Interpenetration of solid

The other option would be to use a bonded interface - that would prevent interpenetration, but would also prevent separation.

In some other analysis package there's a sort of 'tied' interface, which keeps parts in contant but allows them to slide, but I'm not aware of anything equivalent in Mechanica.

By definition, if you want surfaces to be able to separate but not penetrate, that's a non-linearity...