Review of the evolution and prevention of friction, wear, and noise for water-lubricated bearings used in ships

With the development of green tribology in the shipping industry, the application of water lubrication gradually replaces oil lubrication in stern bearings and thrust bearings. In terms of large-scale and high-speed ships, water-lubricated bearings with high performance are more strictly required. However, due to the lubricating medium, water-lubricated bearings have many problems such as friction, wear, vibration, noise, etc. This review focuses on the performance of marine water-lubricated bearings and their failure prevention mechanism. Furthermore, the research of marine water-lubricated bearings is reviewed by discussing its lubrication principle, test technology, friction and wear mechanism, and friction noise generation mechanism. The performance enhancement methods have been overviewed from structure optimization and material modification. Finally, the potential problems and the perspective of water-lubricated bearings are given in detail.


Introduction
The applications of water-lubricated bearings (WLBs) originated from the early days of ship propulsion.As shown in Fig. 1, WLBs have evolved with the developing materials and related technologies to form a rich spectrum of products, which mainly belong to radial sliding bearings and are used for ship propulsion system stern bearing.With the advancement and application of shaftless rim-driven thruster (RDT), the WLBs in ships began to extend their application fields from journal bearings to thrust bearings.
It is necessary to protect the environment and resources that oil-lubricated bearings need to be replaced because they might consume massive mineral oil and precious metal resources.It has been a trend to replace oil-lubricated bearings with WLBs in the marine field because they meet the requirements of green shipping [1].The complex structure makes it difficult for oil-lubricated metal bearings to reduce friction, wear, vibration, shock, and noise.But the simple structure WLBs with polymer bearing materials such as rubber have significant advantages in these areas.However, as shown in Fig. 2, due to the low viscosity of lubrication water and its harsh operating environment and working conditions [2,3], there are some problems with friction wear and vibration noise.Further research on the materials and structure of WLBs is required for the lower vibration and noise and less friction and wear to improve the operational performance and service life.
In general, WLBs need the performance of low 2 Lubrication theories and characteristic simulations of WLBs

Lubrication theories
Under the low speed and heavy load, the water-lubrication film of WLBs is thin, which is difficult to form an effective lubrication film.Moreover, the bearing bush generally adopts polymer composite materials, which is easy to be large deformation.Therefore, it is necessary to establish elastohydrodynamic lubrication (EHL) and mixed lubrication (ML) models.

Hydrodynamic lubrication theories
The differential equation derived by Reynolds for fluid film pressure distribution provided a solution for hydrodynamic lubrication (HD) theory.The engineering application of HD theory was promoted by the infinite length bearings theory [4] and the analytical formula for short bearings [5].Subsequently, the models of EHL, thermohydrodynamic lubrication (THD), thermoelastic hydrodynamic lubrication (TEHD), and soft elastohydrodynamic lubrication (SEHL) were further developed with material The theoretical studies were shown in Table 1.
Table 1 shows that the hydrodynamic lubrication theories are gradually developed by considering practical influencing factors on the basis of a simplified and idealized theoretical model.But it cannot fully reflect the actual situation.When the peak contact appears at the rough micro-body of the lubrication surface, it would be difficult to analyze the performance of the bearings by the EHL model, which requires partial elastohydrodynamic lubrication (PEHL) or ML theory.

Mixed lubrication theories
ML introduces a rough surface contact model to consider hydrodynamic and surface contact pressures based on the EHL or TEHD model, so it is difficult for ML to describe the rough surface and simultaneously solve these two pressures.According to the description method of surface roughness, the ML model could be divided into statistical and deterministic models, and the related studies [11,[23][24][25][26][27][28][29][30][31][32][33][34][35] were shown in Fig. 3.
The Patir-Cheng (PC) flow model is a typical representative of the statistical model.From this model, the average contact pressure can be obtained by the average clearance from the average Reynolds equation.The contact factor of the PC flow model makes the mean Reynolds equation more suitable for partial hydrodynamic (PHD) and PEHL.Venner et al. [33] argued that the PC flow model failed to consider the elastic deformation of the microgeometry, and the orientation parameter cannot uniquely define the roughness direction so that different roughness profiles might have the same statistical properties.These two disadvantages caused that the "averaging treatment" of the PC flow method could not solve the ML model well.The improving surface topography observation capability has enabled techniques such as optical interferometry [36,37], and thin film colorimetric interferometry [38] to be applied for surface roughness, which has attracted attention to deterministic models of roughness for micro-EHL studies [33,34].
Conventional ML models are incompletely suitable for marine water lubricated stern bearings (WLSBs) because those only consider microscopic ML in line or point contact.As shown in Fig. 4, shaft tilt and large deformations could also lead to macroscopic surface contact in the partial area [2].In this case, it is meaningful to distinguish the contact zone by lubrication state as mentioned in Refs.[39,40].Ouyang et al. [41] firstly proposed that a distributed parameter lubrication model should be established  for partial fluid film lubrication.It is meaningful to introduce distribution characteristic parameters into the basic lubrication equation to analyze the different lubrication zones.Li et al. obtained the film thickness distribution of the full bearing by the circumferential film thickness of three interfaces, which identified the distributed lubrication characteristics of the WLBs [42].

Solution algorithms
The lubrication theories of WLBs and their control equations could be summarized as shown in Fig. 5.A variety of solution algorithms for lubrication models have been developed just as shown in Table 2.
As one of the representatives of the ML model, more rigorous mathematical derivation was needed by the PC flow model to calculate the flow factor with the effects of micro-cavitation and elastic-plastic deformation of rough surfaces [53].The deterministic models have been developed with the surface profile recognition technologies, its further development depends on more accurate surface identification, better surface roughness mathematical models, and solution methods.The researches on distributed lubrication only present the modeling concept and simple calculation, which needs to be further proposed by the complete equations and algorithms.It is necessary to develop the solving algorithms with high computational speed, low grid dependence, and high accuracy of results.Meanwhile, the thin water film and large deformation of the stern bearing bring great difficulties to modeling, meshing, and boundary conditions setting of computational fluid dynamics (CFD) calculations.Thus, it is necessary to strengthen the scientific and reproducibility in the coupled thermo-hydro-solid analysis of WLSBs [2].

Simulation of WLBs characteristics
The research of WLBs mainly focuses on their characteristics of lubrication, loading, and dynamic performance.The parameters of its structures and materials directly determine the performance of WLBs as internal factors, while the effects of in-use conditions and environments cause the change in service life.This section would review the evolution mechanism of WLBs through the simulation studies.
Numerical analysis and finite element analysis were usually used to simulate the characteristics of WLBs.The numerical analysis method was suitable for the calculation of loading capacity and liquid film characteristics with high precision, but with low calculation efficiency and poor adaptability to complex structures.The finite element method could be used to analyze the lubrication characteristics with better Friction 12(1): 1-38 (2024) | https://mc03.manuscriptcentral.com/frictionadaptability to complex structures and higher computational efficiency.However, it has poor adaptability to multi-structure and multi-working conditions.To analyze the performance of WLBs more comprehensively, these two methods are generally combined [54].A review of the simulation studies for different WLBs is shown in Table 3.
As shown in Table 3, the studies of WLBs simulation have covered all types (e.g., stable and transient, cavitation and non-cavitation, laminar and turbulent flow, alignment and misalignment, textured and non-textured, etc.) to simulate the lubrication, loading, and dynamic characteristics.It should also be noted that the study of dynamic characteristics needs to consider the coupling relationship with the shaft system.
This section reviewed the research on lubrication theory and characteristics simulation of WLBs.The models of hydrodynamic lubrication and mixed lubrication and their algorithms were compared to summarize the advantages and disadvantages of each model, and some possible improvement directions were proposed.Sommefeld or half-Sommefeld condition Ocvirk [5]; Dousti [7] HD model

Distributed ML Consider lubrication partition and distribution characteristics
Computational strategies for decomposition and synthesis Ouyang [41] Notes: *AICV: asymmetric integrated control volume discretization scheme.

Performance test techniques for WLBs
It is an effective method to measure and evaluate the performance of WLBs by parameters from the WLBs performance test.This section would review the typical WLBs test devices and parameter identification methods.

Test devices of WLBs
As shown in Fig. 6, according to the different objects, WLBs test devices could be classified into three types just like basic tribological test, bearing model test, and appraisal test.
The basic tribological properties test devices for specimen could perform different types of friction tests on specimens such as rotations, reciprocating, block on ring drive, micro-movement, scratching, and high-speed long stroke.It is necessary for WLBs to consider the corrosion of water on metals and the simulation of the sediment environment, so the basic tribological test devices of test block are generally customized, such as the ring-block test device [78], ring-plate test device [79], etc.These devices could simulate different operating conditions by changing the frictional wear method and time, motor speed, test environment, and preload force on the specimen.The friction coefficient of the specimen could be calculated by friction force.The surface morphology information of the specimen could be identified by scanning electron microscope (SEM), atomic force microscope (AFM), or X-ray diffraction (XRD) after the test to analyze the basic tribological properties such as the wear state [80] and stick-slip phenomenon of the materials [81].
The model test is used to acquire the performance test of shrinkage bearings or full-size bearings [42], the appraisal test device [82] is used to simulate the real-life condition of a real propulsion shaft system and evaluate whether the bearing meets the design requirements.According to different loading methods, the radial bearing test device could be divided into positive [83] and inverted types.The load of the positive is located on the shafts on both sides of the test bearing with a downward direction, which could simulate bias load.The load of the inverted acts on the test bearing in an upward direction.The thrust bearing test device could be divided into vertical and horizontal types according to the structure form.Compared with the horizontal, the vertical is simpler because the lubrication seal structure is cancelled.4 and Fig. 7.

Parameter identification methods
The WLBs performance could be determined by the water film quality, including pressure distribution, temperature, film thickness distribution, and kinetic parameters.An overview of the identifications for WLBs lubrication water film is shown in Table 5.
In this paper, the difficulty level of the test methods is comprehensively evaluated by the maturity of the sensor itself, the universality of use in the field of water/oil lubricated bearings, test accuracy, test environment, technical maturity (such as test principle, laboratory verification and practical application, etc.), and the number of literature reports.The difficulty level of the test method is considered to be lower when the method has the characteristics of the more mature sensor, the wider application in related fields, the lower test environment requirements, the more literature reports, the simpler test principle, and the more reliable verification test method.In Table 5, the difficulty level is represented by the number of ★, and the difficulty level is positively correlated with the number of ★.
As shown in Table 5, the measurements for vibration, temperature, and pressure have been quite developed to meet the requirements of various tests.However, it is difficult to measure the film thickness and the distribution of all performance parameters, which are closer to the characteristic parameters of bearing service and more directly reflect the bearing performance.In addition, the in-situ measurement of wear is also a challenge.The stiffness and damping identification of WLBs is derived from the measured values of force and displacement, which belongs to indirect measurement with low accuracy and large error.This section focused on the performance test technology and test device of WLBs, and summarized the characteristics of different test technologies and devices.It is a great challenge to realize the distributed and online test of WLBs performance and improve its test accuracy.

Friction-reduction technologies for WLBs
The marine WLBs are easily under the mixed lubrication.Especially, in certain extreme conditions such as the start/stop phase or at low speed and heavy load, the bearings are easy to suffer from the boundary lubrication or even a dry friction state.It would reduce the efficiency of the shaft system and cause serious wear, or even threaten the safety of the ships.
To achieve the WLBs' friction reduction, bearing design and material selection should be carried out to avoid the bearings working in poor lubrication.This section would review the research and application of friction-reduction technologies for WLBs from two aspects: structural optimization and material modification.

Structural optimization
Structural optimization could improve the tribological properties of bearings by optimizing the structural form or applying the appropriate surface treatment.The optimization research of basic structures (e.g., the L/D, clearance, and groove structure, etc.) [130][131][132][133][134][135] has shown that the structural parameters interact with each other to jointly influence the bearing performance.In addition, the lining structure of WLBs could be divided into integral type and slab type [2].The surface type of slab bearing (such as flat type, concave type, and convex type) would affect the bearing performance [78,136].Therefore, it is essential to perform multi-factor coupled optimization [137] and synergistic optimization with operating parameters [138] when bearing is designed.
This section would review the friction-reduction applications of surface texture techniques and magneto-hydraulic composite structures on WLBs.

Surface texture
Surface texture is fabricated by precision machining techniques to form different morphologies such as pits, grooves, or micro-convex on the friction surface and improve the tribological properties.As shown in Table 6, a series of researches on surface texture has been carried out by the Wuhan University of  FBG sensors [96,97]; PDMS sensors [98]; Continuous pressure distribution along the circumferential direction

Less application ★★ Temperature
Non-contact measurement Spectroscopic analysis of CeTe quantum dots films [110] The temperature inside the bearing Not applied to water film of WLBs

★★ Electricity
Eddy current method [112][113][114] Minimum Technology.Among them, the basic tribology studies on texture types of Guo et al. are shown in Fig. 8.
The studies indicate that surface textures enhance the performance of wedge-shaped gaps or local stepped structures between contact surfaces to improve the effects of the wedge, converge, and cavitation or pressure gradient [146].These effects would promote the formation of hydrodynamic lubrication to reduce the adhesive effect and prevent the accumulation of abrasive particles [145].Surface textures could significantly enhance the level of fluid lubrication to improve the tribological characteristics of WLBs by designing the correct form and setting appropriate locations [139].
To resolve slow computing rate and limited computing scale of the traditional numerical methods for multiscale surface texture hydrodynamic lubrication, the finite cell method (FCM) was proposed by Pei   [147].This method is successfully applied to solve the problem of the static and dynamic performances of oil lubricated offset bearing with surface texture [148], and further improved to improve robustness and computational efficiency [149].The FCM and its derivative method can be employed for the simulation study of WLBs with surface texture.

Magneto-hydraulic double-suspension bearings
The magneto-hydraulic double-suspension bearing technology combines the advantages of liquid hydrostatic/dynamic pressure and magnetic suspension.This structure can play a role of magnetic force loading at low speed, and the joint loading of magnetic and hydraulic force at high speed to prevent frictional surfaces from contact.Therefore, it could reduce friction, improve operational stability, and increase loading capacity. Figure 9 illustrates the application of magnetohydraulic double-suspension technology for bearings.One of the important was the artificial blood pump [150].Elsewhere, this technology was used to improve start/stop characteristics [151], dynamic performance [152], or as a maintenance-free solution [153], etc., but some designs were unsuitable for marine WLBs because of their application scenarios [154,155] or structural forms [156].Ouyang et al. introduced it to the applications of water-lubricated thrust bearings [157] and radial bearings with the permanent magnets adopted and proposed the fixed/tilting pad thrust bearings and high-load radial bearings with magneto-hydraulic double-suspension structure [158,159].
It is beneficial for WLBs to employ magnetohydraulic double-suspension technology because it could improve their friction-reduction performance, loading capacity, and operational stability.However, the current research exhibits unclear mechanisms of multi-physical fields coupled and weak proactive design ability.It is necessary for magnetic coupling structure to explore its internal synergistic evolution mechanism between bearing force and deformation in thermal-fluid-solid-magnetic multi-physical fields.

Material modification
The structural optimization could enhance the formation   and retention ability of the lubrication water film to reduce the contact of the friction interface.However, under specific conditions such as the bias load or at start/stop phase and low-speed heavy-load, the inevitable frictional surface contact would result in boundary lubrication or even dry friction which could cause an increased friction coefficient.Therefore, it is necessary to modify the bearing materials to improve the lubrication capacity when contact occurs.

Blending modification
The blending modification of friction reduction employs a more stable interlayer to functionate as stable self-lubrication and separate the contact surfaces [160].The formation mechanism of the interlayer has been explained and classified in detail, which involved the friction process caused by mechanical, physical, or chemical effects [161].Moreover, the synergistic lubrication phenomenon of the interlayer has been observed by the other studies [162][163][164][165][166].
As a widely used WLBs material, nitrile butadiene rubber (NBR) has been a great concern for material modification to improve its tribological properties.Zhou et al. modified NBR by blending MoS 2 nanoparticles [164], UHMWPE and graphene powder [167], and SiO 2 [168] or nano-SiO 2 [169] to obtain good friction properties at a low speed.Qu et al. [170] blended polyurethane (PU) and epoxy (EP) into NBR to produce novel ternary NBR/PU/EP interpenetrating polymer networks (IPNs) and obtained a minimum friction coefficient lower 75.4% than that of pure NBR.Blending modification with suitable material could provide a better friction-reduction effect for NBR.
More and more polymer materials have been used for marine WLBs to improve their performance.To reduce the friction of these materials, scholars began to pay attention to the research on material modification.Figure 10 shows the blending modification of polymer materials.Figure 10(a) is the direct blending modification with a different mechanism.The ① is the utilization of micro-convexity and rolling formed by particles [164,168,169]; the ② is by reducing the material hardness and increasing the load bearing area [170]; the ③ is the weakening of deformation and sticking [80,[171][172][173]; and the ④ is the promotion of water-film formation by the hydrophilic/absorbent nature of the materials [174][175][176][177].But it may be difficult for direct blending to achieve an ideal lubrication effect due to the material compatibility limitation.In this case, the self-lubricated microcapsule structure as shown in Fig. 10(b) could be considered for material modification [178,179].The urea-formaldehyde resin Fig. 10 The blending modification of polymer materials: cases and mechanism.

Bionic modifications
It is greatly significant for the achievement of ships' green friction goal to apply bionic principles and design materials with self-lubricated structures to develop new WLBs with low friction factor, high loading capacity, low noise, and good self-lubrication performance.Guaiac resin as oily resin could promote the formation of lubrication water film and thus improve the friction and wear properties of the material [181].Guo et al. prepared the HDPE/guaiacum to reduce the friction coefficient of the material, which was a self-lubrication bionic material referred to the composition and surface structure of lignum vitae [182] and lignin [183].The self-lubrication microcapsules shown in Fig. 10(b) are also bionic applications for the structure of lignum vitae.
In addition, some researchers have also focused on the surface morphology of snakes [184] and water snail [185] etc., to develop surface textures with similar structures to reduce friction.Human joints have excellent lubrication and a very low friction coefficient at high loads [186].This superior interfacial lubrication mechanism is also a major research topic in bionic water lubrication.The skin of earthworm exhibits low friction characteristics when it moves in the soil.This low friction is not only from the annular micro-groove structure on the skin surface of earthworms [187] but also from the coupling effect of surface structure and interfacial lubrication [188,189].The review of Li et al. [190] illustrated the bionic modifications in WLBs more systematically and enumerated several preparation methods of typical materials or structures.Bionic modification is expected to promote the performance of WLBs.

Ion implantation
The friction pairs of WLBs are mainly made of metal, ceramic, and polymer materials.Ion implantation is an advanced technique to modify these materials' surface properties without changing the original properties of the matrix material [191].This technology can obtain a high alloy concentration on the surface of the material only by injecting a small amount of alloy elements, thereby effectively improving the wear resistance, corrosion resistance, oxidation resistance, and fatigue resistance of the material surface [192,193].The studies on surface modification of metal materials [194,195], ceramic materials [192], and polymer [193] by ion implantation technology indicate that it can effectively improve the wear resistance and surface hardness of matrix materials, and the friction coefficient of some materials also decreases.
Ion implantation has great application potential in the field of WLBs.
Currently, ion implantation has been widely used in the fields of photovoltaic materials [191], electronic devices [196], and medical devices [195], but research or application in the field of WLBs has not yet been seen.The specific effects of ion implantation on the performance of WLBs need to be explored by combining the materials and application conditions of WLBs to further guide the practical applications.

Wear mechanism and wear resistance technology for WLBs
It is significant for service performance enhancement of ships to improve the WLBs wear resistance because of its great influence on operational performance and the service life of WLBs.This section would overview the influencing factors of WLBs wear from wear mechanism and review the wear resistance enhancement methods from material modification and friction matching pairs optimization.

Wear mechanism
It is important for WLBs wear research to reveal the mechanism of wear failure and propose available life prediction methods.Liu et al. [197] summarized the failure forms of water-lubricated radial bearings and found that most of the rubber bearing failures were related to wear.

Main wear forms of WLBs
Adhesion wear is a major wear form of marine rubber stern bearings.Drummond et al. [198] explained most stick-slip friction by the model based on the Furtherly, an improved model of the stick-slip friction was proposed [199,200].Dong et al. [201,202] revealed the adhesion wear mechanism of NBR by pin-disc test and pointed out that severe adhesion tearing wear occurred between the friction pairs under water lubrication conditions.This wear process was a cyclic process of adhesionstretched or deformed -tearing -re-adhesion -stretched again -tearing again.Moreover, high lubricating water temperature in some extreme conditions could lead to a separation of some composite components (such as the polymer resin (PR) substrate component) to cause strong adhesion wear [203].Abrasive wear is another major form of WLBs mechanical wear.It usually occurs in open-type WLBs working in turbid water by the influence of sediment or hard suspensions.In this process, the hard particles wear the bearing surface and produce microforms such as grooves, scratches, and impact pits through the impact, shoveling, plowing, and cutting action.The hard particles with appropriate particle size [204] would destroy the water film and retain in the friction pairs [205] to exacerbate the wear process.Zhou et al. [206] obtained similar conclusions and the main wear mechanisms were shown in Fig. 11.The research pointed out that the sediments entering the WLBs would mainly result in furrow wear, but clay might reduce the wear because it might adhere to the surface of the friction pairs to prevent direct contact and convert particle-NBR wear into clay-NBR wear.
Adhesive wear and abrasive wear would occur simultaneously and abrasive particles formed by adhesive wear could cause abrasive wear [201].During the friction process, when the load is too large and the interface heat dissipation is insufficient, the surface of the friction pair of the metal-polymer material would adhere, and the metal protrusion would be torn off and embedded into the surface of the polymer material.After further friction, the metal abrasive particles embedded in the surface of the polymer material would appear hardening effect, and then grinding with the metal surface of the friction pair like "sandpaper".Two-body abrasive wear is converted into three-body abrasive wear [207], forming a so-called "soft wear hard".
Concurrent multiple forms of wear could cause complex interactions.Ning et al. [208] analyzed the worn surface morphological characteristics of the Aluminum bronze (ZCuAl9Mn2)/PEEK friction pairs.Then, the wear evolution mechanism was proposed as shown in Fig. 12 by considering the effects of frictional heat effect and oxidative decomposition on wear based on adhesion and abrasive wear.It involved the process of contact-adhesion-thermal expansion-thermal oxidative decomposition-avulsionplowing.| https://mc03.manuscriptcentral.com/frictionmorphology of the thrust ring with aluminum bronze/Throden SXL friction pairs [86] and compared the wear results with stainless steel/PEEK friction pairs [209], which concluded that harder frictional subsets could increase the loading capacity of thrust bearings and reduce wear.Therefore, six sets of friction pairs with different hardness combinations as shown in Table 7 were tested and the results indicated that the increasing hardness of thrust rings would reduce the production possibility of abrasive particles and improve wear resistance [207].

Influence factors of bearing wear
(

2) Influence of operation conditions on bearing wear
The speed and load could significantly influence the wear process of WLBs.The water-lubricated NBR pin-disc tests observed a large number of spalling particles at low-speed operation and a lot of tiny pits and particles at high-speed operation.The wear increased dramatically when the load increased [202].
Material aging which is caused by increasing temperature would lead to performance degradation of friction and wear.The rubber is sensitive to temperature that the increasing temperature would significantly reduce its mechanical property and further affect its tribological property [210].Frictional heat may lead to a temperature rise of 5-10 °C on Fig. 12 Wear evolution mechanism of water-lubricated ZCuAl9Mn2/PEEK friction pairs.Reproduced with permission from Ref. [208], © IEEE 2021.  the wear surface compared with the environment due to the extremely poor heat dissipation properties of rubber [202].It would further aggravate the temperature rise of the rubber bearing because of the surface contact under heavy load, low speed, and muddy water conditions [211].Marine stern bearings are required to operate constantly for a long time, which would accelerate the rubber aging.The NBR-pins under different accelerated aging times indicated that the resistance to fracture and tearing of NBR decreased sharply when the large cracks and wear particles formed due to a certain degree of aging, which further led to a decrease in wear resistance [212].In summary, the lubrication conditions, load, and speed change the frictional wear state by influencing the adhesion and hysteresis friction of the friction pairs which further affect the service performance of the bearing.Service safety needs to assess the reliability life of bearings.Dong et al. fitted the material wear test data to obtain 1) an equation for the wear of NBR as a function of mechanical parameters, load, and speed [201] as shown in Eq. ( 1), and 2) an empirical equation for the volumetric wear of NBR under pure water lubrication [202] as shown in Eq. ( 2 where A is the wear mass loss rate (mg/h), H is the shore hardness of the NBR (A), N is the load (MPa), and v is the velocity (m/h); V X is the average stroke wear volume (mm 3 /km), V is the wear volume (mm 3 ), L is the sliding distance of stainless steel plate on NBR (km), t is the aging time (d), and T is the aging temperature (K).The prediction error of Eq. ( 1) is basically below 10% and could be applied in engineering practice.

Methods for wear resistance enhancement
It is possible to improve the wear resistance of bearings from both structures and materials.Structural modifications are similar to the groove design and surface texture [213] methods used in Section 4 for friction reduction, which aims to promote the formation and maintenance of lubricating water films to reduce frictional surface contact and keep the flow of lubricating water smooth to take away the hard particles and frictional heat.This section would focus on the material aspects including material modification and friction matching pairs optimization.

Material modifications
Wear-resistant modification is mainly based on fiber enhancement by using fiber materials with dimensional stability, high strength, high toughness, and environmental resistance to share the load on the contact surface.Besides, the presence of high-strength fibers could hinder the expansion of the fatigue cracks formed.Fiber materials used commonly are carbon fiber (CF), carbon nanofiber (CNF) or carbon nanotube (CNT), and glass fiber (GF).The wearresistant modifications by fibers [79,80,165,175,176,[214][215][216][217] and solid particles [164,168,171,172,218] are shown in Fig. 13, and some blending modifications [164,165,168,171,172,175,176] can simultaneously achieve the purpose of improving the friction reduction and wear resistance of the material.
In addition, the orientation of the fibers in the polymer material also affects the wear resistance of the material, it would have a better effect when the fiber arrangement is perpendicular to the sliding direction [219].To make the blended fiber orientation controllable, one or more fibers could be woven into a fabric with higher order, tightness, and integrity to improve the mechanical strength of the modified material which could bring better structural stability and higher loading capacity.Fabric reinforced technology Fig. 13 Blending modification for wear resistance improvement.

20
Friction 12(1): 1-38 (2024) | https://mc03.manuscriptcentral.com/frictionhas already been used in several commercial bearings [220].Meanwhile, the water swelling of the filler should not be ignored, because it is easier for some hydrophilic or water-absorbing particles to fall off from the composite substrate in the wear process [168].Falling materials would destroy the surface condition of the material and cause abrasive wear to increase the wear rate [221].
In some cases, the multi-composite modification of multiple fillers could take advantage of the synergistic effect between the components to obtain better tribological properties.This effect has been observed when CF is reinforced by nano-SiO 2 [222] or TiO 2 particles [223], CF and PTW synergistically reinforce PEEK [224], and 10 vol% Gr-10 vol% CF-5 vol% CNT modifies polyimide (PI) [225].The components could share the load, and improve the interfacial combination between fillers and substrate to reduce stress concentration and achieve synergistic efficiency.However, it may also lead to increased wear, such as the addition of both CNT and SCF in PPS composites because the fillers cannot play a good interaction [226].

Friction matching pairs optimization
Abrasive wear in fluid lubrication depends on the hardness of the wear surface, the opposing surface, and the abrasive particles.It is effective for friction pairs to reduce the wear volume and improve the performance of abrasive wear resistance by decreasing the hardness of the abrasives or increasing the hardness of the wearing surface [227].The research reviewed in Section 5.1.2also indicates the same conclusion about improving the wear resistance of WLBs by increasing the hardness of the friction pairs.As shown in Table 8, some special surface treatments can be applied to improve the surface hardness.
However, Yuan et al. [238] showed that it was not enough to enhance the wear resistance only by increasing the surface hardness because the wear was affected by multiple factors (e.g., load, speed, lubrication water quality, etc.).Golchin et al. [239] supported that the deformation caused by plowing action could further lead to adhesive wear on the contact surface when the difference of the elastic modulus and surface free energy between the friction pairs was large.Quaglini et al. [240] concluded that polymers with low elastic modulus exhibited better sliding behavior on smooth interfaces, while polymers with high modulus could obtain better tribological properties when it was sliding on rough interfaces.The wear resistance improvement of bearings requires  an effective match of highly wear-resistant polymer bearings with surface hardened shafts [241].The commonly used water-lubricated friction pairs are organized as shown in Table 9, which could provide guidance for engineering applications.

Generation mechanism and suppression methods of friction noise
Noise is generated by vibration.WLSBs are an important part of the marine propulsion system.It can be seen as a vibration component in shafting operation.Under special conditions (e.g., low speed, heavy load, or poor lubrication), it is easy to produce abnormal vibration and noise.Bharat Bhushan named the vibration noise at higher frequencies (560-1,670 Hz) as "Squeal" and lower frequencies (300-600 Hz) as "Chatter" in his research about WLRBs [249].Besides, WLSBs can also be considered as a vibration isolation element in shafting system.It can weaken the excitation on the shaft (such as the excitation from the propeller or prime motor, etc.) to be transmitted outward to the bearing base or even the hull, and also reduce the external excitation (such as the excitation generated by waves on the hull, etc.) to be transmitted to the shaft.
The vibration-induced noise generated by these internal or external excitations would threat the stealthiness, reliability, and passenger comfort of the ship.The evolution mechanism of friction noise is required to improve the quiet level and survivability of ships.In this section, the evolution mechanism of the friction noise would be represented from the theoretical research and experiment test to propose the prevention methods from structure optimization and material modification.

Generation mechanism of friction noise
Theoretical research is mainly to establish the model to study the influencing factors or system coupling by reasonably simplifying the friction system; experimental research is to collect and analyze the signals (e.g., torque, displacement, vibration, etc.) to capture the vibration noise characteristics and analyze the generation mechanism.

Theoretical studies
The current research is mainly focused on automotive brakes, among which the relatively well-established and highly recognized explanatory mechanisms are shown in Table 10.The stick-slip mechanism and Friction | https://mc03.manuscriptcentral.com/frictionnegative friction-velocity slope mechanism were used to explain the friction vibration and noise of WLBs.For example, the research on self-excited vibration of the shaft-bearing coupling system of Wu et al. [250] verified the negative friction-velocity slope mechanism.
Ibrahim believed that the factors considered in various mechanisms were not comprehensive enough.Therefore, the friction-induced phenomena (e.g., vibration, chatter, squeal, and chaos, etc.) because of the contact and frictional mechanics were systematically studied [261], which pointed out that the friction models of any dynamical system should be established according to the operating conditions and geometry of the system.The friction models adopted for different engineering applications were reviewed [262] and pointed out that it was necessary for WLBs to study the effect of friction on nonlinear modal coupling and explore the role of nonlinear coupling in the elimination of friction excitation.Simpson et al. [263] established a nonlinear dual-degree-of-freedom model for a submarine tailpipe bearing to analyze the nonlinear response of the coupling system and concluded that the bearing instability was mainly caused by the friction force varying nonlinearly with time and velocity.
In the real shaft system, the friction vibration of WLBs does not exist independently, which needs to pay attention to the coupling relationship between the non-linear friction excitation and the shaft system vibration generated by the bearing and the shaft or even the shaft system.Zhang et al. considered the torsional-lateral coupling through bearing-shaft interaction [264], and further investigated the effect of nonlinear friction on the propeller-shaft system [265].
The results showed that friction-induced system vibrations were caused by the coupling effects of interface nonlinear friction and system components [266].The role of nonlinear coupling models in the study of friction vibration of WLBs is becoming more and more important [267].Therefore, theoretical research must establish and solve the coupling models according to the actual influencing factors by considering the hydrodynamic effect, thermal effect, and the deformation effect of the bearing and shaft.

Experimental studies
Some experimental studies on frictional vibration noise are shown in Table 11.It shows that stick-slip and nonlinear frictional vibrations are considered as the main induced factors of vibration noise.In addition, Peng et al. [268] compared the spectrum characteristics of WLRBs when frictional vibration occurred and during normal operation, which provided a reference for the identification of abnormal friction noise in stern bearings.Dong et al. revealed the relationship between the lubrication properties of polymers and friction noise performance, which contributed to the material selection in bearing design [269].
Only through reasonable experimental design and visualization of friction vibration information can the characteristic information be captured, and the mechanism of friction vibration can be further revealed through experiments, which can provide verification and guidance for theoretical research and engineering application.

Suppression methods for friction noise
The vibration and noise problems of WLBs should be

Ref. Features
Stick-slip mechanism [251] Negative friction-velocity slope mechanism [252] Friction-induced vibrations exist in the intrinsic properties of the friction interface, especially the dependence of the friction coefficient on the sliding velocity.
Sprag-slip mechanism [253] Modal coupling mechanism [254,255] Friction time-lag theory [256,257] Hammering theory [258] The geometry-induced or the kinematic-constrained instability caused by the system intrinsic structure contributes to the vibration and noise.
Unified theory [259,260] / www.Springer.com/journal/40544| Friction solved by reducing bearing vibration and improving the vibration isolation capacity of bearing.The previous section indicates that the generation of friction noise is related to the self-excited vibration caused by the stick-slip motion and lubrication state change between friction pairs.The friction coefficient [281] and the ratio of static and dynamic friction coefficients [282] could also have an impact on friction noise.The prevention and control of friction noise need to be approached from both structure and material to improve the lubrication performance and reduce the friction coefficient fluctuations.

Structure optimization
Qin et al. pointed out that the significance of each factor on the effect of friction vibration was in the order of lubrication conditions, contact pressure, hardness, and rubber layer thickness.It is possible to reduce the friction vibration intensity of rubber bearings by increasing the rubber layer hardness, decreasing contact pressure or rubber layer thickness [280].Jiang et al. [283] indicated that it was effective to control the abnormal vibration of rubber bearings by increasing the damping coefficient and Stribeck curve attenuation coefficient.Otherwise, the cooling water temperature [284] and the shape the rubber slat surface [285] also could influence the friction vibration.The surface texture enabled the waterlubricated rubber slat bearings to reduce the critical speed and suppress the high-frequency friction vibration [275].
The above traditional structural optimization can play a certain vibration suppression effect.In addition, porous structure is one of the important means to improve the vibration and noise reduction performance of materials.Jin et al. designed a fluid-saturated perforated slab, which effectively reduced the verticaldirection vibration amplitude of WLSBs [286].

Material modification
It is possible for WLBs to suppress the friction noise by using the materials with better hydrophilicity, self-lubricating and viscoelasticity [269].The self-excited vibration could be suppressed by large damping, high Friction vibration coupled with lateral vibration [91] The interference of lateral vibration accelerated the instability of the system.
The influence of the lateral vibration on the friction-induced vibration of WLSBs [273] The normal vibration would aggravate the amplitude, and narrow the speed range of friction-induced vibration.
The friction noise of bearings could also be reduced by improving the lubrication performance through material modification.Zhou et al. modified NBR with a blending of UHMWPE and Gr powder to obtain a new composite rubber with a small negative slope of velocity change, which was not easy to occur stick-slip phenomena and suppressed the friction noise at low speed.The polymer material of WLSBs modified by GNS could improve the self-lubrication capability to reduce the friction vibration and noise during start/stop process [173].The material modification could also improve the surface hardness and weaken the surface deformation to reduce the magnitude of the friction fluctuation.HDPE modified by MWCNTs [80] or tetrapod-like zinc oxide whisker (T-ZnOw) powder [171] could improve the hardness and compression strength to effectively reduce the friction coefficient and fluctuation.

Damping-enhanced structure
It is not sufficient to achieve the reduction goal of the frictional vibration and noise only by material modification, because the soft lining and rubber layer would deform and lose the damping effect under the heavy or bias load.The contradiction between high load capacity and high damping needs to be solved by carrying out the decoupling design, which requires adding an attachment structure to the bearing bushing or on the outer wall of the bearing to isolate vibration [287].
A damping-enhanced structure separating stiffness from damping can decouple the load carrying and damping functions.As shown in Fig. 14, the attachment structure must have a high stiffness k 2 to provide high load carrying capacity, and a large damping c 2 to achieve energy dissipation [287].
As shown in Fig. 15, the integral squeeze film damper (ISFD) technology is one of the effective methods for WLBs to achieve this damping-enhanced structure [287].It is effective for the retained material S-type ISFD with the compact structure to solve the problem of nonlinear vibration and low critical speed of the rotor system [288].The Wuhan University of Technology further proposed the application of magnetic fluid to SFD to achieve adjustable and controllable damping of the bearing [287,289]; Mayank Tiwari et al. [290] made an ISFD filled with magnetorheological fluid and verified its vibration damping effect under different operating conditions through experiments.
In addition, metamaterials based on locally resonant sonic materials [291] have been applied in mechanical vibration control of rolling bearing box [292] and marine power equipment mount [293], and achieved good results.It is feasible to apply metamaterials to water-lubricated bearings.The reasonable design of metamaterials to make them suitable for waterlubricated bearings will be a breakthrough point to improve the bearing capacity and damping capacity of water-lubricated bearings.
In general, it is difficult for bearings to improve lubrication and reduce vibration only by traditional methods such as material modification and surface texture.It requires to combine with innovative structural designs such as damping-enhanced structure or magneto-hydraulic double floating to achieve the reduction of bearing vibration and noise.

Future study
Although the WLBs has seen a great deal of research in recent years some of the results have been applied on ships, but green and high-performance ships put forward new requirements for WLBs.There are still www.Springer.com/journal/40544| Friction some urgent problems to be solved in theoretical research, test technology and intelligent operation and maintenance, etc.

Theory and mechanism research
To reflect the real lubrication state, the thermo-soft elastomeric transient mixed lubrication model should be developed by considering the thermal-fluid-solid coupling with material nonlinearity.The distributed mixed lubrication theory should be further developed by the identification of the lubrication state.Moreover, the complex interaction between lubrication, friction, wear, and vibration noise requires the following research to consider their evolution mechanism comprehensively, which would also become a theoretical basis for the research of intelligent operation and maintenance.In addition, it is necessary to establish the multi-constraint frictional vibration model of the bearing system.The shafting dynamics theory should be considered by analyzing partial contact states to describe the complex mechanism of vibration noise.

Develop more advanced test technology
New measurement technology for wear and load is an important development trend in the measurement of water-lubricated bearings.Combined with vibration monitoring, it will provide the possibility of intelligent bearings and shaft system.The single-point measurement should be replaced by distributed measurement with the application of sensor array testing technology to reflect the real lubrication state.In addition, the current test technologies are difficult to separate the friction vibration well, which requires deeper dynamic friction test methods with faster response rate and higher identification accuracy.Furthermore, it is necessary for WLBs to establish the mapping relationship between performance test and real ship operation to improve the mutual verification ability and residual life prediction.

Collaborative design of structure and material
At present, the material and structural design of WLBs is relatively independent.The collaborative design should be carried out by considering both material modification and structural optimization, especially when new materials are introduced into the WLBs.It should be taken load-carrying, friction, wear, and vibration noise as the overall optimization Friction 12(1): 1-38 (2024) | https://mc03.manuscriptcentral.com/frictiongoal to develop prevention and control methods.Magneto-hydraulic double-suspension technology should be further developed to improve the performance stability and robustness of WLBs.The metamaterials technology and biomimetic technology combined with WLBs will be a beneficial attempt.Furthermore, the damping control of ISFD and other active control technologies, and embedded sensor technology will become an important trend in smart WLBs and shafting.

Establish standards and regulations
Compared with American standards with detailed provisions of the bearing (e.g., material performance indexes, test methods, and the test bench, methods, conditions, samples, indexes, etc.), the current standards in China only briefly regulate the structure and material of the integrated rubber bearings, which lacks of the standards of specific process planning procedure, performance testing methods, and index evaluations.Therefore, it is urgent to deal with the lack standards of WLSBs test and evaluation.In order to better ensure its service performance, a systematic and highly operational regulations should be established to enhance the system management.

Address the challenges of new-type ships
Due to the development of the electric direct-driven and integrated marine propulsion technology, the RDT would be more advantageous than the traditional shaft propulsion device because of its compact structure and higher efficiency.The water-lubricated thrust bearing with excellent performance is the key for RDT power to break through the megawatt-class.Moreover, the development of intelligent and less manned ships requires the establishment of intelligent operation and maintenance and health management of bearing/shafting in the whole life cycle to realize the intelligence of ship engine room.

Conclusions
With the trend of large-scale and high-speed development of ships, WLBs as a key component of ship propulsion shaft system play a more and more important role in the efficient, smooth, and safe operation of ships.The research of WLBs has distinct characteristics of demands, problems, and goals.
This paper summarized the lubrication principle and testing technology of WLBs, and focused on the mechanism and prevention methods of friction, wear, and noise of WLBs.While summarizing the performance improvement methods of WLBs with traditional structural optimization and material modification, the new methods for performance improvement were also reviewed.
(1) In the section of friction-reduction, the structure optimization technology of surface texture was mainly introduced, and the development and application of magneto-hydraulic double-suspension bearing technology were summarized.The main content of material modification was blending modification.In addition, the related research of bionic modification was introduced, and the potential application of ion implantation technology in WLBs was pointed out.
(2) According to different wear forms, the main wear mechanisms of WLBs were reviewed, and the influence of the two main factors of friction pairs hardness and operating conditions on bearing wear was summarized.Based on these, the material blending modification and friction matching pairs optimization technology were proposed to improve the wear resistance of bearing.
(3) The generation mechanisms of friction noise were reviewed and explained from two aspects of theoretical and experimental studies.In addition to the friction noise suppression methods of structural optimization and material modification, the related research of damping-enhanced structure was also summarized.
Extensive efforts have been carried out to improve the performance of WLBs, but there are still many bottlenecks to be overcome (e.g., the lubrication theory, wear and vibration mechanism, test technology, collaborative optimization ability, etc.).According to the research status of marine WLBs and the development trend of green and intelligent, the possible research directions in the future were proposed.With the continuous development and improvement of relevant research, WLBs can be reasonably expected to play a more critical role in ship propulsion systems in the future, and may have a profound influence on the green shipping industry as well.

Fig. 2
Fig. 2 Application of WLBs in ship propulsion systems and main problems.

Fig. 4
Fig. 4 Mixed lubrication state on the interface of WLSBs.

Fig. 6
Fig. 6 Test devices and performance tests of WLBs.

Fig. 7
Fig. 7 WLBs test rigs of Wuhan University of Technology.

Fig. 15
Fig.15ISFD structure and its application in bearings.

Table 2
Model and solution methods for lubrication of WLBs.

Table 3
A review of simulation studies on the characteristics of different WLBs.

Table 4
Information of the bearing test rigs of the Wuhan University of Technology.
[91]aft alignment and vibration and noise reduction in shaft systems[91]

Table 5
Test methods for water film properties.

Table 6
Researches on surface texture at Wuhan University of Technology.
www.Springer.com/journal/40544| Friction et al., which was proved to significantly improve solution efficiency of large-scale texture

Table 7
The effect of hardness on friction pairs wear.
Notes: WRB means the wear rate of the bearing; DWF means the deepest wear furrows on the thrust collar.

Table 8
Surface treatment methods to improve the surface hardness of the friction pairs.

Table 9
Commonly used water-lubricated friction pairs.

Table 10
Mechanism of friction noise.

Table 11
Experimental study of friction and vibration noise for WLBs.