The role of specimen thickness in the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

Most empirical studies and numerical simulations of fatigue damage accumulation mechanisms for metals with nanoscale grains usually invoke grain boundary-mediated processes. However, our recent studies of ∼500 nm platinum films showed that when the grain morphology is stable in the presence of high stresses and/or elevated temperatures the slip of dislocations is of primary interest. These thin film metals exhibit low fracture toughness and inferior resistance to fatigue crack growth when compared to conventional microscale grained bulk forms of the materials. In this manuscript we explore how sample form (thickness) contributes to the fracture and fatigue crack growth behavior of platinum thin films. When the thickness of the films was doubled to ∼1 μm the tensile yield and ultimate strengths were slightly depressed (yield strength ∼1.3 GPa, ultimate tensile strength ∼1.45 GPa). However, the fracture toughness markedly improved (Kq value 25.4 MPa √m) and the fatigue crack growth resistance was very similar to bulk forms (power law exponent m ≈ 3.9). The progression and type of damage shows that sample form makes a critical and distinct contribution to the fatigue crack growth mechanisms in nanograined metals.