Authors 
Simonov Nikolay Petrovich, Applicant, Penza State University (40 Krasnaya street, Penza, Russia), Email: artemov@pnzgu.ru

Abstract 
Background. The functional characteristics of machine parts are largely determined by the properties of their surface. A special role is played by the defective structure of the surface, which in most cases is the cause of the fatigue failure of metal. In this regard, the task of developing technologies for modifying the defective structure of the surface of parts becomes very urgent. The aim of this work is to theoretically and experimentally substantiate the possibility of using nanomodified cutting fluid in the conditions of cavitation development as a strengthening technology for the formation of wearresistant surface layers.
Materials and methods. To calculate the time of the collapse of the cavitation bubble, the NoltingNepires equation was used. The calculation of the number of metal nanoparticles falling into the channels of microcracks (MC) due to the action of the cumulative jet was performed using the energy conservation law. The calculation of the effective Young's modulus of the surface layer of metal was performed using the formula obtained by modifying the formula for the Young's modulus of a porous material. For experimental studies of the effect of nanomodified coolant on the surface characteristics, turbocharger blades were used, the sample material was ZhS6K alloy. The samples were processed on a surface grinding machine with highly porous circles.
Results. In the approximation of an incompressible fluid and neglecting viscous losses, an analytical formula has been obtained for the time of collapse of a cavitation bubble. It is shown that the collapse time of a cavitation bubble depends on the vibration frequency, bubble radius, coolant density, amplitude of pressure fluctuations in the liquid, and also on the damping decrement. Taking into account the identified limitations on the given parameters, it was found that the collapse time ≥10−8 с is most effective from the point of view of the influence of the cumulative jet. An analytical formula has been obtained for calculating the number of metal nanoparticles falling into the channels of microcracks due to the action of a cumulative jet. The number of nanoparticles trapped in MCs was estimated; it is shown that this value substantially depends on the time of collapse of a cavitation bubble and the geometric parameters of MCs. An analytical formula has been obtained for the effective Young's modulus of the metal surface layer. It is shown that the effective Young's modulus is a function of the number of nanoparticles entering the MC channel, as well as the size of the nanoparticles and MCs. The assessment showed that the effective Young's modulus, depending on the value of the time of collapse of a cavitation bubble, could exceed its initial value by about an order of magnitude. A qualitative agreement was found between theoretical results and experimental results: when using coolant with Ni nanoparticles when grinding samples of a turbocompressor blade, a 10% decrease according to Ra parameter in surface roughness and an increase in surface microhardness by more than 20% were achieved.
Conclusions. By using cavitation technologies to modify the defective structure of the surface during finishing, it is possible to provide the required strength characteristics of the surface layer of metal.

Key words 
cavitation technology, nanomodified cutting fluid, time of collapse of a cavitation bubble, metal nanoparticles, effective Young's modulus of the surface metal layer, roughness, microhardness

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