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Surface geometry of slide bearings after percussive burnishing
ARTICLE IN PRESS
Tribology International 40 (2007) 1516–1525 www.elsevier.com/locate/triboint
Surface geometry of slide bearings after percussive burnishing Lidia Galda, Waldemar Koszela, Pawel Pawlus Department of Manufacturing Processes and Production Organisation, Rzeszow University of Technology, 8 Powstan´co´w Warszawy Street, 35-959 Rzeszow, Poland Received 18 July 2006; received in revised form 9 November 2006; accepted 10 January 2007 Available online 1 March 2007
Abstract A review of literature about the effect of oil pockets on improvement of sliding elements tribological performance as well as about the changes of surface topography during ‘‘zero-wear’’ process is shown. The paper presents also the results of experimental investigations done in the Department of Manufacturing Processes and Production Organisation of Rzeszow University of Technology, connected with the creation of oil pockets on sliding surfaces. In order to simulate a deterministic surface a program for the visualisation of pits was written. The procedures for assessment of the oil pocket size of specific shape and oil pockets coverage are presented. The tendencies of changes of surface topography and oil pockets dimensions during ‘‘zero-wear’’ process are also described. r 2007 Elsevier Ltd. All rights reserved. Keywords: Percussive burnishing; Oil pocket; Zero-wear
1. Introduction The effect of the initial surface topography of machine elements on their performance is very substantial during the operating process. The need to reduce friction and the amount of wear on machine element components involved in sliding contact is very important. The efficiency, durability and reliability of components depend on the friction between sliding surfaces. It was found that presence of oil pockets on contacting surfaces could improve some tribological properties of the components. It is easy to create oil pockets on contacting surfaces using burnishing technique. Special tools act as hammers to form dimples on metal surfaces. The methods of creating that surface type and the analysis of its changes during ‘‘zero-wear’’ will be described. 2. A survey of the literature 2.1. The effect of oil pockets on component performance The surface topography in tribological contact may have a significant effect on friction and wear properties. The Corresponding author. Tel.: +48 178651904; fax: +48 178651184.
E-mail address: [email protected] (L. Galda). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.01.010
presence of artificially created microstructures (surface depressions) may affect the friction and wear behaviour of lubricated surfaces. The surface depressions may be beneficial by supplying lubricant into the contact zone during sliding. The oil pockets (micropits, holes, microdimples or cavities) may reduce friction by providing lift themselves by a cavitation mechanism or/and by acting as a reservoir of lubricant [1]. The amount of deformation can be minimised, the lubricant endurance prolonged and lifetime of the contact increased. The oil pockets are also micro-traps to capture wear debris. Such depressions should be large enough to trap and store the generated debris. Surface texturing emerged as an option of surface engineering resulting in significant improvement in load capacity, coefficient of friction and wear resistance. A laser ablation technique was applied to modify the tribological properties of a hard material by the fabrication of surface microstructures. The effect of the patterns on the sliding friction was examined with a pin-on disk tribometer against the sliding bar [2]. These structures may act as reservoirs for a liquid lubricant. The test results revealed that the wear could be reduced and the sliding life extended by an appropriate size and form of oil pockets. Friction reduction in actual production of piston rings and cylinder liner was demonstrated [3]. The potential
ARTICLE IN PRESS L. Galda et al. / Tribology International 40 (2007) 1516–1525
benefit of surface texturing under conditions of lubricant starvation was also presented. The authors of paper [4] proved that optimum surface texturing may substantially reduce the friction losses in reciprocating automotive components. The laser beams allowed holes, on the cylinder bore structure, to be obtained. The results indicated that these holes could improve oil lubrication time [5]. The results obtained by the authors of Ref. [6] highlighted significant modification of the oil film EHL thickness distribution due to micro-dimples passing through the contact, acting as lubricant micro-reservoirs. The potential use of laser surface texturing in parallel thrust bearings was investigated [7,8]. Optimum parameters of the dimples were found in order to obtain maximum load carrying capacity for a thrust bearing having parallel mating surfaces [7]. The authors of Ref. [8] showed the benefits of laser surface texturing in terms of increasing clearance and reducing friction. Texturing of sensitive parts is an effective approach to improve tribological performance and component lifetime by overcoming stiction problems in micro-electronic system (MEMS) surfaces [9]. The authors of Ref. [10] verified the effect of the microdimples on the frictional properties of silicon nitride ceramics mated with hardened steel. Compared to smooth surfaces without texturing, some samples with microdimples successfully realized a reduction in friction coefficient. The tribological characteristics depended greatly on the size and density of the micro-dimples, but the dimple shape did not significantly affect the friction coefficient. A commercial laser was adapted to produce microstructures on steel surfaces. The laser-structured substrates were tribologically tested by a sliding tribometer under standard conditions. The test results showed that the microstructures improved the lifetime of the samples [11]. In sheet-forming processes the tribological system is characterised mainly by sheet-surface structure. It has the function of storage, transport and distribution of the lubricant. A new type of surface topography was developed by the authors of Ref. [12] by the combination of a deterministic and a stochastic sheet surface structure. Various techniques are currently employed for texturing including machining, ion beam texturing, etching techniques and laser texturing [13]. Examples include honed cylinder surfaces in combustion engines and modern aerodynamically lubricated magnetic heads against hard disks. Micro-texturing techniques based on photolithography and anisotropic etching of silicon have been shown successful in making textured surfaces [14,15]. The method of electrodischarge texturing of sheet rolls is described by the authors of Ref. [16]. However laser texturing has recently attracted much interest [2,3,6,7,9,11].
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2.2. Change of surface topography during ‘‘zero-wear’’ During the ‘‘zero-wear’’ process the wear volume or wear loss is within the limits of the original surface topography of the component and is hard to determine [17]. A lot of papers were concerned with change of the surface topographies during a ‘‘zero-wear’’ regime. Whitehouse and Archard [18] and Hirst and Hollander [19] showed that during sliding there was little effect on the main structure apart from the elimination of finer details. However Wu and Cheng [20] found that running in of the lubricated surface hardly changes the surface wavelength. The authors of paper [21] found a dramatic change of the ordinate distribution of the samples during the ‘‘zerowear’’ process. Particularly, the use of skewness as a means of quantifying the changes of height distribution shape is promising providing that the main valleys remained unaltered. The authors of Ref. [22] studied the changes in surface topography during running in of plain bearings. Their conclusion was that the major portion of the wear process was due to material loss by abrasion. Qualitative 3D characterisation of cylinder surface wear was done by Dong and Stout [23]. Visual plots provide very useful information about the surface wear status. There were marked changes in skewness and kurtosis. As long as the honing texture exists, the texture direction does not exhibit significant change. The authors of Ref. [24] analysed the changes of piston skirt surfaces during ‘‘zero-wear’’. The worn piston skirt surfaces were observed to be smoothed. The ordinate distribution became asymmetric during ‘‘zero-wear’’. Summit density increased. Lay direction changed from circumferential to axial. The ratio of average slopes in axial and circumferential directions can monitor piston skirt wear. Piston skirt surface wear can be diagnosed also by changes in Ssk and Sku parameters and increase in summit density Sds. The statistical height parameters of the cylinder surface decreased during wear. The tendencies of Ssk to decrease (up to 3) and kurtosis Sku to increase (up to 15) were observed. The surface slope decreased, and the summit density increased during wear. Changes of the texture direction Std parameter are consequences of the formation of a new structure during the wear. The ratio of summit curvatures in perpendicular directions can monitor cylinder ‘‘zero-wear’’. The surface of a cylinder sample without honing tracks seemed to be a one-process surface. The formed random texture is one-directional [25]. The fundamental aim of the investigation is to present the burnishing method of oil pockets creation on the sliding surfaces. Two methods will be mentioned. The second aim is to analyse the changes of surface topography of the components with oil pockets during ‘‘zero-wear’’.
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3. The methods of oil pocket creation From the presented literature analysis one can see that little information was given about creating oil pockets using a burnishing technique. But it is relative easy to form them. As was mentioned above there are many ways to texture surfaces in a regular determined array. At Rzeszow University of Technology several technologies based on burnishing processes were developed and they were used mainly for strengthening the surface layer. An impulsive burnishing head with a electromagnetic drive was the first tool. A percussive burnishing (embossing) machine having stamps with specially formed tips working as a hammer to create oil pockets on metal elements is the other possibility. 3.1. Determination of quantity and lay-out of oil pockets on the bearing sleeve surface The impulsive burnishing head is a universal tool which allows us to create oil pockets on outer, inner cylindrical and plane surfaces. This tool has a special ending with a spherical shape but one could change it arbitrarily [26]. In order to determine the oil pockets array on the bearing sleeve surface the diagram from Fig. 1 was used. Developed surface was obtained by cutting it parallel to axis of machining object. Nomenclature: R—length of machining patch after one full rotation of object pd—circumference of machining surface l—length of object xi and yi—displacements of oil pocket a—angle between path of machining marks and development surface f—feed per revolution 2a—minor axis of an ellipse 2b—major axis of an ellipse k—frequency of machining (quantity of marks per min) n—rotational speed of machining object (rpm)
The array of oil pockets on the machining surface is unequivocally specified by geometric coordinates xi and yi. The beginning of the first oil pocket was fixed on point 0 with the following coordinates: x0 ¼ 0, y0 ¼ 0.
ð1Þ
Geometric coordinates of next oil pocket were specified by the formulae qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (2) R ¼ f 2 þ p2 d 2 ,
tg a ¼
pd . f
(3)
The following formula for forward speed was used: vf ¼ fn.
(4)
We calculated x1 ¼
nf , k
y1 ¼ x1 tg a ¼
nf pd npd ¼ . k f k
ð5Þ
Next coordinates xi were calculated by multiplication of the x1 value by the number of consecutive pit marks. Values of xi were varied within the following range: 0pxi pl.
(6)
Determinations of the successive yi were conducted similarly but only for yi opd. The coordinate ymax value was in the range pd y1 oyi max ppd.
(7)
The coordinate yi for the first oil pocket from next path of burnishing was given by the formula yi ¼ yi max þ y1 pd.
(8)
if yimax ¼ pd then yi ¼ 0. We proceeded in the same way with consecutive marks and burnishing paths. Values yi for the machining path were situated in the following range: 0oyi ppd.
(9)
Length of machining could be defined by multiplying the feed by the number of revolutions (the number of machining pathss) α
Fig. 1. Developed surface with the machining marks.
l sf ¼ l ! s ¼ . f The total length of the machining path was qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l f 2 þ p2 d 2 . sR ¼ f
(10)
(11)
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Fig. 2. Screen from program for array of oil pockets visualization on bearing surface.
0.758 mm
0.485 mm
6 mm
6 mm 7 mm
7 mm
Fig. 3. Examples of surfaces with burnished oil pockets.
The oil pocket quantity c was 2 3 6 sR 7 c ¼ 4qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi5, x21 þ y21
pockets can be determined. An example of calculation with the aid of this program is presented in Fig. 2. (12)
or: c¼
l x1
(13)
and finally c¼
kl . nf
(14)
Using the relations presented in (1)–(14) a computer program for the determination of the array of oil pockets on machining surface was developed. With the aid of this program the coordinates of burnishing marks, the number of marks for each of the paths and the total number of oil
3.2. Estimation of the degree of oil pocket coverage The kinematics of the other burnishing process allowed optimisation of the array of oil pockets and the textured area ratio on the outer cylindrical surface [27,28]. Because of process parameter regulation one can obtain separate oil pockets of determined shape and dimension having the proper ending of a stamp [29]. Fig. 3 shows examples of textured surfaces after percussive burnishing (embossing) and grinding. Surfaces after burnishing were ground to remove bulges. To estimate the size of specific shaped oil pocket measuring points around oil pocket were used (see Fig. 4). The surface of a discrete oil pocket was divided into two equal parts and the surface area of one part was calculated. The measuring points were acquired using
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(X3,Y3)
y [mm]
(X1,Y1) (X2,Y2)
1
0.0
2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
x [mm]
Fig. 4. View of a separate drop shaped oil pocket with measuring points.
a microscope and then we obtained the described outline of the half oil pocket using the Grapher 3 program. The obtained graph was approximated by a polynomial. An example is given below. block
T
Y 1 ¼ 4:30458 105 þ 1:69211 x 12:94016 x2 þ 99:45043 x3 383:94587 x4 þ 741:88889 x5 696:85185 x6 þ 253:96825 x7 .
ð15Þ
The equation given above was integrated using the program Mathematica 5.0 and the unknown surface area was given: Z 0:8 Y 1 dX ¼ 0:21 ½mm2 . (16) Surface 1 ¼ 0
Total surface area was obtained by multiplication of Surface 1 by 2: Total surface ¼ 2 Surface 1 ¼ 0:42 ½mm2 .
(17)
Total surface area of one oil pocket should be multiplied by the number of micro-pits in assigned zone I and 100% and divided by the zone area in order to obtain the degree of oil pockets coverage Aop: Aop ¼
Total surface I 100% ¼ 4:6%. Zone area
(18)
4. Change of surface topography with oil pockets during ‘‘zero-wear’’ process Machined surface topography can influence the compatibility of the sliding surfaces with a very great significance. In a lot of cases the ‘‘zero-wear’’ can affect the wear amount during normal exploitation. Therefore the analysis of changes of surface topography during ‘‘zero-wear’’ is a task of great practical significance. The effect of oil pockets on improvement of the tribological properties of: a ring-on block assembly was analysed using a special stand (see Fig. 5). The oil pockets were formed on a block surface specimen (bearing sleeve)
n ring
Fig. 5. The scheme of tested assembly.
made from bronze B101 (CuSn10P) of hardness 138 HB. It was in contact with a 40 HM steel counter-specimen of hardness 40 HRC. The load was 1500 N, speed was 0.22 m/s. The tested assembly was lubricated by oil L-AN 46 [30,31]. The sliding distance was 5060 m. A few block surfaces having oil pockets after machining and operating were analysed. The oil pockets existed on the worn surfaces. The area of the measured surface was 5 mm 5 mm, the sampling intervals were 15 mm in perpendicular directions. Table 1 presents parameter symbols and their descriptions. The 3-D surface topography parameters definitions are given in Ref. [32]. The height parameters decreased during wear, the changes of statistical parameters were bigger than those of maximum height parameters (for example, the average relative change of Sq was 78%, of St 60%). SHtp and SDHtp (differences between heights corresponding respectively to range 20–80% and 5–95% of the material ratio) decreased, their average changes were comparatively big (85% and 82%). The change of Sv parameter was the smallest (56%). Sk family parameters Sk, Spk and Svk decreased, but the change of Svk was the smallest. The mean decrease of oil volume Sa2 during wear was rather big (85%). Before the wear the analysed surfaces were characterised by the following parameters: Ssk (2.2)–(1.3), Sku 3.3–7.2. During the ‘‘zero-wear’’ process the tendencies of Ssk to
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Table 1 Description of surface topography parameters
Table 2 Changes of surface topography parameters during wear
Parameter
Description
Parameter symbol
Parameter range before wear
Parameter range after wear
Sa Sq Sv Sp St Sz SHtp
Arithmetical mean deviation of the surface Root-mean-square deviation of the surface Maximum depth of valleys Maximum height of summits Total height of the surface Ten-point height of the surface Surface section height difference corresponding to 20–80% of the material ratio Surface section height difference corresponding to 5–95% of the material ratio Core roughness depth Reduced summit height Reduced valley depth Oil capacity Skewness of the surface Kurtosis of the surface Root-mean slope of the surface Arithmetic mean summit curvature of the surface Developed interfacial area ratio Texture aspect ratio of the surface Fastest decay autocorrelation length Density of summits of the surface
Sa (mm) Sq (mm) Sv (mm) Sp (mm) St (mm) Sz (mm) SHtp (mm) SDHtp (mm) Sk (mm) Spk (mm) Svk (mm) Sa2 (mm3/mm2) Ssk Sku Sdq Ssc (1/mm) Sdr (%) Str Sal (mm) Sds (1/mm2)
15.7–29.1 23.8–35.8 90.5–99.6 40.8–43.1 97.1–143.2 118.0–136.2 13.7–56.9 81.5–111.1 15.8–28.1 3.9–9.1 74.2–97.3 7839–15468 (2.2)–(1.3) 3.3–7.3 0.16–0.19 0.0043–0.0115 1.31–1.82 0.82–0.91 0.41–0.43 79.2–180.1
1.1–6.2 2.6–11.1 24.3–67.9 5.8–24.7 30.9–92.6 26.5–68.2 1.3–5.4 3.2–37.9 1.9–7.1 1.9–4.0 13.1–56.2 415–2883 (2.5)–(5.1) 11.9–33.1 0.038–0.114 0.0017–0.0049 0.073–0.64 0.82–0.84 0.25–0.33 329–443
SDHtp Sk Spk Svk Sa2 Ssk Sku Sdq Ssc Sdr Str Sal Sds
decrease (up to 5) and kurtosis Sku to increase (up to 33) were observed. The hybrid parameters decreased (slope Sdq, mean summit curvature Ssc and developed interfacial area ratio Sdr), but changes of Sdr were the biggest (83%). The summit density Sds increased during wear, the fastest decay autocorrelation length Sal decreased. The texture parameter Str was constant for small wear values, when the wear amount was big—it decreased. Periodicity level decreased during wear. The isotropy level (82–90% after burnishing) decreased slightly for a large wear amount. Table 2 presents the ranges of analysed surface topography parameters. One can obtain important information about the wear process based on the parameters values and their variation in perpendicular directions. During the wear, the repeatability of height parameters and slope in the direction perpendicular to sliding was smaller than in the orthogonal direction. When the wear was finished an anisotropic surface had been formed. The lay direction was the sliding direction. Fig. 6 presents an axonometric view of the analysed block surface before and after wear. The local wear values were measured as the changes of oil pocket dimensions such as depth, average diameter and oil capacity (see Table 3). The degrees of oil pockets coverage are also given in column 1. Surfaces 1 and 2 were created by cutting along axes of ellipses in parallel and perpendicular to the movement directions. In this case the measured area was 2 mm 2 mm, the sampling intervals were 5 mm in perpendicular directions. We found out that the effect of the degree of oil pockets coverage on wear values (measured as the changes of
parameters given in Table 3) is substantial. The Sample no. 1 with 20% of oil pockets coverage had the biggest wear resistance in comparison to variants nos. 2 and 3. The relative change in oil pockets depths is similar to the decrease of Sz (its relative changes for Sample nos. 1–3 were, respectively, 43%, 74% and 77%) or Sv parameters (30%, 73%, 65%). The decreases of oil capacities of individual holes were similar to changes of Sa2 oil capacity (from the Sk family) which amounted, respectively, to: 63%, 96% and 96%. One should know that separate oil pockets and surface topographies were measured on the different areas. The effect of oil pockets existence on wear resistance was also analysed. In this case a steel-cast iron assembly: was considered. Contrary to the experiment described above, the oil pockets were created on the counter-specimen surface (ring-outer cylindrical surface). The specimen (block) was made from spheroidal cast iron, but the counter-specimen (ring) was made from 40 HM steel of 32 HRC hardness [29]. The steel counter-specimens had textured surfaces with 10% coverage of oil pockets of different shapes (see Fig. 7). During the test for wear resistance the load was 2400 N and speed was 0.27 m/s. The tested assembly was lubricated by oil L-AN 46. The sliding distance for each variant was 2000 m. The analysis of several ring surfaces having oil pockets was also carried out. The area of the measured surface was 6 mm 7 mm, the sampling intervals were 15 mm in the perpendicular directions. The wear amount was comparatively small (big material hardness, the oil pocket existence). The height parameters decreased during
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82.9 μm
130 μm
5.01 mm
5.01 mm 5 mm
5 mm
Fig. 6. View of the block surface before (a) and after wear (b).
Table 3 Characteristic of oil pockets on selected test specimens B—before wear, A—after wear, D—change Max depth (mm)
Average diameter (mm)
Oil capacity (mm3)
Surface 1 (mm2)
Surface 2 (mm2)
B
A
D (%)
B
A
D (%)
B
A
D (%)
B
A
D (%)
B
A
D (%)
95
53
44
1.1
0.75
32
0.043
0.016
63
0.066
0.031
53
0.068
0.032
53
93
25
73
1.1
0.6
45
0.04
0.003
92
0.065
0.009
86
0.067
0.009
86
93
26
72
1.1
0.6
45
0.045
0.003
93
0.066
0.0095
85
0.066
0.0085
87
Test no.
1 20.4% 2 27.8% 3 25.9%
‘‘zero-wear’’ (however for small wear amount, some of them increased). The average decreases of Sa and Sq were, respectively, 35% and 26%, of Sp 46%, of Sv 15%, of St 23%, and of Sz 18%. The change of Sp parameter was the biggest, of Sv the smallest. Sk parameter decreased (the mean value was 39%), Spk too (45%). The average changes of SHtp and SDHtp were comparatively big (35–40%). The parameters characterising the shape of the ordinate distribution changed during ‘‘zero-wear’’. Ssk decreased, but Sku increased. The summit density increased 4.5–5.5 times during the wear. The hybrid parameters Sdq and Sdr decreased during wear (by 18% and 24%, respectively). However a tendency to increase summit curvature SSc was found. The oil volume Sa2 decreased during ‘‘zero-wear’’ (average change was 32%). For small wear amounts systematic changes of Str, Sal parameters were not found. The variations of profile parameters of worn surfaces were similar to those of machined surfaces. Fig. 8 presents the axonometric view of analysed ring surface before and after wear. Table 4 presents the ranges of analysed surface topography parameters. Wear resistance was also measured by the change of oil pocket dimensions such as depth, length, width and oil capacity (see Table 5). The area of measured surface was 2 mm 2 mm, the sampling intervals were 5 mm in the perpendicular directions.
Intersection areas could be important in the case of an asymmetric oil pocket on a shaft neck (see Fig. 9). Oil pockets of tear drop shape situated wider and deeper relative to the rotational direction of the shaft neck could cause a hydrodynamic lift force which would compensate the outer load. The biggest changes in dimensions were noticed for oil pockets of short tear drop shape. The smallest wear intensity was obtained in the case of pits twice as long. Pits in variant 2 could stay on the surface for longer periods and a surface with such micro-geometry seemed to have adequate oil capacity to ensure discontinuous slide contact. These oil pockets were not so deep as others, d0 ¼ 55 mm, however they remained deeper at the end of the test, d1 ¼ 52 mm. Although pits of spherical and short tear drop shapes were at the beginning similar in all dimensions and oil capacity they changed differently during wear. Probably the short drop shaped pit did not give rise to a hydrodynamic lift force. We confirmed, that the change of oil depth during wear was proportional to change of height parameters like Sz or Sa. Results of experimental investigations concerning seizure resistance were presented in [26,33]. It was found that texture area ratio, shape and dimensions of oil pockets were significant for a decrease of seizing. It is especially important in steel–steel and steel–cast iron contact because they are sensitive to adhesion. In those experiments an optimum texture area ratio of 10% was found and it is
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beneficial at low sliding velocity in a steel–cast iron assembly. 5. Conclusions The presented paper examines the use of a burnishing tool to give good surface characteristics such as surface
a 50
30
1
20
μm
40
57.1 μm
10 1.5 mm
2.6 mm
0
b 50 54.8 μm
30
1
μm
40
20 10 1.5 mm
0
1.5 mm
c
50 50.5 μm
40
20
μm
30
10 1.5 mm
0
1.5 mm
Fig. 7. Examples of burnished oil pockets of tear drop (a), (b) and spherical (c) shapes. (a) tear drop shape, length 1.8 mm, (b) tear drop shape, length 0.9 mm and (c) spherical shape, length 0.8 mm.
a
topography having oil pockets. The proposed technique is very easy and allows us to obtain a deterministic surface on hard materials. The computer program developed helps us in surface topography creation. The described procedure of oil pocket’s size assessment gives us information about degree of pits coverage. We analysed the changes of surface topographies having oil pockets of small and big amounts of wear. Height parameters and slope decreased. The changes of parameter (describing material volume) were the smallest. The changes of average parameters are bigger than those of parameters describing maximum height. Summit density increased. Ssk decreased and Sku increased. For a small wear amount the change of Sp (describing void volume) was the biggest. When the wear amount was big, the surface topography directionality changed. The sliding direction became the lay direction. The spatial parameter Sal decreased due to the smaller number of oil pockets. During ‘‘zero-wear’’ the local wear amount is hard to determine. Therefore a profilometer should be used to determine it. The local wear value can be obtained as the changes of oil pockets dimensions. We found that the area of oil pockets coverage on inner cylindrical surface made from bronze had a significant effect on wear resistance. The shape of oil pocket is important with regard to wear resistance of steel outer cylindrical surfaces. The relative change in oil pockets depths during wear is similar to the decrease of surface topography height parameters. We found that creation of oil pockets on co-acting surfaces by burnishing technique might cause increase of wear and seizure resistances under mixed lubrication. But, under different kind of lubrication, the recommended texture may impair the tribological characteristics. Much work is necessary before we can design the surface topography for specific tribosystems. The selection of materials for sliding pair and operating conditions seem to be very important. In the future, the tribological property improvement by oil pocket creation by electromechanical etching using other tribological testers will be carried out.
b
V
1523
V
53.9 μm
65.1 μm
5 mm
5 mm 5 mm
5 mm
Fig. 8. View of ring surface before (a) and after wear (b), v—sliding velocity.
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Table 4 Changes of surface topography parameters during wear Parameter symbol
Parameter range before wear
Parameter range after wear
Sa (mm) Sq (mm) Sv (mm) Sp (mm) St (mm) Sz (mm) SHtp (mm) SDHtp (mm) Sk (mm) Spk (mm) Svk (mm) Sa2 (mm3/mm2) Ssk Sku Sdq Ssc (1/mm) Sdr (%) Str Sal (mm) Sds (1/mm2)
4.7–5.2 9.5–9.9 56.1–65.3 11.2–25.9 67.2–91.2 62.3–68.2 3.75–4.25 23.3–25.8 5.2–6.6 5.7–6.7 25.4–29.7 2184–2581 (4.1)–(3.5) 15.5–19.7 0.064–0.074 0.00084–0.0011 0.20–0.27 0.36–0.94 0.33–0.46 87.5–93.3
3.1–3.8 7.4–8.2 50.8–55.1 7.5–10.7 58.3–65.9 50.1–60.7 2.54–2.64 9.1–17.8 2.9–3.9 3.7–4.2 24.4–27.5 1210–2115 (4.5)–(4.1) 22.2–24.6 0.055–0.062 0.0015–0.0019 0.16–0.19 0.36–0.89 0.38–0.44 438–464
Table 5 Characteristic of oil pockets on selected test specimens B—before wear, A—after wear, D—change Test No. Shape
Depth (mm)
Length (mm)
B
A
Oil capacity (mm3)
Width (mm)
A
D (%)
B
D (%) B
A
D (%) B
50 52
23 5
0.82 0.72 12 1.8 1.45 19
0.85 0.75 11.5 0.65 0.58 10
42
35
0.9
0.95 0.75 21
Surface 1 (mm2) A
Surface 2 (mm2)
A
D (%)
B
D (%) B
A
D (%)
0.015 0.024
0.01 0.015
33 37
0.035 0.025 28 0.056 0.041 26
0.032 0.023 28 0.023 0.019 17
0.017
0.0083
51
0.027 0.019 31
0.04
1 2 3
Sphere 65 Long 55 drop Short 65 drop
0.65 27
0.021 47
Fig. 9. Areas of long shaped oil pocket intersections, parallel (a) and perpendicular to the movement direction (b).
References [1] Nilsson B, Rosen B-G, Thomas TR, Wiklund D, Xiao L. Oil pockets and surface topography: mechanism of friction reduction. In: Proceedings of the XI international colloquium on surfaces. 2004. Chemnitz (Germany), Addendum. [2] Blatter A, Maillat M, Pimenov SM, Shafeev GA, Simakin AV, Loubnin AN. Lubricated sliding performance of laser-patterned sapphire. Wear 1999;232:226–30.
[3] Ryk G, Kligernman Y, Etsion I. Experimental investigation of laser surface texturing for reciprocating automotive components. Tribol Trans 2002;45/4:444–9. [4] Ronen A, Etsion I, Kligernman Y. Friction-reducing surface texturing in reciprocating automotive components. Tribol Trans 2001;44/3:359–66. [5] Duffet G, Sallamand P, Vannes AB. Improvement in friction by cw Nd: YAG laser surface treatment on cast iron cylinder bore. Appl Surf Sci 2003;205:289–96.
ARTICLE IN PRESS L. Galda et al. / Tribology International 40 (2007) 1516–1525 [6] Mourier L, Mazuyer D, Lubrecht AA, Donnet C, Audouard E. Action of a femtosecond laser generated micro-cavity passing through a circular AHL contact. In: Proceedings of the 10th international conference on metrology and properties of engineering surfaces. Saint-Etienne, France 2005. p. 231–40. [7] Brizmer V, Kligerman Y, Etsion I. A laser surface textured parallel thrust bearing. Tribol Trans 2003;46/3:397–403. [8] Etsion I, Halperin G, Brizmer V, Kligerman Y. Experimental investigation of laser surface textured parallel thrust bearings. Tribol Lett 2004;17/2:295–300. [9] Tan B, Ventakrishnan K, Tok KG. Selective surface texturing using femtosecond pulsed laser induced forward transfer. Appl Surf Sci 2003;207:365–71. [10] Waluda M, Yamauchi Y, Kanzaki S, Yasuda Y. Effect of surface texturing on friction reduction between ceramic and steel material under lubricated sliding contact. Wear 2003;254:356–63. [11] Dumitru G, Romano V, Weber H-P, Haefke H, Gerbig Y, Pfuger E. Laser microstructuring of steel surfaces for tribological applications. Appl Phys A 2000;70:485–7. [12] Steinhoff K, Rasp W, Pawelski O. Development of deterministicstochastic structure to improve the tribological conditions of sheet metal forming. J Mater Process Technol 1996;60:355–61. [13] Etsion I. State of the art in laser surface texturing. ASME J Tribol 2005;125:248–53. [14] Peterson U, Jacobson S. Tribological texturing of steel surfaces with a novel diamond embossing tool technique. Tribol Int 2006; 39:695–700. [15] Pettersson U, Jacobson S. Influence of surface texture on boundary lubricated sliding contact. Tribol Int 2003;36:857–64. [16] McGeough JA, Rasmusse H. A theoretical model of electrodischarge texturing. J Mater Process Technol 1997;68:172–8. [17] Kaliszer K, Rowe GW, Trmal G. On the relationship between wear and surface topography. CIRP Ann 1973;22/2:284–90. [18] Whitehouse DJ, Archard JF. The properties of random surface of significance in their contact. Proc R Soc A 1970;316:97–121. [19] Hirst W, Hollander AE. Surface finish and damage in sliding. Proc R Soc London 1974;A337:379–94. [20] Wu C, Zheng L. Effect of waviness and roughness on lubricated wear related to running-in. Wear 1991;147:323–34.
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[21] King G, Whitehouse DJ, Stout KJ. Some topographic feature of the wear process—theory and experiment. CIRP Ann 1972;21:119–23. [22] Roy Chowdury SK, Kaliszer H, Rowe GW. An analysis of changes in surface topography during running-in of plain bearings. Wear 1980; 44:170–80. [23] Dong WP, Stout KJ. An integrated approach to the characterization of surface wear. I: Qualitative characterization. Wear 1995;181–183: 700–16. [24] Krzyz˙ ak Z, Pawlus P. ‘Zero-wear’ of piston skirt surface topography. Wear 2006;260(4–5):554–61. [25] Pawlus P. Characterizastion of engine parts surface wear. In: Proceedings of NORDTRIB 2006, Helsingor, Denmark, 2006. [26] Koszela W, Galda L, Pawlus P. Tribological properties improvement by oil pockets creation on co-acting surfaces. International Conference Insycont, Cracow 2006, submitted. [27] Galda L, Stadnicka D. Contrib Title: Possibilities of creation of regular microgeometry of crankshafts in percussive burnishing process. Tribologia 2004;3:111–9 [in Polish]. [28] Stadnicka D. The effect of percussive burnishing on steel surface layer. PhD dissertation. Rzeszow, Poland: Rzeszow University of Technology; 2004 [in Polish]. [29] Galda L. Selection of stamp geometry for surface texturing using. In: Proceedings of the conference on automation and CA systems in technology planning and in manufacturing. Herlany 2005. p. 75–9. [30] Galda L, Koszela W, Stadnicka D, Pawlus P. The improvement of machine elements functional properties by oil pockets creation on collaborating surfaces. International Conference Austrib 2006, Brisbane, Australia, submitted. [31] Koszela W. Impulse burnishing of cylindrical surfaces in friction condition. PhD dissertation. Rzeszow, Poland: Rzeszow University of Technology; 2003 [in Polish]. [32] Stout KJ, Sullivan PJ, Dong WP, Mainsah E, Luo N, Mathia T, et al. The development of methods for the characterisation of roughness in three dimensions. Publication EUR 15178, EN Commission of the European Communities, 1993. [33] Galda L, Dzierwa A. The influence of surface microgeometry in slide bearings on resistance to seizing. In: Proceedings of the conference Ło´dz´ 2005 on problems of unconvencional bearing systems. p. 56–62 [in Polish].
Tribology International 40 (2007) 1516–1525 www.elsevier.com/locate/triboint
Surface geometry of slide bearings after percussive burnishing Lidia Galda, Waldemar Koszela, Pawel Pawlus Department of Manufacturing Processes and Production Organisation, Rzeszow University of Technology, 8 Powstan´co´w Warszawy Street, 35-959 Rzeszow, Poland Received 18 July 2006; received in revised form 9 November 2006; accepted 10 January 2007 Available online 1 March 2007
Abstract A review of literature about the effect of oil pockets on improvement of sliding elements tribological performance as well as about the changes of surface topography during ‘‘zero-wear’’ process is shown. The paper presents also the results of experimental investigations done in the Department of Manufacturing Processes and Production Organisation of Rzeszow University of Technology, connected with the creation of oil pockets on sliding surfaces. In order to simulate a deterministic surface a program for the visualisation of pits was written. The procedures for assessment of the oil pocket size of specific shape and oil pockets coverage are presented. The tendencies of changes of surface topography and oil pockets dimensions during ‘‘zero-wear’’ process are also described. r 2007 Elsevier Ltd. All rights reserved. Keywords: Percussive burnishing; Oil pocket; Zero-wear
1. Introduction The effect of the initial surface topography of machine elements on their performance is very substantial during the operating process. The need to reduce friction and the amount of wear on machine element components involved in sliding contact is very important. The efficiency, durability and reliability of components depend on the friction between sliding surfaces. It was found that presence of oil pockets on contacting surfaces could improve some tribological properties of the components. It is easy to create oil pockets on contacting surfaces using burnishing technique. Special tools act as hammers to form dimples on metal surfaces. The methods of creating that surface type and the analysis of its changes during ‘‘zero-wear’’ will be described. 2. A survey of the literature 2.1. The effect of oil pockets on component performance The surface topography in tribological contact may have a significant effect on friction and wear properties. The Corresponding author. Tel.: +48 178651904; fax: +48 178651184.
E-mail address: [email protected] (L. Galda). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.01.010
presence of artificially created microstructures (surface depressions) may affect the friction and wear behaviour of lubricated surfaces. The surface depressions may be beneficial by supplying lubricant into the contact zone during sliding. The oil pockets (micropits, holes, microdimples or cavities) may reduce friction by providing lift themselves by a cavitation mechanism or/and by acting as a reservoir of lubricant [1]. The amount of deformation can be minimised, the lubricant endurance prolonged and lifetime of the contact increased. The oil pockets are also micro-traps to capture wear debris. Such depressions should be large enough to trap and store the generated debris. Surface texturing emerged as an option of surface engineering resulting in significant improvement in load capacity, coefficient of friction and wear resistance. A laser ablation technique was applied to modify the tribological properties of a hard material by the fabrication of surface microstructures. The effect of the patterns on the sliding friction was examined with a pin-on disk tribometer against the sliding bar [2]. These structures may act as reservoirs for a liquid lubricant. The test results revealed that the wear could be reduced and the sliding life extended by an appropriate size and form of oil pockets. Friction reduction in actual production of piston rings and cylinder liner was demonstrated [3]. The potential
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benefit of surface texturing under conditions of lubricant starvation was also presented. The authors of paper [4] proved that optimum surface texturing may substantially reduce the friction losses in reciprocating automotive components. The laser beams allowed holes, on the cylinder bore structure, to be obtained. The results indicated that these holes could improve oil lubrication time [5]. The results obtained by the authors of Ref. [6] highlighted significant modification of the oil film EHL thickness distribution due to micro-dimples passing through the contact, acting as lubricant micro-reservoirs. The potential use of laser surface texturing in parallel thrust bearings was investigated [7,8]. Optimum parameters of the dimples were found in order to obtain maximum load carrying capacity for a thrust bearing having parallel mating surfaces [7]. The authors of Ref. [8] showed the benefits of laser surface texturing in terms of increasing clearance and reducing friction. Texturing of sensitive parts is an effective approach to improve tribological performance and component lifetime by overcoming stiction problems in micro-electronic system (MEMS) surfaces [9]. The authors of Ref. [10] verified the effect of the microdimples on the frictional properties of silicon nitride ceramics mated with hardened steel. Compared to smooth surfaces without texturing, some samples with microdimples successfully realized a reduction in friction coefficient. The tribological characteristics depended greatly on the size and density of the micro-dimples, but the dimple shape did not significantly affect the friction coefficient. A commercial laser was adapted to produce microstructures on steel surfaces. The laser-structured substrates were tribologically tested by a sliding tribometer under standard conditions. The test results showed that the microstructures improved the lifetime of the samples [11]. In sheet-forming processes the tribological system is characterised mainly by sheet-surface structure. It has the function of storage, transport and distribution of the lubricant. A new type of surface topography was developed by the authors of Ref. [12] by the combination of a deterministic and a stochastic sheet surface structure. Various techniques are currently employed for texturing including machining, ion beam texturing, etching techniques and laser texturing [13]. Examples include honed cylinder surfaces in combustion engines and modern aerodynamically lubricated magnetic heads against hard disks. Micro-texturing techniques based on photolithography and anisotropic etching of silicon have been shown successful in making textured surfaces [14,15]. The method of electrodischarge texturing of sheet rolls is described by the authors of Ref. [16]. However laser texturing has recently attracted much interest [2,3,6,7,9,11].
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2.2. Change of surface topography during ‘‘zero-wear’’ During the ‘‘zero-wear’’ process the wear volume or wear loss is within the limits of the original surface topography of the component and is hard to determine [17]. A lot of papers were concerned with change of the surface topographies during a ‘‘zero-wear’’ regime. Whitehouse and Archard [18] and Hirst and Hollander [19] showed that during sliding there was little effect on the main structure apart from the elimination of finer details. However Wu and Cheng [20] found that running in of the lubricated surface hardly changes the surface wavelength. The authors of paper [21] found a dramatic change of the ordinate distribution of the samples during the ‘‘zerowear’’ process. Particularly, the use of skewness as a means of quantifying the changes of height distribution shape is promising providing that the main valleys remained unaltered. The authors of Ref. [22] studied the changes in surface topography during running in of plain bearings. Their conclusion was that the major portion of the wear process was due to material loss by abrasion. Qualitative 3D characterisation of cylinder surface wear was done by Dong and Stout [23]. Visual plots provide very useful information about the surface wear status. There were marked changes in skewness and kurtosis. As long as the honing texture exists, the texture direction does not exhibit significant change. The authors of Ref. [24] analysed the changes of piston skirt surfaces during ‘‘zero-wear’’. The worn piston skirt surfaces were observed to be smoothed. The ordinate distribution became asymmetric during ‘‘zero-wear’’. Summit density increased. Lay direction changed from circumferential to axial. The ratio of average slopes in axial and circumferential directions can monitor piston skirt wear. Piston skirt surface wear can be diagnosed also by changes in Ssk and Sku parameters and increase in summit density Sds. The statistical height parameters of the cylinder surface decreased during wear. The tendencies of Ssk to decrease (up to 3) and kurtosis Sku to increase (up to 15) were observed. The surface slope decreased, and the summit density increased during wear. Changes of the texture direction Std parameter are consequences of the formation of a new structure during the wear. The ratio of summit curvatures in perpendicular directions can monitor cylinder ‘‘zero-wear’’. The surface of a cylinder sample without honing tracks seemed to be a one-process surface. The formed random texture is one-directional [25]. The fundamental aim of the investigation is to present the burnishing method of oil pockets creation on the sliding surfaces. Two methods will be mentioned. The second aim is to analyse the changes of surface topography of the components with oil pockets during ‘‘zero-wear’’.
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3. The methods of oil pocket creation From the presented literature analysis one can see that little information was given about creating oil pockets using a burnishing technique. But it is relative easy to form them. As was mentioned above there are many ways to texture surfaces in a regular determined array. At Rzeszow University of Technology several technologies based on burnishing processes were developed and they were used mainly for strengthening the surface layer. An impulsive burnishing head with a electromagnetic drive was the first tool. A percussive burnishing (embossing) machine having stamps with specially formed tips working as a hammer to create oil pockets on metal elements is the other possibility. 3.1. Determination of quantity and lay-out of oil pockets on the bearing sleeve surface The impulsive burnishing head is a universal tool which allows us to create oil pockets on outer, inner cylindrical and plane surfaces. This tool has a special ending with a spherical shape but one could change it arbitrarily [26]. In order to determine the oil pockets array on the bearing sleeve surface the diagram from Fig. 1 was used. Developed surface was obtained by cutting it parallel to axis of machining object. Nomenclature: R—length of machining patch after one full rotation of object pd—circumference of machining surface l—length of object xi and yi—displacements of oil pocket a—angle between path of machining marks and development surface f—feed per revolution 2a—minor axis of an ellipse 2b—major axis of an ellipse k—frequency of machining (quantity of marks per min) n—rotational speed of machining object (rpm)
The array of oil pockets on the machining surface is unequivocally specified by geometric coordinates xi and yi. The beginning of the first oil pocket was fixed on point 0 with the following coordinates: x0 ¼ 0, y0 ¼ 0.
ð1Þ
Geometric coordinates of next oil pocket were specified by the formulae qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (2) R ¼ f 2 þ p2 d 2 ,
tg a ¼
pd . f
(3)
The following formula for forward speed was used: vf ¼ fn.
(4)
We calculated x1 ¼
nf , k
y1 ¼ x1 tg a ¼
nf pd npd ¼ . k f k
ð5Þ
Next coordinates xi were calculated by multiplication of the x1 value by the number of consecutive pit marks. Values of xi were varied within the following range: 0pxi pl.
(6)
Determinations of the successive yi were conducted similarly but only for yi opd. The coordinate ymax value was in the range pd y1 oyi max ppd.
(7)
The coordinate yi for the first oil pocket from next path of burnishing was given by the formula yi ¼ yi max þ y1 pd.
(8)
if yimax ¼ pd then yi ¼ 0. We proceeded in the same way with consecutive marks and burnishing paths. Values yi for the machining path were situated in the following range: 0oyi ppd.
(9)
Length of machining could be defined by multiplying the feed by the number of revolutions (the number of machining pathss) α
Fig. 1. Developed surface with the machining marks.
l sf ¼ l ! s ¼ . f The total length of the machining path was qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l f 2 þ p2 d 2 . sR ¼ f
(10)
(11)
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Fig. 2. Screen from program for array of oil pockets visualization on bearing surface.
0.758 mm
0.485 mm
6 mm
6 mm 7 mm
7 mm
Fig. 3. Examples of surfaces with burnished oil pockets.
The oil pocket quantity c was 2 3 6 sR 7 c ¼ 4qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi5, x21 þ y21
pockets can be determined. An example of calculation with the aid of this program is presented in Fig. 2. (12)
or: c¼
l x1
(13)
and finally c¼
kl . nf
(14)
Using the relations presented in (1)–(14) a computer program for the determination of the array of oil pockets on machining surface was developed. With the aid of this program the coordinates of burnishing marks, the number of marks for each of the paths and the total number of oil
3.2. Estimation of the degree of oil pocket coverage The kinematics of the other burnishing process allowed optimisation of the array of oil pockets and the textured area ratio on the outer cylindrical surface [27,28]. Because of process parameter regulation one can obtain separate oil pockets of determined shape and dimension having the proper ending of a stamp [29]. Fig. 3 shows examples of textured surfaces after percussive burnishing (embossing) and grinding. Surfaces after burnishing were ground to remove bulges. To estimate the size of specific shaped oil pocket measuring points around oil pocket were used (see Fig. 4). The surface of a discrete oil pocket was divided into two equal parts and the surface area of one part was calculated. The measuring points were acquired using
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(X3,Y3)
y [mm]
(X1,Y1) (X2,Y2)
1
0.0
2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
x [mm]
Fig. 4. View of a separate drop shaped oil pocket with measuring points.
a microscope and then we obtained the described outline of the half oil pocket using the Grapher 3 program. The obtained graph was approximated by a polynomial. An example is given below. block
T
Y 1 ¼ 4:30458 105 þ 1:69211 x 12:94016 x2 þ 99:45043 x3 383:94587 x4 þ 741:88889 x5 696:85185 x6 þ 253:96825 x7 .
ð15Þ
The equation given above was integrated using the program Mathematica 5.0 and the unknown surface area was given: Z 0:8 Y 1 dX ¼ 0:21 ½mm2 . (16) Surface 1 ¼ 0
Total surface area was obtained by multiplication of Surface 1 by 2: Total surface ¼ 2 Surface 1 ¼ 0:42 ½mm2 .
(17)
Total surface area of one oil pocket should be multiplied by the number of micro-pits in assigned zone I and 100% and divided by the zone area in order to obtain the degree of oil pockets coverage Aop: Aop ¼
Total surface I 100% ¼ 4:6%. Zone area
(18)
4. Change of surface topography with oil pockets during ‘‘zero-wear’’ process Machined surface topography can influence the compatibility of the sliding surfaces with a very great significance. In a lot of cases the ‘‘zero-wear’’ can affect the wear amount during normal exploitation. Therefore the analysis of changes of surface topography during ‘‘zero-wear’’ is a task of great practical significance. The effect of oil pockets on improvement of the tribological properties of: a ring-on block assembly was analysed using a special stand (see Fig. 5). The oil pockets were formed on a block surface specimen (bearing sleeve)
n ring
Fig. 5. The scheme of tested assembly.
made from bronze B101 (CuSn10P) of hardness 138 HB. It was in contact with a 40 HM steel counter-specimen of hardness 40 HRC. The load was 1500 N, speed was 0.22 m/s. The tested assembly was lubricated by oil L-AN 46 [30,31]. The sliding distance was 5060 m. A few block surfaces having oil pockets after machining and operating were analysed. The oil pockets existed on the worn surfaces. The area of the measured surface was 5 mm 5 mm, the sampling intervals were 15 mm in perpendicular directions. Table 1 presents parameter symbols and their descriptions. The 3-D surface topography parameters definitions are given in Ref. [32]. The height parameters decreased during wear, the changes of statistical parameters were bigger than those of maximum height parameters (for example, the average relative change of Sq was 78%, of St 60%). SHtp and SDHtp (differences between heights corresponding respectively to range 20–80% and 5–95% of the material ratio) decreased, their average changes were comparatively big (85% and 82%). The change of Sv parameter was the smallest (56%). Sk family parameters Sk, Spk and Svk decreased, but the change of Svk was the smallest. The mean decrease of oil volume Sa2 during wear was rather big (85%). Before the wear the analysed surfaces were characterised by the following parameters: Ssk (2.2)–(1.3), Sku 3.3–7.2. During the ‘‘zero-wear’’ process the tendencies of Ssk to
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Table 1 Description of surface topography parameters
Table 2 Changes of surface topography parameters during wear
Parameter
Description
Parameter symbol
Parameter range before wear
Parameter range after wear
Sa Sq Sv Sp St Sz SHtp
Arithmetical mean deviation of the surface Root-mean-square deviation of the surface Maximum depth of valleys Maximum height of summits Total height of the surface Ten-point height of the surface Surface section height difference corresponding to 20–80% of the material ratio Surface section height difference corresponding to 5–95% of the material ratio Core roughness depth Reduced summit height Reduced valley depth Oil capacity Skewness of the surface Kurtosis of the surface Root-mean slope of the surface Arithmetic mean summit curvature of the surface Developed interfacial area ratio Texture aspect ratio of the surface Fastest decay autocorrelation length Density of summits of the surface
Sa (mm) Sq (mm) Sv (mm) Sp (mm) St (mm) Sz (mm) SHtp (mm) SDHtp (mm) Sk (mm) Spk (mm) Svk (mm) Sa2 (mm3/mm2) Ssk Sku Sdq Ssc (1/mm) Sdr (%) Str Sal (mm) Sds (1/mm2)
15.7–29.1 23.8–35.8 90.5–99.6 40.8–43.1 97.1–143.2 118.0–136.2 13.7–56.9 81.5–111.1 15.8–28.1 3.9–9.1 74.2–97.3 7839–15468 (2.2)–(1.3) 3.3–7.3 0.16–0.19 0.0043–0.0115 1.31–1.82 0.82–0.91 0.41–0.43 79.2–180.1
1.1–6.2 2.6–11.1 24.3–67.9 5.8–24.7 30.9–92.6 26.5–68.2 1.3–5.4 3.2–37.9 1.9–7.1 1.9–4.0 13.1–56.2 415–2883 (2.5)–(5.1) 11.9–33.1 0.038–0.114 0.0017–0.0049 0.073–0.64 0.82–0.84 0.25–0.33 329–443
SDHtp Sk Spk Svk Sa2 Ssk Sku Sdq Ssc Sdr Str Sal Sds
decrease (up to 5) and kurtosis Sku to increase (up to 33) were observed. The hybrid parameters decreased (slope Sdq, mean summit curvature Ssc and developed interfacial area ratio Sdr), but changes of Sdr were the biggest (83%). The summit density Sds increased during wear, the fastest decay autocorrelation length Sal decreased. The texture parameter Str was constant for small wear values, when the wear amount was big—it decreased. Periodicity level decreased during wear. The isotropy level (82–90% after burnishing) decreased slightly for a large wear amount. Table 2 presents the ranges of analysed surface topography parameters. One can obtain important information about the wear process based on the parameters values and their variation in perpendicular directions. During the wear, the repeatability of height parameters and slope in the direction perpendicular to sliding was smaller than in the orthogonal direction. When the wear was finished an anisotropic surface had been formed. The lay direction was the sliding direction. Fig. 6 presents an axonometric view of the analysed block surface before and after wear. The local wear values were measured as the changes of oil pocket dimensions such as depth, average diameter and oil capacity (see Table 3). The degrees of oil pockets coverage are also given in column 1. Surfaces 1 and 2 were created by cutting along axes of ellipses in parallel and perpendicular to the movement directions. In this case the measured area was 2 mm 2 mm, the sampling intervals were 5 mm in perpendicular directions. We found out that the effect of the degree of oil pockets coverage on wear values (measured as the changes of
parameters given in Table 3) is substantial. The Sample no. 1 with 20% of oil pockets coverage had the biggest wear resistance in comparison to variants nos. 2 and 3. The relative change in oil pockets depths is similar to the decrease of Sz (its relative changes for Sample nos. 1–3 were, respectively, 43%, 74% and 77%) or Sv parameters (30%, 73%, 65%). The decreases of oil capacities of individual holes were similar to changes of Sa2 oil capacity (from the Sk family) which amounted, respectively, to: 63%, 96% and 96%. One should know that separate oil pockets and surface topographies were measured on the different areas. The effect of oil pockets existence on wear resistance was also analysed. In this case a steel-cast iron assembly: was considered. Contrary to the experiment described above, the oil pockets were created on the counter-specimen surface (ring-outer cylindrical surface). The specimen (block) was made from spheroidal cast iron, but the counter-specimen (ring) was made from 40 HM steel of 32 HRC hardness [29]. The steel counter-specimens had textured surfaces with 10% coverage of oil pockets of different shapes (see Fig. 7). During the test for wear resistance the load was 2400 N and speed was 0.27 m/s. The tested assembly was lubricated by oil L-AN 46. The sliding distance for each variant was 2000 m. The analysis of several ring surfaces having oil pockets was also carried out. The area of the measured surface was 6 mm 7 mm, the sampling intervals were 15 mm in the perpendicular directions. The wear amount was comparatively small (big material hardness, the oil pocket existence). The height parameters decreased during
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82.9 μm
130 μm
5.01 mm
5.01 mm 5 mm
5 mm
Fig. 6. View of the block surface before (a) and after wear (b).
Table 3 Characteristic of oil pockets on selected test specimens B—before wear, A—after wear, D—change Max depth (mm)
Average diameter (mm)
Oil capacity (mm3)
Surface 1 (mm2)
Surface 2 (mm2)
B
A
D (%)
B
A
D (%)
B
A
D (%)
B
A
D (%)
B
A
D (%)
95
53
44
1.1
0.75
32
0.043
0.016
63
0.066
0.031
53
0.068
0.032
53
93
25
73
1.1
0.6
45
0.04
0.003
92
0.065
0.009
86
0.067
0.009
86
93
26
72
1.1
0.6
45
0.045
0.003
93
0.066
0.0095
85
0.066
0.0085
87
Test no.
1 20.4% 2 27.8% 3 25.9%
‘‘zero-wear’’ (however for small wear amount, some of them increased). The average decreases of Sa and Sq were, respectively, 35% and 26%, of Sp 46%, of Sv 15%, of St 23%, and of Sz 18%. The change of Sp parameter was the biggest, of Sv the smallest. Sk parameter decreased (the mean value was 39%), Spk too (45%). The average changes of SHtp and SDHtp were comparatively big (35–40%). The parameters characterising the shape of the ordinate distribution changed during ‘‘zero-wear’’. Ssk decreased, but Sku increased. The summit density increased 4.5–5.5 times during the wear. The hybrid parameters Sdq and Sdr decreased during wear (by 18% and 24%, respectively). However a tendency to increase summit curvature SSc was found. The oil volume Sa2 decreased during ‘‘zero-wear’’ (average change was 32%). For small wear amounts systematic changes of Str, Sal parameters were not found. The variations of profile parameters of worn surfaces were similar to those of machined surfaces. Fig. 8 presents the axonometric view of analysed ring surface before and after wear. Table 4 presents the ranges of analysed surface topography parameters. Wear resistance was also measured by the change of oil pocket dimensions such as depth, length, width and oil capacity (see Table 5). The area of measured surface was 2 mm 2 mm, the sampling intervals were 5 mm in the perpendicular directions.
Intersection areas could be important in the case of an asymmetric oil pocket on a shaft neck (see Fig. 9). Oil pockets of tear drop shape situated wider and deeper relative to the rotational direction of the shaft neck could cause a hydrodynamic lift force which would compensate the outer load. The biggest changes in dimensions were noticed for oil pockets of short tear drop shape. The smallest wear intensity was obtained in the case of pits twice as long. Pits in variant 2 could stay on the surface for longer periods and a surface with such micro-geometry seemed to have adequate oil capacity to ensure discontinuous slide contact. These oil pockets were not so deep as others, d0 ¼ 55 mm, however they remained deeper at the end of the test, d1 ¼ 52 mm. Although pits of spherical and short tear drop shapes were at the beginning similar in all dimensions and oil capacity they changed differently during wear. Probably the short drop shaped pit did not give rise to a hydrodynamic lift force. We confirmed, that the change of oil depth during wear was proportional to change of height parameters like Sz or Sa. Results of experimental investigations concerning seizure resistance were presented in [26,33]. It was found that texture area ratio, shape and dimensions of oil pockets were significant for a decrease of seizing. It is especially important in steel–steel and steel–cast iron contact because they are sensitive to adhesion. In those experiments an optimum texture area ratio of 10% was found and it is
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beneficial at low sliding velocity in a steel–cast iron assembly. 5. Conclusions The presented paper examines the use of a burnishing tool to give good surface characteristics such as surface
a 50
30
1
20
μm
40
57.1 μm
10 1.5 mm
2.6 mm
0
b 50 54.8 μm
30
1
μm
40
20 10 1.5 mm
0
1.5 mm
c
50 50.5 μm
40
20
μm
30
10 1.5 mm
0
1.5 mm
Fig. 7. Examples of burnished oil pockets of tear drop (a), (b) and spherical (c) shapes. (a) tear drop shape, length 1.8 mm, (b) tear drop shape, length 0.9 mm and (c) spherical shape, length 0.8 mm.
a
topography having oil pockets. The proposed technique is very easy and allows us to obtain a deterministic surface on hard materials. The computer program developed helps us in surface topography creation. The described procedure of oil pocket’s size assessment gives us information about degree of pits coverage. We analysed the changes of surface topographies having oil pockets of small and big amounts of wear. Height parameters and slope decreased. The changes of parameter (describing material volume) were the smallest. The changes of average parameters are bigger than those of parameters describing maximum height. Summit density increased. Ssk decreased and Sku increased. For a small wear amount the change of Sp (describing void volume) was the biggest. When the wear amount was big, the surface topography directionality changed. The sliding direction became the lay direction. The spatial parameter Sal decreased due to the smaller number of oil pockets. During ‘‘zero-wear’’ the local wear amount is hard to determine. Therefore a profilometer should be used to determine it. The local wear value can be obtained as the changes of oil pockets dimensions. We found that the area of oil pockets coverage on inner cylindrical surface made from bronze had a significant effect on wear resistance. The shape of oil pocket is important with regard to wear resistance of steel outer cylindrical surfaces. The relative change in oil pockets depths during wear is similar to the decrease of surface topography height parameters. We found that creation of oil pockets on co-acting surfaces by burnishing technique might cause increase of wear and seizure resistances under mixed lubrication. But, under different kind of lubrication, the recommended texture may impair the tribological characteristics. Much work is necessary before we can design the surface topography for specific tribosystems. The selection of materials for sliding pair and operating conditions seem to be very important. In the future, the tribological property improvement by oil pocket creation by electromechanical etching using other tribological testers will be carried out.
b
V
1523
V
53.9 μm
65.1 μm
5 mm
5 mm 5 mm
5 mm
Fig. 8. View of ring surface before (a) and after wear (b), v—sliding velocity.
ARTICLE IN PRESS L. Galda et al. / Tribology International 40 (2007) 1516–1525
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Table 4 Changes of surface topography parameters during wear Parameter symbol
Parameter range before wear
Parameter range after wear
Sa (mm) Sq (mm) Sv (mm) Sp (mm) St (mm) Sz (mm) SHtp (mm) SDHtp (mm) Sk (mm) Spk (mm) Svk (mm) Sa2 (mm3/mm2) Ssk Sku Sdq Ssc (1/mm) Sdr (%) Str Sal (mm) Sds (1/mm2)
4.7–5.2 9.5–9.9 56.1–65.3 11.2–25.9 67.2–91.2 62.3–68.2 3.75–4.25 23.3–25.8 5.2–6.6 5.7–6.7 25.4–29.7 2184–2581 (4.1)–(3.5) 15.5–19.7 0.064–0.074 0.00084–0.0011 0.20–0.27 0.36–0.94 0.33–0.46 87.5–93.3
3.1–3.8 7.4–8.2 50.8–55.1 7.5–10.7 58.3–65.9 50.1–60.7 2.54–2.64 9.1–17.8 2.9–3.9 3.7–4.2 24.4–27.5 1210–2115 (4.5)–(4.1) 22.2–24.6 0.055–0.062 0.0015–0.0019 0.16–0.19 0.36–0.89 0.38–0.44 438–464
Table 5 Characteristic of oil pockets on selected test specimens B—before wear, A—after wear, D—change Test No. Shape
Depth (mm)
Length (mm)
B
A
Oil capacity (mm3)
Width (mm)
A
D (%)
B
D (%) B
A
D (%) B
50 52
23 5
0.82 0.72 12 1.8 1.45 19
0.85 0.75 11.5 0.65 0.58 10
42
35
0.9
0.95 0.75 21
Surface 1 (mm2) A
Surface 2 (mm2)
A
D (%)
B
D (%) B
A
D (%)
0.015 0.024
0.01 0.015
33 37
0.035 0.025 28 0.056 0.041 26
0.032 0.023 28 0.023 0.019 17
0.017
0.0083
51
0.027 0.019 31
0.04
1 2 3
Sphere 65 Long 55 drop Short 65 drop
0.65 27
0.021 47
Fig. 9. Areas of long shaped oil pocket intersections, parallel (a) and perpendicular to the movement direction (b).
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