How do you model the changes in the mechanical properties of materials coated in with nano-materials? If I coat abs plastic with amorphous diamond how does that change the sheer, tensile and other strengths of the object?
Diamond like coatings have been used for a long time to improve wear characteristics of materials. The important thing to note is that most of the time, "wear" happens because some local force exceeds the material's intrinsic strength. A coating can act to spread out the force so that there is no local stress concentration - think of it as making "a sharp point a little bit blunter".
Now in the case of very thin coatings (you stated 4 - 6 nm) you will see some benefit, but it will be small. Sometimes, the more significant advantage is the reduction of friction (see for example this article). A nice treatment is given in this paper.
There is no simple answer...