Coating of Silicon Nitride with Sintering Aid
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COATING OF SILICON NITRIDE WITH SINTERING AID
P. N. JOSHI AND R. A. McCAULEY Rutgers University, Dept. of Ceramic Science and Engineering, Piscataway, NJ 08855-0909 ABSTRACT Commercially available silicon nitride powders were coated with various metal oxides with the help of metal-organic surfactants in an aqueous medium. A surfactants could also act as a dispersant. The coatings were studied by XPS, TEM and accoustophoresis and the mechanism for formation of these coatings is proposed. The amount of sintering aid incorporated was determined by the surface area and surface chemistry of the nitride powder, the pH of the solution, the structure of the surfactant and the surface charge behavior of the powder and the coating. Multicomponent coatings were also possible by precise control of these parameters. The sintering behavior of the coated powders and the properties of the sintered material were promising. INTRODUCTION Most advanced ceramics are formed as powder compacts densified by heat treatment. In spite of being prone to heterogeneities and nonuniform phase distribution, powder processing is still the most efficient method to form ceramics. If reliable ceramics are to be produced, methodologies must be developed to ensure, with a high probability, that the heterogeneities will be eliminated from powders and they will not be reintroduced in subsequent processing steps [1]. Agglomerates are a major heterogeneity stemming from the powder itself. This can be overcome by colloidal processing where repulsive interparticle forces are introduced to break apart weak agglomerates and sedimentation is used to remove hard agglomerates. But segregation of additives can still remain a difficulty and if the additive is a powder, intimate mixing may still be difficult because of the finite size of the additive powder particles [2]. The introduction of sintering aids to silicon nitride powder is thus a crucial step in the fabrication of silicon nitride. These sintering aids react with the oxygen surface layer of silicon nitride to form a high temperature liquid phase that promotes densification and is a necessary requirement for pressureless sintering of silicon nitride. This liquid phase may remain as a glassy phase at grain boundaries and affects the high temperature mechanical properties in a detrimental way [3]. Homogeneous distribution of the sintering aid over the entire micorstructure will allow complete densification with a minimum amount of liquid phase present, because of short diffusion distances. Therefore it is desirable to find powder processing routes that produce green compacts of exceptional homogeneity in order to produce materials with optimum properties. The most common way of adding and mixing the additives into the silicon nitride powder is by ball milling. This may affect the particle size distribution, powder surface characteristics and purity of the system, but will not allow control over the distribution of the additives. Over the past few years some attempts have been made to improve the homogeneity
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