Roberto Tomasi

Reaction sintering of titanium carbide and titanium silicide prepared by high-energy milling

Powders of titanium, silicon and carbon black with a molar ratio of 3:1:2 were milled in a high-energy ball mill in order to produce titanium silicon carbide (Ti3SiC2). The milling process initiated an exothermic reaction and the starting material was completely converted into a mixture of mainly titanium carbide (TiC) and minor amounts of TiSi2 ,T i 5Si3Cx and Ti3SiC2. Using off-stoichiometric mixtures or SiC as a source of silicon did not affect the amount of TiC in the product. To obtain Ti3SiC2 the as-milled powders were pressed to compacts and sintered for 2 h. The thermal treatment caused the formation of large amounts of Ti3SiC2. Using a slight excess of silicon in the range of 10–30%, pure samples of the ternary phase could be obtained.

Synthesis of Al2O3–NbC by reactive milling and production of nanocomposites

Reactive high-energy milling can lead to self-sustaining reactions in the synthesis of a variety of systems, with the reaction being observed after an induction or ignition time which produces a temperature increase in the reactants. The products of such reactions are usually very strong agglomerates which, would be not very useful for further processing; to obtain fully dense sintered samples, it would be necessary an additional and difficult desagglomeration procedure. Therefore, the knowledge and control of the mechanism during reactive high-energy milling may be important in preventing or reducing this agglomeration and improve the physical properties of the powder reaction products. In the present work, high-energy ball milling of the powder mixture Nb2O5–Al–C was performed in a SPEX 8000 shaker/mill. The reaction was monitored by a thermocouple fixed in the external surface of the vial. With the condition of ball to mass ratio of 4:1 ignition occurs after 190 min milling. Powder mixtures were characterized after different milling times before and after the reaction. Also, alumina powder was added as diluent to the reactant mixture aiming to decrease the reaction temperature. The powders were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The reaction products after 1 h were desagglomerated and mixed with commercial ultra-fine alumina powder, producing alumina matrix nanocomposites with 5 wt.% of NbC.

Characterization of high-energy milled alumina powders

Tem sido amplamente divulgada a utilizacao da moagem reativa de alta energia para a sintese de pos cerâmicos de oxidos de metais, carbetos, boretos, nitretos ou misturas de compostos cerâmicos ou compostos cerâmicos e metalicos. Neste trabalho, a moagem de alta energia (nao reativa) foi utilizada para a reducao de pos de alumina para particulas de dimensoes nanometricas. As caracteristicas cerâmicas dos pos obtidos foram analisadas a partir de resultados de comportamento durante a desaglomeracao, curvas de densificacao, sinterizacao e caracterizacao de microestrutura. Observou-se que a moagem de alta energia tem forte efeito de aglomeracao dos pos com particulas em dimensoes nanometricas. Esse efeito e explicado pelo impacto de alta energia das bolas, os quais podem fraturar as particulas ou apenas causar a compactacao das mesmas. Nesse ultimo caso, que sempre ocorre, sao formados aglomerados de alta resistencia. O aumento da area superficial do po produz aglomerados mais resistentes.

Spark plasma sintering (SPS) de nanocompósitos de Al2O3-ZrO2

A recent alternative to sintering nanometric ceramics is Spark Plasma Sintering - SPS. This process permits the sintering at lower temperatures and short times producing materials with density near the theoretical density with small grain sizes. In this work alumina powder with well dispersed 5%vol addition of nanometric zirconia inclusions were obtained and sintered using SPS method by heating to temperatures ranging from 1250 to 1400 oC and different holding times were applied to determine the best condition for obtaining dense material with minimal grain growth. The samples were characterized by apparent density measurement, high-resolution SEM, and microhardness. The results show microstructural evolution for different sintering temperatures and holding times and the effect of inclusions on the alumina matrix grain growth, which is related to the results of densification under SPS conditions. Using the SPS method it was possible to obtain samples with full density at 1300 oC and holding time of 2 min with homogeneous microstructure, and microhardness near 22 GPa.

Mechanisms of Microstructure Control in Conventional Sintering

The manner and mechanisms involved on the sintering process are essential investigation to achieve the required microstructure and final properties in solids. During the conventional sintering of a compacted powder, densification and grain growth occur simultaneously through atomic diffusion mechanisms. Many researchers have been working on reducing the grain size below 1 μm aiming to improve some properties, such as strength, toughness and wear resistance in ceramics (Greer, 1998; Inoue & Masumoto, 1993; Morris, 1998). In order to obtain ultra-fine ceramic microstructures, nanocrystalline powders can be used. Although the sinterability of nanoparticles is superior to that of fine particles due to the higher sintering stress, densification of these powders is often accompanied by grain growth (Suryanarayana, 1995).

Dispersão de nanopartículas de ZrO2 visando produção de nanocompósitos de ZrO2 em matriz de Al2O3

Nanostructured materials systems have at least one microstructural characteristic with nanometric dimensions (up to 150 nm). The interest on nanocomposites with ceramic matrix can be associated to the improvement of mechanical properties and wear resistance. Dispersion of the small quantities of zirconia provides at least one benefit: the inhibition of grain growth during sintering. One problem related to the dispersion process is the tendency of powder agglomeration. In this work different dispersion procedures of nanometric powders of zirconia in the alumina matrix were investigated. These powders were dispersed (1, 3 and 5% vol) in an alumina matrix. The samples obtained were uniaxial and isostatic pressed, sintered and characterized by measurements of their physical and microstructural properties. The results showed that the improved dispersion of the zirconia particles leads a refinement of the microstructure and densification upon pressureless sintering.

Effects of the Pin-on-Disc Parameters on the Wear of Alumina

There is a growing interest in the application of ceramic as high wear resistance materials due to the unique properties. Although brittleness and low toughness, recent improvements in alumina processing have lessen this restrictions and suggest improvements in wear resistance which in turn have driven some research on this. Since a direct comparison between the many published works regarding this wear improvement is complicated due to the fact that wear resistance is a response of the microstructure, material and testing condition, the work herein presented aims to first do a literature review on the main parameters to be controlled in a pin-on-disc apparatus on the wear of alumina and then discuss preliminary test results and analyze the influence of critical parameters as load and sliding speed in a pin-on-disc wear test in a dense and sub micrometer grain size alumina.

X-ray photoemission spectroscopy and secondary-ion mass spectroscopy applied to the compositional study of pre-colonial pottery from Pantanal, Brazil

X-ray photoemission spectroscopy (XPS) and time-of-flight (TOF) secondary-ion mass spectroscopy (SIMS) have been applied to investigate potsherd samples from Pantanal, Brazil. One of the potsherds presented burnt bone as an additive, which was characterized by XPS as carbonate hydroxyapatite. For shell-tempered ceramics the phase present in shells after firing was identified by X-ray diffraction. TOF-SIMS was used to study the distribution of quartz as temper in one of the sherds. XPS was also applied to the characterization of the finishing external layer of the ceramic vessels. In this case, based on the Fe 2p spectra of the sherds interior and outermost layers, it was possible to prove that their difference in coloration is due to black heart formation. Hierarchical clustering analysis and principal component analysis of the XPS elemental data enabled the potsherds to be classified with respect to their clay composition.

Densification of Reactive Milled Al2O3-TiC Composite Powders

The synthesis of Al 2 O 3 - TiC composite powder can be achieved by reactive milling of powder mixtures of TiO 2 , Al and C. In this case the reactive milling occurs through a sudden highly exothermic self-sustaining reaction started after a certain milling time. Due to the high temperatures reached during the reaction, the products are formed as strong agglomerates. For the synthesis of the Al 2 O 3 + TiC, probably due the high melting point of the reaction products, particularly of the TiC, these agglomerates are formed by crystallites in the nanometric range. In the present work, Al 2 O 3 - TiC composite was synthesized by reactive milling in a shaker/mill apparatus, followed by a deagglomeration processing. The products of reactive milling were mixed with a commercial ultra-fine alumina powder to produce alumina matrix composites with 5 wt% of TiC. The powder densification was performed by hot pressing and by pressureless sintering between 1400 and 1600 °C. We report on the microstructure characterization of the dense TiC+Al 2 O 3 composites by SEM and TEM. It was observed that TiC is dispersed in the alumina matrix microstructure as both agglomerated inclusions and also dispersed nanometric inclusions. Vickers Hardness measurements of the sintered composites indicated values of about 13.0GPa.

Al2O3-Ni3Al Composites Obtained by Reactive Milling and Reactive Sintering

Ceramic-intermetallic composites, especially Al 2 O 3 -Ni 3 Al, can exhibit good values of mechanical properties, mainly associated with wear resistance. Fine and homogeneous precursors powders to produce Al 2 O 3 -Ni 3 Al can be obtained through the reaction 3NiO + 3Al →Ni 3 Al + Al 2 O 3 during high-energy ball milling processing. If there is no control, the reaction can occur in a SHS (self-sustaining high temperature synthesis) type. In this work, we report on the effect of the amount of Al 2 O 3 which was added as diluent in the mixture of the reactants, and also on the effect of milling time, to result in the final products. We have observed that there is a critical amount of diluent that inhibits the reaction to proceed in the SHS manner. For this reaction, the limit amount of alumina which keeps the reaction still in the SHS type was observed to be 0.7 moles. Using concentrations of alumina above this critical value, the reaction proceeds gradually, and if desired it may be further completed during sintering of the milled powders. The phase evolution during milling and sintering was studied by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. The results are discussed in terms of the occurrence or not of the SHS reaction during milling, the fine structure of the precursors powders when the reaction is gradual and the microstructure of the sintered samples.