Abstract:
A surface coating film includes, a base material which is a hard material; and an oxidation-resistance coating layer containing, as a main component, a complex oxide of Li and at least Al and covering a surface of the base material. Further, a method of manufacturing a surface coated member, includes, supporting a base material which is a hard material in a hermetic container with the use of a holder arranged in the container; arranging a complex oxide forming target containing Li and at least Al, as main components, in the container; feeding oxygen into the container; and forming an oxidation-resistance coating layer that covers the base material to obtain the surface coated member by effecting electric discharge between the complex oxide forming target as an anode and the holder as a cathode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surface coating film formed on a base material which is a hard material, direct or indirect there onto, a method of manufacturing thereof, a cutting tool and a cutting machine. 
     2. Description of the Related Art 
     These years, the consciousness as to environmental problems have been stimulated even in the technical field of cutting, and in particular, environmental adaptability with respect to lubrication oil has become a great issue. Accordingly, in order to reduce detrimental affection upon the environment, and further, in view of cost reductions, the need of a dry cutting technology which can completely eliminate the necessity of lubrication oil have been increased. 
     The merits as to a dry cutting process are as follows: occurrence of environmental problems caused by cutting oil or additives thereof can be eliminated, recycling can be facilitated, the costs caused by using and treating lubrication oil can be reduced, and so forth. Meanwhile since no lubrication oil is used, advantages obtained by lubrication, cooling and the like which have been obtained by the lubrication oil cannot be obtained, and severe cutting conditions are required. 
     A technology for forming a coating film having a high degree of hardness on a base material with the use of a process of physical vapor deposition has been developed so as to accept the dry cutting technology, that is, a TiN group coating film, a TiAlN group coating film or the like has been applied to a tool. However, since the TiN group coating film starts its oxidation at about a temperature of 500 deg. C. while the TiAlN group coating film starts its oxidation at a temperature of about 800 deg. C., there has been raised such a problem that these films cannot be applied to a tool adapted to be used at a high temperature. 
     Recently, a TiAlN group coating film disclosed in Patent Document 1 (Japanese Patent Laid-Open No. H10-25565) has been developed, which starts its oxidation at a high temperature in a range from about 1,000 deg. C. to 1,200 deg. C., and which can accept cutting conditions under a high temperature. 
     However, although the TiAlN group coating film disclosed in the Patent Document 1, can accept the cutting condition under a high temperature, it would soon peel off. Further, its oxidation-resistance is insufficient in the case of using it under a high temperature. 
     SUMMARY OF THE INVENTION 
     Accordingly, in view of the above-mentioned problems inherent to the above-mentioned technology, an object of the present invention is to provide a surface coating film having a higher degree of hardness and an excellent oxide-resistance, a method of manufacturing thereof, a cutting tool and a cutting machine. 
     To the end, according to the present invention, there is provided a surface coating film characterized in that an oxidation-resistance coating layer mainly composed of a complex oxide of Li and at least Al is coated over a surface of a base material which is a hard material, direct to thereto or indirect thereto through the intermediary of a highly hard coating layer. 
     The structure of the surface coating film containing a complex oxide, as a main component, of Li and at least Al can exhibit, in addition to a high degree of hardness and an excellent oxidation-resistance, an effect of restraining temperature rise, and as well exhibits an excellent wear-resistance since its frictional coefficient is small and its heat generation is less. Further, in the structure of the surface coating film, the base material is preferably a hard material such as cemented metal alloy or high-speed steel (also including high speed tool steel). 
     Further, the surface coating film according to the present invention has a high degree of hardness and an excellent oxidation-resistance, and accordingly, is preferably used as a coating film material for processing the surface of a tool or dies. Thus, it is industrially valuable. Further, in addition to the high degrees of hardness and oxidation-resistance, the surface coating film according to the present invention has an excellent wear-resistance due to high adherence, and accordingly, it is preferably used as a coating film material for a cutting tool used in a dry cutting process (that is, cutting with no use of cutting fluid). 
     Further, the present invention as stated above is characterized in that the oxidation-resistance coating layer made of a complex oxide formed by containing Li and other metal oxide, is coated over the surface of a highly hard coating layer formed on a base material such as the above-mentioned hard material. 
     Further, more preferably, the complex oxide is formed by containing Li and Al, or Li, Al and Mg or Si metal oxide. 
     Further, the structure of the surface coating film as stated above is characterized in that the highly hard coating layer contains, in addition to Al, Si, a nitride of at least one kind of an element selected from a group consisting of Zr, Hf, Pd, Ir and rare earth elements. 
     Further, the structure of the surface coating film is characterized in that the highly hard coating layer further contains, in addition to Al and Si, a nitride of at least one kind of an element which is either Cr or Ti. 
     Further, the structure of the surface coating film as stated above, is characterized in that the thickness A of the oxidation-resistance coating layer is not greater than the thickness B of the highly hard coating layer, that is, A≦B. 
     Li contained in the oxidation-resistance coating layer has a high degree of affinity with respect to the nitride contained in the highly hard coating layer, that is, it has a synthesis of a crystal structure with respect to the nitride. Thus, the adherence between the oxidation-resistance coating layer and the highly hard coating layer is increased, and accordingly, they can hardly peel off from each other. 
     Further, the above-mentioned structure of the surface coating film, is characterized in that an intermediate layer containing at least one of a nitride of at least one kind of an element selected from a group consisting of Al, Ti and Cr, a carbide and a carbon nitride is provided between the base material and the highly hard coating film. 
     With the provision of the above-mentioned intermediate layer, the adherence between the base material and the highly hard coating layer becomes higher, and accordingly, the highly hard coating layer can hardly peel off, thereby it is possible to enhance the wear-resistance of the oxidation-resistance coating layer. 
     Further, according to the present invention, there is provided a method of manufacturing a surface coating film comprising the steps of: 
     supporting a base material which is a hard material, as it is, or a highly hard coating layer coated substance in which the base material is coated over its outer surface with a highly hard coating layer in a hermetic container with the use of a holder set in the container, 
     arranging a complex oxide forming target formed of Li and at least Al as main components in the container, feeding oxygen into the container, 
     causing electric discharge between the target as an anode and the holder as a cathode so as to form a surface oxide-resistance coating layer on the base material or the highly hard coating layer coated substance in order to obtain the surface coating film. 
     With this manufacturing method, there can be formed a surface coating film having an excellent wear-resistance in addition to a high degree of hardness and an excellent oxidation-resistance. 
     Further, according to the present invention, there is provided a cutting tool characterized by using a surface coating film in which an oxidation-resistance coating layer composed of, as a main component, a complex oxide of Li and at least Al is coated over an outer surface of a tool base material which is either a high speed steel (or high speed tool steel) in which metal components including chromium, tungsten, molybdenum and vanadium are added to the steel that is then subjected to heat-treatment such as quenching, or cemented metal alloy which is formed by sintering a powder of hard metal carbide, or over an outer surface of a highly hard coating layer formed on the tool base material. 
     Thus, there can be obtained a cutting tool having a high degree of hardness, an excellent oxidation-resistance and an excellent wear-resistance. 
     Further, the cutting tool may be used for a machine tool for fabricating a workpiece in a dry cutting process (which is cutting with no use of cutting fluid), and may be preferably used, in particular, for a machine tool for gear cutting, in which a desired workpiece is fabricated by a gear cutting tool such as a hob cutter, a pinion cutter or a broach. 
     The above-mentioned cutting machine tool can be used for a cutting process which requires, for a cutting tool, a high degree of hardness, an excellent oxidation-resistance and an excellent wear-resistance, and in particular for a dry cutting process (which is a cutting process with no use of cutting fluid), and accordingly, it is excellent in view of the environmental protection and the costs. 
     It is noted that the above-mentioned configurations can be combined unless the concept of the present invention is departed. 
     As stated above, the surface coating film according to the present invention, having a high degree of hardness and an excellent oxidation-resistance, may be preferably used for a tool or dies. Further, it has a high degree of adherence with respect to the highly hard coating layer containing a nitride so as to hardly peel off, and further, it has an excellent wear-resistance, thereby it is possible to preferably use the surface coating film for a cutting tool and a cutting machine tool for a dry cutting process in which no cutting fluid is used. 
     Further, according to the present invention, a surface coating film having an excellent wear-resistance in addition to a high degree of hardness and an excellent oxidation-resistance can be manufactured. 
     In particular, according to the present invention, a high degree of hardness and an excellent oxidation-resistance can be also obtained by the physical vapor deposition, the thus obtained surface coating film can be preferably used as a cover film for surface-treating a tool and dies, and accordingly, it is industrially valuable. Further, according to the present invention, the surface coating film can have a more excellent wear-resistance due to a high degree of adherence in addition to a high degree of hardness and a high oxidation-resistance, and accordingly, it may be preferably used not only for manufacturing a tool and dies, but also for a cover film for a cutting tool for a dry cutting process (which is a cutting process with no use of cutting fluid). 
     In particular, it may be preferably used for a gear cutting tool such as a hob cutter, a pinion cutter or a broach. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view illustrating an embodiment 1 of the present invention; 
         FIG. 2  is a schematic sectional view illustrating an embodiment 2 of the present invention; 
         FIG. 3  is a schematic sectional view illustrating a embodiment 3 of the present invention; 
         FIG. 4  is a schematic view illustrating a coating apparatus for forming an oxidation-resistance coating layer on a base material or a substance coated thereover with a high hard coating layer; and 
         FIG. 5  is a view for exhibiting results of film forming tests for a comparison example and reference examples 1 to 6 among which target compositions of a high hard coating layer (nitride) and an oxidation-resistance coating layer (oxide) are changed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Explanation will be hereinbelow made of preferred embodiments of the present invention. It is noted that dimensions, materials, shapes, relative positions and the like of components explained in these embodiment should be intended to limit the technical scope of the present invention unless otherwise specifically explained, that is, they are merely used for explanation purpose. 
     (Embodiment 1) 
     Referring to  FIG. 1  which is a schematic sectional view illustrating an embodiment 1 of the present invention, a surface coating film in this embodiment has a base material  1  and an oxidation-resistance coating layer  2  made of a complex oxide formed by containing Li and another metal oxide. It is noted that a hard material such as a cemented metal alloy or a high speed steel (or a high speed tool steel) is used as the base material  1 . 
     (Embodiment 2) 
     Referring to  FIG. 2  which is a schematic sectional view illustrating an embodiment 2 of the present invention, a surface coating film in the second embodiment, is composed of a base material  1  formed on its outer surface with a highly hard coating layer  3 , and an oxidation-resistance coating layer  2  coated over the outer surface of the highly hard coating layer  3 . 
     In this configuration, the film thickness of the oxidation coating layer  2  is preferably less than that of the high hard coating layer  3 , but their thicknesses may be set to be equal to each other in view of various conditions. 
     (Embodiment 3) 
     Referring to  FIG. 3  which is a schematic sectional view illustrating an embodiment 3 of the present invention, a surface coating film in the third embodiment, is composed of a base material  1  formed thereon with an intermediate layer  4  containing at least one of a nitride of at least one kind of an element selected from a group consisting of Al, Ti and Cr, a carbide or a carbon nitride, and a high hard coating layer  3  formed on the outer surface of the intermediate layer  4 , and an oxidation-resistance coating layer  2  formed on the high hard coating layer  3 . 
     In the embodiments 2 and 3, the highly hard coating layer  3  and the intermediate layer  4  are preferably formed with the use of a physical vapor deposition process such a high frequency sputtering or an ion plating in order to form the surface coating film having a high degree of adherence and an excellent wear-resistance. 
       FIG. 4  is a schematic view illustrating a film forming apparatus for forming the oxidation-resistance coating layer on the base material or the highly hard coating layer coated substance coated thereover with the highly hard coating layer. 
     The film forming apparatus  10  is composed of a casing  12  which is hermetic against the atmospheric air, a target  13  arranged to the ceiling of the container while a table-like holder  17  is located in a chamber  20  of the casing  12 . The holder  17  is coupled to a motor  18  through the intermediary of a rotary shaft  19  so that the holder  17  can be rotated in its circumferential direction. Further, a DC power source  11  is connected between the target  13  and the holder  17 , the target  13  being connected to the positive (+) side of the power source  11  while the holder  17  is connected to the negative (−) side of the power source  11 . 
     Although only one target  13  is schematically shown in  FIG. 4 , there may be arranged not less than two targets. In this case, the not less than two targets  13  may be located at positions which have a substantially equal distance to the holder  17 . 
     A vacuum pump  14  is connected to the chamber  20  of the casing  12  through the intermediary of a control valve  22  in order to evacuate the chamber  20 . Further, an argon gas source  15  is connected to the chamber  20  through the intermediary of a control valve  23  so as to feed an inert gas into the chamber  20 . Further, an oxygen gas source  16  is connected to the chamber  20  through the intermediary of a control valve  24  in order to feed oxygen gas into the chamber  20 . 
     In the above-mentioned embodiment, the number and kinds of targets  13  are adjusted in accordance with a kind of a layer formed on the base material or a substance  21  coated thereover with a high degree coating layer. 
     In the film forming apparatus  10 , the base material as it is, or the highly hard coating layer substance  21  in which the high hard coating layer is formed on the surface of the base material is set on the holder  17 , and the control valves  22 ,  23  are at first opened so as to feed an argon gas into the chamber  20  while the chamber  20  is evacuated to be vacuum. 
     After the completion of the vacuum evacuation, when an argon atmosphere is effected in the chamber  20 , the holder  18  is rotated by the motor  18 . Next, the control valves  22 ,  23  are closed, and a DC voltage is applied between the target  13  and the holder  17  so as to generate plasma in order to raise the temperature in the chamber  20 . When the temperature in the room is settled at a constant temperature, the control valve  24  is opened in order to feed oxygen from the oxygen gas source  16  into the chamber  20  so as to cause electric discharge. 
     Thus, an oxidation-resistance coating layer is formed on the outer surface of the base material or the outer surface of the substance in which a base material is covered with a highly hard coating layer. Thus, a surface coating film having a high degree of hardness, and an oxidation-resistance and a wear-resistance which are excellent is obtained. 
     The surface coating film in this embodiment has a single layer structure in which an oxidation-resistance coating layer (oxide) is formed on a base material which is a cemented metal alloy (WC), or a multilayer structure in which an oxidation-resistance coating layer is formed on the outer surface of a highly hard coating layer (nitride) formed on the base material. 
     The oxidation-resistance coating layer (oxide) is adapted to be formed on the outer surface of the high hard coating layer (nitride) which is rigid, in order to improve its properties such as a friction coefficient and an oxidation starting temperature. In the case of the single layer structure, the oxidation-resistance coating layer was built up to a film thickness of about 3 μm, and in the case of the multilayer structure, the oxidation-resistance coating layer was built up to a film thickness of about 1 μm on the highly hard coating layer after the latter was built up to a film thickness of about 3.5 μm. 
       FIG. 5  is a view which shows test results for coatings in a comparison example and reference examples 1 to 6 among which the target compositions of the oxidation-resistance layer (oxide) and the rigid high hard coating layer (nitride) are changed. 
     It is noted that the nitride film was formed in such a way that a disc-like compact of a nitride material powder (having a diameter of 50 mm and a thickness of about 3 mm) was set up on a 4 inch diameter AlN target, and a high frequency sputtering process was carried out. As to the preparation composition of the compact for forming the nitride film, Al 0.7 Zr 0.2 Si 0.1 N which can exhibit a high degree of hardness and a high oxidation starting temperature was used. 
     The oxide film was formed in such a way that a disc-like compact (having a diameter of 50 mm and a thickness of about 3 mm) of an oxide material powder is set on a 4-inch diameter n Al 2 O 3  target, and a high frequency sputtering was carried out. The base material which was used for forming the film was cemented metal alloy (WC) having dimensions of about 12 mm×12 mm×5 mm. 
     The nitride compact for the sputtering was produced by molding a mixture of material powders of AlN, ZrN and Si 3 N 4  which are mixed so as to have a predetermined composition, in dies having an inner diameter of 50 mm. Similarly, the oxide compact was formed by molding a mixture of material powders of LiAlO 2 , MgO, Al 2 O 3 , Cr 2 O 3 , SiO 2  and the like which were mixed so as to have a predetermined composition, in dies having an inner diameter of 50 mm. The powder of LiAlO 2  was synthesized by calcining material powders of LiCO 3  and Al 2 O 3  at a temperature of 1,250 deg. C. for two hours. 
     Further, in the reference examples, the mixing ratio of Li and Al for Li x Al y O z  was set to y/x=0.9 to 40. If y/x is not greater than 0.9, a sole Li 2 O layer was possibly formed, and accordingly, it is not preferable. 
     For example, as in the reference example 7, an oxide layer having a film composition of Li 2.5 Mg 0.5 Al 37 O 59.7  was formed by a sputtering process so as to obtain a target composition, (nitride/oxide):(Al 0.7 Zr 0.2 Si 0.1 N/Li 0.17 Mg 0.05 Al 2.804 ) in the reference example 7. 
     The hardness of the thus formed surface coating film was measured with the use of a micro Vickers hardness meter under a load condition of 10 g in 10 sec. 
     Further, the oxidation starting temperature of the surface coating film was determined in such a way that a nitride coating film formed on a Pt plate having dimensions of about 12 mm×5 mm×0.05 mm was heated in a differential thermobalance (TG-DTA) so as to analyze a temperature at which an increase in weight and heat generation that were caused by oxidation were initiated. During the TG-DTA analysis, a maximum heating temperature was 1,400 deg. C. while a temperature rising rate was 10 deg. C./min. The crystal structure of the coating film was measured by an X-ray analysis. 
     The testing was carried out with the use of AlZrSiN group nitride for a typical nitride layer as a lower layer. In view of results of measurements for the harnesses and the oxidation starting temperatures of the surface coating films shown in  FIG. 5 , it was found that the oxidation-resistance coating layers each composed of a complex oxide formed by containing an oxide of Li and the other metal, (reference examples 1 to 6) were hard to peel off in comparison with an oxide coating layer which does not contain Li (comparison example 1). 
     Further, in the case of reference example 6 in which Si was added, it has been found that the adherence with respect to the nitride can be enhanced while the frictional coefficient thereof is maintained substantially at a value obtained in reference example 1. 
     The above-mentioned effects can be similarly obtained by carrying out the oxide coating according to the present invention on a coating film of another nitride such as a nitride of Al, Cr or Si or a nitride of Al, Ti, Si, that is, a coating having a low frictional coefficient and a long service life can be obtained. 
     Further, the oxidation starting temperature is high, that is for example, 1,310 deg. C. in reference example 1 and 1,350 deg. C. in example 2 in comparison with 1,200 deg. C. in comparison example 1. Thus, even though it is hard to be oxidized even though the cutting temperature increases, and accordingly, the service life of a cutting tool or the like becomes longer. Moreover, the frictional coefficient is low, that is, 0.2 in reference example 1 and 0.23 in reference example 2 in comparison with 0.3 in comparison example 1, and accordingly, the heat value caused by friction can be decreased. Thus, even though the cutting speed (revolution speed) is increased, the service life of a cutting tool can become longer. 
     Further, in reference example  7 , the oxidation starting temperature becomes higher, that is, 1,340 deg. C. Thus, oxidation can hardly occur even though the cutting temperature increases, and accordingly, the service life of the cutting tool becomes longer. Further, the friction coefficient is extremely low in comparison with 0.3 in the comparison example 1, that is, it is 0.164 which is lower than 0.2 in reference example 1. In other words, the friction coefficient in reference example 7 is not greater than ⅔ of the friction coefficient (0.3) in comparison example 1, that is, about ½. Thus, in view of such a low frictional coefficient which is comparable with that of boric oxide, there can be provided a long service life tool coating. 
     Thus, with the formation of the oxidation-resistance coating layer composed of the complex oxide formed by containing an oxide of Li and other metal, the oxidation starting temperature can be increased so that oxidation can hardly occur even at a high temperature, and the friction coefficient is low. Thus, it has been found that the oxidation-resistance coating layer can hardly peel off. It is noted that the same result can be obtained even though Cr, Ti, Hf, Pd, Ir and rare earth elements are used, instead of Zr in the AlZrSiN group compound which were used a typical example of the nitride layer since the lattice constant is not changed appreciably. 
     Industrial Applicability 
     According to the present invention, there can be provided a surface coating layer having a high degree of hardness, an excellent oxidation-resistance, and as well having an excellent wear-resistance, and a method of manufacturing thereof. The surface coating film can be also used for a cutting tool or a cutting machine with the use of its property.