J. Cent. South Univ. (2020) 27: 2210-2219
DOI: https://doi.org/10.1007/s11771-020-4442-7
Effect of laser textured surface with different patterns on tribological characteristics of bearing material AISI 52100
PAUL JOSHUA S, DINESH BABU P
School of Mechanical Engineering, SASTRA Deemed University, Thanjavur 613401, Tamilnadu, India
Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract: Chrome steels are used in bearings since they possess high strength and wear resistance. However, when those parts are in service, failure happens due to sliding friction before the lifetime. To improve the durability of the American Iron and Steel Institute (AISI) 52100 chromium steel, in this work, the effect of laser surface texturing (LST) was analyzed. With the different patterns of circle and ellipse comparing with the untextured samples, the wear behavior was investigated using the pin-on-disc tribometer. The lubricant used for wear analysis is semisolid lithium grease National Lubricating Grease Institute lubricant (SKF NLGI-3). Sliding wear analysis was conducted at different loads of 10 N, 30 N and 50 N for the sliding speed of 750 r/min and 1400 r/min. The wear morphology was analyzed using a scanning electron microscope(SEM). The roughness of the samples was found using a white light interferometer. The effect of different patterns like circle and ellipse, alter the friction and wear properties of chromium alloy was observed compared with the untextured samples. LST shows considerable reduction in friction and wear for ellipsoidal pattern compared with the circular pattern because of wear debris and lubricant getting trapped.
Key words: laser surface micro texturing; AISI52100; 3D surface roughness; wear; scanning electron microscopy
Cite this article as: PAUL JOSHUA S, DINESH BABU P. Effect of laser textured surface with different patterns on tribological characteristics of bearing material AISI 52100 [J]. Journal of Central South University, 2020, 27(8): 2210-2219. DOI: https://doi.org/10.1007/s11771-020-4442-7.
1 Introduction
One of the most important components in the majority of machines is bearings whose exact demands are found satisfactory for increasing performance as well as efficiency, reliability, and durability. The bearing which is used in railway axle boxes, constant velocity joints in automobiles and ball screws is of bearing material AISI 52100. AISI 52100 is a chrome steel material having high strength and extensively preferred for its corrosion resistance. It is very common that rolling bearing has made an impact, and throughout the years it was the subject of broad research. Sometimes it happens that a bearing does not attain its calculated rating life. LU et al [1] created an unavoidable loss of material when there is a direct contact between metallic surfaces of the bearings. Even though the lubricants applied to the bearings reduce the friction rate, still there will be a considerable loss of material which should be taken into account. Hence the wear rate study plays a vital role in determining the life of bearing materials. Wear rate is an important factor in deciding the durability of the bearing. Minimizing friction is essential for efficiency improvement, to reduce wear, to operate at high speeds without overheating and to avoid unexpected failure of the bearing.Fundamentally, friction can diminish greatly by its shape, by its material, by applying a lubricant between surfaces or by confining the surfaces with an electromagnetic field.
ETISON et al [2] used surface texture method to create a particular kind of pattern onto the surface to change its tribological properties. Laser offers very good control while creating texture and also producing low environmental impact compared to other surface etching processes. An application of LST includes mechanical seals, piston rings and thrust bearings. The earlier application involves cylinder liner honing, which reduces adhesion wear and friction. MEMS devices recently developed a method with the help of laser to texture mechanical seal surfaces. SHARMA et al [3] found that the textured metal surface improved the bearing performance. SHARMA et al [4] used surface texture to give ideal outcomes for both load- carrying capacity and the coefficient of friction. Laser surface texturing (LST) reduces the surface- to-surface contact between the bearings and hence reduces the wear rate. That is the reason for incorporating laser surface texturing in bearing materials. DLC coatings, lubricant modification and solid lubricants offer improvement in tribological properties. GACHOT et al [5] used LST to a surface texturing procedure to improve the tribological properties of materials. Using a laser to make desired microstructures on the surface of the materials can improve load limit, wear rates, grease lifetime, and reduce friction coefficients. QIU et al [6] revealed that six diverse surface shapes, namely, round, ellipsoidal, roundabout, curved, triangular, and chevron, affect the lubricant film thickness. They detailed that the load-carrying capacity of the bearing increases due to lubricant film formation, which is accomplished with the ellipsoidal shape, an ideal geometry for these working conditions. LST can alter the nearby geometry and microstructure of substrate material. In this way, it will change the surface properties and also mechanical performance. Required texture created at the surface can trap wear particles, change pressure conveyance, save lubricant and reduce contact area. DINESH BABU et al [7] conducted wear analysis on Ti–6Al–4V titanium alloy using laser surface texturing. They concluded that wear resistance is improved substantially for the laser textured surfaces for 10 N load comparing with 30 N and 50 N loads. Their results clearly show that the use of suitable surface texturing with appropriate wear parameters can reduce the wear rate of a titanium alloy. Also changing the texture pattern affects the wear behavior. GIULIA et al [8] produced circular dimples by laser texture on steel pins. Textured pin showed a lower coefficient of friction compared with the untextured pin. When density was increased, the coefficient of friction would be reduced when compared with the untextured pin. BAI et al [9] thought that at high velocities, the number of bonds formed between contact surfaces would be reduced. So that there will be a reduction in friction forces and wear rate. Large surface roughness can increase friction force but cause a minimum wear rate by causing the transition of wear from adhesive wear to abrasive wear.
ZHANG et al [10] found that the friction coefficient of the non-textured surface was high when compared with a textured surface. At Tohoku University Japan, a CO2 laser was used to texture SiC surfaces to study the behaviour of LST on the transition from hydrodynamic to mixed lubrication regime. The results show significant improvement in the wear resistance, load capacity and friction coefficient of mechanical seals. Tribological performance depends on the shape of the pattern, geometry, and also the density of the textured pattern. Those samples, including micro dimples and micro-grooves, had been studied to improve the friction behaviour and wear characteristics. BURSTEIN et al [11] demonstrated that the hydrodynamic weight of the lubricants increased in the region of the dimples. They clarified that the lubricant began to turn out and increasingly gathered in the region of dimples. It could lead to the expansion of hydrodynamic weight close to the dimples.NANBU et al [12] showed that the impact of various surface dimple shapes alters the lubricant collection. The outcomes recommended that the surface base shapes, including a small scale wedge or potentially a smaller scale step bearing, will result in thicker lubricant film. WANG et al [13] found that there had been an impact of surface textural shape on the lubrication property of steel pair. CHEN et al [14] studied the tribological performance of gourd-shaped surface texture using a numerical optimization approach based on the finite element (FE) simulation. Their findings showed that the irregular texture pattern was most suitable to create greater fluid dynamic pressure because of better coordination and combination of the geometry. In turn, the lubrication conditions were improved and also the frictional co-efficient was reduced. ZHAI et al [15] analyzed the direct contact between opposite asperities, as well as contact among asperities was avoided by superlubricity, which leads to the achievement of an ultralow friction coefficient. A thin film was covering nanodiamonds in the lubricants, forming hydrogen bonding films adhering to steel surfaces. Stable superlubricity is maintained by the nanodiamonds, ensuring less wear as a result of the ball-bearing effect between contacting interfaces. The effects of surface defects on the super low friction behaviour at the carbon-water interface are investigated. Nanomaterials with heterogeneous structures having an edge atom can prevent the edge pinning effect during sliding contact, thus significantly reducing edge friction. This is because of the high intralayer strength and low interlayer bonding energy of nanomaterials structural.
A large quantum of work had already been carried out in texture pattern to reduce the friction and also to improve wear resistance of different materials. Still, for chrome steel (AISI52100) the influences of different texture patterns and its densities are yet to be clearly studied.This paved the pathway for the current study which involves laser surface texturing of chrome steel with the dimple shapes of the circle and ellipse whose texture densities are 14% with the spacing between the pattern being 300 μm. To understand the influence of these patterns on friction behaviour and wear morphology, the pin-on-disc test had been conducted and discussed elaborately.
2 Experimental procedure
2.1 Material
The material AISI 52100, which is of chrome steel, is used for bearings. The chemistry of the AISI52100 is shown in Table 1. The specimens used for texturing were cut into a length of 50 mm and diameter of 10 mm. Before conducting laser surface texturing, all specimens were ground and polished using different grit SiC papers and then cleaned ultrasonically in acetone. The average surface roughness Ra was 3.21 μm when measured using a profilometer. The hardness values of the polished samples were HRC 60.
Table 1 Chemical composition of AISI 52100 samples
2.2 Micro texturing
Micro-dimples of the ellipsoidal and circular pattern were made using Nd: YAG laser having a wavelength of 1064 nm. The power source was used which offered 20 W power and scanning speeds which can be kept between 1 mm/s to 10 m/s. The laser beam spot diameter was 7 μm, and the pulse frequency was 15 kHz. The texture density for the ellipsoidal pattern and the circular pattern was 14%.
Figure 1 Schematic sketch of Nd: YAG laser texturing (a) and laser textured sample (b)
2.3 White light interferometer
The white light interferometer is an optical instrument used in the measurement of surface profiles across a few micrometres. The white light will be passed to the sample. The condenser lens will be collimating the light from the light source. A beam splitter will be separating the light into reference and measurement beams. The returning beams will be relayed or will be sent to the image sensor by the beam splitter. There would have various interference patterns such that each and every pattern will be studied and it would be in the form of individual pixels. After the image is studied, the required data from the image can be analyzed, and the required value of the surface roughness can be obtained. All these are done using the computer-integrated software where various tools are available for analyzing the image. Selecting a particular boundary on an image will make the software calculate the average roughness over that region. These graphs will show the fall of white light along the surface of the metal as well as the dimples to incur the surface roughness and waviness along that coordinate. So the average value of roughness will be calculated along with the maximum height, peak height, roughness valley depth, and waviness.
2.4 Wear test
The sliding wear test was conducted with the help of pin-on-disc set up to understand the wear and friction character of the textured surface as well as the not textured surface. A pin-on-disc tribometer consisted of a stationary pin under an applied load in contact with a rotating disc. As the disc rotates contact was established between pin and disc, hence wear occurred on the pin. We were keeping the textured surface in the disc area to compare the loss of material between textured shapes and base metals. Figure 2 shows the pin-on- disc setup image in which the test was conducted. The pin sample was oriented perpendicular to rotating disc so that the pin can make contact with the flat disc without any error.
Figure 2 Pin-on-disc sliding wear
The tribometer used can be operated within the disc rotation speed of 200 to 2000 r/min. The load can also be varied from 5 N to 200 N depending upon the need. Friction study test, as well as wear test, was carried out at varying loads of 10 N, 30 N and 50 N while disc rotation speeds were 750 and 1400 r/min for a constant sliding distance of 1500 m as mentioned. In the load part, the required load was added, and a separate control mechanism was used to record the wear rate in the pin. The speed of the disc can be controlled according to our requirement. A timer would be there which was manually set in relation to the sliding distance kept. Wear rate and coefficient of friction could be obtained from the graphs and data from the computer which was connected to a pin-on-disc control system. With the help of the data acquisition system, the information about the frictional force that is generated on the pin which slides against the disc material and the respective values was stored in a computer. Here the sliding distance was kept constant as 1500 m and two speeds were calculated. National Lubricating Grease Institute lubricant NLGI-3 was applied on the surface of the pin before setting it in the sample holder and it was held tight with the disc. Approximately 0.01 g of lubricant was applied for each sample. NLGI-3 is a light colour heavy-duty grease based on a lithium-12-hydroxy stearate soap for long time lubrication under high load. It contains anti-wear additives, and it is a good corrosion protection agent. It is a relatively rigid grease which is used for long term lubrication of rolling and sliding bearings. It has a very good resistance to high pressure and good temperature resistant. The parameter combination selected for wear analysis is shown in Table 2.
Table 2 Parameters chosen for wear test
All these experiments were carried out for ellipsoidal pattern, circular pattern and the results are compared with each other to wear morphology, and hence the wear rate was studied. When each sliding test was finished, the wear produced wear in those specimens was determined by measuring the weight of the samples before the test and after the test using a weight scale. The mass loss and amount of volume loss and their density were calculated.
△V=△m/ρ (1)
L=πDNt (2)
where △V is the volume loss; △m is the mass loss; ρ is the density; D is wear track diameter, mm; N is revolution per minute (RPM); t is time, s.
The wear rate y is calculated:
η=△V/L (3)
where L is the sliding distance.
2.5 Scanning electron microscopy
SEM gives detail surface data by following a sample in a raster design with an electron beam. At the point when the incident electrons interact with the example, vivacious electrons are discharged from the outside of the sample. The disperse designs made by the connection yield data on size, shape, surface, and organization of the example. SEM produces high contrast, pictures of a textured surface with dimple dimensions. After the friction and wear tests, the worn topology of the samples was observed with an optical microscope and a scanning electron microscope (VEGA3-TESCAN).
3 Results and discussion
3.1 Assessments of textured surface
The texture pattern with dimensions is shown in Figures 3(a)-(e). The geometrical dimensions for the LST of the ellipsoidal pattern have been measured and the averages of those values are computed. The ellipse pattern major axis is found to be 150 μm, with its minor axis measure 100 μm and also the spacing between the ellipse being 200, 300 and 400 μm. For circle pattern, the diameter is 100 μm, and the distance from center to center is noticed as 300 μm. From that, it can be observed that the spattering of the metal surface overlays one over another, and hence there is an interference of two dimples occurring. When compared to 200 μm, dimple spacing 300 μm or 400 μm gives better layout in the textured surface. The surface area of a circle having 100 μm dimple diameter is 0.007 mm2. The surface area of a pin having 10 mm diameter is 78.54 mm2. The number of circular dimples is 1500, which gives 14% texture density. The surface area of an ellipse whose major axis and minor axis are 150 μm and 100 μm, respectively, is 0.047 mm2. The number of ellipse is 235, which gives the same texture density as that of the circular pattern.
3.2 Effect of surface roughness
Surface roughness is an influential factor while predicting the tribological behaviour. Friction coefficient induced by the deformation and collision of asperities is related to the surface roughness.The roughness of ellipsoidal, circle and base metal was subjected to analysis. In ellipsoidal sample, roughness was taken in major axis, minor axis, and center to center distance. In the circle, it was taken along the diameter and center to center distance. In the case of the un-textured sample, it was taken as a random as it did not have any textured surface.
The average roughness of the un-textured surface was found to be 3.21 μm. Wear happened due to adhesive and abrasive wear in the untextured surface. Adhesive wear of material was dominated by the shear modulus while the abrasive wear closely related with elastic modulus.For homogeneous material, elastic modulus is larger than the shear modulus.The roughness of the laser surface textured circle was found to be 6.8 μm. Since adhesion strength at the contact is influenced by surface roughness by altering the real contact area, friction and wear increase with the increase in the root mean square roughness.There had been a transition from adhesive wear to abrasive wear when there is an increase in the surface roughness. The roughness of laser surface textured ellipsoidal was 23.6 μm. The results of the roughness show that the roughness values are the lowest for the un-textured surface. In comparison with other shapes, ellipsoidal has the highest roughness and circle one.
The surface roughness increases the friction underwear due to the interlocking between the surface asperities, leading to increased trapping ability. So that lubricant will be stored as well as wear debris. The dimples aid in the collecting process of lubricant and debris. From the results, we conclude that even though surface roughness increased for the ellipsoidal pattern than the base metal, due to the above-mentioned mechanism, the coefficient of friction and wear reduces for the textured ellipsoidal pattern than the circular and even to a larger extent when compared with the untextured surface.
3.3 Wear characteristics
Wear test was conducted for ellipsoidal, circle pattern and base metal in a pin-on-disc tribometer to understand the wear behaviour. The average value of the co-efficient of friction was taken from the graph with a maximum possible error of 5% obtained through the software in pin-on-disc wear test apparatus. In load vs wear rate graph, for a speed of 750 r/min, the rate of wear almost increases linearly for circle whereas it increases slowly in the case of ellipsoidal. In every load, the ellipsoidal performs better than the circle, which means that there is larger wear in the circle. Also, it is a general trend that the wear rate will have an increase with an increase in speed, which is also true here and further other outcomes are also obtained. In the case of 1400 r/min wear rate increases for both ellipsoidal and circular patterns. Here also in every load ellipsoidal pattern performs better than the circle one. In going through both the cases, we can infer that, for the same speed and load, wear rate for the circle is noted higher than ellipsoidal. The ellipsoidal shape has more volume for the wear debris to be accumulated than the circle, which reduces the wear created by the debris particles. Also, the lubricant film thickness is maintained because of shallow dimples, and those dimples will act as a reservoir for lubricants. A single dimple temporarily increases the film thickness, but to sustain it, subsequent dimples were presented. Due to the fact that elastic deformation happening in the contact area for the metals lubricant was released from the dimples, high viscous lubricants were unable to flow freely from the dimples; hence, they are retained. Wear reduces because of the improved lubrication condition existing in the textured surface.
Figure 3 SEM image:
3.4 Coefficient of friction
In load vs co-efficient of friction graph, at speed of 750 r/min, the coefficient of friction falls at a good rate on increasing the load in the case of a circular dimple. The trend in ellipsoidal is different that the coefficient of friction decreases at a very low rate, and it looks almost constant. In the speed of 1400 r/min, the coefficient of friction of the circle almost decreases in a considerable fashion. Whereas in ellipsoidal, it shows a very low decrease and is not constant. When comparing the circle and ellipsoidal dimples under every load at 750 and 1400 r/min speed, there has been a huge reduction in the coefficient of friction for the ellipsoidal pattern. The reason for this behaviour is that the ellipsoidal pattern holds more wear particles than the circular pattern.
Figure 4 3D surface profile:
Figure 5 Effect of load with varying wear rate at speeds of 750 and 1400 r/min
The inference is that for the same speed, the value of the coefficient of friction decreases on increasing the load. This indicates that the effect of the dimple on tribological performance due to lubricant film could be with the existence of the dimples, with much easier film formation at higher sliding speeds. So on comparing wear rate and coefficient of friction, the relation between them is that on increasing the load, the wear rate increases with a decrease with a coefficient of friction on taking single speed into account.
Figure 6 Effect of load with varying coefficient of friction at 750 and 1400 r/min
There are two stages in the frictional process of laser-textured specimens. The coefficient of friction increases initially and then reaches an average constant value after some time. At times that laser textured dimples have bulges nearby, average CoF values may not always be low for all textured samples, even though CoF decreases for textured surfaces after some time. It means that coefficient of friction will at times exhibit high or than the as-received sample at the initial stages. Since there is no possibility to collect wear debris in untextured surface, the friction coefficient is higher.Whereas the textured surface generates less contact area, which aids the sliding to be smooth. Therefore, the CoF of textured specimens is lower than that of the untextured specimen. However, increasing the texture density will lead to an increase in surface roughness and thereby increase the friction coefficient. Lowering the density, on the other hand, causes more contact area as well as no possibility to collect wear debris.
3.5 Wear morphology
The SEM images of base metal, textured circle and textured ellipsoidal with dimple spacing 300 μm are shown in Figure 7. The base metal has a huge difference in microstructure, before and after the wear test. The base metal surfaces (Figure 7(b)) are almost having an abrasive and adhesive mode of wear, and there is no chance of lubricant staying as dimples. In the case of the textured circle, the surface is almost clean before the wear test (Figure 7(c)) and surface has considerable wear along with the surface (Figure 7(d)) as well as in the dimples after the wear. Dimples almost become extinct (Figure 7(d)), which means that the chance of lubricant staying is very less. In the case of textured ellipsoidal (Figures 7(e) and (f)), the microstructure of dimples is clearly visible in both cases because wear particles are collected in the dimple, which prevents further wear.
Perhaps the dimples are not 100% clear, but it has some sort of wear and disturbances in the dimple structure. The chance of lubricant staying is considerably high as there are enough spaces in the dimples which can accommodate them comfortably; when compared with the circle, it is once again evident that even with the same spacing, ellipsoidal pattern is able to hold the lubricant inside the dimples, so the rate of wear will be the minimum. The ellipsoidal shape has a micro-wedge or micro-step bottom, so its film thickness is more than the flat bottom, even the volume of the ellipsoidal is more than that of the circle, which enhances the accommodation of lubricant. Finally, the trend would be such that wear is the highest for the circle shape of 0.841 mm3/m and lowest for the ellipsoidal shape of 0.833 mm3/m. Hence, ellipsoidal shape can be preferred over circle and base metal in the case of laser surface texturing of AISI52100 steel.
Figure 7 SEM morphology of specimens before (a) and after wear test on untextured surface (b), circular pattern (c, d) and ellipsoidal pattern (e, f)
Untextured surface was observed more scratch and delamination, which confirms that there had been a mixed wear mechanism of abrasive and adhesive wear happening. WHEREAS the textured surface exhibits less wear and delamination is almost zero. The reason is that the dimples trap the small wear debris so that plowing of the surface is avoided. Also, the substrate of textured specimen absorbs energy during laser processing, leading to improved microhardness. The local quenched zone is used to collect the wear debris. At last, the stress concentration is uniform throughout the textured surface, and there is an excellent wear resistance.
4 Conclusions
1) Laser surface textured specimen has shown a considerable reduction in wear rate compared with the base metal due to the reduction in coefficient of friction for a textured surface. This is because of the capability to trap the wear debris, reduction in surface contact and lubricant entrapment in the dimples.
2) The wear rate of base metal is obviously more than the circular texture as it does not have any dimples after wear and chance of staying of lubricant is very less.
3) The wear rate of the circle is more than that of the ellipsoidal pattern. The wear results confirm that the wear rate is based on dimple geometry; ellipsoidal pattern has lower wear rate than circle or base metal.
4) Lubricant is able to stay in the textured dimples in the case of ellipsoidal pattern, whereas it is not possible in circle and base metal for the sliding distance selected. Also, the metal-to-metal contact is considerably reduced in ellipsoidal pattern. Also, the ellipsoidal shape has a micro- wedge or micro-step bottom, so its film thickness is more than the flat bottom, even the volume of the ellipsoidal is more than that of the circle, which enhances the accommodation of lubricant. Taking all these factors into account, the ellipsoidal shape with 300 μm spacing has the lowest wear rate.
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(Edited by YANG Hua)
中文导读
不同激光表面纹理对轴承材料AISI52100摩擦性能的影响
摘要：铬钢因具有高强度和耐磨性而用于轴承。 然而，在实际使用中，因滑动摩擦导致的故障而使寿命缩短。 为了提高AISI52100铬钢材料的耐久性，本文分析了激光表面纹理(LST)化的效果。 利用圆盘摩擦，对不同形状，如椭圆和圆，与未纹理化处理试样的磨损行为进行了对比研究。用于磨损分析的润滑剂是SKFNLGI-3半固体锂基润滑脂。在10、30和50 N的不同载荷下，用750 r/min和1400 r/min 2种滑动速度进行滑动磨损分析。用扫描电子显微镜(SEM)对磨损形貌进行了分析。样品的粗糙度用白光干涉仪测量。与未纹理化处理试样相比，观察到不同形状如圆和椭圆对铬合金摩擦磨损性能的影响。由于磨损碎片和润滑剂被捕获，激光表面纹理显示椭圆的摩擦和磨损与圆形图案相比，明显减少。
关键词：激光表面微纹理；轴承材料AISI52100；三维表面粗糙度；磨损；扫描电镜
Received date: 2020-02-18; Accepted date: 2020-05-22
Corresponding author: DINESH BABU P, PhD, Senior Assistant Professor; Tel: +91-9442451792; E-mail: dineshbabu@mech. sastra.edu, pdineshbabu81@gmail.com; ORCID: https://orcid.org/0000-0002-9606-3289
