In the study, an attempt was made to increase durability of copper by creating a surface layer saturated or supersaturated with chromium only in the area of the highest thermo-mechanical loads during the welding process. There was developed a two-stage technological process, consisting of: application of chrome coating of the thickness approx. 1 µm on the Cu-ETP copper surface using the PVD method, and then performing the process of its diffusion by annealing at a temperature of 950°C in the vacuum. As a result, a diffusion CuCr layer with a thickness of approx. 20 µm was obtained, with hardness of approx. 120 HV0,01

Drajewicz, M.

Góral, M.

Poreba, M.

Pytel, M.

Ziaja, W.

English

Hrčak - Portal of scientific journals of Croatia

673METALURGIJA 61 (2022) 3-4, 673-676M. PORĘBA, M. PYTEL, M. DRAJEWICZ, W. ZIAJA,  M. GÓRALMODIFICATION OF THE Cu-ETP COPPER SURFACE  LAYER WITH CHROMIUM BY PHYSICAL VAPOR  DEPOSITION (PVD) AND DIFFUSION ANNEALINGReceived – Primljeno: 2021-11-06Accepted – Prihvaćeno: 2022-03-10Original Scientific Paper – Izvorni znanstveni radIn the study, an attempt was made to increase durability of copper by creating a surface layer saturated or super-saturated with chromium only in the area of the highest thermo-mechanical loads during the welding process. There was developed a two-stage technological process, consisting of: application of chrome coating of the thick-ness approx. 1 μm on the Cu-ETP copper surface using the PVD method, and then performing the process of its diffusion by annealing at a temperature of 950 °C in the vacuum. As a result, a diffusion CuCr layer with a thickness of approx. 20 μm was obtained, with hardness of approx. 120 HV0,01.Keywords: Cu-ETP and Cu-OF copper, surface, CuCr layer, PVD, annealing, x-ray analysis, scanning electron micros-copy (SEM), energy dispresive spectroscopy (EDS)ISSN 0543-5846METABK 61(3-4) 673-676 (2022)UDC – UDK 546.056:621.9.015:546.56.76.002.4:621.785:53926=111INTRODUCTIONChromium copper contains no more than 0,5-1 wt.% of Cr. Small amounts of chromium added to copper sig-nificantly boost its strength and technological proper-ties. One of the most important features of chromium copper is its ability to maintain strength properties ob-tained by dispersion, hardening up to a temperature of approx. 400 °C. High hardness and slightly lower elec-trical conductivity make chromium copper suitable for point electrodes used in spot welding or shielding elec-trodes used in welding, thus replacing pure copper. This has resulted in an increase in durability of spot welding electrodes, which is one of most important technologi-cal issues, especially in relation to welding galvanized sheets, mainly in the automotive industry [1-3].Currently sought after are the possibilities of in-creasing the functional properties of copper and copper alloys through the use of intensive plastic working methods or introduction of alloying additives which re-fine the microstructure [4-7].In the study, an attempt  was made to increase dura-bility of copper by creating a layer saturated or super-saturated with chromium (chromium bronze) only on its surface in the area of the highest thermo-mechanical loads during the welding process. It was assumed that the diffusively produced chromium-rich coating would increase the surface hardness whilst maintaining good adhesion to the substrate. There was developed a two-stage technological process, consisting of: application of chrome coating on the Cu-ETP copper surface using the PVD method, followed by performing the process of its diffusion annealing at a temperature of 950 °C.The homogenization annealing process of CuCr al-loy copper (most often produced by casting and cold or hot plastic working) is usually carried out in the tem-perature range from approx. 950 to 1 000 °C. It was as-sumed that carrying out the heat treatment process in this temperature range would enable dissolution of chromium in the copper solid solution and would also cause its partial supersaturation. The annealing process was carried out in air and in a protective atmosphere.EXPERIMENTAL PROCEDUREChromium coatings on Cu-ETP and Cu-OF copper substrate were deposited in the process of physical va-pour deposition in a vacuum chamber, under pressure of 1 Pa, using nano PVD apparatus from Moorfield Nano-technology. There were produced pure chromium coat-ings with a thickness of approx. 0,5, 1,0, and 2,0 µm (Figure 1). Prior to the process, the surface of the sam-ples was polished using MD-Mol polishing cloths and 9,6, and 3 µm polycrystalline DP-type diamond suspen-M. Poręba email:poreba@prz.edu.pl, M. Pytel, M. Drajewicz, W. Ziaja, M. Góral, Department of Materials Science, Rzeszow University of Technology, Rzeszow, Poland.Figure 1  Cross-section of the chromium coating applied on Cu-ETP by the PVD method674  METALURGIJA 61 (2022) 3-4, 673-676M. POR BA et al.: MODI I ATION O  T E u ETP OPPER SUR A E LAYER IT  ROMIUM BY P YSI AL...sions from Struers. The polished samples were washed in ultrasonic cleaner in isopropyl alcohol for 5 minutes and then placed in the working chamber of the PVD device. A target containing 99,95 wt. % of chromium was used as the source of chromium in the process.Adhesion of the chromium coating to the CuETP copper substrate was determined by the “scratch-test” method. Scratching was performed with a load increas-ing from 0,9 N to 10 N in 30 seconds, over a distance of 5 mm, with simultaneous recording of acoustic emis-sion signal and friction force. The analysis of these val-ues showed that the critical load for the chromium coat-ing was 1,5 N. In the next stage of the research, the samples were subjected to diffusion annealing process at 950 °C for 1 h using the Vacuum Heat Treatment Mono Therm HK.446.N.20.gr furnace in order to intro-duce chromium atoms into the copper crystal lattice. Two variants of the annealing were applied – first in the air at atmospheric pressure and second in high vacuum (Table 1).Table 1  Conditions of the diffusion annealing processes for Cu-OF and Cu-ETPThickness of Cr coating / µmSub-strateTempera-ture / ºCTime / hPressure / PaAtmo-sphere1 Cu-ETP 950 1 105 Air950 1 10-3 Vacuum0,5 Cu-OF 950 2 10-3 Vacuum2,0 10-3 VacuumIt was found that the annealing process in the air at-mosphere of Cu-ETP copper with chromium coatings with thickness of 1 μm, led to formation of a copper oxide layer Cu2O with thickness of approx. 20 μm on the copper surface as a result of oxidation processes and in its subsurface zone (as a result of internal oxidation and simultaneous inward oxygen diffusion). Formation of oxygen and chromium enriched crystals in the near-surface zone was also observed, especially at the grain boundaries of copper (Figure 2, Table 2). Their pres-ence may result both from oxidation caused by inward diffusion of atmospheric oxygen, and also as a result of the reaction of diffusing chromium atoms with oxygen atoms dissolved in the copper matrix.As a result of the use of a protective atmosphere during the annealing of Cu-ETP copper at 950 °C/1 h, a diffusion CuCr layer with a thickness of approx. 20 µm was obtained on its surface (Figure 3, Table 3) having hardness of 120 HV0,01 while a substrate hardness was 50 HV1. Chromium diffused into the substrate, dissolved in the copper matrix and formed a saturated copper solid solution. As a result of further saturation of copper with chromium, crystals rich in chromium and oxygen were formed from the super-saturated solid solution.Figure 2  Cross-section of Cu-ETP copper surface layer (SEM) following the high temperature diffusion annealing process at 950 °C/1 h. The microstructure of the oxide layer formed due to the oxidation process is visible.Table 2  Chemical composition determined by Scanning Electron Microscopy-Energy Dispersive Spectro-scopy (SEM-EDS) in micro-areas of the surface layer and near-surface zone of Cu-ETP copper after its annealing at 950 °C/ 1 h in airMicro-area(Figure 2)Element content/ wt. % / at. %O Cu O Cu Pt 1  11,20    88,8 33,3 66,7 Pt 2  11,20    88,8 33,3 66,7 Pt 3 -   100,0 - 100,0 Pt 4 -   100,0 - 100,0 Figure 3  Cross-section of Cu-ETP copper: a) PVD chrome plating; b) diffusion layer formed as a result of chromium diffusion from the coating during annealing at 950 °C/1 h in a protective atmosphere.Table 3  Chemical composition (SEM-EDS) in micro-areas of the chromium diffusion layer produced on the Cu-ETP copper substrate in the PVD process and then annealed at 950 °C/1 h (Figure 3)Micro-area(Figure 3)Element content/ at. % / wt. %Cu Cr O Cu Cr O1 74,01 4,92 21,06 88,80 4,83 6,362 15,85 15,81 68,34 34,46 28,1 37,413 99,43 0,57 - 99,53 0,47 -4 99,54 0,46 - 99,63 0,37 -5 99,47 0,58 - 99,57 0,43 -6 99,58 0,47 - 99,62 0,38 -7 100,0 - - 100,0 - -8 100,0 - - 100,0 - -675M. POR BA et al.: MODI I ATION O  T E u ETP OPPER SUR A E LAYER IT  ROMIUM BY P YSI AL...METALURGIJA 61 (2022) 3-4, 673-676Then, in accordance with the same methodology as for the PVD chromium coatings produced on the Cu-ETP copper substrate, there were performed annealing processes of Cu-OF copper at the temperature of 950 °C and for 2 hours with the deposited chromium coatings with thickness of 0,5 µm and 2 µm.As a result of the diffusion annealing process in a high vacuum (0,0133 Pa) at 950 °C/2 h of chromium coatings deposited on the surface of the Cu-OF oxygen-free copper, an outer coating consisting of copper and chromium was obtained. The thickness of the PVD chromium coating did not change during the diffusion annealing, but as a result of outward diffusion of copper from the substrate, a copper-enriched chromium coat-ing was formed (unlike in the case of electrolytic copper containing significantly more oxygen) (Figures 4-7 Ta-bles 5-6).RESULTS AND DISCUSSIONAs a result of the research, it was found that the dif-fusion annealing processes (carried out in the air atmo-sphere) of PVD chromium coatings produced on the Table 4  Vickers hardness of the chromium diffusion layer produced on the Cu-ETP copper substrate in the PVD process and then annealed at 950 °C/1 hVickers scale Vickers hardnessLayer SubstrateHV0,01 119 -HV0,02 112 -HV1 - 50Figure 4  Cross-section (SEM) of Cu-OF copper with a chromium coating with thickness of 0,5 µm –  thickness of the coating following the high vacuum annealing process at 950 ºC/2 h is markedFigure 5  Cross-section (SEM) of Cu-OF copper with a chromium coating with thickness of 2,0 μm – thickness of the coating following high vacuum annealing process at 950 ºC/2 h is markedFigure 6  Cross-section (SEM) of Cu-OF copper with a 0,5 µm PVD chromium Cu-enriched coating after high vacuum annealing at 950 °C/2 h, with marked areas of SEM-EDS microanalysis (Table 5)Table 5  Chemical composition (SEM-EDS) in micro-areas of the chromium Cuenriched coating (Figure 6)Element contentMicro-areano.6Cr Cu / wt. % / wt. %1 76,1 23,9 2 70,2 29,8 3 54,6 45,4 4 23,7 76,3 5 1,1 98,96 0,8 99,2Figure 7  Cross-section (SEM) of Cu-OF copper with a 2.0 µm PVD chromium Cu-enriched coating following high vacuum annealing at 950 °C/2 h, with marked areas of SEM-EDS microanalysis (Table 6).Table 6  Chemical composition (SEM-EDS) in micro-areas of the chromium Cu-enriched coating (Figure 7)Element contentMicro-areano.Cr Cu / wt.%  / wt.%1 97,4 2,6 2 83,8 16,23 94,4 5,54 18,9 81,15 1,0 99,06 0,7 99,37 27,0 73,0676  METALURGIJA 61 (2022) 3-4, 673-676M. POR BA et al.: MODI I ATION O  T E u ETP OPPER SUR A E LAYER IT  ROMIUM BY P YSI AL...copper substrate, led to formation of a Cu2O oxide layer on its surface, as a result of internal oxidation processes. As a result of the simultaneous inward diffusion of oxy-gen and chromium, there was observed formation of crystals enriched in oxygen and chromium in the near-surface zone, especially at the boundaries of copper grains. It was found that as a result of the diffusion an-nealing process under high vacuum conditions (at 10-3 Pa pressure) of Cu-ETP copper with a chromium coat-ing deposited using the PVD method, a diffusion CuCr layer with thickness of approx. 20 μm and hardness of approx. 120 HV0,01 was obtained, while hardness of the copper substrate was about 50 HV1. During anneal-ing, chromium diffused inward into the substrate, dis-solved in the copper matrix and formed a saturated cop-per based solid solution. As a result of the further diffu-sion process and saturation of the copper matrix with chromium, crystals rich in chromium and oxygen were formed from the supersaturated solid solution.CONCLUSIONSAnalysis of the results of the studies indicates that in order to obtain diffusion surface layer enriched with chromium on the copper sample, electrolytic copper should be used as a substrate rather than oxygen free grade – solubility of chromium in copper is a function not only of temperature but also of oxygen content. In the electrolytic copper of the Cu-ETP grade oxygen content can reach 400 ppm, which is even a several hun-dred higher value than in the oxygen-free Cu-OF grade (usually < 5 ppm). Therefore, because of high chemical affinity of chromium for oxygen inward diffusion of chromium atoms is facilitated and internal oxidation of chromium results in formation of fine dispersed crystals build of oxygen, chromium and copper. They are locat-ed both inside copper grains – with a diameter of sev-eral dozens of nanometers – and on the grain boundaries – with diameter of several hundreds of nanometers [7,8]. The conclusion was drawn that appropriate oxy-gen content in the copper facilitates diffusion of chro-mium atoms into the substrate.AcknowledgementsThe authors are grateful for the financial support of this research provided by The National Centre for Re-search and Development – Research Project No. TECH-MATSTRAEG1/347960/6/NCBR/2017 .REFERENCES [1] Z. Rdzawski. Alloy copper. [in Polish]. Wydawnictwo Po-litechniki Śląskiej. Gliwice 2009.[2] S. A. Ajeel, R. S. Yaseen, A. K. Eqal. Characterization of Microstructure and Mechanical Properties of CuCr Alloy Produced by Stir Casting.  Diyala Journal of Engineering Sciences 12(2019)4, 82-91. DOI: 10.24237/djes.2019. 12409[3] J. Godek, J. Senkara. Life test methodology for resistance spot welding electrodes with the application of mecha-nized stand. Przegląd Spawalnictwa 6(2014), 55-93.[4] W. Kwaśny, Z. Rdzawski, W. Głuchowski M. Pawlyta. In-fluence of the repetitive corrugation on the mechanism oc-curing during plastic deformation of CuSn6 alloy, Archives of Metallurgy and Materials 61(2016)3, 1261-1264. DOI: 10.1515/amm-2016-0208.[5] Z. Rdzawski, W. Głuchowski, J. Stobrawa, J. Sobota. Ef-fect of rare-earth metals addition on microstructure and properties of selected copper alloys, Archives of Metal-lurgy and Materials 59(2014)2, 641-648. DOI: 10.2478/amm-2014-0105.[6] P. Ostachowski, W. Bochniak, M. Lagoda, S. Ziołkiewicz. Strength properties and structure of CuCrZr alloy subject-ed to low-temperature KOBO extrusion and heat treat-ment. International Journal of Advanced Manufacturing Technology 105(2019)12, 5023-5044. DOI: 10.1007/s00170-019-04602-4[7] K. Franczak, P. Kwasniewski, G. Kiesiewicz, M. Za-sadzinska, B Jurkiewicz, P. Strzepek, Z. Rdzawski. Re-search of mechanical and electrical properties of Cu-Sc and Cu-Zr alloys. Archives of Civil and Mechanical Engi-neering 20(2020)28, 1-15. DOI: 10.1007/s43452-020-00035z [8] Z. Rdzawski. The influence of aging on the operational properties of the chromium copper. [in Polish].  PhD The-sis. AGH Kraków (1977). Note:  The responsible for English language is Dagmara Pędrak-Pelczyk, Translation Office FABRYKA JĘZYKA, Bochnia, Poland.

Modification of the Cu-ETP copper surface layer with chromium by physical vapor deposition (PvD) and diffusion annealing

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia