Effect of various surface conditions on fretting fatigue behavior of Ti-6Al-4V

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti-6Al-4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti-6Al-4V with a highly polished finish, bare Ti-6Al-4V that was low-stress ground and polished to RMS #8 (designated as 'as-received'), bare Ti-6Al-4V that was grit blasted to RMS #64 (designated as 'roughened') and stress relieved, and Cu-Ni plasma spray coated Ti-6Al-4V. Behavior against the Cu-Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 10⁷ cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu-Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements. An increase in fretting fatigue strength of 20-25% was observed for specimens tested against Cu-Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ∼0.3 for the Cu-Ni coated pad against an as-received specimen and ∼0.7 for the bare as-received Ti-6Al-4V. Specimens tested against the polished Ti-6Al-4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu-Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.