Magnetic recording medium manufacturing method and manufacturing apparatus and magnetic recording medium

Abstract: The present invention is directed to methods and apparatus for preparing a magnetic recording medium having a nonmagnetic substrate layer, a back-coat layer containing a metal or semi-metal which has been deposited in a vacuum on one surface of the substrate, a magnetic layer which has been deposited in a vacuum on the other surface of the substrate, and further a top-coat layer which has been sprayed also in a vacuum onto the magnetic layer. Oxygen gas is introduced during depositing of the back-coat layer to obtain a suitable surface resistance of 5 to 10⁸ Ω.

Description: The present invention relates to a magnetic recording medium manufacturing method, a magnetic recording medium manufacturing apparatus and a magnetic recording medium. A magnetic recording medium such as a magnetic tape may include the following two types in regard to a magnetic layer: a conventional application-type tape fabricated by applying magnetic coating material, which is made by dispersing magnetic powder in a binder, to a film serving as a nonmagnetic substrate; the other is a deposition-type tape fabricated by depositing metal on a film without any binder, in a vacuum. The deposition-type tape is desirable for high-density recording because it does not contain binder in a magnetic layer, so that the density of the magnetic material can be increased. Fig. 3 shows the main constitution of a magnetic recording tape that is currently commercially available. A nonmagnetic support (51), e.g. a film, is made of PET (polyethylene terephthalate), polyimide or aramid. The film thicknesses are, for example, 2 to 50 µm. A magnetic layer (52) is formed by depositing Co-Ni alloy (80-20%) on the nonmagnetic support (51) by a vacuum deposition method to have a thickness of 1,500 Å. An undercoat layer optionally may be formed between the nonmagnetic support (51) and the magnetic layer (52) in order to improve the adhesive property of the magnetic layer (52). A top-coat layer (53) is formed to protect the magnetic layer (52) and serves as a lubricant for the smooth contact with a recording or a replaying head. The coating method may be either a gravure technique or a die-coating technique. A fluorine-containing lubricant such as perfluoropolyether (e.g. FOMBLIN (tradename) produced by Montedison S.p.A.) is generally used as the component for the top-coat layer. A back-coat layer (54) is made of a coating material prepared by dispersing carbon black (grain diameter of 10 to 100 nm) in a binder (e.g. polyvinylchloride, urethane or nitrocellulose; used alone or in combination). The coating material is applied to the surface of the nonmagnetic support (51) opposite to the deposition surface of the magnetic layer (52) by a gravure, a reverse coating or a die-coating technique, so that the thickness after drying is 0.4 to 1.0 µm. The back-coat layer mainly has the following functions: (1) To prevent dust from adhering to the tape because of the antistatic effect obtained by imparting an electrical conductivity to the layer. (2) To stabilize the tape transport by controlling the surface property (friction coefficient). (3) To prevent warpage by balancing the properties of both sides of the tape in view of hardness and so on. Function (1) is met by the conductivity of carbon black; function (2) is met by selecting the grain diameter of carbon black and by controlling the application condition, and function (3) is met by selecting the binder and by controlling the thickness. However, when applying the back-coat layer before vacuum-depositing the magnetic layer, gas generation from residual solvent in the coated layer occurs in the vacuum system and the degree of vacuum is lowered. As a result, the deposition of the magnetic material is not well performed. Therefore, conventionally, after the magnetic layer is deposited in a vacuum, the tape is taken out into the air and the back-coat layer is coated in the air. However, this method involves a problem with air borne dusts adhering to the film and contaminating the magnetic layer in the step of applying the back-coat layer, increasing the number of dropouts found in the dropout inspection (inspection for detecting dropouts in the reproduced signals due to scratches of the tape surface or foreign matter adhering to the surface, wherein the magnetic tape is loaded in an inspection cassette deck and predetermined signals are recorded and reproduced simultaneously). Moreover, the conventional method causes a problem in that the electrical conductivity of the back-coat layer is not sufficient and consequently the antistatic effect thereof is marred, because carbon black is mixed with an organic binder. The desirable electric conductivity is, as a resistance value (surface resistance), 10 7 Ω or less. Although the surface resistance for a marketed 8-mm video tape is about 10 6 Ω, a higher electric conductivity is still demanded. On the other hand, a fluorine-containing lubricant, which is frequently used for the top-coat layer of a metallic-thin-film-type magnetic recording medium, is partially decomposed at high temperature (for example at 200°C) and its vapour pressure is low. Therefore, the preparation of the top-coat layer is carried out in the air separately, which is also a factor with respect to the productivity of the magnetic recording medium. The problem of dust also occurs in this process. JP-A-60 191435 refers to a magnetic recording medium including a non-magnetic substrate (polyester film), a ferromagnetic film layer deposited under vacuum on the substrate as well as a top-coat layer and a back-coat layer which are prepared by a plasma polymerization process under vacuum. In consideration of the above disadvantages in the prior art, an object of the present invention is to provide a magnetic recording medium having excellent performance by solving the problems discussed above. Another object of the present invention is to further improve the productivity of a metallic-thin-film-type magnetic recording medium. The present invention provides a method for manufacturing a magnetic recording medium having a magnetic layer on one side of a nonmagnetic substrate, a back-coat layer on the opposite side of the nonmagnetic substrate and a top-coat layer on the magnetic layer, wherein the back-coat layer, the magnetic layer and the top-coat layer are formed by being deposited in a vacuum, and the vacuum is maintained throughout the process for providing said layers, characterized in that the top-coat layer is formed by spraying a liquid lubricant. According to a preferred embodiment a metal or semi-metal layer is formed, in a vacuum, onto a surface other than the magnetic layer-deposited surface of a nonmagnetic substrate to form a back-coat layer. It is preferable that the metal or semi-metal is deposited in a vacuum on the surface, while an oxidizing gas is introduced. Preferably, the back-coat layer is provided by depositing aluminum in a vacuum under an introduction of the oxidizing gas. According to the invention a method for manufacturing a magnetic recording medium is provided, where a top-coat layer is formed on a magnetic layer by spraying a liquid lubricant in a vacuum. Preferably, the lubricant is a fluorine-containing lubricant; the lubricant can be atomized by being exposed to ultrasonic waves and the resulting lubricant liquid particles are sprayed on the surface of the magnetic layer. The invention also provides an apparatus for manufacturing a magnetic recording medium according to the process of the present invention. The invention also provides a magnetic recording medium obtainable by the process of the present invention. Preferably, the back-coat layer is a vacuum deposited layer of a metal or an oxide thereof having a surface resistance of from 5 to 10 5 Ω, or that of a semi-metal or an oxide thereof having a surface resistance of from 10 5 to 10 8 Ω. The present invention also provides a magnetic recording medium having a back-coat layer of aluminum oxide. Following is a brief description of the drawings: Fig. 1 is a schematic view of the vacuum deposition apparatus used for an embodiment of a manufacturing method of the present invention; Fig. 2 is a schematic view of an example of the magnetic recoridng medium manufacturing apparatus of the present invention; and Fig. 3 is a cross section of a magnetic recording tape. The invention provides a method for manufacturing a magnetic recording medium having a magnetic layer on one side of a nonmagnetic substrate, a back-coat layer on the opposite side of the nonmagnetic substrate and a top-coat layer on the magnetic layer, wherein the back-coat layer, the magnetic layer and the top-coat layer are formed by being deposited in a vacuum, and the vacuum is maintained throughout the process for providing said layers, characterized in that the top-coat layer is formed by spraying a liquid lubricant. The process of the invention is carried out by providing the magnetic layer, the back-coat layer and the top-coat layer in a vacuum under a condition that the vacuum is maintained throughout the process for providing any two or all the layers. Furthermore, any two or all of the magnetic layer, the back-coat layer and the top-coat layer, can be provided in a vacuum simultaneously or sequentially in arbitrary order. From a view-point of productivity and performance of the product, the present invention is preferably carried out by providing all of said layers while transporting the nonmagnetic substrate in a vacuum in such a manner that the vacuum is maintained during the whole steps of forming the layers. Although the degree of vacuum can be selected for each step of providing the layers according to the forming method as described below, it is advantageous to carry out the whole process without interruption of atmospheric pressure. In case where those layers are sequentially formed, the order of the forming can be chosen arbitrarily; namely (1) by providing a back-coat layer, providing a magnetic layer and then providing a top-coat layer, (2) by providing a magnetic layer, providing a top-coat layer and then providing a back-coat layer, or (3) by providing a magnetic layer, providing a back-coat layer and then providing a top-coat layer, while transporting the nonmagnetic substrate in a vacuum. The nonmagnetic support of the present invention is usually made of a plastic. Suitable plastics include polyester, such as polyethylene terphthalate (PET) or polyethylene naphthalate; polyolefin, such as polyethylene or polypropylene; cellulose derivative such as cellulose triacetate or cellulose diacetate; polycarbonate; polyvinyl chloride; polyimide; and aromatic polyamide. Generally, the thickness of these nonmagnetic supports ranges from 2 to 50 µm. The back-coat layer according to the present invention is advantageously formed by vacuum deposition or sputtering technique; e.g. by depositing a metal or a semi-metal in a vaccum. Thus, the problems associated with the prior art, including adhesion of dust etc., do not occur when the back-coat layer is formed in a vacuum according to the present invention. It is thereby possible to perform vacuum deposition of the magnetic layer and the back-coat layer simultaneously or sequentially. It is also possible to perform vacuum deposition of the layers in separate steps, that is, to perform vacuum deposition of one layer on the nonmagnetic substrate and wind the substrate on a roller, and then to perform vacuum deposition of the other layer while unwinding the tape already having one layer deposited thereon. When performing vacuum deposition of the layers in separate steps, the problems of dust adhesion may not occur unless the roll is unwound in the air atmosphere even though the roll is exposed to the air. In other words, at least the unwound portion had better be unexposed to the air. When the back-coat layer is formed through a simple vacuum deposition, it may happen that the transport stability of the tape is impaired because the surface is too smooth. To solve the problem, oxidizing gas is peferably introduced during the vacuum deposition in order to control the friction coefficient of the back-coat layer by partly oxidizing and roughening the surface. The oxidizing gas may include oxygen gas, air and ozone gas, or a mixture thereof. In this case, not only the surface roughness but also the surface resistance of the back-coat layer can be controlled by selecting the amount of the oxidizing gas to be introduced. It is understandable that the metallic bond can be weakened and the conductivity decreased by oxidization. If the quantity of the oxidizing gas is too small, the coefficient of friction and the surface resistance of the back-coat layer are still small. On the other hand, if too much oxidizing gas is introduced, the friction coefficient and also the surface resistance extremely increase. This may cause adhesion of dust. In case of vacuum deposition of a metal, the preferred range for resistance is from 5 to 10 5 Ω (ohms), preferably about 10 2 to 10 3 Ω. In this case, the center-line average roughness (Ra) is about 8 to 20 nm. Although there are various metals to be deposited as the back-coat layer, Al, Cu, Zn, Sn, Ni, Ag or alloys thereof are used. However, Al or Cu is most preferable in view of the price, deposition rate and stability after oxidation. Semi-metals to be deposited are, for example, Si, Ge, As, Sc and Sb. Si is preferable in view of economics and evaporation rate. The amount of the fed oxygen gas may be preferably adjusted so that the back-coat layer may have a surface resistance in the range of 10 5 to 10 8 Ω, preferably of 10 6 to 10 7 Ω. The thickness of the back-coat layer ranges preferably from 0.2 to 1.0 µm. The degree of vacuum for the deposition to form the back-coat layer ranges from 1.33·10 -2 to 1.33·10 -5 Pa (10 -4 to 10 -7 Torr), preferably 1.33·10 -3 to 1.33·10 -4 Pa (10 -5 to 10 -6 Torr). Those metals and semi-metals described above can be used for forming a back-coat layer. It is especially preferable to evaporate Al by an electron beam and, at the same time, inroduce oxidizing gas into Al vapour through a gas inlet tube, and form the back-coat layer of a thin film of aluminum oxide. According to the magnetic recording medium manufacturing method of the invention a top-coat layer can be formed by spraying a liquid lubricant on the magnetic layer while transporting a nonmagnetic support in a vacuum. It is preferable to spray a liquid lubricant on the magnetic layer formed on the nonmagnetic support by using an atomizer equipped with an ultrasonic wave generator (hereinafter referred to as an ultrasonic atomizer). By using the ultrasonic atomizer to spray a lubricant in the form of fine particles, it is possible to spray, in a vacuum, a fluorine-containing lubricant such as perfluoropolyether, though such a fluorine-containing lubricant has been conventionally applied in the air to form a top-coat layer because the lubricant is unstable at high temperature, e.g. 200°C or above, and whose vapour pressure is low. Perfluoropolyether with a molecular weight of 2,000 to 5,000 is preferable. For example, marketed lubricants "FOMBLIN Z DIAC" (modified with carboxylic groups, produced by Montecatini Edison S.p.A.) and "FOMBLIN Z DOL" (modified with an alcohol(s), produced by Montecatini Edison S.p.A.) are available. Because these products have a hydroxyl group or a carboxyl group in the molecule, the adhesion between the lubricant and magnetic layer is increased. Therefore, these lubricants are particularly preferable for the present invention. In addition to the above lubricants, fluorine-containing lubricants further comprising benzene rings, double bonds and branched chains, fatty-acid derivatives and other lubricants can be used. However, the fluorine-containing lubricant is particularly preferably used for the present invention, because it improves not only durability but also corrosion resistance as compared with the fatty-acid-based lubricants. To spray a lubricant, it is preferable to use a solution of 0.001 to 10 wt.%, preferably 0.02 to 2.0 wt.%, of the lubricant made by dissolving the lubricant in an adequate solvent such as a fluorine-containing inactive solvent (e.g. perfluorocarbon such as "Fluorinert" produced by Sumitomo 3M Ltd.), an aromatic-hydrocarbon-type solvent such a toluene, an alcohol-type solvent or a ketone-type solvent. When using perfluoropolyether as a lubricant, perfluorocarbon can be used as a solvent. In this case, it is preferable that the solvent concentraiton is in a range of 0.001 to 1.0 wt.%, preferably 0.05 to 0.2 wt.%. In the manufacturing method of the present invention it is desirable that the fine particles of the lubricant to be sprayed (lubricant solution) have a diameter as small as possible. Though the ultrasonic wave frequency to be applied is determined by the type and viscosity of the lubricant, it is generally selected from a range of 20 kHz to 10 MHz. In the manufacturing method of the present invention it is possible to suitably determine the quantity of lubricant to be sprayed by considering the purpose of the magnetic recording medium and the type of the lubricant. However, it is preferable that the thickness of the formed top-coat layer is adjusted to be in the range of 50 to 200 Å. The degree of vacuum to spray the lubricant ranges from 67 to 6 7·10 -2 Pa (5 x 10 -1 to 5 x 10 -4 Torr), preferably 13 to 1.3 Pa (10 -1 to 10 -2 Torr). The method of the present invention is characterized by forming a back-coat layer, a magnetic layer and a top-coat layer of a magnetic recording medium in a vacuum. The steps other than the above steps, including dust removal and smoothing, surface treatment, cutting, winding and assembling of the magnetic layer, are performed by known methods. Ferromagnetic metallic materials used for manufacturing a normal metallic-thin-film-type magnetic recording medium may be used for forming the magnetic layer of the present invention. For example, ferromagnetic metal such as Fe, Co and Ni or ferromagnetic alloy such as Fe-Co, Fe-Ni, Co-Ni, Fe-Co-Ni, Fe-Rh, Fe-Cu, Co-Cu, Co-Au, Co-Y, Co-La, Co-Pr, Co-Gd, Co-Sm, Co-Pt, Ni-Cu, Mn-Bi, Mn-Sb, Mn-Al, Fe-Cr, Co-Cr, Ni-Cr, Fe-Co-Cr and Ni-Co-Cr. Especially, it is preferable to use a thin film of iron or a thin film of ferromagnetic alloy, containing mainly iron, for the magnetic layer. For example, iron nitride, iron carbide, or nitride or carbide of ferromagnetic alloy containing mainly iron. For high-density recording, it is preferable to form the magnetic layer of a magnetic recording medium on a base material, e.g. a nonmagnetic substrate, by means of oblique deposition. The technique of oblique deposition is not restricted, and any known technique can be adopted. The degree of vacuum for deposition ranges from 1.33·10 -2 to 1.33·10 -5 Pa (10 -4 to 10 -7 Torr). The magnetic layer formed by deposition can be of a single layer structure or a multilayer structure. Particularly, the durability can be improved by forming an oxide on the magnetic layer surface by introducing oxygen gas or air. According to the present invention, the magnetic layer can be formed in a single layer or a multilayer. To form a two-layer magnetic layer through deposition, it is preferable that the inner magnetic layer thickness is in the range of 100 to 2,000 Å and the outer magnetic layer thickness is in the range of 50 to 1,000 Å. To form a three-layer magnetic layer, it is preferable that the bottom magnetic layer thickness is in the range of 100 to 2,000 Å, the middle magnetic layer thickness is in the range of 100 to 1,000 Å, and the top magnetic layer thickness is in the raqnge of 50 to 1,000 Å. A larger number of magnetic layers is better for high-frequency recording. Two to five layers may be adequate for practical use. Moreover, the present invention provides a magnetic recording medium manufacturing apparatus for manufacturing a magnetic recording medium having a magnetic layer on one side of a nonmagnetic substrate, a back-coat layer on the opposite side of the nonmagnetic substrate and a top-coat layer on the magnetic layer, said apparatus comprising a housing provided with first, second and third chambers and vacuum means for keeping each of the chambers in a vacuum, the first chamber having a first cooling can for transporting a nonmagnetic substrate and a first deposition means for depositing a back-coat layer disposed under the first cooling can; the second chamber having a second cooling can for transporting the nonmagnetic substrate and a second deposition means for depositing a magnetic layer disposed under the second cooling can; the third chamber having a heating can for transporting the nonmagnetic substrate and a spraying means for forming a top-coat layer on the magnetic layer by spraying a liquid lubricant disposed under the heating can, and the first, second and third chambers or the second, third and first chambers or the second, first and third chambers are adjacently arranged in this order, the adjacent chambers being made to communicate with each other through an opening for passing a nonmagnetic substrate. In the manufacturing apparatus of the present invention, a conventional device such as a vacuum pump is used as the vacuum means, and the chambers generally communicate through slits for passing the nonmagnetic support. The manufacturing apparatus of the present invention makes it possible to start forming either of the magnetic layer and back-coat layer in the first and second chambers. Therefore, to form the magnetic layer by the first chamber, a magnetic metal, as mentioned before, is selected as the metallic material. For the process in the second chamber, a metal or semi-metal to form the back-coat layer previously mentioned is selected. To reverse the order of formation of the layers, the metals are reversely selected for the first and second chambers. In the case of the manufacturing apparatus of the present invention, each oxygen gas introduction means comprises an oxygen gas introduction tube and a proper opening. The manufacturing apparatus of the present invention is provided with transport rollers and a shielding plate for demarcating the inside of the chambers when necessary. The following are embodiments of the manufacturing apparatus of the present invention but do not limit the scope thereof. (1) In one embodiment of the magnetic recording medium manufacturing apparatus of the present invention, the first and/or second deposition means include(s) a metallic material vessel opening to the first or second cooling can and an electron beam generator for applying an electron beam to a metallic material in the vessel. (2) In another embodiment of a magnetic recording medium manufacturing apparatus of the present invention, the first or second chamber is further provided with a first nitrogen-ion supply means for supplying nitrogen ions or a mixture of nitrogen ion and nitrogen gas during the deposition. The nitrogen-ion supply means is installed in a chamber for forming a magnetic layer, nitrogen ions supplied from the nitrogen-ion supply means react with the vapour of a magnetic metal to produce metal nitride, and a magnetic layer of a thin film of the metal nitride is formed on a nonmagnetic support. The previously mentioned oxygen-gas introduction means is disposed in a position where the surface of the magnetic layer is oxidized. (3) In another embodiment of a magnetic recording medium manufacturing apparatus of the present invention, the first or second chamber is further provided with third deposition means disposed under the first or second cooling can, a second nitrogen-ion supply means for supplying nitrogen ions during the deposition by the third deposition means, a third oxygen-gas introduction means for introducing oxygen gas after the deposition by the third deposition means, and a means for demarcating deposition regions of the first or second deposition means and the third deposition means. The third deposition means, second nitrogen-ion supply means, and third oxygen-gas supply means are arranged in a chamber where a magnetic layer is formed. (4) In another embodiment of a magnetic recording medium manufacturing apparatus of the present invention, the lubricant spray means in the third chamber includes an ultrasonic wave generator and a spray nozzle. (5) In another embodiment of a magnetic recording medium manufacturing apparatus of the present invention, the third chamber is further provided with a means for heating a nonmagnetic support on which a lubricant is sprayed. A heating lamp is suitable as the means for heating the nonmagnetic support on which the lubricant is sprayed, and its installation position is not restricted as long as the nonmagnetic support on which the lubricant is sprayed can e heated. The heating lamp heats the support preferably at 80 to 100°C. The manufacturing apparatus of the present invention makes it possible to form a back-coat layer, a magnetic layer, and a top-coat layer on the magnetic layer without transferring the tape to another vacuum deposition line or coating line, and thereby losing the vacuum to the atmospheric pressure. The manufacturing apparatus of the present invention has a very high productivity and makes it possible to prevent dust from adhering to the magnetic medium during production in the air. Moreover, the present invention provides a magnetic recording medium having a back-coat layer with a surface resistance of from 5 to 10 5 Ω, which is formed by depositing metal on the surface opposite to the magnetic layer deposition surface of the nonmagnetic support, while introducing oxidizing gas. The magnetic recording medium is obtainable by the above-mentioned manufacturing method of the present invention and also by the manufacturing apparatus of the present invention. The second embodiment using a semi-metal is also obtainable by the above shown method and manufacturing apparatus. Surface resistance of the back-coat layer is preferably from 10 5 to 10 8 Ω. The recording medium having a semi-metal back-coat layer is improved in view of the enveloping property probably by providing the medium with a suitable elasticity and good contact with a recording or a reproducing head. The present invention will now be described in more detail with reference to the following examples which should not be considered to limit the scope of the present invention. Reference Example 1 A layer of Co-Ni alloy (80%-20%) with a thickness of 1,500 Å is formed on a PET film with a thickness of 9.8 µm by means of vacuum deposition, and a magnetic tape on which a magnetic layer is thus formed is wound on a roller. Then, the Al layer with a thickness of 1,000 Å is deposited on the back surface of the magnetic tape by using a vacuum evaporator shown in Fig. 1, to form a back-coat layer. The following is the description of the vacuum evaporator shown in Fig. 1. A vacuum vessel (1) is evacuated by a turbo pump (2) and a rotary pump (3). An unwinding roller (4) and a winding roller (5) are disposed in the vacuum vessel (1). A magnetic tape (6) runs along the bottom surface of a cooling can (7) until the tape is wound on the winding roller (5) after it is drawn from the unwinding roller (4). A crucible (8), into which Al is put, made of MgO is placed under the cooling can (7). An electron beam is emitted from an electron beam gun (9), located in a position diagonally above the Al, into the crucible (8), to heat and evaporate Al. A shielding plate (10) to limit the deposition area on the magnetic tape (6) is installed between the crucible (8) and the cooling can (7). An oxygen introduction tube (11) leading from the outside to the inside of the vacuum vessel (1) is so provided that the end of the tube faces the deposition surface through the hole of the shielding plate (10). The quantity of oxygen to be introduced can be adjusted by a flow rate control valve (not illustrated). Thus, the roll of magnetic tape on which the magnetic layer is formed is set to the unwinding roller (4) to transport the tape on the cooling can (7) with the back of the magnetic tape (6) outside. Al heated and evaporated by the electron beam gun (9) is deposited on the tape while introducing oxygen through the oxygen introduction tube (11). The degree of vacuum is maintained at 5.3·10 -3 Pa (4 x 10 -5 Torr) and the power of the electron beam gun is 30 kW. Oxygen with a purity of 99.998 % is introduced at a rate of 25 cc/min. Then, the magnetic tape (6) on which a back-coat layer is formed is wound on the winding roller (5). Then, the magnetic tape is taken out into the air, slitted into 8 mm wide tapes, and loaded into an 8 mm cassette to measure the number of dropouts and the surface resistance. Dropouts are counted with a dropout counter and a defect such that the output is decreased by 16 dB or more in 10 µs is judged to be a dropout. As a comparative example, a tape is used which is fabricated as follows: A film on which a magnetic layer is formed like in Example 1 is coated with a back-coat layer of 0.5 µm thickness. The back-coat layer is of a coating material such that carbon black powder is dispersed in a binder of a mixture of polyvinyl chloride and urethane at 1:1 mixture ratio. The carbon black powder is a mixture of two kinds of carbon black, of 20 nm grain diameter and of 60 nm grain diameter, respectively, at a mixture ratio of 1:1. Then, the number of dropouts and the surface resistance of the back-coat layer are measured in a manner similar to that used in Example 1. These measurement results are shown below. Number of dropouts per min. Surface resistance (Ω) Reference Example 1 8 7 x 10 2 Comparative Example 11 5 x 10 6 From the above results it is found that the present invention yields a product in which the number of dropouts is approximately 30 % lower than that of the Comparative Example, wherein a tape was prepared by a conventional method. It is assumed that attachment of dust or scratch formation are prevented for the tape of the present invention because the back-coat layer is provided in a vacuum without being exposed to the air and the resistance value is remarkably low. The vacuum evaporator shown in Fig. 1 has been described as the one which is used only for deposition of the back-coat layer. However, it is also possible to deposit a back-coat layer on the back of the magnetic tape by first depositing Co-Ni alloy instead of Al by using the apparatus of Fig. 1 with or without the introduction of oxygen to perform vacuum deposition of a magnetic layer, thereafter setting the magnetic tape of the widing roller (5) on the unwinding roller (4), and then depositing the back-coat layer on the back of the tape as described before. It is also possible to perform vacuum deposition of the magnetic layer and back-coat layer simultaneously or sequentially by installing both of a magnetic layer deposition means and a back-coat layer deposition means in a line in one vacuum vessel. Reference Example 2 Another 8 mm magnetic recording tape was produced and was tested in the same manner as shown in Example 1, except for using Si for the back-coat layer. The results are: Number of dropouts per min. Surface resistance (Ω) 7 7 x 10 2 This 8 mm cassette was tested in view of the enveloping property by installing it in an apparatus modified from a commercially available Hi8VTR (tradename; manufactured by Sony Corporation), recording signals of 7 MHz and determining an output variation of a reproducing output in one track defined by the following equation: output variation (%) = B/A x 100 A: the maximum output (V) in one track B: the minimum output (V) in one track The larger the result of this calculation, the smaller the output variation and the better the enveloping property. The result obtained is 80 %, which should be taken as good. Example 1 Fig. 2 is a schematic view of an example of a magnetic recording medium manufacturing apparatus of the present invention. In this apparatus, a back-coat layer is first formed, a magnetic layer is then formed, and a top-coat layer is finally formed. A housing (21) is demarcated by a first chamber (22), a second chamber (23) and a third chamber (24). Each chamber is connected to vacuum means (not illustrated) so as to maintain a proper degree of vacuum. The appropriate degree of vacuum of each chamber is determined depending on the material to be attached. Therefore, it is possible for the chambers to have the same or different degrees of vacuum. The first chamber (22) has an unwinding roller (25), a first cooling can (26) for transporting a nonmagnetic support (28), a feed roller (27), a first deposition means, and a first oxygen-gas introduction tube (33). Bombardment devices (29, 29') are so installed that fine dust does not adhere to the nonmagnetic support coming out of the unwinding roller (25) and the nonmagnetic support going to the feed roller (27). The first deposition means of the first chamber (22) comprises a crucible (30) and an electron gun (32) arranged under the first cooling can (26). The crucible (30) is open to a cooling can (26) and is charged with aluminum metal (31) for the back-coat layer in its recess. An electron beam is emitted from the electron gun (32) and applied to aluminum (31) in the crucible (30). Oxygen gas is introduced into the evaporation area of aluminum vapor through the first oxygen-gas introduction tube (33) to oxidize aluminum, and a thin film of aluminum oxide is formed on the nonmagnetic support as a back-coat layer. The quantity of oxygen to be introduced can be adjusted by a flow rate control valve (not illustrated). The nonmagnetic support (28) on which the back-coat layer is formed is transported on the feed roller (27) and sent into the second chamber (23) through a slit (46), which is an opening for passing the nonmagnetic support. The second chamber (23) has feed rollers (34, 34') and a second cooling can (35) where the nonmagnetic support (28) travels. Fig. 2 shows an arrangement in which the second chamber is provided with two (the second and third) deposition means. The second deposition means comprises a crucible (36) and an electron gun (38) arranged under the second cooling can (35). The crucible (36) opens toward the second cooling can (35), and the metal iron (37) to serve as a magnetic layer is contained in its recess. An electron beam is emitted from the electron gun (38) and applied to the metal iron (37) in the crucible (36). Nitrogen ions are supplied into the iron evaporation area by first nitrogen-ion supply means (39) to nitride evaporated iron, and iron nitride adheres the nonmagnetic support and thereby a thin film of iron nitride is formed on the nonmagnetic support. Then, oxygen gas is supplied through a second oxygen-gas introduction tube (33') after the deposition to oxidize the magnetic layer surface and its vicinity. In the second chamber, it is desirable to dispose the oxygen-gas introduction tube in a position where the magnetic layer surface can be oxidized immediately after iron nitride has adhered to the nonmagnetic support. The third deposition means, in similar manner to the second deposition means, comprises a crucible (36') and an electron gun (38') arranged under the second cooling can (35). The crucible (36') opens toward the second cooling can (35), and metal iron (37') to serve as a magnetic layer is contained in its recess. Nitrogen ions are supplied into the evaporation region of iron by a second nitrogen ion supply means (39'), and evaporated iron is nitrided. Iron nitride adheres the nonmagnetic support to form a thin film of iron nitride further on the support. Then, oxygen gas is supplied through a third oxygen-gas introduction tube (33") after the deposition to oxidize the magnetic layer surface and its vicinity. In this arrangement, the second oxygen-gas introduction tube (33') and the third oxygen-gas introduction tube (33") constitute a branch tube leading to the source of oxygen gas. The means for partitioning the areas of metallic vapour produced by the two deposition means is a shielding plate (47) which is normally made of metal. In this apparatus, two magnetic layers are formed in which the corundum inclination directions differ from each other. The nonmagnetic support (28) on which the magnetic layers are formed is transported on the roller (34') and sent to the third chamber (24) through a slit (46'). The third chamber (24) has feed rollers (40, 40'), a heating can (41) for transporting the nonmagnetic support (28), a winding roller (42) and a lubricant spray means. In this case, the lubricant spray means comprises an ultrasonic atomizer (43) and a spray nozzle (44), which are arranged under the heating can (41) on the left of third chamber (24). A lubricant is atomized by ultrasonic waves and sprayed from the spray nozzle (44) on the nonmagnetic support (28) moving on the heating can (41) to form a top-coat layer on the magnetic layer. Moreover, in this arrangement the third chamber has a heating lamp (45) to accelerate the drying of the lubricant sprayed on the magnetic layer. Thus, the nonmagnetic support (28) on which the top-coat layer is formed is transported on the feed roller (40') and finally wound on the winding roller (42). As described above, the present invention solves the problems present in the prior art using liquid lubricants, including attachment of air-borne dust, because a back-coat layer is formed through deposition in a vacuum, namely without being exposed to air atmosphere. Moreover, the present invention makes it possible to continuously form a back-coat layer, a magnetic layer and a top-coat layer of a magnetic recording medium of metallic-thin-film-type, in a single line. Therefore, the productivity of said product is greatly improved. Furthermore, though a back-coat layer formed only through vacuum deposition tends to have too smooth a surface and to impair the transport stability, the present invention also makes it possible in a specific embodiment to maintain the transport stability by introducing oxygen for deposition, and thereby controlling the friction coefficient of the back-coat layer. The present invention makes it possible not only to control the surface roughness but also to improve the corrision resistance of the back-coat layer by introducing oxygen. Generally the metallic bond is weakened and the conductivity is decreased by the introduction of oxygen. However, the present invention further makes it posible to prevent undesirable decrease in the conductivity and to maintain the appropriate conductivity by controlling the quantity of oxygen to be introduced.

Claims: A method for manufacturing a magnetic recording medium having a magnetic layer on one side of a nonmagnetic substrate, a back-coat layer on the opposite side of the nonmagnetic substrate and a top-coat layer on the magnetic layer, wherein the back-coat layer, the magnetic layer and the top-coat layer are formed by being deposited in a vacuum, and the vacuum is maintained throughout the process for providing said layers, characterized in that the top-coat layer is formed by spraying a liquid lubricant. The method as claimed in claim 1, characterized in that said layers are provided in a vacuum simultaneously or sequentially. The method as claimed in claim 2, characterized in that said layers are provided in the order of: (1) a back-coat layer, a magnetic layer and then a top-coat layer; (2) a magnetic layer, a top-coat layer and then a back-coat layer; or (3) a magnetic layer, a back-coat layer and then a top-coat layer, while transporting the substrate in a vacuum. The method as claimed in any of the preceding claims, characterized in that the back-coat layer is provided by depositing a metal or a semi-metal. The method as claimed in claim 4, characterized in that the metal or semi-metal is deposited on the surface of the nonmagnetic substrate while an oxidizing gas is introduced. The method as claimed in claim 4 or 5, characterized in that aluminum is used as the metal. The method as claimed in any of the preceding claims, characterized in that the liquid lubricant is atomized by means of ultrasonic waves before being sprayed. The method as claimed in any of the preceding claims, characterized in that the lubricant is a fluorine-containing lubricant. The method as claimed in any of the preceding claims, characterized in that the magnetic layer is composed of iron, a ferromagnetic alloy of iron, an iron nitride, an iron carbide, a nitride of a ferromagnetic alloy of iron or a carbide of a ferromagnetic alloy of iron. An apparatus for manufacturing a magnetic recording medium having a magnetic layer on one side of a nonmagnetic substrate, a back-coat layer on the opposite side of the nonmagnetic substrate and a top-coat layer on the magnetic layer, said apparatus comprising a housing (21) provided with first, second and third chambers (22, 23, 24) and vacuum means for keeping each of the chambers in a vacuum, the first chamber (22) having a first cooling can (26) for transporting a nonmagnetic substrate (28) and a first deposition means (30, 32) for depositing a back-coat layer disposed under the first cooling can (26); the second chamber (23) having a second cooling can (35) for transporting the nonmagnetic substrate (28) and a second deposition means (36, 38) for depositing a magnetic layer disposed under the second cooling can (35); the third chamber (24) having a heating can (41) for transporting the nonmagnetic substrate (28) and a spraying means for forming a top-coat layer on the magnetic layer by spraying a liquid lubricant disposed under the heating can (41), and the first, second and third chambers (22, 23, 24) or the second, third and first chambers (23, 24, 22) or the second, first and third chambers (23, 22, 24) are adjacently arranged in this order, the adjacent chambers being made to communicate with each other through an opening (46, 46') for passing a nonmagnetic substrate. The apparatus for manufacturing a magnetic recording medium as claimed in claim 10, wherein the apparatus further comprises an oxidizing gas supply means (33,33') in the first and/or the second chambers (22,23). The apparatus for manufacturing a magnetic recording medium as claimed in claim 10 or 11, wherein the spraying means includes an ultrasonic atomizer (43) and a spray nozzle (44). A magnetic recording medium being obtainable by a process as defined in any of the claims 1 to 9. The magnetic recording medium as claimed in claim 13, wherein the back-coat layer is a vacuum deposited layer of aluminum oxide. The magnetic recording medium as claimed in claim 13, wherein the back-coat layer is a vacuum deposited layer of metal and has a surface resistance of from 5 to 10 5 Ω. The magnetic recording medium as claimed in claim 13, wherein the back-coat layer is a vacuum deposited layer of semi-metal and has a surface resistance of from 105 to 10 8 Ω.

Inventors: KITAORI NORIYUKI, MIZUNOYA HIROHIDE, YOSHIDA OSAMU, KITAORI, NORIYUKI, MIZUNOYA, HIROHIDE, YOSHIDA, OSAMU, Kitaori, Noriyuki, c/o Kao Corporation, Mizunoya, Hirohide, c/o Kao Corporation, Yoshida, Osamu, c/o Kao Corporation
Applicants: KAO CORP, KAO CORPORATION
Application year: 1993
Main classification: G11B5
IPC codes: G11B5