A bionic rhombic-skin drag-reducing surface by ball-end milling and a preparation method thereof

Abstract

The embodiment of the application relates to a preparation method of a ball-end milling bionic rhombic scale drag-reducing surface, comprising the following steps: adopting a ball-end milling cutter to perform three-way cross milling processing on a metal substrate surface, a spherical surface cutting edge of the ball-end milling cutter forms a circular-arc cross-section groove on the metal substrate surface; a first oblique tool path is located on one side of a set fluid flow direction and forms a first oblique circular-arc groove array at a first included angle with the set fluid flow direction; a second oblique tool path is located on the other side of the set fluid flow direction and forms a second oblique circular-arc groove array at a second included angle with the set fluid flow direction; and a longitudinal tool path is located on the set fluid flow direction and forms a longitudinal circular-arc groove array; wherein a region between the three groups of grooves which is not removed by milling forms a rhombic convex array, a low-surface-energy fluoropolymer coating is applied on surfaces of the rhombic convex array and the groove array, and an underwater drag-reducing surface is obtained. The embodiment of the application further provides a drag-reducing surface.

Description

Technical Field

[0001] This application relates to the field of surface microstructure processing and biomimetic drag reduction technology, and in particular to a ball-end milled biomimetic rhomboid scale drag-reducing surface and its preparation method. Background Technology

[0002] When ships, underwater vehicles, fluid transport pipelines, and pump impellers operate in underwater or liquid environments, frictional resistance is generated between the fluid and solid surfaces. This frictional resistance increases the energy consumption of the equipment and affects its speed, transport efficiency, or the working efficiency of fluid machinery. Therefore, constructing drag-reducing surface structures on metal components is an important technical direction for reducing underwater frictional resistance.

[0003] In nature, some aquatic organisms possess regularly arranged microstructures on their body surfaces. For example, fast-swimming creatures like sharks and mackerel have scale-like or groove-like microstructures on their bodies, which can alter the flow state of fluids near the substrate surface to some extent. Inspired by this, related technologies include methods such as laser etching, chemical etching, imprinting, spraying, and attaching flexible films to form biomimetic grooves, ribs, or scale-like textures on substrate surfaces. There are also methods that apply low surface energy coatings to substrate surfaces to reduce solid-liquid interfacial friction.

[0004] However, existing biomimetic drag-reducing surfaces still have some shortcomings. On the one hand, laser etching, chemical etching, or micro / nano fabrication processes have high requirements for equipment and process conditions, and are easily limited by processing efficiency, cost, and material adaptability when processing large-area metal components. Biomimetic surfaces formed by film application or transfer printing are prone to introducing an interface between the film layer and the metal substrate, which poses a risk of desorption under long-term water flow erosion or bending service conditions. On the other hand, some trench structures are flat-bottomed, sharp-angled, or V-shaped, with obvious geometric abrupt changes between the trench bottom and the trench wall. The coating is prone to local accumulation or discontinuous coverage at these locations, and the morphology at the trench intersections is also difficult to maintain consistency.

[0005] Furthermore, existing technologies also include solutions that rely solely on surface trench structures or low surface energy coatings for drag reduction. The drag reduction effect of a single trench structure is closely related to the flow direction, trench shape, and surface condition. When the trench direction does not match the actual fluid flow direction, the drag reduction effect is prone to decrease. Although a single low surface energy coating can change surface wettability, its ability to regulate the near-wall flow field is limited if it lacks microstructure guiding units corresponding to the fluid direction, and its drag reduction performance is easily degraded after the coating wears down. Summary of the Invention

[0006] One objective of this application is to provide a ball-end milled biomimetic rhomboid scale drag-reducing surface and a method for preparing the same, at least to solve the aforementioned problems.

[0007] To achieve the above objectives, some embodiments of this application provide a method for preparing a ball-end milled biomimetic rhomboid scale drag-reducing surface, comprising: Cleaning and pretreatment of the metal substrate surface; A ball end mill is used to perform three-way cross milling on the surface of the metal substrate, and the spherical cutting edge of the ball end mill forms a groove with an arc-shaped cross section on the surface of the metal substrate. The three-way cross milling process includes: forming a first oblique arc-shaped groove array by moving the tool along a first oblique direction located on one side of the set fluid flow direction and forming a first angle with the set fluid flow direction; forming a second oblique arc-shaped groove array by moving the tool along a second oblique direction located on the other side of the set fluid flow direction and forming a second angle with the set fluid flow direction; and forming a longitudinal arc-shaped groove array by moving the tool along the set fluid flow direction. The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array intersect each other, so that the area between the three sets of grooves that has not been milled away forms a rhomboid protrusion array, and the long diagonal of each rhomboid protrusion in the rhomboid protrusion array is arranged along the set fluid flow direction. A low surface energy fluoropolymer coating is applied to the surfaces of the rhomboid protrusion array, the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array to obtain the underwater drag-reducing surface.

[0008] Some embodiments of this application provide a ball-end milled biomimetic rhomboid scale drag-reducing surface, including: A metal substrate having a drag-reducing surface for contact with underwater fluids; The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array are all formed on the drag-reducing surface; And a low surface energy fluoropolymer coating, covering the drag-reducing surface; The first oblique arc-shaped groove array includes multiple first oblique arc-shaped grooves, each of which is inclined to a first side relative to the set fluid flow direction and forms a first angle with the set fluid flow direction. The second oblique arc-shaped groove array includes multiple second oblique arc-shaped grooves, each second oblique arc-shaped groove being inclined to a second side relative to the set fluid flow direction and forming a second angle with the set fluid flow direction; The longitudinal arc-shaped groove array includes multiple longitudinal arc-shaped grooves extending along the set fluid flow direction; The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array intersect each other to define a diamond-shaped protrusion array on the drag-reducing surface; The rhomboid protrusion array includes multiple rhomboid protrusions, each of which is formed by the metal substrate area that has not been removed between the first oblique arc groove, the second oblique arc groove and the longitudinal arc groove, and the long diagonal of each rhomboid protrusion is arranged along the set fluid flow direction. The cross-sections of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove are all arc-shaped; The low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusion, and also covers the bottom and walls of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove.

[0009] Compared with related technologies, the solution provided in this application removes oil, particles, and machining residues from the surface of the metal substrate through pre-treatment cleaning. This makes the contact between the tool and the substrate surface more stable during subsequent ball end milling, reducing burrs or local chipping at the groove edges caused by surface impurities. When using a ball end mill for three-way cross milling, the spherical cutting edge can directly form a groove with a circular arc cross-section on the surface of the metal substrate. Compared with sharp-angled or rectangular grooves, the bottom of the circular arc groove is less likely to form abrupt steps when fluid passes through, resulting in a more continuous fluid transition within the groove. After the first oblique circular arc groove array, the second oblique circular arc groove array, and the longitudinal circular arc groove array intersect, the uncut area between the three sets of grooves forms a diamond-shaped protrusion array, with the long diagonal of each diamond protrusion arranged along the set fluid flow direction. Thus, the diamond protrusions can guide the near-wall fluid in sections along the flow direction, while the oblique and longitudinal grooves provide constraint paths for lateral disturbances and downstream flow, respectively. After applying a low surface energy fluoropolymer coating to the diamond-shaped protrusion array and the surface of each groove, the coating covers the already formed three-dimensional microstructure surface, so that the fluid contact interface has both a geometric flow guiding structure and a low surface energy interface, avoiding the problem of being more sensitive to flow conditions and coating wear when relying on only a single groove structure or a single coating. Attached Figure Description

[0010] One or more embodiments are illustrated by way of example with reference numerals in the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0011] Figure 1 This is a schematic flowchart of the preparation method provided in the embodiments of this disclosure.

[0012] Figure 2 This is a schematic diagram of a three-way cross milling process provided in an embodiment of this disclosure.

[0013] Figure 3 This is a schematic diagram of the microstructure of the ball-end milled biomimetic rhomboid scale drag-reducing surface provided in the embodiments of this disclosure.

[0014] Figure label: 1: First oblique arc-shaped groove; 2: Second oblique arc-shaped groove; 3: Longitudinal arc-shaped groove. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0016] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this disclosure are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this disclosure described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0017] In this disclosure, the terms "upper," "lower," "inner," "middle," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for better description of the embodiments of this disclosure and their implementations, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to require them to be constructed and operated in a specific orientation. Furthermore, some of the aforementioned terms may be used to indicate other meanings besides orientation or positional relationship; for example, the term "upper" may in some cases indicate a dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in the embodiments of this disclosure according to the specific circumstances.

[0018] Furthermore, the terms "set up," "connect," and "fix" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this disclosure according to the specific circumstances.

[0019] Unless otherwise stated, the term "multiple" means two or more.

[0020] In this embodiment of the disclosure, the character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0021] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in the present disclosure can be combined with each other.

[0023] Combination Figures 1 to 3 As shown in the embodiments of this disclosure, a method for preparing a ball-end milled biomimetic rhomboid scale drag-reducing surface includes: Cleaning and pretreatment of the metal substrate surface; A ball end mill is used to perform three-way cross milling on the surface of a metal substrate. The spherical cutting edge of the ball end mill forms a groove with an arc-shaped cross section on the surface of the metal substrate. The three-way cross milling process includes: forming a first oblique arc-shaped groove 1 array by moving the tool along a first oblique direction located on one side of the set fluid flow direction and forming a first angle with the set fluid flow direction; forming a second oblique arc-shaped groove 2 array by moving the tool along a second oblique direction located on the other side of the set fluid flow direction and forming a second angle with the set fluid flow direction; and forming a longitudinal arc-shaped groove 3 array by moving the tool along the set fluid flow direction. Among them, the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array intersect each other, so that the area between the three sets of grooves that has not been milled away forms a rhomboid protrusion array, and the long diagonal of each rhomboid protrusion in the rhomboid protrusion array is arranged along the set fluid flow direction. A low surface energy fluoropolymer coating is applied to the surface of the rhomboid protrusion array, the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array to obtain an underwater drag-reducing surface.

[0024] The preparation method provided in this disclosure removes oil, particles, and machining residues from the surface of the metal substrate through a pre-treatment cleaning process. This makes the contact between the tool and the substrate surface more stable during subsequent ball end milling, reducing burrs or local chipping at the groove edges caused by surface impurities. When performing three-way cross milling with a ball end mill, the spherical cutting edge can directly form a groove with a circular arc cross-section on the surface of the metal substrate. Compared to sharp-angled or rectangular grooves, the bottom of the circular arc groove is less prone to abrupt steps when fluid passes through, resulting in a more continuous fluid transition within the groove. After the first oblique circular arc groove array, the second oblique circular arc groove array, and the longitudinal circular arc groove array intersect, the uncut area between the three sets of grooves forms a rhomboid protrusion array, with the long diagonal of each rhomboid protrusion arranged along the set fluid flow direction. Thus, the rhomboid protrusions can guide the near-wall fluid in sections along the flow direction, while the oblique and longitudinal grooves provide constraint paths for lateral disturbances and downstream flow, respectively. After applying a low surface energy fluoropolymer coating to the diamond-shaped protrusion array and the surface of each groove, the coating covers the already formed three-dimensional microstructure surface, so that the fluid contact interface has both a geometric flow guiding structure and a low surface energy interface, avoiding the problem of being more sensitive to flow conditions and coating wear when relying on only a single groove structure or a single coating.

[0025] In this embodiment, the three-way cross milling does not directly etch closed rhomboid grooves onto the surface of the metal substrate. Instead, it uses a ball end mill to sequentially form a first oblique arcuate groove array, a second oblique arcuate groove array, and a longitudinal arcuate groove array, and forms a rhomboid protrusion array through the unremoved area between the three sets of open grooves. Thus, the rhomboid protrusions are the milling residue structure, and the grooves are arcuate grooves directly formed by the spherical cutting edge. This forming method differs from laser etching to form micro-nano rough structures or closed grooves, and also differs from forming biomimetic textures through imprinting, transfer, or film lamination. It can directly obtain a biomimetic scale array with arcuate groove bottoms and continuous protrusion boundaries on the metal substrate.

[0026] In some embodiments, a low surface energy fluoropolymer coating is applied to the surfaces of the rhomboid protrusion array and the first oblique arcuate groove array, the second oblique arcuate groove array, and the longitudinal arcuate groove array. The low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusions, and also covers the bottom and walls of the first oblique arcuate groove, the second oblique arcuate groove, and the longitudinal arcuate groove, thus obtaining an underwater drag-reducing surface. The low surface energy fluoropolymer coating is a solid film layer, and no free lubricant layer is provided on the underwater drag-reducing surface.

[0027] Optionally, the feed directions of the first and second oblique cutters are symmetrically arranged with respect to the set fluid flow direction, so that the two sets of oblique arc-shaped grooves form corresponding boundaries on both sides of the flow direction. This arrangement allows the oblique groove walls on both sides of the rhomboid protrusion to have similar geometric constraints, avoiding the biased guidance of one side of the groove on the fluid and causing an uneven distribution of the flow field in the lateral direction. For the rhomboid protrusion arranged along the set fluid flow direction, the symmetrical oblique grooves also help maintain the repeatability of the rhomboid boundary, facilitating the formation of a periodically consistent biomimetic scale structure on a large area metal substrate surface.

[0028] Optionally, the first included angle and the second included angle are equal, and both the first included angle and the second included angle are 30° or both are 60°. This angle limitation ensures that a defined intersection angle is formed between the first and second oblique arc-shaped groove arrays, thereby keeping the ratio of the long and short diagonals of the rhomboid protrusions within a controllable range. When the included angle is limited to 30° or 60°, the rhomboid protrusions will not be too narrow or too close to a square, retaining the guiding characteristics of the long diagonal along the flow direction while facilitating repeated machining via CNC toolpath. This feature solves the problem of unstable control of the rhomboid unit shape in three-way cross cutting, allowing the geometric parameters of the biomimetic rhomboid scale array to be directly determined by the toolpath angle.

[0029] Optionally, the first and second oblique arc-shaped groove arrays intersect to form multiple rhomboid boundaries arranged along the set fluid flow direction. The longitudinal arc-shaped groove array extends along the set fluid flow direction and passes through the area between adjacent rhomboid boundaries, so that the rhomboid protrusion arrays are spaced apart along the set fluid flow direction. Through this arrangement, the longitudinal arc-shaped grooves not only form downstream grooves but also participate in separating the residual areas between adjacent rhomboid boundaries, so that the rhomboid protrusion arrays are spaced apart along the set fluid flow direction. This avoids the problem of forming continuous rhomboid frames but interconnected protrusion areas after the two sets of oblique grooves intersect, so that each rhomboid protrusion has a relatively independent contour boundary in the flow direction, which is closer to the discrete arrangement of scale arrays.

[0030] Optionally, adjacent rhomboid protrusions in the rhomboid protrusion array are defined by a first oblique arcuate groove array, a second oblique arcuate groove array, and a longitudinal arcuate groove array. This structure ensures that the separation between adjacent rhomboid protrusions does not depend on grooves in a single direction, but rather on the boundaries formed by the oblique and longitudinal grooves. The oblique grooves determine the morphology of the beveled edges of the rhomboid protrusions, and the longitudinal grooves determine the spacing of the rhomboid protrusions along the flow direction, thereby reducing the possibility of incomplete boundaries or partial connections between rhomboid protrusions. This embodiment further clarifies the way in which the three-directional grooves jointly constitute the scale array unit, so that the rhomboid protrusions are not simply residual areas accidentally left by intersecting grooves, but repeatable microstructural units defined by three sets of arcuate grooves.

[0031] The longitudinal arc-shaped grooves are not used solely as flow-direction grooves, but rather, together with two sets of oblique arc-shaped grooves, define the front and rear boundaries of the rhomboid protrusions and the flow-direction spacing between adjacent protrusions. The two sets of oblique arc-shaped grooves determine the direction of the oblique edges of the rhomboid protrusions, while the longitudinal arc-shaped grooves define the segmented relationship of the rhomboid protrusions along the fluid flow direction, giving the rhomboid protrusion array a scale-like morphology with discrete arrangement along the flow direction. The resulting surface differs from the mesh texture formed by only two sets of intersecting grooves, and also from the unidirectional ribbed groove structure.

[0032] Optionally, each arc-shaped groove in the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array has an arc-shaped groove bottom formed by the spherical cutting edge of a ball end mill. Since the groove bottom is directly formed by the same type of spherical cutting edge, there are no bends or steps formed by multiple planar cuts at the bottom of the groove. The curvature of the groove bottom corresponds to the tool geometry, facilitating control through tool radius and depth of cut. This embodiment ensures that the three sets of grooves in different directions maintain consistent cross-sectional morphology, avoiding inconsistent local fluid contact states due to differences in the cross-sections of the grooves in different directions. It also facilitates continuous coverage of the low surface energy fluoropolymer coating along the arc-shaped groove bottom.

[0033] The arc-shaped groove bottom creates a continuous transition surface between the groove bottom and the groove wall, reducing local flow separation points at the bottom edges of flat-bottomed or sharp-cornered grooves. For three-way confluence grooves, the arc-shaped groove bottom also creates a continuous curved surface transition at the confluence nodes, reducing the number of abrupt edge changes at the groove junctions. This facilitates the continuous spreading of the low surface energy fluoropolymer coating at the groove bottom, groove walls, and confluence nodes. This structure allows the drag reduction effect to originate simultaneously from the geometric transition of the arc-shaped groove and the low surface energy of the coating interface, rather than solely relying on surface roughening or unidirectional groove guidance.

[0034] In some embodiments, the ball end mill has a tool radius of 0.5 mm, a depth of cut of 0.1 mm to 0.5 mm for three-way cross milling, and a row spacing of 2.4 mm to 4 mm.

[0035] The tool radius, depth of cut, and row spacing collectively determine the width, depth, and residual protrusion size between adjacent grooves of the arc-shaped groove. Limiting the ball end mill's tool radius to 0.5mm and the depth of cut to the range of 0.1mm-0.5mm ensures the groove has a recognizable arc-shaped cross-section, while avoiding either an excessively shallow cut leading to unclear groove flow boundaries or an excessively deep cut resulting in an undersized top area for the rhomboid protrusions. When the row spacing is set to 2.4mm-4mm, the three-way cross-cutting path allows sufficient metal substrate area to be retained between adjacent grooves to form the rhomboid protrusions, thus maintaining a dimensionally compatible relationship between the groove array and the protrusion array.

[0036] In this embodiment, the row spacing of the three-way cross milling process is greater than the width of a single arc-shaped groove, allowing a continuous metal substrate area to be preserved between adjacent arc-shaped grooves. This metal substrate area forms a rhomboid protrusion under the combined action of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove. If the row spacing is less than or close to the width of a single arc-shaped groove, the preserved area between adjacent grooves will be significantly weakened, the top area of ​​the rhomboid protrusion will decrease, and even excessive overlap of adjacent grooves may occur. If the row spacing is significantly increased, the separation relationship between the oblique grooves and the longitudinal grooves will be weakened, and the periodicity of the rhomboid protrusion array will decrease. Therefore, the row spacing, depth of cut, and tool radius together define the dimensional matching relationship between the groove network and the rhomboid protrusion array.

[0037] In this embodiment, the formation of the diamond-shaped protrusion array depends on the coordination between the ball end mill radius, depth of cut, row spacing, and feed angle. During three-way cross milling, the row spacing is made greater than the theoretical width of a single arc-shaped groove to retain a metal substrate area between adjacent grooves. This retained area is then defined by the first oblique feed, the second oblique feed, and the longitudinal feed, thereby forming the diamond-shaped protrusion array. Thus, the diamond-shaped protrusions are not irregular residual structures generated after random cross-cutting, but rather designable microstructures determined by the tool geometry, depth of cut, row spacing, and feed angle. This structure allows for control of the long diagonal, short diagonal, protrusion spacing, and groove depth of the diamond-shaped protrusions by adjusting the aforementioned parameters.

[0038] In some embodiments, the spindle speed for three-way cross milling is 8000 r / min-15000 r / min, and the feed rate is 400 mm / min-800 mm / min.

[0039] Spindle speed and feed rate are used to control the material removal state during the ball end mill cutting process. Limiting the spindle speed and feed rate within the aforementioned range allows for the formation of continuous arc-shaped grooves on the metal substrate surface and reduces burrs and tearing marks at the groove edges caused by insufficient cutting or excessive feed rate. For three-way cross milling, these machining parameters also ensure relatively consistent cutting quality at the intersection of the first oblique feed, the second oblique feed, and the longitudinal feed, reducing the possibility of groove wall breakage or incomplete raised edges in the intersection area.

[0040] Optionally, the first oblique cutter, the second oblique cutter, and the cutter along the set fluid flow direction are all machined using the same ball end mill, and the depth of cut and the spacing between the three are the same.

[0041] This method ensures that the three sets of grooves have consistent bottom curvature and depth reference, avoiding height differences at the intersection of grooves in different directions due to tool changes or alterations in cutting depth. For a diamond-shaped protrusion array, using the same cutting depth and row spacing for the three sets of grooves makes it easier to maintain consistency in the top surface height, sidewall transition, and array period of the protrusions, thereby improving the consistency of microstructure morphology in different areas during large-area machining.

[0042] Optionally, a low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusion array, as well as the walls of the first oblique arcuate groove array, the second oblique arcuate groove array, and the longitudinal arcuate groove array. Through this coverage relationship, the coating not only covers the exposed top surface of the rhomboid protrusions but also extends to the sides of the protrusions and the groove walls, allowing the fluid to contact a continuous low surface energy interface as it passes over the top of the protrusions, crosses the edges of the protrusions, and enters the groove region. This feature avoids the situation where the coating only covers the top surface of the protrusions while the exposed metal surface remains within the grooves, ensuring that the drag-reducing regions of the structure and the drag-reducing regions of the coating correspond spatially, rather than being separate from each other.

[0043] Optionally, a low surface energy fluoropolymer coating covers the bottom and walls of each arc-shaped trench, forming a continuous coating layer in the trench area between adjacent rhomboid protrusions. Since the trench area between adjacent rhomboid protrusions is a location prone to local shearing and disturbance when fluid flows close to the surface, the continuous coating layer ensures continuous surface chemistry of the trench bottom, walls, and trench junctions, reducing local wetting differences caused by uncoated discontinuities. This feature also keeps the low surface energy coating at the junctions of the three-dimensional trenches connected, avoiding exposed metal points or discontinuous coating areas at the junction nodes.

[0044] In some embodiments, the low surface energy fluoropolymer coating is an amorphous fluoropolymer coating. Specifically, the amorphous fluoropolymer coating is an AF1600 amorphous fluoropolymer coating.

[0045] Amorphous fluoropolymer coatings can form thin, low-surface-energy capping layers on the surfaces of arc-shaped grooves and rhomboid protrusions. AF1600 amorphous fluoropolymer has low surface energy, making it suitable for coating metal substrates with microstructure undulations. When used in the triaxial arc-shaped groove and rhomboid protrusion array of this embodiment, it can alter the interfacial properties of the microstructure surface without filling in the groove morphology, allowing the grooves and protrusions to retain their original geometric contours.

[0046] After the coating is formed, the thickness of the low surface energy fluoropolymer coating is less than the depth of the arc-shaped grooves, so that the geometric contours of the first oblique arc-shaped groove, the second oblique arc-shaped groove, the longitudinal arc-shaped groove, and the diamond-shaped protrusion array are still preserved. The coating is used to change the interfacial properties of the groove bottom, groove wall, and diamond-shaped protrusion surface, rather than to fill the groove structure formed by three-way cross milling.

[0047] In this embodiment, the low surface energy fluoropolymer coating is a solid film layer, and no free lubricant layer is provided on the underwater drag-reducing surface. After curing, the low surface energy fluoropolymer coating adheres to the rhomboid protrusion array, the bottom and walls of the arc-shaped grooves, and the intersection of the three-dimensional grooves to provide a stable low surface energy interface. Compared with drag-reducing surfaces that rely on lubricant storage and replenishment, this embodiment does not require maintaining flowing lubricant in the grooves or micropores, which can reduce the performance degradation caused by lubricant loss during underwater scouring. The three-dimensional arc-shaped grooves and the rhomboid protrusion array provide geometric guidance and interface retention space, while the solid low surface energy fluoropolymer coating provides a low surface energy interface. Together, they constitute a composite drag-reducing surface that does not require an external lubricant layer.

[0048] In some embodiments, the method for applying the low surface energy fluoropolymer coating is a dip coating method, which includes: immersing the metal substrate after three-way cross milling into a low surface energy fluoropolymer emulsion for 5 min-10 min, removing it, allowing it to stand, and drying it.

[0049] The dip-coating method involves immersing the entire metal substrate, after three-way cross-milling, into a low surface energy fluoropolymer emulsion. This allows the emulsion to penetrate the first, second, and longitudinal arc-shaped grooves, and contact the sides of the rhomboid protrusions and the intersection areas of the grooves. Compared to spraying from only one direction, dip-coating has less impact on the sidewalls and bottom of the grooves, making it suitable for covering surface structures with multi-directional intersecting grooves. After immersion for 5-10 minutes, the substrate is removed, allowed to stand, and dried. This allows the coating to form a covering layer on the microstructure surface, while reducing the possibility of excessive emulsion buildup within the grooves, which could weaken the contour of the arc-shaped groove bottom.

[0050] For drag-reducing surfaces with multi-directional intersecting grooves, dip coating allows the low surface energy fluoropolymer emulsion to simultaneously enter the first oblique arc-shaped groove, the second oblique arc-shaped groove, the longitudinal arc-shaped groove, and the intersection of the three-directional grooves. During the static drying process, excess emulsion is discharged along the arc-shaped grooves, while the low surface energy fluoropolymer remains on the top surface, sides, bottom, and walls of the rhomboid protrusions. Subsequently, an anhydrous ethanol solvent replacement treatment is performed to remove unstable free components within the grooves, reducing the accumulation of low surface energy fluoropolymer at the bottom of the grooves and the converging recesses. After this treatment, the coating primarily covers the microstructured surface in a thin layer, preserving the cross-sectional profile of the arc-shaped grooves.

[0051] The intersection of the three-dimensional arc-shaped grooves forms a confluence recess composed of multiple arc-shaped groove bottoms. During dip coating, the low surface energy fluoropolymer emulsion can enter the confluence recess and form a continuous coating layer after static drying and solvent replacement. This continuous coating layer ensures a consistent low surface energy interface in the groove regions between the rhomboid protrusions, the intersection regions of the oblique and longitudinal grooves, and the arc-shaped transition regions at the groove bottoms, reducing wetting differences caused by local uncoated areas or discontinuous coatings. Because the confluence recess is located between adjacent rhomboid protrusions, it can form a local interface retention region in an underwater environment, allowing the low surface energy interface to spatially coincide with the geometry of the three-dimensional grooves, thereby enhancing the stability of the composite drag-reducing surface.

[0052] In some embodiments, after drying, the metal substrate is placed in anhydrous ethanol for solvent replacement treatment for 3-8 minutes, and then removed and dried again.

[0053] After the coating dries, the metal substrate is placed in anhydrous ethanol for solvent replacement for 3-8 minutes. This removes some residual solvent or unstable deposits from the coating surface and within the trenches. This process helps reduce coating residue buildup at the bottom, walls, and edges of the diamond-shaped protrusions of the trenches, allowing the low surface energy fluoropolymer coating to better conform to the contours of the formed arc-shaped trenches. Re-drying after removal ensures the solvent-replaced coating surface remains dry, facilitating its subsequent use as an underwater drag-reducing surface.

[0054] In some embodiments, the metal substrate is an aluminum alloy substrate.

[0055] Aluminum alloy substrates are suitable for surface microstructure machining using ball end mills, enabling the formation of continuous arc-shaped grooves with set depth of cut and row spacing. When using aluminum alloy substrates, three-way cross milling can directly form a diamond-shaped array of protrusions on the substrate body without the need for additional film layers or prefabricated templates, thus eliminating interface delamination issues between the microstructure and the substrate. This feature is suitable for scenarios requiring the direct formation of drag-reducing microstructures on the surface of metal components.

[0056] In some embodiments, the cleaning pretreatment includes: ultrasonically cleaning the metal substrate in anhydrous ethanol for 10-20 minutes, and drying it for 20-40 minutes after cleaning.

[0057] Ultrasonic cleaning of the metal substrate in anhydrous ethanol for 10-20 minutes removes surface oil, fine particles, and contaminants acquired during handling. Drying for 20-40 minutes after cleaning minimizes the impact of residual liquid on subsequent milling positioning and coating adhesion. This pretreatment step, performed before triaxial cross-milling, provides a more consistent substrate surface condition for the ball end mill to create continuous arc-shaped grooves and for subsequent low surface energy fluoropolymer coating application.

[0058] This disclosure provides a ball-end milled biomimetic rhomboid scale drag-reducing surface, comprising: a metal substrate having a drag-reducing surface for contact with underwater fluid; a first oblique arcuate groove array, a second oblique arcuate groove array, and a longitudinal arcuate groove array, all disposed on the drag-reducing surface; and a low surface energy fluoropolymer coating covering the drag-reducing surface; wherein, the first oblique arcuate groove array includes multiple first oblique arcuate grooves, each first oblique arcuate groove being inclined to a first side relative to a set fluid flow direction and forming a first angle with the set fluid flow direction; the second oblique arcuate groove array includes multiple second oblique arcuate grooves, each second oblique arcuate groove being inclined to a second side relative to the set fluid flow direction and forming a second angle with the set fluid flow direction. Angle; The longitudinal arc-shaped groove array includes multiple longitudinal arc-shaped grooves extending along a set fluid flow direction; The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array intersect each other, and the area on the drag-reducing surface not occupied by the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array constitutes a rhombus-shaped protrusion array; The rhombus-shaped protrusion array includes multiple rhombus-shaped protrusions, and the long diagonal of each rhombus-shaped protrusion is arranged along a set fluid flow direction; The cross-sections of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove are all arc-shaped; A low surface energy fluoropolymer coating covers the top surface and side surface of the rhombus-shaped protrusions, as well as the groove walls of each first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove.

[0059] The drag-reducing surface provided in this embodiment includes a ball-end milled biomimetic rhomboid scale drag-reducing surface comprising a metal substrate, a first oblique arcuate groove array, a second oblique arcuate groove array, and a longitudinal arcuate groove array disposed on the drag-reducing surface, and a low surface energy fluoropolymer coating covering the drag-reducing surface. After the three sets of grooves intersect, the area on the drag-reducing surface not occupied by the grooves forms a rhomboid protrusion array, with the long diagonal of each rhomboid protrusion arranged along a predetermined fluid flow direction. This structure ensures that the drag-reducing surface is not simply a parallel groove surface, nor merely a smooth surface with a coating, but rather forms rhomboid protrusions along the flow-directed long diagonal, oblique arcuate grooves on both sides, and longitudinal arcuate grooves on the same metal substrate surface. The low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusions and the walls of each groove, ensuring that the main microstructure surfaces in contact with the underwater fluid all have low surface energy interfaces. Therefore, the product structure itself can reflect the combination of geometric flow guidance and low surface energy interface on the same drag-reducing surface, which makes it easy to form an underwater drag-reducing surface without relying on continuous replenishment of additional drag-reducing agents.

[0060] The present application will be further described below with reference to specific embodiments. In the following embodiments, room temperature refers to 23±2℃, and relative humidity is 45%±10%. Unless otherwise stated, the metal substrate is 6061 aluminum alloy plate with dimensions of 100mm×100mm×3mm and a surface roughness Ra of 0.8μm±0.2μm; the anhydrous ethanol purity is 99.7%; the low surface energy fluoropolymer emulsion is AF1600 amorphous fluoropolymer emulsion with a solid content of 6.0wt%±0.5wt%. Ball end milling is performed using a three-axis CNC machining center, model VMC850E, with spindle radial runout ≤0.005mm; the ball end mill is a carbide double-flute ball end mill with a tool radius of 0.5mm and cutting edge circular runout ≤0.003mm. Ultrasonic cleaning is performed using a KQ-300DE ultrasonic cleaner with a working frequency of 40kHz and an adjustable power range of 100W-300W. Contact angle testing was performed using a DSA100 contact angle measuring instrument with a water droplet volume of 5 μL. Five locations were tested for each sample, and the average value was taken. Drag reduction testing was conducted using a closed circulating water tank with a sample installation section length of 300 mm, an inlet flow velocity of 3.0 m / s ± 0.05 m / s, and a water temperature of 25 ± 1℃. A 6061 aluminum alloy plate of the same material, without microstructure processing or coating treatment, was used as the reference sample.

[0061] The fabrication method described in this application utilizes the spherical cutting edge of a ball end mill to form arc-shaped grooves on the surface of a metal substrate. Through a first oblique cutter, a second oblique cutter, and a longitudinal cutter along a predetermined fluid flow direction, three sets of arc-shaped grooves intersect, leaving an array of rhomboid protrusions arranged along the fluid flow direction in the unmilled areas. After microstructure machining, a low surface energy fluoropolymer coating is formed on the surfaces of the rhomboid protrusions and grooves, resulting in a drag-reducing surface that simultaneously possesses an arc-shaped groove flow-guiding structure, a rhomboid scale-like protrusion array, and a low surface energy interface. This method can be applied to ship outer plating, underwater vehicle hulls, the inner walls of fluid transport pipelines, pump impellers, and the surfaces of metal components requiring reduced underwater friction resistance. This process does not require laser etching equipment or prefabricated flexible film templates; microstructure machining can be performed on a conventional CNC machining center, and the toolpath can be adjusted according to the mainstream fluid flow direction.

[0062] In this embodiment, the forming process of the biomimetic rhomboid scale structure is based on the material retention area of ​​the metal substrate. A first and second oblique cutting tool first form intersecting oblique arc-shaped grooves on the surface of the metal substrate, with the two sets of oblique arc-shaped grooves jointly defining the oblique boundaries of the rhomboid protrusions. Then, a longitudinal cutting tool is moved along a predetermined fluid flow direction, causing the longitudinal arc-shaped grooves to pass through the area between adjacent rhomboid boundaries, dividing the adjacent metal substrate retention areas into rhomboid protrusions arranged along the predetermined fluid flow direction. Thus, the rhomboid protrusion array is not formed by the accumulation of external materials, film adhesion, or coating buildup, but is directly formed from the area of ​​the metal substrate that has not been removed after three-way cross milling. The boundaries of the rhomboid protrusions are defined by the walls of the arc-shaped grooves, and the long diagonal direction of the rhomboid protrusions is determined by the predetermined fluid flow direction.

[0063] Using the above forming method, the oblique arc-shaped grooves are used to determine the oblique edge contour of the rhomboid protrusions, and the longitudinal arc-shaped grooves are used to determine the spacing and arrangement direction of the rhomboid protrusions along the flow direction. The three sets of grooves do not simply superimpose, but jointly participate in the forming and separation of the rhomboid protrusion array, so that the drag-reducing surface simultaneously has rhomboid protrusions arranged in the flow direction, oblique groove boundaries on both sides, and longitudinal groove channels.

[0064] Example 1: Basic Process Example This embodiment illustrates the basic process of preparing a biomimetic rhomboid scale drag-reducing surface using ball-end milling with three-way cross machining combined with a low surface energy fluoropolymer coating.

[0065] Take one 6061 aluminum alloy plate, measuring 100mm × 100mm × 3mm. Immerse the aluminum alloy plate in 500mL of anhydrous ethanol and ultrasonically clean it for 15 minutes at a power of 150W and a cleaning temperature of 25±2℃. After cleaning, remove the aluminum alloy plate with clean tweezers and blow off any residual liquid on the surface with clean compressed air at 0.4MPa for 30 seconds. Then, place it in a clean, ventilated environment to dry for 30 minutes. After drying, there should be no visible oil, water stains, or particle residue on the surface of the aluminum alloy plate.

[0066] The cleaned aluminum alloy plate was fixed on the worktable of a three-axis CNC machining center. A vacuum suction fixture and two mechanical pressure plates on both sides were used for positioning. After clamping, the flatness of the machining area was checked with a dial indicator, and the height difference of the machining area was controlled to be ≤0.02mm. A workpiece coordinate system was established, with the center point of the aluminum alloy plate surface as the origin, and the Y-axis direction was defined as the set fluid flow direction. A 0.5mm radius carbide double-flute ball end mill was clamped. The spindle speed was set to 10000 r / min, the feed rate to 600 mm / min, the depth of cut to 0.3mm, the travel distance to 3.2mm, the cutting mode to climb milling, and the cooling method to air cooling with a compressed air pressure of 0.25MPa. Water-based cutting fluid was not used during the machining process to avoid cutting fluid residue affecting subsequent coating coverage.

[0067] First, a first oblique cutter path is performed. This path is tilted 30° to the left of the Y-axis, covering a 100mm x 100mm machining area with a path spacing of 3.2mm. The starting and ending points of each path are located 5mm outside the machining area. The ball end mill sequentially processes along the first oblique cutter path, forming a first oblique arc-shaped groove array. Next, a second oblique cutter path is performed. This path is tilted 30° to the right of the Y-axis, symmetrically positioned about the Y-axis to the first oblique cutter path, with other machining parameters remaining unchanged, forming a second oblique arc-shaped groove array. Finally, a longitudinal cutter path is performed. This path is aligned with the Y-axis, extending along the set fluid flow direction with a path spacing of 3.2mm, forming a longitudinal arc-shaped groove array. After the three sets of grooves intersect, the areas not removed by the ball end mill form a diamond-shaped protrusion array, with the long diagonal of the protrusions arranged along the Y-axis.

[0068] After three-way cross milling, a first oblique arc-shaped groove array, a second oblique arc-shaped groove array, and a longitudinal arc-shaped groove array are formed on the surface of the metal substrate. Between adjacent arc-shaped grooves, areas of the metal substrate that have not been milled away are retained, forming the top surface of a rhomboid protrusion. The three sets of arc-shaped grooves are interconnected at their intersection points, forming a confluence recess composed of multiple arc surfaces. The confluence recess continuously transitions to the groove bottom and walls of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove.

[0069] The cross-section of the milled grooves was inspected using a three-dimensional white light interferometer. The first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove all exhibited arc-shaped cross-sections, and no right-angle transitions, characteristic of flat-bottomed end milling, were formed between the groove bottom and the groove wall. This intermediate structure allows subsequent low surface energy fluoropolymer emulsions to enter the groove bottom, groove wall, and the intersection of the three-way grooves.

[0070] After milling, the sample was removed from the fixture and ultrasonically cleaned in 500 mL of anhydrous ethanol for 10 min at an ultrasonic power of 150 W and a cleaning temperature of 25 ± 2 °C to remove chips and surface powder from the grooves. After cleaning, it was purged with clean compressed air at 0.4 MPa along the groove direction for 60 s, and then dried at 23 ± 2 °C for 30 min. This post-processing does not involve filtration, calcination, or sieving; since the product is a monolithic metal sheet, there are no powder filtration, powder washing, or sieving steps.

[0071] The dried microstructured sample was vertically clamped and immersed in AF1600 amorphous fluoropolymer emulsion at a descent speed of 5 mm / s to a depth of 110 mm, ensuring the entire processed area was submerged. The immersion time was 7 minutes. After immersion, the sample was vertically pulled out of the liquid at a speed of 5 mm / s and held vertically for 2 minutes, allowing excess emulsion to flow down the grooves and plate surface. The sample was then dried in a clean, well-ventilated environment at 23±2℃ and 45%±10% relative humidity for 6 hours. After drying, the sample was again immersed in 500 mL of anhydrous ethanol for solvent replacement treatment for 5 minutes. After removal, it was purged with clean compressed air at 0.25 MPa for 30 seconds and then dried at 23±2℃ for 2 hours to obtain a ball-end milled biomimetic rhomboid scale drag-reducing surface.

[0072] After coating treatment, the low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusions, and also covers the bottom, walls, and confluence recesses of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove. Cross-sectional microscopic observation shows that the low surface energy fluoropolymer coating adheres as a thin layer to the inner walls of the arc-shaped grooves, but does not fill the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove. The bottom contours of the arc-shaped grooves are still identifiable after coating treatment, and the original microstructural undulations are maintained between the top and sides of the rhomboid protrusion array and adjacent groove areas.

[0073] In this embodiment, the thickness of the low surface energy fluoropolymer coating is less than 10% of the depth of the arc-shaped trench. Therefore, the low surface energy fluoropolymer coating is mainly used to modify the interfacial properties of the diamond-shaped protrusions and the arc-shaped trench surfaces, rather than acting as a filling layer to smooth out the trench structure. This coating state allows the triaxial arc-shaped trenches and diamond-shaped protrusion array to continue participating in the regulation of the underwater fluid boundary layer after the coating is formed.

[0074] The samples obtained in Example 1 were characterized. The cross-section of the grooves was measured using a three-dimensional white light interferometer. The average depth of the arc-shaped grooves was 0.296 mm, the average width was 0.714 mm, the average length of the long diagonal of the rhomboid protrusions was 6.42 mm, and the average length of the short diagonal was 3.18 mm. The thickness of the low surface energy fluoropolymer coating was measured to be 4.8 μm ± 0.7 μm by cross-sectional microscopy. The water contact angle was 151.6° ± 2.1°, and the roll-off angle was 8.4° ± 1.3°. Under an inlet flow velocity of 3.0 m / s, the average reduction rate of wall friction resistance was 15.8% compared to the untreated 6061 aluminum alloy plate. After immersion in artificial seawater for 72 h, the water contact angle was 148.2° ± 2.4°, and the average reduction rate of wall friction resistance was 14.1%.

[0075] In this embodiment, the three-way cross milling is performed in the order of first oblique tool path, second oblique tool path, and longitudinal tool path. Machining the two sets of oblique grooves first can pre-form diamond-shaped boundaries, and then separating the areas between adjacent diamond-shaped boundaries by longitudinal tool path is beneficial for obtaining an array of diamond-shaped protrusions spaced apart along the fluid flow direction. If the longitudinal grooves are machined first, and then the two sets of oblique grooves are machined, a three-way cross groove structure can still be formed, but chip removal at the edges of the diamond-shaped protrusions and boundary integrity may be affected.

[0076] Example 2: Lower limit of cutting depth and lower limit of row spacing This embodiment illustrates the process of preparing a biomimetic rhomboid scale drag-reducing surface under conditions of small cutting depth and small row spacing.

[0077] Take one piece of 6061 aluminum alloy plate, measuring 100mm × 100mm × 3mm. Immerse the aluminum alloy plate in 500mL of anhydrous ethanol and ultrasonically clean it for 10min at a power of 100W and a cleaning temperature of 25±2℃. After cleaning, blow off any residual liquid from the surface with clean compressed air at 0.4MPa for 30s, and then dry it at 23±2℃ for 20min.

[0078] The dried aluminum alloy plate was clamped in a three-axis CNC machining center. The fluid flow direction was set to the Y-axis. A carbide ball end mill with a tool radius of 0.5 mm was used. The spindle speed was set to 8000 r / min, the feed rate to 400 mm / min, the depth of cut to 0.1 mm, and the feed distance to 2.4 mm. The first oblique feed direction was tilted 30° to the left relative to the Y-axis, the second oblique feed direction was tilted 30° to the right relative to the Y-axis, and the longitudinal feed direction was consistent with the Y-axis. The same tool, depth of cut, and feed distance were used for all three feed directions. Air cooling was used during machining, with a compressed air pressure of 0.25 MPa.

[0079] After three-way cross milling, a diamond-shaped protrusion array is formed on the surface of the metal substrate, defined by a first oblique arc-shaped groove, a second oblique arc-shaped groove, and a longitudinal arc-shaped groove. Because the same ball end mill, the same depth of cut, and the same line spacing are used in all three directions, the groove depth reference and groove bottom curvature remain consistent, and no abrupt height changes due to tool changes or variations in cutting depth occur at the intersection of the three grooves. Adjacent diamond-shaped protrusions are separated by three sets of arc-shaped grooves, and the long diagonal of the diamond-shaped protrusions is arranged along the Y-axis.

[0080] After milling, the sample was ultrasonically cleaned in 500 mL of anhydrous ethanol for 10 min at an ultrasonic power of 100 W. After removal, it was purged with clean compressed air at 0.4 MPa for 60 s and dried at 23±2℃ for 20 min. Subsequently, a dip-coating treatment was performed. The sample was vertically immersed in AF1600 amorphous fluoropolymer emulsion at a speed of 3 mm / s for 5 min; then vertically lifted at a speed of 3 mm / s and left to stand vertically for 1 min; and dried at 23±2℃ for 4 h. After drying, the sample was immersed in anhydrous ethanol for solvent replacement treatment for 3 min, and then dried at 23±2℃ for 2 h.

[0081] After coating treatment, the low surface energy fluoropolymer coating is continuously distributed along the top and sides of the rhomboid protrusions, the bottom and walls of each arc-shaped groove, and the intersection of the three-dimensional grooves. Cross-sectional observation shows that the arc-shaped grooves are not filled by the low surface energy fluoropolymer coating, and the coated grooves retain their arc-shaped cross-sectional contours. After anhydrous ethanol solvent replacement treatment, no continuous blocky residues were observed at the bottom of the grooves or in the concave areas where they intersect; the low surface energy fluoropolymer coating adheres to the microstructure surface as a capping layer.

[0082] The samples obtained in Example 2 were characterized. The average depth of the arc-shaped groove was 0.098 mm, the average width of the arc-shaped groove was 0.436 mm, the average length of the long diagonal of the rhomboid protrusion was 4.82 mm, and the average length of the short diagonal was 2.38 mm. The coating thickness was 3.9 μm ± 0.6 μm. The water contact angle was 149.3° ± 2.3°, and the roll-off angle was 10.2° ± 1.5°. Under the condition of an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 12.6%. After immersion in artificial seawater for 72 h, the water contact angle was 146.5° ± 2.7°, and the average wall friction resistance reduction rate was 11.2%.

[0083] Example 3: Upper limit of cutting depth and upper limit of row spacing This embodiment illustrates the process of preparing a biomimetic rhomboid scale drag-reducing surface under conditions of large cutting depth and large row spacing.

[0084] Take one piece of 6061 aluminum alloy plate, measuring 100mm × 100mm × 3mm. Immerse the aluminum alloy plate in 500mL of anhydrous ethanol and ultrasonically clean it for 20 minutes at a power of 200W and a cleaning temperature of 25±2℃. After cleaning, purge with clean compressed air at 0.4MPa for 30 seconds and dry at 23±2℃ for 40 minutes.

[0085] The aluminum alloy plate was clamped on a three-axis CNC machining center, with the Y-axis set as the fluid flow direction. A carbide ball end mill with a tool radius of 0.5 mm was used. The spindle speed was set to 15000 r / min, the feed rate to 800 mm / min, the depth of cut to 0.5 mm, and the feed distance to 4.0 mm. The first oblique feed direction was tilted 60° to the left relative to the Y-axis, the second oblique feed direction was tilted 60° to the right relative to the Y-axis, and the longitudinal feed direction was consistent with the Y-axis. All three-axis cross-milling was air-cooled with a compressed air pressure of 0.30 MPa. After each oblique feed, the spindle was paused and the surface was purged with clean compressed air at 0.30 MPa along the groove direction for 30 seconds before proceeding to the next direction to reduce chip accumulation in the intersection area.

[0086] After three-way cross milling, a diamond-shaped protrusion array is formed on the surface of the metal substrate, defined by a first oblique arc-shaped groove, a second oblique arc-shaped groove, and a longitudinal arc-shaped groove. Because the same ball end mill, the same depth of cut, and the same line spacing are used in all three directions, the groove depth reference and groove bottom curvature remain consistent, and no abrupt height changes due to tool changes or variations in cutting depth occur at the intersection of the three grooves. Adjacent diamond-shaped protrusions are separated by three sets of arc-shaped grooves, and the long diagonal of the diamond-shaped protrusions is arranged along the Y-axis.

[0087] After milling, the sample was immersed in 500 mL of anhydrous ethanol and ultrasonically cleaned for 15 min at an ultrasonic power of 200 W. After removal, it was purged with clean compressed air at 0.4 MPa for 60 s and dried at 23±2℃ for 40 min. Subsequently, the sample was vertically immersed in AF1600 amorphous fluoropolymer emulsion at a speed of 8 mm / s for 10 min; then vertically lifted at a speed of 8 mm / s and left to stand vertically for 3 min; and dried at 23±2℃ for 8 h. After drying, the sample was immersed in anhydrous ethanol for solvent replacement treatment for 8 min, and then dried at 23±2℃ for 2 h.

[0088] After coating treatment, the low surface energy fluoropolymer coating is continuously distributed along the top and sides of the rhomboid protrusions, the bottom and walls of each arc-shaped groove, and the intersection of the three-dimensional grooves. Cross-sectional observation shows that the arc-shaped grooves are not filled by the low surface energy fluoropolymer coating, and the coated grooves retain their arc-shaped cross-sectional contours. After anhydrous ethanol solvent replacement treatment, no continuous blocky residues were observed at the bottom of the grooves or in the concave areas where they intersect; the low surface energy fluoropolymer coating adheres to the microstructure surface as a capping layer.

[0089] The samples obtained in Example 3 were characterized. The average depth of the arc-shaped groove was 0.493 mm, the average width of the arc-shaped groove was 0.998 mm, the average length of the long diagonal of the rhomboid protrusion was 8.05 mm, and the average length of the short diagonal was 4.05 mm. The coating thickness was 5.6 μm ± 0.8 μm. The water contact angle was 153.1° ± 1.9°, and the roll-off angle was 7.6° ± 1.1°. Under the condition of an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 16.9%. After immersion in artificial seawater for 72 h, the water contact angle was 149.0° ± 2.2°, and the average wall friction resistance reduction rate was 14.8%.

[0090] Example 4: Optimal Angle and Optimal Process Parameters This embodiment illustrates the process of preparing a drag-reducing surface using a preferred tool path angle and intermediate machining parameters.

[0091] Take one piece of 6061 aluminum alloy plate, measuring 150mm × 150mm × 3mm. Immerse the aluminum alloy plate in 800mL of anhydrous ethanol and ultrasonically clean it for 15min at an ultrasonic power of 180W and a cleaning temperature of 25±2℃. After cleaning, purge with 0.4MPa clean compressed air for 45s and dry at 23±2℃ for 30min.

[0092] The aluminum alloy plate was clamped in a three-axis CNC machining center, and the Y-axis was set as the fluid flow direction. A carbide ball end mill with a tool radius of 0.5 mm was used. The spindle speed was set to 12000 r / min, the feed rate to 600 mm / min, the depth of cut to 0.3 mm, and the feed distance to 3.2 mm. The first oblique feed direction was tilted 60° to the left relative to the Y-axis, the second oblique feed direction was tilted 60° to the right relative to the Y-axis, and the longitudinal feed direction was consistent with the Y-axis. The three-axis cross-milling was performed in the order of the first oblique feed, the second oblique feed, and the longitudinal feed. The starting point and the retraction point of each feed direction were both located 5 mm outside the machining area to prevent the entry and retraction marks from being located within the effective drag reduction area.

[0093] After three-way cross milling, a diamond-shaped protrusion array is formed on the surface of the metal substrate, defined by a first oblique arc-shaped groove, a second oblique arc-shaped groove, and a longitudinal arc-shaped groove. Because the same ball end mill, the same depth of cut, and the same line spacing are used in all three directions, the groove depth reference and groove bottom curvature remain consistent, and no abrupt height changes due to tool changes or variations in cutting depth occur at the intersection of the three grooves. Adjacent diamond-shaped protrusions are separated by three sets of arc-shaped grooves, and the long diagonal of the diamond-shaped protrusions is arranged along the Y-axis.

[0094] After milling, the sample was ultrasonically cleaned in 800 mL of anhydrous ethanol for 12 min at an ultrasonic power of 180 W. After removal, it was purged with clean compressed air at 0.4 MPa along the first oblique direction, the second oblique direction, and the longitudinal direction for 20 s each, and then dried at 23±2℃ for 30 min. Subsequently, the sample was vertically immersed in AF1600 amorphous fluoropolymer emulsion at a speed of 5 mm / s for 8 min; after being vertically lifted at a speed of 5 mm / s, it was allowed to stand vertically for 2 min; and then dried at 23±2℃ for 6 h. After drying, the sample was solvent-displaced in 800 mL of anhydrous ethanol for 5 min, then purged with clean compressed air at 0.25 MPa for 30 s, and then dried at 23±2℃ for 2 h.

[0095] After coating treatment, the low surface energy fluoropolymer coating is continuously distributed along the top and sides of the rhomboid protrusions, the bottom and walls of each arc-shaped groove, and the intersection of the three-dimensional grooves. Cross-sectional observation shows that the arc-shaped grooves are not filled by the low surface energy fluoropolymer coating, and the coated grooves retain their arc-shaped cross-sectional contours. After anhydrous ethanol solvent replacement treatment, no continuous blocky residues were observed at the bottom of the grooves or in the concave areas where they intersect; the low surface energy fluoropolymer coating adheres to the microstructure surface as a capping layer.

[0096] The samples obtained in Example 4 were characterized. The average depth of the arc-shaped groove was 0.301 mm, the average width of the arc-shaped groove was 0.719 mm, the average length of the long diagonal of the rhomboid protrusion was 6.38 mm, and the average length of the short diagonal was 3.23 mm. No continuous burrs were observed at the intersection of the three-way grooves. The water contact angle was 154.0°±1.6°, and the roll-off angle was 6.9°±1.0°. Under the condition of an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 18.3%. After soaking in artificial seawater for 72 h, the water contact angle was 150.4°±2.0°, and the average wall friction resistance reduction rate was 16.2%.

[0097] Comparative Example 1: No microstructure was set and only a low surface energy coating was applied. This comparative example illustrates the effect of using only a low surface energy fluoropolymer coating without performing triaxial cross-ball milling.

[0098] Take one 6061 aluminum alloy plate with dimensions of 100mm × 100mm × 3mm. Immerse the aluminum alloy plate in 500mL of anhydrous ethanol and ultrasonically clean it for 15min at an ultrasonic power of 150W and a cleaning temperature of 25±2℃. After removal, purge with clean compressed air at 0.4MPa for 30s and dry at 23±2℃ for 30min. Do not perform ball end milling on this sample. Then, vertically immerse the sample in AF1600 amorphous fluoropolymer emulsion at a speed of 5mm / s for 7min; vertically lift it at a speed of 5mm / s and let it stand vertically for 2min; dry at 23±2℃ for 6h. After drying, immerse the sample in anhydrous ethanol for solvent replacement treatment for 5min, and then dry at 23±2℃ for 2h.

[0099] The sample obtained in Comparative Example 1 did not form the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, or the longitudinal arc-shaped groove array, nor did it form a rhomboid protrusion array. The coating thickness was 4.5 μm ± 0.6 μm. The water contact angle was 119.8° ± 2.8°, and the roll-off angle was greater than 30°. Under an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 4.7%. After immersion in artificial seawater for 72 h, the water contact angle was 113.6° ± 3.1°, and the average wall friction resistance reduction rate was 3.2%.

[0100] Comparative Example 2: Performing only three-dimensional cross-ball milling without applying a low surface energy coating This comparative example illustrates the effect of forming only a diamond-shaped protrusion array and arc-shaped grooves without applying a low surface energy fluoropolymer coating.

[0101] A 6061 aluminum alloy plate with dimensions of 100mm × 100mm × 3mm was taken. It was machined according to the same cleaning pretreatment, clamping method, and three-way cross-milling parameters as in Example 1. Specifically, the ball end mill radius was 0.5mm, the spindle speed was 10000r / min, the feed rate was 600mm / min, the depth of cut was 0.3mm, and the feed distance was 3.2mm. The first and second oblique feeds were tilted 30° to the left and right of the Y-axis, respectively, with the longitudinal feed direction consistent with the Y-axis. After machining, the sample was immersed in 500mL of anhydrous ethanol for ultrasonic cleaning for 10min at an ultrasonic power of 150W. After removal, it was purged with 0.4MPa clean compressed air for 60s and dried at 23±2℃ for 30min. This sample was not coated with AF1600 amorphous fluoropolymer emulsion and was not subjected to anhydrous ethanol solvent replacement.

[0102] The sample obtained in Comparative Example 2 formed a three-way intersecting arc-shaped groove and a diamond-shaped protrusion array. The average depth of the arc-shaped groove was 0.298 mm, and the average width was 0.716 mm. The water contact angle was 82.4°±2.5°, and the roll-off angle was greater than 45°. Under the condition of an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 7.9%. After immersion in artificial seawater for 72 h, the water contact angle was 80.7°±2.9°, and the average wall friction resistance reduction rate was 7.1%.

[0103] Comparative Example 3: A unidirectional longitudinal arc-shaped groove array with a low surface energy coating was used. This comparative example illustrates the effect of using only a unidirectional groove structure along the fluid flow direction.

[0104] One 6061 aluminum alloy plate, measuring 100mm × 100mm × 3mm, was used. Cleaning and pretreatment were performed according to Example 1. A carbide ball end mill with a tool radius of 0.5mm was used, with a spindle speed of 10000 r / min, a feed rate of 600 mm / min, a depth of cut of 0.3mm, and a pass spacing of 3.2mm. Only longitudinal cutting was performed along the Y-axis to form a longitudinal arc-shaped groove array; the first and second oblique cutting were not performed. After machining, the sample was immersed in 500 mL of anhydrous ethanol for ultrasonic cleaning for 10 min at an ultrasonic power of 150 W. After removal, it was purged with 0.4 MPa clean compressed air for 60 s and dried at 23±2℃ for 30 min. Subsequently, an AF1600 amorphous fluoropolymer coating was applied following the same dipping, settling, drying, solvent replacement, and re-drying steps as in Example 1.

[0105] The sample obtained in Comparative Example 3 only had a longitudinal arc-shaped groove array, lacking the rhomboid protrusion array defined by the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array. The average depth of the arc-shaped grooves was 0.297 mm, and the average width was 0.715 mm. The water contact angle was 147.5°±2.2°, and the roll-off angle was 11.8°±1.6°. Under an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 10.9%. After immersion in artificial seawater for 72 h, the water contact angle was 144.1°±2.6°, and the average wall friction resistance reduction rate was 9.5%.

[0106] Comparative Example 4: Cross-machining with a flat-bottom milling cutter and application of a low surface energy coating This comparative example is used to illustrate the effect when the cross-section of the groove is not circular.

[0107] Take one 6061 aluminum alloy plate with dimensions of 100mm × 100mm × 3mm. Perform cleaning pretreatment and clamping according to Example 1. Use a 1.0mm diameter carbide flat-bottom end mill instead of a ball end mill. The spindle speed is 10000 r / min, the feed rate is 600 mm / min, the depth of cut is 0.3mm, and the feed distance is 3.2mm. The first oblique feed direction is tilted 30° to the left relative to the Y-axis, the second oblique feed direction is tilted 30° to the right relative to the Y-axis, and the longitudinal feed direction is consistent with the Y-axis. After machining, perform anhydrous ethanol ultrasonic cleaning, drying, AF1600 emulsion immersion coating, standing, drying, anhydrous ethanol solvent replacement, and further drying using the same method as in Example 1.

[0108] The sample obtained in Comparative Example 4 formed an array of intersecting grooves and diamond-shaped protrusions, but the groove cross-section was approximately flat-bottomed, with an angle between the groove bottom and the groove wall. The average groove depth was 0.302 mm, and the average groove width was 1.01 mm. The water contact angle was 145.6°±2.4°, and the roll-off angle was 13.7°±1.9°. Under the condition of an inlet flow velocity of 3.0 m / s, the average wall friction resistance reduction rate was 9.6%. After soaking in artificial seawater for 72 h, the water contact angle was 141.9°±2.8°, and the average wall friction resistance reduction rate was 8.2%.

[0109] Although Comparative Example 4 also forms a cross-groove and diamond-shaped protrusion array, the bottom of the groove formed by the flat-bottomed end mill is approximately flat, with an angle between the groove bottom and the groove wall. This angle location is prone to localized coating buildup or uneven coverage during dip coating, and causes significant geometric abrupt changes at the groove intersections. In Examples 1 to 4, the spherical cutting edge of the ball end mill forms an arc-shaped groove, with a continuous arc transition between the groove bottom and the groove wall, and an arc-shaped concave intersection formed at the intersection of the three-way grooves. This difference allows the low surface energy fluoropolymer coating to continuously cover the inner wall of the groove and the concave intersection, while retaining the arc-shaped groove profile after coating treatment.

[0110] The drag reduction rate test was conducted using a circulating water tank or a flat plate friction resistance testing device. The drag-reducing surface of the test sample was installed facing the incoming flow direction, ensuring the fluid flow direction was aligned with the water flow direction. The inlet flow velocity was 3.0 m / s. The wall friction resistance of the sample under steady flow conditions was recorded, using the wall friction resistance of an untreated 6061 aluminum alloy flat plate of the same size as a benchmark. The drag reduction rate was calculated using the following formula: Drag Reduction Rate = (F0 - F1) / F0 × 100%, where F0 is the wall friction resistance of the untreated 6061 aluminum alloy flat plate, and F1 is the wall friction resistance of the test sample.

[0111] The test results for each embodiment and comparative example are shown in the table below.

[0112] Table 1. Structural parameters and performance results of the embodiments and comparative examples.

[0113] As shown in Table 1, under the same conditions of using the AF1600 amorphous fluoropolymer coating, the drag reduction rates of Examples 1 to 4 are all higher than those of Comparative Example 1, which only has the coating applied. This indicates that the arc-shaped grooves and rhomboid protrusion array formed by the three-way cross ball end milling contribute to the reduction of underwater frictional resistance. Under the same conditions of having three-way arc-shaped grooves and rhomboid protrusion arrays, the drag reduction rate of Example 1 is higher than that of Comparative Example 2, which has no coating applied. This indicates that the low surface energy fluoropolymer coating and the processed microstructure work together at the underwater contact interface. Under the same conditions of applying the AF1600 amorphous fluoropolymer coating, the drag reduction rate of Example 1 is higher than that of Comparative Example 3, which only has unidirectional longitudinal grooves. This indicates that the rhomboid protrusion array defined by the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array is different from the unidirectional groove structure. Under the same conditions of forming a diamond-shaped protrusion array and applying a coating, the drag reduction rate of Example 1 is higher than that of Comparative Example 4, which is machined using a flat-bottom end mill, indicating that the arc-shaped groove cross section formed by the spherical cutting edge of the ball end mill has the effect of reducing local flow abrupt changes in the groove region.

[0114] As shown in Comparative Examples 1 and 2, both the low surface energy fluoropolymer coating and the individual three-dimensional arc-shaped grooves and rhomboid protrusion arrays can produce a certain drag reduction effect, but the drag reduction rate is lower than that of Examples 1 to 4. As shown in Comparative Example 3, even when combined with a low surface energy fluoropolymer coating, the unidirectional longitudinal grooves are difficult to achieve the drag reduction effect of the three-dimensional arc-shaped grooves jointly defining the rhomboid protrusion array. As shown in Comparative Example 4, when both cross grooves and rhomboid protrusion arrays are formed and a coating is applied, the arc-shaped grooves formed by ball end mills are superior to the flat-bottomed grooves formed by flat end mills. These results indicate that the drag reduction effect of this application is not solely contributed by the coating, unidirectional grooves, or ordinary cross grooves, but is obtained through the combined effect of the arc-shaped grooves formed by three-dimensional ball end milling, the rhomboid protrusion array arranged along the flow direction, and the low surface energy fluoropolymer coating covering the groove bottom and walls.

[0115] In other embodiments, the metal substrate can be replaced with 5052 aluminum alloy, 7075 aluminum alloy, titanium alloy, or stainless steel. When the substrate is replaced with stainless steel, the spindle speed can be set to 8000 r / min-12000 r / min, the feed rate can be set to 300 mm / min-600 mm / min, and clean compressed air at 0.20 MPa-0.30 MPa is used for cooling. The ball end mill can be a carbide ball end mill, a diamond-coated ball end mill, or a titanium nitride-coated ball end mill, and the tool radius can be selected from 0.3 mm to 1.0 mm according to the target groove size. In addition to AF1600 amorphous fluoropolymer, the low surface energy fluoropolymer can also be a fluoropolymer coating material with carbon-fluorine segments that can form a film on the surface of metal microstructures; the dip coating equipment can be replaced with a controllable speed pull-out coating machine, with the pull-out speed controlled at 3 mm / s-8 mm / s. The organic solvent used for cleaning can be anhydrous ethanol or isopropanol. When the purity of isopropanol is 99.5%, the ultrasonic cleaning time should be controlled at 10-20 minutes and the drying time at 20-40 minutes.

[0116] When the metal substrate is at least a portion of the surface of a ship's hull, underwater vehicle shell, pipe inner wall, or pump impeller, the fluid flow direction is determined based on the mainstream direction of the metal substrate under service conditions. Before machining, a machining coordinate system is established using the mainstream direction, arranging the long diagonal of the rhomboid protrusion along the mainstream direction. For curved components, the surface can be divided into multiple machining areas, and within each machining area, a three-way cross-milling process is performed using the local mainstream direction of that area as the set fluid flow direction. A 2mm-5mm transition zone can be set at the boundary of the groove array between adjacent machining areas to reduce abrupt changes between different local flow directions.

[0117] The foregoing description and accompanying drawings fully illustrate embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Individual components and functions are optional unless explicitly required, and the order of operation may vary. Parts and features of some embodiments may be included or substituted for parts and features of other embodiments. Embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from its scope. The scope of the present disclosure is limited only by the appended claims, and the foregoing embodiments should be considered exemplary and non-limiting.

Claims

1. A method for preparing a ball-end milled biomimetic rhomboid scale drag-reducing surface, characterized in that, include: Cleaning and pretreatment of the metal substrate surface; A ball end mill is used to perform three-way cross milling on the surface of the metal substrate, and the spherical cutting edge of the ball end mill forms a groove with an arc-shaped cross section on the surface of the metal substrate. The three-way cross milling process includes: forming a first oblique arc-shaped groove array by moving the tool along a first oblique direction located on one side of the set fluid flow direction and forming a first angle with the set fluid flow direction; forming a second oblique arc-shaped groove array by moving the tool along a second oblique direction located on the other side of the set fluid flow direction and forming a second angle with the set fluid flow direction; and forming a longitudinal arc-shaped groove array by moving the tool along the set fluid flow direction. The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array intersect each other, so that the area between the three sets of grooves that has not been milled away forms a rhomboid protrusion array, and the long diagonal of each rhomboid protrusion in the rhomboid protrusion array is arranged along the set fluid flow direction. A low surface energy fluoropolymer coating is applied to the surfaces of the rhomboid protrusion array, the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array to obtain the underwater drag-reducing surface.

2. The preparation method according to claim 1, characterized in that, The feed directions of the first and second oblique cutters are symmetrically arranged with respect to the set fluid flow direction.

3. The preparation method according to claim 2, characterized in that, The first included angle is equal to the second included angle, and both the first included angle and the second included angle are 30° or both are 60°.

4. The preparation method according to claim 1, characterized in that, The first oblique arc-shaped groove array and the second oblique arc-shaped groove array intersect to form a plurality of rhomboid boundaries arranged along the set fluid flow direction. The longitudinal arc-shaped groove array extends along the set fluid flow direction and passes through the area between adjacent rhomboid boundaries, so that the rhomboid protrusion array is arranged at intervals along the set fluid flow direction.

5. The preparation method according to claim 1, characterized in that, The adjacent rhomboid protrusions in the rhomboid protrusion array are defined by the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array.

6. The preparation method according to claim 1, characterized in that, Each arc-shaped groove in the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array has an arc-shaped groove bottom formed by the spherical cutting edge of the ball end mill.

7. The preparation method according to claim 1, characterized in that, The first oblique cutter, the second oblique cutter, and the cutter along the set fluid flow direction are all machined using the same ball end mill, and all three have the same depth of cut and the same line spacing.

8. The preparation method according to claim 1, characterized in that, The low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusion array, as well as the groove walls of the first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array.

9. The preparation method according to claim 8, characterized in that, The low surface energy fluoropolymer coating covers the bottom and walls of each arc-shaped groove, and forms a continuous coating layer in the groove area between adjacent rhomboid protrusions.

10. A ball-end milled biomimetic rhomboid scale drag-reducing surface, characterized in that, include: A metal substrate having a drag-reducing surface for contact with underwater fluids; The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array are all formed on the drag-reducing surface; And a low surface energy fluoropolymer coating, covering the drag-reducing surface; The first oblique arc-shaped groove array includes multiple first oblique arc-shaped grooves, each of which is inclined to a first side relative to the set fluid flow direction and forms a first angle with the set fluid flow direction. The second oblique arc-shaped groove array includes multiple second oblique arc-shaped grooves, each second oblique arc-shaped groove being inclined to a second side relative to the set fluid flow direction and forming a second angle with the set fluid flow direction; The longitudinal arc-shaped groove array includes multiple longitudinal arc-shaped grooves extending along the set fluid flow direction; The first oblique arc-shaped groove array, the second oblique arc-shaped groove array, and the longitudinal arc-shaped groove array intersect each other to define a diamond-shaped protrusion array on the drag-reducing surface; The rhomboid protrusion array includes multiple rhomboid protrusions, each of which is formed by the metal substrate area that has not been removed between the first oblique arc groove, the second oblique arc groove and the longitudinal arc groove, and the long diagonal of each rhomboid protrusion is arranged along the set fluid flow direction. The cross-sections of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove are all arc-shaped; The low surface energy fluoropolymer coating covers the top and sides of the rhomboid protrusion, and also covers the bottom and walls of the first oblique arc-shaped groove, the second oblique arc-shaped groove, and the longitudinal arc-shaped groove.

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