Micro-scale investigation on belt finishing cutting mechanisms by scratch tests

Wear 308 (2013) 17–28

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Micro-scale investigation on belt finishing cutting mechanisms by scratch tests A. Khellouki a,b,n, J. Rech b, H. Zahouani b a Equipe de Recherche: Mécanique Avancée et Applications Industrielles, Ecole Nationale Supérieure d'Arts et Métiers de Meknès, Marjane II, B.P. 4024, Beni Mhamed, Meknès, Morooco b Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5531, Ecole Nationale d'Ingénieurs de Saint-Etienne, 58 rue Jean Parot, Saint-Etienne 42023, France

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2013 Received in revised form 23 September 2013 Accepted 28 September 2013 Available online 16 October 2013

Belt finishing is an abrasive process that involves various complicated tribological phenomena. To understand the micro-mechanisms in belt finishing, simulation by scratch tests on AISI 52100 steel alloy was proposed. Firstly, scratching using perfect indenter geometry was performed by simulating the real movements and the cutting conditions of belt finishing. Multi-pass scratch and parallel interacting scratch tests were carried out. The influence of the attack angle and the sliding cycle on the deformation behavior and the overall friction coefficient were systematically studied. It was found that with repeated scratches, material is cutting by the fatigue of the instable wedges. A three-dimensional analytical model was established to determine the relationship between the adhesion friction coefficient and the plastic deformation friction coefficient. It was found that the adhesion effect is more influential at small attack angles than at high attack angles. Secondly, the simulation of belt finishing was made by scratch tests at low and high speed with a real grain. With the same conditions, a grain cut less material than a perfect indenter which could be due to the adhesion phenomenon. At high speed, a single grain produces more scratches on the surface. Flow stresses and plastic deformation are more severe. & 2013 Elsevier B.V. All rights reserved.

Keywords: Belt finishing Abrasion Scratch tests Multi-pass scratch High speed scratching

1. Introduction Belt finishing is a new super finishing process in industry especially in automotive manufactures (polishing of crankshafts, camshafts, valves, etc). Belt finishing is different from the standard belt grinding where the belt is running at high speed. In belt finishing an oscillating abrasive belt is pushed with a defined pressure against a rotating workpiece (velocity around 120 m/min for steel alloy) by means of a polymer roller as shown in Fig. 1a. The belt is regenerated automatically with an extremely low feed rate (some millimeters/min). The abrasive belt consists of three main components: the polyester backing, the abrasive grits (Al2O3 or SiC) and the resin bond (Fig. 1b). In belt finishing, the size of the grits is usually between 9 mm and 100 mm. Belt finishing has been successfully tested as a complementary process of hard turning of hard materials [1–6]. Studies developed

n Corresponding author at: Equipe de Recherche: Mécanique Avancée et Applications Industrielles, Ecole Nationale Supérieure d'Arts et Métiers de Meknès, Marjane II, B.P. 4024, Beni Mhamed, Meknès, Morooco. Tel.: þ 21 264 437 0393; fax: þ21 253 546 7163. E-mail address: [email protected] (A. Khellouki).

0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.09.016

in recent years have shown that belt finishing process leads to an optimal and stable surface roughness, no matter what the original topography was [1–4]. Belt finishing induces also high compressive residual stresses in the external layer of steel alloys which could improve the fatigue resistance of the material [5]. A study about the effect of lubrication has demonstrated that with minimum quantity lubrication (MQL) at low belt feed rate the belt finishing is similar to lapping. In these conditions the surface topography is optimal [6]. All these studies are limited to the macroscopic approach of belt finishing. Belt finishing is a complicated process in term of movements and tribological phenomena. The cutting mechanisms induced were not been fully explained. This paper is a simple contribution to the understanding of belt finishing micro-mechanisms at the interface between grains and machined surface through different simulations that take into account many simplifying assumptions as explained later in this introduction. To investigate the mechanisms of material removal in belt finishing, analyses must focus on the microscopic cutting phenomena at the scale of a grain. However, the microscopic study confronts many difficulties. One of these difficulties is the irregularity and the randomly distribution of the grains on the belt.

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Fig. 1. Belt finishing process: (a) belt finishing movements and (b) abrasive belt.

Fig. 2. Difference in the action of abrasive grains on the belt finishing surface.

Grains have different orientations and their shapes are also very different, thus their actions on the surface are not the same everywhere as shown in Fig. 2. To simplify the investigation only a single isolated asperity was considered which leads to the technique of scratching. Several researchers made contributions to the modeling of plowing, adhesion and total friction coefficients of cones, spheres and square pyramids plowing along the soft metal surface by scratch tests on different metallic materials like steel alloy, aluminum, copper, nickel, chromium, etc. The mode of the material response is mainly dependent on the geometry of the indenter, the strain hardening ability of the surface and the interfacial friction and has been investigated by many others [7–22]. In this paper, the simulation of belt finishing process begins with scratch tests with a perfect indenter. The knowledge of the indenter geometry could help to exploit the results and verify the correspondence with the scratching conditions. In this regard, an abrasive grain is simulated by a conical indenter with hemispherical end tip because even the sharp grains have not in reality a perfect acute angle. In order to define the adhesion contribution and the plastic deformation contribution in scratch event a threedimensional analytical model is proposed. In a second part, scratch tests with a real grain are proposed. The comparison between the ideal scratch approach and the actual scratch process could give interesting information about the plastic deformation between the indenter and the soft surface. The random distribution of grains will pose another problem concerning the force transmitted to each grain. In fact, there is a macroscopic force applied to all the surface area of the abrasive belt in contact with the workpiece. However, grains are not loaded in the same way. If the grains are simulated by juxtaposed hemispheres with a diameter equal to the grain size, the force applied to each elementary particle can be calculated as shown in Fig. 3b. For example, for grains of 100 mm and an average pressure

of 4 N/mm2 (which are the maximum conditions used in previous experiments on belt finishing [3,4,6]) the force applied to each hemisphere particle is equal to 0.04 N only. Scratch tests cannot be done easily with so little load. In this study large values of applied force were used for most plausible results. Another difficulty is the movement of abrasive particles through the surface. Belt finishing surface is a network of scratches performed in a complex manner by a plurality of randomly distributed grains. Some studies have been developed in the literature to investigate the scratching interaction on different materials like aluminum alloy, copper and hard steel [23–27]. These studies have shown that there is a change in the material's deformation behavior between single and repeated scratching. Williams and Xie [24] have observed that asperities with small attack angles which would lead normally to plowing with single scratching could lead, by interaction of parallel grooves, to material detachment and thus enhanced wear by a form of micromachining or cutting. In order to simulate the multipassage of grains as well as the interaction of the grain's paths in belt finishing, an investigation on multi-pass scratch and on interaction of parallel scratches is proposed. The sliding speed is also one of the limits of this study. In belt finishing process the grain is running at high speed through the surface (cutting speed around 120 m/min for steel alloy). Standard scratch tests are conducted only at low speed. Many authors have found that sliding speed has little effect on wear in low speed abrasion [25], but at high speed the effect could change. Researches in high speed scratch tests are limited. Hamdi et al. [28] have observed that at high speed a single grain produce several scratches on a hard steel alloy. The deformation mode is mainly plastic. In this paper a simulation of high speed scratching is proposed. A special setup is used to simulate the scratch tests by a real corundum grain. This paper is structured as follows. Section 2 presents singlepass scratch analyses by an experimental and a three-dimensional analytical approach. In Section 3 experimental simulations of grains movements by parallel and repeated scratch tests are given. Finally, Section 4 shows a simulation by scratch tests with real grain at low and high speed.

2. Single-pass scratch with a perfect indenter 2.1. Experimental procedure Tests are conducted by a scratch tester which allows scratching flat surfaces by varying sliding speed, penetration depth or load

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Fig. 3. Simulation of the grain's shape: (a) SEM image of a 30 mm alumina abrasive belt and (b) simulation of the grains by hemisphere particles. P: average pressure, FN: elementary normal force, rs: hemisphere radius.

Fig. 4. Scratch process: (a) scratch tester, (b) principle of scratch test and (c) geometry of the Indenter end tip.

Table 1 Height of the hemispherical indenter end tip hs corresponding to different attack angles α. Attack angle, α (deg) Height, hs (lm)

10 0.08

30 0.67

45 1.46

(Fig. 4a). Normal and tangential forces are measured using piezoelectric sensors located under the sample holder (Fig. 4b). A conical diamond indenter with a microscopic hemispherical end tip is used. The radius of the hemisphere rs is equal to 5 mm. Three different attack angles were used: 101, 301 and 451. The height of the hemispherical end tip hs can be calculated geometrically from the attack angle α and the hemispherical radius rs, as explained in Fig. 4c, by the following formula: hs ¼ r s ð1  cos αÞ

ð1Þ

The correspondence between attack angle α and hemispherical height hs is listed in Table 1. AISI 52100 disc specimens with 10 mm thickness are used. To avoid interaction between scratches and surface roughness, the specimens were ground and carefully polished to a fine roughness of Ra ¼0.01 mm. AISI 52100 steel alloy (100Cr6) is hardened and tempered to 62 HRC. This alloy is the same material used for previous experimental

studies on belt finishing [3,4,6]. The main mechanical properties of this alloy are: – Yield strength, se ¼ 1714 MPa. – Elastic modulus, E¼ 210 GPa. – Poisson's ratio, υ ¼0.3. As explained in the introduction, the elementary force applied to each grain is very low. To have convincing results larger values, up to 1 N, are proposed. For each experiment, five tests were conducted. The average and the precision of each test are calculated and plotted in the different curves through this paper. Scratch tests are conducted without lubrication with the following conditions: – Sliding speed, V¼0.1 mm/s. – Normal applied force, FN ¼1, 5, 10 and 20 N. – Scratch length, l¼ 5 mm. 2.2. Representative strain and indentation index The deformation behavior in conical indentation or scratch test at constant temperature is characterized by a parameter called the indentation index [29]. This adimensional parameter is defined as X¼

εi εe

ð2Þ

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where εe is the elastic strain defined by the ratio between yield stress se and Young's modulus E εe ¼

se E

ð3Þ

And εi the indentation strain defined in the case of a conical indenter as a function of the attack angle α [29] εi ¼ tan α

ð4Þ

Table 2 Indentation index X corresponding to different attack angle for AISI 52100 steel alloy. Attack angle, α (deg) Indentation index, X

10 26

30 86

45 149

From Eqs. (2) and (3), the indentation index X can be rewritten as X¼

E tan α se

ð5Þ

So we can expect that the elastic effects become more and more negligible as X increases. If X is greater than 100 the deformation could be fully plastic for steel alloy [30]. Table 2 shows the indentation index X in function of the attack angles for AISI 52100 steel alloy. The indentation index increases with the attack angle and exceeds 100 for 451 which means that the deformation becomes probably fully plastic. 2.3. Scratch deformation behavior In order to characteristic the wear mode of AISI 52100 steel during scratch tests at different attack angles, scanning electronic microscope observations and 2D/3D profiles measured with a

Fig. 5. SEM images and 2D/3D profiles for scratches at different attack angles (FN ¼ 20 N, V¼ 0.1 mm/s). (a) Attack angles, α=101, (b) attack angles, α=301 and (c) attack angles, α=451.

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Table 3 Wear mode of AISI 52100 steel as a function of indenter attack angle. Attack angle, α (deg) Wear mode

10 –Plowing (wedge only): þ þ þ –Cutting: 0

30 –Plowing (wedge and shear tongue): þ þ –Cutting: þ

contact profilometer were performed. The results are presented in Fig. 5. With an angle of 101 (Fig. 5a), accompanied by a low indentation index X of 26 (Table 2), the deformation mode is a wedge formation. The material is pushed as a regular and uniform wedge in the sides and in front of the scratch. There is no trace of material removal. In this case of obtuse indenter, the elastic recovery and the adhesion mechanisms play an important role. An analytical model will clarify this aspect later. With an angle of 301 (Fig. 5b) and an indentation index X of 86 the yield stress point is presumably exceeded. Observations show a dominant ductile plowing with a transition to cutting. All the material is pushed at the sides of the scratch as instable wedges and shear tongues. However, a small amount of material is cutting as a helical micromachined chips as it has seen visually. With an angle of 451 (Fig. 5c) and a high indentation index X exceeding 100, the deformation mode is fully plastic. A steady state cutting regime was established. The instable wedges in the scratch sides are clearly fractured and removed as micromachined chips. Several debris are cumulated at the front of the scratch. In fact with acute indenter the plastic zone breaks out to the free surface, so that the displaced material is free to escape by plastic flow and to break out as chips as observed also by Xie et al. [25]. Table 3 summarizes these qualitative results.

2.5. Modeling scratch test with an analytical model and comparisons with experimental result The total tangential force in scratching can be decomposed into the adhesion force Fa which corresponds to the adhesion energy between the indenter and the material, and the deformation plastic force noted Fpdefo necessary to plastically deform the material F T ¼ F a þ F p def o

Fig. 6 represents the indenter penetration depth versus the applied normal force for single-pass scratch tests. Penetration depth increases linearly with the applied force. This penetration is always very superior to the hemispherical height hs (see Table 1) which means that scratch tests are conducted mainly by the conical portion of the indenter. Furthermore, when the attack angle increases the penetration depth increases which could enhance the material deformation. From the recorded tangential force FT and the normal applied force FN, the overall friction coefficient can be calculated by the relation μ¼

FT FN

ð6Þ

Fig. 7 represents the overall friction coefficient versus the attack angle. The overall friction coefficient increases significantly with the attack angle. Its value is quadrupled from 101 to 451. This result is comparable to those obtained for other steel alloys and metals and developed by many authors such as Black et al. [10], Challen and Oxley [7] and Kopalinsky [31]. The increase in the friction coefficient is due in particular to the increase of the plastic deformation and the flow stress of material. In fact, increasing the attack angle generates the transition of the wear mechanism from plowing to cutting. This transition is accompanied by severe plastic deformation necessary to generate material removal as it has already observed by the SEM images of scratches at different attack angles. Although with the experimental overall friction coefficient it is not possible to distinguish the adhesion contribution to the plastic deformation contribution. For this purpose a three-dimensional analytical model is proposed.

ð7Þ

The deformation plastic force can be also decomposed into two parts, namely, plowing term Fp responsible of plastic plowing and cutting term Fc necessary for material removal F pdef o ¼ F c þF p

ð8Þ

The overall friction coefficient is the ratio of the total tangential force FT to the total normal force FN μ¼

F a þ F pdef o FT ¼ FN FN

ð9Þ

Thus μ ¼ μa þ μpdef o

2.4. Experimental overall friction coefficient

45 –Cutting: þ þ þ –Plowing: 0

ð10Þ

where μa is the adhesion friction coefficient related to the interfacial friction which may be a function of the asperity condition and the relative velocity between the contact surface [32], and μpdefo is the plastic deformation friction coefficient which reflects the resistance as the indenter plows into the material. It has also been found to depend on the geometry of the indenter, the depth of cut and the material properties [33]. In this paragraph, the adhesion friction coefficient is determined by an analytical model inspired from Subhash and Zhang [34]. The effect of the hemispherical tip is secondary in the present paper because the height of the hemispherical tip is negligible compared to the depth of scratching as shown in Fig. 6. For this reason the model proposed use a perfect conical indenter. Fig. 8 developed from Subhash and Zhang shows one half of the contact surface with the material flowing toward the indenter from the right. α is the attack angle, β is the included angle between the planes OAC and OAB, and R is an arbitrary point along the line OC on the contact surface. The material flows toward and around the conical indenter. The normal stress induced during scratching at point R is noted s (on the plane OAC and perpendicular to the line OC), and the tangential shear stress τ given by τ ¼ μa s

ð11Þ

where ma is the adhesion friction coefficient. τ is assumed to be composed of τr along the direction of line OC and τβ normal to plane OAC as illustrated in Fig. 8b. The angle between τ and the line OC is assumed to be ηβ, where η is a parameter introduced to characterize the material flow around the indenter during scratching [34]. From the geometry, these two components are noted τr ¼ μa s cos ηβ

ð12Þ

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τβ ¼ μa s sin ηβ

ð13Þ

The total stresses applied by the indenter on the material along the scratch direction and along the axial direction at point R are: sT ¼ ðs sin α þ τr cos αÞ cos β þ τβ sin β

ð14Þ

sN ¼ s cos α  τr sin α

ð15Þ

By introducing Eqs. (12) and (13) into Eqs. (14) and (15), the total stresses become

Fig. 6. Indenter penetration versus normal applied force for different attack angles.

sT ¼ s½ sin α cos β þ μa ð cos α cos β cos ηβ þ sin β sin ηβÞ

ð16Þ

sN ¼ sð cos α  μa sin α cos ηβÞ

ð17Þ

An elementary area dS on the contact surface at point R (Fig. 8d) can be defined as dS ¼ u cos α dβ du

ð18Þ

By integrating sT and sN the total maximum tangential force FT and the total maximum normal force FN can be calculated as follows: Z Z u0 Z π=2 sT dS ¼ 2 sT u cos α dβ du ð19Þ FT ¼ 0

Z FN ¼ Fig. 7. Overall friction coefficient m versus indenter attack angle α.

Z sN dS ¼ 2

0

u0 0

Z 0

π=2

sN u cos α dβ du

ð20Þ

where u0 ¼h/sin α is the length of the line OC.

Fig. 8. (a) Schematic of the contact surface and the associated variables. (b) Relationship between the relative material flow direction and the angle ηβ. (c) Schematic of the induced shear stress and material flow for α-01. (d) Elementary surface dS (figure developed from Subhash and Zhang [34]).

A. Khellouki et al. / Wear 308 (2013) 17–28

Assuming that the normal stress s is independent of angle β, the result of the integration gives   Z u0 μ ðη þ cos αÞ cos ðηπ=2Þ s u du ð21Þ F T ¼ 2 cos α sin α  a 2 η 1 0  F N ¼ 2 cos α

π cos α μa sin α sin ðηπ=2Þ  2 η

Z

u0 0

s u du

ð22Þ

The overall friction coefficient can be calculated as μ¼

FT 2η½ sin α  fμa ðη þ cos αÞ cos ðηπ=2Þ=ðη2  1Þg ¼ ηπ cos α  2μa sin α sin ðηπ=2Þ FN

ð23Þ

To determine the parameter η, Subhash and Zhang consider an example for the attack angle α-01, i.e. the conical indenter is tending to be a flat plane. All the material will flow relatively to the left when the flat plane is sliding toward the right and the shear stress exerted by the indenter on the material is exactly in the sliding direction (see Fig. 8c). Thus the angle ηβ must be equal to β. This means that η¼ 1 for α¼ 0. This could be also generalized to small attack angle [34]. With this assumption the overall friction coefficient of Eq. (23) becomes μ¼

2 sin α þ μa πð cos ðα=2ÞÞ2 π cos α 2μa sin α

μπ cos α  2 sin α 2μ sin α þπð cos ðα=2ÞÞ2

The experimental overall friction coefficient μ is plotted also for comparison. The adhesion friction coefficient is very low from the overall coefficient friction and increases slightly with the attack angle. The difference between the two curves corresponds to the plastic deformation friction coefficient (plowingþcutting) which means that plastic deformation is important especially at high attack angles. Fig. 9b shows the ratio between the adhesion friction coefficient and the overall friction coefficient. The percentage of plastic deformation is between 54% (at α¼ 101) and 82% (at α¼451). One can say that for scratch event with obtuse grains the adhesion and the elastic contribution are important which confirms the findings made during SEM scratch observations. Thus, material cutting is likely to be very small in these conditions. With the increasing of attack angle, maximum stresses occur below the surface, so that when the yield point is first exceeded, the resulting small plastic zone is fully contained by material which is still elastic [25]. When the attack angle becomes important a steady fully plastic regime is reached, the greater part of the total energy is dissipated in plastic deformation which would be destined essentially to cutting as shown by SEM observations in Section 2.3. The ascending evolution of indentation index X with attack angle (see Table 1) agreed with these findings.

ð24Þ

The adhesion friction coefficient ma can be deduced from Eq. (24) μa ¼

23

ð25Þ

By adopting the assumption of η ¼1 for all the attack angles used in the experiments (α¼ 101, 301 and 451), the adhesion friction coefficient is plotted versus the attack angle in Fig. 9a.

3. Multi-pass scratch and scratch interaction Belt finishing surface texture is a network of scratches superposed in a complicated way. Fig. 10, from Khellouki et al. [35], shows SEM observations of two belt finishing surfaces with different conditions. Without oscillation the surface is composed of straight and rough superposed scratches. With oscillation the surface is a network of fine crossed scratches. Surface roughness is clearly enhanced by oscillation movement as shown in Fig. 10. Single-pass scratch tests performed previously is a great simplification of what actually occurs

Fig. 9. (a) Adhesion friction coefficient and overall friction coefficient versus attack angle in single pass scratch tests. (b) Ratio between adhesion friction coefficient and overall friction coefficient.

Fig. 10. SEM images for two belt finishing surfaces with and without oscillation [35]. N: workpiece revolution speed, nosc: oscillation's frequency. (a) nosc=30 HZ; N=300 rev/ min and (b) nosc=0 HZ; N=300 rev/min.

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in belt finishing process when scratches are superposed on one another to give a statistically stable surface profile. In the present investigation the simulation of crossed scratches is not treated, only the situation of interaction of parallel grooves is elaborated by first a repeated scratch tests and then by a simulation of the interaction of near parallel scratches.

3.1. Multi-pass scratch tests Repeated scratch tests were conducted by a scratch tester. For each test, the scratch section area below original surface was measured using a tactile profilometer. The measures along the scratch show a constant cross-section. The scratching cycle is shown in Fig. 11a. Indeed, 10 repeated scratches are eventually not sufficient to reach a steady state of deformation, although it could give some interesting primary information about the deformation behavior in repeated scratch event.

Fig. 11b shows the evolution of the scratch section versus the sliding cycles for an attack angle of 301. The scratch section increases with the number of sliding cycles with a logarithmic tendency. This increase is great from the first to the second sliding cycle because the indenter sinks more deeply into the surface but weak from the second to the tenth sliding cycle because the indenter penetration slow down gradually. This phenomenon could be probably the result of the work hardening of the external layers which makes the material harder and more resistant to wear for future scratches, or could be the result of the increasing of the scratch contact area and hence of the decreasing of the contact pressure between the indenter and the material. Fig. 12 shows SEM images corresponding to the first and the tenth scratch for an attack angle of 301. During the first pass (Fig. 12a) the deformation is a plowing with a transition to cutting as it has already observed in Section 2.3. Indeed, an attack angle of 301 is not sufficient to induce a dominant cutting. Instable wedges and shear tongues are created while remaining adhered to the

Fig. 11. Multi-pass test scratch: (a) sliding cycle and (b) scratch section versus number of sliding cycle.

Fig. 12. SEM images for the first and the tenth scratching cycle (α ¼ 301, V ¼0.1 mm/s, FN ¼ 20 N). (a) First sliding cycle and (b) tenth sliding cycle.

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Fig. 13. SEM images for parallel interacting scratches (α¼ 301, V ¼ 0.1 mm/s, FN ¼20 N). (a) Single scratch, (b) parallel scratches, d=50 mm and (c) parallel scratches, d=50 mm.

Fig. 14. Scratching with real grain at low speed: (a) optical microscope image of the grain and (b) principle of the test.

material. When the indenter passes several times at the same scratch, the indenter sinks more deeply into the already worn surface (Fig. 12b), the instable wedges and the shear tongues of the previous scratches are removed either on the sides or on the end of the scratch as chips. A large accumulation of chips on the end of the tenth pass is observed. However, the newly deformed wedges from the last sliding cycle remain adhered to the material as if it is a single-pass scratch. One can say that with multi-pass scratch instable wedges become less resistant and are removed even if the attack angle of the indenter is not able to induce cutting initially. Fatigue process would be expected to lead to cutting eventually as a result of the repeated plastic working of the surface by the passage of plastic waves across it. From this, one can say that during belt finishing, where scratches are superposed in a complicated manner, material could be eventually removed by the fatigue of instable wedges even with obtuse grains. 3.2. Interaction of parallel scratches To investigate the effect of the scratch interaction on the material deformation behavior of AISI 52100 steel, near parallel interacting scratch tests were conducted. The procedure consists to make a first scratch (1) with an indenter of 301 attack angle and a second scratch (2) parallel to the first and displaced from it by a distance d. The lateral displacement d takes two values: 50 mm which is inferior to the scratch width and 80 mm which is superior to the scratch width. Fig. 13 shows the scratch SEM observations for the two cases. When the lateral displacement is smaller than the scratch width (case of d¼ 50 mm) the second scratch is made on the already worn surface of the first scratch. Thus, scratch (1) loses one of its two wedges bordering the second scratch as shown in Fig. 13b. With d¼ 80 mm (Fig. 13c), the distance between the two scratches is barely superior to the scratch width. SEM observation shows that the instable wedges are completely fractured at the junction of the two parallel scratches. One can say that with the interaction of scratches, the material deformation mode can pass from plowing to cutting by the fatigue of instable wedges. Thus,

Fig. 15. Overall friction coefficient versus normal applied force for scratching with real grain.

during belt finishing, much of the material is removed not by the isolated action of each grain but by the simultaneous actions of several grains close to each other. The oscillating motion of the abrasive belt plays a crucial role in this regard.

4. Scratching with real grain To better approach the belt finishing mechanisms at the scale of a grain, scratch tests with real grain was proposed, first at low speed with a scratch tester and then at high speed with a special experimental setup. In belt finishing the grains do not exceed generally 100 mm. However, for a best control of the experimental conditions, larger grains are used in the tests. Indeed, with this configuration, the scale of the experiments is not the same as the reality but the deformation mode as well as the information about scratching results would not be very different. 4.1. Scratch tests at low speed Optical microscope image of the grain used in scratch test is shown in Fig. 14a. It is a prismatic blue corundum grain with the

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following approximate dimensions: length E3 mm, width E 1.5 mm, and height E1 mm. The attack angle α is estimated to be around 801. The grain is carefully pasted on the indenter holder (Fig. 14b). To perform scratch tests we use the same scratch tester used previously (see Fig. 4), the same material (AISI 52100 steel) and the same scratch conditions. Fig. 15 represents the overall friction coefficient versus the normal applied force for scratch tests with real grain. This coefficient oscillates around 0.25 which is very low for an acute attack angle of 801. SEM observations and 2D/3D profiles for the scratches show that the wear mode is a wedge formation with uniform and regular wedges on each side of the scratch (Fig. 16). There is no trace of material removal. However, referring to the previous scratching results with a perfect conical indenter and with the same scratching conditions, one can observe that scratch with a real grain (α¼801) is strangely similar to scratch with a perfect obtuse indenter of α¼ 101 (see Fig. 5a). Normally with an attack angle of 801 the material deformation behavior must be a pure cutting with an overall friction coefficient exceeding 0.9. This large difference between the perfect experimental model and the real one is eventually due to the irregularity and the roughness of the grain compared to the uniformity and the smoothness of conical indenter. This irregularity enhances material adhesion at the expense of material plastic deformation. Another reason of this phenomenon could be the loading by wear debris on the rough and irregular edges of the grain as observed visually in the term of scratch tests. Thus the grain covered by broken micro-chips loses the ability to cut material. These results could be extrapolated to the real situation of belt finishing. The abrasiveness and the angularity of grains as well as the orientation of grains on the belt play a crucial role in the cutting capacity of the abrasive belt. Abrasive manufacturing and

grains application method must be carefully chosen to get the best results from belt finishing process. The phenomenon of belt loading by wear debris has been observed in previous macroscopic study on belt finishing. Khellouki et al. [6] have observed that at dry conditions the belt is loaded rapidly by micromachined chips which limit the possibilities of cutting. With lubrication the belt is cleaned continuously which keeps longer the cutting ability of the grains. Thus, to avoid the limitation of cutting ability by adherence of micromachined chips at the irregular wedges of grains a suitable lubrication method must be used.

4.2. Scratch tests at high speed Researches in high speed scratching with real grain are limited. Hamdi et al. [28] have conducted high speed scratch-test (velocity about 37.3 m/s) on a hard steel alloy with a blue corundum grain pasted on a specific grinding wheel. The authors have observed that one grain produces several manufacturing scratches. The deformation mode observed is a plowing with a plastic behavior. In this paragraph scratch tests at high speed with real grain are proposed. Fig. 17a shows the experimental setup. Scratch tests are carried out on a high-precision CNC lathe. The specimen is a AISI 52100 cylinder of 45 mm diameter polished to have a roughness of Ra ¼0.01 mm. Rotation speed of the specimen is 60 m/min (n ¼424 rpm). To avoid repetitive scratching, the grain is animated by a longitudinal turning movement with a feed rate large enough to avoid scratch interaction. The indenter is a prismatic grain of blue corundum carefully selected to have a regular form and a sharp angularity with an attack angle around 451 (Fig. 17b). The approximate dimensions

Fig. 16. SEM image and 2D/3D profiles for a scratch at low speed with real grain (V ¼ 0.1 mm/s, FN ¼ 20 N).

A. Khellouki et al. / Wear 308 (2013) 17–28

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Fig. 17. (a) Experimental setup for high speed scratch-tests. (b) Optical microscope image of the grain.

Fig. 18. SEM image of the scratch at high speed (V ¼ 60 m/min, FN ¼ 20 N).

The mode of deformation is a plowing with a transition to cutting. Some wear debris are observed. This situation is due eventually to the increase in strain rate with high sliding speed which increases the flow stress of the material. Thus, during belt finishing process, where the cutting speed is important and the contact roller supporting the belt is elastic, the strain rate increases and each grain produces several scratches with different random widths and depths. The association of this phenomenon with the belt oscillation movement leads to a tight network of interacting scratches with severe plastic stresses. Fig. 19 shows the overall friction coefficient versus the normal applied force at high speed scratch tests. The overall friction coefficient is almost independent of the applied force. This friction coefficient of around 0.26 is almost the same as this obtained with scratch tests at low speed. As it has already observed for scratch tests at low speed, the value of the friction coefficient is very low despite the sharpness of the grain which could be due to the adherence phenomenon due to the irregularity of the grain as explained previously.

5. Conclusions

Fig. 19. Overall friction coefficient versus normal applied force for scratching at high speed with real grain.

are: length E2 mm, width E1.5 mm, and height E1 mm. To approach the realty of belt finishing, the grain is pasted to a backing of polyurethane of 90 Shore A hardness. This elastic polymer is the same material used for contact roller supporting the abrasive belt in belt finishing process (see Fig. 1). During the test, the normal and tangential forces are recorded by the way of a piezoelectric dynamometer Kistler 9257 A. Observation by optical microscope shows that high speed scratching by a corundum grain produces many parallel scratches with different random widths as shown in Fig. 18. These results are consistent to those of Hamdi et al. [28]. An abrasive grain has several cutting edges and not only one as it may seem. Each cutting edge behaves as an isolated grain. This aspect is different from scratching at low speed where the grain produces only one scratch. It seems that at high speed the grain is more embedded on the material, thus most of the cutting edges of the grain come into contact with the surface which produce several scratches. The elasticity of the backing could also play a role in this phenomenon.

Belt finishing surface is constituted by a complicated structure made of superposed miniscule grooves which is the result of both the random distribution of grains on the substrate and the oscillating movements of the belt. This paper gives a simulation of belt finishing micro-mechanisms at the interface grains/soft surface by scratch tests. Many assumptions have been made to simplify the complicated interaction of grains action. These assumptions concern the load, the grain's shape, the random movements and the sliding speed. Experimental analysis was developed with a perfect indenter and with a real grain. An analytical model has permitted to get the adhesion contribution in scratch process. The principal findings of this study are: – With obtuse indenter the adhesion contribution and the elastic recovery were dominant and lead to plowing or friction behavior without material removal. On the contrary, with acute indenter the proportion of plastic deformation was found to be important which leads to material detachment and thus enhanced wear. – With multi-pass scratch testing or with interaction of near parallel scratch, material detachment by micromachining was observed even with indenter with small attack angle which cannot cut material initially. Fatigue process of the instable wedges of previous scratches would be expected to lead to such a situation. This suggests that during belt finishing the material could be removed by the fatigue of interacted instable wedges even with obtuse grains. In this sense, the complicated movement

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of the belt caused by the combination between belt feed rate and longitudinal oscillation is a critical factor. – Scratching with real grain is different from scratching with prefect indenter. With the same conditions, the grain cut less material than a perfect indenter. The irregularity and the roughness of the real grain as well as the loading of the grain wedges by wear debris are eventually responsible of this phenomenon. In belt finishing, a suitable lubrication could limit this phenomenon. – At high speed scratching with a real grain, which better approach the reality of belt finishing, more severe plastic deformation has been observed that at low speed. The wear mode is a plowing with a transition to cutting. Strain rate increases which increase the flow stress of the material. Furthermore, each cutting edge of the grain behaves as an isolated indenter which leads to produce several parallel scratches on the soft surface. This paper is a simple contribution to the understanding of micromechanisms of belt finishing by simplifying this process. These findings could instigate further researches on this topic with many perspectives like: further investigation on high speed scratching, cross scratch interaction analysis, simultaneous action of a plurality of grains, effect of the grain's loading by mico-chips, etc. References [1] A. Grzesik, J. Rech, T. Wanat, Surface finish on hardened bearing steel parts produced by superhard and abrasive tools, Int. J. Mach. Tools Manuf. 47 (2007) 255–262. [2] J. Rech, A. Moisan, Belt Grinding: A Way to Optimize the Surface Integrity of Cut Surfaces, in: Proceedings of International Conference MMSS, 23–26 September 2003, Krakow, Poland, pp. 125–132. ISBN 83-912887-5-7. [3] A. Khellouki, J. Rech, H. Zahouani, Influence of the belt finishing process on the surface texture obtained by hard turning, Proc. Inst. Mech. Eng., Part B: J. Eng. Manuf. 221 (7) (2007) 1129–1137. [4] A. Khellouki, J. Rech, H. Zahouani, The effect of abrasive grain's wear and contact conditions on surface texture in belt finishing, Wear 263 (2007) 81–87. [5] J. Rech, G. Kermouche, C. Claudin, A. Khellouki, W. Grzesik, Characterization and modelling of the residual stresses induced by belt finishing on a AISI 52100 hardened steel, J. Mater. Process. Tech. 208 (1–3) (2008) 187–195. [6] A. Khellouki, J. Rech, H. Zahouani, The effect of lubrication conditions on belt finishing, Int. J. Mach. Tools Manuf. 50 (10) (2010) 917–921. [7] J.M. Challen, P.L.B. Oxley, An explanation of the different regimes of friction and wear using asperity deformation models, Wear 53 (1979) 229–243. [8] T. Sasada, M. Oike, N. Emori, The effect of abrasive grain size on the transition between abrasive and adhesive wear, Wear 97 (1984) 291–302. [9] A. Misra, I. Finnie, Some observations on two-body abrasive wear, Wear 68 (1981) 41–56. [10] A.J. Black, E.M. Kopalinsky, P.L.B. Oxley, An investigation of the different regimes of deformation which can occur when a hard wedge slides over a soft surface: the influence of wedge angle, lubrication and prior plastic working of the surface, Wear 123 (1988) 97–114.

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