Mechanical Properties of Woodceramics: A Porous Carbon Material

The mechanical properties of woodceramics, which are new porous carbon materials utilizing the natural structure of wood, were investigated. The effects of burning temperature and amount of impregnated phenol resin on Young's modulus, compressive strength and fracture toughness were measured. The fracture morphology was then observed, and simplified mechanical models of the woodceramics were discussed to explain the mechanical properties. The fracture was initiated at the cell walls that were located in vertical direction against the applied stress. The effect of impregnated phenol resin on the Young's modulus and the compressive strength was reasonably explained by a wall-bending model.     

The comparison of mechanical behavior of MgO-MgAl2O4 with MgO-ZrO2 and MgO-MgAl2O4-ZrSiO4 composite refractories

Mechanical properties of different compositions obtained from the additions of 5, 10, 20 and 30 wt.% zircon (ZrSiO₄) into the MgO-spinel composite refractories and ZrO₂ into MgO have been examined, the variations that occurred have been determined, and the parameters affecting those factors have been investigated with the reasons. The density, strength, Young's modulus, fracture toughness, fracture surface energy and work of fracture were measured and evaluated. Microstructural variations and fracture surfaces have been examined and the formation of new phases has been identified depending on the additive type and quantity. The relationships between mechanical properties and structural variations for different compositions have been examined. In MgO-spinel materials, strength, Young's modulus and fracture toughness values decrease up to 20% spinel addition and stay almost constant for further loads. ZrO₂ addition displays same trend but not as effective as spinel. Besides, since ZrO₂ is stable in cubic form, it does not show any toughening mechanism. Forsterite formation is the most important factor for 2-fold improvement in the mechanical properties of MgO-spinel-zircon refractories. The more the zircon addition, the more the mechanical properties improve. The generation of natural bonding between matrix particles with forsterite formation, on the other hand, causes the fracture path to turn to transgranular fracture with an increase in fracture surface energy and a decrease in work of fracture, among which the latter is considered as an indicator of thermal shock resistance of the materials being high.     

Evolution of Mechanical Properties of Porous Alumina during Free Sintering and Hot Pressing

This comparative study addresses the influence of microstructural evolution on mechanical properties of porous alumina sintered with and without the application of uniaxial pressure. A complete set of data on Young's modulus, long-crack fracture toughness, and fracture strength for two alumina powders as a function of density was obtained. The evolution of fracture strength with increased density was modeled using a porosity-dependent crack-tip fracture toughness, linked to the contact area of grains and a porosity-dependent size of the largest defect. The defect shape factor was found independent of porosity. A uniaxial pressure of 13 MPa during densification had negligible effect on the relation of strength to porosity.     

Microstructure and Mechanical Properties of Silicon Nitride Ceramics with Controlled Porosity

Porous silicon nitride ceramic with a porosity from 0–0.3 was fabricated by partial hot-pressing of a powder mixture of α-Si₃N₄ and 5 wt% Yb₂O₃ as sintering additive. Irrespective of the porosity, the samples exhibited almost the same microstructural features including grain size, grain aspect ratio, and pore size. Porosity dependences of Young's modulus, flexural strength, and fracture toughness ( K _{I C} ) were investigated. All these properties decreased with increasing porosity. However, because of the fibrous microstructure, the decreases of flexural strength and fracture toughness were moderate compared with the much greater decrease of Young's modulus. Thus, the strain tolerance (fracture strength/Young's modulus) increased with increasing porosity. The critical energy release rate also increased slightly with an increasing volume fraction of porosity to 0.166 and remained at the same level with that of the dense sample when the porosity was 0.233. They decreased as porosity increased further.     

Mechanical Properties of Dry-Pressed Powder Compacts: Case Study on Alumina Nanoparticles

Young's modulus and fracture toughness of dry-pressed powder compacts of nanocrystalline alumina powders have been determined for different relative densities and average grain size. The powder with a grain size of 40 nm yielded a maximum Young's modulus of 15 GPa and a peak fracture toughness of 0.12 MPa·m^1/2. These high values are rationalized using a model based on contact flattening between particle spheres, which allows a discussion of the influence of grain size and particle packing on the mechanical properties of green bodies.     

Influence of porosity on the mechanical behaviour of single phase β-TCP ceramics

Processing and mechanical behaviour of fine grained (diameter ≈0.5–3 µm) and pure β-TCP materials with different levels of porosity (up to 19%) is described. Pores with diameters, d₅₀ ≈13–14 µm were formed fromcorn starch during sintering. Comprehensive mechanical characterisation –Young´s modulus, strength and toughness– has been done paying special attention to toughness determined in stable fracture tests. The dependence of Young´s modulus and strength with porosity was well fitted to the minimum solid area models while toughness values did not. The competitive processes occurring during fracture impede the degradation of toughness associated to the decrease in Young´s modulus as porosity increases. Materials present similar values of the critical energy release rate G_IC, which describes crack initiation. A maximum of the specific fracture energy, G_F, which averages crack propagation, has been obtained for the material with the highest porosity.     

Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R -curve behavior in these materials.     

Porosity and mechanical properties of cement mortar

The effect of volume fraction porosity on the mechanical properties of cement mortar is studied. It is shown that both the Young's modulus and fracture toughness decrease with porosity. Although the flexural strength also decreases with porosity the linear relationship is largely fortuitous. Maximum size pores do not act as critical flaws in controlling flexural strength. The critical crack size is several times larger than the maximum pore size due to stable crack growth according to the crack growth resistance curve concept applied to cement mortar.     

Silicon Carbide Platelet/Alumina Composites: II, Mechanical Properties

The mechanical properties, i.e., Young's modulus, fracture toughness, and flexural strength, of SiC-platelet/Al₂O₃ composites with two different platelet sizes were studied. Both Young's modulus and the fracture toughness of composites using small platelets (12 μm) increased with increasing SiC volume fraction. Maximum values for toughness and Young's modulus of 7.1 MPa·m^1/2 and 421 GPa were obtained for composites containing 30 vol% platelets. Composites fabricated using larger platelets (24 μm), however, showed spontaneous microcracking at SiC volume fractions of ≤0.15. The presence of microcracks decreased Young's modulus and the fracture toughness substantially. Two types of radial microcracks were identified by optical microscopy and found to be consistent with a residual stress analysis. Anisotropy in fracture toughness was identified with a crack length indentation technique. Cracks propagating in a plane parallel to platelet faces experienced the least resistance, which was the the lowest toughness plane in platelet composites with preferred orientation. Enhanced fracture toughness was found in the plane parallel to the hot-pressing direction, but no anisotropy in toughness was observed in this plane. The flexural strength of alumina showed a decrease from 610 to 480 MPa for a 30 vol% composite and was attributed to the presence of the platelets.     

Effects of Neutron Irradiation on the Mechanical Properties of Magnesium Aluminate Spinel Single Crystals and Polycrystals

The effect of fast neutron irradiation on the mechanical properties of magnesium aluminate spinel single crystals and polycrystals, such as bending strength, fracture toughness, Young's modulus, and hardness, was summarized based on the reported data. Essentially, the changes in these properties are dependent on the nature and concentration of neutron-irradiation-induced defects. Furthermore, the efficiency of damage accumulation is known to be dependent on temperature. Spinel shows superior resistance to the formation of defect aggregates, and the recombination of point defects occurs efficiently under neutron irradiation; therefore, the change in mechanical properties is not critical, up to very high neutron fluences, and the spinel maintains its structural integrity.     

Thermomechanical properties of Y-substituted SrTiO3 used as re-oxidation stable anode substrate material

Re-oxidation robustness is important to warrant a reliable operation of anode-supported solid oxide fuel cell systems. The current work concentrates on the mechanical properties of re-oxidation stable Y-substituted SrTiO₃ ceramic for the use as anode substrate material. Room temperature micro-indentation yielded Young's modulus and hardness of 160 and 7 GPa, respectively, whereas the temperature-dependent modulus was measured with a resonance-based method up to ∼950 °C. The effective Young's modulus as a function of porosity was measured at room temperature and compared with fracture strength data. The fracture toughness was assessed using a combination of pre-indentation cracks and bending test. Creep rates were measured at 800 and 900 °C in a 3-point bending configuration. Post-test fractographic analysis performed using stereo, confocal and scanning electron microscopy, revealed important information on fracture origins and critical defects in the material. A methodology to assess the mechanical properties of porous materials is suggested.     

Mechanical Properties of PMMA‐Sepiolite Nanocellular Materials with a Bimodal Cellular Structure

Bimodal cellular poly(methyl methacrylate) with micro‐ and nano‐sized (300–500 nm) cells with up to 5 wt% of sepiolite nanoparticles and porosity from 50% to 75% are produced by solid‐state foaming. Uniaxial compression tests are performed to measure the effect of sepiolite concentration on the elastic modulus and the yield strength of the solid and cellular nanocomposites. Single edge notch bend tests are conducted to relate the fracture toughness of the solid and cellular nanocomposites to sepiolite concentration. The relative modulus is independent of sepiolite content to within material scatter when considering the complete porosity range. In contrast, a mild enhancement of the relative modulus is observed by the addition of sepiolite particles for the foamed nanocomposites with a porosity close to 50%. The relative compressive strength of the cellular nanocomposites mildly decreases as a function of sepiolite concentration. A strong enhancement of the relative fracture toughness by the addition of sepiolites is observed. The enhancement of the relative fracture toughness and the relative modulus (at 50% porosity) can be attributed to an improved dispersion of the particles due to foaming and the migration of micro‐sized aggregates from the solid phase to the microcellular pores during foaming.     

Influence of porosity on mechanical properties of tetragonal stabilized zirconia

3YSZ specimens with variable open porosity (1–57%) were fabricated, and the stiffness, strength and fracture properties (fracture toughness and R-curve) were measured to investigate their potential use as support structures for solid oxide fuel or electrolysis cells. The ball-on-ring test was used to characterize Young's modulus and Weibull strength. The variation of fracture toughness with porosity was investigated and modelled using the results from fracture mechanical testing. A distinct R-curve behaviour was observed in dense 3YSZ specimens, in samples with a porosity around 15% and in some of the highly porous samples (porosities ～45%) reflecting a transformation toughening in the material. For the most porous samples, the “R-curve behaviour” disappeared and subcritical crack growth was observed. The studies indicate that even highly porous 3YSZ structures (porosities exceeding 40%) are feasible supports for SOFC/SOECs from a mechanical point of view.     

Mechanical Behavior of Lightweight Ceramics Based on Sintered Hollow Spheres

A simple micromechanical model, describing the Young's modulus, fracture toughness, and density of a lightweight ceramic structure produced by the sintering of hollow spheres and based on shell theory, was developed. The approach combined an analysis of the elastic deformation using shell theory and a simple analysis of the densification in terms of the micro-structural parameters. The results were compared with experimental data for materials produced by the sintering of hollow glass spheres and gave reasonable to good agreement, especially in the prediction of the nonlinear variation of Young's modulus and fracture toughness with density. The model should provide a useful basis for rationalizing the relation between mechanical properties and fabrication procedure for such materials.     

Mechanical behaviour of a low-clay translucent whiteware

A novel low-clay translucent whiteware body, using mostly non-plastic prefired materials and only a small amount of clay, was fabricated by slip casting and the effect of slip's solid content and sintering temperature on the mechanical behaviour was investigated. The degree of densification in the sintered specimens was determined by measuring the bulk density. The mechanical behaviour was determined by measuring the flexural strength and fracture toughness. Young's modulus and hardness were also measured. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies were carried out to analyse the microstructure.The flexural strength and fracture toughness increase with both increasing the slip's solid content and the sintering temperature up to a certain level, but further increase in solid content and sintering temperature had an adverse effect on the properties. The maximum flexural strength (∼135 MPa) and fracture toughness (∼1.85 MPa m^1/2) values were attained with specimens produced from a slip having 45 vol.% solid content at a sintering temperature of 1350 °C. It was found that the amount and distribution of closed pores, their size and possible link with each other control the flexural strength and fracture toughness of the low-clay translucent whiteware.     

Determination of hardness, Young's modulus and fracture toughness of lanthanum tungstates as novel proton conductors

Lanthanum tungstate is a promising material to be used as electrolyte in proton conducting fuel cells, or as a mixed proton-electron conducting membrane for hydrogen separation, and its mechanical properties are crucial for these applications. Lanthanum tungstates with a La/W atomic ratio between 4.8 and 6.0 have been investigated at room temperature at micro/nanoindentation range. Lanthanum tungstates exhibit a strain gradient plasticity at the vicinity of the imprints, which implies that the hardness presents an indentation size effect that was corrected using the Nix and Gao approach. The hardness and Young's modulus have therefore been determined to be 8-9 GPa and 130 ± 15 GPa, respectively. The fracture toughness was estimated to be ∼2 MPa m^1/2 for LWO56 using the Palqmvist equation. Both hardness and Young's modulus did not present a significant dependence with neither the sintering temperature nor the composition. The different imprints were visualized by Atomic Force Microscopy.     

Mechanical properties of solid oxide fuel cell glass-ceramic sealants in the system BaO/SrO-MgO-B2O3-SiO2

Planar solid oxide fuel cells (p-SOFCs) require materials that can satisfy the high mechanical demands related to their utilization in stationary and, especially, in mobile applications. Two suitable glass-ceramic sealants based on the system BaO/SrO-MgO-B₂O₃-SiO₂ have been characterized with respect to their mechanical properties such as hardness, Young’s modulus, flexural strength at room and elevated temperature, fracture toughness as well as creep behavior at relevant operation temperatures (800 °C). Fracture toughness was calculated from crack opening displacements (COD) and the results were compared with fracture toughness measured by bending tests of notched bar samples. The mechanical behavior has been discussed regarding different thermal aging times of the glass-ceramics and their microstructural evolution. The glass-ceramics containing SrO revealed a better mechanical behavior than glass-ceramics with BaO. In particular, several superior properties were found in comparison to previously reported materials for this application.     

Porcelain tile microstructure: Implications for polished tile properties

The present study was undertaken to determine the influence of sintered porcelain tile microstructure on mechanical properties (fracture strength, modulus of elasticity and fracture toughness) and surface properties (gloss and stain resistance). To obtain sintered specimens with different microstructures the peak firing temperature was varied for bodies made with industrial spray-dried powder, and sets of test compositions were also made in which quartz content and quartz particle size were varied.Liquid-phase sintering is the typical densification mechanism involved in the achievement of minimum porosity, which is characterised by isolated round pores. Bloating occurred above the firing temperature for minimum porosity. Increases in quartz content and quartz particle size in the starting composition led to reduced body sinterability, and thus gave rise to higher porosity in the fired tile.Mechanical properties were adversely affected by an increase in fired tile porosity. For the same variation in porosity, mechanical properties were more sensitive to the change in quartz content than to changes in particle size. No toughening effect was observed with a rise in quartz content or a decrease in particle size: mechanical properties depended primarily on sintered specimen porosity.Gloss and stain resistance (which characterise polished surface quality) varied with surface porosity, both showing the highest values for lowest porosity. The relationship between porosity and gloss was close to linear.     

Temperature Dependence and Fracture Toughness and Elastic Moduli of a Waste Glass

The temperature dependence of elastic moduli, of the crack lengths formed by Vickers indentations, and of the hardness of the nuclear waste borosilicate glass GP 98/12 has been measured up to 300°C. The temperature dependence of the fracture toughness K_IC exceeds that predicted by the variations of Young's modulus E and hardness H with temperature.