The best-polished metal surface, when examined under electron microscope showed rough surface having peaks and valleys of different heights and depths, is surface as surface roughness. The highest peaks are known as asperities. When two such flat surfaces are placed one over other, the asperities of upper surface rests on those of lower surface. Thus the surfaces come in contact only at these points as shown generally, the real area of contact under is 1/10,000th of the applied load ( due to small surface area) leading to local melting of melts and formation of welded junction such welded junctions lead to wear and tear of the metal parts in motion with respects to each other.
Surface roughness