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.

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.

Modular hip implants are a clinically successful and widely used treatment for patients with arthritis. Despite ongoing retrieval studies the understanding of the fundamental physico-chemical mechanisms of friction and wear within the head-taper interface is still limited. Here, we Raman-spectroscopically analyze structural features of the biotribological material which is formed within the taper joint between Ti6Al4V and low-carbon cobalt alloy or high-nitrogen steel surfaces in in vitro gross-slip fretting corrosion tests with bovine calf serum. As a function of the fretting duration, we investigate short and long aliphatic chains and their adsorption behavior on the cobalt- and steel-type surfaces. Using the intensity and frequency shifts of the amide I and III Raman bands, we furthermore identify progressive protein folding and unfolding including the secondary structures of α-helix, β-sheet, and random-coil configuration as well as the formation of proteinaceous clusters depending on the hydrophilicity of the metallic surfaces. We additionally find a mixture of chromates and iron oxides with tryptophan and tyrosine at the worn cobalt alloy and high-nitrogen steel surfaces, respectively. Also, for long fretting duration, sp² hybridized amorphous carbon is formed due to fretting-induced cleavage of proteins. STATEMENT OF SIGNIFICANCE: Despite efforts enhancing the biomedical tribology of hip implants, the impact of the organic environment on friction&wear at the femoral head-stem taper interface is limitedly understood. Using Raman spectroscopy we resolve structural changes within the biotribological material agglomerated at biomedical-grade metal alloys due to organo-metallic interactions during in vitro fretting corrosion tests. Adsorption of short and long aliphatic chains, progressive protein (un)folding and proteinaceous cluster formation depend to a distinguishable extent on the fretting duration and type of alloy. Chromates and iron oxides are mixed with tryptophan and tyrosine, and amorphous carbon is formed resulting from a fretting-induced cleavage of serum proteins. Such information spectroscopically gleaned from biotribological material are vital to improve the design and performance of taper junctions.

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.