Nanoindentation induced thin film fracture

Kevin R. Morasch

Nanoindentation was utilized to induce fracture of brittle thin oxide films on compliant substrates. During the indentation, a load is applied and the penetration depth is continuously measured. A sudden discontinuity, indicative of film fracture, was observed upon the loading portion of the load-depth curve. The mechanical properties of thermally grown oxide films on various substrates were calculated using two different numerical methods. The first method utilized a plate bending approach by modeling the thin film as an axisymmetric circular plate on a compliant foundation. The second method measured the applied energy for fracture. The crack extension force and applied stress intensity at fracture was then determined from the energy measurements. The plate bending approach estimated the applied radial stress for fracture. The 63 nm thick films were determined to have ultimate strengths between 4.8 and 8.9 GPa. The ultimate stress is a superposition of the bending and membrane stress. A stress intensity at fracture for each of the films was developed by approximating the resulting bending moment and various cracks sizes. At a constant ratio of crack size to oxide thickness of 0.3, the applied stress intensity at fracture of these aluminum oxide films were between 0.46 and 1.20 MPam½. The residual stress in the films was assumed to be negligible in the stress intensity calculations. The energies were calculated from integrating the resulting load-depth curves from indentation. The total energy applied to the system is a superposition of the energy to deform the substrate and the energy to fracture the film. The applied energy to deform the compliant substrate was separated from the energy applied to the film/substrate system resulting in the energy to fracture the film. The energy for fracture was then converted to a crack extension force and a stress intensity using linear elastic fracture mechanics. The crack extension force and stress intensity at fracture for the thermally grown oxide films on aluminum substrates ranged from 0.32 to 1.67 J/m2 and 0.37 to 0.83 MPam1/2, respectively. The titanium oxide had an average crack extension force and stress intensity at fracture of 19.8 J/m2 and 2.51 MPam1/2, respectively.

Nanoindentation induced thin film fracture

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