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.     

Effects of nano-ZrO2 content on microstructure and mechanical properties of GNPs/nano-ZrO2 reinforced Al2O3/Ti(C,N) composite ceramics

Dense Al₂O₃/Ti(C,N) composite ceramics reinforced with GNPs/nano-ZrO₂ were fabricated by hot-press sintering. The effects of nano-ZrO₂ content on the microstructure and mechanical properties of the prepared Al₂O₃/Ti(C,N)/GNPs/ZrO₂ composites were investigated. Results showed that nano-ZrO₂ inclusions refined the matrix grains significantly and resulted in the formation of intra-granular structure. Excellent comprehensive mechanical properties were achieved via addition of combined GNPs and nano-ZrO₂. In particular, the fracture toughness of composites incorporating GNPs (0.4 wt%)/ZrO₂ (1 wt%) exceeded 11 MPa m^1/2, which was increased by more than 86 % compared with that of Al₂O₃/Ti(C,N) ceramic composites without GNPs/ZrO₂. The main toughening mechanisms have been identified as stress-induced phase transformation, crack bridging, deflection and pull-out of GNPs. The toughening effects originated from GNPs were enhanced with the introduction of nano-ZrO₂ because of not only the residual stress resulted from phase transformation but also the formation of intra-granular structure with uneven surface around GNPs.     

Grain growth kinetics in microwave sintered graphene platelets reinforced ZrO2/Al2O3 composites

The grain growth kinetics and mechanical properties of graphene platelets(GPLs) reinforced ZrO₂/Al₂O₃(ZTA) composites prepared by microwave sintering were investigated. The calculated grain growth kinetics exponent n indicated that the GPLs could accelerate the process of the Al₂O₃ columnar crystal growth. And the grain growth activation energy of the Al₂O₃ columnar crystal indicated that the grain growth activation energy of the GPLs doped ZTA composites is much higher than those of pure Al₂O₃ and ZTA in microwave sintering. The optimal mechanical properties were achieved with 0.4?vol% GPLs, whose relative density, Vickers hardness and fracture toughness were 98.76%, 18.10?GPa and 8.86?MPa?m^1/2, respectively. The toughening mechanisms were crack deflection, bridging, branching and pull-out of GPLs. The results suggested that GPLs-doped are good for the Al₂O₃ columnar crystal growth in the ZTA ceramic and have a potentially improvement for the fracture toughness of the ceramics.     

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.     

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.     

Fabrication and properties of monolayer graphene reinforced Al2O3/TiN ceramic tool material

Al₂O₃/TiN/graphene ceramic tool materials were prepared by spark plasma sintering technology and the strengthening and toughening mechanisms were studied. The influence of monolayer graphene content on the mechanical properties and microstructure of the composite material were analyzed and the strengthening and toughening mechanisms were researched. The results showed that with an addition of .5 vol.% graphene the mechanical properties of the material reached the best. The bending strength, hardness, and fracture toughness were 624 MPa, 23.24 GPa, and 6.53 MPa·m^1/2, respectively. Graphene existed in the forms of few-layer and multilayer. The toughening mechanism of few-layer graphene was mainly graphene breaking, and that of multilayer graphene included graphene breaking and pulling-out. Graphene could contribute to the uniform growth of grains due to the excellent electrical conductivity and the high thermal conductivity. The addition of nano-TiN introduced many endocrystalline structures and graphene promoted this phenomenon. Micro-TiN grains made the crack extension show a combination of transgranular fracture, intergranular fracture, crack bridging, and crack deflection, while graphene introduced weak grain interfaces and made the crack appear more branches. The layered graphene made the material fracture change from two-dimension to three-dimension.     

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.     

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.     

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.     

Role of CeAl11O18 in reinforcing Al2O3/Ti composites by adding CeO2

This work discusses the reinforcement effect of elongated CeAl₁₁O₁₈ phase on multifunctional Al₂O₃/Ti composites by adding CeO₂ to inhibit interfacial reaction and strengthen interface for inducing optimized performances. For this purpose, Al₂O₃/Ti composite with different contents of CeO₂ was fabricated and the microstructure, mechanical and electrical properties were studied. Results indicated that after CeO₂ was added, elongated CeAl₁₁O₁₈ phase was formed within these composites. Owing to inhibited interfacial reaction between Al₂O₃ and Ti, Ti content was increased and compositions of composites were calculated using Rietveld method based on X-ray diffraction patterns. Attributed to the strengthening and toughening effects of CeAl₁₁O₁₈ phase, 2 mol% CeO₂ added composite showed the highest flexural strength and fracture toughness of 576 MPa and 5.15 MPa·m^1/2, which increased by 21% and 20% compared to Al₂O₃/Ti composite without CeO₂ addition. In this case, crack bridging by both Ti and CeAl₁₁O₁₈ particles was the major toughening mechanism and the additional fracture toughness caused by CeAl₁₁O₁₈ toughening effect reached a maximum. The role (crack bridging or particle pull-out mechanism) of CeAl₁₁O₁₈ in toughening Al₂O₃/Ti composites depended on the aspect ratio of these elongated particles, which was directly related to CeO₂ content. Because of the inhibition of interfacial reaction and the increase in Ti content, excellent electrical resistivity of composites after CeO₂ addition was maintained in spite of the formation of insulated CeAl₁₁O₁₈ phase. All samples showed relatively low electrical resistivity of ~10⁻³ Ωcm.     

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.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

The toughening mechanism and mechanical properties of graphene-reinforced zirconia ceramics by microwave sintering

ABSTRACT

The graphene/ZrO₂ composites were fabricated by impregnating graphene dispersion into the ZrO₂ ceramic matrix and sintered by microwave, and the microstructure and mechanical properties were investigated. The results showed that the graphene was well dispersed in the ceramic matrix and refined the grain size. The fracture toughness reached 8.62?MPa?m^1/2, confirmed by single-edge notched beam, which was 42% higher than that of the pure ZrO₂. Also, the toughening mechanisms were investigated by micro-hardness testing and showed that a combination of crack deflection, micro-crack and crack bridging increased the fracture toughness.     

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.     

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.     

Synergistic effects of TiB2 and graphene nanoplatelets on the mechanical and electrical properties of B4C ceramic

Monolithic B₄C, B₄C–TiB₂, and B₄C–TiB₂–graphene nanoplatelets (GNPs) were fabricated by hot pressing (HP) at 1900 °C for 1 h under an axial pressure of 30 MPa. The microstructures and mechanical and electrical properties of the B₄C composites were investigated. The results show that the GNPs are distributed homogeneously in B₄C-based ceramic composites. Compared with monolithic B₄C, the TiB₂–GNPs-containing B₄C composite exhibits an approximately 68 % increase in flexural strength and a 169 % increase in fracture toughness due to the synergistic effects of TiB₂ particles and GNPs. The toughening mechanisms mainly include TiB₂ crack deflection, crack branching, transgranular fracture and GNPs crack deflection, crack bridging, and GNPs pull-out. Additionally, the electrical conductivity of the B₄C composite reinforced with dual fillers is three orders of magnitude higher than that of monolithic B₄C due to the establishment of a conductive network. The addition of GNPs can efficiently connect the isolated conductive TiB₂ particles in the B₄C matrix and provides an additional channel for electron migration.     

Alumina ceramic tool material with enhanced properties through the addition of bionic prepared nano SiC@graphene

Graphene-coated SiC nanoparticles containing graphene floating bands (SiC@G) were prepared by a liquid-phase laser irradiation technique, and SiC@G nanoparticles with high dispersivity were incorporated into an Al₂O₃ matrix. An Al₂O₃-based composite ceramic tool was prepared by spark plasma sintering (SPS), and the effects of SiC@G nanoparticles on the mechanical and cutting properties and microstructure of the materials were further investigated. Analysis of the cross-sectional morphology shows that SiC@G nanoparticles containing graphene floating bands were homogeneously dispersed in the composite, which resulted in tighter bonds between the Al₂O₃ particles. This particular core-shell structure increased the contact area between the graphene and the matrix due to the formation of a graphene 3D mesh by extrusion, which enhanced the difficulty of relative sliding of graphene. Second, this special core-shell structure also made the crack propagation path more tortuous, further increasing the energy consumed in the fracture process, which is conducive to improving the mechanical properties of ceramic tools. The addition of SiC@G nanoparticles improves the mechanical properties of Al₂O₃-based composite ceramic tools. The fracture toughness (7.2 Mpa·m^1/2) and flexural strength (709 MPa) increased by 75.6% and 28.7%, respectively. Cutting experiments with Al₂O₃/SiC/G composite ceramic tool and Al₂O₃/SiC@G composite ceramic tools on 40Cr hardened steel were performed. The results prove that the addition of SiC@G nanoparticles improves the cutting life by 18.1% and reduces the cutting force and friction coefficient by 6.3% and 14.8%, respectively.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.     

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.     

The role of TiO2 incorporation in the preparation of B4C/Al laminated composites with high strength and toughness

We prepared B₄C/Al laminated composites via ice-templating and gas-aided pressure infiltration and investigated the effects of TiO₂ addition on the microstructures and mechanical properties of the composites. The incorporation of TiO₂ led to the formation of TiB₂ after sintering, reduced the formation of harmful phases and increased the strength of ceramic architectures. However, its excessive addition resulted in the cracking of ceramic layers and the formation of metal strips after Al infiltration. The bending strength, fracture toughness and work of fracture of the composites first increased and then decreased with increasing initial TiO₂ content, reaching maxima of 420?±?20?MPa, 44?±?2?MPa?m^1/2 and 5002?±?175?J?m^?2, respectively. The specific strength and toughness are comparable to those of titanium alloys. Furthermore, fracture modes and toughening mechanisms were thoroughly addressed by analyzing crack propagation paths and fracture surface morphologies. Crack deflection and metal bridging are two primary extrinsic toughening mechanisms.