Papers by Keyword: Pressureless Sintering

Abstract: Due to their better mechanical and physical properties diamond tools have largely replaced cemented carbide tools for machining of mineral materials like concrete and rocks. The decomposition tendency of diamond has to be taken into consideration during the manufacturing process as well as during their employment in machining tools. By using water cooling the diamond decomposition is reduced, but the contamination of occupied buildings by concrete/rock-watermixture and the need of water supply units on building sites are unfavourable. However, absence of water cooling lead to an increased tribological and thermal wear of conventional diamond tools. Due to the heat development the diamonds in direct contact with mineral materials as well as the diamonds in deeper layers are deteriorated. The Institute of Materials Engineering pursues a novel thermal protection shield concept, in which thermal insulating materials such as Al2O3, ZrO2 or glass in diamond impregnated composite structures act as heat shield, which protects diamonds in deeper layers against high temperature and graphitisation. Before the effectiveness of this concept could be investigated suitable composites have to be manufactured. In this paper the powder metallurgical production processes of diamondalumina- cobalt-composites with varying alumina and cobalt particle sizes, their microstructures and porosities are described. In comparison to composites with larger alumina particle sizes it could be observed that the distribution of alumina particles with particle sizes below 70 ,m in the cobalt matrix is uniform and the porosity of the composite decrease.

Abstract: The effects of hBN content on microstructure, mechanical properties, and machinability of the pressureless-sintered Si3N4 ceramics were investigated. Flexural strength, Young’s modulus, and hardness decreased with increasing h-BN content. The mechanical properties are decreased mainly because of increased porosity of composite, and the much lower Young's modulus of BN compared to that of Si3N4. Pressureless-sintered Si3N4/hBN composites exhibit strong texture of BN grains oriented with the c-axis parallel to the cold-pressing direction. Cutting resistance of Si3N4 ceramic composites with more than 10 vol% hBN decreased with increasing hBN content, demonstrating a good machinability of the composites. The residual pores can be attributed to improved machinability of pessureless-sintered Si3N4-BN composite.

Abstract: Attempts have been made to sinter hexagonal boron nitride (h-BN) by pressureless-sintering in air. In order to achieve this purpose, feldspar has been selected as a sintering aid to produce sintered compacts at a temperature above the melting point of feldspar. Even though h-BN was not wetted by the molten feldspar in nitrogen, the wettability was significantly improved in air. Through the heating process, the h-BN powder disappeared owing to the oxidation of the powder followed by sublimation of the boron oxide. The decomposition of BN was prevented effectively by the presence of molten feldspar in the h-BN/feldspar (30 vol.%) compact. It was shown that h-BN/feldspar composite can be sintered in air under normal pressure, although the bulk density of h-BN/feldspar composites (1.40 to 1.51 g/cm3) was not satisfactory enough.

Abstract: In this paper, single-phase Yb α-SiAlON ceramics were prepared by pressureless sintering and the densification, phase transformation as well as microstructure development were studied. It was demonstrated that the phase transformation could be completed at 1700 °C and elevated temperature was beneficial to grain growth. Additionally, effect of the addition of seed crystals prepared by combustion synthesis was discussed. From the experimental results, it was found that the added seed crystals facilitated the phase transformation and improved the microstructure development, resulting in the formation of elongated grains.