Erhan Ayas

Pressureless Sintering of Al2O3-TiCN Composites

A composition of 70 wt % Al2O3 + 30 wt % TiCN was prepared and pressurelesssintered at temperatures ranging from 1700 to 1840°C. In order to achieve densification, a powder bed was also employed. The resultant microstructures were investigated using XRD and SEM techniques. In addition, Vickers hardness and fracture toughness of the representative samples were measured. The results showed that sintering in powder bed at 1840°C made it possible to obtain 99 % of the theoretical density. Introduction Historically alumina has been the first ceramic material used as a cutting tool insert. The application areas of this material are, however, limited due to its poor fracture toughness and low thermal conductivity. These disadvantages of alumina were improved by means of additives such as ZrO2, SiC, TiC and TiCN [1]. Nowadays, Al2O3-TiC/TiCN composites have widely been used as cutting tools due to combination of physical properties such as high thermal shock resistance, fracture toughness and high wear resistance [2]. These materials also find applications as sliding head substrates and high precision wear-resistant parts [3]. Particularly, titanium carbonitrides are widely applied as the hard phase in sintered ceramic-metal composites and protective coatings on conventional hard metal substrates [4]. The current study involves using TiCN instead of TiC in alumina matrix as an alternative reinforcement and studying its effect on densification behaviour during pressureless sintering. Experimental Procedure In the present work, a 70 wt % Al2O3 + 30 wt % TiCN composition was selected. In order to compare densification behaviour, 70 wt % Al2O3 + 30 wt % TiC batch was also prepared. Properties of the starting commercial powders are given in Table 1. The starting powders were mixed by wet milling in a planetary ball mill in 2-propanol alcohol for 3 hours using ZrO2 media. Then, the slurry was dried in an evaporator (Heidolph WB2000, Germany) at 60°C and uniaxially pressed at 20 MPa to form pellets of 2 cm in diameter. Following the uniaxial pressing, the pellets were further cold isostatically pressed at a pressure of 300 MPa. Sintering was carried out in BN crucible at temperatures of 1800°C and 1840°C for one hour in a pressureless sintering furnace (Thermal Tech., Astro). The composition of the powder bed was chosen to be the same of that of the composites. The powder bed was only employed at 1840°C. Density and XRD analyses of the samples were carried out after grinding the surfaces in order to remove the TiB2 layer. Densities were calculated according to Archimedes principle. The theoretical density of the samples was determined by the rule of mixtures, using the theoretical densities for Al2O3, 3.986 gr/cm 3 and for TiC, 4.92 gr/cm 3 . Fracture toughness was determined from radial cracks produced by Vickers indentation under a load of 10 kg [5]. Each data point was obtained from the average of six indentations. Direct phase analysis was achieved using Rigaku Rint series X-Ray Difractometer (XRD). Polished and fracture surfaces of the samples were examined using a scanning electron microscope (SEM-Camscan S4 Series, UK) after gold coating and representative secondary electron (SE) and back-scattered electron (BE) images of the surfaces were obtained. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 849-852 doi:10.4028/www.scientific.net/KEM.264-268.849

Spark Plasma Sintering of Silicon Nitride-Boron Carbide Composites

Si3N4-B4C composites containing fine and coarse B4C particles were produced using Al2O3 and Y2O3 as sintering additives via spark plasma sintering (SPS) technique. Phase assemblages of the produced composites were determined by XRD analysis. Si3N4, B4C and in situ formed SiC, h-BN and Si phases were observed. Even when incorporated in significant amounts, B4C was consumed readily in the Si3N4 based system. Consequently, full densification of these composites was found to be a very difficult task due to the simultaneous in-situ reactions, even in fast sintering process. Electrical resistivity measurements carried out at room temperature indicated that addition of both fine and coarse B4C particles decreased the electrical resistivity by several orders of magnitude due to the formation of electrically conductive in-situ phases, mainly SiC and metallic Si.

EBSD Characterisation of SPSed CeB 6 Thermionic Electron Emitter

Cerium hexaborides (CeB6) with simple cubic structure has attracted growing interests owing to its special electronic and magnetic performance. As an electron source, CeB6 has low work function (ɸ) (2.5 eV), operation temperature and volatility which means a longer service life when used as a thermionic electron emitter [1]. Recently, many researchers have focused attention on the field emission properties of nanowires obtained by chemical vapor deposition (CVD) method. Compared to the nanowires and films, the polycrystalline bulk materials can provide large size, low cost, simple preparation and can be fabricated to various devices [1,2]. For this reason, in this study, polycrystalline CeB6 was produced as a cathode material by using spark plasma sintering (SPS) technique. X-ray diffraction (XRD) result not given here showed that CeB6 was detected as the only crystalline phase of any sintering temperature. However, backscatter electron images (BSE) indicated that three different phases might exist. At the same time, particle size and distributions could not be clearly identified from BSE images (Figure 1 (a)). Therefore, the aim of the study is to improve the results obtained from XRD and BSE imaging techniques by using energy dispersive xray spectroscopy (EDX) and electron backscatter diffraction (EBSD) techniques.

A Novel Approach for Preparing Electrically Conductive SiAlON-TiN Composites by Spark Plasma Sintering

An effective approach for preparing electrically conductive SiAlON-TiN composites was developed. Granules of a designed composition of α- SiAlON was obtained by spray drying and coated with varying amounts of TiO2 powder homogenously by mechanical mixing. Fully dense composites were obtained by spark plasma sintering (SPS) under a pressure of 50 MPa at 1650°C for 5min. According to the SEM analysis, unique microstructures containing continuously segregated in-situ formed TiN phase in 3-D were achieved. Additionally, XRD studies revealed that all TiO2 was successfully converted to TiN. The resistivity of the α-β SiAlON (1x1011 .m) was drastically reduced with the addition of only 5 vol. % TiO2 (2x10-4 .m).

Characterisation of Dorylaion (Turkey) Pottery Sherds from Hittite Period

In this study, representative samples were taken from eleven pottery sherds of Hittite period recovered during the archaeological excavations of different years in the ancient city Dorylaion, which is nowadays called as Sarhöyük (Eskisehir/Turkey). The similarities and differences of the samples were evaluated from the microstructural and microchemical characterisation point of view using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Energy dispersive X-ray spectrometry (EDS) attached to SEM was also employed for qualitative and semi-quantitative analysis. The morphological and compositional features of each item were then compared; conclusions were drawn.