The effect of incorporation Al2O3-NbC nanopowders to reinforce 3Y-TZP matrix and the influence on mechanical properties of 3YTZP-Al2O3-NbC nanocomposites obtained by conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of Al2O3-NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, and added to the 3Y-TZP matrix in the proportion of 5 vol%. The final powders were dried under airflow, compacted and sintered at the temperature range of 1300-1500 °C. The effect of sintering technique and final temperature on the microstructure and mechanical properties, such as hardness, toughness and Young's modulus were analyzed. An important mechanical value obtained in all materials reinforced with Al2O3-NbC nanopowders is the fracture toughness, which differ significantly of the 3Y-TZP monolithic material (5.2 MPa•m 1/2). The nanocomposites sintered conventionally at 1450°C show the higher fracture toughness (8.7 MPa•m 1/2). Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within 3Y-TZP matrix and limited their grain growth. However, the partial oxidation of the NbC in the surface of the nanocomposites found a limit in the conventional sintering temperature, since the samples sintered at 1500°C showed a reduction in fracture toughness.
Ceramic materials have limited use due to their brittleness. The inclusion of nanosized particles in a ceramic matrix, which are called nanocomposites, and ceramic processing control by controlling the grain size and densification can aid in obtaining ceramic products of greater strength and toughness. Studies showed that the zirconia nanoinclusions in the matrix of alumina favor an increase in mechanical properties by inhibiting the grain growth of the matrix and not by the mechanism of the transformation toughening phase of zirconia. In this work, the microstructural evolution of alumina nanocomposites containing 15% by volume of nanometric zirconia was studied. From the results it was possible to understand the sintering process of these nanocomposites.
High performance ceramic composites have been the subject of frequent studies in recent decades, aiming at improving mechanical properties and increasing their range of applications in technological products. This work consisted in studying the preparation, the conventional and non-conventional sintering and the mechanical properties resulting from two t-ZrO2 matrix composites: the t-ZrO2/Al2O3 system and the t-ZrO2/Al2O3-NbC system. In the t-ZrO2/Al2O3 system, the compositions of 0, 5 and 15% by volume of Al2O3 using commercial powders were studied, while in the t-ZrO2/Al2O3-NbC system, an Al2O3-NbC nanocrystalline powder obtained by high energy reactive milling, deagglomerated, leached in HCl and added in the proportion of 5% by volume to the t-ZrO2 matrix. The obtained powders were uniaxially and isostatically pressed and sintered in conventional furnace and using flash sintering (t-ZrO2/Al2O3) and spark plasma sintering (SPS) (t-ZrO2/Al2O3-NbC). Conventionally sintered t-ZrO2/Al2O3 and conventionally sintered t-ZrO2/Al2O3-NbC composites were characterized by measurements of apparent density, dilatometry, SEM, and mechanical properties: hardness, Young's modulus and fracture toughness. The t-ZrO2/Al2O3 composites sintered by FS were characterized by measurements of apparent density, in situ dilatometry and SEM. t-ZrO2/Al2O3-NbC nanocomposites were also characterized for wear strength by the ball-in-disc method, using Al2O3 and WC-6%Co beads as countermaterials. The results showed that the high energy reactive milling was complete and effective in obtaining nanometric powders of Al2O3-NbC, with crystallite sizes equal to 9.1 and 9.7 nm, for Al2O3 and NbC, respectively. The deagglomeration after high energy reactive milling was effective in reducing the size of agglomerates. Conventionally sintered t-ZrO2/Al2O3 and t-ZrO2/Al2O3-NbC composites and SPS-sintered t-ZrO2/Al2O3-NbC showed high densification (> 97% TD), good dispersion of the inclusions in the matrix and good mechanical properties. The t-ZrO2/Al2O3 nanocomposites sintered by FS presented an ultrafast densification (<1 min) with linear shrinkage superior to the sintered samples in conventional furnace, occurring at temperatures lower than 1000°C, with relative densities higher than 90% TD in some compositions. The t-ZrO2/Al2O3-NbC nanocomposites presented competitive properties between conventionally sintered and SPS-sintered composites with higher hardness and fracture toughness than monolithic t-ZrO2. The wear resistance of these conventionally sintered nanocomposites, however, was markedly higher than those of SPS-sintered ones. The oxidation of NbC in the composites sintered conventionally influenced negatively the properties, leading to the suggestion of a "window" of temperatures in which the sintering of the t-ZrO2/Al2O3-NbC nanocomposite is interesting without the degradation of the mechanical properties. The results allowed concluding that the studied materials present potential for industrial applications that require high mechanical performan...
ResumoA alumina é um material muito estudado devido às suas excelentes características refratárias e propriedades mecânicas. A fase α, mais estável, tem uma temperatura de formação de ~1200 °C. Devido à sua elevada temperatura de formação, muitas pesquisas vêm buscando sua redução através da adição de sementes (seeds) da fase α em processos químicos de síntese. Este trabalho teve como objetivo sintetizar pós de α-alumina pelo método Pechini com adição de sementes e comparar a calcinação ao ar e com um fluxo passante de O 2 , verificando sua influência nas características dos pós obtidos. Tais pós foram caracterizados por difração de raios X, espectroscopia no infravermelho e microscopia eletrônica de varredura. Os resultados foram comparados com os dos mesmos pós calcinados ao ar. A adição de sementes favoreceu a diminuição da temperatura de transformação em fase α, sendo que esta já é identificada a 1000 ºC. A presença de oxigênio na atmosfera de calcinação também favoreceu a formação de fase α a 900 ºC, temperatura inferior à dos pós calcinados ao ar. Palavras-chave: síntese, Pechini, alumina, seeds, oxigênio. Abstract Alumina is a largely studied material due to its excellent refractory characteristics and mechanical properties. The α-phase
The chemical methods allow obtaining powders with high reactivity and chemical homogeneity. This work studied the sintering of Y2O3-stabilized ZrO2 powders produced by polymerization routes. In the three methods used were obtained powders via formation of gels, which were characterized by spectroscopy in the infrared. After the gel calcinations, the powders obtained were characterized by X-ray diffraction and scanning electronic microscopy (SEM). Among the different chemical methods, there were differences in the coordination of the metallic ions, which caused differences in the thermal behavior, reactivity, size and form of the particles of the powders. The powders were pressed and sintered at 1400 and 1600°C for 2 hours. The bodies sintered were characterized by SEM, apparent density and dilatometry. The average size of the particles varied with the method, and the smallest particles were obtained by Pechini method. PCS method showed a strong reactivity of the powder and it was already partially sintered even in the step of elimination of organic substances. Similarly, the sintered compacts presented different properties with each other, and the compacts obtained by PEG/FA method had the best microstructure.
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