Synthesis and characterization of (HfMoTiWZr)C high entropy carbide ceramics

(HfMoTiWZr)C high entropy carbides (HEC) were prepared from the commercial carbide powders of IVB (Hf, Ti, Zr) and VIB (Mo, W) group metals of the periodic table via high energy ball milling (HEBM) and spark plasma sintering (SPS). Metal carbide powders (HfC, TiC, ZrC, Mo₂C and WC) were HEBM’d for 3 h in a vibratory ball mill, and then SPS’d at different temperatures (1800, 1900, 2000 and 2100 °C). The HEBM’d powders and SPS’d ceramics were characterized in composition, density and microstructure using an X-ray diffractometer (XRD), a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), a particle size analyzer and a pycnometer. Also, microhardness and sliding wear tests were conducted on the SPS’d ceramics. Based on the performed characterization, a single-phase FCC structure was observed at all sintering temperatures indicating a high entropy carbide ceramic, and they all have high hardness and wear resistance values.     

Effect of residual excess carbon on the densification of ultra-fine HfC powder

The residual carbon content of ultra-fine hafnium carbide (HfC) powder was controlled by the optimization of the synthesis process, and the effect of residual carbon on the densification of HfC powder was analyzed. The amount of residual carbon in the HfC powder could be reduced by the de-agglomeration of HfO₂ powder before the carbo-thermal reduction (CTR) process. The average particle size of HfO₂ powder decreased from 230 to 130 nm after the de-agglomeration treatment. Ultra-fine (d₅₀: 110 nm) and highly pure (metal basis purity: >99.9 % except for Zr) HfC powder was obtained after the CTR at 1600 °C for 1 h using the C/Hf mixing ratio of 3.3. In contrast, the C/Hf ratio increased to 3.6 without the de-agglomeration treatment, indicating that a large amount of excess carbon was required for the complete reduction of the agglomerated HfO₂ particles. HfC ceramics with high relative density (>98 %) were obtained after spark plasma sintering at 2000 °C under 80 MPa pressure when using the HfC powder with low excess carbon content. In contrast, the densification did not complete at a higher temperature (2300 °C) and pressure (100 MPa) when the HfC powder contained a large amount of residual carbon. The results clearly indicated that residual carbon suppressed the densification of HfC powder in case the carbide powder had low oxygen content, and the residual carbon content could be controlled by the optimization of the synthesis process. The average grain size and Vickers hardness of the sintered specimen were 6.7(±0.7) μm and 19.6 GPa, respectively.     

Synthesis of HfB2 powders by mechanically activated borothermal reduction of HfCl4

HfB₂ powders were synthesized via a borothermal reduction route from mechanically activated HfCl₄ and B powder blends. Mechanical activation of the powder blends was carried out for 1 h in a high-energy ball mill using hardened steel vial and balls. Mechanically activated powders were subsequently annealed at 1100 °C for 1 h under Ar atmosphere. Then, purification processes such as washing with distilled water and leaching in HCl solution were applied for the elimination of the undesired boron oxide (B₂O₃) phase and the probable Fe impurity. The effect of boron amount on the microstructure of the resultant powders was investigated. The boron amount in the starting blends plays an important role in the formation of the HfO₂ phase. HfB₂ powders without any detectable HfO₂ were prepared by adding 20 wt% excess amount of boron. Microstructural analyses of the mechanically activated, annealed and purified powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM).     

Preparation and high-temperature performance of HfC-based nanocomposites derived from precursor with Hf-(O,N) bonds

A novel precursor was synthesized by reacting hafnium chloride with dicyandiamide and dimethylformamide. The precursor was characterized via FT-IR and NMR, as well as TG. Subsequently, the precursor was annealed in Ar over a range of temperatures from 1000 °C to 2000 °C, and the microstructural evolution of the ceramics was investigated by XRD, XPS, and TEM. The results show that the carbothermal reduction of the precursor starts at 1150 °C and the ceramic yields at 1500 °C reach 44.6 wt%. The obtained powders exhibit a uniform distribution and are composed of N-doped HfC and graphite. The N-doped structure postponed the oxidation of the HfC(N) ceramics. The HfC(N) ceramics were first oxidized to yield HfO₂, carbon, and nitrogen, and then the carbon was oxidized with the evolution of CO₂. The presented synthesis method is believed to be applicable to the preparation of other high-performance ceramics.     

Synthesis and characterization of nanometric yttrium-doped hafnia solid solutions

Nanometric-sized yttrium doped HfO₂ powders were obtained by applying metathesis and combustion reactions. The tailored composition of solid solutions was: Hf_1?xY_xO_2?δ with concentration “x” ranging from 0 to 0.2. HfCl₄ was used as a source of hafnium whereas Y(NO₃)₃·6H₂O was used as a source of yttrium. The obtained powders were annealed at different temperatures in order to induce crystallization of HfO₂. The influence of dopant concentration, annealing temperature and annealing time on powder properties was examined. The XRD analysis revealed that the crystal structure of HfO₂ depends on the dopant concentration. The samples doped with 20 mol% of yttrium and annealed at 1500 °C had high-temperature, cubic structure even after cooling to room temperature. The presence of relatively large amount of dopant was beneficial in stabilizing highly desirable cubic phase of HfO₂. It was found that the crystallite size lies in the nanometric range (<10 nm).     

Carbothermal production of ZrB2–ZrO2 ceramic powders from ZrO2–B2O3/B system by high-energy ball milling and annealing assisted process

Zirconium diboride (ZrB₂)-zirconium dioxide (ZrO₂) ceramic powders were prepared by comparing two different boron sources as boron oxide (B₂O₃) and elemental boron (B). The production method was high-energy ball milling and subsequent annealing of powder blends containing stoichiometric amounts of ZrO₂, B₂O₃/B powders in the presence of graphite as a reductant. The effects of milling duration (0, 2 and 6 h), annealing duration (6 and 12 h) and annealing temperature (1200–1400 °C) on the formation and microstructure of ceramic powders were investigated. Phase, thermal and microstructural characterizations of the milled and annealed powders were performed by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). The formation of ZrB₂ starts after milling for 2 h and annealing at 1300 °C if B₂O₃ is used as boron source and after milling for 2 h and annealing at 1200 °C if B is used as boron source.     

Strong and tough HfC-HfB2 solid-solution composites obtained by reactive sintering with a SiB6 additive

HfC-HfB₂ composite ceramics were successfully reactively spark plasma sintered with a unique SiB₆ additive. The incorporation of the SiB₆ not only promotes the densification of HfC (up to ~99%), but also significantly enhances the toughness from 4.3 ± 0.5 MPa m^1/2 to 14.2 ± 1.4 MPa m^1/2. The flexural strength of the HfC-HfB₂ composite ceramics was simultaneously improved to 529 ± 48 MPa, which is about 1.4 times higher than that of HfC. This improvement is attributed to the dense composite microstructure comprising an in-situ formed HfB₂ and a solid solution of Si and O in HfC and HfB₂ grains.     

Effects of high-energy ball milling and reactive spark plasma sintering on the densification of HfC-SiC composites

The combined effects of high-energy ball milling (HEBM) and reactive spark plasma sintering (R-SPS) of HfSi₂ and C powder mixture on the densification and microstructure of nanostructured HfC-SiC composites were investigated. HEBM significantly promoted the densification and improved the microstructure of the HfC-SiC composites. In contrast, the reactions between HfSi₂ and C did not directly promote the densification of the HfC-SiC composites. While the reaction was mostly completed at 1300 °C, the onset temperature of significant densification was 1610 °C. Fine and homogeneously distributed HfC and SiC particles formed by HEBM and R-SPS were the key factors for promoting the densification of the HfC-SiC composites. The fine particles had high surface energy, which provided enough driving force for densification. In addition, the homogeneously distributed SiC particles effectively suppressed the growth of HfC matrix grains during densification.     

Microstructure refinement and mechanical properties improvement of HfB2–SiC composites with the incorporation of HfC

HfB₂ and HfB₂–10 vol% 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 HfB₂–20 vol% SiC (HS) and HfB₂–8 vol% HfC–20 vol% SiC (HHS) ceramics were obtained by hot pressing at 2000 °C/30 MPa. 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 (5.09 MPa m^1/2, 863 MPa) increased significantly compared with HS (3.95 MPa m^1/2, 654 MPa). Therefore, it could be concluded that the incorporation of HfC refined the microstructure and improved the mechanical properties of HfB₂–SiC ceramics.     

Polymer precursor‐derived HfC–SiC ultrahigh‐temperature ceramic nanocomposites

Due to poor mechanical properties and antioxidation properties, etc of single phase ultrahigh‐temperature ceramics (UHTCs), the second phase such as SiC was usually introduced for improving those properties. Herein, a novel stratagem for synthesis of binary HfC–SiC ceramics has been presented. A Hf–O–Hf polymer as a HfO₂ precursor has been synthesized for preparing soluble HfC–SiC precursors with high solid content and low viscosity solutions without additional organic solvents. The structure of PHO was characterized by FTIR and ¹H‐NMR, the crystalline behavior and morphologies of polymer‐derived ceramics were identified by XRD, SEM‐EDS, and TEM. It was shown that PHO firstly transformed into HfO₂, and then reacted with in situ carbon derived from DVB and PCS thus producing cubic HfC through carbothermal reduction. In addition, the obtained HfC–SiC nanopowders exhibited spherical morphology with a diameter less than 100 nm, while the Hf, Si, and C are homogeneously distributed.     

Phase evolution and microstructure characteristics during the carbothermal reduction of Hf(C,N,O) ceramics derived from a precursor

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl₄ with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf₂ON₂ and HfO₂ nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 °C, when HfC first appeared, and continued to completion at 1500 °C, resulting in irregularly shaped crystallites measuring 50–150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 °C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas–solid reaction between CO and Hf₂ON₂. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.     

Magnesiothermic synthesis of HfB2–HfC-SiC nanocomposite by spark plasma technique

The HfB₂-HfC-SiC nanocomposite was produced using H₃BO₃, HfO₂, Si, C, and Mg as starting materials by spark plasma for the first time. The reactions during synthesis indicate that the synthesis process progressed in self-propagating mode. The reaction mechanisms were investigated by the displacement-temperature-time (DTT) diagram, which was obtained during spark plasma cycles. The synthesis process of the composite was completed at the temperature of 400 °C in less than 30 min. The tendency to form the composite was investigated by thermodynamic calculations, and the formation of HfB₂, HfC, and SiC phases was observed by X-ray diffraction. Finally, using the Rietveld method, the mean crystallites sizes of about 54, 26, and 43 nm were calculated for HfB₂, SiC, and HfC phases, respectively.     

Low-temperature synthesis and oxidation resistance of random combination of Hf,Nb, and Ta carbides microcuboids

Based on the dissolution-precipitation mechanism, this article successfully synthesized binary and ternary transition metal carbide microcuboids with random combinations of Hf, Nb, and Ta by annealing monocarbides/cobalt powders. Accelerated mass transport rate through the flow of molten alloys (Co-Hf-Nb-Ta) instead of slow solid diffusion made the low-temperature pressureless sintering technique (1500°C) a reality. Furthermore, the equilibrium morphology was driven by the gradient Gibbs potential of carbides induced by the different local curvature of powders and anisotropic interfacial energy. (Hf_0.5Ta_0.5)C possessed the optimal oxidation resistance among all mentioned carbides, even competed with (Hf_1/3Nb_1/3Ta_1/3)C. During the isothermal oxidation at 800∼1200°C, the doping of Nb and Ta in carbides assisted the monoclinic-orthorhombic HfO₂ transition at ambient pressure, besides, TaC can also restrain the orthorhombic-monoclinic transition of Nb₂O₅. Moreover, oxidation kinetics parameters concluded that the addition of HfC and TaC contributed to the decreasing reaction order and the increasing activation energy, respectively.     

Evolution of the composition,microstructure and electromagnetic properties of HfOC ceramics with pyrolysis temperature

The evolution of phase composition, microstructure, and dielectric characteristics of HfOC ceramics pyrolyzed at various temperatures was studied in this work. When the pyrolysis temperature increased from 900 to 1500 °C, the composition of HfOC ceramics varies from HfO₂ and amorphous carbon (C_amp) at 900 °C to coexistence of HfO₂, C_amp, and HfC at 1100–1300 °C, and HfC and C_amp at 1500 °C. With the continuous consummation of C_amp, its distribution is transformed from a slice-like structure accumulating around the particles to a shell-like structure wrapping around the particles. The atomic ratios of as-obtained HfOC ceramics are HfO_2.0C_2.8, HfO_1.9C_2.7, HfO_1.0C_1.8, and HfO_0.1C_1.1, respectively, after being pyrolyzed at 900, 1100, 1300, and 1500 °C. As the pyrolysis temperature increases, the average value of the real part increases from 13.5 to 16.5, and the imaginary part rises from 12 to 14. The microwave absorption properties of HfOC ceramics need to be enhanced further in the future work.     

Reaction Sintering of HfC/W Cermets with High Strength and Toughness

Hafnium carbide/tungsten (HfC/W) cermets were prepared by an in situ reaction sintering process, using hafnium oxide (HfO₂) and tungsten carbide (WC) as the raw materials. The reaction path, densification behavior, microstructure development, and mechanical properties of the cermets were comprehensively investigated. It was found that WC decomposed to tungsten semicarbide (W₂C) and tungsten (W) in sequence, and meanwhile HfC was formed by carbothermal reduction between HfO₂ and as‐released carbon from the dissociation of WC. The solid solution formation between HfC and W during sintering was also studied. The obtained cermets (>98% TD) have a Vickers' hardness of 8.16 GPa, a fracture toughness of 14.45 MPa m^1/2, and a high flexural strength of 1211 MPa.     

Effect of incorporation of hafnium diboride nanofibers on thermomechanical properties of carbon fiber–phenolic composites

Carbon fiber/phenolic (C/Ph) composites were modified with different weight ratios of hafnium diboride (HfB₂) nanofibers to apperceive thermomechanical properties of C/Ph–Hf nanocomposites. Mechanical properties, thermal stability, and ablation resistance of C/Ph–Hf nanocomposites were found to be optimum when the weight percentage of HfB₂ was equal to one. Maximum flexural strength and modulus were obtained with 118 MPa and 1.9 GPa for C/Ph–1%Hf nanocomposite, respectively. Increasing the proportion of HfB₂, by delaying the temperature of thermal degradation of nanocomposites, enhanced the thermal stability and residual of C/Ph–Hf relative to C/Ph in both nitrogen and air environments. In the oxyacetylene flame test at 2500°C for 160 s, the optimum mass ablation rate of C/Ph–1%Hf nanocomposites was found to be 0.0150 g/s compared to 0.068 g/s for blank C/Ph, along with reducing the back surface temperature by 51%. The ablation mechanism of C/Ph–Hf nanocomposites after the oxyacetylene torch test was concluded from the derivations obtained from X-ray diffraction, energy dispersion spectroscopy, and microstructure analyses. These clarified that the formation of high-temperature species, such as HfO₂, HfC, and B₄C owing to oxidation of HfB₂ and subsequent reaction products with char, resulted in an increased ablation resistance of the nanocomposites.     

Aluminum diboride synthesis from elemental powders by mechanical alloying and annealing

In this study, AlB₂ powders were synthesized by using a combined method of mechanical alloying (MA) and annealing of elemental aluminum (Al) and boron (B) powders. Milling was performed in a planetary ball-mill (Fritsch? Pulverisette 7 Premium Line) up to 15 h under argon (Ar) atmosphere. Annealing process was carried out in a tube furnace at 650 °C for 6 h under Ar atmosphere. The effects of MA durations on the annealing process and AlB₂ formation were investigated. The conversion of Al and B powders to AlB₂ starts after only MA for 3 h or after MA for 1 h and subsequent annealing. A slight formation of AlB₁₂ occurs at 242 °C for as-blended powders and it shifts to about 272 °C for MA’d powders. Al–B powder blends MA’d for 9 h and annealed have AlB₂ particles in size between 35 and 75 nm in the presence of Al₁₃Fe₄, Fe₃B and Fe₂B contaminations.     

In-situ synthesis of tungsten boride-carbide composite powders from WO3-B2O3–Mg–C quaternary system via a mechanochemical route

In this study, in-situ synthesis of tungsten boride-carbide composite powders using a mechanochemical processing (MCP) method was investigated on the WO₃–B₂O₃–Mg–C quaternary powder system. The raw blends of WO₃–B₂O₃–Mg–C were processed in a high-energy ball mill for between 8 and 24 h. In addition to the MCP duration, excess B₂O₃ or C reactants and ball-to-powder weight ratio (BPR) were tested as important process parameters which affected the resultant phases. After the mechanochemical reaction, the products were purified using 6 M HCl solution to eliminate the MgO by-product. FactSage 7.1 and HSC Chemistry Ver.4.1 thermochemical software was used to predict the thermodynamically possible reactions and products. Microstructural properties of the fabricated powders were inspected through X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and energy dispersive spectroscopy (EDS) techniques. Additionally, changes in particle size with duration of the MCP were determined for the synthesized powders. According to the XRD results, tungsten boride and tungsten carbide phases were obtained as the main reaction products for all reaction durations and reactant stoichiometries. Distribution of the total amounts of tungsten boride and tungsten carbide phases were determined respectively as 79.8 and 20.2 wt% for the 8 h-processed stoichiometric powders. However, the total tungsten boride amount decreased with the increasing MCP duration and its respective percentage was calculated as 57.1 wt% for the 24 h-processed stoichiometric powders. In addition, by increasing the processing duration to over 12 h, W₂B, W₂C and B₄C phases were detected. When 200 wt% B₂O₃ was added, the B:W ratio and total amount of tungsten boride phases in the synthesized powders increased. Similarly, by utilizing 200 wt% C, a significant increase in the B₄C peaks together with W₂C peaks was detected. Therefore, the properties of tungsten boride-carbide composite powders synthesized by the mechanochemical route are highly affected by the process parameters.     

Fabrication of ultra-high-temperature nonstoichiometric hafnium carbonitride via combustion synthesis and spark plasma sintering

In this study, nonstoichiometric hafnium carbonitrides (HfC_xN_y) were fabricated via short-term (5 min) high-energy ball milling of Hf and C powders, followed by combustion of mechanically induced Hf/C composite particles in a nitrogen atmosphere (0.8 MPa). The obtained HfC_0.5N_0.35 powder exhibited a rock-salt crystal structure with a lattice parameter of 0.4606 nm. The melting point of this synthesized ceramic material was experimentally shown to be higher than that of binary hafnium carbide (HfC). The nonstoichiometric hafnium carbonitride was then consolidated under a constant pressure of 50 MPa at a temperature of 2000 °C and a dwelling time of 10 min, through spark plasma sintering. The obtained bulk ceramic material had a theoretical material density of 98%, Vickers hardness of 21.3 GPa, and fracture toughness of 4.7 MPa m^1/2.     

Oxidation/ablation behaviors of hafnium carbide-silicon carbonitride systems at 1500 and 2500 C

The oxide scales of hafnium carbide (HfC) typically exhibit a porous structure after oxidation/ablation due to the release of gas oxidation products, which allows oxygen penetration to promote the rapid oxidation of the HfC matrices. Here, we report that the oxidation/ablation resistance of HfC was enhanced by the incorporation of amorphous silicon carbonitride (SiCN). HfC-SiCN ceramics with 10 vol % SiCN showed a significant improvement in the oxidation/ablation resistance compared with pure HfC. The HfC-10 vol % SiCN ceramic has a higher density with good mechanical properties. After being oxidized at 1500 °C for 2 h, a dense and homogeneous HfO₂-HfSiO₄ layer with low oxygen permeability is formed. The ablation resistance of the HfC-10 vol % SiCN ceramic is improved due to the formation of the triple-layer structure oxide with good thermal stability and mechanical scouring resistance. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of HfC-10 vol % SiCN ceramic are −0.019 mg cm⁻² s⁻¹ and -0.156 μm s⁻¹, respectively.