MPa M0 Research Articles

Synthesis and enhanced mechanical properties of compositionally complex MAX phases

A novel strategy of fabricating compositionally complex MAX phases was successfully developed. Multicomponent 413 MAX phase solid solutions (Ti0.36Nb0.27Ta0.37)4AlC2.8 and (Ti0.28Nb0.26Ta0.28V0.18)4AlC2.9 simultaneously containing 3 and 4 transition-metal elements at M site were experimentally synthesized via hot pressing equimolar mixture of 211 type MAX phase powders. By elemental analysis and structural characterization, it can be verified that those uniform compositionally complex solid solutions can be obtained only in the presence of Cr2AlC in raw powders. Those compositionally complex MAX phase solid solutions exhibit typical layered structures with distinct elongated grains. This discovery further enriches the MAX phases family and provide a new avenue for tailoring the properties of these materials. MAX phase composites containing around 87.5 vol.% (Ti0.36Nb0.27Ta0.37)4AlC2.8 exhibit a high flexural strength of 720 MPa and a high fracture toughness of 9.5 MPa m0.5.

Characteristics of hydroxyapatite-reduced graphene oxide composite powders synthesized via hydrothermal method in the absence and presence of diethylene glycol

The synthesis of nanocomposites containing hydroxyapatite and graphene sheets has received much attention recently. Therefore, strategies that improve their properties will be useful. The aim of this study was to investigate the effect of diethylene glycol on the characteristics of hydroxyapatite-reduced graphene oxide powders synthesized by a hydrothermal method (180 ​°C, 5 ​h) and the nanocomposites obtained by spark plasma sintering (950 ​°C, 50 ​MPa). Graphene oxide, calcium nitrate tetrahydrate and diammonium hydrogenphosphate were used as precursors. The characterization methods included electron microscopy, X-ray diffraction, Raman spectroscopy, and indentation technique. Obtained results have shown that the presence of diethylene glycol reduced the average size of hydroxyapatite crystallites from 27 to 16 nm, reduced the particle size distribution, and increased the graphene oxide reduction rate. The presence of diethylene glycol during the synthesis process increased the elastic modulus (from 116.4 ​± ​4.9 to 125 ​± ​3.8 ​GPa), hardness (from 4.08 ​± ​0.7 to 6.32 ​± ​0.4 ​GPa), and fracture toughness (from 1.66 ​± ​0.7 to 1.87 ​± ​0.9 ​MPa ​m0.5) of the composite.

Improved Fracture Toughness and Crack Arrest Ability of Graphene–Alumina Nanocomposite

In this work, high fracture toughness graphene–alumina composite was developed through a novel chemical method using boehmite and graphene, which is followed by extrusion and consolidation. The mixed precursors were consolidated by sintering at 1550 °C in a nitrogen atmosphere. The plate-like structures of boehmite form α-alumina; meanwhile, graphene particles at the grain boundaries hinder the growth of alumina grains. The graphene reinforcement was bonded to α-alumina matrix by van der Waals forces. The XRD pattern reveals the presence of graphene with a plane (002) along with α-alumina. Properties such as fracture toughness (5.6 ± 0.01 MPa m0.5), Vickers hardness (1872 ± 25 kgf/mm2) and true density (3.8 g/cm3) were achieved in 0.5 wt.% graphene–alumina composite when compared to α-alumina with fracture toughness (5.3 ± 0.1 MPa m0.5), Vickers hardness (1984 ± 28 kgf/mm2) and true density (3.91 g/cm3). The bridging and deviation of cracks in 0.5 wt.% graphene–alumina composite are attributed to the anchoring and dissipation of energy during crack growth, which enhances the fracture toughness, whereas α-alumina exhibits failure caused by linear crack propagation. Meanwhile, the slight decrease in Vickers hardness and true density of 0.5 wt.% graphene–alumina composite is due to the tribological and low-density properties of graphene. The obtained properties of composite could be suitable in high-temperature, wear-resistant applications such as crucibles, bearings, etc.

Highly‐stable, nondegradable M2A‐reinforced YSZ ceramic composites prepared by SPS

AbstractMagnesia‐rich spinel (M2A)‐reinforced yttria‐stabilized zirconia (YSZ) ceramic composites have been fabricated by SPS using m‐ZrO2, Y2O3, and waste‐derived M2A powders. The results showed that m‐ZrO2 in M2A‐free YSZ0 specimen is partially stabilized upon temperature rising into tetragonal phase whereas YSZ10‐50 composites containing 10‐50 wt% M2A demonstrated variant behavior where their m‐ZrO2 is stabilized into a cubic form. YSZ10‐50 composites SPSed at 1400°C for 30 minutes revealed greater than 99% relative density, 10‐13 GPa Vickers hardness, 3.5‐5 MPa m0.5 fracture toughness and less than 0.5% apparent porosity. Aging hydration test for five hours soaking at 134°C and water vapor pressure of 2 bar, resulted in a conversion of ~2.8% of t‐ZrO2 to m‐ZrO2 for M2A‐free YSZ0 composite but it does not affect the YSZ10‐50 composites. The outcomes indicate that M2A has significantly improved both the densification and stabilization behavior of m‐ZrO2 through facilitating the diffusion of Y3+, Mg2+, and Al3+ ions inside zirconia lattice structure. In this regard, this study opens the door wide for producing fully stabilized c‐ZrO2 ceramics that could be potentially utilized as industrial ceramics for a broad scope of structural applications of high temperature, high‐stress and corrosive environmental conditions.

Evolution of stress fields during crack growth and arrest in a brittle-ductile CrN-Cr clamped-cantilever analysed by X-ray nanodiffraction and modelling

In order to understand the fracture resistance of nanocrystalline thin films, it is necessary to assess nanoscopic multiaxial stress fields accompanying crack growth during irreversible deformation. Here, a clamped cantilever with dimensions of 200 × 23.7 × 40 μm3 was machined by focused ion beam milling from a thin film composed of four alternating CrN and Cr layers. The cantilever was loaded to 460 mN in two steps and multiaxial strain distributions were determined by in situ cross-sectional X-ray nanodiffraction. Characterization in as-deposited state revealed the depth variation of fibre texture and residual stress across the layers. The in situ experiment indicated a strong influence of the residual stresses on the cross-sectional stress fields evolution and crack arrest capability at the CrN-Cr interface. In detail, an effective negative stress intensity of −5.9 ± 0.4 MPa m½ arose as a consequence of the residual stress state. Crack growth in the notched Cr layer occurred at a critical stress intensity of 2.8 ± 0.5 MPa m½. The results were complemented by two-dimensional numerical simulation to gain further insight into the elastic-plastic deformation evolution. The quantitative experimental and modelling results elucidate the stepwise nature of fracture advancement across the alternating brittle and ductile layers and their interfaces.

Magnetic Structure and Nanomechanical Properties of Sintered Permanent Magnets Nd–Dy–Fe–B USC-20L

Using scanning electron microscopy (EDS analysis), magnetic force microscopy, and nanoindentation, a metallographic study of the magnetic structure and nanomechanical properties of sintered rare-earth magnets Nd–Dy–Fe–B of the USC-20L brand has been carried out (Ural Strip Casting Technology). The microstructure of the sintered Nd–Dy–Fe–B magnet of the USC-20L grade includes the following components: grains of the Nd2Fe14B phase are separated by lamellas of the phases enriched in neodymium. Inclusions Nd–29.1% Fe–6.2% C–2.2% O–1.4% Dy are located at the triple junctions of Nd2Fe14B grains. Inclusions Nd–4.5% Fe–9.1% O–6.7% C–4.5% Fe–2% Dy are along grain boundaries containing Nd and Dy oxides. The chemical composition of grains is Fe–25% Nd–6.9% C–1.6% Dy–1.4% B. It is found that, due to irregular grain growth, the interlayers of the enriched Nd phase are connected to each other in the region of grain junctions, causing a concentration of internal stresses and the appearance of a crack. The crack propagates along the grain boundary from one wetted joint of the grains to another, due to mechanical stresses. The phenomenon of intergranular wetting by a phase of Nd-enriched Nd2Fe14B/Nd2Fe14B grain boundaries is observed. It has been established that phases enriched with Nd can pseudo-incompletely (or pseudo-partially) wet such grain boundaries, i.e., form a nonzero contact angle along the grain boundaries and in triple joints. Based on the results of Magnetic Force Microscopy (MFM), a conclusion is made about the presence of a one-dimensional domain structure; domains cross grain boundaries. The presence of pores and inclusions of Nd and Dy oxides localized along grain boundaries is noteworthy. The average transverse domain size of the banded structure is ~1 μm, the energy of the domain wall γ ~ 14 kJ/m2, and the width of the domain wall δ ~ 0.6 × 10–9 m. The nanoindentation method is used to measure the values of nanohardness (H, GPa), elastic modulus (E, GPa), contact stiffness (S, N/m), elastic work (We, nJ) and plastic strain (Wp, nJ) in submicrovolumes of Nd2Fe14B grains. According to the measurement results, the minimum value of Nd2Fe14B grain adhesion Kint = 0.539 MPa m0.5 is estimated.

Crack growth in borate and silicate glasses: Stress-corrosion susceptibility and hydrolytic resistance

A double cantilever beam technique in air equipped with ultrasound modulation was used to measure the crack velocity v in borate and silicate glasses. In all glasses v and the stress intensity KI followed the empirical correlation v ~ KIn. Indicated by its smallest KI at v = 1 µm s − 1, KI* = 0.27 MPa m0.5, the silicoborate glass containing 70 mol% B2O3 was found most susceptible to stress-corrosion enhanced crack growth. Contrarily, the sodium calcium magnesium silicate glass appeared least susceptible with KI* = 0.57 MPa m0.5. No clear correlation is evident between KI*, reflecting the stress-corrosion susceptibility, and the hydrolytic resistance for all glasses under study, but values of n obtained from the present study and taken from previous literature for 35 glasses tend to decrease with increasing network modifier ion fraction. Energy dissipation during stress-corrosion enhanced crack propagation is assumed to cause this trend.

Optimization of Novel Sphere Sandwich Structure for Impact Requirements

Sphere Sandwich Structures (SSS) are a new structure which may have the potentials to provide the energy absorption as compared with monolithic specimen material. By using innovation carving wax method, fifteen specimens have been prepared according to RSM optimize method and it machined by CNC-3axis machine with sphere end screw. Three factors sphere diameter(D), distance between spheres(X) and skin thickness(K) have been studied. The impact test has been achieved by Izod test to calculate specific fracture toughness. The results show the sample 642 has the highest specific fracture toughness value with an improvement of 300% compared with the monolithic sample. The optimal value of this test is 35.37 MPa m0.5 g-1 of the sample 682 (diameter = 6, distance between spheres = 8, skin thickness = 2 mm) with 10% error when compared with practice result at the same geometry. According to ANOVA analysis, the diameter of sphere(D) has greatest effect than other factors and then skin thickness(K).

Micro-scale evolution of mechanical properties of glass-ceramic sealant for solid oxide fuel/electrolysis cells

The structural integrity of the sealant is critical for the reliability of solid oxide cells (SOCs) stacks. In this study, elastic modulus (E), hardness (H) and fracture toughness (KIC) of a rapid crystallizing glass of BaO–CaO–SiO2 system termed “sealant G” are reported as determined using an indentation test method at room temperature. A wide range of indentation loads (1 mN–10 N) was used to investigate the load-dependency of these mechanical properties. Values of 95 ± 12 GPa, 5.8 ± 0.2 GPa and 1.15 ± 0.07 MPa m0.5 were derived for E, H and KIC using the most suitable indentation loads. An application relevant annealing treatment of 500 h at 800 °C does not lead to a significant change of the mechanical properties. Potential self-healing behavior of the sealant has also been studied by electron microscopy, based on heat treatment of samples with indentation-induced cracks for 70 h at 850 °C. Although the sealant G is considered to be fully crystallized, evidence indicates that its cracks can be healed even in the absence of a dead load.

Ultra-low thermal conductivity and enhanced mechanical properties of high-entropy rare earth niobates (RE3NbO7, RE = Dy, Y, Ho, Er, Yb)

A new entropy-stabilized rare earth niobates (RE3NbO7, RE = Dy, Y, Ho, Er, Yb) system with disordered defective fluorite structure was prepared by solid-state method. XRD, Raman and EDS results confirmed the homogeneous distribution and equimolar doping of rare earth cations. High-entropy RE3NbO7 ceramics showed smaller grain size compared with single rare earth niobates owing to the sluggish grain diffusion. Due to the increased configuration entropy, (Dy0.2Ho0.2Er0.2Y0.2Yb0.2)3NbO7 (5RE3NbO7) has enhanced mechanical properties (Hv = 9.51 GPa, KⅠC = 2.13 MPa m0.5), a high thermal expansion coefficient (10.2 × 10−6 K-1, 1200℃) and ultra-low thermal conductivity (0.724 W m-1 K-1, 25℃). These results suggest that the material can be used as a new type of thermal barrier coatings as alternative to yttria-stabilized zirconia.

Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 4: sporadic fatigue crack propagation

Abstract Fatigue tests were carried out on compression mouldings supplied by a leading polymer manufacturer. They were made from three batches of ultra-high molecular weight polyethylene (UHMWPE) with weight-average relative molar masses, M ¯ W ${\overline{M}}_{\mathrm{W}}$ , of about 0.6 × 106, 5 × 106 and 9 × 106. In 10 mm thick compact tension specimens, crack propagation was so erratic that it was impossible to follow standard procedure, where crack-tip stress intensity amplitude, ΔK, is raised incrementally, and the resulting crack propagation rate, da/dN, increases, following the Paris equation, where a is crack length and N is number of cycles. Instead, most of the tests were conducted at fixed high values of ΔK. Typically, da/dN then started at a high level, but decreased irregularly during the test. Micrographs of fracture surfaces showed that crack propagation was sporadic in these specimens. In one test, at ΔK = 2.3 MPa m0.5, there were crack-arrest marks at intervals Δa of about 2 μm, while the number of cycles between individual growth steps increased from 1 to more than 1000 and the fracture surface showed increasing evidence of plastic deformation. It is concluded that sporadic crack propagation was caused by energy-dissipating crazing, which was initiated close to the crack tip under plane strain conditions in mouldings that were not fully consolidated. By contrast, fatigue crack propagation in 4 mm thick specimens followed the Paris equation approximately. The results from all four reports on this project are reviewed, and the possibility of using fatigue testing as a quality assurance procedure for melt-processed UHMWPE is discussed.

Effect of Tribochemical Coating on Composite Repair Strength.

Tribochemical treatment of existing composite surfaces is highly effective for composite repair. When repairing an old composite restoration, the clinician should try to use the same composite originally used for the restoration. If the information about the original restoration is not known, a composite with strong mechanical properties should be used for the repair restoration. This study evaluated the effect of tribochemical coating on composite-to-composite repair interfacial fracture toughness (iFT). Sixty beam-shaped specimens (21×4×3 ± 0.2 mm) were prepared with a nanofill composite (Filtek Supreme Ultra [FSU]) and a nanohybrid composite (Clearfil Majesty ES-2 [CME]) and aged for 50,000 thermocycles (5°C-55°C, 20-second dwell time) and then sectioned in half. The resulting 120 hemispecimens (60 for each composite) were randomly assigned to different repair methods (n=10): universal adhesive (Clearfil Universal Bond Quick [CUB]), sand-blasting followed by CUB, or tribochemical coating (CoJet, CoJet sand, Espe-Sil, and Visio-Bond). The repair surface was prepared with a diamond bur (Midwest #471271), rinsed, and dried. Each aged composite brand (FSU, CME) was repaired with either the same composite or the opposite composite. All adhesives and composites were light cured with a high-irradiance LED curing light (Elipar DeepCure-S). After postrepair storage in 100% humidity and at 37°C for 24 hours, iFT was measured as KIc (MPa m½). Data were analyzed for statistical significance using two-way analysis of variance (ANOVA) and the Tukey honest significant difference post hoc test (α=0.05). Regardless of the substrate composite, ANOVA showed significant differences for surface treatment (p<0.0001) and repair composite (p<0.0001). Mean iFT values (SD) ranged from 0.91 (0.10) MPa·m½ to 2.68 (0.12) MPa·m½. Repairs made with FSU after CoJet resulted in significantly higher iFT (FSU: 2.68 MPa·m½; CME: 2.21 MPa·m½) when compared to the other experimental groups. The repair iFT was higher with CoJet treatment and when the nanofill composite FSU was used as the repair composite.

Effect of B4C reinforcement on microstructure, residual stress, toughening and scratch resistance of (Hf, Zr)B2 ceramics

Effect of B4C addition on the lower damage tolerance of spark plasma sintered (Hf, Zr)B2 based ceramics has been elucidated. Increased densification (from 89% to 95%) with B4C addition has elicited hardness and elastic modulus increase from 25 GPa to 38 GPa (52%) and 439 GPa–635 GPa (45%), respectively. Systematic 3–9 vol% B4C addition resulted in an increased fracture toughness (from 2.2 MPa m0.5– 4.4 MPa m0.5) and flexural strength (from 320 MPa to 357 MPa) compared to that of (Hf, Zr)B2 due to synergistic effect of solid solutioning and compressive residual stresses generated around reinforcement. The decreased wear rate (from 0.163 mm3/N.m to 0.061 mm3/N.m) with optimal B4C addition, affirms enhanced damage tolerance of (Hf, Zr)B2 composite. Enhanced cracking and fracture in (Hf, Zr)B2 shifts to abrasion induced restricted micro-cracking as micro-scratch wear mechanism with B4C addition. Estimated elastic-modulus and extracted scratch-hardness are correlated with microstructural attributes of porosity, phase content and residual stress in (Hf, Zr)B2 composites.

Development of novel alumina-containing bioactive glass-ceramics in the CaO-MgO-SiO2 system as candidates for dental implant applications

This paper reports on synthesis and characterization (sintering, crystallization, microstructure and mechanical properties) of novel alumina-containing (1 mol%) bioactive glass-ceramics (GCs) in CaO-MgO-SiO2 system with additives of K2O, Na2O, P2O5, CaF2, and Al2O3. Dense and well-sintered GCs were produced by glass-melt quenching and glass-powder compact sintering. The K-free GCs were comprised of diopside, wollastonite, and fluorapatite, and the K-containing ones consisted of diopside, fluorapatite, and alpha-potassium magnesium silicate (α-PMS). The mechanical properties of the produced GCs were better than those of the titanium and zirconia dental implant materials, and their modulus of elasticity (27–34 GPa), microhardness (6.0–6.7 GPa), and fracture toughness (2.1–2.6 MPa m0.5) are a good match to those of human jaw bone and dentine. The produced GCs were also bioactive, which was witnessed by the formation of hydroxyapatite on their surface after their immersion in simulated body fluid at 37 °C.

Thermodynamical evaluation, microstructural characterization and mechanical properties of B4C–TiB2 nanocomposite produced by in-situ reaction of Nano-TiO2

This work discusses the pressureless sintering of a boron carbide-titanium diboride (B4C– TiB2) nanocomposite via in-situ reaction of the boron carbide/titanium dioxide/carbon system. Attempting to sinter pure boron carbide leads to poor mechanical properties. In this work, the effect of adding TiO2 to B4C on mechanical properties of the boron carbide was investigated. Thermodynamic simulations were performed with HSC chemistry software to determine the phases which were most likely to form during the sintering process. The reaction thermodynamics suggested that during the sintering process, formation of TiB2 occurs preferentially over formation of TiC. For examination of the microstructural evolution of the samples, Scanning Electron Microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized. The density, porosity, Young's modulus, microhardness and fracture toughness of the specimens were compared. Optimum properties were achieved by adding 10 wt% TiO2. In the sample possessing 10 wt% TiO2, the relative density, Young's modulus, hardness and fracture toughness were 94.26%, 428 GPa, 23.04 GPa and 5.19 MPa m0.5, respectively, and the porosity was decreased to 5.73%. Furthermore, phase analysis via XRD confirmed that the final product was free of unreacted TiO2 or carbon.

Fracture of sigma phase containing Co–Cr–Ni–Mo medium entropy alloys

Sigma phase is well known for its excellent creep resistance and superb strength; however, its engineering applications are limited due to low temperature brittleness. In this work, we developed a series of dual phase CoCrNiMox (x = 0.4, 0.6, 0.7, 0.83 and 1.0) medium entropy alloys (MEAs) containing sigma and face centered cubic (FCC) phases and performed a systematic study of their fracture behavior at room temperature. Our experimental data clearly show that, as the volume fraction of the sigma phase increases from 4% up to 72%, the fracture toughness (KIc) of the dual phase MEA reduces from 72 to 8 MPa m0.5. In the meantime, these dual phase MEAs clearly exhibit a R-curve behavior, i.e. fracture toughness increasing with crack length. Through a quantitative analysis, we demonstrate that the fracture toughness of the sigma phase containing MEAs is derived from a combination of intrinsic (crack tip blunting, crack-tip deflection and crack-tip micro-crack nucleation) and extrinsic toughening mechanisms (distributed micro cracking and crack bridging). In sharp contrast to conventional sigma phase containing alloys, our work indicates that sigma phase induced embrittlement could be effectively mitigated in multi-principal element alloys, thereby opening a new window for designing strong yet tough metallic composites.

Enhanced fracture toughness of ZrB2–SiCw ceramics with graphene nano-platelets

In this study, the spark plasma sintering technology was implemented to produce hybrid zirconium diboride ceramics comprising of silicon carbide whiskers (SiCw: 25 vol%) and various amounts of graphene nano-platelets (Gnp: 0–7.5 wt%). The effects of nano-graphene addition on the structural evolution, hardness, fracture toughness, and densification behavior of ZrB2–SiCw nanocomposites were investigated. Though all produced hybrid ceramics reached the full densification level, with relative densities of ≥99.9%, a negligible drop was observed in the relative density of the specimen reinforced with 7.5 wt% graphene nano-platelets. After the sintering step and due to the lack of any chemical interaction in the ZrB2–Gnp–SiCw system, no in-situ formed phases were detected by X-ray diffraction or electron microscopy investigations. The incorporation of nano-scale graphene additives resulted in grain coarsening inhibition significantly. The hardness slightly increased from 21.9 GPa for the specimen with no graphene to 22.2 GPa for the sample doped with 2.5 wt% graphene. However, with rising the nano-graphene amount, the hardness remarkably dropped to 16.6 GPa and 19.1 GPa for the samples doped with 7.5 wt% and 5 wt% nano-graphene, respectively. The fracture toughness remarkably enhanced, with a linear trend, by the addition of graphene nano-platelets from lower 5 MPa m½ in the carbon-free ceramic to above 6 MPa m½ in the specimen doped with 7.5 wt% nano-graphene. The identified dominant toughening mechanisms were crack bridging, crack arresting, and crack deflection.

Determination of hydrogen embrittlement stress intensity threshold by fractography

Electroplating of aerospace parts generates hydrogen embrittlement, which is controlled by 200-h sustained-load tests performed on cylindrical notched samples. Those tests are costly and offer only a pass-or-fail assessment, with limited interpretability. In this article, we propose a new fractographic method to obtain a quantitative indication of the embrittlement severity of electroplated samples that failed the sustained-load test. On the fracture surfaces of failed samples, intergranular (IG) zones were observable at fracture initiation sites, surrounded by ductile zones, with intermediate regions containing mixed (ductile+IG) features. Within the framework of the recently proposed Hydrogen-Enhanced Plasticity-Mediated DEcohesion mechanism (HELP-mediated HEDE), we argue that there is a correlation between the intergranular appearance of the fracture surfaces and the subcritical nature of the crack propagation. Using this argument, we computed the stress intensity factor K for each crack and loading condition, with the input of intergranular zone sizes and finite element analysis. Minimum and maximum K values were identified and statistics of the extremes was used to calculate the minimum expected K values for various embrittlement conditions. It is proposed that this value is an estimate of the threshold for embrittlement, Kth. Values of Kth between 9 MPa m0.5 and 30 MPa m0.5 were measured for various plating conditions. Those estimations are more conservative than the thresholds reported in the literature, which were measured using more complex experimental procedures. Moreover, the fracture toughness estimated with the maximum K values was possibly affected by high initial hydrogen concentrations as per the HELP+HEDE embrittlement mechanism.

Nanoceramic composites with duplex microstructure break the strength-toughness tradeoff

There is a strength and fracture toughness tradeoff in nanoceramic composite. The strength varies reciprocally with the grain size whereas the toughness contributed by compressive residual stress increases with the dimension of the second phase. In this work, a novel duplex microstructure with reinforced clusters composing of nanosized grains was proposed and validated using a model system of B4C-TiB2 ceramics densified by carbide boronizing. As-obtained ceramics exhibit excellent combined mechanical properties at room temperature, including Vickers hardness, Young’s modulus, flexural strength and fracture toughness (by surface crack in flexure method) of 32.1 ± 2.7 GPa, 506.9 ± 2.0 GPa,1175 ± 71 MPa and 5.1 ± 0.4 MPa m0.5, respectively. Both strength and toughness are at least ∼30 % higher than the counterparts with similar composition but homogenously distributed TiB2 grains. Graphite onion was confirmed to be an intermediate product during reactive sintering, it facilitated the grain pullout during fracture and retained the nanometric TiB2 grain in the cluster, both of which also contribute the toughening and strengthening mechanisms in the B4C-TiB2 ceramics.

Fracture toughness of a metal\u2013organic framework glass

Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm3) from zeolitic imidazolate frameworks. Here, fracture toughness (KIc) of a representative glass, namely ZIF-62 glass (Zn(C3H3N2)1.75(C7H5N2)0.25), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics. KIc is determined to be ~0.1 MPa m0.5, which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson’s ratio to that of many ductile metallic and organic glasses.