Laser-Fabricated Micro-Dimples for Improving Frictional Property of SKH51 Tool Steel Surfaces

Abstract

Friction involved in metal-forming processes typically leads to the wear of tool and die surfaces, and in turn shortens the tool’s service life. A thriving need for reducing surface friction requires the tool surface to be modified. This paper presents the surface modification of SKH51 tool steel, on which the hexagonal array of micro-dimples is fabricated by a nanosecond pulse laser. Using the average laser power of 25 W can create decent dimples for trapping lubricant and enabling hydraulic pressure at the surfaces in contact. The effect of dimple density and sliding speed on the coefficient of friction was examined in this study through the pin-on-disc test, in which a stainless steel pin was applied against the tool steel disc with a constant load. The laser-textured tool steel surface with a dimple density of 35% had a friction coefficient of 0.087, which was lower than that of the untextured surface by 12.6% when using a sliding speed of 15 cm/s. In addition to friction reduction, there was no substantial wear found on the laser-textured surface compared to the untextured sample. The findings of this study can be a processing guideline and benefit the treatment of tool and die surfaces for friction and wear reduction in metal-forming and related processes.

1. Introduction

The friction and wear of metal-forming tools are significant factors affecting the surface quality of the formed parts and tool life in metal-forming processes [1]. Coating the tool surface with low-friction coatings and/or elevating lubrication effectiveness are vital in reducing friction and wear in the processes [2,3,4]. The roughness of tool surfaces is also a key parameter that influences the kinetic friction force between the surfaces in contact. Sigvant et al. [5] noted that the decrease in tool surface roughness substantially decreases the coefficient of friction. In addition to these approaches, micro-scale surface texturing has recently become a method for enhancing lubrication and reducing friction in tribological pairs [6,7,8]. The surface texturing can be achieved by using electrochemical machining [9,10], electrical-discharge micromachining [11], and laser-texturing processes [12,13,14,15]. Among these techniques, laser is the most preferred as it offers a high material removal rate, high texturing resolution, and fully automated processing with good reliability. Two texturing structures, i.e., micro-groove and micro-dimple, are normally fabricated on the tool surface to help reduce friction. The different sizes and geometries of the texturing structures have been found to cause different tribological properties [16]. Therefore, this area of interest is still open and requires in-depth investigations to further reveal the underlying mechanisms and influences of the texturing parameters on friction and wear improvement.

Micro- or submicro-features are created on the tool surface through the vaporization mechanism induced by the laser surface texturing process. The influence of laser-textured surfaces and speed-load parameters on the transition from a boundary to a hydrodynamic lubrication regime was studied by Kovalchenko et al. [17]. Laser surface texturing extends the range of speed-load parameters for hydrodynamic lubrication and lowers the friction coefficient of surfaces in contact. At greater speeds, higher loads, and with higher viscosity oil, the advantages of laser-textured surfaces are more noticeable. Thrust bearings, journal bearings, and mechanical seals are examples of friction pairs where surface texturing can substantially improve their tribological characteristics [18,19,20]. The textured surface containing micro-dimples has the ability to generate micro-hydrodynamic pressure, retain and distribute lubricant [21], increase lubricant thickness [22], reduce contact area, and capture wear debris [23]. This directly extends the tool’s service life in addition to the friction force reduction.

Abe et al. [24] employed TiCN-based cermet dies with micro-dimples to enhance wear resistance during stainless steel cup ironing. The dimples enable lubricant flow and reduce friction through liquid transfer at the die–cup interface. The influence of surface texturing on friction reduction in a silicon nitride ceramic tool was clarified by Wakuda et al. [25]. Their results show that micro-dimple size and dimple density have a significant impact on tribological behaviors. However, dimple shape has less influence on the change in friction coefficient. The study suggests a dimple diameter of around 100 μm with a dimple density ranging from 5 to 20% to provide a significant reduction in friction. Daodon and Saetang [12] applied a laser texturing process to reduce the surface friction of a cold-work steel tool sliding against an advanced high-strength steel. The laser-textured surface with a dimple density of 5.6% has a lower coefficient of friction compared to the untextured surface. In addition, the wettability of the textured surface still remains unaffected, so the lubricant can wet the surface and effectively facilitate friction reduction. Schneider et al. [26] studied the influence of dimple aspect ratio, dimple density, and arrangements on friction in mixed lubrication. The lowest friction is obtained when using 10% dimple density and 0.1 aspect ratio associated with a hexagonal arrangement of dimples on the work surface. Abe et al. [27] demonstrated that laser-textured dies can enhance seizure resistance in aluminum alloy sheet and stainless steel cup ironing. The micro-dimples arranged in a grid array increase the load-carrying capacity of the lubricant and improve the ironing limits. According to many past studies, the dimple diameter and aspect ratio of about 100 μm [12,25,28,29,30,31,32] and 0.1 [26,30,32], respectively, are usually suggested to be fabricated on the tool surface for improving its tribological properties. However, there have been few discussions on the effect of dimple density on friction reduction. In addition, most of the past studies examined the tribological performance of textured surfaces with a dimple density less than 20%. An increase in dimple density greater than this value can be of a high potential for further decreasing the coefficient of friction. This issue needs to be elaborated to provide a better understanding of the relationship between the dimple density and friction improvement.

This paper aims to investigate the effect of dimple density on the friction coefficient. Due to the high pulse energy, low photon cost, and localized ablation with a small heat-affected zone offered by nanosecond pulse lasers, this type of laser was employed in this study to texture micro-dimples with different dimple densities on SKH51 tool steel surface. This tool steel is a common grade for tools and dies used in metal-forming processes. AISI304 stainless steel representing a workpiece material in the forming processes pressed against the textured tool steel surface in a pin-on-disc test for determining the coefficient of friction of the two surfaces in contact. The findings of this study will further unveil the tribological performance of micro-dimpled textured surfaces and provide a guideline for friction reduction in metal forming as well as cutting processes.

2. Materials and Methods

2.1. Micro-Dimple Fabrication

A nanosecond pulse laser (IPG YLP-1-100, IPG Laser GmbH & Co. KG, Burbach, Germany) emitting a wavelength of 1064 nm and a constant pulse duration of 100 ns was applied in this study to create micro-dimples on the surface of SKH51 tool steel. The hardness of the metal was 64 HRC. To achieve this, the steel was heat treated in a vacuum heat treatment furnace, first preheated to temperatures of 850 and 1050 °C for holding times of 60 and 30 min, respectively, heated to a hardening temperature of 1220 °C for a short period of 15 min and then quenched in nitrogen gas at a pressure of 280 kPa. The workpieces were subsequently tempered at 560 °C for 90 min and cooled to room temperature under a nitrogen atmosphere at a pressure of 80 kPa. The thermophysical properties and chemical composition of the tool steel are listed in

Table 1. Thermophysical properties and chemical composition of SKH51 tool steel.

Thermophysical Properties					Value
Density, $\rho$ (kg/m3)					8138
Specific heat capacity, cp (kJ/kg °C)					0.46
Melting temperature, Tm (°C)					1430
Vaporization temperature, Tv (°C)					2861
Thermal conductivity, k (W/m °C)					24
Chemical composition (wt%)
C	Si	Mn	P	S	Cr	Mo	W	V
0.80–0.88	0.45	0.40	0.030	0.030	3.80–4.50	4.70–5.20	5.90–6.70	1.70–2.10

. The distribution of laser beam profile was Gaussian with the beam quality factor (M²) of 1.65. The collimated laser beam was focused by an f-theta lens having a focal length of 100 mm. The laser beam irradiated on the top surface of workpiece at the off-focused position where the beam diameter at 1/e² (d_b) of 100 μm was attained as shown in Figure 1a. This beam diameter was expected to produce a dimple diameter of about 100 μm, which is recommended by previous studies [12,25,28,29,30,31,32] as a proper size of dimple for trapping lubricant and enabling the hydraulic pressure at the contact surface to reduce the friction. The laser pulse repetition rate (f) and irradiation time (t) for each dimple ablation were kept constant at 100 kHz and 0.1 s, respectively. According to past literature, a suitable aspect ratio of micro-dimples is about 0.1 [26,30,32], so the expected depth of the dimple having the diameter of 100 μm is 10 μm. Thereby, the determination of average laser power (P) for achieving this dimple depth was performed in this study through a set of experiments. After a number of trial tests, the average laser power ranging from 10 to 50 W was able to provide a micro-dimple on the tool steel surface without the excessive melting of workpiece and substantial formation of recast structures around the laser-ablated dimple. The laser parameters used in the fabrication of micro-dimples are summarized in

Table 2. Laser parameters used for fabricating micro-dimples.

Parameter	Value
Laser wavelength (nm)	1064
Average laser power, P (W)	10, 15, 20, 25, 30, 35, 40, 45 and 50
Laser pulse repetition rate, f (kHz)	100
Irradiation time, t (s)	0.1
Laser beam diameter at 1/e2, db (μm)	100

.

After the laser ablation, the workpiece surface was ground by emery papers with grit numbers of 500, 800, 1000, 1500, 2000, and 2500 to remove bulges of recast structures depositing at and around the dimple edge. The diameter and depth of dimples obtained under the different laser power were measured by a 3D laser confocal microscope (Olympus OLS5000, Olympus Corp., Tokyo, Japan). The laser power that caused the dimple aspect ratio of about 0.1 was then selected and applied in the surface texturing of tool steel for friction test. The micro-dimples were arranged in the hexagonal pattern as shown in Figure 1b. This pattern has been proven by past studies to help the reduction of surface friction [26,33]. The dimples were evenly spaced according to the hexagonal pattern. The closer the distance between the adjacent dimples is employed, the higher the dimple density is obtained. The dimple density (ρ_d) is calculated by using:

$${\rho }_d=\frac{\pi }{2\sqrt{3}}{\left(\frac{d}{p_d}\right)}^2$$

(1)

$$L=\frac{p_d}{2\tan 30°}$$

(2)

where d, p_d, and L are the diameter of dimple, horizontal and vertical distances between the adjacent dimples, respectively. In this study, four levels of dimple density, i.e., 5%, 15%, 25%, and 35%, were made on the tool steel surface whose coefficient of friction was subsequently quantified by a tribometer. The micrograph of laser-textured surfaces with the different levels of dimple density taken by an optical microscope is presented in Figure 1c–f.

2.2. Measurement of Friction

The pin-on-disc test was carried out in this study to determine the friction coefficient of the untextured and laser-textured SKH51 tool steel surfaces by using a tribometer. The tool steel was assigned to be the disc having the diameter and thickness of 41 mm and 4 mm, respectively. AISI304 stainless steel with the hardness of 15 HRC was applied as the flat-ended pin, on which the diameter of the flatten area of 3 mm and the average surface roughness (R_a) of 0.04 µm were prepared. The surface of tool steel disc possessing the dimple density of 0% (untextured surface), 5%, 15%, 25%, and 35% were tested under a normal load of 10 N against the 304 stainless steel pin as shown in Figure 2. Before the test, the laser-textured surfaces were ground by emery papers with grit numbers of 500, 800, 1000, 1500, 2000, and 2500 to remove the recast depositing around the dimples. The R_a of the final surface after the 2500-grit polishing was about 0.04 µm. The samples were cleaned by acetone for 10 min in an ultrasonic cleaner and then dried. The stainless steel pin was fixed in a stationary position, while the disc was rotated at a constant speed. Two sliding speeds, i.e., 5 and 15 cm/s, were conducted in the test of each sample. The sliding speed was changed by adjusting the sliding radius. By selecting sliding radii of 5 mm and 15 mm, consistent sliding velocities of 5 cm/s and 15 cm/s were achieved, respectively. This setting allowed the test to maintain a stable rotational speed, thereby ensuring that only the linear speed at the contact point varied. This approach was chosen to minimize potential variations in temperature that could arise from changing the rotational frequency, and the sliding distance was 500 m. The pin-on-disc test was performed at room temperature of 25 °C and lubricated by Castrol Iloform TDN81 (BP–Castrol (Thailand) Ltd., Bangkok, Thailand), whose kinematic viscosity at 40 °C is 157–180 cSt.

3. Results and Discussion

3.1. Effects of Laser Power on the Diameter and Depth of Micro-Dimples

In this study, a micro-dimple was created by irradiating a laser beam for 0.1 s duration on the surface of SKH51 tool steel. The nine levels of average laser power ranging from 10 to 50 W were employed in the experiments, and the dimple dimensions were measured accordingly. Figure 3 presents the 3D morphology of micro-dimples fabricated by using the different laser powers, and the cross-sectional profile of the dimples is shown in Figure 4a. The increase in the laser power apparently increased the dimple size. The maximum depth of the dimples was located at its center where the laser intensity was highest. In addition to the enlarging of the dimples, the recast structures and heat-affected zone were found inside the dimple and around its edge as shown in Figure 4b. Furthermore, although this study did not specifically evaluate the composition and microstructural changes, our previous research noted that increased laser power caused a reduction in hardness near the edge of the pocket. Such a variation in hardness may be indicative of microstructural changes resulting from retempering [12]. The deposition of recast at the edge of the dimples was a result of molten flow induced by the recoil pressure and Marangoni effect. The molten metal was forced to flow and/or eject out of the laser-irradiated region and then solidified at the dimple edge. This thereby led to the protrusion of dimple rim due to the recast deposition. Although the most recast was able to be removed by the surface polishing prior to the dimple measurements, some still remained around the dimple edge. The total removal of recast protruding from the metal surface is possible by increasing the polishing time. However, some dimples are subjected to over polishing which makes them too shallow or even disappeared. A trade-off between the recast removal and maintaining the dimple depth as well as the existence of the dimple array was made in the surface polishing step, in which a small number of recasts with a protrusion height of a few microns were allowed to remain on the metal surface. The amount of recasts was found to increase with the laser power applied in the ablation. This also affected the surface polishing time and effort involved in the recast removal.

The diameter, depth, and aspect ratio of micro-dimples caused by the different laser powers are presented in Figure 5a–c. The dimple diameter was about 80 μm when using the laser power of 10 W, and it linearly increased with the laser power. The relationship between laser power and dimple depth was also linear until the laser power reached 40 W. The depth sharply increased with a large deviation when the laser powers of 45 and 50 W were applied. Using higher laser power typically promotes a thicker molten layer where its high flow velocity and high recoil pressure substantially push a large portion of molten material out of the dimple. This thereby results in a deeper dimple with more recasts depositing at its edge. Since the behavior of the molten layer becomes more dynamic under higher laser power, a high deviation of dimple depth can thus be expected. According to the results, the transition from the static to the dynamic regimes of laser ablation occurred at the average laser power of 40 W. In the static regime, the dimple depth can be predicted by a heat conduction model associated with the conservation of energy. The temperature field induced by the irradiation of a Gaussian laser beam with the average power P and spot radius ω_b is expressed as:

$$T\left(r,z\right)=\frac{\left(1-R\right)P}{2\pi kz}\exp \left(-\frac{2r^2}{{\omega }_b^2}\right)+T_o$$

(3)

where R, k, r, z, and T_o are the reflectivity (0.7) and thermal conductivity of the metal, the radius and depth of the temperature field, and room temperature, respectively. When considering the vaporization temperature of the metal (T_v) to be the ablation point, the dimple depth (z) is predicted by using:

$$z(r)=\frac{\left(1-R\right)P}{2\pi k\left(T_v-T_o\right)}\exp \left(-\frac{2r^2}{{\omega }_b^2}\right)$$

(4)

The predicted dimple depth subjected to the vaporization point is also plotted in Figure 5b. The prediction provided a good agreement with the measured depth under the static regime, in which the applied laser power was less than 40 W. A comparison between the measured and predicted profile of micro-dimples caused by the laser power of 25 W is also shown in Figure 5d. Since the dimple formation is more induced by the outward flow of the molten layer due to the recoil pressure and thermocapillary effect in the dynamic regime of ablation (P > 40 W), the melting point of the metal (T_m), which is able to indicate the depth of the molten layer, can be applied in Equation (4) to approximate the dimple depth. The predicted depth regarding the melting point is also shown in Figure 5b. The prediction was found to be in line with the dimple depth caused by the 45 and 50 W laser powers.

According to the suggested dimple dimensions [25,26], the diameter and aspect ratio of micro-dimples should be about 100 μm and 0.1, respectively, to introduce the effective reduction of surface friction. Among the laser ablation conditions performed in this study, the average laser power of 25 W was selected since it was able to provide the desired dimple dimensions with fewer recast structures, good repeatability, and a small deviation of dimples across the textured area. This processing condition was further applied to texture the hexagonal array of micro-dimples on the SKH51 tool steel surface for the friction test using the pin-on-disc method.

3.2. Effects of Micro-Dimple Density on the Friction and Wear of SKH51 Tool Steel Surfaces

Five levels of dimple density including the untextured surface and two levels of sliding speed were carried out in the pin-on-disc test. The plots of the friction coefficient reading from the tribometer against the sliding distance under the test speed of 5 and 15 cm/s are presented in Figure 6a,b. The friction coefficients of all surfaces exhibited a sharp increase at the initial stage, followed by a subsequent decline. This phenomenon is attributed to the initial roughness of the fresh friction pair that is high during the initial running-in period. The coefficient of friction obtained at the sliding speed of 5 cm/s showed more fluctuation than at the other speed. This is owing to the unstable friction force at the contact surface [34], where the adhesion between the two metals likely occurs and interrupts. This situation is due to the transition between boundary lubrication and mixed lubrication regimes. At lower speeds, the contact time between the surfaces increases, causing more intimate contact and adhesive interactions between the tool steel and stainless steel surfaces. This thereby results in stick–slip behavior and increased friction fluctuations [34,35]. In addition, the coefficient of friction taken at 5 cm/s sliding speed was higher than that at 15 cm/s by 8.3% on average regardless of the percentage of dimple density used. The high surface friction at slow sliding speed is caused by a diminished degree of separation facilitated by the lubricant. The lubrication is typically under a mixed condition consisting of both hydrodynamic and boundary lubrication regimes [36]. The relative motion between the two surfaces is minimal when using slow sliding speed, and the balance between the two lubrication regimes is altered. The lubrication tends to be dominated by the boundary lubrication regime, where a direct contact between asperities on the surfaces becomes more prominent and the lubricant is unable to separate the surfaces effectively. The micro-dimples, which are designed to retain lubricant and reduce direct metal-to-metal contact, are therefore ineffective for the surfaces in contact under the boundary lubrication condition. This finding also corresponds to the study of Vilhena et al. [37], noting that the friction tends to be high and to fluctuate when a low sliding speed is applied. On the other hand, as the sliding speed increases, the shear forces acting on the lubricant become stronger. This induces a better distribution of lubricant within the micro-dimples, thus promoting the mixed lubrication condition. The hydrodynamic effect involved in the mixed lubrication contributes to the separation of the surfaces in contact to a greater extent. This substantially leads to friction reduction [17,38,39]. Mizuno and Okamoto [40] explored the influence of lubricant properties and sliding velocity on lubrication. Employing a surface with micro-dimples to capture lubricant proves advantageous for friction reduction during a metal-forming process. The boundary film thickness can also be enhanced by maximizing the lubricant entrapment within the interface in addition to the use of high-viscosity lubricants.

The average coefficient of friction of each sample is shown in Figure 6c, indicating a decreasing trend with the increased dimple density for both tested sliding speeds. The friction coefficient of the untextured surface (0% dimple density) subjected to the sliding speed of 5 and 15 cm/s was 0.107 and 0.099. The lowest coefficient of friction obtained at both sliding speeds was 0.096 and 0.087 when the 35% dimple density was used. This is equivalent to the reduction of 10.5% and 12.6%, respectively. With the aid of regression analysis, the decreasing rate of the friction coefficient against the dimple density at the sliding speed of 5 and 15 cm/s was about 0.0003 and 0.0002. This implies that the contribution of the dimple density to friction reduction is about the same rate for both sliding speeds tested in this study.

Comparing the friction coefficient of the laser-textured surface to that of the untextured surface, it was observed that the friction values of all textured surfaces, except for those with a dimple density of 5% tested at a sliding speed of 5 cm/s, were significantly lower than those of the untextured surface. The decreased friction coefficient induced by the increase in dimple density is caused by micro-hydrodynamic lubrication [41,42]. This lubricating mechanism is apparent when the lubricant is entrapped, pressurized, and extracted from the micro-dimples. The presence of an asymmetric hydrodynamic pressure distribution or wedging effect above the dimples provides load-carrying forces [43], which facilitate the interface separation [37,43]. The wedging effect occurs when the pressure distribution in the lubricant film generates positive hydrodynamic pressure in the convergent part of the micro-dimples and negative hydrodynamic pressure in the divergent part as shown in Figure 7. This leads to a situation in which the magnitude of the positive pressure is greater than that of the negative pressure and thus results in a net positive pressure within the micro-dimple unit. The positive pressure offers an extra carrying force, which helps increase the load-carrying capacity of the friction pair. The numerical simulation performed by Abe et al. [27] also reveals the change in pressure distribution of lubricant flow on the dimpled surface. A higher dimple density or shorter flat portion length between the adjacent dimples likely leads to an adequate supply of lubricants into the interface, thus enhancing the micro-hydrodynamic lubrication. Our finding also corresponds to the studies of Shimizu et al. [22], Liu et al. [36], and Brizmer et al. [44], noting that a shorter length of the flat portion or a higher dimple density promotes more lubricant transfer between the adjacent dimples. This, in turn, increases the load-carrying capacity and reduces the coefficient of friction accordingly.

Following the pin-on-disc test using the tribometer at the sliding speed of 5 and 15 cm/s, the wear scars on the sliding track were macroscopically observed for the textured and untextured surfaces as illustrated in Figure 8. The observed difference in surface color in Figure 8a compared to the other images could be attributed to factors such as oxidation. It should be noted that any surface oxidation in Figure 8a might affect the reported frictional properties when compared with the other pictures. Although the AISI304 stainless steel pin is softer than the SKH51 tool steel disc, its surface is naturally covered by a passive layer primarily consisting of chromium oxide (Cr₂O₃). This passive layer is harder than the base metal. When some elements of stainless steel are detached from its surface during the sliding test, these hard chromium oxide particles behave like abrasive agents (third bodies) between the contact pairs, leading to abrasive wear on the sliding track. As the surfaces continue to slide against each other, the hard chromium oxide particles cause material removal and wear on both stainless steel and tool steel surfaces. However, no substantial seizures were found on any of the surfaces, and only thin scratches were observed. At the sliding speed of 5 cm/s, where the boundary lubrication tended to dominate, the untextured and textured surfaces with a dimple density of 5% struggled to provide the effective lubrication for separating the surfaces in contact. In the boundary lubrication regime, the lubricant film is not well developed and a direct metal-to-metal contact is likely occurred, thus leading to the formation of shallow scratches as shown in Figure 8a,b. This is also reflected in the friction coefficient-sliding distance curves, which show high friction coefficients and increasing trends for the tested surfaces (Figure 6a). At the sliding speed of 15 cm/s, the textured surface having a dimple density of 15% exhibited thin scratches as shown in Figure 8c. Although using a higher sliding speed likely affects the lubrication regime, the dimple density of 15% may not be sufficient to provide a decent lubrication. This could lead to a situation that the lubricant film is unable to effectively prevent the couple surfaces in contact and the thin scratches are thereby obtained as a result. The laser-textured surfaces with the dimple densities of 25% and 35% were of high potential for promoting the ingress of the lubricant into the interface of the mating components. This significantly improved the lubrication performance and only tiny traces of wear are apparent on the sliding track as shown Figure 8d,e.

A comparison between the morphology of laser-textured surfaces before and after the pin-on-disc test is shown in Figure 9, indicating that the shape of the micro-dimples remained unchanged even after testing for a distance of 500 m. This is apparently a positive outcome, as the micro-dimples are able to provide the lubrication-enhancing function over an extended period of use. Higher dimple densities can hold a greater volume of lubricant within the dimples. As a result, a robust and continuous lubricating film is formed, preventing direct contact and minimizing wear.

SEM micrograph of the worn track on the untextured and textured surfaces is shown in Figure 10. The thin scratches are noticeable on the untextured surface when using the sliding speed of 15 cm/s as depicted in Figure 10a. This is because of the breakdown of the thin lubricant film that was unable to support the load, so the direct contact between the friction pairs occurred accordingly. Regarding the chemical composition listed in

Table 3. Chemical composition taken from the EDS mapping of untextured and laser-textured surfaces.

Element (wt%)	O	V	Cr	Fe	Ni	Mo	W
Untextured surface	0.80	2.17	4.31	78.73	0.15	5.44	8.41
Textured surface	5.66	1.35	2.80	76.27	0.02	6.34	7.55

, a greater amount of Ni is found on the untextured surface compared to the other after the pin-on-disc test. Since Ni is not present in the composition of SKH51 tool steel as noted in Table 1, it can be implied that a number of Ni were transferred from the AISI304 stainless steel counterpart to the tool steel surface during the sliding test. This thereby indicates a combination of adhesion and abrasion during the sliding contact, and the adhesion of Ni occurred more on the untextured surface. In contrast, the textured surface exhibits only slight wear with a smooth appearance in the plateau area as shown in Figure 10b,c. The dimples remained intact after the sliding test. This is attributed to the lubricating oil entrapped in the dimples that is continuously supplied to the tribo-contacts during sliding. Therefore, the presence of an oil film on the textured surface facilitates wear reduction to a great extent. Additionally, the textured surface can capture wear debris in the micro-dimples, preventing surface scratching due to the metallic debris. The EDS mapping of the textured surface before the sliding test shown in Figure 10d reveals a high oxygen content in the micro-dimples, where surface oxidation is typically induced by laser. The oxygen content is higher after the pin-on-disc test (Figure 10e), where the debris of metal oxides is trapped. This can limit the amount of wear particles as well as oxide debris generated under the mixed lubrication conditions to circulate in the tribo-contact. By trapping wear debris in the micro-dimples, surface scratching caused by the metallic debris is thus minimized. In addition to reducing surface friction through micro-hydrodynamic lubrication, the textured surface containing the micro-dimples has the ability to capture wear debris and limit surface scratching.

4. Conclusions

A nanosecond pulse laser was employed in this study to fabricate micro-dimples on the surface of SKH51 tool steel in an attempt to reduce its friction coefficient when sliding against AISI304 stainless steel. The effects of average laser power on the dimple sizes were examined and analyzed together with the laser ablation model. The untextured and micro-dimple textured surfaces having the different dimple densities underwent the pin-on-disc test to determine their coefficient of friction. The major findings and implications of this study can be summarized as follows:

(1)

The diameter and depth of the micro-dimples were found to increase with the laser power applied in the texturing process. However, a clean dimple with less recast deposition was obtained when using the laser power less than 40 W. The dimple diameter of about 100 μm with the aspect ratio of 0.1 and less formation of recast structures was achievable by using the laser power of 25 W.

(2)

Regarding the laser ablation model, the predicted dimple profile had a good agreement with the measured profile. The prediction was, however, accurate when the flow of the molten layer was minimal. The proposed model can be of help in defining the laser texturing conditions to create the desired dimple dimensions.

(3)

The friction coefficient of the surfaces in contact was reduced from 0.099 to 0.087 and from 0.107 to 0.096 at the sliding speed of 5 and 15 cm/s, respectively, when the tool steel surface was textured with the 35% dimple density. The micro-dimples can induce the positive pressure on the surfaces in contact, where the load-carrying force assists the separation of the surfaces through the hydrodynamic effect and reduces the friction accordingly.

(4)

In addition to friction reduction, there was no substantial wear found on the micro-dimple textured surfaces. The micro-dimples are able to continuously supply lubricant into the contact interface and also trap wear debris to prevent surface scratching during sliding. According to the findings, it is apparent that the texturing of tool and die surfaces with high density of micro-dimples can reduce friction and wear on the surfaces. This leads to the prolongation of the service life of tools and dies employed in metal forming and other related manufacturing and mechanical applications, e.g., cutting tools used in machining processes, devices in material handling, and transmission systems.