Role of friction and loading parameters in four-point bend adhesion measurements

The effects of salient testing parameters on four-point adhesion measurements of thin-film structures on silicon substrates were systematically studied. These included specimen geometry, applied displacement rate, and load point separation. Measured fracture energy values, G_c, were observed to increase as the ratio of applied moment arm to specimen thickness was decreased beyond a value of ∼14, particularly for specimens with G_c > 5 J/m². Testing parameters that affect the steady-state crack velocity were also found to affect reported G_c. values. The resulting trends in G_c, values are shown to be related to loading-point friction and environmentally assisted cracking effects. Good practice testing guidelines are suggested to improve the accuracy and precision of four-point bend measurements.