Theoretical Study on the Rheology-Driven Lubrication Synergy of Gel on Textured High-Entropy Alloy Coatings

Abstract

This work is a theoretical study aimed to address the lubrication failure of Tom-Pac TP-2557 gel lubricant on the textured high-entropy alloy coatings (THEACs) under wide-temperature-range operating conditions; the rheological lubrication properties of the surface are investigated in this work. Based on lubrication theory and non-Newtonian fluid mechanics, a gel lubrication viscosity model considering temperature dependence and a thermo-mechanical coupled constitutive relationship for the THEACs are established. The results show a significant shear-thinning behavior of the gel within a moderate low-to-medium temperature range, and the onset temperature of thermal degradation is identified. Optimal geometrical and distributional parameters of the surface textures, along with a favorable surface energy range, are determined to achieve desirable interfacial shear strength. Moreover, an anchoring-slip synergistic mode arising from surface energy heterogeneity is found to further enhance lubricating film stability. This research provides a theoretical basis for the gel lubrication design of the THEACs.

1. Introduction

Textured high-entropy alloy coatings (THEACs) have become ideal protective materials for critical components under extreme conditions [1,2,3], such as aerospace and nuclear power equipment, owing to their excellent high-temperature strength, wear resistance, and thermal stability [4,5,6]. However, a stable lubricating film cannot be maintained by conventional lubricants within the full temperature range under a wide temperature range, which leads to an increase in friction and wear [7,8]. Recently, gel lubricant has been widely concerned in the field of lubrication because of its unique thixotropy, shear-thinning characteristics, and three-dimensional network structure [9]. Tom-Pac TP-2557-bearing lubricating gel is a type of gel lubricant material compounded with 100% polyalphaolefin (PAO) synthetic base oil and non-melting thickener, which makes its application possible in a wide temperature range.

The lubrication characteristics of gel are determined by its internal multi-level structure, which can form a three-dimensional network structure within a specific temperature range, and viscoelastic behavior was exhibited. However, it transforms into liquid when the temperature exceeds the critical range, so as to achieve the adaptive regulation of the lubrication mechanism [10,11]. The relevant research indicated that the molecular structure design of gel factor has a decisive influence on gel-forming ability, thermal stability, and lubrication performance [12]. The gel lubricant formed by the interaction of polyvinyl alcohol, MXene, and polyol shows excellent shear-thinning and creep recovery properties at room temperature and low temperature [13,14]. The gel lubricant formed by the supramolecular polymer gel PMUS-P in 500SN base oil has a dense network structure and significantly reduces friction and wear [15,16,17]. Additionally, the antifriction performance is improved by 69% via polyvinylpyrrolidone/phytic acid hydrogel lubricant [18]. The friction coefficient of urea-based supramolecular gel lubricant decreased from 0.168 to 0.102, and the bearing capacity increased from 50 N to 450 N [19]. Furthermore, the study on the space atomic oxygen radiation resistance of the MAC-based supramolecular gel lubricant containing silsesquioxane provides a new idea for the application of space lubrication [20]. The study of supramolecular perfluoropolyether gel lubricant in irradiation and high vacuum environment shows that the gel has excellent anti-crawling ability [21]. However, the gel mechanism and tribological properties of supramolecular gel lubricants were analyzed, and the structure–activity relationship between gel network structure and lubricating properties was revealed [22,23]. Moreover, further research indicates that the synergistic lubrication effect of gel lubricant with nano-materials and surface texture is an important direction for future development [24,25]. To sum up, scholars have done a lot of research on the material design, performance regulation, and lubrication mechanism of gel lubricants. However, the research on the system performance of commercial gel lubricating products is relatively weak, especially the lubricating performance of PAO-based industrial-grade-bearing lubricating gel lubricant such as Tom-Pac TP-2557 (Tom-Pac Inc., St-Laurent, QC, Canada), and methods for maximizing the lubricating advantages of Tom-Pac TP-2557 gel lubricant by optimizing texture parameters remain to be further explored.

The purpose of this work is to reveal the lubrication performance of Tom-Pac TP-2557 gel-lubricated micro-pit-textured FeCoCrAlCuNi high-entropy alloy coating at extreme temperatures. The rheological properties of gel lubricant and its influence on lubrication performance were systematically analyzed by establishing the non-Newtonian model and the elastohydrodynamic lubrication model of gel-textured alloy coating, and the optimal combination of textured parameter is determined, which provides guidance for the cooperative application of Tom-Pac TP-2557 gel-lubricated THEACs under extreme working conditions.

2. Theoretical Model

2.1. Rheological Model of Gel Lubricant

The core characteristic of TP-2557 gel lubricant lies in its temperature-induced gel–sol transition, which endows it with unique rheological behavior near the gel transition temperature [26]. To accurately describe the temperature-dependent non-Newtonian behavior, the Carreau viscosity model was employed for characterization. The dynamic viscosity η of the gel lubricant can be expressed as a function of shear rate $\dot{\gamma }$ and temperature T:

$$\eta \left(T,\dot{\gamma }\right)={\eta }_0\left(T\right){\left[1+{\left(\lambda \dot{\gamma }\right)}^2\right]}^{{\displaystyle \frac{n-1}{2}}}$$

(1)

where η₀(T) is the zero-shear viscosity, λ is the relaxation time, and n is the power-law exponent (n < 1 is shear-thinning). The relationship between zero-shear viscosity η₀(T) and temperature is described by the Arrhenius equation, as follows:

$${\eta }_0\left(T\right)={\eta }_{\infty }\exp \left[{\displaystyle \frac{E_{\eta }}{R}}\left({\displaystyle \frac{1}{T}}-{\displaystyle \frac{1}{T_{ref}}}\right)\right]$$

(2)

where η_∞ is the intrinsic viscosity, E_η is the flow activation energy, and R is the gas constant. To describe the viscosity jump near the gel transition temperature T_g, the viscosity model of gel transition factor ψ(T) is introduced as follows:

$$\Psi \left(T\right)={\displaystyle \frac{1}{1+\exp \left[k_g\left(T-T_g\right)\right]}}$$

(3)

Thus, the modified effective viscosity η_eff can be expressed as follows:

$${\eta }_{eff}\left(T,\dot{\gamma }\right)=\Psi \left(T\right)·{\eta }_{gel}\left(T,\dot{\gamma }\right)+\left[1-\Psi \left(T\right)\right]·{\eta }_{sol}\left(T,\dot{\gamma }\right)$$

(4)

where η_gel and η_sol are the viscosities of the gel state and sol state, respectively, and k_g is the transition rate coefficient.

2.2. Governing Equation

2.2.1. Lubrication Equation

The unique rheological behavior of gel lubricants directly governs the formation of the optimal depth-to-diameter ratio (micro-pit depth to diameter ratio) of lubricant film and pressure distribution on THEACs [27]. The Reynolds equation for the lubricant film pressure distribution, considering the non-Newtonian behavior of the gel lubricant, is expressed as:

$${\displaystyle \frac{\partial }{\partial x}}\left({\displaystyle \frac{\rho h^3}{{\eta }_{eff}}}{\displaystyle \frac{\partial p}{\partial x}}\right)+{\displaystyle \frac{\partial }{\partial y}}\left({\displaystyle \frac{\rho h^3}{{\eta }_{eff}}}{\displaystyle \frac{\partial p}{\partial y}}\right)=6U{\displaystyle \frac{\partial \left(\rho h\right)}{\partial x}}$$

(5)

where p is the lubricant film pressure, h is the film thickness, U is the rotational speed, and ρ is the density. The local lubricant film thickness h(x, y) for circular micro-pit THEACs can be expressed as [28]:

$$h\left(x,y\right)=h_0+\delta \left(x,y\right)+{\displaystyle \frac{x^2+y^2}{2R_c}}$$

(6)

where h₀ is the central film thickness, δ(x, y) is the geometric profile function of the micro-dimple, and R_c is the radius of curvature. The geometric profile of the circular micro-pit THECAs can be expressed as:

$$\delta \left(x,y\right)=\left\{\begin{array}{l}{h}_p\left[1-{\displaystyle \frac{{\left(x-x_c\right)}^2+{\left(y-y_c\right)}^2}{r_p^2}}\right],\begin{array}{c}\sqrt{{\left(x-x_c\right)}^2+{\left(y-y_c\right)}^2}\le r_p\end{array}\\ 0,\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{otherwise}\end{array}\right.$$

(7)

where h_p, r_p, and (x_c, y_c) are the depth, radius, and center coordinate of the micro-pit THEACs, respectively. The coefficient of friction of lubricant film can be expressed as:

$$\mu ={\displaystyle \frac{{\displaystyle {\iint }_{\Omega }\tau \left(x,y\right)}dxdy}{{\displaystyle {\iint }_{\Omega }p\left(x,y\right)}dxdy}}$$

(8)

where the shear stress τ can be calculated according to the constitutive relation of non-Newtonian fluids:

$$\begin{array}{cc}\tau ={\eta }_{eff}\left(T,\dot{\gamma }\right)·\dot{\gamma },& \dot{\gamma }={\displaystyle \frac{\partial u}{\partial z}}\end{array}$$

(9)

The dynamic pressure field on the surface of the THEACs with a single circular pit is shown in Figure 1. In Figure 1, the maximum film thickness gradient is observed at the texture edge, where a local high-pressure region is generated. A relatively low pressure is observed inside the dimple, where a negative pressure region is formed, which is helpful to capture gel and realize secondary lubrication.

2.2.2. Interfacial Contact of Gel Lubrication

The adhesion work of the solid–liquid interface directly determines the extrusion resistance and interface energy dissipation characteristics of lubricating film. According to the Young–Dupré equation, the solid–liquid interfacial adhesion work W_adh is expressed as:

$$W_{\mathrm{adh}}={\gamma }_{\mathrm{sv}}+{\gamma }_{\mathrm{lv}}-{\gamma }_{\mathrm{sl}}$$

(10)

where γ_sv, γ_lv, and γ_sl are the solid–vapor, liquid–vapor, and solid–liquid interfacial tensions, respectively. According to Young’s equation, γ_sv − γ_sl = γ_lv cosθ (where θ is the solid–liquid contact angle), the following expression is obtained:

$$W_{\mathrm{adh}}={\gamma }_{\mathrm{lv}}\left(1+\cos \theta \right)$$

(11)

For a gel lubrication system, the intermolecular interaction at the interface enhances W_adh in order to lubricate the layer to be attached to the solid surface with greater stability.

The ability of the solid–liquid interface to resist relative motion during sliding is reflected by the interfacial shear strength τ_int. When the contribution of bulk viscosity is neglected, a positive correlation is observed between τ_int and W_adh, which can be described by a modified molecular dynamics model, as follows:

$${\tau }_{\mathrm{int}}={\tau }_0+k·W_{\mathrm{adh}}$$

(12)

where τ₀ is the non-adhesive shear value and k is the proportional constant.

2.3. Thermo-Mechanical Coupling Constitutive Relation

The mechanical properties of the micro-dimple THEACs under extreme temperature directly govern the elastoplastic deformation behavior within the lubricated contact region. It has been reported that the dependence of elastic modulus on temperature can be expressed as:

$$E\left(T\right)=E_0\left[1-{\alpha }_E\left(T-T_0\right)\right]$$

(13)

where E₀ is the elastic modulus and α_E is the temperature coefficient of the elastic modulus. The temperature dependence of yield strength σ_y(T) can be described by the thermal activation mechanism:

$${\sigma }_y\left(T\right)={\sigma }_{y0}\left[1-{\left({\displaystyle \frac{T-T_0}{T_m-T_0}}\right)}^m\right]$$

(14)

where σ_y0 is yield strength, T_m is melting point, T₀ is reference temperature (298 K), and m is the temperature sensitivity exponent. The coating hardness H(T) is proportional to its yield strength. During the lubrication contact process, the actual contact stress σ_c at the contact interface can be expressed as:

$${\sigma }_c\left(x,y\right)=p\left(x,y\right)+{\displaystyle \frac{\tau \left(x,y\right)}{{\mu }_s}}$$

(15)

where μ_s is the friction coefficient under boundary lubrication condition.

The coupling characteristics of gel lubricant on the surface of THEACs are shown in Figure 2. In Figure 2a, the elastic modulus decreases linearly with the increase of temperature, as 225 GPa at −40 °C to 195 GPa at 150 °C, with a decrease of 13.3%. In Figure 2b, an S-shaped nonlinear decrease in yield strength is observed from 1275 MPa at −40 °C to 926 MPa at 150 °C, which corresponds to a reduction of 27.4%. In Figure 2c, high coating hardness but high gel viscosity is observed in the low-temperature region, whereas low coating hardness and low gel viscosity are observed in the high-temperature region. Near the gel transition temperature (40 °C), an optimal balance between coating hardness (about 3.4 GPa) and gel viscosity (about 0.2 Pa·s) is achieved, which serves as the basis for the gel to achieve the optimal lubrication performance.

3. Comparative Analysis

To explain the predictive capability of the proposed theoretical model, the viscosity and shear stress variation with shear rate is shown in Figure 3. In Figure 3, the viscosity and shear stress curve predicted by this model is consistent with the ref. [29], and the relative errors are 1.5% and 2.5%, respectively (

Table 1. The relative error between theoretical value and ref. [29].

Item	Theoretical Value	Ref. [29]	Relative Error/%
Viscosity	4833	4906	1.5
Shear stress	0.762	0.743	2.5

). Theoretical predictions indicate that the present model can describe the steady-state rheological behavior of the investigated system. Consequently, this model is applicable to the prediction of the tribological properties of THEACs lubricated by gels.

4. Results and Discussion

4.1. Theoretical Prediction of Rheological Properties of Gel Lubricant

The theoretical prediction of variation of zero-shear viscosity and transformation factor is shown in Figure 4. In Figure 4a, a reduction of over 99% in the zero-shear viscosity is observed, as it is decreased from 8.5 Pa·s at −40 °C to 0.015 Pa·s at 150 °C. A sufficiently thick lubricating film, which prevents direct contact, is facilitated by low temperature and high viscosity. However, the reduction of viscous resistance and energy loss is achieved by high temperature and low viscosity. In Figure 4b, the gel transition factor ψ(T) is observed to decrease rapidly from 0.9 to 0.1 within the temperature range of 20–60 °C, and the transformation width is about 40 °C, which helps to avoid the lubrication instability caused by viscosity mutation. Moreover, the optimal lubrication performance is achieved within the gel–sol transition zone (approximately 40 °C), rather than after the gel has fully transformed into a sol. Within this transition window, the coexistence of gel and sol phases provides an optimal balance between load-carrying capacity and flowability, which is essential for stable lubrication film formation.

The theoretical prediction variation of equivalent viscosity with shear rate under various temperatures is shown in Figure 5. In Figure 5, the shear thinning is most significant near the gel transition temperature (40 °C). A reduction of more than 95% in equivalent viscosity, from 0.85 Pa·s to 0.037 Pa·s, is achieved when the shear rate is raised from 0.01 s⁻¹ to 100,000 s⁻¹. Under low-temperature conditions, the shear-thinning effect is relatively weakened, and the viscosity is maintained at a relatively high level (0.05–8.5 Pa·s), which facilitates the rapid establishment of a load-bearing oil film during the start-up phase. Under high-temperature conditions, the overall viscosity is low (0.008–0.05 Pa·s), the shear-thinning effect tends to be weakened, and the lubricant exhibits behavior similar to that of a Newtonian fluid. The variation in gel viscosity and thermal degradation under high-temperature conditions are shown in Figure 6. In Figure 6, the thermal degradation starts at about 247 °C, which corresponds to the thermal fracture temperature of chemical cross-linking points in the gel network. The degradation factor is close to 1 (network integrity) below 227 °C, which is about 0.4 at 227 °C (60% cross-linking point breaks) and close to 0.1 at 327 °C (network almost completely collapses). When the effective viscosity η_eff of thermal softening and degradation is lower than 0.1 Pa·s, the gel loses the ability to maintain lubricating film (similar to low-viscosity base oil).

4.2. Theoretical Analysis of the Influence of Surface Energy on the Gel Anchoring Effect

4.2.1. Surface-Energy-Regulated Wetting Behavior

The influence of the textured FeCoCrAlCuNi high-entropy alloy coating surface on the gel wetting behavior predicted theoretically is shown in Figure 7. In Figure 7, the contact angle decreases linearly with increasing surface energy. When γ_sv = 30 mJ/m², the contact angle is about 70°, and the gel is partially wetted. When γ_sv = 45 mJ/m², the contact angle is about 48°, and the spreading coefficient S = 0, which corresponds to the wetting transition point. Meanwhile, the surface energy γsv = 55 mJ/m², the contact angle decreased to about 27°, and the gel spread spontaneously. Additionally, with the increase of the surface energy to γ_sv ≥ 65 mJ/m², the contact angle is less than 10, which is almost complete wetting. The spreading coefficient S is found to increase from negative values (non-wetting) through zero (critical wetting) to positive values (spontaneous spreading). The S value is moderate in the range of γ_sv = 55–65 mJ/m², which corresponds to the optimal wetting state, not only ensuring the gel fully spreads, but also avoiding the gel from losing from the surface texturing due to excessive spreading. The adhesion work Wad increases with the increase of surface energy, which reflects the improvement of bonding strength of gel-coating interface. For the textured high-entropy alloy coating, the surface energy is controlled within the range of 50–60 mJ/m² by adjusting the Cr/Al ratio, so that the gel can achieve good wetting and establish stable physical anchorage.

4.2.2. Interfacial Bonding Strength

The stability of the lubricating film, as well as the resistance against shear-induced delamination, is directly determined by the interfacial bonding strength between the gel and the textured coating. The relationship between the theoretically predicted interface shear strength and surface energy is shown in Figure 8. In Figure 8, the interfacial shear strength τint gradually increases with the surface energy. When γ_sv = 35 mJ/m² and τ_int ≈ 0.35 MPa, the bonding strength is relatively weak. However, the bonding strength is excellent as γ_sv = 60 mJ/m² and τ_int ≈ 1.45 MPa. The critical peeling stress τ_crit (red dotted line) is lower than the interfacial shear strength, and the difference between them reflects the tolerance of the transition from local debonding to overall peeling. When the surface energy is less than 45 mJ/m², τint is less than 0.65 MPa, and the interface between gel and coating is insufficient, so it is easy to peel off under high-shear stress, which leads to lubrication failure. When the surface energy is higher than 62 mJ/m², although the interfacial strength is higher, too strong of an adhesion may inhibit the slip effect induced by texture, which is not conducive to the further reduction of the friction coefficient. Therefore, there is an optimal surface energy window (about 50–58 mJ/m²) in which the interface bonding and lubrication performance reach the best balance.

4.2.3. Elemental Segregation Effect

The heterogeneous distribution characteristics of surface energy of the THEACs are predicted theoretically, as shown in Figure 9. In Figure 9a–c, Cr-enriched regions and Al-depleted regions are distributed as random clusters on the substrate. In Figure 9d, a main peak is located at γ ≈ 52 mJ/m² (substrate), and two secondary peaks are observed on its left and right sides, which are assigned to the Cr-enriched region (62 mJ/m²) and the Al-depleted region (44 mJ/m²), respectively. A dual effect of surface energy heterogeneity on gel anchoring is exhibited. Preferential adsorption of the gel is induced in the Cr-enriched region (high surface energy), where anchoring points are formed to enhance the delamination resistance of the overall lubricating film. In contrast, a slip region is created in the Al-depleted region (low surface energy), where local slip is facilitated during shearing to reduce frictional resistance. The textured high-entropy alloy coating is distinguished from homogeneous surface energy materials by this anchoring-slip synergistic mode, wherein a low-friction coefficient is achieved while the overall integrity of the lubricating film is preserved.

To clarify the physical mechanism by which the anchoring-slip synergistic mode achieves low overall friction, an energy dissipation decomposition model is introduced. The frictional work per unit area is expressed as the weighted contribution of anchoring and sliding regions:

$$W_f={\phi }_{anch}·{\tau }_{high}·\delta +{\phi }_{slip}·{\tau }_{low}·\delta +{\eta }_{eff}·{\dot{\gamma }}^2·t$$

(16)

where ϕ_anch and ϕ_slip are the area fractions of anchoring and sliding regions, respectively. The Cr-enriched anchoring regions occupy less than 30% of the surface; although the local interfacial shear strength is relatively high, its contribution to total frictional work is constrained by the limited area fraction. The Al-depleted sliding regions occupy a larger fraction, providing low-shear-resistance pathways. The anchoring points prevent overall delamination of the lubricating film, while the slip zones accommodate local shear to reduce resistance. The spatially separated coexistence of both effects yields an overall friction coefficient as low as 0.036. This mechanism is consistent with the conclusions of Léger et al. [30] that polymer-anchored layers can simultaneously promote adhesion and slip, and the experimental findings of Hirata et al. [31] confirm the friction-reducing effect of surface energy heterogeneity.

4.3. Theoretical Analysis of Lubrication Characteristics

4.3.1. Effect of the Textured Area Density Ratio

The surface pressure field distribution of high-entropy alloy coatings with different texture area ratios is shown in Figure 10. In Figure 10, when the area ratio is small (5%), the texture distribution is sparsely distributed, a low pressure is observed, and weak coupling is exhibited. When the texture area density ratio is increased to 15% with the moderate texture, a high-pressure network is formed in the pressure field, and the bearing capacity is improved by about 60% compared with 5% texture area density ratio. When the area ratio is 30%, the texture is too dense, the oil film is uniform, and the pressure drops. Consequently, the optimal area ratio is determined to be controlled by the competitive mechanism between texture spacing and the hydrodynamic effect.

The velocity fields and streamline distributions on the surface of THEACs under different area ratios are shown in Figure 11. In Figure 11, a recirculation zone is formed in the texture due to the gel retention effect, a solid-like state is maintained in the low-shear region by the yield stress, and microcirculation is generated to facilitate lubricant renewal and heat transfer. When the area ratio is 15%, the velocity field is the most uniform, and the flow coupling is the best. When it reaches 30%, the channel is blocked, the refluxes interfere with each other, and a decrease in fluidity is observed. It indicates that a balance is required to be achieved between texture spacing and flow smoothness in texture design.

4.3.2. Effect of Depth–Diameter Ratio

The influence of aspect ratio on pressure field is shown in Figure 12. In Figure 12, when the depth–diameter ratio is 0.5%, the pit is too shallow, the dynamic pressure effect is weak, and the pressure distribution is gentle. When the depth–diameter ratio increases to 2.0%, a significant change in film thickness is observed, and the pressure peak and bearing capacity reach the maximum. When the depth–diameter ratio is increased to 3.0%, the flow resistance is greatly increased due to the pit being too deep, and the dynamic pressure effect is weakened in the internal low-velocity region, and the bearing capacity is also reduced.

The flow behavior of gel in texture pits varies with depth–diameter ratio, as shown in Figure 13. In Figure 13, when the depth–diameter ratio is small, the flow is dominated by Couette shear, and the vorticity is uniformly distributed with low intensity. When the depth–diameter ratio is increased to 2.0%, a regular dual-vortex recirculation structure is generated inside the dimple, which is beneficial for lubricant renewal, heat transfer, and wear debris removal. When the depth–diameter ratio exceeds 2.5%, the vorticity intensity is further increased; however, the vortex structure is destabilized, and multi-vortex and chaotic flows are induced, which are found to be detrimental to the formation of a stable lubricating film.

4.3.3. Optimal Parameter

The two-parameter interaction between the depth–diameter ratio and the area density ratio is shown in Figure 14. In Figure 14, a unimodal-valley structure in the friction coefficient is shown by the contour plot within the ε-Sp plane, and the optimal region is located within a rectangular area defined by ε = 0.005–0.03 and Sp = 0.05–0.30. A 70% reduction in the friction coefficient is achieved at the optimal parameter configuration (Figure 14a) compared with that of the smooth surface (Figure 14d), and a further reduction of approximately 10–15% is obtained compared with single-parameter optimization (Figure 14b). This fully demonstrates the necessity of two-parameter synergistic optimization.

The streamline distribution under the optimal parameter condition is shown in Figure 15. Under the optimal parameter, a recirculation vortex is shown by the streamlines in a single micro-dimple cross-section, which occupies most of the space, and the existence of a gel-like plug region is confirmed. Because of the minimal yield stress velocity gradient and low dissipation in the vortex core region, the mainstream is squeezed into the narrow channel between the vortex and the upper wall to pass at high speed. A rolling-bearing-like effect is exhibited by this structure, where the load is supported by the stationary vortex core, and lubrication as well as heat dissipation are provided by the boundary layer flow. When the depth–diameter ratio deviates from the optimal value, the vortex either fails to form stably or becomes too large and blocks the main flow channel.

5. Conclusions

In this paper, the lubrication behavior of gel lubricant on the surface of textured FeCoCrAlCuNi high-entropy alloy coating is investigated. The effects of gel rheological properties, surface energy regulation, and textured parameter on lubrication performance are systematically analyzed, and the conclusions are shown as follows:

(1)

The gel lubricant exhibits significant shear-thinning behavior within its gel–sol transition temperature range, with a strong temperature-dependent reduction in zero-shear viscosity as the temperature rises. The onset temperature of thermal degradation further defines the upper limit of its applicable temperature on the surface of the THEACs.

(2)

A positive correlation was found between the solid–liquid interfacial adhesion work and the surface energy of the coatings. Within an optimal surface energy window of the THEACs, favorable gel wettability is achieved, accompanied by a desirable interfacial shear strength. This suitable surface energy range not only promotes sufficient spreading of the gel over the coating surface but also facilitates stable physical anchoring, thereby ensuring effective load-bearing capacity and resistance to lubrication failure of the lubricating film.

(3)

A heterogeneous distribution of surface energy is exhibited on the THEACs, wherein Cr-enriched regions and Al-depleted regions are randomly distributed on the substrate. The Cr-rich region with high surface energy is used as the preferential adsorption site to enhance the stability of the lubricating film, and the Al-poor region with low surface energy is used as the sliding region to reduce the friction resistance. Therefore, the two work together to achieve overall stability and low friction of the lubricating film.

(4)

A non-monotonic effect on lubrication performance was observed for the aspect ratio and area density ratio. The optimal parameters were identified as an aspect ratio of 2% and an area density ratio of 15%, under which the friction coefficient reached as low as 0.036. This value corresponds to a 70% reduction compared with that of the smooth surface, and a further decrease of 10–15% relative to single-parameter optimization, which explains the necessity of dual-parameter synergistic optimization.

(5)

The existence of the gel plug zone was confirmed by the stable recirculating vortex structure formed within the micro-dimples under the optimally textured parameters. The load is sustained by the almost motionless vortex core with exceptionally low dissipation, and lubrication together with heat removal is provided by the boundary layer flow. The formation of a ‘rolling-bearing-like’ effect is thus observed, which suggests a new paradigm for the underlying low-friction lubrication mechanism.