## 3 Results and Discussion  
Figure 2 shows CoF of ZrO\({}_{2}\)/DLC films with sliding time under various environments at room temperature. In ambient air, CoF is 0.08 at the initial stage, then increases rapidly to 0.2, and finally decreases to 0.16 with the increase of sliding time. It is interesting to find that with the increase of sliding time CoF is relatively stable, which is different from the typical CoF curve of DLC films in ambient air [15]. This means that ZrO\({}_{2}\) as the friction mate material for DLC films is beneficial to the antifriction behavior of DLC films in ambient air. In dry nitrogen, CoF is 0.13 at the initial stage, then increases to 0.22 rapidly, and finally decreases to around 0.04 in subsequent tribotest. In dry hydrogen, CoF is 0.11 at the initial stage, then increases to 0.20 abruptly, and finally decreases. Average CoF is about 0.05 in hydrogen. In vacuum, CoF is 0.05 at the initial stage, then increases rapidly to 0.19, decreases to 0.14, and finally increases to around 0.17, which fluctuates typically in vacuum [16]. The low and stable CoF indicates that this friction pair is applicable used to the vacuum environment. In alcohol environment, CoF is 0.15 at the initial stage, and then CoF fluctuates during the tribotest. It is concluded from the above experimental results that ZrO\({}_{2}\)/DLC films exhibit excellent antifriction and stability behaviors under different environments at room temperature. Figure 1: The calculation mode of specific wear rate of ball. Figure 3 shows CoF of ZrO\({}_{2}\)/DLC films with sliding time under different environments at the temperature of 100 \({}^{\circ}\)C. It is found from the experimental results that CoF is low at initial stage, then increases to the maximum value, decreases, and finally increase again with the increase of sliding time. In ambient air, CoF is 0.15 at the initial stage, then increases to 0.19, and finally CoF decreases to the minimum value of 0.04, which exhibits ultralow friction. Moreover, the CoF is lower than that at room temperature. It is interesting to find that the friction pair exhibits ultralow friction in ambient air. This also means that ZrO\({}_{2}\) is excellent friction mate material for DLC films in ambient air. In dry nitrogen, CoF is 0.11 at the initial stage, then increases to 0.18, and finally decreases slowly along with the sliding time. Average CoF is 0.04. In dry hydrogen, CoF is 0.09 at the initial stage, then increases to 0.19, and decreases to 0.03, and this ultralow friction value is maintained. The whole sliding process is divided into four stages: the running-in process, the low friction stage, the ultralow friction stage and at stable stage. The first fall of CoF from 0.19 to 0.15 happens at around 200 s, whereas the second dramatic decrease from 0.15 to about 0.03 happens at around 700 s, entering ultralow friction regime. The ultralow friction state in the present test is stable at about 500 s. Comparing with the tribotest results at room temperature, the stability of CoF is significantly improved. In vacuum, CoF is 0.10 at the initial stage, then increases to 0.19, decreases to 0.05, and finally increases slightly to 0.08. The low and stable CoF indicates that the friction pair exhibits excellent antifriction behaviors in vacuum environment at the temperature of 100 \({}^{\circ}\)C. It is concluded that the maximum and minimum values of CoF are both lower than those at room temperature, and the stability of the antifriction behavior is improved at the temperature of 100 \({}^{\circ}\)C.  
Figure 3: CoF of ZrO\({}_{2}\) ball/DLC films under different environments at 100 \({}^{\circ}\)C.  
Figure 2: CoF of ZrO\({}_{2}\) ball/DLC (Diamond like carbon) films under various environments at room temperature.