Surface indentation test (SIT) for friction prediction in mixed lubrication of coated sheets

Boundary and Mixed Lubrication: Science and Applications D. Dowson et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

461

S u r f a c e I n d e n t a t i o n T e s t (SIT) for F r i c t i o n P r e d i c t i o n in M i x e d L u b r i c a t i o n o f C o a t e d S h e e t s A. W i h l b o r g a, D. W i k l u n d b, a n d B - G R o s r n b'e a Epsilon Development AB, G6teborg, Sweden b Chalmers University of Technology, Department of Production Engineering, G6teborg, Sweden c Halmstad University, Sweden

In order to investigate the possibility to predict the frictional response of coated steel sheet materials, a combination of the Surface Indentation Test (SIT) and Bending-Under-Tension (BUT) friction test was used. Topographical data from the SIT test was combined with the friction data from the BUT test to verify the usability of SIT topographies for predicting frictional behaviour. The study included 4 different electron beam-textured (EBT) Electro Zinc coated steel sheets. The surface topography was measured with an interference microscope close to the centre of the indentation mark from the SIT. The BUT friction testing was performed in mixed lubrication under equal conditions for all the materials used. In order to describe the frictional behaviour caused by the two main lubrication mechanisms, Micro Plasto Hydro Dynamic Lubrication (MPHDL) and Micro Plasto Hydro Static Lubrication (MPHSL), a topography index was calculated from the surfaces 3D features-the WC (Wihlborg-Crafoord) index. The index consider the actual area fraction of contact area (0t), the number of isolated oil pockets (NIOPt) in the contact area, and finally the border length of the lubricant area at the fraction of contact (BLalfa). The index describes how effective the supply of lubricant is at the contact zone and correlates with the frictional behaviour of the 4 coated EBT. Further, the results correlate with previously published results for less severe lubrication conditions as well as studies performed on uncoated steel sheets. The importance of the determination of a true contact area for the determination of a relation between texture and lubrication is highlighted in the definition of the WC index. The need for further studies to develop the SIT method and methods to simulate the contact area derive from the virgin sheet surface is discussed.

1. Introduction Today, when manufacturing auto-body panels, mainly steel sheets are used. When stamping sheets several parameters are influencing the friction and thereby the punch force needed for the operation. The friction is a very important parameter, since the tension in the sheet is dependent on friction. In some stamping operations a low friction is desired for the whole sheet, e.g. in stretching operations. In other operations a high friction is desired in some sections of the sheet and a low friction in other sections, e.g. deep-drawing. Parameters influencing the results of the stamping process are: tool design (tool radius and shape), blankholder force, drawbeads, drawing speed, lubrication, lubricant viscosity, material properties in the sheet, and sheet surface topography. Moreover, in the production of cold-rolled steel sheets, the final roughness is the result of a mixture of roughnesses obtained from the tandem mill and the temper mill. In steel sheet manufacturing today, five commercial texturing methods are available: shotblasting (SB), electrical discharge-texturing (EDT), laser-texturing (LT), electron beam-texturing (EBT), and electro-chromium deposition (ECD). When manufacturing steel sheet materials, rolls produced by different texturing methods may be used; an example of this is the Lasertex, where the rolls in the tandem mill

are shotblasted and the rolls in the temper mill are lasertextured. In order to investigate the possibility to predict the friction from the Bending Under Tension (BUT) friction test to the surface topography a so-called Surface Indentation Test was used. The first main goal was to investigate the possibilities evaluating surface measurements from a SIT by using a histogram to predict the area fraction of contact. This investigation is here compared to a method proposed by Wihlborg et al. [1] i.e. using the same surface measurement. This method is based on a study reported by Gunnarsson et al. [2] and Jonasson et al. [3] using an area bearing curve and a normal probability paper in order to identify the area fraction of contact was used. Gunnarsson et al. [2] reported a good correlation for the area fraction of contact derived from the method to numbers from the traditional method using a images from an optical microscope, see example in Figure 3. Secondly, to compare friction coefficient and the WC index [1] of different approaches to determine the area fraction contact. 2. Experiments

2.1 Material properties Four Electro Zinc coated sheet materials of deepdrawing quality were included in the study. The texturing method for the sheets was EBT. The sheets were of similar grades with similar mechanical properties (see Table 1).

462

2.2 Friction tests The friction tests were performed in a BUT friction rig; see Figure 1. The sliding direction was perpendicular to the rolling direction of the sheet. The test material was cut into 6 0 0 x 5 0 m m strips. This study used the parameter combinations of a contact pressure of 30 MPa, a dynamic viscosity of 360 mm2/s, and a sliding speed of 100 mm/s. -

j~

Figure 1. Principle of BUT test. 2.3 Surface Indentation Test (SIT) SIT was originally developed at Volvo Technological Development in order to investigate results from galling studies [1 ]. SIT uses a predetermined force F applied to a flattened Brinell ball and thereby causes plastic deformation of the steel sheet surface; see Figure 2. The SIT was performed with a Brinell tester, a Dia Testor 3a from Amsler Otto Wolpert-Werke GMBH, Germany. The Brinell ball was ground fiat and slightly crowned with a top angle of roughly 179.6 ~ The pressure used for the SIT method was 88.8 MPa. No lubricant was used.

exchangeable magnification objectives. The vertical measurement range is 0.5 mm with a resolution better than 10 nm. A 10x objective, producing a 0.9xl.2 mm field of view with an x-sampling of 3.2 ~rn and a ysampling of 3.8~xn was used in this study. The surface topography was measured in the centre of the indentation mark on the sheet. In order to have contact conditions close to the actual ones, a ball filter was applied to 3D topographical data for the method [ 1]. In order to examine the contact conditions for the different textures involved, the true area of contact, o~, between the tool and the sheet material was estimated. The contact area was determined by identifying the height in surface topography where plateaus change to valleys. The technique is based on the fact that a bearing-area curve composed of various Gaussian distributions appears as a straight line when plotted on a normal probability paper. To achieve the level of the contour line separating plateaus from valleys, the intersection point of the two lines was def'med as being at the bearing level. The estimated real contact area was strongly influenced by the slope of the line fitted to the valley component of the topography. Hence, this line slope was held constant and chosen to one standard deviation and the line was set as a tangent to the bearing area curve, as used by Jonasson et al. [3].

2.4 Roughness measurements and evaluation For the sheets tested in the SIT the surface topography was measured with an interference microscope, WYKO RST Plus, from Veeco Instruments Inc., USA. The interference microscope is a white-light vertical scanning instrument that works with one of several

Sheet

Figure 2. Principle of SIT.

Figure 3. Example of the optical method for evaluating an EDT steel sheet after a BUT tesi.-Image a. is the optical microscope image of the sheet surface and image b. shows the manually traced in contacts from image a.

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Table 1. Sheet thickness, yieM point at 0.2 % strain (Rpo.z)and average roughness (Ra) of the tested materials. Material

Sheet thickness

Rp0.2 (MPa)

Ra 0tm)

128 157 163 179

0.85 1.16 1.87 2.50

~mm) EBT-S 1 EBT-S2 EBT-S3 EBT-FF

0.81 0.80 0.80 0.68

The restrictions for the calculations of the number of isolated oil pockets (NIOPt) were: a pocket has at least two points that are connected and should have no contact with the edge of the measured area. The pockets are calculated from the top of the surface down to the area fraction of contact; a truncating level of 10nm has been used [ 1]. In studies performed by Wihlborg and Crafoord [1] a so-called Wihlborg-Crafoord index (WC index) is used to describe the frictional behaviours in a contact between sheet and tool. The WC index is defined as the number of isolated oil pockets (NIOPt) multiplied by the border length of the lubricant area at the area fraction of contact (BLalfa) and divided by the area fraction of contact ((t); see equation 1.

WC index = NIOPt * BLar

(1)

The WC index in theory is based on the results presented by Mizuno [4], Azushima et al. [5], and Beck et al. [6, 7], where NIOPt is a figure for how many pockets are needed to achieve Micro Plasto HydroStatic Lubrication (MPHSL). BL~aea describes the amounts of possible sources for Micro Plasto HydroDynamic Lubrication (MPHDL) to set in. tx is the amount of material that will be supplied with lubricant by these two components.

3. Results a n d discussion In order to find a robust and computer-friendly method to isolate the level of contact, the truncation level, a method based on the shape of the height distribution curve (Histogram) was introduced. Microscope images indicate the worn parts of the steel sheets as areas with similar greyscale values (see Figure 3). Those areas are a result of the truncation wear. Note that the result of the contact between the sheet and the tool is a tnmcated surface. The truncation does not necessarily mean 100%

abrasive removal of material but also a plastic redistribution of material from the peak areas to lower regions. In the histogram, a clearly identified peak verifies the truncation model (see Figure 4 al-cl). In theory it should be simple to use the position of the peak as an indicator of the height level of contact, hence the height level in the surface where the WC index and other parameters characterising features of the contacting surfaces should be calculated. In practice there doesn't exist an infinitely sharp peak in the histogram indicating the shift at the truncated plateaux. Instead there exists a rather wide height region where the change from the original unworn surface reaches a maximum (see Figure 4 al-c 1). In order to investigate a height level robust enough to ensure that the contacted area will be above in the evaluation, a study based on three comparative methods was employed. Firstly, measurements were made with the interference microscope over the contacted area. After this the measurements were evaluated in the normal probability paper to find the true area fraction of contact (tx) according to Wihlborg et al. [ 1]. Secondly, a histogram was made based on the interference measurements and tx was plotted (indicated as a circle and cross in the histograms). By judging the histograms in Figure 4 ale l, the conclusion is that the true o~ is below the histogram maximum and between the root of the maximum and the peak as indicated by the crosses in Figure 4 al-c 1. Figure 4 a2-c4 shows the surfaces at the height corresponding to the area fraction of contact derived from each method. Note that the measurement shown in Figure 4 a3-c3 has been ball-filtered. The numbers of area fraction of contact derived from the maximum and from the root of the maximum in the histogram show no correlation to the true tx calculated from the normal probability paper described above; see Tables 2-4. However, the histogram may still be used for other applications.

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Figure 4. Height distribution curves for the three EBT-textured materials S1, $2, and $3 (from left to righO. The crosses indicate area fraction level of contacts derived from the three methods. Frames a2-c2 show the three sheets at the area fraction of contact derived from the peak of the histogram. Frames a3-c3 show the three sheets at the area fraction of contact derived from method proposed by Wihlborg et al [1]. Frames a4-c4 show the three sheets at the area fraction of contact derived from the root of the maximum of the histogram. (The black areas indicate steel and the white oil pockets.)

465

Table 2. Friction coefficients, area fractions of contact, NIOPt, BLalfa and WC index for the sheets derived from the peak of the histogram. Material

BUT Friction coefficient

EBT-S1 EBT-S2 EBT-S3 EBT-FF

0,10 0,13 0,12 0,10

SIT area fraction of NIOPt [1/mm2] contact a [%]

28 14 22 14

BLalfa [ ~ m m 2]

564 407 604 496

8590 7217 7295 7836

WC index [ ~ m m 2] 0,74 0,90 0,66 0,80

Table 3. Friction coefficients, area fractions of contact, NIOPt, BLalfa and WC index for the sheets derived from the methods proposed by Wihlborg et al. [1]. Material

BUT Friction coefficient

SIT the true area fraction of contact

[l/ram z]

BLalfa [rn/1-n~ 2]

576 461 646 639

8722 7193 7387 8710

WC index

Ira/ram 2]

[%]

a

EBT-S1 EBT-S2 EBT-S3 EBT-FF

NIOPt

30 24 32 36

0,10 0,13 0,12 0,10

0,68 0,56 0,59 0,62

Table 4. Friction coefficients, area fractions of contact, NIOP~ BLatfa and WC index for the sheets derived from the root of the maximum of the histogram. Material

BUT Friction coefficient

EBT-S 1 EBT-S2 EBT-S3 EBT-FF

0,10 0,13 0,12 0,10

SIT area fraction of NIOPt [1/mm2] contact a [%]

BLalfa [m/ram 2]

622 498 698 690

9420 7768 7979 9407

50 36 36 35

Due to the strong influence of the selection of the level for the area fraction of contact when calculating the WC index, (see Table 2-4 and equation 1), no correlation was found for the numbers evaluated by using the histograms and the friction coefficient from 0,14

WC index [lr,./mm 2]

0,94 0,68 0,63 0,66

the BUT friction test. However, when using the method proposed by Wihlborg et al. [ 1] a very good correlation was achieved for the EBT materials; see Figure 5.

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466 REFERENCES

4. Conclusions 1. 9 In order to evaluate the area fraction of contact for deformed surfaces by identifying the peak and the root of the maximum from the histogram no correlation was found with the method proposed by Wihlborg et al.[ 1].

2.

9 The WC index indicates whether high or low friction will be obtained for EBT materials in a mixed lubrication BUT test. 9 SIT in combination with a WC index is a very promising method in order to predict the friction in mixed lubrication from a BUT friction test.

5. Future research The need for finding a more robust and computerfriendly method to isolate the level of contact is high. Therefore, future studies will focus on developing the SIT using microscope-image-processing techniques. Further, numerical simulations of the deformation of the peaks are of importance in order to understand the deformation of the contact region, and thereby understand how to determine the area fraction of contact from deformed sheet materials as highlighted in this paper. The combination of these two will give the possibility to optimise the surface topography in order to achieve a stable friction in deep-drawing processes.

Acknowledgements The authors wish to thank the Swedish Institute for Metal Research, Volvo Car Corporation (Volvo), Swedish Steel (SSAB), Surface Topography Optimisation for the Automotive Industry (AUTOSURF), and The Swedish Programme for Production Engineering Education and Research (PROPER) for their support in this work.

3.

4.

5.

6.

7.

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