Delamination analysis of metal–ceramic multilayer coatings subject to nanoindentation

• Indentation-induced cracking at the multilayer coating–substrate interface is analyzed.
• Finite element modeling with cohesive elements is employed to simulate delamination.
• Hardness and measured modulus can be significantly affected by delamination.
• Delamination can actually occur during the unloading phase of the indentation.

Internal damage has been experimentally observed in aluminum (Al)/silicon carbide (SiC) multilayer coatings subject to nanoindentation loading. Post-indentation characterization has identified that delamination at the coating/substrate interface is the most prominent form of damage. In this study the finite element method is employed to study the effect of delamination on indentation-derived hardness and Young's modulus. The model features alternating Al/SiC nanolayers above a silicon (Si) substrate, in consistence with the actual material system used in earlier experiments. Cohesive elements with a traction–separation relationship are used to facilitate delamination along the coating/substrate interface. Delamination is observed numerically to be sensitive to the critical normal and shear stresses that define the cohesive traction–separation behavior. Axial tensile stress below the edge of indentation contact is found to be the largest contributor to damage initiation and evolution. Delamination results in a decrease in both indentation-derived hardness and Young's modulus. A unique finding is that delamination can occur during the unloading process of indentation, depending on the loading condition and critical tractions.