Fatigue behavior of freestanding nickel-molybdenum-tungsten thin films with high-density planar faults

Abstract

This research addresses the fatigue behavior of freestanding nickel-molybdenum-tungsten (Ni-Mo-W) thin films with high-density planar faults. The as-deposited Ni-Mo-W thin films demonstrate an unprecedented fatigue life, withstanding over a million cycles at a Goodman stress amplitude (Sa,Goodman) of 2190 MPa - nearly 80% of the tensile strength. The texture, columnar grain width, planar fault configuration (spacing and orientation), and tensile strength were unchanged after annealing at 500 degrees C for 24 hours, and the film endured over 2 x 105 cycles at Sa,Goodman of 1050 MPa. The fatigue life of annealed Ni-Mo-W thin films is comparable to those of nanocrystalline Ni-based alloys, but has deteriorated significantly compared to that of the as-deposited films. The high fatigue strength of Ni-Mo-W thin films is ascribed to extremely dense planar faults suppressing fatigue crack initiation, and planar fault-dislocation interaction and grain boundary plasticity are proposed as mechanisms responsible for the fatigue failure. Provisionally the latter is a more convincing account of the experimental results, in which changes in the grain boundary characteristics after annealing cause higher susceptibility to stress concentration during cyclic loading. The fatigue behavior revealed in this work consolidates the thermal and mechanical reliability of Ni-Mo-W thin films for potential nano-structural applications.