Transmission electron microscopy investigation of pulsed-laser deposited hydroxylapatite thin films prepared by tripod and focused ion beam techniques

Hydroxylapatite-based ceramics are largely applied as coatings and thin films on metallic components of prostheses for inducing osteoblasts apposition and subsequent bone re-growth. We report for the first time the focused ion beam technique (FIB) as a milling method to prepare specimens for transmission electron microscopy investigations of HA thin films grown on metallic substrates. New FIB technique results were compared to those obtained by the thinning method of mechanical polishing with tripod. HA films were obtained by pulsed laser deposition on Si wafers and polished Ti–5Al–2.5Fe substrates coated with TiN buffer layers. A KrF* excimer laser (λ=248 nm, τFWHM≥20) was used for deposition. HA films were grown in vacuum (10−4 Pa) on room temperature substrates, subsequently annealed at 550 °C in ambient air for 1 h. Processed structures were investigated by transmission electron microscopy (TEM), grazing incidence X-ray diffraction, selected area electron diffraction, and energy dispersive X-ray spectroscopy. We show that both tripod polishing and focused ion beam milling techniques well preserve the structural and chemical integrity of the thin layers, and their interfaces. Additionally, focused ion beam milling allows for preparation of long specimens (approx. 12 μm in our case). HA films were stoichiometric and had a nano-crystalline structure with an average grain size of 35 nm. Cross-section TEM patterns show a granular growth morphology of the films. Particulates trapped within the film were visualized. High resolution TEM analysis shows that the transition from cubic TiN to hexagonal HA is intermediated by a very thin amorphous calcium phosphate interlayer of ∼3–4 nm thickness.