Indentation stress-based models to predict fracture properties of brittle thin film on a ductile substrate

• Understand the fracture mechanism in brittle film on a ductile substrate by nanoindentation tests and FE analysis
• Develop two stress-based models to obtain the stress distribution and fracture toughness in brittle films on a ductile substrate
• Conduct an FE analysis using a conical indenter and a spherical indenter to validate the stress based models
• Utilize the stress-based models to measure the fracture toughness of alumina film and DLC film on PEEK substrate

In the present work, nano-indentation experiments were performed on the surface of alumina film and diamond-like carbon (DLC) film on PEEK substrates using an axisymmetric indenter. After indentation, ring cracks were noticed in the film by scanning electron microscope. Stress analysis results indicated that radial tensile stress on the surface of the film outside the contact region of the indenter caused the ring crack. Then, two stress-based models were proposed to predict fracture stress and fracture toughness of brittle thin film on a ductile substrate based on the crack channeling criterion. The models were expressed regarding critical indentation load, ring crack radius, plastic zone radius, and film thickness. The stresses in film predicted by the present models were validated using axisymmetric finite element analysis. Finally, the models were utilized to evaluate fracture toughness of the alumina film and DLC film in experiments.