Hydroxyapatite layers (Ca<sub>10</sub>(PO<sub>4</sub>)6(OH)<sub>2</sub>) were deposited by means of laser ablation method using an ArF excimer laser
(193 nm). The influence of substrate temperature on the structure of deposited layers was studied. The layers were
deposited on Ti6Al4V titanium alloy which temperature varied from 250 °C to 700 °C. The characteristics of the
hydroxyapatite coatings were determined by means of Fourier Transform Infrared spectroscopy (FTIR). The obtained
spectra reveal that the presence and abundance of the PO<sub>4</sub> absorption bands depend on the substrate temperature. The
topography of the deposited layers were analyzed with the use of an Atomic Force Microscope.
Effects of process parameters on pulsed laser deposition of hydroxyapatite (Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>) were studied. Process parameters like laser energy and ambient atmosphere influence both the expansion dynamics of a laser ablated plasma plume and topography of deposited layers. The plume created using 193 nm, 20 ns pulses from ArF laser was analysed
by means of space and time resolved optical emission spectroscopy. The velocity of the plasma plume at several distances from the target in different ambient conditions was determined. The deposited hydroxyapatite layers were analyzed by means of atomic force microscopy and X-ray diffractometry in order to determine the film topography, its structure and mechanical and physical properties. The results show that the plume expansion velocity as well as the topography of deposited films depend on the sort of ambient gas and its pressure.
Laser welding process is unstable because the keyhole wall performs oscillations which results in the oscillations of plasma plume over the keyhole mouth. The characteristic frequencies are equal to 0.5-4 kHz. Since plasma plume absorbs and refracts laser radiation, plasma oscillations modulate the laser beam before it reaches the workpiece. In this work temporary electron densities and temperatures are determined in the peaks of plasma bursts during welding with a continuous wave CO<sub>2</sub> laser. It has been found that during strong bursts the plasma plume over the keyhole consists of metal vapour only, being not diluted by the shielding gas. As expected the values of electron density are about two times higher in peaks than their time-averaged values. Since the plasma absorption coefficient scales as ~N<sup>2</sup><sub>e</sub>/T<sup>3/2</sup> (for CO<sub>2</sub> laser radiation) the results show that the power of the laser beam reaching the metal surface is modulated by the plasma plume oscillations. The attenuation factor equals 4-6% of the laser power but it is expected that it is doubled by the refraction effect. The results, together with the analysis of the colour pictures from streak camera, allow also interpretation of the dynamics of the plasma plume.
The analysis of the fluctuations of pressure and plasma radiation observed during welding with a cs CO<SUB>2</SUB> laser is presented. Welding was done with a continuous wave CO<SUB>2</SUB> laser. Photon Sources VFA 2500, operating at the power of 1.75-2 kW. The welded materials were mild and stainless steel sheets 0.8-2 mm thick. Shielding gas was argon or helium. Plasma fluctuations were registered in monochromatic radiation with the use of a monochromator and photomultiplier while the pressure variations - with a microphone in the frequency range of 20-2x10<SUP>4</SUP> Hz. T he results show that the optical and acoustic signals emitted during the welding process can be used for process monitoring. The most promising method is a frequency analysis based on the Fourier techniques. The Fourier spectra (power spectrum densities - PSD) of pressure waves and plasma emission change with the welding conditions and hence contain information about the process of welding. Power spectra of both signals and their coherence are studied in dependence on the welding conditions. Generally two intrinsic frequency peaks are always present but the relative peak amplitude, exact frequency and width depend on welding conditions. In the case of mild steel 0.8 mm thick when helium is used as the shielding gas these peaks correspond to the frequencies of approximately 1.3 kHz and approximately 3.7 kHz on the PSD of both signals, optical and acoustic. The clear dependence of the measured signals on process parameters shows that they can be used to find the best welding conditions.