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Finite-element modeling (FEM) was used to predict the contact stress at which the transition from elastic to plastic deformation occurs in a metallic substrate underneath a hard tribological coating. Using model systems of diamond-like nanocomposite (DLN) coatings on electroformed Ni, NiMn alloy and Inconel 718, friction measurements were made at contact stresses ranging from 540 to 1720 MPa. Cross-sections of wear scars suitable for visualization of friction-induced plastic deformation in the substrate were prepared by focused ion beam microscopy and analyzed by electron backscatter diffraction. At contact stresses below the FEM-predicted elastic–plastic limit, the coefficient of friction decreased linearly with increase in contact stress, suggesting that interfacial shear is the major mechanism of friction in DLN. Contact stresses above the FEM-predicted elastic–plastic limit resulted in plastic deformation of the metallic substrate, and in extreme cases fracture and removal of the coating resulting in a sudden increase in friction. |
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