**2.2. Friction tests**

Friction tests were conducted to evaluate the tribological properties of hard TiN film covered the grit-blasting steel substrate against four different material ball counterbodies using a ballon-disk tribo-tester under starved lubrication regime. The four different upper balls were made of bearing steel, silicon nitride, alumina, and zirconia, which diameter is 8 mm. And the hardness of bearing steel was lower than those of TiN film and other materials. The diameter of the lower disk covered with TiN film was 24 mm. The surface roughness of the test sample was measured with the Micro-Map surface mapping microscope. The initial average surface roughnesses (Ra) of all the upper balls are less than 0.03 μm, and the roughness of lower disk covered TiN film was 8.2 μm which was much higher than upper balls. Friction tests were carried out in ambient temperature and atmospheric environment for 120 min of duration under 5 N of normal load and sliding speed of 31 m/min. The coefficient of friction was recorded automatically during the friction test.

In order to obtain the starved lubrication regime, the small amount of lubricating oil was injected the surface of lower TiN film-covering disk. The calculated thickness of oil film was lower to 3 μm. After friction tests, all the balls and disks were cleaned with acetone by ultrasonic washer for three times. The typical surface morphologies and chemical compositions of friction tracks in lower disks and wear scars in upper balls were analyzed by scanning electron microscopy (SEM), which attached an energy dispersive spectrometer.
