What Is a Sintering Aid?
A sintering aid is an additive used to enhance and stabilize the sintering process of metal and ceramic powders. These aids usually have a lower melting point than the primary material being sintered.
Sintering aids are carefully added to the powders in amounts that don’t compromise the material’s properties. They are particularly useful for sintering materials that are otherwise difficult to sinter or to improve density and strength post-sintering. Commonly used sintering aids include boron, carbon, alkaline earth metals, and rare earth oxides, which are particularly effective for ceramic sintering, enhancing densification and thermal conductivity.
Uses of Sintering Aids
Sintering aids are extensively used in the production of ceramics. Metals with high melting points and ceramics that are unfeasible to produce by conventional methods like cutting or melting are manufactured through sintering. For materials like silicon carbide and silicon nitride, which are challenging to sinter, sintering aids help facilitate densification. They melt at lower temperatures during the heating process, aiding the compacting of the powder.
In the case of fine ceramics, which are even more challenging to sinter, smaller particle sizes and additives like boron and carbon are used. Silicon beryllium nitride is sometimes employed as a sintering aid in silicon nitride ceramics production. Additionally, some ceramic household items are made by mixing ceramic clay with sintering aids and sintering them in a microwave oven.
Principle of Sintering Aids
In the sintering process, raw materials are first pulverized and mixed, with sintering aids added at this stage if needed. The powder is then placed in a mold and pressurized to form the desired shape. Wax or similar materials may be added for easier molding. To ensure even density, particularly with fine powders, a molding aid might be used to create granular material for molding.
Before high-temperature sintering, any organic matter added during molding is slowly heated and removed. For oxide ceramics like alumina and zirconia, most sintering occurs in air, while metals and non-oxide ceramics like silicon nitride are sintered in inert gas or vacuum environments to prevent oxidation.
Sintering involves heating the powder compact, causing raw material particles to bond, reducing voids, and solidifying the structure. Diffusion is a key mechanism in this process, especially for solid metals and ceramics. Materials like SiC and Si3N4 require sintering aids for efficient densification, as high grain boundary energy can hinder sintering. Sintering aids reduce this intergranular energy, enhancing the process. Additionally, low-temperature sintering is crucial for high-temperature structural ceramics to prevent strength loss due to particle growth at high temperatures.