Effect of surface treatments on the fatigue life of titanium for biomedical applications

Many surface treatments that are used in cementless and endosseous implants modify the topography and the roughness to increase the implant-bone contact area and thus favor bio-mechanical anchorage, shortening the period of osseointegration. Nevertheless, the effects that the surface treatments can have on the fatigue life of the material are not generally considered. In this sense, the superficial condition of the component is one of the features that affect the fatigue strength, specially the fatigue crack nucleation. The fatigue behaviour of annealed commercially pure titanium grade 4 was studied. The surface treatments used were acid etching, shot blasting and a dual treatment of blasting + acid etching. An as-machined surface condition was used as a reference. Topography, roughness, surface defects, microstructural changes and residual stresses were characterized in each case. Rotating-bending fatigue tests of each surface condition were conducted at room temperature with a frequency of 33 Hz. S-N curves and Basquin equations were obtained based on the results of these tests. Tested samples were also characterized to evaluate fatigue damage. The acid etching decreases the fatigue endurance, while the blasting and blasting + acid etching treatments showed a similar behaviour with respect to the reference condition. For acid etching, the modifications introduced (stress raisers) contributed to accelerate the nucleation of cracks. On the other hand, the treatments with a blasting stage besides generating stress raisers, introduced compressive residual stresses and superficial plastic deformation that tend to improve the fatigue endurance of the material.