A METHOD FOR TRIBOLOGICAL TESTING OF THIN HARD COATINGS

Transcription

1 A METHOD FOR TRIBOLOGICAL TESTING OF THIN HARD COATINGS R. MICHALCZEWSKI, W. PIEKOSZEWSKI, M. SZCZEREK and W. TUSZYNSKI Institute for Terotechnology (ITeE), ul. Pulaskiego 6/1, 26-6 Radom, POLAND; SUMMARY The authors have elaborated a new method for tribological testing of thin, hard antiwear coatings, using a ball-on-disc tribosystem, under conditions of dry sliding. Al 2 O 3 ball is pressed against the disc rotating in the horizontal plane. The investigated coatings are deposited on steel discs. A novelty of the method is the removal of wear debris from the contact zone through a draught of dry argon. This helps to reduce the scatter of results. All test conditions have been precisely defined, particularly: the kind of motion, air relative humidity, ambient temperature, sliding speed, load, substrate material, substrate hardness and roughness, and coating thickness. The authors used the method to investigate various PVD coatings (deposited by the vacuum-arc method), i.e. single layer: TiN, Ti(C,N), CrN, Cr(C,N), and multilayer: Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr 2 N)/CrN/Cr. It has been shown that CrN coatings exhibit the best antiwear properties. Friction coefficients for CrN and Cr(C,N) coatings are much smaller than for widely used TiN. Multilayer coatings have better antiwear properties than single layer ones. Keywords: Test method, ball-on-disc tribosystem, wear, friction, PVD coatings 1 INTRODUCTION Thin hard antiwear coatings deposited by the PVD process (physical vapour deposition) are used mainly to enhance life of friction pairs. Apart from traditional areas of their successful application, i.e. for enhancing of life of cutting tools, coatings are also applied in various technical devices like transmissions, engines, pumps and compressors [1]. In view of completely different working conditions in such devices, a great diversity of PVD coatings is in use; needless to say that all the time new coatings appear in the market [2]. But by now there has been a lack of a uniform, widely accepted method for investigation of tribological properties of thin hard coatings. This brings about the incomparability of results obtained at various laboratories; e.g. for TiN - steel couples friction coefficient values vary between,1 to [3]. Also measures of antiwear properties of PVD coatings significantly differ from one another. For example, in the work [4] the researchers report larger wear for CrN than TiN coating. Completely different results are presented in [5], [6] and [7], where CrN coating exhibits better antiwear properties than TiN. Due to the above described situation, tribological testing of coatings deposited on cutting tools [8], under real working conditions, may prove much more reliable than laboratory testing. However, such an approach may be unacceptably costly in case of a necessity for testing of coatings deposited on e.g. casting moulds [9]. Thus, to compare and assess thin hard coatings, it is highly required to elaborate and disseminate a uniform method, carried out under laboratory conditions. The first step towards achieving of this goal was the international research project VAMAS (Versailles Advanced Materials and Standards) realised in three stages by the G-7 countries. Experiments conducted in the eighties on ceramics shown that to obtain good reproducibility of test results, one must keep the same test conditions, especially the relative humidity, which has later been expressed in the ASTM G 99-9 and DIN 5324 standards [1], [11]. Poor reproducibility of test results obtained at various laboratories comes from an influence of many factors on measures of tribological properties. This matter - with respect to PVD coatings - is considered in [4]. From that work it is apparent that friction and wear characteristics depend among others on the kind and preparation of the substrate, coating deposition parameters, kind of motion during tribological tests and test conditions. As a result of an overview of the literature, the authors gathered in four groups such factors as have the most significant influence on the tribological results for thin hard coatings. Factors relating to the substrate: substrate hardness [12], [13], substrate roughness [14], [15], [16]. Coating thickness [17]. Kind of motion (sliding or rolling) [18]. Test conditions: counterbody material [19], action of wear debris [19], [2], air relative humidity [21], [22], ambient temperature [12], [16], sliding speed [12], [21], [23], [24], [25], load [25], [26], friction pair vibrations (depending on the stiffness and construction of the loading unit, tribosystem spatial configuration, axial run-out and sliding speed) [22], [27], [28].

2 The particular analysis of the results reported in the literature suggests that all of the mentioned factors have a significant effect on the friction coefficient and/or wear. This practically means that only results obtained under identical or close test conditions are comparable. Thus, the coatings to be compared should have close thickness, be deposited on the substrates of the approximate hardness and roughness, be rubbed against the same counterbody material, at identical motion (e.g. sliding) and under close conditions (speed, load, relative humidity, etc.). Unfortunately, the existing standards like ASTM G 99-9 or DIN 5324 [1], [11], concerning tests using a pin-on-disc or ball-on-disc tribosystem, do not precise all of the above mentioned factors. These standards advise only the kind of motion (sliding), and suggest the range of rotational speeds, pin (ball) diameters and general requirements referring to the construction of the tester loading unit to reduce vibrations. Taking the above remarks into consideration, the authors have elaborated a method for tribological testing of thin hard engineering coatings. They have precisely defined almost all of the mentioned factors. The method is based on the experience gained during the realisation of the VAMAS-TWA 1 and COST 516 Tribology projects [3], [29]. In the present work the authors have employed the new method to compare tribological properties of various coatings deposited by the PVD process (vacuum-arc method) like TiN, Ti(C,N), CrN, Cr(C,N), Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr 2 N)/CrN/Cr. 2 TEST METHODOLOGY 2.1 Substrate and coating preparation Four single layer coatings were employed: TiN, Ti(C,N), CrN and Cr(C,N) as well as two multilayer ones: Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr 2 N)/ CrN/Cr; Cr(C,N) was the outer layer. All of them were deposited by the PVD process (vacuum-arc method) on the steel substrate (discs). The single layer coatings were 2 µm thick. The thickness of particular layers of Cr(C,N)/CrN/Cr was 1,5, 1,5 and,3 µm respectively. In the Cr(C,N)/(CrN+Cr 2 N)/CrN/Cr coating the Cr(C,N) layer was 1,5 µm thick, (CrN+Cr 2 N) together with CrN 5 µm, and Cr,3 µm. The discs were made of hardened and tempered hotwork tool steel (WCL) with hardness not lower than HRC 5. Such steel is used for manufacturing of casting moulds, forging dies and punches. Prior to coating deposition the discs had been ground and lapped to the roughness of,5 µm (R a ). 2.2 New test method In general, the developed method conforms to the requirements during the realisation of the VAMAS- TWA 1 project [3] (performed in 1993) as well as the standards: ASTM G 99-9 [1] and DIN 5324 [11]. The unlubricated ball-on-disc tribosystem was chosen for tribological investigations. A novelty of the method is the removal of wear debris from the contact zone through a draught of dry argon (2 dm 3 min -1 ) - Figure 1. Ar Figure 1: The ball-on-disc tribosystem with an Ar jet The stationary ball, 1 mm in diameter, was pressed from above at the load 1 N against the disc rotating in the horizontal plane at the speed of,1 m s -1. Track radius was 18 mm. Ambient temperature was 23 C and the air relative humidity 5%. Before testing, the specimens had been washed in an ultrasonic washer with toluene (for 1 min) and then hexane (5 min). For tribological assessment of the tested coatings, the wear volume and friction coefficient were computed. The wear volume (in mm 3 ) was calculated by multiplication of the average cross sectional area (measured using stylus profilometry in three planes every 12 - Figure 2) by the wear track perimeter. The friction coefficient was calculated from actual values in this part of the friction characteristic that corresponded to steady friction. Profile depth [µm] 1,5,5, -,5 - -1,5,,5 1,5 2, 2,5 Traverse length [mm] Figure 2: Example of the wear track cross section measured on TiN coating It was assumed that the coatings should be investigated at such a distance as there is no sign of their wearing away. For this purpose, prior to final investigations, the linear wear of each coating versus sliding distance was analysed; the linear wear was obtained through profiling in various sectors of the wear track and its maximum value was assessed. For example, some TiN coatings at about 1 m distance were worn away, i.e. measured linear wear exceeded the coating thickness (2 µm), Figure 3. To prevent from this situation, the maximum sliding distance for TiN coatings was assumed 5 m; from Figure 2 it can be seen that at this distance the maximum linear wear of TiN coating is c.a. µm.

3 Lin. wear (µm) TiN thickness Slid in g distance (m ) Figure 3: The linear wear of 2 µm thick TiN coating versus the sliding distance The similar procedure of maximum sliding distance determination was carried out for the other investigated coatings. It must be added here that the maximum sliding distance extremely differed between them; the shortest value was observed for Ti(C,N) coatings: 15 m, and the longest for CrN: over 6 m. So, to compare the tested coatings, the wear rate (in mm 3 km -1 N -1 ) was calculated for each of them by dividing the coating wear volume by sliding distance and load. For each coating tests were repeated five times. To discard any outlying result, Dixon s test was performed (at 5 % significance level). 2.3 Test rig The experiments were conducted using a special ballon-disc tester, known as T-1, intended for precise investigation of thin hard coatings. The tester was equipped with a computer-aided specialised system of control and measurements, which made it possible to acquire on-line data from different transducers. The T-1 tester and its control-measurement system have been designed and are manufactured at ITeE in Radom. They are shown in Figure 4. Friction coefficient f Friction coefficient f,2,,2 f f Sliding distance (m), W Sliding distance (m) W Total lin. wear W (µm) Total lin. wear W (µm) Figure 5: The influence of wear debris on the friction and wear characteristics for the Al 2 O 3 - TiN tribosystem: wear debris present in the contact zone, wear debris removed (by draught of dry Ar) from the contact zone Thus, the new test method for investigation of thin hard coatings assumes the wear debris removal. The authors tried two ways of achievement of this goal - through a draught of dry Ar and by changing of the tribosystem spatial configuration (changing of the position of the disc axis from vertical to horizontal) Wear debris removal through the draught of dry Ar The removal of wear debris from the contact zone caused a significant reduction in the TiN coating wear and decrease in the scatter of results, Figure 6. An influence of a wear debris action on the friction coefficient value was not observed. In this and the next figures the confidence intervals, calculated at 95% probability, have been added Wear debris removal by changing of the tribosystem spatial configuration Figure 4: The ball-on-disc T-1 tester together with the control-measurement equipment 3 RESULTS AND DISCUSSION 3.1 Investigation of an effect of wear debris removal The presence of wear debris in the contact zone made tribological characteristics unstable (Figure 5. When they were removed, the stability was significantly improved (Figure 5. Changing of the tribosystem spatial configuration did not make wear debris remove from the contact zone; at 5% relative humidity it adheres to the disc surface. As can be seen from Figure 7, the TiN coating wear and friction coefficient, in spite of changing of the position of the disc axis from vertical to horizontal, remains the same as in case of the vertical axis and no Ar draught (Figure 6). This enables the authors to conclude that the Ar draught is a much more efficient way of getting rid of wear debris than changing of the spatial configuration.

4 Wear rate (mm 3 km -1 N -1 ) Friction coefficient,12,9,6,3, Ar Figure 6: The influence of the wear debris action on the ( TiN coating wear rate, ( friction coefficient Wear rate (mm 3 km -1 N -1 ) Friction coefficient,12,9,6,3, Figure 7: The removal of wear debris by changing of the tribosystem spatial configuration: ( TiN coating wear rate, ( friction coefficient Ar 3.2 Comparison of tribological characteristics of the tested coatings The wear rates and friction coefficients for the tested coatings are shown in Figures 8 and 9 respectively. Wear rate (mm 3 km -1 N -1 ) *1-5 Friction coefficient TiN Ti(C,N) CrN Cr(C,N) Cr(C,N)/CrN/Cr Cr(C,N)/(CrN+Cr2N)/CrN/Cr Figure 8: The wear rates of the tested coatings,2 TiN Ti(C,N) CrN Cr(C,N) Cr(C,N)/CrN/Cr Cr(C,N)/(CrN+Cr2N)/CrN/Cr Figure 9: The friction coefficients for the tested coatings From Figure 8 it is undoubtedly apparent that CrN coatings exhibit the best antiwear properties. Similar results, obtained under laboratory conditions, are published in [6] and [7], as well as in [5] (exploitation tests). Additionally, CrN and Cr(C,N) coatings show much lower friction coefficient than TiN (Figure 9). The very good behaviour of CrN and Cr(C,N) coatings is an argument for their wider and wider application in the industry [5], [7]. The lowest observed friction coefficient for Ti(C,N) coatings comes from the necessity for their testing - due to intensive wear - at a very short sliding distance (15 m), at which the roughness of the Al 2 O 3 ball surface does not increase yet (this takes place for the other tested coatings). The greatest wear intensity observed for Ti(C,N) coatings results from their surface defects (Figure 1, which, going along with their highest nanohardness (hence large brittleness) and poor adhesion to the substrate [3], causes very fast coating cracking and then breaking off. Completely different behaviour exhibit TiN coatings - in spite of similar nanohardness, owing to much lower number of surface defects (Figure 1 and better adhesion [3], they show incomparably lower wear. It is also observed (Figure 8) that multilayer coatings have better antiwear properties than single layer ones, which is particularly apparent when comparing Cr(C,N)/(CrN+Cr 2 N)/CrN/Cr and Cr(C,N). This proba-

5 bly results from a reduction in the crack propagation intensity possible in multilayer coatings [31]. precise control of the air relative humidity and ambient temperature; 5% and 23 C respectively have been suggested, carrying out of experiments at close sliding speeds and similar loads (e.g.,1 m s -1 and 1 N respectively). Figure 1: Optical microscope images of the surface of ( Ti(C,N) coatings, ( TiN coatings 4 CONCLUSIONS On the ground of the literature overview and results of their own work, the authors state that to improve reproducibility during the testing of thin hard coatings, the following requirements should be met or actions undertaken: the substrate material should have approximate hardness (e.g. HRC 5) and similar roughness (e.g. R a =,5 µm), the thickness of investigated coatings should be close to one another (e.g. 2 µm), setting up of the identical kind of the motion (e.g. sliding friction), and tribosystem kind and configuration (e.g. ball-on-disc; 1 mm ball; vertical disc axis; ball above the disc), proper selection of the counterface material; Al 2 O 3 has been suggested, determination of the optimum sliding distance (even local wearing away of the coating ought to be avoided), removal of wear debris from the contact zone; the draught of dry Ar (2 dm 3 min -1 ) has been proposed, Using the developed method for testing of PVD deposited coatings (arc-vacuum method), i.e.: single layer TiN, Ti(C,N), CrN, and Cr(C,N), as well as multilayer Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr 2 N)/ CrN/Cr, the authors state also that: CrN coatings exhibit the best antiwear properties, CrN and Cr(C,N) coatings show much lower friction coefficient (rubbed against Al 2 O 3 ) than widely employed TiN, multilayer coatings have better antiwear properties than single layer ones. To sum up, owing to relatively small scatter of obtained results and very good resolution of the new method, it can be used at coaters laboratories for investigation of tribological properties of various thin hard coatings. 5 ACKNOWLEDGEMENTS The authors wish to express their thanks to the scientific group and their head Dr J. Walkowicz from the Plasma Technology Centre in Radom for coatings deposition and valuable advice concerning the substrate preparation. The work is partly financed by the Polish Committee for Scientific Research; the Chairman s Decision No. 476/E-15/SPUB/COST/T-7/DZ46/99. 6 REFERENCES [1] Hedenqvist, P., Hansson, G.: PVD-coating of machine components. Proc. 8 th Int. Conf. NORDTRIB 98, 1998, Ebeltoft, Denmark [2] Hogmark, S.: Materials and structures of tomorrow s PVD coatings. Proc. 2 nd Symp. COST 516 Tribology, 1999, Antwerpen, Belgium [3] Santner, E.: Comparison of wear and friction measurements of TiN coatings. Tribologia. 139 (1995), 7-29 [4] Hedenqvist, P., Jacobson, S., Hogmark, S.: Tribological PVD coatings - characterisation of mechanical properties. Surf. and Coat. Techn. 97 (1997), [5] Navinsek, B., Panjan, P., Milosev, I.: Industrial applications of CrN (PVD) coatings, deposited at high and low temperatures. Surf. and Coat. Techn. 97 (1997), [6] Su, Y.L., Yao, S.H., Leu, Z.L., Wei, C.S., Wu, C.T.: Comparison of tribological behavior of three films - TiN, TiCN and CrN-grown by physical vapor deposition. Wear. 213 (1997),

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