Effect of gas pressure on tribological properties and tribochemical reactions of alumina sliding against zirconia in HFC-134a

Friction tests were performed in a vacuum chamber with an alumina (Al2O3) ball sliding against a zirconia (ZrO2) disk during exposure to CF3CH2F (HFC-134a) gas at pressures between 10−5 and 105 Pa. Surface analysis was carried out by using X-ray photoelectron spectroscopy (XPS). It was found that friction and wear behaviors of Al2O3-ZrO2 pairs strongly depended on the pressure of HFC-134a gas. With an increasing pressure, the friction and wear significantly decreased and then slightly increased when pressure exceeded 104 Pa, showing transition features. The XPS analysis showed that more fluorinated compounds and organic matters were formed on the worn surfaces at a higher pressure. In addition, completely fluorinated compounds of ZrF4 and AlF3, and fluorine-containing organic matters were identified existing at the wear interface when the pairs were rubbed in HFC-134a at 105 Pa. It can be concluded that the friction and wear transitions are due to the degree of tribochemical reactions.