## 2 Experimental Details  
Hydrogenated DLC films were deposited on AISI H13 steel discs in the capacitive coupled plasma enhanced chemical vapor deposition system with a gas feed of methane. The deposition details of DLC films are available in Ref. [12]. The hydrogen content in DLC films is expected to be high in the methane environment. The thickness of DLC films is about 1 \(\upmu\)m. The surface roughness of DLC films is about 0.04 \(\upmu\)m.  
The tribotests of the uncoated ZrO\({}_{2}\) ball and DLC films-coated disc were performed in rotating ball-on-disc configuration tribometer to evaluate the tribological properties of ZrO\({}_{2}\)/DLC films friction pair under various environments from room temperature to 200 \({}^{\circ}\)C. There is mainly zircon sand (ZrSiO\({}_{4}\)) and few yttria as the stabilizing agent in ZrO\({}_{2}\). The surface roughness of ZrO\({}_{2}\) ball counterpart is 0.02 \(\upmu\)m. The nitrogen and hydrogen atmospheres with high purity of 99.99% are introduced by the gas introduction system. The tribotests were carried out in ambient air, dry nitrogen, hydrogen and vacuum environment. Vacuum pressures of around 0.2 Pa were achieved by controlling the pump during the tribotest in the testing chamber. The relative humidity is about 20 RH% in ambient air at room temperature. The rotating flat was heated to the set temperatures and then kept the constant temperature during sliding. ZrO\({}_{2}\) ball was not heated and the ball is contacted with flat until the tribotests starting and during sliding. The environmental gas was controlled by gauge valve to ensure the same friction conditions for the same group tribotests. DLC films-coated steel disc was slid against ZrO\({}_{2}\) ball with the diameter of 9.5 mm at the speed of 0.05 m/s and normal load of 10 N. The initial Hertzian pressure is about 1.3 GPa and this maximum contact pressure is calculated according to the classical Hertz theory for elliptical contacts. The specimens were cleaned first ultrasonically in acetone and then blown by dry nitrogen before the tribotests and measurements. The new disc and ball are used for each tribotest. The sliding time is 1 h. The volume loss, \(W_{\text{b}}\) for the ball was calculated, \(W_{\text{r}}\) in terms of specific wear rate was calculated by dividing the sliding distance \(L\) and the normal load, \(N\). The volume loss, \(W_{\text{b}}\) was calculated by Equation (1):  
\[W_{\text{b}}=\frac{\pi d^{4}}{64r} \tag{1}\]  
where \(W_{\text{b}}\) is the volume of wear in mm\({}^{3}\), \(d\) is the width of wear scar in mm and \(r\) is the radius of ball in mm, as shown in Figure 1.  
The specific wear rate, \(W_{\rm r}\) was calculated by Equation (2):  
\[W_{\rm r}=\frac{W_{\rm b}}{NL} \tag{2}\]  
where \(W_{\rm r}\) is the specific wear rate in mm\({}^{3}\)/(N\(\cdot\)m), \(N\) is the normal load in N and \(L\) is the sliding distance in m.  
After the tribotests, the worn surfaces of the friction pair were observed by 3D non-contact optical profiler. The microstructure of the worn surface on ZrO\({}_{2}\) ball was investigated by Raman spectroscopy using a 633 nm He-Ne laser excitation source with a resolution of 1 cm\({}^{-1}\). Raman spectra were collected within the contact area of the worn surface.