It is well known that during sliding friction the properties of rubbing surfaces are changed owing to transformations caused by applied loads, friction and wear processes. In the last years many research works were devoted to reveal the properties of the surfaces transformed during the friction and wear at the Budapest University of Technology and Economics in the Department of Machine Design. During these investigations the conditions of developing beneficial transformed surface layers and their operating boundaries were determined by experiments on metal sliding pairs in laboratory. The conditions of evolving large-scale adhesion junctions, scuffing and seizure were investigated on heavy-loaded surfaces at slow sliding velocity. During the experiments hard steel balls slid on dry or lubricated metal disks. Coefficient of friction and wear tracks were measured and structures and damages of sliding surfaces were evaluated using profilometry, SEM and normal microscope photographs, and micro-hardness measurement.