## 3 Results and Discussion
Figure 12 shows Raman spectroscopy of DLC films on flat before and after the tribotest test in dry hydrogen at 100 \({}^{\circ}\)C. There is no large different between as-is DLC films and DLC films after the friction test, however, the luminescence (in the range of 1600 cm\({}^{-1}\) and 1800 cm\({}^{-1}\)) of DLC films after the friction test is higher than that of as-is DLC films, which indicates that there is more hydrogen in DLC films after the tribotest in hydrogen. As shown in Figure 9, the same phenomena in DLC films is observed at 200 \({}^{\circ}\)C, which is helpful to improve the antifriction and anti-wear behaviors of ZrO\({}_{2}\)/DLC films friction pair under various environments. Figure 11: Raman spectra of the transferred films on the worn surface of ZrO\({}_{2}\) ball: (**a**) before the friction test; (**b**) after the friction test. Figure 12: Raman spectra of DLC films before and after friction test. Figure 10: Raman spectra of ZrO\({}_{2}\) ball: (**a**) before the friction test; (**b**) after the friction test. At room temperature, CoF is low and stable in the application of solid lubrication of DLC films. CoF is ultralow when the transferred films are formed on ball in dry nitrogen, hydrogen and even vacuum environment. CoF in dry nitrogen is lowest because the chemically passive surface cannot to be oxidized in inert gas environment. At 100 \({}^{\circ}\)C, there is the slight graphitization in DLC films (as shown in Figure 9) under high friction heating and pressure simultaneously. DLC films are transferred to the contact surface of ball, and the friction interface of ball is partially covered by the transferred films. According to Raman spectroscopy analysis, the transferred films on the worn surface on ball are carbon-related materials, which result in low CoF compared with the tribotest in ambient air. At 200 \({}^{\circ}\)C, CoF is as ultralow as 0.02 in ambient air due to the tribochemistry reaction during sliding. However, CoF increases in dry nitrogen, hydrogen and vacuum at 200 \({}^{\circ}\)C. The experimental results show that the antifriction behavior of ZrO\({}_{2}\)/DLC films is influenced significantly by the environment temperature. From the tribotest results, it is found that the environment gas influences strongly the tribological properties of ZrO\({}_{2}\)/DLC films. CoF is almost the same in dry nitrogen from room temperature to 200 \({}^{\circ}\)C. CoF is low and stable because the tribotests are performed in the inert environment and there are also the transferred films on the worn surface of ZrO\({}_{2}\) ball. However, CoF is unstable in vacuum in the absence of gas molecules. It is shown that the antifriction behavior of ZrO\({}_{2}\)/DLC films is influenced by the environment gas. CoF decreases from 0.16 to 0.02 from room temperature to 200 \({}^{\circ}\)C in ambient air. However, the microstructure of DLC films is almost the same from room temperature to 200 \({}^{\circ}\)C according to Raman measurements (as shown in Figure 9) and our previous work [12]. It is surprising that there are almost no transferred films on the worn surface of ZrO\({}_{2}\) ball after the tribotests from room temperature to 200 \({}^{\circ}\)C. Therefore, there should be another possible factor influencing the antifriction behavior of ZrO\({}_{2}\)/DLC films in ambient air. It is well known that hydrogen plays an important role in the shielding action in DLC films. CoF is almost the same as 0.05 in dry hydrogen from room temperature to 200 \({}^{\circ}\)C. It is also found that there are the transferred films on the worn surface of ZrO\({}_{2}\) ball in dry hydrogen. Like the friction behavior, the wear resistance behavior follows the tendency.