Stress-sensitive Fatigue Crack Initiation Mechanisms of Coated Titanium Alloy

The presence of hard coatings can reduce the fatigue life of metal substrates to an unacceptable level, causing the catastrophic failure of load-bearing structures. Unfortunately, the present fatigue crack initiation mechanism is difficult to explain some existing experimental phenomena, and the preventive guidance theory remains absent. Here, by performing tension–tension axial fatigue experiments of TiN coated Ti-6Al-4V alloys, we found that the fatigue crack initiation mechanisms of the coated alloys were stress sensitive, showing two-fold characteristics in S–N curve, which was never reported in previous studies. As the applied stress higher than critical stress, coating fracture induced cleavage cracking of substrate is responsible for the fatigue crack initiation, leading to the fatigue crack source appear at the interface. As the applied stress lower than critical stress, slip step causes coating fracture, forming additional stress concentration on brittle α phase where has already been pressed by dislocations pile-up, thus accelerating fatigue crack initiation at the subsurface. Based on the proposed mechanisms, a ductile chromium interlayer was introduced to decrease coating crack velocity and absorb dislocations escaped from substrate, leading to the increase in the percentage reduction from 40% to 20%. The proposed mechanisms in this work will provide theoretical guidance for designing hard coatings to reduce detrimental effects on metal loading-structure's fatigue life to an acceptable level, even improving substrate's the fatigue properties.