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a-C:H:Zr-0 a-C:H:Zr-0.2 a-C:H:Zr-0.3 a-C:H:Zr-0.4 a-C:H:Zr-0.5 Coatings

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Hardnesss I(d)/I(g)

Influences of various Zr target current on tribological behavior of a-C:H:Zr-x coatings

W.H. Kao 1,a

1 Institute of Mechatronoptic Systems, Chienkuo Technology University, Changhua, Taiwan.an18851158@yahoo.com.tw

1.Introduction Diamond-like carbon films exhibit a number of excellent mechanical and tribological

properties, these coatings arrest abrasive wear, improve friction environment, increase tool life and biocompatibility[1-7]. Diamond-like carbon (DLC) coatings are increasingly being used to improve the tribological performance, due to the structure of DLC consists of both diamond-like sp3 bonding and graphite-like sp2 bonding, so that DLC coatings had high hardness, high electrical resistance, chemical inertness, low friction and high wear resistance have been reported in numerous publications reviewed [8-13].In light of these developments, a-C:H:Zr-x coatings with varying metal-Zr doping were deposited on a AISI M2 substrate by a Closed Field Unbalanced Magnetron (CFUBM) sputtering (Teer Coating UDP450, UK). An a-C:H:Ti/a-C:Ti/Ti/ interlayer was deposited on the disk under a a-C:H:Zr-x topcoat. In order to find the optimum a-C:H:Zr-x coatings/counterbodies as wear pairs, the tribological experiments of these variously coated AISI M2 disks were performed AISI 52100 steel and Si3N4 ceramic balls, respectively.

Nantou taiwan,

November 21-23,2014

3.2 Adhesion of coatingWith the increase of the current from 0.0 A to 0.5 A, the adhesion quality raking is improve. The typical results were shown in Fig.2. Figure 2(a) displayed a typical A grade of a-C:H:Zr-0.4 coating with excellent adhesion performance. On the other hand, Figure 2(b) displayed a typical E grade of a-C:H:Zr-0 coating with poor adhesion performance. The a-C:H:Zr-0.3, a-C:H:Zr-0.4 and a-C:H:Zr-0.5 coatings have an excellent adhesion performance (A grade ),whereas the a-C:H:Zr-0, and a-C:H:Zr-0.2 coatings have a poor adhesion performance (E grade).

3.3 Tribological properties Fig. 3 and 4illustrate the influence of the current of Zr target on the tribological properties of the coatings. The results show that wear lifetime increase and the wear depth reduce as the Zr target current is increased. Figures 3 and 4 display that the a-C:H:Zr-0.4 coating provides the optimal tribological properties for sliding against 52100 and Si3N4, respectively. Furthermore, Hertz contact stress of the a-C:H:Zr-0.4 coating sliding against Si3N4 is 755.6 MPa, it is higher than sliding against AISI 52100 is 582.6 MPa. But the values of friction coefficient against the both balls was the same, and the were lifetime sliding against Si3N4 is 5143 seconds more longer than sliding against AISI 52100 (2840 second). It is due to a carbon-rich transfer layer is formed on the Si3N4 counterbody surface during wear testing. This transfer layer acts as a solid lubricant between the contact surfaces, and therefore further reduces the wear and friction effects.

Figures 5(a) and (b) respectively display the worn surfaces of Si3N4 ball and 52100 ball after sliding against a-C:H:Zr-0.4 coating ( testing 24 minutes). The chemical elements analysis of circular marked region of Si3N4 ball was shown in Fig. 5(a), it displayed a carbon element transformed from coating to Si3N4 ball about 52 at.%. The carbon is a good lubrication material reducing wear and friction during wear test. The chemical element of circular marked region of worn surface of 52100 steel ball as shown in Fig.5 (b), it displayed a very low carbon content about 4.37 at%, i.e. no lubricated material was formed on 52100 steel ball. Compare to the Fig. 5 (a) and (b), the Si3N4 ball can be formed a transformed layer with a carbon-rich, but on the other hand the worn surface of 52100 steel ball was content of a major elements of Fe (74.3 at.%), O(21 at.%) and a little C (4.7 at.%) existed. All of the wear pairs for a-C:H:Zr-x coatings sliding against Si3N4 and 52100 steel possessed the similar wear behavior described as above. Therefore, the lifetime of a-C:H:Zr-x coatings sliding against Si3N4 ball were longer than a-C:H:Zr-x coatings sliding against AISI52100 ball.

4. ConclusionsBased on the results of this study, we can conclude the following:(1).The a-C:H:Zr-0.4 coating possesses the best adhesion property with A grade.(2).All of a-C:H:Zr-x coatings, can be formed a rich-carbon on Si3N4 ball while sliding against Si3N4

ball induced their wear lifetime were longer than sliding against AISI52100 ball.(3).The a-C:H:Zr-0.4 coating possesses the excellent tribological properties including low wear depth

and long wear lifetime as sliding against Si3N4 ball or AISI 52100 steel ball.

Fig. 1 The hardness and I(d)/I(g) ratio of various current of a-C:H:Zr-x coatings.

(a) (b)Fig. 2 The results of adhesion properties of (a) a-C:H:Zr-0.4 (b) a-C:H:Zr-0

Fig. 3 Wear lifetime of a-C:H:Zr-x coatings with x=0.0, 0.2, 0.3, 0.4, and 0.5A sliding against AISI 52100 and Si3N4.

Fig. 4 Wear depth of a-C:H:Zr-x coatings sliding against AISI 52100 and Si3N4

(a) (b)Fig. 5 SEM image of the worn surfaces of counter balls that sliding against a-C:H:Zr-0.4 coating after tested 24 min: (a) Si3N4 ball (b) AISI 52100 ball.

Keywords: Diamond-like carbon; Tribological property; Lifetime Abstract. A series of a-C:H:Zr-x coatings (the x in the term a-C:H:Zr -x is the Zr target current varied in the deposition process) have been deposited on AISI M2 steel used unbalanced magnetron sputtering system (UBMS). During deposition process, different a-C:H:Zr-x topcoats were deposited by varying the Zr target current from 0.0 A to 0.5A while maintaining the remaining process conditions at the constant settings. The microstructure, adhesion and tribological properties of the a-C:H:Zr-x coatings were found to vary with the Zr content. The tribological properties of the coatings had been tested against AISI 52100 or Si3N4 counterbody under ball-on-disk point contact wear mode using an oscillating friction and wear tester. Of the various coatings, the a-C:H:Zr-0.4 coating provided the best tribological properties, including the lowest wear depth, the friction coefficient and the longest lifetime. Compare to the coatings sliding against both counterbodies, all of coatings possessed the high wear depth as sliding against Si3N4, but it displayed longer wear lifetime than sliding against AISI 52100

2. Experimental2.1. Coating deposition In the present study, a-C:H:Zr-x /a-C:H:Ti/a-C:Ti/Ti coatings are deposited on AISI M2 steel disks using a CFUBM sputtering method with a zirconium (Zr) metal target, a titanium (Ti) metal target, two graphite (C) targets and reactive gases of methane (CH4) and acetylene (C2H2). The various currents (0, 0.2, 0.3, 0.4 and 0.5 A) were respectively applied Zr metal target during topcoat a-C:H:Zr-x deposition (deposited time:75 minutes) and a constant current 0.4 A was applied Ti target during interlayer a-C:H:Ti/a-C:Ti (deposited time 45 minutes). In order to enhance the adhesion between coating and substrate a 0.2 μm Ti layer was deposited (1 A current applied Ti target and deposition time 20 minutes). A constant current 2.3 A was applied C targets during a-C:H:Zr-x /a-C:H:Ti/a-C:Ti deposition. The gas flow rates of methane (CH4) and acetylene (C2H2) were a contact 4 sccm during deposited topcoat a-C:H:Zr-x and interlayer a-C:H:Ti/a-C:H. The typical operating pressure of the source with the Argon plasma turned on was on the order of 10-4 Torr. The substrate bias -42V was applied to the substrate holder at pulse repetition rate of 50 KHz. The substrate holder was rotated at 3 rpm during the deposition. The total deposition time was 140 minutes. 2.2. Adhesion test The adhesion characteristics of the coatings were investigated using a standard Rockwell-C hardness tester with an applied load of 150 kgf. In [14], the authors identified five distinct cracking and flaking failure modes for thin coatings, raking from A to E grade. The A grade has only radial cracks around the indentation. This failure mode is generally indicative of a thin coating with good adhesion characteristics. However, thin coatings with poor adhesion properties typically fail with a E grade, i.e. a large annular area of the coating flakes off around the indentation2.3. Coatings structure The Raman spectroscopy was used to analysis the structure of a-C:H:Zr-x coatings. I(d)/I(g) was the intensity ratio of the peak integral, was obtained from the peak of Raman spectra. When the I(d)/I(g) ratio is small, it indicates more amount of sp3 contents in the coating. 2.4. Mechanical properties of the coatings The hardness and elastic modulus of the coatings were measured by nano-indentation with a Berkovich diamond tip under the applied load of 5mN. 2.5. Tribological properties test Tribological properties were performed on a wear tester. Both typical balls (AISI 52100 and Si3N4) with 10.0 mm in diameter, a load of 10N was applied to the tribological system slide on coatings at a constant frequency of 50 Hz, stroke 1 mm. Each test was performed for a total of 24 minutes and was conducted at room temperature, atmospheric pressure, and under unlubricated conditions. Following each test, the maximum depth of the wear scar on the coated sample was measured using an optical profilometer (BMT, Germany). Each coating was tested twice (using a different substrate each time), and three different wear measurements were taken from each worn specimen. The six measurements were then averaged to obtain a representative wear measurement for the coating. The wear lifetime, defined as the elapsed time between the start of the wear test and the point at which the friction coefficient suddenly increased. For some wear pairs, however, the friction coefficient curve remained low and stable throughout the entire 24-minute sliding test. In this case, the coating lifetime was evaluated by conducting a new wear test and recording the time at which the friction coefficient increased and started to fluctuate. The wear surface evolution was examined progressively using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).3. Results and discussion3.1 Hardness and I(d)/I(g) ratio of a-C:H:Zr-x coatings The Raman curves can be fitted with G-band and D-band components, and the intensity ratio of I(d)/I(g) of different coatings have been calculated with the results as shown in Fig.1.With the increase of the current of the Zr target, the I(d)/I(g) ratio decreases. According to the reports, the G-band and the D-band intensity ratio I(d)/I(g) give important information on the structure of the DLC coatings, which the I(d)/I(g) is proportional to the ratio sp2/sp3 bonds[15].The 0.4 A coating has the smallest I(d)/I(g) ratio with 1.04. The decreased in I(d)/I(g) ratio due to the increase in sp3 component. Fig.1. illustrates the hardness and the I(d)/I(g) ratio of a-C:H:Zr-x coatings. With the I(d)/I(g) ratio decreased the hardness of the coatings is increased. The highest hardness was a-C:H:Zr-0.4 coating with the lowest 1.04 value of I(d)/I(g). The current in the 0.4 A coating with the highest hardness that it was forecasted it would be a excellent tribological performance.