Tribological behavior of nanotextured metallic surfaces by femtosecond lasers

Transcription

1 Tribological behavior of nanotextured metallic surfaces by femtosecond lasers Ana Beatriz Silvestre Ferreira Instituto Superior Técnico, Universidade de Lisboa, Portugal 1. Abstract MEMS ( Microelectromechanical Systems ) are devices with very small dimensions, typically ranging from 100 nm to 1 mm. This causes high adhesion and friction forces in these mechanisms surfaces, reducing their wear resistance and eventually causing their catastrophic failure. In this work we tried to reduce the friction and wear between the surfaces of a tool steel, through surface laser-texturing by femtosecond laser with LIPSS ( Laser Induced Periodic Surface Structures ) and lubrication with molybdenum disulphide (MoS2). Tribological tests were carried under the ball-on-flat configuration, using linearly reciprocal sliding and rotation conditions. These tests allowed the measurement of friction and wear coefficients of the different surfaces: polished and laser-textured with LIPSS, the last ones with and without lubricant. In the laser-textured samples, the wear tests were made under the perpendicular and parallel sliding directions to the LIPSS. The worn areas of the surfaces were analyzed by scanning electron microscopy. In the LIPSS covered surfaces the friction and wear coefficients have shown a lower value than that of the smooth surface, being the lowest values observed in the perpendicular sliding direction. It was also in this sliding direction that LIPSS could resist wear for longer time. Even lower friction and wear coefficient values were observed in the lubricated laser-textured samples, and the sliding direction was shown to be indifferent (the friction coefficient was practically the same in both directions). The worn areas indicated that it had occurred delamination wear in the dry surfaces, with the formation of a tribolayer consisting of oxide particles. Keywords: LIPSS; Tribology; MoS2; Steel; MEMS 2. Introduction Microelectromechanical systems (MEMS) are microscopic devices with a characteristic length ranging from 100 nm to 1 mm and they combine mechanical and electrical components [1]. Due to their very small dimensions, the surface-to-volume ratio is quite high, making the superficial forces (van der Waals, capillarity, electrostatic and chemical bonds) to become dominant over the inertial and electromagnetic forces [2]. Consequently, these types of mechanisms normally don t have a good tribological behavior, that is, the friction generated between the moving surfaces is high and the wear resistance is poor. In this situation, macro lubrication and wear mitigation methods of surfaces, like fluid films and coating in the micro range, become ineffective [3]. Therefore, there is a need to develop new types of nanoengineered surfaces and lubrication methods for the movable components of MEMS. It was in this sense that the laser-texturing 1

2 of surfaces appeared as the method with the most promising concept, in relation to the other methods of surface texturing, since the laser (i) is extremely fast enabling short processing times, (ii) is an environmentally clean technology and (iii) allows an excellent control of the microstructures shape and size, improving the design quality of surfaces [4]. By laser texturing a surface what occurs at the surface is a reduction of its real area of contact, and because of that the adhesion and friction between surfaces will be lower. In addition, if this laser-textured surface is coated with a lubricant, friction and adhesion will become even much lower. The main effect mechanisms of surface texturing, besides reducing the real area of contact, are to trap wear particles, store lubricants and increase load carrying capacity [5]. By trapping lubricant in the topographic valleys, these surface textures are capable of continuously feeding the lubricant directly into the contact zone of the sliding surface, and by trapping the wear debris, they eliminate these particles from the interface, reducing the plowing component of friction [6]. Laser induced periodic surface structures (LIPSS), also called ripples, are a phenomenon that occur in solids after being irradiated with a linearly polarized laser beam, and consist of surface patterns made of parallel ripples with a sub-wavelength period [7]. These structures have micro or nano dimensions, with periodicities and heights in the micron and submicron range [8]. Their periodicity and orientation will vary accordingly with the processing parameters employed (like, the radiation s angle of incidence, number of pulses of the incident laser beam, fluence and polarization of the laser beam, among others). In the present work, the tribological behaviour of tool steel laser-textured surfaces with LIPSS by femtosecond laser was investigated, with and without lubricant. The lubricant used was molybdenum disulfide, MoS2, which is a solid lubricant. Linear and reciprocating sliding wear tests were made, under the ball-on-flat configuration, to measure the friction and wear coefficients of the samples with different surface finishes: polished and laser-textured with LIPSS (with and without MoS2 lubricant). In the laser-textured samples, the wear tests were made under two sliding directions: perpendicular and parallel to the LIPSS orientation, to study if the steel s friction and wear behavior changes. The worn areas of the surfaces were examined by SEM (Scanning Electron Microscopy) to evaluate the wear mechanisms that occurred. 3. Methods Materials Table 1 Properties of DIN 90MnCrV8 tool steel and AISI e AISI 440C steel balls. Material Diameter (mm) Young s Modulus, E [GPa] Poisson Coefficient, ν Yield Strength, σ y [MPa] Hardness Tool steel DIN 90MnCrV8 - ~200 [9] ~0,3 [10] 1750 [11] HRc[12] ~ HV Nanowear balls Chrome steel AISI , HRc ~ HV Microwear balls Stainless steel AISI 440C 25, , HRc ~ HV 2

3 The material used for the tests was a DIN 90MnCrV8 tool steel and two different balls were used in the wear tests. The main properties of these materials are listed in Table 1. The steel samples were cut with dimensions 1,5 cm x 1,5 cm x 2 mm from a block of DIN90MnCrV8 steel. Before surface texturing, the steel samples were cold mounted in resin and afterwards they were ground and polished to a mirror finish. Surface laser-texturing The plane steel samples were mounted on a xyz stage which was precisely controlled by a computer for laser texturing. A Yb:KYW pulsed laser (Amplitude Systèmes s-pulse HP) was used for the laser surface texturing and its characteristics and processing parameter used are listed in Table 2. Table 2 Laser properties and processing parameters used in the laser-texturing of the surfaces. Parameters Values Laser central wavelength 1024nm Pulse duration 560 fs Distribution of the laser energy Gaussian Average beam power 125 mw Scanning average fluence 130 mj/cm² Stage scanning speed 1 mm/s Laser pulse frequency 100 Hz Lateral movement of the xyz stage 50 µm Processing environment Ambient atmosphere A half-wavelength plate was placed in the optical path to produce s-polarization at the sample surface. The laser beam was perpendicular to the specimen s surface and focused 10 mm above that surface by a 100 mm focal length lens. Consecutive linear tracks were partially overlapped by moving the sample 50 µm in a direction perpendicular to the scanning direction after the writing of each track. In Figure 1 are represented images of the resulting surface topography after laser texturing. (a) (b) Figure 1 Surface texturing with LIPSS of a polished tool steel sample. Magnifications: (a) x100, (b) x5000. Friction and wear tests Reciprocating ball-on-flat tests were executed using a nanotribometer (CSM Instruments) to investigate the friction and wear behavior of smooth, textured and textured and lubricated DIN 90MnCrV8 tool steel samples against AISI chrome steel balls. The steel samples were mounted on the flat, while the balls were 3

4 fixed on a fixture. The sliding velocity was 1 cm/s, the loads used were 10, 25, 50, 100 and 200 mn and the cycles were 50 (10 cm), 100 (20 cm), 250 (50 cm), 500 (1 m), 1000 (2 m) and 2500 (5 m). For each test, the stroke length was 1 mm, corresponding to a wear track with 2 mm width. On the laser-textured surfaces, the sliding orientation was perpendicular and parallel to the LIPSS orientation, to investigate the sliding direction influence on the LIPSS tribological behavior. Sliding ball-on-flat tests were executed using a micro-scale abrasion wear tester (PLINT TE 66SLIM model) to measure the wear coefficient and investigate the wear behavior of the different samples. In this case, on the laser-textured surfaces, wear tests were only made along the sliding orientation perpendicular to the LIPSS orientation. The sample is fixed to a lever arm and put in contact parallel with a rotating sphere. In the lever arm were applied two different weights to apply two different forces to the systems. The forces used were 0,5 and 1 N, the sliding velocity was 100 mm/s ( 75rpm) and the sliding distance was 200 m (2506 rotations). For each force, three tests were made in order to have a medium value of the craters size and these were separated by a 3,5 mm distance. Between tests, the sphere was cleaned with alcohol and a paper cloth. In the end of the tests, the craters were ultrasonically cleaned to facilitate their visualization and measurement in the optical microscope. Wear surfaces analysis After the wear tests, the worn areas in each sample were analyzed by scanning electron microscopy, to investigate which type of wear mechanism occurred. To identify the elements present in these worn areas, chemical composition maps were made along the samples surfaces using an INCA software program. The microscope used in this work was a FEG-SEM JEOL. 4. Results and discussion Friction coefficient ഥμ force 1,4 1,2 1 0,8 0,6 0,4 0, Force (mn) Steel+LIPSS-0º+MoS2 Steel+LIPSS-90º+MoS2 Polished Steel Steel+LIPSS-90º Steel+LIPSS-0º Figure 2 Evolution of the average coefficient of friction with the normal force. In Figure 2 is represented the evolution registered for the coefficient of friction of all the samples tested. It is possible to see that below 100 mn, polished steel has the highest friction value and that the laser-textured sample tested in the perpendicular sliding direction in relation to the orientation of the LIPSS 4

5 (steel+lipss-90º) has the lowest. In between, we have the lubricated samples with lower values of friction. This demonstrates that as initially discussed, LIPSS can reduce the real contact area between surfaces and thus allowing a reduction in friction. However, a higher reduction is reached with the sample steel+lipss-90º than with steel+lipss-0º (parallel sliding direction in relation to LIPSS orientation). To explain these friction results, the main three components of the friction coefficient have to be taken into consideration. According to Wang et al. [5], [13], friction coefficient is composed of three components: adhesion, plowing by asperities and wear particles, and asperity deformation caused by the applied load. There is no obvious difference in the surface global contact area of steel+lipss-90º and steel+lipss-0º, because the dimension of ripple structure is much smaller than that of the wear track [5]. However, the LIPSS direction does profoundly modify the way by which occurs the surface plastic deformation. In fact, the LIPSS deformation is discontinuous in the perpendicular sliding direction and continuous in the parallel direction since, in the last case, plastic deformation occurs along the crests. According to Yu et al. [14], a longer stiction length (the contact length prior to macroslip initiation) exists in the parallel configuration comparatively to the discontinuous contact occurring in the perpendicular direction, between the ball and the sample. Based in this theory, the longer stiction length results in a higher adhesion component of friction in the parallel direction. Finally, regarding the friction component due to plowing caused by wear particles, when the sliding direction is perpendicular to the orientation of LIPSS, the wear particles can be effectively trapped in the valleys, but when the sliding occurs along the parallel direction, these particles may stay in the interface and slide continuously with the movement of the ball [5]. Consequently, the friction component due to plowing by wear particles is higher in the parallel direction. Since the adhesion and plowing components of friction are higher in the parallel sliding direction to the orientation of LIPSS, we can conclude that the friction coefficient will be higher in this direction and thus explaining the results obtained. Above 100 mn, the friction coefficient was similar for all the samples because the type of wear that occurred was oxidative, resulting in lower friction coefficients. Up until 100 mn, the wear mechanisms that occurred in the dry samples varied with load and number of cycles. The examination of the worn areas of the dry samples showed that occurred wear by delamination. The wear regimes were moderate and severe, depending on the load and number of cycles. The transitions of friction coefficients registered in Figure 2, from 10 to 100 mn, were due to transitions from moderate (lower values of friction coefficient) to severe (higher values of friction coefficient) wear by delamination. In the moderate regime, the oxide film protects the metal surface against wear, and in the severe regime the oxide film is penetrated and occurs metallic contact along the interface asperities, causing higher adhesion between the surfaces. Consequently, a tribolayer composed of iron oxides is formed and wear debris are released, some of them causing abrasion of the surface. Concerning the influence of the sliding direction in relation to the LIPSS orientation, the worn areas showed that the perpendicular direction is more advantageous since LIPSS could withstand wear for longer periods. This is because of the lower adhesion and plowing components of friction attained in this direction and 5

6 because the wear particles become trapped in the ripples (consequently, they are disabled from sliding or rolling along the interface and causing abrasion of the surface). In the lubricated laser-textured samples, the lubricant film was only penetrated for 200 mn and 1000 and 2500 cycles, in both sliding directions. The worn areas showed that the lubricant film was compressed against the surface, causing the film to become stored in the ripples and so, the ripples act like lubricant reservoirs. The lubricant stored in the ripples acts as a secondary source of lubricant and can bed ran into the rubbing surface. Wear coefficient In Table 3 are represented the calculated values of the diameters, d, depth, h, wear volume, V, wear rate, Q, and wear coefficient, K, of the worn areas of the micro-abrasion tests. These values were calculated using the following equations: h = d2 8R (1) V = πd4 64R (2) Q = V L = πd4 64RL (3) K = Q F N = πd4 64RLF N (4) where d is the average diameter of the crater, R is the ball radius, L is the total sliding distance and FN the normal force applied to the system. Table 3 Microwear test results. Steel samples d (µm) h (µm) V (*10-4 mm³) Q (*10-6 mm³/m) K (*10-15 m²/n) 0,5N 1N 0,5N 1N 0,5N 1N 0,5N 1N 0,5N 1N Polished steel ,5 9,9 44,4 38,1 22,2 19,0 44,4 19,0 Steel+LIPSS-90º ,0 5,6 6,4 12,6 3,2 6,3 6,4 6,3 Steel+LIPSS-90 +MoS ,3 2,7 2,1 2,8 1,0 1,4 2,1 1,4 We can see that major reductions in the wear coefficients were attained in the laser-textured surfaces, being the one lubricated with molybdenum disulfide the one with the lowest value. This difference is more notorious in Figure 3. 6

7 Figure 3 Wear coefficients. In all the samples tested in the micro-scale abrasion tests, the wear mechanisms that occurred in their surfaces was oxidative, resulting in the formation of a large quantity of oxides. In fact, in the polished steel sample, the reduction of wear coefficient that occurred when the force increased from 0,5 to 1 N was precisely due to higher oxidation rates that resulted in a higher formation of oxides in the contact zone protecting the steel sample from wear. In the laser-textured samples, the lower wear coefficients were due to the reduced contact area, oxidation of the surfaces (that protect LIPSS from wear) and the use of lubrication. Besides oxidation of the surfaces, it was also observed in all the samples abrasion tracks, caused by the oxide wear particles that became trapped under the ball. 5. Conclusions Concerning friction, it was observed that, in general, LIPSS reduced the surface friction coefficient, in both sliding directions (Figure 2). These results were expected since LIPSS reduce the real area of contact, causing less adhesion between the surfaces and, so, lower friction. A major reduction of the friction coefficient was observed in the perpendicular sliding direction of the laser-textured samples tested without lubricant, comparatively to the polished steel samples. In the parallel direction, the friction coefficients measured for the laser-textured samples were higher than in the perpendicular direction. This is because the adhesion and the plowing components of friction are higher in the parallel direction than in the perpendicular direction, and so the higher friction coefficients measured in the parallel direction are in agreement with what was theoretically expected. MoS2 lubricant can reduce friction between surfaces even more, due to the lower shear strength of the formed films. It was also observed in the lubricated laser-textured samples that the sliding direction was indifferent since the friction coefficients measured along the perpendicular and parallel directions were practically the same. Friction coefficient reductions (comparatively to the polished sample) in the laser-textured samples, dry and lubricated with MoS2, were only observed until 100 mn. When the force increased up until 200 mn, the friction coefficient for all the samples decreased practically in the same way, reaching a similar value, between 0,3 and 0,4. That is, the tribological advantage of LIPSS and MoS2 lubricant in reducing friction is only achieved for loads lower than 100 mn. 7

8 Concerning wear coefficients, it was observed that lower wear coefficients were attained in the laser-textured samples (K=6,4x10-15 m²/n for 0,5 N and K=6,3x10-15 m²/n for 1 N), being the ones lubricated with molybdenum disulfide with the lowest wear coefficient (K=2,1x10-15 for 0,5 N and K=1,4x10-15 m²/n for 1 N). The wear coefficient was higher for the polished sample (K=44,4x10-15 m²/n for 0,5 N and 19x10-15 m²/n for 1 N). Since LIPSS reduce the surface contact area and the lubricant reduce the interface shear strength, the adhesion between surfaces will be lower, leading to lower wear in the laser-textured samples (with and without lubricant), comparatively to the polished sample. In all the samples tested in the micro-scale abrasion tests, the wear mechanisms that occurred in their surfaces was oxidative, resulting in the formation of a large quantity of oxides. In fact, in the polished steel sample, the reduction of wear coefficient that occurred from 0,5 to 1 N was precisely due to higher oxidation rates, that resulted in a higher formation of oxides in the contact zone protecting the steel sample. In the laser-textured samples, the lower wear coefficients observed for 1 N were due to higher oxidation of the surfaces (that protect LIPSS from wear) and the use of lubrication (that reduces the interface shear strength). As suggestions for work to be developed in the future, the following ideas are indicated: (i) apply higher loads and cycles to the laser-textured samples, with and without lubricant, in order to see how much longer can the LIPSS stay in the surface and to see the joint behavior of LIPSS with lubricant inside its valleys; (ii) apply other type of solid lubricants, like graphene, because it also has promising tribological properties; (iii) try to make a pre-oxidation of the laser-textured surfaces or add oxide particles before the wear tests (several researchers [15] [18] found out that this measure enables the elimination of the run-in period and allows a more rapid formation of the protective oxide layer, resulting in lower friction and wear of the surfaces); (iv) apply different laser-patterned textures to the steel surface (for example, increase the distance between laser ablated tracks) and apply lubricant, in order to see if the friction and wear behavior improves (in the literature [6], [13], [19] researchers have only investigated the effect of different spacing between laser textures without lubricants). 6. References [1] B. Bhushan, «Nanotribology and nanomechanics of MEMS/NEMS and BioMEMS/BioNEMS materials and devices», Microelectronic Engineering, vol. 84, n. 3, pp , [2] S. H. Kim, D. B. Asay, e M. T. Dugger, «Nanotribology and MEMS», nanotoday, vol. 2, n. 5, pp , [3] B. Bhushan, «Nanotribology and nanomechanics in nano/biotechnology», Philosophical Transactions of The Royal Society A, vol. 366, pp , [4] I. Etsion, «State of the Art in Laser Surface Texturing», Journal of Tribology- Transactions of the ASME, vol. 127, n. 1, p. 248, [5] Z. Wang, Q. Zhao, e C. Wang, «Reduction of Friction of Metals Using Laser-Induced Periodic Surface Nanostructures», Micromachines, vol. 6, n. 11, pp , [6] Z. Wang, Q. Zhao, C. Wang, e Y. Zhang, «Modulation of dry tribological property of stainless steel by femtosecond laser surface texturing», Applied Physics A: Materials Science and Processing, vol. 119, n. 3, pp , [7] L. T. Cangueiro, A. J. Cavaleiro, J. Morgiel, e R. Vilar, «Mechanisms of formation of low spatial frequency LIPSS on Ni/Ti reactive multilayers», Journal of Physics D: Applied Physics, vol. 49, pp. 1 9, [8] A. Cunha, «Multiscale Femtosecond Laser Surface Texturing of Titanium and Titanium Alloys for 8

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