Aluminum diboride synthesis from elemental powders by mechanical alloying and annealing

In this study, AlB₂ powders were synthesized by using a combined method of mechanical alloying (MA) and annealing of elemental aluminum (Al) and boron (B) powders. Milling was performed in a planetary ball-mill (Fritsch? Pulverisette 7 Premium Line) up to 15 h under argon (Ar) atmosphere. Annealing process was carried out in a tube furnace at 650 °C for 6 h under Ar atmosphere. The effects of MA durations on the annealing process and AlB₂ formation were investigated. The conversion of Al and B powders to AlB₂ starts after only MA for 3 h or after MA for 1 h and subsequent annealing. A slight formation of AlB₁₂ occurs at 242 °C for as-blended powders and it shifts to about 272 °C for MA’d powders. Al–B powder blends MA’d for 9 h and annealed have AlB₂ particles in size between 35 and 75 nm in the presence of Al₁₃Fe₄, Fe₃B and Fe₂B contaminations.     

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     

Oxidation resistance of ZrB2 and ZrB2-SiC ultrafine powders synthesized by a combined sol-gel and boro/carbothermal reduction method

ZrB₂ and ZrB₂-SiC powders were prepared by a combined sol-gel and boro/carbothermal reduction method, and their oxidation kinetics was studied by using a non-isothermal thermogravimetric technique. The results showed that the Mample power law (n=1) was the most probable mechanism function, and the incorporation of SiC into ZrB₂ greatly enhance the latter's oxidation resistance. The oxidation activation energy values of phase pure ZrB₂ and ZrB₂-SiC powders were respectively 249 and 308 kJ/mol.     

Seed-mediated growth of ZrC whisker and its toughening effect on ZrB2-SiC-C ceramic

ZrC whiskers (ZrC_w) hold great promise in improving the strength and toughness of ultra-high temperature ceramics (UHTCs) without reducing their high-temperature stability. However, obtaining high quality ZrC_w has been challenging. Herein, we propose a novel method for easily synthesizing catalyst-free ZrC_w by seed-mediated growth technique, in which single crystal ZrC nanoparticle (ZrC_np) was used as seed crystal and ZrO₂-C-NaF mixture was used as precursor system. The effect of ZrC_np, NaF, and, synthesis temperature on the growth of ZrC_w was studied and the reaction process was analyzed based on the experimental results. The vapor-solid (V-S) growth mechanism was proved to be the dominating growing mechanism. Subsequently, the synthesized ZrC_w were added to a ZrB₂-SiC-C ceramic. Compared with the baseline, ZrC_w reinforced ZrB₂-SiC-C exhibited a remarkable combination of high strength and high toughness (592 ± 30 MPa and 7.1 ± 0.8 MPa·m^1/2). This novel synthesis method of ZrC_w may be applicable to the synthesis of other carbide ceramic whiskers and enrich the design of high performance ultra-high temperature ceramics.     

In situ fabrication and characterization of laminated C/ZrC ceramic via filter papers and zirconia powders

Capillary infiltration and an in situ reaction between filter papers and zirconia powders were employed to synthesize laminated C/ZrC composite via vacuum impregnation and hot-pressing sintering at 1700 °C for 90 min under a pressure of 30 MPa. The microstructures and mechanical properties of the laminated C/ZrC composite were characterized via XRD, SEM, TEM analyses and the three-point bending test. The results indicated that the obtained composite exhibited a distinct laminar structure with alternating carbon and zirconium carbide layers. The composite had a bulk density of 1.89 g/m³, an open porosity of 21.6%, and a bending strength of 128 MPa. Typical non-brittle fracture behaviors are observed, and the composites show an elastic deformation at the beginning of the test, exhibiting a zigzagging rise until the maximum stress is reached.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Densification,microstructure and mechanical properties of ZrB2–SiCw ceramic composites

ZrB₂–SiCw composites were prepared through hot-pressing at a low temperature of 1800 °C, and Al₂O₃ plus Y₂O₃ were added as sintering aids. Analysis revealed that additives may react with impurities (i.e. surface oxygen impurities and residual metallic impurities) to form a transient liquid phase, thus promote the sintering and densification of ZrB₂–SiCw composites. The content of additives was found to have a significant influence on the sinterability, microstructure and mechanical properties of ZrB₂–SiCw composites. ZrB₂–SiCw composite prepared with a small amount of additives (3 vol.%) provided the optimal combination of microstructure (relative density of 98.3%) and excellent properties, including flexural strength of 783 MPa and fracture toughness of 6.7 MPa m^1/2. With further addition of additives, SiC whiskers were inclined to gather together and be enveloped by excessive liquids to form core-rim-like structures, which lead to little decrease in mechanical properties.     

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.     

The fabrication and mechanical properties of SiC/ZrB2 laminated ceramic composite prepared by spark plasma sintering

Laminated SiC/ZrB₂ ceramic was fabricated by roll-compaction and spark plasma sintering at 1600 °C. A maximum fracture toughness of 12.3 ± 0.3 MPa m^1/2 was measured for the sintered SiC/ZrB₂ laminated ceramic. This significant improvement in fracture toughness can be attributed to the crack deflection along the interfacial layer and the presence of residual stresses in the sample. The effect of interlayer composition on the residual stresses was discussed in detail. It is observed that the residual thermal stress could be reduced by addition of ZrB₂ particles to the SiC interlayer. The bending strength can be increased to 388 ± 44 MPa with the addition of 20 vol% ZrB₂ to the SiC interlayer.     

Microstructure,Mechanical, and Thermal Properties of (ZrB2 + ZrC)/Zr3[Al(Si)]4C6 Composite

The microstructure, mechanical, and thermal properties of in situ hot‐pressed 30 vol% (ZrB₂+ZrC)/Zr₃[Al(Si)]₄C₆ composite have been investigated and compared with monolithic Zr₃[Al(Si)]₄C₆ ceramic. The composite is composed of ZrB₂ and ZrC grains embedded in a Zr₃[Al(Si)]₄C₆ matrix. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 415 GPa), strength (bending strength of 621 MPa), and toughness (fracture toughness of 7.37 MPa·m^1/2) compared with monolithic Zr₃[Al(Si)]₄C₆. The composite retains high modulus of 357 GPa at 1430°C (86% of that at ambient temperature) due to clean grain boundaries with no glassy phase. In addition, the composite exhibits higher specific heat capacity and thermal conductivity but slightly lower coefficient of thermal expansion compared with monolithic Zr₃[Al(Si)]₄C₆. The calculation of the thermal stress fracture resistance parameter (R) predicts a much improved thermal shock resistance of the composite. Based on these results, (ZrB₂+ZrC)/Zr₃[Al(Si)]₄C₆ composites show promising potential for high‐temperature and ultra high‐temperature applications.     

Fabrication of near-infrared transparent Dy2Zr2O7 ceramic by controlled pre-calcination of powders

Fabrication of transparent or pore-free oxide ceramics usually requires sintering in a vacuum or reducing atmosphere. In the present paper, a near-infrared transparent Dy₂Zr₂O₇ ceramic was prepared by ordinary pressureless sintering under the air atmosphere, with a controlled pre-calcining of raw powders process. The key point of the process is to control the content of the phases by pre-calcination of the starting mixture of ZrO₂ and Dy₂O₃ powders. The influence of pre-calcining temperature on the content of phases, the particle sizes, on the sinterability of the mixture of Dy₂O₃ and ZrO₂ powders has been investigated. It was found that when the pre-calcination was conducted at 1200°C for 2 h, the powder contains 4 mol% Dy₂Zr₂O₇, 18 mol% m-ZrO₂, 30 mol% Dy₂O₃, and about 50 mol% t/c-ZrO₂ which is beneficial for the sintering of pore-free Dy₂Zr₂O₇ ceramics. The mechanism is discussed in the paper. The transparency of the sintered Dy₂Zr₂O₇ ceramic was measured, and their transmittance is about 63% at 1450 nm and 73% at 1900 nm. This work provides a simple and convenient way to prepare the near-infrared transparency ceramic without special sintering facilities.     

Structural and electrochemical characterization of Fe–Si/C composite anodes for Li-ion batteries synthesized by mechanical alloying

A Fe–Si (FeSi₂ + Si)/C composite was prepared by mechanical ball milling and investigated as a new inserting anode for use in Li-ion batteries. The composite so prepared has a sandwich structure with the alloy particles as middle cores and the graphite layer as outer shells. The charge-discharge measurements revealed that the Fe–Si/C composite not only had a quite high initial capacity of approximately 680 mAh g⁻¹, but also exhibited greatly improved capacity retention with a reversible capacity of approximately 500 mAh g⁻¹ after 15 cycles in comparison with pure Si and Fe–Si alloy. Based on XRD, XPS, SEM, Raman and EIS analysis of the composite electrode in different lithiated states, the mechanism for improved cycleability is found to be due to the effective buffering of the volumetric changes of the Fe–Si particles by the graphite shell.     

Influence of mechanical alloying on Ti3SiC2 formation via spark plasma sintering technique from Ti/SiC/C powders

Ti₃SiC₂ was elaborated by two different methods: (i) Spark plasma sintering of 5Ti/2SiC/C powders and (ii) mechanical alloying of powders followed by Spark plasma sintering. The results showed that mechanical alloying was not advantageous for pure Ti₃SiC₂ formation but it can significantly improve the density of the obtained bulk material via the particles refinement as well as the microhardness by increasing the TiC content. It was found that the relative density was increased up to 98.58% for the sintered mechanically alloyed sample whereas it was not more than 96.04% for the sintered 5Ti/2SiC/C starting powders. The Vickers microhardness measured for both bulk samples demonstrates a high improvement for the previously mechanically alloyed powder mixture, as it was of about 1282 Hv and only 581.2 Hv for the alloy obtained from 5Ti/2SiC/C starting powders.     

Fabrication method and mechanical properties of biomimetic Cf/ZrB2-SiC ceramic composites with bouligand structures

Herein, biomimetic C_f/ZrB₂-SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, C_f/ZrB₂-SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-C_f/ZrB₂-SiC ceramic films are coated with ZrB₂ and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly C_f/ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic C_f/ZrB₂-SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-C_f/ZrB₂-SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of C_f/ZrB₂-SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging.     

Dynamic oxidation resistance and residual mechanical strength of ZrB2-CrSi2-SiC-Si/SiC coated C/C composites

To improve oxidation resistance of carbon/carbon (C/C) composites, a multiphase double-layer ZrB₂-CrSi₂-SiC-Si/SiC coating was prepared on the surface of C/C composites by pack cementation. Thermogravimetry analysis showed that the as-prepared coating could provide effective oxidative protection for C/C composites from room temperature to 1490 °C. After thermal cycling between 1500 °C and room temperature, the fracture behaviors of the as-prepared specimens changed and their residual flexural strengths decreased as thermal cycles increased. The specimen after 20 thermal cycles presented pseudo-plastic fracture characteristics and relatively high residual flexural strength (83.1%), while the specimen after 30 thermal cycles failed catastrophically without fiber pullout due to the severe oxidation damage of C/C substrate especially the brittleness of the reinforcement fibers.     

Characterization of pop-in phenomena and indentation modulus in a polycrystalline ZrB2 ceramic

Low-load nanoindentation tests were carried out on a polycrystalline ZrB₂-based ceramic. Pop-in phenomena were observed when indentation marks were placed in the interior of the ZrB₂ grains. Both pop-in loads and pop-in extents were statistically distributed with a mutual strong correlation. The critical shear stresses at pop-in were in good agreement with the theoretical shear strength of ZrB₂. The experimental pop-in extents were also compared to a simplified model developed for homogeneous dislocation nucleation. The influence of the grain orientation on the indentation modulus was derived from the model of Delafargue and Ulm (2004)⁵⁰ and compared to the experimental results. Some results were definitely influenced by the polycrystalline structure of the investigated ceramic.     

Rapid synthesis,electrical, and mechanical properties of polycrystalline Fe2AlB2 bulk from elemental powders

Polycrystalline Fe₂AlB₂ bulk including minor Al₂O₃ is synthesized by reactive hot pressing from Fe, Al, and B powders at 1200°C and 30 MPa for 30 minutes, with a relative density of 96%. The present approach enables a markedly reduced holding time compared with previous studies. The derived Fe₂AlB₂ shows an electrical resistivity of 2.27±0.01 μΩ·m, Vickers hardness of 10.2±0.2 GPa, flexural strength of 232±25 MPa, compressive strength of 2101±202 MPa, fracture toughness of 5.4±0.2 MPa·m^1/2 and work of fracture of 117±12 J/m². No dominant indentation cracks are observed, indicating that Fe₂AlB₂ may be quite damage tolerant. Interestingly, a noncatastrophic failure is present in the SENB test, with a high work of fracture. The energy‐absorbing mechanisms in inhibiting crack formation are delamination and pullout of Fe₂AlB₂ grains.     

Characterization of Al2024-CNTs composites produced by mechanical alloying

In the present work, the 2024 aluminum alloy (Al₂₀₂₄) alloy has been produced by mechanical alloying (MA). The alloy was then strengthened by dispersion of carbon nanotubes (CNTs) during different times. Thus, the effect of CNTs concentration and milling time on the microstructure of the Al₂₀₂₄-CNTs composites was studied. The results show a homogeneous dispersion of CNTs into the Al-matrix phase by mechanical milling (MM). It was observed that the increment in the milling time, for a fixed amount of CNTs, causes a reduction of the particle size of powders resulting from MA. The finest particle size was obtained at 20 h of milling. These observations were confirmed by scanning and transmission electron microscopy. After 10 h of milling, Cu, Mg and other alloying elements constituting the Al₂₀₂₄ alloy, form a solid solution and only some remnant Mn particles were observed but not detected by X-ray diffraction.     

Influence of ZrB2 hard ceramic reinforcement on mechanical and wear properties of aluminum

In this work, the effect of ZrB₂ (0, 5, 10 and 20?vol%) ceramic reinforcement on densification, structure, and properties of mechanically alloyed Al was investigated. The milling of Al-ZrB₂ powder compositions resulted in formation of agglomerates with varied size. In particular, the size of agglomerates was reduced considerably with increased addition of ZrB₂ to Al. Interestingly, the densification of hot pressed Al increased from 96.06% to 99.22% with ZrB₂ addition. The reduction of agglomerates size was attributed to the enhanced densification of Al-ZrB₂ composites. Pure Al showed relatively low hardness (0.94?GPa) and it was improved to 1.78?GPa with the addition of 20?vol% ZrB₂. The mechanical properties have significantly been improved for Al-ZrB₂ composites. Especially Al - 20?vol% ZrB₂ possessed a very high yield strength (529?MPa), compressive strength (630?MPa) and compressive strain of 19.25%. Realization of such a good combination of mechanical properties is the highest ever reported for Al composites so far in the literature. The coefficient of friction (COF) of Al-ZrB₂ varied narrowly between 0.33 and 0.40 after dry sliding wear against steel disc. The wear rate of Al-ZrB₂ composites was within mild wear regime and varied between 98.88?×?10^?6 and 34.66?×?10^?6 mm³/Nm. Among all the compositions, Al - 20?vol% ZrB₂ composite exhibited the lowest wear rate and high wear rate was noted for pure Al. Mild abrasion, tribo-oxidation, third body wear (wear debris) and delamination were the major material removal mechanisms for Al-ZrB₂ composites. Overall the hardness, strength and wear resistance of Al - 20?vol% ZrB₂ composite was improved by 84.3%, 84.3% and 64.2%, respectively when compared to pure Al.     

Characterization of Al18B4O33 ceramic synthesized by the sol-gel,precipitation, and combustion methods

In this study, Al₁₈B₄O₃₃ aluminum borate is synthesized by the sol-gel, precipitation, and combustion methods, which use similar starting materials. Simultaneous thermal analyzer and Fourier transform infrared spectroscopy are employed to determine the thermal behavior and the chemical bonding structures. In addition, the crystal structure and microstructure are identified by X-ray diffraction and scanning electron microscope. The results reveal that Al₁₈B₄O₃₃ nanorods are produced by sol-gel method and citric acid controls the morphology of powders. Additionally, Al₁₈B₄O₃₃ phase is obtained by decomposing Al₄B₂O₉ unstable phase at 1000°C by sol-gel method. However, Al₁₈B₄O₃₃ phase is immediately obtained from calcination of an amorphous phase at 1000°C by precipitation and combustion methods.