Effects of marine environment on frictional properties of porous oil-impregnated ɑ-Al₂O₃/UHMWPE composites and molecular dynamics simulation

The frictional performance of modified porous ultra-high molecular weight polyethylene (UHMWPE) oil-containing composite materials under pure water and seawater conditions is studied using nano aluminum oxide (nano-Al₂O₃) as filler and three different particle sizes of sodium chloride powder as the pore-forming agents. Experimental results showed that compared to pure UHMWPE, the friction coefficient (COF) of a 100/Al₂O₃/UHMWPE oil-containing composite material decreases by 68%. Further, using the friction behavior of the 100/Al₂O₃/UHMWPE oil-containing composite material under oil-only conditions as the control, it is observed that water molecules disrupt the transfer oil film formed during friction when pure water and seawater are present. However, due to the presence of water molecules, the abrasion debris generated during the friction process is promptly flushed away, effectively reducing the surface roughness and wear rate of the 100/Al₂O₃/UHMWPE oil-containing composite material. Based on experimental research, molecular dynamics (MD) simulation analysis was conducted to elucidate the main reasons for the variation in COF of the 100/Al₂O₃/UHMWPE in water and seawater. The simulation results indicated that the presence of water molecules and seawater components during the friction process reduces the interfacial interaction energy between the oil-containing polymer material and the friction pair, leading to the formation of an unstable friction transfer film. Therefore, the higher COF of the 100/Al₂O₃/UHMWPE oil-containing composite material in pure water and seawater environments compared to pure 100/Al₂O₃/UHMWPE oil-containing composite material is explained.