Sebastiano Tosto

Functionally graded materials by laser sintering

This article concerns functionally gradient materials (FGM) realized by laser sintering. Layers of Fe rich and Cu rich powders were compacted first and then laser sintered to obtain multilayer samples with smooth composition gradients. Microanalysis profiles on the cross sections of sintered samples show gradual increasing of Cu content and decreasing of Fe content. The laser sintered FGM contain several phases such as free graphite, ferrite, pearlite, Cu-Ni solid solutions, austenite, f -Cu phase, i -phase (Cu 31 Sn 8 ), k -phase (Cu 3 Sn) as well as m -phase (Cu 6 Sn 5 ). The experimental results show good wear resistance and toughness properties of the laser sintered FGMs.

A thermodynamic model of plasma generation by pulsed laser irradiation in vacuum

This paper introduces a thermodynamic model to determine composition, temperature and pressure of the plasma cloud induced by pulsed laser irradiation in the case where a relevant thermal sputtering mechanism is operating at the surface of a molten layer. The model concerns in particular pulse lengths of the order of several nanoseconds and completes the results of a previous paper concerning the physics of the evaporation and boiling driven thermal sputtering (Tosto S 2002 J. Phys. D: Appl. Phys. 35); the recession rate and temperature at the molten surface are linked to the pulse fluence and plasma properties in the frame of a unique physical model. This paper shows that the plasma properties depend critically on the non-equilibrium character of the surface evaporation and boiling mechanisms. The extension of the model to the case of continuous laser irradiation is also discussed. Some examples of computer simulation aim to show the results available in the particular case of a metal target; the comparison between calculated and experimental data supports the validity of the model.

Ablation of hard dental tissues by ArF and XeCl excimer lasers

The paper investigates the effects of UV laser irradiation on hard dental tissues. 12 sagittal cross sections 1 mm thick of teet extracted have been irradiated with pulsed ArF (A.=193 nm) and XeCl (A.=308 nm) excimer lasers. The beam has been focused to obtain a spot of about 1 mm on the enamel and dentin of the teet. The depth and the internal profile of the holes have been measured with a profilometer and correlated with the number of pulses and beam energy for each wavelength. No cracks were induced by thermal stresses nor preferential concentrations of Ca and F have been evidenced on the enamel and dentin after UV irradiation. These results show that the ablation of hard dental material can be successfully carried out with excimer laser irradiation without heat damage of the underlying tissues.

Assessment of the boundary conditions for a thermal model of pulsed laser ablation

An assessment of the boundary conditions for pulsed laser ablation is presented in the case where a thermal sputtering mechanism is relevant at the surface of the molten pool formed by the effect of irradiation. This paper deals with, in particular, the evaporation and boiling mechanisms when superheating effects are allowed to occur. The non-equilibrium character of the process at fluences enabling superheating plays a critical role in the rise of a large temperature gradient and subsurface temperature maximum even in the presence of surface boiling by vapour bubble nucleation.

Proposed Model of Water Adsorption/Desorption in a PEM Membrane

This paper describes the processes of water adsorption and desorption in PE membranes for fuel cells. A simple equation is inferred assuming that the surface of the membrane is uniformly covered by adsorbed molecules to an average depth of some monolayers. The adsorption depth is only controlled by diffusion of adsorbate from the surface towards the bulk through a two-layer or multi-layer mechanisms; so the empty sites formed at the surface can accept further molecules of water. If the diffusion rate is fast enough, cumulative water uptake occurs. The uptake kinetics is described considering the average penetration depth, i.e. neglecting the local concentration spikes below a random number and position of empty sites statistically formed at the surface of the membrane. The model also describes the desorption process, assumed to start at a prefixed time.

Thermodynamic modeling of pulsed-laser-induced ablation

The concept of equilibrium has a fundamental importance in thermodynamics and is related to the reversibility of any chemical and physical process involving a temperature cycle or a phase change. It is of major interest also for the models of high energy beam processing; a recent ablation model has shown that the non-equilibrium character of laser irradiation with pulse lengths τ of the order of some ns is essential to understand the mechanisms of thermal sputtering, in particular when overheating of a molten layer of metal surface is allowed to occur. It is known in fact that if the heating or surface recession rates overcome the nucleation rate of vapor bubbles, then the surface temperature Ts of the molten layer can increase above the thermodynamic boiling point Tb. This situation entails by consequence the rising of a temperature gradient within the boiling layer, whose thickness λ is controlled by the condition T ≥ Tb.

Mo surface enrichment on a Fe-Cr alloy by electron beam irradiation

Mo alloying has been carried out on a Fe–Cr alloy by electron beam irradiation. It is shown that a Mo surface enrichment can be obtained in the range 2–8 at %. Scanning and transmission electron microscopy, X ray microanalysis and electron energy loss spectroscopy have been used to investigate the microstructures obtained after irradiation. Mo–Cr carbide and intermetallic X phase have been identified after alloying. The intermetallic phase is preferentially formed when the Mo content increases. It is concluded from electrochemical studies and potentiostatic attacks that the active and transpassive dissolution rates are closely related to the Cr content in the ferritic phase. This work must be also regarded as a first step towards the surface preparation of 316 S. S. by Mo incorporation to 304 S. S.

Auger electron spectroscopy on a laser-welded 6061 Al alloy

Abstract Auger spectroscopy has been utilized to investigate the mechanisms of formation of porosity in a laser-welded 6061 Al alloy. The analyses carried out on the internal surface of the porosity of samples cleaved under ultra high vacuum have shown evidence for preferential enrichment of volatile impurities.