Obrabotka Metallov 2018 Vol. 20 No. 4

OBRABOTKAMETALLOV Vol. 20 No. 4 2018 94 MATERIAL SCIENCE Electrical Insulation Properties of Aluminum Oxide Detonation Coatings Vladimir Ulianitsky a , Alexandr Shtertser b,* , Igor Batraev c Lavrentyev Institute of Hydrodynamics of the Siberian Branch of the RAS, 15 Ac. Lavrentieva ave., Novosibirsk, 630090, Russian Federation a http://orcid.org/0000-0002-3538-1486, ulianv@mail.ru , b http://orcid.org/0000-0003-4973-0437, asterzer@mail.ru, c http://orcid.org/0000-0002-0364-144X , ibatraev@gmail.com Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science. 2018 vol. 20 no. 4 pp. 83–95 ISSN: 1994-6309 (print) / 2541-819X (online) DOI: 10.17212/1994-6309-2018-20.4-83-95 Obrabotka metallov - Metal Working and Material Science Journal homepage: http://journals.nstu.ru/obrabotka_metallov ARTICLE INFO Article history : Received: 01 September 2018 Revised: 13 September 2018 Accepted: 22 October 2018 Available online: 15 December 2018 Keywords : Detonation spraying Electro-insulating coating Specific resistivity Dielectric strength Funding The reported research was partially funded by Russian Foundation for Basic Research and the government of the region of the Russian Federation, grant № 18-43-540008. ABSTRACT Introduction. There are different thermal spray techniques used for formation of electrically insulated oxide ceramic coatings: flame spraying, plasma spraying, HVOF, detonation spraying, etc. The nature of electrical conductivity of thermal spray coatings, including those made by detonation spraying, as well as the impact of composition of the used detonating mixture, are important and not yet fully understood questions. It is found experimentally, that electric conductivity of alumina coating depends not only on the spraying mode and coating structure, but on the humidity and temperature of surrounding atmosphere as well. However, there is no physical model describing a mechanism of coating electrical conduction with regard to the said factors; thus, there is a topical problem of theoretical explanation of experimental data. The purpose of the work is to investigate the electrical insulation properties of alumina coatings made by detonation spraying, to study an impact of detonating mixture composition on coating electrical conduction, and to build a physical model enabling a quantitative estimation of specific volume resistivity of coating. Detonation coatings made using the CCDS2000 detonation device from the М40 Super corundum powder are investigated, it is produced using acetylene-oxygen mixtures with different content of components. The methods of investigation. Research techniques included measurements of coating porosity, specific volume resistivity and dielectric strength. The obtained data is used for the development of the model, describing the conductivity of detonation coating considering defects of structure. Results and discussion. The difference in properties of the coatings formed in a wide range of oxygen-to-fuel molar ratios (from 1.0 to 5.0) is not observed. The hypothesis is suggested, that electrical conduction of coatings is due to the presence of defects – microchannels filled with adsorbed water. Specific resistivity of coatings is (0.3-1.3)·10 10 Ohm·cm, conditional dielectric strength is 5-6 kV for the coating thickness of 240-300 mcm. Here we call the dielectric strength a conditional one, because the samples are not brought to the breakdown in the usual sense, when the values of breakdown current are greater than hundreds of milliampere, and even dozens of ampere. It is considered that there is a breakdown, when a current passing through the probe is greater than 1 mA, this current is already perceptible for a human. Based on the experimental data and on the suggested hypothesis the model is proposed, according to which there are defects in a coating volume in a form of through microchannels, the area of which covers 0.5-2% of coating surface, and its transverse size is of 24-105 mcm. Microchannels are filled with water, which is adsorbed from atmosphere and the main current passes through these microchannels when voltage is applied. Specific resistivity of water at conditional breakdown has a value of the order of 10 5 Ohm·cm. Scientific importance of the obtained results lies in explaining the reasons of lower resistance of thermally sprayed coatings compared to the sintered non- porous aluminum oxide ceramics (more than 10 14 Ohm·cm). Practical significance of the obtained results lies in the possibility to use acetylene-oxygen mixtures with different combination of components without compromising the quality of electrically insulated coatings. For citation: Ulianitsky V.Yu., Shtertser A.A., Batraev I.S. Electrical insulation properties of aluminum oxide detonation coatings. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) =Metal Working and Material Science , 2018, vol. 20, no. 4, pp. 83–95. doi: 10.17212/1994- 6309-2018-20.4-83-95. (In Russian). ______ * Corresponding author ShtertserAlexandr A. , D.Sc. (Physics and Mathematics), Associate Professor, Leading researcher Lavrentyev Institute of Hydrodynamics of the Siberian Branch of the RAS, 15 Ac. Lavrentieva ave., 630090, Novosibirsk, Russian Federation Tel.: 8 (383) 333-00-03, e-mail: asterzer@mail.ru References 1. Fauchais P.L., Heberlein J.V.R., Boulos M.I. Thermal spray fundamentals: from powder to part. New York, Springer Science+Business Media, 2014. 1565 p. ISBN 978-0-387-28319-7.