Abstract
Silicon nitride based ceramics have been widely used as structural ceramics, due mainly to their thermo-mechanical properties such as high density, high thermal shock resistance, corrosion resistance and chemical stability. The aim of this study was to determine the influence of rare earth and aluminum oxide additions as sintering aids on densification, microstructure and mechanical properties of silicon nitride. Silicon nitride mixtures with 91 wt. (%) Si3N4 and 9% wt. (%) additives were prepared and sintered. The density, microstructure and mechanical properties of the sintered specimens of these mixtures were determined. In most specimens, scanning electron microscopic examination and X ray diffraction analysis revealed elongated grains of β-Si3N4 with aspect ratio of about 2.0 and dispersed in a glassy phase. The density of the sintered specimens was higher than 94% of the theoretical density (td) and specimens with La2O3 and Al2O3 additions exhibited the highest value. The results of this investigation indicate that the rare earth ion size influences densification of silicon nitride, but this correlation was not observed in specimens containing two different rare earth oxides. The hardness values varied in direct proportion to the density of the specimens and the fracture toughness values were influenced by the composition of the intergranular glassy phase.
Highlights
Silicon nitride based ceramics have been used as structural materials in a wide range of applications due to their high hardness, high heat resistance, high fracture toughness and excellent wear resistance
The results obtained with samples containing alumina and a single rare earth oxide (SN-Y, SN-La, SN-Nd, SN-Dy and SN-Yb) suggest that the density values tend to increase with increasing ionic radius of the rare earth
Our results demonstrate the influence of intergranular phase composition on fracture toughness of silicon nitride-based ceramics
Summary
Silicon nitride based ceramics have been used as structural materials in a wide range of applications due to their high hardness, high heat resistance, high fracture toughness and excellent wear resistance. Even though silicon nitride ceramics have found a variety of applications, a number of issues still remain to be addressed for it to reach its full potential This includes optimization of processing routes, choice of additives and sintering parameters.[4,5,6,7,8]. The type and total amount of additives used can be adjusted to optimize the process and to increase the desired properties of the final silicon nitride-based ceramics, depending on the application. These modifications render controlled microstructures that are composed of elongated grains. It has been mentioned that the glassy phase deteriorates high temperature creep and strength of the ceramics[19]
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