Ion-beam densification of hydroxyapatite thin films

The goal of this study is to demonstrate the feasibility of using ion irradiation to densify a porous ceramic film. Ion irradiation is a room-temperature process, and thus may be preferable to the use of conventional high temperature sintering, which is typically performed at temperatures in excess of 1000°C. Thin films of the bio-ceramic hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂, were deposited on silicon substrates using a sol-gel technique. The films processed in this study were 600 nm thick and had a density of 36% of fully dense HA, after drying at 620°C for 3 min. The dried films were irradiated with 1 and 2 MeV Si⁺⁺ ions with fluences ranging from 10¹⁴ to 6×10¹⁵ ions/cm². Samples irradiated with the largest fluence reached densities of 83% of that of fully dense HA. Rutherford backscattering spectrometry was used to verify the HA stoichiometry of the films and to obtain the areal density. Scratch testing showed that implantation led to a substantial improvement in scratch resistance. Nano-indentation was also used to characterize the mechanical properties of the films. The hardness was increased by a factor of 15 by the irradiation. X-ray diffraction was used to characterize the crystalline phases present in the film. Ion irradiation caused some decrease in the already small degree of crystallinity of the film. The advantage of ion-implantation over high temperature sintering is shown as no secondary crystalline phases appear after densification.