Tribological Properties of Polyoxymethylene, Polyethylene Terephthalate and Polyetherimide Under Water-Lubricated Conditions

To tackle environmental issues caused by traditional oil-lubricated bearings，water-lubricated bearings are increasingly used in clean energy equipment. However，water’ s low viscosity often leads to lubrication film rupture，causing bearings to operate in mixed or boundary lubrication regimes and posing severe challenges to material wear resistance. Current research on water-lubricated bearing materials mainly focuses on thermosetting composites，and there is a lack of systematic evaluation of recyclable and easily processable thermoplastics. This study systematically evaluated the tribological properties of three typical thermoplastic materials，namely polyoxymethylene (POM)，polyethylene terephthalate (PET)，and polyetherimide (PEI)，under water-lubricated conditions through full-scale bearing tests simulating actual operating conditions，providing a basis for their engineering application.A self-developed high-performance comprehensive test bench for water-lubricated bearings was used to conduct systematic full-scale bearing tests on bearings made of three thermoplastic materials (POM，PET，and PEI) with a circumferential eight-narrow-groove structure. The tests strictly simulated the actual operating conditions of horizontal hydro-turbine guide bearings:constant load of 4 050 N (specific pressure 0.5 MPa)，rotational speed of 255 r/ min (corresponding to a journal linear speed of 1.2 m/ s)，and an 8 h formal test under tap water lubrication. An integrated sensor network on the test bench collected real-time data of friction torque and normal load. Based on this data，the friction coefficient curve over time was calculated and plotted. Meanwhile，embedded temperature sensors continuously monitored the temperature evolution of the critical interface of the friction pair，with the water tank temperature as the environmental reference. After the test，a precision electronic balance was used to measure the mass difference of the bearing before and after the test，and a feeler gauge was used to quantitatively detect the change in bearing fit clearance. This method accurately measured the mass loss rate of the material. At the micro level，a scanning electron microscope (SEM) was used to conduct high-resolution morphological observation of the bearing load-bearing surface to reveal the micro characteristics of the worn surface. Further，laser confocal microscopy was used to obtain the 3D topography data of the worn surface，and parameters such as arithmetic mean roughness (Ra) and maximum profile height (Rz) were calculated to quantitatively evaluate the degree of surface damage. Based on systematic test methods and multi-dimensional data analysis，clear and significantly different quantitative results were obtained.In terms of friction performance:POM and PET showed excellent adaptability. After the initial running-in stage，the friction coefficients of both rapidly decreased and entered a stable stage，with steady-state values of 0.110 and 0.109，respectively. Further analysis of data stability found that POM exhibited better operational stability，with a fluctuation range of its friction coefficient of about 0.1 within each 24 s data cycle at the end of the test. In contrast，the friction coefficient of PEI continued to rise throughout the test，eventually reaching a high value of about 0.300 with severe fluctuations，indicating an extremely unstable friction state. In terms of wear characteristics:comparison of test data showed that PET exhibited the most prominent wear resistance，with a mass loss rate of only 0.71％ and the smallest change in bearing fit clearance. POM was the second (mass loss rate of 1.31％). PEI suffered severe wear，with a mass loss rate as high as 4.68％ and an increase in vertical clearance of 2.27 mm，far exceeding the previous two materials. The results of microscopic observation of surface morphology and quantitative analysis of roughness were consistent with the wear data:the worn surface of PET was the smoothest (Ra value of 4.6 μm)，and SEM observation showed that plastic deformation occurred on the bearing surface，along with the generation of fine wear debris. The Ra value of the POM surface was 5.7 μm，and the morphology showed plastic deformation characteristics and flaky debris adhesion. The PEI surface was the most severely damaged，with deep and wide ploughing grooves，significant brittle material spallation zones and abundant large-size wear debris，and the highest surface roughness Ra value (7.9 μm). In terms of thermal behavior response:the temperature rise data of the friction interface directly reflected the friction interface conditions of different materials. POM had the lowest temperature rise (8.82 ℃)，PET was slightly higher(11.33 ℃)，and PEI had the most significant temperature rise (21.21 ℃)，indicating that its friction process generated a large amount of heat and had poor heat dissipation. POM and PET have appropriate hardness and significant crystalline structures. Under the action of water lubrication and frictional heat generated during the test，the surface layer of the materials could undergo a certain degree of viscoelastic deformation，effectively dissipating part of the mechanical energy. Among them，the POM bearing was mainly dominated by abrasive wear and plastic deformation，and the PET bearing was mainly dominated by fatigue wear，with hydrolytic wear generated due to local high temperature. In contrast，PEI has the highest hardness (85.5 HD) and an extremely high glass transition temperature (217 ℃). This high hardness and high rigidity made it difficult for PEI to adapt and buffer through viscoelastic deformation under cyclic contact stress and shear force，and it was more prone to brittle fracture，leading to flaky or blocky spallation of the surface layer material. Bearing tests confirmed that POM and PET exhibited excellent comprehensive tribological performance under water-lubricated conditions:low friction coefficient (about 0.11)，low wear rate (mass loss rate≤1.31％)，moderate temperature rise (≤11.33 ℃)，and stable operation. This made them suitable for horizontal hydro-turbine water-lubricated guide bearings. In contrast，due to its brittle delamination wear mechanism，PEI exhibited high friction，high wear，and significant heat accumulation，making it unsuitable for this operating condition. The research results provided direct experimental basis for the selection of thermoplastic materials for water-lubricated bearings and had practical significance for promoting green bearing technology. Future research can focus on the composite modification of POM and PET to improve their adaptability to extreme operating conditions.