Alumina is not as hard as zirconia, but it is easier to process.

The raw materials for alumina are also more affordable compared to those for zirconia.

For plungers that don't require extremely high precision and smoothness, especially those used in conjunction with metal sleeves, alumina is well-suited due to its wear-resistant, high-temperature-resistant, and corrosion-resistant properties.

Therefore, alumina is more suitable for such applications.