Key Engineering Materials Vols. 512-515

Abstract: The sinterability of ZrB₂-20vol.% SiC ceramics by high-energy ball milling as well as introduction of Zr and Al as sintering additives. Densification process and microstructure of ZrB₂-SiC ceramics were investigated. After high-energy ball milling, the average particle size decreased to about 500 nm-2 μm, and ZrB₂-SiC powder can be sintered to 98.92% theoretical density at 1800 °C, but a trace of amount of oxidation (ZrO₂) were detected in sintered sample. Introduction of Zr, Al and C combined with high-energy ball milling enhanced the densification of ZrB₂-SiC ceramics and reduced the particle sizes, and the relative density of obtained ceramic reached up to 99.49% at 1800 °C. The additions of Zr, Al and C can remove the oxide impurities of the surface of ZrB₂ particles and form a reaction between oxide impurities. The fracture toughness increased of the 40% when ZrB₂ powders were milled by high-energy ball milling, and increased to 4.77±0.18 MPa•m^1/2. However, the attrition-milled composites had lower hardness and Young’s modulus, which was attributed to the presence of a second phase in the grain boundaries.

Abstract: Zirconium diboride and silicon carbide are thought to have a low intrinsic sinterability due to their strong covalent bonds, low bulk and grain boundary diffusivities. ZrB₂-SiC ceramic composites were prepared by a field assisted pressureless sintering process in the present work. The densification behavior and the effect of sintering temperature on microstructure and properties of sintered samples were studied. Pellets were in-situ formed by dry uniaxial pressing in the graphite die at a pressure of 50MPa for 3min and then sintered at a sintering temperature ranged from 1650 °C to 1950 °C with fixed heating rate and holding time. The current, voltage, temperature and displacement data were all collected by the real-time acquisition system. The bulk densities were determined by Archimedes method and the microstructure of samples was characterized by SEM. The onset of some measurable shrinkage of the green body was recorded at around 1400 °C regardless of the sintering temperature and significant shrinkage took place at higher temperature of around 1600 °C. For the sample sintered at 1950 °C, no shrinkage occurred after ~2min holding time. The relative density increased significantly with increasing temperatures and samples could be densified to a relative density of more than 99% at 1950 °C by the field assisted sintering process without obvious grain growth.

Abstract: ZrB2 belongs to a class of ceramics defined ultra-high-temperature ceramics with extremely high melting temperatures, but ZrB2 ceramics is difficultly sintered and easily oxidized. To make ZrB2 ceramics possess the high relative density and the better oxidation resistance. The effects of adding phase on the sintering and oxidation resistance mechanism of ZrB2 based high-temperature multi-phase ceramics were investigated. YAG and Al2O3 help for the densification of ZrB2 based ceramics. The oxidation layer thickness of sintered ceramics adding YAG or YAG-Al2O3 phase is thinner than that of sintered pure ZrB2 ceramics under the same oxidation condition, the oxidation layer thickness of sintered ceramics adding YAG-Al2O3 phase is thinner than that of sintered ceramics adding YAG phase, the oxidation layer thickness of sintered ceramics is decreased with an increased Al2O3 content.

Abstract: For high thermal conductivity and high electrical conductivity, copper is a good electrode material. The wearing resistance and spark resistance of Cu can be improved with the addition of ZrB₂. ZrB₂-Cu composites with high Cu volume fraction was successfully prepared by spark plasma sintering (SPS) process in this paper. The microstructure and properties of the sintered samples were characterized. The effect of the sintering temperature and the ZrB₂ content in composites on the relative density and properties of the composites were investigated. The results show that the relative density and mechanical properties increase with the sintering temperature increasing. The optimum sintering temperature is 900 °C for 10wt.% ZrB₂-Cu, 1000 °C for 20wt.% ZrB₂-Cu and 1050 °C for 30wt.% ZrB₂-Cu. With the ZrB₂ content in composites increasing from 10wt.% to 30 wt.%, the electrical resistivity increases from 2.25×10^-6 Ω.cm to 8.82×10^-6 Ω.cm, the flexural strength decreases from to 539.1 MPa to 482.2 MPa and the fracture toughness decreases from to 15 MPa.m ^1/2 to 9 MPa.m ^1/2. The hardness (HV) of ZrB₂-Cu composites is significantly enhanced by the ZrB₂ particulate reinforcement, increasing from 1410 MPa for 10 wt.% ZrB₂ to 2480 MPa for 30wt.% ZrB₂.

Abstract: The ZrB₂-SiC composite ceramic was prepared using Al₂O₃ and Y₂O₃ as additives through pressureless sintering under 1800°C. The electrical and mechanical properties were studied with the different mass ratio of ZrB₂ and SiC. The microstructure and phase transformation of the ZrB₂-SiC composites ceramic were characterized by scanning electron microscopy. Results show that the densities of the samples are decreased with the decrease of the mass ratio of ZrB₂ and SiC while the resistivities are increased. When the mass ratio of ZrB₂ and SiC is 4 the resistivity reaches 14μΩ•mm while the density is 4.85g/cm³. When the mass ratio of ZrB₂ and SiC is13:7, the flexural strength reaches the max value and toughness reaches the minimum value. The flexural strength reaches 233Mpa and the toughness reaches 4.51 Mpa• m ^1/2. The changes of the properties of the ceramic were analyzed through the microstructure and the conduction mechanism was studied.

Abstract: The B4C/BN composites were fabricated by hot-pressing process. The B4C/BN composites included the B4C/BN microcomposites and B4C/BN nanocomposites. The B4C/BN microcomposites were fabricated by hot-pressing process, and the B4C/BN nanocomposites were fabricated by chemical reaction and hot-pressing process. In this research, the phase composition, microstructure, mechanical property and thermal shock resistance of the B4C/BN microcomposites and B4C/BN nanocomposites were investigated. The B4C/BN microcomposites and the B4C/BN nanocomposites exhibited the homogenous and compact microstructure, and the h-BN particles were homogenously distributed in the B4C matrix. The mechanical property of the B4C/BN microcomposites and B4C/BN nanocomposites decreased gradually with the increase of h-BN content, but the B4C/BN nanocomposites exhibited the higher mechanical property than that of the B4C/BN microcomposites. The thermal shock resistances of the B4C monolith and the B4C/BN composites were measured by water-quenching method. The thermal shock resistances of the B4C/BN microcomposites and the B4C/BN nanocomposites were remarkably improved in comparison with the B4C monolith. The thermal shock resistance of the B4C/BN nanocomposites was much better than that of the B4C/BN microcomposites. The thermal shock temperature difference (ΔTc) of the B4C monolith was about 300oC, the ΔTc of the B4C/BN microcomposites was about 500oC and the ΔTc of the B4C/BN nanocomposites was about 600oC. The B4C/BN composites exhibited the high thermal shock resistance due to the high fracture strength and low elastic modulus. The microstructure showed that the weak interface of B4C/BN and cleavage behavior of laminate structured h-BN particles would remarkably improve the thermal shock resistance of the B4C/BN composites.

Abstract: CBN particles coated with borosilicate glass-aluminum composite coating (Glass-Al-CBN) were prepared by a multilayer coating method. The coated composite particles were sintered at 800°C for 2h. XRD, DTA-TG, SEM techniques were used to characterize the samples. It was found that AlN was formed by the reaction of Al and CBN. The mechanical properties and the oxidation resistance of the composite was improved due to the presence of the glass coating and alumina.

Abstract: In order to improve the oxidation resistance property of carbon/carbon composites, the C/C matrix was modified with a borate sol precursor and B₄C micro-powders by a sol-gel integrating with a solvothermal process. The phase compositions, surface and cross-section microstructures of the C/C matrix modified by different B₄C content were particularly investigated. Results show that the surface of the modified composites is covered by a coating composed of B₂O₃ and B₄C, meanwhile, the internal micro-holes of the C/C composites are occupied by B₂O₃ and B₄C. After oxidation in air at 973 K, the B₂O₃ glassy phase, due to the oxidation of B₄C, seals the cracks and holes and effectively prevents C/C composites from oxidation. The weight loss of the modified C/C composites is only 2.21 % after oxidation in air at 973 K for 20 h.

Abstract: Surface modification of carbon fibers(CF) by physicochemical methods directs an attractive approach for improvement of metal uptake from solutions. We investigated pretreatment of carbon fibers by HNO₃ with different time on absorption of catalysts, which is related to the coverage of carbon nanotubes (CNTs) grown on Carbon fibers. The effects of surface modifications on the properties of carbon fibers were studied by X-Ray photoelectron spectroscopy. The modifications bring about variation in the chemical properties. The modification increased a large number of surface functional groups such as hydroxyl and carbonyl. The HNO₃ modification increases the catalysts absorption. The coverage of CNTs on CF increases with pretreatment time, which was studied by SEM.

Abstract: βThe B₄C and phenolic resin were used as sintering additives to study the different β-SiC（cubic silicon carbide) addition to the performance of α-SiC pressureless sintering ceramics in this paper, and the optimum sintering temperature and the best β-SiC addition amount were determined. By XRD, SEM,density testing and other tests showed that, all the β-SiC transform 6Hα-SiC during the sintering process ,the ceramics have a Vickers hardness of 18.04 GPa,a frature toughness of 4.51MPa ,a density of 3.13g/cm³ by adding 15wt% β-SiC powders.