A ceramic that bends instead of shattering

The microstructure of the ceramic silicon nitride can be tuned to create plasticity.acceptedVersionNon peer reviewe

Dependence between glass transition and plasticity in amorphous aluminum oxide : A molecular dynamics study

Aluminum oxide (Al2O3) is known to form amorphous structures that exhibit a unique plastic deforming ability at room temperature. However, alumina is considered a poor glass former, and it has been unclear whether alumina undergoes a glass transition during solidification from melt, and what effects such a transition would have on the plastic deform ability of the material. Here, we show using molecular dynamics simulations that a melt-quenched alumina indeed exhibits a glass transition, and that the glass transition greatly affects the observed material ductility. The glass transition temperature is found to positively correlate with the used cooling rate and we observe that maximum stress correlates with varying quench cooling rates in tensile test simulations, indicating that profound structural differences are formed during the glass transition. Significantly, we show that inducing plastic deformation allows erasing the structural memory of the material, and at 50% strain, all samples quenched at different rates shift again to exhibit similar flow stress. Characterizing methods that include medium-range structural information show a better ability to capture the structural differences formed during the glass transition. Our analysis results indicate that lower glass transition temperature imposes deeper potential wells of atoms and, therefore, a ’colder’ structure. The mechanical work input plays a similar role as input thermal energy to the structure. A ’colder’ structure needs more mechanical energy to get activated, thus showing a higher maximum stress. At a steady state flow, all samples show similar flow stress, indicating a similar structure.Peer reviewe

Room temperature plasticity in amorphous SiO2 and amorphous Al2O3 : A computational and topological study

Requirements for room temperature plasticity in oxides glasses have been only recently established. While atomistic mechanisms of this type of plasticity have been reported, it remains challenging to translate this knowledge between different structures and predict what other oxide glasses can be ductile and by which principle. Here we show that a coarse-grained analysis at the polyhedral level gives valuable information to accompany the atomistic characterization of plasticity, and we propose the analysis of polyhedral neighbor change events (PNCE) as a tool to allow comparison of the room temperature plasticity in various oxide glasses. Classical atomistic simulations with around 1 million atoms provided primitive data for coarse-grained analysis. Based on the PNCE analysis, the edge-sharing polyhedra are found to be up to 2 orders of magnitude more active in enabling plasticity, and combined with the occurrence of edge-sharing polyhedra, is shown to explain the brittle to ductile transition in a-SiO2 and the intrinsically high ductility of a-Al2O3. Finally, the coarse-grained analysis enables the benefit of using additional topological constraint theory analysis to yield more in-depth information regarding the ductile features of each glass structure. Quantitative comparison between amorphous Al2O3 and SiO2 shows a consistent trend between the materials and shows that the approach can be extended to the designing of other damage tolerant oxide glass materials.Peer reviewe

Evolution of alumina phase structure in thermal plasma processing

Alumina (Al2O3) remains one the most important engineering ceramic for industrial applications. In addition to the α phase, transition alumina phases have interesting characteristics. Controlling the obtained phase structure from alumina melt requires processes with extreme cooling rates and therefore limits the tailoring capabilities. This study investigates how the cooling rate of pure alumina affects its microstructural properties and phase structure in plasma-based processing. The paper reports phase changes in micron sized granulated alumina particles in high-temperature plasma spheroidization and compares the results to plasma sprayed alumina coatings. Both plasma processes involve melting of the material followed by subsequent rapid cooling. Direct comparison on the alumina phase transitions is obtained for the two methodically distinct processing routes, creating unique microstructures due to difference in their cooling rates.</p

Uncovering exceptional micro-scale plasticity accommodation mechanisms in amorphous aluminum oxide through experimental and simulation results

Please click Additional Files below to see the full abstract

Chitosan/collagen/Mg, Se, Sr, Zn-substituted calcium phosphate scaffolds for bone tissue engineering applications : A growth factor free approach

According to the biomimetic bone scaffold design paradigm, a scaffold resembling natural bone tissue with molecular, structural and biological compatibility is needed to allow effective regeneration of bone tissue. Continuing our previous studies regarding scaffolds with chitosan matrix containing Mg, Se, Sr, Zn-substituted calcium phosphates (CaPs), the focus of this work was to further improve the properties of these growth factor-free scaffolds. By addition of collagen into the chitosan matrix at weight ratios of 100:0, 75:25, 50:50, 25:75 and 0:100, we aimed to better resemble natural bone tissue. Highly porous composite scaffolds based on chitosan and collagen, with 30 wt% of Mg, Se, Sr, Zn-substituted CaPs, were prepared by the freeze-gelation method. The scaffolds show a highly porous structure, with interconnected pores in the range of 20–350 μm and homogeneously dispersed CaPs. The added collagen further enhanced the stability measured during 28 days in simulated biological conditions. Live/dead and CyQUANT assays confirmed good viability and proliferation of human bone marrow-derived mesenchymal stem/stromal cells, while successful osteogenic differentiation was confirmed by alkaline phosphatase quantification and type I collagen immunocytochemical staining. Results indicated that the addition of collagen into the chitosan matrix containing Mg, Se, Sr, Zn-substituted CaPs improved the physicochemical and biological properties of the scaffolds.publishedVersionPeer reviewe

Three-dimensional printing of zirconia: characterization of early stage material properties

Objective: The aim of this study was to evaluate the mechanical properties of 3D printed zirconia (ZrO2). Materials and Methods: The test specimens were produced with a 3D printer that uses lithography-based ceramic manufacturing (LCM) technique with two different parameters in horizontal and vertical printing orientations. Altogether four groups of nine specimens were printed and examined. Mechanical characterization was performed using 3-point bending test (ISO 10477) and surface microhardness (Vickers) test. Grain structure, porosity and printing layer morphology were examined with optical and scanning electron microscopy (SEM). Additionally fractography analysis was done to investigate and evaluate features of fracture initiation site. Numeric results were statistically analyzed with ANOVA (a = 0.05).Results: The average flexural strength reached for printed zirconia was 499 MPa (+/−75 MPa) for specimens printed in horizontal orientation and 575 MPa (+/−69 MPa) for specimens printed in vertical orientation. Optical microscopy and SEM analysis revealed that fractures initiated between the printing layers or from a local porosity. Printing layer thickness varied from under 13 μm to over 20 μm.Conclusions: The study revealed that 3D printed zirconia has challenges in regards to layer integration. Based on this study, 3D printed zirconia still suffers from low mechanical strength, which together with long carbon-debinding time, does not make 3D printed zirconia a potential material for dental appliances at this stage. Further research is needed to create more suitable zirconia precursor slurries and to optimize printing parameters and sintering conditions to be able to 3D print zirconia with higher mechanical properties.</p

Vat photopolymerization of biomimetic bone scaffolds based on Mg, Sr, Zn-substituted hydroxyapatite : Effect of sintering temperature

In response to the urgent demand for innovative bone regeneration solutions, the focus of this study is to develop and characterize Mg, Sr, Zn-substituted calcium phosphate scaffolds that replicate the trabecular architecture of cancellous bone. Ion substitution represents a promising approach to improve the biological effectiveness of calcium phosphates and composite materials used in bone tissue engineering applications. Porous scaffolds mimicking the natural bone structure were additively manufactured from the photosensitive ceramic suspensions for vat photopolymerization using digital light processing. The impact of the selected trace elements (0, 1 and 5 mol.% substitution) and the sintering temperature (900, 1000, 1100, 1200, and 1300 °C) was investigated in relation to the obtained crystalline phase content, microstructure, elemental distribution, thermal stability, and mechanical properties. After sintering, in addition to hydroxyapatite, β-tricalcium phosphate was detected as a result of the added trace elements in the calcium-deficient hydroxyapatite used as a starting powder. The obtained scaffolds exhibited uniform distribution of the trace elements, and they feature 3D-designed porosity predominantly ranged from 10 to 900 μm in diameter, with an average pore size of 546.25 ± 10.95 μm. The total porosity of scaffolds was 76.24 ± 1.32 vol% and an average wall thickness of 217.03 ± 8.98 μm, closely resembling the morphology of cancellous bone tissue. The mechanical properties of the scaffolds sintered at 1100 °C, 1200 °C, and 1300 °C were in line with those typically observed in trabecular bone. The study demonstrates the feasibility of using custom made bioactive hydroxyapatite powders together with vat photopolymerization to design the porosity and properties of the bone scaffolds on demand, based on the requirements of individual bone defects.Peer reviewe

Exceptional Microscale Plasticity in Amorphous Aluminum Oxide at Room Temperature

Oxide glasses are an elementary group of materials in modern society, but brittleness limits their wider usability at room temperature. As an exception to the rule, amorphous aluminum oxide (a-Al2O3) is a rare diatomic glassy material exhibiting significant nanoscale plasticity at room temperature. Here, it is shown experimentally that the room temperature plasticity of a-Al2O3 extends to the microscale and high strain rates using in situ micropillar compression. All tested a-Al2O3 micropillars deform without fracture at up to 50% strain via a combined mechanism of viscous creep and shear band slip propagation. Large-scale molecular dynamics simulations align with the main experimental observations and verify the plasticity mechanism at the atomic scale. The experimental strain rates reach magnitudes typical for impact loading scenarios, such as hammer forging, with strain rates up to the order of 1 000 s−1, and the total a-Al2O3 sample volume exhibiting significant low-temperature plasticity without fracture is expanded by 5 orders of magnitude from previous observations. The discovery is consistent with the theoretical prediction that the plasticity observed in a-Al2O3 can extend to macroscopic bulk scale and suggests that amorphous oxides show significant potential to be used as light, high-strength, and damage-tolerant engineering materials.Peer reviewe