Graphene nanosheets toughened TiB2-based ceramic tool material by spark plasma sintering

One kind of TiB₂/TiC composite ceramic tool material toughened by graphene nanosheets was fabricated by spark plasma sintering. Effects of graphene nanosheets on microstructure, mechanical properties and toughening mechanisms were investigated. The results indicated that TiB₂/TiC with 0.1?wt% graphene nanosheets sintered at 1800?°C with the holding time of 5?min obtained full densification and optimal mechanical properties. Its fracture toughness and Vickers hardness were 7.9?±?1.2?MPa?m^1/2 and 20.0?±?0.7?GPa, respectively. Excess graphene nanosheets had no effects to toughness improvement. Fracture toughness was increased by 31.7% in comparison with the TiB₂/TiC without graphene nanosheets. Toughness enhancement mainly benefited from crack bridging, also slip-stick effect of graphene made it hard to detach and effectively restrained crack extension.     

Enhancing Toughness in Boron Carbide with Reduced Graphene Oxide

Ceramics typically have very high hardness, but suffer from poor toughness. Here, we use graphene to enhance the toughness of bulk boron carbide ceramics. The reduced graphene oxide (rGO) platelets are homogenously dispersed with boron carbide particles after sintering at 1350°C, under high pressure of 4.5 GPa with a multi‐anvil apparatus. Fracture toughness of the composites is increased ~131% (from ~3.79 to ~8.76 MPa·m^1/2) at 1.5 vol% rGO platelets as a result of a toughing effect of graphene along with a little sacrificing of the hardness and elastic modulus, compared with those of pure boron carbide. The remarkably enhanced fracture toughness in the boron carbide ceramics is associated with graphene sheets crack bridging and graphene interface sliding effect. This study holds much significance for the understanding and development of high‐performance graphene reinforcing ceramics.     

Electroconductive composite of zirconia and hybrid graphene/alumina nanofibers

Novel type of hybrid nanofillers representing graphene encapsulated alumina nanofibres was selected as an additive to develop toughened electroconductive partially stabilized zirconia. The sinterability, mechanical and electrical properties of the produced nanocomposites were studied as function of the filler/graphene content. Composites containing just 0.6 vol.% of graphene corresponding to 3 vol.% of hybrid nanofibres exhibited high electroconductivity of 58 S/m without deterioration of mechanical properties. They also showed a slight toughening effect that is reflected by an increase in the indentation fracture toughness by 20% as compared to monolithic zirconia.     

In situ study on microwave sintering of ZTA ceramic: Effect of ZrO2 content on densification,hardness, and toughness

Better understanding of the effect of multimode‐microwave sintering of zirconia‐toughened alumina (ZTA) was investigated. A comparative dilatometric analysis was conducted between conventional and microwave heating processes, to clarify the influence of zirconia on the densification of ZTA under electromagnetic field. The thermal gradient on sample measurements indicates the change to the microwave volumetric heating is improved by zirconia which adsorbs microwave energy better, thus acting as a susceptor. The most beneficial effect on microstructure, toughness, and hardness were observed at the optimal zirconia content of 10 vol%. The results with both microwave and conventional sintering illustrate the strengthening effect on the composite by zirconia. Of special interest, multimode microwave sintering creates a finer homogeneous microstructure, with resulting hardness and toughening comparable to those obtained for conventional sintering, as well as improved densification, and at lower cost.     

Effect of zirconia addition amount in glaze on mechanical properties of porcelain slabs

Applying toughened glaze layer on porcelain slabs can improve the fracture toughness of slabs and greatly reduce the production cost. In this study, porcelain slabs glaze with high toughness was fabricated by the processes of impregnation glazing and single firing method, using opaque frits, kaolin clay as the main raw materials, zirconia as an additive, and the effect of the addition amount of zirconia in glaze on fracture toughness of porcelain slabs was investigated. The results showed that the type and content of crystal phase of the glaze were greatly influenced by the addition amount of zirconia. Meanwhile, compared with the base glaze, the hardness and fracture toughness of the sample with zirconia glaze were significantly improved. Porcelain slabs with 10 wt% zirconia in glaze, sintering at 1200 °C, exhibited higher quality glaze and outstanding properties, including a water absorption of 1.95%, a Vickers hardness of 6.36 GPa, and a fracture toughness of 2.71 MPa m^1/2. The toughening mechanism of the glaze layer was as follows: a large number of zirconium silicate grains with high hardness were generated by the reaction of added zirconia with silica in the glass phase, which increased the content of crystal phase and then prevented the propagation of cracks; moreover during the martensitic transformation of the tetragonal zirconia grains, the volume and shear strain were generated to offset the stress field generated by the crack tip, thus toughening the material.     

Microstructure and properties of a graphene platelets toughened boron carbide composite ceramic by spark plasma sintering

A kind of B₄C/SiC composite ceramic toughened by graphene platelets and Al was fabricated by spark plasma sintering. The effects of graphene platelets and Al on densification, microstructure and mechanical properties were studied. The sintering temperature was decreased about 125–300?°C with the addition of 3–10?wt% Al. Al can also improve fracture toughness but decrease hardness. The B₄C/SiC composite ceramic with 3?wt%Al and 1.5?wt% graphene platelets sintered at 1825?°C for 5?min had the optimal performances. It was fully densified, and the Vickers hardness and fracture toughness were 30.09?±?0.39?GPa and 5.88?±?0.49?MPa?m^1/2, respectively. The fracture toughness was 25.6% higher than that of the composite without graphene platelets. The toughening mechanism of graphene platelets was also studied. Pulling-out of graphene platelets, crack deflection, bridging and branching contributed to the toughness enhancement of the B₄C-based ceramic.     

Microstructure and fracture toughness of SiAlCN ceramics toughened by SiCw or GNPs

Mechanical alloying and spark plasma sintering (SPS) were used to prepare dense SiAlCN ceramic and SiAlCN ceramic toughened by SiC whiskers (SiC_w) or graphene nanoplatelets (GNPs). The influences of different reinforcements on the microstructure and fracture toughness were investigated. The SiAlCN ceramic exhibited a fracture toughness of 4.4 MPa m^1/2 and the fracture characteristics of grain bridging, alternative intergranular and transgranular fracture. The fracture toughness of SiC_w/SiAlCN ceramic increased to 5.8 MPa m^1/2 and toughening mechanisms were crack deflection, SiC_w bridging and pull-out. The fracture toughness of GNP/SiAlCN ceramic increased significantly, which was up to 6.6 MPa m^1/2. GNPs played an important role in grain refinement, which resulted in the smallest grain size. Multiple toughening mechanisms, including crack deflection, crack branch, GNP bridging and pull-out could be found. The better toughening effect could be attributed to the larger specific surface area of GNPs and the appropriate interface bonding between GNPs and matrix.     

High‐temperature fracture toughness of mullite with monoclinic zirconia

Reactive sintering of zircon and alumina and zirconia additions to mullite are well‐established methods for improving the poor fracture toughness of mullite. While it is clear that transformation toughening is responsible for the improved toughness by addition of partially stabilized zirconia, it is not clear why adding unstabilized zirconia increases the toughness although microcracking and crack deflection have been suggested. Therefore, the fracture toughness of a mullite composite with 20 vol% unstabilized zirconia and a monolithic mullite were investigated at ambient conditions and at temperatures up to 1225°C. It was found that monoclinic zirconia increases the toughness at ambient conditions from the monolithic mullite value of 1.9 to 3.9 MPa·m^1/2. The toughness of the composite with zirconia remains relatively constant from ambient to 600°C but then decreases rapidly. The mechanism for the toughness enhancement as well as the reason for its variation with temperature are explained using changes in residual stress state as deduced using the sphere in shell model from the measured thermal expansion behavior.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Preparation and characterization of reduced graphene oxide-reinforced boron carbide ceramics by self-assembly polymerization and spark plasma sintering

A self-assembly polymerization process was used to prepare graphene oxide/boron carbide (GO/B₄C) composite powders, spark plasma sintering (SPS) was used to fabricate reduced graphene oxide/boron carbide (rGO/B₄C) composites at 1800 °C and 30 MPa with a soaking time of 5 min. The effects of rGO addition on mechanical properties of the composites, such as Vickers hardness, flexural strength and fracture toughness, were investigated. The results showed that GO/B₄C composite powders were successfully self-assembled and a network structure was formed at high GO contents. The flexural strength and fracture toughness of rGO/B₄C composites were 643.64 MPa and 5.56 MPa m^1/2, respectively, at 1 and 2.5 wt.% rGO content, corresponding to an increase of 99.11% and 71.6% when compared to B₄C ceramics. Uniformly dispersed rGO in rGO/B₄C composites played an important role in improving their strength and toughness. The toughening mechanisms of rGO/B₄C composites were explained by graphene pull-out, crack deflection and bridging.     

Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO₂ phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness.     

Enhancing toughness and strength of SiC ceramics with reduced graphene oxide by HP sintering

In this paper, the silicon carbide-reduced graphene oxide (SiC/rGO) composites with different content of rGO are investigated. The hot pressing (HP) at 2100?°C for 60?min under a uniaxial pressure of 40?M?Pa resulted in a near fully-dense SiC/rGO composite. In addition, the influence of graphene reinforcement on the sintering process, microstructure, and mechanical properties (fracture toughness, bending strength, and Vickers hardness) of SiC/rGO composites is discussed. The fracture toughness of SiC/rGO composites (7.9MPam^1/2) was strongly enhanced by incorporating rGO into the SiC matrix, which was 97% higher than the solid-state sintering SiC ceramics (SSiC) by HP. Meanwhile, the bending strength of the composites reached 625?M?Pa, which was 17.3% higher than the reference materials (SSiC). The microstructure of the composites revealed that SiC grains were isolated by rGO platelets, which lead to the toughening of the composite through rGO pull out/debonding and crack bridging mechanisms.     

SiBCN ceramic aerogel/graphene composites prepared via sol-gel infiltration process and polymer-derived ceramics (PDCs) route

The SiBCN ceramic aerogel/graphene composites were synthesized by combining a simple sol-gel infiltration process with CO₂ supercritical drying technology and polymer-derived ceramics route. In order to select the best preceramic sample for sintering, the micromorphology of PSNB aerogel/graphene composites fabricated with different graphene oxide solution concentrations were investigated. The microstructure evolution of the prepared SiBCN ceramic aerogel/graphene composites and phase composition were studied by SEM, TEM and XRD, the pore structure of the preceramic composites pyrolyzed at 1200 °C was tested by specific surface area and pore size analyzer. Furthermore, the compressive strain-stress curve and toughening mechanisms of composites were also investigated in detail. The results showed that all the preceramic composites and obtained ceramic aerogel composites possessed the mesoporous structure. The basic structure of SiBCN aerogel network changed from the initial spherical particles accumulation to the nanowires lapping with the sintering temperature increased from 800 °C to 1200 °C. After pyrolyzing at 1200 °C, the specific surface area and pore volume for the sample were 101.61 m² g⁻¹ and 1.43 cm³ g⁻¹, respectively, and a small amount of β-SiC crystalline phases were formed in amorphous ceramic matrix and had an relatively uniform distribution. Moreover, the paepared ceramic aerogel composites possessed a certain degree of toughness, the toughening mechanisms of composite samples mainly included the crack deflection, graphene pull-out, graphene bridging and graphene crumpling.     

Preparation and properties of reduced graphene oxide/fused silica composites

An in situ strategy for fabrication of reduced graphene oxide/fused silica (rGO/FS) composites using 3-aminopropyltriethoxysilane as surfactant is reported. GO nanosheets were bound to FS particles by an electrostatic assembly between ultra thin negatively charged GO sheets and positively charged amino-modified FS particles. After spark plasma sintering, rGO/FS bulk composites have been produced from the GO and FS composite particles with GO being reduced to rGO in vacuum at high temperatures. Results show that rGO sheets were well dispersed in the matrix, and conductivity of these rGO/FS composites at room temperature was strongly dependent on the rGO nanosheet concentration. i.e., the conductivity of rGO/FS was increased to 10⁻⁴ S/cm when a conducting network was formed inside the composites. The effect of GO nanosheets on the mechanical properties of rGO/FS bulk composites was also investigated. The addition of 1 wt.% GO sheets to FS resulted in 72% increase in Vickers hardness, indicating the stress transfering from the FS matrix to the rigid rGO sheets. With the same rGO content, the fracture toughness of the as-prepared composites was increased by 74%. The main toughening mechanisms were thought to be crack deflection, crack branching, pulling-out and bridging of the rGO sheets.     

Processing and mechanical properties of new hierarchical metal-graphene flakes reinforced ceramic matrix composites

Hierarchical tantalum-graphene flakes reinforced zirconia (3Y-TZP) ceramic matrix composites were fabricated by wet processing route and freeze drying followed by spark plasma sintering (SPS). The microstructures and mechanical properties were investigated. The results show that graphene and Ta particles are homogeneously dispersed in the ceramic matrix and the optimum sintering temperature for complete densification of composites and thermal reduction of the graphene oxide is 1500 °C. The addition of dual reinforcements of tantalum microflakes and graphene nanoflakes results in significant improvement in the mechanical properties of the ZrO₂ matrix. Approximately a 30% increase in flexural strength vs the zirconia-Ta composite and a 175% increase in fracture toughness vs the monolithic zirconia have been achieved by introducing 0.5 vol% GO and 20 vol% Ta particles.     

Microstructural and mechanical properties of Al2O3 and ZTA foams prepared by thermo-foaming technique

Alumina and zirconia toughened alumina foams were fabricated through a thermo-foaming method using varying amounts of powder to sucrose weight ratio. All the fabricated foams were characterized for their compressive strength, bending strength, and fracture toughness under static loading conditions. The compressive strength increases with an increase in a weight ratio from 0.4 to 1.2. The maximum compressive strength values were found to be 1.9 MPa, and 1.8 MPa for alumina and zirconia toughened alumina foams, respectively. Further increase in powder to sucrose weight ratio led to a decrease in the compressive strength due to the partial collapse of the cell walls during the foaming process. The 3-point bend test results revealed an improvement of bending strength and fracture toughness values of zirconia toughened alumina foams compared to alumina foam, which can be attributed to the transformation toughening mechanism.     

Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics

Fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with reduced graphene oxide (RGO) were fabricated by spark plasma sintering (SPS), and their electrical, thermal, and mechanical properties were investigated. Graphene oxide (GO) was exfoliated by a short sonification in dimethylformamide (DMF)/water solution and uniformly mixed with ZrO₂ powders. The microstructure of the composites showed that undamaged RGO sheets were homogeneously distributed throughout matrix grains. The electrical conductivity of YSZ composites drastically increased with the addition of RGO, and it reached 1.2 × 10⁴ S/m at 4.1 vol.%. However, the thermal diffusivity increased only 12% with RGO addition. The hardness decreased slightly with RGO addition, whereas the fracture toughness significantly increased from 4.4 to 5.9 MPa^1/2. The RGO pull-out and crack bridging contributed to the improved fracture toughness.     

聚酰胺共混改性研究进展

综述了近年来聚酰胺（PA）在增韧、增强以及增强增韧等方面的最新研究进展，重点介绍了增韧PA中反应性挤出的方法，指出在弹性体的分子链上接枝小分了，能与PA中的氨基或酰胺基反应生成原位共聚物，不相容的弹性体与PA间的相容性大为提高，达到增韧目的，聚苯醚和聚砜等性能优异的工程塑料与PA共混，加入适当的增容剂，在增韧的同时，共混物的强度下降较小，在合适的条件下，液晶聚合物（LCP）与PA共混，LCP产生原纤化作用，得到原位聚合物复合材料，具有很高的强度，加入增容剂，能提高PA和LCP的相容性，达到增韧的目的。PA与纳米级的无机填料共混，能得到刚性，强度和韧性综合性能较好的改性PA。     

Mechanical properties of biomimetic ceramic with Bouligand architecture

Biomimetic Bouligand architecture is constructed in the ceramic to improve its toughness. Firstly, unidirectional carbon fiber-reinforced ZrB₂-SiC ceramic films are achieved through a vacuum-assisted filtration method using graphene oxide. Then, ceramic films are helically assembled at a fixed angle of 30° in the graphite die based on the fiber orientation. Finally, the spark plasma sintering method was utilized to densify helical assembly carbon fiber/ceramic films. By constructing Bouligand structure, high fracture toughness (7.4 MPa·m^0.5) and work of fracture (∼1055 J/m²) are achieved in ZrB₂-based ceramic. The toughening mechanisms mainly are crack deflection, twisting and branching, carbon fiber pulling out, and bridging.     

Toughening of zirconia/alumina composites by the addition of graphene platelets

A study on graphene platelet/zirconia-toughened alumina (GPL/ZTA) composites was carried out to evaluate the potential of the new structural materials. GPL–ZrO₂–Al₂O₃ powders were obtained by ball milling of graphene platelets and alumina powders using yttria stabilized ZrO₂ balls. Samples were sintered at different temperatures using spark plasma sintering. Fracture toughness was determined by the single-edge notched beam method. The results show that the GPLs are uniformly distributed in the ceramic matrix and have survived high temperature sintering processes. Several sintering experiments were carried out. It is found that at 1550 °C, GPL/ZTA composites were obtained with nearly full density, maximum hardness and fracture toughness. A 40% increase in fracture toughness in the ZTA composite has been achieved by adding graphene platelets. The toughening mechanisms, such as pull out, bridging and crack deflection, were observed and are discussed.