Processing,microstructure, and mechanical properties of hot-pressed ZrB2 ceramics with a complex Zr/Si/O-based additive

Densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) ceramics modified with a complex Zr/Si/O-based additive were studied. ZrB₂ ceramics with 5–20 vol.% additions of Zr/Si/O-based additive were densified to >95% relative density at temperatures as low as 1400°C by hot-pressing. Improved densification behavior of ZrB₂ was observed with increasing additive content. The most effective additive amount for densification was 20 vol.%, hot-pressed at 1400°C (∼98% relative density). Microstructural analysis revealed up to 7 vol.% of residual second phases in the final ceramics. Improved densification behavior was attributed to ductility of the silicide phase, liquid phase formation at the hot-pressing temperatures, silicon wetting of ZrB₂ particles, and reactions of surface oxides. Room temperature strength ranged from 390 to 750 MPa and elastic modulus ranged from 440 to 490 GPa. Vickers hardness ranged from 15 to 16 GPa, and indentation fracture toughness was between 4.0 and 4.3 MPa·m^1/2. The most effective additive amount was 7.5 vol.%, which resulted in high relative density after hot-pressing at 1600°C and the best combination of mechanical properties.     

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2-SiC composites

A study has been carried out to examine the effect of LaB₆ addition on the compressive creep behavior of ZrB₂-SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB₂-20 vol% SiC composites containing LaB₆ (10% in ZSBCL-10 and 14% in ZSBCL-14) besides 5.6% B₄C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL-14 composite has exhibited a lower steady-state creep rate at 1300°C than the ZSBCL-10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB₂ grain boundaries and ZrB₂-SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL-10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL-14 composite at 1300–1400°C suggest that ZrB₂ grain boundary diffusion is the rate-limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation-induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep-based deformation and damage at the inner core of samples.     

Volatility diagram of ZrB2‐SiC‐ZrC system and experimental validation

A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB₂ and SiC, the volatility diagram of a ternary ZrB₂‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB₂‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed.     

Influence of angle of incidence,temperature and SiC content on erosive wear behavior of ZrB2-SiC composites

This research work deals with the investigation of erosive wear of spark plasma sintered ZrB₂-SiC composites with variation in angle of incidence (30°, 60°, and 90°), test temperature (room and 800°C) and SiC content (10, 20, and 30 vol.%). Results indicate a large variation in erosion rate from 2.13 to 75.45 mm³/kg with change in angle of incidence, test temperature, and SiC content. Erosion rate decreased with the decrease in angle of incidence, increase in temperature, and increase in SiC content. With increase in SiC content from 10 to 30 vol.%, a maximum reduction of 68% in erosion rate obtained at shallow incidence and room temperature, and a maximum reduction of 78% in erosion rate obtained at shallow incidence and 800°C. SEM-EDS and XRD analyses indicate that formation of B₂O₃ and SiO₂-rich protective surface is responsible for high temperature erosion resistance of ZrB₂-SiC composites.     

Effect of HfSi2-induced intergranular liquid phase on densification behavior of ZrB2

The densification behavior and mechanical properties of ZrB₂-based composites were investigated. The results demonstrated that the fully dense ZrB₂-based composites could be obtained at lower sintering temperature (1600°C) and pressure (30 MPa) when the content of HfSi₂ was above 20 vol.%. The as-sintered composite was a special core–shell structure, with ZrB₂ as the core and (Zr, Hf)B₂ solid solution as the shell. The core–shell structure resulted from the diffusion of Hf atom into the boride matrix, which could accelerate the densification. In addition, the intergranular liquid phase induced by the HfSi₂ addition filled the micropores of the composites effectively during the sintering. When the content of HfSi₂ increased to 20 vol.%, its compressive strength, hardness, and fracture toughness all reached the maximum values, which were 1617 MPa, 15.99 GPa, and 2.44 MPa m^1/2, respectively.     

On the oxidation behavior of ZrB2–SiC–VC composites

In this study, near-fully dense ZrB₂–SiC–VC (75-20-5 vol%) composite was manufactured through hot pressing at 1850°C under the pressure of 40 MPa for 60 min. Then the oxidation examination of the composite was carried out under different durations and temperatures. The microstructure and phase evolution after hot pressing and oxidation processes were examined by scanning electron microscopy, and X-ray diffractometry. The VC addition led to the formation of ZrC and VSi₂ phases, which assisted the densification of the composite by removing ZrO₂ from the particles’ surface. The oxides of ZrO₂, SiO₂, ZrSiO₄, V₂O₅, and VO₂ formed distinct layers on the sample during the oxidation at 1700°C for 4 h with a parabolic regimen and activation energy of 177.5 kJ/mol.     

Oxidation of ZrB2 Nanoparticles at High Temperature under Low Oxygen Pressure

The oxidation of ZrB₂ nanoparticles was observed at high temperature of 1500°C under low oxygen partial pressure of 5 × 10^?2 Pa by an environmental transmission electron microscope. The results demonstrate that the oxidation starts on the surface of ZrB₂ nanoparticles with decomposition of ZrB₂ into ZrO₂ and B₂O₃. The nucleation and growth of ZrO₂ on the surface of ZrB₂ proceed with B₂O₃ being evaporated.     

Effect of the surface microstructure of SiC inner coating on the bonding strength and ablation resistance of ZrB2-SiC coating for C/C composites

The present study has been conducted in order to investigate the effect of the surface morphology of SiC inner coating on the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating for C/C composites. The microstructure of SiC inner coatings prepared by chemical vapor deposition and pack cementation at different temperatures were analyzed by X-ray diffraction, scanning electron microscopy, and 3D Confocal Laser Scanning Microscope. Tensile bonding strength and oxyacetylene ablation testing were used to characterize the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating, respectively. Results show that SiC inner coating prepared by chemical vapor deposition has a smooth surface, which is not beneficial to improve the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating. SiC inner coating prepared by pack cementation at 2000 °C has a rugged surface with the roughness of 72.15 µm, and the sprayed ZrB₂-SiC coating with it as inner layer exhibits good bonding strength and ablation resistance.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.     

Effect of silicon/graphite ratio and temperature on oxidation protective properties of SiC/ZrB2–SiC coatings prepared by pack cementation

To investigate the silicon/graphite ratio and temperature on preparation and properties of ZrB₂–SiC coatings, ZrB₂, silicon, and graphite powders were used as pack powders to prepare ZrB₂–SiC coatings on SiC coated graphite samples at different temperatures by pack cementation method. The composition, microstructure, thermal shock, and oxidation resistance of these coatings were characterized and assessed. High silicon/graphite ratio (in this case, 2) did not guarantee higher coating density, instead could be harmful to coating formation and led to the lump of pack powders, especially at temperatures of 2100 and 2200 °C. But residual silicon in the coating is beneficial for high density and oxidation protection ability. The SiC/ZrB₂–SiC (ZS50-2) coating prepared at 2000 °C showed excellent oxidation protective ability, owing to the residual silicon in the coating and dense coating structure. The weight loss of ZS50-2 after 15 thermal shocks between 1500 °C and room temperature, and oxidation for 19 h at 1500 °C are 6.5% and 2.9%, respectively.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Elevated temperature thermal properties of ZrB2-B4C ceramics

The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B₄C led to a decrease in the ZrB₂ grain size from 22 µm for nominally pure ZrB₂ to 5.4 µm for ZrB₂ containing 15 vol% B₄C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB₂ to 80 W/m·K for ZrB₂ containing 15 vol% B₄C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB₂, the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB₂ containing 15 vol% B₄C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB₂ and less than 6 W/m·K when 15 vol% B₄C was added.     

Long-term oxidation and ablation behavior of Cf/ZrB2–SiC composites at 1500, 2000, and 2200°C

Although C_f/ZrB₂–SiC composites prepared via direct ink writing combined with low-temperature hot-pressing were shown to exhibit high relative density, high preparation efficiency, and excellent flexural strength and fracture toughness in our previous work, their oxidation and ablation resistance at high and ultrahigh temperatures had not been investigated. In this work, the oxidation and ablation resistance of C_f/ZrB₂–SiC composites were evaluated via static oxidation at high temperature (1500°C) and oxyacetylene ablation at ultrahigh temperatures (2080 and 2270°C), respectively. The thickness of the oxide layer of the C_f/ZrB₂–SiC composites is <40 μm after oxidizing at 1500°C for 1 h. The C_f/ZrB₂–SiC composites exhibit non-ablative properties after oxyacetylene ablation at 2080 and 2270°C for >600 s, with mass ablation rates of 3.77 × 10⁻³ and 5.53 × 10⁻³ mg/(cm² s), and linear ablation rates of −4.5 × 10⁻⁴ and −5.8 × 10⁻⁴ mm/s, respectively. Upon an increase in the ablation temperature from 2080 to 2270°C, the thickness of the total oxide layer increases from 360 to 570 μm, and the carbon fibers remain intact in the unaffected region. Moreover, the oxidation and ablation process of C_f/ZrB₂–SiC at various temperatures was analyzed and discussed.     

Synthesis of a (Ti,Mo)B2 coating by electro-codeposition in NaCl–KCl–AlCl3–MoO3 melt containing TiB2 nanoparticles

This paper demonstrates the availability of electro-codeposition (i.e., the simultaneous occurrence of electrophoretic deposition of nanoparticles and electrochemical deposition of metal ions) in molten salts without the assistance of stirring of the bath. In molten NaCl–KCl–AlCl₃–MoO₃ system containing TiB₂ nanoparticles at 710°C, the electro-codeposition of TiB₂ nanoparticles and Mo(VI) ions has been achieved, and a (Ti, Mo)B₂ coating has been prepared.     

Fully dense ZrB2 ceramics by borothermal reduction with ultra-fine ZrO2 and solid solution

The effects of ZrO₂ particle size (55 nm and 113 nm) and borothermal reduction routes (borothermal reduction with water-washing (BRW) and in situ 5 mol% TaB₂ solid solution, BRS) on synthesis and densification of ZrB₂ were investigated. Irrespective of reduction routes, the use of finer ZrO₂ powders as raw materials resulted in finer ZrB₂ powders. Compared to the powders derived from BRS, the powders derived from BRW had smaller particle size with higher oxygen content, especially the powders synthesized with finer ZrO₂. Irrespective of ZrO₂ particle size, the oxygen contents of ZrB₂ powders prepared by the BRS route were similar. Because of the high oxygen content, the ZrB₂ ceramics synthesized by BRW with finer ZrO₂ demonstrated the lowest relative density (90.5%), which resulted in the lowest Vickers’ hardness (14.2 ± 0.9 GPa). Due to the low oxygen content and small particle size of ZrB₂ powders, fully dense ZrB₂ ceramics (relative density: 99.6%) with highest Vickers’ hardness (16.0 ± 0.2 GPa) were achieved by BRS with finer ZrO₂ powders.     

Evaluation of oxy-acetylene flame angle effect on the erosion resistance of SiC/ZrB2–SiC/ZrB2 multilayer coatings fabricated by the shielding shrouded plasma spray technique

In the present study, the effect of oxy-acetylene flame angle on the erosion resistance of SiC/ZrB₂–SiC/ZrB₂ multilayer coatings with the gradient structure was investigated. To this aim, first, the SiC inner layer was applied by the reactive melt infiltration (RMI) technique; then ZrB₂ and ZrB₂–SiC layers with 10, 20 and 30%wt. SiC were applied on graphite by the plasma spraying technique. To prevent the oxidation of ZrB₂ and SiC particles, the plasma spraying process was performed by a solid protective shield. To evaluate the performance of the coatings in erosive environments, the samples were exposed to oxy-acetylene flame at the angles of 30°, 60° and 90° for 360 s; the destruction mechanism of SiC/ZrB₂–SiC/ZrB₂ multilayer coatings appeared to be controlled mechanically and chemically. The results of the erosion test showed that at the low flame angles of about 30°, due to the shear forces of oxy-acetylene flame, mechanical erosion overcame the chemical one. With increasing the flame angle, due to raising the surface temperature, chemical erosion overcame the mechanical one; so, most chemical destruction occurred at the flame angle of 90°. Also, the results of the erosion test showed that the total chemical and mechanical destruction at the angle of 60° was greater than that in other angles. Also, among the coatings tested, SiC/ZrB₂- 20% wt. SiC/ZrB₂ coatings had the best erosion resistance; so, the weight changes under the oxy-acetylene flame at the angles of 30° and 60°, respectively, were about ?0.038%. and ?0.355%; meanwhile, at the angle of 90°, it was about +4.3%.     

Electrical and thermophysical properties of ZrB2 and HfB2 based composites

Electrical resistivities, thermal conductivities and thermal expansion coefficients of hot-pressed ZrB₂–SiC, ZrB₂–SiC–Si₃N₄, ZrB₂–ZrC–SiC–Si₃N₄ and HfB₂–SiC composites have been evaluated. Effects of Si₃N₄ and ZrC additions on electrical and thermophysical properties of ZrB₂–SiC composite have been investigated. Further, properties of ZrB₂–SiC and HfB₂–SiC composites have been compared. Electrical resistivities (at 25 °C), thermal conductivities (between 25 and 1300 °C) and thermal expansion coefficients (over 25–1000 °C) have been determined by four-probe method, laser flash method and thermo-mechanical analyzer, respectively. Experimental results have shown reasonable agreement with theoretical predictions. Electrical resistivities of ZrB₂-based composites are lower than that of HfB₂–SiC composite. Thermal conductivity of ZrB₂ increases with addition of SiC, while it decreases on ZrC addition, which is explained considering relative contributions of electrons and phonons to thermal transport. As expected, thermal expansion coefficient of each composite is reduced by SiC additions in 25–200 °C range, while it exceeds theoretical values at higher temperatures.     

Final-stage densification kinetics of direct current–sintered ZrB2

Final-stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high-purity ZrO₂. Analysis was conducted on ZrB₂ ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB₂ ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB₂. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10⁻⁶ cm²/s at 1900°C and 5.1 × 10⁻⁵ cm²/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final-stage sintering of ZrB₂ ceramics is dislocation motion.     

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.     

Laser modified microstructures in ZrB2, ZrB2/SiC and ZrC

Microstructural evolution of spark plasma sintered ZrB₂, ZrB₂/20 vol.% SiC (ZS20) and ZrC ultra high temperature ceramics (UHTCs) during laser heating has been investigated. Laser heating at temperatures between 2000 and 3750 °C for up to 300 s, in air or vacuum, resulted in extensive bubble and crater formation on the surfaces of 10 mm diameter samples. However, even after exposure to ultra high temperatures, samples did not disintegrate. X-ray diffraction of exposed faces of ZrB₂ and ZS20 samples laser heated in air up to 2700 °C detected only crystalline zirconia. A wide range of morphologies, including nodules, needles, nanofibres and lamella, were observed. The surface of ZrC samples, laser heated in vacuum up to 3750 °C, were characterised by dendritic and eutectic morphologies. Other features associated with melting, such as solidification cracks and trapped porosity, were also observed. A complex array of mechanisms involving solid, liquid and vapour phases led to formation of these various morphologies including melting, oxidation, volatilisation and liquid flow