Excellent Fracture Toughness Research Articles

Effect of grinding on subsurface modifications of pre-sintered zirconia under different cooling and lubrication conditions.

Pre-sintered zirconia is preferred as a restoration material in dental applications due to its excellent strength and fracture toughness. When abrasive processes were used to obtain the required shape of (Y-TZP) yttria-stabilized tetragonal pre-sintered zirconia, it resulted in material strength degradation in the presence of coolant. Therefore, experiments were carried out on pre-sintered zirconia with diamond grinding wheel to evaluate the performance of cooling conditions such as dry, wet and minimum quantity lubrication (MQL). The effects of different environments on the grinding performance were studied based on the temperature distribution, phase transformation, flexural strength, microhardness and edge chipping damage. The Raman spectroscopy and X-ray diffraction analysis were used to estimate the quantity of monoclinic phase in pre-sintered zirconia. The temperature rise of the workpiece material during the grinding experiment was not higher and insufficient to cause the thermal stresses. The microstructural changes induced by grinding under different cooling strategies were associated with the quantitative assessment of monoclinic phase. The flexural strength of ground components was improved in the dry condition compared to the other process due to the absence of the defective layer and the occurrence of Y3+ ions segregation. After grinding, there was a slight decrease in the hardness value by (1-8 HV), which was due to the formation of microcracks in the subsurface layer of the ground surface. In addition, to ensure the presence of microcracks, the edge chipping depth was measured. The damage depth obtained from the wet condition showed a higher value of 30 µm compared to the dry and MQL conditions.

Composition, microstructure and mechanical properties of cBN-based composites sintered with AlN- Al- Ni binder

Cubic boron nitride (cBN)-based composites were fabricated using AlN-Al-Ni binders at high pressure and high temperature (HPHT). Effects of binder content and temperature on phase composition, microstructure and mechanical properties of cBN-based composites were investigated. Abrasive ratio was measured using a new method. Almost all Al reacted with cBN during sintering, resulting in AlN as well as AlB2 phases, while remaining Al and Ni formed intermetallic compounds. At 1500 °C, the reaction reached its maximum. Excellent relative density, flexural strength and fracture toughness of composites, sintered with 17 wt% binder, were 99.6%, 661 MPa and 7.19 MPa m1/2, respectively. However, the optimum value of hardness and abrasive ratio of composites at binder content of 13 wt% were 47.92 GPa and 2.99, respectively.

The remarkably enhanced particle erosion resistance and toughness properties of glass fiber/epoxy composites via thermoplastic polyurethane nonwoven fabric

In this work, a new method of preparing glass fiber/epoxy (GF/EP) composites with excellent solid particle erosion resistance and fracture toughness properties via thermoplastic polyurethane nonwoven fabric (TNF) was reported. TNF was prepared by virtue of the melt-blown (MB) process, and the composites were fabricated by vacuum-assisted resin transfer molding (VARTM) technique. Solid particle erosion characteristics of the composites were investigated in a confined space by impinging angular silica particles with a size about 300 μm. Compared with conventional GF/EP composites, the erosive wear resistance of TNF/glass fiber/epoxy (TNF/GF/EP) composites were improved by 99%, 108%, 205% and 493% at the impingement angles of 20°, 30°, 45° and 90°, respectively. Meanwhile, increase of 78% in GIC and 115% in GⅡC were achieved. Microstructure characterizations, including scanning electron microscopy (SEM) and optical microscopy were performed to investigate the variations of microstructure and further to establish the relationship between epoxy matrix and glass fibers. This work suggests a promising routine to improve particle erosion resistance and fracture toughness by interleaving the laminates with lightweight thermoplastic polyurethane nonwoven fabrics.

Improved fracture toughness of a Mo-12Si-8.5B-3Zr alloy by grain coarsening and its multiple toughening mechanisms

Mo-12Si-8.5B-3Zr alloy was manufactured via a mechanical alloying (MA) followed by hot pressing (HP) sintering technology. The correlation between microstructure and mechanical properties of the alloy annealed at different temperatures (i.e., 1600 °C, 1700 °C and 1800 °C) for 3 h was investigated. The microstructure of the as-HPed alloy exhibited that the Mo3Si/Mo5SiB2 (T2) particles were distributed dispersedly in α-Mo matrix. Additionally, spherical or ellipsoidal ZrO2/Mo2Zr particles with nano-scale and submicro-scale were located at the grain boundaries (GBs) and partially in grain interiors (GIs). Increasing annealing temperature resulted in the growth of grains including α-Mo, Mo3Si and T2. The α-Mo average grain size of 1800 °C-annealed alloy reached the maximum value (∼1.2 μm) which was more than double that of as-HPed alloy. Consequently, compared with the as-HPed alloy, the 1800 °C-annealed alloy containing the coarsest α-Mo phase demonstrated excellent fracture toughness values of 13.9 MPa·m1/2 and 15.3 MPa·m1/2, which were determined from single edge notch bend tests and indentation tests, respectively. Based on theoretical analysis, α-Mo phase coarsening provided remarkable improvement in fracture toughness primarily by promoting crack trapping mechanism. Besides, coarsened Mo3Si/T2 particles elevated cracks deflection and interfacial debonding effects, which provided further resistance to cracks growth. Although the compression strength of alloy decreased with increasing annealing temperature, it maintained a high value of 2.86 GPa at 1800 °C.

Surface modification and fatigue properties of electrical discharge machined aluminum alloy 6061 using plasma etching

Aluminum (Al) alloy 6061 is often used in engineering applications due to its high strength, excellent fracture toughness, and corrosion resistance. Lots of material removal or machining processes would induce some undesirable side effects like surface damages, fractures, and erosions. In this study, the surface defects in the workpieces caused by the electrical discharge machining process were eliminated with the plasma etching treatment. Meanwhile, the surface integrity and the fatigue properties of the workpiece improved significantly. Surface roughness, contact angle, and morphology were investigated by varying plasma etching parameters such as the etching power, oxygen gas flow, and the etching time. Furthermore, the optimum process conditions were evaluated using statistical analyses. The workpiece treated with the plasma etching had the smallest roughness of 12.2 µm, the largest contact angle of 92°, and 25% increase in the fatigue life. These experimental results indicated that the surface characteristics of the workpiece had a great influence on the fatigue life of electrical discharge machined Al alloy.

Mechanics and nanovoid nucleation dynamics: effects of polar functionality in glassy polymer networks.

We use molecular simulations and experiments to rationalize the properties of a class of networks based on dicyclopentadiene (DCPD), a polymer with excellent fracture toughness and a high glass transition temperature (Tg), copolymerized with 5-norbornene-2-methanol (NBOH). DCPD is a highly non-polar hydrocarbon, while NBOH contains a hydroxy group, introducing polar functionality and hydrogen bonds (H-bonds). NBOH thus represents a possible route to improve the chemical compatibility of DCPD-based networks with less-hydrophobic materials. We systematically vary the NBOH content (polar chemistry) in DCPD networks, while keeping other network parameters nearly constant, including the molecular weight between cross-links, chain rigidity, and Tg. Using molecular dynamics (MD) simulations, we quantify the thermovolumetric and mechanical properties, including Tg, cohesive energy density, stiffness, and yield strength. We compare these results with experiments on networks of similar composition, finding good agreement. The relation between these properties and polar chemistry are studied by examining a secondary network of physical cross-links, formed by hydrogen bonds between NBOH units. Further, we examine nanovoid formation, an energy dissipation mechanism hypothesized to contribute to the toughness of pDCPD. Using metadynamics to accelerate sampling, we quantify the nanovoid nucleation rate under hydrostatic tension, similar to the stress state in the plastic zone preceding a crack tip. Small additions of NBOH have minimal effect, but the rate drops steeply with larger amounts. Several properties are mapped at nanometer scales, including stiffness and mobility, and associated with void nucleation. Estimates of the length- and time-scale of the plastic zone near a crack tip are used in discussing nanovoid formation as a plausible toughening mechanism in these materials. Overall, the results suggest that pDCPD tolerates the addition of some polar chemistry without degrading its excellent mechanical properties.

Effect of calcia co-doping on ceria-stabilized zirconia

Ceria-stabilized zirconia ceramics are characterised by excellent hydrothermal stability and high fracture toughness, but the fracture strength and hardness are lower than that of conventional 3Y-TZP, which is sensitive to low temperature degradation in humid environments. In the present work, the influence of small concentrations of calcia on the microstructure, mechanical properties and hydrothermal ageing resistance of 10 and 12 mol% CeO2 stabilised ZrO2 has been assessed. The addition of only 1 mol% of CaO had a strong refining effect on the microstructure resulting in an increased hardness and strength but reduced stress activated tetragonal-to-monoclinic transformability. The addition of 3 mol% CaO however enhanced the transformability with respect to 1 mol% CaO and preserved the high resistance to hydrothermal degradation of Ce-TZP.

Densification and structural transformation during spark plasma sintering of WC-Co-YSZ-cBN systems

The demand for enhanced materials with high hardness, strength and wear resistance have become critically important in the cutting tool industry. WC-Co cermets are hard metals that possess a combination of excellent mechanical properties and high fracture toughness. However, WC-Co has specific limits in application as a structural component were high temperature strength, oxidation and corrosion resistance are required. Efforts have been made to replace cobalt with nickel or iron binder. WC-Ni has shown to possess better corrosion and oxidative resistance than WC-Co and WC-Fe, however it has lower mechanical properties compared to WC-Co. WC-Fe has inferior oxidative and corrosion to WC-Co and WC-Ni. Recent studies have recently been reported on the use of zirconia, having good binding strength and adhesion with the matrix, as a novel binding phase for WC. This study focusses on the partial replacement of cobalt with milled ZrO2 stabilized with 7.5wt% Y2O3 (YSZ) reinforced with cubic boron nitride (cBN). The synthesis focuses on full densification of 4 different compositions (WC-Co (control sample), WC-Co-YSZ, WC-Co-cBN and WC-Co-YSZ-cBN) using spark plasma sintering all at 1300°C and 50MPa with the goal of developing a WC-YSZ-cBN composite with no or little sintering aid. The effect of isothermal holding time on the densification, mechanical properties and microstructural & phase evolution were studied and will be presented. The results show that full densification, retention of tetragonal-YSZ phase, no cBN to hBN phase transformation and improved hardness & fracture toughness was obtained on the newly developed composite.

Medium entropy alloy CoCrNi coatings: Enhancing hardness and damage-tolerance through a nanotwinned structuring

Medium entropy alloys (MEA) are defined as alloys consisting of three equiatomic elements, such as CoCrNi. MEAs are reported to have superior mechanical properties and high thermodynamic stability, as well as excellent fracture toughness at cryogenic temperatures. Here, we investigate a series of equiatomic medium entropy alloy coatings, containing three elements, Co, Cr, Ni. These coatings were deposited onto M2 steel substrates with a range of coating thicknesses using a DC magnetron sputtering system using a CoCrNi alloy target (1:1:1at.%). The microstructure and mechanical properties were examined by a number of characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. XRD analysis showed that the coatings were dominated by a fcc CoCrNi phase, with a smaller amount of hcp Co. TEM analysis demonstrated that the elongated grains contained a high density of {111} nanotwins. In addition, the residual stresses in the coatings were analysed using X-ray diffraction by adopting the conventional sin2ψ method. A high hardness value, ~10GPa, was determined by nanoindentation of these coatings. Exceptional damage-tolerance was also found in these coatings under contact loading. It is believed that the nanotwinned structure is responsible for the high hardness and damage tolerance observed in the new coatings.

Reinforcing effect of graphene on the mechanical properties of Al2O3/TiC ceramics

Multilayer graphene (MLG)-reinforced Al2O3/TiC ceramics were fabricated through hot pressing sintering, and the reinforcing effect of MLG on the microstructure and mechanical properties of the composites was investigated by experiment and simulation. The simulation of dynamic crack initiation and propagation was investigated based on the cohesive zone method. The results show that the composite added with 0.2wt% MLG has excellent flexural strength and high fracture toughness. The major reinforcing mechanisms are the synergistic effect by strong and weak bonding interfaces, MLG pull-out, and grain refinement resulting from the addition of MLG. In addition, the aggravating of crack deflection, branching, blunting, and bridging have indispensable contribution to the improvement of the as-designed materials.

Evolution of Primary α Phase Morphology and Mechanical Properties of a Novel High-Strength Titanium Alloy during Heat Treatment

Abstract The evolution of primary α phase morphology and mechanical properties of a novel high-strength titanium alloy during heat treatment were investigated. The results show that the primary α phases exhibit a globular growth feature on the basis of the equiaxed α phases under double solution treatment at air cooling process of α/β zone. But abnormal growth of primary α grains occurs in the 0.5 °C/min furnace cooling process except that some primary α remains spherical growth, i.e. the other part of α grains evolve into similar “fork” shaped dendritic growth rather than keeping nearly spherical growth trend. The novel titanium alloy has attractive combinations of strength and ductility (≈1300MPa of ultimate strength with 10.5% of elongation) owing to the microstructures of equiaxed or thin billet-like primary α phase and fine needle-like secondary α phase after α/β solution treatment followed by air cooling plus aging. After α/β solution treatment followed by furnace cooling to low temperature and then aging heat treatment, the new type alloy has an excellent fracture toughness (≥80 MPa·m1/2), and the elongation and reduction of area remain at about 19% and 45%, respectively, and its corresponding tensile strength is maintained at about 1000 MPa. It can be assumed that the alloy may be a usable structural material.

Epoxy resins toughened with in situ azide–alkyne polymerized polysulfones

ABSTRACTTo simultaneously improve the fracture toughness and heat resistance of a cured toughened epoxy resin along with a reduction in its viscosity during the mixing process, two novel polysulfone‐type polymers are synthesized via azide–alkyne polymerization for use as toughening agents. The epoxy resin toughened with these polymers by in situ azide–alkyne polymerization during the cure process, which shows excellent processibility and based on the significantly lower viscosity (61 and 62 cP) during epoxy mixing process than that of commonly commercial polyethersulfone (PES, 127,612 cP). The novel polysulfone‐type polymer toughened epoxy resin showed the advantage in excellent fracture toughness than the PES toughened epoxy. In addition, the glass transition temperature of the novel polysulfone‐type polymer toughened epoxy resin is similar to that of the neat one (∼230 °C) and does not decrease, which implies excellent heat resistance of the toughened epoxy. These phenomena can be attributed to the formation of semi‐interpenetrating polymer networks comprising the epoxy network and the linear polysulfone‐type polymers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45790.

Microwave sintering of zirconia-toughened alumina (ZTA)-TiO2-Cr2O3 ceramic composite: The effects on microstructure and properties

This paper focuses on the development of a zirconia-toughened alumina ZTA-TiO2-Cr2O3 ceramic composite by means of microwave sintering at 2.45 GHz within the range 1200 °C–1400 °C, with a dwell time of 5–20 min. It is aimed at attaining improved microstructure and properties at a lower sintering temperature and shorter soaking time, compared to using a conventional heating method. Consequently, the effects of sintering temperature and soaking time on densification, properties and microstructural behaviour of the composite, are investigated. XRD analysis reveals that the microwave-sintered samples possess a higher crystallinity at a higher sintering temperature. Microstructural analysis confirms the uniform distribution of particles and controlled grain growth; with the lowest AGI value being 1.28 grains/μm. The sample that is microwave-sintered at 1350 °C with 10 min of soaking time achieves a high density (95.74% of the theoretical density), elevated hardness (1803.4 HV), and excellent fracture toughness (9.61 MPa m1/2), and intergranular cracks. This proves that the microwave sintering technique enhances densification, microstructural evolution and the properties of the ceramic composite at a lower temperature and shorter soaking time, compared to conventional heating. Overall, the improved mechanical properties of the microwave-sintered ceramics, compared to conventionally-sintered ceramics, are attributed to the enhanced densification and finer and more homogeneous microstructure that is achieved through the use of a microwave sintering method. The results reveal that microwave sintering is effective in improving the microstructure and density of materials, and will be useful for enhancing the mechanical properties of ZTA-TiO2-Cr2O3 ceramic composites.

On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades

Acicular ferrite nucleates intragranularly on nonmetallic inclusions, forming a microstructure with excellent fracture toughness. The formation of acicular ferrite is strongly affected by the size, content, and composition of nonmetallic inclusions, but also by the composition of the steel matrix. The potential of inclusions in medium carbon HSLA (high-strength low-alloyed) steels has been the main focus in the literature so far. The current study evaluates the acicular ferrite capability of various inclusions types in four different steel grades with carbon contents varying between 0.04 and 0.65 wt pct. The investigated steels are produced by melting experiments on a laboratory scale and subsequent heat treatment in a High-Temperature Laser Scanning Confocal Microscope. Inclusions are exclusively formed by deoxidation and desulfurization reactions. No synthetic particles are added to the melt. The inclusion landscape is analyzed by Scanning Electron Microscopy. Final ductility of the samples is evaluated based on performed tensile tests. Inclusion types in every steel grade are assessed regarding their nucleation potential always considering the interaction with the steel composition, especially focusing on the role of manganese. The effects of (Ti,Al)Ox-, MnS-, and MgO-containing inclusions are discussed in detail.

Negative Poisson's ratio in rippled graphene.

In this work, we perform molecular dynamics (MD) simulations to study the effect of rippling on the Poisson's ratio of graphene. Due to the atomic scale thickness of graphene, out-of-plane ripples are generated in free standing graphene with topological defects (e.g. heptagons and pentagons) to release the in-plane deformation energy. Through MD simulations, we have found that the Poisson's ratio of rippled graphene decreases upon increasing its aspect ratio η (amplitude over wavelength). For the rippled graphene sheet η = 0.188, a negative Poisson's ratio of -0.38 is observed for a tensile strain up to 8%, while the Poisson's ratio for η = 0.066 is almost zero. During uniaxial tension, the ripples gradually become flat, thus the Poisson's ratio of rippled graphene is determined by the competing factors of the intrinsic positive Poisson's ratio of graphene and the negative Poisson's ratio due to the de-wrinkling effect. Besides, the rippled graphene exhibits excellent fracture strength and toughness. With the combination of its auxetic and excellent mechanical properties, rippled graphene may possess potential for application in nano-devices and nanomaterials.

A Multi Scale Approach for Analysis of Fiber Reinforced Composites

Fiber reinforced polymer resin composite materials are finding wide variety of applications in automotive and aerospace industry because of their high specific stiffness and specific strengths when compared to metals. New class of 2D and 3D textile and woven fabric composites offer excellent fracture toughness as they bridge the cracks formed during fracture. Due to complexity of their fiber architectures and its resulting composite microstructures, optimized design and analysis of these structures is very complicated. A traditional homogenization approach is typically used to analyze structures made up of these materials. This approach usually fails to predict damage initiation as well as damage propagation and ultimate failure of structure made up of woven and textile composites. This study demonstrates a methodology to analyze woven and textile composites by using the multi-level multi-scale modelling approach.In this approach, a geometric repetitive unit cell (RUC) is developed with all its constituents to develop a representative volume element (RVE) with all its constituents and their interaction modeled correctly. The structure is modeled based on the RUC/RVE and analyzed at different length scales with desired levels of fidelity incorporating the damage and failure. The results are passed across (up and down) the scales qualitatively as well as quantitatively from the perspective of material, configuration and architecture.

Study of in vitro bioactivity and mechanical properties of diopside nano-bioceramic synthesized by a facile method using eggshell as raw material

In this study, diopside bioceramic was synthesized using a mechanical milling process and subsequent heat treatment. The simplicity of experiments and also the high energy available in ball milling lead to rapid synthesis of the products in comparison with other synthesis methods. Magnesium oxide (MgO), silicon dioxide (SiO2) and eggshell (as the calcium source) powders were weighted in stoichiometric conditions and milled to initial activation of the surface of the powder's mixture. Then a sintering process was conducted to complete formation of diopside nanopowder and also evaluates its thermal stability. The mechanisms occurred during the synthesis of this bioceramic were carefully investigated. X-Ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetry (TG), differential thermal analysis (DTA), and inductive coupled plasma atomic emission spectroscopy (ICP-AES) were used for gathering and analyzing data. The ability and rate of apatite formation on the sample surface were evaluated by Simulated Body Fluid (SBF) test, a method that is well recognized to characterize the in vitro bioactivity of ceramic materials. According to the results obtained, the diopside samples had a significant potential to form apatite layer on their surface during soaking in the SBF solution. Besides, the bonding strength of this bioceramic was about 350±7MPa which was almost more than three times of that reported for hydroxyapatite. An excellent fracture toughness of 4±0.3MPam0.5 was also obtained for this ceramic which was higher than that of previously reported works.

Investigation of mechanical properties and proton irradiation behaviors of SA-738 Gr.B steel used as reactor containment

The proton irradiation behaviors of two kinds of SA-738Gr.B steels prepared by different heat treatment used as AP1000 reactor containment were investigated by transmission electron microscopy and positron annihilation lifetime spectrum (PAS). The mechanical properties of as-received steels were also measured. In the unirradiated conditions, the SA-738Gr.B steels had high tensile strength and excellent impact fracture toughness, which met the performance requirements of ASME codes. Both kinds of SA-738Gr.B steels were irradiated by 400keV proton from 1.07×1017H+/cm2 to 5.37×1017H+/cm2 fluence at 150 ºC. Some voids and dislocation loops with several nanometers were observed in the cross-section irradiated samples prepared by electroplating and then twin-jet electropolishing technology. The number of irradiation defects increased with increasing of displacement damage, as well as for the mean positron lifetimes. The stress-relief annealing treatment improved irradiation resistance based on open volume defect analysis from proton irradiation. SA-738Gr.B (SR) steel had higher proton irradiation resistance ability than that of SA-738Gr.B (QT) steel. The mechanism of irradiation behaviors were also analyzed and discussed.

The capability of hibonite elongated grains to influence physical, microstructural, and mechanical properties of zirconia toughened alumina–CeO2–MgO ceramics

This work investigated the capability of hibonite (CaAl12O19) phase on structure and microstructure of zirconia toughened alumina–CeO2–MgO ceramics. Three different additives (CeO2, MgO and CaCO3) were introduced into zirconia toughened alumina ceramics prepared by solid state reaction. X-ray diffraction and FESEM analyses were employed to observe the role of secondary phases especially hibonite and its influence on the microstructural features and other properties. Among the secondary phases present, EDX analysis revealed that hibonite phase contributed to elongated grains. Vickers indentations hinted a strong difference in the efficiency of compositional adjustment among the composites. The excellent Vickers hardness and fracture toughness results obtained for 3wt.% CaCO3 additions showed the appearance of 5.9% hibonite with the value of 1485HV and 7.10MPa·√m, respectively.

Hybrid Polymer-Network Hydrogels with Tunable Mechanical Response.

Hybrid polymer-network gels built by both physical and covalent polymer crosslinking combine the advantages of both these crosslinking types: they exhibit high mechanical strength along with excellent fracture toughness and extensibility. If these materials are extensively deformed, their physical crosslinks can break such that strain energy is dissipated and irreversible fracturing is restricted to high strain only. This mechanism of energy dissipation is determined by the kinetics and thermodynamics of the physical crosslinking contribution. In this paper, we present a poly(ethylene glycol) (PEG) based material toolkit to control these contributions in a rational and custom fashion. We form well-defined covalent polymer-network gels with regularly distributed additional supramolecular mechanical fuse links, whose strength of connectivity can be tuned without affecting the primary polymer-network composition. This is possible because the supramolecular fuse links are based on terpyridine–metal complexation, such that the mere choice of the fuse-linking metal ion adjusts their kinetics and thermodynamics of complexation–decomplexation, which directly affects the mechanical properties of the hybrid gels. We use oscillatory shear rheology to demonstrate this rational control and enhancement of the mechanical properties of the hybrid gels. In addition, static light scattering reveals their highly regular and well-defined polymer-network structures. As a result of both, the present approach provides an easy and reliable concept for preparing hybrid polymer-network gels with rationally designed properties.