Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.     

Multicomponent synergistically affected mechanical properties,microstructure, and oxidation resistance of Zr–Al(Si)–C based composites

Herein, in-situ Zr₃[Al(Si)]₄C₆-based composites with 10–40 vol% ZrB₂–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB₂–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB₂–SiC, while the hardness increases near linearly. Zr₃[Al(Si)]₄C₆–ZrB₂–SiC shows high strength (623 MPa), toughness (7.59 MPa m^1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB₂–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10⁻⁵ kg² m⁻⁴ s⁻¹ when oxidized at 1400 °C.     

High-temperature oxidation behavior of monolithic Zr3[Al(Si)]4C6 and its composite incorporating 30vol% ZrB2-SiC: Providing a basis for broadening the service temperature

To provide a basis for the high-temperature oxidation of ultra-high temperature ceramics (UHTCs), the oxidation behavior of Zr₃[Al(Si)]₄C₆ and a novel Zr₃[Al(Si)]₄C₆-ZrB₂-SiC composite at 1500 °C were investigated for the first time. From the calculation results, the oxidation kinetics of the two specimens follow the oxidation dynamic parabolic law. Zr₃[Al(Si)]₄C₆ exhibited a thinner oxide scale and lower oxidation rate than those of the composite under the same conditions. The oxide scale of Zr₃[Al(Si)]₄C₆ exhibited a two-layer structure, while that of the composite exhibited a three-layer structure. Owing to the volatilization of B₂O₃ and the active oxidation of SiC, a porous oxide layer formed in the oxide scale of the composite, resulting in the degradation of its oxidation performance. Furthermore, the cracks and defects in the oxide scale of the composite indicate that the reliability of the oxide scale was poor. The results support the service temperature of the obtained ceramics.     

High‐Strength ZrC Ceramics Doped with Aluminum

High‐strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC_1?x, Zr₃Al₃C₅ and Zr–Al compound such as AlZr₃ and Al–C–Zr. The intermediate product AlZr₃ below 1600°C and remained Al–C–Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C–1900°C) could be attributed to the formation of nonstoichiometric ZrC_1?x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC_0.85–Zr₃Al₃C₅ and ZrC_0.80–Zr₃Al₃C₅ based ceramics had 3‐point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2–18.3 GPa and 3.3–3.5 MPa m^1/2, respectively.     

Reactive Processing of a ZrB2 / ZrC / Zr–Si Ceramic Composite with a Controlled Oxygen Potential

A Zr–Si liquid reacted with B₄C in a graphite enclosure was configured to control the oxygen potential (10^?45 kPa) to form a ZrB₂ / ZrC / Zr – Si ceramic composite. The graphite enclosure was placed in a temperature gradient with the hot zone at >2133 K to react Zr – Si with B₄C and with the opposite end approximating 933–1000 K at the position of an aluminum melt. A ZrB₂ / ZrC / Zr – Si composite forms with the scanning‐electron microscope (SEM), microstructures showing rectangular ZrB₂ precipitates and hexagonally shaped ZrC precipitates embedded in a Zr – Si matrix.     

Elastic,mechanical, electronic,and defective properties of Zr–Al–C nanolaminates from first principles

By means of first principles calculations, Zr–Al–C nanolaminates have been studied in the aspects of chemical bonding, elastic properties, mechanical properties, electronic structures, and vacancy stabilities. Although the investigated Zr–Al–C nanolaminates show crystallographic similarities, their predicated properties are very different. For (ZrC)_nAl₃C₂ (n = 2, 3, 4), the Zr–C bond adjacent to the Al–C slab with the C atom intercalated in the Zr layers is the strongest, but the one with the C atom intercalated between the Zr layer and Al layer is the weakest. In contrast, the situation for (ZrC)_nAl₄C₃ (n = 2, 3) is just the opposite. For Zr–Al–C nanolaminates, the calculated bulk, shear and Young's modulus increase in the sequence of Zr₂AlC < Zr₃AlC₂ < Zr₂Al₄C₅ < Zr₃Al₄C₆ < Zr₂Al₃C₄ < Zr₃Al₃C₅ < Zr₄Al₃C₆. The (ZrC)_nAl₃C₂ (n = 2, 3, 4) series exhibit the most outstanding elastic properties. In the presence of the external pressure, the bulk and shear moduli exhibit a linear response to the pressure, except for Zr₂AlC and Zr₃AlC₂, both of which belong to the so‐called MAX phases. The two materials also exhibit very distinct properties in the strain‐stress relationship, electronic structures and vacancy stabilities. As the intercalated Al layers increase, the formation energy of V_Zr and V_Al increases, while the formation energy of V_C decreases.     

Reactive Hot Pressing and Properties of Zr1−xTixB2–ZrC Composites

Reactive hot pressing was used to prepare Zr_1?xTi_xB₂–ZrC composites with advantageous microstructure and mechanical properties from ZrB₂–TiC powders. The reaction mechanisms and the effects of different levels of TiC on the physical and mechanical properties of the resulting composite were explored in detail and compared to conventionally hot‐pressed ZrB₂ and ZrB₂–ZrC. Incorporation of 10 to 30 vol% TiC enabled full densification and restrained grain growth, reducing the final average grain size from 5.6 μm in pure ZrB₂ to a minimum of 1.4 μm in samples with 30 vol% TiC. The flexural strengths and hardnesses of the composites sintered with TiC were consequently greater than the conventionally processed ZrB₂–ZrC materials, increasing from 440 MPa and 17.4 GPa to a maximum of 670 MPa and 24.2 GPa at 10 vol% TiC. However, despite a decrease in the total average grain size, the flexural strength at higher TiC levels was limited by an increase in ZrC grain growth, which was observed to determine the flexural strength of the reaction sintered composites similar to the case of ZrB₂–SiC.     

Mechanical,Tribological, and Thermal Properties of Hot‐Pressed ZrB2‐B4C Composite

The effect of addition of submicrometer‐sized B₄C (5,10 and 15 wt%) on microstructure, phase composition, hardness, fracture toughness, scratch resistance, wear resistance, and thermal behavior of hot‐pressed ZrB₂‐B₄C composites is reported. ZrB₂‐B₄C (10 wt%) composite has V_H1 of 20.81 GPa and fracture toughness of 3.93 at 1 kgf, scratch resistance coefficient of 0.40, wear resistance coefficient of 0.01, and ware rate of 0.49 × 10^?3 mm³/Nm at 10N. Crack deflection by homogeneously dispersed submicrometer‐sized B₄C in ZrB₂ matrix can improve the mechanical and tribological properties. Thermal conductivity of ZrB₂‐B₄C composites varied from 70.13 to 45.30 W/m K between 100°C and 1000°C which is encouraging for making ultra‐high temperature ceramics (UHTC) component.     

Microstructure and mechanical properties of reaction-hot-pressed zirconium diboride based ceramics

ZrB₂ ceramics were prepared by in-situ reaction hot pressing of ZrH₂ and B. Additions of carbon and excess boron were used to react with and remove the residual oxygen present in the starting powders. Additions of tungsten were utilized to make a ZrB₂-4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB₂-10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB₂ and ZrB₂–ZrC composition contained a residual oxide phase and ZrC inclusions, while the W-doped composition contained residual carbon and a phase that contained tungsten and boron. All three compositions exhibited similar values for flexure strength (~520 MPa), Vickers hardness (~15 GPa), and elastic modulus (~500 to 540 GPa). Fracture toughness was about 2.6 MPa m^1/2 for the W-doped ZrB₂ compared to about 3.8 MPa m^½ for the ZrB₂ and ZrB₂–ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W-doped ZrB₂ compared with the other compositions. The study demonstrated that reaction-hot-pressing can be used to fabricate ZrB₂ based ceramics containing solid solution additives or second phases with comparable mechanical properties.     

Zirconium diboride-mullite composite form mineral: Combustion synthesis,consolidation, characterizations and properties

Zirconium diboride-mullite composite powder was synthesized in-situ by combustion in argon of a zircon sand/B₂O₃/Al reactant system in a 3 : 3: 10 M ratio. Zircon sand with a particle size less than 45 μm was activated by high-energy milling for 360 min. The optimum reactant system included the addition of 0.01 mol of Si. The product of the synthesis of this system contained 34 wt% ZrB₂ and 50 wt% mullite. The obtained zirconium diboride-mullite powder was consolidated by hot pressing at 25 MPa in an argon environment, ramping at 10 °C/min to 1,450, 1550 and 1650 °C and holding for 60 min. The sintered composite hot-pressed at 1650 °C had a density of 3.39 g/cm³, flexural strength of 153.25 ± 1.19 MPa, hardness of 10.66 GPa and fracture toughness of 4.23 MPa.m^1/2. The flexural strength and hardness of the composite was significantly influenced by the grain size of the reinforced ZrB₂. The predominantly intergranular fracture observed in surface micrographs confirmed the high toughness of the composite. The coefficient of thermal expansion of the product hot-pressed at 1650 °C was 6.53 × 10⁻⁶/°C: much lower than reported coefficients of existing Al₂O₃, ZrO₂ ZrB₂, and ZrB₂–SiC refractory ceramics.     

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.     

Preparation,microstructure, and mechanical properties of ZrB2-based ceramics with layered Zr2Al4C5 inclusions

ZrB₂/Zr₂Al₄C₅ composite ceramics with different volume contents of Zr₂Al₄C₅ formed in situ were fabricated by the spark plasma sintering technique at 1800 ^°C. The content of Zr₂Al₄C₅ was found to have an evident effect on the preparation, phase constitution, microstructure as well as the mechanical properties of ZrB₂/Zr₂Al₄C₅ ceramics. The results indicated that sinterability of the composites was remarkably improved by the addition of Zr₂Al₄C₅ compared to the single-phase ZrB₂ ceramic. The microstructure of the resulting composites was fine and homogeneous, the average grain size of the ZrB₂ decreased, and the average aspect ratio of the Zr₂Al₄C₅ increased with the increase in the amount of Zr₂Al₄C₅. As the content of Zr₂Al₄C₅ increased, both the Vickers hardness and Young's modulus of the composites first increased and then decreased. The fracture toughness of the ZrB₂–40 vol% Zr₂Al₄C₅ composite was 4.25 MPa m^1/2, which increased by approximately 70% compared to the monolithic ZrB₂ ceramic. The improvement was mainly attributed to the toughening mechanisms such as the layered structure toughening, crack deflection and crack bridging, caused by the in situ formed layered Zr₂Al₄C₅ inclusions.     

Investigation of the density and microstructure homogeneity of square-shaped B4C-ZrB2 composites produced by spark plasma sintering method

Square-shaped monolithic B₄C and B₄C-ZrB₂ composites were produced by spark plasma sintering (SPS) method to investigate the effect of 5, 10, 15 vol% ZrB₂ addition on the densification, mechanical and microstructural properties of boron carbide. The relative density of B₄C increased with the increasing volume fraction of ZrB₂ and density differences in different regions of the sample narrowed down. Homogeneous density distribution and microstructure were accomplished with the increasing holding time from 7 to 20 min for the B₄C-15 vol% ZrB₂ composites, and the highest overall relative density was achieved as 99.23%. The hardness and fracture toughness of composites were enhanced with the addition of ZrB₂ compared to monolithic B₄C. The enhancement in fracture toughness was observed due to the crack deflection, crack bridging and crack branching mechanisms. The B₄C-15 vol% ZrB₂ composite exhibited the combination of superior properties (hardness of 33.08 GPa, Vickers indentation fracture toughness of 3.82 MPa.m^1/2).     

Processing and mechanical properties of hot-pressed zirconium diboride – zirconium carbide ceramics

ZrB₂ was mixed with 0.5 wt% carbon and up to 10 vol% ZrC and densified by hot-pressing at 2000 °C. All compositions were > 99.8% dense following hot-pressing. The dense ceramics contained 1–1.5 vol% less ZrC than the nominal ZrC addition and had between 0.5 and 1 vol% residual carbon. Grain sizes for the ZrB₂ phase decreased from 10.1 µm for 2.5 vol% ZrC to 4.2 µm for 10 vol% ZrC, while the ZrC cluster size increased from 1.3 µm to 2.2 µm over the same composition range. Elastic modulus was ~505 GPa and toughness was ~2.6 MPa·m^½ for all compositions. Vickers hardness increased from 14.1 to 15.3 GPa as ZrC increased from 2.5 to 10 vol%. Flexure strength increased from 395 MPa for 2.5 vol% ZrC to 615 MPa for 10 vol% ZrC. Griffith-type analysis suggests ZrB₂ grain pullout from machining as the strength limiting flaw for all compositions.     

Aqueous Gelcasting and Pressureless Sintering of Zirconium Diboride Ceramics

Dense (97.3%) zirconium diboride (ZrB₂) ceramics were obtained via gelcasting and pressureless sintering. Four wt% B₄C was used as sintering aid. ZrB₂, SiC, and B₄C can codisperse well in the alkaline region, using a polyacrylate dispersant. Compared with monolithic ZrB₂ (Z), the mechanical properties of ZrB₂‐SiC (ZS) were enhanced. The Vickers hardness and fracture toughness of ZS were (13.1 ± 0.6) GPa and (2.5 ± 0.4) MPa m^1/2, respectively.     

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.     

Mechanical,electrical and thermal properties of ZrC-ZrB2-SiC ternary eutectic composites prepared by arc melting

ZrC-ZrB₂-SiC composites were prepared by arc-melting in Ar atmosphere using ZrC, ZrB₂ and SiC as starting materials. The ternary eutectic composition of 20ZrC-30ZrB₂-50SiC (mol%) was first identified. SiC about 7?μm in length and 500?nm in diameter, ZrC about 4 μm in length and 1 μm in diameter, in rod-like microstructure, were uniformly dispersed in ZrB₂ matrix of eutectic composite. The eutectic temperature of ZrC-ZrB₂-SiC composite was approximately 2550?K. The Vickers Hardness and fracture toughness of eutectic composite was 23?GPa and 6.2?MPa?m^1/2, respectively. The electrical conductivity decreased from 7.2?×?10⁷ to 1.75?×?10⁶?S?m^?1 with the temperature increasing from 287 to 800?K. The thermal conductivity decreased from 85 to 61?W?K^?1?m^?1 with increasing temperature from 287 to 973?K.     

Fabrication and properties of reactively hot pressed ZrB2–SiC ceramics

ZrB₂–SiC ceramics with relative densities >99% were fabricated by ‘in situ’ reactive hot pressing from ZrH₂, B₄C and Si. The reaction was studied using two processes, (1) powder reactions at temperatures from 1150 to 1400 °C and (2) reactive hot pressing between 1600 and 1900 °C. The products from the reaction of a 2ZrH₂:1B₄C:1Si molar mixture were ZrB₂, SiC, ZrO₂ and ZrC. Modification of the composition to 2ZrH₂:1.07B₄C:1.16Si resulted in the elimination of the undesired ZrO₂ and ZrC phases. The final composition was approximately ZrB₂–27 vol% SiC with no undesired phases detected by X-ray diffraction, and only low concentrations of B₄C detected by scanning electron microscopy. Elimination of the undesired phases was accomplished by removing surface oxides through chemical reactions at elevated temperatures. Reactively hot pressed samples consisting of ZrB₂ with 27 vol% SiC had a Young's modulus of 508 GPa, a flexure strength of 720 MPa, a fracture toughness of 3.5 MPa m^1/2 and a Vickers’ hardness of 22.8 GPa.     

High-strength ZrB2-based ceramics prepared by reactive pulsed electric current sintering of ZrB2–ZrH2 powders

Fully densified ZrB₂-based ceramic composites were produced by reactive pulsed electric current sintering (PECS) of ZrB₂–ZrH₂ powders within a total thermal cycle time of only 35 min. The composition of the final composite was directly influenced by the initial ZrH₂ content in the starting powder batch. With increasing ZrH₂ content, ZrB₂–ZrO₂, ZrB₂–ZrB–ZrO₂ and ZrB₂–ZrB–Zr₃O composites were obtained. The ZrB₂–ZrB–ZrO₂ composite derived from a 9.8 wt% ZrH₂ starting powder exhibited an excellent flexural strength of 1382 MPa combined with a Vickers hardness of 17.1 GPa and a fracture toughness of 5.0 MPa m^1/2. The high strength was attributed to a fine grain size and the removal of B₂O₃ through reaction with Zr. Higher ZrH₂ content starting powders were densified through solution-reprecipitation resulting in the formation of coarser angular ZrB₂–ZrB composites with a Zr₃O grain boundary phase with a fracture toughness of 5.0 MPa m^1/2 and an acceptable strength in the 852–939 MPa range.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.