книги / FISMA and the risk management framework the new practice of federal cyber security

260	Scratching of materials and applications

DISCUSSION

A TiN coating was deposited onto a steel and a Ti alloy substrate in the same coating run. Each substrate had similar surface polished finish and the same coating thickness applied. Hardness measurements showed that the Ti alloy substrate had a greater influence on the composite hardness than the steel substrate, which is not surprising given the comparative hardness of the two materials. Also, a Rockwell indent performed on each sample showed slight chipping around the indent on the coated Ti alloy substrate. Each coated substrate was tested for adhesion using a conventional progressive load scratch test under standard conditions. For each substrate it was observed that the coating failure events were of a similar type. It was also noted, firstly, that the friction coefficient recorded just before the failure point was higher for the Ti alloy than for the steel, secondly, the AE showed that the failure event occurred while the sample was under load and not during the elastic recovery of the coating I substrate system and, finally, that the coating on the softer Ti alloy substrate failed under a lower load than the coating on the steel substrate. The latter observations could be due to the fact that the shear forces applied on the coating are more important in the case of soft substrates, as shown by the higher recorded friction force. This could in turn be explained by the fact that the ploughing component of the friction force is more important in the case of soft substrates. Even if the failure modes were of similar types, stopping a progressive load scratch test at the first significant AE event identified that, on the steel substrate, the coating showed cohesive failure and, on the Ti alloy substrate, the failure was adhesive.

Constant load tests were performed on each coated substrate at the same load for which the first failure event had been previously seen (15 N and 40 N for the Ti alloy and steel substrate respectively), so that more of the scratch track could be analysed. For the coating on the Ti alloy considerable cracking and adhesive failure was observed, but for the steel substrate only slight cracking and cohesive failure was observed. Multiple constant load scratch tests, performed at 15 Nand 40 N for the Ti alloy and steel substrate respectively, showed that, after 10 passes, the coating failed in the track exposing the substrate. The Lo type failure of the coating happened at a lower load than previously seen for the progressive load scratch tests. This could be explained by the fact that the coating was weakened during the first few passes by the occurrence of cracking events, as shown during the constant load scratch tests. As chipping can originate from a surface or an internal crack and these cracks can appear at very low loads it could explain how it was possible to observe 1..0 type adhesive failure of the coating during the multipass test, even when it was carried out at a load lower than the Lc3 critical load determined during progressive load scratch test. Indentations in the coating on the Ti alloy substrate showed that at a load of 15 N the test did not show any cracking inside or around the subsequent indent. Indents were also performed at loads of 40 Nand 100 N. Only the 100 N load indent showed slight cracking inside and at the edge of the indent. As it was possible to observe cracking events at loads lower than 15 N during progressive load scratch test, it can be concluded that the failure events are not only due to the deformation of the substrate but also to the shear forces applied to the coating I substrate system.

CONCLUSIONS

It has been shown that, even if the residual indent depth is similar for scratches on the TiN coated Ti alloy substrate and the TiN coated steel substrate, more failure events can be observed in the case of scratches on the Ti alloy substrate. This may be due to the interface bond strength of the deposited Ti layer being less important with the Ti alloy than with the steel. It could also

The study of the adhesion of a tin coating on steel and titanium alloy substrates

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be due to the fact that the elastic recovery for the Ti alloy substrate is more important than for the steel and so the actual indent depth in the loaded state was more important on the Ti alloy coated sample than on the steel coated sample, hence generating a higher ploughing effect. It has also been shown that adhesive failure events appear at lower load for the TiN coated Ti alloy. This could be explained by the fact that, because of the greater deformation observed in the case of the Ti alloy substrate, the shear forces are applied at the interface between the coating and the substrate.

REFERENCES

I.Chapman, B. N. (1974)J. Vac. Sci. Techno/. 11, 106

2.Chalker, P.R., Bull, S. J. and Rickerby, D. S. (1991) Mater. Sci. Eng., A, (40), 583

3.Jacobson, R. (1976) Thin Solid Films, 34, 191

4.Valli, J. (1986) J. Vac. Sci. Tchnol., A 4 (6), 3007

5.Hedenquist, P., Olsson, M., Jacobson, S. and Soderberg, S. (1990) Surf Coat. Techno/., 41,31

6.Perry, A. J., (1983) Thin Solid Films 107, 167.

7.Steinmann, P. A. and Hintermann, H. E. (1985) J. Vac. Sci. Techno!., A3, 2394

8.Ichimura, H. and Rodrigo, A. (2000) Surf Coat. Techno!., 126, 152

9.Burnett, P. J. and Rickerby, D. S. (1988) Thin Solid Films 157,233

10.Steinmann, P. A., Tardy, Y. and Hintermann, H. E. (1987) Thin Solid Films 154,333

11.Bull, S. J., Rickerby, D. S., Matthews, A., Leyland, A., Pace, A. R. and Valli, J. (1988)

Surf Coat. Techno/. 36, 503

12.Valli, J. and Makela, U. (1987) Wear 115,215

13.Jindal, P. C., Quinto, D. T. and Wolfe, G. J. (1987) Thin Solid Films 154,361

14.European commissionStandards, Measurements and Testing Programme, Project 'Multi-mode Scratch Testing (MMST): Extension of Operation Modes and Update of Instrumentation', contract SMT4-CT97/2150

15.Heike, W., Leyland, A., Matthews, A., Berg, G., Friedrich, C. and Broszeit, E. (1995)

Thin Solid Films 270, 431

16.Antunes, J. M., Cavaleiro, A., Menezes, L. F., Simoes, M. I. and Fernandes, J. V. (2002)

Surface and Coatings Tech. 149, 27

17.Saha, R. and Nix, W. D. (2002) Acta Materialia 50, 23

18.ASTM standard E 1182-93

19.Walls, J. M., Hall, D. D. and Sykes, D. E. (1979) Surface and Interface Analysis, 1 (6), 204.

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CHAPTER14

STUDIES ON FRICTION AND TRANSFER LAYER USING INCLINED SCRATCH

Originally published in Tribo/ogv International vol 39. February 2006

PRADEEP L. MENEZES, KISHORE

Department ofMetallurgy

&

SATISH V. KAlLAS

Department ofMechanical Engineering Indian Institute ofScience Bangalore 560 012 INDIA.

E-mail: [e-mail]

ABSTRACT

Friction influences the nature of transfer layer formed at the interface between die and sheet during forming. In the present investigation, basic studies were conducted using Inclined Scratch Tester to understand the effect of surface topography on friction and transfer layer formation. A tribological couple made of an Al-Mg alloy pin against steel flat was used in the tests. Tests were conducted at a sliding speed of 2 mm/sec in ambient conditions under both dry and lubricated conditions. The inclination angle of the steel flat was 1.0 ± 0.1 0. Normal loads varied from 0 to 135 N during the scratch test. Two surface parameters of steel flatsroughness and texture - were varied in tests. Care was taken to ensure that the surface roughness, measured along the scratch direction, had similar values for different textures, namely Unidirectional, 8-ground, and Random. Grinding the ENS flat in a uni-directional manner and a criss-cross manner on emery sheets produced the uni-directional and S-ground surfaces. While the random surfaces were produced by polishing the ENS flats using various abrasive powders. Scanning Electron Microscopy was used to reveal the pin damage and morphology of the transfer layer formed on flats. It is observed that the coefficient of friction, and the transfer layer formation, is controlled by the nature of surfaces and is independent of surface roughness. Moreover, the coefficient of friction, which has two components - the adhesion and plowing, is controlled by the nature of surfaces. The plowing component of friction was highest for the surface that promotes plane strain conditions near the surface and was lowest for the surface that promotes plane stress conditions near the surface.

KEYWORDS

Coefficient of Friction, Nature of surface, inclined scratch, surface roughness, surface topography, Transfer Layer

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INTRODUCTION

The knowledge of various parameters, which control the friction forces are important in metal working operations. Friction is one such important parameter which controls the tool load, product quality (geometry, tolerance and surface finish) and tool wear. The coefficient of friction, if controlled properly, could generate the required stresses to deform the metal to the required shape. It could also lead to fracture of the sheet if not controlled properly. Since the pioneering work by Bowden and Tabor [I] various aspects that contribute to the friction have been extensively studied [2-7]. Kim and Suh [8] studied frictional forces generated by plowing of surfaces, where they concluded that it is extremely difficult to eliminate micro structural changes caused by mechanical interactions between the surfaces at a microscopic scale. Lovell et al. [9] studied the variation of sliding friction as a function of normal load by sliding a hard pin on a soft surface. They found that the coefficient of friction increases with apparent contact pressure due to increased plowing effects. The initial rise in friction was found to be rapid, due to change from elastic to plastic contact, and then levels off once all the contacting asperities deform plastically.

Many researchers have carried out experiments on friction to characterize the metal forming process and to study the effect of surface roughness on friction during metal forming. Bello and Walton [10] studied the interaction of surface roughness and lubrication at the tool-metal interface in sliding contact. In their experiment, strips of commercial pure aluminum were pulled through steel dies designed to give partial simulation of the conditions, which exist in the flange and die radius profile regions of the deep drawing process. They found that the conventional surface roughness parameters do not provide a satisfactory functional characterization of the surface in the context of the friction developed in sliding contact. Rasp and Wichern [II] studied the effect of surface topography on frictional resistance using different kinds of surfaces. In their experiment, the specimen surfaces were as received, mirror polished, chemically etched, abrasively scratched parallel and perpendicular to the simulated rolling direction. They found that the arithmetic roughness value and lubrication regime had greater influence than the directionality. The influence of surface topography of the sheet material on the frictional characteristics of 3104 AI alloy sheet were investigated by Saha et al. [12] by stretching a strip around a cylindrical pin. They found that friction increased with the strain occurring during the contact, which supports the model relating friction to flattening of strip asperities and real area of contact. They also found that the coefficient of friction depends on the rolling direction of the strip. Schedin [13] conducted experiments using U-bending test and strip drawing test to study the formation of transfer layer during forming processes. His experimental conditions resemble the contact conditions in sheet metal forming, where a hard and smooth tool surface will make repeated contact with a soft and rough sheet surface. He concluded that it is impossible to completely avoid sheet metal transfer in sheet metal forming operation but the growth of the transfer layer could be controlled.

But in all the above cases the test material is deformable, so the surface topography of deformable material cannot explain true friction values during metal forming, and thus it is important to have knowledge about the surface topography of harder material. Considerable amount of work has also been done to study the effect of surface topography of harder material on softer deformable material during sliding in sheet metal forming operations. Lakshmipathy and Sagar [14] studied the influence of die grinding marks directionality on friction in open die forging under lubricated conditions. They used commercial pure aluminium as the work piece material and Hll steel as the die material. Two sets of dies, one with unidirectional grinding marks and the other with criss-cross grinding marks, were used. It was found that, for the same

264	Scratching of materials and applications

percentage of deformation, the criss-cross ground dies required lesser forging loads compared with the die of uni-directionally ground ones. The friction factor was also lesser during the forging process when the die with the criss-cross surface pattern was used. They concluded that the lubrication breakdown tendency is more when pressing is done with unidirectional ground die than with criss-cross ground die. The relation between friction and surface topography using various lubricants was studied by Hu and Dean [15]. They found that a random smoother surface could retain more lubricant and reduce friction. Maatta et a!. [16] studied the friction and adhesion of stainless steel strip against different tool steels. They concluded that the composition of the tool steel does not have a marked effect on the friction between the tool and the work piece. However, the surface roughness and topography of the tool have a marked effect, where polishing of the tool surface reduces the surface roughness which in-tum reduces the friction between the tool and the work piece. Xie and Williams [17] proposed a model in order to predict the value of the overall coefficient of friction and wear rate, when the soft surface slid against a rough harder surface. This model points to both friction and wear to depend essentially on the roughness characteristics of the harder surface, the mechanical properties of both surfaces, nominal contact pressure or load, and the state of lubrication. Malayappan and Narayanasamy [18] studied the bulging effect of aluminium solid cylinders by varying the frictional conditions at the interface between cylinder and flat die surfaces. Different machining processes like grinding, milling, electro-spark machining, and lathe turning with emery finish were produced on the flat dies to vary the surface roughness which in tum varied the frictional conditions. They concluded that the barreling depends on friction and thus surface finish.

Many researchers used scratch test to study the effect of various parameters on coefficient of friction [19-21]. Liu eta!. [22] carried out experimental and analytical study of plowing and friction for commercially available metals using a nano-indenter, where the indenter is used to make scratches on the surface of metals under different normal loads. They found that the hardness of the scratched surface dominates the plowing friction mechanism and the contribution of the plowing component to the total friction coefficient is predominant.

Considerable amount of work has also been done by FEM and other computational methods to study the effect of various parameters on the contact between two surfaces [23-27]. The experimental and computational results are vast. Many fundamental issues have been reviewed, and new issues "discovered" [28-35].

Most of the experiments were based on vanatton in roughness values than the nature of surfaces, which cannot explain the true friction values during metal forming processes. The exact description of the contact surface and the nature of surfaces are important to understand the tribological system. The characterization of technical surfaces with traditional surface roughness parameter is insufficient to describe the tribological behavior. To mitigate this inadequacy, surfaces are to be simulated to those witnessed in actual conditions. Therefore, inclined scratch test is used here to study the effect of surface roughness and nature of surfaces on the coefficient of friction. The investigation is conducted by sliding an Al-Mg alloy pin on EN8 steel flat. The surface morphology of the flat is varied using various grinding and polishing methods. In the following sections, we present the experimental results and discuss the nature of contact and friction.

Studies on friction and transfer layer using inclined scratch

265

EXPERIMENTAL DETAILS

Experiments were conducted using an inclined scratch-testing machine, the schematic of which is shown in Fig. I.

Vertical slide

r-..o:'-";_---\-- EN8 Flat

IF-~'-- Holder

Direction of Motion of EN8 Flat

Fig. 1. Schematic diagram of scratch testing machine and inclined sample.

Description ofscratch tests

The scratch-testing machine has a vertical slide and a horizontal slide, which are driven by stepper motors with step size of 2.5 /J11I and 10 IJ11I respectively. A 2-D load cell is mounted on the vertical slide to measure both the normal and traction forces. A LVDT with a resolution of

± 1 IJ11I mounted on the vertical slide was used to measure the angle of inclination of EN8 flat. The pins were made of an Al-3.5%Mg alloy with a total Fe, Mn, and Si content of about 1 wt. %. The pins were 10 mm long, 3 mm in diameter with a tip radius of 1.5 mm. ENS steel flats were of dimensions 2S mm x 20 mm x 10 mm (thickness). The pins were first machined, and then were electro-polished to remove any work-hardened layer that might have formed during machining.

The ENS steel flat was ground against emery papers of grit size 220, 400, 600, SOO or 1000 to generate 2 kinds of surfaces with varying roughness. For the first surface, care was taken so that the grinding marks were unidirectional in nature. S-ground surfaces were generated by moving the ENS steel flat on the emery papers in a path having profile of the number "S" for about 500 times. The third kind of surfaces with random grinding marks were generated using a polishing wheel with abrasive medium as SiC powder (600 or 1000 grit), Aiz03 powder (0.017 microns), or diamond paste (1-3 microns). Figures 2 (a) and 2 (b) show 3-D profiles of surfaces generated by uni-directional grinding and random grinding, respectively.

Before each experiment, the pins and ENS steel flats were thoroughly cleaned in a soap solution and then in an ultrasonic cleaner with acetone. Then ENS steel flat and a pin were mounted on the horizontal slide and vertical slide respectively. Tests were performed to obtain five parallel scratches on the same flat. It was observed that the initial sphere-on-flat contact essentially became a flat-on-flat type contact even before the end of the first scratch. But at the same time, it was observed that scratch width varies considerably in first three tests. Hence, all the results presented were of the fourth scratch. It was observed that the coefficient of friction did not vary much for all these five scratches. For the lubricated tests, a drop of commercially available engine lubricant was applied on the surface before the experiment. Both the dry and lubricated