PROCESS FOR PRODUCING PERMANENT MAGNETS AND PERMANENT MAGNET

Abstract: Fe-B-R type permanent magnet is produced by: forming an anticorrosive coating film layer on a Fe-B-R base perma­ nent magnet material body by means of vapor deposition to thereby improve the corrosion resistance thereof. The anticorrosive thin film is formed of metal, oxides, nitrides, carbides, borides, silicides, composite compositions thereof, or a mixture thereof. Additionally shot blasting, peening, heat treatment for forming an interdiffusion layer, and/or resin impregnation may be applied.

Description: This invention relates to sintered, permanent magnet bodies and methods for producing same and lies especially in the field of rare earth-boron-iron base permanent magnet containing as the main components R (R standing for at least one of rare earth elements including Y), B and Fe. Currently typical permanent magnet materials are alnico, hard ferrite and rare earth-cobalt magnets. The rare earth-cobalt magnets have been used in various fields due to their very excellent magnetic properties. However, it is now expected to encounter difficulty in stable supply of them in greater amounts and over an extended period of time, since the key components SM and Co are both scarce and expensive. For that reason, there has been a keen desire for permanent magnet materials excelling in magnetic properties and comprising compositional elements that are abundant and inexpensive, and will stably be supplied. The present applicant has already proposed Fe-B-R base (wherein R is at least one of rare earth elements including Y) permanent magnets as novel high-performance permanent magnets (Japanese Patent Kokai-Publication Nos. 59-46008, 59-64733, 59-89401 and 59-132104; EP publication of application Nos. 0101552, 0106948, 0126179, 0126802, 0124655 and 0125347). The permanent magnets disclosed therein are excellent permanent magnets in which abundant rare earth elements, mainly Nd and/or Pr, are used as R, and R, B and Fe constitute the main components, and which show a practical energy product of at least 31.8 k·TA/m or 79.6 k·TA/m, as well as an extremely high energy product of 159.2, 199.0, 238.8, 278.6 k·TA/m or higher. With the recent trend to high performance and diminishing sizes of magnetic circuits, increasing attention has been paid to Fe-B-R base permanent magnet materials. In the production of permanent magnet materials for that purpose, formed (compacted) and sintered magnet bodies have to be cut on the entire surface or the required surface portion for removing surface irregularities or strains or surface oxide layers, in order to incorporate them in magnetic circuits in later steps. For cutting, use is made of outer blade cutters, inner blade cutters, surface grinders, centerless grinders, lapping machines, etc. However, since the Fe-B-R base permanent magnet materials contain as the primary components rare earth elements and iron which undergo oxidation in the air so easily that stable oxides are immediately formed, they generate heat or form oxides due to the contact thereof with the cutting surface upon being processed by the aforesaid machines, thus offering a problem of deterioration of the magnetic properties. Furthermore, when permanent magnets comprising Fe-B-R based, magnetically isotropic sintered bodies are incorporated into magnetic circuits, reductions and variations in the outputs of the magnetic circuits will occur. A problem also arises that surrounding devices may be contaminated by the separation of surface oxides. To solve the aforesaid problems, the present applicant has already proposed permanent magnets in which anticorrosive metal layers are coated on the surface on magnet bodies by the electroless plating or electroplating technique (Japanese Patent Application No. 58-162350), or anticorrosive resin layers are coated on the surface of magnet bodies by the spray or immersion technique (Japanese Patent Application No. 58-171907) with a view to improving the corrosion resistance of the aforesaid Fe-B-R base permanent magnets. In the former plating technique, however, there is the danger that the resulting magnets may rust with changes of age, since the base bodies are sintered, porous masses, in the pores of which an acidic or alkaline solution remains in the pre-plating treatment. Also, there is a problem that the magnets may be corroded on the surface during plating with the resulting drops of adhesiveness and corrosion resistance. In the latter spray technique, on the other hand, a number of steps and much labor are needed to apply a uniform resin coating on the entire surface of the bodies to be treated, since the resin is sprayed in a certain direction. In particular, difficulty is involved in the application of a uniform coating on complicatedly and irregularly shaped magnets. In the immersion technique, there is also a problem that the resin coating becomes irregular in thickness, thus resulting in a drop of the dimensional accuracy of the products. From the Patent Abstracts of Japan, Vol. 6, No. 144 (E-122) [1022] of August 3, 1982, the covering of intermetallic compounds such as R₂M₁₇ or RM₅ (R=Y, La, Ce and/or Sm; M=Fe, Co, Ni and/or Mn) with metallic plating is known. Finally, it is described in Patent Abstracts of Japan, Vol. 5, No. 149 (E-75) [821] of September 19, 1981, to copper-plate and nickel-plate a rare earth-cobalt-magnet. This technique, however, requires degreasing the surface of the magnet prior to the plating with the fluid solvent triclean, so that in this case the problems as already mentioned above for acidic or alkaline solutions remaining on the surface are likely to occur. It is an object of the invention to provide a permanent magnet body composed mainly of rare earth elements, boron and iron, which introduce improvements into the corrosion resistance of the Fe-B-R base permanent magnet materials. This object is solved by the permanent magnet bodies of independent Claims 1 and 7. Further advantageous features of such permanent magnet bodies are evident from the dependent Claims 2, 3 and 8 to 14. A further object of the invention is to provide a process for producing the Fe-B-R base permanent magnets in which an anticorrosive thin film showing excellent adhesiveness and corrosion proofness can be applied in a uniform thickness on the surface of a magnet material without using any corrosive chemicals and hence with no possibility that they may remain. This object is solved by the processes according to independent Claims 4 and 15. Further advantageous features of the processes are evident from the dependent Claims 5, 6 and 16 to 41. This invention provides a method for processing the Fe-B-R permanent magnets which eliminates deteriorations of the magnetic properties thereof taking place in association with the oxidation and cutting processing of the sintered magnet material bodies. Stillmore, this invention provides a process for the production of the Fe-B-R base permanent magnet materials containing as the main components rare earth, boron and iron, which can eliminate deteriorations of magnetic properties taking place in association with the cutting processing of magnet material bodies (particularly sintered bodies). Generally, according to the present invention, there is provided a process for producing a permanent magnet material characterized by: providing an Fe-B-R base permanent magnet material wherein at least 50 Vol % of the entire material (i.e. major phase) consists of an Fe-B-R type tetragonal crystal structure, and forming an anticorrosive coating film layer on said material body by means of vapor deposition, to thereby improve the corrosion resistance thereof. According to the first aspect of the present invention there is provided a sintered permanent magnet body of 10 to 30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2 to 28 at % B and at least 42 at % Fe and wherein at least 50 Vol % of the entire magnet material body consists of a Fe-B-R type tetragonal crystal structure, with an anticorrosive coating film layer consisting of a ceramic material on the body. In the first aspect, Fe is preferably at least 52 at % (more preferably 65 at %) and more preferably no more than 80 at %. A permanent magnet material body which comprises 12 to 24 at % R wherein at least 50 at % of R is Nd and/or Pr, 4 to 24 at % B and at least 52 at % Fe, is preferred for providing magnetic energy product of at least 79.6 k·TA/m, wherein Fe of up to 82 at % is preferred. In the present disclosure, the symbol "R" generally represents rare earth elements in the broad sense, i.e. lanthanide and yttrium. However, in the following "R" specifically represents the selected elements hereinabove defined in the first aspect, if not otherwise specified. The anticorrosive film is preferably made up of at least one selected from the group consisting of oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. More preferably, said anticorrosive film is formed of at least one selected from the group consisting of oxides of Si, Al, Cr, Ti, Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof and mixtures thereof. According to a second aspect, a sintered permanent magnet body is provided with 10 to 30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2 to 28 at % B and at least 42 at % Fe and wherein at least 50 Vol % of the entire sintered magnet material body consists of a Fe-B-R type tetragonal crystal structure, obtainable by blasting or reverse sputtering the body and by vapor depositing an anticorrosive coating film layer of low gas permeability on the said treated body. The anticorrosive film is preferably made up of at least one selected from the group consisting of metals, oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. More preferably, the anticorrosive film is formed of at least one selected from the group consisting of Al, Zn, Ni, Cr, Cu, Co, Ti, Ta, Si, Ag, Au, Pt, Rh and alloys thereof, oxides of Si, Al, Cr, Ti and Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof and mixtures thereof. According to a preferred embodiment of this aspect, the anticorrosive coating film layer is formed of a metal having a melting point not higher than the sintering temperature of the permanent magnet material body. The following preferred embodiments refer to the first and the second aspect of the present invention. Preferably, the interdiffusion layer is provided between the magnet material body and the anticorrosive coating film layer. Furthermore, the anticorrosive coating film layer is preferably impregnated with a resin. The permanent magnet bodies contain preferably 50 at % or more of the R as Nd and/or Pr. The permanent magnet bodies may further comprise at least one of the following additional elements not exceeding the values specified below: provided that, when two or more of said additional elements are contained, the total amount thereof does not exceed the highest value of the atomic percent of the element of said additional elements actually added. This invention further provides production processes for forming a uniform, firm and stable anticorrosive thin film layer on the surfaces of the Fe-B-R base permanent magnets to inhibit the oxidation of the magnet material. By the anticorrosive thin film formed according to the present invention, the surface oxidation of magnet bodies is inhibited. Further, since any corrosive chemicals, etc. are not used and, hence, there is no possibility that they may remain, the magnetic properties are stably maintained over an extended period of time without deterioration. According to a third aspect of the present invention there is given a process for producing a permanent magnet, by providing a permanent magnet material body of 10 to 30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2 to 28 at % B and at least 42 at % Fe and wherein at least 50 Vol % of the entire magnet material body consists of a Fe-B-R type tetragonal crystal structure, and forming an anticorrosive coating film layer consisting of a ceramic material on the permanent magnet body by means of vapor deposition to improve the corrosion resistance thereof. The anticorrosive film is preferably formed of at least one selected from the group consisting of oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. More preferably, the anticorrosive film is formed of at least one selected from the group consisting of oxides of Si, Al, Cr, Ti and Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof, and mixtures thereof. According to a fourth aspect of the present invention, there is given a process for producing a permanent magnet by providing a sintered permanent magnet material body of 10 to 30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2 to 28 at % B and at least 42 at % Fe and wherein at least 50 Vol % of the entire magnet material body consists of a Fe-B-R type tetragonal crystal structure, blasting or reverse sputtering the sintered magnet body and forming an anticorrosive coating film layer of low gas permeability by means of vapor deposition on the treated magnet surface. The anticorrosive film is preferably formed of at least one selected from the group consisting of metals, oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. More preferably, the anticorrosive film is formed of at least one selected from the group consisting of Al, Zn, Ni, Cr, Cu, Co, Ti, Ta, Si, Ag, Au, Pt, Rh and alloys thereof, oxides of Si, Al, Cr, Ti and Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof, and mixtures thereof. The blasting involves preferably blasting hard particles having a mean particle size of 20 to 350 µm and a Mohs hardness of at least 5. More preferably, said blasting is effected by blasting said particles together with a pressurized gas of 0.98 to 5.88 bar. Grit may be used as the suitable hard particles. Blasting is applied to the surface of the permanent magnet material body prior to the application of said vapor deposition, thereby removing oxide layer and/or machining strain layer (or Bailby layer). This blasting is effected typically as follows: A hard powder comprising at least one of powders having a mean particle size of 20 to 350 µm and a Mohs hardness of no lower than 5, together with a gas pressurized to 0.98 to 5.88 bar, is blasted onto the surface of a sintered permanent magnet body for 0.5 to 60 minutes for the removal of an oxide or distortion layer thereon. A vapor deposition film layer is thereafter deposited onto the surface of said magnet body. The sand or grit blasting using sand or grit of random shape is preferred as the blasting. Usually, cutting is required for manufacturing the end products of the Fe-B-R base magnets. However, the usual cutting processing offers a problem that the magnetic properties of the magnet products deteriorate, partly because of the occurrence of cutting strains, and partly because of the formation of oxides due to the generation of heat during machining and the contact of surface to be machined with the atmosphere. Therefore, if such unpreferred surface layers are removed from the surfaces of the magnets in a proper manner, followed by the provision of protective coating layers, the magnets are then expected to be further improved in terms of magnetic properties and durability. This is achieved by the above described blasting procedure. More specifically, this method contemplates eliminating or preventing deteriorations of the magnet properties of sintered magnet bodies due to oxidation and machining strain (deterioration due to machining) by blasting a hard powder having the required properties, together with a pressurized gas, onto the surfaces thereof for the removal of surface layers such as black skin, oxide and machining strain layers. By the application of this processing method, it is possible to obtain at low costs Fe-B-R base permanent magnet materials which are free from any deterioration of the magnet properties due to cutting and oxidized layers. The hard powders having a Mohs hardness of no lower than 5, which are used in this invention, may be based on Al₂O₃, silicon carbide, ZrO₂, boron carbide, garnet and the like. Preference is given to Al₂O₃ base powders having a high hardness. The powders used are preferably of random shape. Powders having a Mohs hardness below 5 are unpreferred, since then the blast-grinding force is so small that an extended period of time is required for blast-grinding. The reason why the mean particle size of the hard powders is limited to 20 to 350 µm is that, at below 20 µm, the blast-grinding force is so small that an extended period of time is needed for cutting, while, at a mean particle size of higher than 350 µm, the surface of sintered magnet bodies becomes uneven. Referring to the blasting conditions of the hard powders, a prolonged period of time is needed for grinding at a pressure of below 0.98 bar, while, at a pressure exceeding 5.88 bar, there is a fear that the surface roughness of magnet bodies may drop due to the fact that the blast-grinding leads to uneven surfaces. When the blasting time is below 0.5 minutes, the result of the blast-grinding is limited and uneven surfaces occur, and when it exceeds 60 minutes, the blast-grinding of the surfaces of magnet bodies increases with a resulting drop of surface roughness. Air or inert gases such as Ar, N₂ and like gases may be used as the pressurized fluids for blasting the hard powders. However, preference is given to the use of inert gases for the purpose of preventing oxidation of magnet bodies. It is also preferred that air is dehumidified for use. Suitably, the deposition of vapor deposition layers on the surfaces of sintered magnet bodies, which have been cleaned of surface oxide layers, may be effected relying upon the thin-film formation techniques such as vacuum deposition, sputtering, ion plating, etc. The thin film layers should have a thickness of, preferably not more than 30 µm, most preferably 5 to 25 µm, in view of their peeling, a drop of their mechanical strength and the assurance of their corrosionproofness. According to a preferred embodiment, the anticorrosive coating film layer is formed of a metal having a melting point not higher than the sintering temperature of the permanent magnet material body. It is also preferred to form the anticorrosive coating film layer of metal and to subject the surface of said coating film layer to passivation. A further preferred embodiment involves a heat treatment which is effected at a temperature ranging from 250 °C to the melting point of the metal employed. It is also preferred to carry out a heat treatment at a temperature ranging from 250°C to the aging temperature of the permanent magnet material body. The following preferred embodiments refer to the third and fourth aspect of the present invention. In this invention, the formation of the anticorrosive vapor, deposition layer on the surfaces of magnet materials relies upon the vacuum deposition, physical vapor deposition (ion sputtering, ion plating, ion-deposition thin-film formation (IVD)), chemical vapor deposition (plasma-deposition thin-film formation) and like vapor deposition techniques. Plasma vacuum deposition may be classified as the chemical vapor deposition (CVD). Note, however, that the CVD such that employs halogen compound gas is not preferred. The film layer which is produced may be crystalline, however, a glassy or amorphous layer may be deposited, if desired. Generally, the anticorrosive thin film layer may be a stable layer such that can be deposited through the vapor deposition technique, has a resistance to oxidation or other harmful gas and has a low gas permeability. The vacuum deposition is effected in that a coating substance is heated in vacuum by means of resistance heating, ion beam heating, induction heating, etc. to put it into an atomic, molecular or finely particulate state, whereby on the surface of the permanent magnet body to be coated the anticorrosive thin film is formed comprising the metals, alloys or compositions (or compounds) as mentioned in the foregoing. According to the ion sputtering technique, an argon gas is admitted into a vacuum vessel, and electrical discharge is produced therein by means of a sputter power source. The ionized argon gas is accelerated by an electrical field into collision with a target material that constitutes a cathode comprising a coating substance, thereby emitting atoms out of the target material, and the emitted atoms are deposited on the surface of the permanent magnetic material body. In this manner, the aforesaid anticorrosive thin film is formed on the surface of a permanent magnet body forming an anode. According to the ion plating technique, a coating material is heated by means of resistance heating, electron beam heating, induction heating, etc. to bring it into an atomic, molecular or finely particulate state. Thermoelectrons are then collided with the thus obtained particulated coating material for ionization. The ionized particles, travelling along an electrical field, are collided with other evaporated particles to increase the number thereof. These ionized particles are attracted by an electrical field, and are deposited onto the surface of a permanent magnet body forming a cathode, thereby forming the aforesaid anticorrosion thin film layer. According to the ion vapor deposition thin-film formation technique, evaporated substances by means of an electron gun, arc discharge, etc. and ions supplied from an ion source and accelerated by a high-accelerating voltage are simultaneously deposited and ion-radiated in a certain proportion, whereby the aforesaid anticorrosive thin film is formed on the surface of a permanent magnet body. According to the plasma vapor deposition thin-film formation technique (a sort of CVD), the starting gas for the formation of thin films is introduced into a vacuum vessel, and is maintained at a constant pressure with the use of a vacuum pump. Discharge is then effected by the application of high-frequency power on the electrodes, whereby the aforesaid anticorrosive thin film is formed on the surface of a permanent magnet material body through a plasma chemical reaction. In accordance with this invention, the anticorrosive thin film formed on the surface of a permanent magnet material body by any one of the aforesaid vapor deposition techniques can provide a thickness suitable for anticorrosion resistance. The alloys for permanent magnets according to this invention are also characterized by containing at least 50 Vol % of a compound of a Fe-B-R type tetragonal crystal structure having a crystal grain size of 1 to 100 µm (preferably 1 to 80 µm) and 1 to 50 % (in volume ratio) of nonmagnetic phases (except for oxide phases). Based on this invention, therefore, it is possible to obtain at low costs improved permanent magnets having an extremely high energy product of no lower than 199.0 k·TA/m and excelling in residual magnetic flux density, coercive force and corrosion resistance by using as R resourceful light rare earth, primarily Nd and/or Pr, and as the main components Fe, B and R. According to the vacuum deposition used in this invention, the substance required to form a thin film is charged in a reduced-pressure vessel having a vacuum of about 1.33 x 10⁻⁷ to 1.33 x 10⁻¹⁰ bar, and is heated for evaporation or sublimation. The resulting vapor is then condensed on the surface of a magnet material body placed in the same vessel to form and coat the thin film. The substance to be evaporated in accordance with the vacuum deposition technique may be heated in the crucible heating system or the direct heating system such as, e.g., resistance heating, high-frequency induction heating, electron beam heating, which may suitably be selected depending upon the composition and thickness of the coating substance to be deposited, the shape of the permanent magnet on which it is to be deposited, workability, etc. Preferably, the coating substances to be evaporated include metals, alloys, ceramics and compositons (or compounds), e.g., nitrides, oxides, carbides, borides, silicides of metals (or composite composition), which can improve the corrosion resistance of the present permanent magnets. The substances include metals, for instance, Al, Zn, Ni, Cr, Cu, Co, Ti, Ta, Si, Au, Ag, Pt, Rh, etc., or their alloys, or SiO₂, Al₂O₃, Cr₂O₃, TiN, AlN, TiC,etc. When noble metals are used, however, it will entail the disadvantage of high costs. Not only a single layer but also a superposed layer may be deposited. For instance, a metal layer (e.g. Al or Si) may be first deposited, then oxide layer (e.g. Al₂O₃ or SiO₂) may be deposited thereon. The oxide layer may be formed by oxidation of the deposited metal layer. The deposited film is preferably 30 µm or less thick. According to the ion plating technique used in this invention, vapor deposition is carried out in a vacuum vessel having a degree of vacuum of, e.g., 1.33 x 10⁻⁷ bar to 1.33 x 10⁻¹⁰ bar, as mentioned previously. The substance to be ionized may be heated in the crucible heating system or the direct heating system such as resistance heating, high-frequency induction heating, electron beam heating, etc., which may suitably be selected depending upon the composition and thickness of the coating substance to be deposited, the shape of the permanent magnet on which it is to be deposited, workability, etc. The coating substances to be evaporated include metals, alloys, ceramics and compounds which are mentioned previously. The ion plating is suitable for depositing a plurality of elements simultaneously. Where nitride, oxide or carbide films are formed on the surfaces of permanent magnet bodies by the ion plating technique, it is preferred that a reactive gas such as O₂, N₂, CO₂, acetylene or the like is introduced into a vacuum vessel. In the case of forming alloy coating films, a plurality of evaporation sources are provided for the respective alloy components, which are evaporated in a certain proportion for the formation of alloy coating films having a certain composition. According to the sputtering technique used in this invention, the coating material to be formed and coated and the permanent magnet body on which it is to be deposited are used as a cathode target material and an anode, respectively, in an inert (e.g., argon) atmosphere of a reduced pressure. A voltage is then applied between at least two electrodes for the ionization of the atmosphere gas. The resulting cations are accelerated by an electrical field to collide with the surface of the cathode with a large kinetic energy for the cathode sputtering of atoms present thereon, whereby the thus emitted atoms are condensed on the surface of the magnet material body forming the anode to form and coat a thin film. The sputtering techniques used include D.C. sputtering such as bipolar sputtering, bias sputtering and the like, A.C. sputtering such as asymetrical A.C. sputtering, high-frequency sputtering or the like, and other sputtering such as getter sputtering, plasma sputtering or the like. Referring particularly to D.C. bipolar sputtering, high-frequency sputtering and plasma sputtering, there is the so-called reactive sputtering, according to which at least one of reactive gases such as N₂, O₂, C₂H₂, CO₂ and the like is introduced in a high-vacuum inert (argon) atmosphere for reaction with the released atoms, thereby forming a thin film of a composition (or compound) such as a nitride, oxide or carbide of the target metal. These techniques may suitably be selected depending upon the composition and thickness of the coating substance to be deposited, the shape of the permanent magnet on which it is to be deposited, workability, etc. The target materials, viz., the coating materials preferably include metals, alloys, ceramics and compounds which have been mentioned previously. It is desired in the sputtering techniques that a single target be used in the case where the anticorrosive thin films to be formed on permanent magnet bodies are formed of single metal, and a plurality of targets be applied in the case where they are formed of alloys. The vapor deposition may also be effected by ion plating which is characterized by heating a coating material forming an anode in a vacuum vessel in the presence or absence of a reactive gas to bring it into an atomic, molecular or particulate state (vapor), followed by colliding thermoelectron with the resulting vapor for ionization, accelerating by an electrical field and further colliding the thus ionized particles of said coating material with other evaportated particles to increase the number thereof, and depositing the resulting ionized particles onto the surface of a permanent magnet forming a cathode, whereby an anticorrosive thin film comprising said coating material is formed and coated on said body. It is further preferred to apply shot peening after the anticorrosive thin film has been formed on the surface of said permanent magnet. The shot peening may be applied without or with the blasting as given in the third and fourth aspect, respectively, to be applied prior to the vapor deposition. The shot peening typically involves jetting or blasting spherical particles having a mean particle size of 30 to 3,000 µm and a Mohs hardness of at least 3. Preferably shot peening is effected by jetting said particles together with a pressurized gas of 0.98 to 4.90 bar. Shot peening is usually effected for 1 to 60 minutes. Preferably after shot peening, the surface of said permanent magnet can further be treated with chromating, thereby providing an improved anticorrosion resistance. The Fe-B-R base permanent magnets are considerably improved in terms of corrosion resistance by the deposition of vapor deposition layers if applicable after the blasting. Since the evaporated metal particles deposited on the surfaces of magnet bodies in the case of the deposition techniques, however, the aforesaid vapor deposition layers may be sometimes deficient in density. This may further lead to a problem that local separation or cracking of the coated thin films occurs, resulting in local rusting. Such a problem is solved by depositing a vapor deposition thin film layer on the surface of a magnet material body which has, if applicable, been cleaned in the foregoing manner, and, thereafter, blasting a spherical powder comprising at least one of powders having a mean particle size of 30-3000 micrometers and a Mohs hardness of no lower than 3, together with a gas pressurized to 0,98 - 4,90 bar, onto that surface for, e. g., 1-60 minutes for shot peening. More specifically, a certain powder having the required properties, together with a pressurized gas, is blasted (shot-peened) onto the surface of a vapor deposition thin film layer to densify said thin film layer and enhance the adherance between the magnet body and said film layer. Like in the foregoing, the coating materials used in this treatment preferably include metals, alloys, ceramics and compounds e.g., nitrides, oxides or carbides of metals, which can improve the corrosion resistance of the present permanent magnets, such as metals, for instance, Al, Ni, Cr, Cu, Co, etc., or their alloys, or SiO₂, Al₂O₃, Cr₂O₃, TiN, AlN, TiC, etc. However, particular preference is given to Al (alminium). The shot peening powders used include spherical hard powders having a Mohs hardness of no lower than 3, such as steel balls, glass beads, etc., and may have a hardness equal to or higher than that of the thin film layer on which they are to be deposited. Preference is given to glass heads. Spherical peening powders having a Mohs hardness of below 3 are unpreferred, since they produce no sufficient peening effect due to the fact that their hardness is lower than that of the vapor deposition thin film layer. The reason why the mean particle size of the spherical peening powders is limited to 30-3000 micrometers is that, at below 30 µm, the force to be applied on the thin film layer is so small that a prolonged period of time is needed for peening, while, at a larger size than 3000 micrometers, the surface roughness of the sintered permanent magnet bodies is so large that the finished surfaces becomes uneven. A more preferable mean particle size ranges from 40 to 2000 micrometers. Referring to the blasting conditions for spherical powders, a force to be applied on the thin film layer at a pressure of below 0,98 bar, is so small that a prolonged period of time is needed for peening. At a pressure exceeding 4,90 bar on the other hand, the force to be applied on the thin film layer is so strong that same becomes uneven, resulting in a deterioration surface roughness. Further, when the blasting time is below 1 minute, it is impossible to treat uniformly the entire surface of the thin film layer. Although the upper limit of the blasting time is determined depending upon the peening amount and conditions, a time exceeding 60 minutes is unpreferred, since there is then a drop of surface roughness. For the same reason as mentioned in the foregoing, the thin film layer should have a thickness of, preferably no higher than 30 µm, most preferably 2 to 25 µm. If required, the magnet body having said thin film layer deposited thereon may further be treated by chromating to form a chromate coating film on the surface of said thin film layer, thereby further improving the corrosion resistance of the magnet body. In this manner, it is possible to further improve durability of the Fe-B-R base permanent magnets. The chromate coating film deposited on the thin film layer should preferably have a thickness from a few tenth of a nm to one micrometer, and have preferably its appearance finished to a color of light iridescence to yellowish brown assuming golden color. According to a further preferred embodiment of the present invention, it is possible to further improve or enhance the magnetic properties and corrosion resistance of the coated permanent magnets prepared according to the processes of the present invention by heat-treating said magnet to form an interdiffusion layer on the interface of the deposited coating and the magnet material body. The vapor deposition coating is usually deposited physically onto the surface of the present permanent magnet bodies into a firm film having a uniform thickness. Under the general conditions, however, the deposited coating film grows in the columnar form during deposition, so that there occur gaps between the growing particles. In some cases, water may enter these gaps, resulting in rusting. This gives rise to a drop of mechanical and thermal strength stability over an extended period. Such a problem can be eliminated by the heat treatment to be described below. More specifically, the vapor deposited permanent magnet material of the present invention is subjected to the predetermined heat treatment to fill up the aforesaid gaps through the melting effects and form a diffusion layer on the interface of the deposited phase and the permanent magnet body, thereby promoting diffusion, into the crystal grain boundaries, of the deposited coating layer-forming elements, not to speak of the crystal grains of the magnet layer. This results in great improvements in the corrosion resistance of the grain boundaries as well as the mechanical and thermal strength of the vapor-deposited coating, whereby peeling-off of the thin film and rusting can be avoided. According to that heat treatment, a stable passivated oxide is formed on the surface of the vapor-deposited coating. Thus, the permanent magnet of the present invention can be used under extremely severe environmental conditions for an extended period. There is also an advantage that the present permanent magnet including on the surface the vapor-deposited coating and the interdiffusion layer is improved in terms of coercive force for the following reasons. Namely, the Fe-B-R base sintered permanent magnet has its major phase (at least 50 vol %) consisting of Nd₂Fe₁₄B crystal grains of ca. 10 micrometers (e.g., if Nd is used as R), surrounding bcc phase and Nd-rich phase and a small amount of a B-rich phase. Among others, the presence of the Nd₂Fe₁₄B and bcc phases takes a great part in the generation of coercive force. However, in the Fe-B-R base permanent magnet, the bcc phase is formed, only when the Nd-rich phase and the tetragonal Nd₂Fe₁₄B phase exist. However, on the surface of the permanent magnet there is present only the tetragonal Nd₂Fe₁₄B crystal phase without the surrounding bcc phase. This results in a lowering of the coercive force of the magnet surface layer, and is responsible for the degradation of the magnetic properties of the permanent magnet, when it is machined into a small or thin product or article. According to this preferred embodiment of the present invention, however, the interdiffusion layer formed on the interface of the vapor-deposited coating layer and the permanent magnet material body serves to enhance the crystal magnetic anisotropy appearing at the above-mentioned portion, so that any drop of the coercive force appearing on the magnet surface is avoided with improvements in the magnetic properties. Another factor of rusting is that the formation of thin films (vapor-deposited coating films) does not well proceed at the crystal boundary due to the presence of the R-rich phase at the crystal grain boundaries of the permanent-magnet. However this problem is eliminated by the formation of the aforesaid interdiffusion layer. In this embodiment of the present invention, the formation of the interdiffusion layer on the interface of the vapor-deposited coating and the permanent magnet material is achieved by heat treatment in the atmosphere or in vacuum. It is preferred, however, that, when the heat treatment is carried out after vapor deposition has been applied to the aged permanent magnet, its treatment temperature ranges from 250°C to the aging temperature. This is because only insufficient diffusion takes place between the vapor deposited coating and the permanent magnet at a temperature of lower than 250°C, while the effect of the aging treatment previously applied disappears at a temperature higher than the aging temperature. Where the heat treatment is carried out after a vapor-deposited layer has been applied to the permanent magnet which has not been aged, it is preferred that the heat treatment temperature ranges from 250°C to the melting point of the vapor-deposited metal used while the melting point should not exceed the sintering temperature. It should be noted that, depending upon the temperature conditions for heat treatment, aging may be carried out simultaneously with heat treatment, and so the subsequent aging can be omitted. Turning to the aging temperature for the permanent magnet body of the present invention, a temperature of 350°C to the sintering temperature (900-1200°C) of that magnet body is preferably applied when one-stage aging is applied. For two-stage aging, it is preferred that a temperature of 750-1000°C is applied at the first staged and a temperature of 480-700°C is applied at the second stage. For the one-stage aging, it is preferred that the heat treatment is carried out at a temperature of 250°C to the aging temperature, and for two-stage aging, it is preferred that the heat treatment is effected at a temperature of 250°C to the first-stage aging temperature. Where the heat treatment is carried out without application of aging at a temperature of 250°C to the melting point of the vapor-deposited metal, it is desired in view of the resulting magnetic properties that aging be conducted after that heat treatment. Furthermore, where the heat treatment temperature to be applied after aging is higher than the temperature for that aging, it is required that the aging be again conducted. To obtain the required interdiffusion layer, the heat treatment is preferably carried out for 5 minutes to 5 hours, although it may be effected for a suitable period of time, depending upon the type of vapor deposited metals, the amount to be treated and the temperature condition. In this embodiment of the present invention, it is preferred that the diffusion layer formed on the interface of the vapor-deposited coating layer and the permanent magnet body by heat treatment has a thickness of 0.01 to 10 micrometers in view of the corrosion resistance and adhesion strength with respect to the underlying magnet body. In the case that the vapor-deposited metals are aluminium, chromium, titanium, etc., a layer of the oxide thereof is formed on the surface of the vapor deposited coating layer during heat treatment, and is then passivated (e.g., oxidized) to introduce further improvements in corrosion resistance. The resulting magnet can be used for a prolonged period under the conditions that are more severe than applied in the prior art. Where both the heat treatment and the shot peening treatment are carried out, it is preferred that, after the peening treatment, the diffusion layer is formed by the heat treatment. As will be discussed later, further improvements are introduced into corrosion resistance by the application of resin impregnation following the heat treatment. The coated permanent magnets prepared according to the above described embodiments of the present invention excel in corrosion resistance. However, it would be unavoidable that extremely fine micropores are present in the coating film. Thus, there is still a fear that local peeling-off or cracking of the coating film (layer) may take place, while the magnet is used over an extended period and/or under severe conditions, leading to local rusting. The possibility of the aforesaid magnet being rusted under such severe conditions is reduced or limited to a considerably little degree by impregnating a coating film layer to the magnet which has been subjeced to vapor deposition (or further shot peening or further chromate treatment) with a resin (preferably heat-resistant resin). For instance, a thermosetting resin is impregnated in the surface of the magnet, which has been washed with a solvent (or water). After the solvent (or water) is dried off, the resin remaining in the pores of the coating film is thermally set. As the resins to be impregnated into the micropores of the vapor-deposited thin coating film, use may be made of (general thermosetting resins such as), e.g., urea resin, melamine resin, phenol resin, epoxy resin, unsaturated polyester, alkyd resin, urethane resin, ebonite, etc. Particular preference is given to a thermosetting phenol resin soluble in alcohol and having a low molecular weight. The thermosetting conditions and the solvents used may be selected depending upon the type of thermosetting resins used. In addition to the thermosetting resins, it may be possible to use any resin suitable for impregnation of the coating film layer and having a certain heat resistance (for instance 100°C, preferably 100 to 150°C or higher selected depending upon the purpose) such as, for instance, polyamide, silicone resin, fluorine-containing resin, chlorinated vinyl chloride, polycarbonate and the like. The thermosetting resins may be impregnated into the fine pores of the thin coating film by means of dip impregnation, vacuum impregnation, vacuum/pressure impregnation. Resin impregnation may also be carried out (e.g., in vacuo) by other suitable means under suitable conditions, provided that any impregnation of impurities, etc. into the pores should be avoided. It is most preferred to apply all the embodiments of the present invention, however, only the vapor deposition is obligatory and any of the subsequent embodiments may be eliminated depending upon the ultimate purpose in use. With respect to the third and fourth aspect of the present invention the following compositional ranges of the elements are preferred. It is preferred that 50 at % or more of R is Nd and/or Pr. It is further preferred that the composition of the permanent magnet material body comprises 12 - 24 at % R wherein at least 50 at %of R is Nd and/ or Pr, 4-24 at % B and at least 52 at % Fe, most preferably 65 at % Fe. A specific amount of the Fe can also be substitued by Co according to another preferred composition. This composition may comprise Co which is substituted for Fe up to 45 at % of the magnet material body, provided that the resultant Fe is at least 27 at %. According to a further preferred embodiment, Co is substituted for Fe up to 35 at % of the magnet material body, more preferably up to 25 at %, and most preferably up to 20 at %. According to the first til the fourth aspect of the present invention the rare earth element(s) R used in the permanent magnet material bodies of the present invention amounts to 10-30 at % of the overall composition wherein R represents at least one of Nd, Pr, Dy, Ho and Tb or a mixture of at least one of said five and at least one of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y. Usually, it suffices to use one of said five R, but use may be made of mixtures of two or more R (mishmetal didymium, etc.) for the reasons of their easy avialability, etc. It is noted that R (as the starting material) may not be pure rare earth elements, but may contain impurities to be inevitably entrained from the process of production, as long as they are industrially available. R is an element or elements inevitable in the novel permanent magnet materials based on the foregoing systems. However, in an amount of below 10 at % it is impossible to obtain permanent magnets having high magnetic properties, in particular high coercive force, since the cubic system of the same structure as alpha-iron begins to occur. In an amount of higher than 30 at %, on the other hand, no excellent permanent magnets are obtained, since the proportion of R-rich nonmagnetic phases is increased, resulting in a drop of residual magnetic flux density (Br). Therefore, the amount of the rare earth element(s) is limited to a range of 10-30 at %. B (boron) is an inevitable element in the permanent magnet materials of this invention. However, in an amount of lower than 2 at % it is impossible to obtain permanent magnets having high coercive force (iHC), since their major phase is of the rhombohedral structure. In an amount of higher than 28 at %, on the other hand, no practical permanent magnets are obtained, since the proportion of B-rich nonmagnetic phases is increased, resulting in a drop of residual magnetic flux density (Br). Therefore, the amount of B is limited to a range of 2-28 at %. Note, however, these limitations are made in view of the practical level of the energy product of 31,8 k·TA/m. Fe (iron) is an inevitable element in the novel permanent magnets based on the aforesaid systems and the balance is Fe (at least 42 at %). For (BH)max of at least 79.6 k·TA/m a composition of 10-24 at % R wherein 50 at % of R is Nd and/or Pr, 4-24 at % B and the balance Fe (at least 52 at %) is suitable. Still higher (BH)max may be achieved in the preferred compositions. 12.5-20 at % R, 5-15 at % B and 65-82.5 at % Fe provide (BH)max of at least 159,2 k·TA/m. 13-18 at % R, 5-11 at % B and 67-82 at % Fe provide (BH)max of at least 238.8 k·TA/m. 6-11 at % B, 13-16 at % R and the balance Fe provide (BH)max of at least 278.6 k·TA/m. 6.5-7 at % B, 13.5-14 at % R and the balance being Fe provide (BH)max of at least 318.4 k·TA/m, ranging up to 358.2 k·TA/m. At least 80 at % of R should be Nd and/or Pr. For the highest energy product R should be Nd. An Fe amount of lower than 65 at % leads to a drop of residual magnetic flux density (Br) and at least 65 at % is preferred. An Fe amount of higher than 80 at % gives no further increase in coercive force. Thus, the amount of Fe is preferably 65-80 at % in view of the coercivve force. In the permanent magnet materials of this invention, the substitution of a part of Fe with Co yields magnets having an improved temperature dependence (i.e., less dependent on temperature) through increase in the Curie temperature and the improved temperature coefficient of Br. However, it is unpreferred that Co exceeds 20 at %, since there is then gradual deterioration of magnetic properties. To obtain high residual magnetic flux density, it is most preferred that the amount of Co is in a range of 5-15 at %, since Br is higher than that obtained in the absence of Co. However, in view of the temperature dependence, Curie temperature, and the corrosion resistance Co may be incorporated up to 45 at % substituted for a part of Fe wherein the remaining Fe should be at least 27 at %. Co may be present up to 35 at %, or 25 at % subject to the gradual change in Br. By the same token, the permanent magnet materials according to this invention may contain, in addition to R, B and Fe, impurities which are inevitably entrained from the industrial process of production. Such impurities include C, P, S, Cu etc. which should be as little as possible, however, may be present up to about 1 at % in total, or strictly up to 0.1 at % in total. At least one of the following additional elements M may be added to the R-B-Fe base permanent magnets, since they are effective in improving the coercive force, loop squareness of demagnetization curves and productivity thereof, or cut down the price thereof. The additional elements M are: The preferred amounts of the additional elements M are: However, when two or more of the additional elements are contained, the highest total amount thereof is no higher than the at % of the element of the additional elements, that is actually added in the largest amount. It is thus, possible to enhance the coercive force of the permanent magnets of this invention. The former amounts of M are defined to provide (BH)max of at least 31.8 k·TA/m, while the preferred amount of M are defined at (BH)max of at least 79.6 k·TA/m. Most preferred amounts of M are 0.1-3 at % in total. In the production of sintered permanent magnets having excellent magnetic properties from finely divided and uniform alloy powders, it is inevitable that their crystal phase has its major phase consisting of the Fe-B-R type tetragonal crystal structure. The Fe-B-R type tetragonal crystal structure of the present invention has a central composition of R₂Fe₁₄B, or R₂(Fe,Co)₁₄B. It is understood that the permanent magnets of this invention are made magnetically anisotropic by compacting in a magnetic field, and magnetically isotropic by compacting in the absence of any magnetic field. The permanent magnet materials according to this invention show a coercive force iHc of at least 79.6 k·A/m, a residual magnetic flux density of at least 4 x 10⁻¹ T, and a maximum energy product (BH)max of at least 31.8 k·TA/m and reaching a high of 238.8, 278.6, 318.4 k·TA/m or more. It is clear that all process features mentioned above and in the following do not only apply to the process claims 4 to 6 and 15 to 41, but also to production of the sintered permanent magnet bodies of claims 1 to 3 and 7 to 14. The present invention will now be explained in detail with reference to the following examples. EXAMPLES Example 1 The starting materials used were electrolytic iron of 99.9 % purity, a ferroboron alloy containing 19.4 % B with the remainders being Fe and impurities such as Al, Si, C, etc., and Nd of 99.7 % or higher purity. These materials were melted by high frequency melting, and were thereafter cast in a water-cooled copper casting mold to obtain a cast ingot having a composition of 15Nd-8B-77Fe (in at %). The ingot was coarsely pulverized in a stamp mill, and was then finely pulverized in a ball mill to obtain fine powders having a particle size of 3 micrometers. The powders were charged into a metal mold, oriented in a magnetic field 955.2 k.A/m, and were compacted in the direction parallel with the magnetic field at a pressure of 1471 bar. The thus obtained compact was sintered at 1100°C for 1 hour in Ar, was then cooled off, and was further aged at 600°C for 2 hours in Ar to prepare a permanent magnet. Test pieces, each being 20 mm in outer diameter, 10 mm in inner diameter and 1.5 mm in thickness, were cut out of that permanent magnet. One of the aforesaid test pieces was placed in a vacuum vessel with the degree of vacuum being 1.33 x 10⁻⁸ bar, and was pre-treated by heating at 350°C for 30 minutes. The test piece was cooled down to 300°C. Thereafter, a Ni piece measuring 100 mm diameter X 10 mm and having a purity of 99.99 % or higher for a coating material was irradiated with electron beams of 0.6 A and 8 kV for heating and evaporation, whereby a thin film of Ni was vacuum-deposited onto the test piece. The Ni thin film formed on the surface of the permanent magnet according to this invention was found to have a thickness of 5 micrometers. With this test piece, corrosion resistance testing was carried out, and adhesion strength testing of the thin film was thereafter done. The magnetic properties of the test piece were also measured before and after corrosion resistance testing. The results of testing and measurement are set forth in Tables 1 and 2. For the purpose of comparison, another test piece was solvent-degreased with trichlorethylen for 3 minutes, and was alkali-degreased with 5 % NaOH at 60°C for 3 minutes. Thereafter, the piece was washed with 2 % HCl at room temperature for 10 seconds, and was electroplated with nickel in a Watt bath at a current density of 4 A/dm² and a bath temperature of 60°C for 20 minutes to obtain a control test piece (Comparison Example 1) having thereon a nickel plating layer of 10 µm in thickness. The same tests and measurement as in Example 1 were carried out with this control piece. Table 1 also shows the results. In the corrosion resistance testing, the test pieces were allowed to stand for 500 hours in an atmosphere of a temperature of 60°C and a humidity of 90 % for the visual appreciation of the appearance thereof. In the adhesion strength testing, an adhesive tape was applied on the test pieces which had been subjected to the corrosion resistance testing and provided with cells at pitch of 1 mm, and was peeled off to estimate whether or not the thin film layers were separated off (unpeeled cells/whole cells). Example 2 The same test pieces as used in Example 1 was placed in a vacuum vessel having a degree of vacuum of 1,33 x 10⁻⁸ bar, and an Ar gas was introduced therein to a degree of vacuum of 1,60 x 10⁻⁵ bar. Discharge was then effected in the Ar gas at 150 W to sputter a target material formed of a Co-18.5Cr alloy piece for 5 hours, whereby a thin film of the same composition as the target material was formed on the surface of the test piece. The thin film formed on the surface of the test piece was found to have a thickness of 5 micrometers. With this test piece, corrosion resistance testing and adhesion strength testing of the thin film were carried out in the procedures of Example 1. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Tables 1 and 4. Example 3 The same test piece as used in Ex. 1 was placed in a vacuum vessel having a degree of vacuum of 1,33 x 10⁻⁸ bar, and reverse sputtering was effected at a voltage of 400 V for 1 minute in an Ar gas of 1,07 x 10⁻³ bar. The test piece was then pre-treated by heating at 350°C for 30 minutes, and cooled down to 300°C. Thereafter, a target material formed of particulate molten quartz of 3-5 mm in size was heated to put that molten quartz into a molecular state. Thermoelectrons were collided with the molecular quartz for ionization. The ionized SiO₂ particles, traveling by an electrical field distribution, were collided with other evaporated particles to increase the number thereof. These ionized SiO₂ particles were attracted by an electrical field for deposition onto the test piece constituting a cathode, whereby a SiO₂ thin film was formed on the surface of the test piece. That film had a thickness of 5 micrometers. Referring to the foregoing ion plating conditions, the test piece was treated at a voltage of 1 kV, an ionization voltage of 100 V and 80-90 mA for 40 minutes. With this test piece, corrosion resistance testing and adhesion strength testing of the thin film were effected in the procedures of Ex. 1. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Tables 1 and 3. Example 4 The starting materials used were electrolytic iron of 99.9 % purity, a ferroboron alloy containing 19.4 % B with the remainders being Fe and impurities such as Al, Si, C, etc., and Nd and Dy each having a purity of 99.7 % or higher. These materials were melted by high-frequency melting, and were thereafter cast in a water-cooled copper casting mold to obtain a cast ingot having a composition of 15Nd-1.5Dy-8B-75.5Fe (in at %). Thereafter, the ingot was coarsely pulverized in a stamp mill, and was then finely pulverized in a ball mill to obtain fine powders having particle size of 3 microns. The powders were placed into a metal mold, oriented in a magnetic field of 955.2 k.A/m, and were compacted in the direction normal to the magnetic field at a pressure of 1471 bar. The thus obtained compact was sintered at 1100°C for 1 hour in Ar, was then allowed to cool, and was further aged at 600°C for 2 hours in Ar to prepare a permanent magnet. Test pieces, each being 20 mm in outer diameter, 10 mm in inner diameter and 1.5 mm in thickness, were cut out of the obtained permanent magnet. A Ti piece for a coating material was evaporated by arc discharging at a degree of vacuum of 1,33 x 10⁻⁵ bar or less in a vacuum vessel into which one of the aforesaid test piece was placed. In the meantime, a N₂ gas was accelerated as N₂ gas ions at an extraction voltage of 40 kV, an ionization current of 100 mA and a beam size of 4 X 10 cm² for Ti-evaporation and N₂ gas ion-irradiation for 3 hours, whereby a TiN thin film was formed on the surface of the test piece. The TiN thin film was then found to have a thickness of 5 micrometers. The same corrosion resistance testing and adhesion strength testing of the thin film as mentioned in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 1. For the purpose of comparison, another test piece was solvent-degreased with trichloroethylene for 3 minutes, and was alkali-degreased with 5 % NaOH at 60°C for 3 minutes. Thereafter, the piece was acid-washed with 2 % HCl at room temperature for 10 seconds, and was electroplated with nickel in a Watt bath at a current density of 4 A/dm² and a bath temperature of 60°C for 20 minutes to obtain a control test piece (Comparison Example 2) having thereon a nickel deposited layer of 10 micrometers in thickness. Like in Ex. 4, the same tests and measurement as in Ex. 1 were carried out with this control piece. Table 1 also shows the results. Example 5 With the same test piece as used in Ex. 4, the plasma vapor deposition thin-film formation technique was applied for 3 hours to form a SiO₂ thin film of 5 micrometers in thickness on the surface thereof. More exactly, SiH₄ and N₂O gases were simultaneously fed at a flow rate of 100 ml/min into a vacuum vessel having therein the test piece, and discharge was effected at 200 W with a high-frequency plasma of 13.56 MHz, thereby forming a SiO₂ thin film on the surface of the test piece preheated to 200°C. The same corrosion resistance testing and adhesion strength testing of the thin film as mentioned in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 1. As clearly appreciated from the results of tests and measurement set forth in Table 1, the anticorrosive vapor-deposited layers according to this invention have the required thickness and show a uniformity much better than do the control layers. It is thus appreciated that the permanent magnets of this invention are steadily protected against oxidation without any deterioration of the magnetic properties, and have the magnetic properties considerably improved over those of the control magnets. Example 6 The same test piece as used in Ex. 1 was placed in a vacuum vessel having a degree of 4,00 x 10⁻⁹ bar, and was pre-treated by heating at 100°C for 30 minutes. Thereafter, a Cr piece measuring 3 mm diameter X 5 mm and having a purity of 99.99 % or higher for a coating material was irradiated with electron beams of 0.02 A and 5 kV for 1 hour for heating and evaporation, whereby a Cr thin film was formed on the surface of the test piece, which was found to have a thickness of 5 micrometers. The same corrosion resistance testing and adhesion strength testing of the thin film as mentioned in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 2. Example 7 The same test piece as used in Ex. 1 was placed in a vacuum vessel having a degree of vacuum of 6,67 x 10⁻⁹ bar, and was pre-treated by heating at 100°C for 30 minutes. Thereafter, a molten quartz piece measuring 3 mm diameter X 5 mm for a coating material was irradiated with electron beams of 0.04 A and 5 kV for 1 hour for heating and evaporation, thereby forming a SiO₂ thin film on the surface of the test piece, which was found to have a thickness of 5 micrometers. The same corrosion resistance testing and adhesion strength testing of the thin film as applied in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 2. Example 8 The same test piece as used in Ex. 1 was placed in a vacuum vessel having a degree of vacuum of 6,67 x 10⁻⁹ bar, and was pre-treated by heating at 100°C for 30 minutes. Thereafter, an Al piece measuring 3 mm diameter X 5 mm and having a purity of 99.99 % or higher for a coating material was irradiated with electron beams of 0.28 A and 5 kV for 1 hour for heating and evaporation, thereby forming an aluminium thin film on the surface of the test piece, which was found to have a thickness of 5 micrometers. The same corrosion resistance testing and adhesion strength testing of the thin film as applied in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 2. As clearly appreciated from the results of tests and measurement set forth in Table 2, the anticorrosive thin films according to the vacuum deposition technique have the required thickness and show a uniformity much better than do the control film. It is thus appreciated that the permanent magnets of this invention are steadily protected against oxidation without any deterioration of the magnetic properties, and have the magnetic properties considerably improved. Example 9 The same test piece as used in Ex. 1 was placed in a vacuum vessel having a degree of vacuum of 1,33 x 10⁻⁸ bar, and reverse sputtering was effected at a voltage of 400 V for 1 minute in a N₂ gas of 1,33 x 10⁻⁵ bar. The test piece was then pre-treated by heating at 350°C for 30 minutes, and was cooled down to 300°C. A coating material formed of a Ti piece measuring 5 mm diameter X 3 mm and having a purity of 99.99 % was heated to put it into an atomic state. Thermoelectrons were collided with the atomic Ti for ionization. The ionized TiN particles, traveling by an electrical field distribution, were collided with other evaporated particles to increase the number thereof. These ionized TiN particles were attracted by an electrical field for deposition onto the test piece constituting a cathode, thereby forming a TiN thin film found to have a thickness of 5 micrometers. Referring to the aforesaid ion plating conditions, the test piece was treated at a voltage of 1 kV, an ionization voltage of 100 V and 40-60 mA for 20 minutes. The same corrosion resistance testing and adhesion strength testing of the thin film as applied in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 3. Example 10 The same test piece as used in Ex. 1 was placed in a vacuum vessel having a degree of vacuum of 1,33 x 10⁻⁸ bar, and reverse sputtering was effected at a voltage of 400 V for 1 minute in a CO₂ gas of 1,33 x 10⁻⁵ bar. The test piece was then pre-treated by heating at 350°C for 30 minutes, and was cooled down to 300°C. A coating material formed of a Ti piece measuring 5 mm diameter X 3 mm and having a purity of 99.99 % was heated to put it into an atomic state. Thermoelectrons were collided with the atomic Ti for ionization. The ionized TiC particles, traveling by an electrical field distribution, were collided with other evaporated particles to increase the number thereof. These ionized TiC particles were attracted by an electrical field for deposition onto the test piece defining a cathode, thereby forming a TiC thin film. The thin film formed on the surface of the test piece was found to have a thickness of 5 µm. Referring to the aforesaid ion plating conditions, the test piece was treated at a voltage of 1 kV, an ionization voltage of 100 V and 40-60 mA for 20 minutes. The same corrosion resistance testing and adhesion strength testing of the thin film as applied in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 3. As clearly appreciated from the results of tests and measurement set forth in Table 3, the anticorrosive thin films according to the ion plating technique have the required thickness and show a uniformity much better than do the control film. It is thus appreciated that the permanent magnets of this invention are well protected against oxidation without any deterioration of the magnetic properties, and have the magnetic properties considerably improved. Example 11 The same test piece as used in Ex. 1 was placed as an anode in a vacuum vessel having a degree of vacuum of 6,67 x 10⁻⁹ bar, and an Ar gas was introduced therein to a degree of vacuum of 1,07 x 10⁻⁶ bar. A voltage of 150 V was applied between electrodes for discharge to sputter a cathode target material formed of a Ni material measuring 100 mm diameter X 5 mm and having a purity of 99.99 % for 5 hours, whereby a thin film having the same composition as the target material was formed on the surface of the test piece. The thin film formed on the surface of the test piece was found to have a thickness of 5 micrometers. The same corrosion resistance testing and adhesion strength testing of the thin film as mentioned in Ex. 1 were carried out with this coated test piece. The magnet properties of test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 4. Example 12 The same test piece as used in Ex. 1 was placed as an anode in a vacuum vessel having a degree of vacuum of 6,67 x 10⁻⁹ bar, and an Ar gas was introduced therein to a degree of vacuum of 1,60 x 10⁻⁵ bar. A voltage of 170 V was applied between electrodes for discharge to sputter a cathode target material formed of a SiO₂ material measuring 100 mm diameter X 5 mm and a purity of 99.99 % for 3 hours, whereby a thin film having the same composition as the target material was formed on the surface of the test piece. The thin film formed on the surface of the test piece was found to have a thickness of 5 micrometers. The same corrosion resistance testing and adhesion strength testing of the thin film as applied in Ex. 1 were carried out with this test piece. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 4. As clearly appreciated from the results of tests and measurement set forth in Table 4, the anticorrosive thin films according to sputtering have the required thickness and show a uniformity much better than do the control film. It is thus appreciated that the permanent magnets of this invention are well protected against oxidation without any deterioration of the magnetic properties, and have the magnetic properties considerably improved. Example 13 The starting materials used were electrolytic iron of 99.9 % purity, a ferroboron alloy and Nd of 99.7 % or higher purity. These materials were formulated and melted by high-frequency melting. Thereafter, the melt was cast in a water-cooled copper casting mold to obtain a cast ingot having a composition of 16.0Nd-7.0B-77.0Fe. Thereafter, the ingot was coarsely pulverized in a stamp mill, and was finely pulverized in a ball mill to obtain fine powders having a mean particle size of 2.8 micrometers. The fine powders were placed in a metal mold, oriented in a magnetic field of 1194.0 k·A/m, and were compacted at a pressure of 1177 bar in the direction parallel with the magnetic field. The obtained compact was sintered at 1100°C for 1 hour in an Ar atmosphere to obtain a sintered body of 25 mm in length, 40 mm in width and 30 mm in thickness. The sintered body was further subjected to a two-stage aging treatment at 800°C for 1 hour and at 630°C for 1.5 hours. Test pieces, each being 5 mm in length, 10 mm in width and 3 mm in thickness, were cut out of the thus obtained permanent magnet at 2400 rpm and a feed rate of 5 mm/min in the atmosphere, using a diamond No. 200 grinder. For blasting, glass beads (Comparison Examples) and Al₂O₃ powders (Examples) - the powders for blasting - were blasted together with air pressurized to 3.92 bar onto the test pieces for 7-10 minutes, as stated in Table 5, to remove surface layers therefrom. The magnetic properties of each test piece were measured before and after blasting. Table 5 shows the processing conditions and the results of measurement. The results of Table 5 clearly indicate that the processings according to this invention eliminate any deterioration of the magnetic properties of the sintered magnet body, which otherwise takes place due to black skins remaining thereon and oxidation or deteriorated layers formed by finish machining. Example 14 The starting materials used were electrolytic iron of 99.9 % purity, a ferroboron alloy and Nd of 99.7 % or higher purity. These materials were formulated and melted by high-frequency melting. Thereafter, the melt was cast in a water-cooled copper casting mold to obtain a cast ingot having a composition of 16.0Nd-7.0B-77.0Fe (atomic %). Thereafter, the ingot was coarsely pulverized in a stamp mill, and was finely pulverized in a ball mill to obtain fine powders having a mean particle size of 2.8 micrometers. The fine powders were placed in a metal mold, oriented in a magnetic field 1194.0 k·A/m, and were compacted at a pressure 1177 bar in the direction parallel with the magnetic field. The obtained compact was sintered at 1100°C for 1 hour in an Ar atmosphere to obtain a sintered body of 25 mm in length, 40 mm in width and 30 mm in thickness. The sintered body was further subjected to a two-stage aging treatment at 800°C for 1 hour and at 630°C for 1.5 hours. Test pieces, each being 5 mm in length, 10 mm in width and 3 mm in thickness were cut out of the thus obtained permanent magnet at 2400 rpm and a feed rate of 5 mm/min in the atmosphere, using a diamond No. 200 grinder. Al₂O₃ hard powders having a mean particle size of 50 µm and a Mohs hardness of 12 were blasted together with a N₂ gas pressurized to 2.94 bar onto one of the test pieces for 15 minutes for grit blasting to remove a surface layer therefrom. Next, the aforesaid test piece was placed in a vacuum vessel having a degree of vacuum of 6,67 x 10⁻⁸ bar into which an Ar gas was fed. Subsequently to 20 minute-discharge at a voltage of 400 V in an Ar gas of 1,33 x 10⁻⁵ bar, a coating material formed of an Al plate of 99.99 % purity was heated for the ionization of evaporated Al. The thus ionized particles were attracted by an electrical field for deposition onto the test piece forming a cathode, thereby forming an Al thin film. The thin film formed on the surface of the test piece was found to have a thickness of 20 micrometers. The foregoing ion plating conditions were a voltage of 1.5 kV and a treating time of 15 minutes. With this test piece, corrosion resistance testing was carried out, and adhesion strength testing of the thin film was done thereafter. The magnetic properties of the test piece were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 6. For the purpose of comparison, another test piece was solvent-degreased with trichorethylene for 3 minutes, and was alkali-degreased with 5 % NaOH at 60°C for 3 minutes. Thereafter, the piece was acid-washed with 2 % HCl at room temperature for 10 seconds, and was electroplated with nickel in a Watt bath at a current density of 4 A/dm² and a bath temperature of 60°C for 20 minutes to obtain a control test piece (Comparison Example 7) having thereon a nickel plating layer of 20 µm in thickness. The same tests and measurement as in Ex. 14 were carried out with this control piece. The results are also given in Table 6. In the corrosion resistance testing, the test pieces were allowed to stand for 200 hours in an atmosphere of a temperature of 60°C and a humidity of 90 % for the visual estimation of the appearance thereof. In the adhesion strength testing, the test pieces which had been subjected to the corrosion resistance testing were ruptured for the visual estimation of the rupture cross-sections thereof. The results of Table 6 clearly indicate that the method of this invention eliminate any deterioration of the permanent magnets, which otherwise takes place in association with cutting or grinding, and are thus very effective in providing permanent magnets having improved corrosion resistance. Example 15 The starting materials used were electrolytic iron of 99.9 % purity, a ferroboron alloy and Nd of 99.7 % or higher purity. These materials were formulated and melted by high-frequency melting. Thereafter, the melt cast in a water-cooled copper casting mold to obtain a cast ingot having a composition of 16.0Nd-7.0B-77.0Fe. Thereafter, the ingot was coarsely pulverized in a stamp mill, and was finely pulverized in a ball mill to obtain fine powders having a mean particle size of 2.8 micrometers. The fine powders were placed in a metal mold, oriented in a magnetic field of 1194 k·A/m, and were compacted at a pressure of 1177 bar in the direction normal to the magnetic field. The obtained compact was sintered at 1100°C for 1 hour in an Ar atmosphere to obtain a sintered body for 25 mm in length, 40 mm in width and 30 mm in thickness. The sintered body was further subjected to a two-stage aging treatment at 800°C for 1 hour and at 630°C for 1.5 hours. Test pieces, each being 5 mm in length, 10 mm in width and 3 mm in thickness, were cut of the thus obtained permanent magnet at 2400 rpm and a feed rate of 5 mm/min in the atmosphere, using a diamond No. 200 grinder. Al₂O₃ hard powders of random shape having a mean particle size of 50 micrometers and a Mohs hardness of 9 were blasted together with a N₂ gas pressurized to 2.45 bar onto one of the test pieces for 20 minutes for blasting to remove a surface layer therefrom. Next, the aforesaid test piece was placed in a vacuum vessel having a degree of vacuum of 6,67 x 10⁻⁸ bar, into which an Ar gas was fed. Subsequently to 15 minute-discharge at a voltage of 500 V in an Ar gas of 1,33 x 10⁻⁵ bar, a coating material formed of an Al plate of 99.99 % purity was heated for the ionization of evaporated Al. The thus ionized particles were attracted by an electrical field for deposition onto the test piece forming a cathode, thereby forming an Al thin film. The thin film formed on the surface of the test piece was found to have a thickness of 15 micrometers. The aforesaid ion plating conditions were a voltage 1.5 kV and a treating time of 10 minutes. Powders of spherical glass beads having a mean particle size of 120 micrometers and a Mohs hardness of 6 were blasted together with a N₂ gas pressurized to 1,47 bar onto the Al thin film-deposited test sample for 5 minutes for shot peening, thereby preparing a test piece (Example 15-1). After shot peening, the magnet sample was immersed in a 2 % AROGINE No. 1200 (Trade Name; manufactured by Nippon Paint) solution maintained at 30°C for 1 minute to deposite a golden chromate thin film onto the surface of the Al thin film layer after peening, thereby obtaining a test piece (Example 15-2). With these test pieces, corrosion resistance testing was carried out, and adhesion strength testing of the thin films was thereafter done. The magnetic properties of the test pieces were also measured before and after the corrosion resistance testing. The results of tests and measurement are set forth in Table 7. For the purpose of comparison, the as-cut test piece (Comparison Example 8) and the aforesaid test piece were solvent-degreased with trichlorethylene for 3 minutes, and was alkali-degreased with 5 % NaOH at 60°C for 3 minutes. Thereafter, the pieces were acid-washed with 2 % HCl at room temperature for 10 seconds, and was electroplated with nickel in a Watt bath at a current density of 4 A/dm² and a bath temperature of 60°C for 20 minutes to obtain control test pieces (Comparison Example 9) having thereon a nickel plating layer of 20 micrometers in thickness. The same tests and measurements as in Ex. 15 were carried out with these control test pieces. The results are also given in Table 7. In the corrosion resistance testing, the test pieces were allowed to stand for 500 hours in an atmosphere of a temperature of 70°C and a humidity of 90 % for the visual estimation of the appearance and adhesion thereof. This testing was also estimated in terms of the magnetic properties of the test pieces before and after the corrosion resistance testing. Measurement was made of a time by which the test pieces were rusted under the aforesaid conditions. In the adhesion strength testing, the test pieces 15-1 and 15-2 of this invention and the control test pieces 9, which had been subjected to the corrosion resistance testing, were ruptured to examine the rupture cross-sections thereof. Table 7 clearly indicates that the method of this invention eliminates any deterioration of the permanent magnets, which otherwise takes place in association with cutting or grinding, and are thus very effective in providing permanent magnets having improved corrosion resistance. Example 16 Electrolytic iron of 99.9 % purity, a ferroboron alloy and Nd of 99.7 % or higher purity used as the starting materials were formulated together, molten by high-frequency induction, and were thereafter cast in a water-cooled copper casting mold to obtain an cast ingot having composition of 15.0Nd8.0B77.0Fe (at %). Thereafter, the ingot was coarsely pulverized in a stamp mill,and was then finely pulverized in a ball mill into fine powders having a mean particle size of 3 micrometers. The thus obtained powders were charged in a mold, oriented in a magnetic field of 955.2 k·A/m, and were compacted at a pressure 1471 bar direction normal to the magnetic field. The obtained green compact was sintered at 1100°C for 1 hour in an Ar atmosphere to obtain a sintered body measuring 25 mm in length, 40 mm in width and 30 mm in thickness. The thus sintered body was subjeced to the two-stage aging of 800°C x 1 hour and 630°C x 1.5 hours in Ar. With the use of a grinding wheel of diamond No. 200, the thus obtained permanent magnet body was cut at 2400 rpm and a feed rate of 5 mm/min in the atmosphere to prepare a sample of 5 mm in length, 10 mm in width and 3 mm in thickness. The sample was then subjeted to blasting by blasting hard powders of Al₂O₃ of random shape having a mean particle size of 50 micrometers and a Mohs hardness of 9 along with a N₂ gas pressurized to 2.45 bar to remove the surface layer therefrom. Subsequently, the sample was placed in a vacuum vessel having a degree of vacuum 6,67 x 10⁻⁸ bar into which an Ar gas was supplied for 15 minute-discharge at a voltage of 500 V. Subsequently thereafter, a coating material formed of an Al plate of 99.99 % purity was heated for the ionization of vaporized Al. The thus ionized particles were attracted by an electric field, and were deposited onto the test piece defining a cathode to form an Al thin film, which was found to have a thickness of 15 micrometers. The aforesaid ion plating was carried out at a voltage of 1.5 kV for 10 minutes. Further, the magnet sample with the deposited Al thin film layer was subjected to shot peening for blasting powders of spherical glass beads having a mean particle size of 120 µm and a Mohs hardness of 6 along with a N₂ gas pressurized to 1,47 bar for 5 minutes to obtain a test piece. The thus obtained test piece was impregnated with a thermosetting resin (manufactured by Hitachi Kasei K.K. under the trade name of HITANOL) for 3 minutes (Ex. 16-1) and 5 minutes (Ex. 16-2) in a vacuum vessel of 1,33 x 10⁻⁵ bar. After impregnation, the test piece was washed on the surface with a solvent, dried at 25°C, and was thermally set at 140°C for 30 minutes in the atmosphere. Tests were conducted to measure the corrosion resistance of the test pieces and the adhesion strength of the thin films after the corrosion resistance test. Measurement was also made of the magnetic properties of the test pieces after and before the corrosion resistance test. The results of testing and measurement are set forth in Table 8. For the purpose of comparison, provision was made of a test piece (Comparison Example 16-1) prepared under the same conditions as in the present invention, except that any resin impregnation was not carried out, and an as-cut test pieces (Comparison Example 16-2) as mentioned in the foregoing. A further comparison test piece (Comparison Example 16-3) was obtained by degreasing the test piece of (Comparison Example 16-2) with a solvent trichloroethylene for 3 minutes and 5 % NaOH (alkali-decreasing) at 60°/C for 3 minutes, washing the thus degreased piece with 2 % HCl (acid-washing) at room temperature for 10 seconds, and electroplating the thus washed piece with nickel at a current density of 4 A/dm² and a bath temperature of 60°C for 20 minutes in a Watt bath to give a nickel-Plated coating layer having a thickness of 20 micrometers. Tests were conducted to measure the corrosion resistance of the test pieces and the adhesion strength of the thin films after the corrosion resistance test. Measurement was also made for the magnetic properties of the test pieces before and after the corrosion resistance test. The results are also set forth in Table 8. Estimation of corrosion resistance testing was made in terms of the appearance and adhesion strength of the test pieces allowed to stand in an atmosphere of a temperature of 70°C and a humidity of 90 % for 1000 hours as well as the magnetic properties of the test pieces before and after the corrosion resistance test. Estimation of adhesion strength testing was made in terms of visual appreciation of the rupture section of each of the test pieces of Example 16-1 and 2 and Comparison Examples 16-1 and 3 after the corrosion resistance test. Example 17 A magnet sample was obtained by repeating the procedures of Example 16, followed by shot peening. That sample was dipped into a 2 % arosin No. 1200 (trade name, manufactured by Nippon Paint K.K.) solution maintained at 30°C to deposite a golden chromate coating film onto the surface of the Al thin film layer treated by shot peening to thereby obtain a test piece. The thus obtained test piece was impregnated with a thermosetting resin (manufactured by Hitachi Kasei K.K. under the trade name of HITANOL) for 3 minutes (Ex. 17-1) and 5 minutes (Ex. 17-2) in a vacuum vessel of 1,33 x 10⁻⁵ bar. After impregnation, the test piece was washed on the surface with a solvent, dried at 25°C, and was thermally set at 140°C for 30 minutes in the atmosphere. Tests were conducted to measure the corrosion resitance of the test pieces and the adhesion strength of the thin films after the corrosion resistance test. Measurement was also made of the magnetic properties of the test pieces before and after the corrosion resistance test. The results of testing and measurement are set forth in Table 9. For the purpose of comparison, provision was made of a test piece (Comparison Example 17-1) prepared under the same conditions as in the present invention, except that any resin impregnation was not carried out, and an as-cut test piece (Comparison Example 17-2) as mentioned in the foregoing. A further comparison test piece (Comparison Example 17-3) was obtained by degreasing the test piece of (Comparison Example 17-2) with a solvent trichloroethylene for 3 minutes and 5 % NaOH (alkali-degreasing) at 60°C for 3 minutes, washing the thus degreased piece with 2 % HCl (acid-washing) at room temperature for 10 seconds, and electroplating the thus washed piece with nickel at a current density of 4 A/cm² and a bath temperature of 60°C for 20 minutes in a Watt bath to give a nickel-plated coating layer having a thickness of 20 µm. Tests were conducted to measure the corrosion resistance of the test pieces and the adhesion strength of the thin films after the corrosion resitance test. Measurement was also made of the magnetic properties of the test pieces before and after the corrosion resistance test. The results are set forth in Table 9. Estimation of corrosion resistance testing was made in terms of the appearance and adhesion strength of the test pieces allowed to stand in an atmosphere of a temperature of 80°C and a humidity of 90 % for 1000 hours as well as the magnetic properties of the test pieces before and after the corrosion resistance test. Estimation of adhesion strength testing was made in terms of visual appreciation of the rupture section of each of the test pieces of Examples 17-1 and 2 and Comparison Examples 17-1 and 3 after the corrosion resistance test. Example 18 Electrolytic iron of 99.9 % purity, a ferroboron alloy consisting of 19.4 % B and the balance being Fe and impurities such as Al, Si and C and Nd of 99.7 % or higher purity used as the starting materials were formulated together, molten by high-frequency induction, and were thereafter cast in a water-cooled copper casting mold to obtain a cast ingot having a composition of 16.0Nd7.0B77.0Fe (by atomic %). Thereafter, the ingot was coarsely pulverized in a stamp mill, and was then finely pulverized in a ball mill into fine powder having a particle size of 2.8 micrometers. The powders were placed in a mold, oriented in a magnetic field of 1194.0 k·A/m, and were compacted at a pressure of 1177 bar in the direction normal to the magnetic field. The thus obtained green compact was sintered at 1100°C for 1 hour in an Ar atmosphere, was thereafter cooled off, and was further aged at 600°C for 2 hours to prepare a permanent magnet. Seven test pieces of 20 mm in outer diameter, 10 mm in inner diameter and 1.5 mm in thickness, were cut out of the thus obtained permanent magnet. The test pieces were each placed in a vacuum vessel having a degree of vacuum of 1.33 X 10⁻⁸ bar, and reversely sputtered at a voltage of 400 V for 1 minute in an Ar gas of 1.07 bar. Thereafter, the test piece was heated to 350°C for 30 minutes, and was cooled down to 300°C as the pre-treatments. Further, a coating material formed of an Al piece of 99.99 % or higher purity and 10 mm diameter x 10 mm was exposed to electron beams of 0.6 A and 8 kV for 30 minutes for heating and evaporation, whereby an aluminium thin film was deposited onto the test piece. The aluminium thin film formed on the surface of the permanent magnet was found to have a thickness of 10 micrometers. The test piece with the deposited Al thin film was heat-treated for 1.5 hours under the conditions specified in Table 10. Testing was conducted to measure the corrosion resistance of the test piece and the adhesion strength of the Al thin film after the corrosion resistance test. A magnetic flux drop (%) of the test piece was also measured after the corrosion resistance test. The results of testing and measurement are set forth in Table 10. In the case of Sample Nos. 18-4 and 18-5, the respective tests and measurements were carried out after aging had been again at 600°C for 2 hours following the heat treatment. For the purpose of comparison, the same tests were carried out with a test piece (Comparison Example 18-1) prepared under the same conditions as mentioned above, except that no heat treatment was effected, and an as-cut test piece (Comparison Example 18-2). The results of testing and measurement are also set forth in Table 10. Estimation of corrosion resistance testing was made in terms of the appearance of the test pieces allowed to stand in atmosphere of a temperature of 80°C and a humidity of 90 % for 175 hours. Estimation of adhesion strength testing was made in terms of whether or not the thin film layer was peeled off, when the test piece provided thereon with 1 mm pitch cells were pulled up by an adhesive tape after the corrosion resistance test (i.e., unpeeled cells/all the cells). The thickness of the diffusion layer was measured with an X-ray microanalyzer. As can clearly be understood from the results of Table 10, the permanent magnets of the present embodiment are positively prevented from oxidation, suffer no deterioration of the magnetic proeperties, and are more considerably improved in terms of the magnetic properties, as compared with the comparison examples, since the corrosion-resistant vapor-deposited coating layer of the present invention includes a diffusion layer obtained by the heat treatment. It should be noted that the Fe-B-R base sintered permanent magnet per se is disclosed in the European Publications of Applications as mentioned hereinbefore thus not disclosed herein in detail. The disclosure in those European publications should be referred to if further information is necessary with respect to the detailed description subject to the prevailing nature of the disclosure of the present application.

Claims: A sintered permanent magnet body of 10-30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2-28 at % B and at least 42 at % Fe and wherein at least 50 vol % of the entire magnet material body consists of a Fe-B-R type tetragonal crystal structure; with an anticorrosive coating film layer consisting of a ceramic material on the body. A permanent magnet as defined in claim 1, in which said anticorrosive film is made up of at least one selected from the group consisting of oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. A permanent magnet as defined in claim 1 or 2, in which said anticorrosive film is formed of at least one selected from the group consisting of oxides of Si, Al, Cr, Ti, Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof and mixtures thereof. A process for producing a permanent magnet, by providing a permanent magnet material body of 10-30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2-28 at % B and at least 42 at % Fe and wherein at least 50 vol % of the entire magnet material body consists of a Fe-B-R type tetragonal crystal structure, and forming an anticorrosive coating film layer consisting of a ceramic material on the permanent magnet body by means of vapor deposition to improve the corrosion resistance thereof. A process as defined in claim 4, in which said anticorrosive thin film is formed of at least one selected from the group consisting of oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. A process as defined in claims 4 or 5 in which said anticorrosive thin film is formed of at least one selected from the group consisting of Oxides of Si, Al, Cr, Ti and Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof, and mixtures thereof. A sintered permanent magnet body of 10-30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2-28 at % B and at least 42 at % Fe and wherein at least 50 vol % of the entire sintered magnet material body consists of an Fe-B-R type tetragonal crystal structure obtainable by blasting or reverse sputtering the body and by vapor depositing an anticorrosive coating film layer of low gas permeability on the said treated body. A permanent magnet as defined in Claim 7 in which said anticorrosive film is made up of at least one selected from the group consisting of metals, oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. A permanent magnet as defined in one of claims 7 or 8 in which said anticorrosive film is formed of at least one selected from the group consisting of Al, Zn, Ni, Cr, Cu, Co, Ti, Ta, Si, Ag, Au, Pt, Rh and alloys thereof, oxides of Si, Al, Cr, Ti and Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof and mixtures thereof. A permanent magnet as defined in one of the claims 7 to 9 in which the anticorrosive coating film layer is formed of a metal having a melting point not higher than the sintering temperature of the permanent magnet material body. A permanent magnet as defined in one of the claims 1 to 3 or 7 to 10 in which an interdiffusion layer is provided between the magnet material body and the anticorrosive coating film layer. A permanent magnet as defined in one of the claims 1 to 3 or 7 to 11, in which the anticorrosive coating film layer is impregnated with a resin. A permanent magnet as defined in one of the claims 1 to 3 or 7 to 12, wherein 50 at % or more of R is Nd and/or Pr. A permanent magnet as defined in one of the claims 1 to 3 or 7 to 13, in which said permanent magnet material of the body further comprises at least one of the following additional elements not exceeding the values specified below: provided that, when two or more of said additional elements are contained, the total amount thereof does not exceed the highest value of the atomic percent of the element of said additional elements actually added. A process for producing a permanent magnet by providing a sintered permanent magnet material body of 10-30 at % R wherein R is at least one element selected from the group consisting of Nd, Pr, Dy, Ho and Tb, or a mixture of said at least one element and at least one selected from the group consisting of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, Pm and Y, 2-28 at % B and at least 42 at % Fe and wherein at least 50 vol % of the entire magnet material body consists of a Fe-B-R type tetragonal crystal structure; blasting or reverse sputtering the sintered magnet body and forming an anticorrosive coating film layer of low gas permeability by means of vapor deposition on the treated magnet surface. A process as defined in claim 15 in which said anticorrosive film is formed of at least one selected from the group consisting of metals, oxides, nitrides, carbides, borides, silicides, composite compositions thereof, and mixtures thereof. A process as defined in one of claims 15 or 16, in which said anticorrosive film is formed of at least one selected from the group consisting of Al, Zn, Ni, Cr, Cu, Co, Ti, Ta, Si, Ag, Au, Pt, Rh and alloys thereof, oxides of Si, Al, Cr, Ti and Ta, nitrides of Si, Ti, Ta and Al, carbides of Si, Ti and W, boron nitride, composite compositions thereof and mixtures thereof. A process as defined in one of claims 15 to 17, in which said blasting involves blasting hard particles having a mean particle size of 20-350 micrometers and a Mohs hardness of at least 5. A process as defined in claim 18, in which said blasting is effected by blasting said particles together with a pressurized gas of 0.98 - 5.88 bar. A process as defined in one of claims 15 to 19, in which the anticorrosive coating film layer is formed of a metal having a melting point not higher than the sintering temperature of the permanent magnet material body. A process as defined in one of claims 15 to 20 in which the anticorrosive coating film layer is formed of metal and the surface of said coating film layer is subjected to passivation. A process as defined in one of claims 15 to 21 in which a heat treatment is effected at a temperature ranging from 250°C to the melting point of the metal employed. A process as defined in one of claims 15 to 21 in which a heat treatment is effected at a temperature ranging from 250°C to the aging temperature of the permanent magnet material body. A process as defined in one of claims 4 to 6 or 15 to 23, in which said vapor deposition is effected by means of vacuum deposition, physical vapor deposition or chemical vapor deposition. A process as defined in claim 24, in which said physical vapor deposition is effected by means of ion plating. A process as defined in one of claims 4 to 6 or 15 to 25, in which shot peening is applied after said anticorrosive thin film has been formed on the surface of said permanent magnet material body. A process as defined in claim 26 in which said shot peening involves blasting spherical particles having a mean particle size of 30-3000 micrometers and a Mohs hardness of at least 3. A process as defined in one of the claims 26 or 27, in which said shot peening is effected by blasting said particles together with a pressurized gas of 0.98 - 4.90 bar. A process as defined in one of the claims 26 to 28, in which said shot-peened surface of said permanent magnet material body is further treated with chromating. A process as defined in one of claims 4 to 6 or 15 to 29, in which an interdiffusion layer is provided between the magnet material body and the anticorrosive coating film layer by heat treating the resultant mass. A process as defined in one of claims 4 to 6 or 15 to 30, which further comprises impregnating the anticorrosive coating film layer with a resin. A process as defined in claim 31, in which said resin is a heat resistant resin. A process as defined in one of claims 4 to 6 or 15 to 32, wherein 50 at % or more of R is Nd and/or Pr. A process as defined in claim 33, in which said permanent magnet material body comprises 12-24 at % R wherein at least 50 at % of R is Nd and/or Pr, 4-24 at % B and at least 52 at % Fe. A process as defined in claim 34, in which Fe is present in an amount of at least 65 at %. A process as defined in one of the claims 4 to 6 or 15 to 34, in which Co is substituted for Fe up to 45 at % of the magnet material body provided that the resultant Fe is at least 27 at %. A process as defined in claim 36, in which Co is substituted for Fe up to 35 at % of the magnet material body. A process as defined in claim 37, in which Co is substituted for Fe up to 25 at % of the magnet material body. A process as defined in Claim 38, in which Co is substituted for Fe up to 20 at % of the magnet material body. A process as defined in one of claims 4 to 6 or 15 to 39, in which said permanent magnet material of the body further comprises at least one of the following additional elements not exceeding the values specified below: provided that, when two or more of said additional elements are contained, the total amount thereof does not exceed the highest value of the atomic percent of the element of said additional elements actually added. A process as defined in claim 40, in which said permanent magnet material of the body contains at least one of the following additional elements not exceeding the values specified below: provided that, when two or more of said additional elements are contained, the total amount thereof does not exceed the highest value of the atomic percent of said additional elements actually added.

Inventors: HAMADA, TAKAKI, HAYAKAWA, TETSUJI, MATSUURA, YUTAKA
Applicants: SUMITOMO SPECIAL METALS CO., LTD.
Application year: 1985
Main classification: H01F1
IPC codes: H01F41, H01F1