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Experimental study on effect of micro textured surfaces generated by ultrasonic vibration assisted face turning on friction and wear performance
Accepted Manuscript Title: Experimental study on effect of micro textured surfaces generated by ultrasonic vibration assisted face turning on friction and wear performance Author: S. Amini H. Nouri Hosseinabadi S.A. Sajjady PII: DOI: Reference:
S0169-4332(16)31500-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.064 APSUSC 33632
To appear in:
APSUSC
Received date: Revised date: Accepted date:
26-2-2016 15-6-2016 10-7-2016
Please cite this article as: S.Amini, H.Nouri Hosseinabadi, S.A.Sajjady, Experimental study on effect of micro textured surfaces generated by ultrasonic vibration assisted face turning on friction and wear performance, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Experimental study on effect of micro textured surfaces generated by ultrasonic vibration assisted face turning on friction and wear performance S. Amini,, H. Nouri Hosseinabadi, S.A. Sajjady Manufacturing Department, Faculty of Engineering, University of Kashan ,Kashan, Iran
Corresponding author.Address: University of Kashan, Kashan, Iran.Tel.:+98 31 5912497, Fax: .:+98 31 5912424
E-mail address: [email protected]
Corresponding author.Address: University of Kashan, Kashan, Iran.Tel.:+98 31 5912497 E-mail address: [email protected]
Highlights
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Owing to the unique nature of UVAT processes in creating micro-dimples on the surface, surface geometric properties and 3D parameters of roughness improve and surface hardness increases. These factors cause increased wear resistance and decreased friction coefficient. 3D ultrasonic vibration assisted face-turning (3D-VT) has the lowest friction coefficient. Compared with CT, the average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes at a maximum level decrease 13%, 18%, and 21%, respectively. 3D parameters of roughness are the main factors in decreasing friction coefficient caused by increased cutting speed. The results obtained from topography of surface texture and micro-hardness shows that the increased feed rate leads to the improvement of 3D parameters of roughness and surface hardness. As a result, wear resistance and friction coefficient decrease. ANOVA results for average friction coefficient indicate that type of UVAT process with 32.32% effectiveness has the most effect on average friction coefficient. Feed rate and cutting speed with the effectiveness of 17.46% and 11.73% are other parameters which affect output parameter of average friction coefficient.
Abstract Ultrasonic vibration-assisted turning process (UVAT) is one of the effective methods in improving the tribological properties. In this research, the effect of different machining parameters such as cutting speed and feed rate as well as the effect of three vibration modes of one-dimensional (LVT), elliptical (EVT), and three-dimensional (3D-VT) on the tribological properties is examined. In order to validate the results of UVAT process, conventional faceturning operation was performed as well. Wear and friction tests were performed using pin-ondisk wear and friction machine with identical vertical load and sliding speed. The results of the tests show that the surfaces upon which micro-dimples have been created by UVAT processes reduce the average friction coefficient, wear rate and adhesion between pin and samples surface compared with conventional machined surfaces. Compared with conventional face-turning surfaces, average friction coefficient of the surfaces face-turned by LVT, EVT, and 3D-VT processes, show a maximum decrease of 13%, 18%, and 21% respectively. Moreover, compared
with CT process, because of the unique features of UVAT process in creating micro-dimples, the contact between chrome steel pin and the sample surface decreases; this in turn leads to further reduction in wear rate for the processes of LVT, EVT, and 3D-VT respectively.
Keywords: Ultrasonic vibration-assisted turning, Tribological property, Micro-textured surface, Friction coefficient, wear rate.
1. Introduction Commonly a surface is described as the outer boundary of an object regardless of the depth of textures on the surface. However, in describing engineering surfaces it should be noted that depth of textures on the surface is also important that is due to its physical and chemical properties as well as its efficiency .
Depending on the method of creating surface textures, there are surfaces with a variety of depths ranging from a few microns to a few nanometer. That attribute of the surface that is determined by three parameters of topography, surface roughness, and waviness is called surface texture that is fundamentally important in studying tribological behaviors [1]. Surface texture is considered as an influential factor in the displacement of layer structure during wear operation and friction control. According to the researches have been conducted so far, it can be inferred that surface texture is studied in order to evaluate the effect of it on tribological properties during the sliding conditions. In the field of manufacturing, each machining process, which is used for finishing, leaves special effects which are related to the topic of surface texture and can affect the appearance and performance of the components. No engineering surface is ideally smooth and flat, even if it is polished perfectly. Therefore, when two surfaces are in contact with each other, only their summit points come in contact with each other and must bear the entire load; hence, local contact stresses will be high even at relatively low loads [1]. Today, with the progress of modern industry, demand has widely increased for the equipment and materials which, in terms of tribological behavior and surface quality, have appropriate performances. These performances include self-cleaning properties [2], wettability [3, 4], and optimized friction coefficient and wear resistance [5-8]. In this regard, several processes have been so far proposed to create micro-textures which include electrical discharge machining [9],
micro grinding/cutting/drilling [10-12], etching [13], ion beam texturing [14], and energy beam techniques [15]. Any process of creating surface texture has limitations and no process can fulfill all properties of the surface texture. Ultrasonic vibration-assisted machining (UVAM) process is a method that combines conventional machining (CM) with ultrasonic vibrations to create precise surfaces. This process, through applying vibration components to the tool, results in oscillatory changes during the machining process and finally to the creation of some textures on the workpiece surface. Finally, these created surfaces lead to the integrity of the workpiece surface. Surfaces created by UVAT process are different from surfaces created by other processes. The reason is that this process would achieve high dimensional precision and high surface quality that in turn reduces machining forces and lowers the temperature and tool wear without any limitation in machining of materials [16-19]. In 1950s and 1960s, Japanese researchers proposed ultrasonic vibration assisted machining theory. Then, American researchers confirm the potential of ultrasonic vibration assisted cutting process. In UVAM process the tool vibrates in a range of several micrometers to tens of micrometers and with vibrational frequencies in an ultrasonic range. One-dimensional UVAM process first was used in macro-scale turning operations on various kinds of materials in 1950s. Experimental tests of UVAM process were carried on materials such as steel, glass, and brittle ceramics in order to validate this process. Experimental results demonstrated that this process, compared with conventional turning (CT), can significantly affect output parameters such as diamond tool life and surface quality. In 1D-UVAM process, system performance is such that through assembly of the tool to an acoustic structure and stimulation of the acoustic structure in its natural frequency by using a low-amplitude and high-frequency transducer, the tool tip vibrates linearly. The original plan of 2D-UVAM systems first was developed in 1990s to reduce machining forces and improve tool life and surface quality in comparison with 1D-UVAM process. Structure design of 2D-UVAM systems is such that the tool tip vibrates in resonant frequency, in two dimensions and in an elliptical path [20, 21]. Gua et al., [22, 23] presented a new plan. This plan, in addition to applying vibratory motion to cutting and feed direction, added a tertiary vibratory motion, in radial direction, to the tool tip in an elliptical vibration cutting process.
Until 1960s, there was a traditional view stating that the more is the smoothness of the surface, the better is the performance of the surface in terms of tribological behaviors. Finally, the idea of creating micro textures was proposed to improve the performance of tribological properties. Hamilton proposed a theory which stated that asperity surfaces with regular micro features are usable to improve tribological properties. Moreover, asperity surfaces are considered as a place for storing lubricant [24, 25]. A variety of manufacturing processes have been so far proposed to create micro textures and evaluate their effects on wear and friction behaviors. Lithography and anisotropic etching process [26], abrasive jet machining and different laser processes such as LBM, LST, and LPT [6] are among these processes. Xing et al. [8] examined tribological properties of aluminum alloy surfaces resulted from ultrasonic vibration assisted milling method. In the current research, ultrasonic vibration assisted face-turning process was performed to create micro-dimple arrays on Al7075-T6 surfaces. Given the unique nature of ultrasonic vibration assisted machining in creating micro-dimple arrays on different surfaces, the impact of surface 3D structure was examined on tribological properties. Moreover, doing micro-hardness tests, the impact of surface hardness of UVAT process was evaluated on surface microstructure and wear and friction properties. Therefore, the effect of parameters such as cutting speed, feed rate and different modes of vibratory motion was examined on 3D geometrical properties of surface, 3D parameters of surface roughness, and surface hardness. Through experimental examination of the effects of each parameter, a valid argument was achieved to improve wear and friction properties.
2. Surface engineering and mechanisms of wear and friction In surface engineering, even under the best conditions and the most advanced manufacturing processes, no flat and smooth surface can be achieved. Investigation of 3D structure of the surface and its geometric characteristics can help to describe engineering surface, 3D parameters of surface roughness, and surface tribological properties. Geometric characteristics depend on processing operation, shape and distribution of asperities resulted from processing operation. Therefore, the created surface structure shows interaction between the surface processing parameters including geometry and movements of tool, tool’s/mating surface’s roughness rate during the processing operation.
3D structure of the surface is formed by repetitive summits and depressions. The parameters that characterize topography of the surface texture are known as 3D parameters of roughness. One of the most important functionalities of roughness is to examine tribological properties of surfaces (friction, wear, and lubrication). The most effective and practical 3D roughness parameters which affect wear and friction greatly and are examined in this research are as follows [1, 27, 28]: The Surfaces Area Ratio (Sdr), the Density of Summits (Sds) and the Mean Summit Curvature (Ssc). Sdr, expresses the increment of the interfacial surface area relative to the area of the projected (Fig. 1). Roughness parameter of Sdr is a useful factor in applications related to tribological properties such as adhesion and wetting [28].
(1)
Sds, is the number of local summits per area. Summits are derivatives of peaks. A peak can be defined as above all eight nearest points of neighbors. When under the sliding and loading conditions summits are deformed elastically and plastically, this parameter is consider as a key parameter. In sliding applications, there should be a number of summits in order to prevent optical contacting [28].
(2) Ssc, is defined as the average of the principal curvature of the summits, local maximums, on the surface. This parameter has a pivotal role in forecasting a surface’s elastic and plastic deformation. Moreover, it can be used to predict wear and friction applications [28]. Peripheral layer structure of the surface depends on the processing operation and consequently on its microstructure. Hence, the primary structure of the surface can be improved through selecting an appropriate surface operation. One of the most influential and widely used surface operations is surface hardness operation. Hardness refers to resistance and permeability of an object against the penetration of another one. Peripheral layer hardness is a function of object’s
material and microstructure and changes by changing the microstructure during processing operation. Surface hardness is another factor that has an impact on surface microstructure and the improvement of surface wear and friction properties. Generally, in UVAT process, because of the intermittent nature of cutting, the insert, according to the type of ultrasonic vibration process, is moved away from the cutting area at the end of each cutting cycle. Moreover, according to the feet rate and spindle speed, in the next cutting cycle, a contact is made between the insert and a portion of the previously machined surface. Hence, a portion of the surface becomes crushed. As a result, surface hardness increases in the crushed areas. Consequently, increased surface hardness caused by the type of UVAT process, increases wear resistance of the two frictional components and decreases friction coefficient. Friction phenomenon is caused because of contact between two surfaces and mechanical conflict in the summit points of the two frictional components. Among the theories proposed to explain friction, phenomenon adhesion theory and deformation theory of ploughing are more in agreement with experimental observations. When two surfaces come to contact with each other, they contact first in the highest points of asperity. Total area of these points is called real contact area that is a tiny proportion of the apparent contact area. These points are not able to withstand a high amount of force and, therefore, plasticity deformation and thereby adhesive bond occurs in them. Then, in order to break the welded areas, friction force is created. Wear phenomenon is defined as the destruction and separation of material particles from contact surfaces because of mechanical factors. Adhesive wear and abrasive wear are two effective mechanisms to control the wear rate. Adhesive wear occurs by material transfer from one surface to another one due to the adhesion of two surfaces during the operation. Topography of many surfaces is such that when two surfaces contact each other, only the summit points contact with each other and have to bear the whole load. Over time or increasing the load, the lower points of the surfaces come in contact that is due to the plastic deformation of the asperity areas. In other words, the number of contact points increases. Thus, while sliding, adhesive rate increases and adhesive force will damage the surface. Abrasive wear is made by hard particles isolated from the surfaces and because of abrasion and scratch of the contact surfaces which are moving against each other. This kind of wear can be controlled by factors such as increased hardness of the contact surfaces and decreased roughness of the surfaces [1, 27, 28].
Wear and friction phenomena are among the complex phenomena which are not predictable through general principles and should be tested experimentally. Wear and friction are among the most important properties of the surface during the contact of the surfaces. Moreover, friction effect shows itself on the cost of energy and, thus, for economic reasons, there is an urgent need to reduce friction. Accordingly, the focus of this research has been on wear and friction properties.
3. Experimental Details 3.1. Ultrasonic equipment and machine tools In this research, ultrasonic equipment were installed on TNC50 turning lathe made in Machine Sazi Tabriz. The device used to create ultrasonic vibrations has been MPI model made in Sweden. This device searches excitation frequency and provides the best frequency for the tool stimulation and performs auto resonance as feedback (close loop). UVAT tool used for the machining test include a set of full-ring stack for excitation of linear mode and two sets of halfring stack for bending mode excitation. Two sets of half-ring stack are excited by 180˚ phase shift to enhance the amplitude of flexural vibration. The reason is that during the expansion of one set, another set is contracting and vice versa. For linear mode, the stimulating signal has a 90˚ phase shift, compared with the flexural mode, to create an elliptical path at the tool tip. Inserts used in all machining processes are made of Polycrystalline Diamond insert (PCD) with tool tip radius of 0.4 mm, rake angle of 0˚ and clearance angle of 0˚. Since the wear of tool’s cutting edge is one of the factors affecting the precision of the results, cutting edge was replaced for machining of each sample. Furthermore, in all machining operations, cutting depth was considered as equal as 0.25 mm.
3.2. Experiment design and measurement Ultrasonic vibration assisted face-turning process was performed in three vibrational modes of linear (LVT), elliptical (EVT), and 3D micro-features and conventional turning as well. Working
axis of LVT process together with working planes of EVT and 3D-VT machining processes are shown in Fig. 3.
According to Fig 3, in LVT process, vibration direction is linear and in cutting direction. In EVT process, two vibratory direction along the axis z (depth direction) and axis x (cutting direction) lead to the creation of an elliptical motion. Finally, in 3D-VT process, in addition to vibration directions along the z and x axes, a tertiary direction is applied along the y axis (feed direction). To determine the number of required samples, full factorial method was used in this research. Factors include feed rate at three levels (0.14, 0.18, 0.22 mm/rev) and spindle speed in three levels (22.4, 31.5, 45 rpm). In general, 9 tests were performed for each machining mode. Hence, taking into account 3 vibration turning modes (LVT, EVT, and 3D-VT) together with the conventional turning (CT), a total number of 36 samples were machined. In this research, pin on disk wear and friction testing machine was employed to investigate tribological behaviors under the condition of identical vertical load and sliding speed. 3D optical microscope also was used to take 3D images of the surface texture.
3.3. Friction and wear tests In this research, in order to investigate wear and friction behaviors of the micro textured surfaces made by UVAT and to compare them with un-textured surfaces, dry wear and friction test were performed using the pin on disk testing method. Fig. 4 shows the pin on disk wear and friction testing machine and a schematic of its settings. Chrome steel pins with a diameter of 6 mm and a fixed position were positioned under a load of 10 N against the rotating disk. All tests were performed at a constant distance of 300 m and before each test, all of the samples’ surfaces were cleaned by alcohol. Wear and friction parameters are given in Table 1. In order to analyze the coefficient of friction, dynamic data were recorded during the test. Moreover, using an optical microscope, the surface topography in the worn areas of the Al7075-T6 and the chrome steel pin was studied.
4. Results and discussion 4.1. ANOVA results for average friction coefficient Analysis of variance for average coefficient of friction was performed by Minitab software with 95% confidence level. Contribution percentages of the Table 2 show the influence of each parameter on the results of the average friction coefficient. It can be seen that UVAT process type with the effectiveness of 32.32% has the largest impact on average friction coefficient. Then, feed rate with 17.46% and cutting speed with 11.73% effectiveness are other parameters that affect the output parameter of average friction coefficient.
4.2. Influences of UVAT parameters on the behavior of antifriction 4.2.1. Influences of vibration parameters on friction coefficient In order to evaluated the effect of UVAT process on friction coefficient, the changing trend of friction coefficient at different spindle speeds with constant parameters of the friction and wear tests (mentioned in Table 1) and feed rate of 0.14 mm/rev were evaluated in the three sample surfaces machined by UVAT process (LVT, EVT, and 3D-VT) and the sample obtained from conventional turning. The results are shown in Fig. 5a-c. Under experimental machining conditions and mentioned tribology tests for the samples of CT, LVT, EVT, and 3D-VT, the following conclusion is obtained: the samples machined by UVAT process have the lowest friction coefficient compared with the samples machined by CT process. Hence, 3D ultrasonic vibration assisted face-turning process (3D-VT) has the lowest friction coefficient. Furthermore, linear ultrasonic vibration assisted face-turning (LVT) and elliptical ultrasonic vibration assisted face-turning (EVT) processes, compared with CT process, further reduce the friction coefficient. Considering the constant spindle speed, average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes, compared with the sample machined by CT process, show a decrease of about 3-11%, 6-15%, and 8-17% respectively. Owing to the unique nature of UVAT process in creating micro-textures on surface, compared with un-textured surfaces created by CT process, in these processes, surface area increases in the depth and width of the surface and summits are decreased. According to Fig. 6, LVT process, due to its linear vibration nature in cutting direction, leads to the creation of micro-dimple arrays.
This process leads to the expansion of micro-textures in depth and width of the surface compared with un-textured surfaces. Surface texture topography of EVT and LVT processes has been shown in Figs. 6 and 7. By comparison of these figures, it can be understood that in EVT process, compared with LVT process, micro-dimples created on the surface are arrayed with a greater deviation angle in cutting direction. This is due to the 2D vibration nature of EVT process in depth direction (axis z) and cutting direction (axis x) associated with feed rate and rotational speed of machine tool. Hence, these factors lead to the expansion of micro-dimples in depth direction and cutting direction, compared with LVT process. Surface texture topography of 3D-VT process is shown in Fig. 8 and compared with surfaces texture topography of LVT and EVT processes (Figs. 6 and 7). From this comparison, and considering feed rate and spindle speed of machine tool, it can be concluded that in 3D-VT process, micro-dimples created on the surface are arrayed with a greater deviation angle than EVT process. This is due to the nature of vibration of 3D-VT process that in addition to vibrating in depth and cutting direction, can simultaneously apply the third vibration in y axis direction (feed direction). Therefore, microdimples created by this process extend in feed and cutting directions compared with EVT process, and in depth, feed and cutting directions compared with LVT process. In this regard, it can be concluded that surface area increases more in LVT, EVT, and 3D-VT respectively. Referring to Eq. (1), it is concluded that the further the surface area increases, the more the ratio between the interfacial and projected area, Sdr, will be increased. As a result, the contact ratio between two frictional components decreases. Moreover, LVT, EVT, and 3D-VT processes respectively lead to a further reduction in summits density. This, in return, reduces the contact of summit points in two frictional components and plastic deformation of asperity areas during the sliding. Hence, adhesion decreases and the least damaging effects appear on the sample surface. Observing the topography of textured surfaces and un-textured surfaces in Figs. 6c, 7c, and 8c, it is concluded that textured surfaces, compared with un-textured surfaces, have a higher radius of curvature at the summit curvature. Thus, LVT, EVT, and 3D-VT processes respectively result in micro-dimple arrays with greater mean summit curvatures which in turn reduce the stress concentration. In general, given the unique properties of the surfaces machined by LVT, EVT, and 3D-VT processes, by increasing the applied vibration directions to vibrating tools, friction coefficient decreases more compared with CT process.
Surface hardness is another factor that has an impact on surface microstructure and the improvement of surface wear and friction properties. Generally, in UVAT process, because of the intermittent nature of cutting, the insert, according to the type of ultrasonic vibration process, is moved away from the cutting area at the end of each cutting cycle. Moreover, according to the feed rate and spindle speed, in the next cutting cycle, a contact is made between the insert and a portion of the previously machined surface. Hence, a portion of the surface becomes crushed. As a result, surface hardness increases in the crushed areas. As mentioned, 3D-VT process, due to vibration in three directions of depth, cutting, and feed, creates more expansion micro-dimples in feed and cutting direction compared with EVT. In comparison with LVT, it creates more expansion micro-dimples in three directions of depth, feed, and cutting. Therefore, LVT, EVT, and 3D-VT processes respectively increase micro-dimples width and, hence, the width of crushed areas and lead to increased surface hardness. Thus, increased surface hardness caused by the type of UVAT process, increases wear resistance of the two frictional components and decreases friction coefficient (Fig. 9).
4.2.2. Influence of spindle speeds on friction coefficient Likewise, the effect of spindle speed was examined on friction coefficient. In this regard, changing trend of friction coefficient was investigated in the three sample surfaces machined by UVAT process (LVT, EVT, and 3D-VT) and the sample obtained from CT under the following conditions: spindle speeds of 22.4, 31.5, and 45 rpm with constant vertical load of 10 N; fixed velocity of 0.2 m/s; distance of 100 m and feed rate of 0.18 mm/rev. The results are shown in Fig. 5a-c. Under experimental machining conditions and mentioned tribological tests, average friction coefficient of samples face-turned by CT, LVT, EVT, and 3D-VT processes is calculated in different spindle speeds. The results are shown in Fig. 5d. According to Fig. 5d, it is concluded that the changing trend of spindle speed has a reverse relationship with average friction coefficient. Therefore, with the increase of spindle speed, average friction coefficient decreases. In this research, it has been tried to achieve a valid argument on the effectiveness of vibration and machining parameters. This is done through examining 3D surface structure and unique features of the surfaces created by UVAT process including 3D parameters of surface roughness
and hardness. In examining the impact of spindle speed on friction coefficient, 3D parameters of surface roughness and hardness have been evaluated. Hence, the impact of each parameter on wear resistance of the two frictional components and friction coefficient has been investigated. Moreover, surface hardness was investigated that the results are shown in experimental results of micro-hardness tests (Fig. 10) and topography of the textured surfaces affected by the changes in spindle speed (Figs. 11, 12, and 13). As the results imply, the increase of cutting speed leads to the expansion of micro-dimples in cutting direction. However, since conflict and separation cycles of the workpiece and vibration tool extremely depend on cutting speed, the effects of ultrasonic vibrations decrease greatly because of increased cutting speed. Hence, the overlapping areas caused by the effects of ultrasonic vibrations or, rather, the crushed areas are reduced and, thereby, surface hardness decreases.
In evaluating 3D parameters of roughness, as shown in Figs. 11, 12, and 13, the increase in cutting speed expands the width of micro-dimples on textured surfaces. As a result of this, surfaces area increases. According to Eq. (1), the increase in surface area leads the ratio between the interfacial and projected area, Sdr, to increase and hence conflict between the surfaces of the two frictional components decreases. In investigating two parameters of summits density, Sds, and mean summits curvature, Ssc, it can be concluded that the increase of cutting speed leads to the decrease of summits density and the increase of mean summits curvature. These arguments suggest that optimal conditions of stress concentration in local maximums, low surface contact, and low plastic deformation in local maximums, all caused by increased cutting speed, decrease damaging effects of adhesion and heat accumulation. As a result, frictional force (ploughing force), friction coefficient and wear rate of two contacting surfaces are reduced. As a final conclusion, it can be said that 3D parameters of surface roughness are the main factors in reducing friction coefficient caused by increased cutting speed.
4.2.3. Influence of feed rate on friction coefficient Similarly, the effect of feed rate was examined on friction coefficient. In this regard, changing trend of friction coefficient was investigated in the three sample surfaces machined by UVAT
process (LVT, EVT, and 3D-VT) and the sample obtained from CT under the following conditions: fixed velocity of 0.2 m/s, constant vertical load of 10 N, constant distance of 100 m and spindle speeds of 22.4 rpm, and feed rates of 0.14, 0.18, and 0.22 mm/rev. The results are shown in Fig. 14a-c. Average friction coefficient of the samples face-turned by CT, LVT, EVT, and 3D-VT processes has been calculated in different feed rates (Fig. 14d). According to Fig. 14d, there is a reverse relationship between the changing trend of feed rate and average friction coefficient. Thus, the increase of feed rate will reduce average friction coefficient. With regard to a constant feed rate, average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes respectively is lower than those of CT process. Values of average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes decrease 5-13%, 7-18%, and 9-21% respectively compared with CT process.
As seen in Figs. 15, 16, and 17, the increase of feed rate has a direct impact on the increased width of micro-dimples in feed direction; it also affects expansion of overlapping areas of ultrasonic vibration effect in feed direction. By increasing the extent of the crushed areas in feed direction, surface hardness increases in these areas. The results obtained from the micro-hardness tests (Fig. 18) confirm this argument that: the increase of feed rate increases surface hardness and hence leads to increased wear resistance in two contacting surfaces and decreased friction coefficient. In evaluating 3D parameters of surface hardness caused by changes in feed rate, it can be concluded that through expansion of micro-dimples in feed direction, surface area and consequently Sdr increases. Therefore, decreased interference of the contacting surfaces and summits density together with increased mean summit curvature leads to the improvement of friction and wear conditions.
4.3. Morphologies of worn scars
4.3.1. The wear rates of the worn sample’s surfaces Morphology of the worn surface of the samples face-turned by CT, LVT, EVT, and 3D-VT processes is shown in Figs. 19a-d respectively. Images are taken by an optical microscope with
20-200 magnification. According to these photos, wear effects are observable on both textured surfaces and un-textured surfaces. However, intensity of the wear effects is different on each surface. As shown in Fig. 19a, the surface of un-textured samples is almost completely worn. Hence, the wear effects left on these surfaces are wider. The width of wear effects indicates that both abrasion and adhesion phenomena have a significant impact on the creation of wear effects on the samples surfaces. As shown in Figs. 19b-d, wear effects of the surfaces obtained from UVAT process are narrower than those of CT surfaces. So that, on the surfaces face-turned by LVT process (Fig. 19b), wear effects are less and milder than those of CT process. Moreover, for EVT surfaces, wear effects are far less than LVT surfaces. Finally, the least wear effects can be observed on the surfaces face-turned by 3D-VT process (Fig. 19c and d). Although un-textured surfaces are smoother, the density of asperity areas is more in these surfaces. Topography of many surfaces is such that when two surfaces contact each other, only the summit points contact each other and have to bear the whole load. Over time or increasing the load, the lower points of the surfaces come in contact that is due to the plastic deformation of the asperity areas. In other words, the number of contact points increases. Thus, while sliding, adhesion rate increases and adhesion force will damage the surface. However, when UVAT process is performed, the density of asperity areas decreases that is due to the creation of microtextures on surface. Several factors are involved in the creation of micro-dimples by using UVAT process. Vibratory motion modes and vibration amplitudes are two factors affecting the formation of micro-dimples and the density of asperity areas. Compared with CT process, LVT process, due to the creation of micro linear grooves in only one direction, can decrease the density of asperity areas on the surface. EVT process, due to the creation micro-dimples in two directions, can decrease the density of asperity areas in two dimensions and, hence, has less asperity areas compared with LVT process. 3D-VT process has the ability to create micro-dimples in three directions and, hence, can decrease the density of asperity areas in three dimensions. Thus, compared with CT, LVT, and EVT process, creates less asperities on the surface. When the density of asperity areas decrease on the surface, adhesion decreases during the sliding and surface’s wear resistance increases.
Wear rate of worn areas on surfaces face-turned in different vibration modes, spindle speeds and feed rates are calculated that are shown in Figs. 20 and 21. Obviously, wear rate decreases during the wear test for all samples including both those obtained from UVAT and conventional turning. Under similar conditions of tribological tests, the wear rate of the worn surfaces obtained from UVAT process is less than conventional turning samples. Therefore, wear rate of the samples in 3D-VT is less than EVT and in EVT is less than LVT.
4.3.2. Analysis of wear loss of chrome steel pin’s surfaces Morphology of worn surfaces of chrome steel pins under the influence of sliding contact with the samples surfaces after a distance of 300 m is shown in Fig. 22. According to this figure, wear areas of chrome steel pins can be seen in all ultrasonic vibration processes (CT, LVT, EVT, and 3D-VT). However, the extent of pins worn areas varies for each process. Extent of the worn areas for the pins which have been in contact with the surfaces face-turned by UVAT processes is less than the pins which have been in contact with CT surfaces. Thus, extent of the worn areas for the pins in contact with the machined surfaces in 3D-VT process is less than EVT, and in EVT is less than LVT.
4. Conclusion In this research, tribological properties were evaluated in ultrasonic vibration assisted faceturning. In order to get valid arguments in relation to the influence of machining and vibratory parameters of UVAT process on wear and friction properties, 3D parameters of roughness and surface hardness have been studied experimentally. Hence, the results of the impacts of each parameter on wear resistance and friction coefficient are as follows: -
Owing to the unique nature of UVAT processes in creating micro-dimples on the surface, surface geometric properties and 3D parameters of roughness improve and surface hardness increases. These factors cause increased wear resistance and decreased friction coefficient.
-
3D ultrasonic vibration assisted face-turning (3D-VT) has the lowest friction coefficient. Compared with CT, the average friction coefficient of the samples face-turned by LVT,
EVT, and 3D-VT processes at a maximum level decrease 13%, 18%, and 21%, respectively. -
3D parameters of roughness are the main factors in decreasing friction coefficient caused by increased cutting speed; whereas, the increase of the cutting speed reduced the overlapping areas caused by the effects of ultrasonic vibrations or, rather, the crushed areas to the cutting length in the cutting direction and, thereby, surface hardness decreased.
-
The results obtained from topography of surface texture and micro-hardness shows that the increased feed rate leads to the improvement of 3D parameters of roughness and surface hardness. As a result, wear resistance and friction coefficient decrease.
-
ANOVA results for average friction coefficient indicate that type of UVAT process with 32.32% effectiveness has the most effect on average friction coefficient. Feed rate and cutting speed with the effectiveness of 17.46% and 11.73% are other parameters which affect output parameter of average friction coefficient.
References [1] Arnell D. Mechanisms and laws of friction and wear. Tribology and Dynamics of Engine and Powertrain: Fundamentals, Applications and Future Trends. 2010:41. [2] Li X-M, Reinhoudt D, Crego-Calama M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chemical Society Reviews. 2007;36:1350-68. [3] Matsumura T, Iida F, Hirose T, Yoshino M. Micro machining for control of wettability with surface topography. Journal of Materials Processing Technology. 2012;212:2669-77. [4] Lu Y, Guo P, Pei P, Ehmann KF. Experimental studies of wettability control on cylindrical surfaces by elliptical vibration texturing. The International Journal of Advanced Manufacturing Technology. 2015;76:1807-17. [5] Abdollahi A, Alizadeh A, Baharvandi HR. Dry sliding tribological behavior and mechanical properties of Al2024–5wt.% B 4 C nanocomposite produced by mechanical milling and hot extrusion. Materials & Design. 2014;55:471-81. [6] Li K, Yao Z, Hu Y, Gu W. Friction and wear performance of laser peen textured surface under starved lubrication. Tribology International. 2014;77:97-105.
[7] Sudeep U, Pandey R, Tandon N. Effects of surface texturing on friction and vibration behaviors of sliding lubricated concentrated point contacts under linear reciprocating motion. Tribology International. 2013;62:198-207. [8] Xing D, Zhang J, Shen X, Zhao Y, Wang T. Tribological properties of ultrasonic vibration assisted milling aluminium alloy surfaces. Procedia CIRP. 2013;6:539-44. [9] Abbas NM, Solomon DG, Bahari MF. A review on current research trends in electrical discharge machining (EDM). International Journal of Machine Tools and Manufacture. 2007;47:1214-28. [10] Denkena B, Kästner J, Wang B. Advanced microstructures and its production through cutting and grinding. CIRP Annals-Manufacturing Technology. 2010;59:67-72. [11] Pujana J, Rivero A, Celaya A, de Lacalle LL. Analysis of ultrasonic-assisted drilling of Ti6Al4V. International Journal of Machine Tools and Manufacture. 2009;49:500-8. [12] Barani A, Amini S, Paktinat H, Tehrani AF. Built-up edge investigation in vibration drilling of Al2024T6. Ultrasonics. 2014;54:1300-10. [13] Wang X, Kato K. Improving the anti-seizure ability of SiC seal in water with RIE texturing. Tribology letters. 2003;14:275-80. [14] Zhou L, Kato K, Umehara N, Miyake Y. Nanometre scale island-type texture with controllable height and area ratio formed by ion-beam etching on hard-disk head sliders. Nanotechnology. 1999;10:363. [15] Ryk G, Etsion I. Testing piston rings with partial laser surface texturing for friction reduction. Wear. 2006;261:792-6. [16] Sajjady S, Abadi HNH, Amini S, Nosouhi R. Analytical and experimental study of topography of surface texture in ultrasonic vibration assisted turning. Materials & Design. 2015;93:311–23. [17] Silberschmidt VV, Mahdy SM, Gouda MA, Naseer A, Maurotto A, Roy A. Surface-roughness improvement in ultrasonically assisted turning. Procedia CIRP. 2014;13:49-54. [18] Celaya A, Lopez de Lacalle LN, Campa FJ, Lamikiz A. Ultrasonic Assisted Turning of mild steels. International Journal of Materials and Product Technology. 2009;37:60-70. [19] Celaya. A LdLLN, Campa F.J, Lamikiz A. Application of Ultrasonics as Assistance in Machining Operations: University of the Basque Country; (pp. 159-172). [20] Brehl DE, Dow TA. Review of vibration-assisted machining. Precision Engineering 2008;32:153–72. [21] Negishi N. Elliptical vibration assisted machining with single crystal diamond Tools. 2003. [22] Guo P, Ehmann KF. Development of a new vibrator for elliptical vibration texturing. ASME Conference Proceedings2011. p. 373-80.
[23] Guo P, Ehmann KF. Development of a tertiary motion generator for elliptical vibration texturing. Precision Engineering. 2013;37:364-71. [24] Shen X-H, Tao G-C. Tribological behaviors of two micro textured surfaces generated by vibrating milling under boundary lubricated sliding. The International Journal of Advanced Manufacturing Technology. 2015:1-8. [25] Hamilton D, Walowit J, Allen C. A theory of lubrication by microirregularities. Journal of Fluids Engineering. 1966;88:177-85. [26] Pettersson U, Jacobson S. Influence of surface texture on boundary lubricated sliding contacts. Tribology International. 2003;36:857-64. [27] Burakowski T, Wierzchon T. Surface engineering of metals: principles, equipment, technologies: CRC press; 1998. [28] Cohen DK. Glossary of surface texture parameters. Michigan metrology. 2004.
Fig. 1. Schematic of the Developed Interfacial Area Ratio.
Fig. 2. Experimental setup for UAT tool used for the machining tests.
Fig. 3. Schematic shape of elliptical ultrasonic vibration process.
Fig. 4. (a) Pin on disk wear and friction testing machine, (b) Schematic of surface sliding against pin.
Fig. 5. The variations of friction coefficient of machined surfaces by CT, LVT, EVT and 3D-VT at different spindle speeds (a) 22.4 rpm, (b) 31.5 rpm, (c) 45 rpm, (d) Average friction coefficient.
Fig. 6. Evaluation of surface texture obtained from LVT under the feed rate of 0.18 mm/rev and spindle speed of 31.5 rpm.
Fig. 7. Evaluation of surface texture obtained from EVT under the feed rate of 0.18 mm/rev and spindle speed of 31.5 rpm.
Fig. 8. Evaluation of surface texture obtained from 3D-VT under the feed rate of 0.18 mm/rev and spindle speed of 31.5 rpm.
Fig. 9. Hardness of machined surfaces at UVAT processes.
Fig. 10. Hardness of machined surfaces at spindle speeds.
Fig. 11. Comparison of surface topography for LVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.14mm/rev, spindle speed of 45rpm.
Fig. 12. Comparison of Surface Topography for EVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5 rpm, (b) Feed rate of 0.14mm/rev, spindle speed of 45 rpm.
Fig. 13. Comparison of surface topography for 3D micro-features (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.14mm/rev, spindle speed of 45rpm.
Fig. 14. Variations of friction coefficient of machined surfaces by CT, LVT, EVT and 3D-VT at feed rate of (a) 0.14 mm/rev, (b) 0.18 mm/rev, (c) 0.22 mm/rev, (d) Average friction coefficient.
Fig. 15. Comparison of surface topography for LVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.18mm/rev, spindle speed of 31.5rpm.
Fig. 16. Comparison of Surface Topography for EVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5 rpm, (b) Feed rate of 0.18mm/rev, spindle speed of 31.5 rpm.
Fig. 17. Comparison of surface topography for 3D micro-features (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.18mm/rev, spindle speed of 31.5rpm.
Fig. 18. Hardness of machined surfaces at feed rates.
Fig. 19. Morphologies of wear scars of four kinds of un-textured surface and textured surfaces after distance of 300 m at velocity of 0.2 m/s; (a) CT, (b) LVT, (c) EVT, (d) 3D-VT.
Fig. 20. (a) Weight loss and (b) Wear rates of the worn sample’s surfaces with four kinds of samples (CT, LVT, EVT and 3D-VT) at distance. (Spindle speed of 31.5 rpm; Feed rate of 0.18 mm/rev).
Fig. 21. (a) Wear rates of the worn sample’s surfaces with variation of feed rates at distance. (EVT process; cutting speed of 31.5 rpm), (b) Wear rates of the worn sample’s surfaces with variation of cutting speeds at distance. (EVT process; Feed rate of 0.18 mm/rev).
Fig. 22. 2D images of the worn surfaces of the chrome steel pins after distance of 300 m at velocity of 0.2 m/s; (a) CT, (b) LVT, (c) EVT, (d) 3D-VT.
Table 1. Parameters of the friction and wear tests. Parameters
Values
Units
Rotational speed
rpm (m/s)
85 (0.2)
Distance
m
300
Normal load
N
10
Temperature
C ̊
Ambient temperature
Table 2. ANOVA results of average friction coefficient. Source
DF
Seq SS
Adj MS
F
P
Contribution (%)
Vc (rpm)
2
0.0011787
0.0005894
4.27
0.005
11.73
Vf (mm/rev)
2
0.0017547
0.0008774
6.35
0.024
17.46
Process
3
0.0032473
0.0010824
7.84
0.001
32.32
Error
28
0.0038674
0.0001381
-
-
38.49
Total
35
0.0100482
-
-
-
100
S0169-4332(16)31500-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.064 APSUSC 33632
To appear in:
APSUSC
Received date: Revised date: Accepted date:
26-2-2016 15-6-2016 10-7-2016
Please cite this article as: S.Amini, H.Nouri Hosseinabadi, S.A.Sajjady, Experimental study on effect of micro textured surfaces generated by ultrasonic vibration assisted face turning on friction and wear performance, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Experimental study on effect of micro textured surfaces generated by ultrasonic vibration assisted face turning on friction and wear performance S. Amini,, H. Nouri Hosseinabadi, S.A. Sajjady Manufacturing Department, Faculty of Engineering, University of Kashan ,Kashan, Iran
Corresponding author.Address: University of Kashan, Kashan, Iran.Tel.:+98 31 5912497, Fax: .:+98 31 5912424
E-mail address: [email protected]
Corresponding author.Address: University of Kashan, Kashan, Iran.Tel.:+98 31 5912497 E-mail address: [email protected]
Highlights
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Owing to the unique nature of UVAT processes in creating micro-dimples on the surface, surface geometric properties and 3D parameters of roughness improve and surface hardness increases. These factors cause increased wear resistance and decreased friction coefficient. 3D ultrasonic vibration assisted face-turning (3D-VT) has the lowest friction coefficient. Compared with CT, the average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes at a maximum level decrease 13%, 18%, and 21%, respectively. 3D parameters of roughness are the main factors in decreasing friction coefficient caused by increased cutting speed. The results obtained from topography of surface texture and micro-hardness shows that the increased feed rate leads to the improvement of 3D parameters of roughness and surface hardness. As a result, wear resistance and friction coefficient decrease. ANOVA results for average friction coefficient indicate that type of UVAT process with 32.32% effectiveness has the most effect on average friction coefficient. Feed rate and cutting speed with the effectiveness of 17.46% and 11.73% are other parameters which affect output parameter of average friction coefficient.
Abstract Ultrasonic vibration-assisted turning process (UVAT) is one of the effective methods in improving the tribological properties. In this research, the effect of different machining parameters such as cutting speed and feed rate as well as the effect of three vibration modes of one-dimensional (LVT), elliptical (EVT), and three-dimensional (3D-VT) on the tribological properties is examined. In order to validate the results of UVAT process, conventional faceturning operation was performed as well. Wear and friction tests were performed using pin-ondisk wear and friction machine with identical vertical load and sliding speed. The results of the tests show that the surfaces upon which micro-dimples have been created by UVAT processes reduce the average friction coefficient, wear rate and adhesion between pin and samples surface compared with conventional machined surfaces. Compared with conventional face-turning surfaces, average friction coefficient of the surfaces face-turned by LVT, EVT, and 3D-VT processes, show a maximum decrease of 13%, 18%, and 21% respectively. Moreover, compared
with CT process, because of the unique features of UVAT process in creating micro-dimples, the contact between chrome steel pin and the sample surface decreases; this in turn leads to further reduction in wear rate for the processes of LVT, EVT, and 3D-VT respectively.
Keywords: Ultrasonic vibration-assisted turning, Tribological property, Micro-textured surface, Friction coefficient, wear rate.
1. Introduction Commonly a surface is described as the outer boundary of an object regardless of the depth of textures on the surface. However, in describing engineering surfaces it should be noted that depth of textures on the surface is also important that is due to its physical and chemical properties as well as its efficiency .
Depending on the method of creating surface textures, there are surfaces with a variety of depths ranging from a few microns to a few nanometer. That attribute of the surface that is determined by three parameters of topography, surface roughness, and waviness is called surface texture that is fundamentally important in studying tribological behaviors [1]. Surface texture is considered as an influential factor in the displacement of layer structure during wear operation and friction control. According to the researches have been conducted so far, it can be inferred that surface texture is studied in order to evaluate the effect of it on tribological properties during the sliding conditions. In the field of manufacturing, each machining process, which is used for finishing, leaves special effects which are related to the topic of surface texture and can affect the appearance and performance of the components. No engineering surface is ideally smooth and flat, even if it is polished perfectly. Therefore, when two surfaces are in contact with each other, only their summit points come in contact with each other and must bear the entire load; hence, local contact stresses will be high even at relatively low loads [1]. Today, with the progress of modern industry, demand has widely increased for the equipment and materials which, in terms of tribological behavior and surface quality, have appropriate performances. These performances include self-cleaning properties [2], wettability [3, 4], and optimized friction coefficient and wear resistance [5-8]. In this regard, several processes have been so far proposed to create micro-textures which include electrical discharge machining [9],
micro grinding/cutting/drilling [10-12], etching [13], ion beam texturing [14], and energy beam techniques [15]. Any process of creating surface texture has limitations and no process can fulfill all properties of the surface texture. Ultrasonic vibration-assisted machining (UVAM) process is a method that combines conventional machining (CM) with ultrasonic vibrations to create precise surfaces. This process, through applying vibration components to the tool, results in oscillatory changes during the machining process and finally to the creation of some textures on the workpiece surface. Finally, these created surfaces lead to the integrity of the workpiece surface. Surfaces created by UVAT process are different from surfaces created by other processes. The reason is that this process would achieve high dimensional precision and high surface quality that in turn reduces machining forces and lowers the temperature and tool wear without any limitation in machining of materials [16-19]. In 1950s and 1960s, Japanese researchers proposed ultrasonic vibration assisted machining theory. Then, American researchers confirm the potential of ultrasonic vibration assisted cutting process. In UVAM process the tool vibrates in a range of several micrometers to tens of micrometers and with vibrational frequencies in an ultrasonic range. One-dimensional UVAM process first was used in macro-scale turning operations on various kinds of materials in 1950s. Experimental tests of UVAM process were carried on materials such as steel, glass, and brittle ceramics in order to validate this process. Experimental results demonstrated that this process, compared with conventional turning (CT), can significantly affect output parameters such as diamond tool life and surface quality. In 1D-UVAM process, system performance is such that through assembly of the tool to an acoustic structure and stimulation of the acoustic structure in its natural frequency by using a low-amplitude and high-frequency transducer, the tool tip vibrates linearly. The original plan of 2D-UVAM systems first was developed in 1990s to reduce machining forces and improve tool life and surface quality in comparison with 1D-UVAM process. Structure design of 2D-UVAM systems is such that the tool tip vibrates in resonant frequency, in two dimensions and in an elliptical path [20, 21]. Gua et al., [22, 23] presented a new plan. This plan, in addition to applying vibratory motion to cutting and feed direction, added a tertiary vibratory motion, in radial direction, to the tool tip in an elliptical vibration cutting process.
Until 1960s, there was a traditional view stating that the more is the smoothness of the surface, the better is the performance of the surface in terms of tribological behaviors. Finally, the idea of creating micro textures was proposed to improve the performance of tribological properties. Hamilton proposed a theory which stated that asperity surfaces with regular micro features are usable to improve tribological properties. Moreover, asperity surfaces are considered as a place for storing lubricant [24, 25]. A variety of manufacturing processes have been so far proposed to create micro textures and evaluate their effects on wear and friction behaviors. Lithography and anisotropic etching process [26], abrasive jet machining and different laser processes such as LBM, LST, and LPT [6] are among these processes. Xing et al. [8] examined tribological properties of aluminum alloy surfaces resulted from ultrasonic vibration assisted milling method. In the current research, ultrasonic vibration assisted face-turning process was performed to create micro-dimple arrays on Al7075-T6 surfaces. Given the unique nature of ultrasonic vibration assisted machining in creating micro-dimple arrays on different surfaces, the impact of surface 3D structure was examined on tribological properties. Moreover, doing micro-hardness tests, the impact of surface hardness of UVAT process was evaluated on surface microstructure and wear and friction properties. Therefore, the effect of parameters such as cutting speed, feed rate and different modes of vibratory motion was examined on 3D geometrical properties of surface, 3D parameters of surface roughness, and surface hardness. Through experimental examination of the effects of each parameter, a valid argument was achieved to improve wear and friction properties.
2. Surface engineering and mechanisms of wear and friction In surface engineering, even under the best conditions and the most advanced manufacturing processes, no flat and smooth surface can be achieved. Investigation of 3D structure of the surface and its geometric characteristics can help to describe engineering surface, 3D parameters of surface roughness, and surface tribological properties. Geometric characteristics depend on processing operation, shape and distribution of asperities resulted from processing operation. Therefore, the created surface structure shows interaction between the surface processing parameters including geometry and movements of tool, tool’s/mating surface’s roughness rate during the processing operation.
3D structure of the surface is formed by repetitive summits and depressions. The parameters that characterize topography of the surface texture are known as 3D parameters of roughness. One of the most important functionalities of roughness is to examine tribological properties of surfaces (friction, wear, and lubrication). The most effective and practical 3D roughness parameters which affect wear and friction greatly and are examined in this research are as follows [1, 27, 28]: The Surfaces Area Ratio (Sdr), the Density of Summits (Sds) and the Mean Summit Curvature (Ssc). Sdr, expresses the increment of the interfacial surface area relative to the area of the projected (Fig. 1). Roughness parameter of Sdr is a useful factor in applications related to tribological properties such as adhesion and wetting [28].
(1)
Sds, is the number of local summits per area. Summits are derivatives of peaks. A peak can be defined as above all eight nearest points of neighbors. When under the sliding and loading conditions summits are deformed elastically and plastically, this parameter is consider as a key parameter. In sliding applications, there should be a number of summits in order to prevent optical contacting [28].
(2) Ssc, is defined as the average of the principal curvature of the summits, local maximums, on the surface. This parameter has a pivotal role in forecasting a surface’s elastic and plastic deformation. Moreover, it can be used to predict wear and friction applications [28]. Peripheral layer structure of the surface depends on the processing operation and consequently on its microstructure. Hence, the primary structure of the surface can be improved through selecting an appropriate surface operation. One of the most influential and widely used surface operations is surface hardness operation. Hardness refers to resistance and permeability of an object against the penetration of another one. Peripheral layer hardness is a function of object’s
material and microstructure and changes by changing the microstructure during processing operation. Surface hardness is another factor that has an impact on surface microstructure and the improvement of surface wear and friction properties. Generally, in UVAT process, because of the intermittent nature of cutting, the insert, according to the type of ultrasonic vibration process, is moved away from the cutting area at the end of each cutting cycle. Moreover, according to the feet rate and spindle speed, in the next cutting cycle, a contact is made between the insert and a portion of the previously machined surface. Hence, a portion of the surface becomes crushed. As a result, surface hardness increases in the crushed areas. Consequently, increased surface hardness caused by the type of UVAT process, increases wear resistance of the two frictional components and decreases friction coefficient. Friction phenomenon is caused because of contact between two surfaces and mechanical conflict in the summit points of the two frictional components. Among the theories proposed to explain friction, phenomenon adhesion theory and deformation theory of ploughing are more in agreement with experimental observations. When two surfaces come to contact with each other, they contact first in the highest points of asperity. Total area of these points is called real contact area that is a tiny proportion of the apparent contact area. These points are not able to withstand a high amount of force and, therefore, plasticity deformation and thereby adhesive bond occurs in them. Then, in order to break the welded areas, friction force is created. Wear phenomenon is defined as the destruction and separation of material particles from contact surfaces because of mechanical factors. Adhesive wear and abrasive wear are two effective mechanisms to control the wear rate. Adhesive wear occurs by material transfer from one surface to another one due to the adhesion of two surfaces during the operation. Topography of many surfaces is such that when two surfaces contact each other, only the summit points contact with each other and have to bear the whole load. Over time or increasing the load, the lower points of the surfaces come in contact that is due to the plastic deformation of the asperity areas. In other words, the number of contact points increases. Thus, while sliding, adhesive rate increases and adhesive force will damage the surface. Abrasive wear is made by hard particles isolated from the surfaces and because of abrasion and scratch of the contact surfaces which are moving against each other. This kind of wear can be controlled by factors such as increased hardness of the contact surfaces and decreased roughness of the surfaces [1, 27, 28].
Wear and friction phenomena are among the complex phenomena which are not predictable through general principles and should be tested experimentally. Wear and friction are among the most important properties of the surface during the contact of the surfaces. Moreover, friction effect shows itself on the cost of energy and, thus, for economic reasons, there is an urgent need to reduce friction. Accordingly, the focus of this research has been on wear and friction properties.
3. Experimental Details 3.1. Ultrasonic equipment and machine tools In this research, ultrasonic equipment were installed on TNC50 turning lathe made in Machine Sazi Tabriz. The device used to create ultrasonic vibrations has been MPI model made in Sweden. This device searches excitation frequency and provides the best frequency for the tool stimulation and performs auto resonance as feedback (close loop). UVAT tool used for the machining test include a set of full-ring stack for excitation of linear mode and two sets of halfring stack for bending mode excitation. Two sets of half-ring stack are excited by 180˚ phase shift to enhance the amplitude of flexural vibration. The reason is that during the expansion of one set, another set is contracting and vice versa. For linear mode, the stimulating signal has a 90˚ phase shift, compared with the flexural mode, to create an elliptical path at the tool tip. Inserts used in all machining processes are made of Polycrystalline Diamond insert (PCD) with tool tip radius of 0.4 mm, rake angle of 0˚ and clearance angle of 0˚. Since the wear of tool’s cutting edge is one of the factors affecting the precision of the results, cutting edge was replaced for machining of each sample. Furthermore, in all machining operations, cutting depth was considered as equal as 0.25 mm.
3.2. Experiment design and measurement Ultrasonic vibration assisted face-turning process was performed in three vibrational modes of linear (LVT), elliptical (EVT), and 3D micro-features and conventional turning as well. Working
axis of LVT process together with working planes of EVT and 3D-VT machining processes are shown in Fig. 3.
According to Fig 3, in LVT process, vibration direction is linear and in cutting direction. In EVT process, two vibratory direction along the axis z (depth direction) and axis x (cutting direction) lead to the creation of an elliptical motion. Finally, in 3D-VT process, in addition to vibration directions along the z and x axes, a tertiary direction is applied along the y axis (feed direction). To determine the number of required samples, full factorial method was used in this research. Factors include feed rate at three levels (0.14, 0.18, 0.22 mm/rev) and spindle speed in three levels (22.4, 31.5, 45 rpm). In general, 9 tests were performed for each machining mode. Hence, taking into account 3 vibration turning modes (LVT, EVT, and 3D-VT) together with the conventional turning (CT), a total number of 36 samples were machined. In this research, pin on disk wear and friction testing machine was employed to investigate tribological behaviors under the condition of identical vertical load and sliding speed. 3D optical microscope also was used to take 3D images of the surface texture.
3.3. Friction and wear tests In this research, in order to investigate wear and friction behaviors of the micro textured surfaces made by UVAT and to compare them with un-textured surfaces, dry wear and friction test were performed using the pin on disk testing method. Fig. 4 shows the pin on disk wear and friction testing machine and a schematic of its settings. Chrome steel pins with a diameter of 6 mm and a fixed position were positioned under a load of 10 N against the rotating disk. All tests were performed at a constant distance of 300 m and before each test, all of the samples’ surfaces were cleaned by alcohol. Wear and friction parameters are given in Table 1. In order to analyze the coefficient of friction, dynamic data were recorded during the test. Moreover, using an optical microscope, the surface topography in the worn areas of the Al7075-T6 and the chrome steel pin was studied.
4. Results and discussion 4.1. ANOVA results for average friction coefficient Analysis of variance for average coefficient of friction was performed by Minitab software with 95% confidence level. Contribution percentages of the Table 2 show the influence of each parameter on the results of the average friction coefficient. It can be seen that UVAT process type with the effectiveness of 32.32% has the largest impact on average friction coefficient. Then, feed rate with 17.46% and cutting speed with 11.73% effectiveness are other parameters that affect the output parameter of average friction coefficient.
4.2. Influences of UVAT parameters on the behavior of antifriction 4.2.1. Influences of vibration parameters on friction coefficient In order to evaluated the effect of UVAT process on friction coefficient, the changing trend of friction coefficient at different spindle speeds with constant parameters of the friction and wear tests (mentioned in Table 1) and feed rate of 0.14 mm/rev were evaluated in the three sample surfaces machined by UVAT process (LVT, EVT, and 3D-VT) and the sample obtained from conventional turning. The results are shown in Fig. 5a-c. Under experimental machining conditions and mentioned tribology tests for the samples of CT, LVT, EVT, and 3D-VT, the following conclusion is obtained: the samples machined by UVAT process have the lowest friction coefficient compared with the samples machined by CT process. Hence, 3D ultrasonic vibration assisted face-turning process (3D-VT) has the lowest friction coefficient. Furthermore, linear ultrasonic vibration assisted face-turning (LVT) and elliptical ultrasonic vibration assisted face-turning (EVT) processes, compared with CT process, further reduce the friction coefficient. Considering the constant spindle speed, average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes, compared with the sample machined by CT process, show a decrease of about 3-11%, 6-15%, and 8-17% respectively. Owing to the unique nature of UVAT process in creating micro-textures on surface, compared with un-textured surfaces created by CT process, in these processes, surface area increases in the depth and width of the surface and summits are decreased. According to Fig. 6, LVT process, due to its linear vibration nature in cutting direction, leads to the creation of micro-dimple arrays.
This process leads to the expansion of micro-textures in depth and width of the surface compared with un-textured surfaces. Surface texture topography of EVT and LVT processes has been shown in Figs. 6 and 7. By comparison of these figures, it can be understood that in EVT process, compared with LVT process, micro-dimples created on the surface are arrayed with a greater deviation angle in cutting direction. This is due to the 2D vibration nature of EVT process in depth direction (axis z) and cutting direction (axis x) associated with feed rate and rotational speed of machine tool. Hence, these factors lead to the expansion of micro-dimples in depth direction and cutting direction, compared with LVT process. Surface texture topography of 3D-VT process is shown in Fig. 8 and compared with surfaces texture topography of LVT and EVT processes (Figs. 6 and 7). From this comparison, and considering feed rate and spindle speed of machine tool, it can be concluded that in 3D-VT process, micro-dimples created on the surface are arrayed with a greater deviation angle than EVT process. This is due to the nature of vibration of 3D-VT process that in addition to vibrating in depth and cutting direction, can simultaneously apply the third vibration in y axis direction (feed direction). Therefore, microdimples created by this process extend in feed and cutting directions compared with EVT process, and in depth, feed and cutting directions compared with LVT process. In this regard, it can be concluded that surface area increases more in LVT, EVT, and 3D-VT respectively. Referring to Eq. (1), it is concluded that the further the surface area increases, the more the ratio between the interfacial and projected area, Sdr, will be increased. As a result, the contact ratio between two frictional components decreases. Moreover, LVT, EVT, and 3D-VT processes respectively lead to a further reduction in summits density. This, in return, reduces the contact of summit points in two frictional components and plastic deformation of asperity areas during the sliding. Hence, adhesion decreases and the least damaging effects appear on the sample surface. Observing the topography of textured surfaces and un-textured surfaces in Figs. 6c, 7c, and 8c, it is concluded that textured surfaces, compared with un-textured surfaces, have a higher radius of curvature at the summit curvature. Thus, LVT, EVT, and 3D-VT processes respectively result in micro-dimple arrays with greater mean summit curvatures which in turn reduce the stress concentration. In general, given the unique properties of the surfaces machined by LVT, EVT, and 3D-VT processes, by increasing the applied vibration directions to vibrating tools, friction coefficient decreases more compared with CT process.
Surface hardness is another factor that has an impact on surface microstructure and the improvement of surface wear and friction properties. Generally, in UVAT process, because of the intermittent nature of cutting, the insert, according to the type of ultrasonic vibration process, is moved away from the cutting area at the end of each cutting cycle. Moreover, according to the feed rate and spindle speed, in the next cutting cycle, a contact is made between the insert and a portion of the previously machined surface. Hence, a portion of the surface becomes crushed. As a result, surface hardness increases in the crushed areas. As mentioned, 3D-VT process, due to vibration in three directions of depth, cutting, and feed, creates more expansion micro-dimples in feed and cutting direction compared with EVT. In comparison with LVT, it creates more expansion micro-dimples in three directions of depth, feed, and cutting. Therefore, LVT, EVT, and 3D-VT processes respectively increase micro-dimples width and, hence, the width of crushed areas and lead to increased surface hardness. Thus, increased surface hardness caused by the type of UVAT process, increases wear resistance of the two frictional components and decreases friction coefficient (Fig. 9).
4.2.2. Influence of spindle speeds on friction coefficient Likewise, the effect of spindle speed was examined on friction coefficient. In this regard, changing trend of friction coefficient was investigated in the three sample surfaces machined by UVAT process (LVT, EVT, and 3D-VT) and the sample obtained from CT under the following conditions: spindle speeds of 22.4, 31.5, and 45 rpm with constant vertical load of 10 N; fixed velocity of 0.2 m/s; distance of 100 m and feed rate of 0.18 mm/rev. The results are shown in Fig. 5a-c. Under experimental machining conditions and mentioned tribological tests, average friction coefficient of samples face-turned by CT, LVT, EVT, and 3D-VT processes is calculated in different spindle speeds. The results are shown in Fig. 5d. According to Fig. 5d, it is concluded that the changing trend of spindle speed has a reverse relationship with average friction coefficient. Therefore, with the increase of spindle speed, average friction coefficient decreases. In this research, it has been tried to achieve a valid argument on the effectiveness of vibration and machining parameters. This is done through examining 3D surface structure and unique features of the surfaces created by UVAT process including 3D parameters of surface roughness
and hardness. In examining the impact of spindle speed on friction coefficient, 3D parameters of surface roughness and hardness have been evaluated. Hence, the impact of each parameter on wear resistance of the two frictional components and friction coefficient has been investigated. Moreover, surface hardness was investigated that the results are shown in experimental results of micro-hardness tests (Fig. 10) and topography of the textured surfaces affected by the changes in spindle speed (Figs. 11, 12, and 13). As the results imply, the increase of cutting speed leads to the expansion of micro-dimples in cutting direction. However, since conflict and separation cycles of the workpiece and vibration tool extremely depend on cutting speed, the effects of ultrasonic vibrations decrease greatly because of increased cutting speed. Hence, the overlapping areas caused by the effects of ultrasonic vibrations or, rather, the crushed areas are reduced and, thereby, surface hardness decreases.
In evaluating 3D parameters of roughness, as shown in Figs. 11, 12, and 13, the increase in cutting speed expands the width of micro-dimples on textured surfaces. As a result of this, surfaces area increases. According to Eq. (1), the increase in surface area leads the ratio between the interfacial and projected area, Sdr, to increase and hence conflict between the surfaces of the two frictional components decreases. In investigating two parameters of summits density, Sds, and mean summits curvature, Ssc, it can be concluded that the increase of cutting speed leads to the decrease of summits density and the increase of mean summits curvature. These arguments suggest that optimal conditions of stress concentration in local maximums, low surface contact, and low plastic deformation in local maximums, all caused by increased cutting speed, decrease damaging effects of adhesion and heat accumulation. As a result, frictional force (ploughing force), friction coefficient and wear rate of two contacting surfaces are reduced. As a final conclusion, it can be said that 3D parameters of surface roughness are the main factors in reducing friction coefficient caused by increased cutting speed.
4.2.3. Influence of feed rate on friction coefficient Similarly, the effect of feed rate was examined on friction coefficient. In this regard, changing trend of friction coefficient was investigated in the three sample surfaces machined by UVAT
process (LVT, EVT, and 3D-VT) and the sample obtained from CT under the following conditions: fixed velocity of 0.2 m/s, constant vertical load of 10 N, constant distance of 100 m and spindle speeds of 22.4 rpm, and feed rates of 0.14, 0.18, and 0.22 mm/rev. The results are shown in Fig. 14a-c. Average friction coefficient of the samples face-turned by CT, LVT, EVT, and 3D-VT processes has been calculated in different feed rates (Fig. 14d). According to Fig. 14d, there is a reverse relationship between the changing trend of feed rate and average friction coefficient. Thus, the increase of feed rate will reduce average friction coefficient. With regard to a constant feed rate, average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes respectively is lower than those of CT process. Values of average friction coefficient of the samples face-turned by LVT, EVT, and 3D-VT processes decrease 5-13%, 7-18%, and 9-21% respectively compared with CT process.
As seen in Figs. 15, 16, and 17, the increase of feed rate has a direct impact on the increased width of micro-dimples in feed direction; it also affects expansion of overlapping areas of ultrasonic vibration effect in feed direction. By increasing the extent of the crushed areas in feed direction, surface hardness increases in these areas. The results obtained from the micro-hardness tests (Fig. 18) confirm this argument that: the increase of feed rate increases surface hardness and hence leads to increased wear resistance in two contacting surfaces and decreased friction coefficient. In evaluating 3D parameters of surface hardness caused by changes in feed rate, it can be concluded that through expansion of micro-dimples in feed direction, surface area and consequently Sdr increases. Therefore, decreased interference of the contacting surfaces and summits density together with increased mean summit curvature leads to the improvement of friction and wear conditions.
4.3. Morphologies of worn scars
4.3.1. The wear rates of the worn sample’s surfaces Morphology of the worn surface of the samples face-turned by CT, LVT, EVT, and 3D-VT processes is shown in Figs. 19a-d respectively. Images are taken by an optical microscope with
20-200 magnification. According to these photos, wear effects are observable on both textured surfaces and un-textured surfaces. However, intensity of the wear effects is different on each surface. As shown in Fig. 19a, the surface of un-textured samples is almost completely worn. Hence, the wear effects left on these surfaces are wider. The width of wear effects indicates that both abrasion and adhesion phenomena have a significant impact on the creation of wear effects on the samples surfaces. As shown in Figs. 19b-d, wear effects of the surfaces obtained from UVAT process are narrower than those of CT surfaces. So that, on the surfaces face-turned by LVT process (Fig. 19b), wear effects are less and milder than those of CT process. Moreover, for EVT surfaces, wear effects are far less than LVT surfaces. Finally, the least wear effects can be observed on the surfaces face-turned by 3D-VT process (Fig. 19c and d). Although un-textured surfaces are smoother, the density of asperity areas is more in these surfaces. Topography of many surfaces is such that when two surfaces contact each other, only the summit points contact each other and have to bear the whole load. Over time or increasing the load, the lower points of the surfaces come in contact that is due to the plastic deformation of the asperity areas. In other words, the number of contact points increases. Thus, while sliding, adhesion rate increases and adhesion force will damage the surface. However, when UVAT process is performed, the density of asperity areas decreases that is due to the creation of microtextures on surface. Several factors are involved in the creation of micro-dimples by using UVAT process. Vibratory motion modes and vibration amplitudes are two factors affecting the formation of micro-dimples and the density of asperity areas. Compared with CT process, LVT process, due to the creation of micro linear grooves in only one direction, can decrease the density of asperity areas on the surface. EVT process, due to the creation micro-dimples in two directions, can decrease the density of asperity areas in two dimensions and, hence, has less asperity areas compared with LVT process. 3D-VT process has the ability to create micro-dimples in three directions and, hence, can decrease the density of asperity areas in three dimensions. Thus, compared with CT, LVT, and EVT process, creates less asperities on the surface. When the density of asperity areas decrease on the surface, adhesion decreases during the sliding and surface’s wear resistance increases.
Wear rate of worn areas on surfaces face-turned in different vibration modes, spindle speeds and feed rates are calculated that are shown in Figs. 20 and 21. Obviously, wear rate decreases during the wear test for all samples including both those obtained from UVAT and conventional turning. Under similar conditions of tribological tests, the wear rate of the worn surfaces obtained from UVAT process is less than conventional turning samples. Therefore, wear rate of the samples in 3D-VT is less than EVT and in EVT is less than LVT.
4.3.2. Analysis of wear loss of chrome steel pin’s surfaces Morphology of worn surfaces of chrome steel pins under the influence of sliding contact with the samples surfaces after a distance of 300 m is shown in Fig. 22. According to this figure, wear areas of chrome steel pins can be seen in all ultrasonic vibration processes (CT, LVT, EVT, and 3D-VT). However, the extent of pins worn areas varies for each process. Extent of the worn areas for the pins which have been in contact with the surfaces face-turned by UVAT processes is less than the pins which have been in contact with CT surfaces. Thus, extent of the worn areas for the pins in contact with the machined surfaces in 3D-VT process is less than EVT, and in EVT is less than LVT.
4. Conclusion In this research, tribological properties were evaluated in ultrasonic vibration assisted faceturning. In order to get valid arguments in relation to the influence of machining and vibratory parameters of UVAT process on wear and friction properties, 3D parameters of roughness and surface hardness have been studied experimentally. Hence, the results of the impacts of each parameter on wear resistance and friction coefficient are as follows: -
Owing to the unique nature of UVAT processes in creating micro-dimples on the surface, surface geometric properties and 3D parameters of roughness improve and surface hardness increases. These factors cause increased wear resistance and decreased friction coefficient.
-
3D ultrasonic vibration assisted face-turning (3D-VT) has the lowest friction coefficient. Compared with CT, the average friction coefficient of the samples face-turned by LVT,
EVT, and 3D-VT processes at a maximum level decrease 13%, 18%, and 21%, respectively. -
3D parameters of roughness are the main factors in decreasing friction coefficient caused by increased cutting speed; whereas, the increase of the cutting speed reduced the overlapping areas caused by the effects of ultrasonic vibrations or, rather, the crushed areas to the cutting length in the cutting direction and, thereby, surface hardness decreased.
-
The results obtained from topography of surface texture and micro-hardness shows that the increased feed rate leads to the improvement of 3D parameters of roughness and surface hardness. As a result, wear resistance and friction coefficient decrease.
-
ANOVA results for average friction coefficient indicate that type of UVAT process with 32.32% effectiveness has the most effect on average friction coefficient. Feed rate and cutting speed with the effectiveness of 17.46% and 11.73% are other parameters which affect output parameter of average friction coefficient.
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Fig. 1. Schematic of the Developed Interfacial Area Ratio.
Fig. 2. Experimental setup for UAT tool used for the machining tests.
Fig. 3. Schematic shape of elliptical ultrasonic vibration process.
Fig. 4. (a) Pin on disk wear and friction testing machine, (b) Schematic of surface sliding against pin.
Fig. 5. The variations of friction coefficient of machined surfaces by CT, LVT, EVT and 3D-VT at different spindle speeds (a) 22.4 rpm, (b) 31.5 rpm, (c) 45 rpm, (d) Average friction coefficient.
Fig. 6. Evaluation of surface texture obtained from LVT under the feed rate of 0.18 mm/rev and spindle speed of 31.5 rpm.
Fig. 7. Evaluation of surface texture obtained from EVT under the feed rate of 0.18 mm/rev and spindle speed of 31.5 rpm.
Fig. 8. Evaluation of surface texture obtained from 3D-VT under the feed rate of 0.18 mm/rev and spindle speed of 31.5 rpm.
Fig. 9. Hardness of machined surfaces at UVAT processes.
Fig. 10. Hardness of machined surfaces at spindle speeds.
Fig. 11. Comparison of surface topography for LVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.14mm/rev, spindle speed of 45rpm.
Fig. 12. Comparison of Surface Topography for EVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5 rpm, (b) Feed rate of 0.14mm/rev, spindle speed of 45 rpm.
Fig. 13. Comparison of surface topography for 3D micro-features (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.14mm/rev, spindle speed of 45rpm.
Fig. 14. Variations of friction coefficient of machined surfaces by CT, LVT, EVT and 3D-VT at feed rate of (a) 0.14 mm/rev, (b) 0.18 mm/rev, (c) 0.22 mm/rev, (d) Average friction coefficient.
Fig. 15. Comparison of surface topography for LVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.18mm/rev, spindle speed of 31.5rpm.
Fig. 16. Comparison of Surface Topography for EVT (a) Feed rate of 0.14mm/rev, spindle speed of 31.5 rpm, (b) Feed rate of 0.18mm/rev, spindle speed of 31.5 rpm.
Fig. 17. Comparison of surface topography for 3D micro-features (a) Feed rate of 0.14mm/rev, spindle speed of 31.5rpm, (b) Feed rate of 0.18mm/rev, spindle speed of 31.5rpm.
Fig. 18. Hardness of machined surfaces at feed rates.
Fig. 19. Morphologies of wear scars of four kinds of un-textured surface and textured surfaces after distance of 300 m at velocity of 0.2 m/s; (a) CT, (b) LVT, (c) EVT, (d) 3D-VT.
Fig. 20. (a) Weight loss and (b) Wear rates of the worn sample’s surfaces with four kinds of samples (CT, LVT, EVT and 3D-VT) at distance. (Spindle speed of 31.5 rpm; Feed rate of 0.18 mm/rev).
Fig. 21. (a) Wear rates of the worn sample’s surfaces with variation of feed rates at distance. (EVT process; cutting speed of 31.5 rpm), (b) Wear rates of the worn sample’s surfaces with variation of cutting speeds at distance. (EVT process; Feed rate of 0.18 mm/rev).
Fig. 22. 2D images of the worn surfaces of the chrome steel pins after distance of 300 m at velocity of 0.2 m/s; (a) CT, (b) LVT, (c) EVT, (d) 3D-VT.
Table 1. Parameters of the friction and wear tests. Parameters
Values
Units
Rotational speed
rpm (m/s)
85 (0.2)
Distance
m
300
Normal load
N
10
Temperature
C ̊
Ambient temperature
Table 2. ANOVA results of average friction coefficient. Source
DF
Seq SS
Adj MS
F
P
Contribution (%)
Vc (rpm)
2
0.0011787
0.0005894
4.27
0.005
11.73
Vf (mm/rev)
2
0.0017547
0.0008774
6.35
0.024
17.46
Process
3
0.0032473
0.0010824
7.84
0.001
32.32
Error
28
0.0038674
0.0001381
-
-
38.49
Total
35
0.0100482
-
-
-
100