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ISSN: 1402-1544 ISBN 978-91-86233-XX-X Se i listan och fyll i siffror där kryssen är DOCTORA L T H E S IS Department of Applied Physics and Mechanical Engineering Division of Machine Elements M Tribology of elastomeric seal materials o ISSN: 1402-1544 ISBN 978-91-86233-26-6 h a m Luleå University of Technology 2009 m a d r e z a M o f i d i T r i b o lo g y o f e la s t o m e Mohammadreza Mofidi r i c s e a l m a t e r i a ls Luleå University of Technology Tribology of elastomeric seal materials Mohammadreza Mofidi Luleå University of Technology Department of Applied Physics and Mechanical Engineering Division of Machine Elements Tryck: Universitetstryckeriet, Luleå ISSN: 1402-1544 ISBN 978-91-86233-26-6 Luleå (cid:17)(cid:15)(cid:15)(cid:24) www.ltu.se Tribology Of Elastomeric Seal Materials iii ____________________________________________________________________________ Preface The work presented in this thesis has been carried out at the Division of Machine Elements, Department of Applied Physics and Mechanical Engineering at Luleå University of Technology (LTU) in Luleå, Sweden. I would like to express my deep gratitude to my supervisors, Professor Braham Prakash and Professor Elisabet Kassfeldt, for their wholehearted support and guidance throughout this work. I have learnt a lot from the various courses I attended at this university and would like to thank all my teachers, especially Professor Braham Prakash and Professor Roland Larsson. My sincere thanks also go to my previous teachers at BSc and MSc levels especially, Professor Mansour Nikkhah Bahrami at the University of Tehran, Dr. Mohammad Reza Ghazavi and Professor Gholamhosein Liaghat at Tarbiat Modarres University. I wish to thank the “Ministry of Science, Research and Technology of IRAN” for awarding me the scholarship to pursue research at Luleå University of Technology. This work would not have been possible without this support and I am really grateful to the Government of Iran for this. All my colleagues at the Division of Machine Elements, especially Dr. Marika Torbacke, Gregory F. Simmons, Jens Hardell and Donald McCarthy have been very helpful whenever I had any difficulty and I sincerely acknowledge their support. A special thanks to my wife, Sedigheh and my son Aref for their support and patience. My sincere gratitude goes to my parents-in-law for their affection and kindness who passed away during the last three years. I would like to extend my appreciation to all my brothers and sisters for their profound kindness. Finally, I am deeply indebted to my parents, Parviz and Soghra, and feel a tremendous sense of appreciation for their genuine support, care, encouragement, patience and eternal dedication. Luleå, April 2009 Mohammadreza Mofidi iv Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ Tribology Of Elastomeric Seal Materials v ____________________________________________________________________________ Abstract Elastomers are the most commonly used materials for various sealing applications owing to their low modulus of elasticity, large elongation-at-break, and high Poisson’s ratio. Most seals operate in the presence of lubricants, therefore the sealing elastomer- oil interaction plays an important role in determining the tribological performance of elastomers. Furthermore, at times, such as during start-up periods, the seals may also operate under dry conditions and the seal material can be affected by high friction coefficient and wear. In this work, the tribological behaviour of different sealing elastomers has been studied. The influence of the ageing of a sealing elastomer in different lubricants on its tribological behaviour has been investigated. Further studies pertaining to the influence of lubrication on the abrasive wear of a sealing elastomer have also been carried out. The results show that the friction coefficient of an elastomer in lubricated sliding against a hard counterface, at low contact pressure depends on the surface topography of the elastomer but at high contact pressure, it is mainly a result of the viscoelastic deformation of the rubber by the counterface surface asperities. Even if the hard surface appears smooth to the naked eye, it may exhibit short-wavelength roughness, which may make the dominant contribution to rubber friction. Ageing of the nitrile rubber in ester base fluids leads to a reduction of the friction coefficient. In unidirectional dry sliding of an elastomer against a counterface, the friction coefficient decreases during the running-in period. The longest running-in periods have been observed when the elastomers slide against relatively smooth surfaces. Depending on the elastomer-lubricant compatibility, abrasive coarseness, geometry of sliding contact area and contact pressure, the two-body abrasive wear of elastomers may increase or decrease in the presence of lubricants. Ageing of nitrile rubber in lubricating fluids increases the abrasive wear both in dry and lubricated conditions. The abrasive wear of nitrile rubber in ester base fluids and rapeseed oil is higher than that in the mineral oils. vi Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ Appended papers A. G. F. Simmons. M. Mofidi. B. Prakash. Friction evaluation of elastomers in lubricated contact: a comparison of different test methodologies. Submitted for publication. B. M. Mofidi, B. Prakash, Influence of counterface topography on sliding friction and wear of some elastomers under dry sliding conditions, Proc. Inst. Mech. Eng. Part J.-J. Eng. Tribol. 222(5) (2008) 667-673. C. M. Mofidi, E. Kassfeldt, B. Prakash, Tribological behaviour of an elastomer aged in different oils, Tribol. Int. 41 (2008) 860-866. D. M. Mofidi, B. Prakash, B. N. J. Persson, O. Albohr, Rubber friction on (apparently) smooth lubricated surfaces, J. Phys.-Condes. Matter 20 (2008) 085223. E. M. Mofidi, B. Prakash, The influence of lubrication on two-body abrasive wear of sealing elastomers under reciprocating sliding conditions. Submitted for publication (A part of this paper was presented at NordTrib 08, Tampere, June 2008 and published in the conference proceedings). F. M. Mofidi, B. Prakash, Two body abrasive wear and frictional characteristics of sealing elastomers under unidirectional lubricated sliding conditions. Submitted for publication. Tribology Of Elastomeric Seal Materials vii ____________________________________________________________________________ List of symbols Units of Variable Meaning Measurement E((cid:550)) Complex dynamic modulus Pa E The real part of E((cid:550)) Pa 1 E The imaginary part of E((cid:550)) Pa 2 (cid:550) Oscillation frequency s-1 (cid:547) Poisson’s ratio - (cid:540) wavelength of the surface roughness m q The wavevector corresponding to the wavelength (cid:540) = 2(cid:652)/q m-1 q The low-wavevector cut-off to the wavelength (cid:540) m-1 0 0 q The large-wavevector cut-off to the wavelength (cid:540) m-1 1 1 C(q) The power spectrum of the surface roughness 1/m A The contact area observed at the highest magnification M2 A The nominal or apparent contact area M2 0 p Mean normal pressure Pa v Sliding velocity ms-1 (cid:536) Dynamic viscosity Pa s viii Tribology Of Elastomeric Seal Materials ____________________________________________________________________________