Fatigue crack growth rate in self-standing plasma sprayed refractory metals

Fatigue crack growth in self standing plasma sprayed Molybddenum and Tungsten beams with artificially introduced notches subjected to pure resonant bending was observed. Beams width, thickness and length was 4 mm, 3 mm and 32 mm respectively. Distributed nature of the fatigue damage was observed with multiple fatigue cracks spanning over a growing volume during the fatigue lifetime. Equivalent crack length was established using the differential compliance method and fatigue crack growth rate was established as a function of equivalent stress intensity factor. Fractographic analysis was performed after the fatigue test. At near threshold range, the fatigue crack propagation was controlled by splat decohesion, leading to crack deflection from the notch plane. At higher values of stress intensity factor, the trans-splat intergranular cracking become more common. The performed experimets show, that the classical fracture mechanics approach is able to describe crack growth in thermally sprayed coatings. The observed crack growth resitance of both coatings was relativelly low, indicating the importance of the underlying substrate and its surface for the fatigue resitance od coated parts.