Progress in Research on Structural Ceramic Materials Toughened by Graphene

Graphene has excellent mechanical properties and unique physical/chemical properties,which make it have a good strengthening and toughening effect on structural ceramic materials.In recent years,it has received widespread attention and research.This article reviews the mixing and sintering processes in the preparation of graphene/ceramic com-posites,as well as the toughening mechanism of graphene on ceramic materials.It also looks forward to how to further enhance the toughening effect of graphene.

Ionomer Toughened Polyolefine

The morphology and properties of HDPE blends with Zn-SEPDM and GR were studied through SEM and mechanical property test. The results show that as Zn-SEPDM/GR content amounts to 20%, the blend becomes an IPN in structure, and that a rather high impact and tensile strength of HDPE may be obtained after blending. The antistatic effect, the softening point,and HDT of the blend are higher as compared to HDPE/Zn-SEPDM/ZnSt (zinc stearate).The effect of Zn-SEPDM on the compatibility the morphology and properties of IPP blends were studied by DSC, TEM and mechanical properties test. The results show that as Zn-SEPDM content exceeds 20%. Zn-SEPDM in the blend becomes continuous and an abrupt change in impact strength is incurred there from. Owing to the incorporation of ionic groups into EPDM.the strong interactions betWeen the chains make both the impact and the tensile strength of IPP remarkably higher

Influence of the morphology on mechanical properties of toughened epoxies with CTBN

Presents a toughened epoxy risen based on diglycidylether of bisphenol A (DGEBA) cured with ethylene diamine (EDA) and modified with a carboxyl terminated acrylonitrile butadiene (CTBN), studies morphologies with different rubber content and analyses the influence of rubber phase morphology corresponding to different rubber contents on mechanical properties (elastic modulus, uniaxial compression yield stress, shearing strength, uneven tensile strength, critical stress intensity factor K ic  and strain energy release rate G ic ) of toughened epoxies.

Wear Characteristics of Zirconia Toughened Ceramic Drawing Dies

Wear resistance of several zirconia toughened ceramics in comparison with a metal-ceramic Co-WC has been studied in drawing wire field test. Result indicates that the harder the ceramic die, the longer the service life. Excellent wear resistance of ceramic die is obtained with a very high hardness (19 GPa). The service life is nearly three times that of Co-WC die. SEM observation on wear surfaces showed that material removal is mainly caused by plastic flow and ploughing process. But when the ceramic is composed of zirconia, alumina and some titanium carbide, micro-chipping and tribochemical reaction take place, and wear rate increases. Wear and friction induced martensite was detected by XRD. The T-M (tetragonal to monoclinic) phase transformation has a contribution to inhibiting microfracture.

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.

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.

Mechanical and Thermal Properties of Muscovite and Density Polyethylene-reinforced and–toughened Polypropylene Composites

The mechanical and thermal properties of polypropylene （PP）/muscovite/ low-density polyethylene （LDPE）/ polypropylenegraftmaleic anhydride （PP-g-MAH） ternary composites were investigated. In PP matrix, muscovite, LDPE, and PP-g-MAH were added as strengthening agent, toughening agent, and compatibilizer, respectively. The effects of dosages of the added materials were analyzed. The - experimental results show that the optimum recipe of PP/muscovite/LDPE/PP-g-MAH composites is 100/10/6/20 （mass ratio）. Compared with the pure PP, the mechanical properties of PP/muscovite/LDPE/PP-g-MAH composites, including notched impact strength, Rockwell hardness and flexural strength, are improved. Although tensile strength is slightly decreased, they have better toughness. Filled with muscovite, the heat-resistance and heat-decompostion of the composites are improved.

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.

CNT toughened aluminium and CFRP interface for strong adhesive bonding

This study presents a simple technique for strengthening the adhesive-bond strength between aluminium(Al)substrate and carbon fibre reinforced polymer(CFRP) utilising resin pre-coating(RPC) with carbon nanotubes(CNTs). The CNT-containing RPC solution with 90 wt% acetone and 10 wt% resin(without hardener) was applied onto Al substrates, where micro-/nano-vertical channels had been created by chemical or mechanical surface treatments to accommodate CNTs. RPC was able to fill all micro-/nano-cavities over the Al substrate surface, then CNTs were pulled into those vertical micro-channels by the capillary action generated from acetone evaporation.Normal epoxy adhesive(resin + hardener) was applied after the CNT-containing RPC treatment. CNTs bridging across the interface between the adhesive joint and Al substrate and sealing of micro-/nano-cavities by RPC effectively enhanced the interfacial shear bond strength between the Al substrate and CFRP by 30–100%depending on the Al substrate surface profiles. Al substrates with two different chemical treatments were compared in this study for the effectiveness of CNT interfacial reinforcement. Results from a steel substrate after sandblasting were also included for comparison.

La_2Zr_2O_7 TBCs toughened by Pt particles prepared by cathode plasma electrolytic deposition

La2Zr2O7 thermal barrier coatings（TBCs） with dispersed Pt particles were prepared by cathode plasma electrolytic deposition（CPED） with ceramic balls added to the cathode region. Compared with the conventional CPED, when ceramic balls are used in the cathode region, the plasma discharge ignition current density decreases approximately 62-fold and the stable plasma discharges occur at the whole cathode surface. Such TBCs with a thickness of 100 μm exhibit a crack-free surface and are composed of pyrochlore-structured La2Zr2O7. Cyclic oxidation, scratching, and thermal insulation capability tests show that such TBCs not only exhibit high resistance to oxidation and spallation but also provide good thermal insulation. These beneficial effects are attributed to the excellent properties of TBCs, such as good thermal insulation because of low thermal conductivity, high-temperature oxidation resistance because of low-oxygen diffusion rate, and good mechanical properties because of the toughening effect of Pt particles.

SiC WHISKER-TOUGHENED ALUMINIUM NITRIDE CERAMICS

Aluminium nitride ceramics containing 15 -30 w/o of SiC whiskers can be sintered to full density (by hot pressing at 1800℃)using 2-3 w/o of Y2O3 additions. The whiskers increase the toughness and strength of the composite, KIC increasing from 2. & to about 5.0 and flexural strength increasing by 30-50%. However, the whiskers must be well dispersed and if the dispersion is not satisfactory, toughness may increase but the strength decreases. The hot-pressing temperature can be reduced by up to 100℃ if Y(NO3)3 . 5H2O is used as the sintering additive instead of Y2O3, but some oxidation of the AIN occurs during heating. Isopropan-2-01 is a better dispersing agent than cyclohexane, but again some oxidation of the AIN does occur.The best sample prepared during this work contained 20 w/o of SiC whiskers and 2 w/o Y2O3 added as Y(NO3)3. 5H2O and mixed in isopropanol. This exhibited a mean strength of 453MPa (maximum 522MPa, measured by disc flexure) and a fracture toughness of5. 5MPam1/2.

TOUGHNESS INCREMENT BY CRACK GROWTH IN TOUGHENED STRUCTURAL MATERIALS

Material toughening could be furnished by the energy dissipating wakes and bridging segments during crack growth.According to their contributions to the energy integral applicable to a growing crack,the toughening mechanisms are categorized as: dilatational plasticity and induced shear yielding in the crack wakes,bridging due to second inclusion phases,and the matrix bridging caused by wavy crack front.Detailed toughening analysis is pursued for structural polymers and composite materials reinforced by short aligned fibers.

Effect of toughened epoxy resin on partial discharge at solid-solid interface

A series of solid-solid interfaces, consisting of ceramic-epoxy resin interface samples with a tip-plate electrode, were investigated by performing partial discharge tests and realtime electrical tree observations. A toughening agent was added to the epoxy resin at different ratios for comparison. The impact strength, differential scanning calorimetry （DSC） and dielectric properties of the cured compositions and ceramic were tested. The electric field strength at the tip was calculated based on Maxwell＇s theory. The test results show that the addition of a toughener can improve the impact strength of epoxy resin but it decreases the partial discharge inception voltage （PDIV） of the interface sample. At the same time, toughening leads to complex branches of the electrical tree. The simulation result suggests that this reduction of the PDIV cannot be explained by a change of permittivity due to the addition of a toughening agent. The microstructural change caused by toughening was considered to be the key factor for lower PDIV and complex electrical tree branches.

CTBN-TOUGHENED EPOXY RESINS——EFFECT OF CURING MECHANISM ON NETWORK STRUCTURE OF THE RUBBER PHASE

Epoxy resins toughened with carboxyl-terminated butadieneacrylonitrile copolymers (CTBN) are two-phase thermosets. The network of the in situ formed rubber particles depends upon the curing mechanism of the resin. When a primary polyamine such as triethylene tetramine was used as curing agent, the network of the rubber phase was quite incomplete, whereas a perfect rubber network was formed with 2-ethyl-4-methyl imidazole as the curing agent.

Effect of Grain Size of β-Si_3N_4 and Glassy Phase on Strength and Fracture Toughness of In-Situ Toughened Si_3N_4

An in-situ toughened Si_3N_4 ceramic is obtained by hot-pressing.Its flexural strength and fracture toughness are 960MPa,12.74MPa·m^(1/2)at room temperature and 720MPa,23.94 MPa·m~_(1/2)at 1350℃,respectively.The relation between grain size of β-Si_3N_4 and mechanical properties is investigated.The glassy phase containing Y and La plays an important role in increasing high-temperature mechanical properties because of its high viscosity and softening temperature.Crack deflection,crack branching and pullout of rodlike β-Si_3N_4 grains are ob- served in this material by SEM,and the toughening mechanisms are discussed.

Strengthened and toughened SiHfBCN-based hightemperature resistant adhesive with SiC NWs

To further improve the performance of binders,a SiHfBCN-based high-temperature resistant adhesive was successfully synthesized by Polymer-Derived Ceramics(PDC)route using TiB2,Polysiloxane(PSO)and short SiC nanowires as fillers.The effect of short SiC nanowires on the adhesive strength at room temperature and high temperature,as well as the reinforcing mechanism was studied.Compared with the adhesive without SiC nanowires,after curing(at 170℃)and pyrolysis(at 1000℃)in air,the appropriate adding of SiC nanowires upgrades the room temperature and high temperature(at 1000℃ in air)adhesive strength to(12.50±0.67)MPa(up by about 32%)and(13.11±0.79)MPa(up by about 106%),respectively.Attractively,under the synergistic impact of the nanowire bridging,nanowire breaking,nanowire drawing and crack deflection,the optimized adhesive exhibits multi-stage fracture,causing the increscent fracture displacement.

Boron nitride microribbons strengthened and toughened alumina composite ceramics with excellent mechanical,dielectric,and thermal conductivity properties

Aluminum oxide(Al_(2)O_(3))ceramics have been widely utilized as circuit substrates owing to their exceptional performance.In this study,boron nitride microribbon(BNMR)/Al_(2)O_(3)composite ceramics are prepared using spark plasma sintering(SPS).This study examines the effect of varying the amount of toughened phase BNMR on the density,mechanical properties,dielectric constant,and thermal conductivity of BNMR/Al_(2)O_(3)composite ceramics while also exploring the mechanisms behind the toughening and increased thermal conductivity of the fabricated ceramics.The results showed that for a BNMR content of 5 wt%,BNMR/Al_(2)O_(3)composite ceramics displayed more enhanced characteristics than pure Al_(2)O_(3)ceramics.In particular,the relative density,hardness,fracture toughness,and bending strength were 99.95%±0.025%,34.11±1.5 GPa,5.42±0.21 MPa·m^(1/2),and 375±2.5 MPa,respectively.These values represent increases of 0.76%,70%,35%,and 25%,respectively,compared with the corresponding values for pure Al_(2)O_(3)ceramics.Furthermore,during the SPS process,BNMRs are subjected to high temperatures and pressures,resulting in the bending and deformation of the Al_(2)O_(3)matrix;this leads to the formation of special thermal pathways within it.The dielectric constant of the composite ceramics decreased by 25.6%,whereas the thermal conductivity increased by 45.6%compared with that of the pure Al_(2)O_(3)ceramics.The results of this study provide valuable insights into ways of enhancing the performance of Al_(2)O_(3)-based ceramic substrates by incorporating novel BNMRs as a second phase.These improvements are significant for potential applications in circuit substrates and related fields that require high-performance materials with improved mechanical properties and thermal conductivities.

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 Performance Polylactide Toughened by Supertough Polyester Thermoplastic Elastomers:Properties and Mechanism

It is a challenge to develop a biodegradable toughener to toughen polylactic acid(PLA)with both high strength and high toughness,since toughness and strength are mutually exclusive.Here,a series of supertough polyester thermoplastic elastomers(TPEs),poly(L/D-lactide)-b-poly(ε-caprolactone-co-δ-valerolactone)-b-poly(L/D-lactide)s(PLLA-PCVL-PLLA,L-TPEs or PDLA-PCVL-PDLA,D-TPEs),were prepared and blended with a PLLA matrix to toughen PLLA.The mechanical properties of PLLA could be regulated in a wide range by changing blending ratios and TPE structures.For PLLA blends toughened by L-TPEs,the highest elongation at break is up to 425%with the tensile strength of 33.1 MPa and the toughness of 104 MJ/m3.By the stereocomplex crystallization of PLA(sc-PLA),the tensile strength of the PLLA/D-TPE blends further increased to 41.8 MPa with a similar elongation at break(418%)and the toughness up to 128 MJ/m3.The detailed characterizations revealed a toughening mechanism:(I)the added soft segments increased the ductility of the PLLA matrix,(II)the PLLA segments of L-TPEs increased the compatibility between TPEs and PLLA matrix,and(III)the formation of sc-PLA between the PDLA segments in D-TPE and PLLA provided higher tensile strength by enhancing the strength of the crystal skeleton.The toughened PLA using TPEs can maintain original non-toxic and degradable properties,and be applied potentially in surgical sutures,and 3D-printed scaffolds.

Instrumented and Vickers Indentation for the Characterization of Stiffness,Hardness and Toughness of Zirconia Toughened Al_2O_3 and SiC Armor

Instrumented and Vickers indentation testing and microstructure analysis were used to investigate zirconia toughened alumina （ZTA） and silicon carbide （SIC）. Several equations were studied to relate the Vickers indentation hardness, Young＇s modulus and crack behavior to the fracture toughness. The frac- ture in SiC is unstable and occurs primarily by cleavage leading to a relatively low toughness of 3 MPa m1/2, which may be inappropriate for multi-hit capability. ZTA absorbs energy by plastic deformation, pore collapse, crack deviation and crack bridging and exhibits time dependent creep. With a relatively high toughness around 6.6 MPa m1/2, ZTA is promising for multi-hit capability. The higher accuracy of median equations in calculating the indentation fracture toughness and the relatively high c/a ratios above 2.5 suggest median type cracking for both SiC and ZTA. The Young＇s modulus of both ceramics was most accurately measured at lower indentation loads of about 0.5 kgf, while more accurate hardness and fracture toughness values were obtained at intermediate and at higher indentation loads beyond 5 kgf, respectively. A strong indentation size effect （ISE） was observed in both materials. The load independent hardness of SiC is 2563 HV, putting it far above the standard armor hardness requirement of 1500 HV that is barely met by ZTA.