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Graphene reinforced Al 2 O 3 -based composite ceramic tool materials were synthesized using hot pressing (HP) under sintering conditions of 1550–1750 °C sintering temperature, 5–30 min dwell time and 25–40 MPa applied pressure in order to investigate the effects of sintering parameters on microstructure, graphene structure stability and mechanical properties of composite ceramic tool materials. Sintering parameters of 1600 °C sintering temperature, 15 min dwell time and 30 MPa applied pressure were demonstrated to be the optimal sintering parameters to obtain dense and refined microstructure. The wrapping mechanism of graphene on ceramic grains was believed to play a significant role in the formation process of this optimized microstructure. Based on Raman spectrum analysis, graphene structure stability was subjected to sintering temperature and dwell time. Graphene structure could be destructed under the high sintering temperature or extended dwell time. And the shortened dwell time was also detrimental to graphene structure stability due to the lower crystallinity. Mechanical properties of composite ceramic tool materials were also optimized under the optimal sintering parameters. Grain refinement and weak bonding interface induced by graphene were responsible for the improved fracture toughness. [ABSTRACT FROM AUTHOR]

Abstract The low fracture toughness of Al 2 O 3 -based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al 2 O 3 -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 m1/2, 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. [ABSTRACT FROM AUTHOR]

The paper studies the phase transition of ingot with the composition (Ce 50 Nd 50 ) 30 Fe bal Co 4 Ga 0.2 B 0.92 after the annealing treatment at 1050 °C. The melt-spun ribbons which is prepared by the two treatment status ingots. The phase structure and microstructure morphologies of the ingots and melt-spun ribbons were analysed and observed by XRD and SEM. It was found that the grain size of the ribbons is on the nanometer scale. The EDS results show that there are four different phases in the ingot: (CeNd) 2 Fe 14 B, α-Fe, Ce-rich phase and Nd-rich phase. After the annealing treatment, α-Fe, Ce-rich phase, and Nd-rich phase were obviously reduced and the contents of the main phase was significantly increased in the annealed ingot compared with the unanneal treatment ingot. The VSM results show that there is a peak waist in the ribbon which is prepared by the untreated ingot. Because the ingot is uneven, the ribbons may have the secondary phase, the Hcj is 8394 Oe. But the demagnetization curves of the ribbons, which is prepared by the annealed ingot, is relatively smooth and without the soft magnetic phase and the Hcj is 12,528 Oe, which is higher than the unanneal treatment ingot. We can know that the ingot with fine organization is the key factors to preparing high-performance ribbons. [ABSTRACT FROM AUTHOR]

Fabrication of rare-earth-based permanent magnets with both high electrical resistivity and excellent magnetic properties for the traction motor applications is highly challenging. In this paper, Nd-Fe-B/Dy2O3 composite magnets have been fabricated by hot-pressing and die-upset. The influence of Dy2O3 filler on the microstructures, electrical resistivity and magnetic properties of the Nd-Fe-B magnets have been investigated by scanning electron microscope, four-probe resistivity meter and hysteresis graph. The results showed that the electrical resistivities of the Nd-Fe-B/Dy2O3 composite magnets increased with the increase of Dy2O3 amount and reached 1270 \mu \Omega cm when the Dy2O3 amount increased to 20 wt.%. Whereas the electrical resistivity of the nondoped Nd-Fe-B magnets is 230 \mu \Omega cm. The cross-section SEM images of the composite magnets showed that the Dy2O3 phase formed laminated structure and some agglomerate phenomenon. The agglomerate areas became larger with the increase of Dy2O3 amount. The residual magnetic flux density (Br) and maximum energy product of the Nd-Fe-B/Dy2O3 composite magnets decreased due to the nonmagnetic Dy2O3 filler. It is interesting to find that coercivity of the Nd-Fe-B/Dy2O3 composite magnets increased from 7.8 kOe to 9.64 kOe when the Dy2O3 amount increased from 10 wt.% to 20 wt.%. The reason may be that some of the dysprosium diffused into the Nd-Fe-B matrix and formed (Nd,Dy)2Fe14B compounds. In the mean time, some of neodymium-rich phase diffused to Dy2O3 phase, replaced dysprosium and formed Nd2O3. This probably happened during the die-upset process at 850 ^\circC. [ABSTRACT FROM AUTHOR]

Graphene additives with different sizes were employed to strengthen and toughen Al 2 O 3 -WC-TiC ceramic composites. The effect of graphene sizes on interfacial characteristics, microstructures and mechanical properties was investigated. The results indicated that the interfacial chemistry reaction products and sizes of the products were subject to graphene sizes. With the increase of graphene thickness, the interfacial reaction products between graphene and Al 2 O 3 changed from Al 4 O 4 C to Al 4 C 3 and the sizes of products were increased. Based on first-principles calculation results, it has been found that Al 4 O 4 C and Al 4 C 3 interfacial phases contributed to the formation of the strong-weak staggered interfaces in ceramic composites. Graphene with low thickness and large lateral size was more beneficial to improve mechanical properties of ceramic composites. Especially, the flexural strength and fracture toughness of the composite incorporating single layer graphene were increased by 67.59% and 54.10%, respectively, compared to the composite without graphene. The strong-weak staggered interfaces induced by graphene were responsible for the improved flexural strength and fracture toughness. [ABSTRACT FROM AUTHOR]

Graphene and nano-TiC, which have good reinforcing effects on Al2O3-based ceramic-tool materials, are generally used as additive phases for ceramics. In this study, nine kinds of samples were sintered, to investigate the effects of graphene and nano-TiC on the reinforcing mechanisms of Al2O3-based ceramics. The experimental results indicated that adding 0.5 vol% graphene and 10 vol% nano-TiC can obtain the optimum flexural strength, fracture toughness, and Vickers hardness, which were 705 ± 44 MPa, 7.4 ± 0.4 MPa m1/2, and 20.5 ± 0.8 GPa, respectively. Furthermore, the reinforcing mechanisms of crack bridging, pull-out of graphene, and pull-out of nano-TiC are identified, which are contributed to improving the mechanical properties of ceramics. Meanwhile, other reinforcing mechanisms induced by graphene (graphene break, crack guiding, and 3D propagation) and nano-TiC (crack branching, crack deflection, and peeling) are discussed. These reinforcing mechanisms are coupled together, while decoupling is hard to work out. Thus, further quantitative studies of reinforcing effects of graphene and nano-TiC on Al2O3-based ceramic-tool materials are necessary to be carried out. [ABSTRACT FROM AUTHOR]

Dense Al 2 O 3 /Ti(C,N) composite ceramics reinforced with GNPs/nano-ZrO 2 were fabricated by hot-press sintering. The effects of nano-ZrO 2 content on the microstructure and mechanical properties of the prepared Al 2 O 3 /Ti(C,N)/GNPs/ZrO 2 composites were investigated. Results showed that nano-ZrO 2 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-ZrO 2. In particular, the fracture toughness of composites incorporating GNPs (0.4 wt%)/ZrO 2 (1 wt%) exceeded 11 MPa m1/2, which was increased by more than 86 % compared with that of Al 2 O 3 /Ti(C,N) ceramic composites without GNPs/ZrO 2. 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-ZrO 2 because of not only the residual stress resulted from phase transformation but also the formation of intra-granular structure with uneven surface around GNPs. [ABSTRACT FROM AUTHOR]

Al 2 O 3 -Ti(C, N) ceramic composites reinforced with graphene nano-platelets (GNPs) were fabricated via hot-pressing sintering (HP). The effects of GNPs content on the microstructure, mechanical properties, thermal conductivity and reinforcing mechanisms of the composites were investigated. To make GNPs dispersed homogeneously in ceramic matrix, N-Methyl-2-pyrrolidone (NMP) was used as dispersing media and alcohol solution of polyvinyl pyrrolidone was used as reference. Results show NMP is an effective dispersing media for powder mixing. The mechanical properties of Al 2 O 3 -Ti(C, N) composites are improved with the addition of GNPs, and when GNPs content is 0.4 wt%, significant improvements in fracture toughness (40.13%) and flexural strength (40.78%) are achieved in comparison with those of specimens without GNPs. However, there is no obvious effect of GNPs content change on the thermal conductivity of the composites when GNPs content is below 0.8 wt%. Many reinforcing mechanisms induced by GNPs are related to the interfacial bonding strength between GNPs and ceramic matrix. Pull-out of GNPs, crack deflection and branching are caused by weak interfaces between GNPs and matrix while crack bridging and alteration of fracture mode are caused by strong interfaces. Furthermore, interleaving distribution of weak and strong bonding interfaces between GNPs and matrix is essential for the reinforcement of ceramic composites. [ABSTRACT FROM AUTHOR]

To toughen the Al 2 O 3 matrix ceramic materials, Al 2 O 3 /(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. [ABSTRACT FROM AUTHOR]