Phase Field Modeling of Crack Growth in Shape Memory Ceramics

Shape memory ceramics (SMCs) are promising candidates for actuators in extreme environments such as high temperature and corrosive applications. Despite outstanding energy dissipation, compared to metallic shape memory materials, SMCs suffer from a sudden brittle fracture. While the interaction of crack propagation and phase transformation in SMCs has been the subject of several experimental and theoretical studies, mainly at the macroscale, the fundamental understanding of the dynamic interaction of crack propagation and martensitic transformation is poorly understood. This dissertation attempts to provide a mathematical model for crack propagation in transformable zirconia to address the shortage of classical methods. This dissertation uses the phase field framework to fully couple the martensitic transformation to the variational formulation of brittle fracture. Firstly, the model is parameterized for single crystal zirconia, which experiences tetragonal to monoclinic transformation during crack propagation. For mode I of fracture, the opening mode, crack shows an unusual propagation path that is in good agreement with the experiments and indicates the significant role of phase transformation on the crack propagation path. The investigation on the effect of lattice orientation on crack propagation shows that the lattice orientation has a significant influence not only on the crack propagation path but also on the magnitude of the transformation toughening. Secondly, the model is parameterized for tetragonal polycrystalline zirconia, and the experimental data from literature were used to validate the model. The model predicts the three dominant crack propagation patterns which were observed experimentally, including the secondary crack initiation, crack branching, and grain bridging. The model shows the critical role of texture engineering in toughening enhancement. Polycrystalline zirconia samples with grains that make low angles between the a-axis in the tetragonal phase and the crack plane, show higher transformation toughening, due to maximum hydrostatic strain release perpendicular to the crack tip. The model also shows the grain boundary engineering as a way to enhance the transformation toughening. The maximum fracture toughness occurs at a specific grain size, and further coarsening or refinement reduces the fracture toughness. This optimum grain size is the consequence of the competition between the toughening enhancement and MT suppression with grain refinement. Finally, we parameterized the model for the 3D single crystal zirconia, which experienced stress- and thermal-induced tetragonal to monoclinic transformation. The developed 3D model considers all 12 monoclinic variants, making it possible to acquire realistic microstructures. Surface uplifting, self-accommodated martensite pairs formation, and transformed zone fragmentation were observed by the model, which agrees with the experimental observations. The influence of the crystal lattice orientation is investigated in this study, which reveals its profound effects on the transformation toughening and crack propagation path.

Revolutionary High-entropy Zr-Y-Yb-Ta-Nb-O Oxides for Next Generation Thermal Barrier Coating Applications

We report a revolutionary ceramic material with exceptional high temperature stability and superior thermo-mechanical properties for next generation thermal barrier coatings (TBCs) for aeroengines. The multicomponent oxides (Zr1 − 4xYxYbxTaxNbxO2) designed via a high entropy concept could exhibit a double tetragonal phase. The optimized composition breaks the limitation of intrinsic brittleness in previously reported TBC candidate materials and shows a superior toughness up to ~ 4.59 MPa m1/2 due to ferroelastic and phase transformation toughening mechanisms. It also shows a remarkable high temperature stability at 1600 ºC, which is almost 400 ºC higher than the state-of-the-art yttria stabilized zirconia TBC material. In addition, it also exhibits a significantly lower thermal conductivity (~ 1.37 W∙m− 1∙K− 1 at 900 ºC) and a higher coefficient of thermal expansion (~ 11.3 × 10− 6 K− 1 at 1000 ºC), as well as excellent corrosion resistance to molten silicate (~ 2.9 µm/h at 1300 ºC). This work provides a new approach to design ceramics by extending the high-entropy concept to both medium-entropy and high-entropy compositions searching for multifunctional properties.

Physico-mechanical properties and microstructure of multi-phase ceramic composites based on zircon and dolomite mixtures

Four composites containing zircon and dolomite were designed by adding dolomite from 5wt% to 35wt% at the expense of zircon content. Densification parameters in terms of bulk density, apparent porosity and linear change were determined at different firing temperatures (1200°C–1400°C). Cold crushing strength of sintered composites, phase composition and microstructure were investigated. Samples contain 35wt% of dolomite and fired at 1200°C for 2 hours exhibited higher porosity (AP ∼ 51.25%) than other samples and can be used as porous ceramics. This is due to CO2 evaporation through the thermal decomposition of dolomite. Dense ceramics can be obtained by adding 5wt% of dolomite and fired at 1400°C for 2 hours (bulk density ∼3.67 g/cm3 and apparent porosity ∼4.5%). Only zirconia and diopside crystalline phases were detected at composite containing 35wt% of dolomite and fired at 1400°C. Due to the liquid phase sintering process, the densification parameters of the sintered samples have been enhanced by increasing the temperature. The mechanical properties of the sintered samples were improved due to the transformation toughening mechanism of tetragonal zirconia. Microstructure and EDAX analysis of the sintered composites show the presence of sub-prismatic zircon and rounded zirconia crystals as well as irregularly dark diopside crystals.

Cyclic fatigue behaviour of hydrothermally aged 3Y-TZP ceramics in 4-point bending tests

Fatigue is one of the most important properties to be considered in ceramic dental implants due to cyclic mechanical stresses arising from the chewing process. In this work, the fatigue behaviour of hydrothermally degraded ZrO2-based ceramics stabilized with 3mol% Y2O3 (3Y-TZP) was studied in 4-point bending tests. Samples of 3Y-TZP were compacted (100MPa), sintered at 1475 ?C for 2 h, polished and hydrothermally degraded in an autoclave as described in the ISO-13356 standard. The samples were characterized by their relative density, crystalline phase composition, microstructure and surface roughness. The highly dense (>99.6%TD) sintered 3Y-TZP ceramics has only tetragonal t-ZrO2 phase, even after hydrothermal ageing. Furthermore, the ceramic materials presented a Vickers hardness of 12.7?0.2GPa, a fracture toughness of 7.1?0.3MPa?m1/2 and a 4-point bending strength of 940.1?67MPa. Based on the bending test results 5 different stress levels for the fatigue tests were selected and conducted by cyclic 4-point bending obtaining the S-N curve. Weibull statistics was used for the statistical analysis. The fatigue tests indicate that the limit of fatigue resistance of this 3Y-TZP ceramics is around 550MPa, i.e. higher than the limits established in the ISO-13356 standard for the use of Y-TZP ceramics for the manufacture of implants. The fatigue behaviour of the investigated 3Y-TZP ceramics was related to the toughening mechanisms acting in Y-TZP ceramics, such as transformation toughening related to t?m phase transformation and microcracking.

An Evaluation on Fracture Toughness in SUS304 at High Strain Rate Considering Process Zone

Some research works report the relationships between transformation toughening and process zone. In SUS304, the reduction of transformation toughening at high strain rate is expected from the result of a small punch test. Currently, the process zone in SUS304 is fuzzily defined. Therefore, a consideration of damaging process is necessary in order to understand a fracture mechanism associated with transformation toughening and process zone. In this study, at first, tensile tests of pre-cracked sheet specimens made of SUS304 are conducted by the split Hopkinson pressure bar in order to understand the fracture mechanism phenomenologically at high deformation rate. During the test, a DC potential difference method is introduced to capture onset time of fracture.

Investigation of the Mechanical Behavior of Electroless Ni–P–Ti Composite Coatings

To improve the toughness of Ni–P coatings, NiTi superelastic particles were introduced into the Ni–P matrix through the electroless co-depositing of Ni–P and Ti particles and annealing Ni–P–Ti coatings. The mechanical properties of the coatings were determined through bend testing bilayer specimens and tensile testing the standalone coating. The effects of Ti content and annealing on Young’s modulus, toughness, and fracture strength were investigated. After annealing, the toughness and strength improved considerably. The formation of the superelastic NiTi phase after annealing led to the improvement of toughness and fracture strength of the composite coating through transformation toughening, crack deflection, bridging, and shielding. Different toughening mechanisms interacted with each other and operated together. This contributed to the enhancement of toughness and fracture strength.

Review Article: Zirconia surface bone graft implant with Ultraviolet Stimulation works toward Accelerating Bone Healing

Biomedical zirconia was introduced in 1969 into medicine to solve the problem of alumina brittleness in hip replacement procedures and has since been used for various joint replacement appliances in orthopedic surgery. The most frequently-studied material is yttrium-stabilized zirconia, which is also known as tetragonal zirconia polycrystal (TZP). Y-TZP presents various interesting characteristics, such as low porosity, high density and high bending and compression strength, proving that it is suitable for biomedical application. UV-treated zirconia surfaces exhibited an enhanced osteoblast response, which was characterized by an accelerated and augmented cell attachment, accelerated cell spread and cytoskeletal development with increased proliferation. The purpose of this paper is to identify which method of treatment of zirconia material implant & ultraviolet stimulation effect for bone healing is the most effective and efficient based on literature review. Bone grafts are available in a variety of substances. These bone substitutes can be biological (natural) or synthetic. Re-absorption is also essential for bone growth. Specific cells continuously break down bones and rebuild them. Substitutes that break down too quickly are not suitable for bone grafts, as they do not allow enough time for the new bone to grow. From our literature review, Zirconia is one of the biomaterials that have a bright future because of its high mechanical strength and fracture toughness. Zirconia ceramics have several advantages over other ceramic materials due to the transformation toughening mechanisms operating in their microstructure that can be expressed in components made out of them. UV treatment substantially enhances the osteogenesis process, resulting in a greater amount of peri-implant bone, as well as an increased strength of bone-zirconia integration.