METHOD FOR PRODUCING SINTERED HARD METAL WITH DIAMOND FILM

Abstract: The present invention is directed to a method of producing a sintered hard metal having a diamond film wherein a gas that is obtained by activating a raw material gas containing 2 to 60 mol % of carbon monoxide and hydrogen gas is brought in contact with a sintered hard metal to form a diamond film, thereby a diamond film can be formed on said sintered hard metal with the adhesion of the diamond excellent.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention This invention relates to a method of producing a sintered hard metal with a diamond film, and more particularly to a method of producing a sintered hard metal with a diamond film suitable for, for example, various cutting tools such as bits, end mils, drills, and cutters, draw dies, blanking dies, abrasion resistant machine parts, and the like. (2) Description of the Related Art Conventionally, in the field of various cutting tools such as bits, end mils, and cutters, since cutting tools excellent abrasion resistance can be obtained by using a diamond film as a protective film for cutting tools made of a sintered hard metal, the demand for cutting tools made of a sintered hard metal on which a diamond film is formed is increasing. Conventionally, as methods of forming a diamond film on a sintered hard metal have been suggested, for example, a method wherein, prior to the formation of a diamond film, a sintered hard metal is subjected to ion etching treatment thereby activating the surface of the sintered hard metal, and then the sintered hard metal is contacted with an activated raw material gas (see Japanese Laid-Open Patent Application No. 204695/1985), and a method wherein prior to the formation of a diamond film on a substrate of a cermet, the cermet is etched with an acid solution to remove the bonding phase of the surface, and then the cermet is contacted with an activated raw material gas (see Japanese Laid-Open Patent Application No. 52363/1986). However, the cutting tools of a sintered hard metal formed with a diamond film that are obtained by these methods still are accompanied by such problems that the cutting edge is still abraded, that chipping (breaking part of the cutting edge) will occur, and that it is still hardly said that the adhesion of the diamond film is satisfactory. Thus, generally the adhesion between the surface of a sintered hard metal and a diamond film is poor. Therefore, in order to improve the adhesion between the surface of a sintered hard metal and diamond, techniques for forming an intermediate layer between a sintered hard metal and a diamond thin film are developed. For example, Japanese Laid-Open Patent Application No. 126972/1983 discloses a method wherein after an intermediate layer made of one or more compounds selected from a carbide, a nitride, a boride, and an oxide of a metal of Groups IVa, Va, and VIa of the periodic table is first formed on the surface of a sintered hard metal, a diamond thin film is formed on the intermediate layer. Also, Japanese Laid-Open Patent Application No. 159981/1984 discloses a tool that includes on a metal such as W, Nb, Ta, V, Cr, and Hf a diamond thin film formed by the CVD method or the like. However, in either of the above Patent Applications, since the thin film is formed stepwise by forming first an intermediate layer and then the diamond thin film, it is hardly said that the adhesion between the sintered hard metal and the diamond thin film has been improved sufficiently enough to the practical level although the methods are allegedly intended to improve the adhesion. Further, as a tool member coated with artificial diamond, a tool member coated with artificial diamond is suggested wherein at least the tool working surface of a cermet substrate member is coated with an artificial diamond film having an average layer thickness of 1 to 1.2 µm formed by an artificial diamond deposition formation process via a deposited layer having an average layer thickness of 0.05 to 1.2 µm made of any one of W, Mo, and Nb and their alloys [see Japanese Patent Publication No. 15347/1988 (Japanese Laid-Open Patent Application No. 208473/1985)]. However, since, in the method disclosed in the Japanese Patent Publication mentioned above, the composition of the reactant mixed gas for the formation of diamond is 1 vol. % or below, the deposited metal layer is not effectively carbonized. Therefore, the adhesion between the diamond layer and the cermet substrate member is accordingly poor. SUMMARY OF THE INVENTION The object of the present invention is to provide a method of producing a sintered hard metal with a diamond film excellent in the adhesion between the sintered hard metal and the diamond thin film. In order to solve the above problems, the inventors have intensively studied, and have found that after a surface layer of a specified metal is formed on a sintered hard metal, when a gas that comprises carbon monoxide and hydrogen gas mixed in a specified ratio and has been activated is brought in contact with the surface layer, a sintered hard metal having a diamond film adhered firmly thereon can be obtained, leading to the present invention. That is, the present invention is directed to a method of producing a sintered hard metal with a diamond film, wherein a gas obtained by activating a raw material gas containing carbon monoxide and hydrogen gas is brought into contact with a sintered hard metal to form a diamond film, characterized in that said raw material gas contains 2 to 60 mol % of carbon monoxide and that the surface of said sintered hard metal has a surface layer consisting of the same metal species as the metal species of said sintered hard metal made of at least one element selected from the group consisting of Si and metals of groups IVa, Va and VIa. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a general view of a reaction apparatus used in the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Sintered Hard Metal: The sintered hard metal used in the present method is employed to serve as a base on which a diamond film will be formed. Use can be made of one containing at least one carbide comprising Si or a metal selected from the group consisting of metals of Groups IVa, Va, and VIa. For example one suitably selected from WC/Co alloys, WC/TiC/Co alloys, WC/TiC/TaC/Co alloys, WC/TiC alloys, WC/TiN alloys, and WC/TiCN alloys. In the present invention, a sintered hard metal containing at least 80 wt. % of a WC component is preferable. In addition to the above sintered hard metals, WC alloys, TiC alloys, TiN alloys, TiCN alloys, TiC/ Mo/Ni alloys, TiC/TiN/Mo alloys, and TiC/TiN/Ni/Mo alloys can be exemplified. The Surface Layer: In the present invention a surface layer of a specified element is previously formed on the surface of the sintered hard metal. This is because if a surface layer of a metal is previously formed on the surface of a sintered hard metal, when a gas obtained by activating a raw material gas that will be described later is brought in contact with said surface layer, the adhesion between the sintered hard metal and the diamond film is more increased than in the case of the conventional sintered hard metal comprising a sintered hard metal, an intermediate layer, and a diamond layer. The reason why the adhesion in the case of the conventional sintered hard metal having a diamond film is not increased is unknown for the present. It is supposed that in the conventional sintered hard metal having a diamond film diamond crystals having a large particle diameter are formed on the intermediate layer, and subtle clearances are formed between the intermediate layer surface and the diamond particles adjacent the intermediate layer, which clearances lower the adhesion. In the present sintered hard metal having a diamond film, the reason why the diamond film has great adhesion will be presumably attributed to the following: when a specific excited gas is brought in contact with a specified metal layer of the present invention to start the reaction, since carbon monoxide is used as a raw material gas immediately after the start of the contact, and the content of the carbon monoxide is relatively high, carbide of said metal is formed and a reaction of the formation of very fine diamond particles also takes place after the start of the reaction so that the carbonizing reaction of the metal and the synthesis reaction of diamond proceed simultaneously, then, since, after the completion of the carbonizing reaction, fine crystalline diamond is formed, there are no clearances between the intermediate layer of the carbide of the metal and the diamond film, and in addition since the formed diamond is in the state of fine crystals, the contact area between the diamond and the surface of the metal carbide is very large. It is important that fine crystalline diamond is formed. Since the diamond is fine crystalline, formation of a diamond film becomes possible, and the stress induced by the difference between the thermal expansion of the diamond and the thermal expansion of the sintered hard metal can be reduced, which also will contribute to the improvement of the adhesion of the diamond film. Further although it has not yet been confirmed, such a possibility is considered that near the surface of the layer of the carbide of the metal formed by the contact of the excited gas, fine crystals of part of the diamond bite into said surface. Anyway, the surface layer of the sintered hard metal before coated with diamond is made of at least one metal selected from the group consisting of Si and metals of Groups IVa, Va, and VIa of the periodic table. As the metals of Groups IVa, Va, and VIa of the periodic table that will form the metal layer can be exemplified Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. Further, in the present invention the species of the metal that forms the sintered hard metal and the species of the metal that forms the metal layer are the same. In this case, the adhesion between the surface layer and the sintered hard metal becomes high. Preferably, the major component of the metal species in the sintered hard metal is tungsten. The layer of Si or a metal of Group IVa, Va, or VIa of the periodic table that will be placed on the sintered hard metal can be formed by a coating process. The coating process includes, for example, vacuum deposition process, ion plating process, CVD process, plasma CVD process MOCVD process, and sputtering process. The thickness of the layer of Si or a metal of Group IVa, Va, or VIa that is placed on the sintered hard metal is 0.05 to 1 µm, and preferably 0.1 to 0.5 µm. When the thickness of the metal layer is less than 0.05 µm, the adhesion between the sintered hard metal and the diamond film cannot be made sufficient, and when the thickness exceeds 1 µm, the metal layer cannot be carbonized completely, and therefore the adhesion cannot be increased sufficiently. For instance, in the case wherein the metal layer is made of tungsten, an imperfect carbide such as W₂C is produced, and it is presumed that this W₂C is brittle in comparison with WC or the like. Further, the particles of the intermediate layer formed by carbonizing the metal layer have a particle diameter of 0.5 µm or below, and particularly preferably on the order of 0.05 to 0.1 µm. When the particles of the intermediate layer have a particle diameter of 0.5 µm or below, the surface of the intermediate layer becomes smooth, the particles of the diamond film that is formed become small, and the adhesion is improved. Raw Material Gas: In the present method, a gas obtained by activating a raw material gas containing hydrogen gas and 2 to 60 mol % of carbon monoxide is brought in contact with the sintered hard metal. The raw material gas may contain an inert gas to such a degree that it does not hamper the object of the present invention. There is no particular limitation on the carbon monoxide to be used, and for example produced gas or water gas that will be obtained by reacting coal or coke with hot air or hot steam followed by sufficient purification can be employed. There is no particular limitation on the hydrogen to be used, and for example hydrogen that is obtained by gasification of petroleums, denaturing of natural gas or water gas, hydrolysis of water, reaction between iron and steam, or complete gasification of coal followed by sufficient purification can be used. In the present method, by bringing a gas obtained by exciting, as a raw material gas, a mixed gas containing carbon monoxide in an amount of 2 to 60 mol %, preferably 3 to 40 mol %, and more preferably 3 to 30 mol % in contact with a surface layer formed on a sintered hard metal, a diamond film can be deposited on the sintered hard metal with the adhesion favorably. If the content of carbon monoxide in the mixed gas is lower than 2 mol % when diamond film is formed on a surface layer made of at least said one element on a sintered hard metal, carbide of said element is not formed enough and the adhesion of the diamond film becomes worse. If the content of carbon monoxide in it is higher than 60 mol % in the above same case, there is no effect by increasing the content of it. Although hydrogen gas is used as a gas for diluting the carbon monoxide gas and also is used as a gas for producing a plasma when the raw material gas is to be plasma-decomposed, the hydrogen gas also has a function for cutting the C-H bond that will be formed on the surface of the sintered hard metal when diamond is formed thereby facilitating the formation of diamond. When this hydrogen gas is used, the concentration of the hydrogen gas in the raw material gas is 98 to 40 mol %, preferably 97 to 60 mol %, and more preferably 97 to 70 mol %. An inert gas can be used as a carrier gas for the mixed gas of carbon monoxide and hydrogen gas. As the inert gas can be mentioned, for example, nitrogen gas, argon gas, neon gas, and xenon gas. Activation of the Raw Material Gas and Formation of Diamond: As means of activating the raw material gas use can be made of any one of various techniques conventionally used for synthesizing diamond. Specifically, examples of means of activating the raw material include various plasma decomposition processes including a process wherein a DC voltage is applied between electrodes to effect plasma decomposition, a process wherein high frequency is applied between electrodes to effect plasma decomposition, a process wherein microwaves are used for plasma decomposition, and an ion beam process wherein plasma decomposition is carried out in an ion chamber or by an ion gun, and ions are taken out by an electric field, and a thermal decomposition process wherein thermal decomposition is effected by heating by a hot filament, with preference given to plasma decomposition process. In the present method, generally a diamond film is formed on a sintered hard metal under conditions given below. That is, although the surface temperature of the sintered hard metal depends on the means of activating the new material gas and therefore cannot be determined generally, the surface temperature is generally 400 to 1,200 o C, and preferably 450 to 1,100 o C for example in the case of plasma decomposition. If this temperature is lower than 400 o C, sometimes the formation rate of the diamond film is decreased. On the other hand, if this temperature exceeds 1,200 o C, sometimes the diamond film deposited on the sintered hard metal is removed by etching, making the formation of the diamond film slow. The reaction pressure is 1.3x10⁻¹ to 1.3x10⁵Pa (10⁻³ to 10³ Torr), and preferably (1 to 800 Torr) 1.3x10² to 1x10⁵Pa. If the reaction pressure is less than 1.3x10⁻¹Pa (10⁻³ Torr), sometimes the formation rate of the diamond film becomes slow. On the other hand, if the reaction pressure is higher than 1.3x10⁵Pa (10³ Torr), the corresponding effect cannot be exhibited, and in some cases it will result in the lowering of the formation rate of the diamond film. The reaction time can be set suitably depending on the desired thickness of the diamond film and on the formation rate of the diamond film. Further, the plasma output in the case wherein the raw material gas is plasma-decomposed is generally 0.1 kW or over. If the plasma output is less than 0.1 kW, sometimes the plasma is not produced sufficiently. The reaction under the above conditions can be carried out, for example, by a reaction apparatus as shown in Fig. 1. Fig. 1 is a general view of a reaction apparatus that can be used in the present method. That is, the raw material gas containing carbon monoxide gas and hydrogen gas is introduced into a reaction vessel 2 via a raw material gas inlet 1. The raw material gas introduced into the reaction vessel 2 is a plasma-decomposed by microwaves or high-frequency waves from a waveguide 3 to be activated, and the carbon in the excited state contained in the activated gas deposits on a substrate made of a sintered hard metal. Since a layer of a specified metal is previously formed on the surface of a sintered hard metal, when the gas obtained by activating the raw material gas is brought in contact with the layer, the layer is carbonized, and formation of diamond occurs partially, so that a diamond film is formed on the carbonized metal layer. As a result, a diamond film good in adhesion is formed. In the present invention, the diamond film formed in the manner mentioned above is a film of diamond and/or diamond-like carbon (DCL). The sintered hard metal having a diamond film obtainable by the present invention can be used suitably for various cutting tools such as bits, cutters, and end mils, draw dies, blanking dies, abrasion resistant machine parts, and the like. Now, the present invention will be described particularly with reference to Examples according to the present invention and Comparative Examples. Examples 1 to 7, Comparative Example 1, and Reference Examples 1 to 3 Each of cutting tips of a sintered hard metal [WC/Co (95/5) (JIS K 10 SPGN 422) was washed with Rangel E (diluted 10 times and having a temperature 50 o C; manufactured by Nikkaseiko Kabushiki-kaisha) three times, then with pure water three times and with isopropyl alcohol three times to remove the soil, grease, rust, etc. from the surface. The period of each washing was 60 sec, the second washing in the three washing operations was carried out by using ultrasonic treatment additionally. Then the surface of the cutting tip was coated with tungsten by vacuum deposition. The conditions of the vacuum deposition were as follows: Degree of vacuum: (10⁻⁵ Torr) 1.3x10⁻³Pa Temperature of the cutting top: 300 o C Deposition speed: (10 Å/sec) 1nm/sec Then the cutting tip was placed as a substrate in the reaction chamber, the output of the microwave power source having a frequency of 2.45 GHz was set at 350 W under such conditions that the substrate temperature was 900 o C, and the pressure in the reaction chamber was 6.6x10³Pa (50 Torr), the total amount of carbon monoxide gas and hydrogen gas as the raw material gas flowed into the reaction chamber was set at 100 sccm, the concentration of the carbon monoxide gas was set as shown in Table 1, and the reaction was carried out for the periods as shown in Table 1, thereby forming a deposit on the substrate controlled to the above temperature. By XRD, the metal layer on the cutting tip after contact with the raw material gas and the intermediate layer of said deposited layer were analyzed to determine the component. When the surface of the deposit was subjected to Raman spectroscopy analysis, a peak due to diamond was observed near 1,333 cm⁻¹ of the Raman spectrum, and thereby it was confirmed that it was a diamond thin film free from impurities. The intermediate layer formed on each cutting tip, and the thickness and the particle diameter (in µm) of the diamond thin film were measured by the SEM image. The cutting tip having a diamond thin film through an intermediate layer was subjected to a cutting test under the following conditions. Material that was cut: Aℓ/8 wt. % Si alloy Cutting speed: 800 m/min Feed: 0.1 mm/rev. Depth of cut: 0.25 mm Cutting time: 10 min, 100 min, 1,000 min After the test, fused matter of the material that was cut was removed by using diluted hydrochloric acid, and the state of the cutting edge of the cutting tip was observed by a scanning type electron microscope (JSM840: manufactured by Nihon Denshi Kabushiki-kaisha). The test results and the observation results thus obtained are shown in Table 1. Example 8 Example 2 was repeated, except that as the sintered hard metal of the cutting tip, JIS K20 [WC/Co/TiC/TaC (93/6/0.5/0.5) was used. The results are shown in Table 1. Comparative Example 2 On the surface of the same washed cutting tip as in Example 1 was formed a WC film having a thickness of 0.5 µm by flowing a mixed gas stream of WF₆, CH₄ and H₂ (having a mixing ratio of 1/2/97) at (50 Torr) 6.6x10³Pa for 30 min while heating it to 900 o C. The cutting tip having the WC film was treated in the same manner as Example 1 to form a diamond thin film thereon. The results are shown in Table 1. According to the present invention: (1) A sintered hard metal having a diamond thin film can be provided wherein the adhesion between the sintered hard metal and the diamond thin film is higher than that of conventional sintered hard metals having a diamond thin film; (2) A tool that is made of a sintered hard metal having a diamond thin film with the adhesion of the diamond film excellent, and is excellent in abrasion resistance and free from exfoliation can be provided; and (3) If the surface of a sintered hard metal is rough, since the surface is coated with a metal layer, the surface on which a diamond thin film will be formed can be made uniform, therefore a dense diamond thin film can be formed, and the above-mentioned sintered hard metal having a diamond thin film with the adhesion of the diamond film excellent can be produced easily by simple operations.

Claims: A method for producing a sintered hard metal having a diamond film wherein a gas obtained by activating a raw material gas containing carbon monoxide and hydrogen gas is brought into contact with a sintered hard metal to form a diamond film, characterized in that said raw material gas contains 2 to 60 mol % of carbon monoxide and that the surface of said sintered hard metal has a surface layer consisting of the same metal species as the metal species of said sintered hard metal made of at least one element selected from the group consisting of Si and metals of groups IVa, Va and VIa. The method according to claim 1, characterized in that the major component of the metal species in said sintered hard metal is tungsten. The method according to claim 1, characterized in that the thickness of said surface layer is 0.05 to 1 µm. The method according to claim 1, characterized in that said surface layer is formed by a vacuum deposition process. The method according to claim 1, characterized in that the activation of said raw material gas is effected by a plasma decomposition process.

Inventors: IIO, SATOSHI, ITO, TOSHIMICHI, KATSUMATA, SATOSHI, WATANABE, MASAKAZU
Applicants: IDEMITSU PETROCHEMICAL CO. LTD., NGK SPARK PLUG CO. LTD.
Application year: 1989
Main classification: C23C16
IPC codes: C23C16, C30B25