Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Design of spark plasma sintering parameters and preparation of Al2O3/TiB2/TiC micro–nano composite ceramic tool material

A microstructure evolution model for the ceramic materials was constructed, and the spark plasma sintering parameters were optimized using the model to shorten the designing period and reduce the consumption of the material. Based on the optimized sintering parameters, the ceramic tool material with a composition of Al₂O₃, TiB₂, and TiC proved to be a success. It verified that the materials prepared under the optimized sintering parameters exhibited excellent mechanical properties. The results showed when sintered at 1600°C, under the pressure of 40 MPa and with the holding period of 7 min, the materials with 70% Al₂O₃, 20% TiB₂, and 10% nano-TiC possess the relatively best performance, with the hardness, fracture toughness, and flexure strength being 20.3 GPa, 10.5 MPa/m², and 839.5 MPa, respectively.     

Microstructure and mechanical properties of titanium aluminum carbides neutron irradiated at 400–700 °C

This work reports the first mechanical properties of Ti₃AlC₂-Ti₅Al₂C₃ materials neutron irradiated at ∼400, 630 and 700 °C at a fluence of 2 × 10²⁵ n m⁻² (E > 0.1 MeV) or a displacement dose of ∼2 dpa. After irradiation at ∼400 °C, anisotropic swelling and loss of 90% flexural strength was observed. After irradiation at ∼630–700 °C, properties were unchanged. Microcracking and kinking-delamination had occurred during irradiation at ∼630–700 °C. Further examination showed no cavities in Ti₃AlC₂ after irradiation at ∼630 °C, and MX and A lamellae were preserved. However, disturbance of (0004) reflections corresponding to M-A layers was observed, and the number density of line/planar defects was ∼10²³ m⁻³ of size 5–10 nm. HAADF identified these defects as antisite Ti_Al atoms. Ti₃AlC₂-Ti₅Al₂C₃ shows abrupt dynamic recovery of A-layers from ∼630 °C, but a higher temperature appears necessary for full recovery.     

Electrodeposited TiB2 on graphite as wettable cathode for Al production

Titanium diboride (TiB₂) is considered as a promising cathode material for Al production. However, the manufacture of TiB₂ cathodes is facing numerous challenges. In this study, electrodeposition of TiB₂ on graphite was performed in molten fluoride (FLiNaK) electrolyte at 600°C by using a periodically interrupted current technique for various electrodeposition times (from 10 to 75 minutes) and at two different current densities (−0.12 and −0.5 A/cm²). It is shown that the TiB₂ coating morphology/microstructure strongly depends on the applied current density. Denser coatings were obtained at j_on = −0.12 A/cm² with a growth rate of ca. 0.7 µm/min. The thicker films display a preferential crystallographic orientation along the [110] plan. At j_on = −0.5 A/cm², TiB₂ coatings are deposited at a growth rate of ca. 6 µm/min with no crystallographic texture. They present a porous and stratified morphology with numerous transversal macrocracks. All TiB₂ coatings show excellent wettability for molten Al as confirmed by sessile drop experiments. However, significant molten Al infiltration occurs in the TiB₂ coatings, which accumulates at the coating/graphite interface, inducing the coating delamination.     

Radiation Damage of LaMgAl11O19 and CeMgAl11O19 Magnetoplumbite

Studies of radiation damage in magnetoplumbite‐type LaMgAl₁₁O₁₉ and CeMgAl₁₁O₁₉ are reported. Ion irradiation was conducted on ceramic composites containing a LaMgAl₁₁O₁₉ phase at 500°C with 10 MeV Au⁺ ions and on ceramic composites containing CeMgAl₁₁O₁₉ phase at 800°C with 92 MeV Xe⁺ ions. The radiation response of these similar LnMgAl₁₁O₁₉ (Ln = La and Ce) hexaaluminate magnetoplumbite phases was evaluated using transmission electron microscopy (TEM) and X‐ray diffraction (XRD). LaMgAl₁₁O₁₉ was amorphized by 10 MeV Au ions with swelling of the structure within an approximate 2 μm radiation depth from the irradiation surface. CeMgAl₁₁O₁₉ did not amorphize after 92 MeV Xe‐ion irradiation, but ion track damage contrast is seen in approximately 5 μm of the irradiated depth. SRIM Monte‐Carlo simulations of nuclear displacements correlate with the experimental results.     

Hard polycrystalline eutectic composite prepared by spark plasma sintering

A polycrystalline eutectic B₄C–TiB₂ composite was prepared by spark plasma sintering. The starting eutectic powder was obtained by mechanical grinding of the directionally solidified eutectic B₄C–TiB₂ alloy. The microstructure of the polycrystalline composite exhibited randomly oriented eutectic grains with an average size of about 50–100 μm. Eutectic grains consisted of boron carbide matrix reinforced by titanium diboride inclusions. The secondary eutectic structure in the grain boundary is formed at sintering temperature higher than 1700 °C. XRD analysis revealed that the eutectic B₄C–TiB₂ composite consist mainly of B₄C and TiB₂ phases. The measured Vickers hardness was in the range of 32.35–54.18 GPa and the average fracture toughness of the samples was as high as 4.81 MPa m^1/2. The bending strengths of the composite evaluated at room temperature and at 1600 °C were 230 and 190 MPa, respectively.     

Wear mechanisms of TiN–TiB2 ceramic in sliding against alumina from room temperature to 700 °C

TiN–TiB₂ ceramic was prepared by the reactive hot-pressing method using titanium and BN powders as raw materials. The friction and wear properties of TiN–TiB₂ ceramic were evaluated in sliding against alumina ball from room temperature to 700 °C in air. The TiN–TiB₂ ceramic has a relative density of 98.6%, a flexural strength of 731.9 MPa and a fracture toughness of 8.5 MPa m^1/2 at room temperature. The TiN–TiB₂ ceramic exhibits a distinct decrease in friction coefficient at 700 °C as contrasted with the friction data obtained at room temperature and 400 °C. Wear mechanisms of TiN–TiB₂ ceramic depend mainly upon testing temperature at identical applied loads. Lubricious oxidized products caused by thermal oxidation provide excellent lubrication effects and greatly reduce the friction coefficient of TiN–TiB₂ ceramic at 700 °C. However, abrasive wear and tribo-oxidation are the dominant wear mechanisms of TiN–TiB₂ ceramic at 400 °C. Mechanical polishing effect and removal of micro-fractured grains play important roles during room-temperature wear tests.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Synthesis and properties of conductive B4C ceramic composites with TiB2 grain network

High electrical resistance and low fracture toughness of B₄C ceramics are 2 of the primary challenges for further machining of B₄C ceramics. This report illustrates that these 2 challenges can be overcome simultaneously using core‐shell B₄C‐TiB₂&TiC powder composites, which were prepared by molten‐salt method using B₄C (10 ± 0.6 μm) and Ti powders as raw materials without co‐ball milling. Finally, the near completely dense (98%) B₄C‐TiB₂ interlayer ceramic composites were successfully fabricated by subsequent pulsed electric current sintering (PECS). The uniform conductive coating on the surface of B₄C particles improved the mass transport by electro‐migration in PECS and thus enhanced the sinterability of the composites at a comparatively low temperature of 1700°C. The mechanical, electrical and thermal properties of the ceramic composites were investigated. The interconnected conductive TiB₂ phase at the grain boundary of B₄C significantly improved the properties of B₄C‐TiB₂ ceramic composites: in the case of B₄C‐29.8 vol% TiB₂ composite, the fracture toughness of 4.38 MPa·m^1/2, the electrical conductivity of 4.06 × 10⁵ S/m, and a high thermal conductivity of 33 W/mK were achieved.     

B4C‒TiB2 composite with modified microstructure and enhanced properties from optimal size coupling of raw powders

B₄C‒15 vol% TiB₂ composites were fabricated by in situ reactive spark plasma sintering with B₄C, TiC, and amorphous B powders as the raw materials. The size coupling of initial B₄C and TiC particles was optimized based on the reaction mechanism to derive B₄C‒TiB₂ composites with enhanced microstructure and properties. During the reactive sintering, fine B₄C–TiB₂ particles were firstly formed by an in situ reaction between TiC and B. Then, large B₄C particles tended to grow at the cost of small B₄C particles. The in situ TiB₂ grains gradually grew up and interconnect, distributing around the large B₄C grains to form an intergranular TiB₂ network. The results showed that the B₄C‒15 vol% TiB₂ composite prepared from 3.12 μm B₄C powder and 0.80 μm TiC powder had the optimal comprehensive properties, with a relative density of 99.50%, a Vickers hardness of 31.84 GPa, a flexural strength of 780 MPa, a fracture toughness of 5.77 MPa·m^1/2, as well as an electrical resistivity of 3.01 × 10⁻² Ω·cm.     

Kr ion irradiation study of polymer-derived SiFeOC–C–SiC nanocomposite

This study focuses on the pyrolysis and ion irradiation behaviors of polymer-derived SiFeOC–C–SiC ceramic. The pyrolyzed material is composed of SiO₂ and SiOC (amorphous), carbon (amorphous and turbostratic), and Fe₃Si and β-SiC (nanocrystalline). Irradiation was carried out at both room temperature and 600°C using 400 keV Kr ions with fluences of 4 × 10¹⁵ and 1 × 10¹⁶ ions cm⁻², respectively. The Fe₃Si and SiC nanocrystals are stable against irradiation up to 3 displacement per atom (dpa) at room temperature and up to 12 dpa at 600°C. The SiOC tetrahedrals show phase separation and minor carbothermal reduction. The high irradiation resistance and the dense, defect-free amorphous microstructure of SiFeOC–C–SiC after prolonged irradiation demonstrate its great potential for advanced nuclear reactor applications.     

Neutron transmutation of 10B doped diamond

Free standing ¹⁰B isotope doped diamond films deposited by chemical vapor deposition in a microwave chamber were irradiated to thermal neutron fluence values of 0.32 × 10¹⁹, 0.65 × 10¹⁹, 1.3 × 10¹⁹, and 2.6 × 10¹⁹ n/cm². Cooling of the diamond films was maintained during irradiation. In a separate experiment, neutron irradiation to a total fluence of 2.4 × 10²⁰ n/cm² with equal fast and thermal neutrons was also performed on a diamond epilayer without cooling during irradiation. The formation of defects in the diamond films was characterized using Raman, FTIR, photoluminescence, electron paramagnetic resonance spectroscopy, and X-ray diffraction. It was found that defect configurations in diamond responsible for an increase in continuum background in the one-phonon region of Raman spectrum were absent in the films that have been cooled. The FTIR peak at 1530 cm^− 1 annealed in the sample irradiated to a fluence of 2.6 × 10¹⁹ n/cm² indicating that the sample reached a temperature of 300 °C during irradiation. Absence of characteristic infrared absorption peaks that were observed only upon annealing neutron irradiated diamond is used to conclude that the temperature of the sample during neutron irradiation to a fluence of 2.6 × 10¹⁹ n/cm² was well below 650 °C needed for mobility of defects and accumulation of stable unrecoverable damage. On the other hand, results from diamond epilayer subjected to equal thermal and fast neutron fluence of 2.4 × 10²⁰ n/cm² and without cooling showed that defects formed from displaced carbon atoms became mobile and formed complex configurations of irrecoverable damage. Electrical conductance of the unirradiated and irradiated diamond samples was measured as a function of temperature to determine the compensation of the p-type by the n-type charge carriers.     

Mechanical properties of fusion welded ceramics in the SiC-ZrB2 and SiC-ZrB2-ZrC systems

Mechanical properties of welded SiC-ZrB₂ and SiC-ZrB₂-ZrC ceramics were measured up to 1700 °C. Commercial powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics using either tungsten inert gas welding or plasma arc welding. Toughness of the parent materials was 3–4 MPa*m^1/2 which decreased after welding to 2–2.5 MPa*m^1/2. Strength of the SiC-ZrB₂-ZrC parent material was ~700 MPa at 25 °C, ~300 MPa at 1700 °C, and retained 40–60% of this strength once welded. Strength of the SiC-ZrB₂ parent material was ~600 MPa at 25 °C and 1700 °C and retained 20–30% of this strength once welded. Griffith analysis indicated that the strength in the parent materials was controlled by the size of SiC clusters while strength of welds was controlled by the size of pores in fusion zones. Therefore, removal of pores in produced fusion zones should be investigated to improve strength of future ceramic welds.     

Zirconium diboride laminates for improved damage tolerance at elevated temperatures

The mechanical response was studied for dense laminates containing layers of ZrB₂ (~145 µm) and graphite—10 vol% ZrB₂ (~20 µm). Individual layers were formulated by mixing starting powders with thermoplastic polymers and pressing into sheets. Laminates were produced by stacking and warm pressing the sheets, debinding, and hot pressing at 2050°C, 32 MPa, in Ar. The laminates were fractured at temperatures up to 2000°C in Ar. Laminates exhibited room temperature flexure strength of 260 MPa, increasing to 300 MPa at 1600°C, and then decreasing to 160 MPa at 2000°C. Inelastic work of fracture was 0.6 kJ/m² at room temperature, reached a maximum of 1.3 kJ/m² at 1400°C, and reverted to linear elastic failure at 2000°C. During fracture, cracks were deflected at the interfaces between the strong ZrB₂ layers and the relatively weak C-ZrB₂ layers, which led to an increased inelastic work of fracture by more than an order of magnitude compared to conventional ZrB₂ ceramics. This study demonstrated that laminate architectures are a promising approach for improving the damage tolerance of ZrB₂-based ceramics at elevated temperatures.     

Preparation and properties of B6O/TiB2-composites

B₆O/TiB₂ composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B₆O material showed that the addition of TiB₂ in a non-reactive sintering route promotes the B₆O densification. Further improvement was obtained by sintering reactive B–TiO₂ mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m^1/2 (SEVNB) and 2.6–5.0 MPa m^1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV_0.4) and 28 GPa (HV₅) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10^?6/K, respectively.     

A low temperature approach for photo/cathodoluminescent Gd2O2S:Tb (GOS:Tb) nanophosphors

Titrating the aqueous solution of equimolar RE(NO₃)₃ and (NH₄)₂SO₄ with NH₄OH to pH~9 at ~4°C produced an amorphous precursor that yielded phase-pure and well-dispersed RE₂O₂S nanopowder (RE = Gd_0.99Tb_0.01; GOS:Tb) via a RE₂O₂SO₄ intermediate upon annealing in H₂. The powders calcined at the typical temperatures of 700/1200°C exhibited unimodal size distributions and have the average crystallize sizes of ~17/55 nm, average particle sizes of ~284/420 nm, and specific surface areas of ~14.62/4.53 m²/g (equivalent particle sizes: ~56/180 nm). The 1200°C product exhibited sharp green luminescence at ~544 nm (FWHM = 2.3 nm; λ_ex = 275 nm), with an absolute quantum yield of ~24.8% and a fluorescence lifetime of ~1.34 ms at room temperature. It was also shown that the powder possesses favorable thermal stability (the activation energy for thermal quenching of luminescence ~0.305 eV) and is stable under electron beam irradiation up to 7 kV and 50 μA. The synthetic technique has the advantages of scalability and favorable dispersion and high chemical/phase purity for GOS powder, which may allow the sintering of scintillation ceramics at lower temperatures.     

Synthesis of hydrophobic W18O49 nanorods for constructing UV/NIR-shielding and self-cleaning film

Nanomaterials with ultraviolet/near-infrared (UV/NIR) shielding property have great potential for developing energy-saving windows. In this work, we report low-cost W₁₈O₄₉ nanorods as UV/NIR shielding material. W₁₈O₄₉ nanorods with the length of ~20 or ~60 nm were prepared by simple solvothermal method, and they exhibited strong size-dependent absorption in the UV/NIR region. By mixing W₁₈O₄₉ nanorods with polydimethylsiloxane (PDMS), W₁₈O₄₉@PDMS films were constructed and they could shield 55.58% of UV and 75.89% of NIR light while transmit 58.03% of visible light. A sealed box with W₁₈O₄₉@PDMS-coated glass as the window exhibited a minimal temperature elevation (△T = 9.2 °C) compared to those coated with pure glass (△T = 18.2 °C) or ITO glass (△T = 12.1 °C), under the irradiation of solar light (0.6 W cm⁻²). Additionally, the films had a contact angle of 122 ± 2°, showing self-cleaning ability. Therefore, W₁₈O₄₉@PDMS films can act as cost-efficient UV/NIR-shielding and self-cleaning film.     

Development of spark plasma sintered conductive SiC–TiB2 composites for electrical discharge machining applications

Highly dense electrically conductive silicon carbide (SiC)–(0, 10, 20, and 30 vol%) titanium boride (TiB₂) composites with 10 vol% of Y₂O₃–AlN additives were fabricated at a relatively low temperature of 1800°C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed β→α phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB₂ content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m^1/2. The electrical conductivity increased to a remarkable 2.72 × 10³ (Ω cm)^–1 for SiC–30 vol% TiB₂ composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC–TiB₂ composites toward extending the application regime of conventional SiC-based ceramics.     

Synthesis and performance of TiB2–B4C composites by high pressure of TiC–B mixture

TiB₂–B₄C composites were in situ synthesized and consolidated by high pressure synthesis method from a mixture of TiC and B powders at the pressure and temperature of 5.0 GPa and 1500℃-1900℃. The phase composition, microstructure, density, hardness, thermal conductivity, and electrical resistivity of TiB₂–B₄C composites were analyzed. As the increase in the synthesis temperature, the products were TiB₂ and B₄C phases and that crystallinity improved. TiB₂–B₄C composites were dense without obvious pores. TiB₂–B₄C composites synthesized at 1800℃ obtained the optimized performance, including the relative density of 98.2%, the Vickers hardness of 31.7 ± 1.2 GPa with the load of 9.8 N, the thermal conductivity of 30.3 ± 0.7 W/(m K), and the electrical resistivity of 3.3 × 10⁻³ Ω cm, respectively. The grain size of the TiB₂–B₄C composites changed with the increase in synthesis temperature, leading to the changes in hardness, thermal conductivity, and electrical resistivity.     

Oxidation resistance of AlN–(TiB2–TiSi2) ceramics in air up to 1450 °C

With the aid of DTA, TG, XRD, SEM and EPMA methods the kinetics and mechanism of oxidation of both composite powders and monolithic ceramics of AlN–(TiB₂–TiSi₂) system with different content of components were studied in the air up to 1450 °C under isothermal and non-isothermal conditions. It was established that the oxidation isotherms of monolithic ceramics follow the parabolic and paralinear rate law. According to the kinetic data and results of investigation of composition, morphology and structure of oxide films that are formed at different temperatures, the AlN–based ceramics containing up to 30% (TiB₂–TiSi₂) solid solutions are the corrosion-resistant and have the high adhesion of oxide layer in relation to substrate material. The activation energies of oxidation calculated for ceramics with 10% (TiB₂–TiSi₂) are: E₁ =180 kJ/mol for the temperature range up to 1300 °C and E₂=630 kJ/mol at 1350–1450 °C. The change of activation energy value is associated with the change of oxide layer composition: at the oxidation till 1300 °C the oxides of individual elements are, mainly, formed on the samples while above 1350 °C the formation of very dense surface film containing β-tialite Al₂TiO₅ takes place.