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Zugl.: Darmstadt, Techn. Univ., Diss., 2003 / Download lizenzpflichtig

Zugl.: Darmstadt, Techn. Univ., Diss., 2003 / Download lizenzpflichtig

In this thesis, a synthesis concept has been developed, which uses nitrogen-rich liquid phases for sintering of Ca-α-sialon ceramics. First, keeping the Si/Al ratios constant, the effects of N/O ratio on the properties and microstructure were investigated through a liquid phase sintering process. Second, nitrogen-rich Ca-α-sialon ceramics, with nominal compositions: CaxSi12-2xAl2xN16, x < 2.0, was synthesized and characterized. Third, mechanical and thermal properties of nitrogen-rich Ca-α-sialons were investigated in terms of high temperature deformation resistance,reaction mechanism, phase stability and oxidation resistance, and further correlated to their phase assemblage and microstructure observation. It has been found that increasing the N/O and Ca/Al ratio simultaneously in the materials could result in development of a microstructure with well shaped, high-aspect-ratio Ca-α-sialon grains, and an improvement in both toughness and hardness. For the nitrogen-rich Ca-α-sialon, mono-phasic α-sialon ceramics were obtained for 0.51 ≤ x ≤ 1.32. The obtained Ca-α-sialon ceramics with elongated-grain microstructures show a combination of high hardness and high fracture toughness. Compared with the oxygen-rich Ca-α-sialons, the nitrogen-rich Ca-α-sialons exhibited approximately 150 oC higher deformation onset temperatures and decent properties even after the deformation. The α-sialon phase was first observed at 1400 oC, however the phase pure Ca-α-sialon ceramics couldn’t be obtained until 1800 oC. The nitrogen-rich Ca-α-sialons were thermal stable, no phase transformation observed in the temperatures range1400-1600 oC. In general, mixed α/β-sialon showed better oxidation resistance than pure α-sialon in the low temperature range (1250-1325 oC), while α-sialons with compositions located at α/β-sialon border-line showed significant weight gains over the entire temperature range (1250-1400 oC).

Dans ce travail, deux matériaux réfractaires à base de SiC et à liaison nitrurée sont caractérisés au niveau microstructural et thermomécanique. Ces matériaux sont utilisés dans les Unités de Valorisation Energétique d'Ordures Ménagères (U.V.E.O.M.). Dans cette application, les revêtements réfractaires sont soumis à de fortes sollicitations comme des hautes températures, des gradients thermiques, des gradients de composition chimique ainsi que de la corrosion. Les matériaux SiC-Si3N4 et SiC-SiAlON étudiés sont constitués d'agrégats de SiC reliés entre eux par une phase liante. Cette dernière présente un fort taux de porosité ouverte sous la forme d'un réseau complexe. L'analyse microstructurale de la phase liante a été réalisée par microscopie électronique à balayage et le comportement thermomécanique de chaque matériau a été caractérisé en flexion 4-points et par échographie ultrasonore. Lors de l'utilisation dans les U.V.E.O.M., les réfractaires sont exposés à des phénomènes d'oxydation, en particulier en présence d'humidité. Ainsi, le comportement à l'oxydation de ces matériaux a été étudié sur la plage 800°C - 1200°C sous air sec, sous air ambiant et sous air humide. Les évolutions microstructurales liées à l'oxydation montrent l'influence du réseau de porosité sur les régimes d'oxydation qui caractérisent ces matériaux en fonction du niveau de température et des conditions environnementales. Par ailleurs, le comportement thermomécanique a été déterminé après des vieillissements dans chacun de ces régimes. Cette démarche de caractérisation a permis l'établissement de relations entre le comportement thermomécanique et les évolutions microstructurales associées à l'oxydation. Des modèles phénoménologiques d'oxydation sont présentés. Pour finir, des traitements sont proposés afin d'améliorer la durabilité des réfractaires SiC-Si3N4 et SiC-SiAlON dans les U.V.E.O.M. / In this work, the microstructure and the thermomechanical behaviour of two SiC-based refractories are studied.In waste-to-energy plants, these materials are subjected to severe sollicitations such as thermal gradients, chemical composition gradients and corrosion. SiC-Si3N4 and SiC-SiAlON refractories are composed by SiC agregates bonded with silicon nitride based phases. A complex porosity network characterises the bonding phase. Its microstructure is analysed with Scanning Electron Microscopy (SEM). The thermomechanical behaviour of each refractory is characterised with 4-points bending tests and ultrasonic pulse echo measurement of the Young's modulus. During the running of waste-to-energy plants, refractories are exposed to oxidation phenomenons. They are more critical with an increase of a water content in the air. Thus, the oxidation behaviour is studied between 800°C and 1200°C under dry air, ambiant air and humid air. The microstructural evolutions due to oxidation are influenced by the porosity network, which leads to two oxidation stages depending on temperature level and environnemental conditions. Thermomechanical behaviour is also determined for each oxidation stage. This characterisation approach permits to establish relationships between the thermomechanical behaviour and the microstructural evolutions during oxidation. Phenomenological modellings are presented. Finally, thermal treatments are proposed in order to improve the durability of SiC-Si3N4 and SiC-SiAlON refractories in waste-toenergy plants.

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General perspective of this diploma thesis was aimed at machining of hard-to-machine materials. Theoretical part contains division and specification of these types of materials. Ways of classification and evaluation were also listed. Causes of worsened machinability were defined with the design of possible technological solutions, which eliminates these features. The main target of experimental part was turning nickel-based superalloy Inconel® 718 with the purpose of increase process productivity. Knowledge about appropriate types of cutting tool material and his coating were necessary for process optimization. It was achieved by replacing reference cutting tool material (coated carbide inserts) with SiAlON ceramic. Cutting tool edge was tested with an intention to discover appropriate deal between tool life and volume of material removed. Measured cutting forces, generated by machining of reference and optimize technology, were processed by using simulation software. Total deformation of workpiece material was statistically evaluated. The conclusion of diploma thesis is technical-economic evaluation of both options.