Nie, Meng

Stokes, Keith

Wang, Ling

Watson, Simon

Southampton (e-Prints Soton)

Development Of Polymer Brushes For The Lubrication Of Silicon Nitride–Steel ContactsSimon Watson a, Ling Wang a, Mengyan Nie a, Keith Stokes a, b aNational Centre for Advanced Tribology at Southampton (nCATS), University of Southampton, Southampton SO17 1BJ, UKbPlatform Systems Division, Dstl, Porton Down, Salisbury, Wiltshire SP4 0JQ, UKSTLE Annual Meeting, May 15-19 2016Contact:S.Watson@soton.ac.uk 1. IntroductionSilicon nitride rolling element bearings have seen great success as hybrid bearing systems in applicationsincluding automotive, aerospace, renewables and the railway industry. The main sought after property isthe relative low density of the ball bearing, with a 60% reduction in weight resulting in an 80% reductionin friction compared to classic steel bearings [1]. Compared to steel on steel contacts the coefficient offriction (COF) of hybrid systems is reduced to approximately 0.04-0.09 under oil lubricated slidingconditions and between 0.1-1.0 for dry sliding conditions [2, 3]. Current lubrication and protectionsolutions are focussed on lubricating metal surfaces thus are not optimised for hybrid contacts. Thisstudy aims to develop polymer brush based lubrication solutions that are optimised for silicon nitride-steel contacts through self-assembling initiators and subsequent polymerisation techniques.Polymer brushes can be attached to surfaces either by “grafting from” or “grafting to” method usingprefabricated polymers that can be attached to a surface using physisorption or chemisorption much likeself-assembled monolayers (SAMs) are produced [4]. The film thicknesses and brush density of polymersproduced through the “grafting to” method are limited by the molecular weights of the preformedpolymer in solution which can limit the tribological properties [5]. Atom Transfer Radical Polymerisation(ATRP) is the most popular type of polymerisation for producing “grafting from” brushes due to therelative robustness of the technique [6]. For example, unlike the other techniques, rigorously dryworking conditions are not needed and reactions are very tolerant of a variety of monomers, ligands andcatalysts [4].The mechanism in which solvated polymer brushes lubricate is thought to be as follows [4, 7, 8].3. ATRP Initiator Self-Assembly2. Synthesis of Polymer Brushes2.1 ATRP initiator SynthesisMethod Synthesis Steps2.2 Polymer SynthesisMethodSynthesis Steps4. Polymer GrowthAllyl 2-bromo-2-methylpropionate (1.6 mL),dimethylchlorosilane (10 mL) and platinum oncarbon (10 mg) were added to a 25 mL roundbottom flask attached to a microscale rig. Thesystem was purged with nitrogen and then themixture was refluxed under nitrogen for 18hrs. The excess silane was then distilled off.The resultant solution was quickly filtered overanhydrous sodium sulphate to produce acolourless oil which was confirmed by 1H and13C NMR. The attachment of the ATRP initiatorwas completed in toluene as well as in a poly-alpha-olefin (PAO) synthetic oil atconcentrations of 1, 2.5 and 5 mM solutionsfor both 2 and 18 h. A PAO 4 from ExxonMobilwas selected due to its similarities withmilitary defence standards, there are a fewlubricating oils that are used, namely OMD-55,OMD-90 and OX-90 [9].To conduct the surface initiated polymerisation a 22 mLvial containing a silicon wafer with pre attached initiatorwas charged with: methyl methacrylate (MMA, 15 mL,150 mmol), CuBr2 (0.00558 g, 0.025 mmol) and PMDETA(0.043 g, 0.25 mmol) in anisole (2 mL) in addition to EBIB(0.048 g, 0.25 mmol). The vial was then bubbled withnitrogen for 5 mins then sealed. A solution of sodium L-ascorbate (0.0495 g, 0.25 mmol) in anisole (2 mL) wasthen syringed through the septum and the vial wasplaced in a thermostatic water bath at 70°C. Thepolymerisation was stopped when the vial wasunscrewed and therefore exposing the catalyst to air. Themodified wafer was removed and sonicated intetrahydrofuran (THF) and dried in nitrogen.Reproduced from Ref. 4 with permission from the Royal Society of Chemistry.Activators Re-Generated by Electron Transfer (ARGET) is a development of ATRP and has been used to reduce the concentration of metal catalysts up to 1000 times to ppm levels [10].The contact angle data show that in all cases amonolayer of the initiator was formed on silica waferto some degree. Self-assembled deposition of theinitiating monolayer was expected to take a similaramount of time to that of octadecyltrichlorosilane, ≈2h [1]. However, it was observed that the maximumcontact angle was achieved over a longer duration ofca. 18 h at the initiator concentration 2.5 mM intoluene. It is likely this is due to stearic hindrance ofthe two methyl groups on the silane. A monolayer wasalso successfully formed in PAO, but the same contactangle was not achieved as in toluene. On the otherhand, it is noteworthy that a maximum contact angle (85°) was unlikely to be achieved as the PAO is animperfect solvent [11]. In comparison to the tests in toluene, the difference of the measured contact anglesbetween 2.5 mM and 5 mM in PAO is insignificant, and slightly higher contact angles were achieved for 2 htests in PAO than in toluene. This may be due to the fact that an overall lower quality monolayers wereformed, i.e., multilayers or agglomerations due to a higher water content in the PAO.The graph to the right shows polymethyl methacrylate (PMMA)growth with respect to time. Thedry thickness of the polymerbrushes was measured by aWoolham M-2000 spectroscopicellipsometer at three intervals, 2h,4h, and 6h. The data was modelledwith the correspondingCompleteEASE software using athree component model consistingof Si/SiO2/Cauchy [12, 13]. It canbe seen that the thickness of thepolymer film grew linearly withimmersion time from 3 nm to 11.5nm over 6 h.7. References and Acknowledgements The authors acknowledge the financial support from the Defence Science and Technology Laboratory (DSTL, DSTLX1000093632), which is part of the UK's Ministry of Defence and from EPSRC (Grant EP/M50662X/1) to this project.0.010.020.030.040.050.060.070.080.090.02hrs 18hrs 2hrs 18hrsToluene PAOContact Angle (°)1mM 2.5mM 5mMReproduced from Ref. 4 with permission from the Royal Society of Chemistry.[1] L. Wang, M. Nie and J. Rumbol, Tribology - Materials, Surfaces & Interfaces, 2012, 6, 75-83.[2] L. Wang, R. J. K. Wood, T. J. Harvey, S. Morris, H. E. G. Powrie and I. Care, Wear, 2003, 255, 657-668.[3] L. Wang, R. W. Snidle and L. Gu, Wear, 2000, 246, 159-173.[4] S. Watson, M. Nie, L. Wang and K. Stokes, RSC Advances, 2015, 5, 89698-89730.[5] S. Edmondson, V. L. Osborne and W. T. S. Huck, Chemical Society Reviews, 2004, 33, 14-22.[6] K. Matyjaszewski and J. Xia, Chemical Reviews, 2001, 101, 2921-2990. [7] R. Bielecki, E. Benetti, D. Kumar and N. Spencer, Tribology Letters, 2012, 45, 477-487.[8] D. Irfachsyad, D. Tildesley and P. Malfreyt, Physical Chemistry Chemical Physics, 2002, 4, 3008-3015[9] M. o. Defence, in DEF STAN 01-5 Issue 17, DSTAN, GLASGOW, Editon edn., 2011.[10] K. Matyjaszewski, H. Dong, W. Jakubowski, J. Pietrasik and A. Kusumo, Langmuir, 2007, 23, 4528-4531 [11] M. G. Santonicola, G. W. de Groot, M. Memesa, A. Meszyńska and G. J. Vancso, Langmuir, 2010, 26, 17513-17519.[12] O. Kahle, U. Wielsch, H. Metzner, J. Bauer, C. Uhlig and C. Zawatzki, Thin Solid Films, 1998, 313–314, 803-807.[13] N. Vourdas, A. G. Boudouvis and E. Gogolides, Journal of Physics: Conference Series, 2005, 10, 405.1.The resistance to rearrangement of the grafted chains with the repulsive nature of the brushes;2. Lubricant entrapment in the polymer brushes;3.High concentrations of lubricant in the outer polymer brush creates a low shear area protecting the brush system.024681012140 1 2 3 4 5 6 7Thickness (nm)Polymerisation time (h)5. NanotribologyNanotribology experiments wereconducted on the polymer brushmodified silicon wafers using anatomic force microscope (MAC ModeIII, 5500 Scanning Probe MicroscopyAgilent Technologies, USA).Nanotribological tests wereperformed under ambient conditionsin air. The results show that areduction in friction force wasachieved by all polymer brushes, seefigure on the right. However, theinfluence of the polymer thickness isnot obvious or unidirectional, i.e. thethicker polymer may not provide alower friction force. However,comparing those with the silicon andsilicon nitride surfaces withoutpolymer brush layers, a significantreduction in friction force wasachieved. Hence, the polymer hasprovided a protection to the siliconsurface. It is interesting that thepolymers formed in 4 and 6 hbehaved in a similar way but differentfrom that of the thinner layer andthis will be further investigated.00.511.522.533.544.550 10 20 30 40 50 60 70 80Friction force, Ff(nN)Applied Load, Fn (nN)Silicon Polymer 2hr Polymer 4hr Polymer 6hr Silicon Nitride6. ConclusionsIn the current work it has been shown that polymer brushes have been synthesized on silicon via ARGETATRP in a controlled manner and have the potential to from silicon nitride. The grafted structures havethe promising characteristic in that they are covalently attached to the surface in question providing astable coating for tribological contacts. It is likely that the polymer will synergize well with oil lubricatedcontacts and the solvated polymer brushes would trap lubricant therefore decreasing COF. Tribologicalproperties of the PMMA brushes were analysed at the nanoscale and compared to bare substrates. Thepolymers formed have shown to reduce the frictional force in all thicknesses tested. However, there weretwo distinct behaviours to the increased normal load, with a thinner polymer surprisingly performingbetter. This may be due to longer chains repeated interaction with the probe. Nevertheless the reductionin friction force is promising for future applications. In addition, it is a good indicator that in anappropriate solvent or oil, it may be possible to exploit the lubricating properties of polymer brushes to ahigher degree.

Development of polymer brushes for the lubrication of silicon nitride–steel contacts

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