The nano-laminated DLC coating was RF-sputtered at P = 0.5 Pa in ambient temperature onto the SKD11 specimen with the diameter of 25 mm and the thickness of 5 mm. The bias voltage (VB) was pulsewise controlled in the program; e.g. in case of nano-lamination with L = 10 nm and the sublayer ratio of 35:35, VB = -200 V for 35 s in formation of the high density sublayer and VB = 0 V for 35 s in formation of low density sublayer, respectively, in the pulse width of 70 s. Before DLC-coating, the metallic chromium was deposited as the bottom interlayer onto the SKD11 specimen and followed by the transient interlayer of CrN for matching with the main layer of nano-laminated DLC coating.
When the prehardened layer by the plasma nitriding is absent, the hard counter material seizes with the nano-laminated coating in two seconds after starting. This is because the raw SKD-11 substrate makes plastic deformation by indentation of the hard counter ball and both are adhesive at the moment.
On the other hand, the original SKD-11 specimen was plasma nitrided at 753 K at 200 Pa for 14.4 ks. Under the same coating conditions as stated in the above, the nano-laminated DLC was coated onto this prehardened SKD-11 specimen. Figure 9 depicts the time variation of measured friction coefficient under the same condition as stated in 2.3. Except for the initial running out period, the friction coefficient becomes constant by = 0.15 with less deviation in time. In order to investigate the reliability of this tribological property among specimens, two duplexed SKD11 specimens were prepared and subjected to ball-on-disc testing. As compared in Fig. 9, both transients of friction coefficient are equal to each other; the measured tribological property is characteristic to this duplexed SKD11.
With comparison to the tribological behavior of the nano-laminated WC (Co), the time history of friction coefficient in Fig. 9 is nearly the same even by using the prehardened SKD11 instead of WC (Co).