Second-order normal force and extension brought about by sliding friction of a rubbery polymer

By rheology experiments we show that the torque created by sliding friction between a polyurethane foam planarizing pad and patterned silicon dioxide wafer in contact creates a large second-order tensile normal force previously found only in the torsion of bulk specimens. The normal force is a linear function of the square of the torque. Such tensile force reduces the applied compressive force. With elaborate polishing experiments we found second-order normal extension of the polymer into the valleys of the opposing surface which by interlocking the interface may provide a new mechanism for friction. © 2003 Plenum Publishing Corporation.