Dense ZrB2 Research Articles

Enhanced high temperature ablation resistance of dense ZrB2–Al2TiO5 composite coating by very low-pressure plasma spraying

ZrB2-based composite coatings with addition of Al2TiO5 were prepared via very low-pressure plasma spraying using mechanically mixed powder of ZrB2 and Al2O3-40 wt%TiO2 (AT40). The ablation behavior of the composite coating was investigated using a high temperature plasma jet at a temperature of ∼2500oC. Results demonstrate the addition of AT40 improved the ablation resistance of the ZrB2 coatings at high temperatures. The coating with 17 vol%AT40 addition exhibited the lowest ablation rate, approximately 1.33 μm/s, which is only one-third of that of the pure ZrB2 coating. The added AT40 filled the pores between ZrO2 crystals during ablation, reducing oxygen penetration and lowering the ablation rate. However, excessive addition of AT40 hindered gaseous B2O3 release and thus led to large pore formation, impairing the protective effect. The ZrB2-AT40 coating is a promising candidate for high-temperature applications.

A TEM study of nanostructures and interfaces in the hot-press sintered ZrB2–SiC–Si3N4 composites

A fully dense ZrB2–30 vol% SiC composite containing 5 wt% Si3N4 and 4 wt% phenolic resin (1.6 wt% carbon) was sintered using the hot-pressing route under the external pressure of 10 MPa at 1900 ºC for 2 h. The microstructural evolution and interfacial phenomena were scrutinized using advanced microscopy facilities such as high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The FESEM images showed the ZrB2 and SiC grains without any evidence of Si3N4. The formation of the hexagonal BN (hBN) phase was proven in the sintered composite. The hBN nanosheets had a graphite-like morphology with an average thickness of 20 nm. This phase has a perpendicular orientation to the pressure direction and prevents abnormal ZrB2 grain growth. Two types of ZrB2/SiC interfaces were detected, which exhibited an amorphous phase along with the grain boundary and a clean/smooth interface, resulting from the Si3N4 addition. HRTEM and inverse fast Fourier transform (IFFT) observations disclosed that the d-spacing value in the ZrB2 grain (0.335 nm) is higher than those reported in the literature. Furthermore, it was found that the exerted pressure during the sintering distorted atomic planes. The presence of numerous dislocations within the ZrB2 grains confirmed dislocation creep as the main densification mechanism.

Electrophoretic deposition of ZrB2 coatings in NaCl‐KCl‐AlF3 melt containing synthesized ZrB2 nanoparticles

AbstractA novel method for preparation of ZrB2 coatings has been proposed by combination of molten salt synthesis of ZrB2 nanoparticles and subsequent electrophoretic deposition of the as‐synthesized ZrB2 nanoparticles in the same molten system in this paper. Nanoscale ZrB2 particles have been produced by borothermal reduction of ZrO2 in NaCl‐KCl‐AlF3 melt at 980°C. Then electrophoretic deposition of the as‐synthesized ZrB2 nanoparticles has been achieved in the resulting molten suspension at a reduced temperature of 900°C, yielding relatively dense ZrB2 coatings on graphite substrates with a thickness of around 25 μm. Moreover, the effect of different cell voltages ranging from 0.8 V to 1.4 V (i.e., electric field 0.4–0.7 V/cm) on the prepared ZrB2 coatings has been investigated. Finally, during the tests at 600°C–800°C in air, ZrB2 coatings deposited at a cell voltage of 1.2 V have exhibited good high‐temperature oxidation resistance.

Investigation of the Effect of Supersonic Flow of Dissociated Nitrogen on ZrB2-HfB2-SiC Ceramics Doped with 10 vol.% Carbon Nanotubes.

The method of fabricating dense ultra-high temperature ceramic materials ZrB2−HfB2−SiC−CCNT was developed using a combination of sol-gel synthesis and reaction hot pressing approaches at 1800 °C. It was found that the introduction of multilayer nanotubes (10 vol.%) led to an increase in the consolidation efficiency of ceramics (at temperatures > 1600 °C). The obtained ZrB2−HfB2−SiC and ZrB2−HfB2−SiC−CCNT materials were characterized by a complex of physical and chemical analysis methods. A study of the effects on the modified sample ZrB2−HfB2−SiC−CCNT composition speed flow of partially dissociated nitrogen, using a high-frequency plasmatron, showed that, despite the relatively low temperature established on the surface (≤1585 °C), there was a significant change in the chemical composition and surface microstructure: in the near-surface layer, zirconium−hafnium carbonitride, amorphous boron nitride, and carbon were present. The latter caused changes in crucial characteristics such as the emission coefficient and surface catalyticity.

Crucible-free fabrication of ZrB2–SiC composite ceramics with ultra-fine eutectic microstructure by laser surface zone-melting: The selection of lasers

Here we report the crucible-free fabrication of ZrB2–SiC composite ceramics by laser surface zone-melting (LSZM, a rapid solidification method) using Nd: YAG laser, ytterbium-doped fiber laser, and CO2 laser, respectively. Based on the comparative investigation of the thickness and the microstructure of the laser processed zone, this work highlights that the CO2 laser is the most suitable laser resource for the fabrication of highly dense and large-scale ZrB2–SiC composite ceramics with the eutectic colonies consisting of maze-shaped eutectic microstructure by LSZM. The high temperature gradient up to ∼4 × 105 K/m using CO2 laser extremely reduces the phase size down to submicron-scale, which contributes to the high relative density (∼99%), Vickers hardness (24.18 GPa) and fracture toughness (7.4 MPa m1/2) of the ZrB2–SiC composite ceramics. This work demonstrates for the first time that LSZM is a promising method for the rapid fabrication of dense ZrB2 based ultra-high temperature bulk ceramics.

Synthesis of ZrB2–ZrC hybrid powders via boro-carbothermal reduction of ZrO2 by B4C and carbon black

ZrB2–ZrC hybrid powders were synthesized by a novel two-step reduction on basis of ZrO2 + B4C + C→ ZrC + ZrB2 + CO reaction in Ar atmosphere, using ZrO2, B4C, and carbon black powders as starting materials. Thermodynamics of relevant reactions were evaluated. Effects of excess additions of B4C and C on phase constituents were investigated. Morphology and chemistry of the powder products were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM). The results showed that ZrB2–ZrC hybrid powders with no obvious impurity content could be obtained after heating at 1350 °C for 1 h followed by further reaction at 1700 °C for 1 h with 16 wt% B4C + 8 wt% C excess addition. Relative contents of the ZrB2: ZrC phase in the product powders could be conveniently regulated by varying the B4C and C content in the starting compositions. The resultant powders had good oxidation resistance with an oxidation activation energy value of 433 kJ/mol. Good sinterability of the powder products was demonstrated by hot pressing at 1950 °C for 60min under 30 MPa pressure, which resulted in fully dense ZrB2–ZrC composite ceramics with Vickers hardness value larger than 18.3 ± 0.6 GPa.

Microstructure evolution, enhanced hardness and toughness in the solid-solution ceramic composite by reaction pressureless sintering of ZrB2 and TiC powders

Fully dense ZrB2–TiC composites were produced by reactive pressureless sintering using ZrB2 and TiC as the starting reactants. The effect of TiC content on microstructure and mechanical properties of ZrB2–TiC composites was investigated. The densification and grain refinement were achieved due to the reaction and solid solution coupling effect. Besides, elongated (Ti, Zr)B2 plate-like grains with a high aspect ratio were obtained for the first time by the reaction of ZrB2 and TiC. The ZrB2-70 mol%TiC composite exhibited good comprehensive mechanical properties of high hardness of 29.0 GPa, flexural strength of 555 MPa, and fracture toughness of 5.4 MPa m1/2 due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects.

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

AbstractThe 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.

Enhanced Mechanical Properties and Oxidation Resistance of Zirconium Diboride Ceramics via Grain-Refining and Dislocation Regulation.

Zirconium diboride (ZrB2) is considered as one of the most promising ultra‐high temperature materials for the applications in extreme environments. However, the difficulty in fabrication of ZrB2 limits its industrial applications. In this study, fully dense and grain‐refined ZrB2 is prepared under ultra‐high pressure of 15 GPa at low temperature of 1450 °C. The as‐prepared ZrB2 exhibits excellent mechanical and oxidation‐resistant properties. Compared with raw powder, the grain size decreases 56%. Compared with high‐temperature sintered control specimen beyond 2000 °C, the hardness and fracture toughness increase about 46% and 69%, respectively, the dislocation density increase 3 orders of magnitude, while the grain size considerably decrease 96%. According to work hardening, Hall–Petch and Taylor dislocation hardening effects, the refined grains, substructures, and high dislocation density caused by plastic deformation during sintering can enhance the mechanical properties. The unique structure contributes to a threshold oxidation temperature increase of ≈250 °C relative to the high‐temperature sintered ZrB2, achieving one of the highest values (1100 °C) among the reported monolithic ultra‐high temperature ceramics. A developed densification mechanism of dislocation multiplication with grain refining is proposed and proved to dominate the sintering, which is responsible for simultaneous improvements in mechanical and oxidation‐resistant properties.

Microstructure and Ablation Behavior of Low-Pressure Plasma Sprayed ZrB2 Coatings Down to 100 Pa.

The effect of chamber pressure on the microstructure and ablation behavior of ZrB2 coatings deposited by low-pressure plasma spraying was investigated. The results showed that as the spray chamber pressure further was reduced to less than 50 kPa, the porosity of the coating deposited at the same distance decreased with the chamber pressure, and the coating prepared under 100 Pa presented the lowest porosity of about 0.89%. The ablation performance test subjected to high-temperature plasma jet revealed that the linear ablation rate of ZrB2 coating increased with the porosity of the coating. As a result, among the ZrB2 coatings deposited at chamber pressures of 100 Pa, 5 kPa, 10 kPa and 50 kPa, the dense coating deposited at 100 Pa showed the lowest ablation rate of 0.33 µm/s. The dense ZrB2 coating with a thickness of about 100 μm was able to withstand 300 s ablation by a plasma flame with a net power of 25 kW resulting in an ablating coating surface temperature of about 2000 °C. The ablation mechanism of the coating was also examined.

Synthesis and Sintering of Nanostructured ZrB2-Al2O3 Composite

ZrB2 is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, ZrB2 applications are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites, and to fabricate nanostructured materials. In this study a dense nanostructured ZrB2-Al2O3 composite was rapidly sintered using the pulsed current activated heating (PCAH) method within 3 min in one step, from mechanically synthesized powders of ZrB2 and Al2O3. Consolidation was accomplished using an effective combination of current and mechanical pressure. A highly dense ZrB2- Al2O3 composite with a relative density of up to 97.4% was fabricated using the simultaneous application of 70 MPa pressure and a pulsed current. The fracture toughness and hardness of the ZrB2-Al2O3 composite were 3.9 MPa.m1/2 and 1917 kg/mm2, respectively. The fracture toughness of the composite was higher than that of monolithic ZrB2.

Synthesis and Sintering of Nanostructured ZrB2-Al2O3 Composite

ZrB2 is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, ZrB2 applications are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites, and to fabricate nanostructured materials. In this study a dense nanostructured ZrB2-Al2O3 composite was rapidly sintered using the pulsed current activated heating (PCAH) method within 3 min in one step, from mechanically synthesized powders of ZrB2 and Al2O3. Consolidation was accomplished using an effective combination of current and mechanical pressure. A highly dense ZrB2- Al2O3 composite with a relative density of up to 97.4% was fabricated using the simultaneous application of 70 MPa pressure and a pulsed current. The fracture toughness and hardness of the ZrB2-Al2O3 composite were 3.9 MPa.m1/2 and 1917 kg/mm2, respectively. The fracture toughness of the composite was higher than that of monolithic ZrB2.

On the oxidation behavior of ZrB2–SiC–VC composites

AbstractIn 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.

Review: Effect on physical, mechanical, and wear performance of ZrB2‐based composites processed with or without additives

AbstractBecause of unique combination of properties, ultra high temperature ceramics (UHTCs) are considered the most suitable material for applications in extreme environments as in hypersonic flights, atmospheric reentry, and rocket propulsion system. Processing of UHTCs especially ZrB2‐based ceramic composites with additives offer advantages in terms of simple processing methodology and excellent properties. Processing route highly controls the ceramic properties. Present review share out systematically and explain the processing strategies of ZrB2‐based ceramic composites––conventional, hot press or spark plasma sintering and their effect on microstructure features, physical, and mechanical properties and tribological performance. Present review suggests that it is possible to process full dense ZrB2–SiC ceramic composite with ultrafine or nano size particles via fast sintering technique like spark plasma sintering and gives better mechanical and wear resistant properties.

Oxidation and cracking/spallation resistance of ZrB2–SiC–TaSi2–Si coating on siliconized graphite at 1500 °C in air

A ZrB2–SiC–TaSi2–Si coating on siliconized graphite substrate was prepared by a combination process of slurry brushing and vapor silicon infiltration. The high-temperature oxidation behavior and cracking/spallation resistance of the as-prepared coating were investigated in detail. It was revealed that the oxidation kinetics at 1500 °C in static air followed a parabolic law with a relatively low oxidation rate constant down to 0.27 mg/(cm2·h0.5). The crack area ratio of the as-prepared coating was determined as 3.8 × 10−3 after severe thermal cycling from 1500 °C to room temperature for 20 times. Apart from the formation of ZrO2 as skeleton phase with SiO2 as infilling species, the good oxidation and cracking/spallation resistance of the coating also could be attributed to its unique duplex-layered structure, i.e., a dense ZrB2–SiC–TaSi2 major layer filled with Si and an outermost Si cladding top layer. Meanwhile, the strong adhesion strength of the SiC transition layer with the graphite substrate and the outer ZrB2–SiC–TaSi2–Si layer was a vital factor as well.

Thermal stability under laser heating of hot-pressed (Hf1-X ZrX)B2/SiC powder mixtures obtained by mechano-synthesis

The paper studied ultra-refractory (Hf1-X ZrX)B2/SiC ceramics fabricated by hot-pressing (HP) of mechano-chemically assisted precursors synthesis, and tested their thermal stability under laser heating. Fully dense materials were obtained after HP. Microstructures of the sintered materials were analyzed by XRD and SEM-EDS, while 4-pt flexure strength in air at room temperature up to 1773 K was measured. The thermal stability was tested using a diode laser source. A SiC-free fully dense ZrB2 ceramic was used as benchmark to identify the potential of the laser heating technique and separate the effects related to the addition of SiC into a diboride ceramic matrix. Infrared thermo-camera and 2-color pyrometer provided the real-time variation of the surface temperature vs time. For surface temperatures below 1900 K reached by the SiC-containing samples, SiC acted as a key player and provided excellent protection to the diboride matrices against oxidation: extensive post-test microstructural analyses documented this response.

Preparation and properties of dense ZrB2 composite reinforced by elongated SiC and Al3BC3 grains

AbstractDense ZrB2‐SiC‐Al3BC3 ultra‐high temperature ceramic composite was fabricated by hot pressing sintering at 1900°C for 1 hour under a pressure of 20 MPa using Zirconium diboride (ZrB2) as the raw material and a powder mixture of SiC, B4C, Al, and carbon as the sintering additive. Al and B4C underwent in situ reaction with carbon powder to produce Al3BC3, which promoted the densification of ZrB2 ceramic. SiC grains were found to be elongated during sintering. The ZrB2‐SiC‐Al3BC3 composite exhibited excellent mechanical properties, such as high flexural strength of 589 ± 147 MPa and fracture toughness of 7.81 ± 1.09 MPa m1/2. Oxidation behavior of the ZrB2‐SiC‐Al3BC3 composite was studied in air at 1500°C for 1 hour. A continuous layer of oxides consisting of a mixture of SiO2, Al2SiO5, and Al2O3 was formed on the surface of the ZrB2‐SiC‐Al3BC3 composite. This layer of oxides efficiently prevented oxygen from diffusing into the specimens during oxidation, which improved the oxidation resistance of the ZrB2 ceramics.

The Effect of Ball Milling on Properties and Consolidation of Nanostructured ZrB₂.

ZrB₂ powders were milled using high-energy ball for various durations and consolidated using the pulsed current activated sintering (PCAS). The effects of milling on the sintering behavior and crystallite size ZrO₂ powders were investigated. A nanostructured dense ZrB₂ specimen with a relative density of up to 97% was readily achieved within 3 min. The ball milling effectively refined the crystallite structure of ZrB₂ powders and facilitated the subsequent consolidation. The sinter-onset temperature was reduced appreciably by the prior milling for 10 h. Accordingly, the relative density of ZrB₂ compact increases as the milling time increases. The hardness and fracture toughness of sintered ZrB₂ increased as the density increases.

ZrB2 particles reinforced glass coating for oxidation protection of carbon/carbon composites

A dense ZrB2 particles reinforced glass (ZrB2/SiO2) coating was prepared on the SiC coated carbon/carbon composites by a facile sol-dipping approach. The prepared ZrB2/SiO2 coating could protect the composites from being oxidized for 160 h at 1773 K with a weight loss of 6.9 mg/cm2. The flexural strength retention ratio of the ZrB2/SiO2 coated composites is 87% after oxidation for 160 h at 1773 K. The continuous SiO2 glass layer embedded with the submicron ZrSiO4 particles was formed during oxidation. This was helpful to lower the diffusion rate of oxygen and improve the stability of SiO2 glass film, thus improving the oxidation resistance of the coated samples. After thermal cycles between 1773 K and room temperature for 15 times, penetrated cracks formed in the coating. The weight loss of the ZrB2/SiO2 coated sample presented linear relationship, and the final weight loss per unit area was 6.35 mg/cm2. The generation of the penetrative cracks and the debonded coating interface resulted in the failure of the ZrB2/SiO2 coating.

Микроструктурные особенности СВС-прессования композитов ZrB2-B4C и TiB2-B4C

In the present paper dense ceramic composites TiB2-B4C and ZrB2-B4C have been produced by means of pressure-assisted self-propagating high temperature synthesis (SHS). The method includes combustion synthesis of refractory compounds and its consolidation under high mechanical pressure. An equilibrium SHS product formed during the exothermic interaction in the mixture of Ti, Zr, B and C powders contains TiB2 or ZrB2 as a dispersed phase, and B4C as a ceramic binder. The influence of content of the ceramic binder (B4C) on the formation of the microstructure of SHS composites was studied. It is shown at the B4C content of 20 wt.% in the TiB2-B4C composite and at 5 wt.% in the ZrB2-B4C composite were formed dense TiB2 and ZrB2 particles. At content B4C 40 wt.% in the TiB2-B4C-composite and 20 wt.% in the ZrB2-B4C-composite the particles of the dispersed phase were formed in hollow shells form. The size and thickness of the shells depend on the initial Ti and Zr particles size. It had been proposed formation mechanism of hollow shells, are including stages formation the TiB2 and ZrB2 layers on the metallic particles surface, melting of the internal unreacted part of the metallic particles, and spreading of the melt on the outer surface of the product layer. The experimental results showed the “chemical furnace” had provided thermal regime needed to efficient consolidation SHS composites to minimum residual porosity. Physical and mechanical characteristics of SHS composites were studied depending on content of ceramic binder. It is shown that the maximum microhardness of the TiB2-B4C and ZrB2-B4C composites are 39.1– 44.8 GPa, and 20.4 – 24.5 GPa, accordingly. The flexural strength of the TiB2-B4C-composites is 140 – 210 MPa.