Continuous microscratch measurements of thin film adhesion strengths

The adhesion strengths of metal/ceramic, metal/polymer, and polymer/polymer interfaces have been characterized using the continuous microscratch technique. In these experiments, a conical diamond indenter was driven simultaneously into a thin film at a rate of 15 nm/s and across the film surface at a rate of 0.5 µm/s until a load drop or other discontinuity occurred, indicating film failure. The critical load at failure of the thin film was taken as a measure of the adhesion strength. For metal/ ceramic systems such as Cr thin films on A1₂O₃, and for diamond-like-carbon (DLC) films on glass, clear load drops provided an accurate measure of the adhesion strengths. For metal/polymer systems such as Cu thin films on PET, a change in the loading pattern and periodic cracking events along the scratch track provided evidence of film delamination. For DLC films on polycarbonate substrates, the carbon thin film cracked before it delaminated. For bulk polymers such as polycarbonate and polystyrene/polypropylene, crack growth occurred by a stick-slip mechanism. Using a model developed in an earlier paper, the practical work of adhesion for the Cr/Al₂O₃ system was determined to be 0.09 J/m² and that for the DLC/polycarbonate system was 0.05 J/m². The fracture toughnesses of the polycarbonate and polystyrene were 0.81 and 0.2 MPa m^½, respectively. These numbers are in good agreement with those obtained by other methods for these systems.