WC/Co coating sprayed via HVOF has been widely used in industries due to its high adhesive strength, good abrasion- and corrosion-resistance. However its fatigue performance after, especially fatigue fracture mechanism, was seldom studied. The fatigue and its fracture mechanism were analyzed at different stress levels. Investigations showed that, precedingly, the cracks in the HVOF coating sinuously extended to the interface during fatigue test and deflected thereat along the interface owing to its lower fracture toughness and elastic modulus than substrate at different stress levels, and finally led to formation of free-standing coatings. However, higher stress level means more delaminations for coatings. Therefore, the cracks in the HVOF coating produced no negative on the fatigue life of substrate. The analysis revealed that only a single main crack initiator, which was 0.2~0.5mm away from the substrate surface, was found in the substrate at low stress levels, and the crack propagated at the subsurface crack initiator to substrate surface. Comparatively, a single main crack initiator, which was nearer to the substrate surface, in substrate was also found accompanied by cracks on substrate surface at higher stress levels, and the higher the stress level was, more cracks on the substrate occurred. While, multicrack initiators took place in substrate combined with cracks on substrate surface, also, plastic deformation and microcracks were observed in substrate. Finally, three fatigue fracture physical modals were constructed to directly explicate the fracture mechanism of thermally spayed low alloy steel at different stress levels.
Keywords: fatigue fracture mechanism HVOF coating