118 results found
Gavalda-Diaz O, Manno R, Melro A, et al., 2021, Mode I and Mode II interfacial fracture energy of SiC/BN/SiC CMCs, Acta Materialia, Vol: 215, Pages: 1-11, ISSN: 1359-6454
Quantifying the mixed mode fracture toughness of interfaces in ceramic matrix composites (CMCs) is crucial for understanding their failure. In this work we use in situ micromechanical testing in the scanning electron microscope to achieve stable interfacial crack propagation in Mode I (Double Cantilever Beam) and Mode II (Push out) and measure the corresponding fracture resistances. We use this approach to measure the interfacial fracture resistance in SiC/BN/SiC CMCs and compare it to the fracture energy of the fibres. During in-situ testing, fracture paths can be observed while data is acquired simultaneously. We clearly observe debonding at the BN-fibre interface (i.e. inside adhesive debonding). The critical energy release rate of the BN-fibre interface for Mode I and II (GIc ≈ 2.1 ± 1.0 J/m2 and GIIc ≈ 1.2 ± 0.5 J/m2) are equivalent and is lower than that measured for the fibre using microscopic DCB tests (GIc ≈ 6.0 ± 2.0 J/m2). These results explain the generalized fibre debonding and pull out observed in the fracture of these CMCs. By enabling direct observation of crack paths and quantifying the corresponding fracture energies, we highlight possible routes for the optimisation and modelling of the new generation of CMC interphases.
Humphry-Baker S, Vandeperre L, 2021, Creep deformation of WC hardmetals with iron-based binders, International Journal of Refractory Metals and Hard Materials, Vol: 95, Pages: 1-8, ISSN: 0263-4368
Iron is a candidate to replace cobalt in WC hardmetals, due to its lower cost and toxicity. A WC-FeCrhardmetal was compression tested at 900-1200 °C. Particular attention is paid to the steady-state creeprates and stress-exponents (n) during isostress treatments. Three regimes of n are observed. Two ofthese were previously reported for WC-Co: power law creep (n»3) at stresses below ~100MPa; andgrain boundary sliding (n»1) at higher stresses. A previously unreported regime at very low stresses(<10MPa), with an exponent of n»2, is also observed. By combining electron microscopy with X-raydiffraction texture measurements, the low stress regime is attributed to viscous flow of the binder,which is accommodated by diffusional creep in the WC skeleton. The mechanism may be applicableto other hardmetals. Compared to analogous WC-Co materials, WC-FeCr shows improved creepresistance below 1000 °C, which can be explained by its lower self-diffusivity, and a lower solubilityfor WC than Co. However, at temperatures corresponding to liquid eutectic formation (~1140 °C), itscreep resistance becomes inferior. These results indicate FeCr may be a suitable replacement for Coprovided the eutectic temperature is not exceeded.
Diaz OG, Marquardt K, Harris S, et al., 2020, Degradation mechanisms of SiC/BN/SiC after low temperature humidity exposure, Journal of the European Ceramic Society, Vol: 40, Pages: 3863-3874, ISSN: 0955-2219
The environmental degradation of SiC/BN/SiC CMCs under low temperature water exposure is still an unexplored field. This work shows how the effect of low temperature humid environments can be detrimental for turbostratic BN interphases, leading to a drop in mechanical properties. Furthermore, initial low-temperature humid environments can induce a faster degradation during subsequent thermal exposure. In order to understand how low temperature water exposure affects the CMC and how these changes affect the material response to subsequent exposures, intermediate temperature (800 °C) exposures have been studied before and after the low temperature humidity tests. The main challenge of this work consists of understanding how different constituents of the CMC structure (e.g. fibres and interphases) are degrading and consequently affecting the overall bulk mechanical performance and failure modes of the material. For this, linking the change in morphology and chemistry of the interphases with the micromechanical properties each constituent has been crucial.
Jones LD, Vandeperre LJ, Haynes TA, et al., 2020, Theory and application of Weibull distributions to 1D peridynamics for brittle solids, Computer Methods in Applied Mechanics and Engineering, Vol: 363, Pages: 1-11, ISSN: 0045-7825
Peridynamics is a continuum mechanics modelling method, which is emerging as a solution for – in particular – the modelling of brittle fracture. The inherent variability of brittle fracture is captured well by the Weibull distribution, which describes the probability of fracture of a given material at a given stress. Recreating a Weibull distribution in peridynamics involves adjusting for the fact that the body is made up of a large number of bonds, and the distribution of strengths associated with these bonds must be different to the distribution of strengths associated with the peridynamic body. In the local case, where the horizon ratio, m=1 is used, Weibull’s original simple size scaling gives exact results, but the overlapping nature of non-local bonds that occurs in higher m cases, typically used in the peridynamics literature (such as m=3), causes a significant distortion of Weibull distributions. The cause of these distortions is spurious toughening and partial component failures as a result of the reduced localisation associated with larger horizon ratios. In order to remove these distortions, appropriate size scaling is used for the bonds, and a methodology that is capable of reflecting the heterogeneity of the material in the model, is proposed. The methodology described means Weibull parameters measured at specimen or component level can be reproduced for higher values of m.
Elizarova I, Vandeperre L, Saiz Gutierrez E, 2020, Conformable green bodies: plastic forming of robocasted advanced ceramics, Journal of the European Ceramic Society, Vol: 40, Pages: 552-557, ISSN: 0955-2219
Robocasting, or the additive manufacturing of ceramics by continuous extrusion of a ceramic paste, has limited capabilities when printing complex unsupported structures such as overhangs or free standing thin artefacts. In this paper we address this limitation using a new type of paste, which allows for shaping of the green bodies after printing. To illustrate the flexibility of the paste, it was used to produce both alumina and silicon carbide parts. The paste consists of a solution of phenolic resin in methyl ethyl ketone and ceramic powders. Fabricated parts can be cut, bent, folded and draped over various objects. Once dry and fully solid, the parts become rigid and can be processed further by slow pyrolysis and sintering. Sintered samples exhibit flexural strength comparable to both conventionally produced and robocasted ceramics and shaping of the green bodies after printing does not affect the mechanical strength of the sintered parts.
Jones LD, Vandeperre LJ, Haynes TA, et al., 2020, Modelling of Weibull Distributions in Brittle Solids Using 2-Dimensional Peridynamics, 1st European-Structural-Integrity-Society (ESIS) Virtual European Conference on Fracture (ECF), Publisher: ELSEVIER, Pages: 1856-1874, ISSN: 2452-3216
Giuliani F, Ciurea C, Bhakhri V, et al., 2019, Deformation behaviour of TiN and Ti–Al–N coatings at 295 to 573 K, Thin Solid Films, Vol: 688, ISSN: 0040-6090
Temperature-dependent nanoindentation testing was employed to investigate the deformation behaviour of magnetron sputtered (100) TiN and Ti1-xAlxN (x = 0.34, 0.52, 0.62) coatings in the temperature range from 295 to 573 K. The maximum temperature is sufficiently below the deposition temperature of 773 K to guarantee for stable microstructure and stress state during testing. The TiN coating displayed the same hardness as bulk single crystal (SC) TiNbulk. The addition of aluminium to TiN (to form single-phase face centred cubic structured Ti1-xAlxN coatings) increased the room temperature hardness due to increased bond strength, lattice strain and higher activation energy for the dislocation slip. For coatings with a low aluminium content, Ti0.66Al0.34N, the decrease in hardness with temperature was similar to the TiN coating and SC-TiNbulk. In contrast, the hardness of coatings with moderate, Ti0.48Al0.52N, and high, Ti0.38Al0.62N, aluminium contents varied little up to 573 K. Thus, the Ti1-xAlxN matrix is mechanically more stable at elevated temperatures than its TiN relative, by providing a lower decrease in lattice resistance to the dislocation flow with increasing temperature. The findings suggest that the addition of Al to TiN (to form Ti1-xAlxN solid solutions) not only improves the hardness but also leads to stable hardness with temperature, and emphasizes the importance of bonding states and chemical fluctuations, next to structure and morphology of the coatings that develop with changing the chemistry.
Feilden E, Glymond D, Saiz E, et al., 2019, High temperature strength of an ultra high temperature ceramic produced by additive manufacturing, Ceramics International, Vol: 45, Pages: 18210-18214, ISSN: 0272-8842
In this study hafnium diboride was fabricated using the additive manufacturing technique robocasting. Parts have been successfully produced with complex shapes and internal structures not possible via conventional manufacturing techniques. Following pressureless sintering, the monolithic parts reach densities of 94–97% theoretical. These parts exhibit bending strength of 364 ± 31 MPa at room temperature, and maintain strengths of 196 ± 5 MPa up to 1950 °C, which is comparable to UHTC parts produced by traditional means. These are the highest temperature mechanical tests that a 3D printed part has ever undergone. The successful printing of the high density HfB2 demonstrates the versatile range materials that can be produced via robocasting using Pluronic pastes.
Rocher ME, Hermann T, McGilvray M, et al., 2019, Testing a transpiration cooled zirconium-di-boride sample in the plasma tunnel at irs
Transpiration cooling is an active thermal protection system (TPS), in which a coolant gas is fed through a porous material. This requires a material that stays structurally stable at high temperatures, while having the desired permeability. This paper explores transpiration cooling of Ultra-High-Temperature-Ceramics (UHTCs). A stagnation probe with transpiration cooled ZrB2 was tested in a plasma wind tunnel at a null point heat flux of 3.59 MW/m2 in steady state and transiently at 2 MW/m2. The aim is to understand whether transpiration cooling can increase the UHTC operating temperature by shielding it from oxygen and reducing the heating from surface re-combination. Several diagnostics are applied, including an Echelle spectrograph that explores outgassing of oxidation products from the surface. Infrared thermography is employed to track the surface temperature at the front and the back surface temperature is measured by a pyrometer. Furthermore, the Planck radiation background of the emission spectra is used to assess the front surface temperature. The testing included a variation in the injectant species and mass flux. While the uncooled sample fully oxidised at a surface temperature of 2150 K, 20.25 g/m2 s of helium and 620.11 g/m2 s of nitrogen prevented oxidation of the transpiration cooled samples. At a blowing parameter of 0.1094, the helium cooled probe reached a front surface temperature of 1428 K and reduced the incident heat flux by 77% compared to the uncooled sample. The nitrogen cooled sample had a maximum front surface temperature of 1128 K with a blowing parameter of 1.958 and an 83.3% lower incident heat flux than the uncooled sample.
Glymond D, Vandeperre LJM, 2018, Robocasting of MgO-doped alumina using alginic acid slurries, Journal of the American Ceramic Society, Vol: 101, Pages: 3309-3316, ISSN: 0002-7820
The benefits of MgO doping of alumina for maintaining a homogeneous grain structure have long been established. Therefore in this work a bespoke ink for Robocasting of alumina is developed based on the gelation of alginic acid using magnesium ions, thereby ensuring homogeneous MgO doping of the alumina green body. The shear thinning behavior of alginic acid based solutions was paired with the rheological properties of a partially coagulated colloidal suspension to allow high solid loading inks (up to 50 vol%) with good extrusion behavior. Shear thinning coefficients of n ~ 0.2 were recorded, with yield stresses of 250 Pa and stiffness values in the range 100-1000 kPa. The printed alumina bars reached densities of >98% and unpolished strengths reached up to 326 ± 16 MPa after sintering at 0.4 mol/L magnesium chloride and 45 vol% alumina.
Gasparrini C, Chater RJ, Horlait D, et al., 2018, Zirconium carbide oxidation: kinetics and oxygen diffusion through the intermediate layer, Journal of the American Ceramic Society, Vol: 101, Pages: 2638-2652, ISSN: 0002-7820
Oxidation of hot‐pressed ZrC was investigated in air in the 1073‐1373 K range. The kinetics were linear at 1073 K, whereas at higher temperature samples initially followed linear kinetics before undergoing rapid oxidation leading to a Maltese cross shape of the oxide. The linear kinetics at 1073 K was governed by inward oxygen diffusion through an intermediate layer of constant thickness between ZrC and ZrO2 which was comprised of amorphous carbon and ZrO2 nanocrystals. Diffusion of oxygen through the intermediate layer was measured to be 9 × 10−10 cm2 s−1 using 18O as a tracer in a double oxidation experiment in 16O/18O. Oxidation at 1073 and 1173 K produced samples made of m‐ZrO2 and either t‐ or c‐ZrO2 with an adherent intermediate layer made of amorphous carbon and ZrO2, whereas oxidation at 1273 and 1373 K produced samples with a voluminous oxide made of m‐ZrO2 showing a gap between ZrC and the oxide. A substoichiometric zirconia layer was found at the gap at 1273 K and no carbon uptake was detected in this layer when compared with the top oxide layer. The loss of the intermediate layer and the slowdown of the linear rate constant (g m−2 s−1) at 1273 K compared to 1173 K was correlated with the preferential oxidation of carbon at the intermediate layer which would leave as CO and/or CO2 leaving a gap between ZrC and substoichiometric zirconia.
Cal E, Qi J, Preedy O, et al., 2018, Functionalised magnetic nanoparticles for uranium adsorption with ultra-high capacity and selectivity, Journal of Materials Chemistry A, Vol: 6, Pages: 3063-3073, ISSN: 2050-7496
The removal of radioactive contaminants from the environment for safe and efficient waste disposal is a critical challenge, requiring the development of novel selective and high-capacity sequestering materials. In this paper the design of superparamagnetic iron oxide nanoparticles (SPIONs) as highly efficient magnetic-sorbent structures for uranium (U(VI)) separation is described. The nanosorbent was developed by surface functionalisation of single crystalline magnetite (Fe3O4) nanoparticles with a phosphate-based complex coating. This new design allowed for the development of a magnetically separable ultra-effective sorbent, with a measured U(VI) sorption capacity of ∼2333 mg U per g Fe (1690 mg U per g Fe3O4 NP), significantly higher than everything previously reported. Based on TEM analysis, it is proposed that these properties are the result of a multi-layer ligand structure, which enables a high degree of U-incorporation compared to conventional surface-ligand systems. Moreover, the phosphate-NP construct ((PO)x-Fe3O4) shows exceptionally high specificity for the sequestration of U(VI) in solution at pH 7. Adsorption tests in the presence of competing ions, such as Sr(II), Ca(II) and Mg(II), showed high selectivity of the nanoparticles for U(VI) and extremely rapid kinetics of contaminant removal from solution, with the total amount of uranyl ions being removed after only 60 seconds of contact with the NPs. The results presented in this paper highlight the potential of such a phosphate-functionalised magnetic nanosorbent as a highly effective material for the remediation of U(VI) from contaminated water and industrial scenarios.
Natural structural materials like bone and shell have complex, hierarchical architectures designed to control crack propagation and fracture. In modern composites there is a critical trade-off between strength and toughness. Natural structures provide blueprints to overcome this, however this approach introduces another trade-off between fine structural manipulation and manufacturing complex shapes in practical sizes and times. Here we show that robocasting can be used to build ceramic-based composite parts with a range of geometries, possessing microstructures unattainable by other production technologies. This is achieved by manipulating the rheology of ceramic pastes and the shear forces they experience during printing. To demonstrate the versatility of the approach we have fabricated highly mineralized composites with microscopic Bouligand structures that guide crack propagation and twisting in three dimensions, which we have followed using an original in-situ crack opening technique. In this way we can retain strength while enhancing toughness by using strategies taken from crustacean shells.
Hauth M, Lawlor V, Cartellieri P, et al., 2017, Production and reliability oriented SOFC cell and stack design, 15th International Symposium on Solid Oxide Fuel Cells (SOFC), Publisher: Electrochemical Society, Pages: 2231-2249, ISSN: 1938-5862
The paper presents an innovative development methodology for a production and reliability oriented SOFC cell and stack design aiming at improving the stacks robustness, manufacturability, efficiency and cost. Multi-physics models allowed a probabilistic approach to consider statistical variations in production, material and operating parameters for the optimization phase. A methodology for 3D description of spatial distribution of material properties based on a random field models was developed and validated by experiments. Homogenized material models on multiple levels of the SOFC stack were established. The probabilistic models were related to the experimentally obtained properties of base materials to establish a statistical relationship between the material properties and the most relevant load effects. Software algorithms for meta models that allow the detection of relationships between input and output parameters and to perform a sensitivity analysis were developed and implemented. The capabilities of the methodology is illustrated on two practical cases.
Feilden E, Giovannini T, Ni N, et al., 2017, Micromechanical strength of individual Al2O3 platelets, Scripta Materialia, Vol: 131, Pages: 55-58, ISSN: 1359-6462
Optimising the properties of platelet reinforced composites requires the strength of the reinforcing phase to be known, however strength measurements at such small scales are difficult and therefore data is sparse. In this work the flexural strength and Weibull modulus of microscopic, alumina platelets has been measured as 5.3 ± 1.3 GPa and 3.7 respectively, using an in-situ micro 3-point bend test. A general approach to correct for the effect of variation in sample size on the Weibull modulus is presented, and the internal structure of the platelets is revealed by TEM.
Glymond D, Vick M, Giuliani F, et al., 2017, High Temperature Fracture Toughness of Mullite with Monoclinic Zirconia, Journal of the American Ceramic Society, Vol: 100, Pages: 1570-1577, ISSN: 1551-2916
Reactive sintering of zircon and alumina and zirconia additions to mullite are well establishedmethods for improving the poor fracture toughness of mullite. While it is clear that transformationtoughening is responsible for the improved toughness by addition of partially stabilised zirconia, it isnot clear why adding unstabilised zirconia increases the toughness although microcracking and crackdeflection have been suggested. Therefore the fracture toughness of a mullite composite with 20vol% unstabilised zirconia and a monolithic mullite were investigated at ambient conditions and attemperatures up to 1225 oC. It was found that monoclinic zirconia increases the toughness atambient conditions from the monolithic mullite value of 1.9 MPa m1/2 to 3.9 MPa m1/2. Thetoughness of the composite with zirconia remains relatively constant from ambient to 600 °C butthen decreases rapidly. The mechanism for the toughness enhancement as well as the reason for itsvariation with temperature are explained using changes in residual stress state as deduced using thesphere in shell model from the measured thermal expansion behaviour.
Feilden E, Giovannini T, Ni N, et al., 2017, Micromechanical strength of Al2O3 platelets, Scripta Materialia, ISSN: 1359-6462
Optimising the properties of platelet reinforced composites requires the strength of the reinforcing phase to be known, however strength measurements at such small scales are difficult and therefore data is sparse. In this work the flexural strength and Weibull modulus of microscopic, alumina platelets has been measured as 5.3±1.3 GPa and 3.7 respectively, using an in-situ micro 3-point bend test. A general approach to correct for the effect of variation in sample size on the Weibull modulus is presented, and the internal structure of the platelets is revealed by TEM.
Hafnium diboride (HfB2) is one of a family of ultra-high temperatureceramics (UHTCs) which are being considered forapplication in environments with a substantial heat flux such ashypersonic flight. In order to characterize transitions in thematerial response with heat flux and therefore predict the inservicebehavior of UHTCs, a range of tests were conducted inwhich small cylindrical bars of HfB2 were laser heated usingheat fluxes from 25 to 100 MW/m2. After testing, the externaldamage as well as damage observable in cross sections throughthe cylinders was characterized using photography, optical, andscanning electron microscopy. Experimental results were comparedwith finite element modeling of the heat flow, temperaturedistribution, and phase transition. Heat flux rather thantotal deposited heat was found to be the strongest determinantof the way in which damage develops in samples; for lower heatfluxes, the main damage mechanism is oxidation, progressingto oxidation-induced melting and finally, at the highest heatfluxes, substantial ablation by melting irrespective of oxidation.The agreement between calculations and experimental observationsindicates that such calculations can be used with confidenceto guide the design of components.
Jia Y, Wang B, Wu Z, et al., 2016, Role of sodium hexametaphosphate in MgO/SiO2 cement pastes, Cement and Concrete Research, Vol: 89, Pages: 63-71, ISSN: 0008-8846
The extent of reaction between magnesium oxide (MgO) and silica fume (SiO2) is normally limited and mixes require high water contents to give suitable rheology. The use of considerably lower water contents and the formation of magnesium silicate hydrate (M-S-H) gel as a binding phase is made possible by adding sodium hexametaphosphate (Na-HMP) to the mix water prior to the addition of MgO and SiO2. This results in the formation of extensive reaction products and cured samples with high compressive strength and low porosity. In this work, the effect of Na-HMP on the hydration of MgO/SiO2 mixes is investigated using high water to solids ratio samples to allow monitoring of pH and the solution chemistry during hydration. It is shown that a relatively small amount of Na-HMP inhibits the formation of Mg(OH)2 when MgO is hydrolyzed. It is proposed that this is due to adsorption of phosphate species on the MgO which inhibits the nucleation of the Mg(OH)2. This gives rise to high Mg2 + species in solution and elevated pH (> 12) conditions relative to when Mg(OH)2 forms. In contrast, the phosphate does not suppress formation of M-S-H gel. In combination with the enhanced dissolution rate of SiO2 at high pH, M-S-H gel can form quickly without competition for Mg2 + ions by Mg(OH)2 precipitation. Incorporating the optimum concentration of Na-HMP into the mix water therefore transforms the properties of cement paste and mortar samples formed by reacting MgO and SiO2.
Zhang T, Liang X, Li C, et al., 2016, Control of drying shrinkage in magnesium silicate hydrate (M-S-H) gel mortars, Cement and Concrete Research, Vol: 88, Pages: 36-42, ISSN: 0008-8846
Magnesium silicate hydrate (M-S-H) gel can be formed by the reaction of MgO with amorphous silica in the presence of sodium hexametaphosphate (Na-HMP). Typical pastes contain 40% MgO and 60% SF and have a w/c ratio of 0.5, but these exhibit shrinkage cracking on drying. The shrinkage characteristics of M-S-H mortar samples containing different additions of sand have been studied using dilatometry. The drying shrinkage was found to decrease with increasing sand addition and to increase with increased water content. Mortars with 60 wt.% sand addition and a w/c ratio of 0.5 had a drying shrinkage of 0.16% and did not show shrinkage cracking. A simple geometrical model based on particle packing is presented that explains the observed changes in drying shrinkage. Based on the geometrical model, the shrinkage of M-S-H mortar system can be reduced to zero when the volume fraction of sand in the mortar is about 0.77.
Zhang T, Liang X, Lorin M, et al., 2016, Control of drying shrinkage of magnesium silicate hydrate gel cements, ICMEN 2016, Publisher: Trans Tech Publications, Pages: 109-113, ISSN: 1013-9826
Cracks were observed when the magnesium silicate hydrate gel cement (prepared by 40% MgO/60% silica fume) was dried. This drying cracking is believed to be caused when unbound water evaporates from the binder. The shrinkage upon forced drying to 200 °C of mortars made up from a reactive magnesium oxide, silica fume and sand was measured using dilatometry. The magnitude of the drying shrinkage was found to decrease when more sand or less water was added to the mortars and can be as low as 0.16% for a mortar containing 60 wt% sand and a water to cement ratio of 0.5, which is of a similar order of magnitude as observed in Portland cement based mortars and concretes. A simple geometrical interpretation based on packing of the particles in the mortar can explain the observed drying shrinkages and based on this analysis the drying shrinkage of the hydration products at zero added solid is estimated to be 7.3% after 7 days of curing.
Feilden E, Garcia Tunon Blanca E, Giuliani F, et al., 2016, Robocasting of structural ceramic parts with hydrogel inks, Journal of the European Ceramic Society, Vol: 36, Pages: 2525-2533, ISSN: 0955-2219
Robocasting is a 3D printing technique that may be able to achieve the much-coveted goal of reliable, complex ceramic parts with low porosity and high strength. In this work a robust hydrogel formulation was optimised for use as the extrusion paste for robocasting. The paste’s rheological properties were characterised and the printing process was optimised with the aim of attaining dense monolithic ceramic parts. The pastes exhibit a characteristic shear thinning behaviour with yield stresses that can reach values above 1 kPa and depend mostly on their solid content and the particle size distribution. It is possible to formulate printable Al2O3 and SiC inks with solid contents as high as 40 vol% that reached densities up to 95th% for SiC and 97th% for Al2O3 with flexural strengths of 300 MPa and 230 MPa respectively after sintering. The sources of strength limiting defects are identified and related to the printing process.
Pettina M, Harrison RW, Vandeperre LJ, et al., 2016, Diffusion-based and creep continuum damage modelling of crack formation during high temperature oxidation of ZrN ceramics, Journal of the European Ceramic Society, Vol: 36, Pages: 2341-2349, ISSN: 0955-2219
ZrN’s good thermal and mechanical properties make it suitable for many commercial applications including in nuclear fuels. An understanding of its oxidation behaviour is essential to prevent catastrophic failures and ensure it is employed safely in nuclear power plants. Based on available experimental results on oxidation of ZrN in the temperature range 1173–1373 K, a continuum damage mechanics-based combined creep and time-dependent material oxidation model is proposed. The model allows for the development of a surface oxide layer combined with damage due to creep under an applied load. A representative grain structure has been modelled according to ZrN microstructural characteristics in order to allow intergranular cracking and individual oxidation damage rates for grains and grain boundaries. The proposed damage model is implemented as a user subroutine and runs in a coupled temperature-displacement analysis using the commercial finite element software Abaqus. Available data on ZrN are used to validate the capability of the model to predict oxidation damage in ceramics at high temperatures.
Nasiri NA, Saiz E, Giuliani F, et al., 2016, Grain bridging locations of monolithic silicon carbide by means of focused ion beam milling technique, Materials Letters, Vol: 173, Pages: 214-218, ISSN: 1873-4979
A slice and view approach using a focused ion beam (FIB) milling technique was employed to investigate grain bridging near the tip of cracks in four silicon carbide (SiC) based materials with different grain boundary chemistries and grain morphologies. Using traditional observations intergranular fracture behaviour and hence clear evidence of grain bridging was found for SiC based materials sintered with oxide additives. More surprisingly, in large grain materials, the FIB technique reveals evidence of grain bridging irrespective of the grain boundary chemistry, i.e. also in materials which macroscopically fail by transgranular failure. This helps to explain why the toughness of large grained materials is higher even if failure is transgranular.
Alex J, Vandeperre L, Lee WE, et al., 2016, Effect of Sodium on Microstructures and Thermoelastic Properties of Calcium Aluminate Cement-Bonded Refractories, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 99, Pages: 1079-1085, ISSN: 0002-7820
Vandeperre LJ, Wang X, Atkinson A, 2016, Measurement of mechanical properties using slender cantilever beams, Journal of the European Ceramic Society, Vol: 36, Pages: 2003-2007, ISSN: 1873-619X
The measurement of mechanical properties of materials only available in the form of thin sheets requires the use of load cells and displacement sensors of high sensitivity at low applied loads. These are available in testing platforms such as instrumented nano-indenters. In the current work, the elastic modulus and fracture toughness of thin cantilever beams of a representative brittle thin sheet material (300 μm thick NiO/YSZ support for a solid oxide fuel cell) were measured using a micro-/nano-indenter. The Young’s modulus and KIC were determined to be 139 ± 4 GPa and 2.13 ± 0.27 MPa m0.5 respectively using this method.
Jayaseelan DD, Xin Y, Vandeperre L, et al., 2015, Development of multi-layered thermal protection system (TPS) for aerospace applications, COMPOSITES PART B-ENGINEERING, Vol: 79, Pages: 392-405, ISSN: 1359-8368
Al Nasiri N, Ni N, Saiz E, et al., 2015, Effect of microstructure and grain boundary chemistry on slow crack growth in silicon carbide at ambient conditions, Journal of the European Ceramic Society, Vol: 35, Pages: 2253-2260, ISSN: 0955-2219
Silicon carbide (SiC) is being used increasingly as a room temperature structural material in environments where moisture cannot always be excluded. Unfortunately, there have been almost no reports on slow crack growth (SCG) in SiC at room temperature. To address this gap, SCG in SiC was studied using constant stress rate and double torsion tests in water. SiC based materials were produced with a wide range of grain boundary chemistries and microstructures, which may affect their slow crack growth behaviour. To clarify the role of chemistry and microstructure respectively, solid state (SS) sintering with carbon and boron along with liquid phase (LP) sintering using oxides additives were used to produce materials with fine and coarse grains. The LP-SiC was three times more sensitive to SCG than SS-SiC materials. Moreover, the larger grained material with a higher toughness was less sensitive to SCG than the materials with fine grains.
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