I got into a PhD Materials Engineering program with a BSc Geophysics, so I don't think it's that much harder, although I think the requirements are more relaxed for the materials sciences because they more or less accept most applicants from the physical sciences or engineering, which is a good thing.
Regarding the transition for me, I think that it's not particularly difficult because most of what I studied during undergraduate years is quite similar to what I am doing right now. Eventhough the fields might not seem related here, I just focused on finding that one part that overlaps both fields, and put it into the application.
