Esteban Broitman

Rolling bearings in modern wind turbines operate under demanding tribological conditions that include high contact stresses, mixed lubrication, transient loads, electrical discharge, corrosion, and hydrogen-assisted damage. These factors contribute to premature failures such as micropitting, scuffing, white etching cracks (WECs), and electrical fluting, which remain major reliability challenges in both onshore and offshore turbines. Surface-engineering technologies have emerged as effective tools to mitigate these failure modes. Carbon-based coatings improve sliding performance and reduce wear under boundary-lubricated conditions; black oxide conversion layers enhance the corrosion resistance, lubricant retention, and early-life running-in behavior; and insulating ceramic coatings protect generator bearings from electrical discharge damage. Additional solutions, including polymer overlays, composite films, and hybrid ceramic architectures, offer further improvements in friction, surface fatigue resistance, and environmental robustness. This manuscript reviews the current state of coated bearing technologies relevant to wind turbine applications, synthesizing findings from tribological research, industrial practice, and emerging material developments. While only a limited number of coating suppliers provide documented evidence of coating use in wind turbine drivetrain bearings, the collective progress in surface engineering demonstrates clear potential for improving reliability and extending service life across the installed turbine base. The analysis highlights the mechanisms by which coatings enhance performance, the conditions under which they are most effective, and the gaps that remain in field validation and large-scale deployment. Coated bearings represent a promising pathway toward higher turbine availability, reduced maintenance costs, and improved drivetrain durability. Continued advances in carbon-based films, multilayer architectures, and insulating coatings, combined with better integration of lubrication strategies and condition-monitoring technologies, will play an increasingly important role in enabling the next generation of high-power wind energy systems.

This Special Issue, “Surface Engineering, Coatings, and Tribology”, was open for submissions from the entire community and incorporates a selection of studies presented at the VIII National Symposium and First Latin American Meeting on Surface Engineering and Tribology

Nowadays, Thin Dense Chromium (TDC) coatings are being industrially used in rolling bearings applications due to their claimed advantages such as high hardness, low wear, and good corrosion resistance. However, despite their broad commercial use, very little has been published in the open scientific literature regarding their microstructure, nanomechanical, and tribological properties. In this paper, TDC coatings with a thickness of about 5 µm were deposited by a customized electrochemical process on ASTM 52100 bearing steel substrates. Surface microstructure and chemical composition analysis of the TDC coatings was carried out by scanning electron microscopy and atomic force microscopy. The results revealed a coating with a dense, nodular, and polycrystalline microstructure. Unlike standard electrodeposited “Hard Chromium” coatings, TDC coatings show no presence of micro/nano-cracks, likely contributing to their superior corrosion resistance. The nanomechanical behavior, studied by nanoindentation as a function of penetration depths, exhibits a pronounced size effect near the coating surface that can be linked to the nodular microstructure. A hard surface with hardness H_IT 14.9 ± 0.5 GPa and reduced elastic modulus E_r = 216.8 ± 3.9 GPa was observed. Tribological characterization under the presence of lubricants was performed by two single-contact tribometers using coated and uncoated steel balls against flat steel substrates. An in-house fretting wear rig was used to measure the lubricated friction coefficient in pure sliding conditions, whilst the friction performance in rolling/sliding lubricated conditions was evaluated using a WAM test rig. In pure sliding, TDC/TDC contacts show ~13% lower friction than for steel. Under rolling/sliding conditions with 5% sliding, the traction coefficient of TDC/TDC coating contact was at least 20% lower than that for steel/steel contact. The tribological results obtained in various contact conditions demonstrate the benefits of applying TDC coatings to reduce bearing friction.

With a growing global railway market which needs to reduce its energy consumption and emissions, railway wheel-end bearing units are being optimized to further reduce power losses with no compromise on reliability. One of the different solutions being evaluated is the selection of the optimum surface engineering. Inner and outer rings of railway bearing units are currently coated with a zinc-calcium phosphate conversion coating designed for anti-corrosion, anti-fretting, and mounting properties. In this study, different conversion layers, like zinc-calcium phosphate, manganese-phosphate and tribological black oxide, have been compared in terms of friction performance using a single-contact tribometer and a grease-lubricated bearing friction test rig. Results demonstrate that an optimum tribological black oxide conversion layer can reduce the bearing torque by up to 30% in both low and intermediate speeds relevant to intercity trains.