The key factor that determines whether the surface of a grinding ball has a bright shine is the smoothness of the spherical surface. A smoother sphere exhibits a stronger mirror effect, while a rougher sphere shows a diminished reflection. Several factors influence the smoothness of the sphere, primarily the material’s internal structure and the degree of external processing applied to the ball. External processing is relatively straightforward to understand. For example, grinding balls are typically matte after sintering, but polishing them can result in a mirror-like surface.
The internal structure of the material—such as the presence of pores, grain size, and the liquid phase—also plays a crucial role in the surface smoothness of the sphere:
1、Pores
Due to the high density of modern sintered grinding balls, the occurrence of pores from the sintering process is minimal. The pores we refer to here are generally large defects caused by issues during the molding process. For instance, the powders used for forming balls, such as air flow powder, spray-dry powder, or granulation powder, are essentially aggregates of many individual grains. Variations in milling processes result in different particle strengths of the powders, causing some to remain partially uncrushed during use. This prevents the grains from fully fusing during the sintering process, leading to defects like atmospheric pores.
2、Grain Size
Differences in raw material types, grinding processes used by different manufacturers, and variations in formula and firing techniques can lead to differing grain sizes within sintered grinding balls. Finer grains result in a smoother spherical surface, while coarser grains cause more surface unevenness—similar to the difference between paving with sand or stones.
Zirconia grinding balls, for instance, typically have grain sizes ranging from 0.3 µm to 0.5 µm, whereas alumina grinding balls tend to have grain sizes between 1 µm and 5 µm. Zirconia grains, being finer, produce a smoother and shinier sphere compared to alumina.
① Alumina grinding ball ② Zirconia ceramic grinding balls
Grain size: ① ＞ ②
3、Liquid Phase
In the production of alumina ceramics, additives are often used to lower the sintering temperature. Some of these additives generate a liquid phase in high-alumina ceramics. Materials such as kaolin, boromagnesite, and silicon powder form binary, ternary, or more complex low-melting-point eutectics in combination with other additives. When these additives are introduced in the right amounts, they reduce the ceramic sintering temperature, resulting in finer grains. Additionally, the liquid phase behaves like asphalt on a road, smoothing out the spherical surface and enhancing its brightness.
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