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Tribological characteristics of a cemented carbide friction surface with chevron pattern micro-texture based on different texture density
Journal Pre-proof Tribological characteristics of a cemented carbide friction surface with chevron pattern micro-texture based on different texture density Li Dan, Yang Xuefeng, Lu Chongyang, Cheng Jian, Wang Shouren, Wang Yanjun PII:
S0301-679X(19)30533-X
DOI:
https://doi.org/10.1016/j.triboint.2019.106016
Reference:
JTRI 106016
To appear in:
Tribology International
Received Date: 10 June 2019 Revised Date:
10 October 2019
Accepted Date: 10 October 2019
Please cite this article as: Dan L, Xuefeng Y, Chongyang L, Jian C, Shouren W, Yanjun W, Tribological characteristics of a cemented carbide friction surface with chevron pattern microtexture based on different texture density, Tribology International (2019), doi: https://doi.org/10.1016/ j.triboint.2019.106016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
The rotation direction of the upper specimen
40rpm 80rpm 120rpm 180rpm
Circular wear area
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Surface scratches on un-textured specimen
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Slight scratches on micro-textured specimen
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Un-textured
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9.5% texture density
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40rpm 80rpm 120rpm 180rpm
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Tribological characteristics of a cemented carbide friction surface with chevron pattern micro-texture based on different texture density Li Dana, Yang Xuefenga,*, Lu Chongyanga, Cheng Jianb, Wang Shourena, Wang Yanjuna a
b
College of Mechanical Engineering, University of Jinan, Jinan 250022, China College of Mechanical Engineering, Hubei University of Technology, Hubei 430068, China
Abstract: The influence of the drawing area on the tribological propert7ies was studied using femtosecond laser processing of four kinds of chevron pockets of cemented carbides with 5.05%, 9.5%, 13.02%, and 15.2% texture density. The results show that the chevron pattern texture with a certain texture density helps decrease the friction of a YT15 cemented carbide surface compared with un-textured specimens. This surface texture minimizes and stabilizes the friction coefficient, while the fluid dynamic pressure increased in the specimen with 9.5% texture density. Wear occurs due to abrasive and adhesive wear. Therefore, designing a reasonable shape and selecting appropriate texture density will be the focus of further research on surface micro-texture. Keywords: Chevron pattern micro-texture; Texture density; Tribological characteristics; Cemented carbides 1. Introduction Theoretical research and experimental research in modern tribology show that friction on a surface can be decreased through the use of some reasonable features, such as a micro-convex surface and narrow groove [1-3]. Some patterned array with certain geometric size and arrangement were prepared on sliding contact friction surfaces such as micro-holes [4], elliptical grooves [5], dimple-textured [6], nano-textures [7]. The texture can reduce the contact surface area of the friction pair and reduce adhesion. The texture can also store wear particles and lubricating oil, making it easier to form a continuous lubricating film on the
1
surface of a friction pair [8]. I.M. Hutchings et al. [9] reported that circular pockets, parallel grooves, chevron patterns can increase hydrodynamic film thickness. This reduces solid contact between the surface of the friction pair, which reduces friction and wear [10-11]. Improving the stability of the lubricating film on the working surface of a conical mold and reducing mold wear while drawing wire are important topics at present [12]. When metal passes through a conical mold cavity, there will be greater pressure and friction between the metal and the mold cavity, which will cause some wear and damage to the conical mold cavity. This will cause the lubrication film to rupture, causing mold wear and affecting surface quality of the product [13]. Therefore, increasing the thickness and stability of a lubricating film on the inner surface of a drawing mold cavity and preventing premature rupture of the lubrication film are the focus of conical mold manufacturing [14]. The working surface of a mold is often subjected to excessive pressures during the wire drawing process. The amount of friction between the inner hole and the metal wire rod, wear in the inner hole, and wear between friction pairs will impact the quality of the drawing product [15-17]. Lubrication in deformation zone and invariable zone can reduce wear and subsequent loss of the wire rod to the mold. The structural layout of cemented carbide mold is shown in the Fig. 1. When local high temperature and high pressure are generated, the extreme pressure wear resistance agent and oily agent in the lubricant can form a high-toughness lubricating film between the mold hole and the wire, separating the two friction surfaces, preventing metal wear and sintering, and greatly reducing the heat generated by friction. Compared with other technologies that are used to decrease friction through direct contact between a friction pair, surface texture treatment decreases friction on the contact surface by providing dynamic fluid 2
pressure, which provides advantages that cannot be gained with other surface treatment technologies [18]. Etsion et al. [19] reported that the presence of surface texture reduces wear on parts and extends service life compared to untextured mechanical seal rings. It has been shown that stainless steel textured surfaces exhibit 80% less friction than untextured surfaces [20]. Kovalchenko et al. [21] found that textured surfaces increase the wear at point contact at increased contact pressure, however the increased wear also increases the contact area which reduces contact pressure promoting a transition from boundary lubrication mode to the mixed lubrication with lower friction. Mitchell et al. [22] showed that a larger size of pit or columnar textured surface can provide a lower coefficient of friction during a unidirectional sliding test. Singh et al. [23] found that the preparation of micro/nano-scale texture on the surface of Si (100) can store wear particles or lubricating oil, which also decrease friction. Bearing surfaces with partial laser-processed texture increased the clearance by about three times and reduced the friction coefficient by more than 50% compared to the un-textured bearing [24]. Laser surface textured bearings are more reliable and efficient. Andreas Rosenkranz et al. [25] summarized the latest techniques applied to the surface texture of piston rings, seals, roller bearings, and gears, and proposed that geometric shapes can be placed along the stroke of the piston ring according to lubrication conditions. C. Gachot et al. [26] reported that most machines operate in different frictional states in one work cycle and multi-scale and multi-shape textures can be designed on contact surfaces to cover different feature sizes and geometries for the respective application. However, not all surface textures can reduce friction, surface defects caused by unreasonable design or machining problems can also cause severe wear on a workpiece. 3
Garrido et al. [27] used a stainless steel probe to perform a reciprocating sliding test on an NiCrBSi laser textured surface. The test results show that the textured surface exhibited decreased friction when the pit diameter is small and incorrect dimple density values lead to disruptive behavior, which arises with relatively high and low dimple ratios. To reduce friction, the relationship between density, dimple diameter, and contact area should be studied. In this paper, due to the unidirectional friction and lubrication environment of the metal wire during drawing, a chevron pattern micro-texture was designed and used in experiments, which provides more unidirectional convergence than the customary circular, rectangular, and strip-concave micro-texture. Hutchings et al. [9] reported that among circular, grooves and chevron patterns, chevron patterns pointing along the sliding direction were the most effective in increasing hydrodynamic film thickness. The influence of the texture density on tribological properties was studied using the chevron pockets with different texture density. These were fabricated using femtosecond laser processing of cemented carbides. Four samples were compared with un-textured samples using a friction and wear tester. The microstructure and morphology of the working surface were also analyzed using a scanning electron microscope (SEM). In spite of difference in temperature and loading conditions during wire drawing (huge plastic deformation of steel) and tribotests at normal temperature and evidently lower contact pressure, according to our previous studies [16] [28], this experimental condition can also partially reflect the effect of texture density on the friction coefficient under real working conditions. 2. Experimental 4
2.1. Selection of material Drawing dies often reach high temperature and exhibit severe abrasion during production. Therefore, a high temperature and wear resistant cemented carbide has significant advantages as the material for a drawing mold. Drawing dies made of cemented carbide materials have been extensively used in the market due to their high wear resistance, hardness, and compressive resistance. A YT15 cemented carbide block with 55 mm diameter and 5 mm thickness was used for friction and wear tests. As a tungsten-cobalt-titanium cemented carbide, YT15 has high thermohardening and wear resistance. Its chemical composition, density, and mechanical properties are listed in Table 1. The quenched 45 carbon steel in the form of a stepped cylinder was used as the mating sliding part, and the wear surface is annular, the outer diameter is R1 = 7.3 mm, and the inner diameter is R2 = 5 mm (88.91 mm2 wear area). 45 carbon steel is a high-quality carbon structural steel with density of 7.85g/cm3, elastic modulus of 210 GPa and Poisson's ratio of 0.269. Its chemical composition is shown in Table 2. 2.2. Selection of surface texture The main purpose of this study was to investigating the effect of the morphology of the cemented carbide disc on friction. Two critical parameters were proposed: the micro-texture shape and texture density. According to previous experimental studies, the tribological characteristics depended greatly on the shape，size and density of the micro-textures [21][29]. The influence of the chevron pockets and texture density on friction on the specimens was explored experimentally. Chevron pattern is an axisymmetric pattern, where the microcell is surrounded by inner
5
and outer equilateral triangles, as shown in Fig. 2. The chevron pattern microcell area was S1 = 0.208 mm2 and the wear zone area was S2 = 88.91 mm2. The texture density is defined as follows: S = n⋅
S1 S2
(1)
Substitution of S1 and S2 into Eq. (1) yields S = n⋅
0.208 88.91
(2)
where n is the number of chevron pattern cells. The wear zone was an annular region and the chevron pattern cells were uniformly distributed in the annular region. Table 3 shows the relationship between the number of chevron pattern microcells with unit area of 0.208 mm2 and their texture density for a given wear zone area.
2.3. Surface texture processing Laser processing was used to style the surface of the specimens. Only the surface of the cemented carbide was sculpted during processing. The frictional surface of the upper 45 steel specimens remained smooth. The surface roughness value Ra was measured to be approximately 0.04 µm. Before laser processing of the lower specimen, the specimen was ground, polished, cleaned with acetone and dried. The surface roughness value Ra of the cemented carbide original before pretreatment was measured to be approximately 0.83 µm. A surface roughness tester (Mitutoyo SJ-410) was used to measure the roughness at three different positions on the surface of a polished specimen. The surface roughness values Ra were 0.043 µm, 0.04 µm, and 0.04 µm. A femtosecond laser (Libra-HE) was used to engrave the cemented carbide surface to a 6
processing depth of 32 µm. The processing parameters are listed in Table 4. Engraving was performed at 10 kHz pulse frequency. Each micro-texture unit was scanned and sculpted 13 times at low power. In this way, the micro-texture unit contained a clear contour with high dimensional accuracy. The micro-texture dimensions of the surface were measured using a Bruker meter while processing each specimen. Meanwhile, the dimensions of the engraved image were controlled by adjusting the parameters of the femtosecond laser. Chevron pattern profiles meeting the size requirements were finally obtained after repeated measurements and machining of the micro-texture on the surface of the specimen.
2.4. Microstructure of specimen After preparing the specimens with chevron pockets, the surface of the sample was cleaned ultrasonically and was microscopically inspected with an optical microscope (DINO-LITE) and a digital microscope with an ultra-high depth of field (VHX-2000). The high depth of field images of the specimens included physical images that were magnified by a factor 500 and a 3D display of the micro-texture, as shown in Fig. 3. The surface roughness value of the specimens was very small, which is beneficial to study the influence of micro-texture on the surface friction characteristics.
2.5. Friction test equipment Drawing Steel 45 wire with cemented carbide drawing dies is the background of this study. The test was carried out using a friction and wear tester (MMG-10, Jinan Hansen Precision Instrument Co., Ltd.). In this experiment, a quenched 45 carbon steel in the form of a stepped cylinder with a toroidal friction surface was used as a mating sliding part. During the test, the Steel 45 followed the main shaft with rotary friction on the surface of the 7
cemented carbide. Sliding friction was present during motion. The setup layout and schematic diagram of experimental apparatus of MMG-10 experimental machine are shown in Fig. 4. 23 ml of Mobil 1 0W-40 lubricant with a viscosity of > 76.4 cSt @ 40 °C [ASTM D 445] was used for tribological experiments. Frictional response was investigated for various textured specimens while varying the pockets density, applied load and rotational speed. The distribution of the chevron pockets on the wear-resistant area of the cemented carbide surface is shown in Fig. 5. During wear, the mating sliding part rotated around the principal axis in a clockwise direction, which is consistent with the convergence direction of the chevron pockets. The equipment was used to evaluate friction on the lubricated surfaces by recording the instantaneous load and the friction force on the surface in real time. The main experimental parameters used in the friction and wear tester are shown in Table 5.
3. Results and Discussion 3.1. Tribological tests for non-textured and polished parts The influence of the chevron pattern texture on friction was investigated by comparing friction on the non-textured and polished areas of the lubricated cemented carbide surface. The rotation speed was kept at 120 rpm to explore differences in friction on the two surfaces under different test forces, as shown in Figs. 6 and 7. Figs. 6 and 7 show that the friction coefficient on the polished interface fluctuates between 0.14 and 0.18 under different bearing capacities and the overall trends are relatively similar. The friction coefficients on the unpolished and polished surfaces are approximately 0.07 and 0.17, respectively, at 120 rpm rotation speed and 180 N load. The main reason for the large difference between the two is that the unpolished specimen has a higher surface 8
roughness than the polished specimen. There is a large number of irregular microscopic humps, troughs, and other features on the surface. These structures exert a certain lubrication effect due to dynamic fluid pressure, thus the unpolished interface exhibits a lower friction coefficient. At 100 and 150 N loads, the friction coefficient of the unpolished interface was greater than 0.1, while the friction coefficient ranges from 0.07 to 0.09 when the load was 180 or 200 N. This occurs because the micro-morphology provides greater positive pressure on the lubricating film at a larger load.
3.2. Tribological study on chevron pattern micro-texture surface In this test, unpolished specimens and chevron pattern micro-textured specimens with 5.05%, 9.5%, 13.02%, and 15.2% texture density were chosen to study tribological characteristics on the micro pattern. 50, 100, 200, and 300 N test loads were applied while rotating at 40, 80, 120, and 180 rpm during the experiment, respectively. By changing the speed and load, the effect of changes in the texture density of the chevron pattern on friction could be studied. Figs. 8 and 9 show friction measurements from un-textured specimens and chevron pattern textured specimens with distinct texture density, respectively, with changes in test force and rotational speed. Fig. 8 shows friction measurements on (a) the unpolished sample and (b) the polished sample in different experimental conditions. Fig. 8 (a) shows that the unpolished sample exhibits large fluctuations in the friction coefficient in different experimental conditions. The surface friction coefficient exhibits larger fluctuations at each rotation speed at 50 N load. In contrast, the friction coefficient of the polished sample in different experimental conditions exhibits smaller fluctuations. Overall, the friction coefficient on the unpolished sample fluctuates between 0.08 and 0.26 in different experimental conditions. The polished sample 9
has similar friction coefficient in different experimental conditions. Changes in the friction coefficient were large because the unpolished sample had high surface roughness. Therefore, chevron pockets were prepared on the polished sample to further investigate the influence of the chevron pattern micro-texture on friction. The experimental error caused by high surface roughness was lowered to some extent. Fig. 9 (a) shows that the friction coefficient of the specimen decreases as the load increases at various fixed rotation speeds. The surface friction coefficient of the specimen is generally small (approximately 0.04) and remains stable at 300 N load. The friction coefficient on the surface of the specimen decreases obviously as the rotation speed increases at 50 N load. Meanwhile, the friction coefficient becomes more stable with larger loads. Fig. 9 (b) shows that the friction coefficient on the surface of the specimen decreases significantly when the load reaches 200 N, regardless of the rotation speed. The friction coefficients corresponding to F = 200 and 300 N loads are similar and reach minimum at 180 rpm. Under any load, the friction coefficient of the specimen decreases as the rotation speed increases. When the rotation speed is between 40 and 120 rpm, the friction coefficient decreases as the load increases. For the chevron pockets with 13.02% texture density, an excessive number of micro-texture units increase the surface roughness of the specimens. Fig. 9 (c) shows that the friction coefficient on the surface of this specimen slightly increases as the load increases at 180 rpm, which contradicts the results from most other surfaces, where the friction coefficient decreases as the load increases. Considering the measurement error in the test and the accuracy of the equipment itself, it seems that the friction coefficient stabilizes to approximately 0.075 at 180 rpm. Fig. 9 (d) shows that the friction coefficient on the chevron 10
pattern micro-textured specimens with 15.02% texture density is generally large and its minimum value reaches 0.1, where excessive concentration of topography is the main reason for the large friction coefficient. Similar to the specimen with 13.02% texture density, the high density of micro pits increases the roughness of the surface. The effect of dynamic fluid pressure and cavitation on friction coefficient reduction is not evident at surface high roughness. The following conclusions can be drawn by analyzing the surface friction characteristics of chevron pattern micro-textured specimens with four different texture density. The friction coefficient of the chevron pattern micro-textured specimens with 15.02% texture density is generally higher than in other specimens. 72 micro-textured elements are available over a circle with 6.15 mm radius, thus the spacing between each element is very small, resulting in a sharp increase in the surface roughness. Although a high texture density means that a larger number of micro pits can provide hydrodynamic pressure, the positive pressure generated by these units and the load provided by the lubricating film are limited. Therefore, roughness is still an important factor affecting the surface friction coefficient. Meanwhile, excessive reduction in the microcell spacing will also cause fluid to flow rapidly from the positive pressure zone to the negative pressure zone in the next unit. From the perspective of the Bernoulli effect, the internal pressure will be smaller when the fluid flow rate is higher and the load on the lubrication film will be lower. A chevron pattern micro-textured sample with a 9.5% texture density exhibited lower friction during testing. The lowest friction coefficient is about 0.02, which occurs under load conditions of 300 N and speed of 120 rpm and 180 rpm. The combined effect of rotation speed and load on the friction coefficient of the chevron pattern with a 9.5% texture density 11
is shown in the Fig.10. The load and the rotation speed have a significant influence on the friction coefficient of the test piece. As the load and speed increase, the coefficient of friction of the surface of the test piece continues to decrease. The maximum friction coefficient is only 0.1, and the difference from the minimum is only 0.08. The area containing the minimum coefficient of friction value remains relatively stable. A 9.5% chevrons texture density can be considered as the optimal density pattern, which works in conjunction with the hydrodynamic pressure generated by the micro-cells to minimize friction. The surface friction coefficient on the specimens with 5.05% and 13.02% texture density were high. The friction coefficients for these specimens become unstable as the working conditions change. In particular, the specimen with 5.05% texture density has a relatively high friction coefficient at low load. The dynamic fluid pressure is insufficient if there are too few surface micro-pits, thus decreasing the load in the fluid.
3.3. Pressure distribution in chevron patterns Fluid simulation analysis of the chevron pattern micro-cell texture was performed using Fluent 12.0 software. Considering the flow of fluid through the micro-pits, there is not only a slid in the velocity direction but also a pulsation in the vertical direction of the flow velocity between adjacent fluid layers, so the fluid type was set to turbulent flow. The standard k-ɛ model was applied, the calculation equations for the relevant parameters in the model are as follows:
ρ
µt ∂k dk ∂ = ( µ + ) + Gk + Gb − ρε − YM σ k ∂xi dt ∂xi
dε ∂ µt ∂ε ε ε2 ρ = ( µ + ) + C1ε (Gk + C3ε Gb ) − C2ε ρ dt ∂xi k k σ ε ∂xi 12
（3 ）
（4 ）
Where: ρ and t are density and time, respectively. Gk and Gb are the turbulent flow energy affected by velocity and buoyancy, respectively. µt is the viscosity coefficient, YM is the extent to which the dissipation rate is affected by pulsation expansion. The values of the constants C1ɛ, C2ɛ and C3ɛ are 1.44, 1.92 and 0.09, respectively. σk and σɛ are the Prandtl numbers of the turbulent flow energy k and the dissipation rate ɛ. Micro-unit modeling was performed using SolidWorks 2014 software, and then imported into ICEMCFD to "automatically divide" the grid. The model was introduced into Fluent and a new fluid was defined in the material property settings. The fluid density and viscosity were now set to 895 kg/m3 and 0.045 Pa·s, respectively. The fluid type was set to turbulent and the standard k-ɛ model was selected. The pressure inlet boundary and the pressure outlet boundary condition were selected, the loading force was 300 N, the wear zone area was 88.91 mm2, the fluid operation reference pressure was about 3.376 MPa. The pressure at the pressure outlet boundary was set to atmospheric pressure (101,325 Pa). The texture pressure distribution and flow velocity vector distribution of the texture unit are shown in Fig. 11. It can be seen from the cross-sectional pressure distribution of Fig. 11 (a) that a significant pressure gradient is produced along the direction of texture convergence and a high positive pressure occurs at the sharp corners of the right side. From the pressure distribution of the longitudinal section of Fig. 11 (b), the section passes through the apex angle of the texture unit and the midpoint of the bottom edge. Significant backflow occurs inside the pit and the outlet portion has a large positive pressure, while the pressure near the inlet is lower than the outlet and produces a significant negative pressure. It can be seen from the longitudinal section flow velocity diagram of Fig. 11 (c) that the fluid flow rate near the 13
outlet is faster than the fluid flow velocity near the inlet, and the entire flow direction is usually distributed clockwise, which promotes the generation of backflow in the pit. As the fluid flows through the surface of the groove microtextured unit with unidirectional convergence, a certain pressure gradient tends to occur in the direction of convergence. The inside of the micro-pits is often accompanied by the reflow phenomenon and produces a certain negative pressure value. The convection effect formed by the convergence of the texture also promotes the generation of backflow inside the pit to some extent. As shown in Fig. 11 (d), when lubricant flows from the inlet to the outlet, the flow per unit length gradually decreases along the direction of fluid flow. The texture is axially convergent and the average pressure at the outlet is greater than the pressure at the inlet. This pressure distribution reduces inflow at the inlet and increases outflow at the outlet. Hydrodynamic lubrication is often accompanied by cavitation. The fluid velocity gradient changes dramatically at the end of the diverging wedge. The internal pressure drops rapidly, producing negative pressure. Vacuoles will form once the negative pressure drops below a critical value. Cavitation will suppress generation of negative pressure. Daniel Gropper et al. [30] reviewed a number of different mass-conserving cavitation algorithms. Cavitation may occur not only globally in divergent contact regions, but also in local of individual pits or in-between asperities. Positive pressure on the lubricant film arises when lubricant flows out of the converging wedge port. This asymmetric pressure gives the lubricating film a certain load carrying capacity. Therefore, every tiny shape can be regarded as a lubricated bearing under dynamic pressure. The dynamic pressure generated by the fluid can separate friction pair surfaces, resulting in a lower friction coefficient and reducing wear. 14
3.4. Wear mechanism analysis of texture working surface During mutual wear of the two working surfaces in the friction pair, micro-scratches, various damage pits, and adhesive protrusions on the surface of the specimens may cause damage to the surface micro-texture. Therefore, the micro-texture should be designed while considering whether it can effectively reduce wear between friction pairs. Design aspects include the length of the furrow, number of exfoliation pits caused by free particles, adhesion of wear debris during shear fracture at the adhesive point, and the degree of cavitation at the micro-texture edge. First, the cemented carbide sample after the friction coefficient test was subjected to wire cutting to facilitate observation with a scanning electron microscope. Subsequently, the sample was cleaned ultrasonically for 30 minutes in a beaker containing acetone to remove surface oil. After drying, the surface wear scar on each sample was examined using an SEM. Figs. 12 and 13 show the wear tracks and worn surfaces of the untextured specimen and the chevron pattern textured specimen with 9.5% texture density at 300 N load and 180 rpm rotation speed, where the wear time reached 2100 seconds. Fig. 12 (a) and (b) show the wear tracks and worn surfaces of the non-textured specimen without polishing. Panels (c) and (d) show the wear tracks and worn surfaces of the polished specimen. The wear scar on the surface of the specimens is apparent in Fig. 12 (a). After magnifying the SEM to a factor 500, obvious scratches were observed on the surface of the non-textured specimens and some micro pits are present near the narrow part of the furrow mark (Fig. 12(b)). Because the unstable solid particles on the surface of the unpolished non-textured specimens easily detach due to friction and hard protrusions on the surface of the friction pair act on the surface of the specimens, these will cause micro-cutting and 15
furrowing on the surface of the specimens. This produces the shedding pit and deep groove shown in the figure. After magnifying the worn polished part to a factor 500, one can see that there are shallow scratches on the surface, as shown in Fig. 12 (c). After magnifying to a factor 1500, as shown in Fig. 12 (d), it is apparent that these shallow scratches form due to an accumulation of discontinuous tiny pits. The unstable particles on the surface of the cemented carbide specimen were removed during polishing and particulate impurities from the friction pairs were effectively reduced. Therefore, the degree of wear during the friction test was lower than that on the unpolished parts. Fig. 13 shows the morphology in the surface wear of a chevron pattern. One can see in Fig. 13 (a)-(c) that the surface of the specimens is much less worn than the non-textured cemented carbide specimens. Fig. 13 (b) and (c) show that there are slight scratches and fine furrows on the surface consisting of shedding pits. The presence of wear debris in a contact accelerates wear and protection afforded by wear debris against adhesion is greater than the damage caused by their presence [31]. Additional mechanisms for texture reduce wear may be debris trapping and replenishing lubricant and reducing preventing starvation [32]. Abrasive particles between the friction pairs peel away from the brittle material on the surface of the specimens during extrusion and the exfoliated abrasive will produce a micro-cutting effect on the adjacent surface at the appropriate angle, leaving a furrow. Meanwhile, when each portion of the friction pair is in relative motion, individual contacts on the surface of the specimens will bond and solder. As shown in Fig. 13(d), there are white spots on carbide surface. Elemental analysis of the white point area is shown in Fig. 13 (j). Element Fe can be detected in the area, confirming that this is transferred steel on carbide 16
surface. Because the mating sliding part is quenched 45 carbon steel, the lower specimen is a cemented carbide block and the softer material falls off and migrates to carbide surface when these bonding points are cut. These adhesives are applied to the surface of the specimens due to friction in the pair and the adhesive area increases in size under high load. This process is often associated with heating due to friction. Gas is produced in the fluid, which then collapses in the high-pressure region, causing cavitation pits near the edge of the micro-texture. The fluid pressure in the experiment was insufficient to cause obvious cavitation. However, only small pits formed locally on the surface and large spongy pits did not appear, as shown in the edges of the micro-texture in Figs. 13 (e) and (f). Therefore, the wear mechanism on the surface of the chevron pattern specimens arises due to abrasive wear and adhesive wear.
4. Conclusions Friction on specimens with chevron pockets at four kinds of texture density was examined. The surface friction coefficient was found to be minimum and stable, while the fluid dynamic pressure increased in the specimen with 9.5% texture density. Too few surface micro-texture units often cannot provide sufficient dynamic fluid pressure, while an excessive number of micro-texture units increase the surface roughness of the specimens. The load does not individually affect the friction properties of the test piece. There are one or more sets of load-speed combinations that enable the textured surface to achieve a minimum friction coefficient. The chevron pattern test piece with 9.5% texture density has two sets of load-speed combinations which enable the test piece to reach a minimum friction coefficient value of 0.02. The wear mechanism on the surface of the chevron pattern specimens arises due to 17
abrasive wear and adhesive wear. Protection afforded by wear debris against adhesion is greater than the damage caused by their presence. Additional mechanisms for texture reduce wear may be debris trapping and replenishing lubricant.
Acknowledgments This work is supported by the National Natural Science Foundation of China (grant No. 51575234,51872122), the Postdoctoral Science Foundation of China [grant No. 2017M620286],
the Key Research and Development
Program of Shandong Province, China [grant No. 2018CXGC0809], the Agricultural Machinery Equipment Research and Development Innovation Plan of Shandong Province [grant No. 2018YF012], and Experts from Taishan Scholars of Shandong Province. *Corresponding author, Xuefeng Yang, major in tribology, email [email protected]
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[16] Yang XF, Ze XB, Wang HY, Wang H. Wear properties and microstructures of alumina matrix composite ceramics used for drawing dies. CERAM. INT 2009;35(8):3495-3502. [17] Lowrie J, Ngaile G. Analytical Modeling of Hydrodynamic Lubrication in a Multiple-reduction Drawing Die. Procedia Manufacturing 2016;5:707-723. [18] Gualtieri E, Borghi A, Calabri L, Pugno N, Valeri S. Increasing nanohardness and reducing friction of nitride steel by laser surface texturing. TRIBOL. INT 2009;42(5):699-705. [19] Etsion I. Improving Tribological Performance of Mechanical Components by Laser Surface Texturing. TRIBOL. LETT 2004;17(4):733-737. [20] Ramesh A, Akram W, Surya PM, Andrew HC. Friction characteristics of microtextured surfaces under mixed and hydrodynamic lubrication. TRIBOL. INT 2013;57(4):170-176. [21] Kovalchenko A, Ajayi O, Erdemir A, Fenske G. Friction and wear behavior of laser textured surface under lubricated initial point contact. WEAR 2011;271(9-10):1719-1725. [22] Mitchell N, Eljach C, Lodge B, Sharp JL, DesJardins JD, Kennedy MS. Single and reciprocal friction testing of micropatterned surfaces for orthopedic device design. J. MECH. BEHAV. BIOMED 2012;7(none):106-115. [23] Singh RA, Yoon E. Friction of chemically and topographically modified Si (100) surfaces. WEAR 2007;263(7):912-919. [24] Etsion I, Halperin G, Barizmer V, Kligerman Y. Experimental Investigation of Laser Surface Textured Parallel Thrust Bearings. TRIBOL. LETT 2004;17(2):295-300. [25] Rosenkranz A, Grützmacher P G, Gachot C, Costa H L. Surface Texturing in Machine Elements -A Critical Discussion for Rolling and Sliding Contacts. Adv. Eng. Mater 2019;1900194. [26] Gachot C, Rosenkranz A, Hsu S M, Cost H L. A critical assessment of surface texturing for friction and
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wear improvement. WEAR 2017;372:21-41. [27] Garrido AH, González R, Cadenas M, Battez AH. Tribological behavior of laser-textured NiCrBSi coatings. WEAR 2011;271(5):925-933. [28] Yang XF, Deng JX, Wang H, Ze XB. Wear behaviors and lubrication medium of TiC/Al2O3 ceramic wire drawing dies. Trans. Nonferrous Met. Soc. China 17(2007)s663-s666. [29] Wakuda M, Yamauchi Y, Kanzaki S, Yasuda Y. Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact. WEAR 2003;254(3):356-363. [30] Daniel Gropper, Ling Wang, Terry J. Harvey. Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings. Tribol. Int 2016;94:509-529. [31] Varenberg M, Halperin G, Etsion I. Different aspects of the role of wear debris in fretting wear. WEAR 2002;252:902-910. [32] Vlădescu S, Olver A V, Pegg I G, Reddyhoff T. Combined friction and wear reduction in a reciprocating contact through laser surface texturing. WEAR 2016;358:51-61.
21
Cemented carbide mold Moving direction of wire
Exit zone
Entrance zone
Invariable zone
Deformation zone Fig.1 The structural layout of cemented carbide mold
Table 1 Performance parameters of YT15 cemented carbide Composition
Component
Density
Hardness
Bending
content(%)
(g/cm3)
(HRA)
strength(MPa)
11-12.7
91
1150
WC
79
TiC
15
Co
6
Table 2 The element content of 45 steel Element
C
Si
Mn
P
S
Cr
Ni
Cu
Content(%)
0.42-0.5
0.17-0.37
0.5-0.8
≤0.035
≤0.035
≤0.25
≤0.25
≤0.25
Table 3 Relationship between the number of chevron pattern units and texture density Chevron pattern unit The number of units(n)
24
45
60
72
Texture density(%)
5.05
9.5
13.02
15.2
Table 4 Processing parameters of Libra-HE femtosecond laser The laser
Pulse width
Pulse
Average power
wavelength(nm)
(fs)
frequency(kHz)
(W)
800
100
1-10
4
Number of scanning 13
Table 5 Technical index of MMG-10 friction wear tester Technical
Axial experimental
Maximum friction
Rotation speed
indicators
force(N)
moment(N·m)
range(rpm)
10-10000
5
0.1-2000
Parameter values
Feeding speed(N/s)
1-200
Fig.2 Dimensions of the chevron pattern
200 µm
50 µm
200 µm
（a）Chevron pattern with 45 units （b）Physical image photography （c）Chevron pattern with 24 units
（d）3D display of micro-texture composition
Fig.3 The actual figure of the chevron patterns in the DINO-LITE(a), (c) and the image (b), (d) of the ultra-depth-of-field digital microscope
Drive motor
Rotating tong Steel 45 Lubricating oil pool
Lifting shaft
Torque sensor
Hydraulic cylinder
Fig.4 Setup layout and schematic diagram of experimental apparatus of MMG-10 experimental machine
Fig.5 The distribution of chevron patterns in the wear zone
0.22 100N 150N 180N 200N
0.20
Friction coefficient
0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0
100
200
300
400
500
600
Text time(s)
Fig.6 Friction curve of unpolished cemented carbide
0.22
100N 150N 180N 200N
Friction coefficient
0.20
0.18
0.16
0.14
0.12
0.10 0
100
200
300
400
500
600
Text time(s)
Fig.7 Friction curve of polished cemented carbide
0.26
0.26
40rpm 80rpm 120rpm 180rpm
0.22
Friction coefficient
0.20
40rpm 80rpm 120rpm 180rpm
0.24 0.22 0.20
Friction coefficient
0.24
0.18 0.16 0.14 0.12 0.10 0.08
0.18 0.16 0.14 0.12 0.10 0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00 0
50
100
150
200
250
Load (N)
(a) the unpolished sample
300
350
0
50
100
150
200
250
300
350
Load (N)
(b) the polished sample
Fig.8 Friction coefficient of (a) the unpolished sample and (b) the polished sample in different experimental conditions
0.26
0.26
40rpm 80rpm 120rpm 180rpm
0.22
Friction coefficient
0.20
40rpm 80rpm 120rpm 180rpm
0.24 0.22 0.20
Friction coefficient
0.24
0.18 0.16 0.14 0.12 0.10 0.08
0.18 0.16 0.14 0.12 0.10 0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00 0
50
100
150
200
250
300
350
0
50
100
150
Load (N)
(a) the sample with 5.05% texture density
250
300
350
(b) the sample with 9.5% texture density
0.26
0.26
40rpm 80rpm 120rpm 180rpm
0.22 0.20
40rpm 80rpm 120rpm 180rpm
0.24 0.22 0.20
Friction coefficient
0.24
Friction coefficient
200
Load (N)
0.18 0.16 0.14 0.12 0.10 0.08
0.18 0.16 0.14 0.12 0.10 0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00 0
50
100
150
200
250
300
350
Load (N)
(c) the sample with 13.02% texture density
0
50
100
150
200
250
300
350
Load (N)
(d) the sample with 15.2% texture density
Fig.9 Friction coefficient of different texture density samples with changes in test force and speed
Fig.10 The combined effect of rotation speed and load on the friction coefficient of a chevron pattern with 9.5% texture density
(a) Cross sectional pressure distribution
(b) Longitudinal section pressure distribution
(c) Longitudinal section velocity vector distribution
(d) fluid dynamic pressure diagram
Fig.11 The texture pressure distribution and flow velocity vector distribution of the texture unit
200 µm (a) Wear tracks of unpolished test pieces
20 µm (b) Surface scratches on unpolished specimens
10 µm (c) Surface scratches on polished specimens
10 µm (d) Tiny pits on the surface of polished specimens
Fig.12 Wear tracks and worn surfaces of untextured specimens
10 µm
10 µm
(b) Tiny furrows
(a) Surface wear morphology
10 µm (c) Slight scratches
100 µm (d) Bonding phenomenon
20 µm (e) Tiny pits
10 µm (f) Local pits
(j) Elemental analysis Fig.13 Surface abrasion of chevron pattern micro-texture specimen surface and elemental analysis of white spots
Highlights:
Chevron pattern micro texture can improve the friction performance of hard alloy. Chevron pattern micro texture has hydrodynamic effect on friction surface with oil. 9.5% texture density has better hydrodynamic effect. The main wear form is abrasive wear and adhesive wear.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0301-679X(19)30533-X
DOI:
https://doi.org/10.1016/j.triboint.2019.106016
Reference:
JTRI 106016
To appear in:
Tribology International
Received Date: 10 June 2019 Revised Date:
10 October 2019
Accepted Date: 10 October 2019
Please cite this article as: Dan L, Xuefeng Y, Chongyang L, Jian C, Shouren W, Yanjun W, Tribological characteristics of a cemented carbide friction surface with chevron pattern microtexture based on different texture density, Tribology International (2019), doi: https://doi.org/10.1016/ j.triboint.2019.106016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
The rotation direction of the upper specimen
40rpm 80rpm 120rpm 180rpm
Circular wear area
Scratch
Shedding pits
Friction coefficient
0.24 0.20 0.16 0.12 0.08
0.00
Surface scratches on un-textured specimen
0
50
100 150 200 250 300 350
Load (N) 0.24
Slight scratch
Slight scratches on micro-textured specimen
Friction coefficient
Chevron pattern micro-texture
Un-textured
0.04
9.5% texture density
0.20 0.16
40rpm 80rpm 120rpm 180rpm
0.12 0.08 0.04 0.00
0
50
100 150 200 250 300 350
Load (N)
Tribological characteristics of a cemented carbide friction surface with chevron pattern micro-texture based on different texture density Li Dana, Yang Xuefenga,*, Lu Chongyanga, Cheng Jianb, Wang Shourena, Wang Yanjuna a
b
College of Mechanical Engineering, University of Jinan, Jinan 250022, China College of Mechanical Engineering, Hubei University of Technology, Hubei 430068, China
Abstract: The influence of the drawing area on the tribological propert7ies was studied using femtosecond laser processing of four kinds of chevron pockets of cemented carbides with 5.05%, 9.5%, 13.02%, and 15.2% texture density. The results show that the chevron pattern texture with a certain texture density helps decrease the friction of a YT15 cemented carbide surface compared with un-textured specimens. This surface texture minimizes and stabilizes the friction coefficient, while the fluid dynamic pressure increased in the specimen with 9.5% texture density. Wear occurs due to abrasive and adhesive wear. Therefore, designing a reasonable shape and selecting appropriate texture density will be the focus of further research on surface micro-texture. Keywords: Chevron pattern micro-texture; Texture density; Tribological characteristics; Cemented carbides 1. Introduction Theoretical research and experimental research in modern tribology show that friction on a surface can be decreased through the use of some reasonable features, such as a micro-convex surface and narrow groove [1-3]. Some patterned array with certain geometric size and arrangement were prepared on sliding contact friction surfaces such as micro-holes [4], elliptical grooves [5], dimple-textured [6], nano-textures [7]. The texture can reduce the contact surface area of the friction pair and reduce adhesion. The texture can also store wear particles and lubricating oil, making it easier to form a continuous lubricating film on the
1
surface of a friction pair [8]. I.M. Hutchings et al. [9] reported that circular pockets, parallel grooves, chevron patterns can increase hydrodynamic film thickness. This reduces solid contact between the surface of the friction pair, which reduces friction and wear [10-11]. Improving the stability of the lubricating film on the working surface of a conical mold and reducing mold wear while drawing wire are important topics at present [12]. When metal passes through a conical mold cavity, there will be greater pressure and friction between the metal and the mold cavity, which will cause some wear and damage to the conical mold cavity. This will cause the lubrication film to rupture, causing mold wear and affecting surface quality of the product [13]. Therefore, increasing the thickness and stability of a lubricating film on the inner surface of a drawing mold cavity and preventing premature rupture of the lubrication film are the focus of conical mold manufacturing [14]. The working surface of a mold is often subjected to excessive pressures during the wire drawing process. The amount of friction between the inner hole and the metal wire rod, wear in the inner hole, and wear between friction pairs will impact the quality of the drawing product [15-17]. Lubrication in deformation zone and invariable zone can reduce wear and subsequent loss of the wire rod to the mold. The structural layout of cemented carbide mold is shown in the Fig. 1. When local high temperature and high pressure are generated, the extreme pressure wear resistance agent and oily agent in the lubricant can form a high-toughness lubricating film between the mold hole and the wire, separating the two friction surfaces, preventing metal wear and sintering, and greatly reducing the heat generated by friction. Compared with other technologies that are used to decrease friction through direct contact between a friction pair, surface texture treatment decreases friction on the contact surface by providing dynamic fluid 2
pressure, which provides advantages that cannot be gained with other surface treatment technologies [18]. Etsion et al. [19] reported that the presence of surface texture reduces wear on parts and extends service life compared to untextured mechanical seal rings. It has been shown that stainless steel textured surfaces exhibit 80% less friction than untextured surfaces [20]. Kovalchenko et al. [21] found that textured surfaces increase the wear at point contact at increased contact pressure, however the increased wear also increases the contact area which reduces contact pressure promoting a transition from boundary lubrication mode to the mixed lubrication with lower friction. Mitchell et al. [22] showed that a larger size of pit or columnar textured surface can provide a lower coefficient of friction during a unidirectional sliding test. Singh et al. [23] found that the preparation of micro/nano-scale texture on the surface of Si (100) can store wear particles or lubricating oil, which also decrease friction. Bearing surfaces with partial laser-processed texture increased the clearance by about three times and reduced the friction coefficient by more than 50% compared to the un-textured bearing [24]. Laser surface textured bearings are more reliable and efficient. Andreas Rosenkranz et al. [25] summarized the latest techniques applied to the surface texture of piston rings, seals, roller bearings, and gears, and proposed that geometric shapes can be placed along the stroke of the piston ring according to lubrication conditions. C. Gachot et al. [26] reported that most machines operate in different frictional states in one work cycle and multi-scale and multi-shape textures can be designed on contact surfaces to cover different feature sizes and geometries for the respective application. However, not all surface textures can reduce friction, surface defects caused by unreasonable design or machining problems can also cause severe wear on a workpiece. 3
Garrido et al. [27] used a stainless steel probe to perform a reciprocating sliding test on an NiCrBSi laser textured surface. The test results show that the textured surface exhibited decreased friction when the pit diameter is small and incorrect dimple density values lead to disruptive behavior, which arises with relatively high and low dimple ratios. To reduce friction, the relationship between density, dimple diameter, and contact area should be studied. In this paper, due to the unidirectional friction and lubrication environment of the metal wire during drawing, a chevron pattern micro-texture was designed and used in experiments, which provides more unidirectional convergence than the customary circular, rectangular, and strip-concave micro-texture. Hutchings et al. [9] reported that among circular, grooves and chevron patterns, chevron patterns pointing along the sliding direction were the most effective in increasing hydrodynamic film thickness. The influence of the texture density on tribological properties was studied using the chevron pockets with different texture density. These were fabricated using femtosecond laser processing of cemented carbides. Four samples were compared with un-textured samples using a friction and wear tester. The microstructure and morphology of the working surface were also analyzed using a scanning electron microscope (SEM). In spite of difference in temperature and loading conditions during wire drawing (huge plastic deformation of steel) and tribotests at normal temperature and evidently lower contact pressure, according to our previous studies [16] [28], this experimental condition can also partially reflect the effect of texture density on the friction coefficient under real working conditions. 2. Experimental 4
2.1. Selection of material Drawing dies often reach high temperature and exhibit severe abrasion during production. Therefore, a high temperature and wear resistant cemented carbide has significant advantages as the material for a drawing mold. Drawing dies made of cemented carbide materials have been extensively used in the market due to their high wear resistance, hardness, and compressive resistance. A YT15 cemented carbide block with 55 mm diameter and 5 mm thickness was used for friction and wear tests. As a tungsten-cobalt-titanium cemented carbide, YT15 has high thermohardening and wear resistance. Its chemical composition, density, and mechanical properties are listed in Table 1. The quenched 45 carbon steel in the form of a stepped cylinder was used as the mating sliding part, and the wear surface is annular, the outer diameter is R1 = 7.3 mm, and the inner diameter is R2 = 5 mm (88.91 mm2 wear area). 45 carbon steel is a high-quality carbon structural steel with density of 7.85g/cm3, elastic modulus of 210 GPa and Poisson's ratio of 0.269. Its chemical composition is shown in Table 2. 2.2. Selection of surface texture The main purpose of this study was to investigating the effect of the morphology of the cemented carbide disc on friction. Two critical parameters were proposed: the micro-texture shape and texture density. According to previous experimental studies, the tribological characteristics depended greatly on the shape，size and density of the micro-textures [21][29]. The influence of the chevron pockets and texture density on friction on the specimens was explored experimentally. Chevron pattern is an axisymmetric pattern, where the microcell is surrounded by inner
5
and outer equilateral triangles, as shown in Fig. 2. The chevron pattern microcell area was S1 = 0.208 mm2 and the wear zone area was S2 = 88.91 mm2. The texture density is defined as follows: S = n⋅
S1 S2
(1)
Substitution of S1 and S2 into Eq. (1) yields S = n⋅
0.208 88.91
(2)
where n is the number of chevron pattern cells. The wear zone was an annular region and the chevron pattern cells were uniformly distributed in the annular region. Table 3 shows the relationship between the number of chevron pattern microcells with unit area of 0.208 mm2 and their texture density for a given wear zone area.
2.3. Surface texture processing Laser processing was used to style the surface of the specimens. Only the surface of the cemented carbide was sculpted during processing. The frictional surface of the upper 45 steel specimens remained smooth. The surface roughness value Ra was measured to be approximately 0.04 µm. Before laser processing of the lower specimen, the specimen was ground, polished, cleaned with acetone and dried. The surface roughness value Ra of the cemented carbide original before pretreatment was measured to be approximately 0.83 µm. A surface roughness tester (Mitutoyo SJ-410) was used to measure the roughness at three different positions on the surface of a polished specimen. The surface roughness values Ra were 0.043 µm, 0.04 µm, and 0.04 µm. A femtosecond laser (Libra-HE) was used to engrave the cemented carbide surface to a 6
processing depth of 32 µm. The processing parameters are listed in Table 4. Engraving was performed at 10 kHz pulse frequency. Each micro-texture unit was scanned and sculpted 13 times at low power. In this way, the micro-texture unit contained a clear contour with high dimensional accuracy. The micro-texture dimensions of the surface were measured using a Bruker meter while processing each specimen. Meanwhile, the dimensions of the engraved image were controlled by adjusting the parameters of the femtosecond laser. Chevron pattern profiles meeting the size requirements were finally obtained after repeated measurements and machining of the micro-texture on the surface of the specimen.
2.4. Microstructure of specimen After preparing the specimens with chevron pockets, the surface of the sample was cleaned ultrasonically and was microscopically inspected with an optical microscope (DINO-LITE) and a digital microscope with an ultra-high depth of field (VHX-2000). The high depth of field images of the specimens included physical images that were magnified by a factor 500 and a 3D display of the micro-texture, as shown in Fig. 3. The surface roughness value of the specimens was very small, which is beneficial to study the influence of micro-texture on the surface friction characteristics.
2.5. Friction test equipment Drawing Steel 45 wire with cemented carbide drawing dies is the background of this study. The test was carried out using a friction and wear tester (MMG-10, Jinan Hansen Precision Instrument Co., Ltd.). In this experiment, a quenched 45 carbon steel in the form of a stepped cylinder with a toroidal friction surface was used as a mating sliding part. During the test, the Steel 45 followed the main shaft with rotary friction on the surface of the 7
cemented carbide. Sliding friction was present during motion. The setup layout and schematic diagram of experimental apparatus of MMG-10 experimental machine are shown in Fig. 4. 23 ml of Mobil 1 0W-40 lubricant with a viscosity of > 76.4 cSt @ 40 °C [ASTM D 445] was used for tribological experiments. Frictional response was investigated for various textured specimens while varying the pockets density, applied load and rotational speed. The distribution of the chevron pockets on the wear-resistant area of the cemented carbide surface is shown in Fig. 5. During wear, the mating sliding part rotated around the principal axis in a clockwise direction, which is consistent with the convergence direction of the chevron pockets. The equipment was used to evaluate friction on the lubricated surfaces by recording the instantaneous load and the friction force on the surface in real time. The main experimental parameters used in the friction and wear tester are shown in Table 5.
3. Results and Discussion 3.1. Tribological tests for non-textured and polished parts The influence of the chevron pattern texture on friction was investigated by comparing friction on the non-textured and polished areas of the lubricated cemented carbide surface. The rotation speed was kept at 120 rpm to explore differences in friction on the two surfaces under different test forces, as shown in Figs. 6 and 7. Figs. 6 and 7 show that the friction coefficient on the polished interface fluctuates between 0.14 and 0.18 under different bearing capacities and the overall trends are relatively similar. The friction coefficients on the unpolished and polished surfaces are approximately 0.07 and 0.17, respectively, at 120 rpm rotation speed and 180 N load. The main reason for the large difference between the two is that the unpolished specimen has a higher surface 8
roughness than the polished specimen. There is a large number of irregular microscopic humps, troughs, and other features on the surface. These structures exert a certain lubrication effect due to dynamic fluid pressure, thus the unpolished interface exhibits a lower friction coefficient. At 100 and 150 N loads, the friction coefficient of the unpolished interface was greater than 0.1, while the friction coefficient ranges from 0.07 to 0.09 when the load was 180 or 200 N. This occurs because the micro-morphology provides greater positive pressure on the lubricating film at a larger load.
3.2. Tribological study on chevron pattern micro-texture surface In this test, unpolished specimens and chevron pattern micro-textured specimens with 5.05%, 9.5%, 13.02%, and 15.2% texture density were chosen to study tribological characteristics on the micro pattern. 50, 100, 200, and 300 N test loads were applied while rotating at 40, 80, 120, and 180 rpm during the experiment, respectively. By changing the speed and load, the effect of changes in the texture density of the chevron pattern on friction could be studied. Figs. 8 and 9 show friction measurements from un-textured specimens and chevron pattern textured specimens with distinct texture density, respectively, with changes in test force and rotational speed. Fig. 8 shows friction measurements on (a) the unpolished sample and (b) the polished sample in different experimental conditions. Fig. 8 (a) shows that the unpolished sample exhibits large fluctuations in the friction coefficient in different experimental conditions. The surface friction coefficient exhibits larger fluctuations at each rotation speed at 50 N load. In contrast, the friction coefficient of the polished sample in different experimental conditions exhibits smaller fluctuations. Overall, the friction coefficient on the unpolished sample fluctuates between 0.08 and 0.26 in different experimental conditions. The polished sample 9
has similar friction coefficient in different experimental conditions. Changes in the friction coefficient were large because the unpolished sample had high surface roughness. Therefore, chevron pockets were prepared on the polished sample to further investigate the influence of the chevron pattern micro-texture on friction. The experimental error caused by high surface roughness was lowered to some extent. Fig. 9 (a) shows that the friction coefficient of the specimen decreases as the load increases at various fixed rotation speeds. The surface friction coefficient of the specimen is generally small (approximately 0.04) and remains stable at 300 N load. The friction coefficient on the surface of the specimen decreases obviously as the rotation speed increases at 50 N load. Meanwhile, the friction coefficient becomes more stable with larger loads. Fig. 9 (b) shows that the friction coefficient on the surface of the specimen decreases significantly when the load reaches 200 N, regardless of the rotation speed. The friction coefficients corresponding to F = 200 and 300 N loads are similar and reach minimum at 180 rpm. Under any load, the friction coefficient of the specimen decreases as the rotation speed increases. When the rotation speed is between 40 and 120 rpm, the friction coefficient decreases as the load increases. For the chevron pockets with 13.02% texture density, an excessive number of micro-texture units increase the surface roughness of the specimens. Fig. 9 (c) shows that the friction coefficient on the surface of this specimen slightly increases as the load increases at 180 rpm, which contradicts the results from most other surfaces, where the friction coefficient decreases as the load increases. Considering the measurement error in the test and the accuracy of the equipment itself, it seems that the friction coefficient stabilizes to approximately 0.075 at 180 rpm. Fig. 9 (d) shows that the friction coefficient on the chevron 10
pattern micro-textured specimens with 15.02% texture density is generally large and its minimum value reaches 0.1, where excessive concentration of topography is the main reason for the large friction coefficient. Similar to the specimen with 13.02% texture density, the high density of micro pits increases the roughness of the surface. The effect of dynamic fluid pressure and cavitation on friction coefficient reduction is not evident at surface high roughness. The following conclusions can be drawn by analyzing the surface friction characteristics of chevron pattern micro-textured specimens with four different texture density. The friction coefficient of the chevron pattern micro-textured specimens with 15.02% texture density is generally higher than in other specimens. 72 micro-textured elements are available over a circle with 6.15 mm radius, thus the spacing between each element is very small, resulting in a sharp increase in the surface roughness. Although a high texture density means that a larger number of micro pits can provide hydrodynamic pressure, the positive pressure generated by these units and the load provided by the lubricating film are limited. Therefore, roughness is still an important factor affecting the surface friction coefficient. Meanwhile, excessive reduction in the microcell spacing will also cause fluid to flow rapidly from the positive pressure zone to the negative pressure zone in the next unit. From the perspective of the Bernoulli effect, the internal pressure will be smaller when the fluid flow rate is higher and the load on the lubrication film will be lower. A chevron pattern micro-textured sample with a 9.5% texture density exhibited lower friction during testing. The lowest friction coefficient is about 0.02, which occurs under load conditions of 300 N and speed of 120 rpm and 180 rpm. The combined effect of rotation speed and load on the friction coefficient of the chevron pattern with a 9.5% texture density 11
is shown in the Fig.10. The load and the rotation speed have a significant influence on the friction coefficient of the test piece. As the load and speed increase, the coefficient of friction of the surface of the test piece continues to decrease. The maximum friction coefficient is only 0.1, and the difference from the minimum is only 0.08. The area containing the minimum coefficient of friction value remains relatively stable. A 9.5% chevrons texture density can be considered as the optimal density pattern, which works in conjunction with the hydrodynamic pressure generated by the micro-cells to minimize friction. The surface friction coefficient on the specimens with 5.05% and 13.02% texture density were high. The friction coefficients for these specimens become unstable as the working conditions change. In particular, the specimen with 5.05% texture density has a relatively high friction coefficient at low load. The dynamic fluid pressure is insufficient if there are too few surface micro-pits, thus decreasing the load in the fluid.
3.3. Pressure distribution in chevron patterns Fluid simulation analysis of the chevron pattern micro-cell texture was performed using Fluent 12.0 software. Considering the flow of fluid through the micro-pits, there is not only a slid in the velocity direction but also a pulsation in the vertical direction of the flow velocity between adjacent fluid layers, so the fluid type was set to turbulent flow. The standard k-ɛ model was applied, the calculation equations for the relevant parameters in the model are as follows:
ρ
µt ∂k dk ∂ = ( µ + ) + Gk + Gb − ρε − YM σ k ∂xi dt ∂xi
dε ∂ µt ∂ε ε ε2 ρ = ( µ + ) + C1ε (Gk + C3ε Gb ) − C2ε ρ dt ∂xi k k σ ε ∂xi 12
（3 ）
（4 ）
Where: ρ and t are density and time, respectively. Gk and Gb are the turbulent flow energy affected by velocity and buoyancy, respectively. µt is the viscosity coefficient, YM is the extent to which the dissipation rate is affected by pulsation expansion. The values of the constants C1ɛ, C2ɛ and C3ɛ are 1.44, 1.92 and 0.09, respectively. σk and σɛ are the Prandtl numbers of the turbulent flow energy k and the dissipation rate ɛ. Micro-unit modeling was performed using SolidWorks 2014 software, and then imported into ICEMCFD to "automatically divide" the grid. The model was introduced into Fluent and a new fluid was defined in the material property settings. The fluid density and viscosity were now set to 895 kg/m3 and 0.045 Pa·s, respectively. The fluid type was set to turbulent and the standard k-ɛ model was selected. The pressure inlet boundary and the pressure outlet boundary condition were selected, the loading force was 300 N, the wear zone area was 88.91 mm2, the fluid operation reference pressure was about 3.376 MPa. The pressure at the pressure outlet boundary was set to atmospheric pressure (101,325 Pa). The texture pressure distribution and flow velocity vector distribution of the texture unit are shown in Fig. 11. It can be seen from the cross-sectional pressure distribution of Fig. 11 (a) that a significant pressure gradient is produced along the direction of texture convergence and a high positive pressure occurs at the sharp corners of the right side. From the pressure distribution of the longitudinal section of Fig. 11 (b), the section passes through the apex angle of the texture unit and the midpoint of the bottom edge. Significant backflow occurs inside the pit and the outlet portion has a large positive pressure, while the pressure near the inlet is lower than the outlet and produces a significant negative pressure. It can be seen from the longitudinal section flow velocity diagram of Fig. 11 (c) that the fluid flow rate near the 13
outlet is faster than the fluid flow velocity near the inlet, and the entire flow direction is usually distributed clockwise, which promotes the generation of backflow in the pit. As the fluid flows through the surface of the groove microtextured unit with unidirectional convergence, a certain pressure gradient tends to occur in the direction of convergence. The inside of the micro-pits is often accompanied by the reflow phenomenon and produces a certain negative pressure value. The convection effect formed by the convergence of the texture also promotes the generation of backflow inside the pit to some extent. As shown in Fig. 11 (d), when lubricant flows from the inlet to the outlet, the flow per unit length gradually decreases along the direction of fluid flow. The texture is axially convergent and the average pressure at the outlet is greater than the pressure at the inlet. This pressure distribution reduces inflow at the inlet and increases outflow at the outlet. Hydrodynamic lubrication is often accompanied by cavitation. The fluid velocity gradient changes dramatically at the end of the diverging wedge. The internal pressure drops rapidly, producing negative pressure. Vacuoles will form once the negative pressure drops below a critical value. Cavitation will suppress generation of negative pressure. Daniel Gropper et al. [30] reviewed a number of different mass-conserving cavitation algorithms. Cavitation may occur not only globally in divergent contact regions, but also in local of individual pits or in-between asperities. Positive pressure on the lubricant film arises when lubricant flows out of the converging wedge port. This asymmetric pressure gives the lubricating film a certain load carrying capacity. Therefore, every tiny shape can be regarded as a lubricated bearing under dynamic pressure. The dynamic pressure generated by the fluid can separate friction pair surfaces, resulting in a lower friction coefficient and reducing wear. 14
3.4. Wear mechanism analysis of texture working surface During mutual wear of the two working surfaces in the friction pair, micro-scratches, various damage pits, and adhesive protrusions on the surface of the specimens may cause damage to the surface micro-texture. Therefore, the micro-texture should be designed while considering whether it can effectively reduce wear between friction pairs. Design aspects include the length of the furrow, number of exfoliation pits caused by free particles, adhesion of wear debris during shear fracture at the adhesive point, and the degree of cavitation at the micro-texture edge. First, the cemented carbide sample after the friction coefficient test was subjected to wire cutting to facilitate observation with a scanning electron microscope. Subsequently, the sample was cleaned ultrasonically for 30 minutes in a beaker containing acetone to remove surface oil. After drying, the surface wear scar on each sample was examined using an SEM. Figs. 12 and 13 show the wear tracks and worn surfaces of the untextured specimen and the chevron pattern textured specimen with 9.5% texture density at 300 N load and 180 rpm rotation speed, where the wear time reached 2100 seconds. Fig. 12 (a) and (b) show the wear tracks and worn surfaces of the non-textured specimen without polishing. Panels (c) and (d) show the wear tracks and worn surfaces of the polished specimen. The wear scar on the surface of the specimens is apparent in Fig. 12 (a). After magnifying the SEM to a factor 500, obvious scratches were observed on the surface of the non-textured specimens and some micro pits are present near the narrow part of the furrow mark (Fig. 12(b)). Because the unstable solid particles on the surface of the unpolished non-textured specimens easily detach due to friction and hard protrusions on the surface of the friction pair act on the surface of the specimens, these will cause micro-cutting and 15
furrowing on the surface of the specimens. This produces the shedding pit and deep groove shown in the figure. After magnifying the worn polished part to a factor 500, one can see that there are shallow scratches on the surface, as shown in Fig. 12 (c). After magnifying to a factor 1500, as shown in Fig. 12 (d), it is apparent that these shallow scratches form due to an accumulation of discontinuous tiny pits. The unstable particles on the surface of the cemented carbide specimen were removed during polishing and particulate impurities from the friction pairs were effectively reduced. Therefore, the degree of wear during the friction test was lower than that on the unpolished parts. Fig. 13 shows the morphology in the surface wear of a chevron pattern. One can see in Fig. 13 (a)-(c) that the surface of the specimens is much less worn than the non-textured cemented carbide specimens. Fig. 13 (b) and (c) show that there are slight scratches and fine furrows on the surface consisting of shedding pits. The presence of wear debris in a contact accelerates wear and protection afforded by wear debris against adhesion is greater than the damage caused by their presence [31]. Additional mechanisms for texture reduce wear may be debris trapping and replenishing lubricant and reducing preventing starvation [32]. Abrasive particles between the friction pairs peel away from the brittle material on the surface of the specimens during extrusion and the exfoliated abrasive will produce a micro-cutting effect on the adjacent surface at the appropriate angle, leaving a furrow. Meanwhile, when each portion of the friction pair is in relative motion, individual contacts on the surface of the specimens will bond and solder. As shown in Fig. 13(d), there are white spots on carbide surface. Elemental analysis of the white point area is shown in Fig. 13 (j). Element Fe can be detected in the area, confirming that this is transferred steel on carbide 16
surface. Because the mating sliding part is quenched 45 carbon steel, the lower specimen is a cemented carbide block and the softer material falls off and migrates to carbide surface when these bonding points are cut. These adhesives are applied to the surface of the specimens due to friction in the pair and the adhesive area increases in size under high load. This process is often associated with heating due to friction. Gas is produced in the fluid, which then collapses in the high-pressure region, causing cavitation pits near the edge of the micro-texture. The fluid pressure in the experiment was insufficient to cause obvious cavitation. However, only small pits formed locally on the surface and large spongy pits did not appear, as shown in the edges of the micro-texture in Figs. 13 (e) and (f). Therefore, the wear mechanism on the surface of the chevron pattern specimens arises due to abrasive wear and adhesive wear.
4. Conclusions Friction on specimens with chevron pockets at four kinds of texture density was examined. The surface friction coefficient was found to be minimum and stable, while the fluid dynamic pressure increased in the specimen with 9.5% texture density. Too few surface micro-texture units often cannot provide sufficient dynamic fluid pressure, while an excessive number of micro-texture units increase the surface roughness of the specimens. The load does not individually affect the friction properties of the test piece. There are one or more sets of load-speed combinations that enable the textured surface to achieve a minimum friction coefficient. The chevron pattern test piece with 9.5% texture density has two sets of load-speed combinations which enable the test piece to reach a minimum friction coefficient value of 0.02. The wear mechanism on the surface of the chevron pattern specimens arises due to 17
abrasive wear and adhesive wear. Protection afforded by wear debris against adhesion is greater than the damage caused by their presence. Additional mechanisms for texture reduce wear may be debris trapping and replenishing lubricant.
Acknowledgments This work is supported by the National Natural Science Foundation of China (grant No. 51575234,51872122), the Postdoctoral Science Foundation of China [grant No. 2017M620286],
the Key Research and Development
Program of Shandong Province, China [grant No. 2018CXGC0809], the Agricultural Machinery Equipment Research and Development Innovation Plan of Shandong Province [grant No. 2018YF012], and Experts from Taishan Scholars of Shandong Province. *Corresponding author, Xuefeng Yang, major in tribology, email [email protected]
References
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21
Cemented carbide mold Moving direction of wire
Exit zone
Entrance zone
Invariable zone
Deformation zone Fig.1 The structural layout of cemented carbide mold
Table 1 Performance parameters of YT15 cemented carbide Composition
Component
Density
Hardness
Bending
content(%)
(g/cm3)
(HRA)
strength(MPa)
11-12.7
91
1150
WC
79
TiC
15
Co
6
Table 2 The element content of 45 steel Element
C
Si
Mn
P
S
Cr
Ni
Cu
Content(%)
0.42-0.5
0.17-0.37
0.5-0.8
≤0.035
≤0.035
≤0.25
≤0.25
≤0.25
Table 3 Relationship between the number of chevron pattern units and texture density Chevron pattern unit The number of units(n)
24
45
60
72
Texture density(%)
5.05
9.5
13.02
15.2
Table 4 Processing parameters of Libra-HE femtosecond laser The laser
Pulse width
Pulse
Average power
wavelength(nm)
(fs)
frequency(kHz)
(W)
800
100
1-10
4
Number of scanning 13
Table 5 Technical index of MMG-10 friction wear tester Technical
Axial experimental
Maximum friction
Rotation speed
indicators
force(N)
moment(N·m)
range(rpm)
10-10000
5
0.1-2000
Parameter values
Feeding speed(N/s)
1-200
Fig.2 Dimensions of the chevron pattern
200 µm
50 µm
200 µm
（a）Chevron pattern with 45 units （b）Physical image photography （c）Chevron pattern with 24 units
（d）3D display of micro-texture composition
Fig.3 The actual figure of the chevron patterns in the DINO-LITE(a), (c) and the image (b), (d) of the ultra-depth-of-field digital microscope
Drive motor
Rotating tong Steel 45 Lubricating oil pool
Lifting shaft
Torque sensor
Hydraulic cylinder
Fig.4 Setup layout and schematic diagram of experimental apparatus of MMG-10 experimental machine
Fig.5 The distribution of chevron patterns in the wear zone
0.22 100N 150N 180N 200N
0.20
Friction coefficient
0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0
100
200
300
400
500
600
Text time(s)
Fig.6 Friction curve of unpolished cemented carbide
0.22
100N 150N 180N 200N
Friction coefficient
0.20
0.18
0.16
0.14
0.12
0.10 0
100
200
300
400
500
600
Text time(s)
Fig.7 Friction curve of polished cemented carbide
0.26
0.26
40rpm 80rpm 120rpm 180rpm
0.22
Friction coefficient
0.20
40rpm 80rpm 120rpm 180rpm
0.24 0.22 0.20
Friction coefficient
0.24
0.18 0.16 0.14 0.12 0.10 0.08
0.18 0.16 0.14 0.12 0.10 0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00 0
50
100
150
200
250
Load (N)
(a) the unpolished sample
300
350
0
50
100
150
200
250
300
350
Load (N)
(b) the polished sample
Fig.8 Friction coefficient of (a) the unpolished sample and (b) the polished sample in different experimental conditions
0.26
0.26
40rpm 80rpm 120rpm 180rpm
0.22
Friction coefficient
0.20
40rpm 80rpm 120rpm 180rpm
0.24 0.22 0.20
Friction coefficient
0.24
0.18 0.16 0.14 0.12 0.10 0.08
0.18 0.16 0.14 0.12 0.10 0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00 0
50
100
150
200
250
300
350
0
50
100
150
Load (N)
(a) the sample with 5.05% texture density
250
300
350
(b) the sample with 9.5% texture density
0.26
0.26
40rpm 80rpm 120rpm 180rpm
0.22 0.20
40rpm 80rpm 120rpm 180rpm
0.24 0.22 0.20
Friction coefficient
0.24
Friction coefficient
200
Load (N)
0.18 0.16 0.14 0.12 0.10 0.08
0.18 0.16 0.14 0.12 0.10 0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00 0
50
100
150
200
250
300
350
Load (N)
(c) the sample with 13.02% texture density
0
50
100
150
200
250
300
350
Load (N)
(d) the sample with 15.2% texture density
Fig.9 Friction coefficient of different texture density samples with changes in test force and speed
Fig.10 The combined effect of rotation speed and load on the friction coefficient of a chevron pattern with 9.5% texture density
(a) Cross sectional pressure distribution
(b) Longitudinal section pressure distribution
(c) Longitudinal section velocity vector distribution
(d) fluid dynamic pressure diagram
Fig.11 The texture pressure distribution and flow velocity vector distribution of the texture unit
200 µm (a) Wear tracks of unpolished test pieces
20 µm (b) Surface scratches on unpolished specimens
10 µm (c) Surface scratches on polished specimens
10 µm (d) Tiny pits on the surface of polished specimens
Fig.12 Wear tracks and worn surfaces of untextured specimens
10 µm
10 µm
(b) Tiny furrows
(a) Surface wear morphology
10 µm (c) Slight scratches
100 µm (d) Bonding phenomenon
20 µm (e) Tiny pits
10 µm (f) Local pits
(j) Elemental analysis Fig.13 Surface abrasion of chevron pattern micro-texture specimen surface and elemental analysis of white spots
Highlights:
Chevron pattern micro texture can improve the friction performance of hard alloy. Chevron pattern micro texture has hydrodynamic effect on friction surface with oil. 9.5% texture density has better hydrodynamic effect. The main wear form is abrasive wear and adhesive wear.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: