24th International Colloquium Tribology - January 2024 137 Implementing the use of Water Based Enviornmentally Acceptable Lubricants in the Ship Industry On the Frictional and Wear performance of SiC-YAG Composite Coating N. Espallargas 1,* , E. Valaker 2 , H. Khanmohammadi 1 1 Norwegian Tribology Center, Dept. of Mechanical and Industrial Engineering, NTNU, Trondheim, Norway. 2 Corrosion and Tribology, SINTEF Industry, SINTEF, Trondheim, Norway. * Corresponding author: nuria.espallargas@ntnu.no 1. Introduction Maritime transportation is increasing worldwide leading to annual lubricant release into the ports of about 32 to 61 million litres. Norwegian ports account for more than 1.6 million litres ranking Norway as the 12 th world nation with the highest lubricant input to the sea. Lubricants reaching the sea waters can be catastrophic to local marine wildlife and aquatic plants and harmful chemical substances can enter the food chain affecting human health [1-3]. Therefore, the aim of this work is to test a new water-based lubricant for the oil-to-sea interfaces of a thruster system introducing optimized surfaces (e.g. SiC coatings) in contact with polymeric seals. Five different seal materials and four different surfaces have been tested for frictional and wear performance in a water-based lubricant formulated for a thruster application. This work is part of an on-going industrial project in Norway funded by the Research Council of Norway and the partner companies. A methodology based on Stribeck curve parameters fitting real thruster conditions was chosen to study the frictional behaviour of each seal-surface pair. The Stribeck number of three main seals (rotational seal, leap seal and axial seal) was calculated and the contact pressures and speeds of the tests were designed to have the Stribeck curves covering all seals. 1.1 Materials and experimental procedure Five seal materials were tested against the different countersurfaces. Table 1 shows the selected seal materials and their properties. Four countersurfaces were selected: hardened steel as the reference material, WC-CoCr coating, Cr 3 C 2 -NiCr coating and ThermaSiC coating were applied on mild steel using a thermal spray technique. Table 2 shows a summary of the tested counter surfaces. Table 1: description of the seal materials used in this work. Short name Shore Hardness Material type Chemical compatibility and sealing performance PE 67D Ultra-high-molecular-weight polyethylene Moderate swelling below 1% and surface plasticization in mineral oil above RT. Exceptional durability and toughness, self-lubricating in dry-running applications, high abrasion resistance, very low friction. HNBR 90A Hydrogenated acronitrile-butadiene rubber Excellent weathering resistance. High abrasion resistance, outstanding dynamic characteristics, very low compression set. EPR 80A Ethylene-propylene-diene rubber, peroxide-cured Best for aqueous fluids, inferior to mineral oil, excellent weathering resistance. High abrasion and tear resistance in hydrocarbon-free environments. PK 78D Aliphatic polyketone Moderate swelling and water-induced plasticizing. High resilience, excellent dynamic sealing characteristics, low friction, no stick-slip behaviour. SWF - Synthetic woven fabric impregnated with phenolic resin Resistant to water uptake. High strength, flexibility and integrity at extreme pressures, excellent gliding characteristics and thermal expansion stability. Table 2: description of the counter surfaces used in this work. Surface Wt % Metallic matrix Coating technique Hardness (HV) Coating Thickness (µm) Hardened steel 100 - 780 - WC-CoCr 14 HVOF* 1165 254 Cr3C2-NiCr 25 HVOF* 890 221 ThermaSiC 0 HVOF* 540 191 * High Velocity Oxygen Fuel The seal samples were provided by Seal Engineering AS (Norway) in the form of pins with a diameter of 8 mm and a height of 6 mm. All the counter surfaces in Table 2 were prepared as discs with a diameter of 40 mm and a thickness of 6 mm. The cermet coatings (WC-CoCr and Cr 3 C 2 -NiCr) were provided by Wear Solutions AS (Norway) and were plane ground to reach a surface average roughness (Ra) of 0.4 µm. The hardened steel and ThermaSiC (provided by Seram Coatings AS, Norway) samples were ground using sandpapers until reaching the same roughness as the cermet coatings. For the Stribeck curves, seven different conditions giving seven points on the coefficient of friction (COF)-Stribeck number plot were designed. Table 3 shows the seven test conditions used in this work and their assigned Stribeck number and the Stribeck number of three components selected by the final user in the project. 138 24th International Colloquium Tribology - January 2024 Implementing the use of Water Based Enviornmentally Acceptable Lubricants in the Ship Industry Table 3: Test conditions for the Stribeck curves Test number Normal load (N) Contact pressure (MPa) Rotational speed (RPM) Assigned Stribeck number 1 45 0.9 8 0.024 2 7.5 0.15 8 0.145 3 45 0.9 76 0.242 4 45 0.9 300 0.969 5 7.5 0.15 76 1.45 6 7.5 0.15 300 5.81 7 4 0.08 300 10.9 Application - - - - Rotational Seal - - 0.87 Leap Seal - - 4.1 Axial Seal - - 10.9 Each test is repeated at least two times. All seal pins were soaked in the lubricant for 7 days at 80 °C before testing. The test duration was 1800 s and the average friction during the last 400 s was considered as the COF for the test point. The wear investigations were performed using TE88 reciprocating tribometer. The normal load was set to 250 N to measure wear without having any mechanical failure in the pins. The wear tests were performed with a linear velocity of 48 mm/ s with a stroke length of 24 mm and for a duration of 23 hours. The seal pins were weighted before and after the tribotests to measure the material loss. In the case of cermet coatings (WC-CoCr and Cr 3 C 2 -NiCr), the sliding direction was set perpendicular to the grinding lines. 2. Results and discussion Figure 1 shows the Stribeck curves obtained for all surfaces tested against the different seals. Stribeck results showed lower frictional numbers for the plastic seals compared to the rubber seals. Figure 1: Stribeck curves of the seal materials against (a) hardened steel, (b) WC-CoCr coating, (c) Cr 3 C 2 -NiCr coating and (d) ThermaSiC coating. Tests against the WC-CoCr coating showed the highest frictional numbers, followed by Cr 3 C 2 -NiCr. The hardened steel and ThermaSiC showed lower numbers compared to the cermets. Adsorption of the lubricant additives to the metallic surfaces are proposed as the dominant mechanism in the boundary regime of the hardened steel and the cermet coatings. In the case of ThermaSiC the formation of a hydrated film due to the water present in the lubricant is proposed as the main frictional mechanism. For the softer seals, the hydrated film formation mechanism was not activated, resulting in higher friction for these samples against ThermaSiC compared to the other surfaces. For harder seals, the hydrated film formation took place resulting in lower friction against ThermaSiC. In terms of wear, the pair ThermaSiC-PE was the one providing with lowest material loss. 3. Conclusions Using water-based lubricants in a thruster system requires of new material selection to get the optimal friction and wear results. In this case, SiC with PE provides the best surface-seal couple to obtain the lowest friction and wear for the application. References [1] D. S. Etkin. Worldwide Analysis of In-Port Vessel Operational Lubricant Discharges and Leakages. Proceedings of the 33rd Arctic and Marine Oil Spill Program Technical Seminar. New York 2010. [2] https: / / www.tribonet.org/ wiki/ environmentally-accept able-lubricants/ [3] https: / / www.dw.com/ en/ exclusive-cargo-ships-dump ing-oil-into-the-sea-go-unpunished/ a-61201989