Adrian Wittrock

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 in vitro 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_x tribomaterial. The resulting tribofilm is substoichiometric (x < 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_x 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_x 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.

MoS₂ not only has unique optoelectronic properties realizing photonic and semiconductor applications but also serves as a promising solid lubricant in tribological three-body contacts due to its advantageous friction and wear behavior. Its functionality is defined by elementary processes including strain, oxidation processes, and material mixing. However, these mechanisms were not elucidated for MoS₂ having transferred from the MoS₂ film synthesized at the main body to a steel counter body during tribological ball-on-disk tests. Using spatially and spectrally high-resolved Raman spectroscopy, we study the compressive and tensile strain within the MoS₂ transfer material and analyze the oxidation of molybdenum, sulfur, and iron. In addition, we elaborate on the impact of transition metals modifying the MoS₂ films on the strain distribution and oxidation processes. Decreasing intensities of the MoS₂ Raman lines are accompanied with enhanced intensities of sulphur and molybdenum oxide Raman signatures which are particularly agglomerated at the edges of the tribological track. The formation of tribochemical oxides, including Mo₄O₁₁ in the Magnéli-phase, depends weakly on the type of modifying element, and an oxidation of a modification element itself is not detected. We also identified a tribologically induced formation of disordered few-layer graphene at counter-body surface areas which experienced weak thermo-mechanical tribological load. Our results characterize structural and chemical features of the MoS₂ transfer material, thus predicting material failure at the microscopic level.

Modular artificial hip joints are a clinical standard today. However, the release of wear products from the head–taper interface, which includes wear particles in the nm size range, as well as metal ions, have raised concerns. Depending on the loading of such taper joints, a wide variety of different mechanisms have been found by retrieval analyses. From these, this paper concentrates on analyzing the contribution of gross slip fretting corrosion at ultra-mild wear rates using a bovine calf serum solution (BCS) as the lubricant. The parameters were chosen based on biomechanical considerations, producing wear rates of some ng/m wear path. In parallel, the evolution of tribomaterial (third bodies) was analyzed as to its constituents and generation rates. It has already been shown earlier that, by an advantageous combination of wear mechanisms and submechanisms, certain constituents of the tribomaterial remain inside the contact area and act like extreme-pressure lubricant additives. For the known wear and corrosion resistance of austenitic high-nitrogen steels (AHNSs), which outperform CoCrMo alloys even under inflammatory conditions, we hypothesized that such steels will generate ultra-mild wear rates under gross slip fretting. While testing AHNSs against commercially available biomedical-grade materials of CoCrMo and TiAlV alloys, as well as zirconia-toughened alumina (ZTA) and against itself, it was found that AHNSs in combination with a Ti6Al4V alloy generated the smallest wear rate under gross slip fretting corrosion. This paper then discusses the wear behavior on the basis of ex situ analyses of the worn surfaces as to the acting wear mechanisms and submechanisms, as well as to the tribological reaction products.

Amorphous carbon (a-C) attracts great attention in tribology research and thin film technologies due to its versatile properties. However, high temperatures and mechanical stresses may cause significant changes in the structural ordering of the a-C network. We present an optical method to initiate structural ordering and to probe solid-to-solid structural transitions of element modified a-C films. A pulsed pump laser introduces heat into the film in a controlled manner, while a second laser probes confocally the first- and second-order Raman scattering signatures of the a-C network. For low pump power, the number of defects and non-sixfold aromatic rings is reduced. A further increase in the laser power leads to sharply evolved changes in the Raman scattering features, indicating a transition from a-C to defected graphite and an effusion of hydrogen. Moreover, graphite-dominant defect relaxation and an enhancement in hexagonal lattice areas occur and, in turn, activate second-order Raman scattering lines. A rising laser power subsequently results in progressive graphitization. Chemical modification of the films with Si or Cu enhances their thermal stability and even shifts the upper thermal limit of the film ablation, while the a-C:W film demonstrates a more efficient enrichment of nanocrystalline graphitic clusters.