Richard Todd
Professor of Materials
+44-1865-273718
Mechanical properties and processing of ceramics and metals. Current interests include processing and properties of ceramic nanocomposites, the use of carbon nanotubes and graphene in ceramic composites, mechanical testing and stress measurement in ceramics at the microscale, high strain rate performance of ceramics for armour applications, "flash sintering" of ceramics, probing of stress and structure using neutrons and synchrotron radiation, improved metal forming for automobiles and mechanisms of superplastic deformation.
New Postgraduate Research Projects Available
Selected Publications
-
MWCNT-coated alumina micro-platelets for nacre-like biomimetic composites
Evers, K, Porwal, H, Todd, RI, Grobert, N2019|Journal article|Carbon© 2019 The Authors A novel building block material for the generation of non-brittle ceramic composites consisting of micron-sized alumina platelets homogeneously coated with multi-wall carbon nanotubes (MWCNTs) bound to their surface is described. The MWCNT phase is grown in situ from immobilised metal catalyst particles using chemical vapour deposition techniques. In-depth Raman and scanning electron microscope studies revealed that this approach solves the typical issue of MWCNT-agglomeration in ceramic matrices and paves the way to excellent control over MWCNT purity and concentration within the resulting composite material. Moreover, we show that the preparation of the catalyst is the most important factor for the generation of uniformly distributed MWCNTs of high-quality on these platelets. With these MWCNT-coated alumina building blocks, we have manufactured nacre-like biomimetic composites using spark plasma sintering. The resulting composites are electrically conductive and three-point bending tests show a transition from brittle/catastrophic failure to graceful failure, holding great promise towards multifunctional, tough and strong lightweight ceramic composite manufacturing. -
Characterisation of damage mechanisms in oxide ceramics indented at dynamic and quasi-static strain rates
Dancer, CEJ, Spawton, JNF, Falco, S, Petrinic, N, Todd, RI2019|Journal article|Journal of the European Ceramic Society© 2019 The Authors Ceramic materials are known to display rate dependent behaviour under impact. Tests to establish the strain-rate dependent variations in damage mechanisms have been carried out on debased alumina, an alumina-zirconia composite, and 3Y-TZP. Materials were indented dynamically and quasi-statically using identical sharp hardened steel projectiles while recording the load profile. Characteristics typical of both sharp and blunt indentation types were observed using scanning electron microscopy and piezospectroscopic mapping. At dynamic strain rates both the depth of the indentation and the residual stress in the material were lower than for quasi-static tests. This was attributed to temperature-induced softening of the projectile. Unusual behaviour was observed in the 3Y-TZP samples due to the reversible transformation from tetragonal to monoclinic crystal structures during mechanical loading. These effects and the observed superior mechanical strength against impact suggest that zirconia or zirconia-composite materials may have advantages over debased alumina for application as ceramic armour materials. -
Ultra-fast firing: Effect of heating rate on sintering of 3YSZ, with and without an electric field
Ji, W, Parker, B, Falco, S, Zhang, JY, Fu, ZY, Todd, RI2017|Journal article|Journal of the European Ceramic Society© 2017 Elsevier Ltd.It has recently been reported that ceramics can be sintered in a few seconds with the aid of an electric field ("flash sintering"). This investigation tests the possibility that the accelerated sintering is a consequence of the rapid heating rate involved rather than a direct effect of the electric field on mass transport. The sintering of 3YSZ powder compacts at a temperature of ∼1300. °C was compared (i) in flash sintering, (ii) with rapid heating rates produced without the application of an electric field, and (iii) with conventional heating rates. The results show that rapid heating can accelerate sintering by over 2 orders of magnitude compared with heating to the same temperature at conventional rates, even without the application of an electric field. It is concluded that the rapid densification in flash sintering of 3YSZ is at least partly a consequence of the rapid heating involved. Possible explanations are discussed. -
Liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing
Candelario, VM, Moreno, R, Todd, RI, Ortiz, AL2017|Journal article|Journal of the European Ceramic Society© 2016 Elsevier Ltd The effects were investigated of the starting particle size (i.e., nanometer or submicrometer powders), content of Y3Al5O12 additives (YAG; in the range 5–20 wt.%), and difference of size scales between the two particle types on the liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing. It was found that flash sintering benefits from the refinement of the particles size, the increase in additive content, and the smaller size scale of the particulate additive. It was also found that under the present flash sintering conditions (i.e., 900 °C furnace temperature, 13 A current, and 50 s in flash state) the resulting ceramics are, despite the formation of liquid phase, porous to a greater or lesser extent, and exhibit decreasing porosity gradients from their surface to the centre. These observations are rationalized to extract guidelines for powder batch design contributing to the pressureless ultrafast sintering of non-oxide advanced ceramics. -
Abrasive wear rate of boron carbide ceramics: Influence of microstructural and mechanical aspects on their tribological response
Moshtaghioun, BM, Gomez-Garcia, D, Dominguez-Rodriguez, A, Todd, RI2016|Journal article|Journal of the European Ceramic Society -
Transient liquid phase spark plasma sintering of B4C-based ceramics using Ti-Al intermetallics as sintering aid
Ji, W, Todd, RI, Wang, W, Wang, H, Zhang, J, Fu, Z2016|Journal article|Journal of the European Ceramic Society© 2016 Elsevier Ltd. Fully-dense B4C-based ceramics were fabricated using Ti-Al intermetallics as sintering aid by Spark Plasma Sintering at a low temperature of 1700 °C whilst applying 32 MPa uniaxial pressure. The influence of the intermetallic additions on the phase composition, morphology and mechanical properties of the ceramics were investigated. The intermetallics flowed and react with B4C to form a three-phase composite containing B4C, TiB2 and Al4C3 during the transient liquid phase sintering process. The TiB2 and Al4C3 particles were located on the boundaries between B4C grains and effectively inhibited the growth of B4C, which improved the mechanical properties. Good bonding between B4C and the phases formed was confirmed by TEM analysis. The specimens with 5 wt.% Ti-Al provide an attractive combination of homogeneous morphology and excellent mechanical properties, including a Vickers hardness of 33.5 ± 0.4 GPa, a flexural strength of 506 ± 16 MPa and a fracture toughness of 5.5 ± 0.1 MPa m0.5. -
Grain size dependence of hardness and fracture toughness in pure near fully-dense boron carbide ceramics
Moshtaghioun, BM, Gomez-Garcia, D, Dominguez-Rodriguez, A, Todd, RI2016|Journal article|JOURNAL OF THE EUROPEAN CERAMIC SOCIETYBoron carbide, Hardness, Fracture toughness, SEVNB test -
Stiffness, strength and interwall sliding in aligned and continuous multi-walled carbon nanotube/glass composite microcantilevers
Otieno, G, Koos, A, Dillon, F, Yahya, NA, Dancer, CEJ, Hughes, GM, Grobert, N, Todd, RI2015|Journal article|ACTA MATERIALIACarbon nanotubes, Glass matrix composites, Mechanical properties, Microcantilevers -
Ultra-fast and energy-efficient sintering of ceramics by electric current concentration.
Zapata-Solvas, E, Gómez-García, D, Domínguez-Rodríguez, A, Todd, RI2015|Journal article|Sci RepElectric current activated/assisted sintering (ECAS) techniques, such as electrical discharge sintering (EDS) or resistive sintering (RS), have been intensively investigated for longer than 50 years. In this work, a novel system including an electrically insulated graphite die for Spark Plasma Sintering (SPS) is described, which allows the sintering of any refractory ceramic material in less than 1 minute starting from room temperature with heating rates higher than 2000°C/min and an energy consumption up to 100 times lower than with SPS. The system alternates or combines direct resistive sintering (DRS) and indirect resistive sintering (IRS). Electrical insulation of the die has been achieved through the insertion of a film made of alumina fibers between the graphite die and the graphite punches, which are protected from the alumina fiber film by a graphite foil. This system localized the electric current directly through the sample (conductive materials) as in DRS and EDS, or through the thin graphite foil (non-conductive materials) as in IRS, and is the first system capable of being used under EDS or RS conditions independently combining current concentration/localization phenomena. -
A Mathematical Model for Flash Sintering
Hewitt, IJ, Lacey, AA, Todd, RI2015|Journal article|Mathematical Modelling of Natural Phenomena© 2015 EDP Sciences. A mathematical model is presented for the Joule heating that occurs in a ceramic powder compact during the process of flash sintering. The ceramic is assumed to have an electrical conductivity that increases with temperature, and this leads to the possibility of runaway heating that could facilitate and explain the rapid sintering seen in experiments. We consider reduced models that are sufficiently simple to enable concrete conclusions to be drawn about the mathematical nature of their solutions. In particular we discuss how different local and non-local reaction terms, which arise from specified experimental conditions of fixed voltage and current, lead to thermal runaway or to stable conditions. We identify incipient thermal runaway as a necessary condition for the flash event, and hence identify the conditions under which this is likely to occur.
