the invention disclosed herein may be variously modified and be in alternative forms . specific embodiments therefor have been shown by way of example in the drawings and detailed description . it should be understood , however , that the drawings and detailed description thereof are not intended to limit the invention to the particular form disclosed , but on the contrary , the invention is to cover all modifications , equivalents and alternatives falling within the scope of the present invention as defined by the claims . [ 0037 ] fig1 is a cross section of a turbine according to an embodiment of the present invention . in fig1 a turbine 1 comprises a rotor 10 and a shroud 20 . the rotor 10 comprises a shaft 15 which extends along a rotational axis x - x , a rotor blade unit 11 which extends from the shaft 15 radially , and an abradable layer 12 which is formed on the tip of the rotor blade unit 11 . numerous abrasive particles 13 are fixed on the abradable layer 12 . parts of abrasive particles 13 protrude from the abradable surface 14 as a surface of the abradable layer 12 so as to form a protruding section 13 a . the rotor blade unit 11 has a screw shape along the rotational axis x - x . this screw shape is not shown in the drawing . a shroud 20 is a jacket which surrounds the rotor 10 in an axial manner with the shaft 15 . on an inner wall of the shroud 20 , a tcb 22 is formed having an intermediate layer 21 between the inner wall of the shroud and the tcb 22 . the surface of the tcb 22 acts as an abrasive surface 23 . accordingly , a clearance c - 0 is formed between the abrasive surface 23 and the abradable surface 14 of the rotor blade unit . the abrasive particle 13 is made from alumina particle ( al 2 o 3 ) diameter of the abrasive particle 13 is in a range of 800 μm to 1000 μm . height h of the protruding section 13 a of the abrasive particle 13 is mostly in a range of 30 % to 70 % of the diameter d of the abrasive particle 13 . the abradable layer 12 is an anti - oxidization coating which is mainly made from mcraly alloy . this point is explained in detail later . the abradable layer 12 is united with the rotor blade unit 11 and the abrasive particle 13 very tightly . the tcb 22 which is formed on the shroud 20 is a ceramic member which is stabilized by adding a stabilizing agent such as mgo or y 2 o 3 to zro 2 . thickness of the tcb 22 is almost in a range of 300 μm to 1000 μm . the intermediate layer 21 is formed by performing a plasma - thermal - spraying process of the mcraly alloy ( m is at least one of fe , ni , coni , nico , and co ) into the shroud . also , mcraly can be craly ( in this case , m is nothing ). the turbine 1 has a screw shape on its rotor blade unit 11 ; therefore , the rotor 10 begins rotating in one direction around a rotational axis x - x when a high - pressure - high - temperature fluid is introduced from an opening section of the shroud 20 . when the rotor 10 begins rotating , condition of the clearance which is disposed between the abrasive surface 23 and the abradable surface 14 changes according to factors such as thermal expansion of the rotor blade unit 11 , centrifugal force on the rotor 10 , and vibrations which occur in the entire turbine 1 . for example , when the length of the rotor blade unit 11 increases due to thermal expansion , the clearance becomes smaller than it initial length c - 0 and becomes smaller than a height h of the protruding section 13 a of the abrasive particle . when the rotor blade unit 11 is under such conditions in the initial phase of the rotation , the tip of the protruding section 13 a slides on the abrasive surface 23 and is ground until the sliding friction becomes almost zero as shown in fig2 . as a result , the clearance changes to a smaller value such as c - 1 than the initial length c - 0 . the clearance c - 1 can be maintained by the rest of the protruding section 13 b of the abrasive particle 13 ; therefore , the rotor blade unit and the shroud do not contact unless conditions change , for example , by the increase of temperature . accordingly , the tip of the rotor blade unit and the inner wall of the shroud are not ground nor cohered to each other . when the turbine 1 is operated for a long period such as 1000 hours , thermal transformation occasionally occurs gradually on the rotor blade unit 11 and the shroud 20 because rotor blade unit 11 and the shroud 20 are exposed to the high - temperature - high - pressure fluid . also a stress which further narrows the clearance c - 1 partially is generated occasionally . in such a case , the rest of the tip of the protruding section 13 b of the abrasive particle 13 slides on the abrasive surface 23 of the shroud secondarily so as to be ground again . as a result of this , the clearance becomes further narrower than that of c - 1 . however , the rotor blade unit and the shroud do not contact by setting the rotor blade unit and the shroud such that the protruding section of the abrasive particle 13 does not become zero . therefore , the tip of the rotor blade unit and the inner wall of the shroud are not ground nor cohered to each other . even if the height of the protruding section becomes zero , it was confirmed that the cohesion of the rotor blade unit and the shroud are hardly cohered to each other because the ratio of the abrasive particles on the contacting surface is large . diameter d of the abrasive particle 13 is in a range of 800 μm to 1000 μm . also , the height of the protruding section 13 a is in a range of 30 % to 70 % of the diameter of the abrasive particle ; thus , initial clearance c - 0 is sufficiently large . therefore , it is possible to compensate for abrasion which is caused by the initial sliding and the secondary sliding . also , the rest of the protruding section can obtain an appropriate clearance . also , the fluid does not pass because of the protrusion of the large diameter abrasive particle 13 ; therefore , there is less pressure loss and no operational efficiency loss even if the initial clearance c - 0 is large . the rotor blade unit of the present embodiment can be manufactured by the following manufacturing method . [ 0047 ] fig3 a and 3b are flowcharts showing a manufacturing process of the rotor blade . according to flow chart 3 a , an abradable layer sheet is made as a preparatory process ( 1 ). next , the abradable layer sheet is cohered to the rotor blade unit ( 2 ). in a process ( 3 ), the abradable sheet is heated together with the rotor blade unit so as to form an abradable layer on the rotor blade unit . in a process ( 4 ), a blasting - operation is performed on the abradable layer so as to protrude the abrasive particle . thus , a rotor blade unit is manufactured . also , according to the flow chart shown in fig3 b , in a process ( 1 ), nickel - plating - operation is performed on the abrasive particle so as to be fixed . consequently , as a process ( 2 ), a thermally - sprayed - coating having mcraly as a main ingredient is formed on the abrasive particle . as a process ( 3 ), heating - operation is performed on the thermal - spray - coating . as a process ( 4 ), the rotor blade unit is finished by performing a thermal - spray - process including a blasting operation . in fig1 the tcb 22 which forms an inner wall of the shroud 20 is made from a ceramic member which is made by adding a stabilizer such as mgo or y 2 o 3 to zro 2 . the stabilized ceramic member is formed by performing a plasma - thermal spraying process such that the intermediate layer 21 having a thickness in a range of 300 μm to 1000 μm is formed on the inner wall of the shroud to which mcraly alloy ( m is at least one of fe , ni , coni , nico , and co ) is thermally sprayed . also , mcraly can be craly ( in this case , m is nothing ) in the present embodiment , the inner wall of the shroud is the abrasive surface 23 , and an abradable surface 14 is formed on the tip of the rotor blade unit . it is certainly acceptable for the inner wall of the shroud to be an abradable surface and the tip of the rotor blade unit to be an abrasive surface . also , a turbine in the present embodiment was explained as a gas turbine . however , it is certain that the present invention can be applied to any turbine structures which have a problem in fluctuating clearance between the rotor blade unit and the shroud .