Abstract
In this paper, the influence of the starting powder characteristics for five commercial Si3N4 powders on sintering behavior, microstructure and mechanical properties of hot-pressed Si3N4 ceramics with and without the addition of La2O3-Yb2O3-MgO sintering additives was investigated. For the system without sintering additives, the high amount of the impurities in the starting powders could facilitate the densification process and promote ?-Si3N4 grain growth. However, large whisker-like particles (? phase) present in the initial powders inhibited the sintering process, which led to a lower density of Si3N4 ceramics. On the other hand, when the sintering additives were introduced in the beginning stage of the powder processing step, the effects of impurities content and particle morphology in the initial powders on the densification and microstructure were not apparent. There was tendency that finer powder size resulted in finer microstructure. However, a high ?-phase ratio in the initial powders could accelerate the abnormal grain growth and thus lead to better mechanical properties.
Highlights
Silicon nitride (Si3N4) ceramic has been one of the most promising materials for applications as high-temperature structural components, which is due to its good mechanical properties at high temperatures, superior thermal shock resistance, wear and corrosion resistance
It is known that the powder characteristics of initial powders employed may have a significant influence on the densification behavior and microstructure of Si3N4 ceramics
The observed difference in the sintered densities might be attributed to impurities content, i.e., A12O3, Fe2O3, CaO and particles morphology in the initial powders
Summary
Silicon nitride (Si3N4) ceramic has been one of the most promising materials for applications as high-temperature structural components (e.g., engine hot-section parts components, cutting tools, and bearing balls etc.), which is due to its good mechanical properties at high temperatures, superior thermal shock resistance, wear and corrosion resistance. It is known that the powder characteristics of initial powders employed may have a significant influence on the densification behavior and microstructure of Si3N4 ceramics. Reported that the metallic impurities, morphology, and size distribution of the Si3N4 particles had significant impact on the densification behavior of four kinds of powder using gas pressure sintering [1]. Wötting etc showed that the densification degree of Si3N4 strongly depend on the particle size, oxygen and carbon content in pressure-less sintering [2]. Tanaka reported that a few hundred PPM impurities (Fe, Ca, Al) were sufficient to achieve full densification by HIP without sintering additives [4]. Many studies investigated the effects of the sintering additives on the microstructure and mechanical
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