Atomic force microscopy is used to show that production of surface defects at the interface of rubbing solids is an important mechanism of energy dissipation in friction. Using mica and Si-tips, we demonstrate that defects produced by the rupture of Si-O bonds at the surface, which are not visible in contact mode AFM images, have a noticeable contribution to friction. When defects accumulate beyond a critical concentration, they grow to form visible wear scars similar to2 Angstrom deep at first and deeper holes later. The contribution of defect production to friction is explained by a simple model, which is based on the stress-induced enhancement of the rate of thermal defect production.