ICMCTF 2025 Session MC1-1-ThA: Friction, Wear, Lubrication Effects, & Modeling I

Tribological contacts play an essential role in the prevalent and required endeavor for increased sustainability and efficient use of resources. Considering the energy losses related to friction and wear, a huge possibility of saving resources, energy, and CO₂ is often overlooked. Here, solid lubricants are an attractive option, especially for applications pushing their conventional liquid counterparts to their thermal and chemical stability limits – typically at elevated temperatures above 200 °C or under extreme conditions excluding liquids (i.e. space industry, semiconductors, or life science). Therefore, this study examines different solid lubrication concepts in thin film materials, classifying them concerning predominant mechanisms, application ranges, and performance.

As a starting point, carbon-containing thin film materials will be discussed comprising diamond-like carbon (DLC) coatings and non-reactively sputter deposited transition metal (TM) carbide thin films (i.e., HfC, TaC, or WC). Here, advances in PVD growth techniques (i.e., HiPIMS) and their impact on tribological performance are in focus. Furthermore, insights on the limits of carbon as the source for solid lubrication will be given by a set of high-resolution characterization techniques (i.e., HR-TEM, APT, etc.). The second part presents an alternative class of TM dichalcogenide coating materials (compared to MoS₂) and their in-situ formation. In detail, in an innovative approach, selenium nanopowders are converted in-situ into lubricious 2D selenides on sliding W and Mo films, achieving a coefficient of friction (COF) down to 0.1 in ambient air. This in-situ formation is an exciting concept, especially for extreme environmental conditions. Nevertheless, further advances in solid lubricants are required to overcome the limitations for high-temperature applications (above 450 °C). Here, a concept on B₂O₃ formation in TM borides (i.e., TiB_2±z or WB_2±z) leads to a drastic reduction of COF from 0.6 to 0.2 at 500 °C (and higher temperatures), highlighting the capabilities of boron-containing thin films in high-temperature tribological contacts.

In summary, the different concepts of solid lubrication in thin film materials emphasize the potential of exploring new materials and the need for an in-depth understanding to push these materials in potential applications.

Although titanium is not a noble metal, it is increasingly attracting interest from the luxury industry (jewelry, watches, packaging) due to its lightweight, hypoallergenic properties, and especially the wide range of colors it can display when coated with a thin layer of TiO₂. However, its application in luxury products remains limited because these colors tend to lack durability. Improving the wear resistance of these colored TiO₂ layers, and in particular preserving the original color, is a critical challenge for luxury jewelry.

The interference-based nature of titanium’s color makes it highly sensitive to changes in oxide layer thickness, as well as to variations in the oxide layer’s chemical composition and internal structure, which can alter its refractive index. Tribological tests conducted on thin titanium oxide layers, using a100Cr6 steel ball in both dry conditions and with artificial sweat, demonstrated a clear correlation between color changes due to friction and a reduction in oxide layer thickness in both environments.

An experimental study of the wear resistance of several potential protective coatings deposited on oxidized titanium samples is carried out in order to preserve the color of the samples. Three coatings—SiAlON, Si₃N₄, and a commercial hydrophobic coating—were examined for their wear resistance in both dry and artificial sweat conditions, as well as for their transparency and surface wettability. The challenge is to have a coating that is not only transparent but also resistant to wear in both dry and sweat-exposed conditions and insensitive to fingerprints.

Thus, the color variation before and after coating, the surface wettability of the coating with water and sebum, as well as its resistance to dry friction and friction in the presence of artificial sweat against a 100Cr6 steel ball, will be analyzed and compared to those of uncoated TiO₂ to assess the performance of the coatings.

Introduction:

Biomedical implants are vital medical devices surgically placed to replace or support damaged tissues and organs. Modular implants, such as hip replacements, improve adaptability for diverse patients but introduce challenges like tribocorrosion—a complex interaction of tribology and corrosion. Tribocorrosion releases debris, ions, and particles into surrounding tissues, causing reactions, systemic toxicity, and infections. Biocompatible materials like Ti6Al4V are commonly used in implants. Although various experimental methods exist to study tribocorrosion, limited mathematical modeling efforts have been undertaken. This study reviews available models to identify those most suitable for implant applications, with two aims: a) validating model efficiency using literature data, and b) conducting experiments to generate data for further validation.

Methodology:

Aim 1: Electrochemical current evolution is a key measure of tribocorrosion. Models like “Olsson and Stemp,” “Feyzi and Hashemi,” and the Uhlig model predict tribocorrosion currents, but their efficiency remains insufficiently tested. Data from M.T. Mathew et al.’s “Tribocorrosion Behaviour of TiCxOy” was selected for its robust dataset, clear graphical representation, and systematic evaluation across varying voltages, ensuring analytical versatility. Aim 2: Fretting-corrosion experiments were conducted using a custom-built tribocorrosion apparatus (Pin- on-flat) to validate models against experimental outcomes. Materials included Ti6Al4V and CoCr bases with a Zr pin. Testing was performed in 0.9% saline at 83N load and ±6mm amplitude at 1Hz frequency.

Results:

Aim 1 demonstrated that Mischler’s model outperformed Olsson and Stemp's in predicting experimental data. While Olsson’s model worked well at -0.5V, it struggled at +0.5V due to assumptions about voltage- dependent oxide film growth, making it better suited for lower voltage predictions. Aim 2 revealed Feyzi and Hashemi’s model best predicted tribocorrosion behavior, though significant variance highlighted the need for refined assumptions. Olsson and Stemp’s model showed promise with adjustments to variables like oxide layer thickness, emphasizing its role in tribocorrosion modeling.

Conclusions:

The study concludes that tribocorrosion current is influenced by multiple factors, and model predictions improve with accurate variable inputs. Further research is needed to refine models, including developing experimental procedures to determine assumed variable values (e.g., asperity radius) and creating real- time computational models to compare experimental and predicted results.

Extensive large molecular dynamics simulations (MD) were conducted to investigate the impact of different Zr/Nb interface orientations on the friction/wear behavior of Zr/Nb multilayers. The primary cause of plastic deformation of the Nb layer was dislocations and BCC twinning, while Zr layers deformed via dislocations and intrinsic stacking faults. The Zr/Nb exhibited better tribological properties, such as lower COF, higher scratch hardness, and improved wear resistance compared to their single-crystal counterparts. The interface structure was analyzed, and its blocking strength was discussed. tailoring them to achieve desired properties for specific applications.

The simulations of friction and wear were compared with experimentally obtained nanoscratches on Zr/Nb multilayers with a periodicity of 6 nm prepared by magnetron sputtering. The wear was evaluated by AFM, structure by STEM and XRD. Qualitative agreement with experiments demonstrates predictive power of MD simulations in tribology.