Covered member and method of producing the same

This invention provides a covered member which possesses a high bond strength of a base material and a covering film, and has a smooth surface. The covered member comprises a base material and a covering film and the surface of the base material to be covered with the covering film is characterized by an uneven surface having projections with an average height in the range from 10 to 100 nm and an average width of not more than 300 nm. The uneven surface can be formed by ion impacting. A surface of a covering film formed on that is smooth since the unevenness of the surface is extremely fine.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a covered member which is formed by
 covering a base material such as irons and steels with a hard film and
 which is improved in bond strength of the base material and the covering
 film, and to a method of producing the same.
 2. Description of the Background
 Widely used is a covered member which is produced by forming a metallic
 film, a ceramic film, a carbon-base film, or the like of several to tens
 of microns in thickness on a surface of irons, steels or the like by
 physical vapor deposition (PVD) or chemical vapor deposition (CVD), and
 which comprises the base material, and the covering film covering a
 surface of this base material. The bond strength of the base material and
 the covering film, which constitute this covered member, becomes a problem
 and in some cases the film is peeled off. For example, in the art of
 employing PVD and applying a hard film to a base material which has a low
 temperature of 600.degree. C. or less, the bond strength of the base
 material and the covering film is small because of low temperature, and
 the film is often peeled off.
 In such a case, as means of increasing the bond strength of the base
 material and the covering film, there have so far been employed a method
 of cleaning a base material surface by sputtering the base material
 surface, a method of roughening a base material surface by blasting the
 base material surface, and so on.
 In using a covered member as a sliding member, it is important that its
 covering film is smooth and bond strength of the covering film and a base
 material is high. When the covering film is not smooth, seizure resistance
 is low and a tendency to attack a mating member is increasing. When the
 bond strength of the covering film and the base material is insufficient,
 the film is peeled off and the covered member cannot be used as a sliding
 member.
 However, in the sputtering method, although organic stains and oxides can
 be removed from the surface, it is often that a desired bond strength is
 not obtained in the case of low temperature film application. In the
 blasting method, the base material surface is greatly roughened into an
 uneven surface with projections and concaves of about several microns at
 the minimum, so a smooth film cannot be obtained. Hence, the surface
 roughness of a hard film thereafter formed deteriorates. For instance,
 when used as a sliding member, the covered member rather abrades a mating
 member and desired sliding characteristics cannot be obtained.
 Accordingly, when the covered member is used, for example, as a sliding
 member which needs to have a smooth surface, repolishing of the hard film
 is sometimes required.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a covered member which
 is free from those problems, possesses a high bond strength of a base
 material and a covering film, and has a smooth surface, and a method of
 producing the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present inventors have found and confirmed that by making a base
 material surface to be covered with a covering film into an uneven surface
 having projections with a predetermined extremely small height and a
 predetermined width, it becomes possible to obtain a covered member which
 possesses a high bond strength of the base material and the covering film
 and has a smooth surface. The present inventors have thus completed the
 present invention.
 The covered member of the present invention is a covered member comprising
 a base material and a covering film covering a surface of the base
 material, and is characterized in that the base material surface covered
 with the covering film is an uneven surface having projections with an
 average height in the range from 10 to 100 nm and an average width of not
 more than 300 nm.
 The present inventive method of producing a covered member is characterized
 in comprising a first step of forming, on a base material surface to be
 covered, projections with an average height in the range from 10 to 100 nm
 and an average width of not more than 300 nm by ion impacting beforehand,
 so as to make the base material surface into an uneven surface, and a
 second step of forming a covering film on the uneven surface.
 Because an interface between the base material and the covering film are
 uneven surfaces having projections with a predetermined height and width,
 the bond between them is strong and a covered member with improved
 integrality is obtained.
 The covered member of the present invention comprises a base material and a
 covering film. The base material constitutes various components or parts
 of apparatus such as a substrate, a sliding member, and a structural
 member, and is formed of metal. On the other hand, the covering film is to
 cover at least part of a surface of this base material and to add such
 functions as corrosion resistance, abrasion resistance, and decoration to
 the surface. The covering film is a metallic film, a ceramic film, or a
 carbon-base film formed by various applying methods.
 Examples of metal constituting the base material are iron-base metals such
 as steels, metals such as titanium, aluminum, copper, and magnesium, and
 alloys of these metals. Examples of a metallic film constituting the
 covering film are chromium, nickel, tungsten and the like. Examples of a
 ceramic film are a nitride film, a carbide film, and an oxide film,
 comprising one of the elements in group IV to group VI of the periodic
 table or composite elements containing one of these elements. Examples of
 a carbon-base film are diamond-like carbon (DLC) and diamond.
 The base material surface to be covered with the covering film is an uneven
 surface having projections with an average height in the range from 10 to
 100 nm and an average width of not more than 300 nm. The projections have
 a hemispherical shape. Note that the height of a projection means a
 distance from the bottom to the vertex of this hemispherical projection,
 and that the width of a projection means a horizontal distance
 corresponding to the maximum length of the bottom of the hemispherical
 projection (a diameter in the case where the projection bottom shape is a
 true circle, and a major axis length in the case where the projection
 bottom shape is an ellipse).
 The reason why the average height should range from 10 to 100 nm is that
 when the average height is less than 10 nm, a mechanical anchoring effect
 cannot be obtained and bond strength becomes insufficient, and that, on
 the other hand, when the average height exceeds 100 nm, a smooth film
 cannot be obtained. It is more preferable that the average height ranges
 from 20 to 70 nm. In this range, bond strength is further improved.
 The reason why the average width should be not more than 300 nm is that
 when the average width exceeds 300 nm, an anchoring effect cannot be
 obtained and bond strength decreases. Note that the size of projections
 cannot be measured by a conventional surface roughness tester (a tracer
 method). For this reason, here, projection size and width is measured by
 microshape measuring methods such as scanning electron microscope (SEM)
 observation and AFM.
 If projection areas are small, no effect on the bond strength of the film
 can be exhibited despite of a predetermined projection size. The ratio of
 the projection areas to the whole area of an uneven surface is preferably
 not less than 30% when the whole area of the uneven surface is assumed as
 100%. When the projection areas are 30% or more, the bond strength of the
 film is high.
 The present inventive method of producing a covered member comprises a
 first step of making a base material surface into an uneven surface, and a
 second step of forming a covering film on this uneven surface.
 As the method of forming an uneven surface in the first step, the ion
 impacting method can be employed. By this ion impacting method,
 projections with an average height in the range from 10 to 100 nm and an
 average width of not more than 300 nm are formed on a base material
 surface to be covered, so as to make the base material surface into an
 uneven surface.
 For ion impacting, a base material to be treated is placed in an air-tight
 chamber and the pressure in the air-tight chamber is controlled to about
 10.sup.-3 to 20 torr. When the pressure is less than 10.sup.-3 torr, the
 material to be treated cannot be heated sufficiently. When the pressure is
 more than 20 torr, the material to be treated can be heated but cannot
 attain ultrafine unevenness.
 Next, ultrafine-unevenness pretreatment gas is introduced into the chamber.
 This ultrafine-unevenness pretreatment gas may be one or more rare gases
 of helium, neon, argon, krypton, xenon, and radon. Further, in the case of
 an iron-base base material, adding hydrogen to the ultrafine-unevenness
 pretreatment gas can prevent oxidation of the material surface to be
 treated.
 Ion impacting is given under these conditions. As the means for giving ion
 impacting, glow discharge or ion beam can be employed. When ion impacting
 is carried out with a discharge voltage of 200 to 1000V, an electric
 current of 0.5 to 3.0 A and a treating time of 30 to 60 minutes, uniform
 ultrafine unevenness on the nano order can be formed. If the material to
 be treated is heated to a temperature at which hardness is not lowered (at
 least 200.degree. C. is needed) while ion impacting is given, more uniform
 and more ultrafine unevenness on the nano order can be obtained.
 In the case of an iron-base material, it is better to form before ion
 impacting a nitriding layer, a carburizing layer or a soft nitriding layer
 on a surface of the material to be treated, by carrying out any one of
 ordinary gas nitriding treatment, carburizing treatment, and gas soft
 nitriding treatment. Formation of the nitriding layer, the carburizing
 layer, or the soft nitriding layer makes it easy to form very uniform and
 ultrafine unevenness on the nano order by the ion impacting method. Note
 that unevenness can be formed by ion impacting subsequently after the
 nitriding layer, the carburizing layer, or the soft nitriding layer is
 formed in an ion impacting apparatus.
 As for the method of forming a covering film in the second step, the
 covering film can be formed by ion plating (an arc process, a hollow
 cathode process, etc.), sputtering, vacuum deposition, plasma CVD and so
 on. Since the base material surface is an uneven surface, the covering
 film thus formed adheres to the surface strongly. Besides, the obtained
 covering film has a smooth surface with little unevenness. In regard to
 CVD, because the treatment is carried out at high temperatures, relatively
 good bond strength is exhibited without forming microfine unevenness in
 the first step of the present invention. Therefore the formation of
 ultrafine unevenness in the first step has little effect.
 After the second step is carried out and a covering film is formed, there
 is no need to apply aftertreatment such as polishing or lapping of the
 formed covering film. Taking the first step and the second step of the
 present invention appropriately enables formation of a smooth covering
 film which does not require polishing or lapping.
 The covered member of the present invention can be used as a sliding member
 which requires abrasion resistance. For instance, the covered member of
 the present invention can be used as machine parts in sliding motion, e.g.
 sliding portions of engine parts (a piston, piston rings, a valve stem,
 etc.), compressor parts (vanes, shoes, etc.), a fuel-injection pump (a
 rotor, a plunger, etc.).
 In the covered member of the present invention, the interface between the
 base material and the covering film is an uneven surface having
 projections with an average height in the range from 10 to 100 nm and an
 average width of not more than 300 nm. Owing to this micro unevenness of
 the interface, the bond area increases. In response to this increase in
 bond area, bond strength increases. In addition, since the base material
 surface has been cleaned and activated by forming this uneven surface, a
 strong covering film is formed. Moreover, owing to the mechanical
 anchoring effect of concaves formed at the boundary of projections, the
 base material and the covering film are mechanically combined, so stronger
 bond can be obtained.
 Furthermore, since the unevenness of the uneven surface of the base
 material is as ultrafine as 10 to 100 nm, the unevenness of the base
 material is not reproduced on a covering film surface, and accordingly the
 covering film surface is smooth.
 Note that by forming a nitriding layer, a carburizing layer, or a soft
 nitriding layer on the base material surface beforehand, projection
 formation by ion impacting becomes easy, and the projections become
 ultrafiner and the ratio of the area occupied by the projections per unit
 area becomes higher. This results in large increase in the contact area of
 the base material and the covering film, so the bond strength is further
 improved.
 BEST MODES FOR CARRYING OUT THE INVENTION
 In the following experiment, nitriding steel SACM645 was used as a base
 material and formed into test specimens of 50.times.10.times.7 mm in
 dimension. The base material had a surface roughness of Rz 0.1 .mu.m.
 Examination on Unevenness-Forming Conditions
 Examination was conducted on conditions for forming unevenness on a base
 material surface. The aforementioned nitriding steel was employed as a
 base material and gas nitriding treatment in NH.sub.3 gas at 520.degree.
 C. for 35 hours was applied, whereby a nitriding diffusion layer of about
 0.4 mm in thickness was formed on the base material surface. Then, the
 base material surface was polished so as to have a surface roughness of Rz
 0.1 .mu.m.
 Argon gas was used as gas for ion impacting treatment for forming
 unevenness, and hydrogen was added as oxidation-preventing gas. The gas
 pressure was 4 torr, and the voltage and electric current were 100 V and
 0.4 A, and 200 V and 0.8 A. The treatment time was 0, 5, and 50 minutes.
 This ion impacting treatment produced projections described in Table 1.
 Note that the surface roughness remained to be Rz 0.1 .mu.m even after the
 ion impacting treatment.
 Thereafter, a hard film of DLC-Si (silicon-contained diamond-like carbon)
 was formed on the base material surface by plasma CVD. The hard film had a
 thickness of 3 .mu.m.
 The bond strength of the obtained hard film was evaluated by an indentation
 test and a scratch test. The indentation test was to evaluate bond
 strength of the hard film by pressing a Rockwell C-scale indenter against
 the film with a pressure of 150 kg and observing whether the film around
 an indented portion was peeled off or not. The scratch test is to scratch
 a surface with a conical diamond having a vertical angle of 120.degree.
 and a point of 0.2 mm in radius with a given load. The load at the time
 when the film is peeled off is called critical load, and the bond strength
 of the hard film is evaluated by the value of that critical load.
 Evaluation results are shown in Table 1.
 TABLE 1
 CONDITIONS FOR FORMING DIMENSIONS OF
 PROJECTIONS PROJECTIONS
 CRITICAL PEELING OFF
 ELECTRIC AVE. AVE.
 LOAD IN AFTER
 SPECIMEN VOLTAGE CURRENT TIME HEIGHT WIDTH OCCUPIED
 SCRATCH INDENTATION
 NO. (V) (A) (min) (nm) (nm) AREA (%)
 TEST (N) TEST
 1 100 0.4 15 0 0 0
 10-20 YES
 2 200 0.8 15 5 5 5
 20 YES
 3 200 0.8 40 50 70 70 50
 NONE
 As apparent from Table 1, sufficient bond strength could not be obtained
 with an average projection height of 5 nm. It is clear that ion impacting
 treatment with a voltage of 200 V, an electric current of 0.8 A and a
 treating time of 40 minutes was required as conditions for forming
 unevenness.
 Examination on Base Material Surface Hardening Conditions Affecting
 Unevenness Formation
 Projections were formed under the same ion impacting conditions as used for
 forming unevenness on Specimen No. 3 in Table 1 by using three kinds of
 test specimens: one which underwent no hardening treatment, one which
 underwent the same nitriding treatment as the specimens in Table 1, and
 one which underwent carburizing treatment. Then, a hard DLC-Si film of 3
 .mu.m in thickness was formed on each base material surface by the same
 plasma-CVD as used for the specimens in Table 1. After that, bond strength
 of each obtained hard film was evaluated by an indentation test and a
 scratch test in the same way as the films of the specimens in Table 1 were
 evaluated. The results are shown together in Table 2.
 Note that the carburizing treatment was carried out by using case hardening
 steel as a base material and placing that steel in a salt bath at
 900.degree. C. for one hour. The obtained carburizing layer had a
 thickness of 0.4 mm.
 TABLE 2
 DIMENSIONS OF PROJECTIONS CRITICAL PEELING
 OFF
 SURFACE AVE. AVE. LOAD IN
 AFTER
 SPECIMEN HARDENING HEIGHT WIDTH OCCUPIED SCRATCH
 INDENTATION
 NO. TREATMENT (nm) (nm) AREA (%) TEST (N) TEST
 11 NONE 30 20 20 or less 25
 OBSERVED
 12 NITRIDING 70 70 80 55 NONE
 13 CARBURIZING 30 20 60 40 NONE
 It is apparent from Table 2 that the base material surface is preferably
 subjected to hardening treatment before ion-impacting treatment for
 forming unevenness. In the case of Specimen No. 11 which underwent no
 surface hardening treatment, even the ion impacting treatment with a
 voltage of 200 V, an electric current of 0.8 A and a treating time of 40
 minutes did not form sufficient unevenness. Hence, bond strength was not
 sufficient.
 It is clear that nitriding or carburizing the base material surface
 beforehand made it easy to form unevenness by ion impacting treatment.
 For reference, SEM photographs of uneven surfaces of Specimen Nos. 11 and
 12 after the ion impacting treatment was applied and before hard films
 were formed are respectively shown in FIGS. 1 and 2. These figures
 demonstrate that hemispherical projections were formed by ion impacting.
 Less projections are formed on Specimen No. 11 as shown in FIG. 1. On the
 other hand, FIG. 2 shows that the projections are formed all over the
 surface of Specimen No. 12.
 Total Evaluation Test
 Four kinds of specimens in total were prepared by employing TiN as hard
 covering films. As for surface hardening by nitriding, two kinds of
 specimens, i.e. untreated and treated ones were prepared. As for
 unevenness formation, two kinds of specimens, i.e. unevenness-formed and
 unformed ones were prepared. Thereafter, an indentation test and a scratch
 test were conducted in the same way as done to the specimens in Table 1,
 thereby evaluating bond strength of the hard film. The results are shown
 together in Table 3.
 Nitriding treatment was carried out in the same way as done to the
 specimens in Table 1. Ion impacting treatment for forming unevenness was
 carried out with a voltage of 400 V, an electric current of 1.5 A and a
 treating time of 1 hour. TiN covering films were formed by arc ion plating
 with a treating temperature of 400.degree. C. and a treating time of 1
 hour. Covering TiN films of 3 .mu.m in thickness were thus formed.
 TABLE 3
 MATERIAL DIMENSIONS OF
 PROJECTIONS CRITICAL
 OF HARD SURFACE AVE. AVE.
 LOAD IN
 SPECIMEN COATING HARDENING UNEVENNESS HEIGHT WIDTH
 OCCUPIED SCRATCH
 NO. FILM TREATMENT FORMATION (nm) (nm)
 AREA (%) TEST (N)
 21 TiN NONE NONE 0 0 0 40
 22 TiN NONE FORMED 40 30 20
 60
 23 TiN TREATED NONE 0 0 0 50
 24 TiN TREATED FORMED 50 40 70
 80
 As apparent from Table 3, unevenness formation resulted in an improvement
 in bond strength, and besides, the specimens which underwent surface
 hardening treatment before unevenness formation were further improved in
 bond strength.