a tip of rotor blade which rotates is an abradable surface . an inner wall of a shroud as a jacket for the rotor blade is an abrasive surface . a part of an abrasive particle protrudes from the abradable surface . when the tip of the rotor blade which rotates contacts the inner wall of the shroud , the protruding section of the abrasive particle slides with the abrasive surface so as to be ground . by doing this , a turbine which can maintain an appropriate clearance between the rotor blade and the shroud and can be used for a long period under high - temperature conditions with easy restoration and remaking thereof .