Evaluation of adhesion strength and toughness of fluoridated hydroxyapatite coatings

Adhesion strength and fracture toughness are two crucial mechanical properties for bioceramic coatings on metal implants directly affecting successful implantation and long-term stability. In this study, the adhesion strength of sol-gel derived FHA coatings on Ti6Al4V substrates was measured by pull-out tensile test, and the toughness was assessed by energy release method. With increase of the degree of fluoridation, the adhesion strength increases up to about 40% and the fracture toughness increases about 200 to 300%. Contrary to the wide-spread belief, it is interesting to note that after soaking in the Tris-buffered physiological saline solution (for 21 days), the adhesion strength increases about 60% as compared with the as-deposited coating, instead of decreasing. The mechanism of the increase is discussed.