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A model for high-temperature plasticity of polycrystals controlled by disclination dipoles is proposed that predict a parabolic dependence of the strain rate versus the applied stress. The presence of a precise stationary disclination density explains the origin of plasticity without microstructural invariance, commonly known as superplasticity. The disclination mechanism is universal, although other processes, such as dislocation glide, are superposed to this one in many systems such as metals or metallic alloys. While, in ceramics it is likely to be the only operative mechanism. Activation of disclination dipoles is a necessary condition for plasticity and sufficient one for superplastic yielding., This work was supported by the Ministerio de Economía y Competitividad (Government of Spain) and FEDER Funds under Grants No. MAT2015-71411-R. BMM wants to acknowledge the support of the Spanish MINECO by means of a “Juan de la Cierva-Incorporación” fellowship during her sabbatical stay in Zaragoza and also the funding by the ‘Junta de Andalucía’ Regional Government through the project P18-RTJ-1972.

This paper is devoted to the study of the electrical conductivity of tantalum-doped zirconia ceramics prepared by spark plasma sintering. In this study, the temperature dependence of conductivity in as-prepared specimens and in those previously annealed in air is determined and compared. A semi-empirical model, which is based on the oxidation states of the cations, has been developed and successfully assessed. According to this, the conductivity is basically controlled by the diffusion of tetravalent zirconium cations in both cases, although the concentration of these species varies drastically with the amount of induced oxygen vacancies. This is a quite unexpected fact, since conductivity is normally controlled by anionic diffusion in zirconia ceramics. This option is forbidden here due to the presence of substitutional pentavalent cations. Therefore, conductivity values are much lower than those reported in trivalent or divalent substitutional cation doped zirconia ceramics., The authors would like to acknowledge provided by several institutions: BMM wants to acknowledge the Spanish Ministry of Science and Innovation through the grant PID2019-103847RJ-I00, as well as the financial support awarded by the Regional government of the ‘Junta de Andalucía’ through the project P18-RTJ-1972. BMM and JIP gratefully acknowledge the financial support provided the Ministry of Science in Spain through the projects MAT2016-77769-R and Gobierno de Aragón, Spain (Grupo Reconocido DGA T02_17R) and the associated EU Regional Development Funds. MLB acknowledges the funding support provided by the projects MAT2015-68078-R and PID2019-107106RB-C32 funded by the Spanish Ministry of Science and Innovation. Finally, DGG and ADR are grateful to the Spanish Ministry of Science for the research funding through the project MAT2015-71411-R.

This paper aims to give an answer to the following question: is the oxidation of graphene a critical issue for high-temperature plasticity in graphene-reinforced ceramics? To give a convincing reply, we will focus on two very different graphene-based ceramic composites: reduced graphene oxide (rGO)-reinforced alumina ( -Al2O3) and reduced graphene oxide (rGO)-reinforced yttria tetragonal zirconia (t-ZrO2). The processing of the powders has been made using a colloidal route, and after that, a spark plasma sintering process was performed in order to densify the samples. Creep tests were performed at temperatures between 1200–1250 C in an argon atmosphere. The microstructure obtained by SEM of the sintered and tested specimens was characterized quantitatively to elucidate the deformation mechanism. Raman spectroscopy was carried out to check the integrity of the graphene. The average grain size was in the order of 1 m and the shape factor was 0.7 for all the studied materials. The integrity of the graphene was checked before and after the creep experiments. The careful analysis of the creep tests shows that graphene oxide or its reduced version are not efficient phases for creep resistance improvement in general, contrary to what is reported elsewhere. However, the results permit the suggestion of a creep improvement in nanocomposites at a very high temperature regime due to an enhanced reactivity of oxygen between carbon and alumina interfaces. In the case of zirconia, the results give us the conclusion that the oxidation of graphene is a highly detrimental issue regarding the improvement of high-temperature plasticity

Mg2SiO4-MgAl2O4 eutectic ceramics have been fabricated by means of the laser floating zone (LFZ) technique. The microstructure has revealed as an unusual one at lower growth rate, composed of broken lamellae of MgAl2O4 distributed randomly along one matrix, composed of Mg2SiO4. At higher growth rates, a cell structure with intra-cell lamella structure is dominant. Contrary to most eutectic systems, hardness is not dependent upon the inter-spacing, but it does depend on one characteristic length of lamellae: their perimeter. One simple model based upon the dislocation is proposed, which successfully accounts for such extraordinary hardness law. Accordingly, Mg2SiO4-MgAl2O4 eutectic ceramics fabricated at 50 mm/h growth rate with the smallest MgAl2O4 lamella perimeter favorably showed more elevated hardness (13.4 GPa from Vickers indentation and 15.3 GPa from nanoindentation) and strength (∼430 MPa) than those found in the monolithic Mg2SiO4 matrix., This work was supported by the Ministerio de Economía y Competitividad (Government of Spain) and FEDER Funds under Grants No. MAT2016-77769-R and MAT2015- 71411-R and Gobierno de Aragon (Grupo Reconocido DGA T02_17R) and the associated EU Regional Development Funds. BMM wants to acknowledge the support of the Spanish MINECO by means of a “Juan de la Cierva-Incorporación” fellowship during her sabbatical stay in Zaragoza and the ‘Junta de Andalucía’ project P18-RTJ-1972.

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.

The search of a competitive ceramic material for structural applications demands several requisites: a simple microstructure with easy reproducibility, good intrinsic mechanical properties and most of all, an optimal oxidation resistance. This later point is a challenging point for most ultrahigh refractory materials. In this work an alumina (AlO) whisker-reinforced AlO composite prepared by spark plasma sintering (SPS) is studied. It will be shown that, although the microstructure is quite similar to that of pure monolithic one, there is a notorious enhancement of the high-temperature deformation resistance, reaching up to one order of magnitude over the pure AlO specimen. On the other hand, the activation energy of these composites increases notably. The results are explained in terms of an original model. A comparison with reported data shows that such composite is as efficient as a SiC-whisker-reinforced AlO composite, with the advantage of its oxidation resistance and much less fabrication cost., The authors acknowledge Nippon Steel & Sumitomo Metal Corporation (Japan) for doctoral thesis financial support through a Spanish-Japanese research joint project (FIUS1967/0679), the project from Spanish Ministry of Science MAT-2015–71411-R and the project from Andalucía Government P12-FQM-1079. EZS also acknowledges support through a contract from the JAE-DOC program of CSIC, Spain, co-funded by the FSE (2012–2015). This work is also supported by Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013C099) and International Science & Technology Cooperation Program of China (No. 2014DFA2014DFA53020).

Hafnium diboride (HfB2) is a highly refractory (melting above 3000°C) ceramic with many potential applications at high temperatures. To enable its use at temperature for extended periods its high-temperature plasticity must be known. This paper examines the mechanical response at temperatures between 900°C and 2000°C in air and in a reducing atmosphere, interpreting the data in the frame of classical models for the plasticity of compact-packed metals at low temperatures. In particular, the Friedel law and the principle of similitude for dislocation patterning are assessed. This reveals that HfB2 is a singular example of a ceramic material with “metal” mechanical behaviour.

In recent years, the interest of graphene and graphene-oxide has increased extraordinarily due to the outstanding properties concurring in this material. In ceramic science, the possibility of combining excellent electrical conductivities together with an enhancement of mechanical properties has motivated the research in fabrication of graphene oxide-reinforced ceramic composites despite the intrinsic difficulties for sintering. In this work a comparison is made between graphene oxide-reinforced alumina composites and carbon nanofiber-reinforced alumina ones. It will be concluded that the improvement of mechanical properties is scarce, if any. Since carbon nanofibers have also a good electrical conductivity their importance for future applications as a replacement of more sophisticated but expensive graphene-based ceramic composites will be stressed., The authors acknowledge the financial support awarded by the Regional project of Excellence of “Junta de Andalucía”P12-FQM-1079 and the Spanish MINECO and FEDER funds through the projects MAT2015-71411-R and MAT2015-67586-C3-2-R.

Alumina (AlO) ceramic composites reinforced with either graphene oxide (GO) or carbon nanofibers (CNFs) were prepared using Spark Plasma Sintering. The effects of GO and CNFs on the microstructure and in consequence on their mechanical properties were investigated. The microstructure of the sintered materials have been characterized quantitatively prior to and after the creep experiments in order to discover the deformation mechanism. Graphene-oxide reinforced alumina composites were found to be more creep resistant than carbon nanofibers-reinforced alumina ones or monolithic alumina with the same grain size distribution. In all the cases, grain boundary sliding was identified as the deformation mechanism., The authors acknowledge the financial support awarded by the Regional project of Excellence of ''Junta de Andalucía'' P12-FQM-1079 and the Spanish MINECO and FEDER funds through the projects MAT2015-71411-R and MAT2015-67586-C3-2-R.

Densification of alumina whisker-reinforced alumina ceramics by spark plasma sintering (SPS) has been investigated with the aim of obtaining a fine-grained microstructure and also studying the effect of whisker addition on the room-temperature mechanical properties. It was found that whisker addition retards slightly the sinterability of alumina by whisker hindering of particle rearrangement. Besides, the internal stress on the alumina matrix particles reduced due to the presence of a whisker network structure of strong rigid boundaries. Nevertheless near fully-dense and fine-grained alumina ceramics with alumina whisker content between 3 wt% and 10 wt% could be obtained under appropriate SPS conditions. The hardness of alumina ceramics with 3 wt% was comparable to that of pure alumina ceramics (∼26 GPa) whereas its fracture toughness (5.6 MPa m) was higher (4.2 MPa m). Crack bridging by well-dispersed whiskers and whiskers pull-out were identified as the main toughening mechanisms., The authors acknowledge Nippon Steel & Sumitomo Metal Corporation (Japan) for doctoral thesis financial support through a Spanish-Japanese research joint project (FIUS1967/0679), the project from Spanish Ministry of Science MAT-2015–71411-R and the project from Andalucia Government P12-FQM-1079.

Boron carbide is among the most promising ceramic materials nowadays: their mechanical properties are outstanding, and they open potential critical applications in near future. Since sinterability is the most critical drawback to this goal, innovative and competitive sintering procedures are attractive research topics in the science and technology of this carbide. This work reports the pioneer use of the laser-floating zone technique with this carbide. Crystallographic, microstructural and mechanical characterization of the so-prepared samples is carefully analysed. One unexpected output is the fabrication of a B6C composite when critical conditions of growth rate are adopted. Since this is one of the hardest materials in Nature and it is achievable only under extremely high pressures and temperatures in hot-pressing, the use of this technique offers a promising alternative for the fabrication. Hardness and elastic modulus of this material reached to 52 GPa and 600 GPa respectively, which is close to theoretical predictions reported in literature., This work was supported by the Ministerio de Economía y Competitividad (Government of Spain) and FEDER Funds under Grants Nos MAT2015-71411-R and MAT2016-77769-R. B.M.M. wants to acknowledge the support of the Spanish MINECO by means of a “Juan de la Cierva-Incorporación” fellowship during her sabbatical stay in Zaragoza.

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.

Grain boundaries are the elementary carriers of mechanical stress and chemical transport in polycrystalline materials. They are responsible for most macroscopic properties of them, particularly the mechanical performance. Most models take into account the ability of grain boundaries to slide each other assuming that surfaces do not suffer any deformation. However, one fact that is normally ignored is the mechanical distortion of flat boundaries under shear stresses. Indeed, such deformation is usually very small to play a significant role on the dynamics of point defects or dislocations. In this work, we will report the presence of a notably mechanical instability of flat boundaries under shear solicitation in zirconium carbide, giving rise to a periodic transversal shift. Such effect is the driving force for dislocation nucleation and annihilation, the main mechanism for high-temperature plasticity in this carbide.

Room temperature fracture toughness and hardness of spark plasma sintered pure B 4 C ceramics with grain sizes ranging from 120 nm to 17 μm have been studied. Vickers indentation and single edge V-notched beam (SEVNB) techniques have been used to measure hardness and fracture toughness, respectively. A critical analysis of the results derived from these two techniques has been carried out and the conditions for proper comparison of the derived results are discussed. The results have shown that hardness follows the Hall–Petch dependence with either grain size or twin spacing when the effect of porosity is corrected for. On the contrary, fracture toughness is found to be essentially grain size independent. The value of this quantity is ∼2 MPa m 1/2 .

B4C-based composites are considered to be a practical option in order to overcome the major limitation of pure boron carbide ceramics, their poor sinterability, without a major degradation of mechanical properties. This work is focused on the analysis and comparison of the high-temperature creep of silicon carbide-reinforced B4C composites sintered with or without graphite. The mechanical tests, together with microstructural characterization, will allow clarifying the micromechanisms into play and will give insight into the capability of these composites to optimise the mechanical resistance of these materials. The results have shown that the presence of graphite plays a key role on the plasticity of B4C-based composites. In the absence of graphite, silicon carbide-reinforced B4C composites have similar plastic behaviour as pure boron carbide polycrystalline ceramics.

Superplastic behavior of fine-grained zirconia ceramics (YTZP) is still a major issue on the ceramic science. This work presents a theoretical approach to the deformation mechanism yielding superplastic response in YTZP. In this paper we prove that, in the absence of any relaxation mechanism of internal stresses, a linear dependence of strain rate with stress is obtained, but the invariance of the microstructure cannot be assured. On the other hand, if the grain dynamics is controlled by the relaxation mechanism, a parabolic dependence is expected. This model predicts a transition between parabolic to linear dependence as the grain size increases, in quantitative agreement with the experimental data reported in the literature for YTZP.

This paper is a brief review of the concept of superplasticity, which has been extensively used to explain the superplastic behavior of yttria-tetragonal zirconia polycrystals. The diverse theories develop to account for the origin of this quantity are outlined. In addition to that, a modern approach to this concept is reported. This new contribution can give rise to a revision of the until-now established concept of invariant microstructure associated to superplasticity.

Boron carbide-based ceramics will be probably the most promising materials during the next decades due to their excellent mechanical properties combined with its chemical stability and low-density. Boron carbide itself is a very challenging system because of its complicated but highly-symmetrical crystallographic structure. Room-temperature mechanical properties of pure boron carbide have deserved considerable attention due to its remarkable hardness and resistance to shock-impact; however, its high-temperature plasticity with unsual ductility remains unexplored. The high-temperature creep of pure B4C polycrystals and the microsturctural observation were performed to find the mechanism of deformation in this material.

Titanium nitride ceramics have an intrinsic interest due to its optical and structural applications. However, the conditions for sintering of dense pieces are not still clarified. This research work is focused on the spark plasma sintering (SPS) of near-fully dense fine-grained TiN. The main goal is giving a response to a longstanding debate: can the external atmosphere favor sintering? Different sintering atmospheres, either vacuum or a nitrogen flow, have been used during SPS heating to this purpose. X ray diffraction analysis has showed the presence of TiN as the main phase with traces of Ti4O7 in optimal SPS conditions (1600 °C, one minute dwell time). Our results show that the use of a nitrogen flow while heating can improve sinterability very slightly, but mechanical properties are essentially unaltered within the experimental uncertainty. The hardness reaches values as high as 20GPa whereas fracture toughness can be evaluated around 4 MPam1/2., This work was supported by the Ministerio de Economía y Competitividad (Government of Spain) and FEDER funds under the Research project MAT2015-71411-R.

Grain growth is a ubiquitous phenomenon in all materials, and it affects both structural and functional properties. Despite its intrinsic importance, a full comprehension of grain growth from a fundamental point of view—i.e., from the nanoscale to the macroscale—is still a pending issue. In practical terms, our knowledge relies on the classical kinetic laws reported sixty years ago. This paper reports the violation of such classical laws in boron carbide ceramics consolidated by spark plasma sintering. The conjunction of high temperature gradients with large compressive stress when a pulse electric current passes through the ceramic powders gives rise to an intense twinning–detwinning formation. These forming steps at the grain boundaries change the grain mobility drastically. Therefore, a new ‘exotic’ law for grain-growth kinetics is found and validated at different temperatures and dwell times., The authors gratefully acknowledge the financial support provided by MINECO and FEDER Funds through the research project MAT2015-71411-R. BMM wants to acknowledge the support of the Spanish MINECO by means of a “Juan de la Cierva-Incorporación” fellowship during her sabbatical stay in Zaragoza.

The sintering of fine-grained α-alumina by spark plasma sintering (SPS) was performed to study grain growth under SPS conditions. Grain growth is found to be extensive at relative densities above 95%. A grain growth versus dwell time analysis during SPS allows for the determination of the grain-boundary diffusion coefficient. This study shows that the remarkable enhancement of grain-boundary diffusion derived from a previous analysis could be a consequence of the presence of the recently discovered “disconnections” at the grain boundaries of α-alumina. Their presence, together with their electric charge and the external electric field at the boundaries, are the key ingredients for a violation of the typical grain growth kinetic law. When they are introduced appropriately, an updated value of the grain-boundary diffusion coefficient is achieved. A comparison with other values reported previously in the literature through other techniques and a critical analysis are also carried out., The authors acknowledge Nippon Steel & Sumitomo Metal Corporation, Japan for the doctoral thesis financial support awarded to Y. Tamura. They also acknowledge the financial support provided by the project from the Spanish ministry of Economy (MINECO) MAT-2015-71411-R as well as through a contract for Dr. Zapata-Solvas from the JAE-DOC program of CSIC, Spain. Dr. B. M. Moshtaghioun wants to acknowledge the financial support provided by the Spanish MINECO through the "Juan de la Cierva-Incorporación" contract during her postdoctoral stay in CSIC-Universidad de Zaragoza, Spain. The authors also acknowledge some financial support from the regional government "Junta de Andalucía" P12-FQM-1079.

The use of allotropic phases of carbon (i.e. nanotubes, graphene or carbon nanofibers) as second phases to design ceramic composites is a hot topic at present. Researchers try to provide a remarkable improvement of the parent ceramic assuming that some of the outstanding mechanical properties of these phases migrate to the resultant composite. This reasonable idea has been questioned severely in the case of nanotubes addition but there is not any analysis for the other two phases cited previously. To elucidate this question, zirconia was selected as a model ceramic. This paper reports the mechanical properties of zirconia composites reinforced either with graphene or carbon nanofibers, with special emphasis on the high-temperature plasticity., The authors acknowledge the financial support awarded by the Regional project of Excellence of “Junta de Andalucía”P12-FQM-1079 and the Spanish MINECO and FEDER funds through the projects MAT2015-71411-R and MAT2015-67586-C3-2-R.

Boron carbide ceramics are the hardest material in Nature after diamond and the cubic phase of boron nitride. Due to this fact, their room-temperature fracture properties are the object of intense research. Paradoxically, high-temperature deformation is essentially unknown, because very high temperatures and stresses are necessarily required and high-quality specimens have not been available until recently. In this paper, the high-temperature compressive creep of fine-grained boron carbide polycrystals is reported. The breakdown of the classical power-law for high-temperature plasticity in ceramics is found. An analytical model is proposed. The model assumes that deformation is produced by dislocation glide. However, since the formation of twins is energetically favorable in this material and they act as strong barriers for dislocation glide, their motion turns to become progressively more difficult as elongation proceeds. The combination of increasing twin barriers and dislocations in mutual interaction is proposed to be the mechanism for high-temperature plasticity in this material. The model is validated with the experimental results. Final elongation of boron carbide specimens is reported to be over 100%, although this material cannot be described as a superplastic ceramic. Keywords: Plasticity, Dislocations, Twinning, Ceramics, Boron carbide

Densification of a commercially available B4C nanopowder by spark-plasma sintering (SPS) with nanograin retention was investigated, with emphasis on elucidating the potential of both single-step and two-step SPS cycles as well as the effect of pre-sintering annealing treatments of the nanopowders. It was found that single-step SPS does not allow a suitable processing window to obtain near-fully-dense B4C nanostructured ceramics from the as-received nanopowder, the cause being attributable to boria impurities producing coarsening. Pre-sintering annealing treatments of the nanopowder eliminate boria impurities thus facilitating the densification by single-step SPS, but at the expense of grain growth. Two-step SPS overcomes these drawbacks of the single-step approach (both without and with pre-sintering annealing treatments), enabling the desired densification of the as-received B4C nanopowder with nanograin retention (but nonetheless with a certain grain size heterogeneity). It was also found that the resulting ceramics are both superhard and toughened, highly desirable attributes in contact-mechanical and tribological applications.

Boron carbide (B 4 C) is a promising ceramic as it possesses a unique combination of properties. While the room-temperature mechanical properties of pure B 4 C have received considerable attention because of its high hardness and resistance to shock-impact, its high-temperature mechanical properties remain unexplored. To that end, creep deformation of coarse-grained, dense B 4 C has been studied under uniaxial compression in the stress range 250–500 MPa and the temperature range 1600–1800 °C. It has been found that B 4 C deforms extensively without failure, with a stress exponent n ∼ 3 and an activation energy Q ∼ 632 kJ/mol. These data, together with the extensive microstructural characterization of the deformed specimens, are used to gain insights into the creep mechanisms operative in B 4 C.

Ceria tetragonal zirconia polycrystals with a content of 12 mol% ceria (CeTZP) have been tested in compression at constant strain rate between 1150 °C and 1300 °C. An accurate analysis of the stress–strain curves has permitted to determine the value of the grain boundary cation diffusion. The results are compared with those reported in literature for this alloy and yttria tetragonal zirconia polycrystals (YTZP). An isotopic effect is found to correlate both grain boundary diffusion coefficients.

Normal grain growth is a ubiquitous process in nature playing a central role in many technological applications of materials. Despite the considerable effort devoted to account for its features over the past years, there remain important aspects which are not fully understood. Among them, the mere existence of a grain distribution function for normal grain growth and its mathematical formulation deserves attention. Although it is commonly assumed that lognormal function is the “expected” grain distribution in materials, there are many examples, particularly in ceramics, in which such assumption has revealed to be not correct. An alternative grain size distribution is proposed after phase field model simulations of static and dynamic grain growth. A simple explanation based upon the local grain size correlation is proposed.

The literature data on the superplastic deformation of high purity yttria stabilised tetragonal zirconia polycrystals is reviewed in detail. It is shown that, based on the existence of a threshold stress, the single mechanism of grain boundary sliding (GBS) accommodated by diffusional processes can explain the superplasticity of these materials over all the ranges of temperature, stress, grain size, and surrounding atmosphere that have been studied. The origin of the threshold stress and its quantitative dependence on temperature and grain size is explained in terms of the segregation of yttrium atoms at the grain boundaries. A new model for GBS accommodated by lattice or grain-boundary diffusion is presented which can explain the transition of the stress exponent from 2 to 1.

A Molecular Dynamics study about the segregation of yttrium at 1500 K to a Σ5 grain boundary in 8 mol% YSZ has been performed. Segregation has been induced by explicitly taking into account the excess energy associated to the elastic misfit effect for yttrium cations located nearby the grain boundary planes. After an initial transient, a steady regime is reached, in which the number of yttrium cations does not increase with time. Accumulation of yttrium cations is accompanied by that of zirconium ones and oxygen vacancies at some distance of the grain boundary planes. The changes in the radial distribution functions for different ionic pairs are discussed, as also the effect of segregation on oxygen diffusion along the grain boundaries and in volume. Finally, the possibility that segregated yttrium located at available free sites at the grain boundaries is pointed out.

Spark plasma sintering (SPS) parameters are reported to have a remarkable effect on microstructure and fracture toughness of boron carbide. Fully dense fine-grained boron carbide samples have been sintered by SPS at 1700 °C for 3 min as optimal conditions. Both temperature and the current density applied during sintering seem to be important parameters for fully dense output and can induce significant changes on the final microstructure. The initial grain size of the powder is a crucial factor to perform fine-grained fully dense specimens. The improvement on room-temperature hardness and toughness is discussed.

Boron carbide ceramics with systematic variations in microstructure were subjected to an abrasive wear test involving an abrasive slurry of SiC particles on a rotating steel ball as counterface and with a load of 5N. The total sliding distance was 70 m and a sliding speed of 0.2 ms −1 was used for all tests. This research work is a pioneering contribution on the analysis of the abrasive wear resistance of boron carbide as a function of grain size and porosity. In contrast to most ceramics, it is concluded that grain size is not directly relevant to the wear resistance, whereas porosity and hardness play the main role. The absence of surface pullout by brittle fracture is a relevant experimental feature in this material which results in mild wear. Our results are rationalized in terms of the classical Rabinowicz model, which gives a reasonable description of the wear response.

Superplasticity is the ability exhibited by some fine-grained materials to be elongated a great deal with no failure. Such phenomenological definition accounts for the engineering view point of this remarkable property. From a fundamental basis, there is a full consensus to admit that it is essentially linked to the grain boundary motion under invariance of microstructure. Despite the great scientific effort carried out during the two last decades, or probably due to that, there is still a gap on the scientific comprehension of the equation describing superplasticity from a basic point of view, not to say about its potential extension to nanostructured materials. This paper presents the essential basis of a new model proposed to account for the main features of structural ceramics.

Carbon nanotubes (CNTs) have been considered as a new promising reinforcement for ceramic matrix composites (CMCs) over the last decade, owing to their exceptional properties. CNT-reinforced CMCs posses a unique microstructure, nanoscale objects dispersed throughout ceramic matrix grain boundaries, which allows tailoring physical properties with an unprecedented combination of remarkable engineered transport properties as well as superior mechanical properties. However, gaining further control over challenging CNTs dispersion is still an important issue with the aim of tailoring multifunctional properties of CNT-reinforced CMCs. This paper reviews the current status of the research and describes all different approaches used to effectively disperse CNTs throughout ceramic matrices, providing an overview of composites microstructure and mechanical, electrical and thermal properties. Besides, all findings reported till date point out a promising approach towards physical properties tailoring of CNT-reinforced ceramic CMCs.

Several physical characteristics of the Σ3, Σ5, Σ11 and Σ13 grain boundaries in model YSZ bicrystals (namely grain boundary energies, radial distribution functions for pairs of ions and diffusion of oxygen along the grain boundaries) are systematically studied as functions of the concentration of yttria and temperature. The concentration of dopants affects the local structure at the grain boundaries; a preferential location of oxygen vacancies around yttrium cations is detected. Regarding the grain boundary energies, they mainly decrease with temperature, although any clear dependence with the amount of yttria was not apparent. Diffusion of oxygen is found to proceed more slowly along the grain boundaries than within the bulk, the differences becoming less important with increasing the temperature. The grain boundary diffusion coefficients for oxygen do not depend much on the concentration of yttria, but the activation energies exhibit a maximum at around 15 mol.% yttria for all the geometries. To the authors' knowledge, this is the first time that such a systematic study has been carried out. It could constitute the basis for a further understanding of the grain boundary structure and diffusion features at atomic scale in the YSZ system.

Single wall carbon nanotube reinforced yttria stabilized zirconia ceramic materials have been obtained by means of spark plasma sintering technique. Single wall carbon nanotubes were treated in an acid solution before mixing with zirconia powders to obtain a uniform distribution of both powders. This method allows obtaining ceramic materials with a grain size between 200 nanometers and 1 micron and with a grain size distribution which depends on processing conditions. This new route opens a new perspective for new ceramic composites tailoring with enhanced mechanical properties as structural materials

A first principles continuum analytical model for cationic segregation to the grain boundaries in complex ceramic oxides is presented. The model permits one to determine the electric charge density and the segregation-induced electric potential profiles through the grain and can be extrapolated to the range of nanostructured grain sizes. The theoretical predictions are compared with existing data for yttria-stabilized tetragonal zirconia polycrystals. The implications for physical properties (mainly high temperature plasticity and hardening behaviour) are then discussed. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved., The authors wish to acknowledge the financial support awarded by the Spanish Research authorities through the Projects MAT2009-14351-C02-01 and MAT2009-14351-C02-02. D.G.G. would like to acknowledge the encouraging scientific atmosphere found in the Tokyo Research Institute during his stays at that institution, hosted by Prof. F. Wakai and sponsored by the Japan Society for the Promotion of Science.

This work attempts to give a unified picture of the microscopic mechanisms which control the plastic deformation in yttria-stabilized cubic zirconia single crystals, particularly for the soft orientation (uniaxial compression along the crystallographic axis) at low, intermediate and high temperatures while also reviewing and updating the existing data for different yttria contents (9.4 – 32 mol.%). The controlling deformation mechanisms are: internal friction within the crystal lattice (Peierls mechanism at low temperatures), pinning of dislocations by localized obstacles and long-range interaction between dislocations (intermediate temperatures) and dislocation viscous glide and climb (recovery creep) at higher temperatures. New aspects of the Portevin–le Chatelier phenomenon during viscous glide, due to the pinning and unpinning of dislocations from their defect clouds (yttrium substitutional atoms), are included.

Superplasticity in ceramics has been the subject of intense research activity for the last two decades. Quite recently, the fabrication of fully dense nanocrystalline oxides with grain size below 100 nm enabled examination of their superplastic behaviour. This work presents a critical analysis of the plasticity of two important nanostructured oxide systems: MgO and yttria tetragonal zirconia polycrystals. A thorough comparison of their plastic deformation reveals that nano-structuring may be a necessary, but not a sufficient condition for superplasticity in ceramics as commonly assumed. Instead, the changes in the chemical composition and the transport properties, through the bulk and at grain boundaries, versus temperature and grain size can induce a rich variety of mechanical responses.

A microstructural modelling of the microstructure in single wall carbon nanotubes reinforced alumina ceramics has been developed. The model accounts for the main microstructural features, being quite useful to describe the carbon nanotube distribution along the ceramic matrix. The microstructural analysis derived from this model is found to give a deeper insight into the high-temperature creep of these composites.

The aim of this work is the determination of the grain growth kinetic law in 4 mol.% yttria tetragonal zirconia polycrystals. To that purpose, several sets of slices of 4 mol.% yttria tetragonal zirconia polycrystals were annealed for different times at a constant temperature as well as at a constant time for different temperatures. The mean grain size was determined by means of scanning electron microscopy observations of the annealed surfaces, after fitting the grain size distributions to lognormal functions. A power law of the mean grain size versus the annealing time was found to describe the experimental results. Surprisingly, the same power law was shown to account for the dependence of the grain size standard deviations versus time. A correlation of these results with a model reported in the literature on the yttrium segregation to grain boundaries is discussed.

Ceria-zirconia ceramic alloys with the following molar composition: 0.12CeO2-0.88ZrO2 have been sintered by high-temperature annealing. Monolithic specimens haven been crept in compression at high temperatures. Creep experiments have been rationalized to an empirical constitutive equation which is consistent with a classical Ashby-Verrall creep regime. This result has been assessed through microstructural characterization of as-received and post-mortem specimens. A pure Ashby-Verrall creep is contrary to the conventional mechanism controlling creep in other zirconia alloys. A discussion on the explanation for such mechanism is outlined.

In this review paper, the macro and microcharacteristics of superplasticity will be analyzed. Based on the macroscopic features of the superplasticity, the joining of ceramics as a new application will be analyzed with special emphasis in the fabrication of functional gradient ceramics materials. Finally, the new trends as high strain rate superplasticity (HSRS) or superplasticity at low homologous temperature will be discussed.

Uniaxial compressive creep behaviour of spark-plasma-sintered Al2O3/graphite particulate composites has been studied at temperature between 1250 and 1350 °C. Values of stress exponent, n, ranging from 1 to 1.4 and, activation energy, Q, of 600 ± 40 kJ/mol have been determined. With 10 vol% graphite in the composite, the creep deformation of the composite is controlled by the fine-grained Al2O3 matrix, where Coble creep has been identified as the dominant creep mechanism.

In this paper, the origin of the threshold stress in pure 3 mol %-yttria tetragonal zirconia polycrystals (YTZP) is analysed in detail. At this regard, the two explanations reported in literature are discussed thoroughly. One of them invokes dislocation activity as the origin for such quantity, whereas the other one is based upon yttrium segregation at the grain boundaries. Critical assessment for both of them is performed, and it has allowed concluding that the dislocation activity observed by Morita and Hiraga is just an artifact created during sample preparation. Thence, segregation at the grain boundaries seems to be the only mechanism accounting for a threshold stress and its temperature and grain size dependence.

The plastic behavior of cermets based on a 3 mol% yttria-stabilized tetragonal zirconia matrix that incorporates nanometric nickel inclusions (3YTZP/n-Ni), with 2.5, 5 and 10 vol.% of nickel content, has been studied by constant load tests in compression carried out in argon atmosphere. The microstructure of these composites consists of nanometric nickel inclusions homogeneously dispersed into a fine-grained zirconia matrix (about 200 nm). The microstructural and mechanical results obtained show that the creep behavior is controlled by the zirconia matrix as in 3YTZP-based cermets with micrometric Ni inclusions (3YTZP/μ-Ni); whereas the stress exponent values are similar to those of high-purity monolithic 3YTZPs, anomalous high values of the activation energy have been measured. The ceramic/metal interface plays a crucial role for creep properties; the strong TZP/n-Ni interface matching can be at the origin of these high values of the activation energies for creep.