Excimer laser deposition of hydroxyapatite thin films

Hydroxyapatite coatings are widely used in biomedical applications due to their biocompatibility and osteoconductivity. Traditional deposition methods often require high temperatures, which can lead to decomposition or phase instability in the final product. Prior research has shown that high-temperature processing can cause phosphorus loss and structural degradation in HA films. This gap motivated the development of alternative deposition techniques that maintain stoichiometry and crystallinity. A key challenge remains the control of phase stability during deposition. Conventional methods lack precise control over process parameters like chamber pressure and water vapour content. The need for a low-temperature, stable HA coating fabrication method is well recognized. This paper introduces a novel approach using excimer laser ablation to address these limitations.

The aim of this study was to develop a new excimer laser deposition method for hydroxyapatite thin films that preserves stoichiometry and crystallinity at lower temperatures. The specific problem addressed is the decomposition of HA coatings during high-temperature processing. The motivation stems from the need for stable, adherent coatings in biomedical applications. The researchers propose using a pulsed excimer laser to ablate a stoichiometric HA target. This approach allows for precise control over deposition parameters. The study focuses on optimizing conditions such as substrate temperature and chamber pressure. The goal is to produce fully crystalline, single-phase HA coatings. The method also aims to prevent phosphorus loss during deposition.

The excimer laser deposition process used a 248 nm wavelength laser with a 25 x 10^-9 s pulse duration. A dense HA target was prepared via coprecipitation, cold pressing, and sintering at 1200°C. The laser ablated the target in a controlled chamber environment. Substrate temperatures were varied between 500 and 600°C to assess their effect on coating composition. Chamber pressure and water vapour presence were also adjusted as process variables. Rutherford backscattering spectrometry confirmed elemental composition. Energy dispersive X-ray analysis and X-ray diffraction assessed crystallinity and phase structure. Transmission and scanning electron microscopy evaluated film morphology and adhesion. Qualitative scratch tests measured film adhesion strength.

The excimer laser deposition produced dense, adherent HA coatings with a Ca:P ratio of 1.65. The coatings exhibited a fully crystalline single-phase structure under optimal conditions. Substrate temperatures above 600°C led to phosphorus-deficient coatings due to re-evaporation. Chamber pressure and water vapour content significantly influenced phase stability. Rutherford backscattering spectrometry confirmed the nominal Ca:P ratio. X-ray diffraction showed no secondary phases in the coatings. Transmission electron microscopy revealed uniform film morphology. Scanning electron microscopy confirmed coating density and adherence. The process successfully avoided HA decomposition by maintaining low temperatures.

The excimer laser deposition method successfully produced dense, single-phase HA coatings at lower temperatures. The process preserved the Ca:P ratio and avoided decomposition typically seen at higher temperatures. Chamber pressure and water vapour content were key factors in stabilizing HA phases. The method offers advantages over traditional high-temperature deposition techniques. The researchers propose that this approach improves coating quality and adhesion. The findings suggest that process parameters can be optimized for specific biomedical applications. The method's ability to control phase stability is a notable contribution. The authors state that this technique provides a reliable alternative for HA coating fabrication.