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Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces
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CIRP-1635; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
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CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces Alessandro A.G. Bruzzone (1)a,*, Henara L. Costa b a b
DIME, University of Genova, via Opera Pia 15, 16145 Genova, Italy Universidade Federal do Rio Grande, Escola de Engenharia, Campus Carreiros 96203900, Rio Grande, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Surface analysis Tribology Energy
While improvement of functional behaviour of engineered surfaces is fundamental, the complexity of surface phenomena in tribology makes difficult to establish consistent models to evaluate and compare functionality of textured patterns. In order to face complexity, a new approach to study textured steel surfaces produced by photochemical texturing in reciprocating sliding under hydrodynamic lubrication is presented. The approach extends hierarchically the description of surface characteristics and provides a systemic perspective that relates textures to the dissipated energy. ß 2017 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction The characterization of surfaces is usually performed according to standards that provide procedures to compute some features capturing particular aspects of the examined surface [1,2]. Although functionality of surfaces has been emphasized [3,4], researches generally focus on specific aspects, such as surface manufacturing [5], microgeometry characterization [4]. A more comprehensive analysis of the surface role in its application is often neglected due to the intrinsic complexity of the surfaces phenomena. This paper presents a new analysis perspective to face complexity and disclose the empirical relationships between surface features and surface functionality. The case used to validate the new approach relates to tribology, specifically the engineered surfaces for lubricated friction. Tribological phenomena are inherently complex; recent reviews on the possible benefits provided by texturing surfaces outline conflicting results [6,7]. Fundamentally, friction reduction due to surface texturing is highly dependent on the lubrication regime and the largest gains seem to occur under mixed lubrication. Under hydrodynamic lubrication conditions, some works show some gain with surface texturing [8,9], whereas others evidence negligible or even detrimental effects [10–12]. The presented analysis is based on data provided by previous experimental tests performed in hydrodynamic regime, considering textured surfaces sliding against a smooth cylindrical counterbody with a reciprocating movement, under diverse normal loads [10]. The proposed approach permits to establish an empirical model between the texture characteristics and energy dissipation;
* Corresponding author. E-mail address: [email protected] (Alessandro A.G. Bruzzone).
in particular the energy and the hydrodynamic lift-off effect for different textures are discussed. 2. Conceptual framework and experimental setup 2.1. Conceptual framework Microgeometry characterization generally studies the surface alone. Conversely real engineering applications exploit the surface characteristics where the surface is one element of a more complex system: each surface relates to the other system components according to the relevant physics of the specific system. The system may include separate solids and their surfaces, fluids, energy fields, etc. The overall system description becomes more and more complex as the number of components increases and different phenomena are involved. The system complexity requires a careful analysis considering the ultimate goal of the study in order to establish which and how controllable factors influence the system goal. The tribological system considered in this study includes: (i) two surfaces: a textured surface, corresponding to a flat sample, and a smooth surface, corresponding to a counterbody (Fig. 1); and (ii) a lubricant fluid.
Fig. 1. Schematic view of the tribological system.
http://dx.doi.org/10.1016/j.cirp.2017.04.077 0007-8506/ß 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
G Model
CIRP-1635; No. of Pages 4 A.A.G. Bruzzone, H.L. Costa / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
2
The relationships established between these elements during reciprocating sliding, require different description levels, specifically:
Table 2 Identifiers and texture parameters of the examined cases.
level 0 (D0): describes materials of the sliding elements, lubricant and geometry of the single pockets of the textures; level 1 (D1): captures static constraints such as the geometrical relationship between the single elements of the texture, time dependent constraints such as the relative motion between the surfaces of the sample, and the loads applied to the counterbody; level 2 (D2): depicts the relationship between friction, velocity, normal load, viscosity of the lubricant; level 3 (D3): addresses the purpose underlaying the tribological system, in this case the energy inputted into the system and its load carrying capacity.
Shape
Circles
Identifier d (mm) w (mm) p (mm) l (mm) Orientation A (103 mm2) V (103 mm3) h1 (mm) f
C1 70 – 100 – – 4.8 20.1 10.3 17%
Lines C4 120 – 460 – – 8.8 5.3 2.2 24%
C5 70 – 510 – – 3.6 10.4 7.0 1%
L1 – 60 340 – = 125.4 388.6 14.5 16%
Chevrons L3 – 40 200 – ? 661.1 91.8 6.3 13%
EV0
EV90 – 40
210 =
280 200 ? 40.6 202.3 12.6 19%
2.2. Experimental setup Table 1 reports the experimental conditions grouped according to the description levels D0 and D1 of the proposed taxonomy. Essentially textures with lines, circle and chevron-like pockets, produced using photochemical etching, have been examined; the counterbody is moved by a crank mechanism according to the reported motion law (Figs. 1–3). Sliding direction is perpendicular to the axis of the cylinder, so that the contact is linear, transverse to the direction of motion. Further details on manufacturing and reproducibility of the textures are in [10]. Table 1 Experimental setup features. Level
Features
D0
Counterbody Material: aluminium, mirror-polished Form: cylinder, radius r = 100 mm, width d = 16 mm Samples Material: AISI 01 GFS gauge plates with HV = 2000 Form: rectangular prism, 35 mm 35 mm 2 mm Shape of the texture elements Circle: diameter d = {70, 120} mm Line: width w = {40, 60} mm Chevron: w = 40 mm, a = 708, l = 200 mm Lubricant Additive-free mineral oil Dynamic viscosity: 1.5 Pa s at 20 8C Temperature 20 2 8C
D1
Texture elements periodicity p Circles: p = {100, 460, 510} mm, square pattern Lines: p = {200, 340} mm Chevrons: p = {210, 280} mm Orientation between texture counterbody axis Lines and chevrons: perpendicular (?) and parallel (=) Sliding motion law Displacement: x(t) = Acos(2pft) mm Velocity: vðtÞ ¼ A2pf sinð2pftÞ mm/s Acceleration: a(t) = (2pf)2x(t) mm/s2 A = 11 mm (stroke length 22 mm) f = 0.55 Hz Applied normal loads Fn = {12.3, 22.1, 31.9, 41.7, 51.5, 61.3, 71.1, 80.9} N
Fig. 4. Geometry of the sliding surfaces.
Table 2 shows the identifiers used to label the texturesorientation for the studied cases. Also, a non-textured sample, with a smooth surface, was considered to evaluate the texture effect. For each sample, several surface parameters were evaluated by analysing square areas of 2 mm 2 mm, acquired with a 3D autofocus laser interferometer with sampling steps dx = 1 mm and dy = 10 mm. Table 2 also reports the mean values of some morphological parameters that characterize each texture element (D0 level), and their relationships (D1 level), specifically: for each pocket (D0 level): area A, volume V, maximum depth of the pockets volume h1; for the analyzed area (D1 level): the coverage f, given by the ratio between of the areas of all the textured elements and the area of the analyzed area. Further details on the parameters computation are in [13]. In order to assess the relationships between friction, velocity, normal load for the examined textures, friction force Ff and the minimum thickness of the dielectric lubricant film h0 were measured: Ff was measured by using a single point load cell with NTEP accuracy class III 5.000S; h0 was evaluated via the measurement of the electric capacity between the sliding bodies (Fig. 4) according to [10]. These measures permit to: assess the lubrication regime, characterize the tribological system performance, and point out the dependence of the system performance on the surface texture, providing the information for the description levels D2 and D3. 3. Experimental results The experimental analysis of the friction force Ff measured during the reciprocation tests outlines a non-reversible, hysteretic behaviour. If the phenomenon were reversible, friction force Ff and velocity v should be related by a bijective function, a one-to-one correspondence: for a given velocity v there should be only one value of the friction force Ff. When velocity v is expressed as a function of displacement x, an elliptical curve is obtained:
Fig. 2. Texture geometries.
Fig. 3. Orientation of counterbody axis motion relative to textures.
x2 2
A
þ
v2 2
ðA2pf Þ
¼1
Accordingly, the same velocity occurs for the same value of the displacements x taken symmetrically respect to the x-axis origin. Moreover, in correspondence of a given displacement x, velocity has the same absolute value with opposite signs v, depending on the approaching direction.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
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Fig. 5. Friction force vs. position (a) and vs. velocity (b) for the smooth surface with Fn = 80.9 N.
Fig. 7. Stribeck curve for the smooth surface.
Fig. 6. Lubricant thickness h0 (smooth surface Fn = 80.9 N).
Measured friction force exhibits however an asymmetrical behaviour for all textures and normal loads. Fig. 5 shows the friction force Ff as a function of the displacement x (a) and velocity v (b) for the smooth surface, with a normal load Fn = 80.9 N. In correspondence of the same displacement x, friction force Ff assumes different absolute values; the arrow indicates the direction of movement. Similarly, for the same velocity v, friction force Ff has two different values depending on the acceleration sign. The asymmetrical behaviour evidences that friction is lower when the surfaces are decelerating than when they are accelerating; this shows that a squeeze film effect is contributing to the hydrodynamic lubrication mechanisms [12,14]. During reciprocating cycles, lubricant thickness h0, ranges from zero in correspondence of the nil velocity, at the stroke boundaries, to maximum values in correspondence of the highest velocity. Fig. 6 shows lubricant thickness h0 for the smooth surface with a normal load Fn = 80.9 N, as a function of displacement x. Together, the left and the right curves relate to a complete cycle. Similar curves were observed for all the textured surfaces.
Fig. 8. Stribeck curve for the C1 texture.
curves for the smooth and the C1 surfaces respectively. The smooth surface operates in the hydrodynamic regime for all the normal loads Fn. Lower loads correspond to thicker lubricant films and therefore higher COF; when load increases, COF decreases. Similar consideration applies for the textured surface with circular C1 pattern and for the other examined textures with the exception of texture L3 that is characterized by a mixed-hydrodynamic transition for the highest load (Fn = 80.9 N). Although Stribeck curves describe the relationship between friction, velocity, normal load and lubricant viscosity (level D2), their use is complex and sometimes unpractical. A system approach focussed on the energy efficiency is more appropriate. With reference to Fig. 9 input energy, EIN, is the energy to slide the counterbody against the textured surface, computed as: EIN ¼
I
F f ds
cycle
4. Discussion The lubrication regime can be assessed by using the Stribeck curves that plot the coefficient of friction, COF = Ff/Fn, versus the Sommerfeld number, So = hv/Fn, where h is the lubricant dynamic viscosity and v is the velocity. Using a Stribeck curve, three major modes of contact can be detected: boundary lubrication, for lower values of So, when the bodies are in intimate contact; COF is high due to the contact; mixed lubrication, for intermediate values of So; a lubricant film develops and partially separates the sliding surfaces; hydrodynamic lubrication, for higher values of So, when the lubricant film becomes thicker than the roughness of the surfaces; the contact between the surfaces is limited and the COF assumes lower values that increase with velocity. Stribeck curves have been computed for all the combinations of textures and loads. Figs. 7 and 8 show two examples of Stribeck
where ds is the displacement between the contacting bodies. The useful effect is the hydrodynamic lift-off effect; its value characterizes the capacity of the textured surfaces to support the applied load during the sliding cycles; it varies during sliding since it depends on several parameters, such as the lubricant, the relative velocity of the sliding bodies, the normal load, the surface microgeometry. The evaluation of the lift-off effect can be made considering lubricant thickness h0, in particular the maximum value observed during the sliding cycle, h0max.
Fig. 9. Energy use approach for the tribological system.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
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Fig. 10. Logarithmic plot of EIN as a function of h0max.
The wasted energy is the difference between the input energy EIN and the energy associated to the lift-off effect ELO given by: I h0 dy ELO ¼ cycle
where dy is the vertical displacement. The direct analysis of the experimental values of EIN and h0max by itself does not provide a clear indication of the texture effect on the system energy efficiency. In order to assess the effect of textures, a relationship between the input and output of the tribological system must be established. Whenever the experimental values of EIN are plotted versus the corresponding values of h0max on a logarithmic diagram (Fig. 10), it is possible to observe a good correlation within the points describing each single texture; moreover the diagram shows clear differences between different textures. On the basis of these observations the following experimental relationship is proposed: EIN ¼ EIN ðh0max ; K; bÞ ¼ Khb0max
(1)
where K and b are parameters that depend essentially on the typology of the textured surfaces. Table 3 reports the parameters b and K for each texture, obtained by fitting the experimental data to Eq. (1). Eq. (1) provides a model to determine the energy necessary to obtain a specific maximum lubricant thickness i.e. lift-off effect: EIN(h0max). Conversely, by expressing the maximum lubricant thickness as a function of the input energy: 1=b EIN (2) h0max ¼ K it is possible the obtain the maximum lubricant thickness corresponding to a given input energy. As the lubricant film thickness h0max decreases, the required energy to move the bodies increases. Furthermore, for each texture, the experimental data point out that higher energy corresponds to higher normal loads. The interpretation of the parameters K and b can be made by considering the logarithmic transform of Eq. (1): log EIN ¼ log K þ b log h0max
(3)
Parameter K gives the position of the line in the logarithmic diagram EIN-h0max: fixed h0max, higher values of K correspond to higher values of EIN, i.e. a lower efficiency of the tribological Table 3 Fitted parameters b and K for the model (1). Texture
b
K
C1 C4 C5 EV0 EV90 L1 L3 Smooth
0.512 0.511 0.497 0.538 0.497 0.444 0.507 0.427
166 135 139 178 152 164 192 120
system. Parameter b represents the slope of the line: it has negative values for all the examined textures, the greater its value the greater is the variation of EIN for the same variation of h0max, i.e. it describes the sensibility of input energy to the variation of the maximum lubricant thickness. Fig. 10 shows that all surface textures are represented by practically parallel lines with a slope b close to 0.5 with the exception of texture L1 and the smooth surface whose slopes are slightly lower. Considering the energy efficiency, the most efficient textures are C4 and C5; the less efficient are L1 and L3. The smooth surface exhibits good efficiency and low sensibility. As pointed out in [6], the effects of surface texturing under full film, hydrodynamic conditions are indeed small. Despite this, the analysis here proposed allows to differentiate the effect of the different patterns and confirms literature results: (i) line patterns result in poorer performance, since lines tend to channel away the lubricant from the contact [10,12]; (ii) circles pockets, with diameter of the same order of magnitude of the elastic contact widths, between 82 and 212 mm according to Hertz equations [14], provide a lift-off effect. Surface texturing generates also stress concentration around the pockets and induces cavitation [11]; texture C1, with very deep and closely spaced pockets, favours cavitation and so shows worse behaviour than C4 and C5. For sliding surfaces operating in hydrodynamic regime, the proposed model gives an example of a level D3 description that relates the inputted energy to the lift-off effect controlled by the texture morphology. The model’s parameters K and b permit to compare different textures, assess their suitability for a specific application and optimize the system’s energy efficiency. 5. Conclusion Starting from an experimental study on reciprocating sliding surfaces with different textures, a novel approach to functionally characterize engineered surfaces has been proposed and evaluated. The approach, using different description levels D0, D1, D2 and D3, provides a conceptual framework that extends the study from the single surfaces features (D0) to a system perspective, where the effect of the interaction between the sliding surfaces on the energy input is controlled by the different textures typologies (D3). References [1] ISO 25178-2 (2012) Geometrical Product Specifications (GPS) – Surface Texture: Areal – Part 2: Terms, Definitions and Surface Texture Parameters. [2] Stout KJ, Sullivan PJ, Dong WP, Mainsah E, Luo N, Mathia T, et al (1993) The Development of Methods for the Characterization of Roughness in Three Dimensions, EUR 15178 EN of Commission of the European Communities, University of Birmingham, Birmingham, UK. [3] Bruzzone AAG, Costa HL, Lonardo PM, Lucca DA (2008) Advances in Engineered Surfaces for Functional Performance. CIRP Annals 57(2):750–769. [4] Jiang X, Whitehouse D (2012) Technological Shifts in Surface Metrology. CIRP Annals – Manufacturing Technology 61(2):815–836. [5] Costa HL, Hutchings IM (2015) Some Innovative Surface Texturing Techniques for Tribological Purposes. Proceedings of the Institution of Mechanical Engineers Part J: Journal of Engineering Tribology 229(4):429–448. [6] Gachot C, Rosenkranz A, Hsu SM, Costa HL (2017) A Critical Assessment of Surface Texturing for Friction and Wear Improvement. Wear 372–373:21–41. [7] Gropper D, Wang L, Harvey TJ (2016) Hydrodynamic Lubrication of Textured Surfaces: A Review of Modeling Techniques and Key Findings. Tribology International 94:509–529. [8] Checo HM, Ausas RF, Jai M, Cadalen JP, Choukroun F, Buscaglia GC (2014) Moving Textures: Simulation of a Ring Sliding on a Textured Liner. Tribology International 72:131–142. [9] Ronen A, Etsion I, Kligerman Y (2001) Friction-Reducing Surface-Texturing in Reciprocating Automotive Components. Tribology Transactions 44:359–366. [10] Costa HL, Hutchings IM (2007) Hydrodynamic Lubrication of Textured Steel Surfaces Under Reciprocating Sliding Conditions. Tribology International 40:1227–1238. [11] Rosenkranz A, Szurdak A, Gachot C, Hirt G, Mu¨cklich F (2016) Friction Reduction Under Mixed and Full Film EHL Induced by Hot Micro-Coined Surface Patterns. Tribology International 95:290–297. [12] Vla˘descu S-C, Medina S, Olver AV, Pegg IG, Reddyhoff T (2016) Lubricant Film Thickness and Friction Force Measurements in a Laser Surface Textured Reciprocating Line Contact Simulating the Piston Ring–Liner Pairing. Tribology International 98:317–329. [13] Bruzzone AAG, Costa HL (2013) Functional Characterization of Structured Surfaces for Tribological Applications. Procedia CIRP 12:456–461. [14] Williams JA (1994) Engineering Tribology, Oxford University Press, Oxford.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
CIRP-1635; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces Alessandro A.G. Bruzzone (1)a,*, Henara L. Costa b a b
DIME, University of Genova, via Opera Pia 15, 16145 Genova, Italy Universidade Federal do Rio Grande, Escola de Engenharia, Campus Carreiros 96203900, Rio Grande, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Surface analysis Tribology Energy
While improvement of functional behaviour of engineered surfaces is fundamental, the complexity of surface phenomena in tribology makes difficult to establish consistent models to evaluate and compare functionality of textured patterns. In order to face complexity, a new approach to study textured steel surfaces produced by photochemical texturing in reciprocating sliding under hydrodynamic lubrication is presented. The approach extends hierarchically the description of surface characteristics and provides a systemic perspective that relates textures to the dissipated energy. ß 2017 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction The characterization of surfaces is usually performed according to standards that provide procedures to compute some features capturing particular aspects of the examined surface [1,2]. Although functionality of surfaces has been emphasized [3,4], researches generally focus on specific aspects, such as surface manufacturing [5], microgeometry characterization [4]. A more comprehensive analysis of the surface role in its application is often neglected due to the intrinsic complexity of the surfaces phenomena. This paper presents a new analysis perspective to face complexity and disclose the empirical relationships between surface features and surface functionality. The case used to validate the new approach relates to tribology, specifically the engineered surfaces for lubricated friction. Tribological phenomena are inherently complex; recent reviews on the possible benefits provided by texturing surfaces outline conflicting results [6,7]. Fundamentally, friction reduction due to surface texturing is highly dependent on the lubrication regime and the largest gains seem to occur under mixed lubrication. Under hydrodynamic lubrication conditions, some works show some gain with surface texturing [8,9], whereas others evidence negligible or even detrimental effects [10–12]. The presented analysis is based on data provided by previous experimental tests performed in hydrodynamic regime, considering textured surfaces sliding against a smooth cylindrical counterbody with a reciprocating movement, under diverse normal loads [10]. The proposed approach permits to establish an empirical model between the texture characteristics and energy dissipation;
* Corresponding author. E-mail address: [email protected] (Alessandro A.G. Bruzzone).
in particular the energy and the hydrodynamic lift-off effect for different textures are discussed. 2. Conceptual framework and experimental setup 2.1. Conceptual framework Microgeometry characterization generally studies the surface alone. Conversely real engineering applications exploit the surface characteristics where the surface is one element of a more complex system: each surface relates to the other system components according to the relevant physics of the specific system. The system may include separate solids and their surfaces, fluids, energy fields, etc. The overall system description becomes more and more complex as the number of components increases and different phenomena are involved. The system complexity requires a careful analysis considering the ultimate goal of the study in order to establish which and how controllable factors influence the system goal. The tribological system considered in this study includes: (i) two surfaces: a textured surface, corresponding to a flat sample, and a smooth surface, corresponding to a counterbody (Fig. 1); and (ii) a lubricant fluid.
Fig. 1. Schematic view of the tribological system.
http://dx.doi.org/10.1016/j.cirp.2017.04.077 0007-8506/ß 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
G Model
CIRP-1635; No. of Pages 4 A.A.G. Bruzzone, H.L. Costa / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
2
The relationships established between these elements during reciprocating sliding, require different description levels, specifically:
Table 2 Identifiers and texture parameters of the examined cases.
level 0 (D0): describes materials of the sliding elements, lubricant and geometry of the single pockets of the textures; level 1 (D1): captures static constraints such as the geometrical relationship between the single elements of the texture, time dependent constraints such as the relative motion between the surfaces of the sample, and the loads applied to the counterbody; level 2 (D2): depicts the relationship between friction, velocity, normal load, viscosity of the lubricant; level 3 (D3): addresses the purpose underlaying the tribological system, in this case the energy inputted into the system and its load carrying capacity.
Shape
Circles
Identifier d (mm) w (mm) p (mm) l (mm) Orientation A (103 mm2) V (103 mm3) h1 (mm) f
C1 70 – 100 – – 4.8 20.1 10.3 17%
Lines C4 120 – 460 – – 8.8 5.3 2.2 24%
C5 70 – 510 – – 3.6 10.4 7.0 1%
L1 – 60 340 – = 125.4 388.6 14.5 16%
Chevrons L3 – 40 200 – ? 661.1 91.8 6.3 13%
EV0
EV90 – 40
210 =
280 200 ? 40.6 202.3 12.6 19%
2.2. Experimental setup Table 1 reports the experimental conditions grouped according to the description levels D0 and D1 of the proposed taxonomy. Essentially textures with lines, circle and chevron-like pockets, produced using photochemical etching, have been examined; the counterbody is moved by a crank mechanism according to the reported motion law (Figs. 1–3). Sliding direction is perpendicular to the axis of the cylinder, so that the contact is linear, transverse to the direction of motion. Further details on manufacturing and reproducibility of the textures are in [10]. Table 1 Experimental setup features. Level
Features
D0
Counterbody Material: aluminium, mirror-polished Form: cylinder, radius r = 100 mm, width d = 16 mm Samples Material: AISI 01 GFS gauge plates with HV = 2000 Form: rectangular prism, 35 mm 35 mm 2 mm Shape of the texture elements Circle: diameter d = {70, 120} mm Line: width w = {40, 60} mm Chevron: w = 40 mm, a = 708, l = 200 mm Lubricant Additive-free mineral oil Dynamic viscosity: 1.5 Pa s at 20 8C Temperature 20 2 8C
D1
Texture elements periodicity p Circles: p = {100, 460, 510} mm, square pattern Lines: p = {200, 340} mm Chevrons: p = {210, 280} mm Orientation between texture counterbody axis Lines and chevrons: perpendicular (?) and parallel (=) Sliding motion law Displacement: x(t) = Acos(2pft) mm Velocity: vðtÞ ¼ A2pf sinð2pftÞ mm/s Acceleration: a(t) = (2pf)2x(t) mm/s2 A = 11 mm (stroke length 22 mm) f = 0.55 Hz Applied normal loads Fn = {12.3, 22.1, 31.9, 41.7, 51.5, 61.3, 71.1, 80.9} N
Fig. 4. Geometry of the sliding surfaces.
Table 2 shows the identifiers used to label the texturesorientation for the studied cases. Also, a non-textured sample, with a smooth surface, was considered to evaluate the texture effect. For each sample, several surface parameters were evaluated by analysing square areas of 2 mm 2 mm, acquired with a 3D autofocus laser interferometer with sampling steps dx = 1 mm and dy = 10 mm. Table 2 also reports the mean values of some morphological parameters that characterize each texture element (D0 level), and their relationships (D1 level), specifically: for each pocket (D0 level): area A, volume V, maximum depth of the pockets volume h1; for the analyzed area (D1 level): the coverage f, given by the ratio between of the areas of all the textured elements and the area of the analyzed area. Further details on the parameters computation are in [13]. In order to assess the relationships between friction, velocity, normal load for the examined textures, friction force Ff and the minimum thickness of the dielectric lubricant film h0 were measured: Ff was measured by using a single point load cell with NTEP accuracy class III 5.000S; h0 was evaluated via the measurement of the electric capacity between the sliding bodies (Fig. 4) according to [10]. These measures permit to: assess the lubrication regime, characterize the tribological system performance, and point out the dependence of the system performance on the surface texture, providing the information for the description levels D2 and D3. 3. Experimental results The experimental analysis of the friction force Ff measured during the reciprocation tests outlines a non-reversible, hysteretic behaviour. If the phenomenon were reversible, friction force Ff and velocity v should be related by a bijective function, a one-to-one correspondence: for a given velocity v there should be only one value of the friction force Ff. When velocity v is expressed as a function of displacement x, an elliptical curve is obtained:
Fig. 2. Texture geometries.
Fig. 3. Orientation of counterbody axis motion relative to textures.
x2 2
A
þ
v2 2
ðA2pf Þ
¼1
Accordingly, the same velocity occurs for the same value of the displacements x taken symmetrically respect to the x-axis origin. Moreover, in correspondence of a given displacement x, velocity has the same absolute value with opposite signs v, depending on the approaching direction.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
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CIRP-1635; No. of Pages 4 A.A.G. Bruzzone, H.L. Costa / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
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Fig. 5. Friction force vs. position (a) and vs. velocity (b) for the smooth surface with Fn = 80.9 N.
Fig. 7. Stribeck curve for the smooth surface.
Fig. 6. Lubricant thickness h0 (smooth surface Fn = 80.9 N).
Measured friction force exhibits however an asymmetrical behaviour for all textures and normal loads. Fig. 5 shows the friction force Ff as a function of the displacement x (a) and velocity v (b) for the smooth surface, with a normal load Fn = 80.9 N. In correspondence of the same displacement x, friction force Ff assumes different absolute values; the arrow indicates the direction of movement. Similarly, for the same velocity v, friction force Ff has two different values depending on the acceleration sign. The asymmetrical behaviour evidences that friction is lower when the surfaces are decelerating than when they are accelerating; this shows that a squeeze film effect is contributing to the hydrodynamic lubrication mechanisms [12,14]. During reciprocating cycles, lubricant thickness h0, ranges from zero in correspondence of the nil velocity, at the stroke boundaries, to maximum values in correspondence of the highest velocity. Fig. 6 shows lubricant thickness h0 for the smooth surface with a normal load Fn = 80.9 N, as a function of displacement x. Together, the left and the right curves relate to a complete cycle. Similar curves were observed for all the textured surfaces.
Fig. 8. Stribeck curve for the C1 texture.
curves for the smooth and the C1 surfaces respectively. The smooth surface operates in the hydrodynamic regime for all the normal loads Fn. Lower loads correspond to thicker lubricant films and therefore higher COF; when load increases, COF decreases. Similar consideration applies for the textured surface with circular C1 pattern and for the other examined textures with the exception of texture L3 that is characterized by a mixed-hydrodynamic transition for the highest load (Fn = 80.9 N). Although Stribeck curves describe the relationship between friction, velocity, normal load and lubricant viscosity (level D2), their use is complex and sometimes unpractical. A system approach focussed on the energy efficiency is more appropriate. With reference to Fig. 9 input energy, EIN, is the energy to slide the counterbody against the textured surface, computed as: EIN ¼
I
F f ds
cycle
4. Discussion The lubrication regime can be assessed by using the Stribeck curves that plot the coefficient of friction, COF = Ff/Fn, versus the Sommerfeld number, So = hv/Fn, where h is the lubricant dynamic viscosity and v is the velocity. Using a Stribeck curve, three major modes of contact can be detected: boundary lubrication, for lower values of So, when the bodies are in intimate contact; COF is high due to the contact; mixed lubrication, for intermediate values of So; a lubricant film develops and partially separates the sliding surfaces; hydrodynamic lubrication, for higher values of So, when the lubricant film becomes thicker than the roughness of the surfaces; the contact between the surfaces is limited and the COF assumes lower values that increase with velocity. Stribeck curves have been computed for all the combinations of textures and loads. Figs. 7 and 8 show two examples of Stribeck
where ds is the displacement between the contacting bodies. The useful effect is the hydrodynamic lift-off effect; its value characterizes the capacity of the textured surfaces to support the applied load during the sliding cycles; it varies during sliding since it depends on several parameters, such as the lubricant, the relative velocity of the sliding bodies, the normal load, the surface microgeometry. The evaluation of the lift-off effect can be made considering lubricant thickness h0, in particular the maximum value observed during the sliding cycle, h0max.
Fig. 9. Energy use approach for the tribological system.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077
G Model
CIRP-1635; No. of Pages 4 A.A.G. Bruzzone, H.L. Costa / CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx
4
Fig. 10. Logarithmic plot of EIN as a function of h0max.
The wasted energy is the difference between the input energy EIN and the energy associated to the lift-off effect ELO given by: I h0 dy ELO ¼ cycle
where dy is the vertical displacement. The direct analysis of the experimental values of EIN and h0max by itself does not provide a clear indication of the texture effect on the system energy efficiency. In order to assess the effect of textures, a relationship between the input and output of the tribological system must be established. Whenever the experimental values of EIN are plotted versus the corresponding values of h0max on a logarithmic diagram (Fig. 10), it is possible to observe a good correlation within the points describing each single texture; moreover the diagram shows clear differences between different textures. On the basis of these observations the following experimental relationship is proposed: EIN ¼ EIN ðh0max ; K; bÞ ¼ Khb0max
(1)
where K and b are parameters that depend essentially on the typology of the textured surfaces. Table 3 reports the parameters b and K for each texture, obtained by fitting the experimental data to Eq. (1). Eq. (1) provides a model to determine the energy necessary to obtain a specific maximum lubricant thickness i.e. lift-off effect: EIN(h0max). Conversely, by expressing the maximum lubricant thickness as a function of the input energy: 1=b EIN (2) h0max ¼ K it is possible the obtain the maximum lubricant thickness corresponding to a given input energy. As the lubricant film thickness h0max decreases, the required energy to move the bodies increases. Furthermore, for each texture, the experimental data point out that higher energy corresponds to higher normal loads. The interpretation of the parameters K and b can be made by considering the logarithmic transform of Eq. (1): log EIN ¼ log K þ b log h0max
(3)
Parameter K gives the position of the line in the logarithmic diagram EIN-h0max: fixed h0max, higher values of K correspond to higher values of EIN, i.e. a lower efficiency of the tribological Table 3 Fitted parameters b and K for the model (1). Texture
b
K
C1 C4 C5 EV0 EV90 L1 L3 Smooth
0.512 0.511 0.497 0.538 0.497 0.444 0.507 0.427
166 135 139 178 152 164 192 120
system. Parameter b represents the slope of the line: it has negative values for all the examined textures, the greater its value the greater is the variation of EIN for the same variation of h0max, i.e. it describes the sensibility of input energy to the variation of the maximum lubricant thickness. Fig. 10 shows that all surface textures are represented by practically parallel lines with a slope b close to 0.5 with the exception of texture L1 and the smooth surface whose slopes are slightly lower. Considering the energy efficiency, the most efficient textures are C4 and C5; the less efficient are L1 and L3. The smooth surface exhibits good efficiency and low sensibility. As pointed out in [6], the effects of surface texturing under full film, hydrodynamic conditions are indeed small. Despite this, the analysis here proposed allows to differentiate the effect of the different patterns and confirms literature results: (i) line patterns result in poorer performance, since lines tend to channel away the lubricant from the contact [10,12]; (ii) circles pockets, with diameter of the same order of magnitude of the elastic contact widths, between 82 and 212 mm according to Hertz equations [14], provide a lift-off effect. Surface texturing generates also stress concentration around the pockets and induces cavitation [11]; texture C1, with very deep and closely spaced pockets, favours cavitation and so shows worse behaviour than C4 and C5. For sliding surfaces operating in hydrodynamic regime, the proposed model gives an example of a level D3 description that relates the inputted energy to the lift-off effect controlled by the texture morphology. The model’s parameters K and b permit to compare different textures, assess their suitability for a specific application and optimize the system’s energy efficiency. 5. Conclusion Starting from an experimental study on reciprocating sliding surfaces with different textures, a novel approach to functionally characterize engineered surfaces has been proposed and evaluated. The approach, using different description levels D0, D1, D2 and D3, provides a conceptual framework that extends the study from the single surfaces features (D0) to a system perspective, where the effect of the interaction between the sliding surfaces on the energy input is controlled by the different textures typologies (D3). References [1] ISO 25178-2 (2012) Geometrical Product Specifications (GPS) – Surface Texture: Areal – Part 2: Terms, Definitions and Surface Texture Parameters. [2] Stout KJ, Sullivan PJ, Dong WP, Mainsah E, Luo N, Mathia T, et al (1993) The Development of Methods for the Characterization of Roughness in Three Dimensions, EUR 15178 EN of Commission of the European Communities, University of Birmingham, Birmingham, UK. [3] Bruzzone AAG, Costa HL, Lonardo PM, Lucca DA (2008) Advances in Engineered Surfaces for Functional Performance. CIRP Annals 57(2):750–769. [4] Jiang X, Whitehouse D (2012) Technological Shifts in Surface Metrology. CIRP Annals – Manufacturing Technology 61(2):815–836. [5] Costa HL, Hutchings IM (2015) Some Innovative Surface Texturing Techniques for Tribological Purposes. Proceedings of the Institution of Mechanical Engineers Part J: Journal of Engineering Tribology 229(4):429–448. [6] Gachot C, Rosenkranz A, Hsu SM, Costa HL (2017) A Critical Assessment of Surface Texturing for Friction and Wear Improvement. Wear 372–373:21–41. [7] Gropper D, Wang L, Harvey TJ (2016) Hydrodynamic Lubrication of Textured Surfaces: A Review of Modeling Techniques and Key Findings. Tribology International 94:509–529. [8] Checo HM, Ausas RF, Jai M, Cadalen JP, Choukroun F, Buscaglia GC (2014) Moving Textures: Simulation of a Ring Sliding on a Textured Liner. Tribology International 72:131–142. [9] Ronen A, Etsion I, Kligerman Y (2001) Friction-Reducing Surface-Texturing in Reciprocating Automotive Components. Tribology Transactions 44:359–366. [10] Costa HL, Hutchings IM (2007) Hydrodynamic Lubrication of Textured Steel Surfaces Under Reciprocating Sliding Conditions. Tribology International 40:1227–1238. [11] Rosenkranz A, Szurdak A, Gachot C, Hirt G, Mu¨cklich F (2016) Friction Reduction Under Mixed and Full Film EHL Induced by Hot Micro-Coined Surface Patterns. Tribology International 95:290–297. [12] Vla˘descu S-C, Medina S, Olver AV, Pegg IG, Reddyhoff T (2016) Lubricant Film Thickness and Friction Force Measurements in a Laser Surface Textured Reciprocating Line Contact Simulating the Piston Ring–Liner Pairing. Tribology International 98:317–329. [13] Bruzzone AAG, Costa HL (2013) Functional Characterization of Structured Surfaces for Tribological Applications. Procedia CIRP 12:456–461. [14] Williams JA (1994) Engineering Tribology, Oxford University Press, Oxford.
Please cite this article in press as: Bruzzone AAG, Costa HL. Engineered surfaces for lubricated friction: A new characterization approach for sliding surfaces. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.077