HfB2 Ceramics Research Articles

Synthesis and growth mechanism of highly crystalized multi-branched HfB2 microrods with self-toughening effect

Development of strategies to toughen HfB2 ceramic, improve their mechanical properties is the key to promote their applications in extreme thermal environments. This study explores a kind of self-toughening method of HfB2 ceramic by incorporation the highly crystalized multi-branched HfB2 microrods. The multi-branched HfB2 microrods were synthesized by sol–gel process and carbothermal reduction, exhibiting length of 6–13 μm and diameter of 0.60–1.50 μm, respectively. Detailed analysis of the formation mechanism of HfB2 microrods reveals that the HfB2 multi-branched structure is a collaborative consequence of orientation attachment mechanism and reactive templates growth, in which process monoclinic HfO2 crystals transform into crystalized HfO2 microrods at ∼1000 °C, and worked as template to induce one-dimensional HfB2 microrods. Furthermore, spark plasma sintering experiments show that the optimal mechanical properties were attained by 6 wt% addition of HfB2 microrods, comparing with HfB2 ceramics free of self-toughening agent, the hardness and fracture toughness of self-toughened HfB2 ceramic improved by 8.82 % and 81.91 % respectively, demonstrating the effectiveness of self-toughening of the microrods without sacrificing the hardness of HfB2 ceramic. Thus, it can be seen that the work has important reference value for the synthesis of HfB2 powder with different morphologies and the toughening of HfB2 ceramics.

Fabrication of high speed steel electrodes with MoSi2–MoB–HfB2 ceramic additives for electrospark deposition on die steel

The electrodes for electrospark deposition (ESD) were fabricated from hot-pressed blanks composed of a mechanically alloyed powder mixture of R6M5K5 high speed steel. This mixture was enriched with a 40 % addition of heat-resistant MoSi2–MoB–HfB2 ceramics, produces through the self-propagating high-temperature synthesis method (resulting in the R6M5K5-K electrode), as well as variant without any ceramic addition (resulting in the R6M5K5 electrode). We examined both the composition and structure of the electrode materials and the coatings derived from them, identifying the characteristics of mass transfer from hot-pressed electrodes to substrates of 5KhNM die steel under various frequencies and energy conditions during processing. The R6M5K5 electrode consists of an α-Fe-based matrix incorporating dissolved alloying elements and contains discrete particles of ferrovanadium, tungsten carbide, and molybdenum. The R6M5K5-K electrode, in addition to the α-Fe-based matrix, includes borides and carbides, as well as hafnium oxide. The use of the R6M5K5 electrode resulted in a consistent weight increase in the cathode throughout the entire 10-minute processing period. In contrast, the application of the ceramicenhanced electrode led to weight gain only during the initial 3 min of processing. Subsequently, ESD produced coatings of 22 and 50 μm thickness on the surface of 5KhNM steel using R6M5K5 and R6M5K5-K electrodes, respectively. The introduction of SHS ceramics escalated the roughness (Ra) of the surface layers from 6 to 13 μm and the hardness from 9.1 to 15.8 GPa. The coating from the R6M5K5 electrode was composed of austenite (γ-Fe) and exhibited high uniformity. Conversely, the coating from the R6M5K5-K electrode consisted of a diverse matrix with both crystalline and amorphous iron, an amorphous phase rooted in the Fe–B alloy, and scattered phases of HfO2, HfSiO4, Fe3Si, and Fe3B. High-temperature tribological testing at 500 °C in an air atmosphere showed that the coatings possess a friction coefficient of 0.55–0.57 when coupled with a counterbody of AISI 440C steel. The integration of heat-resistant ceramics notably enhanced the coating's wear resistance, increasing it by a factor of 13.5.

Synthesis and formation mechanism of HfB2 ultrafine powders with low oxygen via flocculating settling assisted process and carbo/borothermal reduction

In this study, ultrafine HfB2 powders with low oxygen were synthesized by a flocculating settling process which yielded ceramic precursors and subsequent carbo/borothermal reduction of the precursors. The liquid phase precursor method can achieve uniform mixing of components at the molecular level through multiple complexation reactions, and then realize the carbo/borothermal reduction reaction at a lower temperature to obtain ultrapure HfB2 powders. The as-resulted quasi-spherical HfB2 powders under the optimum conditions (atomic molar ratio M:B:C = 1:2.8:10) calcined at 1500 °C for 1 h have an average particle size of 205 nm and an oxygen content of 0.097 wt.%. Detailed analysis of the phase evolution of precursors shows that the formation of HfB2 particles is a mass diffusion mode from the external to internal HfO2 cores. We reveal that below 1300 °C, HfC is not an intermediate product of HfB2 powder during the transition of precursors. Instead, HfC was formed as a by-product at high temperatures in the carbo/borothermal reduction process. The proposed formation mechanism of HfB2 is completely different from the traditional two-step transformation method. After the sintering of the ultrafine powders, the HfB2 ceramics show a relative density of 96.1% and superior mechanical properties compared to other works. Furthermore, by simply replacing the initial metal source, chlorinated group IV and V transitional metals (Ti, Zr, Ta, Nb) can also convert into high-purity and ultrafine diborides. This work shows that flocculating settling assisted carbo/borothermal reduction has potential in lot size production of various high-purity and ultrafine boride powders.

Densification, microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB2 ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB2 ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB2 increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m1/2 and bending strength of 501 ±10MPa belong to the HfB2 ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB2 ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm2 and the lowest work function of 2.92 eV.

Effects of group-VB transition metals diborides substitution on HfB2-based ceramics

In order to study the effects of solid solution of group-VB (VB=V, Nb and Ta) transition metal diborides on HfB2 ceramics, the particle size, sinterability, microstructure, and mechanical properties of Nb substituted HfB2-based ceramics were investigated, and compared with the reported (Hf,V)B2 and (Hf,Ta)B2 ceramics. The results showed that the solid solution of group-VB transition metal diborides could refine the particle size of powder and improve the densification by spark plasma sintering. This might be related to the solubility of diboride phase in B2O3 liquid phase. With the addition of SiC, no obvious difference of densification could be observed between (Hf,VB)B2-SiC ceramics. Instead, after the substitution of V, Nb and Ta, the hardness of the (Hf,VB)B2-SiC ceramics was much improved, alongside the decreasing fracture toughness. The influence on mechanical properties of V, Nb and Ta substitution was discussed in combination of the microstructure analysis.

(Hf0.99Ta0.01)B2-based ceramics prepared by pressureless sintering with boron additive

HfB2-based ceramics were fabricated by pressureless sintering with the combination of TaB2/B. It was found that the incorporation of TaB2 reduced the particle size of the HfB2-based powders prepared by borothermal reduction, but the densification of pressureless sintered HfB2 ceramics was slightly improved (from 63.2% to 69.4%). It was revealed that the combination of TaB2 and 5 wt% B could dramatically improve the sinterability of HfB2 ceramics. In combination of TaB2 and B, the relative density of HfB2-based ceramics increased significantly from 63.2% to 92.3%, and further reached to 97.2% with increasing fraction of TaB2. Finally, (Hf0.99Ta0.01)B2-5 wt% B ceramics displayed the favorable mechanical properties (Hardness: 18.1 ± 0.6 GPa).

Spark Plasma Sintering-Reactive Synthesis of SiC and SiC–HfB2 Ceramics Based on Natural Renewable Raw Materials

Spark Plasma Sintering-Reactive Synthesis of SiC and SiC–HfB2 Ceramics Based on Natural Renewable Raw Materials

Powder synthesis, densification, microstructure and mechanical properties of Hf-based ternary boride ceramics

(Hf1/3Ti1/3Zr1/3)B2, (Hf1/3Ta1/3Zr1/3)B2 and (Hf1/3Ta1/3Ti1/3)B2 (Hf-based ternary boride, HTB) powders were synthesized by boro/carbothermal reduction method, and then densified by SPS at 2000 ℃. Results showed that HTB powders were significantly refined compared to HfB2 powder synthesized by the same process, and HTB powders doped with TaB2 combined with Ti or Zr had finer particle size (0.26−0.35 μm) than doped with TiB2 and ZrB2 (0.76 μm). Due to the fine powders and the effects of solid solution, HTB ceramics presented higher densification compared to HfB2 ceramics in the literature. The microstructure of (Hf1/3Ti1/3Zr1/3)B2 ceramics exhibited a coarse microstructure around 6.62 μm, whereas (Hf1/3Ta1/3Zr1/3)B2 and (Hf1/3Ta1/3Ti1/3)B2 had refined microstructure, in the 1.65–2.45 μm range. (Hf1/3Ti1/3Zr1/3)B2 and (Hf1/3Ta1/3Ti1/3)B2 had higher hardness and fracture toughness than (Hf1/3Ta1/3Zr1/3)B2. HTB sintered ceramics with TaB2 demonstrated finer powder and microstructure, whereas HTB ceramics doped with TiB2 yielded the best mechanical properties. The microstructure and mechanical properties of HTB ceramics can be guided by selection of solid solution element.

Efficient multiscale strategy for toughening HfB2 ceramics: A heterogeneous ceramic–metal layered architecture

AbstractA multiscale structural design was innovatively adopted herein to increase the toughness of monolithic HfB2 ceramics. SiC whiskers (SiCw) and graphene oxide (GO) were used as fillers for the HfB2 matrix, whereas a ductile W foil was introduced as an interlayer to synthesize laminated HfB2‐SiCw‐rGO/W ceramics. Monolithic HfB2‐SiCp (particulate) and laminated HfB2‐SiCp/W ceramics were prepared using the same routes and used as controls. Following tape casting and spark plasma sintering at 1800°C, the toughness of the prepared laminated HfB2‐SiCw‐rGO/W samples was increased to 14.2 ± 0.6 MPa·m1/2, with minimal sacrifice in flexural strength (421 ± 16 MPa). Morphological analysis of the fracture surface revealed the synergistic effects of micro‐toughening (including bridging and pullout of whiskers and rGO) and macro‐toughening (including crack deflection, bifurcation, and delamination) mechanisms responsible for improving the fracture toughness of the laminated HfB2‐SiCw‐rGO/W composites.

Dense HfB2 ceramics fabricated by high-energy ball milling and spark plasma sintering

The effects of high-energy ball milling (HEBM) on the sintering ability and microstructure of hafnium diboride (HfB2) ultrahigh-temperature ceramics (UHTCs) were investigated. After HEBM, the average particle size range of the HfB2 ceramics decreased to 10–20 nm with a slight alteration in the crystal structure. The microscopic grains of the sintered samples were homogeneous and fine in size. WC removed oxide impurities on the surface of the HfB2 particles by reacting with the oxide impurities, which further improved the sintering ability of the HfB2 ceramics. The densification mechanism of the HfB2 ceramics was also investigated, and nearly fully dense HfB2 (99.1%) was fabricated by reactive spark plasma sintering (R–SPS) at 2100 °C for 35 min under a pressure of 40 MPa. HEBM increased the surface energy of the fine particles, which provided a sufficient driving force to improve the sintering ability and significantly increase the densification of HfB2.

Thermo-mechanical simulation of ultrahigh temperature ceramic composites as alternative materials for gas turbine stator blades

The efficiency of the Bryton cycle strongly depends on the maximum temperature of the cycle. However, restrictions on metallurgical problems deprive engineers from the benefit of high temperatures. Ultrahigh temperature ceramics can be considered in such cases, instead of traditional materials like M152 superalloy. In this study, SiC reinforced HfB2 and ZrB2 ultrahigh temperature ceramics were proposed as gas turbine stator blades. The heat transfer and stress-strain equations were solved numerically by the finite element method to obtain temperature and stress distributions. The results showed that the maximum thermal stress occurs in vicinity of the cooling ducts where the temperature gradient is maximum. The maximum displacements of 1.2 mm (for HfB2–SiC) and 1.14 mm (for ZrB2–SiC) occur in the upper wall. It can be noticed that the ZrB2–SiC made blade showed lower maximum stress and displacement than those for the HfB2–SiC made one, as a result of lower expansion coefficient of ZrB2–SiC system. The addition of SiC to monolithic HfB2 and ZrB2 ceramics decreases their thermal conductivity and following that, the temperature uniformity in blades reduces. Although the thermal stresses and the probability of failure in these stator blades enhance, the ZrB2–SiC material presented the best performance among the other investigated samples. Both Coulomb-Mohr and Von Mises failure analyses were employed. It was understood that both blades made of HfB2–SiC and ZrB2–SiC composites simply withstand the applied stresses with the safety factors of about 1.5.

Investigation of Adhesion Mechanism in Molten Silicon/Ceramics Systems

An experimental study was conducted to investigate the interfaciale adhesion between molten silicon and TiB2, ZrB2 and HfB2 ceramics by using the sessile drop technique. Contact angle measurements were performed as a function of wetting time at 1703 °C. The interface between silicon and ceramic was analyzed using SEM-EDS. The dependence of adhesion energy, Wad, on elastic parameters of these systems was investigated. Different approaches are used and semi-empirical relations are deduced for all systems. It is shown that, in all cases, the adhesion energy increases linearly with Rayleigh velocities of ceramic substrate, VRC; it takes the form: Wad. = 0.07 VRC + C. Where the first term of this equation represents the Van der Waals, WVDW, contribution of Wad, it is only depends on VRC. However, the second term represent the chemical equilibrium term, Wchem-equil, is strongly depends on the system combination as well as on the energy gap of the ceramics substrate. Moreover, Wchem-equil is higher for small bandgap ceramic materials due to big density inside ceramic crystal; consequently, ease and height electron transfer through the metal/ceramic interface. On the other hand, a new phenomenon takes place in silicon/large bandgap ceramics interfaces which is the generation of the interface states density that occur from electrons-holes recombination. This interfacial phenomenon depends on energy bandgap of ceramic which explained the variation of Wchem-equil values for large bandgap ceramic materials.

Study of the compression behavior and elastic properties of HfB2 ceramics using experimental method and first-principles calculations

In-situ high pressure synchrotron angle-dispersive x-ray diffraction (AXRD) studies have been performed for HfB2, an ultra high temperature refractory alloy with AlB2 structure, by using a diamond anvil cell (DAC) at room temperature. The mechanical constants (bulk modulus K, shear modulus G, Young's modulus E, Poisson's ratio ν, density ρ, and Debye temperatures ΘD) of bulk HfB2 are measured by ultrasound experiments at room temperature. Furthermore, the compression behavior and elastic properties of HfB2 under pressure are investigated using density functional theory. The experimental results are in good agreement with the theoretical calculations. The pressure dependence of mechanical constants and elastic anisotropy of HfB2 are discussed. The experimental data and theoretical calculations results show that HfB2 has a distinct anisotropic compression behavior along the a- and c-axes, and the anisotropy increases as the pressure is increasing. These markedly elastic properties are attributes to the strongly directional bonding between Hf and B atoms.

Improvement of densification and microstructure of HfB2 ceramics by Ta/Ti substitution for Hf

AbstractUltrafine HfB2 powders were synthesized by the combination of borothermal reduction of HfO2 and solid solution of 5 mol% TiB2 or 5 mol% TaB2, prototypically, (Hf0.95Ti0.05)B2 and (Hf0.95Ta0.05)B2. The influence of substitution on the particle growth, high‐temperature stability, densification, microstructure, and mechanical properties of HfB2 was investigated. Results showed that the particle sizes of HfB2, (Hf0.95Ti0.05)B2 and (Hf0.95Ta0.05)B2 powders prepared by borothermal reduction at 1500°C were 1.73, 0.87, and 0.21 µm, respectively. The substitution of TaB2 led to a greater decrease in particles size than TiB2. After heat treatment at 1800°C, the particle sizes of HfB2, (Hf0.95Ti0.05)B2 and (Hf0.95Ta0.05)B2 powders increased to 2.60, 1.59, and 0.32 µm, respectively, indicative of the good high‐temperature stability of TaB2‐substituted HfB2. The relative densities of HfB2, (Hf0.95Ti0.05)B2 and (Hf0.95Ta0.05)B2 ceramics after spark plasma sintering at 2000°C were 76.1%, 85.2% and 99.8%, respectively. The fully dense (Hf0.95Ta0.05)B2 ceramics with fine microstructure showed comparably high Vickers hardness of 21.1 GPa combined with flexural strength of 521.2 MPa. It was proved that the solid solution of TaB2 could effectively inhibit the grain growth of HfB2 powders, and improve the densification, microstructure, and mechanical properties of HfB2 ceramics.

Fabrication and Characterization of SiC fiber reinforced HfB2 Ceramics for Space Propulsion Components

Fabrication and Characterization of SiC fiber reinforced HfB2 Ceramics for Space Propulsion Components

Microstructure evolution, densification behavior and mechanical properties of nano-HfB2 sintered under high pressure

A series of pure HfB2 ceramics have been prepared by sintering nano-grained powder using high-energy ball milling at 700–1600 °C and 5.5 GPa. The HfB2 ceramics are characterized via various techniques for their residual stress, grain size, density, microstructures and defects, hardness, fracture toughness, thermal stability, and oxidation resistance. All properties strongly depend on the treatment temperature, but the exact manner of dependence for each property varies. The results identified that the HfB2 ceramic sintered at a relatively low temperature of 1000 °C and 5.5 GPa – a bulk pure nano-grained composite for the first time – has the best overall performance. It has a relative density of 99.6%, a Vickers hardness of 26 GPa, a fracture toughness of 5.2 MPa m1/2, and excellent thermal stability and oxidation resistance at high temperatures. Additional strengthening and stabilizing effects are provided by microstructures and defects such as large-angle grain boundaries, stacking faults and twinning. Simultaneous high temperature and high pressure is an effective sintering route for HfB2 ceramics with grain-size ranging from nanometer to micron.

Flash spark plasma sintering of HfB2 ceramics without pre-sintering

Strong HfB2 powder green bodies with compressive strengths exceeding 20 MPa have been produced by incorporating 1 wt% of glassy carbon derived from a pyrolyzed phenolic resin binder. Starting from the green body after debinding in argon, non-textured HfB2 ceramics (diameter ~ 20 mm and height ~ 4 mm) with a relative density of 95.1% were achieved using flash spark plasma sintering (FSPS) with a discharge time of 20 s, peak power of 19 kW and applied pressure of ~16 MPa. The lack of a need for a pre-sintering step resulted in energy savings compared to the densification of HfB2 ceramics by FSPS.

Fabrication of low-density carbon-bonded carbon fiber composites with an Hf-based coating for high temperature applications.

A novel Hf-based anti-oxidation coating has been prepared on the surface of low-density carbon-bonded carbon fiber composites (CBCFs). The coating exhibits a gradient transition structure, with mainly HfB2, Hf2Si and SiC ceramics. Oxyacetylene torch testing has been utilized to evaluate the ablation resistance under the condition ranging from 1.6 MW m−2 to 2.2 MW m−2 for 300 s. The experimental results show that the as-prepared Hf-based coating can effectively protect CBCFs under high-temperature oxidation conditions. The surface maximum temperature can reach 1616–2037 °C, and the mass ablation rates vary from −3.5 × 10−5 g s−1 cm−2 to 1.5 × 10−5 g s−1 cm−2. The formation of a dense SiO2 glass layer embedded with HfO2 grains or particle accumulation in the HfO2 layer is responsible for the good ablation resistance.

In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB2) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO2 and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000°C, which was supposed to be the synthesis temperature of HfB2 particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050°C with a 30min hold time under 60MPa uniaxial pressure leads to obtain monolithic HfB2 ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12GPa) were achieved, which were directly attributed to the low bonding between HfB2 grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12MPam1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06MPam1/2) and the fracture mode changed from transgranular to mixed mode.

Fabrication and properties of HfB2 ceramics based on micron and submicron HfB2 powders synthesized via carbo/borothermal reduction of HfO2 with B4C and carbon

The two batches of commercially available powders were used as raw materials to synthesize HfB2 powders. The synthesized HfB2 powders were heated at 450°C for 10min in the flow of nitrogen and then quenched into liquid nitrogen. The results indicated the refinement effect of liquid nitrogen quenching on synthesized HfB2 powders was very obvious. The relative density values of HfB2 ceramics fabricated using micron and submicron HfB2 powders were less than 89.5% due to the presence of trace carbonaceous compound and more than 98.1% due to higher driving force for densification, respectively. The microstructure, mechanical properties and thermal shock resistance of HfB2 ceramics fabricated using micron and submicron HfB2 powders were investigated in detail. In particular, finite element analysis was used to explore the temperature and thermal stress distribution in the specimen in the course of thermal shock.