Fretting Wear Behaviors of Hoisting Rope Wires in Acid Medium

Materials and Design 55 (2014) 50–57

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Technical Report Fretting wear behaviors of hoisting rope wires in acid medium ⇑ Linmin Xu a,b, Dekun Zhang b, , Yan Yin b, Songquan Wang a, Dagang Wang a a School of Mechatronic Engineering, China University of Mining & Technology, Xuzhou 221116, China b School of Materials Science and Engineering, China University of Mining & Technology, Xuzhou 221116, China article info abstract

Article history: The fretting wear behaviors of hoisting rope wires in acid medium were investigated in this paper. Fret- Received 11 July 2013 ting wear tests of steel wires were conducted on a self-made fretting wear rig, and their fretting running Accepted 18 September 2013 characteristics, coefficient of friction, dissipated energy and wear morphology were analyzed. The results Available online 27 September 2013 show that the relative sliding between steel wires can be promoted in the acid medium. As the contact load increases, the fretting of steel wires changes from a slip regime to a mixed one, and the coefficient of friction decreases significantly. Moreover, the coefficient of friction changes from about 1.2 in the dry friction environment to about 0.5 in the acid medium. Energy loss presents the same variation trend. Wear scar depth is larger in the acid medium than in the dry friction environment. The primary wear mechanism in the dry friction environment is peeling as compared to peeling, particle attrition and corrosion in the acid medium. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction tic deformation, wear and cracking due to fretting. Perier et al. [16] studied fretting wear behavior of wire rope in sodium chloride Wire rope is an important bearing component in mine hoisting solutions and pointed out that the lubrication and galvanized layer systems. Fretting wear and fretting corrosion usually occur be- can effectively reduce corrosion and fretting fatigue caused by the tween the contacting wires of the steel wire rope. It is subjected sodium chloride solution. Ding and Dai [17] studied fretting wear to repeated stretching and bending, which results in fretting wear characteristics of a titanium alloy in seawater and analyzed the im- of the steel wires under different displacements amplitude and pact of sea water on the coefﬁcient of friction and wear. As men- loads [1,2]. Moreover, the humid and oxygen-rich working envi- tioned above, most studies are concentrated on the fretting wear ronment of the wires can cause corrosion. Therefore, the fretting of materials under dry friction and marine environments, few of wear behavior of steel wires will change somewhat in corrosive them have been focused on fretting wear behaviors of wire rope environments [3,4]. According to the published statistics [5,6], in acid medium. According to the literature [18], acid rain is a prob- the failure proportion among scrapped wires caused by corrosion lem in South China and this may be a problem for steel wires in is about 70–80%. Therefore, to prolong the service life of the wire this area. The wire damage caused by the interaction between rope, it is necessary to study the fretting wear behavior of steel the acid medium and fretting wear is far greater than that caused wires under corrosive conditions. by simple corrosion and fretting wear. Therefore, the objective of Zhang [7–10] and others mainly studied the fretting wear this paper is to explore the fretting properties and damage mech- mechanism of hoisting rope between steel wires and fatigue failure anisms of steel wires in acid medium. behavior under dry friction and alkaline corrosive environments. Li [11] studied the fretting corrosion characteristics of the Zr-4 alloy 2. Experimental details in Na2SO4 solution. Han et al. [12] studied the fretting behavior of self-piercing riveted aluminum alloy joints under different interfa- Fig. 1 is a self-made fretting corrosion test rig. This rig consists cial conditions. Ramesh and Gnanamoorthy [13] found that the of a driving device, movement device, load-measuring device and fretting wear damage was frequently reported in the races of roll- lifting platform. The upper wire specimen was installed in the ing element bearings and leaded to the increased noise and vibra- force-measuring device and the lower one was ﬁxed on the speci- tion in the total machinery. Zhou et al. [14,15] studied fretting men slider. The slide moves left/right due to the screw rotation wear and fretting fatigue performance of a single aluminum wire that is driven by the step motor, and this achieves the relative hor- and found that the cable fatigue failure was mainly caused by plas- izontal position adjustment of both the upper and lower wire specimens. The loading device moves under the action of the step ⇑ Corresponding author. Tel.: +86 13952207958. motor and which results in the movement of the horizontal posi- E-mail address: [email protected] (D. Zhang). tion adjustment device and force-measuring device. When the con-

tact load between the upper and lower wire specimens reach the when Fn = 10 N and 15 N, all Ft–D curves in the fretting process present value, the step motor rotates and the eccentric derive de- are irregular parallelograms, which apparently illustrates the rela- vice moves. The displacement amplitude was maintained at a va- tive slip between the friction pairs and the slip regime. As the con- lue proportional to the eccentricity of the driving wheel. tact load exceeds 15 N, the Ft–D curves become perfect A typical steel wire based on the request of GB/T8918-2006 [19] parallelograms during the early period (about 100 cycles), which was used in this paper. Fretting wear tests of perpendicular steel represents complete slipping of the contacting wires. With the in- wires in corrosive environments were performed on the self-made crease of the fretting cycles, the Ft–D curve shifts to an elliptical fretting wear rig. Hoisting rope is made of high-quality carbon shape, indicating that the contact interface experiences signiﬁcant structural steel, of which the components (in wt.%) are 0.84% C, plastic deformation and a mixed regime. However, in the acid med-

94.62% Fe, 4.53% Zn and its allowances are S and P. The hardness ium, the Ft–D curves are parallelograms for the entire testing pro- and tensile strength are 365 HV0.1 and 1600 MPa, respectively. cess, regardless of the load. This illustrates the slip state and slip The main test parameters are as follows: Displacement amplitude regime between contacting wires. The friction force increases with of ±150 lm, contact force of 10–30 N, frequency of 1.2 Hz, fretting the increase of the number of fretting cycles according to all the cycles ranging from 1 to 1 104 and room temperature. The coal figures. mine water in South China was tested and the pH value of the acid The fretting running regimes of steel wires under different con- medium was about 2.97. The friction forces were recorded during tact loads in dry friction and acid medium are shown in Table 1.As the experiment. According to the friction (Ft) – displacement (D)– the contact load increases, the running regime of wires changes cycle (N) curve, the fretting running regional characteristics of steel from a slip regime to a mixed one, which reveals that shear stress wire and the final average coefficient of friction could be obtained. of the friction pair surface increases with the increase of the load In addition, the effect of acid medium on the fretting wear of steel and that the plastic deformation occurs on the contact interface. wire was analyzed. The impact of the acidic medium on the fretting However, in the dry friction environment, the wire fretting regime wear behavior of the steel wire was analyzed. starts to shift into the mixed regime slip regime at Fn = 20 N, while After each test, the length and width of the wire abrasion gap the fretting operates in the slip regime in the acid medium. This were measured using an optical microscope with video imaging shows that the acid medium significantly alters the fretting regime device, and the maximum wear depth of the gap was calculated of steel wires and forces the fretting regime into a full slip state un- according to the following formula [20]: der smaller loads than in the dry friction environment; this illus- rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi trates that the lubrication of the acidic medium allows the a 2 h ¼ R R2 max ð1Þ relative sliding of contact steel wires to occur more easily. max 2 3.2. Coefficient of friction where R is the radius of the wire and amax is the maximum width of the fretting wear scar. An optical microscope and scanning electron Variation of the coefficient of friction vs. fretting cycles in dry microscope were used to observe the sample morphology of the friction environment and acid medium under different loads is wear scar. shown in Fig. 3. Under dry friction conditions, the curve is very low and stable at the beginning of the friction test (a few to hun- 3. Results and discussion dreds of cycles), namely, the running-in period of friction test. This is due to the oxide film and membrane fouling of the steel surface. 3.1. Analysis of wire fretting running characteristics The surface film was destroyed with the relative movement between the friction pair. The friction coefficient increased rapidly Fig. 2 shows the fretting running curves of the steel wires under and reached to the peak caused by the production of the new fric- dry friction and acid medium. In the dry friction environment, tion contact surface. Subsequently, the friction coefficient decreased because of the lubricating action of the third body (wear debris). However, the experimental displacement amplitude was relatively high, so the debris was easy to discharge from the contact area, which made the peak value not easy to determine. At last, the formation and overflow of the wear debris at the contact interface reached a dynamic balance, so the curve became stable. In the acid medium, fretting of steel wires operates in a slip regime, and the curve of the coefficient of friction is typically characterized by five stages: running-in, rapid increase, peak, rapid reduction and stable. An increase of contact load induced a shortened running- in period due to the increase in surface shear stress. The acid medium had a certain influence on the fretting transition phase. Under small loads (10 N and 15 N), the acid medium obviously prolonged the running-in period, but the fretting transition period was short- er under higher loads (20 N, 25 N and 30 N). The coefficient of friction decreased as the load increased, which was mainly affected by the contact area, stress state and wear debris lubrication of the steel surface convex peak. Under small loads, the contacting material between the steel wires was in the elastic state, and the contact pressure and contact area of the surface convex peak were small. In the reciprocating fretting process of steel wires, the Fig. 1. Schematic diagram of fretting corrosion apparatus rig: (1) fixator; (2) step convex peak mutually crisscrossed, which caused the surface shear motor; (3) loading device; (4) horizontal slider; (5) step motor; (6) horizontal stress and coefficient of friction to become relatively higher. Under position adjustment device; (7) force measuring device; (8) fretting amplitude measuring device; (9) setting; (10) specimen slider; (11) scaling-down device; (12), high loads, the micro-convex peak between the steel wires yielded eccentric drive device and (13) step motor. under positive pressure, and two surfaces were in elastoplastic 52 L. Xu et al. / Materials and Design 55 (2014) 50–57

Cycle 1 10 100 1000 10000 cycle 1 10 100 1000 20 30 10000 30 cycle 1 10 100 1000 10000 15

20 20 10

10 10 5

0 0 0 -5 friction force/N friction force/N friction

-10 force/N friction -10 -10

-20 -20 -15

-30 -30 -20 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -200 0 200 F =10N Amplitude/µm F =15N Amplitude/µm Amplitude/µm n n Fn=20N

25 Cycle 1 10 100 1000 10000 cycle 1 10 100 1000 10000 30 20

15 20 10 10 5

0 0

-5 -10 10 force/N friction

15 -20

20 -30 25 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 F =25N Amplitude /µm F =30N Amplitude/µm n n (a) dry friction

cycle 1 10 100 1000 10000 cycle 1 10 100 1000 10000 30 30 20 Cycle 1 10 100 1000 10000

15 20 20 10 10 10 5

0 0 0

-5

-10 force/N friction -10 friction force /N Friction force/N -10 -20 -20 -15

-30 -30 -20 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 F =10N Amplitude/µm F =15N Amplitude/µm F =20N Amplitude /µm n n n

25 Cycle 1 10 100 1000 10000 30 Cycle 1 10 100 1000 10000 20 20 15

10 10 5

0 0

-5 -10 10 Friction Force/N

15 -20 20 -30 25 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 -150 0 150 F =25N Amplitude /µm F =30N Amplitude /µm n n

Fig. 2. Fretting running curve of steel wires in two environments.

states. When the contact area of the micro-convex peak increased, the effect of friction became significant. However, wear debris was Table 1 not easily discharged from the contact surface as the cycle number Fretting running regimes of steel wires in different loads in dry friction and acid medium environment. increased, and this played the role of lubrication in the tests. The friction force decreased and thus the coefficient of friction de- Load 10 N 15 N 20 N 25 N 30 N creased. The coefficient of friction in the acid medium was signifi- Dry friction S S M M M Acid medium S SSSS cantly smaller relative to the dry friction environment. At a load of 10 N, coefficient of friction in the acid medium decreased from 1.2 Annotated: S represents Slip Regime; M represents Mixed fretting Regime (dry friction) to 0.5. L. Xu et al. / Materials and Design 55 (2014) 50–57 53

1.4 (a) dry friction 0.50 (b) acid medium

0.45 1.2 10N 0.40 10N 1.0 15N 15N 20N 0.35 20N 25N 25N 0.30 0.8 30N 30N 0.25 0.6 0.20 Friction coeffecient Friction coefficient 0.4 0.15

0.10 0.2 0.05 0 1 2 3 4 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 Cycles Cycles

Fig. 3. Variation of the coefﬁcient of friction vs. fretting cycles in dry friction and acidic medium environment under different loads.

At the beginning of the test, the properties of the surface film of When loads of 10 N and 15 N are applied in the dry friction envi- steel wires were affected by the acid medium and the thickness of ronment, the fretting runs in the slip regime, and the dissipation the surface film increased. At low loads, the contact area between energy has three-stage characteristics (see Fig. 4a). During the ini- steel wires was small and friction force was not large, thus leading tial fretting, the dissipation energy is relatively stable. As the cycle to an extension of the running-in period. When loads increased, number increases, the dissipation energy increases rapidly and the friction force became large and the running-in period was reaches a maximum. After that, as the cycle number further in- shortened because of the corrosion from the acid medium. During creases, the dissipation energy first declines, then increases slightly the rising period, steel wires contacted each other directly when and finally becomes stable. the surface film was damaged, and the adhesion effect was en- Fretting runs in the mixed regime under loads of 20 N, 25 N and hanced, which made it difficult for the acidic medium to enter 30 N, so the dissipation energy curve is obviously different from that the contact interface. When reaching the peak stage, the acid med- in the slip regime. During the initial stage, because there is relative ium was able to enter the contact interface due to the loads and the slip at the contact interface, the Ft–D curvehasaparallelogram regulatory role of wear debris as well as the isolation effect of wear shape, which induces high dissipation energy. As the number of cy- debris, and this resulted in sustained lubrication as well as a corre- cles increases, the tangential slip severity on the contact interface sponding decrease in the coefficient of friction. In the stable stage, decreases, and the material continuously hardens. The Ft–D curve the fluid lubrication film of the acid medium was formed on the gradually becomes a straight line and thus the friction dissipation contact surface of steel wires. energy decreases rapidly. Then the curve opens in the shape of an ellipse, during which the dissipation energy increases slightly, but 3.3. Friction dissipation energy the energy is still less than that in the early fretting stage. At this stage, the dissipation energy exhibits some instability, and this is

Friction generates energy and most of it is released in the form mainly because the Ft–D curve has multiple conversions between of heat. Fouvry et al. [21–23] introduced the energy method to the linear and elliptical shapes. During the stable stage, with the fur- study fretting behaviors of metallic materials. Energy consumption ther increase of the cycle number, the friction dissipation energy is important in a tangential fretting process, and the differences of tends to be stable and the Ft–D curve maintains the elliptic shape. the friction force–displacement curves primarily manifested in dis- Fig. 4b shows the trends of the dissipation energy with numbers sipation energy differences. Through the calculation of the area of cycles under different loads in the acid medium. At a displace- surrounded by the Ft–D curves, the relationship of the dissipation ment amplitude of 150 lm, the fretting modes are in the sliding energy with the number of cycles can be obtained and the material state regardless of the load, and the dissipation energy trend was damage in the view of energy can be analyzed. The friction work of similar to that of the steel tested in the dry friction environment the friction dissipation energy in a single loop in the tangential in the sliding state. This is because the acid medium promoted fric- fretting mode can be deﬁned as: tion and made relative slip between friction pairs easier. Comparing Fig. 4a with b, during initial fretting in the acid medium, the dissipa- XD tion energy curves are lower than those in the dry friction environ- E ¼ Ft DdD ð2Þ D ment. However, as the cycle number increases, the dissipation energy became higher than that in the dry friction case, which indicates that the change of fretting wear in the acid medium is less sud- From the fretting map theory, the Ft–D curve has three basic den during the initial stage, and the acid medium changes the forms: linear, elliptical and parallelogram. For the linear type of thickness of the surface ﬁlm. Later, corrosion from the acid medium Ft–D curve, the change of displacement relies on the control of accelerated fretting wear. Meanwhile, the cleaning effect of the material elastic deformation, so friction dissipation energy is min- medium promoted relative sliding between contacting surfaces. imal and can be approximated as zero. For elliptic partial slip Ft–D After the operation, the dissipation energy in the acid medium curves and parallelogram type of full slip Ft–D curves with corre- was higher than that in the dry friction environment. Fig. 4 also sponding plastic deformation in the tests, the dissipation energy shows that the dissipation energy increases as the contact load in- is equal to the area of the Ft–D curve. creases, which indicates that the tangential force increases as the Fig. 4 shows the variation of dissipation energy with cycle num- contact load increases. bers in both dry friction and acid medium under various loads. 54 L. Xu et al. / Materials and Design 55 (2014) 50–57

3500 3000 30N 2500 2000 1500 1000 500 0 101 102 103 104 3000 25N 2500 J)

-6 2000 1500 1000 500 0 101 102 103 104 2500 20N 2000 1500 1000 500 0 101 102 103 104 2000 15N 1600 Dissipated energyDissipated (10 1200 800 400 0 101 102 103 104 2000 10N 1600 1200 800 400 0 101 102 103 104 Cycles (a) dry friction

3500 3000 2500 2000 1500 1000 30N 500 0 101 102 103 104 3000 2500 J)

-6 2000 1500 1000 25N 500 0 101 102 103 104 2500 2000 1500 1000 20N 500 0 101 102 103 104 2000

Dissipated energy (10 Dissipated 1500 1000 15N 500 0 101 102 103 104 2000 1500 1000 10N 500 0 101 102 103 104 Cycles (b) acid medium

Fig. 4. Variation trend of dissipation energy in two environments under different loads with cycle change.

3.4. Interaction between fretting wear and corrosion function of corrosion and wear increased the wear of the steel wires, which was in consistent with the result of Ren [20]. In 1949, Zelder first proposed that the interaction between cor- As can be seen from Fig. 5, the wear loss of the steel wires also rosion and abrasion often appeared to accelerate each other [24]. affects the contact load. The wear depth of the steel wires increases Our test results agree with this, i.e. fretting wear loss in the acid as the load increased in both environments. Under low loads, the medium is larger than that in the dry friction environment contact stress between the upper and lower samples is small, (Fig. 5), and both are influenced by lubrication and corrosion of and contact interface produces elastic deformation. In this case, the acid medium. From the above discussion, the acid medium smaller wear debris easily overflow from the fretting zone, and changes the wire running area, which transitions from the mixed the specimen surface becomes slightly scratched. As the contact regime to the slip regime. Moreover, the acid medium promotes loads increase, the contact stress also increases, and plastic defor- the relative sliding of the steel wires. However, the pH value of mation occurs on the two contacting surfaces, which causes en- the acid medium is lower, causing pitting corrosion of the matrix hanced wear loss. material. After corrosion, the wire surface is easily scraped by abra- Fig. 6 shows the optical morphologies of grinding cracks in the sive particles or washed off by liquid flow particles. The alternating steel wires. As can be seen from the images obtained in the dry L. Xu et al. / Materials and Design 55 (2014) 50–57 55

formation of the furrows. Some pits with small diameters can be 60 Acid Specimen observed on the worn surface for the corrosive effect of the acid Dry Specimen medium. Scar size is in contact with the increase of the contact 50 load. The generated debris is continuously dissolved by the H+ ions in acid medium, while the corrosion pits form on the new surface m

µ 40 of matrix material at the same time. The wear loss of the wire is increased due to the positive synergism of corrosion and wear in 30 acid medium. The action of H+ ions causes the excretion of abrasive and grinding debris. A new round of fretting wear occurs between

Wear depth/ 20 the contact interfaces, which results in a slow increase of the friction coefficient. 10 Fig. 7 exhibits the SEM morphologies of wear scar in different contact load conditions under the fretting amplitude of 150 lm. As can be seen from the figure, the wear of the contact interfaces 0 10N 15N 20N 25N 30N becomes more serious, for fretting basically on the mixed regime Different Loads and the slip regime (Fig. 7a–e). In certain load and fretting amplitude, the contact point of the surface plastically flows along the Fig. 5. The depth of wear scar of the fretted steel wires in two environments. sliding direction with a strong plastic deformation. As a result, the scar shows elliptical shape in the direction of motion. Wear surface produces the uneven morphology caused by the strong friction environment, the wear scar surface is covered by reddish plastic deformation (Fig. 7a–e) and there are many pits and parti- brown debris (Fig. 6a and b), which indicates that the friction pairs cles or flake debris on the contact interface (Fig. 7c and d). Owing resulted in an oxidation reaction, while that does not occur in the to a cyclic process of adhesion, shear, and then adhesion, shear acid medium. This is because the friction surface of the metallic occurring on the surface of the material, furrows begin to arise material forms a certain thickness oxidation film for the election after the formation of third substance which makes micro-cutting of the oxygen. The periodically regenerated oxidation film is easy on the metal surfaces. The area and depth of wear scar increase as to break off and then causes wear. Moreover, both the wear scar the contact load increases. Meanwhile, the contact fatigue of sur- size increases with the increase of the contact load, which is con- face becomes serious with the increase of the contact stress be- sistent with the results of wear scar depth. tween wire contact surfaces. The marking off of wear debris and Fig. 6c and d shows the optical microscopy of wear topography fatigue micro-cracks can be found on the wear scar (Fig. 7d and of wires in an acid medium under different contact load conditions. e). The cyclic alternating stress and the friction force lead to plastic As can be seen, under the condition of the acid medium, the more deformation and surface hardening on the friction surface micro- obvious the effect of the ‘‘cleaning’’ of fluid medium is, the smooth- bulge, which result in the generation of fatigue cracks on the defec- er the surface of the wear scar is. There is a small amount of wear tive surface. Meanwhile, the flake and particle debris fall off due to debris adhering to the surface of scar center, which causes the the further expansion of the fatigue cracks. It can be found that the

Fig. 6. Optical morphologies of the fretted steel wire in two environments. (a) Fn = 20 N, dry friction, (b) Fn = 25 N, dry friction, (c) Fn = 20 N, acid medium and (d) Fn =25N, acid medium. 56 L. Xu et al. / Materials and Design 55 (2014) 50–57

Fig. 7. SEM morphologies of wear scars in dry friction environment. (a) Fn = 10 N, (b) Fn = 15 N, (c) Fn = 20 N, (d) Fn = 25 N and (e) Fn =30N.

Fig. 8. SEM morphologies of wear scars in acid medium. Fn = 10 N, (b) Fn = 15 N, (c) Fn = 20 N, (d) Fn = 25 N and (e) Fn =30N.

injurious mechanism contains mainly abrasive wear, surface fati- Above all, as the contact load increases in the dry friction envi- gue and oxidation when the wire acts on the mixed regime and ronment (Fig. 7e) and in the acid medium (Fig. 8c and e), the con- the slip regime. tact stress in the steel wire between the contact surfaces also In the acid medium, there is no adhesion phenomenon on the increases, which also makes the wear scar surface develop micro- wear scar surface (Fig. 8) and there is more severe plastic deforma- cracks and traces of debris fatigue shedding. Therefore, in the case tion, which indicates that there was no adhesive wear during fret- of dry friction, the wear mechanisms are abrasive wear, adhesive ting because the solution acted as a barrier. Due to the ‘‘washing’’ wear, fatigue wear and oxidation as compared to abrasive wear of the ﬂuid medium, the wear scar surface is smooth, and there is and fatigue wear which are dominant in the acid environment. only a small amount of debris that remain adhered (Fig. 8a–d). Acid medium plays the dual role of lubrication and corrosion on Moreover, the typical abrasive wear characteristics of the furrow the steel wire. The wear depth of the steel wire increased because can be seen from Fig. 8e. Because of the corrosive action of the acid the wear debris was constantly dissolved by acid medium and pit- medium, there are small-diameter pits in the grinding trace surface ting corrosion appeared on the surface of the matrix material, (Fig. 8a and e). which showed that the corrosive effect was more pronounced. L. Xu et al. / Materials and Design 55 (2014) 50–57 57

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