ISSN 1220-8434 Volume 17, 2009
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ISSN 1220-8434 Volume 17, 2009 ACTA TRIBOLOGICA Volume 17, 2009 Acta tribologica A Journal on the Science of Contact Mechanics, Friction, Lubrication, Wear, Micro/Nano Tribology, and Biotribology Volume 17, 2009 EDITOR E. Diaconescu, University of Suceava, ROMANIA EDITORIAL BOARD N.N. Antonescu, Petroleum-Gas University of Ploiesti, ROMANIA J.R. Barber, University of Michigan, U.S.A Y. Berthier, INSA de Lyon, FRANCE M. Ciavarella, Politecnico di Bari, ITALY T. Cicone, University Politehnica of Bucharest, ROMANIA S. Cretu, Technical University of Iasi, ROMANIA L. Deleanu, University of Galati, ROMANIA D. Dini, Imperial College London, UNITED KINGDOM V. Dulgheru, Technical University of Moldova, MOLDOVA I. Etsion, Technion, Haifa, ISRAEL M. Glovnea, University of Suceava, ROMANIA R. Glovnea, University of Sussex, UNITED KINGDOM I. Green, Georgia Institute of Technology, U.S.A M. Khonsari, Louisiana State University, U.S.A Y. Kligerman, Technion, Haifa, ISRAEL D. Nelias, INSA de Lyon, FRANCE D. Olaru, Technical University of Iasi, ROMANIA M. Pascovici, University Politehnica of Bucharest, ROMANIA M. Ripa, University of Galati, ROMANIA A. Tudor, University Politehnica of Bucharest, ROMANIA ASSISTANT EDITOR S. Spinu, University of Suceava, ROMANIA Published by the Applied Mechanics Section of the University of Suceava University “Stefan cel Mare” of Suceava Publishing House 13th University Street, Suceava, 720229, Suceava, ROMANIA Phone: (40) – 0230 – 216 – 147 int. 273, E-mail: [email protected] ACTA TRIBOLOGICA VOLUME 17, 2009 CONTENTS 1 E. DIACONESCU, V.F. ZEGREAN, M. GLOVNEA Shear Properties of Molecular Liquid near a Metallic Wall 6 L. PETRARU, F. NOVOTNY-FARKAS, K. BAUMANN New Approach in Oxidation Stability Testing of the Gas Turbine Oils 13 L. BOGATU, I. PETRE, A. MARIN, C. TĂNĂSESCU, D. PETRE, N. N. ANTONESCU, L. MIRCI Hydraulic Oils from Regenerable Feed Stocks with a Low Environmental Impact 17 C. BUJOREANU, V. PALEU Impact Parameters in Rolling Contact Lubrication 21 G. IANUŞ, D. OLARU, P. LORENZ Adhesion in Ball on Flat Microsystems 25 G.V. PUIU, D. OLARU, V. PUIU Friction Torque and Efficiency in Ball - Screw Systems 30 V. VASAUSKAS, J. PADGURSKAS, R. RUKUIZA Surface Modification of Carbon Steel by Deposition of Fluor-Oligomeric Films 34 A. SZUDER, I.D. MARINESCU, T.G. MATHIA, G. MEILLE Abrasion in Finishing Processes 42 S. GHEORGHE, C. TEISANU, A. MERSTALLINGER, I. CIUPITU The Influence of the Sintering Atmospheres on the Wear Behaviour of Copper Based Alloys 46 G. MEILLE, T.G. MATHIA Strategy for Identification of Wear Mode of Engineering Surfaces 52 I. SOVILJ-NIKIC, B. SOVILJ, M. BREZOCNIK, S. SOVILJ-NIKIC, V. PEJIC Analysis of Influence of Gear Hob Geometric Parameters on the Tool Life Using a Genetic Algorithm 58 A. TACHE, C. TACHE Thermal Aspects Regarding the Drilling Process 63 L. PASTRAV, S. JAECQUES, M. MULIER, G. VAN DER PERRE The Detection of Insertion Endpoint and Stability Assessment of Cementless Hip Stems by Vibration Analysis: A Per-Operative Pilot Study ISSN 1220 - 8434 ACTA TRIBOLOGICA Volume 17, (2009), 1-5 Emanuel DIACONESCU SHEAR PROPERTIES OF MOLECULAR LIQUID NEAR A METALLIC WALL Vlad – Flaviu ZEGREAN email: [email protected] Although the thickness of EHD oil films should vanish at very low Marilena GLOVNEA rolling speeds, experimental investigations indicate that often this thickness does not fall below a remnant value of 10-30 nm. Initially, this phenomenon was attributed to a much higher viscosity Department of Applied Mechanics, near the wall than in the bulk, according to an empirical formula. University “Stefan cel Mare” of Suceava, Recently, theoretical physical background of the phenomenon was ROMANIA advanced by means of molecule-wall interaction potential. It was shown that, besides viscosity, shear modulus and limiting shear stress are enhanced in the neighborhood of the wall. These theoretical findings predict correctly experimentally found thin film thickness. New experimental evidence is needed to support theoretical results. This paper provides experimental results concerning the evaluation of structural flow units, minimum intermolecular potential and limiting shear stress in thin layers. Keywords: liquid-wall interaction, viscosity, shear modulus, limiting shear stress, molecular layer 1. INTRODUCTION 3 1 10 EHL theory predicts that if the relative rolling speed of two surfaces tends towards zero, the film thickness formed between them must vanish. 100 Experimental results obtained by Johnson, Waite j and Spikes [1], Cann and Spikes [2], Luo and Wen [3], and others show that a lubricant film of about 10 10…30 nm or thicker is preserved between the surfaces at very low speeds. Zhang, Luo and Wen 1 [4,5] attributed this phenomenon to the existence of 01020 an ordered lubricant film, caused by the interaction j between liquid molecules and the metallic surface, Figure 1. The dependence of viscosity on the which is placed between central EHD film and the number of molecular layer from the metallic surface adsorbed layer. To predict theoretically the thickness of this layer, they used an empirical Using a molecular theory previously equation for the viscosity and a numerical program advanced for the solid like properties of molecular written for one-dimensional flow. They carried out substances, [8], and the molecule-wall interaction experimental investigations of thin film thickness potential, [6], Diaconescu, [7] found that both shear using a ball on disk apparatus and they found a good modulus and limiting shear stress increase in the agreement with the numerical results. wall vicinity with respect to their bulk values in the By noting the insufficiency of an empirical same fashion, as shown in Figure 2. formula for the viscosity in the neighborhood of the Again, the distance from the wall is indicated wall, Diaconescu [6,7] approached the subject by by the number of the molecular layers, whereas using the molecule-wall interaction potential and a dimensionless shear modulus g, is referenced to its general theoretical viscosity equation. The viscosity bulk value. is found to decrease exponentially with the distance The advanced theory shows that the wall from the wall as shown in Figure 1. This distance is viscosity is hundred times higher that in the bulk, expressed by the number of molecular layers from whereas the wall shear modulus as well as the wall the wall and the dimensionless viscosity is limiting shear stress exceed by a few times their referenced by the bulk viscosity. corresponding values in the bulk. the flow units in liquid paraffin, as well as their 10 average dimension. 8 6 gj 4 2 0 01020 j Figure 2. Dimensionless variation of shear modulus and limiting shear stress with the number of molecular layer from the metallic surface These values show that, compared to the bulk, the liquid flow is impeded in the neighborhood of the wall, as well as its elastic or plastic shear. This suggests that the supplementary attraction in the wall neighborhood generates a thin attached layer on each wall, measuring 6 to 14 molecular Figure 3. Typical scanned image of layers. These attached layers are responsible for solid paraffin film remnant oil film. At very low rolling speeds, the thickness of this film is the sum of the thicknesses of To this end, a tiny amount of melted paraffin the layers attached to the wall. If the speed is high was let to drop on a cooled mica surface, thus enough, an EHD film is formed between attached freezing rapidly while preserving the flow structural layers. Consequently, total film thickness is the sum units. In its turn, the mica layer was fixed on a cool of remnant and EHD film thicknesses. The effects metallic surface. Then, the surface of solid paraffin of pressure and temperature upon nanometric film film was scanned by aid of a non-contact AFM. A thickness enter by their influence upon bulk shear typical example for obtained images is depicted in properties of the liquid. This conclusion and the Figure 3. predicted dependence of thin EHD oil films on rolling speed agree very well with the experimental results of Zhang, Luo, and Wen [4,5]. This agreement suggests that the advanced theory is qualitatively correct. Nevertheless, supplementary experimental confirmation is needed, especially concerning the shear properties of molecular liquids in the neighborhood of a metallic wall. This paper aims to provide new experimental evidence into the subject, as well as corresponding experimental procedures. 2. EXPERIMENTAL RESULTS Figure 4. Paraffin surface profile 2.1 Evaluation of structural flow units It can be seen that paraffin molecules and A unanimous point of view upon the their branches tend to pack into nearly spherical structural flow units in EHD oil films was not clusters having a diameter of 20…25 Å. For a better achieved yet. These may be single molecules, view, some of these are circled by black lines. molecular clusters or molecular segments. Nevertheless, individual molecules fill the space According to Hirst and Moore, [9], molecular between the clusters. A 40 Å long surface profile, clustering occurs at low temperatures, which shown in Figure 4, allows assessment of the average dissociate into molecules as the temperature rises. value the diameter of structural flow unit. This was When the liquid possesses branched molecules, at found to be 4.47 Å. certain moments, some of these branches can flow individually. To clarify the subject, new evidence is 2.2 Evaluation of minimum intermolecular needed. potential Experimental investigations