The effect of dissolution on plasma sprayed hydroxylapatite coatings on titanium.

Plasma sprayed hydroxylapatite (HA) coated titanium specimens were immersed into Ca-free Hank's balanced salt solution for periods of 1, 2, 4, and 6 weeks. At each of the respective time intervals the HA coatings were analyzed with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy to determine the effect of dissolution on the structure and composition of the coatings. At the 2- and 6-week intervals, additional samples were removed from solution and shear tested to evaluate the effect of dissolution on the coating-to-substrate interfacial bond strength. XRD results revealed that the plasma sprayed coatings retained their basic apatitic structure throughout the 6-week period in solution. There was, however, a trend towards a more definitive baseline, suggesting a loss of amorphous material by dissolution. FTIR and Raman analyses of the as-sprayed and dissolution specimens showed that the phosphate groups were not lost during the time in solution; however, there was a decrease in hydroxyl content as a result of dissolution mechanisms. FTIR also revealed an increase in carbonate content within the coating during immersion in the simulated physiological balanced salt solution. The average shear bond strengths for the as-sprayed, 2-, and 6-week dissolution specimens were 14.82+/-3.52 MPa, 12.51+/-3.41 MPa, and 12.54+/-2.02 MPa, respectively. Duncan's multiple range test confirmed that the shear bond strength was not significantly reduced after 6 weeks in solution, but there was evidence that suggested a decrease in interfacial bond strength as a result of dissolution mechanisms.