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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 ( 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⋅m0.5). 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.

Superhard B4C parts with microarchitectures constituted by ceramic layers and evenly-spaced rGO-enriched reinforcing interlayers were fabricated for the first time. To this end, a concentrated slurry of B4C with its Ti-Al sintering additive was first prepared by aqueous colloidal processing, optimizing its total solids loading and content of both binder and plasticizer to obtain, by tape casting, handleable and flexible green tapes. A semi-dilute aqueous suspension of B4C with Ti-Al and abundant GO was also prepared to dip-coat those green tapes with a GO-enriched layer, optimizing the withdrawal rate and the dipping time. The bare and GO-coated B4C+Ti-Al tapes were then sequentially laminated, thus yielding green multilayered laminates that finally were appropriately debinded and densified by spark plasma sintering. Vickers indentation tests demonstrated that these multilayered laminates are superhard (∼31 GPa), and that their rGO-enriched reinforcing interlayers are effective in arresting crack propagation.

An ultrahard ZrC–Co cemented carbide was fabricated by high-energy ball-milling (HEBM) of micrometric starting powders and spark plasma sintering (SPS), whose ultrafine-grained microstructure makes it comparatively more sliding-wear resistant. First, it is shown that prolonged HEBM activates mechanically the ZrC+Co powder mixture, enhancing markedly its SPS densifiability (notable reduction of the SPS temperature attributable to the lowered eutectic temperature and speeded kinetics of solution-reprecipitation). The combination of refined particle sizes and enhanced sinterability yields ultrafine-grained ZrC–Co cemented carbides with ultrahigh hardness (~18 GPa) and sufficient fracture toughness (~6 MPa·m1/2). And second, it is shown that dry wear of the ZrC–Co cemented carbides is, regardless of the microstructure, mild (10−7 mm3/(N·m)) and occurs essentially by plasticity-dominated abrasion, but also that their microstructural refinement makes them comparatively much more wear resistant and therefore attractive for tribological applications.

The microstructural development during crystallization firing of a commercially-available dental-grade nanostructured lithia-zirconia glass-ceramic (Vita Suprinity® PC) was unraveled using a wide battery of ex-situ and in-situ characterization techniques. It was found that the milling blocks are slightly crystallized glass-ceramics, with a complex chemical composition and consisting of partially de-polymerized glass plus lithium silicate (Li2SiO3) nanocrystals. It was also found that during crystallization firing the glassy matrix first reacts with part of the Li2SiO3 to form lithium disilicate (Li2Si2O5) at ∼810−820 °C, and then lithium orthophosphate (Li3PO4) precipitates from the glass. This results in glass-ceramics with abundant nanocrystals embedded in a sparse zirconosilicate glass matrix (containing many other cations subsumed) that, due to its high viscosity, inhibited crystal growth. Therefore, these dental glass-ceramics are not reinforced with zirconia (ZrO2) crystals unless over-fired above ∼890 °C and at the expense of its singular nanostructure. Finally, this study opens doors for optimizing the clinical performance of these dental glass-ceramics via microstructural tailoring.

The dry sliding-wear behaviour of a zirconia doped with 3 mol% yttria (3Y-TZP) monolithic and of two 3Y-TZP composites, these latter with the same ultrafine-grained microstructure as the former but reinforced with either 1-D carbon nanofibres (CNFs) or 2-D reduced graphene oxide (rGO) nanoplatelets, was evaluated by ball-on-disk tests at moderate load, and compared critically. It was found that 3Y-TZP, 3Y-TZP/CNFs, and 3Y-TZP/rGO undergo mild wear, in the three cases by abrasion with contributions from both plastic deformation and fracture (with varying severities depending on the sample). It was also found that wear resistance does not correlate with hardness or toughness, but with whether or not there is formation of self-lubricating tribofilms on the contact surface. Specifically, once pulled-out, 2-D rGO nano-reinforcements impose solid-state lubrication that reduces the coefficient of friction (CoF), thus providing 3Y-TZP/rGO with superior wear resistance relative to both 3Y-TZP and 3Y-TZP/CNFs. 1-D CNF nano-reinforcements, however, do not form such tribofilms, or hardly do so, thus having no effect on the CoF and wear resistance. Implications are discussed of both the dimensionality of the carbon-based nano-reinforcements and the testing conditions for the microstructural design of ceramic composites for tribological applications.

The densifiability by spark plasma sintering (SPS) and the dry sliding-wear behaviour of novel cemented carbides based on micrometre-sized ZrC with 16.7 vol.% Co binder (ZrC–Co), compositionally equivalent to the typical WC–10 wt%Co but with ZrC in place of WC, were investigated. Firstly, detailed analysis of both the densification and the shrinkage-rate curves registered during SPS (at 50 MPa) showed that ~1275 °C is the onset SPS temperature for liquid-phase sintering, and ~1600 °C that of full densification. Hardness measurements by Vickers indentation tests confirmed the gradual densification with increasing SPS temperature up to 1600 °C, a condition yielding an extra-coarse-grained (~7–8 μm grain size), ultrahard (~18.2 ± 0.3 GPa) ZrC–Co cemented carbide that is also sufficiently tough (~6.1 ± 0.2 MPa m1/2). And secondly, pin-on-disk tribological tests (at both 20 N for 1000 m and 40 N for 2000 m) and subsequent characterization of the worn surfaces indicated that, under sliding contact, these ZrC–Co cemented carbides wear essentially by two-body abrasion dominated by plastic deformation. Wear increases with increasing severity of the contact conditions, but is nonetheless mild due to the high hardness of these ZrC–Co cemented carbides and to the formation of discontinuous oxide tribolayers.

In the quest for novel ceramics for tribological applications via bioinspired design, the differences in the fracture modes that arise upon scratching relevant locations of ceramic-like tooth enamel are investigated. It is found that fracture initiates from weak rod-sheath interfaces at relatively low loads, independent of the sliding direction. However, the geometry and propagation of the cracks depends on the orientation of the interfaces relative to the maximum tensile stress: scratching along the occlusal surface propagates approximately sinusoidal cracks, parallel to the sliding direction, while scratching along the cross-section produces straight cracks that propagate normal (scratch parallel to occlusal surface) or parallel (scratch perpendicular to occlusal surface) to the sliding direction. The formation of cracks is hindered in scratching near the enamel-dentine junction. Implications for the microstructural design of triboceramics (bulks and coatings) with improved durability are discussed.

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.

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 (

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.

A study was made of the influence of the substrate (fused quartz, sapphire, soda-lime glass, and stainless steel AISI-310) and sintering temperature (300 °C, 500 °C, and 800 °C) on the thickness and number of layers of 3YSZ multilayer sol-gel coatings that can be deposited by dip-coating without the coating cracking. It was found that the thickness and the number of layers increase with the higher sintering temperature and the greater substrate's thermal expansion coefficient. These trends were explained as resulting from the residual in-plane stresses generated in the sol-gel coating when cooled from the sintering temperature down to room temperature due to its thermal expansion coefficient differing from that of the substrate. Thus, the smaller the residual in-plane tensile stress or the greater the residual in-plane compressive stress, the greater can be the thickness and number of layers of the sol-gel coating. The same is the case for higher sintering temperatures because the densification of the layers either predominates over the increased in-plane tensile stresses or combines synergistically with the greater in-plane compressive stresses.

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.

This study clarifies, for the first time, that misplacement of Cr3+ ions at the Na+ site plays a more important role in dictating the specific capacity and capacity retention over cycles of O3-NaCr...

OBJECTIVE. To elucidate the compositional and microstructural developments of a novel lithium silicate glass-ceramic during its crystallization cycle. METHODS. Blocks of a lithium silicate glass-ceramic (Obsidian®, Glidewell Laboratories) were cut into 1 mm thick plates and polished to 1 μm finish. Some of them were crystallized prior to polishing. Firstly, ex-situ compositional and microstructural characterizations of both the pre- and post-crystallized samples were performed by wavelength dispersive X-ray fluorescence, field-emission scanning electron microscopy (FE-SEM), and X-ray diffractometry (XRD). Secondly, the pre-crystallized samples were subjected to in-situ compositional and microstructural characterizations under non-isothermal heating by simultaneous thermogravimetry/differential scanning calorimetry, X-ray thermo-diffractometry, and field-emission scanning electron thermo-microscopy. RESULTS. The microstructure of pre-crystallized Obsidian® consists of an abundant population of perlitic-like/dendritic lithium silicate (Li(2)SiO(3)) nanocrystals in a glass matrix. Upon heating, the residual glassy matrix does not crystallize into any form of SiO(2); elemental oxides do not precipitate unless over-heated above 820°C; and the Li(2)SiO(3) nanocrystals do not react with the glassy matrix to form typical lithium disilicate (Li(2)Si(2)O(5)) crystals. Nonetheless, the Li(2)SiO(3) nanocrystals grow and spheroidize through the solution-reprecipitation process in the softened glass, and new lithium orthophosphate (Li(3)PO(4)) nanocrystals precipitate from the glass matrix. SIGNIFICANCE. The identification of compositional and microstructural developments of Obsidian® indicates that, by controlling the firing conditions, it is possible to tailor its microstructure, which in turn could affect its mechanical and optical properties, and ultimately its clinical performance.

There is growing interest in using 2D graphene-related reinforcements to toughen brittle ceramics in nanocomposites. However, there is a lack of fundamental understanding of the toughening mechanisms and microstructural effects in such nanocomposites. To address this paucity, fully-dense nanocomposites of aluminum oxide (Al2O3) matrix and reduced graphene-oxide (rGO) reinforcements (~5 vol%) of different average-thicknesses and orientations are fabricated and characterized. The interactions between stably propagating cracks and rGO in the Al2O3/rGO nanocomposites are observed in situ inside a scanning electron microscope (SEM). Toughening by pullout of thick rGO in the crack-tip wake in the cross-section orientation is found to be the most effective, which is consistent with the highest fracture toughness (KIC~6.7 MPa.m0.5) measured in those Al2O3/rGO nanocomposites. Interestingly, upon unloading and reloading, the intact rGO crack-bridges appear to crinkle and uncrinkle without a remnant crease, respectively, which is a unique deformation property of multi-layer graphene-like materials. This points to a possible new cyclic-fatigue resistance mechanism in those nanocomposites. Sliding-wear properties of the Al2O3/rGO nanocomposites are also studied, where the hardness and microstructural heterogeneities are found to play dominant roles. The results from this study have implications for the creation of high-toughness, fatigue-resistant, and wear-resistant graphene-reinforced ceramic nanocomposites of the future.

The lubricated sliding-wear resistance of a fine-grained B4C composite fabricated by transient liquid-phase assisted spark-plasma sintering with Ti–Al additives at smooth conditions is critically compared to that of its reference monolithic B4C ceramic. It is shown that the former has an excellent sliding-wear resistance that exceeds that of the later by one order of magnitude (far less specific wear rate, worn volume, and wear damage), attributable to its greater hardness and densification. The wear mode is abrasion dominated by plastic deformation, plus localized microfracture in the monolithic ceramic. Implications for fabricating highly wear-resistant tribocomponents based on superhard ceramics are presented.

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.

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.

13-93 bioglass (BG) scaffolds reinforced with graphene oxide (GO) were fabricated by robocasting (direct-ink-writing) technique. Composite scaffolds with 0–4 vol% content of GO platelets were printed, and then consolidated by pressureless spark plasma sintering at 650 °C. It was found that, despite hampering densification of the bioglass, the addition of GO platelets up to a certain content enhanced the mechanical performance of the 13–93 bioglass scaffolds in terms of strength and, especially, toughness. Best performance was obtained for 2 vol.% GO, which increased strain energy density (toughness) of the scaffolds by ∼894%, and their compressive strength by ∼26%. At higher contents, agglomeration of the nanoplatelets and increased porosity significantly reduced the mechanical enhancement obtained. Implications of the results on the fabrication of novel bioglass scaffolds that may find use in load-bearing bone tissue engineering applications are discussed.

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 (

Analytical expressions are formulated that enable the lognormal distribution of nanocrystallite sizes and the mean-squared crystal lattice microstrain in both doped and undoped nanocrystalline ceramics to be routinely determined by X-ray diffractometry. Importantly, these expressions, which are also applicable to most well-annealed nanocrystalline metals/alloys, are developed under the consideration that both the experimental and instrumental X-ray diffraction peaks are modeled by split Voigt functions to account for the (hitherto widely ignored) intrinsic peak asymmetry. The general analytical expressions of this new single-peak line-broadening method are then particularized to the case of small nanocrystallites with uniform composition for which a reasonable simplification (i.e., symmetrization of the experimental peak) can be made without compromising the accuracy of the results. Next, as an example of application the method developed is applied to the characterization of a nanocrystalline ceramic powder, and the nanocrystallite sizes so-determined are validated by transmission electron microscopy. The more complete and reliable microstructural information offered by this single-peak line-broadening method could contribute towards the accurate characterization of both doped and undoped nanocrystalline ceramics, and of most well-annealed nanocrystalline metals/alloys as well, a topic of particular research interest today.

Some crystallographic considerations on the novel orthorhombic ZrO2 (o-ZrO2) stabilized at ambient conditions by Ta substitutional doping are presented. Specifically, how the Ta-doped o-ZrO2 was observed for the first time by conventional X-ray diffractometry (XRD) is reported, after which the ab initio and non-ab initio procedures used to resolve its crystal structure are detailed. The crystal structure is then explicitly reported (i.e., space group and cell parameters, as well as atomic positions of the asymmetric unit). It is also shown that this novel Ta-doped o-ZrO2 has a crystal structure similar, but not identical, to the high-pressure o-ZrO2. Additionally, given that the crystal structure of the Ta-doped o-ZrO2 is identified as being a symmetry-breaking distortion of the typical tetragonal ZrO2 (t-ZrO2) stabilized with the commonest dopants, a complete analysis is also made of the structural distortions relating both the Ta-doped o-ZrO2 and the high-pressure o-ZrO2 to the t-ZrO2. Finally, a justification for the apparent greater immunity of the Ta-doped o-ZrO2 to aging is provided.

A novel fine-grained orthorhombic ZrO2 ceramic stabilized with 12 mol% Ta doping was fabricated by spark-plasma sintering from home-made powders, and its high-temperature mechanical properties evaluated for the first time by compressive creep tests in both Ar and air. It was found that the high-temperature plasticity of the ceramic deformed in Ar, under which the Ta-doped orthorhombic ZrO2 is a black suboxide with abundant oxygen vacancies in its crystal structure, is controlled by grain boundary sliding (stress exponent ∼2, and activation energy ∼780–800 kJ/mol). However, the high-temperature plasticity of the ceramic deformed in air, under which the Ta-doped orthorhombic ZrO2 is a white oxide due to the elimination in situ of oxygen vacancies, is controlled by recovery creep (stress exponent ∼3, and activation energy ∼750 kJ/mol). It was also observed that black Ta-doped orthorhombic ZrO2 is more creep resistant than its white counterpart with the same grain size, and that the former deforms as the more conventional Y2O3-stabilized ZrO2 does., This work was supported by the Junta de Andalucía under Grant nº P12-FQM-1079 and by the Junta de Extremadura under Grant nº GR15078, co-financed with FEDER Funds. Financial support from the Ministerio de Economía y Competitividad (Government of Spain) and FEDER Funds under Grants nº MAT2015-71411-R and MAT2016-76638-R is gratefully acknowledged as well.

Robocasting and pressureless spark plasma sintering are combined for the first time to fabricate geometrically-complex B4C components. It is shown that robocasting allows B4C green pieces to be printed with near-net shape from inks with suitable rheological properties, and that subsequent pressureless spark plasma sintering permits an ultrafast, energy-efficient, solid-state densification that yields B4C parts with adequate mechanical properties. Furthermore, the usefulness of cold-isostatic pressing to improve the densification of the pieces is evaluated, and the benefits of robocasting over conventional dry powder compaction are identified. Finally, the scalability for the production of large B4C pieces is discussed.

Newtonian nanoparticulate TiO2 sols without and with Pr3+ doping (0.5, 1, 2, 3, 5, and 10 mol%) were synthesized by the aqueous colloidal sol-gel method, next elucidating that the Pr3+ doping increasingly retards their peptization while makes them more viscous at high doping contents. Subsequently, undoped and Pr3+-doped TiO2 xerogels were obtained by drying the colloidal sols, demonstrating that they crystallize as tiny nanocrystals of anatase with some brookite, with the Pr3+ cations in substitutional solid solution within the TiO2 crystal lattice. It was also shown that moderate calcination (i.e., at 400–500 °C) causes only a slight nanocrystal growth, and a minor transformation to rutile in the xerogels with lowest Pr3+ doping proportions (i.e., ≤1 mol%). Finally, a preliminary study of the photocatalytic activity under ultraviolet-visible light irradiation of the undoped and Pr3+-doped TiO2 xerogels calcined at 400 °C for 10 min was performed. The xerogel doped with 0.5 mol% Pr3+ and calcined at 400 °C ranks first and that those with Pr3+ doping proportion above 1 mol% behave worse than the undoped one. Also, calcination at 500 °C was found to worsen the photocatalytic activity. The photocatalysis observations were rationalized on the basis of the dependence of the polymorph composition, crystal size, bandgap, and specific surface area as a function of the dopant content.

In this study, we report the effects of structural defects on the electrochemical cycle stability of Na-ion batteries made of NaCrO2 cathodes. Three processing conditions are used to create NaCrO2 with different structural defects. One condition entails high-energy ball milling of reactants before the solid-state reaction at 900 °C, the second one has no high-energy ball milling but simple mixing of reactants before the solid-state reaction, and the last one has high-energy ball milling after the solid-state reaction. The NaCrO2 crystals derived from the process with high-energy ball milling of reactants before the solid-state reaction exhibit the best bulk conductivity and highest specific capacity with little capacity decay over 46 charge/discharge cycles. In contrast, the NaCrO2 crystals derived from the process without high-energy ball milling have significant capacity decay, whereas the NaCrO2 crystals obtained from the process with high-energy ball milling after the solid-state reaction exhibits the lowest bulk conductivity and very low specific capacities. The different electrochemical properties exhibited by these NaCrO2 crystals have been ascribed to different extents of structural defects present in these crystals. This study unambiguously demonstrates that structural defects in NaCrO2 crystals are critical in achieving NaCrO2 electrodes with high specific capacity and excellent capacity retention.

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.

The effect of the Tb–doping content (in the range 0–100 mol%) on the structure (using Raman spectroscopy) and photoluminescence (PL) properties of both rhabdophane-type LaTbPO·nHO and monazite-type LaTbPO single-crystal nanorods was investigated. LaTbPO·nHO was directly obtained by microwave-assisted hydrothermal synthesis and LaTbPO by calcination of LaTbPO·nHO at 700 °C in air for 2 h. It was found that the characteristic Raman bands shift to higher wavenumbers and become broader with increasing Tb concentration, which is attributed to attendant unit-cell volume reduction. The PL due to the Tb f-f transitions has been studied with continuous and pulsed excitations. The PL intensity increases with doping and is maximum for LaTbPO·nHO and LaTbPO, i.e., for rhabdophane and monazite-type structures, respectively. A quenching effect is detected for concentrations below x=0.05, but constant efficiency is obtained at higher doping. The calcination increases the efficiency by a factor around 2 due to the combined effects of water molecules and defect elimination. The PL decay curves reveal a long lifetime attributable to single Tb ions which is almost independent of doping for monazite nanorods (5–4.5 ms), and a shorter one around 0.2 ms most likely due to Tb dimmers., This work was supported by the Ministerio de Economía y Competitividad (MINECO, Government of Spain) under Grants no. MAT2012-31090, MAT2015-65356-C3-1-R, MAT2016-78700-R and MINECO and FEDER Funds under Grant no. MAT2016-76638-R. Financial support from the Comunidad de Madrid under Grant no. P2013/MIT-2740 is also acknowledged.

45S5 bioactive glass composite scaffolds reinforced with reduced graphene oxide (rGO) were fabricated for the first time by robocasting (direct-writing) technique. Composite scaffolds with 0–3 vol.% content of rGO platelets were printed, and then consolidated by pressureless sintering at 550 or 1000 °C in Ar atmosphere. It was found that the addition of rGO platelets up to 1.5 vol.% content enhanced the mechanical performance of the 45S5 bioactive glass scaffolds in terms of strength and toughness. Best performance was obtained for 1 vol.% rGO, which yielded an enhancement of the fracture toughness of ∼850 and 380% for sintering temperatures of 550 and 1000 °C, respectively, while the compressive strength increased by ∼290 and 75%. rGO addition thus emerges as a promising approach for the fabrication of novel bioglass scaffolds with improved mechanical performance without deterioration of their bioactivity, which may then find use in load-bearing bone tissue engineering applications.

The appropriate conditions for liquid-phase assisted spark-plasma sintering (SPS) were identified for the fabrication of both SiC nanoceramics and their nanocomposites with carbon nanotubes (CNTs). A parametric study of the nanoceramics and nanocomposites with a given type of CNTs showed that the SPS temperature (as measured by the radial optical pyrometer) optimizing their densification, nanograin size, and mechanical properties is 1700 °C (soaking for a few minutes), below which there is incomplete densification, and above which there is obvious grain growth with no benefit in hardness or toughness in the case of the nanoceramics, and prejudicial to both properties in the case of the nanocomposites due to the CNT degradation. It was also shown that the nanocomposites have smaller nanograins than their nanoceramic counterparts, and are softer but tougher. Extension to nanocomposites with different types of CNTs confirmed these trends, and showed that the CNT features do not condition the densification, microstructure or mechanical properties of these nanocomposites.

SiC nanostructured ceramics without and with carbon nanotubes (CNTs) were prepared by aqueous colloidal processing plus liquid-phase assisted spark-plasma sintering, and their sliding wear behaviour compared. It was found that the SiC nanoceramic is highly resistant to wear, with specific wear rates in the order of 10−8 mm3/N m or lower under the tribological conditions used, but also that the SiC/CNT nanocomposite is even more so. Qualitatively, the nanocomposite displayed a simple wear curve with a steady-state wear rate, over the wear curve of the nanoceramic with two wear rates, the second being one order of magnitude greater than the first. Quantitatively, the nanocomposite wore less overall with smaller worn volume, and especially had a lower wear rate (particularly marked for moderate and prolonged sliding distances). It was also found that the superior wear resistance of the SiC/CNT nanocomposite is due to combined effects of lubrication and toughening provided by the CNTs, the former reducing the severity of the sliding-contact conditions and the latter hindering the propagation and coalescence of the short cracks responsible for nanograin pull-out. CNT addition thus emerges as a hopeful line for processing ceramic nanocomposites that are both toughened and highly wear resistant for use in contact-mechanical and tribological applications.

The effects were individually investigated of ZrC grain size and MoSi 2 content on the lubricated sliding-wear resistance of ZrC–MoSi 2 triboceramics densified by spark-plasma sintering (SPS). Microstructures with different average grain sizes were obtained by varying the high-energy ball-milling time of the ZrC–MoSi 2 powder mixtures and their SPS temperature, while the microstructures with different secondary phase contents were obtained by varying the relative concentrations of ZrC and MoSi 2 in the powder batches. Experimentally, it was found that the ZrC–MoSi 2 triboceramics are all highly wear resistant, but also that wear resistance decreases as the ZrC grain size and MoSi 2 content increase. The dominant wear mode was identified as abrasion, and the prevailing wear mechanism was plastic deformation accompanied by localized fracture. Accordingly, the wear trends are explained in terms of the hardness of the ZrC–MoSi 2 triboceramics. Finally, the implications are discussed of providing the electrically-conductive ZrC–MoSi 2 triboceramics with greater wear resistance via controlled processing.

B 4 C nanopowders were subjected to room-temperature methylation to assess its possible beneficial effect on single-step cycle spark-plasma sinterability. It was found that methylation removes the oxidic impurities, thus chemically purifying the B 4 C nanopowders without nanoparticle growth. Next, it was determined that the temperature of spark-plasma sintering optimizing the microstructure and mechanical properties is ∼1500 °C, below which there is insufficient densification and above which there is marked grain growth, scenarios both detrimental for the hardness of these B 4 C ceramics. It was also found that the so-purified nanopowders are more sinterable than the as-received nanopowders, and also than the same nanopowders purified by annealing under an inert atmosphere. This greater sinterability resulting from the elimination of the oxidic impurities is attributable to the enhanced grain-boundary diffusion favouring the densification. Thanks to the methylation, dense, nano-grained, superhard B 4 C ceramics can thus be fabricated with greater energy efficiency. Implications of interest for the ceramics community are discussed.

Thermal barrier coatings (TBCs) used in gas-turbine engines experience severe degradation by calcia-magnesia-alumino-silicate (CMAS) deposits during high-temperature operation. The present study identified and evaluated the chemical and microstructural changes in air plasma-sprayed (APS) 7 wt.% Y2O3 stabilized ZrO2 (7YSZ) TBCs caused by CMAS attack under isothermal conditions at 1340 °C. Additionally, a ‘model’ experimental study was conducted by characterizing 7YSZ ceramic powders immersed in molten CMAS glass at 1300 °C for different exposure times. The combined results from both studies highlight the importance of local CMAS glass composition on the 7YSZ/CMAS interaction. Specifically, low Y-content in the glass, caused by a relatively large glass 'sink,' produces Y-depleted ZrO2 grains that undergo tetragonal (t) → monoclinic (m) phase transformation upon cooling. Alternatively, small pockets of Y-enriched glass induce the formation of t″-ZrO2, a phase characterized by its high stabilizer content. After prolonged high-temperature exposure, solution-reprecipitation induces the formation of both m-ZrO2 and t″-ZrO2 throughout the APS 7YSZ TBC in accordance with the phase diagram. Using a thermomechanical model it is shown that the strain associated with the martensitic t→m phase transformation plays an important role in the delamination failure of TBCs attacked by CMAS.

The conditions under which ZrC triboceramics are fabricated (high-energy ball-milling and spark-plasma sintering (SPS)) lead to major microstructural differences. The effects of these conditions on the resulting ceramic׳s lubricated sliding-wear resistance were investigated. One condition studied entailed SPS of the as-purchased micrometre powder, giving super-coarse-grained ceramics with little residual porosity. Another was SPS at the same temperature for a shorter time, with the powder having been subjected to short high-energy ball-milling, yielding near-dense, coarse-grained ceramics. The last was with the powder subjected to prolonged high-energy ball-milling, followed by SPS in smother conditions in terms of temperature and time, resulting in fully-dense, fine-grained ceramics. It was found that, while all three were highly wear resistant, the ceramic obtained by combining prolonged ball-milling with smooth SPS was the most so, with less damage and a lower steady-state wear rate coefficient, ascribable to the greater hardness and toughness deriving from the microstructural refinement and densification. The results therefore demonstrate the importance of the processing conditions in obtaining electrically-conductive ZrC triboceramics with superior wear resistance.