Tribological Properties of Hybrid Process DLC Coating against Magnesium Alloy

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1 Materials Transactions, Vol. 47, No. 4 (2006) pp to 1012 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, III #2006 The Japan Institute of Metals Tribological Properties of Hybrid Process DLC Coating against Magnesium Alloy Tai Kumagai 1, Kisaburo Shimamura 1, Haruo Okahara 2, Yorinobu Takigawa 3 and Kenji Higashi 3 1 TS Nanocoat Corp., Tokyo , Japan 2 Research and Development Department, Kasatani Co., Ltd., Osaka , Japan 3 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai , Japan The tribological properties of hybrid process DLC (Diamond-like carbon) coating against magnesium alloy are examined in order to understand the basic friction and wear properties of DLC coatings against magnesium alloy and partially to simulate the drawing process in real transfer press line. The tests are conducted using simple ball or pin on disc system with various combination of uncoated/ WC-Co (cemented carbide) and AZ31B specimen in room/elevated temperature and dry/lubricated environments. As the result, DLC coating has excellent tribological properties against AZ31B magnesium alloy at 250/260 C, which is the actual hot-working drawing condition of AZ31B sheet, in both dry and lubricated environment. DLC coating prevents possible adhesion between AZ31B and WC-Co at elevated temperature in less lubricated or nearly dry contact condition. (Received November 29, 2005; Accepted March 7, 2006; Published April 15, 2006) Keywords: magnesium alloy, diamond-like carbon, friction coefficient, wear, tribological properties, drawing 1. Introduction DLC (Diamond-like carbon) coating has very good tribological properties preventing the adhesion of worked material like aluminum and magnesium alloy during deep drawing without lubrication. 1 3) The authors are developing high-quality high-speed hot-working transfer press process of magnesium alloy for laptop PC (personal computer) casing, digital camera and various multimedia device casing in the scheme of 3-year government-funded R&D project. 4) In order to reach the target lifetime of 100,000 shots of AZ31B magnesium alloy plate drawing, various technologies such as removal of degraded WC-Co surface layer after EDM (Electro-Discharge Machining), C or N ion implantation, hybrid process DLC coating and heat-resistant lubricant are used on the WC-Co drawing die. This paper summarizes the tribological test results using simple ball (or pin) on disc system with various combination of uncoated/ WC-Co and AZ31B specimen in room/elevated temperature and dry/lubricated environments. The purpose is to understand the basic friction and wear properties of DLC coatings against magnesium alloy and partially to simulate the drawing process in real transfer press line. 2. Hybrid Process DLC Coating Equipment Figure 1 shows a schematic diagram of the DLC coating equipment used for this experiment. (HEF R&D, Model: TSD400CDD 5) ) This equipment has one magnetron sputtering cathode and one plasma source and enables hybrid PVD and Plasma-assisted CVD process continuous coating. First CrN layer is deposited using Cr target with additional ionization of gas molecule by plasma source in reactive magnetron sputtering mode on negatively biased and rotated WC-Co substrate after ion cleaning process by Ar ion bombardment. Then DLC layer is deposited using hydrocarbon gas precursor in plasma-assisted CVD mode. Typical thickness of CrN underlayer and DLC top layer is 0.5 and 2.5 mm respectively. Typical hardness of CrN and DLC is Fig. 1 Schematic diagram of Hybrid PVD/PA-CVD Coating Equipment and 2500 HV by nano-indentation hardness measurement. (CSM Instruments, model: NHT, load 20 mn Berkovich indenter, Oliver&Pharr method) This underlayer is used for excellent adhesion on WC-Co substrate and also as supporting layer to prevent excessive deformation of DLC layer leading crack formation under higher load. 3. Tribological Test Table 1 summarizes the tribological test combination and condition performed in this paper. Two different combinations of friction partner were used i.e. AZ31B pin and DLCcoated/uncoated WC-Co flat disc (Test 1) and / uncoated and AZ31B flat disc (Test 2). Table 2 shows chemical composition of AZ31B. The purpose of the Test 1 is mainly to assess the basic friction behavior such as adhesion property of magnesium alloy on / uncoated WC-Co at room temperature and heated (250 C) temperature in dry condition. Test 2 is performed in order to partially simulate the real drawing condition, so both dry and lubricated test are used at room temperature and elevated temperature. Figure 2 shows the schematic diagram of the tribometer used in this paper. It is noted that the heating is applied only on disc specimen i.e. ball or pin is heated by heat transfer from the disc specimen only after the contact between the disc and ball or pin.

2 Tribological Properties of Hybrid Process DLC Coating against Magnesium Alloy 1009 Table 1 Test conditions for various Tribometer test. Test Pin/Ball Disc Lubrication Temperature 1 AZ31B pin WC-Co Dry RT/250 C AZ31B pin WC-Co Dry RT/250 C 2 AZ31B Dry/Lubricated RT/260 C AZ31B Dry/Lubricated RT/260 C Test condition: Load 5 N, sliding speed 10 mm/s, rotation radius 3 mm Total revolution 1000 rev. Pin/ball radius 3 mm, AZ31B plate thickness 0.8 mm Lubricant hindered ester-based heat-resistant lubricant (Sugimura Chemical) Table 2 Chemical composition of AZ31B (mass%). Al Zn Mn Fe Ni Mg min max max. Bal. Fig. 2 Schematic diagram of Tribometer test setup. Fig. 3 Friction coefficient of AZ31B pin against uncoated/ WC-Co disc (Test 1, upper: uncoated WC-Co disc, lower: disc). Table 3 Mechanical and thermal properties of WC-Co (C70). Hardness 85.5 HRA Impact strength 64 kj/m 2 Thermal conductivity 71 W/(mk) Thermal expansion coefficient 5: C Test 1 DLC layer was deposited on WC-Co with WC typical grain size about 3 5 mm (Fuji Die C70). Table 3 shows mechanical and thermal properties of WC-Co used in this experiment. Surface roughness after coating is Ra 0.03 mm and the thickness of CrN underlayer and DLC layer is 0.9 and 2.3 mm respectively. disc is WC-Co with WC typical grain size about 1 2 mm (Fuji Die D50) and surface roughness is Ra 0.01 mm. Figure 3 shows friction coefficient during 1000 revolution and Fig. 4 shows the optical microscope observation of AZ31B pin and /uncoated WC-Co disc after the test. In the case of uncoated WC-Co disc, friction coefficient is around 0.4 at room temperature and is increased up to 0.8 at 250 C. The disc wear track shows partial adhesion of pin material at RT while severe adhesion was observed at 250 C, which may caused the increase of the friction coefficient. In the case of WC-Co disc, friction coefficient shows fluctuation between 0.15 and 0.5 at R.T. while stable gradual increase from 0.15 up to 0.2 was observed at 250 C. It should be noted that the minimum value during fluctuation for RT corresponds to the value for 250 C at same revolution. SEM observation of wear track after wiping the wear particle shows no prominent difference and only small AZ31B particles trapped inside the channel of WC grain boundary are observed by EDS. Raman spectroscopy of DLC before and after the test at 250 C showed no structure change. In both cases wear particles are observed beside the disc wear track only at RT. These wear particles and pin wear observation show that pin wear is dominated by abrasive wear at RT and mostly plastic deformation at 250 C, which is also the reason why hot-drawing is necessary for AZ31B. From Test 1 DLC coating is proved to prevent magnesium

3 1010 T. Kumagai, K. Shimamura, H. Okahara, Y. Takigawa and K. Higashi AZ31B pin Disc partial pile-up of disc RT wear particle of disc 250 wear particle of disc RT disc 250 Fig. 4 Optical microscope observation of AZ31B pin wear and disc wear track. alloy adhesion on WC-Co substrate at elevated (250 C) temperature and drastically decrease the friction coefficient against AZ31B. 3.2 Test 2 Figure 5 shows friction coefficient during 1000 revolution in dry at RT/260 C and in lubricated at 260 C. Figure 6 shows optical microscope observation of ball and disc wear track. In dry at RT with uncoated, friction coefficient is about 0.29 throughout the total revolution with some fluctuation, while with ball friction coefficient is as low as 0.1 after running-in and then gradually increased up to On uncoated, large adhesion and pile-up of the disc material are observed, which correspond to the larger width of the disc wear track. On ball, no adhesion or pile-up of the disc material are observed but slight scratches on DLC layer, which correspond to the small disc wear track. In dry at 260 C with uncoated, friction coefficient is about 0.46 throughout the total revolution with some fluctuation, while that with ball is about 0.1 after running-in and after about 410 revolution suddenly increased up to about 0.4 till the end of the test. On uncoated, large adhesion and pile-up of the disc material throughout the whole contact area are observed. On DLCcoated ball, most of the surface has no adhesion but small adhesion and pile-up of the disc material is observed, which Fig. 5 Friction coefficient of uncoated/ against AZ31B plate (Test 2, upper: Dry RT, middle: Dry 260 C, lower: lubricated 260 C). might have caused the sudden increase of the friction coefficient after 410 revolutions. In both case largest wear and also the plastic deformation of the disc are observed. In lubricated condition, average friction coefficient is 0.11 with uncoated and 0.09 with ball. It is noted that friction coefficient is very stable with DLCcoated ball. No adhesion or pile-up of the disc material on the ball is observed in both cases, while different surface aspect was observed inside the wear track of the disc. SEM (scanning electron microscope) and EDS (energy-dispersive X-ray spectroscopy) analysis inside the track shows that black aspect of the wear track with ball is carbon, which is less detected with uncoated. 4. Conclusion 4.1 Friction coefficient Figures 7 and 8 are the summary of the test 1 and test 2. At room temperature for both test 1 (soft pin on hard disc) and test 2 (hard ball on soft disc) friction coefficient of DLC coating and the counterpart is lower than that of WC-Co.

4 Tribological Properties of Hybrid Process DLC Coating against Magnesium Alloy 1011 Ball AZ31B disc plate Dry RT Dry RT Dry 260 Fig. 7 Comparison of average friction coefficient and ball wear diameter /uncoated WC-Co disc against AZ31B pin (Test 1). Dry 260 Lubricated 260 Lubricated 260 Fig. 8 Comparison of average friction coefficient and disc wear track width /uncoated against AZ31B plate disc (Test 2) average friction coefficient before 410 rev. Fig. 6 Optical microscope observation of /uncoated WC-Co ball and AZ31B plate disc wear track (after wiping after test). However the friction coefficient of DLC on the disc (0.3) is higher than DLC on the ball (0.14). This can be caused by the wear particle debris trapped inside the track, which is confirmed by SEM observation and EDS analysis after wiping the wear particle and also by the fact that the minimum friction coefficient value during fluctuation corresponds to the value for 250 C at same revolution. 6) In the case of DLC coating on the ball, friction coefficient is stable at 0.14 because wear debris is maybe less trapped inside the track. At elevated temperature for test 1, friction coefficient of WC-Co against AZ31B becomes rather high (0.79) compared to R.T., while that of DLC coating stays low (0.18). High friction coefficient of WC-Co seems to be due to the transfer of pin material to the disc surface at elevated temperature resulting from the strong bond on WC-Co surface and higher shear strength inside the tip of the pin. For test 2, also DLC coating showed low friction coefficient (0.15) before 410 revolutions. Sudden increase after 410 revolutions could be caused by the partial adhesion of disc material on the ball surface in Fig. 6 in combination with the excessive disc wear/plastic deformation. Further test is planned to confirm this phenomenon. In lubricated condition at elevated temperature both WC- Co and DLC coating had low friction coefficient (0.11 and 0.09) but DLC coating showed more stable friction behavior. EDS analysis of wear track of AZ31B showed more carbon on the surface of wear track for the test with ball. More comprehensive experiments are necessary to understand the friction behavior of DLC coating at elevated and lubricated environment. 4.2 Wear At room temperature for both test 1 and test 2 wear mechanism seems to be dominated by the abrasive wear producing wear particles of softer material i.e. AZ31B. Increase of pin wear against disc in test 1 compared to test 2 could be caused by the different wear particle trapping inside the track by different scheme. At elevated temperature for both test 1 and test 2 wear mechanism seems to be dominated by the adhesive wear and plastic deformation of softer material. In case of DLC coating in test 1 apparent wear of the pin could be attributed to plastic deformation wholly. 4.3 Summary DLC coating has excellent tribological properties against AZ31B magnesium alloy at 250/260 C, which is the actual hot-working drawing condition of AZ31B sheet, in both dry and lubricated environment. DLC coating prevents possible adhesion between AZ31B and WC-Co at elevated temperature in less lubricated or nearly dry contact condition.

5 1012 T. Kumagai, K. Shimamura, H. Okahara, Y. Takigawa and K. Higashi Acknowledgement This research was accomplished with funding assistance provided by the Organization for Small & Medium Enterprises and Regional Innovation, JAPAN. The authors would like to thank N. Nihira of Tokyo Metropolitan Small Business Promotion Agency for SEM/EDS analysis and M. Iwaki and M. Sasaki of RIKEN for Raman spectroscopy of the specimen in this paper. (1995) ) N. Koga and R. Paisarn: Journal of the JSTP 42 no. 481 (2001-2) ) A. Takara and K. Higashi: Mater. Sci. Forum PRICM-5 vol (2005) pp ) H. Okahara: Proc. Robot-Die Technomeeting (2005) pp ) P. Maurin-Perrier, C. Heau, J.-M. Poirson and K. Metzgar: Proc. Society of Vacuum Coaters 43 rd Annual Technical Conference (2000) pp ) Elsevier tribology series 28, Coatings tribology (1998) pp. 42. REFERENCES 1) M. Murakawa, N. Koga and T. Kumagai: Surf. Coat. Technol