Influence of Surface Preparation on the Roughness ... · Influence of Surface Preparation on the ... characteristic microtopography of steel surfaces on the ... impact of surface

BULETIN ŞTIINłIFIC, Seria C, Fascicola: Mecanică, Tribologie, Tehnologia ConstrucŃiilor de Maşini SCIENTIFIC BULLETIN, Serie C, Fascicle: Mechanics, Tribology, Machine Manufacturing Technology ISSN 1224-3264, Volume 2014 No.XXVIII

6

Influence of Surface Preparation on the Roughness Parameters and Tensile

Strength of Steel/Rubber Hybrid Parts

István BARÁNYI1,* - Tamás RENNER2 - Gábor KALÁCSKA3 - Patrick De BAETS4

Abstract: Components of steel/rubber hybrid parts used as vibration dampers are fixed to each other by vulcanization.

The technical reliability of vibration dampers thus constructed is determined by physical and chemical processes at the

rubber/steel interface. Bond zone strength highly depends on surface preparation. This work studied the impact of the

characteristic microtopography of steel surfaces on the strength of bonds established by vulcanization. Steel surfaces

were modified in function of spraying time while applying spraying by EKF 24 quality corundum and GN 50 steel shot

(by industrial procedure). Correlations were shown between the tensile strength of the bond layer and indices to

characterize the surface based on 2D and 3D microtopography measurements. Among 3D surface topography

parameters, the values of Sv (distance between the lowest point of the surface and the median plane) correlate with

tensile strength changes. In the bond zone of parts of optimal strength, electronmicroscopic tests were conducted to

describe steelwork surfaces in contact with rubber and characteristic X-ray emission measurements were performed to

track material structure changes in the surroundings of the boundary layer. Based on 1D and 2D lateral element maps,

sulphur and chlorine were shown to be considerably concentrated near the boundary layer. On the basis of lateral

element maps it was concluded that no harmful material composition modifications were produced in the rubber/steel

bond zone. Keywords: Shot blasting, corundum spraying, microtopography, roughness, tensile strength

1 INTRODUCTION

Rubber spring production basically consists of two sub-processes. First, the steelwork and the caoutchouc mix must be produced, then the caoutchouc must be converted into rubber by vulcanization and proper bonds must be established between the steelwork and the rubber body. According to Naß [1], vulcanization time can be reduced by up to 40% in case of state-of-the-art injection pressing, without any reduction in the strength of the rubber/steel bond. Rubber springs are sensitive machine parts: their lifetime highly depends on the quality of bond formation. Connections are appropriate when products are torn during tensile tests in a way that steelwork surfaces remain rubber-covered, meaning that no plane metallic surfaces can be seen (DIN 53531).

The following sub-processes should be discussed in bond system analysis:

- steelwork preparation [2], - product shaping, - vulcanization tool shaping. Rubber/steel bonds by chemically produced

binders represent a leading-edge method of industrial mass production. The essence of the procedure is that the surfaces of properly pre-treated steelwork must be treated by a two-ply adhesive, which can be followed by the unification of the two components (metal and rubber) into a single machine part using an appropriate rubber mix in compliance with the vulcanization technology prescribed. The first binder layer is the base to be attached to the basic metal and the second layer can establish chemical bonds with the elastomer. Chemical bonds are also formed in the so-called interdiffusion layer between these two layers, thereby ensuring bond layer stability [3].

A number of active places are created along the treated rough metal surface where the binder (primer) can establish linkages with the basic metal [4]. The impact of surface micro boreholes on adhesion bonding was studied by H.C.Man [5], but no correlations were established in respect of vulcanized bonds. Based on investigations by James El Delattre et al [6], there is a special plasma polymerization process by which a strength increasing film can be created between rubber and metal, but it is far from spreading at an industrial scale as yet. Our component analytics tests serve as a basis for further improvements in bond strength.

Even alongside state-of-the-art manufacturing technology, there is no testing procedure to check the quality of hybrid components produced without destruction. Akimasa Tsujimoto et al [7] studied surface energy and bond force, but failed to establish results for industrial-scale standardized production. However, in addition to subjecting finished parts to destruction testing methods, there is also a standard laboratory procedure for specimen bond strength analysis [8]. Tensile test DIN 53531 is frequently used, in the course of which bonds are characterized by rubber residues remaining on the steelwork in proportion of the surface. In the course of spraying, corundum or steel grains are mixed with high-pressure air and sprayed to the workpiece. The maximum speed of spraying as well as the quality and size of the grain are determined by the quality of the material to be treated. Tests by Shaymaa E. Elsaka [9] demonstrated that the time factor was highly important in terms of adhesion in case of surfaces treated by different methods; however, no correlations were found in case of steel/rubber components. According to Nitzsche [3], the following results can be achieved through surface pre-treatment by corundum (Figure 1).

- spraying by steel grain 0.7-0.8 mm - spraying by corundum 0.4-0.7 mm - spraying by steel grain 1.0 mm


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-

Fig. 1. Impact of various spraying materials on tensile

force (Nitzsche, 1983) In the light of the above, it has been set as an

objective to find a surface roughness index in the course of our work which correlates with the extent of the tensile force measured on the machine part. Research results on 3D parameters and micrograph characterization by Á. Czifra, T. Goda, E. Garbayo [10], J. Sukumaran, M. Ando, P. De Baets, V. Rodriguez, L. Szabadi, G. Kalacska, V. Paepegemaare [11], Á. Czifra, K. Váradi, S. Horváth [12] and the roughness parameters definition and usability by V. Rodrigues, M. Ando, J. Sukumaran [13] also used in our work.

2 EXPERIMENTS

Scanning electron microscope measurements

were performed to describe the morphology of steel surfaces and characteristic X-ray emission measurements to produce lateral element maps in the surroundings of boundary layers. Measurements were intended to verify whether the composition of the mix came to be enriched by the elements examined in accordance with references in the literature; another goal was to establish numerically how sulphur content changed in the surroundings of the boundary layer. Specimens were cylindrical rubber/steel parts of 25 mm diameter, cut into half lengthwise; their metal component was Fe-235 quality general structural steel and their rubber component was a natural caoutchouc-based mix coded R155OF1; the contact surface was treated by a binder combination of Chemosil 211+411. For testing purposes, a 6x10 mm2, 2 mm thick part was sawed out of the specimen. The test surface of specimens (Figure 2) was cleaned by ethanol of analytic purity (Ethanol 96%) in order to remove scraps from sawing. The envisaged testing method also required that the surface to be examined of the sample should be coated by an approx. 10 nm thick gold layer as well. Surface tests by scanning

electron microscope (SEM) were performed for the purpose of 2D morphological testing. As a complementary test, optical microscopic interferometry contrast images were produced.

Fig. 2. Specimen geometry (d1 = 25 mm, h = 20 mm, s =

2 mm, l = 18 mm, d2 = M6)

Surfaces were pre-treated by a Büffel 140 injection sprayer in case of EKF-24 corundum spraying and by a STEM TWS 7.5 centifugal sprayer in case of GN 50 shot spraying. The average roughness Ra of original specimens was 2 micrometers; spraying pressure was set to 8 bars; and the angle of incidence was 25° in both cases according to the accepted technology.

Roughness measurement results were recorded by a Mahr Perthen Concept 3D stylus instrument on a 1mmx1mm surface part, at 0.2 mm intervals in both directions. Static tensile tests of the finished product were performed on a Lloyd LR 30K tensile test machine, set at a constant speed of 500 mm/min, subject to 25°C environment temperature and 45% relative humidity.

3 RESULTS

3.1 Impact of surface roughness characteristics on tensile strength in a a case of EKF-24 corundum

On the basis of the tests it was established that the impact of the rubber and metal connection on tensile strength is less characterized by average surface roughness ”Ra” than by microtopographic testing, whose progress clearly follows changes in tensile strength (Figure 3).

Table 1. Correlations between 2D surface roughness, spraying time, and tensile force

Num. Spraying

time [s]

Ra

[µm]

Ry

[µm]

Rz

[µm]

Rq

[µm]

M 1

[N]

M 2

[N]

M 3

[N]

M 4

[N]

AVG tensile

force [N] σ 2 σ

1 3 4,78 35,62 29,63 6,05 3184 3135 3304 3205 3207 55 110

2 5 5,1 42,45 31 6,53 3100 3308 3420 3352 3295 107 214

3 8 4,07 29,5 25,82 5,15 3422 3206 3316 3400 3336 77 154

4 15 4,4 31,07 26,97 5,52 3588 3434 3434 3456 3478 64 128

5 30 4,48 31,49 26,2 5,52 2963 3074 3213 3010 3065 88 176


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Table 2. Correlations between 3D surface roughness, spraying time, and tensile force

Num. Spraying

time [s]

Sq

[µm]

Sv

[µm]

Sz

[µm]

Sa

[µm]

M 1

[N]

M 2

[N]

M 3

[N]

M 4

[N]

AVG tensile

force [N] σ 2 σ

1 3 7 27 61 5,08 3184 3135 3304 3205 3207 55 110

2 5 7 27,5 60,5 5,04 3100 3308 3420 3352 3295 107 214

3 8 7 30,1 54,5 5,4 3422 3206 3316 3400 3336 77 154

4 15 7 34,1 59,08 5,4 3588 3434 3434 3456 3478 64 128

5 30 8 27,8 56,78 5,98 2963 3074 3213 3010 3065 88 176

Results were evaluated as follows: a 5% error range was defined in the negative direction from the maximum value of the curve showing the course of the tensile force, which the tensile force generated should fall within. Then the Sv sections associated with the 5% sections of the curve were projected to the Sv axis, wherefrom optimized microtopography indices could be read directly. Thereby a roughness range describing the surface (31 µm <Sv<34 µm) was identified where the

bond force was at its maximum. On the basis of the tests performed it can be stated that the tensile strength of cylindrical rubber products can be kept within an 5% error range by applying an R155OF1 caoutchouc mixture in the testing system in compliance with the steps and criteria of the optimized manufacturing technology presented. It should be noted that these results are valid only in case of plain plated steelwork and injection moulding.

Fig. 3. Interpretation of error limits for corundum as spraying material

3.2 Impact of surface roughness characteristics on tensile strength in a a case of GN-50 steel shot

Experiments were continued by changing metal pre-treatment technologies and mechanical spraying was performed by applying GN-50 quality steel grain in a closed centrifugal spraying cabin. In the course thereof, another 50 pieces of steelwork were treated, out of which 25 tensile specimens were vulcanized. A Mitutoyo measuring machine was used for specifying the 2D characteristics of the surface, with settings of DIN Pc5 1990 0.8x5. Measurement results are shown in Tables 3-4 and Figure 3. Based on the results it was stated,

similarly to the series of measurements with corundum, that the impact of the rubber and metal connection on tensile strength was less characterized by average surface roughness ”Ra” than by microtopographic testing; therefore the 3D surface characteristics of sprayed surfaces were determined in case of steel grain spraying material as well, whose progress clearly follows changes in tensile strength (Figure 4).


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Table 3. Correlations between 2D surface roughness, spraying time, and tensile strength

Num. Spraying

time [s]

Ra

[µm]

Ry

[µm]

Rz

[µm]

Rq

[µm]

M 1

[N]

M 2

[N]

M 3

[N]

M 4

[N]

AVG tensile

force [N] σ 2 σ

1 3 5,57 45,6 35,65 7,16 2500 2530 2670 2610 2578 79 159

2 5 6,69 49,81 39,76 8,47 2750 2760 2700 2710 2730 37 73

3 8 7,53 58,84 43,83 9,4 3550 3530 3400 3700 3545 108 216

4 15 7,47 48,93 40,82 9,14 3670 3690 3200 3401 3490 181 363

5 30 6,01 42,58 35,07 7,48 3600 3610 3560 3200 3493 157 313

Table 4. Correlations between 3D surface roughness, spraying time, and tensile strength

Num. Spraying

time [s]

Ra

[µm]

Ry

[µm]

Rz

[µm]

Rq

[µm]

M 1

[N]

M 2

[N]

M 3

[N]

M 4

[N]

AVG tensile

force [N] σ 2 σ

1 3 7,2 27,12 57 5,7 2500 2530 2670 2610 2578 79 159

2 5 7,12 27,78 55 5,57 2750 2760 2700 2710 2730 37 73

3 8 5,96 33,45 36,6 4,82 3550 3530 3400 3700 3545 108 216

4 15 6,27 31,11 40,5 5,02 3670 3690 3200 3401 3490 181 363

5 30 6,82 29,23 51,2 5,41 3600 3610 3560 3200 3493 157 313

Fig. 4. Evaluation of measurement results

Fig. 5. Steelwork surface microtopography before

spraying

Fig. 6. Surface microtopography associated with

maximum tensile force


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Figure 5 shows the microtopography of untreated steelwork, and Figure 6 shows the microtopography associated with optimized production technology, that is, with the maximum tensile force.

Results were evaluated similarly to the previous section. It can be observed in Figure 3 that the numerical value of the Sv index number to characterize surface microtopography starts to decrease if spraying time is increased over a certain limit (9 s). Figure 3 shows the values associated with the 5% error limit of the curve representing the course of tensile force. The roughness range to describe the surface can be identified from the figure as 32 µm<Sv<34 µm, where the bond force is at its maximum. It can be stated that surface pre-treatment by EK-24 corundum and GN 50 steel grain yield identical results. The practical significance of this is that considerable productivity improvements can be achieved by applying the spraying times specified in the course of machinery operation in addition to the fact that energy and wages costs can be minimized by reducing cycle times.

3.3 Component analytics survey of the rubber/steel boundary layer

In order to clarify processes within the boundary layer, one-dimensional component distributions perpendicular to the rubber/metal boundary layer were examined. It was concluded from the experimental results that no material composition modifications were produced along the interface, therefore development experiments with nanofibers can be productive.

On the basis of X-ray test results in can be stated that chlorine, sulphur, zinc and iron appear in the bond. Dominant carbon content is due to soot mixed into the rubber. In order to define locations of further enrichment, measurements were performed in 1 mm length perpendicularly to the bond, to check whether enrichment is caused by the adhesive, the rubber or the steelwork (Figure 10).

Fig. 9. Testing range of bond surroundings

Fig. 10. Element distribution in bond layer

4. CONCLUSIONS

- It was established that sulphur and chlorine were

concentrated in the rubber/steel boundary layer with no material composition modifications. The zone is 0.1 mm wide.

- It can be observed that in case of an optimized production technology, the rupture surface is the adhesive/rubber interface caused by the stress collection impact of steelwork.

- It was established by microtopography tests that changes in 2D ”Ra” (average surface roughness) fail to

1 mm

Rubber Steel

Length of the test zone [10-2mm]





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properly characterize changes in the tensile strength of the rubber/metal connection.

- It was demonstrated in the pilot system that the tensile strength established was in correlation with the 3D microtopograhy index number ”Sv” of the steelwork (distance between the lowest point of the surface and the median plane).

- In respect of the rubber/metal bond strength examined, 31 µm<Sv< 34 µm is the optimum roughness range.

REFERENCES

[1] Naß, F, Möglichkeiten der Heizzeitreduzierung

und Eigenschaftsbeeinfl ussung eines dickwandigen Bauteils beim Spritzgießen von Kautschukmischungen mittels EFE-Spritzaggregat, dissertation, Würzburg-Schweinfurt, 2011.

[2] Katona and S.C. Batterman, Surface roughness effects on the stress analysis of adhesive joints, Int J. Adhesion and Adhesives (1983) 90-91.

[3] Nitzsche, C., H., Haftung von Kautschuk an Metalle, Kautschuk und Gummi-Kunststoffe, Bd. 36, Nr.7, Deutschland (1983), 572-576.

[4] Ibrahim Nergiz et al, Effect of alloy type and surface conditioning on roughness and bond strength of metal brackets, American Journal of Orthodontics and Dentofacial Orthopedics 125 (2004), 42-50.

[5] H.C. Man, K.Y. Chiu, X. Guo, Laser surface micro-drilling and texturing of metals for improvement of adhesion joint strength, Applied Surface Science, v. 256, iss. 10 (2010), 3166-3169.

[6] James L. Delattre, Riccardo d’Agostino, Francesco Fracassi: Plasma-polymerized thiophene films for enhanced rubber–steel bonding, Applied Surface Science, v. 252, iss. 10 (2006), 3166-3169.

[7] Akimasa Tsujimoto et al, Enamel bonding of single-step self-etch adhesives: Influence of surface energy characteristics, Journal of Dentistry, v.38, iss. 2 (2010), 123-130.

[8] Yongjun Xu et al, Experimental research on fatigue property of steel-rubber vibrationisolator for offshore jacket platform in cold environment, Ocean Engineering, v36, 588-594.

[9] Shaymaa E. Elsaka: Effect of surface pretreatments on thebonding strength and durability of selfadhesive resin cements to machined titanium, The Journal of Prosthetic Dentistry, v.109, iss. 2 (2013), 113-120.

[10] Á. Czifra, T. Goda, E. Garbayo: Surface characterisation by parameter-based technique, slicing method and PSD analysis, Measurement, v.44, iss. 5 (2011), 909-916.

[11] J. Sukumaran, M. Ando, P. De Baets, V. Rodriguez, L. Szabadi, G. Kalacska, V. Paepegemaare: Modelling gear contact with twin-disc setup, Tribology International 49 (2012) 1–7,

[12] Á. Czifra, K. Váradi, S. Horváth: Three dimensional asperity analysis of worn surfaces, Meccanica, v. 43, iss. 6 (2008), 601-609

[13] V. Rodrigues, M. Ando, J. Sukumaran: Roughness measurement problems in tribological testing, Sustainable Construction & Design, v. 2 (2011), 115-121

Authors’ addresses

1István Barányi, Institute for Mechanical Engineering

Technology, Szent István University, institution, Páter

Károly u. 1. H-2100 Gödöllı, Hungary, +36(28)522000

2Tamás Renner, Institute for Mechanical Engineering



3Gábor Kalácska, Institute for Mechanical Engineering



4Patrick De Baets, Ghent University, Laboratory Soete,

Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium

Contact person

* István Barányi, Institute for Mechanical Engineering


Károly u. 1. H-2100 Gödöllı, Hungary, +36(28)522000,

[email protected]

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Influence of Surface Preparation on the Roughness ... · Influence of Surface Preparation on the ... characteristic microtopography of steel surfaces on the ... impact of surface