Elíria Maria Jesus Agnolon Pallone

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.

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.

Two-Steps Sintering of Alumina-Zirconia Ceramics

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.

Obtained of Diamond Nanometric Powders Using High Energy Milling for the Production of Alumina-Diamond Nanocomposites

The addition of nanometric particles of a second phase into ceramics matrix is one of the most recent alternatives in the development of materials with high mechanical properties and wear resistance. These nanostructured materials can be defined as systems that have at least one microstructural characteristic of nanometric dimensions (less 100nm). In this work aluminadiamond nanocomposites were produced using diamond nanometric powders obtained by high energy milling. Diamond powder was produced in the SPEX shaker/mill during 6h, with a ball-tomass ratio of 4:1. The crystallite size was 30nm. After the elimination of the Fe deriving of the contamination during the milling, and desaglomeration, this nanometric powder was added in the alumina matrix in the ratio of 5wt%. The powder densification was performed by hot pressing sintering. The obtained nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness, and they have promising characteristics regarding abrasion and wear resistance.

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.

Effect of the Two-Steps Sintering in the Microstructure of Ultrafine Alumina

The control of the heating curve to manipulate microstructure during sintering is a way that has being studied and it presents advantages such as simplicity and economy. In this work, it was studied the sintering in two-steps of a commercial ultrafine alumina. For this, the alumina power was deagglomerated in milling ball and the specimens for sintering were pressed. Sintering was performed in a dilatometer, with constant heating rate of 15°C/min up to 1500°C. By these results, heat treatment temperatures for two-step sintering were defined. The sintering specimens were characterized through the apparent density measures using Archimedes method, the grain size measures using image analysis program and microstructural analysis using a scanning electron microscope. The results showed that the two-step sintering influence in the development of the final microstructure and permit the control of the grain size and density.

Nanocompósitos de Al2O3-SiC sinterizados por "spark plasma sintering" (SPS)

(Alumina-silicon carbide) nanocomposite has been extensivelly studied due to its promising results regarding its mechanical properties. The processing of this material usually involves high cost, once the use of hot pressing is necessary for obtaining dense materials. A more recent alternative for sintering nanocrystalline ceramics is the Spark Plasma Sintering - SPS. In this work alumina powders with 5%vol SiC inclusions were sintered using the SPS method at temperatures varying from 1500 to 1600 °C, using different holding times. The effect of temperature and hold time on density and microstructure was investigated. The best results in microestucture and microhardness measurements were shown at 1500 °C and time of landing of 7 min.

Compósitos de Al2O3/ZrO2 recobertos com hidroxiapatita dopada com íons Ag

Al2O3/ZrO2 composites are indicated to play structural functions inside the organism due to their mechanical qualities, being able to be optimized by means of the attainment of a layer of bioactive material. The biomimetic method considers the attainment of this layer using a synthetic solution that simulates the body fluid. However, this does not prevent that these materials in the body interfere with the defense mechanism, besides influence in the necessary doses of antibiotics. A way of prevention and treatment of bacterial infections is the use of silver salts, in this case being incorporated to the coating. The present work optimized the processing of zirconia-toughened alumina (ZTA) composites with bioactive and bacterial characteristics. The production method, as well as the implant coatings, had been efficient in the studied conditions and the morphology of the bioactive layer on the composite suffered alteration when immersed in the AgNO3 solution, with the formation of silver phosphate.

Synthesis of WC Powders by Reactive Milling

One possible route for the production of nanometric powders is the reactive high-energy milling. For a variety of systems of highly exothermic reactions, the milling can lead to self-sustaining reactions, with the reaction being observed after an induction or ignition time, which produces a temperature increase in the reactants. In this work, WC powder was obtained by reactive high energy-milling, performed in a SPEX 8000 shaker/mill. During milling the highly exothermic displacement reaction of reduction of the WO3 by Mg was performed in presence of carbon to produce WC and MgO. The material to ball mass ratio was fixed in 4:1 and the ignition time of the reaction was determined. In order to characterize the transformations from reactant powders to reaction products, the milling was stopped at given times before, immediately after and after the reaction; the powders obtained were characterized by X-ray diffraction, scanning electron microscopy and specific surface area. Depending on the amount of carbon, W and the W2C were also observed as reaction products. The complete formation of WC was achieved with addition of an excess of carbon.