Tribology Characteristics in 300 μm of Hexagonal Array Dimple Pattern

Abstract

In the tribological performance of materials, a textured surface reduces the friction coefficient and wear. This study investigates the effects of a pattern of 300 µm dimples in a hexagonal array on the tribological characteristics. Previous studies investigated 200 µm dimples by using a similar material and method. There are three frictional conditions based on the Stribeck curve: boundary friction, mixed friction, and fluid friction. In this experiment, we investigated the frictional characteristics by conducting frictional tests at sliding speeds ranging from 9.6 rpm to 143.3 rpm and a normal load ranging from 13.6 N to 92 N. We used a photolithography method to create dimples for surface texturing. We used five specimens with different dimple densities 10%, 15%, 20%, 25%, and 30% in this study. The dimple density on the surface area is one of the important factors affecting the friction characteristics. The duty number graph indicates a fully developed fluid friction regime. Fluid friction occurs at a velocity of 28.7-143.3 rpm. We observed the best performance at a dimple density of 10% and a dimple diameter of 300 µm in the hexagonal array, the lowest friction coefficient at 0.0037 with 9.6 rpm 9.6N load, and the maximum friction coefficient at 0.0267 with 143.3 rpm 92N load.

1. Introduction

Surface texturing onto a solid material during sliding contact can modify the friction characteristics of the material. Surface texturing to create square-patterned dimples or pores has been reported to reduce the coefficient of friction due to lubricant retention. In the previous research, the hexagonal pattern of 200 µm dimple has reported by Choi et al. [1], nevertheless the surface texturing method has been studied since 1940’s. Surface texturing can be performed by chemical, mechanical or varied method to create marking in the surface [2].

Improving tribology performance can reduce energy loss from engine, it has reported by Nakada [3], which is approximately 40% of the total energy loss caused by wear and friction. Hence, tribology performance improvement can reduce fuel consumption indirectly.

The improvement of the tribology performances with surface texturing is attributed mainly to the fact that textured surfaces may play a role as lubricant reservoirs, increasing the thickness of the lubricant film between the mating surfaces, thereby decreasing friction. In addition, the dimples also serve as pockets for wear particle embedment, preventing debris from further damaging the substrate surface via plowing and third body abrasion [4]. The third-body abrasion is also known to cause resistance due to the hydrodynamic disturbance of the lubricant during sliding motions, resulting in an increased coefficient of friction.

Scaraggi et al. reported that different surfaces with different micro hole depths exist, minimizing friction at the interface [5]. Wakuda et al. also reported that distribution of micro-dimples is an important factor against frictional characteristic [6].

The condition underlying surface texturing was divided by worn fragments trapping, lubricant reservoir and hydrodynamic, but it also depends on the size and density of the texture, as well as the depth of the pattern even if each theory about friction characteristic on surface texturing are reasonable [7]. Therefore, it can be assumed that the surface texturing density and depth of the hole have roles to play in surface texturing for low-friction materials.

The present study tested the friction coefficient for 300 µm surface dimple patterns. In several studies, grooves were created by a mechanical process or by using a laser. A different method is applied in this study, where photolithography is used to produce the dimple. The mechanism of surface texturing is decided by a worn fragment trapping, lubricant reservoir and hydrodynamic, but each theory has propriety, they depend on the density, depth, dimension of the pattern. Thus, there has been no complete study on surface texturing till date [8-13].

Stribeck showed that the friction of sliding bearings was high at low speeds, decreased to a minimum when metal-to-metal contact was ceased, and then increased again at higher speeds. He declared that friction as a function of load and speed, and published his research in the early 1900 [14] s.

Stribeck systematically studied the variation in friction between two liquid-lubricated surfaces as a function of speed for different loads. The graphs of friction force reported by Stribeck stem from a carefully conducted, wide-ranging series of experiments on journal bearings. They clearly show the minimum value of friction, now known as the transition between full fluid-film lubrication and some solid asperity interactions. The original results published by Stribeck best fit the classical “Stribeck curve”. The friction regimes for sliding of lubricated surfaces are traditionally categorized into solid/boundary friction, mixed friction, and fluid friction, on the basis of the “Stribeck curve” [15]. Accordingly, is possible to identify a point of minimum friction and apply it to a lubrication system.

The dimple diameter of 300 µm was chosen for continuing previous study, which resulted efficient friction coefficient at 10% dimple density with 200µm size in the hexagonal array configuration. The dimples resemble the bulges and are distributed evenly on the surface of the pin. In this study, the friction coefficient in a hexagonal array of 300 µm micro-dimple patterns will be investigated and discussed.

 

2. Materials and Methods

The pin specimen was steel, with 4 mm diameter and 1 mm thickness. Micro-dimple patterns were formed on its surface by photolithography, via wet etching. The dimple diameter of the prepared film photo mask was designed to be 300 µm. The test piece was made by using NaCl electrolyte and the dimple depth was about 2 µm within the hexagonal array pattern. 60 mm diameter and 5 mm thickness bearing steel material was used for disc material. It has 0.039 mm Ra surface roughness after polishing.

Fig. 1 shows the Stribeck curve [14]. Each friction region is clearly described by lubricant parameters. In this study, the duty number was used as the lubricant parameter. Duty number is a dimensionless number as a function of velocity in m/s, viscosity in Pas, load in N, and pin diameter in mm. The duty number is expressed as follows:

Fig. 1.Stribeck’s curve.

Fig. 2 shows the frictional tester and the data recording. The data were recorded in a computer. Seven variations of load were given to the test specimen, and the velocity was varied for each loading. The sensor (load cell) measured the surface friction in 60 s at 0.1 s time intervals for each variation. The test conditions are shown in Table 1, with the density ranging from 10% to 30% and the velocity ranging from 0.06 m/s to 0.34 m/s

Fig. 2.Schematic of experimental apparatus.

Table 1.Test Conditions

Pin and disk contacted perpendicularly. Position adjustment was made using the water level. Maintaining the pin at the level position is very important because the load must be maintained at the normal position, which is the position perpendicular to the contact surface.

The perpendicular position will help in maintaining the normal force distribution on the contact surface. Besides, the stability of the pin affects the load cell readability. For lubricating the pin, about 20 ml of paraffin oil was used. The lubricant must be flooded and cover the contact surface between the pin and the disk.

The dimple density is the ratio of dimple area to the total pin area, and hence, the dimple density is proportional to the number of dimples. The conditions of dimple pin were shown in Fig. 3 under an electron microscope with 25x magnification.

Fig. 3.Dimple pin density (a) 10%, (b) 15%, (c) 20%, (d) 25% and (e) 30%.

Fig. 4 shows the dimple pattern in the hexagonal array, as observed under an electron microscope with 800x magnification. The dimples are arranged in the symmetrically hexagon, and form a honeycomb-like structure when connected by an imaginary line. This configuration provides symmetry in the dimple distribution.

Fig. 4.Hexagonal array observed under an electron microscope.

 

3. Results and Discussion

3-1. Friction characteristics

The experiment results revealed several graphical tendencies in each variation. In the initial stage of the experiment, the sliding speed was 9.6 rpm. Fig. 5 depicts the friction coefficient as a function of load at 9.6 rpm. A dimple density of 15% provided the highest friction force at a load ranging from 13.8N to 83.2N. At the 92N of load, higher friction force was observed at the15%, 20% and 30% of densities. Decrement in the friction force was seen at 10% dimple density at load of 92 N to 25% dimple density of the pin.

Fig. 5.Friction force as a function of load at 9.6 rpm.

At higher rpm, the friction force was stable as seen in Fig. 6 and Fig. 7. No significant fluctuation in the friction coefficient occurred under these conditions. At 15% and 20% dimple density, variations in the load significantly affected the friction force.

Fig. 6.Friction force as a function of load at 56 rpm

Fig. 7.Friction force as a function of load at 143.3 rpm.

For 15% and 20% dimple density, high friction force was occurred and ranging from 0.575N to 1.070N. Three other pin density, 10%, 25% and 30% have lower friction force, which is ranging from 0.307N to 0.637N. This phenomenon indicated that pin density had important to control the friction force in the pin surface.

Velocity and load increment had also important roles to play in this test, as they both affected the friction force [14, 15]. The graph indicated that the friction force differed with the area density of the dimples. Among the five pin specimens considered, the pin with 10% density showed the best performance. The lowest friction force was observed when the sliding speed was in the range 9.6 rpm to 143.3 rpm.

The friction initially increased, reaching a maximum, and then tended to constant with an increment in the velocity and load [14].The dimpled pin with 10% has lowest slope, which is the ratio between friction force and load. Accordingly, it can be concluded that 10% density has carrier load higher than the other pins. Higher carrier load gives the pin ability to maintain the friction force stable with lower slope even the load increase significantly.

The slope calculation was shown in Table 2. The effectiveness of pin density to support load increment were shown by 10%, 25%, 30%, 15% and 20% respectively. It was indicated by the average slope in each load increment, which 0.0311 the lowest slope occurred at 10% pin density.

Table 2.Friction force slope in variation of density

There are three kinds of friction as classified on the basis of the Stribeck curve: boundary friction, mixed friction, and fluid friction [15]. Boundary friction occurs when two surfaces in contact rub against each other. Under this condition, third-body contact may occur.

The dimpled pin surface plays the main role in the friction, under the boundary friction conditions. The third-body contact is potentially reduced [7] in this case, as shown in Fig. 8, on the graph for 15% dimple density. Under a load of 13.8 N and sliding speed of 9.6-143.3 rpm, two regions of friction can be seen. The friction coefficient at the lowest sliding speed is initially high, and then decreases to a minimum with increasing sliding speed. The minimum friction coefficient is observed when the sliding speed s varied between 28.7 rpm to 143.3 rpm. The others pin density showed only one region of friction, which is hydrodynamic lubrication fully developed.

Fig. 8.Friction coefficient as a function of duty number at 13.8 N.

Fig. 9 shows that another condition with a different load, it has similar graph tendency in entire sliding velocity variation. Changes in the graph tendencies showed in Fig. 10, significantly can be seen in 15%, 20% and 30% of pin density.

Fig. 9.Friction coefficient as a function of duty number at 55.4N.

Fig. 10.Friction coefficient as a function of duty number at 92N.

The dimpled pin with 10% density showed the best performance at a higher load and sliding speed. The friction coefficient was the lowest when the load increased and remained almost constant with an increase in velocity as shown in Fig. 8, Fig. 9 and Fig. 10.

The minimum friction coefficient is generally seen in the 143.3 rpm sliding speed, which corresponds to the fluid friction condition. Hydrodynamic or liquid friction is fully developed when the sliding speed higher than 28.7 rpm. Correlation between load and sliding speed with friction coefficient is vice versa. Friction coefficient is decreasing with increment of load and increase with increment of sliding speed. This condition can be seen in the Fig. 11.

Fig. 11.Connection between friction coefficient with load and sliding speed.

The minimum friction coefficient is seen in the highest load. The maximum friction coefficient occurred in the highest sliding speed for each treatment. At 9.6 rpm and 13.6N the minimum friction coefficient 0.0171 occurred and 0.0267 for maximum friction coefficient at 143.3 rpm. The 10% pin has achieved the lowest friction coefficient at 92N load with 9.6 to 143.3 rpm of sliding speed variation. The friction coefficient ranged from 0.0037 to 0.0064. The result of friction coefficient can be seen in Table 3.

Table 3.Friction coefficient of 10% dimple density

3-2. Scanning electron microscope image

Fig. 12 shows the scanning electron microscope (SEM) images of the dimpled pin before and after the test, under 25X and 800X magnification. Fig. 12(a) shows the image, that the pin surface is covered with dimples appearing as dots.

Fig. 12.SEM image of dimpled pin (a) 10% dimple pin density, (b) dimple condition before test and (c) dimple condition after test.

Under solid friction conditions, the dimples are in contact with the disk surface. The free area among the dimples acts as a reservoir. Paraffin oil, the lubricant is trapped in this area, resulting in some advantages [2]. The pressure provided by load increment at lower speeds compresses the lubricant, because of the formation of a thin film between the contact surfaces.

The area that is not covered with dimples may act as a trap for worn fragments, so that these worn fragments are not welded into the contact surface. Small fragments are disadvantageous, potentially allowing for third-body contact. This condition was shown by Fig. 12(b) and (c). The fragment trap can be seen clearly in the Fig. 12(c) and sliding direction was indicated by pin surface scratch. Dimples prevent the worn fragment welded together with contact surface; hence third body contact can be minimized for sure.

 

4. Conclusions

In this study, we obtained the following results by performing frictional tests on a hexagonal array of 300 µm dimples:

1. The friction coefficient increases with an increase in the sliding speed and decrease with an increment of load.

2. The minimum friction coefficient is generally observed at the sliding speed 9.6 rpm with load ranges from 13.8N to 92N

3. The pin with various dimple densities shows two friction regimes: mixed, and fluid friction also known as mixed and hydrodynamic regime lubrication.

4. The friction coefficient of the sample with 10% dimple density is the most effective to reduce the friction for the velocity and load condition studied. The lowest friction coefficient was at 0.0037 with 9.6 rpm 9.6N load and maximum was at 0.0267 with 143.3 rpm 92N load.

5. Hydrodynamic or liquid friction is fully developed when the sliding speed higher than 28.7 rpm.