High-pressure sintering of ultrafine-grained high-entropy diboride ceramics

Herein the ultrafine-grained (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy diboride ceramics were successfully fabricated by high-pressure sintering technology for the first time. The results showed that the grain size, relative density, and Vickers hardness of the as-fabricated samples all increased gradually with increasing sintering temperatures from 1373 K to 1973 K. The relative density and mean grain size of the as-sintered samples at 1973 K were 97.2% and 684 nm, respectively, and simultaneously they exhibited excellent comprehensive mechanical properties, combining a Vickers hardness of 26.2 GPa and a fracture toughness of 5.3 MPa·m^1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism.     

Synthesis of high-purity high-entropy metal diboride powders by boro/carbothermal reduction

Synthesis of high-purity high-entropy metal diboride powders is critical to implementing their extensive applications. However, the related studies are rarely reported. Herein we first theoretically studied the synthesis possibility of high-purity high-entropy diboride powders, namely (Hf_0.25Ta_0.25Nb_0.25Ti_0.25)B₂ (HTNTB), via boro/carbothermal reduction by analyzing the thermodynamics of the possible chemical reactions and then successfully synthesized the high-purity and superfine HTNTB powders via boro/carbothermal reduction for the first time. The as-prepared powders exhibited low-oxygen impurity content of 0.49 wt% and small average particle size of 260 nm. Meanwhile, they possessed good single-crystal hexagonal structure of metal diborides and high-compositional uniformity from nanoscale to microscale. This work will open up a new research field on the synthesis of high-purity high-entropy metal diboride powders.     

Formation of high entropy metal diborides using arc-melting and combinatorial approach to study quinary and quaternary solid solutions

High entropy metal diborides (HEBs) represent a radically new approach to extend the chemical composition window of ultra-high temperature ceramics (UHTCs). In this work, arc-melting was used to produce dense HEBs starting from UHTC powders. In order to understand the influence of each individual diboride within the quinary system (HfB₂, ZrB₂, TiB₂, TaB₂ and CrB₂), we investigated five quaternary equimolar solid solutions e.g. Hf-Zr-Ti-Ta, Hf-Zr-Ti-Cr, Hf-Zr-Ta-Cr, Hf-Ti-Ta-Cr, Zr-Ti-Ta-Cr and the overall quinary equimolar combination. Arc-melting allowed a rapid screening of favorable and unfavorable combinations. The produced HEBs were free from undesired oxides and characterized by linear variation of lattice parameters typical of diborides and binary solid solutions. Because of evaporation during arc melting, CrB₂ was hardly found in the solid solution, suggesting that vapor pressure should be taken into account when designing HEB compositions especially for operating temperatures exceeding 2000 °C. Finally, Vickers microhardness ranged between the typical values of starting diborides.     

Influence of high pressure on the structure,hardness and brittle-to-ductile transition of NbSi2 ceramics

We apply first-principles calculations to study the influence of pressure on the structure, elastic modulus, hardness, brittle-or-ductile behavior and melting point of NbSi₂. Four NbSi₂ phases: C40, C11_b, C54 and C49 are considered based on the structural feature. The results show that the calculated formation enthalpy of the four NbSi₂ phases is negative within the pressure range of 0–60 GPa, indicating that they are thermodynamically stable in whole pressure. In particular, the calculated formation enthalpy of the C54 NbSi₂ is smaller than the other NbSi₂ phases, indicating that the C54 NbSi₂ is more thermodynamically stable than the other NbSi₂ phases. The calculated elastic modulus and Vickers hardness of NbSi₂ increase with increasing pressure. Note that the pressure results in brittle-to-ductile transition of the C40 NbSi₂, C11_b NbSi₂ and C54 NbSi₂ between 30 GPa and 60 GPa. Naturally, the increasing of mechanical properties is that the pressure enhances the electronic hybridization between Nb and Si, which is demonstrated by the chemical bonding such as Nb–Si bond and Si–Si bond.     

Compositional pathways and anisotropic thermal expansion of high-entropy transition metal diborides

The recent discovery of high entropy transition metal diborides (HEBs) has sparked renewed interest in ultra-high temperature ceramics (UHTCs). Presently, transition metal (Me) oxides based boro-carbo/thermal reduction (BCTR) syntheses show great promise as relatively cheap production methods, but also may present limits to attain single phase pure HEBs. Herein, by selectively tuning the concentration of boron and carbon, the reducing agents of Me oxide mixture (Me = Ti, Ta, Nb, Zr and Hf), and exploiting high-resolution synchrotron X-ray powder diffraction, we first identified and quantified the formation of intermediate phases during the BCTR synthesis, with the ultimate intent to achieve a full dense (Ti,Ta,Nb,Zr,Hf)B₂ solid solution (SS). Additional insight was obtained by temperature dependent diffraction, which highlighted, for the first time in this class of materials, anisotropic thermal expansion, most likely at the origin of the SS micro-cracking, as was also observed by electron microscopy.     

Nanoindentation and tribology of a (Hf-Ta-Zr-Nb-Ti)C high-entropy carbide

A (Hf-Ta-Zr-Nb-Ti)C high-entropy carbide was prepared by ball milling and a two-step Spark Plasma Sintering process, achieving a single-phase ceramic sample with a high relative density of 99.4 %. The wear resistance of the sample was measured by tribology and micro-scale mechanical behaviour was studied by nanoindentation on both the non-deformed and worn surfaces. Grains and the vicinity of grain boundaries exhibited high hardness values of 38.5 ± 0.5 GPa and 35.5 ± 1.0 GPa with similar Young’s moduli of 562 ± 11 GPa and 547 ± 16 GPa, respectively. The dominant wear mechanism was mechanical wear with limited grain pull-out and fracture, and with a localized and thin tribo-layer formation. The specific wear rate exhibited an increase with the increasing load from 2.53·10^?6 mm³/Nm at 5 N to 9.03·10^?6 mm³/Nm at 50 N. This was correlated to the decrease of nanohardness of the worn surfaces with increasing wear load, which is attributed to the increased number of microcracks.     

Phase stability,mechanical, and thermal properties of high-entropy transition and rare-earth metal diborides from first-principles calculations

The narrow composition design space of high-entropy transition metal diborides (HE TMB₂) limits their further development. In this study we designed six quaternary and quinary high-entropy transition metal and rare-earth diborides (HE TMREB₂) and investigated their phase stability using the energy distribution of the local mixing enthalpy of all possible configurations. The results show that both quaternary and quinary HE TMREB₂ have higher enthalpic driving forces, which facilitates the formation of single-phase AlB₂-type structures between TMB₂ and REB₂. Calculations of elastic constants show that the TMB₂ component has the greatest effect on the c₄₄ elastic constant and shear modulus G, while REB₂ significantly influences the bulk modulus B. Furthermore, LuB₂ and TmB₂ substantially affect the elastic modulus anisotropy of HE TMB₂. Rare-earth atoms in HE TMREB₂ can enhance the nonharmonic interactions between phonons, which results in a significant hindrance in the thermal transport of low-frequency phonons as well as an increase in the volume thermal expansion coefficients. Thus, the incorporation of REB₂ into HE TMB₂ has a significant impact on the phase stability and properties.     

Rare earth and transition metal based entropy stabilised perovskite type oxides

Multicomponent oxides with perovskite type of structure containing up to 10 different cations in equiatomic amounts have been synthesised for the first time. Out of eleven systems synthesised, only six systems crystallised as single phase perovskite type compounds with random and homogenous cation distribution on the respective sites. The formation of phase pure 10-cationic system, (Gd_0.2La_0.2Nd_0.2Sm_0.2Y_0.2)(Co_0.2Cr_0.2Fe_0.2Mn_0.2Ni_0.2)O₃, in contrast to the multiphase mixtures observed in five of the lower entropy systems (containing 6 cations) indicates a possible role of entropy in the stabilisation of a single phase crystal structure. The entropy driven structural stabilisation effect is further supported by the reversible phase transformation, from single phase to multiple phase upon cyclic heat treatment, observed in the (Gd_0.2La_0.2Nd_0.2Sm_0.2Y_0.2)MnO₃ system. This type of entropic signature has been observed in rocksalt based high entropy oxide systems. However, it has not been reported before for perovskite based compounds, as shown in this study.     

Wettability and interfacial phenomena in the liquid-phase bonding of refractory diboride ceramics: Recent developments

Joining and integration technologies are integral to manufacturing of components based on ultrahigh-temperature ceramics (UHTCs) such as transition metal diborides. Brazing is a particularly attractive joining technique because of its simplicity and versatility, but its use to join the UHTCs demands knowledge of the complex interplay among high-temperature wettability, interfacial reactions, and chemical and thermoelastic compatibilities. This paper summarizes the research and development activities carried out over the last two decades to characterize the wettability and interfacial phenomena in brazing of refractory diboride ceramics. The contact angle data of various metal alloys on diboride-based ceramics have been collected and critically evaluated in conjunction with an analysis of the chemistry and structure of the interface to understand the underlying mechanisms and phenomena that govern interface formation. It explores how solid–liquid interactions impact and are impacted by physical, chemical, and mechanical properties of joined materials. It also describes how this knowledge has been successfully utilized to create liquid-phase bonded diboride-based joints. The paper concludes with a summary of the current state of the art and highlights integration challenges and future research and technology development needs in the area.     

Size effect on hardness for micro-sized and nano-sized YAG transparent ceramics

Traditional micro-sized and nano-sized YAG transparent ceramics were tested by nanoindentation at different peak loads. The micro-sized YAG transparent ceramics show a marked indentation size effect (ISE). However, for the nano-sized YAG transparent ceramics, the hardness was constant in the whole investigated range without any evidence of ISE. We show that the absence of indentation size effect for nano-sized YAG transparent ceramics can be accurately modeled using the plastic deformation mechanism of grain boundary sliding.     

Corrosion and friction resistance of TiVCrZrWNx high entropy ceramics coatings prepared by magnetron sputtering

High entropy alloy coatings have attracted much attention because of their high hardness, low-level fault energy, and chemical stability. Nevertheless, this type of coating would inevitably suffer from wear, corrosion, aging, and so on. Hence, a novel coating with corrosion and friction resistance would be constructed for broadening its application scenarios. In this work, TiVCrZrWN_x high entropy ceramics coatings were prepared by reactive magnetron sputtering. The microstructure, mechanical properties, friction, and corrosion resistance of the coatings deposited at different nitrogen flow rates have been studied. The microstructure of TiVCrZrWN_x coatings is strongly dependent on the nitrogen flow rate and forms a stable FCC structure when the nitrogen flow rate reaches 24 sccm. The pure TiVCrZrW coating is 15.65 GPa, with the increase of nitrogen flow rate (24 sccm), the coating hardness reaches 21.27 GPa. The corrosion resistance of the coatings also increases continuously. According to the results of the impedance spectrum and polarization curve, the charge transfer resistance value of the coating gradually increases with the content of nitrogen, the current density rapidly decreases to a minimum as the potential increases. In terms of tribological behavior, the formation of V₂O₅ during the sliding in seawater could significantly reduce the coefficient of friction from 0.603 to 0.383. Therefore, TiVCrZrWN_x HECs coatings simultaneously possess high hardness, toughness, and excellent resistance to friction and corrosion, which is expected to provide a new and reliable method for the research field of coatings in the maritime field.     

Effect of ball milling on the sintering performance of indium-gallium-zinc oxide ceramics: The diffusion mechanism and lattice distortion of milled powders

The mechanisms inducing lattice distortion and grain boundary diffusion in mixed In₂O₃–Ga₂O₃-ZnO (IGZO) powders were investigated during ball-milling treatment at different rotation speeds. After sintering, the highest relative densities for the sintered IGZO compacts were 92.87%, 98.72%, and 99.12% at rotation speeds of 100 rpm, 200 rpm, and 300 rpm, respectively, and the corresponding grain growth activation energies were 78.78, 67.17, and 49.52 kJ/mol. The sintering process of InGaZnO₄ ceramics was shown to be dominated by grain boundary diffusion. X-ray diffraction line profile analysis indicated that the highest average lattice distortion of IGZO powders milled at 200 rpm and 300 rpm was 2.85 and 10.27 times higher, respectively, than that of IGZO powder milled at 100 rpm. The enhancements of the sintering properties of the IGZO powders induced by the faster milling speeds were attributed to the higher lattice distortion energies and shorter diffusion distances of the elements. This elucidated the mechanism behind the high-energy ball-milling process for achieving enhanced sintering performances in ceramic powders.     

First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: Microstructural,mechanical and thermal characterization

First principles calculations were conducted on (HfTiWZr)B₂ high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B₂ HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB₂ structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10^?4 mm³, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C.     

Influence of graphite on the low-frequency fatigue behavior of zirconium diboride ceramics

The fatigue behavior of a ZrB₂-based ceramic containing SiC and graphite was compared to a ZrB₂-SiC reference material based on bending testing, quantitative calculations as well as crack growth and fracture characterization. The addition of graphite flake makes ZrB₂-SiC-Graphite ceramics exhibit fatigue failure behavior at very high stress level (93% of the characteristic strength, σ₀), owing to the increased K_Ic promoted by crack deflection, bridging, bifurcation and pull-out of graphite, while the fatigue behavior of ZrB₂-SiC appears when the maximum stress is below ~86%σ₀. However, both the slow crack growth exponents of the graphite containing ceramic, n and nc values, which reflect the fatigue resistance in static and cyclic fatigue conditions, respectively, are only 1/4 as compared to the reference graphite-free ceramic. This may be due to the weak boride/graphite interfaces, which lead to the decrease of the initial critical stress intensity factor (K_c-initial) value from 2.6 to 2.0 MPa m^1/2.     

Improved creep resistance of high entropy transition metal carbides

The creep behaviour of (Ta-Hf-Zr-Nb)C high entropy ceramic (HEC) was investigated at temperatures between 1400 and 1600 °C in vacuum under compressive stresses from 150 to 300 MPa. The measured steady-state creep rates ranged from 2 × 10⁻⁹/s to 8 × 10^-8/s, which are approximately 10 times lower than the published creep rates of the corresponding monocarbides. The stress exponent n is in the range of 2.34 ∼ 2.89 and the average activation energy is 212 kJ/mol. The creep mechanisms involve dislocation glide/climb and the formation of voids and cracks. The voids formed at the grain boundaries parallel to the loading direction, which often connected to form cracks at the highest load/temperature The active dislocation slip system during creep was &lt,110&gt,{111}. The reason why (Ta-Hf-Zr-Nb)C has enhanced creep resistance compared to the monocarbides can be explained by lattice distortion and the higher thermodynamic stability of HEC ceramic at high temperatures.     

Simultaneous enhancement of electrical and mechanical properties in CaBi2Nb2O9-based ceramics

CaBi₂Nb₂O₉ (CBN) with Aurivillius phase has an enormous potential in high-temperature piezoelectric devices due to their high Curie temperature and excellent free-fatigue characteristics. Nevertheless, simultaneous enhancement of electrical and mechanical properties in CBN-based ceramics are still a great challenge because of the trade-off between the electrical and mechanical properties. Herein, a strategy, the synergy effect of lattice distortion and oxygen vacancy, is designed to realize the enhanced electrical and mechanical properties of CBN-based ceramics via the domain structure and grain size engineering. The materials can simultaneously deliver a high piezoelectric property of 17.3 pC/N, large hardness of 4.68 GPa, and intensive bending strength of 113.07 MPa, which are enhanced by 346%, 197%, and 141% over those of unmodified CBN ceramics. We believe that the founding of this research opened up a novel and efficient guideline for exploring new bismuth-layered structure ceramics with excellent electrical and mechanical properties.     

Thermal properties of (Zr0.2Ce0.2Hf0.2Y0.2RE0.2)O1.8 (RE= La,Nd and Sm) high entropy ceramics for thermal barrier materials

The high-entropy ceramic materials (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_1.875 (H-0) and (Zr_0.2Ce_0.2Hf_0.2Y_0.2RE_0.2)O_1.8 (H-RE) (RE = La, Nd and Sm) with fluorite structure and homogeneous element distribution were prepared. With fluorite structure, fine grain size and high density, the H-0 and H-RE ceramics displayed low thermal conductivity, suitable thermal expansion coefficient, high hardness and fracture toughness. The effect of La, Nd and Sm on the mechanical, heat conductivity and heat expansion properties of high entropy ceramics were discussed. The single-phase high-entropy ceramic materials in this work are very suitable for application as thermal barrier materials.     

The effect of submicron grain size on thermal stability and mechanical properties of high-entropy carbide ceramics

(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two-step sintering process. Both X-ray and neutron diffractions confirmed the formation of single-phase with rock salt structure in the as-fabricated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine-grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse-grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine-grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine-grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.     

Effect of a lattice distortion strategy on the phase transition and properties in KNN-based ceramics

High performance lead-free piezoelectric ceramics are of great importance to the sustainable development of the environment. To obtain excellent comprehensive performance KNN-based lead-free piezoelectric ceramics, a lattice distortion strategy combined with domain configuration was designed in (1 − x)K_0.5Na_0.5Nb_0.95Sb_0.05O₃–xCaHfO₃ ((1 − x)KNNS–xCH) system by introduced Ca²⁺ into the A-site and Hf⁴⁺ into the B-site. The results demonstrated that the rhombohedral–orthorhombic–tetragonal polymorphic phase boundary (PPB) was constructed in 0.02 ≤ x ≤ 0.04 and significant lattice distortion occurred in R- and T-phase. Moreover, the 0.97KNNS–0.03CH sample exhibited excellent electrical performance (e.g., k_p ∼ 43.8%, d^*₃₃ ∼ 478.6 pm/V, and d₃₃ ∼ 392 pC/N) together with a high Curie temperature (T_C ∼ 295°C) profited from the PPB and domain configurations. The ceramics also showed the optimal thermal stability, which was beneficial to promote the development of KNN-based ceramics.