Influence of Fields on Grain Boundary Mobility in Alumina

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The influence of carbon on the microstructure of sintered alumina

Alumina is one of the most used ceramic materials, and as such understanding its sintering and densification processes is important. It is known that the sintering behavior is strongly affected by dopants, such as MgO, which promotes sintering and limits grain growth. The present study focuses on the influence of carbon on the sintering of alumina. Commercial ready-to-press (RTP) alumina specimens were sintered to full density (98%) at 1600°C for 2 h in air and in a graphite furnace using flowing He, a reducing atmosphere. The specimens sintered in graphite furnace resulted in a black color, an indication of the high carbon content originating from the large amount of organic compounds in RTP powders. In comparison, alumina specimens sintered in air, where the carbon decomposes during sintering at elevated temperatures, were white. Sintering with carbon in under He resulted in specimens with a finer microstructure. The presence of carbon retards grain growth, most probably by solute drag. A uniform segregation of carbon to the grain boundaries of alumina was shown by atom probe tomography [1]. In order to evaluate the wear resistance of the sintered alumina, the time (normalized by area) to section specimens with a diamond wafer blade was determined. The time to section specimens containing carbon was more than 40 times longer compared to the specimens sintered in air. The combination of reducing atmosphere and high carbon content has a positive effect on the microstructure and mechanical properties of alumina. [1] Marquis, E. A., Yahya, N. A., Larson, D. J., Miller, M. K. & Todd, R. I. Probing the improbable: imaging C atoms in alumina. Mater. Today 13, 34–36 (2010)

Solute-drag vs solute-acceleration during microstructural evolution of alumina

The role of dopants in processing ceramics has been an important issue for many years, especially given the contradicting reports of retarded or accelerated grain growth by key dopants and impurities. To fully understand these effects in terms of solute segregation versus enhanced grain boundary (GB) mobility due to liquid phase formation, an effort has been made to quantify the high temperature (1600°C) solubility limit of Mg, Ca, and Si in alumina. Using these values, samples doped below the solubility limit with Mg, or Ca, or C, or co-doped with Mg and Ca were prepared at concentrations which were measured using fully standardized wavelength dispersive spectroscopy. Measurements of GB mobility as a function of measured dopant concentration below the solubility limit has shown that Mg and carbon indeed retard GB mobility by solute-drag. However, Ca impurities increase the GB mobility of alumina at dopant values below the solubility limit (i.e. without forming liquid phases at the grain boundaries or triple junctions). The segregating dopants are associated with 2-D structural and compositional transitions at the GBs, and possible changes in the mechanism of GB migration. This presentation will review recent GB mobility measurements and the concept of 2-D GB transitions and their potential role on the mechanism of GB motion

Calcium and the elongated grain shape of alumina

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A review of wetting versus adsorption, complexions, and related phenomena: the rosetta stone of wetting

This paper reviews the fundamental concepts and the terminology of wetting. In particular, it focuses on high temperature wetting phenomena of primary interest to materials scientists. We have chosen to split this review into two sections: one related to macroscopic (continuum) definitions and the other to a microscopic (or atomistic) approach, where the role of chemistry and structure of interfaces and free surfaces on wetting phenomena are addressed. A great deal of attention has been placed on thermodynamics. This allows clarification of many important features, including the state of equilibrium between phases, the kinetics of equilibration, triple lines, hysteresis, adsorption (segregation) and the concept of complexions, intergranular films, prewetting, bulk phase transitions versus “interface transitions”, liquid versus solid wetting, and wetting versus dewetting.Seventh Framework Programme (European Commission) (Grant FP7-NMP-2009-CSA-23348-MACAN