Influence of Nanoadditives on Lubricants Tribological Properties

Transcription

1 ISSN 9 0 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 4, No Influence of Nanoadditives on Lubricants Tribological roperties Andrzej KOTNAROWSKI Radom University of Technology, al. Chrobrego 45, Radom, oland Received 0 June 008; accepted 7 September 008 The paper deals with the influence of base oils (applied for lubricating and cutting fluids compounding) modification with metal micro- or nanoparticles addition on tribological properties of friction couples. The results of tribological investigations are presented as well as amount of wear measurements, and friction surface structure and composition examination results. Molybdenum nanoparticles added to some base oils modify their tribological properties observed in model sliding friction couples made in this purpose of materials characteristic for machining systems (steel C45 cooperating with sintered carbide S0S). Such field of investigations results from only few reports of some authors about positive influence of metal nanoparticles applied as additives to lubricating oils and mainly from the examination made by the author in this research area. Examination results presented in the paper prove positive influence of molybdenum nanoparticles addition on tribological properties of lubricants consisting in wear decrease in couples lubricated with these lubricants. Keywords: molybdenum nanoparticle, lubricant, tribological properties, nanostructure, nanopowder.. INTRODUCTION The main duty of lubricants used in almost all branches of industry is to minimize friction and wear of rubbing elements and for this reason searching for new powerful antifriction and antiwear additives is an important research field. One of the ways to improve properties of lubricants (e. g. oils) is to introduce to their composition powders of metals or their alloys consisting of micro- or even nanoparticles. This opinion is formulated as yet by only few authors [ ]. Especially interesting seems to be an application of nanoparticles as they developed in extreme extent in last few years [ 9]. As extremely small (diameter from single nanometers to some tens of nanometers) they possess extraordinary physico-chemical properties, quite different from those observed in bulk form of material. At least two aspects of nanostructures nature can be utilized in tribotechnology [0 ]. One of them is quantum aspect, dominating first of all in the nanometric scale, and the other is superficial aspect. The quantum aspect consists in peculiarities connected with electron flow resulting from lack of conduction band characteristic for solid metal. Instead of this band the discrete states occur on nanograin borders. For this reason electrons can be trapped in a small enough nanoparticle like in the quantum trap and such nanoparticles will possess properties of so called quantum dot. The superficial aspect results from the fact that the smaller object, the bigger relation of atoms quantity existing on the object surface to quantity of all atoms composing this object. For this reason, sets of very small particles are characterized by very big surface in relation to the volume and, because many phenomena occur on the surface, this feature can be used to these phenomena intensification. Thanks to high specific surface, nanoparticles possess much more active centres than bigger particles of the same mass and demonstrate very strong catalytic properties. This feature, in the case of nanoparticles addition to lubricants, * Corresponding author. Tel.: ; fax.: address: d.kotnarowska@pr.radom.pl (A. Kotnarowski) 66 can modify tribological process, the more so because that their ability to penetration (on account of small size) is also much bigger what afford possibilities for their closer penetration of friction zone. The tribological process can be also intensified on account of phenomena occurring on quantum level between nanoparticles and rubbing materials. Considering above, some investigations were made to examine possibilities of metal nanoparticles application to lubricating oils improvement. Results of the tests concerning possibilities of molybdenum nanoparticles use as additives to base oils are discussed. These nanoparticles added to oils modify their tribological properties observed in model sliding couples. resented results prove positive influence of molybdenum nanopowders on wear decrease in examined couples lubricated with these lubricants.. EXERIMENTAL.. Test types The examination covered experiments and tests made in three phases. The influence of dispersion degree of metal particles added to lubricating oil on tribological characteristics of friction couple was investigated in the first phase. The second phase consisted in determination of metal nanoparticles concentration in oil, optimum with regard for the best tribological properties of friction coupe. The third phase was divided into two stages. Tribological investigations were made in the first stage to evaluate the influence of oil modification with some metal nanoparticles addition on oil tribological properties in a wide range of sliding couple loads. The second stage covered the evaluation of samples wear amount after tribological tests made in sliding friction condition as well as the examination of oil nanoadditives influence on phenomena occurring during the friction process... Test equipment Tribological tests in the first phase were made with the help of MT- friction machine (designed in Technical

2 University of Radom with author s participation). The model friction couple of the apparatus is presented in Fig., a. It consisted of two samples in the shape of rod of mm diameter pressed against a countersample in the form of ring (of outer diameter equal 40 mm and 0 mm width), immersed in lubricant () This apparatus enables to control the countersample rotational speed ω and the load force. It enables also to measure and record these values as well as friction force, resistance of the contact between the countersample and the samples, and temperature of the samples and of the oil bath. Tribological tests in the second phase were made using widely known, traditional and reliable tribological machine made by Amsler (Schafhausen, Switzerland), enabling measurement of friction torque. A model sliding friction couple of this machine was modified to the form presented in Fig., b. It consisted of a sample () in the shape of square rod pressed against a countersample () in the form of ring (of outer diameter equal 40 mm and 0 mm width), immersed in lubricant (). ϖ measurements after tribological tests were made in the second stage of the third phase using Taylor and Hobson s scanning profile meter Form Talysurf Series. This apparatus enables to obtain isometric pictures of the friction path and to calculate the surface of right section of friction path representing amount of wear (Table ). The scanning electron microscope Hitachi S-460N, with attached X-ray microanalyser Voyager 050 of Noran Instruments with energy dispersion system (equipped with Norvar glass and SiLi crystal of ev resolution), was used for material analyses. The results were obtained in the form of X-ray spectrum, and on their base, the computer program counted atomic and weight share of particular elements (Table [% wt.]). a v v Fig.. T-05 tester and its friction couple: sample, countersample, oil b Fig.. Model friction couple of MT- apparatus (a) and modified friction couple used in the Amsler machine (b): sample, countersample, lubricant In the first stage of the third phase the recent computerized T-05 tester (Fig. ) (made by Institute of Terotechnology, Radom, oland) was used. The tester can measure changes of friction force as well as sample or oil temperature. Temperature of oil can be kept at any level over room temperature. A model sliding friction couple of T-05 tester, presented in Fig., consisted of a sample () in the shape of cuboid (with square mm mm base and mm height) sliding on a countersample () in the form of ring (of outer diameter equal 40 mm and 0 mm width), immersed in lubricant (). The amount of wear 67 Table. Results of wear and material examinations No. Sample/oil Wear [µm ] Mean values Mo content [% wt.] Outside friction path On friction path S0S/SAE0/ S0S/SAE0/95+Mo S0S/SN S0S/SN00+Mo Materials Friction couple used in MT- friction machine consisted of samples and countersample made of the same

3 material steel type 45 (which is often used for machined piece parts of higher quality) and hardened to 4 ± HRC. Friction couples used in Amsler machine and in tester T-05 were made of identical materials. The samples were made of sintered carbide S0S type while the countersamples were made of C45 type steel and hardened to 4 ± HRC. All above materials used in the tribological examination can be considered as typical for machining systems. As model cutting fluid three sorts of basic oil were used: Hydrorafinat II of kinematic viscosity v mm /s, SN00 (v mm /s) and SAE 0/95 (v mm /s). To modify tribological properties of basic oils, nanoparticles of molybdenum of 54 nm mean size (Fig. ) were used. The nanoparticles were produced in an electrochemical way in the laboratory scale. Fig.. Microphotos of molybdenum nanoparticles obtained by SEM LEO 45V.4. rocedure Each test of the tribological examination was carried out at the constant sliding speed v equal 0.4 m/s. The load between sample and countersample in the first phase tests lasting 000 s was constant. The value of load assured mixed friction what was proved by low values of electric resistance between samples and countersample. The load between the sample and countersample in the second phase tests was constant (4 Ma) and the concentration of molybdenum nanoparticles varied from 0.0 % through 0.0,,, 0.5 % up to 0.5 % by weight. In the first stage of the third phase the load was increased stepwise every 00 s by 50 N up to 50 N. The last value of load applied in the first stage was then kept constant in the second stage for.5 hours. The concentration of molybdenum nanoparticles in each kind of oil was 0.5 % by weight (as optimum value determined in the first phase tests).. RESULTS AND DISCUSSION.. Tribological examination The selected results of tribological test are presented in Figs Results of the first phase tests (examination of metal nanoparticles size influence on tribological properties of couples investigated in base oil Hydrorafinat II) are shown in Fig. 4. Results of the second phase tests (aimed on examination of influence of metal nanoparticles concentration in oil on tribological properties of investigated couples) are shown in Fig. 5. The exemplary results of the third phase tribological tests (aimed on influence examination of molybdenum nanoparticles addition to base oils SAE0/95 and SN00 on tribological properties of steel C45/sintered carbide S0S couple) are shown in Fig. 6 and 7 for oil SAE 0/95. The results cited in Fig. 4 testify to the fact that the smaller particles the lower coefficient of friction. Characteristics in Fig. 5 show that the best result in coefficient of friction decreasing gave addition of molybdenum nanoparticles in amount of 0.5 % by weight and this amount was applied in the further examinations as optimum. Increase of molybdenum nanoparticles content over this value showed unremarkable differences Mean particle size, [mm] Fig. 4. Influence of sizes of metal particles (added to oil Hydrorafinat II in amount of 0.5 % by weight) on tribological characteristic of steel C45/C45 friction couple Hydrorafinat II H+0.0%Mo H+%Mo H+0.5%Mo Time t, [s] Fig. 5. Characteristics of the couple steel C45/sintered carbide S0 lubricated with oil Hydrorafinat II with some concentrations of Mo nanoparticles at constant load = 4 Ma SAE S0S SAE S0S 5 Mo nano Load, [N] Fig. 6. Dependence between coefficient of friction µ and load for couple steel C45 / sintered carbide S0S lubricated with oil SAE 0/95 without ( ) and with ( ) addition of 0.5 % (wt) molybdenum nanoparticles (54 nm) 68

4 Characteristics in Figs. 6 and 7 show that modification of SAE 0/95 oil with molybdenum nanoparticles brought increase of friction resistance in oil SAE 0/95. Similar results were obtained for oil SN Amount of wear examination Images of the sample surface in isometric form (Fig. 8) as well as friction path cross-sections in planes perpendicular to the motion direction (Fig. 9) with counted areas of these cross-sections which represent the wear amount were obtained with a scanning profile meter Form Talysurf Series. All wear results are collected in Table. They testify to positive influence of oil modification with molybdenum nanoparticles on wear of samples decrease. Coeficient of friction μ SAE S0 SAE S0 5Mo nano Time t, [s] Fig. 7. Dependence between coefficient of friction µ and time t for couple steel C45 / sintered carbide S0S lubricated with oil SAE 0/95 without ( ) and with ( ) addition of 0.5 % (wt) molybdenum nanoparticles (54 nm) at constant load = 50 N Fig. 8. Isometric image of sample surface Amount of wear Fig. 9. Cross-section through friction path parallel to motion direction with wear area counted.. Material examination Material examination results were obtained using scanning electron microscope Hitachi S-460N equipped with X-ray microanalyser with energy dispersion system. The analyses were made in areas of carbide blank surfaces located on friction path and on the zone not influenced by the friction motion (Fig. 0). Results were obtained as X-ray spectra showing X radiation intensity in the form of peaks corresponding to determined values of particular elements excitation energy. On their base the computer program counted atomic and weight share of each element. As analysed oils were modified with molybdenum nanoparticles, only this element content is important in order to understand a nature of friction processes in modified oils. The other detected elements originate from the sample or countersample material or from the oil. All material examination results, covering molybdenum weight shares as well as amount of wear examination results are collected in Table. Microzone 00x50 μm in the area not affected by friction Fig. 0. Location of areas where the elemental microanalysis was carried out.4. Discussion of the results Microzone 00x50 μm in the area affected by friction To summarise the obtained results, first of all it should be stated according to Fig. 4 that the smaller molybdenum particles are added to oil, the lower friction resistance in friction couple lubricated with this oil. The characteristics from Fig. 5 show that addition of molybdenum nanoparticles to lubricating oil in amount 0.5 % gives the best results regarding frictional resistance in friction couple. Taking this into consideration, the addition of 0.5 % metal nanoparticles was used for oil modification in all subsequent examinations. Analyzing the results of tribological tests, which characteristics are as example presented in Figs. 6 and 7 it can be noticed that addition of 0.5 % molybdenum nanoparticles increases frictional resistance in sliding friction couples consisting of steel C45 and sintered carbide S0S. What concerns wear of samples (Table ), addition of the molybdenum nanoparticles gives wear reduction in the case of couple steel 45/sintered carbide S0S tested in SAE 0/95 and SN00 oils. For better understanding of the mechanism of copper and molybdenum nanoparticles influence on frictionally cooperating surfaces the X-ray spectra were obtained from two zones of each carbide sample: one placed on the friction path and second from the place not affected by friction process. After computing atomic and weight share of molybdenum in examined zones it was stated that distinct remains of this metal were present only on the 69

5 friction path, despite of fact that whole samples were 8. Kotnarowski, A. Tribological roperties of Oils Modified immersed in oils during the tribological tests. It is visible with the Addition of Metals Nanoparticles Solid State in Table in the case of both tested oils (SN 00 and henomena 006: pp SAE 0/95). This leads to the conclusion that durable 9. Kotnarowski, A. Searching for ossibilities of Lubricating layers, reducing wear of the sample and increasing and Cutting Fluids Modification with Copper Micro- and frictional resistance, form only in a friction process course. Nanopowders Materials Science (Medžiagotyra) () 006: pp The fact that these layers form on friction paths only demonstrate tribological nature of the process which can 0. Kotnarowski, A. Metal Nanoparticles as Modifiers of Oil take place under the condition of sufficient amount of Tribological roperties. In: Kajdas, C., Franek, F. (editors): International Conference Tribology of Alternative Fuels and energy delivery. Ecolubricants, May 9, 006, Vienna Region, Austria. ublished by Österreichische Tribologische Gesellschaft, 4. CONCLUSIONS Copy & Druck GmbH, pp On the basis of the examination results it can be stated. Kotnarowski, A. Metal Nanopowders as Additives to that: Lubricants nd Vienna International Conference: Microand Nano-Technology March 4 6, 007, Vienna,. Modification of base oil with molybdenum Austria. ublisher: Österreichische Tribologische nanoparticles effectively improves tested oil properties Gesellschaft, 007: p. 87. manifested in tribological systems in degree dependent. Feldheim, D. L., Foss, C. A. Metal Nanoparticles, on nanoparticles concentration in the oil. Concentration Synthesis, Characterization and Applications. ublisher of molybdenum nanoparticles that, as it was found, Marcel Dekker, New York 00. most effectively reduce friction effects is equal 0.5 %.. Baraton, M. I. Synthesis, Functionalization and Surface. roduction of antiwear layers by molybdenum Treatment of Nanoparticles. American Scientific ublishers, nanoparticles deposition, occurring during carried out Stevenson Ranch, California 00, 0 p. experiments, is of tribological nature. It can only take 4. Cancer Nanotechnology. National Cancer Institute. place owing to sufficient energy delivery in frictional ublication No , January 004, Bethesda, Maryland, process. USA.. Test methods used in presented examinations enable 5. Kotnarowski, A. Nanostructures Materials of Future distinct differentiation of oils, modified with metal X Jubilee Congress of Technical Equipment Operation and nanoparticles, as regards their tribological properties Maintenance Institute of Terotechnology ublishers, manifested in frictional systems. Radom, 005: pp (in olish). 6. Burda, C., et al. Chemistry and roperties of Nanocrystals Acknowledgments of Different Shapes Chemical Reviews : pp The work was supported in the frames of grant 7. ilotek, S., Tabellion, F. Nanoparticles in Coatings. No. N /46 by Ministry of Science and Higher Tailoring roperties to Applications European Coatings Education. Journal 4 005: pp REFERENCES 8. Knowles, T. The New Toolbox. Nanotechnology in aints and Coatings European Coatings Journal 006:. Kajdas, C. Additives for Metalworking Lubricants-Review. pp th Intern. Kolloquium, Technische Akademie Esslingen, 9. rashant, K. J., El-Sayed, I. H., El-Sayed, M. A. Au 988. Nanoparticles Target Cancer Nanotoday 007:. Lesnikovich, A. I., Shpenkov, G.. Ultra Dispersed Metals pp and Oxides as erspective Additives for Metal-plating 0. Roduner, M. Size Matters: Why Nanomaterials are Lubricants Conference roceedings, Taganrog, 99. Different Chemical Society Review 5 005:. Shpenkov, G.. Tribological roperties of Greases and pp Lubricating Water Emulsions with Additives Containing. Borgohain, K., et al. Quantum Size Effect in CuO Ultradispersed articles of Copper Oxide Exploitation Nanoparticles hysical Review B 6 000: pp. 09 roblems 6 995: 08 p. (in olish) Shpenkhov, G. Friction Surface henomena. Elsevier, Amsterdam, Mavi, H. S., et al. Quantum Confinement Effects in Silicon Nanocrystals roduced by Laser-Induced Etching and cw 5. Kotnarowski, A. Copper Nanoparticles Application to Laser Annealing Semiconductor Science and Technology Cutting Oils Modification Tribologia 004: pp. 006: pp (in olish).. Kozub, V. I., et al. Electron Transport in Monodisperse 6. Kotnarowski, A. Use of Molybdenum Nanopowder for Metal Nanostructures Journal of Experimental and Cutting Oils Modification Tribologia 004: pp. 87 Theoretical hysics Letters 8 006: pp (in olish). 7. Kotnarowski, A. Modification of Oils with Use of Metal Micro- and Nano-owders International Journal of Applied Mechanics 9 004: pp resented at the 7th International Conference "Materials Engineering 008" (Kaunas, Lithuania, November 06 07, 008) 70