## 3 Results and Discussion
The lowest wear rate of ZrO\({}_{2}\) ball in ambient air at room temperature is 5.8% comparing to the wear rate of the same friction pair in vacuum at the temperature of 200 \({}^{\circ}\)C. The wear rate of ZrO\({}_{2}\) ball under different environments is very low except for the tribotests in vacuum at 200 \({}^{\circ}\)C. It is also found that there is no visible wear of DLC films on disc. The ultralow wear rate shows that the transferred films between the two contact surfaces are composed of low strength atomic carbon interlayer [10]. Many factors such as temperature, environment and even tribochemistry reaction in the present case are involved. The synergism of the passive behaviors of DLC films and the transferred films on the worn surface of ZrO\({}_{2}\) ball is beneficial to ultralow friction under different environments and temperatures. The di-hydrated carbon atoms on DLC films surfaces are expected to provide the shielding of chemical passivation. Such repulsive forces at sliding interface may act the weak van der Waals attractions. The shielding effect has positive implications for ultralow friction behavior. Overall, these experiments demonstrate that hydrogen has significant effects on the antifriction and anti-wear behaviors of the tribological system. The transferred films on ZrO\({}_{2}\) ball are beneficial to low and stable friction in our work. However, we cannot explain why CoF is super low where there are no transferred films on the worn surface of ZrO\({}_{2}\) ball at 200 \({}^{\circ}\)C and there are large amounts of black wear debris around the worn surface in ambient air. It is concluded from the experimental results that Zr element in ZrO\({}_{2}\) ball is significantly beneficial to improve the antifriction and anti-abrasive behaviors of the friction pair and the formation of the transferred films on the worn surface of ZrO\({}_{2}\) ball under various environments during sliding. There is the tribochemical reaction producing a transferred layer that covers on the worn region of ZrO\({}_{2}\) ball because there are large amounts of black wear debris from the chemical reaction products. It is assumed that there exists the transferred films on ZrO\({}_{2}\) ball at the initial stage, but thin transferred films are easily worn out during sliding in ambient air. According to the experimental results and Raman analysis of the friction system, the transferred films play important roles in the antifriction and anti-abrasive behaviors of ZrO\({}_{2}\)/DLC films. The transferred layer produced by the tribochemical reaction can achieve ultralow CoF, which is in accordance with the friction results of ceramic material in ambient air. The tribochemistry reactions are unavoidable under high temperature and friction heating conditions in ambient air. It is well known that Zr could be reacted with water molecular under high temperature [17]. It is the hydrothermal metal oxidation for ZrO\({}_{2}\) under the environmental temperatures and frictional heating in ambient air [19]. The possible oxidation reactions during sliding are listed as shown in Equation (3). \[\mathrm{Zr}+\mathrm{H_{2}O}\xrightarrow{300^{\circ}\mathrm{C}}\mathrm{ZrO_{2}}+ \mathrm{ZrH_{x}}\xrightarrow{400^{\circ}\mathrm{C}}\mathrm{ZrO_{2}}+\mathrm{H_{2 }}\uparrow \tag{3}\]  
Zr is derived from the wear debris on ZrO\({}_{2}\) ball during the tribotests and it may react chemically with water molecular to form ZrO\({}_{2}\) and liberate hydrogen. Firstly, Zr is oxidized to ZrO\({}_{2}\) and ZrH\({}_{x}\), and then ZrH\({}_{x}\) disappeared and ZrO\({}_{2}\) and hydrogen were produced finally. Equation (3) shows the chemical reaction products of ZrO\({}_{2}\) and hydrogen according to hydrothermal oxidation. The hydrothermal oxidation involves water molecular, high pressure and high temperature. The hydrothermal oxidation of the friction pair would be happening especially under high temperature during sliding. CoF of ZrO\({}_{2}\)/DLC films is 0.16 because the hydrothermal oxidation could not occur at room temperature.