Key Engineering Materials Vol. 655

Abstract: (0, 5, 10, 15, 20) vol% CBN-WC/Co composites were consolidated by ball milling and the following hot-pressing sintering method. WC, Co and CBN powders were used as the starting materials. The effects of the CBN content on the density, microstructure and mechanical properties of CBN-WC/Co composites were investigated. The results showed that the CBN content had remarkable influence on the microstructure and mechanical properties of CBN-WC/Co Composites. With the increasing content of CBN, the density decrease, while Vickers hardness and flexural strength increased initially to the maximum values and then decreased at CBN 10 vol%. When 10 vol% CBN-WC/Co powders were hot-pressing sintered at 1350°C and 20MPa for 90 min, an excellent Vickers hardness of 19.8GPa was achieved, combining a flexural strength of 682MPa.

Abstract: Three kinds of ceramic composites with laminar structure, B₄C-B₄C/Ti₃SiC₂, B₄C-B₄C/Ti₂AlC and B₄C-B₄C/8%Si were synthesized by hot pressing sintering, respectively. The microstructures and properties of the composites were investigated in detail. The experimental results showed the two layers of each composite were in good combination and no spallation phenomenon was observed during the process of fracture. The fracture mechanism of three kinds of composites was similar with each other. The fracture mode of ceramic layers was mainly transgranular fracture. The ductile layers which showed good ductile fracture characteristic exhibited a mixed mode of intergranular and transgranular fracture. The results of mechanical properties showed the comprehensive performance of B₄C-B₄C/8%Si was the best, the flexural strength and fracture toughness of which were 524 MPa and 3.5 MPa∙m^1/2 respectively.

Abstract: BN_w/Si₃N₄ composites were fabricated by gas pressure sintering process using α-Si₃N₄ powder and self-made BN whisker as principal raw materials. The effects of different sintering additives such as YAG, MgO+YAG and RE₂O₃+YAG(RE=La, Sm, Dy) on the apparent porosity, microstructure, phase composition, mechanical performance and dielectric properties of the composites were analysed. The results revealed that composite sintering aids at certain ratio (MgO/RE₂O₃:YAG=1:5) was more conducive to ceramic densification than single YAG additive. The BN_w/Si₃N₄ sintered with La₂O₃+YAG acquired the highest density and the maximum bending strength (272.46 MPa) and fracture toughness(4.9 MPa·m^1/2). It was speculated that ceramic densification process was related to formation of different eutectic liquid phases with different viscosity. Additionally, when the apparent porosity of BN_w/Si₃N₄ composites was 20% or less, dielectric properties of the material were mainly influenced by the porosity and the value of the permittivity and dielectric loss decreased with the increase of ceramic porosity.

Abstract: SiC/Si ceramic composites have been considered as promising candidate materials for heat exchanger due to their good thermal conductivity and corrosion resistance. In this work, the SiC/Si ceramic composites were fabricated by liquid silicon infiltration technique, and the effect of carbon black content and molding pressure on thermal conductivity of the composites was studied. With carbon black content increasing from 5 wt. % to 25 wt. %, the thermal conductivity increased from 42.36 W/(m·K) to 65.60 W/(m·K) when molding pressure was fixed as 50 MPa. With molding pressure increasing from 30 MPa to 110 MPa, the thermal conductivity of SiC/Si ceramic composites with 20 wt. % carbon black content increased firstly, and then decreased, obtaining a maximum thermal conductivity of 71.62 W/(m·K) under 90 MPa. The SiC/Si ceramic composites with an optimal thermal conductivity of 76.55 W/(m·K) at room temperature were obtained when carbon black content and molding pressure were 25 wt. %, 90 MPa, respectively.

Abstract: Fracture toughness is one of the basic properties that determine the usefulness of these materials for structural applications. In this work, an analysis of cracking mechanism and measuring the fracture toughness of selected carbides: NbC, TaC and ZrC were carried out. Tested materials were sintered using Spark Plasma Sintering method (SPS), at 2200 °C, under a pressure of 35 MPa. Sintering time was 5 and 30 min. Commercial powders were used, with a particle size in the range from 1 to 6 microns. The powders were sintered without the addition of sintering activators.Additionally selected physical and mechanical properties have been investigated: apparent density (Archimedes method), Young's modulus (ultrasonic) and hardness (Vickers). The cracking mechanisms analysis was carried out using Scanning Electron Microscopy (SEM). Studies have shown that the highest fracture toughness, about 5.35 MPa·m^1/2, characterized tantalum carbide, sintered at 2200 °C for 5 min. TaC also had the highest resistance to the formation of chipping around the Vickers indentation.

Abstract: Layered ternary compounds Ti₃SiC₂ combines attractive properties of both ceramics and metals, and has been suggested for potential engineering applications. Near-fully dense Ti₃SiC₂ bulks were sintered from commercial Ti₃SiC₂ powders by hot press at 1350°C-1600°C for 60-120min under Ar atmosphere in this paper. The phase compositions and morphology of the as-prepared samples were evaluated by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). And the mechanical properties were measured by Three-Point bending method. It was found that the Ti₃SiC₂ had only a little of decomposition at sintering temperature above 1350°C. And effects of sintering temperature and holding time on the morphology of the bulk Ti₃SiC₂ are not obvious. Relative density of 98% and flexural strength of 480MPa were obtained for the Ti₃SiC₂ samples sintered at 30MPa and 1400°C for 90min.

Abstract: The wetting and spreading behaviors of pure nickel and nickel based alloys on sintered ZrB₂-SiC ceramics and their interfacial microstructures were investigated in this presentation. The nickel-based alloys were mainly the commercial nickel-molybdenum-chromium products. The wetting and spreading properties were observed by a real-time thermal optical measurement system under flowing argon-5%hydrogen atmosphere. As temperature increased, the pure nickel cylinder sample on ZrB₂-SiC substrate had few changes before 1228°C except for the thermal expansion in size. After that, liquid phase formed and spread gradually on the ceramic substrate. The contact angle was about 15^o after holding 15min at 1600°C. Therefore, pure nickel could contact sintered ZrB₂-SiC ceramics well. Meanwhile, the introduction of molybdenum and/or chromium in the pure nickel was beneficial for the wetting of nickel on sintered ZrB₂-SiC ceramics. The contact angles of Ni-28Mo and Ni-16Mo-23Cr alloys on sintered ZrB₂-SiC ceramics after 1600°C/15min were 13^o and 2^o, respectively. In addition, the temperatures of the liquid drop formed rose obviously in contrast to the pure nickel. The SEM images indicated that the interfacial microstructures of Ni-based alloys on sintered ZrB₂-SiC ceramic substrates were uniform and the dissolved boundaries showed that they had a good bonding. However, some cracks were found in the Ni/ZrB₂-SiC system for their high thermal mismatch. On the other hand, the Ni-Mo (-Cr)/ZrB₂-SiC interface had few defections and evident elemental diffusion between the ceramic substrates and the alloys were found at the interface.

Abstract: AlN/SiC composites with 5 wt.% Y₂O₃ addition were fabricated by pressureless sintering at 1700-1950 oC. The influences of sintering temperature and SiC content on the relative density, mechanical property and thermal conductivity were studied. With sintering temperature increasing from 1700 oC to 1750 oC, the relative density increased significantly to about 98.0%, without evident changes from 1750 oC to 1900 oC, and then decreased slightly at 1950 oC. As SiC content increased, the flexural strength of composites sintered at 1750 oC increased firstly, and then decreased, obtaining a maximum flexural strength of 337 MPa at 20 wt.% SiC content. Meanwhile, the thermal conductivity decreased from 60 W/(m∙K) to 40 W/(m∙K) with SiC content increasing from 0 wt.% to 30 wt.%. Moreover, in the sintering temperature range from 1750 oC to 1950 oC, the thermal conductivity increased from 45 W/(m∙K) to 55 W/(m∙K) for AlN-10 wt.% SiC composites, but decreased from 40 W/(m∙K) to 36 W/(m∙K) for AlN-30 wt.% SiC composites.

Abstract: With honeycomb lattice of sp² hybridized carbon atoms, graphene has demonstrated excellent electrical and mechanical properties. One of its promising applications is to fabricate graphene-ceramic composite to improve the mechanical properties. In order to quantify the strength between graphene-ZrB₂ interactions, molecular dynamic method was utilized to simulate typical interface of graphene/ZrB₂ ceramic structure. Berendsen method was used to control the temperature and pressure during the whole simulation process. Universal potential function was employed to simulate the force filed between graphene and ZrB₂ structure. The binding structures of graphene/ZrB₂ (0001) interface were analyzed in detail and the bonding energy of the interface was calculated. The influence of numbers of graphene layer and sandwich structures on the bonding energy of interface is discussed. The study helped to understand the influence of graphene on mechanical properties of ZrB₂ ceramic.

Abstract: High hardness, good thermal and electrical conductivity make carbides technologically important materials. The high melting temperature and low coefficients of self-diffusion make it difficult to obtain full dense material. In this paper the results of Spark Plasma Sintering (SPS) of transition metal carbides: NbC, TaC, TiC, ZrC, VC with the addition of graphene 10-20 nm x 14 microns in an amount of 2.5 mass % are presented. Powders were mixed in isopropyl alcohol in a planetary ball mill for 1h. The sintering processes was carried out at 2200°C at two different times: 5 and 30 min. Microstructure of the samples was analyzed using scanning electron microscopy. The measurements of density, Young's modulus hardness and electrical properties were carried out, also. The best properties were obtained for titanium carbide powder, sintered for 30 min. The most significant density increase of the sintered carbide–graphene composite by about 5.3% (depending on increasing sintering duration) was obtained for niobium carbide, while the smallest densities change for zirconium carbide.