Fretting fatigue properties of Zr-based bulk amorphous alloy in phosphate buffered saline solution

Fretting fatigue properties were studied in air and in a simulated body fluid, PBS (-), using a commercial Zr-based amorphous alloy (7.6Ni-12.3Cu-3.5Al-76.6Zr in mass%). Fretting fatigue tests were carried out under load control using a sinusoidal wave with a stress ratio of 0.1, a frequency of 20 Hz in air and 2 Hz in PBS (-) and a contact pressure of 30 MPa for fretting. There was no difference between the stress-number of cycles to failure (S-N) curves of the plain fatigue test in air and in PBS (-), and the 10 ⁷-cycle fatigue strength was approximately 150MPa for both environments. The 10 ⁷-cycle fretting fatigue strength in air was one-third as high as the plain fatigue strength, while the 2 × 10 ⁶-cycle fretting fatigue strength in PBS (-) was approximately twice as high as that in air. Although the friction coefficient between the specimen surface and the fretting pad, which affects the fretting fatigue strength, was hardly different between air and PBS(-), SEM observations showed that the actual contact area of the fretting pad on the specimen surface tested in air was smaller than that tested in PBS(-). Thus, we considered that the decrease of the fretting fatigue strength in air was caused by the higher contact pressure of the pad in air than that in PBS (-). XPS analyses also showed that the specimens tested in air and in PBS (-) had the different compositions of the oxide films on their surfaces, suggesting that the difference in the composition of the oxide films affect the fretting damage and thus the fretting fatigue strength.