Materials Science Forum Vols. 660-661

Abstract: Clay based ceramic is one of the most used materials in the form of construction blocks, bricks and tiles. Sintering of clay powders is recommended to be carried out above 900°C to benefit from fluxing phase consolidation. However, solid state consolidation of clay bodies made with mixture of powders may occur bellow this temperature by surface diffusion within the nanopores of the layered silicate particles. The objective of this work was to characterize the activation energy for the sintering of plastic clay at temperatures bellow 900°C. Cylindrical samples of sintered kaolinitic clay powders were tested by diametrical compression at 500, 600, 700 and 800°C in the time interval from 5 minutes to 50 hours. The results showed that the value found for the activation energy could be related to the diffusion of the mineral species that compose the clay. Moreover, this activation value corroborates the efficiency of lower sintering temperature for practical fabrication of clay based ceramics.

Abstract: Sintering in two-steps has been applied with success for densification of nanometric ceramic powders without grain growth. Another mechanism that alters the sintering process is the presence of rigid inclusions in the ceramic. In this work it was studied the effect of two-steps sintering and the presence of zirconia inclusions (5% in volume) in the microstructure of a commercial alumina. For this, the powders of alumina and zirconia were desaglomerated in a ball milling and uniaxially pressed at 80 MPa to form cylindrical compacts and isostatically cold pressed at 200 MPa. Temperatures of the steps were chosen starting from the curves of linear rate shrinkage in function of the temperature. The samples were characterized for apparent density, scanning electronic microscopic and mean grain size. The results showed that two-steps sintering and the zirconia inclusions were efficient to control the densification and grain size of alumina.

Abstract: Zirconia is a bioceramic material widely used for dental implants. In this work, the sinterability of nano-crystalline powders has been investigated by dilatometry in the temperature range of 1250 to 1400 0C with isothermal holding times of up to 8h. A slight increase in grain growth and an increasing linear shrinkage have been observed with increasing sintering temperatures. The sintered samples were submitted to Vickers' hardness and KIC tests and the results compared regarding the sintering conditions. It has been verified that satisfactory hardness and fractures toughness have been achieved after sintering above 1300 0C during 8h.

Abstract: The discovery of the superconductivity of MgB2 was of great importance, because this material is one of the few known binary compounds and has one of the highest critical temperatures (39° K). As MgB2 is a granular compound, it is fundamentally important to understand the mechanisms of the interaction of the defects and the crystalline lattice, in addition to the eventual processes involving the grain boundaries that compose the material. In this sense, the mechanical spectroscopy measurements constitute a powerful tool for this study, because through them we can obtain important information about phase transitions, the behavior of interstitial or substitutional elements, dislocations, grain boundaries, diffusion, instabilities, and other imperfections of the lattice. For this paper, the samples were prepared using the PIT method and were characterized by density, X-ray diffraction, scanning electron microscopy, electric resistivity, magnetization, and mechanical spectroscopy. The samples were measured in their as-cast condition and after an ultra-high-vacuum heat treatment. The results showed complex spectra, in which were identified relaxation processes due to dislocation movement, interaction among interstitial elements and dislocations, auto-diffusion, and movement of grain boundaries. Some of these processes disappeared with the heat treatment.

Abstract: Owing to non-stable conditions generated during conventional HPHT sintering of cubic boron nitride (cBN) composites, it is not possible to predict both the yield and quality of the final product. Moreover, in the existing conventional technique, the particle size and amount of binding compounds that are associated with the composite may be responsible for defects and residual stress. In the present work, a novel technique of automatic cycling the pressure and the temperature was applied to the sintering of cBN composites bound with Ti2N and AI. This type of processing permitted to obtain a more uniform structure for the composites with ultra-fine layers of the binding compounds separating dispersed cBN grains. The technique improved the adhesion between the phases and contributed to decrease both the residual stresses and interfacial defects. As a consequence, the productivity related to machining of hard steels using inserts made of cycling HPHT processed cBN composites was increased by 170-180%.

Abstract: It is still in focus the problem of obtaining high quality polycrystalline materials by means of sintering fine diamond powders. The most important task of this problem is the consolidation or improvement of the processed polycrystalline diamond. It is worldwide recognized that the plastic deformation of the diamond particles performs the most important role on the diamond powder consolidation. In some cases, the contact and shear tensions reconstruct the compact structure. In this work the sintering process used a mixture of micro and nanodiamonds. The sintering process was carried out in a toroidal high-pressure device. The effects of nanodiamond addition and sintering conditions on the microstructure and mechanical properties sintered diamond bodies were studied. The sintering parameters were pressure of 6.8 GPa and a temperature of 1850 K, and these conditions were maintained for 1 minute. Homogeneous sintered bodies were obtained, which have a Vickers' hardness over 40 GPa, and fracture toughness around 7.1 – 7.9 MPa.m1/2.

Abstract: One way to develop a new composite material or to improve its performance is to reinforce the matrix with a stronger dispersive phase. In principal, nanodiamonds can be used as reinforcement of composites owing to high surface energy, which allows an effective structure interaction with most utilized matrices. In this work, polycrystalline metallic–based composites and polymeric-based composites were investigated for the effect caused by addition of nanodiamonds. These composites were obtained at high compression pressures and combine with temperatures for certain short processing times. It was found that even small additions of nanodiamonds increase the wear resistance of the investigated composites.

Abstract: This paper presents the results on the strength evaluation of sintered diamond-based composites with Co binder. The method of intensive electrosintering with a short heating pulse was applied by a high pressure of 0.5 GPa and a time of 2-40 s. Composites with different compositions and for different purposes, such as rock drilling and dressing tools, were investigated. The reactive cell was design with the purpose of preserving the diamond properties from the detrimental sintering temperature exposure. The results of this electrosintering technology permitted to obtain improved composites to be used in tools with higher indexes of performance and productivity.

Abstract: This work aims at evaluating the incorporation of eucalyptus firewood ash into clayey ceramic, through the physical and mechanical properties of the produced ceramics. Mixtures of kaolinitic clay from the municipal area of Campos of Goytacazes, in the state of Rio de Janeiro, Brazil, were prepared with additions of 0, 5, 10 and 20% in weight of ashes from eucalyptus firewood. Specimens were 20 MPa uniaxially press-molded and fired at 900°C. The evaluated firing properties were diametrical shrinkage, water absorption and mechanical strength by diametrical compression. The results showed that the incorporation of 10% wt. of firewood ash enhanced the properties of the clayey ceramic and since plasticity was optimised, water absorption decreased while the mechanical strength increased.

Abstract: The porcelains are ceramics whose properties are strongly influenced by the microstructure developed after sintering. This microstructure can be changed, either by changing the composition as in the sintering conditions. New sintering methods, have allowed a greater control of the ceramics properties as a consequence of the final microstructure greater control. Microwave sintering is one of these techniques that have aroused a high interest in ceramics process because this method led to higher densification and the fine microstructure in much shorter time duration compared to conventional procedures. In this work it was studied the porcelain ceramics sintered under microwave and traditional heating conditions. Porcelains disks were prepared under a uniaxial pressure. One set of samples was sintered at in a conventional muffle furnace at range of 1250°C and 1300°C for 2 h. Another set of disks was sintered using a custom-made 2.45 GHz furnace at 1200°C temperature between 15 and 45 minutes. Density and microstructure information were obtained on microwave and convention processed samples by Archimedes principle and SEM (scanning electron microscope, JSM-5510 LV), respectively. Microwave sintering of porcelains promoted densification in much shorter time duration when compared to conventional method. The microstructure was strongly influenced by microwave heating, promoting an abnormal grain growth of the primary mullite.