Screw Dislocation Assisted Spontaneous Growth of HfB2 Tubes and Rods

The mechanism of anisotropic growth of HfB₂ rods has been discussed in this study. HfB₂ powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron respectively, though a small number of comparative experiments involved amorphous boron as the boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of anisotropic growth of HfB₂ has been investigated. It is hypothesized that imperfect oriented attachment of finer HfB₂ particles results in screw dislocations in the coarser particles. The screw dislocation facilitates dislocation‐driven growth of particles into anisotropic HfB₂ rods.     

Synthesis of Ultrafine Hafnium Diboride Powders Using Solution‐Based Processing and Spark Plasma Sintering

Ultrafine hafnium diboride (HfB₂) powders were synthesized by the boro/carborthermal reduction process. Fine‐scale mixing of the reactants was achieved by solution‐based processing using hafnium oxychloride (HfOCl₂·8H₂O) and phenolic resin as the precursor of HfO₂ and carbon respectively. The heat treatment was completed at a temperature range 1300–1500°C for 1h using spark plasma sintering (SPS) apparatus. The crystallite sizes of the synthesized powders were small (<500 nm) and the oxygen content was low (0.85 wt%). The grain growth of HfB₂ could be effectively suppressed using SPS due to the fast heating rate. The effects of temperature and holding time on the synthesis of ultrafine HfB₂ powders were discussed.     

A different chemical route to prepare hafnium diboride-based nanofibers: Effect of chemical composition

This study reports on the synthesis of hafnium diboride (HfB₂)-based nanofibers via electrospinning of polyhafnoxanesal (PHO)-based solution followed by pyrolyzing hafnium-boron containing polyvinylpyrrolidone precursor fibers by a moderate heat treatment at 1500°C under argon atmosphere. The influence of the molar ratios of C/Hf and B/Hf in preceramic polymer method is investigated on the final phase of HfB₂-based nanofibers. Structural, thermal, microstructural, and physical properties of the hafnium-based fibers are evaluated using Fourier transform infrared spectra (FTIR), thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffractometer (XRD), high-temperature X-ray diffraction (HT-XRD), field-emission scanning electron microscope/energy-dispersive spectrometer (FE-SEM/EDS), and Brunauer-Emmett-Teller (BET). The results unveiled that the acidic pH was the optimal condition needed for obtaining the single phase of HfB₂ nanofibers. The precursor fibers with the stoichiometric ratio of 1:4:5 of Hf:B:C prepared under the acidic conditions converted into pure HfB₂ nanofibers having rough and porous surface after pyrolysis at 1500°C for 2 hour in argon, whereas HfB₂-HfC composite nanofibers with smooth surface were produced in the neutral conditions. The HfB₂ nanofibers with a mean diameter of ~100 nm prepared under the acidic conditions showed a higher specific surface area compared to HfB₂-HfC composite nanofibers with a diameter of ~121 nm derived in the neutral conditions.     

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.     

Hot Pressed HfB2 and HfB2–20 vol%SiC Ceramics Based on HfB2 Powder Synthesized by Borothermal Reduction of HfO2*

Phase pure hafnium diboride (HfB₂) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum. The synthesized HfB₂ powder had an average particle size of 1.37 μm with an equiaxed shape, and a low oxygen content of 0.79 wt%. Using the as-synthesized HfB₂ powder and a commercial SiC, HfB₂ monolithic, and HfB₂–20 vol%SiC composite were hot pressed at 2000°C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993±90 MPa for the composite. Fracture toughness of the composite was 6.29±0.65 MPa m^1/2, which was significantly higher than that of the HfB₂ monolithic and the reported values in literature.     

Development and Characterization of Continuous SiC Fiber‐Reinforced HfB2‐Based UHTC Matrix Composites Using Polymer Impregnation and Slurry Infiltration Techniques

This paper discusses the development of continuous SiC fiber‐reinforced HfB₂‐SiC composite laminates. A range of techniques, based on resin‐based precursors and slurries, for infiltrating porous SiC preforms with HfB₂ powder were developed. While resin‐based precursors proved to be ineffective due to low HfB₂ yield and poor adhesion, the slurry infiltration techniques were effective to varying degrees. The greatest pore filling and composite densities were achieved using pressure and vibration‐assisted pressure infiltration techniques. SiC_f/HfB₂‐SiC laminates were subsequently developed via lamination, cure and pyrolysis of fabrics using a HfB₂‐loaded polymeric SiC precursor, followed by HfB₂ slurry infiltration and preceramic polymer infiltration and pyrolysis (PIP). Repeated PIP processing, for 6–10 cycles, resulted in density increases, from the 3.03–3.22 g/cm³ range after HfB₂ slurry infiltration, to 3.97–4.03 g/cm³ after PIP processing. Correspondingly, there was a decrease in open porosity from approximately 52% to less than 11%. The matrix consisted of discreet, lightly sintered HfB₂ particles dispersed in SiC. The PIP SiC matrix was primarily nanocrystalline after 1300°C pyrolysis, but experienced grain growth with further heat treatment at 1600°C.     

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.     

Oxygen barrier resistance of HfB2-MoSi2-TaB2 coatings in a wide temperature region

HfB₂-MoSi₂-based ultra-high temperature ceramic (UHTC) coatings have shown remarkable antioxidant effects owing to the formation of silicate glass layers with low oxygen permeability in high-temperature environments, which shows great potential in the antioxidation of carbon structural materials. To further enhance the oxidation resistance of the HfB₂-MoSi₂-based coating in a wide temperature region, the influence of volume ratio between HfB₂ and TaB₂ on the antioxidant capacity of the HfB₂-MoSi₂-TaB₂ coatings was investigated. The addition of 15 vol% TaB₂ in the 60HfB₂-40MoSi₂ coating delays the initial oxidation temperature of the 60HfB₂-40MoSi₂ sample from 300 °C to 500 °C, which decreased the oxidation loss by 75.85% during dynamic oxidation. In oxidation process at 900 °C and 1700 °C, the weight gains of the 45HfB₂-40MoSi₂–15TaB₂ coating reduced by 78.56% and 63.14%, respectively. Due to the coexistence of 45 vol%HfB₂ and 15 vol%TaB₂, the suitable Ta⁵⁺ promoted the homogenization and dispersion of Hf/Ta-oxides, which forms the coral-like Hf/Ta oxides skeleton in the glass layer, thus preventing the oxygen penetration and decreasing the inert factor of the HfB₂-MoSi₂ coating at 1700 °C by 51.19%. However, excessive TaB₂ weakened the self-healing ability of the Ta-Hf-Si-O glass layer and inhibited the oxygen barrier effect of the HfB₂-MoSi₂-TaB₂ coating.     

Thermochemical degradation of HfSiO4 by molten CMAS

The thermochemical degradation of hafnium silicate (HfSiO₄) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO₄ coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ～35 mg/cm². Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca₂HfSi₄O₁₂ cyclosilicate phase was observed at the HfSiO₄-CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO₄ are presented herein.     

Self‐propagating mechanosynthesis of HfB2 nanoparticles by a magnesiothermic reaction

A mechanically induced self‐sustaining reaction (MSR) was used to synthesize hafnium diboride nanoparticles. Along this route, magnesium was selected as a robust reducing agent for co‐reduction in boron and hafnium oxides in a combustive manner. Combustion occurred after a short milling period of 12 minutes. The hafnium diboride nanoparticles had a polygonal faceted morphology and were 50‐250 nm in diameter. The assessment of the processing mechanism revealed that the initial combustive reduction in B₂O₃ to elemental B by Mg was the major step for progressing the overall reaction. After that, HfO₂ can be reduced to elemental Hf, followed by the synthesis of HfB₂ phase.     

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).     

Anti-oxidation and ablation properties of carbon/carbon composites infiltrated by hafnium boride

To improve the anti-oxidation and ablation properties of carbon/carbon (C/C) composites, they are modified by hafnium boride (HfB₂) using a two-step process of in situ reaction and thermal gradient chemical vapor infiltration. X-ray diffraction is used to monitor the composition of the samples. Scanning electron microscope images show that the carbon fibers are uniformly coated by HfB₂ particles. The oxidation onset temperature of carbon fibers is greatly increased from 300 to 700 °C after HfB₂ coating. After modification with HfB₂, the linear and mass ablation rates of the C/C composites are decreased by 51.80% and 24.32%. During oxidation and ablation, the interface between carbon matrix and fiber is effectively protected by HfB₂ due to the reaction of HfB₂ with the oxygen, and the resultant hafnium oxide may form the liquid film to resist the oxygen at high temperature.     

Densification,solid solution formation,and microstructural investigation of reactive pressureless sintered HfB2-TiB2-SiC-MoSi2 quadruplet composite

Dense HfB₂-TiB₂-SiC-MoSi₂ quadruplet composite was produced by a reactive pressureless sintering method at 2050 °C for 5 h. The relative density was improved and reached 98% by in situ formation of SiC and MoSi₂ phases. Microstructural studies proved that SiC and MoSi₂ second phases were mostly formed during the sintering process. Moreover, the Sintering mechanism of the composite was investigated by HSC software. TiB₂ co-matrix was improved the sinterability of the composite by the formation of (Hf,Ti,Mo)–B and (Hf,Ti,Mo)–C solid solutions.Mechanical properties such as Vickers hardness (23.2 GPa), fracture toughness (5.4 MPa m^1/2), and elastic modulus (430 GPa) were effectively enhanced by tailoring the composite.     

Controlling phase separation of TaxHf1−xC solid solution nanopowders during carbothermal reduction synthesis

Synthesis of single‐phase tantalum hafnium carbide (Ta_xHf_1?xC, 0<x<1) solid solution nanopowders via carbothermal reduction (CTR) reaction is complicated due to the difference in reactivity of parent oxides with carbon and presence of a miscibility gap in TaC‐HfC phase diagram below ~887°C. These can lead to phase separation, ie, formation of two distinct carbides instead of a single‐phase solid solution. In this study, nanocrystalline Ta_xHf_1?xC powders were synthesized via CTR of finely mixed amorphous tantalum‐hafnium oxide(s) and carbon obtained from a low‐cost aqueous solution processing of tantalum pentachloride, hafnium tetrachloride, and sucrose. Particular emphasis was given to investigate the influences of starting compositions and processing conditions on phase separation during the formation of carbide phase(s). It was found that due to the immiscibility of Ta‐Hf oxides and relatively fast CTR reaction, individual nano‐HfC and TaC phases form quickly (within minutes at 1600°C), then go through interdiffusion forming carbide solid solution phase. Moreover, the presence of excess carbon in the CTR product slows down the interdiffusion of Ta and Hf dramatically and delays the solid solution formation, whereas DC electrical field (applied through the use of a spark plasma sintering system) accelerates interdiffusion significantly but leads to more grain growth.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Reactive consolidation and high-temperature strength of HfB2–SiB6

During the spark plasma sintering at 1900 °C, SiB₆ decomposes into cubic silicon carbide and boron carbide, owing to the reducing environment of the furnace. For the HfB₂-SiB₆ ceramic improvement in hardness (24.5 ± 0.7 GPa) was attributed to the formation of the B₁₂(C,Si,B)₃. Fracture toughness by indentation (6.8 ± 2.4 MPa·m^1/2), single-edge notched bend specimens (4.6 ± 0.4 MPa·m^1/2) and room-temperature strength (513 ± 21 MPa) of the HfB₂–SiB₆ composite produced by spark plasma sintering was higher or on the same level as the HfB₂–SiC ceramics. The high-temperature flexural strength tests suggested that the strength would decrease monotonically with an increase in temperature. At or below 1600 °C, only a linear stress-strain response was observed, and resulted into a mean strength of ~320 MPa. During the tests at 1800 °C, we observed a nonlinear deformation indicating ongoing plastic deformation which led to a strength decrease down to 230 ± 30 MPa.     

Synthesis,Characterization, and Microstructure of Hafnium Boride‐Based Composite Ceramics Via Preceramic Method

The ceramic precursor for HfB₂/HfC/SiC/C was prepared via solution‐based processing of polyhafnoxanesal, linear phenolic resin, boric acid and poly[(methylsilylene)acetylene)]. The obtained precursor could be cured at 250°C and subsequently heat treated at relative lower temperature (1500°C) to form HfB₂/HfC/SiC/C ceramic powders. The ceramic powders were characterized by element analysis, thermal gravimetric analysis, X‐ray diffraction, Raman spectroscopy, and Scanning electron microscopy. The results indicated that the ceramic powders with particle size of 200~500 nm were consisted of pure phase HfB₂, HfC, and SiC along with free carbon as fourth phase with low crystallinity.     

SiC fiber strength after low pO2 oxidation

Hi‐Nicalon?‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO₂ of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO₂, 1400°C with 100 ppm pO₂ or higher, and 1300°C with 50 ppm pO₂ or higher. When active oxidation did not occur, a glass SiO₂ scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO₂. Fiber strengths were lowest after heat treatment at 1300°C and a pO₂ of 3.7 ppm, 1400°C and a pO₂ of 20 ppm, and 1500°C and a pO₂ of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.     

Structural Influence on the Thermal Conversion of Self‐Catalyzed HfB2/ZrB2 Sol–Gel Precursors by Rapid Ultrasonication of Oxychloride Hydrates

Sol–gel precursors to HfB₂ and ZrB₂ are processed by high‐energy ultrasonication of Hf,Zr oxychloride hydrates, triethyl borate, and phenolic resin to form precipitate‐free sols that turn into stable gels with no catalyst addition. Both precursor concentration and structure (a sol or a gel) are found to influence the synthesis of the diboride phase at high temperature. Decreasing sol concentration increases powder surface area from 3.6 to 6.8 m²/g, whereas heat‐treating a gel leads to residual oxides and carbides. Particles are either fine spherical particles, unique elongated rods, and/or platelets, indicating particle growth with directional coarsening. Investigation of the conversion process to ZrB₂ indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B₂O₃ or ZrC reacts with B₂O₃ and C to form ZrB₂. At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB₂ becomes rate limiting. ZrC is found to be a direct reducing agent for B₂O₃ at low temperature (~1200°C) to form ZrB₂ and ZrO₂, whereas at high temperatures (~1500°C) it reacts with B₂O₃ and C to form pure ZrB₂.     

Pressureless densification and mechanical properties of hafnium diboride doped with B4C: From solid state sintering to liquid phase sintering

A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B₄C on the microstructure development and mechanical properties of the sintered ceramics were investigated.