## 1 Introduction  
Diamond like carbon (DLC) films exhibit high hardness, chemical inertness and wear-resistance and low coefficient of friction (CoF), which make them ideal choices for engineering applications that involve sliding or rolling contacts between moving mechanical part surfaces [1; 2; 3; 4]. Tribological studies have demonstrated that the tribotest environment plays a significant role in the tribological properties of DLC films. [PERSON] et al. [5] have evaluated the tribological properties of DLC films in various environments (air, O\({}_{2}\), N\({}_{2}\) and vacuum). Low CoF (0.06) was observed in N\({}_{2}\), whereas high CoF was observed in vacuum, and it is shown that the tribological properties of DLC films are dependent on the transferred layer and tribochemical reactions. [PERSON] et al. [6] have found that CoF of DLC films decreases from the vacuum environment to the ambient air. However, [PERSON] et al. [7] have found that CoF of DLC films decreases from 0.12 to 0.035 as the speed increases in vacuum. These published papers have showed that DLC films may exhibit excellent tribological performances under different environments. It is speculated that the ultralov friction behavior of DLC films could be achieved under the appropriate friction conditions. We have observed that CoF of DLC films in Ar environment is in the range of 0.4 to 0.7 and it is worn out; however, CoF reduces to 0.03 in hydrogen environment [8], which means that DLC films can exhibit excellent tribological properties. Therefore, it is necessary to select the suitable friction conditions such as the friction mate materials, temperature and environment gas to achieve ultralov friction and clarify the ultralov friction mechanism of DLC films under various friction conditions such as environments, temperatures and materials. Zirconium oxide (ZrO\({}_{2}\)) has recently become the preferred ceramic material for high speed roller bearing balls, seal rings, cutters, dental application, drawing tools and valves due to the superior hardness and low density [9]. It is to be expected by combining the advantages of ZrO\({}_{2}\) and DLC films that the excellent tribological properties of ZrO\({}_{2}\) balls against DLC films friction pair are beneficial to improve the roller bearing performances and prolong the service life of high speed roller bearing [10]. It is well known that the temperature rise in the high speed roller bearing system is unavoidable when ZrO\({}_{2}\) ball is relatively slid with the bearing inner race or outer race during running-in, even though oil-air lubrication is applied to high speed roller bearing [11]. The typical operating temperature rise of high speed ball bearings is approximately 100 \({}^{\circ}\)C or above. Therefore, it is reasonably acceptable that tribotests are performed around or above 100 \({}^{\circ}\)C in this paper. However, in spite of the strong potential engineering application, the application of DLC films appears to still be limited due to their low thermal stability [12; 13]. It is well known that DLC films are sensitive to the oxide species such as oxygen and water molecular and they are graphitization and oxidized easily in ambient air or the reaction gas [14]. The aim is to design the DLC films-related tribological system with ultralow friction under various environments and temperatures in the present work.  
The influence of the temperature and environment gas on the tribological properties of ZrO\({}_{2}\)/DLC films friction pair is investigated. The tribological properties are investigated systemically in dry nitrogen, hydrogen, vacuum and ambient air environments from room temperatures to 200 \({}^{\circ}\)C. It is expected to achieve ultralow friction of ZrO\({}_{2}\)/DLC films under various environments and temperatures and elucidate the ultralow friction mechanism. The present work will provide the fundamental theory of the ceramic material as high speed ball bearing material and DLC films as solid lubrication materials for engineering application in high speed ball bearings.