High velocity oxygen fuel (HVOF) sprayed stellite coating is often adopted on contact surfaces of the shrouds and stubs of titanium turbine blades, in order to increase fretting fatigue strength and reduce fretting wear. To confirm the effectiveness of the sprayed stellite coating, we conducted friction-type fretting tests originally developed to simulate the load condition of shrouds and stubs, vibratory force was carried only by friction force. We also investigated the fretting fatigue mechanism of coated materials by observing non-propagating cracks in the tested specimens and analyzed crack propagation behavior at the interface using fracture mechanics.
The fretting fatigue tests confirmed that the sprayed stellite coating could double the fretting fatigue strength of Ti-6A1-4V. The fretting fatigue mechanism of Ti-6A1-4V with a sprayed stellite coating was revealed as follows: Final fracture was led by a newly initiated micro-crack at the interface as cracks initiated on the coating surface did not continuously propagate into the substrate but mainly propagated along the interface. The micro-crack propagation at the substrate depends on the length of interface crack. The longer interface crack delays micro-crack propagation to the substrate because the lines of force are less concentrated at the crack tip. Hence, the interface crack makes the fretting fatigue strength of coated Ti-6A1-4V higher than that of uncoated specimens. This finding was supported by the cross-sectional observations of non-propagating cracks in the tested specimens, and by FE analysis which revealed that ΔK of the substrate crack initiated at the interface decreases with an increase in delaminated crack length. Ti-6A1-4V with a sprayed stellite coating offers better wear resistance than uncoated Ti-6A1-4V because the coating’s friction coefficient is lower than that of Ti-6Al-4V, although the wear rates of coated and uncoated materials against the consumption energy per cycle were almost the same.
On the diversity in design for different bending fretting fatigue mechanism