Preparation and Characterization of Alumina based TiN n and SiC n Composites
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Preparation and Characterization of Alumina based TiNn and SiCn Composites Mats Carlsson1, Mats Johnsson1 and Annika Pohl2 1 Dept. Inorg. Chem., Stockholm University, SE-106 91 Stockholm, Sweden 2 Dept. Mater. Chem., The Ångström Lab., Uppsala University, SE-751 21 Uppsala, Sweden ABSTRACT Ceramic composites containing 2 and 5vol. % of nanosized commercially available TiN and SiC particles in alumina were prepared via a water based slurry processing route followed by spark plasma sintering (SPS) at 75 MPa in the temperature range 1200-1600°C. Some of the samples could be fully densified by use of SPS already after five minutes at 1200°C and 75 MPa. The aim was to control the alumina grain growth and thus obtain different nano-structure types. The microstructures have been correlated to some mechanical properties; e.g. hardness and fracture toughness. INTRODUCTION The weakest link in a chain determines the strength. This is an expression, which is applicable to most areas within the subject of materials science. It becomes especially obvious in the case of ceramics since these materials exhibit a brittle behaviour. In order to obtain improved toughness of ceramics one have to find ways to make the material homogeneous and free from cracks, pores and inclusions. Except for these fundamental but also unattainable details introducing reinforcement material in the matrix can also increase the toughness. Particles of a second phase in a matrix can act as reinforcement by different toughening mechanisms depending on e.g. composition, particle size, powder-processing route and thermal expansion properties. Nano-sized particles have higher intrinsic sintering activities than micron sized ones and therefore a lower sintering temperature is expected to be required when sintering such particles. Nano-composites have proved to exhibit very good mechanical properties, with new toughening mechanisms. During the last decade an increasing interest for the nano sized region have been raised. Niihara and co-workers managed in the early 90´ies to obtain nano-sized particles embedded in or in-between larger alumina grains [1]. The mechanical properties were outstanding, but the results have been difficult to reproduce for other research groups. Both TiN and SiC are used in construction ceramics as reinforcement of alumina. However, since the thermal expansion are different; TiN having a higher, and SiC having a lower thermal expansion than that of alumina, the expected properties should be different. For instance, at room temperature after sintering, in the case of SiC/alumina composites, the SiC particles are compressed and the matrix is tensile stressed. The conversed scenario is expected for TiN/alumina. But, however, that implies strong inter granular bonds between TiN and alumina. Therefore the surface properties e.g. the degree of surface oxidation or impurities plays important roles for the internal stress fields and therefore mechanical properties. This ought to be especially crucial for nano composites since the grain b
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