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Abstract
The objective of this research was to determine mechanisms of fatigue failure in porous coated Ti-6Al-4V. First, acoustic emission was employed to detect fatigue crack initiation and analyze fatigue crack propagation. Second, post-sintering treatments, that used hydrogen as a temporary alloying element, were formulated. By testing smooth and porous coated Ti-6Al-4V, each with a range of microstructures, the effects of microstructure and interfacial geometry on fatigue strength of porous coated Ti-6Al-4V were uncoupled.
Acoustic emission can detect incipient fatigue crack extensions of 10$\mu$m, therefore serving as a sensitive warning to material failure. Fatigue of porous coated Ti-6Al-4V is governed by a sequential, multi-mode fracture process of: (i) cracking in the coating, (ii) bead/bead and bead/substrate debonding and (iii) substrate cracking. Because the porous coating is discontinuous, fracture is discontinuous. Therefore, the emission generated is intermittent and the onset of each form of fracture can be detected by changes in acoustic emission event rate.
The microstructures following hydrogen-alloying have $\alpha$-grain sizes less than 1$\mu$m, aspect ratios near unity and discontinuous GB$\alpha$. As a result, the fatigue strength of uncoated hydrogen-alloy treated Ti-6Al-4V is 29-35% greater than the fatigue strength of lamellar Ti-6Al-4V and 15% greater than the fatigue strength of equiaxed Ti-6Al-4V.
The fatigue strength of porous coated Ti-6Al-4V is, however, insensitive to changes in microstructure. The reason is that the notch effect of the surface porosity does not allow the material to take advantage of the superior fatigue crack initiation resistance of a refined $\alpha$-grain size. In other words, sinternecks act as initiated microcracks. Therefore, fatigue of porous coated Ti-6Al-4V is propagation controlled.
These results have two clinical implications. First, since porous coatings act as notches, fatigue crack propagation may occur under compressive loading. Second, porous coated prostheses have fractured in-vivo through some of the same modes observed in-vitro. The detected sequence of in-vitro failure in porous coated Ti-6Al-4V might imply that debonding in-vivo is a precursor to further damage.
