COATING OF TOOL STEELS WITH CrN ENRICHED WITH SILVER. Jana BOHOVIČOVÁ, Mária HUDÁKOVÁ, Peter JURČI,

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1 COATING OF TOOL STEELS WITH CrN ENRICHED WITH SILVER Jana BOHOVIČOVÁ, Mária HUDÁKOVÁ, Peter JURČI, Faculty of Materials and Technology of the STU, Department of Materials, Trnava, Paulínská 16, Trnava, Slovak Republic, Abstract Specimens made from Vanadis 6 cold work tool steel were machined, ground, heat processed by standard regime and finally mirror polished. After that, they were layered with CrN and CrAgN, respectively. The Agcontent in the layers was chosen to 3 wt% and 15 wt%. The CrN-film grew in a typically columnar manner. The CrN- and CrAg3N-coatings had a thickness of 4.3 and the structure lost columnar character. The hardness of the CrN coating was GPa and it was only slightly lower for coatings with 3% Ag addition. The addition of 15% Ag lowered the hardness coating substantially. The addition of 3%Ag in the CrN improved the adhesion, which can be attributed to the capability of the film to store higher work of fracture before failure. On the other hand the addition of 15%Ag reduced the adhesion markedly. The Ag-containing coatings exhibited superior tribological properties at intermediate temperatures. Compared to pure CrN, the friction coefficient is lowered to 50% when measured at 400 and 500 o C, respectively. This is reflected in lowering the specific wear ratio in an order of magnitude. Keywords: Vanadis 6 cold work steel, PVD, chromium nitride, silver addition, tribology 1. INTRODUCTION CrN films can be synthesized in a very wide range of chemistry, phase constitution and properties. The microhardness of coating is normally ranging from 1500 and 3000 HV [1-5]. Tribological properties of CrNbased films, however, cannot be changed in a sufficiently wide range since they are given by the nature of the film compound itself. One of possible way how to improve the tribological behaviour of CrN-films is external lubrication. But, commercially available lubricants (oxides, molybdenum disulfide, graphite) exhibit considerable shortcomings. This is why self-lubricating composite films providing solid lubrication have been a subject of scientific interest in last few years [6-16]. Silver is the most common noble metal used as an addition to the transition metal (TM) nitrides thin films. It posses stable chemical behaviour and can exhibit self-lubricating properties due to its low shear strength. In addition it is known that silver is capable to migrate to the free surface providing lubrication above 300 o C. The current paper deals with the development of nanocomposite CrAgN coatings on the Vanadis 6 Cr-V ledeburitic tool steel. It describes and discusses the basic coating characteristics like wear resistance, friction coefficient, Young s modulus, as a function of the silver content and deposition temperature. 2. EXPERIMENTAL PROCEDURE The experimental substrate samples were made from the ledeburitic steel Vanadis 6 with nominally 2.1 %C, 1.0 %Si, 0.4 %Mn, 6.8 %Cr, 1.5%Mo, 5.4 %V and Fe as balance. After rough machining to the semi-final dimensions, the samples were subjected to standard heat treatment procedure giving a final hardness of 724 HV 10. After the heat treatment, the samples were fine ground and polished up to the mirror finish. The CrNand CrAgN - coatings were deposited in a magnetron sputter deposition system, in a pulse regime with a frequency of 40 khz. Two targets, opposite positioned, were used. For the deposition of CrN, two targets

2 from pure chromium (99.9 %Cr of purity) were used. The target output power was adjusted to 2.9 kw for each cathode. For the deposition of silver containing films, one silver cathode (99.98% of purity) was inserted into the processing chamber instead of one chromium target. In these trials, the cathode output power was 5.8 kw on the chromium cathode. On the silver cathode, the output powers were 0.1 and 0.45 kw in order to produce the silver contents of 3 wt% and 15 wt%, respectively. Two deposition temperatures have been used. The first one was 250 o C. To achieve that, the samples were heated using resistive heaters during the sputter cleaning step. Afterwards, the temperature of 250 o C was kept constant by ion bombardment only. The second deposition temperature was 500 o C. It was achieved by resistive heaters placed on internal walls of the processing chamber. The processes were carried out in a low pressure atmosphere (0.15 mbar), containing pure nitrogen and argon (both of % of purity), in a ratio of 1:4.5. The microstructure of substrate material was documented using light microscope ZEISS NEOPHOT 32. Microstructural analysis of coatings was completed on fracture surfaces of coated specimens, using a field emission scanning electron microscope (SEM) JEOL JSM-7600F operating at an acceleration voltage of 15 kv. The nanohardness and the Young s modulus (E) of the coatings were determined using the instrumented nanoindentation test under a normal load of 20 mn, at a NanoTest (Micro Materials Ltd) nanohardness tester equipped with a Berkovich indenter. The adhesion of the coatings on the substrate has been evaluated using a CSM Revetest scratch-tester. Tribological properties of the coatings were measured using the CSM Pin-on-disc tribometer at a normal load of 1N, at ambient and elevated temperatures, up to 500 o C. Balls 6 mm in diameter, made from sintered alumina and CuSn6 bronze (as-cast structure, hardness of 149 HV 10) were used for testing. After the testing, the wear tracks widths were measured on light microscope ZEISS NEOPHOT 32 and relative wear ratios were calculated according to method described recently [17]. 3. RESULTS AND DISCUSSION The microstructure of the substrate material after the heat treatment is shown in Fig. 1. Light micrograph shows that the material consists of the matrix, formed with tempered martensite and fine carbides, uniformly distributed throughout the matrix. The carbides are of two types: MC and M 7 C 3. Fig 1. Light micrograph showing the microstructure of PM ledeburitic steel Vanadis 6 substrate in asquenched and tempered state Fig. 2 shows cross-sectional secondary electron micrographs from all the developed films. The thickness of CrN film without a silver addition was 4.3

3 thickness of the films, Fig. 2b,c. On the other hand, the addition of 15 wt. % Ag accelerated the growth rate The pure CrN film formed at 250 o C grew in a columnar manner with well visible individual crystallites, Fig. 2a. The addition of 3%Ag into the CrN, formed at the same temperature did not change the growth mechanism of the layer significantly, Fig. 2b. The temperature effect on the layer growth for the films with 3 wt. %Ag addition is visible on the micrograph in Fig. 2c. It is clearly visible that higher deposition temperature does not influence the growth manner. Fig. 2d shows microstructure of the film with 15 wt. %Ag addition. The nanohardness of pure CrN was ± 1.49 GPa, Table 1. The nanohardness of 3 wt% Ag containing films was only very slightly lower than that of the film that does not contain silver. Further, the nanohardness of these films was practically the same, e.g. the deposition temperature plays only very minor role with respect to the coating hardness. The addition of 15 wt% Ag, on the contrary, led to substantial hardness reduction it was only ± 0.61 GPa. This may be explained by the fact that silver is very soft metal and its agglomerates embedded in the CrN-matrix cause softening of the film. The Young s modulus, E, of pure chromium nitride film and CrAg3N films deposited at 250 o C and 500 o C, respectively, were of about 240 GPa, Table 1. The E values ranges, in addition, considerably overlap. Addition of 15 wt% of silver into the basic film, on the other side, tends towards decrease of the Young s modulus. Fig 2. SEM micrographs showing the microstructure of developed films, a CrN, deposition at 250 o C, b CrAg3N, deposition temperature of 250 o C, c - CrAg3N, deposition temperature of 500 o C and d - CrAg15N, deposition temperature of 500 o C.

4 Table 1. Mechanical properties of investigated films Coating/deposition Hardness [GPa] Young s modulus [GPa] temperature CrN/250 o C ± ± 15 CrAg3N/250 o C ± ± 9 CrAg3N/500 o C ± ± 17 CrAg15N/500 o C ± ± 6 After the scratch-test, the failure of the pure chromium nitride film begins with semi-circular tensile cracking, Fig. 3a. The first cracks were observed at a normal load of around 24 N (L c1 ). The total failure of the chromium nitride film is shown in Fig. 3b. It is manifested by many parallel cracks visible in the scratch, where of about 50% of the coating is removed from the substrate. The typical load range when this phenomenon occurred was N. The critical load at which these phenomena first occurred was around 23 N, Table 2. Figure 3d shows the total failure of the CrAgN film grown at 250 o C. It is evident that some of the parallel cracks were stopped their propagation through the film which suggests that the coating can store a higher amount of plastic deformation energy preceding the failure. This assumption is supported by the fact that the total failure of the film was detected at a load higher than that of pure chromium nitride, Table 2. For the film with 3 wt% Ag addition, grown at 500 o C, the first symptoms of damage occurred at the average loading of around 47 N (L c1 ). Coating damage begins with an appearance of semi-circular tensile cracks, e.g. it looks to be similar than that of the film deposited at lower temperature, Fig. 3e. The total failure, detected in the load range N (L c2 ) is in Fig. 3f. The beginning of the failure of the film containing 15 wt% Ag can not be easily found. Fig. 3g shows the scratch track at relatively low loading range (around 6.4 N), where the critical load L c1 was determined from. It is evident that the film underwent local plastic deformation with clearly visible semi-circular deformation zones. These zones are widely spaced, which suggests that the film is capable to store relatively great amount of plastic energy before failing cohesively. Typical symptoms for total failure of the coating have not been detected, in a similar way to the film with 3 wt% Ag. The scratch track contained many parallel microcracks when higher loaded, Fig. 3h. Moreover, first symptoms of chipping were detected adjacent to the scratch track at a normal load of 44.1 N. Fig 3. Light micrographs showing the failures after scratch testing: a CrN, deposition temperature of 250 o C, L c1, b L c2, c CrAg3N, deposition temperature of 250 o C, L c1, d L c2, e CrAg3N, deposition temperature of 500 o C, L c1, f L c2, g CrAg15N, deposition temperature of 500 o C L c1, h L c2

5 Table 2. Critical loads for defined degree of coatings failure Coating/deposition temperature L c1 [N] L c2 [N] CrN/250 o C 24.5 ± ± 4.4 CrAg3N/250 o C 23.4 ± ± 5.9 CrAg3N/500 o C 46.9 ± ± 8.4 CrAg15N/500 o C 6.4 ± ± 6.3 Table 3. Friction coefficients of coatings against two different counterpart s materials Testing temperature [ o C]/coating CrN/250 o C CrAg3N/250 o C CrAg3N/500 o C CrAg15N/500 o C Al 2 O 3 CuSn6 Al 2 O 3 CuSn6 Al 2 O 3 CuSn6 Al 2 O 3 CuSn6 Room temperature CrAg3N films formed at 250 and 500 o C had average friction coefficients of and 0.373, respectively. conclude that almost no positive effect of silver addition can be found when alumina ball has been used Silver addition of 3 wt % tended to lower friction coefficient and the lowering of friction coefficient became even more significant for the composite Ag-containing films grown at a temperature of 500 o C. The testing at 300 o C against alumina yields to different behaviour of the coatings. The friction coefficients for the pure CrN, CrAg3N grown at 250 o C, CrAg3N grown at 500 o C and CrAg15N formed at 500 o C were 0.357, 0.304, and 0.110, respectively. Higher testing temperature lowered the difference in the different coatings, for instance, the friction coefficients recorded by the testing at 400 o C were 0.256, 0.194, and for the pure CrN, CrAg3N formed at 250 o C, CrAg3N formed at 500 o C and CrAg15N formed at 500 o C, respectively. The measurement at 500 o C gave rather similar results. The testing against as-cast CuSn6-bronze at elevated temperatures gave similar results, also. The friction coefficients decreased as the testing temperature was higher. Table 4. Wear ratio at ambient and elevated temperatures, alumina used as a counterpart Testing temperature CrN CrAg3N/250 o C CrAg3N/500 o C CrAg15N/500 o C [ o C]/coating Room temperature 6.947x x x x x x x x x x x x x x x x10-11

6 Lowered friction coefficient m of silver containing films is reflected in wear ratios measured by the pin-on-disc testing, Table 4. This statement is valid for 3 wt% Ag containing films, in particular, where substantial decrease of wear ratios was recorded at elevated temperatures. For the film with 15 wt% Ag, there was lowered wear ratio recorded when tested at both the room temperature and the temperature of 300 o C. Above that, the wear ratio increased noticeable probably due to the fact that the coating became too soft. Figure 4 gives an overview of selected wear scares developed on different coatings by various testing conditions. Testing at room temperature gives generally very narrow wear scare, Fig. 4a, b. The addition of 3%Ag into the CrN operates as an effective lubricant and this effect leads to almost complete preservation of the film on the substrate after testing at high temperature, Fig. 4c, while the addition of 15%Ag makes the film too soft and sensible to wear, which resulted in it s partial removal from the substrate, Fig. 4d. a b c d Fig 4. Light micrographs showing the wear scares of the films: a CrAg3N deposition at 250 o C, room temperature, b CrAg3N, deposition at 500 o C, room temperature, c - CrAg3N, deposition temperature of 500 o C, testing at 500 o C and d - CrAg15N, deposition temperature of 500 o C, testing at 500 o C. CONCLUSIONS Investigations of magnetron sputtered CrN-films without/with Ag-additions have brought the following findings: t it was recognized that higher Ag-content led to greater thickness of the film. No influence of 3%Ag on the growth manner has been recorded the films without Ag and those with 3%Ag grew in columnar manner. Addition of 15 wt% of silver induced substantial changes in the growth mechanism of the films. Moreover, individual Ag became easily visible in the microstructure of the films. The addition of 3 wt% of Ag did practically not influence both the hardness and the E. On the other hand, incorporation of 15 wt %Ag resulted in substantial hardness decrease and decrease of Young s modulus.

7 The adhesion of CrAg3N-films was much better than pure CrN. On the other hand, the adhesion of CrAg15N-film was very poor. Silver containing films exhibited excellent wear properties at temperatures above 400 o C. The friction coefficient was reduced by 70-75% compared to pure CrN. This was reflected in reduction of wear ratio of the films with 3 wt% Ag. The addition of 15 wt% Ag, on the contrary, gave worse wear properties due to overall softening of the film REFERENCES [1] Kondo, A., Oogami, T., Sato, K., Tanaka, Y.: Surf. Coat. Techn (2004) 238. [2] Mercs, D., Bonasso, N., Naamane, S., Bordes, J.M., Coddet, C.: Surf. Coat. Techn. 200 (2005) 403. [3] Odén, M., Almer, J., Hakansson, G., Olsson, M.: Thin Solid Films (2000) 407. [4] Ehiasarian, A.P., Munz, W.D., Hultman, L., Helmersson, U., Petrov, I.: Surf. Coat. Techn (2003) 267. [5] Mayrhofer, P.H., Tischler, G., Mitterer, C.: Surf. Coat. Techn (2001) 78. [6] Aouadi, S.M., Bohnhoff, A., Sodergren, M., Mihut, D., Rohde, S.L., Xu, J., Mishra, S.R.: Surf. Coat. Techn. 201 (2006) pp [7] Muratore, C., Voevodin, A.A., Hu, J.J., Zabinski, J.S.: Wear 261 (2006) pp [8] Hu, J.J., Muratore, C., Vojvodin, A.A.: Compos. Sci. Technol. 67 (2007) pp [9] Aouadi, S.M., Singh, D.P., Stone, D.S., Polychronopoulou, K., Nahif, F., Rebholz, C., Muratore, C., Vojvodin, A.A.: Acta Mater. 58 (2010) pp [10] Wenda, E.: J. Therm. Anal. 30 (1985) p [11] Basnyat, P., Luster, B., Kertzman, Z., Stadler, S., Kohli,P., Aouadi, S., Xu, J., Mishra, S.R., Eryilmaz, O.L., Erdemir, A.: Surf. Coat. Techn. 202 (2007) pp [12] Mulligan, C.P., Blanchet, T.A., Gall, D.: Surf. Coat. Techn. 204 (2010) pp [13] Mulligan, C.P., Blanchet, T.A., Gall, D.: Surf. Coat Techn. 203 (2008) pp [14] Mulligan, C.P., Gall, D.: Surf. Coat. Techn. 200 (2005) [15] Mulligan, C.P., Blanchet, T.A., Gall, D.: Surf. Coat Techn. 205 (2010) pp [16] Kostenbauer, H., Fontalvo, G.A., Keckes, J., Mitterer, C.: Thin Solid Films 516 (2008) pp [17] Jurči, P., Dlouhý, I.: Appl. Surf. Sci. 257 (2011) pp