How does increasing surface roughness affect the friction coefficient between two sliding surfaces?

When two surfaces slide, the roughness of each surface determines how much their microscopic peaks and valleys fit together. As roughness increases, more peaks from one surface lock into valleys of the other, creating more mechanical interlocking. This interlocking raises the resistance to sliding, so the static friction coefficient usually grows with roughness. During sliding, the increased deformation at these contact points can also generate heat, slightly reducing kinetic friction but the overall effect remains an increase. For example, a smooth steel plate on polished steel has a low friction coefficient, while the same steel on a sandblasted plate shows a noticeably higher friction because the rougher surface grips more.