Toughening mechanism of lined Al_2O_3-ZrO_2 multiphaseceramics in SHS composite pipes

Hypoeutectic and hypereutectic Al2O3-ZrO2 multiphase ceramics-lined composite pipes were produced by using the gravitational separation self-propagate high-temperature synthesis （SHS） process. The microstructure of the ceramics was observed by means of SEM and EPMA. The fracture toughness of the multiphase ceramics was tested by using the Vickers indentation method. The fracture toughness of hypoeutectic Al2O3-ZrO2 multiphase ceramics is 15.96 MPa·m^1/2 and that of hypoeutectic Al2O3-ZrO2 multiphase ceramics is 15.23 MPa·m^1/2. The toughening mechanisms were systematically investigated by means of SEM and XRD. The results show that the bridging toughening mechanism, stress induced ZrO2 transformation toughening mechanism, and microcrack toughening mechanism are the predominant toughening mechanism.

AN INVESTIGATION OF HIGH-TEMPERATURE DEFORMATION STRENGTHENING AND TOUGHENING MECHANISM OF TITANIUM ALLOY

The high-temperature deformation strengthening and toughening mechanisms of titanium alloys have been investigated in this paper. The materials processed by this method produce a new tri-modal microstrvcture, which consists of 10-20% equiaxed alpha, streaky alpha and transformed beta matrix. It is found that the higher ductility of tri-modal microstructure is attributed to the equiaxed alpha's coopemtive slip and coordinated deformation with the transformed beta matrix. The streaky alpha phases not only increase the strength and creep properties, but also increase the fracture toughness. Propagating along grain boundaries between two neighboring streaky alpha phases, cracks in tri-modal microstructure make a more tortuous way, and then the materials show a higher fracture toughness. This new method is applicable to α, near α,α+β and near β titanium alloys.

A Review of Experimental Research on the Mode I Fracture Behavior of Bamboo

Bamboo is an eco-friendly material with light weight,high strength,short growth cycle and high sustainability,which is widely used in building structures.Engineered bamboo has further promoted the development of modern bamboo structures due to its unrestricted size and shape.However,as a fiber-reinforced material,fracture damage,especially Mode I fracture damage,becomes the most likely damage mode of its structure,so Mode I fracture characteristics are an important subject in the research of mechanical properties of bamboo.This paper summarizes the current status of experimental research on the Mode I fracture properties of bamboo based on the three-point bending(TPB)method,the single-edge notched beam(SENB)method,the compact tension(CT)method and the double cantilever beam(DCB)method,compares the fracture toughness of different species of bamboo,analyzes the toughening mechanisms and fracture damage modes,discusses the applicability of different theoretical calculation methods,and makes suggestions for future research priorities,aiming to provide a reference for future research and engineering applications in related fields.

Microstructure and Mechanical Properties of CaCO_3 Whisker-reinforced Cement

Composite Portland cement （PC） played an important role in various kinds of construction engineering owing to low hydration heat,low-cost,and application of solid industrial waste,but its brittleness and low strength limited its use in stress-bearing locations.The aim of this study is to improve the toughness and fracture resistance by incorporating CaCO3 whisker in cement matrix.Effect of different content of calcium carbonate whiskers on the mechanical properties of PC was investigated.The results showed that the flexural strength,impact strength and split tensile strength were increased by 39.7%,39.25% and 36.34% at maximum,respectively.Microstructure and elements of the whiskers in hardened cement were observed and analyzed by SEM/EDS.The mechanisms of the reinforcement of CaCO3 whisker on cement were also discussed,and the conclusion was that the improvement could be correlated to energy-dissipating processes owing to crack bridging,crack deflection,and whisker pull-out at the crack tips.

Mechanical Behaviors of ZrO_2-Al_2O_3 Ceramic Composites with Y_2O_3 as Stabilizer

The ZrO2-Al2O3 ceramic composites were prepared by appropriate techniques with commercial ZrO2 and Al2O3 powders as raw materials and Y2O3 as stabilizer. The results indicate that with the introduction of Al2O3 into the ZrO2 matrix where the quantity of additive Y2O3 is 3.5% (mole fraction), the growth of ZrO2 grains is efficiently inhibited, which helps the ZrO2 grains exist in a metastable tetragonal manner; thus higher strength and toughness are acquired. When the content of alumina is 20% (mass fraction), the bending strength and fracture toughness of the composites are 676.7 MPa and 10 MPa·m1/2 respectively, the mechanical behaviors are close to those prepared with ZrO2 and Al2O3 powders synthesized through wet chemical approach. The mechanical behaviors of the composites are well improved owing to the dispersion toughening of alumina grains and phase transformation toughening of zirconia grains.

EPOXY RESIN TOUGHENED BY THERMOPLASTICS

Two kinds of tough ductile heatresisting thermoplastic, namely bisphenol A polysulfone (PSF) and polyethersulfone (PES) were used to toughen thermoset epoxy resin. A systematic study on the relationship between the molecular weight and the terminal group of the thermoplastic modifier and the fracture toughness of the modified resin was carried out. The morphology of PSF modified epoxy resin was surveyed. With the same kind of PSF the structure of the epoxy resin and the toughening effect of PSF was also investigated. The fractography of PSF, particle modified epoxy was examined in detail with SEM. The contribution of every possible energy absorption process has been discussed. Crack pinning mechanism seems to be the most important toughening mechanism for tough ductile thermoplastic PSF particle modified epoxy system.

Synergistically Toughening Effect of SiC Whiskers and Nanoparticles in Al_2O_3-based Composite Ceramic Cutting Tool Material

In recent decades, many additives with different characteristics have been applied to strengthen and toughen Al2O3-based ceramic cutting tool materials. Among them, SiC whiskers and SiC nanoparticles showed excellent performance in improving the material properties. While no attempts have been made to add SiC whiskers and SiC nanoparticles together into the ceramic matrix and the synergistically toughening effects of them have not been studied. An Al2O3-SiCw-SiC np advanced ceramic cutting tool material is fabricated by adding both one-dimensional SiC whiskers and zero-dimensional SiC nanoparticles into the Al2O3 matrix with an effective dispersing and mixing process. The composites with 25 vol% SiC whiskers and 25 vol% SiC nanoparticles alone are also investegated for comparison purposes. Results show that the Al2O3-SiCw-SiCnp composite with both 20 vo1% SiC whiskers and 5 vol% SiC nanoparticles additives have much improved mechanical properties. The flexural strength of Al2O3-SiCw-SiCnp is 730＋ 95 MPa and fracture toughness is 5.6 ± 0.6 MPa.m1/2. The toughening and strengthening mechanisms of SiC whiskers and nanoparticles are studied when they are added either individually or in combination. It is indicated that when SiC whiskers and nanoparticles are added together, the grains are further refined and homogenized, so that the microstructure and fracture mode ratio is modified. The SiC nanoparticles are found helpful to enhance the toughening effects of the SiC whiskers. The proposed research helps to enrich the types of ceramic cutting tool and is benefit to expand the application range of ceramic cutting tool.

Crack deflection occurs by constrained microcracking in nacre

For decades, nacre has inspired researchers because of its sophisticated hierarchical structure and remarkable mechanical properties, especially its extreme fracture toughness compared with that of its predominant constituent,CaCO3, in the form of aragonite. Crack deflection has been extensively reported and regarded as the principal toughening mechanism for nacre. In this paper, our attention is focused on crack evolution in nacre under a quasi-static state. We use the notched three-point bending test of dehydrated nacre in situ in a scanning electron microscope(SEM) to monitor the evolution of damage mechanisms ahead of the crack tip. The observations show that the crack deflection actually occurs by constrained microcracking. On the basis of our findings, a crack propagation model is proposed, which will contribute to uncovering the underlying mechanisms of nacre’s fracture toughness and its damage evolution. These investigations would be of great value to the design and synthesis of novel biomimetic materials.

Room-Temperature Mechanical Properties of Rare Earths Reinforced MoSi_2 Material

MoSi 2 and rare earths/MoSi 2 materials were prepared by mechanical alloying, IP and high temperature sintering techniques. Their room temperature properties such as bending strength, fracture toughness and electric conductivity were measured. The results show that rare earths have better strengthening and toughening effects on the MoSi 2 matrix than SiC does. The room temperature bending strength and fracture toughness of 0 9% rare earths/MoSi 2 material are 419 41 MPa and 5 81 MPa·m 1/2 , which have increased by 46% and 81% than the matrix, respectively. The strengthening mechanisms of rare earths/MoSi 2 are fine grain and dispersion strengthening. The toughening mechanisms are fine grain toughening, crack deflection and bowing toughening. Moreover, the effect of rare earths on the electric conductivity of MoSi 2 is much weaker than that of SiC whiskers. With the addition of 0 9% rare earths, the resistivity of MoSi 2 is only raised by about 13 9%.

In-situ Fabricated TiB_2 Particle-whisker Synergistically Toughened Ti(C,N)-based Ceramic Cutting Tool Material

The mechanical properties of ceramic cutting tool materials can be modified by introducing proper content of nanoparticles or whiskers.However,the process of adding whiskers or nanoparticles has the disadvantages of high cost and health hazard as well as the agglomeration;although a new in-situ two-step sintering process can solve the above problems to some extent,yet the problems of low conversion ratio of the raw materials and the abnormal grain growth exist in this process.In this paper,an in-situ one-step synthesis technology is proposed,which means the growth of whiskers or nanoparticles and the sintering of the compact can be accomplished by one time in furnace.A kind of Ti（C,N）-based ceramic cutting tool material synergistically toughened by TiB_2 particles and whiskers is fabricated with this new process.The phase compositions,relationships between microstructure and mechanical properties as well as the toughening mechanisms are analyzed by means of X-ray diffraction（XRD）and scanning electron microscopy（SEM）.The composite which is sintered under a pressure of 32 MPa at a temperature of 1700℃in vacuum holding for 60 min can get the optimal mechanical properties.Its flexural strength,fracture toughness and Vickers hardness are 540 MPa,7.81 MPa·m（1/2）and 20.42 GPa,respectively.The composite has relatively high density,and the in-situ synthesized TiB_2 whiskers have good surface integrity,which is beneficial for the improvement of the fracture toughness.It is concluded that the main toughening mechanisms of the present composite are whiskers pulling-out and crack deflection induced by whiskers,crack bridging by whiskers/particles and multi-scale particles synergistically toughening.This study proposes an in-situ one-step synthesis technology which can be well used for fabricating particles and whiskers synergistically toughened ceramic tool materials.

Curing Kinetics, Mechanical Properties and Thermal Stability of Epoxy/Graphene Nanoplatelets(GNPs) Powder Coatings

Epoxy/graphene nanoplatelets（GNPs） powder coatings were fabricated using ultrasonic predispersion of GNPs and melt-blend extrusion method. The isothermal curing kinetics of epoxy/GNPs powder coating were monitored by means of real-time Fourier transform infrared spectroscopy（FT-IR） with a heating cell. The mechanical properties of the epoxy/GNPs cured coatings had been investigated, by evaluating their fracture surfaces with field-emission scanning electron microscopy（FE-SEM） after three-point-bending tests. The thermal stability of the epoxy/GNPs cured coatings was studied by thermo-gravimetric analysis（TGA）. The isothermal curing kinetics result showed that the GNPs would not affect the autocatalytic reaction mechanism, but the loading of GNPs below 1.0 wt % additive played a prompting role in the curing of the epoxy/GNPs powder coatings. The fracture strain, fracture toughness and impact resistance of the epoxy/GNPs cured coatings increased dramatically at low levels of GNPs loading（1 wt %）, indicating that the GNPs could improve the toughness of the epoxy/GNPs powder coatings. Furthermore, from FE-SEM studies of the fracture surfaces, the possible toughening mechanisms of the epoxy/GNPs cured coatings were proposed. TGA result showed that the incorporation of GNPs improved the thermal stability of the cured coatings. Hence, the GNPs modified epoxy can be an efficient approach to toughen epoxy powder coating along with improving their thermal stability.

Variations of Microstructure and Mechanical Properties of Si-B-O-N Ceramics with Sintering Temperatures

Si-B-O-N powder without B-O bonds synthesized by polymeric precursor were hot-pressed into ceramics at different tempera-tures. The variations of microstructure and mechanical properties of Si-B-O-N ceramics have been investigated. Crystallization of Si-B-O-N ceramics occurred at about 1400癈. Density, elastic modulus, and flexural strength of the ceramics increased with the increasing sintering temperatures, and reached to their maximum values at 1600癈. By contrast, hardness and frac-ture toughness of the ceramics monotonically changed with increasing sintering temperatures. Hardness decreased, while the fracture toughness increased. The principal toughening mechanisms including crack deflection, crack bridging and plate grain pulling-out effects are discussed

Microstructural characterization and mechanical properties of self-reinforced Si_3N_4 ceramics containing high oxynitride glass

Self-reinforced Si_3N_4 ceramics containing high oxynitride glass have beenfabricated by the control of microstructure evolution and p-Si_3N_4 grain growth. The effects of thesize distribution of the elongated p-Si_3N_44 grains, and the p-Si_3N_4 grain growth as well as theoxynitride glass chemical characteristic on the microstructure and mechanical properties wereinvestigated. The experimental results show that the p-Si_3N_4 grains in high oxynitride glass growto elongated rod-like crystals and form the stereo-network structure. Under the sintering conditionsof 1800 deg C and 60 min, a quite uniform microstructure with an average aspect ratio of 6.5 and anaverage of 1 mu m can be obtained. A large amount of oxynitride glass phase with high nitrogencontent enhances the elevated temperature fracture toughness because of its high softeningtemperature and high viscosity. In the present material, the crack deflection and pullout of theelongated rod-like P-Si_3N_44 grains are the primary toughening mechanisms.

Microstructural characterization and mechanical properties of self-reinforced Si_3S_4 ceramics containing high oxynitride glass

Self-reinforced Si_3N_4 ceramics containing high oxynitride glass have been fabricated by the control of microstructure evolution and β-Si_3N_4 grain growth. The effects of the size distribution of the elongated β-SiaN_4 grains, and the β-Si_3N_4 grain growth as well as the oxynitride glass chemical characteristic on the microstructure and mechanical properties were investigated. The experimental results show that the β-Si_3N_4 grains in high oxynitride glass grow to elongated rod-like crystals and form the stereo-network structure. Under the sintering conditions of 1800℃ and 60 min, a quite uniform microstructure with an average aspect ratio of 6.5 and an average of 1 μm can be obtained. A large amount of oxynitride glass phase with high nitrogen content enhances the elevated temperature fracture toughness because of its high softening temperature and high viscosity. In the present material, the crack deflection and pullout of the elongated rod-like β-Si_3N_4 grains are the primary toughening mechanisms.

Steel Fiber Toughening Mullite-SiC Castables for Coke Dry Quenching Furnace Corbel Pillar

The mullite-SiC castables for coke dry quenching（CDQ）furnace corbel pillar were prepared by taking mullite and silicon carbide as aggregates,silicon carbide powder,fused silica powder,α-Al2O3powder and silica fume as matrix,calcium aluminate cement as binding system,and extra-adding steel fibers to extend the CDQ furnace refractory life.The effects of steel fiber types（melt pumping meniscus,cut straight,wavy indentation,cold-drawn hook）and steel fiber extra-additions（0,1%,2%,3%by volume）on the properties of mullite-SiC castables were investigated.Then the toughening mechanism of steel fiber in the castables was analyzed according to the load-displacement curves.The results show that：（1）the toughening effects of the melt pumping meniscus steel fiber with rough surface and colddrawn hook steel fiber with strong anchoring effect with the matrix are better than those of cut straight steel fiber and wavy indentation steel fiber;（2）the suitable amount of steel fiber（less than 2%）in mullite-SiC castables not only significantly improves the mechanical properties,thermal shock resistance and wearing resistance,but also improves the fracture energy by 49.0%and slows the crack growth;（3）the bond strength and mechanical anchoring force between steel fiber and castable interface are key elements of steel fiber reinforced castables.

Role of Microstructural Features in Toughness Improvement of Zirconia Toughened Alumina

Ceramics constitute an integral part of highly efficient armours due to their low density, high hardness, strength and stiffness. However, they lack toughness and multi-hit capability. Therefore, zirconia toughened alumina is investigated. The hardness is evaluated using Vickers, Knoop and instrumented indentations, while the fracture toughness is evaluated using the indentation technique and Charpy tests. The strength is evaluated using ring-on-ring, four point bend and drop weight tests. The Young’s modulus is evaluated using the unloading instrumented indentation curves. Microstructure, porosity and density are characterised using ultrasonic scanning, Archimedes principle, optical and scanning electron microscopy. Results show an indentation size effect on all mechanical properties. A substantial improvement in toughness is achieved through retardation of crack initiation by tetragonal-to-monoclinic phase transformation in zirconia particles, crack deviation thanks to appropriate grain structure, as well as energy absorption by densification due to remaining porosity. This improved toughness is expected to promote multi-hit capability.

Self-assembling nacre-like high-strength and extremely tough polymer composites with new toughening mechanism

Achieving high strength,deformability and toughness in polymers is important for practical industrial applications.This has remained challenging because of the mutually opposing effects of improvements to each of these properties.Here,a self-assembling nacre-like polymer composite is designed to achieve ex-tremely tough with increasing strength.This special design significantly improved polymer’s mechanical properties,including an ultra-high fracture strain of 1180%,a tensile strength of 55.4 MPa and a toughness of 506.9 MJ/m^(3),which far exceed the highest values previously reported for polymer composites.This ex-cellent combination of properties can be attributed to a novel toughening mechanism,achieved by the synergy of the domain-limiting effect of metallic glass fragments with the strain-gradient-induced orien-tation and crystallisation within the polymer during stretching.Our approach opens a promising avenue for designing robust polymer materials in armour and aerospace engineering for a range of innovative applications.

Preparation and toughening mechanism of Al_(2)O_(3) composite ceramics toughened by B_(4)C@TiB_(2) core–shell units

In this paper, the concept of incorporating core–shell structured units as secondary phases totoughen Al_(2)O_(3) ceramics is proposed. Al_(2)O_(3) composite ceramics toughened by B_(4)C@TiB_(2) core–shellunits are successfully synthesized using a combination of molten salt methodology and spark plasmasintering. The synthesis of B_(4)C@TiB_(2) core–shell toughening units stems from the prior production ofcore–shell structural B_(4)C@TiB_(2) powders, and this core–shell structure is effectively preserved withinthe Al_(2)O_(3) matrix after sintering. The B_(4)C@TiB_(2) core–shell toughening unit consists of a micron-sizedB4C core enclosed by a shell approximately 500 nm in thickness, composed of numerous nanosizedTiB2 grains. The regions surrounding these core–shell units exhibit distinct geometric structures andencompass multidimensional variations in phase composition, grain dimensions, and thermal expansioncoefficients. Consequently, intricate stress distributions emerge, fostering the propagation of cracks inmultiple dimensions. This behavior consumes a considerable amount of crack propagation energy,thereby enhancing the fracture toughness of the Al_(2)O_(3) matrix. The resulting Al_(2)O_(3) composite ceramicsdisplay relative density of 99.7%±0.2%, Vickers hardness of 21.5±0.8 GPa, and fracture toughness6.92±0.22 MPa·m1/2.

High MXene loading, nacre-inspired MXene/ANF electromagnetic interference shielding composite films with ultralong strain-tofailure and excellent Joule heating performance

The high power density and intelligence of next-generation flexible electronic devices bring many challenges to fabricate flexible composite films with electromagnetic interference(EMI)shielding effectiveness(SE)property and excellent toughness via a simple method.Herein,inspired by the layered structure and biopolymer matrix networks in natural nacre,nacre-like layered Ti_(3)C2TX(MXene)/aramid nanofiber(ANF)films were fabricated through sol-gel,vacuum-assisted filtration,and hot-pressing.Three-dimensional(3D)interconnected aramid nanofibers networks between adjacent layered MXene result in an ultralong strain-to-failure of the film.Even though the functional filler MXene contents are as high as 60 wt.%and 70 wt.%,the strain-to-failure of the films could reach astonishing values of 18.34%±1.86%and 14.43%±1.26%,respectively.And the tensile strength could maintain about 85 MPa.Excitingly,with such a high filler,the film can also withstand double folding and vigorous rubbing without damage,which could better adapt to a harsh application environment.The result means that this work provides a convenient way to prepare other high functional filler composite films with excellent mechanical performance.The EMI SE values could reach 45 and 52.15 dB at 60 wt.%and 70 wt.%MXene in 8.2–12.4 GHz.Meanwhile,the films have prominent Joule heating properties,high sensitivity(<15 s),small voltage operation(0.5 V),and high operation constancy(1300 s).Therefore,nacre-inspired MXene/ANF composite films in this work have ability to apply in many areas including communication technology,military,and aerospace.

Improved damage tolerance and oxidation resistance of (Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC by introducing chopped carbon fibers

High-entropy diborides(HEBs)are considered as promising high-temperature structure materials owing to their high melting point and excellent thermal stability.However,the intrinsic brittleness is the main obstacle that seriously limits their practical applications.To overcome with this obstacle,carbon fibers(Cf)with outstanding mechanical properties are used in the present work as a first attempt to improve the damage tolerance of HEBs.The as-prepared C_(f)/(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC composite(C_(f)/HEB–SiC)shows high relative density(97.9%)and good mechanical properties with flexural strength of 411±3 MPa and fracture toughness of 6.15±0.11 MPa·m^(1/2).More importantly,the damage tolerance parameter(Dt)has increased from 0.10 m^(1/2) for HEB–SiC to 0.29 m^(1/2) for C_(f)/HEB–SiC.Through microstructural analysis and Vickers indentation of the composite,the toughening mechanisms are disclosed.The carbon fibers coated with carbon coatings demonstrate unique capacity for prolonging the crack propagation path,which promotes the reliability of the composite effectively.Moreover,the C_(f)/(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC composite also exhibits good static oxidation resistance in the temperature range of 1100–1500℃in air due to the formation of the protective oxide layer constituting of multicomponent oxides(Zr)HfTiO4 and(Zr)Hf_(6)Ta_(2)O_(17) embedded in a continuous SiO_(2) glass.These results are promising,and this primary work can be used as a reference to the synthesis of C_(f)/HEBs for thermal protection materials under hightemperature serving conditions.