VAPOR DEPOSITED DIAMOND SYNTHESIZING METHOD ON ELECTROCHEMICALLY TREATED SUBSTRATE

Abstract: The bond strength of a sintered hard alloy substrate surface is increased by (1) polishing the same electrolytically with a pulse voltage, or further subjecting the resultant substrate surface to electrolytic polishing and thereafter a scratching treatment with abrasive grain, or (2) polishing the substrate surface electrolytically with a steady-state voltage, and further subjecting the resultant substrate surface to electrolytic polishing and thereafter a scratching treatment with abrasive grain. Film type dense diamond having a high bond strength is then deposited on this substrate by a vapor deposition method.

Description: This invention relates to a method for the deposition of a diamond film on a hard metal substrate by a vapor phase diamond synthesis process for the manufacture of a cutting tool or the like, and more particularly to a method for the deposition of dense and high adhesive diamond on a substrate. Known methods of treatment employed in the deposition of a diamond film by vapor phase processes on hard metal substrates for the purpose of enhancing adhesive force between the deposited diamond film and the substrate include; etching with a chemical substance such as an acid (EP-A-251264) or an alkali (48th Scientific Lecture Meeting, 18a-T-4, Japan Applied Physics Society), scratching treatment with diamond powder (48th Scientific Lecture Meeting, 18a-T-5, Japan Applied Physics Society), and etching in a specific gas containing an alcohol (Japanese Patent Public Disclosure HEI 1-145396. However, when, for example, a tool manufactured by forming a diamond film on hard metal lathe tip treated by one of these methods is actually used in cutting, the diamond film quickly exfoliates from the tip. Thus adhesive strength of a practical level cannot be obtained. Therefore, an object of this invention is to provide a method for the deposition of a diamond film with high adhesive strength on a hard metal substrate surface by the vapor phase process for the synthesis of diamond. As a result of their study in search of a way of enhancing adhesive strength between a deposited diamond film and a hard metal substrate, the present inventors found it advantageous to provide protuberances serving as anchors on the surface of the substrate. These protuberances can be readily produced when the substrate surface is subjected to an electrochemical treatment, namely electrolytic polishing. Furthermore, the adhesive strength of the diamond film to the substrate increases with increasing thickness, length, and density of the protuberances. In the present invention, when the hard metal substrate surface is subjected to electrolytic polishing with a pulse voltage, the protuberances formed on the surface of the hard metal substrate can be varied in size and density according to variations of the frequency and pulse width of the pulse voltage applied to said substrate. The invention may further comprise scratching with an abrasive powder the substrate surface produced by the aforementioned electrolytic polishing. Indeed, the electrolytic polishing in this case need only be performed with a constant applied voltage. When a diamond film is deposited by a vapor phase process on the surface of a substrate treated according to the methods just mentioned, a highly adhesive composite is obtained on the substrate, comprising the protuberances and the diamond film. Figure 1 represents a device used in the synthesis of diamond by the combustion flame method in Example 1. Figure 2 represents a device used in the synthesis of diamond by the hot filament method in Examples 2 and 3. Certain embodiments of the present invention will be described in detail below. Hard metal refers to what is obtained by sintering a carbide, nitride, boride, or silicide of a metal of Groups IV, V, or VI in the Periodic Table of Elements by the use of a binder of Co, Ni, or Fe or an alloy thereof. A typical example is WC-Co, and when a substrate of such a hard metal is subjected to electrolytic polishing with a pulse voltage, the surface of this substrate is melted and forms innumerable protuberances. In this case, a group of suitable protuberances is formed because Co is mainly melted and WC is also partially melted. The electrolyte to be used herein is preferably a mineral acid such as, for example, HCl, H₂SO₄, or HNO₃. The concentration of the mineral acid may be approximately in the range of 1 to 50% and an aqueous solution of 5 to 30% by weight of HC1 proves to be particularly desirable. The aqueous solutions of NaOH and KOH and mixtures thereof with aqua ammonia are also usable. Their concentrations are approximately in the range of 1 to 50%. The current density is preferably in the range of 0.1 to 2 A/cm². A polishing time in the range of 1 to 20 minutes with the hard metal substrate as an anode is sufficient for obtaining innumerable protuberances for a variety of solution types, temperatures, solution concentrations and current densities. When the frequency of the pulse voltage is increased and the pulse width decreased, the protuberances formed on the surface of the hard metal substrate tend to increase in size and density. The results of the electrolysis depend to some extent on the kind of hard metal used and the conditions, such as the current density. When WC-6% Co is subjected to electrolytic polishing in an aqueous 20% HC1 solution at a temperature of 20°C and a current density of 0.5 A.cm², protuberances 2µm thick and 3µm in length are formed in a density of 5 x 10³ pieces/mm² with a voltage applied under the conditions of 1/2 Hz in frequency and 120 mS in pulse width. Protuberances 4 µm and 6 µm in length are produced in a density of 1 x 10⁴ pieces/mm² when the pulse voltage is applied under the conditions of 2 Hz in frequency and 80 mS in pulse width. In the latter case, the protuberances are larger and more density than those produced in the former case and the adhesive strength is 1.5 times the magnitude obtained in the former case using the pulse voltage 1/2 Hz in frequency. In the method of this invention, the time required for the formation of protuberances is long when the time of application of the pulse voltage is unduly short. On the other hand, when the time of application of pulse voltage is too long, the result is the same as in constant voltage electrolytic polishing. Further, when the frequency is unduly large, the migration of ions in the electrolytic solution falls short of the required level and no effect is obtained. The protuberances obtained by the method of this invention are not more than 200 µm in thickness, not more than 150 µm in length, and 1 to 1 x 10⁶ pieces/mm² in density. For the hard metal substrate to be effectively coated with a diamond film, the protuberances preferably have a thickness in the range of 1 to 50 µm, a length inthe range of 1 to 50 µm, and a density in the range of 1 x 10³ to 1 x 10⁵ pieces/mm². When a pulse voltage is used for the electrolytic polishing, the pulse width is in the range of 1 m.sec to 1 sec and the pulse frequency is in the range of 500 Hz to 1/10 Hz. When diamond is formed by a vapor phase process on a hard metal substrate having a surface which has been electrolytically polished with a pulse voltage as described above, a diamond film can be deposited with extremely high adhesive strength on the surface of the substrate. The further process by performing a scratching treatment with an abrasive powder on an electrolytically polished surface for enhancing the substrate adhesive strength will be described below. The scratching treatment is especially effected when the electrolytic polishing has been carried out by the use of a constant voltage. When the surface of the substrate is subjected to the scratching treatment with an abrasive powder, the fastness of adhesion of the diamond film to be deposited thereon is heightened. The substances usable herein for the abrasive powder include diamond, CBN, Al₂O₃, and SiC, for example. Among the foregoing substances, diamond proves to be particularly desirable. In this case, the grinding is desired to be performed to an extent such that the protuberances already formed on the surface thereof are not lost. This grinding with the abrasive powder is effectively carried out, for example, by using so-called diamond paste, i.e. a paste containing diamond dust, or by utilizing ultrasonic vibration generated in a suspension of diamond dust or other abrasive powder in alcohol or other solution. In consequence, of this treatment, the surface of the substrate sustains innumerable scratches of a size about 1/5 of the diameter of the particles of the abrasive powder. Since diamond nuclei are generated from the sites of these scratches, the generation of nuclei proceeds uniformly and the diamond film eventually formed enjoys improvement in homogeneity, surface smoothness, and adhesive strength. Though the pressure with which the hard metal substrate is ground with abrasive powder such as diamond dust is variable according to the material of the hard metal substrate and the time of the electrolytic polishing, a very small pressure not exceeding 5 kg/cm² generally suffices for the grinding, said abrasive powder is desired to be approximately in the range of 0.5 to 10 µm, preferably 1 to 5 µm. When the electrolytic polishing of the aforementioned substrate is carried out by the use of constant voltage, the conditions to be used are substantially identical with those used for the electrolytic polishing using the pulse voltage and protuberances are formed on the surface of the substrate. When a diamond film is formed on the substrate having the above-mentioned surface condition, however, the protuberances form the sites for the generation of nuclei and the diamond film consequently formed lacks uniformity and is deficient in adhesive strength. By combining electrolytic polishing with a scratching treatment as mentioned above, however, the diamond film can be formed homogeneously with high fastness of adhesion. When the hard metal substrate treated as escribed above is placed in an environment for the formation of diamond by vapor phase process and then subjected to the process, a diamond film rich in density and adhesive strength is deposited on the substrate. This process is particularly suitable for the combustion flame process which comprises blowing the flame if a gas of a precursor for diamond against the substrate with the aid of a burner. In this combustion flame process, the substrate is preferably set within the region of incomplete combustion in the combustion flame. When a hard metal having undergone the surface treatment as described above is used as a substrate and a diamond film synthesized by the vapor phase process is formed on the substrate, the protuberances and the scratches inflicted by the abrasive powder allow the surface of the diamond film to acquire an increased surface area for contact with the substrate. Owing to the anchoring effect of the surface ruggedness, the diamond film adheres powerfully to the substrate. Now, the present invention will be described more specifically below with reference to working examples and comparative experiments. [Example 1] In an aqueous 10% HC1 solution, electrolytic polishing was performed with a substrate (12 mm x 12 mm x 2 mm) of a WC hard metal containing 6% of Co as an anode and a carbon bar as a cathode. The voltage was 3V, the current 0.5A, and the polishing time 5 minutes. Since the surface of the hard metal substrate was covered with an oxide after the electrolytic polishing, it was washed with an aqueous 10% NaOH solution. On observation the washed surface was found to be formed with protuberances 3 µm in thickness, 5 µm in length, and 3 x 10³ pieces/mm² in density. The surface of the hard metal substrate was subjected to a scratching treatment with a paste containing diamond particles 1 µm in diameter, washed with alcohol, and subjected to synthesis of a diamond film. For this synthesis, the combustion flame process depicted in FIG. 1 was used. The reaction furnace 1 measured 35 cm in diameter, 50 cm in height, and 48 liters in inner volume. This reaction furnace 1 was provided in the upper part thereof with a burner 6 incorporating therein a flame inlet 2 and in the lateral part thereof with an exhaust pipe. A water cooling base 3 was set below the flame inlet 2 and a hard metal substrate 4 was placed on the water cooling base 3. The distance between the flame inlet 2 and the substrate 4 was 1 cm. The inner pressure of the reaction furnace was fixed at 39900 Pa (300 Torrs). By feeding acetylene gas at a flow rate of 5 liters/min and oxygen gas at a flow rate of 4.5 l/min to the flame inlet, a diamond film was synthesized for 30 minutes within the region 7 of incomplete combustion. After that, to determine the adhesive strength between the substrate and the diamond film, a Rockwell pressing piece of diamond having a tip of a spherical surface 0.2 mm in radius and an apical angle of 120° was forced into the diamond film and the load required for the pressing piece to separate this film from the substrate was measured. As a result, this load was found to be 20 kg. [Comparative Experiment 1] The same substrate as used in Example was subjected to electrolytic polishing in an aqueous HCl solution under the same conditions. The oxide formed on the surface of the substrate was removed with an aqueous NaOH solution. Then, the substrate was subjected to synthesis of a diamond film without undergoing a scratching treatment using a diamond paste. The method of diamond synthesis and the conditions used therefor were the same as those in Example. The synthesized diamond film on observation was found to have a notably jagged surface as compared with the diamond film obtained in Example 1 and revealed the underlying hard metal substrate at a few points. In the same test for adhesive strength as in Example, the load required for the separation of the diamond film was 5 kg. [Comparative Experiment 2] The same substrate as used in Example was subjected, without undergoing an electrolytic polishing, to a surface scratching treatment with a paste of diamond particles 1 µm in diameter and then subjected to synthesis of diamond film. The method of this synthesis and the conditions therefor were identical with those of Example 1. After synthesis, the synthesized diamond film was found to be already separated from the substrate. [Example 2] On the hard metal substrate which had undergone the same surface treatment as in Example 1, a diamond film was synthesized by the hot filament process. FIG. 2 is a diagram illustrating a device used for this synthesis. The reaction furnace 1 measured 15 cm in diameter, 30 cm in length and 5 liters in inner volume. It was provided therein with a hot filament 8' as a source of excitation, a hard metal substrate 4', and a substrate supporting base 10'. Denoted by 9' is a raw gas feeding pipe and 5' an exhaust pipe. In this device, a diamond was synthesized by using a feed gas consisting of hydrogen fed at 100 cc/min and ethanol at 3 cc/min, keeping the temperature of the hot filament at 2450°C, maintaining a distance of 5 mm between the substrate and the filament, and setting the inner pressure at 11.97 KPa (90 Torrs). In the same test for adhesive strength as in Example, the load required for separation of the synthesized diamond film was found to be 16 kg. [Example 3] In an aqueous 10% HCl solution as an electrolytic solution using a substrate (12 mm x 12 mm x 2 mm) of a WC hard metal containing 6% of Co as an anode and a carbon bar as a cathode, electrolytic polishing was performed for 30 minutes under the conditions of 3V in voltage, 0.5 A in current, 1/2 Hz in pulse frequency, and 120 mS in pulse width. Since the surface of the hard metal substrate resulting from the electrolytic polishing was covered with an oxide, it was washed with an aqueous 10% NaOH solution for removal of the oxide. Then, the surface of the substrate was subjected to a scratching treatment with a paste containing diamond particles 1 µm in average diameter and then washed with alcohol. Then, on the substrate resulting from the treatment mentioned above, diamond was deposited by the hot filament process using a device constructed as illustrated in FIG. 2. The reaction furnace used in this example measured 25 cm in diameter and 20 cm in height. The distance between the substrate and the hot filament was 5 mm. As raw materials, gasified ethanol was fed at 3 cc/min and hydrogen at 100 cc/min through a feed inlet 5' into the reaction furnace. Under these conditions, the reaction for deposition of diamond was continued for 3 hours under a pressure of 11.97 KPa (90 Torrs). Consequently, a diamond film 15 µm in average thickness was deposited on the substrate. The diamond was identified by observation under an optical microscope and analysis with a Raman spectroscope. Then, to determine the adhesive strength of the diamond film to the substrate, a Rockwell pressing piece of diamond having a tip of a spherical surface 0.2 mm in radius and an apical angle of 120° was forced into the diamond film and the load required for the pressing piece to separate this film from the substrate was measured. Consequently, this load was found to be 25 kg. [Example 4] A diamond film was formed on a hard metal substrate treated in entirely the same manner as in Example 3, except that a constant voltage was used in place of the pulse voltage. The load required for the separation of the diamond film from the substrate was measured in the same manner as in Example. The load was found to be 12 kg. [Example 5] The same substrate as used in Example 3 was subjected to electrolytic polishing using a pulse voltage under the same conditions. On the substrate resulting from this electrolytic polishing, a diamond film was synthesized without scratching treatment using a paste containing diamond. The synthesis was performed by the hot filament CVD process under the same conditions as in Example 3. When the deposited diamond film was observed after synthesis, it was found to contain a somewhat larger number of recesses and protuberances than the diamond film deposited in Example 3. The load required for the separation of this diamond film was 9 kg. [Example 6] The same substrate as used in Example 3 was subjected to electrolytic polishing using a pulse voltage under the same conditions. The substrate was then subjected to a scratching treatment using a diamond paste in the same manner as in Example 3. For the synthesis of a diamond film, the combustion flame process was carried out under the same conditions as in Example 1. The diamond film resulting from the synthesis, on observation, was found to have a thickness of about 20 µm and possess high surface smoothness. In the test for adhesive strength performed in the same manner as in Example 3, the load required for the separation of the diamond film was found to be 27 kg. Industrial Applicability In accordance with this invention, a diamond film can be deposited with extremely high adhesive strength on a hard metal substrate and a tool furnished with a diamond film capable of enduring harsh working conditions can be manufactured.

Claims: A method for the vapor phase synthesis of diamond onto a hard metal substrate surface, characterized by; electrolytically polishing said hard metal substrate surface to cause the formation of a group of protuberances on the surface thereof, thereby heightening the force of adhesion to said surface, and depositing a dense diamond film on said substrate by a vapor phase process, thereby forming a strongly adhered diamond film on said substrate. A method according to claim 1, wherein said electrolytic polishing is effected with a pulse voltage. A method according to claim 1, wherein said electrolytic polishing is effected with a constant voltage. A method according to any preceding claim, wherein said electrolytic polishing is followed by a scratching treatment with an abrasive powder. A method according to claim 2, wherein said pulse voltage has a pulse width in the range of 1 ms to 1 s and a pulse frequency in the range of 500 Hz to 1/10 Hz. A method according to any preceding claim wherein the electrolytic solution to be used for said electrolytic polishing is a mineral acid. A method according to claim 6, wherein said mineral acid is hydrochloric acid and said hydrochloric acid is an aqueous 5 to 30 wt% hydrochloric acid solution. A method according to any preceding claim, wherein said protuberances have a length in the range of 1 to 50 µm, a thickness in the range of 1 to 50 µm, and a maximum density in the range of 1 x 10³ to 1 x 10⁵ pieces/mm². A method according to any preceding claim, wherein said vapor phase process is a combustion flame process and said substrate is disposed in a diamond forming region thereof. A method according to claim 9, wherein said diamond forming region is in the region for incomplete combustion in the combustion flame. A method according to claim 3, wherein said abrasive powder used for said scratching treatment is the powder of at least one member selected from the group consisting of diamond, CBN, A1₂O 3, and SiC. A method according to claim 11, wherein said scratching treatment comprises forming a paste of said abrasive powder and grinding the electrolytically polished surface with said paste. A method for the vapor phase synthesis of diamond according to claim 12, wherein said abrasive powder is diamond and said paste is a diamond paste. A method according to claim 3, wherein said scratching treatment is carried out by immersing said electrolytically treated substrate in a solution having said abrasive powder suspended therein and then imparting ultrasonic vibration to said suspension. A method according to claim 14, wherein said solution comprises a suspension of diamond abrasive powder in alcohol.

Inventors: KOMAKI KUNIO CENTRAL RESEARCH, YANAGISAWA MASAAKI CENTRAL RES, KOMAKI, KUNIO CENTRAL RESEARCH LABORATORY, YANAGISAWA, MASAAKI CENTRAL RESEARCH LABORATORY
Applicants: SHOWA DENKO KK, SHOWA DENKO KABUSHIKI KAISHA
Application year: 1990
Main classification: C23C16
IPC codes: C23C16