Materials Science Forum Vols. 727-728

Abstract: Wollastonite is a calcium silicate mineral natural or synthetic. Commercial wollastonite starts to melt at about 1450°C and can not be considered a "flux" as alkali feldspar. For this function, it depends on the reaction with other raw materials. Faced with this, came the goal of this work which was to investigate the mechanism of action of wollastonite as a ceramic flux. The use of wollastonite in ceramic bodies was investigated by analysis of its reactivity with other materials such as quartz, kaolin, talc and feldspar. It was analyzed the technological properties of the final parts, especially in relation to the firing temperature, phase formation and technological properties (mechanical strength, porosity, etc.). The results of this characterization showed that the technical properties of the parts are developed according to commercial porcelain products.

Abstract: This paper presents results of experiments on pressureless sintering of boron carbide (B₄C) with addition of titânia (TiO₂) and titanium diboride (TiB₂). The TiB₂ powder was added as a second phase and the TiO₂ powder for reactive sintering and in-situ formation of TiB₂. The final concentrations of TiB₂ in the composites were 0 to 10 vol%. Sintering was performed at 2050 °C/30min in argon atmosphere. TiO₂ was completely transformed into TiB₂ with fine equiaxed grains distributed homogeneously. Composites obtained by in-situ reaction showed a densification increase with the concentration increase, while the composites with TiB₂ powder mixture showed low densification in all compositions. Relative Density of the composite with 10 vol% of TiB₂ obtained in-situ was 91% (TD) compared to 86 % for B₄C only. Vickers hardness was about 29 GPa.

Abstract: Sintering and crystallization behaviors of a LZS glass-powder were investigated by means of thermal shrinkage, differential thermal analysis, X-ray diffraction as well as density and mechanical properties measurements. The melted glass, 9.56Li₂O.22.36ZrO₂.68.08SiO₂ (wt%) first was cast into water to provide a frit for milling. The milled glass powder (mean particle size 5.0 µm) was then uniaxially pressed at 100 MPa and the obtained samples were isothermally sintered in the 800-950°C temperature range in air for appropriated time intervals (15-120 min). Sintering was found to start at about 640°C and crystallization took place just after completion of sintering and was almost complete at 920°C. The glass powder compacts crystallized into lithium and zircon silicates so that glass-ceramics with relative densities between 84 and 99% were obtained reaching maximum hardness and bending strength values of 8 ± 0.5 GPa and 214 ± 20 MPa, respectively.

Abstract: Alumina ceramics are used in many areas of modern industry because of their excellent mechanical, thermal and optical properties. Sintering of alumina usually requires high temperatures (> 1600°C), due to the large and extensive pore network developed during the reconstructive transformation to its stable alpha phase. Consequently, it is very difficult to suppress grain growth during sintering. In this work it was studied the sintering behavior of alumina powders obtained through a modified Pechini-type method, with addition of seeds of calcined alpha-alumina. Powders without addition of seeds were synthesized for comparison. After calcinations at the temperature of 1000°C, the powders were uniaxially pressed. The compacts were sintered in air in the range of 1400-1600°C and characterized by density and porosity measurements, scanning electron microscopy, X-ray diffraction and infrared spectroscopy. The behavior in the sintering of the compacts with addition of seeds was clearly different compared with the compacts with no seeds.

Abstract: Compositions of cutting tools based on cubic boron nitride phase added with different ceramics as binders have been the subject of several works. A major problem of these tools during machining of steel is the loss of cutting efficiency due the "chemical weathering" generated by the reaction between iron and cBN at the high temperatures reached by the machining operations. Aiming to eliminate these problems, this paper presents results on the development of composites obtained by sintering process at high pressures and high temperatures in association with a cyclic operation. Si₃N₄ was used as the binder agent. The compacts were analysed by x-ray diffraction as well as optical and scanning electron microscopy. The mechanical properties were evaluated by means of microhardness and fracture toughness tests. The degree of phase transformation of the Si₃N₄, the chemical reaction during the cyclic sintering process, and the sintering mechanism of the additive were discussed.

Abstract: Polycristalline diamond composites (PDC) obtained by sintering of micro sized diamond particles together with silicon as binder is a conventional superhard material for high speed cutting tools. nanosized dispersed diamond particles have been recently used as precursor for a new generation of superhard composites. In the present work, PDCs were produced by sintering at high pressure and high temperature using a mixture of micro and nanosized diamond particles for optimized compactation. The values of density and hardness as well the results of X-ray diffraction suggest a plastic deformation of the larger diamond particles. The compression strength of these composites was found to be close to that of natural diamonds.

Abstract: Non-oxide covalent ceramics such as TiC and TiN display hardness and thermal stability that favor their use as cermet insert for high speed turning operations. The objective of this work was to assess the properties of the TiC combined with W-Mo as binding matrix. The cermet sintering was carried out inside a toroidal anvil type of high pressure device up to 1.0 GPa of pressure and temperatures up to 1800°C. It was found that at 1400°C the Mo reacts with TiC forming Mo₂C while at 1800°C the reaction with W forms WC. Moreover, the cermets density reaches 98% of the theoretically calculated value with strength of 942 MPa and Rockwell A hardness of 94. Efficient turning parameters are reported for machining operation using these cermets as tools insert.

Abstract: The lifetime of drilling tools for petroleum well perforation is strongly affected by the type of insert at the crown of the tool. Usually, this insert is made a of hardmetal (WC-Co) cermet with embedded diamond particles. In the present work an investigation on the effect of Cu and Ti as doping agents on a WC-6%Co cermet was conducted. I was found a change in coercive force in the interval 12 to 34 kJ/m and harness from 73 to 89 HRA with improving adhesion. It was also shown that this could significantly increase the operational capacity of a drilling tool.

Abstract: This work has as its objective to evaluate the influence of a granite waste into a clayey ceramic body for obtaining of rustic wall tiles. As raw materials, a clayey ceramic body for red ceramic production and a granite waste, resulting from ornamental stones cutting with the multi-wire technology were used. Compositions using 0, 10, 20 and 30% of waste incorporated into ceramic body were prepared. Specimens were fabricated by uniaxial press-molding at 20 MPa and sintered at 1050°C. The following properties were determined: linear shrinkage, water absorption and flexural rupture strength. In general, within the error bar, there was no influence of the waste in the values of water absorption of the clayey ceramic body. The results showed that all investigated formulations used in this work for the production of rustic wall tiles attend the standards for water absorption and mechanical strength.

Abstract: ts well known that CaCu₃Ti₄O₁₂ (CCTO) ceramic presents high dielectric constants, which makes it a strong candidate to be used in microelectronic devices. Several routes were proposed to obtain CCTO crystalline phase, influencing in microstructure and sintering conditions of the ceramics. In this study CCTO powders were produced by a new chemical route, providing reduction on hold time and sintering temperature. Furthermore, the sintering was performed in conventional and microwave oven that produced different microstructures. In this way, the microstructure and dielectric properties of these ceramics were evaluated and compared, showing the higher values of dielectric constant due to lower grain size and reduced copper-rich phase on grain boundary presented by microwave sintered ceramics.