One of the most demanding engineering challenges in wave energy is ensuring that mechanical components survive years of continuous operation fully immersed in seawater, under cyclic loading, without requiring frequent maintenance. A new study from the SHY Project consortium addresses this challenge directly, providing the first systematic comparison of two material combinations relevant to the SHY Project’s seawater hydraulic power take-off (PTO): polyoxymethylene (POM) sliding against polyethylene (PE), and POM sliding against duplex stainless steel (DS).
About the Study
The study investigates the tribological performance of POM sliding against PE and duplex stainless steel under artificial seawater conditions. A custom-built reciprocating wear test rig was used to evaluate both tribo-pairs under identical loading and sliding conditions.
The motivation is directly tied to the SHY Project’s core technology. The experimental setup supports the development of a composite seawater pump in which the piston assembly incorporates six POM seals — three on the piston head and three at the rod-end sea-box — sliding against PE or DS liners under seawater conditions.Unlike conventional wear testing approaches, which often produce unrealistic contact conditions, the custom rig allows area contact under controlled loading and full seawater immersion, replicating what components actually experience inside a wave energy converter.
Key Findings
The results reveal a striking difference between the two material combinations. The POM–PE tribo-pair exhibited significantly lower friction (0.12) and wear compared to the POM–DS pair (0.35), due to the formation of a stable transfer film and improved surface conformity. In contrast, the POM–DS pair showed higher wear and roughness due to the absence of film formation and the hardness mismatch at the interface.
What makes the POM–PE combination perform so well? The superior performance of the POM–PE system may be attributed to the formation of stable transfer films and the conformal nature of the polymer–polymer contact, which promote a more uniform contact interface, reduce localised stress concentrations, facilitate smoother sliding, and decrease interfacial shear.
The POM–DS pair tells a different story. The absence of a continuous transfer film suggests that duplex steel may not promote effective polymer film adhesion under seawater, leaving the POM surface more exposed to abrasive action. The hardness mismatch between the soft polymer and hard metal leads to pronounced ploughing effects, where metal asperities cut into the polymer surface, causing severe abrasive wear characterised by deep grooves.
Water absorption behaviour also emerged as an important factor. Water absorption measurements revealed higher absorption in POM due to its polar groups, while PE remained nearly unaffected.
This difference has practical implications for the long-term dimensional stability of components in marine deployments.
A Custom Test Rig Built for Wave Energy Conditions
A notable contribution of this work is the development of the wear test rig itself. The reciprocating configuration was chosen over conventional wear test methods to better replicate the linear sliding motion and contact conditions experienced by components in wave energy converters that use seawater as an energy transport medium.
The rig enables six wear tests to be conducted simultaneously under full seawater immersion, significantly improving testing efficiency and data repeatability.
Implications for the SHY Project and Marine Energy
These findings demonstrate that polymer–polymer tribo-pairs, particularly POM–PE, offer improved tribological performance under seawater lubrication and are promising candidates for marine energy systems. Looking ahead, the study also identifies a clear path for improving polymer–metal interfaces. Controlled modification of the duplex-steel micro-texture — such as polishing, surface smoothing, or engineered micro-textured patterns — represents a promising direction for future design optimisation, as such strategies may reduce asperity-driven damage, promote more stable tribolayer development, and improve the long-term performance of polymer–metal seals and sliding components in seawater-lubricated marine environments
About the Research Team
The study was led by Amir Hamza Siddiqui from the Department of Civil and Mechanical Engineering at the Technical University of Denmark (DTU Construct), with co-authors Jacob P. Waldbjoern, Mehrtash Manouchehr and Christian Berggreen (DTU), and Kristian Glejbøl and Mian Masoud from Wavepiston A/S — a key industrial partner in the SHY Project consortium.
Access the Publication
The full article is available open access in the journal Wear: https://doi.org/10.1016/j.wear.2026.206735
This work was supported by the European Research Council (ERC) under EU Horizon funding — Funded by the European Union under Grant Agreement No 101147456 – SHY. The SHY Project consortium comprises nine organisations across seven European countries, including the Technical University of Denmark (DTU), the National University of Ireland Maynooth, Wavepiston, and the offshore test site PLOCAN (Spain).