Effects of different milling conditions on the properties of lanthanum hexaboride nanoparticles and their sintered bodies

In this study, the preparation and consolidation of nanocrystalline LaB₆ powders originating from powder blends of La₂O₃, B₂O₃ and Mg were reported. A consecutive route of mechanochemical synthesis (MCS) and purification was utilized for the achievement of nano-sized LaB₆ powders. As-synthesized powders were leached out from intermediate reaction products or impurities. Then, a sequential step of cold pressing (uniaxial pressure at 800 MPa) and pressureless sintering (at 1700 °C for 5 h under Ar gas flow) were utilized for the consolidation of the purified LaB₆ powders. The type of mill (vibratory and planetary high-energy ball mills) was employed as a MCS parameter to reveal its effect on the physical, microstructural and mechanical properties of the LaB₆ powders, and their bulk structures. Compositional, physical and microstructural properties of the products after powder processing were determined via X-ray diffractometer (XRD), particle size analyzer (PSA), differential scanning calorimeter (DSC), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS) coupled with both SEM and TEM, and vibrating sample magnetometer (VSM). The bulk properties of the LaB₆ consolidated from nanocrystalline powders with a minimum 99.99% purity, and ∼62 nm (for vibratory ball mill) or ∼74 nm (for planetary ball mill) average particle size were compared according to various properties. LaB₆ powders were synthesized in planetary mill at an approximately six times longer duration than that of in vibratory mill. According to the results, density, surface area and mean particle size values of the vibratory ball-milled samples (containing paramagnetic powders) are better than those of planetary ball-milled (containing diamagnetic powders) ones. However, mechanical properties such as hardness, surface roughness, wear rate, friction coefficient, and also electrical conductivity were improved in the planetary ball-milled LaB₆ bulks.     

Mechanical and tribological properties of TiB2-SiC and TiB2-SiC-GNPs ceramic composites

TiB₂-SiC and TiB₂-SiC-graphene nanoplatelets (GNPs) composites were prepared using field-assisted sintering technology at 2100 °C in argon atmosphere, and the influence of the SiC and different GNPs addition on microstructure development, mechanical and tribological properties has been investigated. Instrumented hardness, bending strength, chevron-notched fracture toughness and ball-on-flat tribological tests were used for the testing and characterization of the composites. The addition of SiC significantly improved the bending strength and elastic modulus with values of 601 MPa and 474 GPa, respectively, but decreased the fracture toughness with a value of 4.8 MPa.m^1/2. The addition of GNPs has a positive effect on fracture toughness and flexural strength but a negative one on the hardness. The increasing amount of both GNPs has a positive influence on wear characteristics of the composites thanks to the described wear mechanisms.     

Enhanced machinability of SiCp/Al composites with laser-induced oxidation assisted milling

In this study, an innovative machining process called laser-induced oxidation assisted milling (LOM) is proposed. A polycrystalline diamond (PCD) cutting tool is applied to machine 55% SiC_p/Al composites. The laser-induced oxidation mechanism is investigated. Under the condition of average laser power of 10 W, laser scanning pitch of 15 μm, laser scanning speed of 6 mm/s and oxygen-rich atmosphere, the effect of laser-induced oxidation is optimal. A loose oxide layer and a sub-layer with the hardness of 160 ± 40 HV are generated where the composition of the oxide layer is mullite (2Al₂O₃·SiO₂). Comparative investigation on the cutting force, surface quality and tool wear are performed. Compared with the conventional milling (CM), the normal force and thrust force of LOM decrease by 39% and 55%, respectively. The reduction of cutting forces is attributed to thermal failure of the interface layer. The dominant surface defects of the machined surface are particle fracture, particle pull-out, matrix tearing and matrix coating. Among the investigated parameters, the minimum surface roughness S_a is 0.37 μm when the feed per tooth and the cutting depth are 0.005 mm/z and 0.2 mm, respectively. The dominant tool wear modes of LOM include diamond spalling and edge chipping. The tool wear modes of CM are diamond spalling, edge chipping, abrasive wear, and adhesive wear. LOM can prolong the tool life and achieve better surface quality under the same cutting length.     

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.     

Effect of SiC content on electrical,thermal and ablative properties of pressureless sintered ZrB2-based ultrahigh temperature ceramic composites

The effects of SiC content (10–40 vol.%) on electrical, thermal and ablation properties of pressureless sintered ZrB₂-SiC composites showing interfacial segregation of W-rich phases have been studied. The electrical resistivity was measured by four-probe method, whereas thermal diffusivity and coefficient of thermal expansion (CTE) were determined using laser-flash method and thermo-mechanical analyzer, respectively. Whereas thermal conductivities calculated from experimentally obtained thermal diffusivity values are found to be the highest for the ZrB₂-20 SiC composite, both electrical conductivity and CTE decrease with increasing SiC content. The specimens were subjected to thermal shock by soaking at 800–1200 °C, followed by water-quenching. Further, some specimens were exposed to oxyacetylene flame (2200 °C) for 10 min. The damage was estimated from changes in mass, Young’s modulus, and hardness. The highest thermal shock and ablation resistance have been observed for the ZrB₂-20 SiC composite, as thermal properties and formation of protective oxide scale play key role.     

High temperature erosion behavior of spark plasma sintered ZrB2-SiC composites

Damage of structural components of hypersonic vehicles by atmospheric particles demands thorough understanding on their wear behavior. In the present work, dense ZrB₂-SiC (10, 20, and 30 vol%) composites are prepared by spark plasma sintering at 55 MPa in two stages: 1400 °C for 6 min followed by 1600 °C for 2 min. With increase in SiC content, microstructures of sintered composites reveal strongly bonded ZrB₂ grains with SiC particles. A combination of maximum hardness of 23 GPa, elastic modulus of 398 GPa and fracture toughness of 5.4 MPa m^1/2 are obtained for the composite containing 30 vol% SiC particles. It is found that cracks are bridged or deflected by SiC particles in the composites. When the composites are subjected to SiC particle erosion at 800 °C, a 14% decrease in erosion rate is obtained with increase in SiC content from 10 to 30 vol%. The formation of large extent of boro-silicate rich viscous surface on eroded surfaces is attributed to reduced fracture or removal of ZrB₂ grains of the composites with increased SiC content.     

Hydroxyapatite/titania nanocomposites derived by combining high-energy ball milling with spark plasma sintering processes

Hydroxyapatite-reinforced nanocomposites with titania nanocrystals addition are prepared by a homogeneous mixing of hydroxyapatite nanoparticles and titania nanocrystals based on high-energy ball milling and spark plasma sintering processes. The microstructural and mechanical properties of the HA/titania composites are studied by X-ray diffractometry analysis, Raman spectrometry, and scanning electron microscopy. The hardness and Young's modulus of the composites are characterized by a nanoindenter and they show that the incorporation of the titania nanocrystals improves the mechanical properties of the composites obviously and the improvement should be ascribed to the main solitary effect of the ceramic as additives as well as a denser composites due to combining high-energy ball milling with spark plasma sintering techniques. The bioactivity of the HA/titania composites is evaluated by immersing the spark plasma sintering (SPS) compact disk in the simulated body fluid (SBF) and the results indicate that the bioactivity of the composites is related to the addition of titania by inducing apatite nucleation on the sample's surface after being immersed in SBF.     

Directional properties and microstructures of spark plasma sintered aluminum nitride containing graphene platelets

Graphene platelets (GPLs) containing aluminum nitride (AlN) composites were produced by using both pressureless sintering and spark plasma sintering (SPS). Poor densifications were obtained when composites were pressureless sintered whereas highly dense composites were successfully produced by using SPS. In addition, the applied uniaxial load in the SPS resulted in the orientation of GPLs in the microstructure of composites, indicating that composites would have anisotropic properties. All the mechanical, thermal and electrical properties in the in-plane direction were better than the through-plane direction. Fracture toughness of composites with the addition of 1 wt% GPLs were increased more than 30% compared to AlN matrix. Increased anisotropic effect with increasing amount of GPLs led to even larger differences on the thermal conductivities in through-plane and in-plane directions. AlN also became an electrically conducting material after ∼1 wt% GPLs addition in both through-plane and in-plane directions.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Enhanced densification of spark plasma sintered TiB2 ceramics with low content AlN additive

In the present study, we incorporated AlN (5 wt%) with TiB₂ ceramic and consolidated the mixture by relatively low temperature sintering method, resulting in a near fully dense composite. Monolithic TiB₂ and TiB₂–AlN (5 wt%) were manufactured by spark plasma sintering (SPS) at 1900 °C for 7 min under 40 MPa. The prepared composites were precisely characterized by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) analyses. In addition, possible chemical reactions during the sintering process were thermodynamically assessed using the HSC Chemistry software. The addition of AlN influenced the sinterability of titanium diboride, yielding a relative density of 99.7%. XRD results revealed the in-situ formation of h-BN during the sintering, whereas thermodynamic studies suggested the formation of both Al₂O₃ and h-BN. Furthermore, the microstructural investigation verified the synthesis of both Al₂O₃ and h-BN phases. Finally, the fractographical study revealed the effective role of AlN additive for refining the microstructure of TiB₂.     

Electrical conductivity and thermal properties of spark plasma sintered Al2O3-SiC-CNT hybrid nanocomposites

In this study, we report the electrical conductivity and thermal properties of Al₂O₃-SiC-CNT hybrid nanocomposites processed via ball milling (BM) and spark plasma sintering (SPS). The initial powders and consolidated samples were characterized using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM), respectively. A multifunction calibrator and a high-resolution digital multimeter were used to measure the electrical conductivity. The thermal properties were measured using a thermal constants analyser. The SiC and CNT-reinforced alumina hybrid nanocomposites exhibited a significant increase in their room-temperature electrical conductivity, which made them suitable for electrical discharge machining. The Al₂O₃-5SiC-2CNTs had a high electrical conductivity value of 8.85 S/m compared to a low value of 6.87×10⁻¹⁰ S/m for the monolithic alumina. The addition of SiC and CNTs to alumina decreased its room-temperature thermal properties. The increase in temperature resulted in a decrease in the thermal conductivity and thermal diffusivity but an increase in the specific heat of the monolithic alumina and the hybrid nanocomposites. These properties were correlated with the microstructure, and possible transport mechanisms were discussed.     

Production and characterization of spark plasma sintered (Ti,Nb)B2 solid solutions with graphene nanoplatelets and hexagonal boron nitride

For this study, (Ti,Nb)B₂ solid solutions were consolidated by spark plasma sintering. In addition, (Ti,Nb)B₂ with graphene nanoplatelets (GNPs) and hexagonal boron nitride (h-BN) were produced to evaluate the potential of the new structural materials. The phase formation, microstructure, mechanical properties, oxidation resistance and room temperature reflectance, and absorbance features of (Ti,Nb)B₂ were investigated. X-ray diffraction and Transmission electron microscopy observations showed that a complete solid solution phase was formed when the samples were sintered at 1850 °C for 5 min under 50 MPa. Ti_0.75Nb_0.25B₂ exhibited a relative density of ～98.6%, a hardness of ～20.5 GPa, and an indentation fracture toughness of ～3.4 MPa·m^1/2. It was found that the presence of 1 vol% h-BN as an additive enhanced the hardness (～10%) and fracture toughness (～30%) of Ti_0.75Nb_0.25B₂ by activating toughening mechanisms. The GNP added Ti_0.75Nb_0.25B₂ proved to have better oxidation resistance and optical absorbance than the other materials used in the study.     

Role of MgO on densification and mechanical properties in spark plasma sintered Os0.9Re0.1B2 ceramic

To promote the densification and therefore the mechanical properties of boride-based ceramics, MgO was added as sintering aid into Os_0.9Re_0.1B₂ powders for densification by using spark plasma sintering (SPS). The Os_0.9Re_0.1B₂ powders were synthesized by mechanochemical method from powder mixture of Os, Re and amorphous B. The role of MgO on densification, phase composition, microstructure and mechanical properties (hardness, fracture toughness and wear behavior) were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), micro indentation and ball-on-disk tribometer. The results show that, with the introduction of MgO as sintering aid, the relative density of the Os_0.9Re_0.1B₂ ceramic samples increased. When the MgO content reached 9 wt%, the as-sintered sample is almost fully dense. No obvious regularity was found from the samples with the addition of different content of MgO. Vickers hardness values of the samples with 0, 3 wt% and 9 wt% MgO are found to be very close with each other within the experimental error (~30 GPa), while the sample with the addition of 6 wt% MgO exhibits the highest hardness of ~35 GPa. The fracture toughness of the samples is decreased slightly with the addition of MgO. The friction coefficient and wear rate of the sample with the addition of 6 wt% MgO was also found to be the lowest among all samples, which indicate best wear resistance. As a whole, with the addition content of 6 wt% MgO, the Os_0.9Re_0.1B₂ ceramic sample performs relatively excellent mechanical properties among four groups of samples.     

Transmission electron microscopy characterization of spark plasma sintered ZrB2 ceramic

Microstructures of ZrB₂ ceramics consolidated by hot-pressing and spark plasma sintering were investigated by transmission electron microscopy (TEM), combining energy dispersive X-ray spectroscopy (EDX). The microstructures of both ceramics were compared. Amount of impurities was lower for ZrB₂ consolidated by spark plasma sintering than for hot-pressed ZrB₂. In particular, oxygen impurity was not detected even at the grain-boundaries in ZrB₂ consolidated by spark plasma sintering. The cleaning effect generated on the powder surfaces during spark plasma sintering cycle was displayed. In addition, dislocations were present only in the spark plasma sintered ZrB₂ ceramic, as a result of localized high stresses.     

High heterogeneous compatibility of HfB2-SiC ceramic and Zr-4 alloy with in-situ assembled interface

HfB₂-based ceramics, such as HfB₂-SiC, are promising materials of neutron control rods. Under nuclear conditions, the interfacial compatibility between the HfB₂-SiC control rod and the guide tube (made of Zr-4 alloy) is critical in the operation. The present study demonstrated the high compatibility of the two heterogeneous materials even at 1400 °C. The formation of an in-situ assembled triple diffusion layer with stable phases and dense structures largely improved the interface compatibility. In addition, the as-produced phases with unique microstructures were organized at the interface. ZrSi formed typical columnar crystals with strong [001] fiber texture, which extended in the direction favorable to the interface. ZrB₂ grains grew as needle shapes and entered the Zr-4 alloy substrate. These unique morphologies clearly revealed an interface with strong stability and high compatibility. The results provide the basis for the application of HfB₂-based ceramics in nuclear infrastructures.     

Influence of Ba2+ doping on the thermoelectric properties of BiCuSeO fabricated by spark plasma sintering

We studied the influence of Ba²⁺ doping on the thermoelectric properties of the p-type Bi_1–xBa_xCuSeO (0?≤?x?≤?0.21) fabricated by spark plasma sintering. The substitution of Ba²⁺ for Bi³⁺ gradually increased the electrical and thermal conductivities and decreased the Seebeck coefficient, which were due to the increased hole concentration. The largest value of dimensionless figure-of-merit (0.57) was obtained for the Bi_0.86Ba_0.14CuSeO at 500?°C, which was over three times greater than that of pristine BiCuSeO (0.18) at 500?°C. We believe that the thermoelectric properties of BiCuSeO were substantially enhanced through the partial substitution of Ba²⁺ for Bi³⁺.     

Novel structured spark plasma sintered thermal barrier coatings with high strain tolerance and oxidation resistance

Titanium alloys play an important role in lightweight aircraft engines owing to their low densities and high specific strengths. However, an increase in the thrust-to-weight ratio causes the engine operating temperature to be much higher than the service temperature, which deteriorates the oxidation resistance and mechanical properties. In this study, yttria-partially stabilised zirconia (8YSZ)/NiCrAlY thermal barrier coatings (TBCs) with a bimodal structure were prepared on Ti–6Al–4V by using spark plasma sintering (SPS) to improve the service temperature. The distinctive bimodal structure possessed dense particle contacts and a uniform distribution of porous nanoparticles, resulting in higher strain tolerance, sintering resistance, and lower thermal conductivity. Therefore, the bimodal structure prepared by lowering the SPS preparation temperature increased the high-temperature service time of TBCs on titanium alloy. The ceramic top coating (TC) and bond coating (BC) were well connected after isothermal oxidation at 800 °C for 100 h. The TBCs only shed 6% of their surface area at high temperature and large-angle bending. In addition, the bimodal-structured TBCs effectively improved the oxidation resistance of the Ti–6Al–4V substrate. The Ti–6Al–4V substrate with bimodal-structured TBCs only gained 0.51 times the mass gained by the bare Ti–6Al–4V after 100 h of isothermal oxidation.     

Influence of long term oxidation on the microstructure, mechanical and electrical properties of pressureless sintered AlN–SiC–MoSi2 ceramic composites

The effects of long term oxidation on the microstructural modification and on the electrical resistivity and mechanical strength of an AlN–SiC–MoSi₂ electroconductive ceramic composite are presented. The microstructure of the pressureless sintered composite is described and the oxidation behaviour is discussed. The formation of protective mullite layer at temperatures above 1000 °C provides good oxidation resistance for use at higher temperatures. At temperatures below 1000 °C, the AlN/SiC matrix disables the “pesting” phenomena and strength degradation, despite the fact that at these temperatures MoSi₂ oxidizes rapidly. The surface modification induced by oxidation on AlN–SiC–MoSi₂ composites does not affect the mechanical strength, while the electrical conductivity strongly decreases.     

Improvement of interfacial adhesion between oxide ceramic nanoparticles and epoxy resin by wet-jet milling

Although ceramic/polymer composites are useful for various applications, such as sensors, electronics, automobiles, and aerospace, the aggregation of nanoparticles can lead to the degradation of the mechanical and functional properties of the composites. To mitigate this, the interfacial adhesion between epoxy resin and the oxide ceramic nanoparticles γ-aluminum oxide (Al₂O₃), silicon dioxide, and magnesium oxide was strengthened by wet-jet milling (WJM) treatment without a chemical modifier. The WJM treatment of the slurry containing nanoparticles and epoxy resin led to the good adsorption of epoxy resin onto the nanoparticle surface, which significantly improved the mechanical properties of the composites. Throughout this process, the amount of epoxy resin adsorbed on the nanoparticle surface and the composite mechanical properties increased with increasing WJM processing pressure, owing to the increased contact between the nanoparticles and epoxy resin droplets and the reduced droplet size. Furthermore, poor solvent was found to be effective for the dispersal of the nanoparticles because the epoxy resin droplets in the slurry were more stable on the nanoparticle surfaces than those in the solvent. When Al₂O₃ nanoparticles were used as a filler, the amount of epoxy resin adsorbed increased from 3.7 to 70.6 mg g⁻¹, and the composite tensile strength increased from 67.1 to 100.3 MPa in poor solvent and under high WJM processing pressure. This optimized WJM treatment will lead to improvements in the mechanical and functional properties of various composite materials.