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 and dielectric properties of spark plasma sintered Si3N4/SiO2/Eu2O3 composite

In this work, effect of Eu₂O₃ (by 0, 3, 5, and 7 wt%) on mechanical and dielectric properties of Si₃N₄–SiO₂ composites (denoted by E0, E3, E5, E7) was studied. Samples were sintered by spark plasma at 1750 °C - 15 min by 100 °C/min heating rate and 40 MPa pressure under a vacuum atmosphere. The SEM micrographs confirmed the β-Si₃N₄ rod-shaped grains growth. The α to β phase transformation was completed in samples. The E5 and E7 samples have about 180.8 MPa flexural strength due to β-Si₃N₄ rod-shaped grains and increased grain aspect ratio compared to other samples. With the increase of Eu₂O₃ additive, due to an increase in grain size and β phase concentration the average dielectric constant and dielectric loss at 10 GHz frequency growth from 6.6 to 8 and from 0.125 to 0.160, respectively. High hardness values due to the dissolution, diffusion, and precipitation mechanism, and the α phase concentration was developed in these samples.     

Design and fabrication of Si3N4/(W,Ti)C graded nano-composite ceramic tool materials

Si₃N₄/(W, Ti)C graded nano-composite ceramic tool materials with different thickness ratios and number of layers were fabricated by hot pressing technology. The flexural strength, fracture toughness and hardness of the sintered composites were tested and the microstructure and indention cracks were observed. The experiment results showed that the five-layer graded nano-composites with a thickness ratio of 0.2, which were sintered under a pressure of 30 MPa at 1700 °C in vacuum condition for 45 min, had the optimum comprehensive mechanical properties with a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa·m^1/2. The formation of elongated β-Si₃N₄ grains contributes to the favorable mechanical properties. The graded structure can induce residual compressive stress in the surface layer and enhance the mechanical properties. The strengthening and toughening mechanisms are a synergistic effect of intergranular and transgranular fracture, crack bridging and deflection.     

Fine grained Al2O3/SiC composite ceramic tool material prepared by two-step microwave sintering

Fine-grained Al₂O₃/SiC composite ceramic tool materials were synthesized by two-step microwave sintering. The effects of first-step sintering temperature (T1), content and particle size of SiC on the microstructure and mechanical properties were studied. It was found that the sample with higher content of SiC was achieved with finer grains, and the incorporation of SiC particles could bridge, branch and deflect the cracks, thus improving the fracture toughness. Higher T1 was required for the densification of the samples with higher content of SiC (>5?wt%). The sample containing 3?wt% SiC particles with the mean particle size of 100?nm, which was sintered at 1600?°C (T1) and 1100?°C (T2) for 5?min had the fine microstructure and optimal properties. Its relative density, grain size, Vickers hardness and fracture toughness obtained were 98.37%, 0.78?±?0.31?μm, 18.40?±?0.24?GPa and 4.97?±?0.30?MPa?m^1/2, respectively. Compared to the sample prepared by single-step microwave sintering, although near full densification can be achieved in both two methods, the grain size was reduced by 36% and the fracture toughness was improved by 28% in two-step microwave sintering.     

Microstructure and mechanical properties of CaAl12O19 reinforced Al2O3-Cr2O3 composites

Al₂O₃-Cr₂O₃ refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al₂O₃-Cr₂O₃ refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al₂O₃-Cr₂O₃ materials, the CaAl₁₂O₁₉ reinforced Al₂O₃-Cr₂O₃ composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Microstructure and mechanical properties of Si3N4f/Si3N4 composites with different coatings

The Si₃N₄ coating and Si₃N₄ coating with Si₃N₄ whiskers as reinforcement (Si₃N_4w-Si₃N₄) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si₃N_4f/Si₃N₄ composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si₃N_4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si₃N₄ coating, Si₃N_4w-Si₃N₄ coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed.     

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.     

Effect of Y2O3-stabilized ZrO2 whiskers on the microstructure,mechanical and wear resistance properties of Al2O3 based ceramic composites

The aim of this study was to improve the mechanical properties of Al₂O₃ ceramics by the addition of Y₂O₃-stabilized ZrO₂ whiskers (designated as Al₂O₃/YSZ_W composite) through the flux method and hot-pressing technology. The effect of YSZ_W content on their microstructure, phase composition and transformability, mechanical properties, and wear resistance was systematically investigated. The Al₂O₃/YSZ_W composites containing 10 wt% YSZ_W exhibited the best mechanical performance, including the highest content of YSZ_W tetragonal phase and transformability as well as the largest values in their relative density (99.5%), hardness (1969 HV), fracture toughness (9.57 MPa m^1/2) and flexural strength (855 MPa). The strengthening and toughening of the Al₂O₃/YSZ_W composites were attributed to the YSZ_W tetragonal-monoclinic phase transformation as well as the whiskers reinforcing effect. Furthermore, the Al₂O₃/YSZ_W composites also showed the highest friction and wearing properties.     

Fabrication of practically pure Si2N2O ceramic with high performance from amorphous BN surface modified nano-sized Si3N4 powders

In this study, amorphous nano-sized Si₃N₄ powders were surface modified by BN. Then a stable and dense Si₂N₂O ceramic was fabricated using the BN surface modified powders, rather than Si₂N₂O-Si₃N₄ composites usually prepared from nano-sized Si₃N₄ powders without surface modification. The effect of BN surface modification on phase transformation, microstructure and mechanical properties were also investigated. Si₂N₂O ceramics obtained by means of the present method have no residual Si, crystal SiO₂ and other oxide additives, which are usually produced by other methods and may seriously influence high-temperature structural and functional applications of Si₂N₂O ceramics.     

Comparison of the homemade and commercial graphene in heightening mechanical properties of Al2O3 ceramic

Graphene has been successfully fabricated by a novel method, using graphite powder and NMP (N-Methyl Pyrrolidone) as the raw materials based on the principles of liquidoid exfoliation and mechanical milling. SEM, TEM and Raman spectrum were utilized to characterize the morphology of the homemade graphene, illustrating the few defects and rare layers were endowed in this study. Afterwards, the homemade and commercial graphene were doped into Al₂O₃ powder with the mass ratio of 0%, 1%, 2%, and 3% to reinforce the mechanical properties of the matrix. The composites were processed at 1600 °C, pressure of 30 MPa and soaking time of 1 h by vacuum hot pressing. The test results illustrated the bending strength and fracture toughness tended to be intensive at first and subdued afterwards, achieving the optimal performance of 625.4±18.2 MPa and 6.07±0.22 MPa m^1/2 at 2 wt% prepared graphene additive, and the commercial grapheme owned the best heighten effect in 3 wt% graphene/Al₂O₃ composites. Compared to the blank Al₂O₃ sintered samples, the graphene/Al₂O₃ specimens (both prepared and commercial additive) behaved evident increase in mechanical properties, even upon 30% enhanced in fracture toughness and bending strength generally by the prepared grapheme. Moreover, the prepared graphene had better improvement effect than commercial graphene in enhancing mechanical properties of Al₂O₃ ceramic.     

(Ti,V)_3AlC_2/Al_2O_3复合材料的制备及力学性能

为了提高Ti_3Al C_2陶瓷的力学性能,本研究以Ti C粉、Ti粉、Al粉和V2O5粉为起始反应原料,采用原位热压技术在1350°C下反应烧结合成出了(Ti,V)_3AlC_2/Al_2O_3复合材料。利用X-射线衍射和扫描电子显微技术对合成产物的物相和微观结构进行了表征,并分析了复合材料的合成机制。最后,对(Ti,V)_3AlC_2/Al_2O_3复合材料的力学性能进行了研究。测试结果表明:(Ti_(0.92),V_(0.08))_3Al C_2/10wt%Al_2O_3复合材料具有最佳的力学性能,其硬度、断裂韧性及抗弯强度分别为5.56 GPa、12.93 MPa·m~(1/2)和435 MPa,相比于单相Ti_3Al C_2材料分别提升了60%、108%和31%。     

Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites

Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 °C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si₃N₄/BN composites. In addition, the fracture toughness of Si₃N₄/micro BN composites was enhanced comparing to monolithic Si₃N₄ because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si₃N₄/BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si₃N₄ matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

Effects of Pr6O11 on the microstructure and mechanical properties of Ti/Al2O3 composites prepared by pressureless sintering

Ti/Al₂O₃ composites with different volume percentages of Pr₆O₁₁ added (0–12.0  vol.%) were prepared by pressureless sintering at 1600 °C for 1.5 h. The influences of Pr₆O₁₁ on the composition, microstructure and mechanical properties of the composites were characterized and investigated. The results showed that Pr₆O₁₁ could promote the sintering of the composites by generating some new interfacial reaction products, such as AlTiO₂, Pr₂Ti₂O₇ and PrAlO₃. Pr₆O₁₁ could also inhibit the production of TiAl and Ti₃Al by the same mechanism. Additionally, Pr₆O₁₁ changed hexagonal alumina to tetragonal alumina. The latter could improve the mechanical properties of the composites by the effects of crack deflection and particle pullout when it was present in proper amounts. Composites showed satisfactory comprehensive properties when the content of Pr₆O₁₁ was no more than 3.0 vol.%.     

The microstructure and mechanical properties of Ni/Al2O3 composites by in-situ generated CaAl12O19 and ZrO2 via hot pressing sintering

Ni/Al₂O₃ composites with a varying mass fraction of CaZrO₃ (0–12 wt%) were prepared by vacuum hot pressing sintering at 1650 °C under a pressure of 30 MPa for 30 min to investigate how CaZrO₃ affect the mechanical properties and morphology of the composites. The results show that CaZrO₃ can react with Al₂O₃ and form new strengthening and reinforcing phases of CaAl₁₂O₁₉ and ZrO₂, which can promote complete densification and solve the problem of uneven distribution due to the poor wettability between Al₂O₃ and Ni. Additionally, composites showed satisfactory mechanical properties when 6.0–9.0 wt% CaZrO3 was added and the major toughening mechanism involved the typical fracture of delamination and the transgranular mode.     

Preparation and properties of an Al2O3/Ti(C,N) micro-nano-composite ceramic tool material by microwave sintering

One kind of Al₂O₃/Ti(C,N) micro-nano-composite ceramic tool material with acceptable properties was prepared by microwave sintering. Effects of sintering temperature and holding time on densification, mechanical properties and microstructure were studied. The optimal relative density, fracture toughness and Vickers hardness were 98.4±0.30, 6.72±0.28 MPa m^1/2 and 18.42±0.59 GPa, respectively, which were obtained at 1550 °C for 10 min. Compared to the conventional sintering, the sintering temperature and holding time of microwave sintering were reduced by 14% and 89%, respectively. The microwave sintering made the sizes of some particles keep in nano-scale, which leaded to the formation of intragranular structures. The residual stress in the intragranular structures increased the ratio of grain boundary toughness to grain toughness of matrix (K_cb/K_cg), and thus the micro-Al₂O₃ grains were more inclined to transgranular fracture.     

Mechanical properties of directionally solidified Al2O3/Y3Al5O12 eutectic ceramic prepared by optical floating zone technique

Al₂O₃/Y₃Al₅O₁₂(YAG) directionally solidified eutectic (DSE) crystal was prepared by optical floating zone technique. Al₂O₃/YAG DSE consists of continuous entangled Al₂O₃ and the YAG forming a three-dimensional networks structure. The volume fraction of porosity is ultra-low (0.013%) and the average equivalent diameters of most pores (>84%) are smaller than 4?μm. The Al₂O₃/YAG DSE shows excellent high-temperature elastic stiffness. The Young’s modulus at 1500?°C maintains more than 85% of the value at room temperature. Bending strength exhibits excellent retention up to high temperature as well. High-temperature ball indentation testing shows plastic deformation involving dislocations and twinning, which predominantly occur in Al₂O₃ phase, while the YAG phase is stable. Evaluation on H_v/E index predicts Al₂O₃/YAG DSE with moderate capability to accommodate damages. Our results highlight Al₂O₃/YAG DSE as excellent high-temperature structural materials.     

Synthesis,microstructure and mechanical properties of Cr2AlC/Al2O3 in situ composites by reactive hot pressing

Al₂O₃ particle-reinforced Cr₂AlC in situ composites were successfully fabricated from powder mixtures of Cr₃C₂, Cr, Al, and Cr₂O₃ by a reactive hot-pressing method at 1400 °C. A possible synthesis mechanism was proposed to explain the formation of the composites in which Al₂O₃ was formed by the aluminothermic reaction between Al and Cr₂O₃, meanwhile, Cr₃C₂, Al, together with Cr reacted to form Cr₂AlC in a shortened reaction route. The effect of Al₂O₃ addition on the microstructure and mechanical properties of Cr₂AlC/Al₂O₃ composites was investigated. The results indicated that the as-sintered products consisted of Cr₂AlC matrix and Al₂O₃ reinforcement, and the in situ formed fine Al₂O₃ particles dispersed at the matrix grain boundaries. The flexural strength and Vickers hardness of the composites increased gradually with increasing Al₂O₃ content. But the fracture toughness peaked at 6.0 MPa m^1/2 when the Al₂O₃ content reached 11 vol.%. The strengthening and toughening mechanism was also discussed.