Frictional Characteristics of IF-WS2 Nanoparticles in Simulated Engine Conditions

The efficacy of inorganic fullerene-like (IF) tungsten disulfide (WS₂) nanoparticles as engine oil friction modifier additive was investigated using a high-stroke reciprocating piston ring–cylinder bore bench test system. Frictional characteristics of various IF-WS₂ nanoparticle concentrations in mineral oil were experimentally analyzed in simulated engine conditions controlling oil temperature, speed, and normal load. Though the effect of IF-WS₂ nanoparticles on piston ring and cylinder bore friction was minor in low concentrations, after a running period in a mixed lubrication regime with 10% additive, a considerable reduction in friction coefficient was observed when the IF-WS₂ formulated oil was used. This reduction remains to some extent with reference mineral oil after solvent cleaning of the mating components. The results show that a thin tribofilm gradually forms on the piston ring and the cylinder bore surfaces, reducing the friction coefficient in a mixed lubrication regime. However, in order to obtain the friction reduction there seems to be a threshold in the concentration of IF-WS₂ in mineral oil and film development period. The effects of lubrication regime, contact load, speed, and surface roughness on tribofilm formation are also discussed. The presence of WS₂ tribofilm formed on the piston ring and the cylinder bore surfaces was further verified using Raman spectroscopy.