applied sciences Article Impact of the Samples’ Surface State on the Glow Discharge Stability in the Metals’ Treatment and Welding Processes Maksym Bolotov 1,* , Gennady Bolotov 1, Serhii Stepenko 2 and Pavlo Ihnatenko 3 1 Department of Welding Technologies and Construction, Chernihiv Polytechnic National University, 95 Shevchenko Str., 14035 Chernihiv, Ukraine; [email protected] 2 Department of Electrical Engineering and Information Measuring Technologies, Chernihiv Polytechnic National University, 95 Shevchenko Str., 14035 Chernihiv, Ukraine; [email protected] 3 Department of Machine Building and Wood Processing Technologies, Chernihiv Polytechnic National University, 95 Shevchenko Str., 14035 Chernihiv, Ukraine; [email protected] * Correspondence: [email protected]; Tel.: +380-633353906 Abstract: The low temperature plasma of glow discharge has found a widespread use as a heating source in welding and surface treatment of metals. The meticulous analysis of glow discharge’s instabilities in these processes allowed us to highlight the physicochemical characteristics of the cathode surface (the welded or treated samples) as one of the main reasons of its transition into an electric arc—as a more stable form of gas discharges. The prolonged arc action on the samples surfaces inevitably leads to the disruption of the technological process and, consequently, to undesirable overheating of samples. In this regard, the main aim of this work is to study the influence of the macro- and micro relief of the cathode on the stable glow discharge existence in the processes of metals treatment and diffusion welding. It has been analytically established and experimentally supported that the glow discharge’s stability is mainly affected by the sharp protrusions generated Citation: Bolotov, M.; Bolotov, G.; on the cathode surface because of samples pre-treatment by machining before welding. It has been Stepenko, S.; Ihnatenko, P. Impact of µ the Samples’ Surface State on the established that the rough surface pre-treatment with the Rz about 60–80 m decreases the pressure Glow Discharge Stability in the range of glow discharge sustainable existence from 1.33–13.3 kPa to 1.33–5.3 kPa compared with the Metals’ Treatment and Welding surface machining with the Rz about 10 µm. Processes. Appl. Sci. 2021, 11, 1765. https://doi.org/10.3390/app11041765 Keywords: diffusion welding; plasma; glow discharge; surface treatment; plasma techniques Academic Editors: Alessandro Belardini and Mariusz Jasi´nski 1. Introduction Received: 10 December 2020 Nowadays, to obtain qualitative permanent joints of heterogeneous materials the Accepted: 10 February 2021 methods of welding in a solid state are widely used. The most prevalent of these is a Published: 17 February 2021 diffusion bonding. The wide nomenclature of compounds creates a complex of specific requirements for diffusion bonding’s energy sources. These requirements are mainly related Publisher’s Note: MDPI stays neutral to the acceptability of a wide range of materials and shapes of products, the accuracy of with regard to jurisdictional claims in the specific heat capacity control and the ability of the wide regulation of the sample’s published maps and institutional affil- iations. temperature [1]. The distributed plasma of a glow discharge burning in a rarefied gas atmosphere at a pressure of 0.1–10 kPa is widely used in processes accompanied by the direct action of charged electric particles on the treated or welded materials. Known works consider the possibility of application of gas discharges technique in the field of thin films deposition, Copyright: © 2021 by the authors. metals surfacing, treatment and modification of metals before welding [2–5]. Still, as Licensee MDPI, Basel, Switzerland. practice has shown, diffusion welding [6] and thermal and chemical-thermal surface This article is an open access article treatment [7] are the most appropriate. This is due to the high technological capabilities distributed under the terms and conditions of the Creative Commons of the glow discharge, which in these processes can serve both a processing tool and a Attribution (CC BY) license (https:// source of thermal energy for their implementation simultaneously. Additionally, a glow creativecommons.org/licenses/by/ discharge has high technical, economic, and environmental indicators, for instance, high 4.0/). Appl. Sci. 2021, 11, 1765. https://doi.org/10.3390/app11041765 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1765 2 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 12 heatingheating productivity,productivity, energy energy savings, savings, and and last last but but not not least least the the absence absence of of environmental environmental pollutionpollution [[8].8]. However,However, during during surfacesurface treatmenttreatment or or welding welding of of metals, metals, the the instabilities instabilities of of samples samples temperaturetemperature (discharge(discharge cathodecathode inin thesethese processes),processes), thethe pressurepressure ofof thethe working working gas, gas, the the voltagevoltage ofof thethe powerpower supply,supply, andand a number of other factors, can can lead lead to to the the transition transition of of a aglow glow discharge discharge into into another another form form of ofgas gas discha discharge—anrge—an electric electric arc arc [9] [(Figure9] (Figure 1).1 In). Inthis thiscase, case, the thedistributed distributed glow glow discharge discharge transforms transforms into intoa contracted a contracted form. form. Its cathode Its cathode spot spotnarrows narrows to a tovery a very small small size, size, increasing increasing the energy the energy concentration concentration dramatically. dramatically. A pro- A prolongedlonged action action of ofa concentrated a concentrated arc arc discharge discharge inevitably inevitably leads leads to to the locallocal meltingmelting andand destructiondestruction ofof samples.samples. (a) (b) FigureFigure 1. 1.The The view view of of the the glow glow discharge discharge on on the the cathode cathode surface surface in stablein stable (a) and(a) and unstable unstable (b) burning(b) burning modes modes during during the ion the ion treatment (the bright spots to the right—arc breakdowns). treatment (the bright spots to the right—arc breakdowns). TheThe probabilityprobability of arcing increases with with the the rising rising of of the the total total and and specific specific power power of ofthe the discharge. discharge. The The energy energy characteristics characteristics of the of the ionic ionic treatment treatment processes processes are aremainly mainly de- determinedtermined by by the the magnitude magnitude of of the the discharge discharge current current and and the gas pressure inin thethe workingworking chamber.chamber. TheThe developmentdevelopment ofof processes’processes’ productivityproductivityleads leadsto tothe thenecessity necessityof of increasing increasing theirtheir values. values. That, That, in in turn, turn, entails entails rising rising of of average average current current density density (j) in(j) thein the cathode cathode spot spot of theof the discharge discharge and and the averagethe average specific specific volumetric volumetric power power (jE) in(jE) the in discharge the discharge plasma plasma [10]. However,[10]. However, with increasing with increasing of discharge of discharge energy characteristicsenergy characteristics in the interelectrode in the interelectrode gap, the short-termgap, the short-term local arc breakdowns local arc breakdowns with a duration with ofa duration one or two of half-periodsone or two ofhalf-periods the rectified of currentthe rectified can form current (Figure can2 form). (Figure 2). Appl. Sci. 2021, 11, 1765 3 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 12 (а) (b) Figure 2. Oscillogram ofof thethe discharge discharge current current in in stable stable (a )(a and) and unstable unstable (b) ( mode.b) mode. (Glow (Glow discharge discharge current current 4 A. Oscillograms4 A. Oscillo- gramswere recorded were recorded from a from shunt a ofshunt 3 Ohm of 3 connected Ohm connected to the dischargeto the discharge circuit). circuit). InIn [11,12], [11,12], the glowglow discharge’sdischarge’s instabilities instabilities are are closely closely associated associated with with its its contraction contrac- tion(compression) (compression) and transitionand transition to a cord to forma cord of form discharge of discharge with the increasingwith the increasing of volumetric of volumetricpower (jE) accordingly.power (jE) Theaccordingly. gas in the The cord gas discharge in the heatscord updischarge extremely heats while up the extremely burning whilevoltage the decreases burning andvoltage eventually decreases the and cord eventual switcheslyinto the ancord arc switches electric. into Such an a arc mechanism electric. Suchof glow a mechanism discharge’s of instability glow discharge’s is associated instability with volumetricis associated processes with volumetric in positive processes column inof positive the discharge column plasma, of the and discharge therefore plasma, is more and characteristic therefore ofis extendedmore characteristic discharges of of ex- the tendedlaser type discharges [13], where of the the laser length type of [13], the where interelectrode the length gap of is the 0.5–1.0 interelectrode m. In the gap processes is 0.5– 1.0of ionm. treatmentIn the processes and welding of ion oftreatment metals, aand