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Overbased detergents are well known in the tribology field as anti-wear additives. In boundary lubrication, they generate a quite thick tribofilm on rubbing surfaces. They were studied by coupling XPS and AES depth profiles with XANES and ToF-SIMS analyses. Under friction, we show by ToF-SIMS analysis that detergent molecules are split into smaller structural units. Moreover, ionic bonds do not resist high pressure and shearing, and sulfur disappears from the contact zone. The overbased calcium carbonate core finally collapses and crystallizes to give a good anti-wear film between rubbing surfaces.