching 29.8 GPa and 3.71 MPa·m1/2, respectively. In-depth studies show that the solid solubility of Cr element in high-entropy carbide ceramics is closely related to the species and content of metal elements. Enhancing the content of Ti element helps to improve the solid solubility of Cr element in high-entropy carbide ceramics, in which the solid solubility of Cr element in the grain of (Ti0.4Nb0.2Ta0.2Cr0.2)C system reaches the maximum value of about 8.68%.
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Transition metal carbides are promising candidates for thermal protection materials due to their high melting points and excellent mechanical properties. However, the relatively high thermal conductivity is still a major obstacle to its application in an ultra-high-temperature insulation system. In this work, the low thermal conductivity of dense (TiZrHfVNbTa)Cx (x = 0.6–1) high-entropy carbides has been realized by adjusting the carbon stoichiometry. The thermal conductivity gradually decreases from 10.6 W·m−1·K−1 at room temperature to 6.4 W·m−1·K−1 with carbon vacancies increasing. Due to enhanced scattering of phonons and electrons by the carbon vacancies, nearly full-dense (97.9%) (TiZrHfVNbTa)C0.6 possesses low thermal conductivity of 6.4 W·m−1·K−1, thermal diffusivity of 2.3 mm2·s−1, as well as electrical resistivity of 165.5 μΩ·cm. The thermal conductivity of (TiZrHfVNbTa)C0.6 is lower than that of other quaternary and quinary high-entropy carbide ceramics, even if taking the difference of porosity into account in some cases, which is mainly attributed to compositional complexity and carbon vacancies. This provides a promising route to reduce the thermal conductivity of high-entropy carbides by increasing the number of metallic elements and carbon vacancies.
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Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot- pressing using different contents of 3Y2O3-5Al2O3 (molar ratio of 3:5) as the sintering additive. During hot-pressing, the liquid Y3Al5O12 (YAG) phase showing good wettability to h-BN grains was in situ formed through the reaction between Y2O3 and Al2O3, and a coherent relationship between h-BN and YAG was observed with [010]h-BN//
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Phase relation and microstructure evolution in the pressure-less sintered TiB2‒TiC ceramics preceded with mechanical alloying were systematically studied by a combination of SEM analysis. WC debris from milling balls promotes sintering by dissolving into the TiC phase to achieve dense microstructures at 1600 ℃. Variation of W solution in TiC grains exposes two types of core‒rim structures, with no or more W in dark and white cores respectively but with common medium W in both rims. Diminishing white-cores reveal an exchange reaction between WC and TiC via mechanical alloying to form the Ti1-zWzC phase prior to sintering. The dark-cores inherit from the as-milled TiC power to further enable the reprecipitation of rims from a mixed liquid-phase, which facilitated also the anisotropic growth of TiB2 grains. The dark-cores grow persistently in the second-step at 2000 ℃ enabled by this liquid-phase, which coarsens the TiB2 grains too. With more alloyed phase, sintering was insufficient at 1500 ℃ with only the surface fluidity from the primary powders, and the second-step sintering increased the fluidity in the liquid-phase to fully densify the binary microstructure. Re-distribution of the alloyed W by two-step sintering rationalizes the evolution process of the binary microstructures and leads to better understanding of the mechanical behaviors.
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