## 3 Results and Discussion
Figure 4 shows CoF of ZrO\({}_{2}\)/DLC films with sliding time under different environments at the temperature of 200 \({}^{\circ}\)C. It is found that the CoF curve is different from that at room temperature and the temperature of 100 \({}^{\circ}\)C. In ambient air, CoF is 0.11 at the initial stage, then decreases to 0.06, and finally reaches CoF 0.02, which exhibits ultralow friction. It is shown that ZrO\({}_{2}\)/DLC films exhibit ultralow friction in ambient air. In dry nitrogen, CoF is 0.09 at the initial stage, then increases rapidly to 0.17, decreases to 0.04 abruptly, and increases slowly along with the sliding time. Average CoF is about 0.07. In dry hydrogen, CoF is 0.09 at the initial stage, then increases to 0.12, decreases to 0.05, and finally increases to around 0.06 along with the sliding time. The whole sliding process can also be divided into four stages, although it is not obvious. In vacuum, CoF is 0.09 at the initial stage, then increases to 0.3, which indicates that DLC films were failed. It is concluded that the friction pair exhibits excellent antifriction behavior in ambient air and hydrogen environment at the temperature of 200 \({}^{\circ}\)C.  
Figures 5-7 show the worn surface topography of ball and flat under different temperatures and environments. It is found that there is no visible wear scar on flat from room temperature to 200 \({}^{\circ}\)C, whereas a wear scar with circle shape as well as transferred films and wear debris within or around the wear scar are found on the surface of ball. Therefore, we only provide the figures of ball, as shown in Figures 6 and 7. For the tribotests at room temperature, there are obvious and deep flow grooves on the ball surface in alcohol environment, as shown in Figure 5a. In dry nitrogen, there are not only glow grooves but also the color transferred from the films onto the worn surface of ball, as shown in Figure 5b. In ambient air, there are only shallow plow grooves on the worn surface of the ball, as shown in Figure 5c. In dry hydrogen, there are not only shallow plow grooves, but also large amounts of the color transferred from the films around the center of the wear scar, and even black wear debris around the wear scar on the ball surface, as shown in Figure 5d. In vacuum, there are also shallow plow grooves and color transferred from the films within the wear scar, as shown in Figure 5e. Figure 4: COF of ZrO\({}_{2}\) ball/DLC films under different environments at 200 \({}^{\circ}\)C.  
Figure 5: _Cont._  
The worn surface topography of ball under different environments at the temperature of 100 \({}^{\circ}\)C is almost the same as that at room temperature. There are shallow plow grooves on the ball surface in ambient air and dry nitrogen, as shown in Figure 6a,b. However, in dry hydrogen and vacuum, there are not only shallow plow grooves but also the color transferred from the films onto the worn surface of the ball, as shown in Figure 6c,d. Figure 5: Worn surface images of ball under different environments at room temperature: (**a**) in alcohol environment, (**b**) in dry nitrogen, (**c**) in ambient air, (**d**) in dry hydrogen, (**e**) in vacuum and (1) ball, (2) flat and there is the worn zone within circle. Figure 6: Worn surface images of ball under different environments at 100 \({}^{\circ}\)C: (**a**) in dry nitrogen, (**b**) in ambient air, (**c**) in dry hydrogen, (**d**) in vacuum and there is the worn zone within circle. The worn surface topography of the ball at the temperature of 200 \({}^{\circ}\)C is slightly different from that at room temperature and 100 \({}^{\circ}\)C. In dry nitrogen, there are shallow grooves and the color transferred from the films onto the worn surface of the ball, as shown in Figure 7a. In ambient air, the wear scar is very smooth and there is no plow groove, however, there is black wear debris outside the wear scar on the worn surface of the ball, as shown in Figure 7b. In ambient air, CoF at the stable stage is as ultralow as 0.02, and then CoF increases to around 0.13.