23rd International Colloquium Tribology - January 2022 281 Tribological and microstructural analysis of PVD coatings: deposited on high chromium steel substrates for cold rolling applications A. Carabillò Politechnic Department of Engineering and Architecture (DPIA), University of Udine, via delle Scienze 208, 33100, Udine, Italy. EUROLLS S.p.A., Udine, Italy carabillo.antonio@spes.uniud.it, Tel. number: (+39) 0433 750500 A. Lanzutti Politechnic Department of Engineering and Architecture (DPIA), University of Udine, via delle Scienze 208, 33100, Udine, Italy. alex.lanzutti@uniud.it F. Sordetti Politechnic Department of Engineering and Architecture (DPIA), University of Udine, via delle Scienze 208, 33100, Udine, Italy. francesco.sordetti@uniud.it M. Magnan Politechnic Department of Engineering and Architecture (DPIA), University of Udine, via delle Scienze 208, 33100, Udine, Italy. michele.magnan@uniud.it M. Querini EUROLLS S.p.A., Udine, Italy matteo.querini@eurolls.com, Tel. number: (+39) 0432796511 O. Azzolini Laboratori Nazionali di Legnaro (INFN), Viale dell’Università, 2, 35020 Legnaro (PD), Italy oscar.azzolini@lnl.infn.it, Tel. number: (+39) 0498068665 L. Fedrizzi Politechnic Department of Engineering and Architecture (DPIA), University of Udine, via delle Scienze 208, 33100, Udine, Italy. lorenzo.fedrizzi@uniud.it, Tel. number: (+39) 0432 558839 1. Introduction Cold rolling is performed on about 30% of the world metal production [1]. The rolls undergo to a rolling-sliding tribo-contact at very hertzian loads. The main wear mechanisms observed for this kind of applications are mainly abrasive and triboxidative, in the worst case it occurs cracking and spalling phenomena [2]. The use of thin films, in particular PVD coatings, is widespread in many technological fields thanks to the improvement of surface properties of the coated components (wear) [3]. This study aims to determine the tribological performances of many coatings deposited on W.Nr. 1.2344 and W.Nr. 1.2379 steels. In detail, the coatings (CrN, CrCN or TiN, TiON, TiCN, AlTiN-Si 3 N 4 ) studied in this work are produced by a combination of magnetron sputtering and cathodic arc evaporation techniques, in order to match high deposition rates, high energetic and ionized plasmas with low structural residual stress and low growth defects. To obtain a high-quality coating, the PDC-SBV (Pulsed Direct Current Substrate Bias Voltage) was varied and its effect on mechanical properties was investigated. 2. Experimental The study is therefore broken down into a first phase of sample preparation and coatings deposition by using an industrial PVD prototype plant. The second phase involves the chemical and microstructural characterization of the samples. A third phase instead investigates the mechanical properties of the samples as microhardness 282 23rd International Colloquium Tribology - January 2022 Tribological and microstructural analysis of PVD coatings: deposited on high chromium steel substrates for cold rolling applications and coating/ substrate adhesion. Finally, the last phase includes the tribological test. In particular the microstructure of the coatings was characterized by means of Scanning Electron Microscope and Energy Dispersive X-ray Spectrometry (SEM+EDXS) in both top view and cross section. The elemental distribution along the coating thickness was evaluated by means of Radio Frequency Glow Discharge Optical Emission Spectroscopy (Rf-GDOES). The mechanical properties of the coating were evaluated through micro-hardness tests with variable load, while the adhesion was evaluated with scratch test performed at variable load using a HRC indenter (0-90 N in 90 sec for a length of 10mm). Figure 1: top and cross section of a 1,2344 sample coated The samples underwent to ball on flat tests at high Hertzian loads (80 and 40N with a stroke length of 10 mm and 10 Hz of frequency), using an alumina sphere (9mm in diameter). The high Hertzian loads are used to roughly simulate the typical loads of cold rolling process. During the tests, the COF was acquired continuously. Afterwards, the wear rate (WR) was measured by stylus profilometer, while the wear mechanism was evaluated by means of SEM+EDXS analysis of the worn area in both cross section and top view. Figure 2: scheme of the ball on flat test 3. Results In this part of the document, only the results obtained on 1.2344 steel substrate are presented. As shown in the HV graphs (Fig.3), the microhardness for low load is close to what has been found in the literature [4], while as the load increases, the contribution of the substrate becomes more and more evident, while obtaining an always higher value. The adhesion is evaluated by two critical lengths, Lc 1 is the first cohesive defect of the coating and Lc 2 is the first coating delamination. The low bias sample has the best mechanical properties. Figure 3: HV of the coated and uncoated 1,2344 samples Figure 4: scratch s of the 1,2344 coated samples As shown in the COF graphs (Fig.3), the coated sample, tested with an applied load of 40N, presents lower COF respect to the bare steel, while the load increase (80N) showed a similar COF between the coated and uncoated specimen. In this case, the COF presents a less noisy signal. The wear mechanism, in specimens tested at 40N (Fig.4), is abrasive with sporadic delamination between substrate and coating. These delaminations can occur from poor local adhesion due to surface defects. The width of the trace is reduced by approximately 50% compared to the uncoated case. At high load (80N) the coating is completely removed and the delaminations are present on wear track side. In this case, the substrate exposed by coating removal is subject to abrasive and triboxidative wear, as in the bare steel. The wear rate (fig.5) shows a decrease in the wear rate of 97% compared to the case of the sample not coated, for the samples tested at 40N. The delaminations does not affect the wear performances of the coating. At high loads (80N) the wear rate reduction is only of the 10%, compared to the bare steel. This is probably due to the fact that the coating is removed and does not protect the underneath substrate. The results showed how the use of multilayer coatings, high bias deposited with the combination of the magnetron sputtering and cathodic arc techniques, improved the wear resistance of the material. This result was also achieved by the control of surface defects. 23rd International Colloquium Tribology - January 2022 283 Tribological and microstructural analysis of PVD coatings: deposited on high chromium steel substrates for cold rolling applications Figure 5: COF for the coated and uncoated 1,2344 samples. Figure 6: SEM analysis of the worn areas for the specimen tested at 40 N and 80 N Figure 7: wear rates of the tested samples coated and uncoated. Bibliography [1] P. Montmitonnet, Y.E. Khalfalla, K.Y. Benyounis , “Metal Working: Cold Rolling”, Encyclopedia of Materials : Science and Technology, 2001, Pages 5500 5506 [2] A. Lanzutti, J.Srnec Novak, F. De Bona, D. Bearzi, M. Magnan, L. Fedrizzi, “ Failure analysis of cemented carbide roller for cold rolling : Material characterisation, numerical analysis, and material modelling”, Engineering Failure Analysis 116 (2020) 104755 [3] Bemporad, E., Pecchio, C., De Rossi, S., & Carassiti, F. (2001). Characterization and hardness modelling of alternate TIN/ TICN multilayer cathodic arc PVD coating on tool steel. Surface and Coatings Technology, 146 147, 363 370. [4] S.J Bull, D.G Bhat, M.H Staia, Properties and performance of commercial TiCN coatings. Part 1: coating architecture and hardness modelling, Surface and Coatings Technology, 2003, Pag. 499-506, Vol. 163-164.