A package mode of laser operation is considered, when packets of several picosecond pulses follow with a lag of each other of nanoseconds order (high-frequency pulsation mode). In this mode removal of matter from the treated surface generates a pulsed reactive force. This force creates a high-pressure region in the laser-matter interaction zone, causing plastic deformations in metals or destruction of brittle materials both inside this area and outside it. In order to minimize the negative effect of the processing mode, an analytical tool has been developed for the deliberate choice of parameters of laser action on brittle and pseudo-brittle materials. In the proposed mathematical model, the study of the stress-strained state of the plate is reduced to determining the deflection of its median plane by solving the dynamic problem of the theory of elasticity. Analysis of the expression for the deflection showed that with the considered nature of the action of laser radiation, a complex high-frequency oscillatory motion arises in the plate. It represents the result of the superposition of natural and forced oscillations. When choosing the technological modes of laser processing during the formation of pulse packets, the multiplicity of the repetition rates of the packets and their constituent pulses should be avoided in order to prevent a destructive shock resonance. The paper shows the application of the developed model to the calculation of the modes of laser processing of glassy carbon and molybdenum when creating micropoint structures.
Metal-porous cathode is an electrovacuum EHF device, providing a high density of emission current. The emitting surface of the device is usually obtained by ion etching. The emitting layer is made of a porous sponge of a refractory metal, usually tungsten. Through the pores of the sponge alkaline earth metals and their oxides are delivered to the emitting surface. One of the most important parameters of the cathode is a uniformity of emission, depending on regularity of the location of pores.
We describe a method of laser modification of metal-porous cathode surface, instead of traditional ion etching. The preferences of laser etching are uniformity of pores, absence of deformation of surface, opportunity to specify the necessary size of the pores and the distance between them.
Laser structuring of the cathode surface was implemented by laser micrograving. The resulting structure is a field of pores, diameter 15 μm, with spacing of 20 μm.
Glass-phase materials, such as glass-carbon, ceramics etc., are a wide class of substances applied in electronic industry.
These materials often need special technologies for their processing. Unlike traditional methods of micromachining,
focused ultrashort laser pulses of sufficiently high fluence makes it possible not only to avoid the majority of side effects,
including temperature, but also to create a qualitatively new laser technology for "hard materials".
When using ultrafast lasers in micromachining processes it is necessary to account the possible negative effects that
occur in the processing of brittle materials. Removing material from the surface in cold ablation process caused by laser
light, in such a short period of time with such a high rate, creates the area of high pressure in the interaction zone that
could cause a microdamage of brittle materials.
To study the stress-strain state arising in brittle materials under the influence of ultrafast lasers, the special physicalmathematical
model of the process was formulated. As a measure of the mechanical action of laser radiation on the
processed material in cold ablation the reactive force was taken. As a mechanical reaction of the treated glass-carbon
substrate a back pressure generated by the reactive force was considered. Brittle materials suffer plastic deformation, as a
rule, only in the areas of high-temperature heating. Hence, in case of picosecond treatment in cold ablation process the
material, from a mechanical point of view, was seen as a perfectly elastic up to its destruction. From a geometrical point
of view, the processed object was presented in the form of a thin rectangular plate, loosely founded on the elastic base.
The spectroscopic study of dental enamel by LIBS (laser induced breakdown spectroscopy), FTIR (Fourier transform infrared) and XRD (X-ray diffraction) are represented. The changes of enamel structure and composition in process of natural (caries) and artificial demineralization and restoration were studied. In comparison of sound and carious enamel LIBS showed a decrease of the content of Ca, P and change of the content of some other macro-and trace elements (Mn,
Na, Fe, Zn etc). The character of the elemental composition variation was stipulated by the concrete disease. Analysis of
FTIR and XRD spectra of dental samples, subjected to artificial demineralization and restoration, showed that restoration
action reveals slower, than demineralization. And in some cases the damage of crystals after restoration is more
significant than after demineralization.
LIBS is one of the best methods of multilayer coatings studying. Special laser technique-scanning sampling-was developed for studying of different kinds of objects (technical and biomedical coatings). The scanning sampling is based on the scanning of analyzed object during the exposition time. The velocity of scanning is defined by the diameter of laser crater and pulse repetition rate. It allows to increase the volume part of a coating substance in a sample. Some special applications of LIBS and scanning sampling with Q-switched Nd:YAG-laser in the field of technics and biomedicine are described. The layer-by-layer elemental analysis of multilayer components was performed for finding-out the probable non-uniformity. That could appear the reason of wrong work of components. Special layer characteristic calculated as a ratio of spectral lines intensities for elements contained in different layers of a coating was defined for estimation non-uniformity. LIBS in investigation of dental tissues allows to define preliminary the nature of pathology. Scanning sampling used for such tissues as debris and odontolith, allows to find out the stage of lesion and to predict carious conditions.
The surface of rejected dental implants was studied. It was clarified that the implant rejection happened in a case of incomplete osteointegration of implants, incomplete proliferation of a bone tissue in holes. In a turn, osteointegration depends on uniformity of deposition of a titanium-hydroxyapatite (Ti/HA) coating and the impurities composition of used materials. The excess of exterior elements imported at technological processes also influences on adhesion.
Multiple chemical elements of clinical and nutritional interest were measured in the hair of a girl - cerebral palsy patient. Sixteen samples of hair were cut and investigated to determine time and nutrition trends by using laser and arc emission spectroscopy.
Laser microspectral analysis is an effective method for determination of composition of thin films and evaporations. In the paper results on investigation of accidental superthin evaporations are described. The composition of evaporations allows one to make conclusion about their source.
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