CHARACTERIZATION OF TRIBOLOGICAL AND THERMAL PROPERTIES OF METALLIC COATINGS FOR HOT STAMPING BORON-MANGANESE STEELS M. Merklein1, J. Lechler1, T. Stoehr1 1. Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, 91058 Erlangen, Germany

ABSTRACT Within the framework of this paper two methodologies, which enable the determination of the thermal as well as the frictional properties of metallic coating systems used as corrosion preventing layers for quenchenable high strength steels during hot stamping will be presented. For the determination of the thermal properties with the focus on the resulting heat transfer coefficient between workpiece and die in dependency of the particular properties of the coating system cooling experiments under process relevant conditions have been carried out. The procedure regarding the evaluation of tribological coating properties hereby characterizes a combined experimental-analytical-numerical method for the evaluation of the friction coefficient µ based on Siebel’s approach for the modeling of the maximum drawing force. KEYWORDS: Hot stamping, Heat transfer coefficient, Friction coefficient, 22MnB5

1. INTRODUCTION One of the main objectives of the automotive industry in the upcoming years is on the one hand to reduce the fuel consumption with respect to minimize the CO2 air pollution and on the other hand not only maintain but even increase the passenger safety standards. In this context, the application of high and ultra high strength steels with a thinner initial sheet thickness in particular for the body in white (BIW) make a significant contribution to meet this challenge /1/. In general, increasing the strength of a material leads to an observable decrease of the material formability. Thus with the goal to enhance the forming limits of such high strength steel grades in the last years thermal assisted forming technologies gained a tremendous dynamic development regarding its industrial importance for the manufacturing of in particular crash relevant components for the BIW /2/. Hereby the hot stamping process of quenchenable boron-manganese steels of the type 22MnB5 was without any doubt the diving force pointing out the potential of thermal assisted forming technologies on the subject to be capable to realize high strength components up to 1500 MPa with complex geometric shapes. Basically, hot stamping represents a non-isothermal hot sheet metal forming process where austenitized blanks are completely formed at elevated temperatures before afterwards directly getting quenched within forming press. Thus a martensitic microstructure is achieved in the final part responsible for the desired high strength regarding the crash performance etc. With respect to a finite element analysis (FEA) of the forming operation of the hot stamping process the heat transfer coefficient α and the friction coefficient µ represent significant input data /3/. Both process characteristics are hereby among the other determined by the specific properties of the applied coating system of the used blanks. Considering all other parameters constant, functional relations between the coating system and the resulting heat transfer as well as the tribological conditions can be deduced experimentally. Regarding the objective to investigate the influence of the coating system on the heat transfer and the frictional properties beProceedings of the 7th International Conference

Coatings in Manufacturing Engineering, 1-3 October 2008, Chalkidiki, Greece

Edited by: K.-D. Bouzakis, Fr.-W. Bach, B. Denkena, M. Geiger, Published by: Laboratory for Machine Tools and Manufacturing Engineering (ΕΕΔΜ), Aristoteles University of Thessaloniki and of the Fraunhofer Project Center Coatings in Manufacturing (PCCM), a joint initiative by Fraunhofer-Gesellschaft and Centre for Research and Technology Hellas

219

tween workpiece and die during direct hot stamping within this publication two different methodologies will be presented for both, the determination of the heat transfer coefficient as well as the friction coefficient. Furthermore, results for the thermal as well as the frictional properties in dependency of the applied coating system will be shown.

2. HOT STAMPING OF BORON-MANGANESE STEELS As already mentioned before hot stamping, also known as press hardening, displays a nonisothermal hot sheet metal forming process combining forming and quenching of boronmanganese steels of the type 22MnB5 in one process step. The hot stamping process currently exists in two main different variants, the direct and the indirect hot stamping method. Within the direct hot stamping process a flat blank is heated up in a furnace, homogeneously austenitized at a temperature above the material specific Ac3-temperature of approximately 850 °C transferred to the press by robotic feeding systems and subsequently formed and quenched within the tool (Figure 1). In contrary, the indirect hot stamping process is characterized by using a nearly complete cold pre-formed part, which is imposed only to a quenching and calibration operation in the press after austenitization. Besides the fundamental operational differences for the one step direct hot stamping pre-coated blanks of the quenchenable boron-manganese steel 22MnB5 are commonly applied, whereas for the indirect variant usually uncoated blanks are used. Therefore the in two steps manufactured hot stamped parts have to be cleaned by blasting in order to remove the scale and to apply an anti-corrosion coating. To prevent the blanks from oxidation and decarburization during direct hot stamping a protecting aluminumsilicon based layer is conventionally used. Due to the lower forming limits of the Al/Si-layer compared to the base material 22MnB5 at room temperature in the initial state, this kind of the oxidation preventing coating is not applicable for the indirect method. In Figure 1, the process chain of the direct hot stamping method is schematically illustrated.

Figure 1: Schematic sketch of the direct hot stamping process chain In contrary to the direct and indirect hot stamping method the steel manufacture voestalpine has invented a comparably hot forming technology of 22MnB5 steels, namely HT1500PS and HT1500PS(+), using a protecting zinc coating layer. The voestalpine-PHS technology is a combination of direct and indirect press hardening trying to use the advantages of each method. Within the voestalpine process chain the Zn-coated boron-manganese blank is cold formed and cut to its final shape. Afterwards it will be austenitized and quenched in one final process step /4/.

220

7th

Coatings – 2008

3.

EXPERIMENTAL PROCEDURE

3.1.

Investigated Material

As test material the quenchenable boron-manganese steel 22MnB5, currently representing the standard steel grade for the manufacturing of hot stamped parts in the automotive industry, had been subject of investigations. In the as-delivered condition the hot stamping steel exhibits a fine grain ferritic-pearlitic microstructure characterized by good formability. After passing through the time-temperature characteristic of the hot forming process a 2.5 until 3 time increase of the materials yield and tensile strength above 1100 MPa and 1500 MPa can be achieved. This effect is related to a martensitic phase transformation occurring during the quenching operation within the forming press assuring a minimum cooling rate of approximately 30 K/s. Regarding the aspired microstructural transformation, a previous homogeneous austenitization of the boron-manganese sheets for a minimum time of five to eight minutes above the material specific Ac3-temperature up to approximately 850 °C is required. /5/ In previous investigations /6/, it was shown that the flow behaviour of the quenchenable high strength steel is determined by a significant influence of the strain, the temperature and the strain rate in the austenitic state. Further the material plastic deformation behaviour is characterized by a temperature dependent anisotropy /7/. Regarding the modeling the thermo mechanical properties of 22MnB5 as input data for finite element analysis (FEA) of hot stamping those aspects have to be considered /8/. To evaluate the influence of the applied coating system on the resulting heat transfer and the tribological properties during direct hot stamping besides the aluminum-silicon pre-coated reference material A (Table 1) one additional 22MnB5 steel grade pre-coated with an Al/Si-based layer as well as one boron-manganese steel pre-coated with a zinc based layer had been investigated. The test materials supplied by different steel manufactures exhibit a comparable sheet thickness between 1.75 mm and 1.8 mm and represent standard semi-finished products for hot stamping in the automotive industry. In the progress of this work, the test materials will be referred to by their classification according to Table 1. Table 1: Investigated materials and their specification and classification Classification

Base material

Sheet thickness

Coating system

Supplier

Material A

22MnB5

1.75 mm

Al/Si

A

Material B

22MnB5

1.75 mm

Al/Si

B

Material C

22MnB5

1.8 mm

Zinc

C

3.2. Determination of heat transfer coefficient For the determination of the heat transfer coefficient α under process relevant conditions between blank and die in dependency of the material coating system one within the FOSTA project P644 at the Chair of Manufacturing Technology constructed and validated tool had been implemented in a universal mechanical testing machine (SchenckTrebel RM400) with a maximum normal force of 400 kN. The quenching device mainly consists of two 10 mm exchangeable water cooled contact plates where rectangular, thermocouple equipped specimens (160 x 58 mm2) are manually placed on four spring seated pins after undergoing a previous homogeneous austenitization at temperatures between 850 °C and 950 °C in a furnace under atmospheric conditions (Figure 2). By using a set-point testing software the specimens can be loaded during quenching with a defined contact pressure up to 40 MPa. During the test, the temperatures of the blank and both contact plates are recorded by using integrated Ni/Cr-Nithermocouples. Thus following Newton’s cooling law according to equation 1 the resulting heat transfer coefficient α between blank and die can be calculated based on the measured cooling curves in dependency of the respective target value as for example contact pressure or coating system.

Coatings in Manufacturing Processes

221

Figure 2: Experimental setup for the determination of the coatings’ thermal properties regarding the heat transfer coefficient

T (t ) = (T0 − Tu ) ⋅ e

−α ⋅ cA ⋅t P

+ Tu

(1)

Hereby T0 and T(t) indicate the initial and the actual temperature of the specimen during the cooling experiment. A quantifies the geometric contact surface and cp the effective heat capacity. The applicability of the methodology explained above have sufficiently shown in previous publications like /9/. 3.3. Determination of friction coefficient For the determination of the friction coefficient µ under process relevant conditions, a modified cup deep drawing test setup for elevated temperatures, which is integrated in a 1000 kN hydraulic press of the type TSP100So (Lasco, Coburg) had been developed and validated at the Chair of Manufacturing Technology /10/ (Figure 3). The tool is equipped with a load cell for the continuous online recording of the punch force in dependency of the punch movement. Integrated heating cartridges in the punch, the blank holder and the drawing die enable a separate regulation of the tool devices, whereby a respective maximum temperature of 500 °C can be realized. The diameter of the punch and the die is 50 mm and 59 mm, the corresponding edge radii 10 mm. With respect to the required previous austenitization of the specimens according to the direct hot stamping process a furnace is placed beside the press. To determine the temperature profile of the hot blanks instantaneously before the complete closing of the tool a thermo camera had been applied at the rear of the press. This assures that for the determination of the friction coefficient only experimental data from specimens with the same initial starting temperature are chosen. Hereby an emission coefficient of ε = 0.87, determined in prior tests with the help of thermocouples attached to test blanks, was applied. To avoid a too distinctive cooling or even quenching of the sheet in the forming zone before the actual forming operation takes place, the cup deep drawing tests have been carried out without applying any blank holder force to the blank in the flange area. Therefore a defined distance between die and blank holder was established by using a distance ring with a thickness of 3 mm. The analytical determination of the friction coefficient is based on the equation of Siebel /11/ for the maximum drawing force (equation 2).

222

7th

Coatings – 2008

Figure 3: Modified cup deep drawing test for elevated temperatures /12/

⎡ μ3 π ⎛ d p (μ1 + μ 2 ) ⋅ FBH + Fdraw ,max = π ⋅ d Z ⋅ t 0 ⎢e 2 ⎜⎜1,1 ⋅ σ fm1 ⋅ ln d π ⋅ dP ⋅ t0 Z ⎝ ⎣⎢

⎞ t ⎤ ⎟⎟ + σ fm 2 ⋅ 0 ⎥ 2 rR ⎦⎥ ⎠

(2)

As the test is being performed with a distand blank holder, μ1 and μ2 describing the friction between blank holder and blank as well as between drawing die and blank respectively, can be neglected. Solving the equation to the remaining friction coefficient μ in the area of the cup flange, the following form can be observed.

⎛ ⎛ t0 2 ⎜F ⎜ draw ,max − π ⋅ d Z ⋅ σ fm 2 ⎜ ⎜ 2 ⎝ 2 rR μ = ⋅ ln⎜ π ⎜ 1,1 ⋅ π ⋅ d ⋅ t ⋅ σ ⋅ ln d p Z 0 fm1 ⎜⎜ dZ ⎝

⎞⎞ ⎟⎟ ⎟⎟ ⎠⎟ ⎟ ⎟⎟ ⎠

(3)

Regarding equation 3, the required variables can be divided into the two categories direct and indirect input data as illustrated in Table 2. Hereby the expression ‘direct’ remarks the parameTable 2: Direct and indirect variables for the determination of the friction coefficient direct variables (measured values)

indirect variables (analytically and numerical determined variables)

initial diameter of the blank d0

blank diameter at the maximum drawing force dp

initial sheet thickness t0

mean true stresses σfm1 and σfm2

diameter of the cup wall dz edge corner radius of the drawing die rR maximum drawing force Fdraw, max punch-force punch-stroke progress F-s direct variables as input data for the FE model heat transfer coefficient α(pc, coating, etc.) true stress σ(ε, dε/dt, T)

Coatings in Manufacturing Processes

223

ters that are provided directly by the cup deep drawing test, indirect means they have to be analytically or numerically be determined. The direct parameters are the diameter of the cup wall dz, the maximum drawing force Fdraw, max, the edge corner radius of the die rR and the initial sheet thickness t0. Regarding the indirect variables, the blank diameter at the maximum drawing force dp as well as the mean true stresses in the flange σfm1 and at the drawing die radius σfm2 at the maximum drawing force respectively are the variables to be named concerning the determination of the friction coefficient. For the determination of these parameters, a FE model has been established in AutoForm HotForm (version 4.1 alpha), which requires as fundamental input data on the one hand the flow behaviour of boron-manganese steel 22MnB5 in dependency of the process relevant parameters strain, strain rate and temperature [8]. The further on the other hand for the numerical reproduction of the thermal conditions during hot stamping the heat transfer coefficient as function of the occurring contact conditions is essential. The diameter dp at the maximum drawing force is revealed by a finite element analysis of the evolution of the cup diameter in dependency of the punch stroke. The mean true stresses σfm1 and σfm2 at the drawing force maximum had been approximated by using the phenomenological approach according to equation 4, which applicability regarding the accurate modelling of the flow behaviour of 22MnB5 in dependency of strain, strain rate and temperature had been shown and approved in previous works /9, 12/. The required mean strains for the modeling of the mean flow stresses can either be determined using geometrical considerations following /11/ assuming volume constancy or via FEA.

σ ( ε , ε ,T ) = K ⋅ exp( β / T ) ⋅ ( b + ε )n0 ⋅exp( −cn ( Ti −T0 )) ⋅ ε m0 ⋅exp( cm ( Ti −T0 ))

(4)

Due to significant temperature dependency of the flow properties of the boron-manganese steel regarding the accurate modelling of σfm1 and σfm2, the cooling of the specimens during the forming process has to be taken into account. Hence the temperatures acquired through thermal measurements directly before the tool is closed cannot be assumed as isothermal values during the forming operation and can therefore not be applied as input parameters for the modeling of the true stresses at the maximum drawing force. The required temperature data can only be obtained via FE analysis of the cup deep drawing test. The feasibility of the experimentalnumerical-analytical methodology explained above was shown among the others in /10, 13, 14/ as well for non ferrous as for ferrous sheet metal materials.

4. RESULTS AND DISCUSSION 4.1. Thermal properties To investigate the pressure dependent heat transfer between blank and die under process relevant conditions for full metallic contact with respect to influence of the blank coating system, quenching experiments have been performed according to the paragraph 3.2. Within the cooling test specimens of the three investigated materials have been imposed to various contact pressures in a range between 0 MPa and 40 MPa after a previous austenitization at Tγ = 900 °C for five minutes (tγ = 5 min). To confirm the experimental data each test run has been performed at least five times. In Figure 4 the within the scope of this work analytically according to equation 1 determined heat transfer coefficients for the three boron-manganese steel grades A to C are plotted as function of the applied contact pressure pc. The displayed characteristics in Figure 4 show, that all three pre-coated quenchenable steels exhibit a significant increase of the occurring heat transfer between tool and workpiece with increasing contact pressure, whereby maximum values of α ≈ 4.000 W/m2K are achieved. This effect is caused by the increase of the effective contact surface between the two contact partners through smoothing of the surfaces /15/. Consequently, more and more real metallic-metallic contact areas occur enforcing direct heat conductance effects, which are capable to transfer more thermal energy between to contact bodies /16/.

224

7th

Coatings – 2008

Figure 4: Evaluation of the heat transfer coefficient as function of the contact pressure pc in dependency of the applied corrosion preventing coating system Furthermore, the results in Figure 4 point out that the development of the pressure dependency of the heat transfer coefficient is strongly influenced by the applied coating system of the blanks. The clearly visible differences between the detected pressure dependency of the test materials A-C can be related to the particular mechanical and thermal properties of the different aluminium-silicon and zinc layer systems, due to the fact, that all investigated semi-finished products base on the same alloying specification 22MnB5 of the base material. In this context, the slight differences regarding the thickness of material C to material A and B with respect to the thermal properties can be neglected. General conclusions like for example, that Al/Si-based layer always leads to a higher or a lower heat transfer can not be deduced from the obtained measured data. Following for the FEA of hot stamping the evolution of the pressure dependent heat transfer coefficient α besides the sheet thickness as shown in /17/ the impact of the applied corrosion preventing coating system has to be taken into account as well. 4.2. Frictional properties For the determination of the friction coefficient within hot stamping blanks of the test materials A-C with a diameter between 80 mm and 90 mm had been homogeneous austenitized at various heat treatment temperatures in a range between 900 °C and 950 °C for five minutes before being subsequently being transferred manually to the press and as fast as possible being drawn. To reveal different thermal conditions during the experiments with the objective to investigate the impact of the temperature on the tribological conditions the temperature of the tool was varied between 25 °C and 500 °C, whereas the temperature of the punch was maintained constant at room temperature. All the tests have been carried out with a punch velocity of 10 mm/s resulting in an average strain rate of 0.1 s-1 /10/. As reference value of the sheet temperature at the moment of the maximum drawing force according to the methodology introduce and explained in paragraph 3.3, the numerical computed average temperature of the specimen in the contact area of main friction between workpiece and tool at the drawing die radius is taken. All the tests have been carried out for at least five times. In Figure 5 the evolution of the friction coefficients of the reference material A, calculated following Siebel’s approach according to equation 3, are shown as function of the specimens temperature. It can be obviously recognized, that the frictional characteristic µ exhibits a significant decrease with increasing blank temperature, whereby the values lie in a range between µ = 0.6 until µ = 0.3. One aspect contributing to the detected evolution could be related to the effect,

Coatings in Manufacturing Processes

225

Figure 5: Influence of the temperature on the friction coefficient within hot stamping of aluminum-silicon pre-coated boron-manganese steels that with increasing temperature the aluminium-silicon coating, alloying to a ternary Al-Si-Fe layer system during heat treatment /3, 14/, shows a more and more doughy constancy. Hence at elevated temperatures the coating acts as a kind of lubricant, what thus results in a decrease of the resistance force of the sliding contact partners and following of the friction coefficient respectively. Furthermore, with the aim to find a basic explanation of the determined development of the tribological properties smoothing effects affecting the real contact area between workpiece and sheet as well as the increased ductility of the blanks’ bulk material has to be taken into consideration. The almost linear trend of the friction coefficient µ in Figure 5 can be interpreted that in this temperature range the real contact area and the resulting frictional properties are mainly driven by elastic deformations of the involved surfaces according to Greenwood and Williamson /18/. For the other both investigated test materials, one coated also with an Al/Sibased layer (material B) and the second one coated with an zinc coated layer (material C), a

Figure 6: Influence of the temperature and the coating system on the friction coefficient within hot stamping

226

7th

Coatings – 2008

comparable development of the determined friction coefficients in dependency of the temperature was obtained within the experiments as shown in Figure 6. The observable differences between the absolute magnitudes of the calculated values for µ emphazise the impact of the coating specific thermal and mechanical properties for the resulting tribological conditions. In particular a significant discrepancy between the frictional behaviour of the material C with a zinc based layer and material A and B provided with Al/Si-based coatings could be determined. For a comparable specimen temperature of around 550 °C (point out in Figure 6) in the computed friction coefficients of the test materials A and B exhibit with µ ≈ 0.53 and µ ≈ 0.45 more than 50% higher values as the material C with µ ≈ 0.53 exemplarily.

5. SUMMARY AND OUTLOOK Within this work two methodologies regarding the determination of the heat transfer α and the friction coefficient µ under process relevant conditions of hot stamping had been presented. The focus of the experimental investigations hereby was set on the impact of the blank coating system on the thermal and the tribological characteristics α and µ respectively. It could be shown, that assuming equal properties of the different applied boron-manganese steels base on the same alloying concept 22MnB5 the coating has a significant influence on the resulting heat exchange between workpiece and die as well as on the tribological properties in the frictional relevant areas. In future research work the impact of tool coating systems in particular developed for hot forming tools (AlCrN-based layer systems) on the thermal and the tribological conditions within hot stamping are subjective of investigations.

6. REFERENCES 1. 2.

3.

4. 5.

Mörsdorf W., Moderne Werkstoffe und ihre Anwendungen in der Karosserieentwicklung, Proceedings of Neuere Entwicklungen in der Blechumformung, Stuttgart, Germany, 2006, pp: 1-11. Steinhoff K., Maikranz-Valentin M., Weidig U., Paar U., Gücker, E., Bauteile mit maßgeschneiderten Eigenschaften durch neuartige thermo-mechanische Prozessstrategien in der Warmblechumformung, In: Geiger, M.; Merklein, M. (Edrs.): Proceedings of 2nd Erlanger Workshop Warmblechumformung, Erlangen, Germany, 2007, pp: 1-12. Hein P., Kefferstein R., Dahan Y., Presshärten von USIBOR 1500P®: Simulationsbasierte Bauteilund Prozessanalyse, In: Liewald, M. (Edrs.): Neuere Entwicklungen in der Blechumformung, Stuttgart, Germany, 2006, pp: 171-183. Faderl J., Radlmayr K. M., ultraform und ultraform_PHS – Innovation made by voestalpine, Proceedings. of 1. Erlanger Workshop Warmblechumformung 2006, Erlangen, Germany, 2006, pp: 130-149. N.N.: UISBOR 1500P pre-coated (Arcelor), 2003

6.

Merklein M., Lechler J., Geiger M., Characterization of the flow properties of the quenchenable ultra high strength steel 22MnB5, Annals of the CIRP 55/1/ (2006), 229-236.

7.

Merklein M., Lechler J., Gödel V., Bruschi S., Ghiotti A., Truetta A., Mechanical properties and plastic anisotropy of the quenchenable high strength steel 22MnB5 at elevated temperatures. In: Micari, F.; Geiger, M.; Duflou, J.; Shirvani, B.; Clarke, R.; Di Lorenzo, R.; Fratini, L. (Hrsg.): Proceedings of the 12th International Conference on Sheet Metal. Zürich: Trans Tech Publications Ltd, 2007, Palermo, Italy, 2007, pp: 79-87. Merklein M., Lechler J., Geiger M., Mathematical Modeling of the Flow Behavior of Hot Stamping Steels for a FE-based Process Design. In: Dimitri Dimitrov (Ed.) Proceeding of the International Conference on Competitive Manufacturing (COMA’07), Kapstadt, Southafrica, 2007, pp: 191-196. Merklein M., Geiger M., Lechler J., Ermittlung von thermischen und mechanischen Werkstoffeigenschaften höchstfester Vergütungsstähle für das Presshärten. In: Steinhoff, K. (Ed.): Tagungsband VDEh-Workshop: "Moderne thermomechanische Prozessstrategien in der Stahlumformung", Kassel, Germany, 2007, pp: 24-36.

8.

9.

Coatings in Manufacturing Processes

227

10. Geiger M., Merklein M., Hecht J.: Investigations on Formability and Tribology with Regard to

Warm Forming of Magnesium Sheets. Annals of the German Academic Society for Production Engineering (WGP), XIII (2006) 2, 101-104. 11. Siebel, E., Beisswänger, H., Tiefziehen, München, Hanser-Verlag (1955). 12. Merklein M., Lechler, J., Determination of Material and Process Characteristics for Hot Stamping Processes of Quenchenable Ultra High Strength Steels with Respect to a FE-based Process Design, SAE World Congress: Innovations in Steel and Applications of Advanced High Strength Steels for Automotive Structures, Paper No. 2008-0853. 13. Tolazzi M., Vahl M., Geiger M., Determination of friction coefficients for the finite element analysis of double sheet hydroforming with a modified cup test, Proceedings of the 6th Esaform Conference on Material Forming, Salerno, Italy, pp: 479-482. 14. Geiger M., Merklein M., Lechler J., Determination of tribological conditions within hot stamp-

ing, Annals of the German Academic Society for Production Engineering (WGP), Vol. XVIII (2008) 2, accepted, in print. 15. Madhusudana C. V., Thermal Contact Conductance, Berlin, Springer-Verlag (1996). 16. Stöcker H., Taschenbuch der Physik, Harri Deutsch (1998). 17. Geiger M., Merklein M., Hoff, C., Roll K., Lorenz D., Auslegung des Prozessfensters für die Blechumformung höchstfester Vergütungsstähle bei erhöhten Temperaturen, Forschungsvereinigung Stahlanwendung (Edrs.), Verlag und Vertriebsgesellschaft GmbH, Düsseldorf, Germany, ISBN: 3-937567-42-9. 18. Greenwood J., Williamson J., Contact of nominally flat surfaces, Proceedings of the Royal Society of London Band 295, 1966, pp: 300-319.

228

7th

Coatings – 2008