Bending Fracture Toughness Research Articles

Fracture behaviors and crack propagation anisotropy in multi-ions modified bismuth titanate ferroelectrics

Bismuth titanate (BIT) is widely considered a promising lead-free piezoelectric material for advanced high-temperature applications. Despite significant advances in high-performance BIT ferroelectrics, there remains little understanding of mechanical properties including fracture failure and crack propagation behaviors, hindering the optimization of stability and reliability for next-generation high-temperature ferroelectrics. In this work, we investigated mechanical fracture and crack propagation behaviors of a type of high-performance bismuth titanate-based ceramic (Bi3.96Ce0.04Ti3–2xWxNbxO12, BCTWN) with different doping contents and electric poling states. High-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD) results reveal that the BCTWN ceramics possess a single ferroelectric phase and exhibit layered crystal structures characterized by periodic fringe features. In addition, the striped domain structures are observed on the surface of plate-like grains. Mechanical measurements indicate that fracture behaviors of BCTWN ceramics are significantly affected by doping contents. The ceramics with the largest grain size present inferior microstructure with more defects, endowing the lowest bending strength and fracture toughness. For the samples with different poling states, strong anisotropy in crack propagation and fracture toughness is observed, attributed to the different domain switching mechanisms driven by local incompatible stress fields, which is experimentally validated by the intriguing evolutions of striped domain structures around crack tips. This work advances our understanding of the fracture and crack propagation mechanisms of BIT-based ceramics and provides insight into tailoring the mechanical behaviors through doping and poling strategies.

Microstructure and Mechanical Properties of Diamond–Ceramic Composites Fabricated via Reactive Spark Plasma Sintering

In order to prepare diamond composites with excellent mechanical properties under non-extreme conditions, in this study, a diamond–ceramic composite was successfully prepared via reactive spark plasma sintering using a diamond–Ti–Si powder mixture as the raw material. The microstructures and mechanical properties of the diamond–ceramic composite sintered at different temperatures were studied. When the sintering temperature was 1500 °C, the diamond–ceramic composite exhibited a volume density of 3.65 g/cm3, whereas the bending strength and fracture toughness were high at 366 MPa and 6.17 MPa·m1/2, respectively. In addition, variable-temperature sintering activated the chemical reaction at a higher temperature, whereas lowering the temperature prevented excessive graphitisation, which is conducive to optimising the microstructure and mechanical properties of the composite.

The network structure and properties of a unique mining solid waste-based glass-ceramics

To alleviate the environmental pollution caused by the stockpile of mining-based, such as coal-based solid waste, mining solid waste-based CaO-MgO-Al2O3-SiO2 (CMAS) glass-ceramics with a diopside phase as the main crystalline phase was developed by one-step sintering method, and the effect of different solid waste content on the network structure and properties were investigated. The results show that, with the diversification of solid waste content, the crystallization temperature (Tp), the amount of Fe3+ concentration, and diopside crystalline, the main structure of glass Q2 changed also. Accordingly, significant changes have occurred in properties such as the bending strength, Vickers hardness, and density. Optimizing the raw material ratio, crystallization mechanism, a mining solid waste-based glass-ceramic with unique network structure is well developed and the bending strength, Vickers hardness, fracture toughness, and density reaches maximum values of 175.19 MPa, 10.47 GPa, 2.03 MPa⋅m1/2, and 3.00 g/cm3, respectively. The findings contribute significantly to the development of durable and reliable materials for the utilization of mining solid waste resources.

C/C-(Hf0.5Zr0.3Ti0.2)C-W-Cu composites: Long-term ablation resistance based on active-passive protection at 2600 °C

Compared with the traditional C/C composites modified by ultra-high-temperature ceramics (C/C-UHTCs), those modified by metal/medium-entropy ceramics have excellent mechanical properties, thermophysical properties, and long-term ablation resistance. These composites have great potential towards improving the high-temperature resistance and service life of thermal protection systems for spacecraft. In this study, a new type of (Hf0.5Zr0.3Ti0.2)C-W-Cu cermet-modified C/C composites (C/C-(Hf0.5Zr0.3Ti0.2)C-W-Cu) was prepared at 1500 °C. Compared with C/C-UHTCs, the bending strength and fracture toughness of C/C-(Hf0.5Zr0.3Ti0.2)C-W-Cu increased by 70% and 110% to 364.25 MPa and 14.64 MPa·m1/2, respectively. Due to the high thermal conductivity of Cu and W, the thermal conductivity of this new composite was 106% higher than that of C/C-(Hf0.5Zr0.3Ti0.2)C (44.26 versus 21.53 W/m·K). Under a high heat flow of 4.18 MW/m2, this material exhibited very low mass and linear ablation rates (−0.163 mg/s and −0.193 μm/s, respectively). Active and passive protection occur during ablation due to the evaporative cooling of Cu, CuO, and WO3 as well as a dense outer oxide layer that inhibits oxygen diffusion. The internal oxide layer forms a Hf-Zr-Ti-C-O framework mingled with Ti-rich Ti-Hf-Zr-C-O and an unoxidised W-Cu structure, effectively reducing the osmotic oxygen content. This work provides a new direction for developing thermal protection materials capable of long-term service in ultra-high-temperature environments.

Cracking resistance behavior of HVAF-sprayed monolayer and hierarchical Fe-based amorphous coatings under bending stress

The bending strength and fracture toughness are two key indicators that determine the service performance of thermally sprayed Fe-based amorphous coatings (AMCs) in harsh environments. Unfortunately, due to the inherent heterogeneous porosity and limited thickness (typically <1 mm) of the coatings, obtaining these properties is constrained by traditional fracture analysis methods. To address this, a three-point bending combined with the digital image correlation (DIC) method, without prefabricating notch or removing substrate, was proposed in the current work, and the effect of hierarchical structures on the fracture performance of the coatings was successfully revealed. Excitingly, the results show that compared to monolayer coating, hierarchical structures comprising two layers with different porosities can realize the synergistic improvement in the fracture strength and fracture strain of the coatings by alleviating local stress concentration and inducing crack deflection. Further, a finite element method (FEM) was employed to gain insight into the intrinsic relationship between the coating microstructures and the crack evolution behavior. This work not only proposes a simple method for evaluating the fracture performance of Fe-based AMCs sprayed with high velocity air fuel (HVAF), but also presents an innovative idea for optimizing the coating's properties.

On the anisotropic fracture behaviour of AA2050-T84 thick plate

Anisotropy in the fracture behaviour of AA2050-T84 was investigated using 2D digital image correlation (DIC)-assisted 3-point bending fracture toughness test on specimens with crack propagation direction oriented along the normal direction (ND) and rolling direction (RD). The tensile properties along the RD indicated slightly higher yield strength (482 ± 8 MPa) than ND sample (469 ± 9 MPa), while the fracture toughness in terms of energy release rate was higher when the notch was oriented along the ND (35.3 ± 1 kJ/m2) compared to the RD (18.5 ± 0.9 kJ/m2). Fractography of the fractured surfaces and detailed electron backscatter diffraction (EBSD) analysis of the microstructure along crack paths unveiled higher crack tortuosity with extensive secondary cracking in the TD-ND specimen than in the TD-RD. DIC measurements clearly showed a larger deformation zone for the TD-ND specimen also as which corroborated the plastic zone size determined using linear elastic fracture mechanics (LEFM). Additionally, the crystallographic model of tilt and twist angle differences between the neighbouring grains was employed to explain the increase in crack path tortuosity and higher dissipation of plastic work contributing to superior fracture toughness for the TD-ND sample.

Effects of adding Si3N4 whiskers to ZrB2–SiC ceramics on microstructure, mechanical properties and oxidation resistance

ZrB2–SiC–Si3N4w ceramics (ZSNw) were prepared via the hot-pressing sintering method. The bending strength and fracture toughness of ZSNw at room temperature reached to 520 ± 24 MPa and 5.2 ± 0.16 MPa m1/2, respectively. The enhancement in mechanical properties is attributed to the grain refinement and attenuation of grain boundary induced by the homogeneous distribution of Si3N4 whiskers. The oxidation resistance of ZSNw were conducted at 1500 °C in an air atmosphere. Remarkably, after 10 h of oxidation, ZSNw remained excellent strength and toughness reaching at 589 ± 26 MPa and 4.8 ± 0.12 MPa m1/2, respectively. This oxidation resistance of ZSNw was mainly attributed to the gain refinement induced by Si3N4 whiskers, which effectively reduced the voids within the ceramic matrix. Therefore, this process promoted the densification and homogenization of the SiO2 protective barriers, accompanied with hindering the inward permeation of oxygen and the outward release of oxides.

High performance Csf/SiC ceramic matrix composites fabricated by material extrusion 3D printing and precursor infiltration and pyrolysis

Material extrusion (ME) 3D printing is an emerging technology to fabricate short carbon fiber reinforced SiC ceramic matrix composites (Csf/SiC CMCs) with highly oriented short carbon fibers. In this study, the Csf/SiC CMCs were fabricated by ME 3D printing combined with the following precursor infiltration and pyrolysis (PIP). The short carbon fibers could be well kept after the PIP process. The effects of solid loading and fiber content on the microstructure and mechanical properties of the Csf/SiC CMCs were also investigated. Higher solid loading was beneficial for obtaining the denser Csf/SiC CMCs with more oriented fibers. The introduction of short carbon fibers brought more pores. The bending strength and fracture toughness of Csf/SiC CMCs first increased and then decreased with the increase in fiber content. The pulling-out and breakage of short carbon fibers contributed to the improvement of mechanical properties. High performance Csf/SiC CMCs with a bending strength of 212.74 MPa and fracture toughness of 5.84 MPa m1/2 were simultaneously achieved when the solid content was 50 vol% and the fiber content was 20 vol%. It is believed that this study can provide some understanding of the 3D printing of fiber reinforced CMCs.

In situ carbon nanonetwork structure based on MOF-derived carbon to manufacture ceramic composites: A case study of zirconia-toughened alumina

Highly robust ceramics materials are promising materials for applications in the automobile, aerospace, and ceramic cutting tool industries. However, conventional ceramics are typically brittle, which limits their suitability for advanced applications. In this study, with zeolite imidazolium salt skeleton-8 (ZIF-8) as a carbon source, discharge plasma sintering is used to create carbon nanonetworks derived from metal-organic frameworks (MOFs) in situ on the grain boundaries of zirconia-toughened alumina (ZTA) ceramic. ZTA exhibits maximum bending strength (719 ± 20.5 MPa) and fracture toughness (7.91 ± 0.7 MPa m1/2) at MOF concentrations of 1.0 wt% and 2.0 wt%, respectively, which are 1.37 and 1.88 times higher than that of the unmodified sample. This demonstrates that the MOF-derived carbon nanonetworks significantly improve the mechanical properties of the ceramic. The finite element analysis results indicate that the carbon nanonetworks can effectively disperse the stress inside the material and transfer the concentrated stress from the internal defects to the two-phase grain boundary on the surface of the sample, thereby contributing to strengthening and toughening. Overall, this work validates that the in situ production of carbon nanonetworks from MOFs is an effective strategy to enhance the mechanical properties of ZTA ceramics. Moreover, it offers new approaches for reinforcing and toughening other ceramic materials.

Effect of silicon carbide content on microstructure, physical and mechanical properties in vat photopolymerization of alumina

AbstractVat photopolymerization (VPP) printing of ceramic parts offers advantages such as low cost, simple operation, and short fabrication cycles. However, drawbacks include low toughness and brittleness in the printed parts. This study explores enhancing the toughness and strength of alumina (Al2O3) ceramics by incorporating silicon carbide (SiC) particles as additives. The impact of varying SiC contents on the quality of VPP‐printed Al2O3 parts is examined, encompassing microstructure, physical properties, and mechanical properties. Results indicate that optimal SiC addition reduces Al2O3 ceramics' porosity, enhances crystalline quality, and boosts mechanical properties. Excessive SiC, however, diminishes these benefits. The most significant strengthening of Al2O3 parts occurred with a 1.5 wt.% SiC content, increasing bending strength and fracture toughness by 239.7% and 564.7%, respectively. This underscore SiC's positive role in enhancing the quality of VPP‐printed Al2O3 parts.

Effect of Si content on microstructure and properties of Ti(C, N)-based cermet

The effects of Si additions (0, 0.5 wt%, 1 wt%, 1.5 wt%) on the microstructure, mechanical properties and oxidation resistance of Ti(C, N)-based cermets were investigated. The results showed that Si additions would combine with Mo, which leaded to the formation of MoSi2. Even though the ceramic black core phase was refined, the increase in Si content decreased the wettability between the binder and hard phases. The bending strength of the cermet grew and subsequently declined as the Si content increased, while the hardness did not change much. The most optimized comprehensive properties of the cermets were achieved with Si addition of 1 wt%, The bending strength, hardness and fracture toughness was 2133.7 MPa, 89.6 HRA and 12.21 MPa m1/2 respectively. In another side, cermets with 1 wt% Si addition had lower oxidized weight increase and a thinner oxide film than that without Si. A denser oxidation film composed of SiO2 and NiTiO3 was generated during the oxidation process on the surface of the cermets with Si addition, and this would prevent O elements from diffusion into the cermets, thus the oxidation resistance was enhanced.

Effects of grain boundary phase distributions on mechanical characteristics of the high thermal conductivity AlN ceramics

AlN powders with different O impurity contents were used to fabricate the high thermal conductivity (TC) AlN ceramics of different grain boundary (GB) phase distributions via pressureless sintering. Bending strength, fracture toughness, R-curve characteristic, and fracture mechanism of the ceramics were then investigated. The results showed that the GB phase distributions had great influences on mechanical characteristics of the ceramics. The ceramics with the continuous/semi-continuous GB phases possessed low strength and toughness. Comparatively, the ceramics with the isolated island-like GB phases, especially the fine GB phases at the triple AlN GB junctions, owned higher strength and toughness. The AlN ceramics principally followed the intergranular fracture mode, but the GB phase distribution had great effects on the fractions of transgranular fracture. The AlN ceramics with the continuous/semi-continuous GB phases had a lower fraction of the transgranular fracture, and thus lower crack propagation resistance and reliability than the others.

Fabrication and characterization of in-situ Ti(C, N) – CoCrFeNi cermets via mechanical alloying and spark plasma sintering

Ti(C, N) cermets were prepared by adding CoCrFeNiTi to g-C3N4 and using mechanical alloying and spark plasma sintering techniques. The evolution of microstructure and the systematic analysis of the influence of different binder content on mechanical properties were investigated. This result demonstrates that with an increase in binder content, the grain size of Ti(C, N) in the cermets decreases, and the formation of spheroidal small grains becomes more prominent while the occurrence of large grains decreases. Additionally, the relative density and hardness of Ti(C, N) metal ceramics have slightly decreased. The fracture toughness of Ti(C, N)/HEA cermet is codetermined by the intrinsic sintering property and deformability of HEA binders. The Ti(C, N) cermets (CH3) exhibits excellently bending strength and fracture toughness，which are 1941 MPa and 11.2 MPa m1/2 respectively. The study aims to enhance the mechanical properties and toughness of Ti(C, N) and proposes a novel approach for the synthesis of Ti(C, N) cermet.

The effect of h-BN content on mechanical properties, microstructure and machinability of hot-pressed h-BN/Si3N4 composites

Si3N4 ceramics now face new applications and challenges in the semiconductor field. The preparation of h-BN/Si3N4 machinable ceramics incorporating 5–30 vol% h-BN via hot-pressing was carried out. This study systematically explored how the varying h-BN content influenced the composite's microstructure, mechanical attributes, and machinability. The results showed that some of the h-BN was encapsulated by growing Si3N4 grains to form an intragranular structure during the process of hot-pressing, while the remaining h-BN exhibited uniform distribution along grain boundaries. The Si3N4 phase transition in composites was inhibited by excessive h-BN, accompanied by a higher decrease in densification. The sample containing 20 vol% h-BN exhibited the best comprehensive performance, with high mechanical strength and excellent machinability. The bending strength and fracture toughness remained high at 862 MPa and 10.3 MPa m1/2, and could be effortlessly machined with cemented carbide drills. Additionally, the machinability of the composites was systematically characterized through observations of crack propagation paths, fitting of R-curves, and mechanical drilling tests.

Investigation of the fracture behavior of cemented waste rock-tailing backfill by digital image correlation technique and discrete element modeling

Cemented waste rock-tailing backfill (CWTB), as an artificial false roof in the downward-filling mining method, with its fracture mechanical properties, directly affects the safety of workers. However, few studies have been conducted on CWTB. Therefore, in this study, three-point bending tests and digital image correlation were used to systematically investigate the effects of waste rock aggregate content (10, 20, 30, 40, and 50 %) on the bending mechanical properties (bending strength, post-peak toughness, and fracture toughness) and deformation characteristics (crack extension length and crack opening length) of CWTB. In addition, based on the partition modelling method, a mesoscopic DEM model of CWTB was proposed to reveal the crack extension mechanism and contact force evolution trends of CWTB beams. The results showed that increasing the waste rock aggregate content decreased the flexural strength and increased the post-peak toughness of CWTB. The waste rock aggregate content affected the contact force distribution characteristics and contact force variation was the mesoscopic reason for the evolution of the mechanical properties of CWTB. This study provided important experimental data and simulation basis for the engineering application of CWTB.

Influence of Oxygen/Argon/Nitrogen multi-component plasma modification on interlayer toughening of UHMWPE fiber reinforced composites

The study reveals the influence of glow discharged three gas sources of Oxygen/Argon/Nitrogen plasma modification on interlayer toughening of ultra-high molecular weight polyethylene (UHMWPE) fiber reinforced composite. The failure modes under bending, tensile and interlayer fracture toughness testing were analyzed through acoustic emission (AE) technology. The results showed that Oxygen/Argon/Nitrogen multi-component plasma modified fiber introduced more hydrophilic groups such as hydrogen and hydroxyl groups, and significantly enhanced the binding of fiber and matrix. The GⅠC (Mode-I interlaminar fracture toughness) of multi-component plasma modified composites exhibited increased by 61.67 %, 40.88 % and 18.01 % compared to untreated, Oxygen and Oxygen/Argon plasma, respectively, while GⅡC (Mode-II interlaminar fracture toughness) increased by 38.57 %, 25.57 % and 23.09 %. AE cluster analysis exhibited that multi-component plasma modified composites formed an energy dissipation mechanism dominated by fiber breakage. Additionally, the oxygen, argon, and nitrogen plasma modification have a synergistic effect on interlayer toughening.

Low temperature and high efficiency synthesis of boron nitride nanosheets (BNNSs) and its application in SiO2 microwave-transparent composites

In this study, Zn8B3H3O14 nanosheets were synthesized through a one-step coprecipitation process and employed as both a boron source and substrate templates for the fabrication of boron nitride nanosheets (BNNSs), which were synthesized efficiently at low temperatures by the borate nitration method using ammonia as the nitrogen source. The thickness of the synthesized BNNSs ranged from 3 to 10 nm with a flake dimension of about 2.56 μm. The prepared BNNSs using this method realized 98.8% boron source utilization and single batch yields up to gram level. In addition, BNNSs/SiO2 microwave-transparent composites were prepared to investigate the potential for large-scale applications of the BNNSs produced by using this method. The composites demonstrated a remarkable increase in bending strength and fracture toughness, reaching 122.02 MPa and 2.31 MPa·m1/2, respectively, with the addition of only 0.5 wt% BNNSs. Compared with SiO2 without BNNSs, the fracture toughness of the composites is increased by 106%. A novel crack plugging strong toughening mechanism of BNNSs is proposed. Simultaneously, the composites exhibit exceptional microwave-transparent properties, with dielectric constants below 4 in the real part and below 0.03 in the imaginary part. Therefore, this study has successfully achieved the low-temperature and efficient synthesis of micron-scale BNNSs, laying the groundwork for their large-scale application.

Design, optimization, and verification of self‐lubricating ceramic tool microstructure by MC method

AbstractThe computer‐aided analysis of microstructure evolution during the sintering process of self‐lubricating ceramic tools can unveil the underlying sintering mechanism and optimize experimental parameters. In this study, a novel self‐lubricating ceramic tool material, Al2O3/SiC/h‐BN@Al2O3, was fabricated with Al2O3 as the matrix, SiC as the reinforcement phase, and h‐BN@Al2O3 as the solid lubricant. The Monte Carlo (MC) method was employed to construct a model of self‐lubricating ceramic tool material, consisting of Al2O3/SiC/h‐BN@Al2O3. Subsequently, the sintering process was analyzed to investigate the influence of particle size and content of reinforced SiC, as well as the content of coated solid lubricant h‐BN@Al2O3, sintering pressure, and temperature on the microstructure of the tool. Optimal composition and sintering parameters were determined: 3 vol% for SiC additive amount with a particle size of 0.05 μm; 5 vol% for h‐BN@Al2O3 additive amount; and an optimal sintering temperature at 1650°C. The development of self‐lubricating ceramic tool materials, Al2O3/SiC/h‐BN@Al2O3, was realized through the employment of the aforementioned parameters. Their mechanical properties and microstructure were comprehensively characterized. The bending strength, fracture toughness, and Vickers hardness of the experimentally‐prepared Al2O3/3 vol%SiC/5 vol%h‐BN@Al2O3 self‐lubricating ceramic tool material were determined to be 722.42 MPa, 6.5 MPa·m1/2, and 20.65 GPa, respectively.

Preparation of h-BN/SiCO ceramic matrix composites with high thermal conductivity and strength by vat photopolymerization 3D printing

In order to improve the performance of polymer-derived ceramics and expand its application field, the formulation of the photosensitive resin was further optimized and on the basis of modification, the two-dimensional filler h-BN was introduced in this paper. It was found that both the content of PVSA in the photosensitive resin and the introduction of h-BN had remarkable impacts on the properties of the polymer-derived ceramics. When the h-BN content in the PVSA photosensitive resin was 1 wt%, the bending strength and fracture toughness of the ceramic sample after pyrolysis reached 252.4 ± 12.2 MPa and 2.7 ± 0.2 MPa·m1/2, respectively. With the increase of h-BN content, the thermal conductivity of the composite ceramics increased from 0.44 W·m−1·K−1 to 5.34 W·m−1·K−1. It is concluded that the thermal, electrical, and mechanical properties of precursor ceramics are improved by the introduction of two-dimensional fillers of h-BN.

Effects of the amount of magnesia on mechanical, thermal, and electrical properties of silicon nitride ceramics

Silicon nitride (Si3N4) ceramics were prepared using various amounts of magnesia (MgO) and a fixed amount of yttria as sintering additives, and their effects on the microstructures and four essential properties were evaluated: bending strength, fracture toughness, thermal conductivity, and dielectric breakdown strength (DBS). The microstructure varied with the MgO content; a smaller amount promoted grain growth, while an excess amount formed pores. The properties varied along with these structural changes. It was revealed that the four properties coexist at a high level in the range of 4–7 mol% of MgO. In addition, the large amount of MgO enhanced the electric conductivity of grain boundary and decreased the DBS. It is speculated that tips of multigrain junctions composed of a sufficiently large amount of glass modifier might work as the severe defects for the dielectric breakdown.