Experimentally derived friction model to evaluate the anti-wear and friction-modifier additives in steel and DLC contacts

•AW and FM additive influence on the boundary lubrication friction has been described.•AW and FM additives are active in both steel-steel and steel-DLC contacts.•AW additive defines general friction trends in both steel-steel and steel-DLC contacts.•Stabilization effect of the FM is shown for both steel-steel and steel-DLC contacts.•Value of the R2 coefficient provides qualitative information about the investigated oil.

Individual and synergistic contributions of Anti-Wear wear (Zinc dialkyldithiophosphate, ZDDP) and Friction-Modifier (Glycerol isostearate, GIS) additives to the friction of steel and DLC contacts across a broad range of boundary-lubrication contact conditions are evaluated. A design-of-experiments was used to enable broad-range experimental matrix and carefully selected regression technique was used to develop friction models and friction maps depending on the most relevant contact parameters (p, v, T, Ra). Apart from many specific conclusions, one of the major conclusions is that statistically identified interactions of the experimental parameters show which of the parameters interfere and compete in their influence on the coefficient of friction, hence, which of the investigated parameters must be simultaneously considered if proper conclusions are to be made.