TRIBOLOGICAL CHARACTERISATION OF SURFACE TOPOGRAPHY USING ABBOTT-FIRESTONE CURVE

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1 208 FASCICLE VIII, TRIBOLOGICAL CHARACTERISATION OF SURFACE TOPOGRAPHY USING ABBOTT-FIRESTONE CURVE Minodora Rîpă, Lorena Tomescu, Mioara Hapenciuc, Ion Crudu University Dunărea de Jos of Galati, România ABSTRACT The majority of the roughness parameters allow general description of the surface topography. For tribological characterisation of surface topography, analyses based on Abbott-Firestone Curve give useful information about some special functioning properties as bearing, sealing and lubricant retaining capabilities. The paper presents the results of roughness analyses for the worn surfaces of steel rollers, obtained after rolling/sliding wear tests. KEYWORDS: topography analysis, Abbott-Firestone curve. 1. INTRODUCTION For many engineering applications the surfaces are manufactured in order to get some special functioning properties as bearing, sealing and lubricant retaining capabilities. These surfaces may be designed to have specific topographical features that are beneficial for specific functioning applications. The majority of the roughness parameters allow general description of the surface topography. Nevertheless it is more efficient and useful to analyse specially defined parameters for describing the particular chraracteristics of a surface, that are important for a certain application). Traditional roughness parameters as R a and R q are seen to be concerned only with relative departures from the mean line in the vertical direction; they do not provide any information about the shape, slopes and sizes of the asperities or about the frequencies of their occurrence [1]. It is possible to have the same R a and R q values for surfaces of widely different materials and profiles (fig. 1). Unworn surface, R a =2,4µm Worn surface, R a =2.4µm Fig. 1 Different worn surfaces, same R a [2]. Measuring digital devices make possible to explore more sophisticated but appropriate parameters in order to obtained quantitative and qualitative criteria for improving selection of working regimes, materials and manufacturing processes The Abbott-Firestone Curve A function named the curve of the bearing ratio (or the Abbott-Firestone curve or bearing area curve) is defined for a profile by national and international standards. The Romanian standard includes this curve among the relative parameters of the roughness and the parameters referring to the irregularities of the profile. Digital devices for roughness measurements automatically plot this curve. One of the advantages for studying surfaces by the help of Abbott-Firestone curve is that it could simulate the wear effects and the running-in process. Furthermore, this curve gives information about the material and void volumes characterising the surface topography and more recently it could be useful for defining and using functional parameters in 3D analysis [1, 3] Analysing the Abbott-Firestone curve The functional analysis of the surfaces with the profile bearing length ratio was initiated in 1933 by Abbott and Firestone. Three parameters were extracted from the Abbott-Firestone curve. The peak roughness was defined as the range of heights embracing 2-25% of the bearing length while the medial and valley roughness were defined to cover 25-75%, 75-98% of the bearing length, respectively [1]. The standard DIN 4776 formally standardised the R k parameter set, based on three zones classification of the Abbott-Firestone curve. This set

2 FASCICLE VIII, 209 of parameters is mainly used for characterising highly stressed surfaces [1]. In paper [1] Dong and co-workers proposed as functional indices of the Abbott-Firestone curve, the t p values characterising three intervals: 5%, 5-80% and 80%. These specific percentages of 5% and 80%, respectively, are selected based on experience. The levels h 0.05 and h 0.80 are delimiting three zones similar to those given in the standard DIN 4776: the peak zone, the core zone and the valley (void) one. This zoning of the Abbott-Firestone curve was applied by the authors [4]. The zoning of the Abbott-Firestone curve presented in papers [5, 6] has five intervals: 5%, 5-10%, 10-50%, 50-95% and 95%. Good results in studying the porous surfaces were obtained. The first zone is characterising the very high and prominent asperities that are worn during the running-in period. The bearing and fluid retention capabilities could be characterised by the second zone, also named microroughness, and the third zone. The forth underlines the voids or the pores of the investigated surface. The sectioning level (depth) corresponding to t p50%, also named levelling depth of the profile, is used as a criterion in evaluating the load capacity and wear resistance of a surface. The zoning of the Abbott-Firestone curve is common property of these methods for characterising the functional surfaces. Although these methods were successfully used for characterising surfaces for some particular engineering applications, they loose their significance when studying other specific surfaces. The parameters derived from these methods are given in length units and they can not offer complete information on the topography shape of different surface types. In fact, the Abbott-Firestone curve contains more information and not only about load capacity, as void and material volumes of the investigated surface, necessary for 3D analysis. Figure 2 presents a profile measured on a ground, unworn surface from a roller-type element, made of steel grade 40Cr10, quenched and tempered (similar to 51430H SAE steel). Figure 3 presents the Abbott-Firestone curve. Figure 2 Profile of the unworn surface Fig. 3 Abbott-Firestone curve of the unworn surface. The main roughness parameters for the roughness profile (Gaussian filter applied on the raw profile) are: R a (µm) R q (µm) R sk R ku R t (µm) R z (µm) For this representative profile the Abbott- Firestone curve was analysed. These curves were plotted by data processing for the unfiltered profile, as presented in figure 3. The height intervals that are also significant parameters of the Abbott-Firestone curve are presented in Table 2, both for the raw and filtered profiles (according to [1] and [5] zoning types). Table 2 Profile h 0.05 h h 0.05 h 1 - h 0.8 Raw Filtered Profile h h 0.05 h h 0.1 h h 0.5 h 1 - h 0.95 Raw Filtered One may notice, from Table 2, that there are lower values of the height ranges for the filtered profiles as compared to those obtained for raw profiles. This is one of the reasons why Abbott- Firestone curve is recommended to be done on the unfiltered profile or applying a special filter, as it was pointed out in [4, 5, 6, 7]. 2. METHODOLOGY OF WEAR TESTS An Amsler rig was adapted for SAE test sets (twin rollers), adjusted and completed for a running in testing program. The test rig ([8], [9]) has been provided with a torque transducer (type T30FN200 - Hottinger Baldwin ) for measuring friction torque and rotational speed, a force transducer, temperature transducers and a data acquisition system (DMCPlus- Hottinger Baldwin ). For all tested pairs the running time was 10 5 cycles, corresponding for the early-stage wear, while the most significant tribolayer parameters changes occur. Lubrication was achieved by splashing and the lubricant was paraffinic mineral base-oil (AGMA lubricant no. 4 EP). The experimental research was done taking into account the following commanding factors influencing the wear process [8]: - relative sliding speed characterised by rollingsliding ratio s=(v 1 -v 2 )/(v 1 +v 2 ): s1=22.5% s2=7.5% and s3=0% (pure rolling); - normal loading of the line contact, by terms of Hertz pressure: p 1 =850MPa, p 2 =700MPa, p 3 =560MPa; - different materials for rollers, steel 1 - OLC 45 (similar to 1045 SAE steel grade) and steel 2-40Cr10 (similar to 5140 SAE steel grade), witch are two common grades of steel for high pitch spur gears, through-hardened: pair M1 roller 1 made of OLC 45 and roller 2 made of 40Cr10, pair M2 roller 1 and roller 2, both made of 40Cr10;

3 210 FASCICLE VIII, - different initial microgeometry of rollers: pair R1 roller 1 and roller 2 both turned, pair R2 roller 1 turned and roller 2 grinded; the necessity of studying this last commanding factor results because the turning manufacturing process generates a periodical roughness profile and the grinding one generates a random roughness profile. 3. TOPOGRAPHY ANALYSES It has to be taken into account the variation of the roughness parameters, the changes in the bearing ratio curves (Abbott-Firestone curves) and the parameters associated to them, for obtaining a complex analysis on the microgeometry of a surface bearing tribological processes but also microscope studies of the investigated surfaces Roughness of unworn and worn surfaces used for tribological studies of the surfaces and the data were processed using the Talyprof soft (both Taylor Hobson ). The acquired data have been processed, analytically and graphically, by use of Matlab and Excel applications [8]. Figure 4 gives examples of profilograms, Abbott-Firestone curves and microscope photos (50x) for a turned surfaces, having periodical roughness. The values of the roughness parameters are presented in table 1. Table 1 Surface R a R q R y R sk R ku Unworn Worn Studying the worn profile, one may notice a smoothening of the peaks that determines the decrease of the height parameters (R a, R q, R y ). Roughness measurements were done by the help of the digital profilometer Surtronic3+, commonly µm µm points=500µm a) points=500µm b) c) d) e) f) Fig. 4. Surface of a tests sample made of steel (grade 40Cr10, similar to steel grade 5140 SAE), manufactured by turning, periodical profile: a) unworn profile; b) worn profile; c) Abbott-Firestone curve - unworn profile; d) Abbott-Firestone curve - worn profile; e) unworn profile (50x); f) worn profile (50x)

4 FASCICLE VIII, 211 The values of the statistical parameters R sk, R ku for the worn profile shows the asymmetry of this profile (R sk = , as compared to , for the unworn profile that is symmetric to the mean line) and the accentuation of the random character of the profile nonregularities (R ku =3.19, compared to value 3, value characteristic for a profile with Gaussian distribution). The Abbott-Firestone curve for the worn profile shows an increase of the fluid retaining capability, representing a positive effect of bearing the running in process on the surface Analyses based on Abbott-Firestone curve Unworn and worn surfaces of rollers 1 (made of OLC 45 and manufactured by turning) were analysed. Figura 5 presents the Abbott-Firestone curve zoning for roller surfaces, after wear tests done at a rollingsliding ratio s 1 =22.5%, and figure 6 presents the Abbott-Firestone curve zoning for roller surfaces, after wear tests at pure rolling, s 3 =0%. There are pointed out weights of relative heights for the zoning based on two specific percentages, 5% and 80%:, and, where h100 is the maximum roughness height (R t ). One may notice that for all running in tests lubricant retaining capabilities are improved, fact suggested by the parameter.the greatest increase of this parameter is noticed for the tests in set 45/M2/R2 (fig. 5, j2, k2, l2), for both rollers made of 40Cr10 (M2), with different genuine roughness (R2): roller 1 turned and roller 2 grinded. Under the load of p 3 =560 MPa; it seems that running should be continued as the value of the parameter is higher after the wear test. The best surface topography after wear test is noticed at the tests from set 45/M1/R1: s 1 =22.5%, rollers made by different steels (M1), roller 1 and roller 2 both turned, (R1), for average and small load, and (fig. 5, b2 and c2). This conclusion is sustained by the roughness profiles analyses. Under the greatest load, p 1 =850 MPa, wear process is very intense, The height of the worn layer overpassing the total height of the initial profile. For this case (fig. 5, a2) The value of the parameter shows that a new surface will bear wear process, after the running. In pure rolling tests the study of the surface topography must be more complex, because of the specific wear process. The surface accommodation is almost exclusively a process of plastic deformation. During the running in, due to the rolling movement and due to the loading of the lubricated contact, roller 1 profile R 1 is imprinted in roller 2 profile, supporting at its turn, plastic deformations and material losses, that determine the diminish of the proeminences heights but also the depth of the void, reflected by diminishe of the paameters R a, R q, R y. Thus the changes of the weights of relative heights are not significant. In such cases, analyses based by Abbott- Firestone curve zoning must be accompanied by others roughness and topography studies. 4. CONCLUSIONS Due to the complex aspects of the phenomena involving friction and wear, taking place during running in tests with different rolling-sliding ratios, the analysis of the surface topography is not limited only to study the variation of the roughness parameters. Depending on the initial surface type and on the expected wear processes, a particular zoning of the Abbott-Firestone curve may be adopted, this one making possible to point out the changes in the specified zones on the (R t ). Studies based on zoning of Abbott-Firestone curve give the best results for the surfaces characterised by a depth of the worn layer less than maximum roughness height.ttopographical studies are also necessary for cases when wear is dominated by plastic deformation. 5. ACKNOWLEDGMENTS The authors are grateful to the Romanian National University Research Council (CNCSIS) for the support of the Research Grant No /2001-theme no. 16 (CNCSIS code 536). REFERENCES 1. Dong W. P. et al., 1994, Comprehensive study of parameters for characterising three-dimensional surface topography. III: Parameters for characterising amplitude and some functional properties, Wear, 178 (1-2), pp Stout K. J. et al., 1977, Analytical Techniques in Surface Topography and their Application to a Running-in Experiment, Wear, 43, pp Stamboliska Z., Kuzinovski M., 1999, Analysis and Mathematical Interpretation of Parameters That Describe the Microstereometry of Machined Surfaces. 3th Inter. Conf. of Tribology, Balkantrib 99, vol. I, 1999, Sinaia (Romania), pp Rîpă M., Tomescu L., Hapenciuc M., 2001, Using the Abbott Curve for Evaluating the Surface Topography, Proc. Conf. Tribology of XXI Century, Sofia, Bulgaria, pp Kjeldsteen P., 1994, Wear resistance of PM materials and its dependency on surface topography, Procs of Powder Metallurgy World Congress (PM '94), Vol 1, Paris, France, 6-9 June 1994, Ed. de Physique (France), pp Kjeldsteen P., 1997, Tribological investigations of P/M Stainless steels mixed with HCX23 against Al 2 O 3 & SiC, Danfoss Research Report MUP Torrance A. A., 1997, A simple datum for measurement of the Abbott curve of a profile and its first derivative, Tribology International, 30(3), pp Rîpă, M., 2002, Studiul pe tribomodel al tribostratului în procesul de uzură specific roţilor dinţate, PhD Thesis, Universitatea "Dunărea de Jos" din Galaţi. 9. Rîpă M., Crudu I., 1999, Studies of Rolling-Sliding Tribosystems Methodology and Preliminary Tests, The Annals of Dunărea de Jos University of Galaţi, Fascicle VIII, Tribology, pp Rîpă M., et al., 2001, Surface profilometry studies in rolling/sliding wear tests in Proc. World Tribology Congress, Vienna, 2001 (paper no. 930 on CD)

5 212 FASCICLE VIII, test 45/M1/R1 test 45/M1/R2 a1) a2) b1) b2) c1) test 45/M2/R2 c2) d1) d2) e1) e2) f1) f2) j1) j2) k1) k2) l1) l2) Fig. 5 Abbott-Firestone curve zoning for roller surfaces, wear test at rolling-sliding ratio s 1 =22.5%. test 00/M1/R1 a1) a2) worn, b1) b2) worn, c1) c2) worn, test 00/M1/R2 test 00/M2/R2 d1) d2) worn, e1) e2) worn, f1) f2) worn, j1) j2) worn, k1) k2) worn, l1) l2) worn, Fig. 6 Abbott-Firestone curve zoning for roller surfaces, wear test at pure rolling s 3 =0.