Characterization of transfer layers forming on surfaces sliding against diamond-like carbon

Metallic and ceramic surfaces may become covered with a carbon-rich transfer layer during sliding against diamond-like carbon (DLC) films. The presence of such layers at sliding interfaces may dominate the long-term friction and wear performance of these films. In this study, we use Raman and infrared spectroscopies to characterize the chemical structure of such transfer layers forming on the surface of magnesia/partially-stabilized ZrO2 (MgO-PSZ) balls. The DLC film (approximate to 22 mu m thick) was prepared by ion-beam deposition at room temperature, with methane used as the source gas. Tribological tests were performed on a ball-ondisk machine in open air at room temperature (approximate to 22+/-1 degrees C) and humidity of 30-50%. Sliding velocity ranged from 1 to 6m s(-1) and the tests were continued until the DLC films were effectively worn through. The results showed that the friction coefficients of DLC against MgO-PSZ were initially 0.08-0.12 but decreased to 0.05-0.06 after about 200000-500000 sliding passes, depending on velocity. They remained constant at 0.05 for the duration of the tests, which was 1.5 million cycles at 6 m s(-1) but >4 million cycles at 1 m s(-1). The low friction coefficients observed in each test coincided with the formation of a carbon-rich transfer layer on the rubbing surfaces of MgO-PSZ balls. Micro-laser-Raman and Fourier transformed infrared (FTIR) spectroscopies confirmed that these carbon-rich transfer layers had a disordered graphitic structure.