Processing of orthotropic and isotropic superhard B4C composites reinforced with reduced graphene oxide

A fabrication route based on aqueous colloidal processing plus transient liquid-phase assisted spark-plasma-sintering (SPS) with Ti-Al additives is described for the environmentally friendly obtention of superhard B₄C composites reinforced with reduced graphene oxide (rGO) having orthotropic and isotropic microstructures. It is shown that the former, which have coarse rGO platelets preferentially aligned perpendicular to the SPS pressing direction, can be prepared from mixtures of B₄C and Ti-Al particles with a source of thick, large rGO nanoplatelets by imposing smooth co-dispersion conditions to avoid platelet re-exfoliation and fragmentation. The latter, which have fine rGO platelets randomly oriented, can be fabricated from mixtures of B₄C and Ti-Al particles with a source of thin, small rGO nanoplatelets by applying intensive sonication to promote platelet re-exfoliation and fragmentation during co-dispersion. Finally, it is shown that these orthotropic and isotropic B₄C/rGO composites are equally superhard, and that, as expected, their microstructures interact differently with the cracks. Finally, this processing route is simple, and easily adaptable/extensible to make other ceramic/rGO composites with orthotropic and isotropic microstructures.     

Fabricating toughened super-hard B4C composites at lower temperature by transient liquid-phase assisted spark plasma sintering with MoSi2 additives

Toughened, super-hard B₄C triplex-particulate composites were densified by spark plasma sintering with MoSi₂ additives (5, 10, and 15 vol.%) at temperatures in the range 1750–1850 °C at which the reference monolithic B₄C ceramics are porous. It is proved that MoSi₂ is a reactive sintering additive that promotes densification by transient liquid-phase sintering, thus yielding fully-dense B₄C-MoB₂-SiC composites at relatively lower temperatures. Specifically, the MoSi₂ first reacts at moderate temperatures (<1150 °C) with part of B₄C to form MoB₂, SiC, and Si. This last is a transient component that eventually melts (at ～1400 °C), contributing to densification by liquid-phase sintering, and then (at 1500–1700 °C) reacts with free C present in the B₄C starting powders to form more SiC, after which densification continues by solid-state sintering. It is found that these B₄C-MoB₂-SiC composites are super-hard (～30 GPa), tough (～3–4 MPa m^1/2), and fine-grained, a combination that renders them very appealing for structural applications. Finally, research opportunities are discussed for the future microstructural design of a novel family of toughened, ultra-hard/super-hard multi-particulate composites based on B₄C plus refractory borides and carbides.     

Unlubricated sliding wear of B4C composites spark-plasma sintered with Si aids and of their reference B4C monoliths

Two fully-dense B₄C–SiC composites were fabricated by spark-plasma sintering (SPS) from B₄C+Si powders, one superhard (i.e., ～28.7(8) GPa) with abundant SiC by SPS of B₄C+20vol%Si at 1400 °C and the other ultrahard (i.e., ～35.1(4) GPa) with little SiC by SPS of B₄C+4.28vol%Si at 1800 °C, and their unlubricated sliding wear was investigated and compared with those of the reference B₄C monoliths. It was found that the two B₄C–SiC composites underwent mild tribo-oxidative wear with preferential removal of the oxide tribolayer, with the one SPS-ed at 1400 °C from B₄C+20vol%Si being, despite its lower hardness and greater proneness to form oxide tribolayer, only slightly less wear resistant than the one SPS-ed at 1800 °C from B₄C+4.28vol%Si (i.e., ～1.0(5)·10⁷ vs 1.37(8)·10⁷ (N?m)/mm³). The reference B₄C monolith SPS-ed at 1400 °C is comparatively two orders of magnitude less wear resistant (i.e., ～1.70(6)·10⁵ (N?m)/mm³), attributable to its undergoing severe purely mechanical wear by microfracture-dominated three-body abrasion due to its very poor sintering (i.e., high porosity of ～33.5 %), poor grain cohesion, and low hardness (i.e., ～3.1(5) GPa). The reference B₄C monolith SPS-ed at 1800 °C, while equally or less hard (i.e., ～28.4(9) GPa) and slightly porous (i.e., ～5.3 %), is somewhat more wear resistant (～1.8(3)·10⁷ (N?m)/mm³) than the B₄C–SiC composites, attributable to its undergoing only mild purely mechanical wear by plasticity-dominated two-body abrasion without porosity-induced grain pull-out, but it requires SPS temperatures well above 1400 °C. Finally, relevant implications for the ceramics and hard-materials communities with interests in tribological applications are discussed.     

Transient liquid-phase assisted spark-plasma sintering and dry sliding wear of B4C ceramics fabricated from B4C nanopowders

With the motivation of developing B₄C composites with superior wear resistance for tribological applications, an ultrafine-grained (～200?300 nm) B₄C composite was fabricated, characterized microstructurally, and tested mechanically and tribologically. First, a well-dispersed powder mixture of B₄C nanopowders (～40 nm) with coarse Ti-Al powders (～38 μm) as transient liquid-phase sintering additives was environmentally-friendly prepared by aqueous colloidal processing, optimized by measurements of the zeta potential of dilute suspensions and rheological studies of concentrated suspensions. Second, the powder mixture obtained by freeze-drying was densified by spark-plasma sintering (SPS), identifying the optimal SPS temperature (1850°C) by measurements of density, hardness, and toughness. Third, the dry sliding-wear behaviour of the optimal superhard B₄C composite (～31.5 GPa) was investigated by pin-on-disk tests and observations of the worn surface, determining its specific wear rate (～4.4·10^?8 mm³/(N·m)) as well as wear mode (two-body abrasion) and mechanism (plastic deformation). And lastly, the wear behaviour of the ultrafine-grained B₄C composite was compared with that of a reference fine-grained (～0.7?0.9 μm) B₄C composite, finding that both have the same mode and mechanism of wear but with the former being more resistant than the latter (～2.3·10⁷ vs 1.9·10⁷ (N·m)/mm³). Implications for the fabrication of B₄C tribocomponents with greater superior wear resistance are discussed.     

Sliding-wear resistance of pure near fully-dense B4C under lubrication with water,diesel fuel,and paraffin oil

The sliding-wear resistance of pure near fully-dense B₄C is investigated, and the wear mode/mechanisms identified, under lubrication with water, diesel fuel, and paraffin oil. It is found that the wear is mild in the three cases, with specific wear rates (SWRs) of 10^?16–10^?17 m³/N m. Nonetheless, the wear resistance of the B₄C ceramic is one order of magnitude greater under oil lubrication (10¹⁶ N m/m³) than under water lubrication (10¹⁵ N m/m³), and twice as great for the specific case of paraffin oil than diesel fuel, attributable to the lubricant’s viscosity. It is also found that the wear mode is always abrasion, and that the wear mechanisms are plastic deformation and localized fracture with grain pullout. However, in agreement with the macro-wear data, the severity of the wear damage is lower under lubrication with paraffin oil, followed by diesel fuel, and lastly water. Finally, microstructural considerations are discussed with a view to enhancing the sliding-wear resistance of B₄C triboceramics.     

Comminution of B4C powders with a high-energy mill operated in air in dry or wet conditions and its effect on their spark-plasma sinterability

The comminution of a typical submicrometre B₄C powder with a high-energy mill (i.e., a shaker mill) operated in air in either a dry or a wet environment was investigated. It was found that dry shaker milling (i.e., high-energy ball-milling) is able to progressively refine the B₄C particles to the nanoscale. While this is accompanied by oxidation and aggregation, these are not serious drawbacks. Wet shaker milling in methanol (i.e., conventional ball-milling) resulted only in a moderate B₄C particle refinement with greater contamination by the milling tools, which limits its usefulness. It was also found that both dry and wet milling modify the B₄C crystal structure, attributable to carbon enrichment. Consequently, dry shaker milling was found to be more recommendable than wet shaker milling to provide B₄C starting powders with superior sinterability. A comparative densification study by spark-plasma sintering confirmed this recommendation, and also showed the usefulness of dry shaker milling to obtain refined B₄C microstructures for structural applications.     

Effect of high-energy ball-milling on the spark plasma sinterability of ZrB2 with transition metal disilicides

A wide suite of powder mixtures of ZrB₂ with five different additions of transition metal disilicides (MeSi₂; 5, 17.5, or 30 vol% MoSi₂; 17.5 vol% TaSi₂; or 17.5 vol% ZrSi₂) were prepared by high-energy ball-milling (HEBM) for different times, and their optimal densification temperatures were evaluated by dilatometric spark-plasma-sintering (SPS) tests to compare their sinterability. SPS densification tests at the so-determined optimal densification temperatures were also performed in selected cases. It was found that HEBM progressively enhanced the sinterability, especially once the ZrB₂ particle sizes were refined to the ultrafine range or below (<250 nm). It was also observed that the sinterability was enhanced (i) with the greater addition of MeSi₂, and (ii) with the lower refractoriness of the MeSi₂. Nonetheless, under long-time HEBM, the addition of harder, more refractory MeSi₂ or of softer, less refractory MeSi₂ is equally effective in terms of sinterability, both enabling low-temperature densification (SPS at ∼1200 °C).     

Processing of ZrB2 tribo-ceramics by reactive spark plasma sintering of ZrH2+2B subjected to high-energy pre-ball-milling

The reactive spark plasma sintering (RSPS) of monolithic ZrB₂ ceramics from ZrH₂+2B powder mixtures subjected to shaker pre-milling was investigated, and compared with other three sintering approaches. It was found that RSPS is optimal at 1850 °C, which results in fully-dense ZrB₂ ceramics with ∼20 GPa hardness. Comparatively, at 1850 °C RSPS from the simply-mixed ZrH₂+2B powder mixture, SPS from the commercial ZrB₂ powder, and SPS from the shaker-milled ZrB₂ powder result in non-dense (76.7–86.7%) and softer (6.0–11.8 GPa) ZrB₂ ceramics. Furthermore, the optimally RSPS-ed ZrB₂ ceramic was subjected to unlubricated sliding-wear tests against diamond under 40 N load for 1000 m of sliding, demonstrating that it is a promising tribo-ceramic that only undergoes mild tribo-oxidative wear at 10^–8 mm³/(N·m) in the form of a slight plasticity-dominated two-body abrasion with eventual formation and partial loss of a self-lubricating and protective oxide tribolayer.     

Pressureless ultrafast sintering of near-net-shaped superhard isotropic B4C/rGO composites with Ti-Al additives

Superhard composites of B₄C reinforced with randomly-oriented reduced graphene oxide (rGO) nanoplatelets are manufactured by a near-net-shape fabrication route based on three successive steps. Firstly, aqueous colloidal processing is used for the environmentally-friendly preparation of a semi-concentrated multi-component slurry (B₄C as main component, Ti-Al as sintering additive, and rGO as toughening reinforcement), whose suitability for wet shaping is demonstrated by rheological measurements. Secondly, slip casting is used to produce robust green parts with shapes on demand and microstructures free of macro- and micro-defects. And thirdly, pressureless spark-plasma sintering (PSPS) is used for the ultrafast and energy-efficient densification of the green parts with shape retention. Measurements of shrinkage and hardness, as well as the microstructural observations, are used to identify suitable PSPS temperatures leading to obtaining isotropic B₄C/rGO composites that are superhard and almost twice as tough as the monolithic B₄C ceramics.     

Fabrication of B4C ultrafiltration membranes on SiC supports

The fabrication of B₄C ultrafiltration membranes is described. Firstly, a semi-dilute B₄C slurry was environmentally-friendly prepared by aqueous colloidal processing, optimizing its dispersion by sonication, and used to deposit B₄C membranes onto SiC macro-porous supports by dip-coating. Secondly, the resulting green membranes were characterised microstructurally by scanning electron microscopy (SEM), and pressureless sintered within the intermediate sintering regime. Thirdly, the sintered membranes were calcined in air to clean them from possible free carbon in the smallest pores, with the optimal calcination conditions having been identified by thermogravimetry coupled with mass spectrometry. Next, the calcined, sintered membranes were characterised microstructurally by SEM, tested mechanically against scratching, and characterised texturally by capillary flow porometry, thus identifying the optimal among them. Lastly, as a complement to the fabrication study, the filtration permeability of the optimal membrane was evaluated using deionized water. This work thus paves the way towards the fabrication of ceramic membranes based on B₄C, lighter and potentially more durable than others, for filtration applications.     

Hard and tough ultrafine-grained B4C-cBN composites prepared by high-pressure sintering

Tough and hard ultrafine-grained B₄C-cBN composites were firstly fabricated by high-pressure sintering mixed B₄C and cBN nanopowders at 6 GPa and 1700 °C. The phase transition from cBN to hBN is avoided by high pressure during the sintering process. The effects of the cBN content on the densification and mechanical properties of B₄C-cBN composites were evaluated. The results indicated that the hardness of the as-fabricated composites increased gradually with the increase of cBN content. The composite composed of 50 wt.% cBN exhibited excellent comprehensive mechanical properties with relative density of 98.6 %, density of 2.9 g/cm³, Vickers hardness of 36.2 GPa and fracture toughness of 6.7 MPa·m^1/2. The introduction of superhard cBN maintained the lightweight and high hardness while enhancing the fracture toughness of the B₄C. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed cBN grains.     

Improving the dry sliding-wear resistance of B4C ceramics by transient liquid-phase sintering

A critical comparison is made between the dry sliding-wear resistance of a B₄C composite fabricated by transient liquid-phase sintering with Ti-Al intermetallic additive and two reference monolithic B₄C ceramics fabricated by solid-state sintering. It is shown that, as a consequence of its full densification and super-hardness, the B₄C composite is, despite containing secondary phases, markedly more wear resistant (significantly lower coefficient of friction, specific wear rate, worn volume, and wear damage) than the reference monolithic B₄C ceramic fabricated under identical spark-plasma-sintering (SPS) conditions, and at least as wear resistant as the reference monolithic B₄C ceramic fabricated at much higher SPS temperature. In all materials, wear is nonetheless mild and occurred by two-body abrasion dominated by plastic deformation at the micro-contact level plus, in the porous reference monolithic B₄C ceramic, three-body abrasion dominated by fracture. Implications for the lower-cost manufacture of superhard B₄C tribocomponents are discussed.     

Flash Sintering of dense alumina ceramic discs with high hardness

MgO-doped-Al₂O₃ ceramic discs were fabricated by flash sintering (FS) and pressureless sintering (PS). The results showed that MgO-doped Al₂O₃ exhibited typical characteristics of flash sintering under an electric field in excess 2500 V/cm. Compared with the PS- fabricated specimen, the flash sintered specimens exhibited sub-micron grains (≤760 nm) and homogeneous microstructures. The relative density of the ?ash sintered MgO-doped Al₂O₃ ceramics increased with current density, reaching 99.91 % when the current density increased to 7 mA/mm². The FS-fabricated sample exhibited higher hardness (21.02 GPa) and fracture toughness (3.46 MPa m^1/2) than PS-fabricated sample.     

Ultra-low wear B4C-SiC-MoB2 composites fabricated at lower temperature from B4C with MoSi2 additives

Seeking to fabricate B₄C composites that are even more superhard (>30 GPa) at lower cost, B₄C was transient liquid-phase assisted spark-plasma-sintered, somewhat counterintuitively, at lower temperature (<1750 °C) and with greater MoSi₂ aid content (>15 vol.%) than ever before. It was found that just 20 vol.% MoSi₂ aid enables the full densification of B₄C at 1700 °C, thereby avoiding the deleterious transformation β-MoB₂→α-MoB₂, having consumed the entire MoSi₂ to form MoB₂ and SiC. This maximizes the hardness (∼33 GPa) of these novel triplex-particulate B₄C-SiC-MoB₂ composites without penalizing their toughness (∼4.1 MPa⋅m^0.5). Also importantly, the dry sliding-wear of these novel composites was investigated for the first time, showing that they undergo only mild wear (specific wear rate of ∼10⁻⁸ mm³/(N⋅m)) by plasticity-dominated two-body abrasion. Moreover, it was demonstrated that they are much more wear resistant than porous B₄C monolithics fabricated under both the same and more demanding conditions, and at least as equally wear resistant as fully-dense B₄C monolithics and composites fabricated under more demanding conditions.     

B4C陶瓷的复合增韧

采用热压工艺制备的B4 C复相陶瓷的断裂韧性达到 12 .88MPa·m1 2 。加入的第二相颗粒SiC、TiC和B4 C基体之间热膨胀系数不匹配。试样冷却后 ,在SiC、TiC颗粒周围的基体内产生的残余应力导致裂纹偏转使材料的韧性得到提高。加入的α -Si3N4 转变为柱状的 β -Si3N4 ,有助于B4 C韧性的提高     

Preparation of dense SiHf(B)CN-based ceramic nanocomposites via rapid spark plasma sintering

Dense SiHf(B)CN-based ceramic nanocomposites were prepared by spark plasma sintering (SPS) using high heating rates (∼450 ^°C/min.) and high pressures (≥100 MPa). The obtained nanocomposites were investigated by X-ray diffraction, Raman spectroscopy and electron microscopy concerning their phase evolution and microstructure.The hardness and the elastic modulus of dense SiHfCN were found to be 26.8 and 367 GPa, respectively. Whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa. The investigation of the oxidation of the prepared dense ceramic nanocomposites at high temperature revealed that the parabolic oxidation rates of SiHfCN were comparable to those of ultra-high temperature ceramics (UHTCs, e.g. HfC-20 vol% SiC); whereas the parabolic oxidation rates of SiHfBCN were several orders of magnitude lower than those. The results obtained within this study indicate the feasibility of SPS for rapid preparation of dense though nano-scaled Hf-containing ceramic nanocomposites that are promising candidates for high-temperature applications in harsh environments.     

掺硼改性碳化硼陶瓷的研究

以碳化硼粉、无定形硼粉为主要原料,热压烧结制备了B4C陶瓷.研究了不同的硼加入量和烧成温度对样品致密度和力学性能的影响.结果表明,当硼的加入量为11.6wt%时,样品的硬度、断裂韧性、抗弯强度和弹性模量分别达到32.9GPa、3.53MPa·m1/2、544.8MPa和405.3GPa.利用XRD、SEM测试手段对样品的物相组成和显微结构进行了分析.     

Controlling anisotropy of porous B4C structures through magnetic field-assisted freeze-casting

Anisotropic porous boron carbide (B₄C) structures were successfully produced, for the first time, using the magnetic field-assisted freeze casting method. The effect of the magnetic field on the structure and mechanical strength of the formed porous B₄C was compared for two different magnetic field directions that were either aligned with ice growth (vertical), or perpendicular to the ice growth direction (horizontal). It was shown that applying even a weak horizontal magnetic field of 0.1–0.3 T noticeably affected the alignment of mineral bridges between lamellar walls. Both the porosity and the channel widths decreased with increasing horizontal magnetic field strength. In the case of a vertical magnetic field, a larger strength of 0.4 T was required for highly aligned lamellar walls and larger channel widths. Compression strength tests indicated that the application of magnetic fields led to more homogeneously aligned channels, which resulted in increased compression strength in the longitudinal (parallel to the ice growth) direction. Applying a vertical magnetic field of 0.4 T with a cooling rate of 2 °C/min during the freezing step of the magnetic field-assisted freeze-casting method was found to result in the best conditions for producing highly anisotropic structures with large channel widths and fewer mineral bridges, which led to an increase in the mechanical strength.     

Manufacturing B4C parts with Ti-Al intermetallics by aqueous colloidal processing

The aqueous colloidal processing of submicrometre B₄C powder (∼0.6 μm) with coarse Ti-Al powder (∼40 μm) as sintering additive was investigated. Firstly, by measuring the zeta potential, pHs were identified that promote the individual colloidal stability of the B₄C and Ti-Al particles as well as their co-dispersion in water with two different deflocculants (one anionic and the other cationic). It was found that the anionic and cationic deflocculants shift the isoelectric points of B₄C and Ti-Al to more acidic and more basic pHs, respectively, making their co-dispersion possible at neutral pH. And secondly, by means of rheological studies, conditions were identified (sonication time, deflocculant type, and deflocculant content) that at quasi-neutral pH yield B₄C + Ti-Al shear-thinning concentrated suspensions (30 vol.% total solids) with low viscosity and small hysteresis loop. Interestingly, those deflocculated with the cationic polyelectrolyte had better rheological behaviour, being also less viscous and almost non-thixotropic. These suspensions were freeze-dried, obtaining powder mixtures that were compacted by conventional spark plasma sintering (SPS), and also slip-cast, obtaining robust green pieces that were densified by pressureless SPS. The two B₄C composites thus obtained are superhard, with Vickers hardnesses greater than 30 GPa.     

Effect of AlN addition on the electrical resistivity of pressureless sintered SiC ceramics with B4C and C

The effect of 0–12 wt% AlN addition on the electrical resistivity of SiC ceramics pressureless sintered with 0.7 wt% B₄C and 2.5 wt% C additives was investigated. The elemental analysis of SiC grains revealed a codoping of Al and N in the SiC lattice with a higher N concentration with 1 wt% AlN addition and a higher Al concentration with 12 wt% AlN addition. The electrical resistivity decreased by four orders of magnitude (1.7 × 10⁵ → 8.3 × 10¹ Ω cm) with 1 wt% AlN addition due to the increased carrier density (1.7 × 10¹⁰ → 2.3 × 10¹⁵ cm⁻³) caused by excess N-derived donors. However, subsequent AlN addition (4 → 12 wt%) led to an increase (2.9 × 10³ → 1.2 × 10⁴ Ω‧cm) in electrical resistivity due to (1) increased Al dopants which act as deep acceptors for trapping N-derived carriers causing a decrease in carrier density (2.3 × 10¹⁵ → 5.9 × 10¹³ cm⁻³), (2) the formation of electrically insulating SiC-AlN solid solution, and (3) the presence of electrically insulating AlN grains at the grain boundaries.