I am always happy to learn something new, and I am not recommending removing
sources from smoke detectors for use in student labs, but I don't think that
americium easily flakes off of its foil. However, I don't
know exactly how the foils are manufactured so will be happy to defer to an
expert. Taking Mark's scenario of abuse of the foil one step further (by
using sandpaper for example) would render the point moot, however.
I do know that NRC regulations require that each foil be wiped tested (dry
filter paper with moderate finger pressure) and rejected if a detectable
removable radioactivity is discovered (< 0.005 microcuries).