Fretting-Induced Formation of Nanocrystalline CoS_<i>x</i> Tribomaterial at the Hip Implant Taper Junction

Gross-slip fretting corrosion occurs frequently inside taper joints of endoprosthetic implants and is expected to be accompanied by the formation of a metal-organic tribomaterial, which may influence the friction and wear behavior of implants. While it is hypothesized that it contains compounds of the surrounding human body fluid and worn particles of the implant materials, its structure, composition, formation mechanism, and distribution inside the wear area remain elusive. In a multiscale structural-chemical study using Raman scattering spectroscopy and transmission electron microscopy, we reveal the tribological formation of nanocrystalline cobalt sulfide for a CoCrMo/TiAlV couple that was subjected to <i>in vitro</i> gross-slip fretting in bovine calf serum. We demonstrate that sulfur atoms released from the mechanochemical decomposition of cysteine/cystine disulfide bonds in serum proteins ─ as evidenced by the complete absence of characteristic S-S Raman modes and concurrent protein unfolding from α-helix to β-sheet structures ─ react with cobalt ions released tribocorrosively from the alloy to form the CoS_<i>x</i> tribomaterial. The resulting tribofilm is substoichiometric (<i>x</i> < 2) with a primary cubic structure partially mixed by a hexagonal phase. It perfectly adheres to the Co alloy through mechanical mixing, thus exhibiting the structural features of extreme-pressure antiwear additives. The CoS_<i>x</i> tribofilm covers 12.2% of the fretting track in the CoCrMo alloy, while maintaining a consistent thickness of approximately 15 nm. Multiple mechanisms driving this transformation are discussed: mechanical protein unfolding exposes disulfide bonds to forces that reduce their cleavage activation energy, local temperatures allow for thermal decomposition, and the acidic crevice environment facilitates chemical cleavage. The observations on the fretting-generated CoS_<i>x</i> tribomaterial provide the first comprehensive structural-chemical insight into tribologically beneficial features of transition metal sulfides formed in medical alloys through protein-derived sulfur. Understanding this mechanism may enable strategies to deliberately promote such protective film formation to improve the longevity of taper junctions in medical applications.