Key Requirements for Ceramic Sintering Crucibles
Sintering is a central process in the ceramic manufacturing cycle. It refers to the so-called “firing” stage where the compressed, non-sintered powder, as a cast or pressed green body (mold), is heated at high temperatures (below the melting point). As a result, the powdered raw materials fuse (bind) together, forming a solid, densified ceramic body.
An integral component of ceramic manufacturing is the custom-shaped sintering crucible that holds the green body. Such crucibles vary in shape depending on the final, sintered ceramic product. It can include products like thin plates, flanges, columnar sleeves and tubes, spools, and washers.
Considering such extreme environments in which sintering occurs, the ceramic sintering crucibles used for high-temperature manufacturing of ceramics must uncompromisingly fulfil the following requirements [1, 2, 3].
- High melting temperature
The sintering temperatures can reach up to 2000C and even higher. Therefore, the sintering crucible must have a high melting temperature, desirably above the operating temperature during sintering. Moreover, it should possess high structural stability at elevated temperatures. For instance, the structural stability is crucial in applications such as the manufacturing of substrate sheets for LEDs to avoid the warping of crucibles.
- High thermal conductivity and thermal shock resistance
A fast and uniform heat transfer across the sintering crucible to the green body is desirable. The sintering process experiences short cycles of rapid heating and cooling rates. Therefore, an uneven heat transfer can result in different expansions along different directions due to transient temperature gradients across the crucible. Such differential expansion builds up thermal-induced stresses (thermal shocks) in the crucible. Besides distorting the finished product, these stresses can also result in internal cracks, making the material susceptible to fracture. The thermal stresses in ceramics, identified in 1838, pose a big challenge for sintering crucibles .
- Low thermal expansion
Materials characteristically tend to expand when heated. The rate of expansion with temperature is quantified as the coefficient of thermal expansion. A low thermal expansion ensures that the sintering crucible does not expand its volume noticeably during the heating cycle, thereby yielding minimal, acceptable changes in the shape of the ceramic being sintered.
- High chemical resistivity
The sintering crucible should be chemically inert with the sintered material it is holding, i.e., it should not chemically react with it to avoid contamination. It should also be chemically stable against corrosion and oxidation at high temperatures. The degradation of crucible due to the formation and growth of the oxide layer at high temperatures could be a limiting factor in ceramic manufacturing .
- Excellent lubricity
A low coefficient of friction (high lubrication) is desirable to avoid sticking of sintered green body to the crucible. Additionally, most ceramics shrink by as much as 20% under sintering conditions, and uneven shrinkage can pose a problem. Having high lubricity ensures uniform part shrinkage and a smooth surface finish of the manufactured ceramic and minimizes the post-production clean up and scrap rate.
- High wear resistance
The crucible should possess a low wear rate even at high temperatures in order to be used multiple times without compromising the surface finish or shape of the final product.
- Ease of machinability
For the manufacturing of complex-shaped ceramics such as washers, spools, etc., the sintering crucibles should be easily machinable to produce complex prototypes.
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4) Kingery, W. D., Journal of The American Ceramic Society, Vol. 38 (1) 1955.
5) Sohn, H. Y. and Sridhar, S. “Descriptions of high-temperature metallurgical processes”, Fundamentals of Metallurgy, Woodhead Publishing, 2005.