JP2987705B2 - Rare earth permanent magnet with excellent oxidation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application fields> RT, B and B magnets represented by Nd, Fe and B have higher magnet properties than Sm-Co permanent magnets that have been widely used. . Therefore its use is expanding. However, the present magnet contains an Nd-Fe solid solution phase that is extremely active in the atmosphere. Therefore, when the present magnet is incorporated in a magnetic circuit or the like and used, there is a serious drawback in that not only deterioration of the characteristics and variations due to oxidation of the magnet are caused, but also the scattered oxide causes contamination of peripheral parts. As a document relating to the improvement of the corrosion resistance, JP-A-60-54406
JP (JPA), JP-A-60-63903 (JPA)
And the like. In these documents, an object is to form an oxidation-resistant film such as a plating chemical conversion film on the surface of a magnet body to improve the corrosion resistance. However, these oxidation resistant films use a large amount of water and an aqueous solution in the process, and thus carbonize during the treatment process.
Therefore, after the film is formed, blistering or peeling of the film occurs due to oxidation proceeding from the inside, so that it is not suitable for improving corrosion resistance. As a measure not to use water, epoxy-based, acrylic-based oxidation-resistant resin coating, or soft plating to improve the corrosion resistance by forming a metal film such as Al, Ni by sputtering, vapor deposition, ion plating, etc. There is a way. However, even in these methods, when the surface of the magnet body comes into contact with the atmosphere due to minute chips, etc., during oxidation or deterioration of the coating due to long-term use, during use, or during product inspection and incorporation into equipment, oxidation proceeds from this part. Therefore, it is not suitable as a measure for improving corrosion resistance.

[Problems to be Solved by the Invention] As described above, in any of the conventional methods for improving corrosion resistance, the oxidation of the sintered body proceeds during the process or from a portion in contact with the atmosphere. It was extremely difficult to impart the original corrosion resistance of the oxidation resistant film.

Further, as a measure for improving the corrosion resistance of the present magnet sintered body itself, there are many reports on the improvement of corrosion resistance by additional elements such as Cr, Co, Ti, and Cu (Japanese Patent Application Laid-Open No. 60-165350, JP-A-60-132754, JP-A-60-162755, JP-A-61-217549, JP-A-60-106108, JP-A-62-136552) Corrosion resistance by these added elements Can be improved, but the effect is not obtained unless a considerably large amount is added, and the magnet properties are linearly degraded with the added amount, which is not suitable as a countermeasure.

Therefore, a technical problem of the present invention is to provide an Nd ₂ Fe ₁₄ B phase constituting a Nd.Fe.B sintered body as a main phase, and to use an Nd-rich phase and an Nd-rich phase with respect to a B-rich phase in an alloy magnet containing the B-rich phase. By limiting the amount, the deterioration of the magnet characteristics is suppressed,
In addition, it is an object of the present invention to provide an R / T / B-based sintered body magnet having excellent corrosion resistance, and to provide the present magnet with the original corrosion resistance of plating, chemical conversion coating, resin coating, and the like.

[Means for Solving the Problems] The present inventor has obtained the following findings mainly as a result of conducting various studies on the corrosion behavior of this Nd.Fe.B sintered body from an electrochemical standpoint. .

1) The corrosion behavior of the sintered body of Nd.Fe.B is governed by electrochemical corrosion due to the formation of an extreme cell in Nd ₂ Fe ₁₄ B and an Nd-rich phase. That is, Nd ₂ Fe ₁₄ B phase and Nd
Corrosion progresses remarkably because the potential difference of the rich phase is large.

2) Since there is no local battery formation only with the Nd ₂ Fe ₁₄ B phase, the original corrosion resistance of the plating or the like can be imparted even by ordinary plating or the like.

3) The potential difference between Nd ₂ Fe ₁₄ B and the B-rich phase is about 0.02 V and Nd ₂
Since it is significantly smaller than about 0.5 V between the Fe ₁₄ B phase and the Nd rich phase, the presence of the B rich phase can reduce the corrosion rate of the sintered body.

4) The B-rich phase also tends to passivate.

(It is considered that this passivation has an effect of suppressing corrosion.) Based on the above results, the present inventors have conducted various studies and found that the volume ratio of the Nd-rich phase present in the sintered body was B It has been found that by setting the content to 70 vol% or less of the rich phase, a sintered body having excellent corrosion resistance and high magnetic properties can be obtained.

That is, according to the present invention, by setting the volume ratio of the Nd-rich phase to 70 vol% or less with respect to the B-rich phase, a local battery formed of the Nd ₂ Fe ₁₄ B phase and the Nd-rich phase is formed. The corrosion rate of the sintered body can be decreased by increasing the amount of the B-rich phase having an extremely small potential difference from the Nd ₂ Fe ₁₄ B phase, and the B-rich phase is not affected. By activating, the corrosion rate is further reduced, so that a rare earth permanent magnet with significantly improved corrosion resistance and excellent oxidation resistance can be obtained.

Therefore, in order to further improve the corrosion resistance, even if a conventional oxidation-resistant coating such as metal plating of Ni, Cu, Cr or the like using a normal aqueous solution, or a chemical conversion treatment such as a phosphate treatment is applied, the process is not performed. Since the sintered body is not oxidized, the original corrosion resistance of these corrosion-resistant coatings can be imparted, which is extremely useful industrially.

Here, in the present invention, the volume ratio of the R-rich phase to the B-rich phase in the sintered body structure is set to 70 vol% or less,
If there is more Nd-rich phase, Nd-rich phase and Nd ₂ F
Since the number of local batteries formed by the e ₁₄ B phase increases and corrosion progresses remarkably, the content must be 70 vol% or less. In order to further improve the corrosion resistance and obtain high magnet properties, the volume ratio of the Nd-rich phase to the B-rich phase is preferably set to 20 to 40 vol%.

Further, in the present invention, the reason why the composition is set to R: 12 to 16 at% is that when R is 12 at% or less, an Fe phase which remarkably deteriorates the magnetic properties appears, so that it is necessary to be 12 at% or more.
Further, in the region where R exceeds 16 at%, the amount of the R-rich phase is too large, corrosion due to local cell formation in the Nd ₂ Fe ₁₄ B phase and the Nd-rich phase is remarkable, and the corrosion resistance is deteriorated. There is a need.

The reason why the B value is 7.5 to 17 at% is that the B value is smaller than 7.5.
With the value, the desired amount of the B-rich phase is too small and the amount of the Nd-rich phase increases, so that the corrosion resistance is significantly deteriorated. Plus 17
In a region exceeding at%, the sinterability is significantly deteriorated and high magnet properties cannot be obtained.
It must be ~ 17at%. This B value is 7.5-17at%,
In the present invention, an R value of 12 to 16 at% corresponds to a volume ratio of the Nd-rich phase to the B-rich phase of 70 vol% or less.

Further, in order to obtain a sintered body excellent in corrosion resistance and magnet properties, it is preferable that R: 12 to 14 at% and B: 8 to 13 at%.

In the present invention, the reason that the volume ratio of the Nd-rich phase to the B-rich phase is set to 70 vol% or less is that the amount of the Nd-rich phase is too large in the region exceeding 70 vol%, and the Nd-rich phase and Nd ₂ Fe ₁₄ B
Corrosion due to the formation of local cells in the phase significantly degrades the corrosion resistance, so it must be 70 vol% or less.

When the volume is 70 vol% or less, corrosion resistance is improved and high magnet properties can be obtained. However, in order to obtain corrosion resistance and magnet properties, the volume ratio of the Nd-rich phase to the B-rich phase is 20 to 40. % Is preferable.

 Hereinafter, examples will be described.

Although only Nd.Fe.B has been described above, a rare earth element R including Y, a transition metal T and B, and an R.T.
It is easily presumed that the same can be expected for the B-based alloy.

<Example 1> Nd _11.8 Fe _82.4 B _5.8 , Nd ₁₂ Fe ₈₂ B ₆ , Nd ₁₂ Fe by high-frequency heating in an Ar atmosphere using Nd, Fe, B having a purity of 95% or more.
Four types of ingots each having a composition of ₇₆ B ₁₂ and Nd ₁₂ Fe ₇₂ B ₁₆ (at%) were obtained. The coarse powders obtained by coarsely pulverizing these ingots were used as (I) -1 to 4 materials, respectively. Next, using the same Nd, Fe, B as described above, the composition of Nd ₃₀ Fe ₆₁ B ₉ (at%) Liquid quenched flakes having an ambient temperature of 30 m / se in an Ar atmosphere
c, Obtained by the Cu roll single roll method. The obtained flake was roughly pulverized to obtain a (II) material.

Next, (I) -1 to 4 materials and (II) material are mixed in appropriate amounts,
Nd _11.5 Fe _81.5 B ₇ , Nd _12.5 Fe _80.5 B ₇ , Nd ₁₃ Fe ₈₀ B ₇ , Nd ₁₄ Fe
_78.5 B _7.5 , Nd ₁₂ Fe _80.5 B _7.5 , Nd _12.5 Fe _79.7 B _7.8 , Nd ₁₃ Fe
₇₉ B ₈ , Nd _13.5 Fe _77.5 B ₉ , Nd ₁₄ Fe ₇₆ B ₁₀ , Nd ₁₄ Fe _74.5 B _11.5 , N
d ₁₄ Fe ₇₃ B ₁₃ , Nd _14.5 Fe _73.5 B ₁₂ , Nd ₁₅ Fe ₇₂ B ₁₃ , Nd _15.5 Fe
_70.5 B _14, the _{Nd 16 Fe 69 B 15, Nd} 16 Fe 67 B 17, Nd 16.5 Fe 66 17 type mixed powder having a composition of B _17.5 respectively obtained (hereinafter the respective compositions Sample No.1~17 Call). These mixed coarse powder samples Nos. 1 to 17 were finely pulverized to 4 to 5 μm by a ball mill.
The obtained fine powder was molded in a magnetic field of 20 KOe at 1.0 ton / cm ² to obtain a molded body. These compacts were sintered at 1000-1150 ° C.

The obtained sintered body was subjected to Ni plating in a wand bath with Cu underplating. And these test piece samples No. 1 ~ 17
Was subjected to a constant temperature and humidity test of 60 ° C. × 95%. Table 1 shows the magnet properties and the results of the temperature and humidity test of these test pieces.
From Table 1, the range of Nd 12 to 16 at% and B 7.5 to 17 at% (sample No.
4 to 16), it can be seen that a rare earth permanent magnet excellent in magnet properties and corrosion resistance can be obtained.

Further, Nd 12 to 14 at%, B 8 to 13 at% sample Nos. 7 to 13
It can be seen that the sample has even more excellent corrosion resistance and magnet properties.

<Example-2> For the samples Nos. 1 to 17 obtained in Example-1, the existence ratio of the Nd-rich phase and the B-rich phase in each metallographic structure was determined using an image analysis processing device, and the ratio to the B-rich phase was determined.
The volume ratio of the Nd-rich phase was calculated.

Table 2 shows the volume ratio of the Nd-rich phase to the B-rich phase, the results of corrosion resistance obtained in Example 1, and the maximum energy.

From Table 2, when the volume ratio of the Nd-rich phase to the B-rich phase is 70 vol% or less (Sample Nos. 1 and 5 to 13), a sintered body excellent in corrosion resistance can be obtained. Samples No. 7 and No. 11 between 40 vol% showed particularly remarkable improvement, and the magnet properties were also extremely excellent.

<Effect of the Invention> As described above, the Nd ₂ Fe ₁₄ B phase is the main phase.
In the Nd-Fe-B based sintered magnet, the amount of the rich phase present in the structure is 20 to 70 vol% with respect to the amount of the B-rich phase.
Nd: 12-16at%, B: 7.5-17at%
(Preferably, Nd: 12 to 14 at%, B8 to 13 at%) makes it possible to obtain a rare-earth permanent magnet having more excellent corrosion resistance and more excellent magnet properties than conventional Nd.Fe.B. This is to minimize the formation of local cells of Nd-rich phase and Nd ₂ Fe ₁₄ B phase, which deteriorated the corrosion resistance of conventional Nd-Fe-B, and to reduce the corrosion resistance by increasing the B-rich phase as much as possible. It seems to be a synergistic effect of improvement.

Claims (1)

(57) [Claims] 1. An R ₂ T ₁₄ B comprising 12 to 16 at% of R, 7.5 to 17 at% of B, and the balance substantially T (where R is a rare earth element including Y, and T is a transition metal). In an R-T-B based alloy magnet having an intermetallic compound as a main phase, the alloy magnet structure contains an R-rich phase and a B-rich phase, and a volume ratio of the R-rich phase to the B-rich phase. Is a rare-earth permanent magnet excellent in oxidation resistance, characterized in that the content is in the range of 20 to 70%.