## 3 Results and Discussion
The results show that ZrO\({}_{2}\)/DLC films exhibit ultralow friction behavior at the stable stage. It is speculated that ultralow friction of the friction pair is attributed to the transferred films, which is formed during sliding. However, the transferred films are worn quickly out in the subsequent tribotests. In dry hydrogen, there are not only shallow plow grooves but also the color transferred from the films onto the wear scar of the ball surface, as shown in Figure 7c. In vacuum, there is black wear debris around the edge of the wear scar on the worn surface the ball, as shown in Figure 7d. Table 1 shows CoF of ZrO\({}_{2}\)/DLC films and wear rate of ZrO\({}_{2}\) ball, respectively. Numbers represent the friction testing conditions of the testing temperature and environment gas, respectively. Figure 8 shows CoF of ZrO\({}_{2}\)/DLC films under different temperatures and environments. The blue circle in the curve represents CoF of the friction pair under different temperatures and environments. The dark yellow block in the curve represents the wear rate of the friction pair under different temperatures and environments. Numbers 1, 2 and 3 represent the temperatures of room temperature, 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C in ambient air, respectively. Numbers of 4, 5 and 6 represent the temperatures of room temperature, 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C in nitrogen environment, respectively. From room temperature to 200 \({}^{\circ}\)C, CoF decreases with the increase of temperatures in ambient air. CoF is almost the same with the increase of  
Figure 7: Worn surface images of ball under different environments at 200 \({}^{\circ}\)C: (**a**) in dry nitrogen, (**b**) in ambient air, (**c**) in dry hydrogen, (**d**) in vacuum and there is the worn zone within circle. temperatures in dry nitrogen and hydrogen environment. CoF is as low as 0.08 at the temperature of 100 \({}^{\circ}\)C, and high at room temperature and the temperature of 200 \({}^{\circ}\)C in vacuum. The tendency of the wear rate is almost the same as that of CoF. It is found from the experimental results that the friction pair exhibits high wear-resistance behaviors, even though CoF increases slowly with the increase of the temperatures in nitrogen and hydrogen environment. The wear rate presented here is relatively low because there is no coating on the ball before the tribotests. It is found that CoF and the wear rate of ZrO\({}_{2}\)/DLC films are ultralow and stable in dry hydrogen because the transferred films are produced on the worn surface on the ball side, especially at the temperature of 100 \({}^{\circ}\)C; therefore, it is necessary to measure the microstructure of the transferred films on the worn surface of the ball, analyze the tribochemical reaction and the formation of the transferred films during sliding and understand the possible ultralow friction mechanism of ZrO\({}_{2}\)/DLC films. For a good understanding of the temperatures and environments influencing the tribological process, the microstructure of the worn surface of the disc and ball is investigated before and after the tribotests by Raman spectroscopy. It is helpful to understand how ZrO\({}_{2}\)/DLC films behave within the contact area. The tribochemistry reactions in this tribological system unavoidably take place under high temperature and friction heating conditions. Raman spectroscopy would add strong evidence to the tribochemistry mechanism of the friction pair. Figure 9 shows Raman spectra of DLC films on the