Investigation of industrial gear oils: insights from complementary surface and subsurface characterisation

  • Aduragbemi Adebogun

Student thesis: Phd


A traditional approach to investigating gear oils involves running a tribo-test followed by characterisation of the tribofilm formed and the surface topography of the worn surface generated. While this provides insights into how gear oils minimise friction and wear, there has been much less focus on how gear oils influence the near-surface microstructure which plays a crucial part in the generation of wear debris. Three industrial gear oils with different base oil-additive combinations were investigated under boundary lubrication sliding conditions. The aim was to understand how these gear oils and their associated tribofilms influence the near-surface metal layer of spheroidised and hardened AISI 52100 steels. Secondly, the study sought to understand how oil temperature and variable contact pressure influences the respective performance of the gear oils. Oil A contains molybdenum dithiophosphate (MoDTP) friction modifier, Oil B contains an amine molybdate complex friction modifier along with zinc dithiophosphate (ZDDP) antiwear additive. Oil C contains an antiwear/extreme pressure phosphonate additive combined with an ashless commercial gear oil package. To achieve this we evaluated the respective frictional performance of the oils by running sliding friction test using HFRR and SRV tribometers, followed by wear measurement on the wear scars generated. The nature of tribofilm formed was evaluated using complementary EDX and Raman spectroscopy techniques. To understand how the gear oils influence the near-surface layer, we measured the hardness-depth profile of the worn surfaces generated using the nanoindentaton technique and the near-surface microstructure was characterised using SEM-FIB. The results showed that the gear oils and the tribofilms they formed influenced the degree of strain-hardening and the extent of grain refinement below the steel surfaces. Significant amount of strain-hardening and grain-refinement in the near-surface of the spheroidised AISI 52100 steel corresponded to higher levels of wear; whereas severe wear of the hardened AISI 52100 steel corresponded to friction-induced softening of the near-surface layer and the formation of adiabatic shear bands. The gear oil formulations and their respective tribofilms also influenced how friction-induced deformation is distributed in the near-surface microstructure. In addition, higher oil temperature and contact pressure increased surface-additive interaction and promoted the formation of tribofilms with better frictional performance. However, these conditions also reduced the oil film thickness, hence promoting near-surface hardening, degradation of the near-surface microstructure and ultimately corresponded to lower wear performance. It is also clear from this thesis that the performance of the gear oils depends on the contact pressure range and tribo-pair material, in particular the starting microstructures and hardness ratio. This reiterated the importance of testing gear oils with operating conditions similar to those of their industrial application.
Date of Award1 Aug 2019
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPhilip Withers (Supervisor) & Allan Matthews (Supervisor)


  • bearing steel
  • Wear
  • Subsurface deformation
  • Mechanical properties
  • Friction
  • Tribology
  • Gear oils
  • Boundary lubrication
  • Surface chemistry

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