G. Sardin

Mechanical properties of calcium phosphate coatings deposited by laser ablation

Amorphous calcium phosphate and crystalline hydroxyapatite coatings with different morphologies were deposited onto Ti-6Al-4V substrates by means of the laser ablation technique. The strength of adhesion of the coatings to the substrate and their mode of fracture were evaluated through the scratch test technique and scanning electron microscopy. The effect of wet immersion on the adhesion was also assessed. The mechanisms of failure and the critical load of delamination differ significantly depending on the phase and structure of the coatings. The HA coatings with granular morphology have higher resistance to delamination as compared to HA coatings with columnar morphology. This fact has been related to the absence of stresses for the granular morphology.

Deposition of hydroxyapatite thin films by excimer laser ablation

Abstract The influence of the pressure of a sole water atmosphere during the pulsed laser deposition of hydroxyapatite thin films on Ti–6Al–4V substrates has been studied. The rest of the technological parameters involved in the process have been fixed near the conditions where the best crystalline coatings are obtained. The pressure of the water atmosphere has been varied between 0.15 and 1.5 mbar. The films properties have been analysed by means of XRD, SEM, FT-IR spectroscopy and SIMS. An optimal region, in order to obtain thin films of highly crystalline hydroxyapatite, has been found near 0.5 mbar for the two excimer laser wavelengths (193 nm and 248 nm) used in this study. These films have a preferential orientation in the (100) direction.

Influence of thickness on the properties of hydroxyapatite coatings deposited by KrF laser ablation

The growth of hydroxyapatite coatings obtained by KrF excimer laser ablation and their adhesion to a titanium alloy substrate were studied by producing coatings with thicknesses ranging from 170 nm up to 1.5 microm, as a result of different deposition times. The morphology of the coatings consists of grain-like particles and also droplets. During growth the grain-like particles grow in size, partially masking the droplets, and a columnar structure is developed. The thinnest film is mainly composed of amorphous calcium phosphate. The coating 350nm thick already contains hydroxyapatite, whereas thicker coatings present some alpha tricalcium phosphate in addition to hydroxyapatite. The resulting coating to substrate adhesion was evaluated through the scratch test technique. Coatings fail under the scratch test by spallating laterally from the diamond tip and the failure load increases as thickness decreases, until not adhesive but cohesive failure for the thinnest coating is observed.

Dissolution behaviour of calcium phosphate coatings obtained by laser ablation

Pulsed laser deposited calcium phosphate coatings on titanium alloy have been tested under simulated physiological conditions in order to evaluate the changes in morphology, composition and structure. The coatings were deposited under different conditions to obtain different crystalline structures, ranging from amorphous and mixed crystalline phases to pure crystalline hydroxyapatite (HA). The coated samples were immersed in a Ca-free Hanks balanced salt solution for up to 5 days. Characterization of the coatings was performed by X-ray diffraction, scanning electron microscopy and Fourier-transform Raman spectroscopy before and after immersion. Their dissolution behaviour was also monitored through their mass loss and calcium release. Coatings of pure HA preserve their morphology and structure during the exposure time in solution. In multiphasic coatings, consisting of HA with tetracalcium phosphate (TetraCP) or beta-tricalcium phosphate (beta-TCP) with a-tricalcium phosphate (alpha-TCP), microporosity is induced by the complete dissolution of TetraCP or gamma-TCP. Amorphous calcium phosphate coatings totally dissolve.

Effects of plasma processing on the microstructural properties of silicon powders

The effects of plasma processing conditions on the microstructural properties of silicon powders are presented. Hydrogenated nanophase silicon powders were prepared using low-pressure and low-temperature square wave modulated RF plasma (13.56 MHz) using pure silane gas. Plasma parameters such as pressure, RF power, plasma modulation frequency, and gas flow rate were varied. In situ analysis by quadrupolar mass spectroscopy and ex situ analysis of the silicon powders by Fourier transform infrared spectroscopy (FTIR) and thermal desorption spectrometry of hydrogen were performed. The thermal desorption spectrometry results show the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films. The FTIR analysis also determined the microstructural characteristics of powders and hence their volume/surface ratio. This parameter was determined from the balance of Pj probabilities of having one of the Hj-Si-Si4-j bond arrangements in the powder particles. These results reveal an increase in hydrogen content and a reduction in volume/surface ratio as the modulation frequency of RF power increases. In consequence, higher compactness of silicon powders is associated with long particle residence times inside the plasma as a result of ion bombardment. TEM analysis indicated a considerable dispersion of particle size and some degree of structure of the silicon powder characterized by intergrain linkage. We point out the dominant presence of hydrogen on the particle surfaces (external voids), which may cause the high reactivity of grains, increasing the degree of intergrain linkage.

Characterization of calcium phosphate coatings deposited by Nd:YAG laser ablation at 355 nm: influence of thickness.

Calcium phosphate coatings were deposited by pulsed laser ablation with a radiation of 355 nm from a Nd:YAG laser. All the coatings were obtained at the same conditions, but deposition was stopped after different number of pulses to get coatings with different thickness. The influence of thickness in the structural and mechanical properties of the coatings was investigated. Coatings structure was characterised by scanning electron microscopy, grazing incidence X-ray diffractometry and Raman spectroscopy. The mechanical properties were evaluated by scratch test. The morphology of the coatings is dominated by the presence of droplets. The coatings are composed mainly of hydroxyapatite, alpha tricalcium phosphate and amorphous calcium phosphate. Thinner coatings withstand higher loads of failure in the scratch test.

Unusual photoluminescence properties in amorphous silicon nanopowder produced by plasma enhanced chemical vapor deposition

The supralinear dependence of visible photoluminescence intensity from amorphous silicon nanopowder produced in plasma enhanced chemical vapor deposition on excitation power and its exponential dependence on pressure is reported. It is shown that this new material emits two different kinds of photoluminescence that dominate at different pressures.

Hydroxyapatite coatings grown by pulsed laser deposition with a beam of 355 nm wavelength

Calcium phosphate coatings, obtained at different deposition rates by pulsed laser deposition with a Nd:YAG laser beam of 355 nm wavelength, were studied. The deposition rate was changed from 0.043 to 1.16 A/shot by modification of only the ablated area, maintaining the local fluence constant to perform the ablation process in similar local conditions. Characterization of the coatings was performed by scanning electron microscopy, x-ray diffractometry, and infrared, micro-Raman, and x-ray photoelectron spectroscopy. The coatings showed a compact surface morphology formed by glassy grains with some droplets on them. Only hydroxyapatite (HA) and alpha-tricalcium phosphate (a‐TCP) peaks were found in the x-ray diffractograms. The relative content of a‐TCP diminished with decreasing deposition rates, and only HA peaks were found for the lowest rate. The origin of a‐TCP is discussed.

Preparation of nanoscale amorphous silicon based powder in a square-wave-modulated rf plasma reactor

Abstract Nanoscale amorphous silicon powder was prepared in a specially designed plasma reactor, operating with square-wave modulation (SQWM) of the rf power at low pressure and low temperature. We have observed that in order to increase the powder yield, the process temperature must be lower and the pressure must be higher than that normally used for thin film deposition of a-Si: H in plasma enhanced chemical vapour deposition (PECVD). The square-wave modulation (SQWM) of the rf generator at low frequencies also improves the generation of powder. The experimental set-up was equipped with a quadrupole mass spectrometer (QMS), which directly analyses the species present in the plasma, and with a time- and spatially- resolved optical-emission spectroscopy system (OES). The characterization of the amorphous silicon powders by electron microscopy (TEM) and thermal desorption spectroscopy (TDS) of hydrogen provides the nanoscale powder size, microstructure and chemical composition.

Application of dissolution experiments to characterise the structure of pulsed laser-deposited calcium phosphate coatings

A dissolution test was performed with pulsed laser (Nd: YAG, 355 nm)-deposited calcium phosphate coatings composed of hydroxyapatite (HA) and alpha-tricalcium phosphate (alpha-TCP) in different proportions, as a result of the use of different deposition rates. During immersion in a Ca2+-free Hanks solution, the dissolution kinetics were determined while other structural and compositional properties of the coatings were derived. It was possible to infer that the alpha-TCP is distributed uniformly and that the coating is of a non-columnar compact grain structure. The mass ratio of the phases for each coating was also determined and was related to the X-ray diffraction intensities. When incomplete, the hydroxylation level of the HA in the coatings is completed after immersion.