Investigation of industrial gear oils: insights from complementary surface and subsurface characterisation A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering 2018 Aduragbemi J. Adebogun School of Materials Contents List of Tables ................................................................................................................. 7 List of Figures ............................................................................................................... 8 Abstract ...................................................................................................................... 13 Declaration ................................................................................................................. 14 Copyright Statement .................................................................................................. 14 Acknowledgments ...................................................................................................... 15 1 Introduction ....................................................................................................... 16 1.1 Research motivation and objectives ........................................................... 16 1.2 Structure of the thesis ................................................................................. 18 2 Literature review ................................................................................................ 20 2.1 Tribology as a scientific field ....................................................................... 20 2.2 Nature of surfaces ....................................................................................... 20 2.2.1 Surface layer ......................................................................................... 20 2.2.2 Surface finishing ................................................................................... 21 2.2.3 Surface texture ..................................................................................... 22 2.3 Bearing steel ................................................................................................ 25 2.3.1 Introduction to bearings and material requirements .......................... 25 2.3.2 Manufacturing ...................................................................................... 27 2.4 Sliding condition in industrial gears and bearings....................................... 28 2.5 Surface and subsurface deformation wear mechanisms ............................ 29 2.5.1 Surface ploughing ................................................................................. 29 2.5.2 Delamination wear ............................................................................... 30 2.5.3 Micropitting wear................................................................................. 31 2.6 Industrial oils: their tribofilms and tribological performance ..................... 32 2 2.6.1 Antiwear and Extreme pressure additives ........................................... 32 2.6.2 Friction modifiers ................................................................................. 36 2.7 Influence of industrial oils and their tribofilms on subsurface deformation 37 2.8 Conclusions .................................................................................................. 39 3 Materials and methods ...................................................................................... 41 3.1 Lubricants .................................................................................................... 41 3.1.1 Application ........................................................................................... 41 3.1.2 Lubricant formulation and rheological properties ............................... 41 3.2 Tribotesting ................................................................................................. 42 3.2.1 HFRR (high frequency reciprocating rig) .............................................. 42 3.2.2 SRV tribometer ..................................................................................... 44 3.3 Wear and surface roughness measurement ............................................... 45 3.3.1 Wear measurement and wear volume estimation .............................. 46 3.3.2 Surface roughness measurement ........................................................ 47 3.4 Nanoindentation ......................................................................................... 50 3.5 Raman spectroscopy ................................................................................... 50 3.5.1 Raman procedure and parameters ...................................................... 51 3.6 Scanning Electron Microscopy (SEM) .......................................................... 52 3.6.1 SEM principle and surface imaging ...................................................... 52 3.6.2 Scanning electron microscopy - energy dispersive X-Ray spectroscopy (SEM-EDX) .......................................................................................................... 53 3.6.3 Scanning electron microscopy – focused ion beam (SEM-FIB) ............ 53 4 Industrial gear oils: tribological performance and subsurface changes ............ 56 4.1 Introduction ................................................................................................. 57 4.2 Materials and methods ............................................................................... 60 3 4.2.1 Lubricant .............................................................................................. 60 4.2.2 Tribotesting .......................................................................................... 61 4.2.3 Ball and disk ......................................................................................... 62 4.2.4 Wear measurement ............................................................................. 62 4.2.5 Raman spectroscopy ............................................................................ 62 4.2.6 Scanning electron microscopy – energy dispersive spectroscopy (SEM – EDX) 63 4.2.7 Nanoindentation .................................................................................. 63 4.2.8 Scanning electron microscopy – focused ion beam (SEM-FIB) ............ 63 4.2.9 Transmission- electron backscatter diffraction (t-EBSD) ..................... 64 4.3 Results and discussion ................................................................................. 64 4.3.1 Friction and wear performance ........................................................... 64 4.3.2 Subsurface changes (mechanical and microstructural) ....................... 72 4.4 Conclusions .................................................................................................. 79 5 Industrial gear oils: influence of bulk oil temperature and contact pressure on tribological performance and subsurface changes .................................................... 82 5.1 Introduction ................................................................................................. 83 5.2 Materials and methods ............................................................................... 85 5.2.1 Lubricants ............................................................................................. 85 5.2.2 Tribotesting .......................................................................................... 86 5.2.3 Ball and Disc. ........................................................................................ 86 5.2.4 Wear measurement ............................................................................. 86 5.2.5 Raman spectroscopy ............................................................................ 87 5.2.6 Scanning electron microscopy – energy dispersive spectroscopy (SEM – EDX) 87 5.2.7 Nanoindentation .................................................................................. 87 4 5.2.8 Scanning electron microscopy – focused ion beam (SEM-FIB) ............ 87 5.2.9 Calculation of lambda ratio .................................................................. 88 5.3 Results and discussion ................................................................................. 88 5.3.1 Tribological performance and tribofilm formation .............................. 88 5.3.2 Subsurface changes and links to tribological performance. .............. 103 5.3.3 Subsurface transformation: differences between the oils ................ 122 5.4 Conclusions ................................................................................................ 126 6 Industrial gear oils: Influence of sliding contact condition and tribo-pair hardness ratio on tribological performance and subsurface transformation of hardened AISI 52100 steel ....................................................................................... 130 6.1 Introduction ............................................................................................... 131 6.2 Materials and methods ............................................................................. 133 6.2.1 Lubricants ........................................................................................... 133 6.2.2 Tribotesting ........................................................................................ 134 6.2.3 Ball and Disc ....................................................................................... 134 6.2.4 Wear scar measurement ...................................................................