Microstructure and mechanical properties of boron carbide/graphene nanoplatelets composites fabricated by hot pressing

In this study, boron carbide (B₄C)-graphene nanoplatelets (GNPs) composites, with enhanced strength and toughness, were fabricated by hot pressing at 1950 °C under a pressure of 30 MPa for 1 h. Microstructure analysis revealed that the GNPs are homogenously dispersed within the B₄C matrix. Raman spectroscopy and electron microscopy showed the orientation of the GNPs in the composites. The effects of the amount of GNPs on the microstructure and mechanical properties of the composites were also investigated. The optimal mechanical properties were achieved using 1 wt% GNPs. The relative density, Vickers hardness, flexure strength, and fracture toughness of the B₄C-GNPs composite ceramic were found to be 99.12%, 32.8 GPa, 508 MPa, and 4.66 MPa m^1/2, respectively. The main toughening mechanisms included crack deflection in three dimensions, GNPs pull-out, and crack bridging. The curled and semi-wrapped GNPs encapsulated individual B₄C grains to resist GNPs pull-out and to deflect propagating cracks.     

Combination of direct ink writing and reaction bonded for rapid fabrication of SiCw/SiC composites

Silicon carbide ceramic matrix composites are widely used in aerospace field due to their advantages of high temperature resistance, high strength and corrosion resistance. However, its application is greatly limited because of the difficulty in preparing complex shape structures by traditional machining methods. Here, a new strategy for preparing SiC_w/SiC complex structure by combining direct ink writing with reaction bonding is proposed. A water-based slurry consisting of silicon carbide, carbon powder and silicon carbide whisker was developed. The influence laws of C content and SiC_w content in slurry on sintering properties of direct-written samples were studied. The reaction bonding mechanism and whisker reinforcing and toughening mechanism were analyzed by means of microstructure and phase composition. The results show that the slurry exhibits shear thinning behavior with stress yield point, and its flow behavior and plasticity meet the requirements of direct writing. When the carbon content is 6.4 wt%, the maximum flexural strength is 239.3 MPa. When 15 wt% SiC_w was added, the flexural strength of the composite reached 301.6 MPa, and when 20 wt% SiC_w was added, the fracture toughness of the composite reached 4.02 MPa m^1/2, which was increased by 26% and 18% compared with single-phase SiC, respectively. The reinforcing and toughening mechanisms of the whiskers mainly include whisker pullout, crack deflection and whisker bridging. After direct ink writing and reaction bonded, the whole process shows good near net forming ability. 3D printed SiC_w/SiC composites have great application prospects in aerospace field.     

Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al₂O₃–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m^1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al₂O₃–TiC composites synergistically reinforced with MWCNTs and GNPs.     

Bioinspired nacre-like 2024Al/B4C composites with high damage tolerance

The bio-inspired 2024Al/B₄C composites with a laminate-reticular hierarchical architecture were constructed by squeeze casting of 2024Al into loose freeze-cast ceramic scaffolds. This pressurized infiltration process provided a clean and well-bonded interface without physical gaps. By regulating the initial suspension concentration (20, 25, 30 and 35 vol%), the effects of different ceramic content on the microstructure, damage-tolerance behavior and toughening mechanisms of the composites parallel and perpendicular to the ice-growth direction were investigated. The strength and toughness in the longitudinal direction were greater than that in the transverse direction. The 2024Al/20 vol% B₄C composite in the longitudinal direction yielded the highest flexural strength of 658 MPa, crack-initiation toughness (K_Ic) of 18.4 MPa m^1/2 and crack-growth toughness (K_Jc) of 27.5 MPa m^1/2. The unique damage-tolerant properties were attributed to multiple toughening mechanisms, including crack deflection, branching and blunting, ductile-ligament bridging and multiple-crack propagation, as evidenced by the stable crack growth and rising R-curve behavior during fracture. The significantly decreased damage tolerance in the transverse direction was mainly due to inadequate toughening tools. On the other hand, both the flexural strength and fracture toughness reduced remarkably as the ceramic content increased. The 2024Al/35 vol% B₄C composite fractured in a single-crack mode and the crack growth path was almost straight, showing a relatively low ?exural strength (502 MPa) and crack-initiation toughness (9.1 MPa m^1/2). The toughening mechanism was discussed in terms of the relationship between structural characteristics and cracking mode.     

Plasma sprayed graphene/carbon nanotube reinforced lanthanum-cerate hybrid composite coating

Lanthanum cerate (LC: La₂Ce₂O₇) is a potential material for thermal barrier coating, whose improved toughness is a crucial necessity for the pathway of its industrialization. Herein, we demonstrated a promising approach to develop graphene/carbon nanotube hybrid composite coating using a large throughput and atmospheric plasma spraying method. Graphene nanoplatelets (GNP: 1 wt %) and carbon nanotube (CNT: 0.5 wt %) reinforced lanthanum cerate (LCGC) hybrid composite coatings were deposited on the Inconel substrate. Addition of 1 wt % GNP and 0.5 wt % CNT in LC matrix has significantly increased its relative density, hardness, and elastic modulus up to 97.2%, 2–3 folds, 3–4 folds, respectively. An impressive improvement of indentation toughness (8.04 ± 0.2 MPa m^0.5) was observed on LCGC coating, which is ～8 times higher comparing the LC coating. The toughening was attributed to the factors: such as the distribution of GNPs and CNTs in the LC matrix, synergistic toughening offered by the GNPs and CNTs; (i) GNP/CNT pull-out, (ii) crack bridging and arresting, (iii) splat sandwiching, mechanical interlocking, etc. Finally, this improved toughness offered an exceptional thermal shock performance up to 1721 cycles at 1800 °C, without any major failure on the coating. Therefore, the GNP and CNT-reinforced LC hybrid composite coating can be recommended to open a path for turbine industries.     

Toughness and R-curve behaviour of laminated Si3N4/SiCw ceramics

Laminated Si₃N₄/SiC_w ceramics were successfully prepared by tape casting and hot-pressing. Its mechanical properties were measured and the impact resistance was discussed. The toughness of the laminated Si₃N₄/SiC_w ceramics was 13.5 MPa m^1/2, which was almost 1.6 times that of Si₃N₄/SiC_w composite ceramics, namely 8.5 MPa m^1/2. Moreover, the indentation strength of laminated Si₃N₄/SiC_w ceramics was not sensitive to increasing indentation loads and exhibited a rising R-curve behaviour, indicating that the laminated Si₃N₄/SiC_w ceramics had excellent impact resistance. The improved toughness and impact resistance of laminated Si₃N₄/SiC_w ceramics was attributed to the residual stress caused by a thermal expansion coefficient mismatch between the different layers, resulting in crack deflection and bridging of SiC whiskers in the interface layer, thus consuming a large amount of fracture work.     

Evaluation of effects of crack deflection and grain bridging on toughening of nanocrystalline SiO2 stishovite

Nanocrystalline SiO₂ stishovite has a fracture toughness higher than 10 MPa m^1/2 due to the toughening mechanism by fracture-induced amorphization. In order to identify other toughening mechanisms which may operate simultaneously, we evaluated effect of crack deflection on fracture toughness. The median deflection angle of nanocrystalline stishovite was lower than polycrystalline ceramics such as silicon nitride and Y-TZP. While the crack deflection can contribute to fracture toughness to some extent, it cannot be the major origin of high fracture toughness of nanocrystalline stishovite. We discussed also the role of grain bridging from the relation between the fracture toughness and grain size.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Improved fracture resistance and toughening mechanisms of GNPs reinforced ceramic composites

The R-curve behavior and toughening mechanisms of graphene nano-platelets (GNPs) reinforced ceramic composites are investigated. A toughening model is developed with the consideration of interface debonding, crack bridging and pull-out of GNPs, which can be used to quantify the contribution of different mechanisms to the improved toughness of ceramic composites. The theoretical results agree well with the experimental data when GNPs homogeneously dispersed in ceramic matrix. All prepared GNPs/ceramic composites exhibit a raising R-curve behavior owing to the toughening mechanisms induced by GNPs, and the curve becomes steeper with increasing GNPs content, indicating that the fracture resistance and flaw tolerance are improved. The dominant toughening mechanism is GNPs pull-out, which is followed by crack bridging and interface debonding. Furthermore, the analytical model suggests that improving GNPs properties, interfacial sheer strength and reducing GNPs thickness can improve the fracture toughness of ceramic composites.     

Microstructure and toughening mechanisms of Al2O3/(W,Ti)C/graphene composite ceramic tool material

To toughen the Al₂O₃ matrix ceramic materials, Al₂O₃/(W, Ti)C/graphene multi-phase composite ceramic materials were fabricated via hot pressing. The effects of the graphene nanoplates (GNPs) content on microstructure and mechanical properties were investigated. Results showed that the fracture toughness and flexural strength of the composite added with just 0.2?wt% GNPs were markedly improved by about 35.3% (~ 7.78?MPa?m^1/2) and 49% (~ 608.54?MPa) respectively compared with the specimens without GNPs while the hardness was kept about 24.22?GPa. However, the mechanical properties degrade with the further increase of GNPs’ content owing to the increased defects caused by agglomeration of GNPs. Synergistic toughening effects of (W, Ti)C and GNPs played an essential role in improving the fracture toughness of composites. By analyzing the microstructures of fractured surface and indentation cracks, besides GNPs pull-out, crack deflection, crack bridging, crack branching and crack arrest, new toughening mechanisms such as break of GNPs and crack guiding were also identified. Furthermore, interface stress can be controlled by means of stagger distributed strong and weak bonding interfaces correlated with the distribution of GNPs.     

Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites

Spark plasma sintering (SPS) of monolithic ZrB₂ ultra-high temperature ceramic and 2–6 vol% graphene nanoplates (GNPs) reinforced ZrB₂ matrix composites is reported. The SPS at 1900 °C with a uni-axial pressure of 70 MPa and soaking time of 15 min resulted in near-full densification in ZrB₂–GNP composites. Systematic investigations on the effect of GNP reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented. Densification mechanisms, initiated by interfacial reactions, are also proposed based on detailed thermodynamic analysis of possible reactions at the sintering temperature and the analysis of in-process punch displacement profiles. The results show that GNPs can be retained in the ZrB₂ matrix composites even with high SPS temperature of 1900 °C and cause toughening of the composites through a range of toughening mechanisms, including GNP pull-out, crack deflection, and crack bridging.     

Simultaneous hardening and toughening of a high-entropy (NbTaZrW)C ceramic carbide using SiC particle

Previously, we have found that (NbTaZrW)C exhibits a good combination of nanohardness and toughness. In this report, we explore the possibility to further increase the overall properties of this high-entropy carbide ceramic (HECC) through introducing SiC particle (SiC_P). To this end, a series of (NbTaZrW)C–xSiC ceramic composites (x = 0/5/15/30/50 vol.%) were fabricated using spark plasma sintering (SPS), their microstructure and mechanical properties were characterized. Our results reveal a grain refinement effects of SiC_P, an agglomeration of SiC_P with (1 0 0) plane preferentially perpendicular to the SPS-pressing direction and the formation of a transition region with various stoichiometric ratio of (NbTaZrW)_xC_1−x in the (NbTaZrW)C–SiC_P vicinity. The elastic modulus, microhardness, and flexural strength of the HECCs show tight positive relations with the SiC_P content and the beneficial effect of SiC_P to the fracture toughness of (NbTaZrW)C becomes evident once the content of SiC_P reaches 30 vol.%. Altogether, (NbTaZrW)C–50%SiC, which has a microhardness of 22 GPa, a flexural strength of 455 MPa, and an indentation fracture toughness of 6.54 MPa m^1/2, presents the optimal combination of mechanical properties among the investigated composites. Mechanistically, the strengthening effect of SiC_P introduction arises from the intrinsic high hardness of SiC_P and the SiC_P-induced grain refinement and the toughening effect is mainly associated with crack bridging mechanism.     

Microstructure,mechanical properties and toughening mechanisms of graphene reinforced Al2O3-WC-TiC composite ceramic tool material

The low fracture toughness of Al₂O₃-based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al₂O₃-WC-TiC composite ceramic tool materials by hot pressing. Microstructure, mechanical properties and toughening mechanisms of the composite ceramic tool materials were investigated. The results indicated that the more refined and denser composite microstructures were obtained with the introduction of graphene. The optimal flexural strength, Vickers hardness, indentation fracture toughness were 646.31?±?20.78?MPa, 24.64?±?0.42?GPa, 9.42?±?0.40?MPa?m^1/2, respectively, at 0.5?vol% of graphene content, which were significantly improved compared to ceramic tool material without graphene. The main toughening mechanisms originated from weak interfaces induced by graphene, and rugged fractured surface, grain refinement, graphene pull-out, crack deflection, crack bridging, micro-crack and surface peeling were responsible for the increase of fracture toughness values.     

Mechanical performance and microstructure of 3Y-TZP/Al2O3/GNPs medical ceramic surgical scalpel material prepared through SPS–HF dual sintering method

In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP)/Al₂O₃/graphene nanoplatelets (GNPs) medical ceramic materials for manufacturing surgical scalpels were sintered in vacuum in an SPS–625HF furnace. The mechanical performances and microstructures of the composites were investigated, and the influence mechanisms of the sintering temperature and amount of added GNPs were studied. During the sintering process at 1400°C and 30 MPa for 5 min, the added GNPs enhanced the mechanical properties of the 3Y-TZP/Al₂O₃ composites. The results showed that the composite with .1 wt.% GNPs had 6.4% (910 ± 11 MPa) higher flexural strength than 3Y-TZP/Al₂O₃. The composite with .4 wt.% GNPs had 38.7% (12.95 ± .22 MPa m^1/2) greater fracture toughness than 3Y-TZP/Al₂O₃. The main toughening mechanisms of 3Y-TZP/Al₂O₃/GNPs were crack bridging, crack deflection, crack branching, GNPs bridging, transgranular fracture structures, and phase transformation of t-ZrO_1.95. The two-stage densification displacement curve appeared at the optimal sintering temperature of the materials, and the 3Y-TZP/Al₂O₃/GNPs composites with a two-stage densification displacement curve had excellent mechanical properties. The added GNPs can inhibit the grain growth during the sintering process, thereby refining the zirconia grains. With the increase in GNPs content, the grain size and flexural strength of the composites decreased gradually. However, higher content of GNPs was beneficial to improve the relative density and thermal diffusivity of 3Y-TZP/Al₂O₃/GNPs composite material.     

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiC_w) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiC_w/RBSC composites in this study. The microstructures and properties of the PyC-SiC_w/RBSC composites under different mass fraction of nano carbon black and PyC-SiC_w were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiC_w, reached a high value of 5.28 MPa m^1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiC_w. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiC_w were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiC_w bridging and pullout to improve the bending strength and toughness of the SiC_w/RBSC composites.     

Design of crack deflection induced high toughness laminated Si3N4 ceramics based on hollow oriented one-dimensional microstructure

A laminated silicon nitride (Si₃N₄) ceramic material with a hollow, oriented, one-dimensional microstructure was successfully prepared based on the tape casting and sacrificial template method. The results show that hollow, oriented, one-dimensional microstructures can effectively induce crack deflection. Different arrangements of the structural design layer and dense layer will have different effects on the material. In particular, bulks with a single-layer orthogonal arrangement of the structural design layer possess high toughness and obvious crack deflection during the fracture process. A kind of multiscale crack deflection mode was realized. Compared with the fracture toughness of the monolithic Si₃N₄ ceramic bulk (5.55 MPa m^1/2), the fracture toughness can reach 8.73 MPa m^1/2, and the flexural strength can still reach 391.47 MPa with only a slight decrease.     

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 C_f/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-C_f/ZrB₂-SiC composite presented the highest mechanical properties with fracture toughness of 4.47?±?0.15?MPa?m^1/2 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.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

Microstructure and mechanical properties of in situ synthesized (TiB2 + TiC)/Ti3SiC2 composites

Based on thermodynamic analysis, highly dense (TiB₂ + TiC)/Ti₃SiC₂ composite ceramics with different TiB₂ volume contents were in situ fabricated in situ by hot-pressing at 1500 °C. Laminar Ti₃SiC₂ grains, columnar TiB₂ grains and equiaxed TiC grains were clearly identified from microstructural observation; grain boundaries were clean. The increase of TiB₂ volume content significantly restrains the grain growth of the Ti₃SiC₂ matrix. As the content of TiB₂ increases from 5 vol.% to 20 vol.%, the bending strength and fracture toughness of the composites both increase and then decrease, whereas the Vickers hardness increases linearly from 6.13 GPa to 11.5 GPa. The composite with 10 vol.% TiB₂ shows the optimized microstructure and optimal mechanical properties: 700 MPa for bending strength; 9.55 MPa m^1/2 for fracture toughness. These are attributed to the synergistic action of strengthening and toughening mechanisms such as particulate reinforcement, crack deflection, grain's pull-out and fine-grain toughening, caused by the columnar TiB₂ grains and equiaxed TiC grains.     

Microstructure and mechanical properties of Ti3SiC2/3Y-TZP composites by spark plasma sintering

Ti₃SiC₂/3Y-TZP (3 mol% Yttria-stabilized tetragonal zirconia polycrystal) composites were fabricated by spark plasma sintering (SPS). The effect of Ti₃SiC₂ content on room-temperature mechanical properties and microstructures of the composites were investigated. The Vickers hardness and bending strength of the composites decreased with the increasing of Ti₃SiC₂ content whereas the fracture toughness increased. The maximum fracture toughness of 9.88 MPa m^1/2 was achieved for the composite with 50 vol.% Ti₃SiC₂. The improvement of the fracture toughness is owing to the crack deflection, crack bridging, the transformation toughening effects.