Toughness Of Ceramics Research Articles

PHASE EQUILIBRIA IN THE Al2O3–TiO2–Y(Er)2O3 SYSTEMS AT 1400 ºС

The aim of this investigation is the construction of isothermal sections for the Al2O3-TiO2-Y(Gd)2O3 phase diagrams at 1400ºС as part of systematic investigations of Al2O3-TiO2-Ln2O3 (Ln=lanthanides, Y) systems. The 1400°C was taken as the temperature, at which no liquid phases are expected in the both systems. The isothermal sections at 1400 °С for the Al2O3– TiO2–Y(Er)2O3 phase diagrams were constructed for the first time. Samples were prepared by a chemical method. Samples were annealed in air at 1400°С for 80 hours and cooled in the furnace. This temperature value was selected with a view to provide possibility of phase transformations character studies. New phases and appreciable solubility regions based on the components and binary compounds were not found, as predicted. Triangulation of the systems is determined by the phase Y(Er)2T2O7, which is in equilibria with compounds Al2TiO5, Y(Er)3Al5O12, Y(Er)AlO3, Y(Er)4Al2O9 and components TiO2 and Al2O3. The structures of isothermal sections of the both systems are similar. The systems are triangulated into six secondary triangles, in which three-phase eutectic are expected. In five quasibinary sections two-phase eutectic should expect to exist. The obtained results will make a significant contribution to the understanding of interactions between the components in the systems studied.This systems offer a number of promising opportunities such as high-temperature structural composites based on directionally solidified two-phase and three-phase eutectic materials, solid electrolytes (SOFCs, oxygen sensors, film for electronic devices, etc.), promising to accumulate water, immobilizing materials for nuclear industry, tough ceramics, catalysts carriers, wear- and corrosion-resistant ceramic coatings and super refractories.

A New Formula for Evaluating Indentation Toughness in Ceramics

The Lawn-Evans-Marshall model for Vickers-induced crack driving force is widely applied for determining fracture toughness of ceramic materials, yet its simplification that the volumes of Vickers impression and equivalent expanding cavity merely depend on the half-diagonal length of residual impression will bring significant error. In this work, the model is modified by considering the influence of elastic recovery in depth after completely unloading on the volumes. As a result, a new expression for crack driving force is obtained and correspondingly a new fracture toughness formula is presented based on the modified model, which is expressed as the well-known Anstis formula with a different constant multiplied by the ratio of residual impression depth to the maximum indentation depth. Both finite element analysis and experimental results demonstrate the effectiveness of the proposed formula and, especially, that it works much better than Anstis formula on materials with large ratio of hardness to elastic modulus.

Design and Processing of Novel Ceramic Composite Structures for Use in Medical Surgery

. In order to fulfill the clinical requirements for strong, tough and stable ceramics for dental applications, we have designed and developed innovative Ceria-stabilized zirconia (Ce-TZP)-based composites. In particular, we have added two kinds of second phases to the Ce-TZP matrix: equiaxed a-Al2O3 grains, for increasing the hardness and the fracture strength, and elongated hexa-aluminates (both SrAl12O19 and CeMgAl11O19), to provide an additional toughening effect by crack deflection/bridging mechanisms. In order to carefully control the composition and the microstructure in those complex composite systems, we have used a novel surface-coating approach for the preparation of the nanostructured composite powders, which allows a perfect tailoring of the microstructural, morphological and compositional features of the composites. Once optimized the sintering cycle for each composite material, both composites reached full densification. Mechanical properties (Vickers hardness, flexural strength and fracture toughness) were evaluated, while the zirconia transformability was followed by means of an optical microscope during load-unload bending tests. The sensitivity to ageing was estimated by autoclave treatments. In spite of a remarkable different behavior – mainly in terms of stress-induced tetragonal to monoclinic zirconia transformability - both materials showed excellent mechanical properties as well as a negligible sensitivity to ageing, thus demonstrating their high potential for new reliable and safe devices for structural biomedical applications.

Picosecond amorphization of SiO2 stishovite under tension.

It is extremely difficult to realize two conflicting properties-high hardness and toughness-in one material. Nano-polycrystalline stishovite, recently synthesized from Earth-abundant silica glass, proved to be a super-hard, ultra-tough material, which could provide sustainable supply of high-performance ceramics. Our quantum molecular dynamics simulations show that stishovite amorphizes rapidly on the order of picosecond under tension in front of a crack tip. We find a displacive amorphization mechanism that only involves short-distance collective motions of atoms, thereby facilitating the rapid transformation. The two-step amorphization pathway involves an intermediate state akin to experimentally suggested "high-density glass polymorphs" before eventually transforming to normal glass. The rapid amorphization can catch up with, screen, and self-heal a fast-moving crack. This new concept of fast amorphization toughening likely operates in other pressure-synthesized hard solids.

Experimental investigation into the crack propagation in multiphase tantalum carbide ceramics

Tantalum carbide ceramics with high volume fractions of the ζ-Ta4C3 phase have been shown to exhibit high fracture strength and toughness as compared to those in absence of this phase. In this work, we investigated how microcracks propagated in this these high toughness ceramics using Knoop and Vickers microindentation. The Knoop indentations demonstrated that cracking preferentially occurred parallel to the lath structure in ζ-Ta4C3; however shorter cracks did form between the laths when a sufficient driving force was present. The resulting crack path was tortuous providing direct evidence for toughening through crack deflection; however, the microscale nature of the work cannot rule out crack bridging as a toughening mechanism as well. Plasticity is also observed under the indents, but is likely a result of the high confining pressures that occurred during indentation allowing for plastic flow.

In situ reactive synthesis and plasma-activated sintering of binderless WC ceramics

ABSTRACTBinderless micrometre tungsten carbide ceramics (∼1.1 μm) were in situ synthesised and densified by plasma-activated sintering (PAS) from mixed powders of tungsten trioxide and carbon black. The influence of sintering process and powders’ composition ratio on the phase composition, microstructure and mechanical properties of as-prepared samples was clarified in detail. The phase evolution was ascertained by X-ray diffraction to be WO3→WO2.72→WO2→W2C→WC, with the formation of CO and CO2 gases. The sample with nearly single WC phase, dense structure and excellent mechanical properties was fabricated under the optimised process with the proper composition ratio. Owing to the micrograin size, the fracture toughness (8.88 MPa m1/2) was enhanced while high hardness (2159 HV10) was maintained, in the absence of any ceramic toughening phase.

Studies for Improved Damage Tolerance of Ceramics Against Ballistic Impact Using Layers

Ceramics are lightweight alternatives to metal armour. However, brittle behavior of monolithic ceramics impedes their application to armour. One approach for improving toughness of ceramics could be by using layered ceramics. In this study, we evaluated layered ceramics for armour applications based on numerical analysis validated with experiments. Consistent to the trends in literature, we observed that layers degrade the resistance to ballistic impact. However, improved energy absorption is demonstrated by layered ceramics. These conflicting dual trends were not presented and quantified in any earlier studies conducted elsewhere. Another new observation not documented earlier is the effect of interface strength. Using an interface material of sufficient strength, penetration resistance of layered ceramics can be improved beyond monolithic ceramics. Using these findings, new layered ceramic armour can be designed that is cost-effective and better performing than monolithic ceramics.

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100°C and 1450°C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO2 transformed into monoclinic ZrO2; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300°C, and the lowest values were observed at 1200°C. The highest flexural strength and fracture toughness of ceramics reached 348.27MPa and 5.23MPam1/2 when sintered at 1450°C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250°C, and in this range, a low brittleness index and moderate strength of 0.74–1.19µm−1/2 and 46.89–120.15MPa, respectively, were realized.

Fracture toughness in some hetero-modulus composite carbides: carbon inclusions and voids

ABSTRACTStructure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B4C powder mixtures at 1800–1950°C under 30 MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX). The results show that during hot pressing solid-phase chemical reaction 2TiC + B4C = 2TiB2 + 3C has occurred with final products like TiB2–TiC–C, TiB2–C or TiB2–B4C–C hetero-modulus composite formation with around one micrometer size carbon precipitates. The fracture toughness depends on the amount of graphite precipitation and has a distinct maximum K1C = 10 MPa m1/2 at nearly 7 vol.-% of carbon precipitate. The fracture toughness behaviour is explained by the developed model of crack propagation. Within the model, it is shown that pores (voids) and low-modulus carbon inclusions blunt the cracks and can increase ceramic toughness in some cases.

Reinforcement of bone china by the addition of sepiolite nano-fibers

The work attempts to investigate the effect of sepiolite nano-fibers addition on mechanical properties of bone china bodies. Compared with the traditional toughening fibers, the sepiolite nano-fiber has two main advantages: One is that the chemical composition of sepiolite mineral and the chemical composition of bone china raw materials are similar. So it can be used as one of the main components of bone china formulations directly. The other is that the sepiolite is a kind of cheap mineral resources, which will decrease the cost of the toughening ceramics. The flexural strength and fracture toughness were tested by three point bending method and single edge notched beam method, and the microstructure of the bone china was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The results showed that 2wt% sepiolite fiber addition could increase the bending strength of the ceramic bodies. There were two toughening mechanisms in the reinforced ceramic. One was the fiber pull-out. The other was the weak interface mechanism. Plenty of micro-cracks came into being at the weak interface to consume most of the elastic strain energy to prevent main crack from spreading.

Influence of Confinement on Ceramic’s Mechanical Properties

The wide use of ceramic material in engineering is restricted by its brittleness, so the strengthening and toughening of ceramics is always a hot spot of research in material area. And in general, the modification of ceramics is achieved by changing its internal microstructure. In this paper the influence of confinement on the mechanical properties of ceramics and the specific use of this method were investigated. Firstly, the influence of confinement on ceramic’s fracture process was analyzed in theory. Then the three-point bending test was conducted using two types of ceramics, viz. Zirconia and Alumina. The experimental results showed that the fracturing load of zirconia increased from 4.3298 to 5.4639KN as the confinement was increased from 0 to 150MPa, 26.19% increase was found in the confined specimen. The same trend was observed in alumina, whose fracturing load increased from 3.0446 to 5.0259KN as the confinement was increased from 0 to 150MPa, 65.07% increase was found. After that, a series of ballistic experiments were performed. The target in this experiment was boron carbide ceramic, and it was confined by 45 steel. The results showed that with the constraint force was bigger, the ballistic efficiency factor was better and the depth of penetration was smaller. In other words, the confinement can increase the defensible performance of the target. In summary, the ceramic’s fracture toughness, defensible performance and ballistic efficiency factor can be increased by adding confinement to it.

Tribological performance of ceramics in lubricated ultrasonic motors

Small ultrasonic motors are driven by the oscillation of piezoelectric transducers. They are primarily applied to equipment components that require precise movements, such as drivers for auto-focus lenses in cameras, and actuators in robots and positioning devices. While lubrication has been proven effective in improving the performance of ultrasonic motors, the friction materials still play a significant role. In this study, the suitability of four different engineering ceramics, which included alumina (Al2O3), zirconia (ZrO2), silicon carbide (SiC), and silicon nitride (Si3N4), were investigated in the self-mated configuration. By testing the candidate materials in the actual ultrasonic motor configuration, the main wear mechanisms of these ceramics were identified under lubricated conditions, with fracture being the most dominant contributor to their wear. Among the four tested ceramics, ZrO2 was found to be the most promising material as it exhibited the mildest wear and a satisfactory level of friction. The obtained results also indicate that the fracture toughness of ceramics is more critical than their hardness. This study provides guidelines for selecting friction materials that can be used in long-lived, lubricated ultrasonic motors.

Numerical Study of Deformation and Fracture of Ceramics Nanocomposite with Different Structural Parameters under Mechanical Loading

Deformation, fracture and effective mechanical properties of sintered ceramics composite under uniaxial compression were studied. To perform this investigation the plain numerical model of ceramics composites based on oxides of zirconium and aluminum with different structural parameters was developed. The model construction was carried out within the frame of particle based method, namely the movable cellular automaton method (MCA). The implementation of the phase transition in the MCA-model composite was carried out on the basis of the phenomenological approach, the main point of which was the formulation of the principle of irreversible mechanical behavior of the material. Increase the fracture toughness of ceramics after (T-M) transition in its structure was realized in the model by introducing transition kinetics of the automata pair from "bound" to an "unbound" state. The structure of model composite was generated on the basis of scanning electron microscope images of micro-sections of real composite. The influence of such structural parameters as geometrical dimensions of layers, inclusions, and their spatial distribution in the sample, volume content of the composite components and their mechanical properties, as well as the amount of zirconium dioxide undergone the phase transformation on the mechanical response were investigated

Application of the coupled stress-energy criterion to predict the fracture behaviour of layered ceramics designed with internal compressive stresses

One novel approach to improve the apparent toughness of ceramics is to design a multilayer architecture with embedded layers having compressive residual stresses. Surface cracks propagating during mechanical loading can be deflected within the compressive layers, in order to delay the final fracture of the whole structure. The design of high toughness laminates requires understanding the effect of residual stresses on the initiation and propagation of cracks in the material. In this work, a coupled stress-energy criterion is used to predict the initiation and propagation of surface cracks in ceramic laminates upon thermo-mechanical loading. Experiments were conducted on V-notched alumina-based laminates to show the effect of residual stresses and mechanical loading on their fracture behavior. The conditions for crack initiation as predicted for notched specimens agreed with the experimental observations. It is shown that the onset of cracks from V-notches is associated with (i) the tensile residual stresses in the first surface layer and (ii) the depth of the notch. The further propagation of the crack into the first embedded compressive layer was also studied. Based upon the coupled criterion, a short penetration of the propagating crack into the first compressive is foreseen. If the mechanical load is increased, the crack finally deflects within the compressive layer propagating with a certain angle which is also predicted with a good accuracy.

Multilayered Ceramic Composites – A Review

Fracture toughness enhancement of ceramic materials through multilayered ceramic composites has been developed since 1990. Toughening mechanisms are based mainly on delamination, deflection, bifurcation or crack arrest effect. Delamination and crack deflection occur by means of weak interfaces. Bifurcation (and deflection as well) and crack arrest effects are result of residual stresses arising from the thermal expansion coefficient mismatch or phase transformation on alternating layers. The main manufacturing methods of these composites are slip casting of two ceramic materials, and stacking and pressing of ceramic tapes obtained by tape casting or rolling technics, followed by suitable sintering process. This review aims to present general aspects of research performed around the theme so far. It is verified that occurs the enhancement of ceramic toughness and reliability with this technic, so it is possible to enlarge its range of application in engineering.

On the measurement of ceramic fracture toughness using single edge notched beams

The single edge notched beam (SENB) method is a popular method for measuring the fracture toughness of ceramics. However, standard 3-point or 4-point bend loading of notched beams is not always suitable, especially for small or micro specimens. Different test geometries may be necessary to meet different requirements of special specimens. Therefore a consistent theoretical analysis of different tests involving notched beams is essential for a better understanding of test results. Here a new approach is developed for calculating KIC from bending tests of notched beams, which is generally applicable to all bending beam configurations. The equations derived using this approach are simple in form and are accurate when compared with finite element simulation. This approach also provides a more intuitive understanding of the effect of notch geometry on measurements using this and other flexural methods.

Critical radius of zirconia inclusions in transformation toughening of ceramics

The paper presents a transformation toughening model of ceramics taking into account an energy barrier the overcoming of which results in phase transformation of zirconia inclusions. Methods based on experimental data analysis are proposed for estimating the energy barrier. The size range of zirconia inclusions in Al2O3 and WC matrices is defined depending on the energy barrier value, working temperature, and external load. It is shown that the introduction of an energy barrier enables an adequate estimation of the size range of inclusions at which transformation toughening occurs in ceramics. The elastic interaction of inclusions is shown to cause a decrease in their critical radii with the growing volume density, which agrees with experimental data.

Synthesis and Characterization of Zirconium Oxides for Use as Antimicrobial Additives in Paints

Zirconia is a widely used multifunctional material. Its interesting properties, such as high chemical resistance, thermal stability and high mechanical toughness, have turned this material into object of study within fields such as optics, electronics, and magnetism, among others. In recent years, the chemical properties of zirconium oxide have been used, mixed with silicon, in order to obtain more stable and robust mixed oxides, with the final application of toughening ceramics. These properties are related to the degree of mixing of the components at molecular level, therefore, the linking between Si-O-Zr. Considering the effective antimicrobial capacity of silver nanoparticles, their incorporation into different materials is very interesting, acting as biocides. The objective of this work is to obtain zirconium oxides by the sol-gel technique, using zirconium n-propoxide as precursor, and also to study the effect of different solvents (water, ethanol) and the use of various acid catalysts in the synthesis [acetic acid and a heteropolyacid (HPA), as the phosphomolybdic acid (H3PMo12O40)] on the physicochemical properties of the obtained solids. Finally, the addition of a silver salt was carried out into the sample with acetic acid and heteropolyacid as catalyst, in order to incorporate it as antimicrobial filler in paints. The obtained materials were characterized by SEM, XRD, FT-IR, textural properties through the absorption of N2 (SBET) and potentiometric titration with n-butylamine. Homogeneous solids were obtained in all the synthesis with acetic acid as catalysts. In addition, x-ray diffraction diagrams of amorphous solids were observed. The values obtained of surface areas are influenced by the variables of synthesis. The characteristic bands of zirconia were observed by FTIR in the solids synthesized. The results show that the solids environmentally synthesized are promising additives for use in paints.

Interfacial stress transfer in a graphene nanosheet toughened hydroxyapatite composite

In recent years, graphene has emerged as potential reinforcing nanofiller in the composites for structural engineering due to its extraordinary high elastic modulus and mechanical strength. As recognized, the transfer of stress from a low modulus matrix to a high-modulus reinforcing graphene and the interfacial behavior at a graphene-matrix interface is the fundamental issue in these composites. In the case of graphene nanosheet (GNS) reinforced hydroxyapatite (HA) composite, this research presented analytical models and simulated that the number of graphene layers of GNSs has little effect on the maximum axial stress (∼0.35 GPa) and the maximum shear stress (∼0.14 GPa) at a GNS-HA interface, and the energy dissipation by GNS pull-out decreases with increasing the number of graphene layers due to weak bonding between them. Also, GNS-HA interfacial delamination and/or GNS rupture were also indentified to be the two key failure mechanisms. The computed results are expected to facilitate a better understanding of the interfacial behavior at a GNS-ceramic interface and to achieve tough ceramics reinforced with GNSs.

Fracture-induced amorphization of polycrystalline SiO2 stishovite: a potential platform for toughening in ceramics.

Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.