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Alumina is one of the most commonly used and researched structural ceramic because of its excellent properties. However, its intrinsic brittleness is the fatal drawback, which hinders it from wider applications. How to improve its fracture toughness as well as the bending strength is always challenging for material researchers. In this paper, alumina matrix composites were fabricated by hot-pressing, in which some additives, including zirconia, alumina platelets, and MXene, were incorporated. The influence of the introduced additives on their microstructure and mechanical properties was investigated. Compare with the monolithic alumina, both bending strength and fracture toughness of all samples were improved greatly. Incorporation of zirconia was beneficial to the mechanical properties due to the phase-transformation strengthening and toughening mechanism. While alumina platelets resulted in high fracture toughness because of the self-toughening of elongated grains. The synergistic effect of alumina platelets and MXene enormously improved the fracture toughness from 2.9 ± 0.3 MPa·m1/2 for monolithic alumina to 7.5 ± 0.4 MPa·m1/2 for the composite, which was increased by 159%. This work will provide useful references for the fabrication of high-strength and high-toughness alumina ceramics by introducing additives properly. [ABSTRACT FROM AUTHOR]

In this study, the nacre-like alumina ceramics, with introduction of zirconia, are fabricated by freeze casting followed by hot pressing. The experimental results show that zirconia particles are uniformly distributed between the layered alumina 'brick' in the form of 'mortar'. Due to the incorporation of zirconia with proper particle size, both flexural strength and fracture toughness of the nacre-like alumina are greatly improved, much stronger and tougher than pure alumina. In regard to the sample containing 13.3 wt% zirconia, the flexural strength reaches 522 MPa, ∼47% increase in comparison with the monolith. Particularly, when zirconia content is 10 wt%, the fracture toughness is increased by ∼66%, from 6.1 MPa m1/2 to 10.1 MPa m1/2. Microstructure observation and fracture behavior analysis reveal that the coupling effects of bioinspired nacre-like structure and stress-induced transformation toughening are responsible for the enhanced mechanical properties. The results show that it is an effective method to increase the mechanical properties of layered alumina ceramics by introducing proper interlayer phase. [ABSTRACT FROM AUTHOR]

MWCNT/alumina composites were prepared by a flocculation method combined with acid treatment of the MWCNTs and spark plasma sintering. The effects of varying the processing conditions, namely the acid treatment temperature (50–80 °C), MWCNT content (0–10 vol.%) and sintering temperature (1200–1600 °C) on the composites’ microstructure, electrical conductivity and mechanical properties were investigated. This found that flocculation processing brings about a good dispersibility of the MWCNTs, especially with higher acid treatment temperature, which enhances their reinforcing and grain refining effect. When combined with a low sintering temperature, this significantly improves the mechanical properties of the composites.As a result, a composite prepared using a sintering temperature of 1200 °C and an acid treatment temperature of 80 °C exhibited a fracture toughness greater than that of a monolithic alumina, even with a MWCNT content as high as 10 vol.%. [ABSTRACT FROM AUTHOR]

Abstract Graphene oxide (GO) is one of the most important ceramic-toughening candidates due to its outstanding physical and chemical properties. In this paper, nacre-like alumina ceramics reinforced with GO have been prepared by freeze casting followed by hot pressing. The experimental results showed that a laminated structure with homogeneous distribution of GO was obtained in the composites. Microstructural analysis, relative densities, flexural strength and fracture toughness of the samples were investigated to reveal the effect of GO. Higher flexural strength and fracture toughness (~380 MPa and ~7.5 MPa m1/2, respectively) were achieved and the multiple length-scale toughening mechanisms of composites contributed to the improved mechanical properties. The results show that it is a promising method to enhance the mechanical properties of nacre-like alumina by the addition of GO. [ABSTRACT FROM AUTHOR]

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5 wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2)B 2 high-entropy boride (HEB) were first predicted by first-principles calculations combined with virtual crystal approximation (VCA). The results verified the suitability of VCA scheme in HEB studying. Besides, single-phase (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2)B 2 ceramics were successfully fabricated using boro/carbothermal reduction (BCTR) method and subsequent spark plasma sintering (SPS); furthermore, the effects of different amounts of B 4 C on microstructure and mechanical properties were evaluated. Due to the addition of B 4 C and C, all samples formed single-phase solid solutions after SPS. When the excess amount of B 4 C increased to 5 wt%, the sample with fine grains exhibited superior comprehensive properties with the hardness of 18.1 ± 1.0 GPa, flexural strength of 376 ± 25 MPa, and fracture toughness of 4.70 ± 0.27 MPa m1/2. Nonetheless, 10 wt% excess of B 4 C coarsened the grains and decreased the strength of the ceramic. Moreover, the nanohardness (34.5–36.9 GPa) and Young's modulus (519–571 GPa) values with different B 4 C contents just showed a slight difference and were within ranges commonly observed in high-entropy diboride ceramics. [ABSTRACT FROM AUTHOR]

Microstructure–property relationships of pressureless-sintered multi-walled carbon nanotube (MWCNT)/alumina composites have been investigated using four types of MWCNTs having different mechanical characteristics as well as almost the same diameter and length; the influences of grain size, location of MWCNTs, fracture patterns, dispersion states of MWCNTs, and crack bridging characteristics of MWCNTs on the composites׳ mechanical properties have been explored. It has been found that the improvement in bending strength and fracture toughness observed for all types of the composites was primarily attributable to the grain refining effect by adding the MWCNTs and to the energy-dissipation by a MWCNT debonding and pullout, respectively. The number and size of clustered MWCNTs were increased in response to the increase in the MWCNT content, which resulted in the decrease in mechanical properties. The pressureless-sintered composites made with MWCNTs having larger load-bearing ability led to ~25% and ~45% improvement in bending strength (742.6±13.1 MPa) and fracture toughness (5.83±0.19 MPa m 1/2 ), respectively, than a MWCNT-free alumina. [ABSTRACT FROM AUTHOR]