Smoothing surface roughness by the use of additives is one of the promising approaches to reduce friction loss and wear under boundary lubrication. Effect of phosphorus anti-wear agents on controlling surface roughness is focused in this study. Zinc dithiophosphate (ZnDTP), generally used as an anti-wear agent for engine oil, formed rough surface by forming inhomogeneous phosphorus and sulfur films. In contrast, a particular neutral phosphorus agent that reduced surface roughness and prevented

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... ear was found, which forms homogeneous phosphoric acid film. An acidic phosphorus agent also reduced surface roughness and reduced friction and promote low shear stress molybdenum disulfide film formation with Molybdenum dithiocarbamate (MoDTC), but it was accompanied with a large amount of wear. Use of the neutral and the acid phosphorus agents together realized low friction, reduced wear, and very smooth surface by forming reaction films composed of sulfur and phosphorus type films. The study indicates that forming a homogeneous phosphoric film of low shear strength by the use of specific phosphorous agents is one of a promising approach to establish and sustainable boundary film of low friction and low wear.