Thank all of you very much for answering my question. I thought Fdmaxey is right. It is implied and emphasized that materials science studies many types of materials - steels, polymers, etc. I found some similiar uses of the word, such as "The Materials Preparation Center" and "National Institute of Materials Phsics" .

Your question is actually a good one, after I thought about it. It is implied that materials science studies many types of material - steel, polymers, etc. It think it is to emphasize this that the word is plural. There is not iron rule why this should have to be so. I talk about a computer science and mechanical engineers - why not material science? I think it is because of the reason I stated above and nobody wants to change it.

I'm not 100% sure of this, but it could possibly be related to avoiding confusion with the other meaning of material: which refers to things that are physical, as opposed to spiritual or abstract. Also, as Eliot said, it is probably also to do with the fact that they are studying materials in the plural form, not just one material. "Materials science" is the most widely accepted name for this field.

Because it is the science of materials. It is not the science of material.