Insights into the tribological behavior of IF-WS2 nanoparticle reinforced mild extreme pressure lubrication for coated chromium/bulk grey cast iron interface

Abstract

This study reports a novel synergetic approach combining nano-lubrication and conventional extreme pressure additivation, to explore the tribological performance of piston ring/cylinder liner conformal contact. Three typical lubricating formulations, viz, (i) PAO (poly-alpha-olefin) base oil (ii) base oil with mild extreme pressure (MEP) additive, and (iii) base oil with composite blend of mild extreme pressure (MEP) additive and WS2 nano-particles, were investigated using an ASTM G-99 Standard customized universal tribometer for evaluating friction and wear behavior of coated chromium/bulk grey cast iron (CI) tribopair. The lubricant formulations were tested at engine working temperature of 200 °C, reciprocated under a normal load of 30 N, stroke length of 10 mm and a constant sliding velocity of 0.2 ms−1. Wear attributes were evaluated by employing non-contact based 3 D optical profilometry. For assessment of tribo-surface morphology and associated surface characterization, Elemental Dispersion Spectral (EDS) incorporated Scanning Electron Microscope (SEM) was utilised. Green Laser Raman spectroscopy investigated tribo-chemistry, synergetic interactions and tribo-film formations. A state approaching super-lubricity and ultralow friction was achieved by sliding grey CI (Plateau Honed) cylinder liner against electroplated chrome stainless steel top compression ring with PAO10 base oil ultrasonically dispersed with 1 vol% of Isopropyl phenyl Phosphate (IPP), a mild extreme pressure (MEP) triaryl phosphate additive & 1 wt.% of WS2. Results obtained therein demonstrated four dominating forms of wear on the cast iron surface, i.e. plowing wear, abrasive wear, adhesive wear and oxidative wear. Counterpart chromium surface exhibited plowing and abrasive wear only. This substantial friction and wear reduction are attributed to the coalesced intrinsic characteristics of MEP and IF-WS2 additives, thus improving the overall lubrication characteristics of base oil.