Crack Deflection Mechanisms Research Articles

(SiC-Si3N4)w/SiBCN composite ceramics with tunable electromagnetic properties

In this study, composite ceramics with tunable electromagnetic properties were developed based on SiC-Si3N4 hybrids. First, SiC-Si3N4 whisker preforms with various SiC whisker (SiCw) concentrations were prepared by the nitridation of Si in SiCw-Si powder compacts. The in-situ-grown Si3N4 whiskers (Si3N4w) were dispersed uniformly in the voids inside the interconnected network formed by SiCw. The whisker reinforcements effectively toughened the SiBCN matrix through interface debonding, crack deflection and whisker pullout mechanisms. The electromagnetic properties of the composite ceramics can be tuned by adjusting the composition of (SiC-Si3N4)w preforms. The composite ceramic with 31.9 vol% SiCw exhibited the broadest effective absorption bandwidth (EAB) of 2.9 GHz in X band. Furthermore, a multilayer radar absorbing structure (RAS) composed of ceramic composites with gradient electromagnetic parameters was constructed to further improve the absorption properties. By adjusting the structure of the RAS, the resulting EAB of the RAS covered the entire X band.

Fabrication of halloysite nanotubes/reduced graphene oxide hybrids for epoxy composites with improved thermal and mechanical properties

Halloysite nanotubes-reduced graphene oxide hybrids (HRGO), prepared by simultaneous reduction and hybridization of graphene oxide with halloysite nanotubes, were incorporated in epoxy resin to improve the thermal and mechanical properties. The presence of HNTs effectively prevented the aggregation of RGO sheets in epoxy matrix. The thermal stability and thermal conductivity of HRGO/epoxy composites were enhanced. The epoxy composites with the addition of HRGO exhibited significant enhancements of tensile strength, storage modulus and fracture toughness. The improved dispersion of HRGO in epoxy matrix resulted in the better interfacial load transfer efficiency. The improvement in fracture toughness was disclosed of crack deflection and crack pinning mechanisms.

Analysis of strengthening and toughening mechanisms of bioinspired mineral bridges on hot-pressed alumina-based ceramics through finite element method

Bioinspired intergranular mineral bridges were fabricated in alumina-based ceramics through in situ synthesis of Al2TiO5 whiskers. The effects of mineral bridges on the mechanical properties of ceramics were investigated. Results showed that the flexural strength and fracture toughness of the alumina-based ceramics increased by 33.97% and 33.52%, respectively, after introducing the mineral bridges under hot pressing. Moreover, the effects of the mineral bridges on the crack tip stress (CTS) of models were calculated by finite element method for the first time. The mineral bridge buds and mineral bridges reduced the CTS by 15.54% and 22.22%, respectively. This finding revealed the root cause of the crack deflection mechanism of the mineral bridges and directly confirmed their strengthening and toughening effects.

Reinforcing ability and bonding characteristics of multiwall carbon nanotubes and silicon carbide nanoparticles in inductively sintered alumina ceramic hybrid nanocomposites

This paper investigates the interfacial bonding, interfacial nanostructure and reinforcing ability of 4 vol% multi-walled carbon nanotubes (mwCNT) and 6 vol% silicon carbide nanoparticles (SiCNP) in inductively sintered alumina ceramic hybrid nanocomposites. Homogenously dispersed reinforcing inclusions led hybrid nanocomposites to three-fold finer microstructure and respective 81% and 25% higher fracture toughness and microhardness over the monolithic alumina. Better mechanical properties were correlated with distinctive toughening mechanisms of crack-deflection as well as crack-bridging induced by reinforcing phases upon establishing sturdy interfaces with the alumina matrix. Moreover, the bonding mechanisms and nanostructural development at the mwCNT/alumina and SiCNP/alumina interfacial regions were probed by means of both qualitative and quantitative approaches. The results show strong interfacial bonding of alumina with mwCNT and SiCNP through an intermediate interfacial layer in the former case and direct physical sticking in the latter one. The bonding chemistry and nanostructural features at the interfaces are meticulously discussed. We also discuss the high temperature stability of the hybrid nanocomposites under air and argon atmospheres.

Processing, microstructure and mechanical properties of HfB2-ZrB2-SiC composites: Effect of B4C and carbon nanotube reinforcements

The fully dense HfB2-ZrB2-SiC composites were processed using spark plasma sintering (SPS) at 1850 °C. The effect of reinforcements (B4C and CNT) on the densification as well as mechanical properties were investigated and compared (with monolithic) in the present study. The study showed that the addition of B4C and CNT were not only beneficial for the densification but also towards enhancing the mechanical properties (hardness, elastic modulus, and fracture toughness) of HfB2-ZrB2-SiC composites. The augmentation in the mechanical properties establish the synergy between solid solution formation (with the equimolar composition of HfB2/ZrB2) and the reinforcements (SiC, B4C, and CNT). The highest increase in the indentation fracture toughness with the reinforcements of B4C as well as CNT is >3 times (~13.8 MPam0.5 when it is 3–4 MPam0.5 for monolithic ZrB2/HfB2) on HfB2-ZrB2-SiC composites, which is attributed to the crack deflection and pull-out mechanisms. An increase in the analytically quantified interfacial compressive residual stresses in the composites during SPS processing with the synergistic addition of reinforcements (SiC, B4C, and CNT) and its effect on the indentation fracture toughness has also been addressed.

Interfacial investigation, mechanical performance and thermal permanence of the inductively hot-pressed alumina ceramic hybrid nanocomposites reinforced by silicon carbide and multilayer graphene

Hybrid alumina (Al2O3) ceramic nanocomposites containing 3 wt% silicon carbide nanoparticles (SiCnp) and 0.5 wt% multilayer graphene (MLG) were consolidated to near theoretical density (99%) by an inductive hot-pressing (IHP) technology. The enhanced fracture toughness and hardness by 97% and 15% respectively, for the hybrid nanocomposites over the reference Al2O3 were attributed to the higher densification, uniformly dispersed reinforcing constituents within the base matrix, toughening mechanisms of crack-deflection by the 0D-SiCnp and grain-anchoring as well as crack-bridging induced by the 2D-MLG. Thermal investigations of the hybrid nanocomposite verified the occurrence of a chemical reaction at the MLG/Al2O3 interface and thermal diffusion at SiCnp/Al2O3 interface during inductive hot-pressing. The bonding characteristics and interfacial microstructure were appraised by infra-red spectroscopy and high-resolution TEM microscope. Furthermore, the thermal stability in different gaseous environments and the ballistic performance of the hybrid nanocomposites were characterized and discussed. The produced hybrid nanocomposites may be suitable for high-temperature aerospace structural parts and may have applications in high-velocity impact technology.

Fracture toughness and R-curve behavior of laminated ZrB2–SiC/SiCw ceramic

Laminated ZrB2–SiC/SiCw (LZSw) was prepared via tap-casting and hot-pressing processes. The R-curve behavior of the ceramic materials was evaluated using indentation-strength method. The flexural strength and fracture toughness values of the materials were found to reach up to 607 MPa and 14.5 MPa·m1/2, respectively, when employing SiC whiskers as a strong interface. The indentation strength was observed to decrease upon an increase in the indentation load, and compared with the steep decreasing trend exhibited by the monolithic ZrB2–SiC (ZS20), LZSw showed a slow decreasing trend. LZSw showed a distinct upward trend in its R-curve, which was found to be dependent on the indentation collapse caused by its laminated structure and the increasing crack propagation path as a result of crack deflection and branching mechanisms.

Characterization and mechanical properties of Cf/ZrB2-SiC composites fabricated by a hybrid technique based on slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing

The continuous carbon fiber reinforced ZrB2-SiC composite was fabricated successfully via a hybrid technique based on nano ceramic slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing. The Cf/ZrB2-SiC composites exhibited non-brittle fracture modes and the chemical interaction at the fiber/matrix interfaces was effectively inhibited owing to the low sintering temperature. The S2-Cf/ZrB2-SiC composite presented the highest mechanical properties with fracture toughness of 4.47 ± 0.15 MPa m1/2 and the work of fracture of 877 J/m2, which was attributed to the multiple length-scale toughening mechanisms including the macroscopic toughening mechanisms of crack deflection and crack branching, the micro toughening mechanisms of fiber bridging and fiber pull-out. This work presented a novel and effective method to fabricate high-performance continuous carbon fiber reinforced ceramic matrix composites.

Tougher and harder zirconia toughened alumina (ZTA) composites through in situ microstructural formation of LaMgAl11O19

Advanced ceramic materials are highly demanded for various modern industrial and practical engineering applications. Nonetheless, the requirement of attainment hard and simultaneously tough materials is a crucial matter for industrial sectors. To accomplish this target, the strategy of designing a novel microstructure of Zirconia Toughened Alumina (ZTA) ceramics was utilised through in situ formation of LaMgAl11O19 (LMA) based on the stoichiometry ratio of constitute oxides inside ZTA composites during sintering process. ZTA Samples were conventionally sintered at 1600 °C for 2 h to ensure fully formation of LMA while their characteristics were analysed using SEM-EDX and XRD. The LMA elongated grains significantly enhanced ZTAs' Indentation Fracture Resistance (KIFR) due to crack deflection and crack bridging mechanisms combined with transgranular and intergranular fracture modes. Additionally, higher hardness was obtained due to high LMA density, creation of low porous ZTA and strong interface bonding between LMA and ZTA grains. The KIFR was stable while the hardness was steadily declined due to more porosity accompanied by LMA in situ formation. Consequently, sustainable high hardness and enhanced KIFR of ZTA ceramics were achieved. The optimum outcomes were superior KIFR (7.8 MPa.m^1/2) and hardness (1660 HV) than the pure ZTA (5.9 MPa.m^1/2, 1568 HV). The investigation provides a new hint for future design and fabrication of economic and high performance ceramics through in situ formation of LMA according to the stoichiometric ratio of the reinforcing oxides.

Microstructure and mechanical properties of hot-pressed Al2O3–mullite–ZrO2–SiC composites

In this study, the effects of SiC content, sintering temperature and Al2O3/SiO2 ratio on the microstructure and mechanical properties of hot-pressed alumina–mullite–zirconia–silicon carbide (Al2O3–mullite–ZrO2–SiC) composite ceramics were investigated systematically. The research results indicate that the composite ceramics have excellent combined properties when SiC content is 20 vol% and sintering temperature is 1530 °C, with Vickers hardness, fracture toughness and flexural strength of 17.5 GPa, 12.3 MPa m1/2 and 970 MPa, respectively. The fracture toughness and flexural strength are higher than those of monolithic Al2O3 ceramics and Al2O3-based composites, which are fairly attributed to the refined microstructures of composites and the synergistic effects of crack deflection, pull-out, branching, and phase transformation toughening mechanisms induced by short columnar Al2O3 grains and SiC platelets formed in the ceramics, as well as the ZrO2 particles existed in the ceramics.

Orientation-Dependent Tensile Behavior of Nanolaminated Graphene-Al Composites: An In Situ Study

We conducted in situ microtension experiments in a scanning electron microscope (SEM) to study the orientation-dependent mechanical behavior of nanolaminated graphene-Al composite. We found a transition from a weak-and-brittle behavior in the isostress composite configuration to a strong-yet-ductile tensile response in the composite under isostrain condition. This is explained by the excellent load-bearing capacity of the graphene nanosheets and a crack deflection mechanism rendered by the laminate structure. These in situ measurements enabled direct observation of the deformation procedure and the exact failure mode, which highlight the importance of microstructural control in tailoring the mechanical properties of advanced metal matrix composites (MMCs).

Microstructure and mechanical properties of SiC joint with an in-situ formed SiC-TiB2 composite interlayer

Diffusion bonding of SiC ceramic with in-situ formed SiC-TiB2 composite interlayer by reactive spark plasma sintering with B4C-Ti3SiC2-Si powder mixture was investigated. It was found that a dense composite interlayer between the SiC ceramic substrate could be obtained at 1600 °C for 10 min. Granular-shaped SiC with submicron-size and elongated TiB2 with micro-size were uniformly distributed in the bonding layer. The joint strength and hardness of the composite interlayer increased as the joining temperature increased up to 1600 °C due to the improved relative density and the higher fraction of elongated TiB2 phase. The grain growth and formation of micro-cracks deteriorated the mechanical strength of the 1700 °C joint. The maximum shear strength of 128.0 ± 7.3 MPa was achieved for the joint bonded at 1600 °C. The toughening mechanisms of crack deflection, crack bridging and crack branching, and hardening mechanism of dual phase strengthening were observed in the morphology of indentation cracks.

Multifold interface and multilevel crack propagation mechanisms of graphene oxide/polyurethane/epoxy membranes interlaminar-toughened carbon fiber-reinforced polymer composites

Graphene oxide/polyurethane/epoxy (GO/PU/EP) membranes were directly fabricated by functionalization of graphene oxide with epoxy-grafted polyurethane (GO-UE), and the interface correlation and crack propagation mechanisms in GO/PU/EP membranes interlaminar-toughened carbon fiber-reinforced polymer composites were investigated. The functionalized GO-UE with corrugation and scrolling nature of graphene sheets was evenly dispersed in GO/PU/EP membranes below 0.50 wt% loading. Mode I fracture toughness, flexural properties and interlaminar shear strength of GO/PU/EP membranes-toughened composites were enhanced in comparison with untoughened composites and PU/EP membranes-toughened composites, which was ascribed to the multifold interface bonding between the GO-UE layers, epoxy matrix and carbon fiber. Schematic models of multilevel crack propagations were proposed based on different crack extension directions to GO-UE and the morphology evolutions of GO-UE in the interlaminar region and at the carbon fiber interface in toughened composites, which highlighted the toughening mechanisms of crack pinning, crack deflection and separation between GO-UE layers.

Toughening effect and mechanism of polyamide 12 and modified montmorillonite in polybenzoxazine resin

Abstract The mechanical properties of polybenzoxazine (PBa) resins were improved by adding polyamide 12 (PA12) and modified montmorillonite (OMMT) as additives. The mechanical properties and thermal stability of PBa and resulting PBa composites were investigated using an Instron universal material testing instrument and dynamic mechanical analysis. The morphologies of the fracture surfaces were characterized by scanning electron microscopy. The results of morphological studies showed that PBa/PA12/OMMT composites exhibit significantly improved mechanical properties and thermal stability compared with that of the pristine PBa. When the OMMT content increased to 1 wt%, the fracture toughness (1.36 MPa·m1/2) and the fracture energy (G IC, 315.76 J·m−2) of PBa/PA12/OMMT-1 composites increased by 67.9% and 181.4%, respectively, compared with those of the pristine PBa. The thermal stability properties demonstrated that the storage modulus and glass transition temperature (T g) of PBa/PA12/OMMT composites gradually increased with the addition of OMMT particles. The scanning electron microscopy results indicated that PBa/PA12/OMMT composites possess a toughening mechanism of crack deflection, with a large bulk of voids and debonding induced by PA12 and OMMT clay particles. Moreover, the OMMT might provide microvoid nucleating sites at its surface to release constrains for shear yielding.

Study on microstructure and properties of Ti(C,N)-based cermets with dual grain structure

A novel Ti(C,N)-based cermet with dual grain structure was fabricated by in situ carbothermal reduction of MoO3 and subsequent liquid sintering in a single heating process, using Ti(Cx,Ny) solid solutions with different particle size and N/C ratios as the ceramic phase. The phase formation, microstructure and properties of the prepared cermet were characterized by XRD, SEM, TEM, etc. It was found that near-fully dense Ti(C,N)-based cermet with both fine and coarse ceramic grains which distributed evenly in the continuous thin binder phase could be obtained. The novel cermet had nearly equivalent hardness to the fine-graded commercial ones and exhibited much higher fracture toughness mainly due to the toughing mechanism of crack deflection by coarse carbonitride grains and the compensation of hardness loss by fine ceramic grains. The novel Ti(C,N)-based cermet showed excellent comprehensive mechanical properties with a transverse rupture strength of 2332 ± 45 MPa, a Rockwell hardness of 92.0 ± 0.2HRA and a fracture toughness of 11.2 ± 0.4 MPa m1/2, respectively.

Mechanical properties of reaction sintered SiC-TiC composite

SiC-TiC composite containing 5–30 vol% TiC particles was fabricated by conversion of TiO2 to TiC during pressureless sintering. The presence of very fine and uniformly distributed, in-situ created TiC particles increased driving force for sintering allowing fabrication of high density SiC-TiC composite without the use of external pressure. The highest mechanical properties were measured in samples sintered at 1885 °C for 1 h. The highest values for hardness (~ 20 GPa) and fracture strength (~ 490 MPa) were measured in samples containing 5 vol% TiC, whereas the highest fracture toughness (~ 5.5 MPa ⋅m1/2) was measured in samples containing 10 vol% TiC. The presence of in-situ created, uniformly distributed TiC particles was beneficial for activation of not only the crack deflection mechanism but also mechanism called toughening by residual stress crated due to the difference of thermal expansion between SiC matrix and TiC particles. The compressive stress field around TiC particles acts to close the propagating crack and thus to increase fracture toughness.

Synthesis of hard and tough calcium stabilized α-sialon/SiC ceramic composites using nano-sized precursors and spark plasma sintering

Silicon carbide (SiC)-reinforced calcium (Ca)-α-sialon ceramic composites were synthesized using nano-sized precursors and spark plasma sintering at 1500 °C for 30 min by adding 10, 20 and 30 wt % of SiC particles into the sialon matrix. Almost complete densification was achieved for all processed samples. The second-phase SiC particles were found to be homogeneously dispersed in the α-sialon matrix. The effect of the amount of loaded SiC on the mechanical properties of the ceramic composites was studied. A remarkable combination of hardness and toughness values, specifically 24.53 GPa (HV10) and 11.0 MPa m1/2, respectively, was obtained for the composite ceramic containing 30 wt% SiC, whereas hardness and fracture toughness values of only 21.1 GPa (HV10) and 7.3 MPa m1/2, respectively, were obtained for the monolithic α-sialon ceramic. The increase in fracture toughness was fairly attributed to the crack deflection, crack bridging and grain pullout mechanisms caused by the finely dispersed SiC particles.

Using macroporous graphene networks to toughen ZrC–SiC ceramic

Graphene is one of the important candidates in ceramic toughening due to its outstanding physical and chemical properties. For the weak interface toughening of large-diameter graphene sheet and alleviation of the interfacial reaction between ceramic precursors and graphene sheets during high-temperature pyrolysis, ZrC–SiC‒Graphene composite was synthesized via a facile technology of infiltrating ceramic slurry instead of ceramic precursor into macroporous graphene network and spark plasma sintering. The incorporation of the graphene network improved fracture toughness, critical crack size, and fracture energy of ZrC–SiC ceramic. The multiple length-scale toughening mechanisms of ZrC–SiC‒Graphene composite include the macroscopic toughening mechanism of crack deflection and bifurcation and the micro toughening mechanism of graphene bridging, ceramic micro zone tearing, graphene pull-out, graphene and ceramic brick slipping.

Effects of glass fiber reinforcement and thermoplastic elastomer blending on the mechanical performance of polylactide

Abstract The purpose of this study was to investigate how optimum mechanical properties (strength-modulus-toughness) of inherently very brittle polylactide (PLA) could be obtained by reinforcing with E-glass fibers (GF) and blending with thermoplastic polyurethane elastomer (TPU). Composites and blends were compounded by twin-screw extruder melt mixing, while specimens were shaped by injection molding. SEM analyses revealed that 15 wt% GF and 10 wt% TPU domains, alone and together, could be uniformly distributed in the PLA matrix leading to significant improvements in properties. Mechanical tests indicated that use of TPU blending alone resulted in enormous increases in the ductility and fracture toughness values, while GF reinforcements led to significant increases in strength and elastic modulus values. When GF and TPU were added together, it was observed that crack deflection, debonding and fiber pull-out toughening mechanisms of GF reinforcements were as effective as the rubber toughening mechanism of TPU blending.

Enhancing mode-I and mode-II fracture toughness of epoxy and carbon fibre reinforced epoxy composites using multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were added to an epoxy resin in an effort to improve the fracture toughness of bulk epoxy and also when used as matrix for carbon fibre reinforced epoxy composites (CFRPs). The incorporation of MWCNTs to bulk epoxy and CFRPs moderately increased the mode-I fracture energy, and significantly increased the mode-II fracture energy, i.e. the average mode-II fracture energy of CFRPs increased from 2026 J/m2 to 3406 J/m2 due to the addition of 0.5 wt% MWCNTs, and further to 5491 J/m2 due to the addition of 1 wt% MWCNTs. The superior toughening performance of MWCNTs in mode-II fracture is attributed to two reasons: 1) increased MWCNT breaking and crack deflection mechanisms under shear load; and 2) large fracture process zone accompanied with extensive hackle markings and micro-cracks ahead of the mode-II crack tip of CFRPs, which resulted in significant number of MWCNTs contributing to toughening mechanisms.