Alpagut Kara

Characterisation of biscuit fired bone china body microstructure. Part I: XRD and SEM of crystalline phases

Abstract As a ceramic material bone china is a highly specialised product and is the basis of the worlds most attractive and expensive types of tableware, mainly due to its translucency, whiteness, bright glaze, decoration quality, and high strength. Despite numerous studies on bone china made by several authors over many years, insufficient is known even today about the chemical reactions and physical processes taking place between the various raw materials which make up the body, during biscuit firing. As a result, there is still controversy about the formation and chemistry of its constituent phases, particularly the glassy matrix phase. In this paper, part of an extended study, the biscuit firing process and the resultant microstructure of the bone china body, studied using a combination of techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), is reported. It is clearly shown that fired bone china body consists of crystals of anorthite, β-TCP, and also a small amount of quartz, rather unevenly dispersed in a complex glassy matrix phase.

Characterisation of biscuit fired bone china body microstructure. Part II: Transmission electron microscopy (TEM) of glassy matrix

Abstract In this second part of the study, electron transparent specimens of the biscuit fired bone china body were prepared for a conventional transmission electron microscopy (TEM) study. The crystalline phases present in the fired material were noted and particular consideration was given to determination of the nature and composition of the glassy phase. Compositional information on the phases was obtained using energy dispersive X-ray spectrometry (EDS) which was available in conjunction with TEM. Additionally, a simulated glass with a composition typical of that present in the bone china body was prepared and examined by TEM in order to compare its structure and chemistry with that of the glassy phase originally present in the bone china body.

Novel SiAlON Ceramics for Cutting Tool Applications

New α-β SiAlON ceramics were produced in aqueous medium using multiple oxides as densifying additives. The sintered materials were c hara terised using a combination of techniques such as XRD, SEM and TEM. The samples machined to cutt ing tool geometries were tested in turning and milling operations of cast iron and superal loy. The results were compared with some commercially available ceramic inserts. Introduction Ceramics have always had a potential as cutting tool material s due to their high hot hardness and chemical stability. Ceramic inserts based on Al 2O3 and Si 3N4 are applied in machining of cast iron, hardened steel and heat resistant alloys. Metal remova l rates of such inserts are significantly higher than those of conventional coated or uncoated ceme nt d carbide tools. The reason to use Al 2O3 as a cutting tool material is its high chemical stability and wear resistance. Al 2O3-ZrO2 cutting tools were developed for machining cast iron and steel a nd have higher strength, toughness and thermal shock resistance than pure Al2O3. Another type of Al2O3 based cutting tool is that of Al 2O3-TiC composites with better hot hardness and thermal conductivity compared to pure Al 2O3. These properties make this material suitable for machining chilled cast iron and hardened steel and also for fine mac hining of cast iron. SiC whisker reinforced Al 2O3 (SiCw-Al2O3) composites are used commercially in superalloy machining due to their high strength, toughness and thermal shock re sistance. Si 3N4 based ceramic inserts have an advantage of high strength and high toughness as a result of elongated β-Si3N4 grains in the microstructure. Therefore, Si 3N4 cutting tools are the most extensively used materials in cast iron machining [1]. SiAlON ceramics have been known for a few decades for their outstanding high temperature mechanical and thermal properties. In single phase form, β-SiAlON has higher toughness and strength and possesses good oxidation and creep resistance up to 1300°C sim ilar to Si3N4. On the other hand, α-SiAlON has excellent hardness, but slightly worse strength, toug hness and oxidation resistance compared to β-SiAlON. α and β-SiAlONs are in thermodynamic equilibrium and by combining two phases it is clearly possible to de fine quite precisely an optimum combination of mechanical properties [2]. Due to the favourable properties of α-β SiAlONs mentioned above, they are attractive materials for cutting tool applications. Such applications require mat rials with high hardness, high fracture toughness and high strength. Si 3N4 ceramics have favourable fracture toughness and strength to meet these requirements, but their hardness is lowe r than α-β SiAlON composites. Although α-β SiAlON composites possess better mechanical properties than Si3N4 ceramics, they have not been commercialised as cutting tool insert s. This was mainly due to two reasons. The first reason is the difficulty in obtaining de sign d microstructure and/or phases. For example; when 75% α and 25% β-SiAlON microstructure with elongated Key Engineering Materials Online: 2003-04-15 ISSN: 1662-9795, Vol. 237, pp 193-202 doi:10.4028/www.scientific.net/KEM.237.193

Interactions between an ABS type leadless glaze and a biscuit fired bone china body during glost firing. Part II: investigation of interactions

Abstract This part of the study involves the investigation of the interactions between an ABS type commercial leadless glaze and a biscuit fired bone china body during glost firing at different temperatures using XRD and SEM techniques. The aim was to obtain detailed and systematic information about the morphological and chemical characteristics of the resultant phases formed at the interfaces. Separate examination of the interactions between the glaze and the experimental crystalline phases of the body prepared earlier, namely anorthite and β-TCP, were also carried out in an effort to show their susceptibility to react with the glaze independently. As a result, the presence of a range of interaction layers with distinctly different morphological and chemical characteristics was successfully demonstrated. It was proposed that β-TCP was the major contributor to the overall interactions by reacting with CaO from the glaze in the presence of water vapour and forming hydroxyapatite crystals at the glaze-body interfaces. Although the exact origin for the development of water vapour in the molten glaze could not determined, several potential sources were suggested.

Interactions between an ABS type leadless glaze and a biscuit fired bone china body during glost firing. Part I: preparation of experimental phases

Abstract In this paper, part of an extended study, crystalline phases of a biscuit fired bone china body, namely anorthite and beta-tricalcium phosphate (β-TCP), were produced experimentally to be used in subsequent interface studies for simulation of the interactions during glost firing at different temperatures between an ABS type commercial leadless glaze and the individual phases originally present in the body. The research was undertaken based on the premise that bone china has been a product providing the greatest challenge in moving to a totally leadless glaze. High bulk density and low apparent porosity values were achieved from both the anorthite and β-TCP samples through the suitable heat treatments. The microstructural and chemical characteristics of the experimental phases were studied using X-ray diffraction (XRD), scanning (SEM), and transmission (TEM) electron microscopy techniques in combination with EDS analysis. The morphological and chemical similarity of the phases to those originally present in a biscuit fired bone china body was clearly demonstrated. This similarity supports the choice of the experimental material for the interface studies undertaken with these phases in the second part of the study.

Utilization of Coal Combustion Fly Ash in Terracotta Bodies

In this present work, coal combustion fly ash from a power plant in Turkey was used in combination with a traditional raw material in terracotta production with the aim of having a product with improved physico-mechanical properties and lower production cost. Several compositions were prepared by adding different amounts of fly ash (ranging from 0 to 20 %) in a yellow firing terracotta formulation and shaped by wet pressing. Following firing at a suitable temperature, some of the physical properties of the resultant tiles were determined as a function of the fly ash content. A combination of XRD, SEM and EDX techniques were also employed to correlate the properties with the phase composition. The results indicated that fly ash could be utilized easily in certain amounts in such an application.

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

Interactions between a leadless glaze and a biscuit fired bone china body during glost firing: part III: effect of glassy matrix phase

As the final part of the study, the interactions between a commercial leadless glaze and a simulated bone china body glass during glost firing at different temperatures have been investigated. Advanced techniques such as electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) have been employed in order to assess the contribution of the glassy phase of a biscuit fired bone china body to the formation of interaction layers at the interfaces independently. The results showed the presence of several calcium phosphate phases with distinct morphology at the glaze-simulated body glass interface for both the glost firing temperatures employed. It was suggested that the formation of the hydroxyapatite crystals at the glaze-biscuit fired bone china body interface may be helped by the glassy phase. TEM was found to be particularly useful since the interaction layers presented a most complicated and also interesting microstructure due in part to their multi-layered nature and small size of the calcium phosphate crystals situated at the interfaces.

Influence of Type of Cations on Intergranular Phase Crystallisation of SiAlON Ceramics

25α:75β SiAlON composition was designed with different cations and at different molar ratios. Effect of the type of cations both on the composition and the type of intergranular phase investigated after gas pressure sintering and further post sintering heat treatment.

Development of Laminar Type Functionally Graded SiAlON Ceramics

In order to combine high hardness with high toughness, laminar type (two layers) functionally graded (FGM) SiAlON ceramics were developed. In a previous study achieved by the same authors, presence of a sharp transition zone between the two layers was demostrated by SEM analyses. The aim of this study is to eliminate this sharp zone. In addition, the effect of sintering time and temperature on the interaction between the layers was also investigated. For this purpose same metal oxides were added to both layers, which have different compositions. Phase characterisation, microstructural investigation and hardness measurements showed that elimination of the sharp zone was achieved and the laminates sintered at 1700°C for 1 hour was found to be good enough to obtain continuous change in composition, microstructure and hardness through the thickness of the sample.