THE COMPOSITION OF LUBRICANT-DERIVED SURFACE-LAYERS FORMED IN A LUBRICATED CAM TAPPET CONTACT .2. EFFECTS OF ADDING OVERBASED DETERGENT ANDDISPERSANT TO A SIMPLE ZDTP SOLUTION/

The surface films formed by a set of oils of related additive chemistr y, but with differing detergent and dispersant contents, have been che mically characterized using a combination of surface analysis techniqu es. The films were formed in a cam/tappet friction apparatus with a di rect acting bucket tappet geometry. In the absence of overbased deterg ent, the amorphous films were composed essentially of inorganic zinc p hosphates formed by the ZDTP anti-wear additive, and evidence of highe r molecular weight phosphates (e.g. metaphosphates) was found. Adding overbased detergent and dispersant resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight ph osphates in favour of ortho- and pyro-phosphates.