In this work, we experimentally study the features of laser drilling of composite highly porous ceramics consisting of aluminosilicates, glass and a small addition of polystyrene (EPS). It was found that the formation of a liquid phase can contribute to the strengthening of ceramics. A list of factors that determine the dynamics of hole formation is proposed. These include not only the processes of heating, melting, ablation, and ejection, well known previously for metals and dense ceramics, but also a number of other specific processes: (a) boiling of the component of the mixture with the lowest temperature of thermal decomposition and its escape from the affected zone; (b) a melt of less volatile components; (c) sealing the ceramics adjacent to the walls due to the mechanical sealing effect of ablative pressure; (c) glass transition of the liquid phase upon cooling after the end of irradiation.
The paper reviews recent results on modeling a viscous liquid flow driven by ablation pressure. Based on the analysis of
the Navier-Stokes equation various strongly different manifestations of this phenomenon are explained. These are: (i) a
"clean" laser ablation, when laser spot has a clean sharp spot border, free from a re-solidified melt dross; (ii) a new form
of material removal in laser ablation - expulsion on a poly(methyl methacrylate) target of long (up to 1 mm) nanofibers
with a radius about 150-200 nm to the exterior of the spot under the action of a single pulse of KrF excimer laser; and
(iii) a new way of laser surface nanostructuring - the formation of a surface foam having a structure of micro-pores
interconnected with nanofilaments of diameters about 100 nm as a result of single pulse KrF laser irradiation of
biopolymer films.
KEYWORDS: Particles, Chemical species, Molecules, Chemical reactions, Thin films, Monte Carlo methods, Tellurium, Pulsed laser deposition, Laser ablation, Helium
Pulsed laser ablation has attracted great attention over the past few years as promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high- temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the simplicity of the experimental set-up. In a common configuration, the laser- generated flux of particles is collected on a planar substrate positioned parallel to the irradiated surface. Several modifications, like simultaneous generation of two plumes from different targets (double ablation), were proposed. Different lasers (e.g. KrF with (lambda) equals 248 nm, Nd-YAG with (lambda) equals 532 nm, etc.) with energy density 2 - 10 J/cm2 were used in the ablation experiments both in vacuum and into diluted ambient gas (pressure up to 750 mTorr). Monte Carlo simulation was found to be a successful technique for theoretical investigations of the laser ablation processes. This method has allowed us to investigate the influence of elastic collisions and chemical reactions in the laser ablated plume on the angular characteristics of the flow. The results of the simulation show that elastic collisions give rise to focusing of particles towards the surface normal and to the redirection of the velocities of the more energetic particles in the direction close to the surface normal. The chemical reactions are found to influence the angular distributions in the way opposite to the one of elastic collisions. The reaction heat contributes to the energy of particles and the velocity distributions are affected by reactions. As result of these processes, the angular distributions are broadened from the surface normal. Additional collisions with the particles of the ambient gas were shown to influence the composition and uniformity of thin films. The study of these processes is of a particular interest for the developing of pulsed laser deposition (PLD) technique.
A new method based on a combination of pyrometer and integrating sphere is proposed, allowing an express registration of temporal dependences of temperature, T(t), reflectivity, Rlambda(t), and absorptivity, Alambda(t), of heat-conducting samples, when heated by continuous wave laser radiation. Computer-controlled acquisition and processing of these signals gives a possibility to obtain temperature dependences Rlambda(T) and Alambda(T). In addition, a processing recorded by this method T(t) signal at cyclic heating and cooling of the 'thermally thin' sample (when laser beam is chopped periodically) allows us to find a temperature dependence of heat capacity Cp(T). Thus both the Alambda(T) and Cp(T) dependences have been obtained simultaneously during the same laser heating in the range of temperatures beginning from the initial (room) temperature up to melting point or even above for iron, steels and other alloys, when heated by cw Nd:YAG laser radiation with wavelength lambda equals 1064 nm in vacuum, air and inert gas atmosphere at heating rates in the range 103 divided by 104 K/s. Experimental set-up and the proposed experimental technique are described. An analytical estimation of laser overheating of front surface compared with rear surface of the sample is given to prove a validity of the proposed method of heat capacity measurement.
High quality (precise) laser ablative processing of materials, e.g. cutting of polymers or biotissues, is characterized by smooth bottom and edges of a crater and absence of droplets, expeffing from the irradiation zone. Until recently, the experimental realization of the above mentioned conditions has been achieved mostly with excimer lasers. The most popular 'chanism of precise organic material removal is photoablation, but it can be applied only to the case of UV radiation. Though recently, quite good quality tissue cuttin was obtained also by means of erbium (X =2.94 m) laser. In both cases we have laser radiation with relatively short pulse duration r 1O s and high absorption coefficients (a 1O"10 cm-i) in biotissues, but for excimer lasers radiation absorption takes place mostly in biomolecules while for erbium laser energy is deposited in water. That is why it could be quite natural to consider thermal mechanisms of precise tissue ablation. We believe that surface thermal stability under intense laser heating is a key factor which determines surface quality after material ablation. Really, numerous experiments with melting, ablation, laser induced chemical reaction on irradiated solid surfaces (metals, dielectrics, semiconductors) have shown that after sufficiently long heat treatment by CW radiation or by multiple pulse action various types of surface reliefs can be formed. In fact, one of the problems in laser materials processing is to find radiation parameters (wavelength, irradiation time or pulse number, intensity, spot size and beam polarization) when surface reliefs are not formed. Typically, under multiple pulsed action such reliefs show essential degree of order and. are called surface structures (see, e.g. [1]).
A theoretical model of surface polishing of materials evaporating by light from solid phase is proposed. On the basis of the model, an explanation is given for the experimentally observed phenomenon of diamond films surface polishing as a result of their graphitization and vaporization from solid phase under the action of multipulse radiation of an excimer XeCl laser. The proposed model of surface polishing by light can also be applied for the case of thermochemical etching by radiation where the rate of etching also has Arrhenius dependence on surface temperature. This new technique has promising possibilities in the processing of hard and superhard materials.
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