Patent Publication Number: US-6337000-B1

Title: Guide bush and method of forming diamond-like carbon film over the guide bush

Description:
This application is a division of prior application Ser. No. 09/299,729 filed Apr. 27, 1999 now U.S. Pat. No. 6,264,209. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structure of a guide bush mounted on a column of an automatic lathe to support a workpiece for rotation and sliding, and a method of forming a diamond-like carbon film over the inner circumference of the guide bush. 
     2. Description of the Related Art 
     A guide bush is a component mounted on a column of an automatic lathe to support a workpiece for rotation and sliding. 
     Further explaining about the guide bush in detail, it is in an approximate cylindrical form having a center bore formed along its center axis, including a taper outer surface formed on one longitudinal end portion and provided with slits to make the end portion elastic, a threaded portion formed on another end portion to be mounted on a column of an automatic lathe, and an inner surface for holding a workpiece inside a portion in which the taper outer surface is formed, mounted on an automatic lathe so as to hold a workpiece inserted into the center bore with the inner surface so that the workpiece can rotate and axially slide at a portion near a cutting tool. 
     Therefore, the inner surface of the guide bush always contacts and rotates with a workpiece, and a workpiece rotates and axially slides on the inner surface. Thus, the inner surface of the guide bush supporting a workpiece can be abraded easily. 
     So, a guide bush is proposed in JP-A No. 4-141303 which has an inner surface, which is in contact with a workpiece because of rotation and sliding, coated with a cemented carbide alloy or a ceramic. 
     When the inner circumference of a guide bush is provided with a cemented carbide alloy or a ceramic excellent in abrasion resistance and heat resistance, the abrasion of the inner surface of the guide bush can be reduced to some extent. 
     However, even when the guide bush is used on an automatic lathe, there was problems because of a large coefficient of friction and a low thermal conductivity of the cemented carbide alloy and the ceramic, that the workpiece is damaged or seizing occurs due to decrease in the diametrical clearance between the guide bush and the workpiece in case of heavy machining in which the depth of cut is large and the cutting speed is high. 
     Thus, it is proposed that the cemented carbide alloy provided on the inner surface of the guide bush is covered with a diamond-like carbon film to reduce abrasion of the inner surface. 
     The diamond-like carbon film is a black coating, and is a hard carbon film having properties close to those of diamond. The diamond-like carbon film has excellent properties of high mechanical hardness, a low coefficient of friction, good electric insulation, and high corrosion-resistance. 
     So, it enables the guide bush to improve abrasion resistance dramatically and to avoid seizing that the surface of the cemented carbide alloy on the inner surface of the guide bush is covered with the diamond-like carbon film. 
     Incidentally, the diamond-like carbon film is a hydrogenated amorphus carbon film and generally called diamond-like carbon film (abbreviated as “DLC film”) because of above-described diamond-like properties, and also called i-carbon film. 
     DLC Film Forming Method on a Guide Bush According to a Conventional Art: FIG. 16 
     A method of forming DLC film over an inner surface of a guide bush, according to the conventional art will be described referring to FIG. 16 hereinafter. 
     As shown in FIG. 16, a guide bush  11  on which DLC film will be formed is mounted in a vacuum vessel  61  which comprises of a gas inlet port  63  and a gas outlet port  65 . The guide bush  11  is quenched and tempered after an outer form and an inner form thereof are made up with an alloy tool steel (SK steel). And a cemented carbide alloy is provided on the inner surface  11   b  of the guide bush  11 . 
     A DC voltage is applied to the guide bush  11  by a DC power source  73 , a positive DC voltage is applied to the anode  79  disposed in the vacuum vessel  61  opposite the guide bush  11  by an anode power source  75 , and an AC voltage is applied to the filament  81  by a filament power source  77 . 
     Further, the vacuum vessel  61  is evacuated by an evacuating means which is not shown through the gas outlet port  65 , and gas containing carbon is introduced through the gas inlet port  63 . Thus, plasma is produced in the vacuum vessel  61  to form DLC film on the surface of the guide bush  11  containing the inner surface  11   b  thereof. 
     In the case of the method of forming DLC film with the unit shown in FIG. 16, plasma caused by the DC voltage applied to the guide bush  11 , and plasma caused by the filament  81  to which the AC voltage is applied and the anode  79  to which the DC voltage is applied, is produced. 
     Then, according to the pressure in the vacuum vessel  61  at the time the DLC film is formed, either the plasma around the guide bush  11  or the plasma beside the filament  81  and the anode  79  becomes a main source of forming DLC film. 
     In the case of the above-described conventional DLC film forming method, the plasma which is produced in a portion around the guide bush  11  becomes a main source of decomposing the gas containing carbon and forming DLC film when the pressure in the vacuum vessel  61  is above 3×10 −3  torr. 
     Then, it is possible to form DLC film on the outer surface of the guide bush  11 , but the DLC film formed on the inner surface of the center bore  11   j  is poor at adhesion, and properties thereof, hardness or the like, are also poor. 
     This is caused by that the same electrical potential is applied to the guide bush  11 , the inside of the center bore  11   j  becomes a space where electrodes having the same potential are opposite to each other, and the plasma in the center bore  11   j  causes abnormal discharge called hollow discharge. 
     The DLC film formed by this hollow discharge is polymer-like and poor at adhesion. Thus it flakes off easily from the inner surface  11   b  of the guide bush  11 , and the hardness thereof is low. 
     On the other hand, the plasma produced beside the filament  81  and the anode  79  contributes to the forming of DLC film more than the plasma around the guide bush  11  when the pressure in the vacuum vessel  61  is below 3×10 −3  torr. 
     In this case, it is possible to form DLC film uniformly on the outer surface of the guide bush  11 , but it is not possible in the direction of the length on the inner surface of the center bore  11   j  of the guide bush  11 . 
     Here, carbon ions ionized by the plasma produced beside the filament  81  and the anode  79  pile up pulled by the negative DC potential applied to the guide bush  11 , and form DLC film. 
     DLC film is formed by chemical vapor deposition process when the above-described pressure in the vacuum vessel  61  is above 3×10 −3  torr. But it is formed by physical vapor deposition process when the pressure is below 3×10 −3  torr. 
     Thus, when DLC film is formed under the contribution of the plasma produced beside the filament  81  and the anode  79 , the thickness of the DCL film on the inner surface of the center bore  11   j  of the guide bush  11  decreases with the distance from the open end. 
     As a result, it is impossible to form DLC film in uniform thickness in the direction of the length of the guide bush  11  on the inner surface  11   b  thereof. 
     Further, in the case of the guide bush on which the above-described conventional DLC film is formed, a cemented carbide alloy made of alloy tool steel (SK steel) is provided on the inner surface of the guide bush base, and DLC film is formed on the surface of the cemented carbide alloy. G-2 (Japanese Industrial Standards: JIS) is frequently used as the cemented carbide alloy. 
     Generally used chemical composition of the G-2 (JIS) is 87-90% of tungsten (W), 5-7% of carbon (C) and 5-7% of cobalt (Co) as a binder. 
     When DLC film is directly formed on the cemented carbide alloy of this composition, the DLC film can not be formed with good adhesion, and the problem that it easily peels off occurs. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a guide bush having excellent durability, and to provide a method of forming a DLC film whereby the DLC film can be formed with good adhesiveness to a uniform thickness over a cemented carbide alloy, provided on the inner surface of the guide bush, for holding a workpiece, by solving such problems as described in the foregoing. 
     With the guide bush according to the invention, the inner surface thereof described above for holding the workpiece rotatably and slidably is provided with the cemented carbide alloy containing at least cobalt, and the DLC film is formed over the cemented carbide alloy. And in order to achieve the object described above, the invention provides the guide bush wherein cobalt contained in the surface of the cemented carbide alloy, adjoining the DLC film, has been removed. 
     Further, marks caused by a grinding process applied are preferably removed from the surface of the cemented carbide alloy, adjoining the DLC film. And the surface of the cemented carbide alloy, adjoining the DLC film, is preferably not specular, but rough. 
     The invention also provides a guide bush wherein a DLC film is formed on the surface of the cemented carbide alloy described above, from which cobalt contained has been removed, with an intermediate layer interposed therebetween. 
     The intermediate layer can be formed of an alloy containing carbon as a constituent material thereof. 
     The intermediate layer can also be made up of a plurality of films, each made of a different material. 
     The intermediate layer is preferably made up of a first intermediate layer made of titanium or chromium, formed directly over the cemented carbide alloy, and a second intermediate layer made of silicon or germanium, formed over the first intermediate layer. 
     A method of forming the DLC film in a guide bush according to the invention comprises the following steps: 
     (1) a step of removing cobalt contained in the surface of the cemented carbide alloy by etching; 
     (2) a step of placing the guide bush in a vacuum vessel with an electrode inserted inside the center bore of the guide bush; and 
     (3) a step of forming the DLC film on the surface of the cemented carbide alloy formed on the inner surface of the guide bush by evacuating the vacuum vessel before introducing a gas containing carbon therein, and by causing a plasma to be generated inside the center bore of the guide bush by applying a DC voltage or RF electric power to the guide bush while keeping the electrode grounded. 
     A method of forming the DLC film over a guide bush according to the invention may also comprise the following steps: 
     (1) a step of removing cobalt contained in the surface of the cemented carbide alloy by etching; 
     (2) a step of forming an intermediate layer made of a constituent material of a target on the surface of the cemented carbide alloy by means of a sputtering method after placing the guide bush and the target in a vacuum vessel such that the center bore of the guide bush is disposed opposite to the target and evacuating the vacuum vessel before introducing argon gas thereinto, 
     (3) a step of disposing the guide bush in the vacuum vessel with the intermediate layer formed on the guide bush such that an electrode is inserted inside the center bore thereof; and 
     (4) a step of forming the DLC film on the surface of the intermediate layer formed on the inner surface of the guide bush by evacuating the vacuum vessel before introducing a gas containing carbon therein, and causing a plasma to be generated inside the center bore by applying a DC voltage or RF electric power to the guide bush while keeping the electrode grounded. 
     In the step of forming the DLC film as described above, instead of grounding the electrode, a positive DC voltage may be applied thereto. 
     In the step of removing cobalt contained in the surface of the cemented carbide alloy by etching as described above, cobalt is preferably removed by wet etching. 
     The wet etching can be executed in a blended solution of sodium hydroxide and aqueous hydrogen peroxide. 
     In the step of forming the intermediate layer as described above, an alloy composed of tungsten and carbon mixed together at a ratio of 1 to 1 can be used as a constituent material for the target, thereby forming the intermediate layer made up of a tungsten carbide film on the surface of the cemented carbide alloy. 
     In the step of forming the intermediate layer as described above, a first intermediate layer made up of a titanium film or chromium film may be formed on the surface of the cemented carbide alloy by using titanium or chromium as a target before forming a second intermediate layer made up of a silicon film or a germanium film over the first intermediate layer by replacing the target with silicon or germanium. 
     In the step of forming the DLC film on the surface of the cemented carbide alloy or the surface of the intermediate layer, the gas containing carbon, introduced in the vacuum vessel, is preferably benzene or methane. 
     The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged view showing a film makeup in the vicinity of the inner surface of a guide bush according to a first embodiment of the invention; 
     FIG. 2 is a perspective view showing an example of the external appearance of the guide bush according to the invention; 
     FIG. 3 is a longitudinal sectional view of the guide bush shown in FIG. 2; 
     FIG. 4 is an enlarged view showing a film makeup in the vicinity of the inner surface of a guide bush according to a second embodiment of the invention; 
     FIG. 5 is an enlarged view showing a film makeup in the vicinity of the inner surface of a guide bush according to a third embodiment of the invention; 
     FIG. 6 is a schematic sectional view for illustrating an embodiment of a method of forming a DLC film in a guide bush according to the invention, showing an example of steps thereof; 
     FIG. 7 is a schematic sectional view similar to FIG. 6, showing another example of the steps thereof; 
     FIG. 8 is a schematic sectional view similar to FIG. 6, showing still another example of the steps thereof; 
     FIG. 9 is a schematic sectional view for illustrating the embodiment of the method of forming the DLC film in the guide bush according to the invention, showing a partially modified example of the step in FIG. 6; 
     FIG. 10 is a schematic sectional view similar to FIG. 9, showing a partially modified example of the step in FIG. 7; 
     FIG. 11 is a schematic sectional view similar to FIG. 9, showing a partially modified example of the step in FIG. 8; 
     FIG. 12 is a graph showing a correlation between voltage applied to an electrode inserted in the center bore of the guide bush and thickness of the DLC film formed in carrying out the method of forming the DLC film in the guide bush according to the invention as shown in FIGS. 9 to  11 ; 
     FIG. 13 is a schematic sectional view for illustrating the embodiment of the method of forming the DLC film in the guide bush according to the invention, showing an example of steps of forming an intermediate layer; 
     FIG. 14 is a sectional view showing the main parts of an automatic lathe wherein a guide bush unit incorporating the guide bush according to the invention is mounted; 
     FIG. 15 is a sectional view showing the main parts of another type of automatic lathe wherein a guide bush unit incorporating the guide bush according to the invention is mounted; and 
     FIG. 16 is a schematic sectional view for illustrating a conventional method of forming a DLC film in a guide bush. 
    
    
     DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS 
     Structures of guide bushes according to the present invention and preferred embodiments of diamond-like carbon film forming method on the guide bush will be concretely described referring to figures, hereinafter. 
     Construction of an Automatic Lathe: FIG. 14 
     First, construction of a guide bush unit according to the guide bush in the present invention and construction of a numerically controlled automatic lathe employing the guide bush unit is described referring to FIG. 14 which shows neighborhood of a spindle thereof. 
     FIG. 14 is a sectional view of a stationary guide bush unit which is operated in the way that the guide bush in the present invention is fixed with it as a guide bush unit and a workpiece is rotated on an inner surface of the guide bush. 
     A spindle stock  17  is mounted on the not shown bed of the numerically controlled automatic lathe for sliding movement in transverse directions, as viewed in FIG. 14. A spindle  19  is supported for rotation in bearings  21  on the spindle stock  17 . 
     And a collet chuck  13  is mounted on the nose of the spindle  19 . 
     The collet chuck  13  is disposed in the center bore of a chucked sleeve  41 . A taper outer surface  13   a  of the collet chuck  13  and a taper inner surface  41   a  of the chucked sleeve  41  are in surface contact with each other. 
     Further, a coil spring  25  is inserted at the back end of the collet chuck  13  in an intermediate sleeve  29 . 
     The collet chuck  13  can be pushed out of the intermediate sleeve  29  by the action of the elastic force of the coil spring  25 . 
     The front end of the collet chuck  13  is in contact with a cap nut  27  fastened to the front end of the spindle  19  with screws and the position thereof is determined by that. 
     The cap nut  27  is provided to restrain the collect chuck  13  from being pushed out of the intermediate sleeve  29  by the force of the coil spring  25 . 
     A chuck operating mechanism  31  is provided on the back end of the intermediate sleeve  29  through the intermediate sleeve  29 . 
     The chuck operating picks  33  are operated to open or close the collet chuck  13  so that the collect chuck  13  can chuck or release the workpiece  51  shown with an imaginary line. 
     That is to say, when the chuck operating picks  33  of the chuck operating mechanism  31  are turned so that the front ends thereof are moved away from each other, operating portions of the chuck operating picks  33  in contact with the intermediate sleeve  29  move to the left as viewed in FIG. 14 to push the intermediate sleeve  29  to the left. 
     Consequently, the chucking sleeve  41  in contact with the left end of the intermediate sleeve  29  moves to the left. 
     The collet chuck  13  is restrained from being pushed out of the spindle  19  by the cap nut  27  fastened to the front end of the spindle  19  with screws. 
     Therefore, when the chucking sleeve  41  is moved to the left, the taper outer surface  13   a  of the collet chuck  13  and the taper inner surface  41   a  of the chucking sleeve  41  are strongly pressed and move along the taper surfaces. Consequently, the inside diameter of the collet chuck  13  is reduced to grip the workpiece  51 . 
     When releasing the workpiece  51  from the collet chuck  13  by expanding the collet chuck  13  by increasing the inside diameter of the collet chuck  13 , the chuck operating picks  33  are turned so that the front ends thereof are moved toward each other to remove the force acting to the left on the chucking sleeve  41 . Then, the intermediate sleeve  29  and the chucking sleeve  41  are moved to the right as viewed in FIG. 14 by the stored energy of the coil spring  25 . 
     Consequently, the pressure applied to the taper outer surface  13   a  of the collet chuck  13  and the taper inner surface  41   a  of the chucking sleeve  41  is removed to allow the collet chuck  13  to expand by its own resilience, so that the inside diameter of the collet chuck  13  increases to release the workpiece  51 . 
     A stationary guide bush unit  37  placed on a spindle center axis of a column  35  is provided in front of the spindle stock  17 . 
     The guide bush unit  37  shown in FIG. 14 is a stationary type fixedly holding the guide bush  11  to support the workpiece  51  rotatably on the inner surface  11   b  of the guide bush  11 , and a bush sleeve  23  is placed in the center bore of a holder  39  fixed to the column  35 . A taper inner surface  23   a  is formed in the front portion of the bush sleeve  23 . 
     The guide bush  11  having a front portion provided with a taper outer surface  11   a  is placed in the center bore of the bush sleeve  23 . 
     The distance between the inner surface of the guide bush  11  and the outer surface of the workpiece  51  can be adjusted by turning an adjusting nut  43  screwed on the back end portion of the guide bush unit  37 . 
     This is because when the adjusting nut  43  is turned, the inner surface  23   a  of the bush sleeve  23  and the taper outer surface  11   a  of the guide bush  11 , similarly to the collet chuck  13 , is compressed with each other, and the inside diameter of the guide bush  11  is reduced. 
     A cutting tool  45  is disposed in further front of the guide bush unit  37 . The workpiece  51  is chucked by the collet chuck  13  mounted on the spindle  19  and supported by the guide bush unit  37 . Further, a portion of the workpiece  51  projecting from the guide bush unit  37  into a machining region is machined for predetermined machining by a combined motion of the cross feed motion of the cutting tool  45  and the longitudinal traverse motion of the spindle stock  17 . 
     Construction of a Rotary Guide Bush Unit: FIG. 15 
     A rotary guide bush unit that supports rotatably a guide bush gripping a workpiece will be described with reference to FIG.  15 . 
     FIG. 15 is a sectional view of a rotary guide bush unit, and the parts like or corresponding to those shown in FIG. 14 are designated by the same reference characters. 
     Rotary guide bush units are classified into those holding a guide bush  11  so as to rotate in synchronism with the collet chuck  13  and those holding a guide bush  11  so as to rotate in asynchronism with the collet chuck  13 . 
     A guide bush unit  37  shown in FIG. 15 is a rotary guide bush unit which holds the guide bush  11  so as to rotate in synchronism with the collect chuck  13 . 
     The rotary guide bush unit  37  is driven by a drive rod  47  projecting from the cap nut  27  mounted on the spindle  19 . 
     A gear mechanism or a belt-and-pulley mechanism may be used instead of the drive rod  47  for driving the guide bush unit  37 . 
     The rotary guide bush unit  37  has a bush sleeve  23  in a rotatable manner in a center bore of a holder  39  fixed to a column  35 , through bearings  21 . And the guide bush  11  is disposed in the center bore of the bush sleeve  23 . 
     The bush sleeve  23  and the guide bush  11  are similar in construction to those illustrated in FIG. 14, respectively. 
     The distance between the inner surface of the guide bush  11  and the outer surface of the not shown workpiece  51  can be adjusted by turning an adjusting nut  43  screwed on the back end portion of the guide bush unit  37  to decrease the inner diameter of the guide bush  11 . 
     The automatic lathe illustrated in FIG. 15 is the same in construction as the automatic lathe illustrated in FIG. 14 except that this automatic lathe is provided with the rotary guide bush unit  37 , and hence the further description thereof will be omitted. 
     As is clear from above description, a stationary guide bush unit has almost the same structure and operates almost in the same way as a rotate guide bush unit. 
     Preferred Embodiments of the Guide Bush in Accordance with the Present Invention: FIG. 1 to FIG. 5 
     Guide bushes in preferred embodiments according to the present invention will be described hereinafter referring to FIG. 1 to FIG.  5 . 
     FIG. 2 is a diagonal view and FIG. 3 is a sectional view along the direction of the length, respectively, of a guide bush in a preferred embodiment according to the present invention, and a rod-like workpiece  51  is shown with an imaginary line. 
     Referring to these Figs., a guide bush  11  is shown in a free state in which it is opened. The guide bush  11  is in an approximate cylindrical form having a center bore  11   j  formed along its center axis, including a taper outer surface  11   a  formed on one longitudinal end portion and a threaded portion  11   f  formed on another end portion to be mounted on a column of an automatic lathe. 
     And an inner surface  11   b  is formed inside the portion where outer taper surface  11   a  is formed, to hold a workpiece  51 , and the center bore  11   j  except the inner surface  11   b  is a stepped center bore  11   g  with a larger inner diameter than that of the inner surface  11   b.    
     Three slots  11   c  are formed from the outer taper surface  11   a  to an elastic bendable portion lid on the guide bush  11  at angular intervals of 120° to make the guide bush flexible. 
     The distance between the inner surface  11   b  and the workpiece  51  can be adjusted by pressing the taper outer surface  11   a  of the guide bush  11  to the taper inner surface of the bush sleeve, so that the elastic bendable portion  11   d  is bent. 
     The guide bush  11  has a fitting portion  11   e  between the elastic bendable portion  11   d  and the threaded portion  11   f . And the fitting portion  11   e  sets the guide bush  11  with its axis in alignment with the center axis of the spindle. 
     The guide bush  11  is made of an alloy tool steel (SKS steel). The guide bush  11  is quenched and tempered after an outer form and an inner form thereof are made up. 
     Further, cemented carbide alloy with 2 mm to 5 mm thickness is attached to the inner surface  11   b  of the guide bush  11  by brazing to be the cemented carbide alloy  12  shown in FIG.  3 . G-2 (JIS) is used as the the cemented carbide alloy  12 . 
     Chemical composition of the G-2 (JIS) is 87-90% of tungsten (W), 5-7% of carbon (C) and 5-7% of cobalt (Co) as a binder. 
     A distance in the range of 5 to 10 μm is formed between the inner surface  11   b  and the workpiece  51  when the guide bush  11  is opened. So it is a problem that the inner surface  11   b  of the guide bush  11  abrades when the workpiece  51  slides. 
     Furthermore, the workpiece  51  supported on the guide bush  11  rotates at a high surface speed relative to the inner surface  11   b  and, when an excessively high pressure is applied to the inner surface  11   b  by the workpiece  51 , seizing may occur. 
     Accordingly, as shown in FIG. 3, the guide bush  11  according to the invention is provided with a diamond-like carbon (DLC) film  15  formed on the surface (the inner surface) of the cemented carbide alloy  12  fixedly attached to the inner surface of the guide bush  11 , for holding the workpiece  51 . 
     FIG. 1 is an enlarged view of a portion enclosed by the circle A in FIG. 3, showing the structure of the guide bush  11  according to a first embodiment of the invention, in the vicinity of the inner surface  11   b.    
     In actual fact, the cemented carbide alloy  12  is not less than 1000 times thicker than the DLC film  15 , however, in FIG. 1 for convinience of illustration, the cemented carbide alloy  12  is illustrated extremely thinner than it actually is. 
     As shown in FIG. 1, with the first embodiment of the guide bush according to the invention, the cemented carbide alloy  12  is fixedly attached to the inner surface  11   b  of the guide bush  11  (base member), coming in contact with the workpiece  51  (refer to FIG.  3 ), and further, the DLC film  15  is formed directly on the surface of the cemented carbide alloy  12 . 
     Cobalt (Co) serving as a binder among constituent materials of the cemented carbide alloy  12  is selectively removed from the surface of the cemented carbide alloy  12 , adjoining the DLC film  15 . That is, the DLC film  15  is formed on the surface of the cemented carbide alloy  12 , from which cobalt (Co) is removed. 
     The inventors have discovered through tests that in the case of a conventional guide bush, poor adhesiveness of the DLC film against the cemented carbide alloy is attributable to the presence of cobalt serving as a binder when forming the cemented carbide alloy by means of a sintering process. 
     More specifically, results of the tests have shown that cobalt contained in the cemented carbide alloy was completely dissolved into carbon contained in the DLC film, and this caused deterioration in adhesiveness of the DLC film, resulting in exfoliation thereof. 
     In view of this fact, with the guide bush  11  according to the invention, cobalt having adverse effects on the adhesiveness of the surface of the cemented carbide alloy  12 , adjoining the DLC film  15 , is removed. 
     As a result, with the guide bush  11 , there will arise no problem of the DLC film  15  peeling off because the DLC film  15  can be formed over the cemented carbide alloy  12  with excellent adhesion strength. In consequence, it is possible to extend the service life of the guide bush much longer than that of the conventional guide bush, thereby enhancing reliability thereof for use over the long term. 
     Further, observations were made on the conditions of the surface of the cemented carbide alloy  12 , adjoining the DLC film  15 , before and after cobalt was removed therefrom, respectively, by use of a scanning electron microscope (SEM). 
     Results showed that marks caused by a grinding process applied to the inner surface of the cemented carbide alloy  12  were observed on the surface of the cemented carbide alloy  12  before cobalt was removed therefrom. 
     On the other hand, no mark caused by the grinding process applied was observed on the surface of the cemented carbide alloy  12  after cobalt was removed therefrom. 
     Observations with the naked eye on the surface of the cemented carbide alloy  12  also indicated an obvious difference in the surface condition. 
     That is, while the surface of the cemented carbide alloy  12  before cobalt was removed was found to be a polished surface and in a specular condition, the surface of the cemented carbide alloy  12  after cobalt was removed was found not to be in a specular condition but was in a rough condition. 
     This is due to a fact that when cobalt serving as a binder is removed from the surface of the cemented carbide alloy  12 , adjoining the DLC film  15 , tungsten and carbon which are bonded with the binder through sintering treatment also peel off, turning the surface thereof to a rough finish without leaving any mark caused by the grinding process as described in the foregoing. 
     Thus, the adhesiveness of the DLC film  15  is further improved due to the fact that the marks caused by the grinding process on the surface of the cemented carbide alloy  12 , adjoining the DLC film  15 , have disappeared not to be observed, and the surface thereof has turned to a rough finish. 
     Particularly, with respect to the marks caused by the grinding process, it is considered that if the DLC film  15  is formed on spots where the marks are present, membrane stress due to elongation or contraction is developed at the spots, thereby causing poor adhesiveness of the DLC film  15 . Therefore, the elimination of the marks can prevent a phenomenon of the membrane stress causing deterioration in the adhesiveness from occurring. 
     Consequently, with the guide bush  11  according to the invention, a phenomenon of the DLC film  15  peeling off does not occur, and it is possible to extend the service life of the guide bush much longer than that of the conventional guide bush, thereby enhancing reliability thereof for use over the long term. 
     The DLC film  15  is a hard carbon film in a blackish color, and has properties very similar to those of diamond as described in the foregoing. That is, the DLC film  15  has high mechanical hardness, low friction coefficient, good electric insulation, high thermal conductivity, and excellent corrosion resistance. 
     Accordingly, with the guide bush  11  according to the invention, it is possible to inhibit wear and tear on the inner surface thereof, coming in contact with a workpiece, even in the case of a heavy cutting operation for executing deep cut at a high cutting speed, preventing surface flaws from occurring to the workpiece, and also to inhibit seizure from occurring between the guide bush  11  and the workpiece. 
     Further, the guide bush  11  is provided with the DLC film  15  on the inner surface lib thereof with the cemented carbide alloy  12  having high hardness and interposed therebetween. 
     If the cemented carbide alloy  12  having high hardness is provided underneath the DLC film  15  as described above, this makes it possible to inhibit localized exfoliation and wear and tear of the DLC film  15 , thereby enhancing reliability of the guide bush  11  for use over the long term. 
     An appropriate thickness of the DLC film  15  provided on the inner surface  11   b  of the guide bush  11  ranges from 1 to 5 μm. 
     Now, second and third embodiments of a guide bush according to the invention are described hereinafter with reference to FIGS. 4 and 5, respectively. 
     FIGS. 4 and 5 are similar to FIG. 1, showing the second and third embodiments of the guide bush according to the invention, respectively. That is, FIGS. 4 and 5 are enlarged views of the portion enclosed by the circle A in FIG. 3, showing the structure of the guide bush  11 , in the vicinity of the inner surface  11   b  thereof. In these figures as well, a cemented carbide alloy  12  is in fact by far thicker than a DLC film  15 , however, the cemented carbide alloy  12  is illustrated to be much thinner than it actually is for convenience of illustration. 
     Alloy tool steel (SKS) is also used for the base member of the guide bush  11 , and after forming an outer shape as well as an inner shape thereof, quenching and tempering treatments are applied thereto. Further, the cemented carbide alloy  12  is formed by fixedly attaching a cemented carbide alloy having a thickness in the range of 2 to 5 mm to the inner surface  11   b  of the guide bush  11  by means of brazing. 
     The cemented carbide alloy used is composed of 83-89% of tungsten (W), 5-8% of cobalt (Co) as a binder, and the balance of carbon (C). 
     With the second embodiment of the invention shown in FIG. 4, the cemented carbide alloy  12  is fixedly attached to the inner surface  11   b  of the guide bush  11 , an intermediate layer  14  is provided on the cemented carbide alloy  12 , and the DLC film  15  is formed on the intermediate layer  14 . 
     And cobalt (Co) serving as a binder of the cemented carbide alloy  12  is selectively removed from the surface of the cemented carbide alloy  12 , adjoining the intermediate layer  14 . That is, the DLC film  15  is formed on the surface of the cemented carbide alloy  12 , where cobalt (Co) contained therein has been removed, with the intermediate layer  14  interposed therebetween. 
     As a result, the DLC film  15  of the guide bush  11  can be formed with excellent adhesiveness on the inner surface  11   b  of the base member for the guide bush  11 , thus preventing a problem of exfoliation from arising. 
     Consequently, it has become possible to extend the service life of the guide bush much longer than that of the conventional guide bush, so that reliability thereof for use over the long term can be enhanced. 
     Excellent features of the guide bush  11  according to this embodiment as described in the foregoing are attributable to the following reasons. 
     Observations were made on the condition of the surface of the cemented carbide alloy  12 , adjoining the intermediate layer  14 , before and after cobalt was removed therefrom, respectively, by use of a scanning electron microscope (SEM). 
     Results showed that marks caused by a grinding process applied to the inner surface of the cemented carbide alloy  12  were observed on the surface of the cemented carbide alloy  12  before cobalt was removed therefrom. On the other hand, no mark caused by the grinding process applied was observed on the surface of the cemented carbide alloy  12  after cobalt was removed therefrom. 
     Observations with naked eyes on the surface of the cemented carbide alloy  12  also indicated an obvious difference in the surface condition between before and after cobalt was removed. 
     That is, while the surface of the cemented carbide alloy  12  before cobalt was removed was found to be a polished surface and in a specular condition, the surface of the cemented carbide alloy  12  after cobalt was removed was found not to be in a specular condition but was in a rough condition. 
     This is due to a fact that when cobalt is removed from the surface of the cemented carbide alloy  12 , adjoining the intermediate layer  14 , tungsten and carbon which are bonded with a binder through sintering treatment also peel off since the cobalt is serving as the binder, turning the surface of the cemented carbide alloy  12  to rough finish without leaving any mark caused by the grinding process as described in the foregoing. 
     Thus, the adhesiveness of the intermediate layer  14  is improved due to a fact that the marks caused by the grinding process on the surface of the cemented carbide alloy  12 , adjoining the intermediate layer  14 , have disappeared not to be observed, and the surface thereof has turned to rough finish. 
     Particularly, with respect to the marks caused by the grinding process, it is considered that if the intermediate layer  14  is formed on spots where the marks are present, membrane stress due to elongation or contraction is considered to be developed at the spots, causing poor adhesiveness of the intermediate layer  14 . Therefore, the elimination of the marks can prevent a phenomenon of the membrane stress causing deterioration in the adhesiveness from occurring. 
     As a result, with the guide bush  11  according to this embodiment of the invention, the phenomenon of the intermediate layer  14  peeling off does not occur. Consequently, the adhesiveness of the DLC film  15  formed on top of the intermediate layer  14  is also improved, so that it becomes possible to extend the service life of the guide bush  11  much longer than that of the conventional guide bush, thereby enhancing reliability of the guide bush  11  for use over the long term. 
     When the intermediate layer  14  is made up of a single layer film as with the case of the second embodiment, it is preferable that same is made up of a film made of an alloy containing carbon such as a silicon carbide (SiC) film or tungsten carbide (WC) film. 
     Thus by interposing the intermediate layer  14  between the cemented carbide alloy  12  and the DLC film  15 , the adhesiveness of the DLC film  15  is enhanced. 
     Close observation made on spots of the conventional guide bush where the intermediate layer or the DLC film was exfoliated indicated that exfoliation occurred at spots where the intermediate layer or the DLC film could not attain good adhesion with cobalt as a binder contained in the cemented carbide alloy. The causes of such a phenomenon are not well known, however, it is presumed as one of the conceivable causes that cobalt underwent oxidation and an oxide film thereof having poor adhesiveness was formed, thereby causing the DLC film or the intermediate layer to peel off from the inner surface of the guide bush. 
     With the guide bush  11  according to the invention, therefore, cobalt as a binder contained in the cemented carbide alloy  12  is removed from the inner surface of the cemented carbide alloy  12 , on which the intermediate layer  14  is formed. Thus, the removal of cobalt causing poor adhesion of the intermediate layer  14  from the inner surface of the cemented carbide alloy  12  enables the intermediate layer  14  and the DLC film  15  to be formed on the inner surface  11   b  of the guide bush  11  with a high adhesion strength maintained, and consequently, exfoliation of the DLC film  15  from the inner surface  11   b  of the guide bush  11  does not occur. 
     The DLC film  15  is formed on the inner surface  11   b  of the guide bush  11  to a thickness in the order ranging from 1 to 10 μm, and the intermediate layer  14  to a thickness in the order of 0.5 μm. 
     Next, as shown in FIG. 5, with the third embodiment of the invention, a cemented carbide alloy  12  is fixedly attached to the inner surface  11   b  of a guide bush  11 , a first intermediate layer  14   a  and a second intermediate layer  14   b , one over the other, are formed on the cemented carbide alloy  12 , and further, a DLC film  15  is formed on the second intermediate layer  14   b . The DLC film  15  is formed to a thickness in the range of 1 to 10 μm. 
     As is the case with the second embodiment, cobalt serving as a binder of the cemented carbide alloy  12  is selectively removed from the surface of the cemented carbide alloy  12 , adjoining the first intermediate layer  14   a.    
     The first intermediate layer  14   a  is made up of a titanium (Ti) film or a chromium (Cr) film, and the second intermediate layer  14   b  is made up of a silicon (Si) film or a germanium (Ge) film. The first and second intermediate layers  14   a ,  14   b  are formed to a thickness in the order of 0.5 μm, respectively. 
     Since the first and second intermediate layers  14   a ,  14   b  are formed by the sputtering method, these are made up of a film with gradient thickness, respectively, such that the film is thicker at the open end of the guide bush  11 , and becomes thinner inwardly away from the open end thereof. 
     The guide bush  11  provided with the first intermediate layer  14   a  and the second intermediate layer  14   b  which are interposed between the cemented carbide alloy  12  and the DLC film  15  has the following features. 
     That is, as the first intermediate layer  14   a  made of titanium or chromium has an excellent adhesive strength with the cemented carbide alloy  12 , adhesiveness therebetween is reinforced, and further, the presence of the first intermediate layer  14   a  is able to inhibit cobalt as a binder contained in the cemented carbide alloy  12  from reacting with carbon of the DLC film  15 . 
     Furthermore, as the second intermediate layer  14   b  made of silicon or germanium, both being elements belonging to the IV group on the periodic table, has excellent adhesiveness with the DLC film  15  made of carbon belonging to the same IV group on the periodic table, strong adhesion is achieved through covalent bond therebetween. 
     As a result, the DLC film  15  can be formed with excellent adhesiveness on the inner surface  11   b  of the guide bush  11 . 
     Meanwhile, for the second intermediate layer  14   b , a film made of an alloy containing carbon such as a silicon carbide (SiC) film or a tungsten carbide (WC) film may be used instead of a silicon (Si) film or a germanium (Ge) film. 
     Since the silicon carbide (SiC) film or the tungsten carbide (WC) film has substantially the same function as that of the silicon (Si) film or the germanium (Ge) film, the silicon carbide (SiC) film or the tungsten carbide (WC) film, when used for the second intermediate layer  14   b , has substantially the same advantageous effect as that of the silicon (Si) film or the germanium (Ge) film. 
     Preferred Embodiments of a Method of Forming a DLC Film According to the Invention: FIGS. 1 to  5  and FIGS. 6 to  13   
     Next, preferred embodiments of a method of forming a diamond-like carbon (DLC) film in a guide bush according to the invention are described hereinafter. 
     First, a method of forming a DLC film in a guide bush for fabricating a film structure of the guide bush according to the first embodiment of the invention as described with reference to FIGS. 1 through 3 is described hereinafter. 
     The method of forming the DLC film is the method of forming the DLC film  15  on the inner surface  11   b  of the guide bush  11 , for holding the workpiece  51  as shown in FIG. 3, provided with the cemented carbide alloy  12  containing at least cobalt, comprises the following steps: 
     (1) a step of removing cobalt contained in the surface of the cemented carbide alloy  12  by etching; 
     (2) a step of placing the guide bush  11  in a vacuum vessel with an electrode inserted inside a center bore  11   j  thereof; and 
     (3) a step of evacuating the vacuum vessel before introducing a gas containing carbon, and causing a plasma to be generated inside the center bore  11   j  by applying a DC voltage or RF electric power to the guide bush  11  while applying a positive DC voltage to the electrode, thereby forming the DLC film  15  on the surface of the cemented carbide alloy  12  formed on the inner surface  11   b  of the guide bush  11 . 
     Hereupon, the step (1) of removing cobalt contained in the surface of the cemented carbide alloy  12  by etching is first described. 
     The removal of cobalt is preferably performed by means of a wet etching process. 
     As an etchant for wet etching to selectively remove cobalt, a blended solution of sodium hydroxide (NaOH) and aqueous hydrogen peroxide (H 2 O 2 ) is used. 
     By use of the blended solution, other constituent materials of the cemented carbide alloy  12  such as tungsten and carbon, and the constituent material of the guide bush  11 , namely, alloy tool steel (SKS), are not etched at all, but only cobalt can be selectively etched. 
     The wet etching process for the removal of cobalt is executed by immersing the guide bush  11  in the blended solution of sodium hydroxide (NaOH) and aqueous hydrogen peroxide (H 2 O 2 ) as the etchant at room temperature for a duration of from 3 to 5 hours. 
     Now, the step (2) of placing the guide bush  11  in the vacuum vessel and the step (3) of forming the DLC film  15  on the surface of the cemented carbide alloy  12  are described hereinafter. 
     There are available three types of apparatuses in which these step are performed. Further, each of them is available in two ways in which the electrode inserted in the center bore of the guide bush is grounded or a positive DC voltage is applied thereto. 
     FIG. 6 is a schematic sectional view showing an example of the apparatuses. In this example, use is made of a vacuum vessel  61  having a gas inlet port  63 , a gas outlet port  65 , an anode  79  and a filament  81  was put to use. 
     The guide bush  11  wherein cobalt contained in the surface of the cemented carbide alloy  12  has been removed and an electrode  71  in a rod-like shape is inserted in the center bore  11   j  thereof, is placed in the vacuum vessel  61  on insulating support  80 . In this case, an open and face of the inner surface  11   b  of the guide bush  11  is disposed opposite to the anode  79 , and a dummy member  53  is disposed on the open end face. 
     The dummy member  53  is formed in a ring-like shape, and has an opening in the center thereof, the diameter of which is substantially the same as that of an opening formed by the inner surface  11   b  of the guide bush  11 . 
     The outside diameter of the dummy member  53  is set to be the same size as that of the open end face of the guide bush  11 . 
     The electrode  71  inserted in the center bore  11   j  of the guide bush  11  is grounded through connection with the vacuum vessel  61  which is grounded. The electrode  71  is disposed so as to be positioned along the center axis of the center bore  11   j  of the guide bush  11  and such that the tip thereof is slightly set back from a level of the upper end of the dummy member  53 . 
     Subsequently, the vacuum vessel  61  is evacuated to not higher than 3×10 −5  torr through the gas outlet port  65  by evacuating means (not shown). Thereafter, benzene (C 6 H 6 ) as a gas containing carbon is introduced into the vacuum vessel  61  through the gas inlet port  63 , controlling pressure inside the vacuum vessel  61  to be at 5×10 −3  torr. 
     Thereafter, a DC voltage at −3 kV is applied to the guide bush  11  from a DC power source  73 , a DC voltage at +50V is applied to the anode  79  from an anode power source  75 , and an AC voltage at 10V is applied to the filament  81  from a filament power source  77  to allow flow of electric current at 30 A. 
     Whereupon, a plasma is generated in a region surrounding the guide bush  11  and inside the center bore  11   j , thereby forming the DLC film  15  (refer to FIG. 3) on the surface of the cemented carbide alloy  12  fixedly attached to the inner surface  11   b  of the guide bush  11 . 
     With the aforementioned method of forming the DLC film, it is possible to cause the plasma to occur not only in the peripheral region of the guide bush  11  but also inside the center bore  11   j  of the guide bush  11 , because the electrode  71  in a rod-like shape, grounded and inserted in the center bore  11   j , is provided. 
     By providing the electrode  71  at ground potential inside the center bore  11   j  of the guide bush  11 , side portions of the guide bush  11 , at the same potential, are prevented from facing each other in a region inside the center bore  11   j . Consequently, abnormal discharge called hollow discharge will not occur, thereby improving adhesiveness of the DLC film to be formed on the inner surface  11   b  of the guide bush  11 . Further, since potential characteristic in the longitudinal direction inside the center bore  11   j  of the guide bush  11  becomes uniform, the DLC film to be formed on the inner surface  11   b  will have even thickness. 
     It is sufficient that the diameter of the electrode  71  is smaller than that of the center bore  11   j  of the guide bush  11 , however, a plasma forming region is preferably provided in a gap in the order of 4 mm formed between the inner surface  11   b  and the electrode  71 . And the electrode  71  is formed of a metal such as stainless steel. 
     Further, the electrode  71  is circular in cross section, and may have a length such that when the electrode  71  is inserted in the guide bush  11 , the top end thereof is flush with the open end face of the dummy member  53  or projects above the open end face of the dummy member  53 . However, in the example shown in the figure, the electrode  71  has a length such that the tip thereof is disposed at a level set back by 1 to 2 mm below the top end face of the dummy member  53 . 
     The dummy member  53  disposed on the open end face of the guide bush  11  fulfills the following function. 
     With the method of forming the DLC film  15  shown in FIG. 6, the plasma is generated inside the center bore  11   j  of the guide bush  11  and in the peripheral region thereof. 
     And electric charge tends to converge on the open end face of the guide bush  11 , which is then caused to be in a state of higher electric charge in comparison with the interior of the center bore  11   j , and the so-called edge effect occurs. 
     Accordingly, the plasma generated in the vicinity of the open end face of the guide bush  11  has higher intensity than that generated in other regions and is yet unstable. Further, the open end face region of the guide bush  11  is under influence of both the plasma generated inside the center bore  11   j  of the guide bush  11  and the plasma generated in the peripheral region of the guide bush  11 . 
     If the DLC film is formed under such conditions, the adhesiveness of the DLC film formed in a region several mm inward away from the open end face of the guide bush  11  differs from that in other regions, and the same can be said of the quality thereof to some extent. 
     Accordingly, if the formation of the DLC film is carried out with the dummy member  53  disposed on the open end face of the guide bush  11 , a region where the DLC film has different quality and adhesiveness does not occur on the inner surface of the guide bush  11 , but occurs on the inner surface of the opening of the dummy member  53 . 
     Results of a test conducted on the formation of the DLC film employing the apparatus shown in FIG. 6, but without employing the dummy member  53 , showed that a region 1 to 2 mm in vertical length, where the quality and adhesiveness of the DLC film differs from those in other regions, occurred about 4 mm inward away from the open end face of the guide bush  11 . 
     Accordingly, a film was formed under the same conditions as those described above under which the DLC film was formed by disposing the dummy member  53 , with vertical length of 10 mm and with the top end face thereof substantially in the same size as that of the guide bush  11 , on the open end face of the guide bush  11 . As a result, the region where the DLC film has different quality and adhesiveness was found on the inner surface of the opening of the dummy member  53 , but was not found at all on the inner surface  11   b  of the guide bush  11 . 
     Next, other examples of a method of forming the DLC film in the guide bush are described hereinafter with reference to FIGS. 7 and 8. In these figures, parts corresponding to those in FIG. 6 are denoted by the same reference numerals. 
     Similarly to the case previously described, cobalt serving as a binder is first removed from a surface region of the cemented carbide alloy  12  formed on the inner surface  11   b  of the guide bush  11  by means of the wet etching process. 
     With the example shown in FIG. 7, a guide bush  11  wherein cobalt contained in the surface of the cemented carbide alloy formed on the inner surface  11   b  thereof has been removed is placed in a vacuum vessel  61  having a gas inlet port  63  and a gas outlet port  65 . 
     As is with the case previously described, an electrode  71  in a rod-like shape which is grounded is inserted in the center bore  11   j  of the guide bush  11 , and a dummy member  53  in a ring-like form is disposed on the open end face of the guide bush  11 . 
     Subsequently, the vacuum vessel  61  is evacuated to not higher than 3×10 −5  torr through the gas outlet port  65  by evacuating means (not shown). Thereafter, methane gas (CH 4 ) as a gas containing carbon is introduced into the vacuum vessel  61  through the gas inlet port  63 , adjusting degree of vacuum therein to be at 0.1 torr. 
     And RF electric power at a frequency of 13.56 MHz is applied to the guide bush  11  from an RF power source  69  via a matching circuit  67 . 
     Thereupon, a plasma is generated in not only the peripheral region but also the center bore  11   j , thereby forming the DLC film on the surface of the cemented carbide alloy formed on the inner surface  11   b  of the guide bush  11 . 
     Since operational effect in this case is the same as that in the case of the example shown in FIG. 6, description thereof is omitted. 
     Similarly to the case of the example previously described, with the example shown in FIG. 8 as well, a guide bush  11  wherein cobalt contained in the surface of the cemented carbide alloy formed on the inner surface  11   b  thereof has been removed is placed in a vacuum vessel  61  having a gas inlet port  63  and a gas outlet port  65 . 
     Hereupon, an electrode  71  in a rod-like shape which is grounded is inserted in the center bore  11   j  of the guide bush  11 , and a dummy member  53  in a ring-like shape is disposed on the open end face of the guide bush  11 . 
     Subsequently, the vacuum vessel  61  is evacuated to not higher than 3×10 −5  torr through the gas outlet port  65  by evacuating means (not shown). Thereafter, methane gas (CH 4 ) as a gas containing carbon is introduced into the vacuum vessel  61  through the gas inlet port  63 , adjusting degree of vacuum therein to be at 0.1 torr. 
     Thereafter, a negative DC voltage at −600V is applied to the guide bush  11  from a DC power source  73 ′. 
     Thereupon, a plasma is generated both in the peripheral region of the guide bush  11  and inside the center bore  11   j , thereby forming the DLC film on the surface of the cemented carbide alloy formed on the inner surface  11   b  of the guide bush  11 . Since operational effect in this case is the same as that in the case of the example shown in FIG. 6, description thereof is omitted. 
     Next, still other examples of a method of forming the DLC film in the guide bush are described hereinafter with reference to FIGS. 9,  10  and  11 . 
     The examples shown in FIGS. 9,  10  and  11  correspond to those previously described with reference to FIGS. 6,  7  and  8 , respectively, and differ from the latter only in that an electrode  71  in a rod-like shape which is inserted in the center bore  11   j  of the guide bush  11  is insulated and supported by an insulating support member  85  such as an insulator or so forth, fitted into the center bore  11 , and a positive DC voltage at +20V is applied to the electrode  71  from a DC power source  83 . 
     In the case of these examples as well, a plasma can be generated in the peripheral region of the guide bush  11 , including the interior of the center bore  11   j  thereof, thereby forming the DLC film having a high adhesion strength on the surface of the cemented carbide alloy formed on the inner surface  11   b  of the guide bush  11 . 
     FIG. 12 is a graph showing a correlation between positive DC voltage applied to the electrode  71  in this case, and thickness of the hard carbon film formed on the inner surface  11   b  of the guide bush 
     In the graph shown in FIG. 12, variation in the thickness of the hard carbon film according as the positive DC voltage applied to the electrode  71  is varied from 0V to 30V is indicated when a gap dimension between the inner surface  11   b  of the guide bush  11  and the electrode  71  is 3 mm and 5 mm, respectively. 
     In the figure, a curve denoted by “a” indicates a characteristic of variation in the thickness when the gap dimension between the inner surface  11   b  of the guide bush  11  and the electrode  71  is 3 mm, and a curve denoted by “b” indicates the characteristic when the gap dimension is 5 mm. 
     As shown by the curves a and b in FIG. 12, a rate at which the DLC film is formed becomes higher according as the positive DC voltage applied to the electrode  71  from the DC power source  83  is increased. 
     It is further indicated that the rate at which the DLC film is formed becomes higher according as the gap dimension between the inner surface  11   b  of the guide bush  11  and the electrode  71  is increased. 
     When the gap dimension described above is 3 mm as indicated by the curve a, a plasma is not generated inside the center bore  11   j  of the guide bush  11  if the voltage applied to the electrode  71  is 0V, that is, the electrode  71  is at ground potential, so that the DLC film is not formed. 
     Even when the gap dimension is 3 mm, however, a plasma is generated around the electrode  71  inside the center bore  11   j  by raising the positive DC voltage applied to the electrode  71 , so that the hard carbon film can be formed. 
     Thus, in the case of the examples shown in FIGS. 9 to  11 , the hard carbon film is formed by applying a positive DC voltage to the electrode  71  disposed in the center of the center bore  11   j  of the guide bush  11 . This has an effect of gathering electrons together in the peripheral region of the electrode  71 , and in consequence, electron density in the peripheral region of the electrode  71  becomes higher. 
     When the electron density becomes higher as described, since there will naturally be a higher probability of molecules of a gas containing carbon colliding against electrons, ionization of the gas molecules is promoted, thereby increasing intensity of the plasma generated inside the center bore  11   j  of the guide bush  11 . 
     As a result, the rate at which the DLC film is formed in this case becomes higher than that in the case of no positive DC voltage being applied to the electrode  71 . 
     Further, in case that the diameter of the center bore  11   j  of the guide bush  11  becomes smaller and the gap dimension between the inner surface  11   b  thereof and the electrode  71  is reduced, an attempt to form the DLC film without applying a positive DC voltage to the electrode  71  will not succeed because no plasma is generated inside the center bore  11   j , thus preventing the formation of a film. 
     If a positive DC voltage is applied to the electrode  71  disposed in the center bore  11   j  of the guide bush  11 , however, it is possible to generate a plasma around the electrode  71  inside the center bore  11   j  by forcing electrons to gather in the peripheral region of the electrode  71 , so that the DLC film can be formed on the inner surface  11   b.    
     Other operational effects are similar to that for the other examples previously described with reference to FIGS. 6 to  8 , and therefore, description thereof is omitted. 
     With reference to the methods of forming the DLC film described in the foregoing, the examples wherein methane gas or benzene gas as a gas containing carbon is introduced have been described, however, other gases containing carbon such as ethylene and vapor evaporated from a liquid containing carbon such as hexane may also be used. 
     Further, with reference to the methods of forming the DLC film described above, the embodiments wherein the DLC film is formed both on the outer periphery of the guide bush  11  and on the inner surface  11   b  thereof have been described, however, it is possible to form the DLC film on the inner surface  11   b  only. 
     In such a case, the DLC film may be formed by adopting a method of disposing a covering member in such a way as to cover the outer periphery of the guide bush  11  or simply by winding aluminum foil around the outer periphery of the guide bush  11 . 
     Furthermore, with reference to the methods of forming the DLC film described above, the formation of the DLC film with the dummy member  53  disposed on the open end face of the guide bush  11  has been described. 
     If the quality of the DLC film formed in the region in close proximity of the open end face is not so different from that in other regions, however, the DLC film may be formed without disposing the dummy member  53  on the open end face of the guide bush  11 . 
     A Method of Forming an Intermediate Layer: FIG. 13 
     Next, in the case where the DLC film  15  is formed on the surface of the cemented carbide alloy  12  fixedly attached to the inner surface  11   b  of the guide bush  11  with the intermediate layer  14  interposed therebetween as previously described with reference to FIGS. 4 and 5, a method of forming the intermediate layer is described hereinafter with reference to FIG.  13 . 
     As shown in FIG. 13, in a step of forming the intermediate layer, a target receptacle  96  is installed inside a vacuum vessel  91 , close to a sidewall  91   a  thereof, and a target  90  made of a constituent material of the intermediate layer, for example, an alloy composed of tungsten and carbon mixed together at a ratio of 1 to 1, is disposed in the target receptacle  96 . 
     And the guide bush  11  is placed inside a vacuum vessel  91  having a gas inlet port  93  and a gas outlet port  95  such that the open end face of the guide bush  11  is disposed opposite to the target  90 . At this point in time, the guide bush  11  is disposed such that the longitudinal direction of the center bore  11   j  of the guide bush  11  is perpendicular to the surface of the target  90 . 
     As shown in FIG. 4, with the guide bush  11 , the cemented carbide alloy  12  containing cobalt is fixedly attached to the inner surface  11   b  of the base member, and cobalt contained in the surface has been removed by etching. 
     The guide bush  11  covered by foil  94  is connected to a DC power source  92 , and the target  90  is connected to a target power source  97 . And the regions, however, the DLC film may be formed without disposing the dummy member  53  on the open end face of the guide bush  11 . 
     Thereafter, argon gas (Ar) as a sputtering gas is introduced into the vacuum vessel  91  through the gas inlet port  93 , adjusting degree of vacuum therein to be at 3×10 −3  torr. 
     Subsequently, a negative DC voltage at −50V is applied to the guide bush  11  from the DC power source  92 , and a DC voltage in the rage of −500 to −600V is applied to the target  90  from the target power source  97 . 
     Thereupon, a plasma is generated inside the vacuum vessel  91 , and the surface of the target  90  is sputtered by ions of the plasma. 
     Atoms of tungsten and carbon, respectively, ejected from the surface of the target  90  are deposited on the inner surface  11   b  of the guide bush  11 , thereby forming the intermediate layer  14 , made up of a tungsten carbide film, on the inner surface of the cemented carbide alloy  12  of the guide bush  11 . The intermediate layer  14 , made up of the tungsten carbide film, is formed to a thickness in the order of 0.5 μm. 
     With the use of any of the methods of forming the DLC film previously described with reference to FIGS. 6 to  11  after forming the intermediate layer  14  over the cemented carbide alloy  12  as described above, the DLC film can be formed over the intermediate layer  14  with excellent adhesiveness. 
     The intermediate layer  14  can also be formed as a laminated layer made up of a plurality of films. In the case where the first intermediate layer  14   a  is made up of a titanium film or a chromium film, and the second intermediate layer  14   b  is made up of a silicon film or a germanium film as shown in FIG. 5, a method of forming the intermediate layers is described hereinafter. 
     In this case, the target  90  made of titanium or chromium is placed in the vacuum vessel  91  shown in FIG. 13, and the guide bush  11  is disposed opposite to the target  90 . Subsequently, as described above, a negative DC voltage at −50V is applied to the guide bush  11  from the DC power source  92 , and a negative DC voltage in the range of −500 to −600V is applied to the target  90  from the target power source  97 . 
     Thereafter, by use of a sputtering method using argon (Ar) as a sputtering gas, titanium atoms or chromium atoms are ejected out of the target  90 , thereby forming the first intermediate layer  14   a  (refer to FIG.  5 ), made up of a titanium film or a chromium film, on the surface of the cemented carbide alloy  12  attached to the inner surface  11   b  of the guide bush  11 . 
     Subsequently, if the formation of another film is carried out under the same conditions as those for forming the first intermediate layer  14   a  except that the target  90  is replaced with one made of silicon or germanium, the second intermediate layer  14   b , made up of a silicon film or a germanium film, can be formed on the surface of the first intermediate layer  14   a.    
     Any of the methods of forming the DLC film previously described with reference to FIGS. 6 to  11  can be adopted for a method of forming the DLC film over the second intermediate layer  14   b , and by doing so, the DLC film  15  can be formed over the second intermediate layer  14   b  with excellent adhesiveness. 
     Further, for the formation of the second intermediate layer  14   b , an alloy containing carbon such as silicon carbide (SiC) or tungsten carbide (WC) can be used as a constituent material for the target instead of silicon or germanium, thereby forming the layer in the vacuum vessel  91  shown in FIG. 13 by means of the sputtering method 
     Since a silicon carbide (SiC) film or a tungsten carbide (WC) film has substantially the same function as that of a silicon (Si) film or a germanium (Ge) film, the silicon carbide (SiC) film or the tungsten carbide (WC) film, when used for the second intermediate layer  14   b , has substantially the same advantageous effect as that of the silicon (Si) film or the germanium (Ge) film.