Tribocorrosion in Hip Modular Taper Junctions: Load-Triggered Transitions in Electrochemical and Mechanical Behavior

Modular hip junctions operate in a challenging environment. This includes exposure to cyclic loads, micromotions at the interfaces, and a shifting local chemical environment. The objectives of this study are to identify the tribocorrosion mechanisms in mixed-metal contacts under a simulated joint environment at various axial loads at two pH levels. Fretting-corrosion tests were conducted in a custom-made apparatus, by articulating a square Ti-6Al-4V-alloy rod with two, axially loaded, CoCrMo pins (15 cm(2) exposed surface area) in a flat-on-flat contact system. Bovine calf serum (BCS) solution was used as an electrolyte (electrolyte volume = 70 mL with 30 g/L protein content). A sinusoidal fretting motion, with a displacement amplitude of +/- 50 mu M was applied to the Ti-6Al-4V-alloy rod. To evaluate the effect of load, a range of normal loads (50-800 N) was applied at two different pH levels (pH 3.0 and pH 7.6). We observed variations in electrochemical potential and dissipated friction energy that were both functions of load and pH. Under gross slip conditions, there was an increase in magnitude of potential drop as a function of load until a critical value was reached. This was likely related to the incremental increase in the tribo-activated area. At high load, the fretting zone was in the stick-slip fretting region, where the sliding amplitude was almost zero. Hence, at high load, the reduction in the magnitude of potential drop is caused by a reduced tribo-activated area. In addition, the presence of a tribofilm was observed at high load, possibly offering electrochemical protection to the surface. The data indicate that mechanistic transitions in the fretting-corrosion behavior of modular junctions depend on applied load and pH.