GREEN COMPACT AND METHOD FOR PRODUCING SAME

Abstract

A green compact according to the present invention is a green compact, which is obtained by compaction-molding raw material powder containing metal powder as a main raw material, the green compact including an oxide film formed between particles of the raw material powder forming the green compact, the oxide film binding the particles of the raw material powder to each other, in which the metal powder to toe used includes metal powder showing a circularity R at a cumulative frequency of 80% of 0.75 or more, the circularity R being expressed by Equation (1), where S represents a two-dimensional projected area of the metal powder and L represents a two-dimensional projected circumferential length of the metal powder.

[00001]$\begin{array}{cc}R=4.Math.\pi \times \frac{S}{L^2}& \left(1\right)\end{array}$

Claims

1. A green compact, which is obtained by compaction-molding raw material powder containing metal powder as a main raw material, the green compact comprising an oxide film formed between particles of the raw material powder forming the green compact, the oxide film binding the particles of the raw material powder to each other, wherein the metal powder to be used comprises metal powder showing a circularity R at a cumulative frequency of 80% of 0.75 or more, the circularity R being expressed by Equation 1, where S represents a two-dimensional projected area of the metal powder and L represents a two-dimensional projected circumferential length of the metal powder. $\begin{array}{cc}R=4.Math.\pi \times \frac{S}{L^2}& \mathrm{Equation}.Math..Math.1\end{array}$

2. A green compact, which is obtained by compaction-molding raw material powder containing metal powder as a raw material, the green compact comprising an oxide film formed between particles of the raw material powder forming the green compact, the oxide film binding the particles of the raw material powder to each other, wherein the metal powder to be used comprises metal powder showing a jaggedness C at a cumulative frequency of 80% of less than 2.90, the jaggedness C being expressed by Equation 2, where S represents a two-dimensional projected area of the metal powder and L represents a two-dimensional projected circumferential length of the metal powder. $\begin{array}{cc}C=\frac{1}{4.Math.\pi }\times \frac{L^2}{S}& \mathrm{Equation}.Math..Math.2\end{array}$

3. The green compact according to claim 1, wherein the metal powder to be used comprises metal powder further showing a jaggedness C at a cumulative frequency of 80% of less than 2.90, the jaggedness C being expressed by Equation 3, where S represents the two-dimensional projected area of the metal powder and L represents the two-dimensional projected circumferential length of the metal powder. $\begin{array}{cc}C=\frac{1}{4.Math.\pi }\times \frac{L^2}{S}& \mathrm{Equation}.Math..Math.3\end{array}$

4. The green compact according to claim 2, wherein the metal powder to be used comprises metal powder further showing a circularity R at a cumulative frequency of 80% of 0.75 or more, the circularity R being expressed by Equation 4, where S represents the two-dimensional projected area of the metal powder and L represents the two-dimensional projected circumferential length of the metal powder. $\begin{array}{cc}R=4.Math.\pi \times \frac{S}{L^2}& \mathrm{Equation}.Math..Math.4\end{array}$

5. The green compact according to claim 1, wherein the green compact has a green density of 5.0 g/cm.sup.3 or more and 7.6 g/cm.sup.3 or less.

6. The green compact according to claim 1, wherein the metal powder comprises iron-based powder.

7. The green compact according to claim 1, wherein the oxide film is formed by subjecting a surface of the raw material powder to steam treatment.

8. A slide bearing, which is formed of the green compact of claim 1, the slide bearing comprising a bearing surface configured to slidably support a shaft.

9. The slide bearing according to claim 8, wherein internal pores of the green compact are impregnated with 12 vol % or more of a lubricating oil.

10. A method of producing a green compact obtained by compaction-molding raw material powder containing metal powder as a main raw material, the green compact comprising an oxide film formed between particles of the raw material powder forming the green compact, the oxide film binding the particles of the raw material powder to each other, the method comprising the steps of: molding the green compact using, as the metal powder, metal powder showing a circularity R at a cumulative frequency of 80% of 0.75 or more; and subjecting a surface of the raw material powder in a state of forming the green compact to steam treatment, to thereby form the oxide film between the particles of the raw material powder, the circularity R being expressed by Equation 5, where S represents a two-dimensional projected area of the metal powder and L represents a two-dimensional projected circumferential length of the metal powder. $\begin{array}{cc}R=4.Math.\pi \times \frac{S}{L^2}& \mathrm{Equation}.Math..Math.5\end{array}$

11. A method of producing a green compact obtained by compaction-molding raw material powder containing metal powder as a main raw material, the green compact comprising an oxide film formed between particles of the raw material powder forming the green compact, the oxide film binding the particles of the raw material powder to each other, the method comprising the steps of: molding the green compact using, as the metal powder, metal powder showing a jaggedness C at a cumulative frequency of 80% of less than 2.90; and subjecting a surface of the raw material powder in a state of forming the green compact to steam treatment, to thereby form the oxide film between the particles of the raw material powder, the jaggedness C being expressed by Equation 6, where S represents a two-dimensional projected area of the metal powder and L represents a two-dimensional projected circumferential length of the metal powder. $\begin{array}{cc}C=\frac{1}{4.Math.\pi }\times \frac{L^2}{S}& \mathrm{Equation}.Math..Math.6\end{array}$

12. The method of producing a green compact according to claim 10, wherein the steam treatment for the surface of the raw material powder is performed in a temperature range of 400° C. or more and 700° C. or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1A is an SEM image of pure iron powder according to Example 1 of the present invention produced by a water atomizing method.

[0030] FIG. 1B is an SEM image of pure iron powder according to Example 2 of the present invention produced by the water atomizing method.

[0031] FIG. 2A is an SEM image of pure iron powder according to Comparative Example 1 of the present invention produced by the water atomizing method.

[0032] FIG. 2B is an SEM image of pure iron powder according to Comparative Example 2 of the present invention produced by a reduction method.

[0033] FIG. 3A is an SEM image of pure iron powder according to Comparative Example 3 of the present invention produced by the reduction method.

[0034] FIG. 3B is an SEM image of pure iron powder according to Comparative Example 4 of the present invention produced by the reduction method.

[0035] FIG. 4 is a graph for showing the cumulative frequency distribution of the circularity R of the pure iron powder according to each of Example 1 and Comparative Example 4.

[0036] FIG. 5 is a graph for showing the cumulative frequency distribution of the jaggedness C of the pure iron powder according to each of Example 1 and Comparative Example 4.

DESCRIPTION OF EMBODIMENTS

[0037] Now, one embodiment of the present invention is described by way of specific Examples.

[0038] First, test pieces according to Examples 1 and 2, and Comparative Examples 1 to 4 were produced using, as base material metal powder serving as a main raw material for raw material powder, six kinds of pure iron powders having shapes different from each other. Here, in each of Examples 1 and 2, and Comparative Example 1, pure iron powder produced by a water atomizing method was used, and in each of Comparative Examples 2 to 4, pure iron powder produced by a reduction method was used. For each kind of powder, only powder having a sieved particle size of 250 μm or less was used.

(Production Procedure for Test Pieces)

[0039] Each kind of pure iron powder described above was blended and mixed with 0.7 wt % of a lubricant, in this case, an amide wax-based lubricant, and the mixture was loaded into a molding mold (alloy tool steel SKD 11) and subjected to uniaxial pressing at a predetermined molding pressure to provide a cylindrical green compact having a green density of 6.0±0.1 g/cm.sup.3. After that, the green compact was subjected to degreasing treatment at 350° C. for 90 minutes to remove a lubricant component in the green compact, and then subjected to steam treatment at 500° C. for 40 minutes. Thus, a cylindrical test piece was obtained. Dimensions in each case were set to inner diameter φ6 mm×outer diameter φ12 mm×axial-direction dimension 7 mm.

(Evaluation of Shapes of Various Pure Iron Powders)

[0040] Now, in order to evaluate differences in various characteristics resulting from a difference in shape of pure iron powder serving as base material metal powder, differences between the shapes of various pure iron powders were expressed in numerical values by the following method. That is, various pure iron powders having the shapes shown in FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B are each embedded in a resin, and then subjected to mirror polishing with sandpaper and buff to prepare a sample. At this stage, the polished surface of each sample is in a state in which a cross-section of each of the various pure iron powders is exposed.

[0041] Next, an image obtained by observing the polished surface of each sample with an optical microscope was subjected to binarization processing with predetermined image processing software (Mitani Corporation, WinROOF), and then the area and the circumferential length of the cross-section of each of the various pure iron powders (the area and the circumferential length in this case correspond to the two-dimensional projected area S and the two-dimensional projected circumferential length L of the metal powder, respectively) were measured for each particle to calculate the circularity R and the jaggedness C of each of the various pure iron powders for each particle. In this operation, measurement was performed for at least 4,000 particles of each kind of pure iron powder. When holes, such as pores, were present inside the cross-section of the pure iron powder, the area and the circumferential length of the cross-section were measured on the assumption that the holes were not present.

[0042] Then, on the basis of the area and the circumferential length obtained by performing measurement as described above (the two-dimensional projected area S and the two-dimensional projected circumferential length L), and Equation 1 and Equation 2, the circularity R and the jaggedness C of each of the various pure iron powders were calculated for each particle. Incidentally, as the circularity R gets closer to 1, the shape gets closer to a perfect circle (perfect sphere). In addition, as the jaggedness C gets closer to 1, the shape gets closer to a perfect circle (perfect sphere), or as the jaggedness C gets further away from 1, the contour shape is distorted, or the shape may be regarded as an elongated shape as a whole. As apparent from Equation 1 and Equation 2, the circularity R and the jaggedness C have a relationship of being the reciprocal of each other.

[0043] After the circularity R and the jaggedness C of each of the predetermined number (4,000 or more for each kind) of particles of pure iron powder had been thus calculated, the circularities R and the jaggednesses C were each arranged in ascending order to create a cumulative frequency distribution, and the circularity R and the jaggedness C at a cumulative frequency of 80%, at which differences among those shapes were considered to be most likely reflected in numerical values, were defined as the typical circularity R and jaggedness C of each of the various pure iron powders. As an example, the cumulative frequency distribution of the circularity R of each of Example 1 and Comparative Example 4 is shown in FIG. 4, and the cumulative frequency distribution of the jaggedness C thereof is shown in FIG. 5. In addition, the circularity R and the jaggedness C of each of Examples and Comparative Examples determined by the above-mentioned method are shown in Table 1.

TABLE-US-00001 TABLE 1 Production method Circularity R Jaggedness C Example 1 Water atomizing 0.8 2.4 method Example 2 Water atomizing 0.75 2.51 method Comparative Water atomizing 0.74 2.9 Example 1 method Comparative Reduction method 0.72 2.96 Example 2 Comparative Reduction method 0.72 3.41 Example 3 Comparative Reduction method 0.71 3.12 Example 4

Evaluation of Radial Crushing Strength

[0044] The strength of the resultant test piece was evaluated on the basis of the result of measurement of radial crushing strength performed in conformity to JIS Z 2507. A testing device used in this case is Autograph AG-5000A manufactured by Shimadzu Corporation. Herein, the “radial crushing strength” refers to the strength of a cylindrical green Compact determined on the basis of a radial crushing load by a certain method, and the “radial crushing load” refers to a load .at which the cylindrical green compact, starts to break when compressed between two planes each parallel to its axis.

[0045] In this test, judgment criteria for the radial crushing strength were defined as described below. That is, the radial crushing strength (unit: MPa) is classified into three levels, i.e., 100 or more and less than 130, 130 or more and less than 150, and 150 or more, and respective corresponding evaluations are represented by Symbols “Δ”, “∘”, and “⊚” in order starting from the lowest value.

(Evaluation of Oil-Impregnated Rate)

[0046] In addition, the oil-impregnated rate -of a test piece was evaluated on the basis of the result of measurement of an oil-impregnated rate performed in conformity to JIS Z 2501. The procedure and method therefore are as described below. First, the weight W1 (unit: g) of the test piece (green compact) before being impregnated with a lubricating oil (hydraulic action oil Shell Tellus S2 M 68, corresponding to ISO viscosity VG 68) is measured. Then, the test piece is immersed in the lubricating oil, and kept in a vacuumed state at 70° C. for 1 hour or more, and then the weight W2 (unit: g) of the test piece (green compact) after being impregnated with the lubricating oil is measured. After the weights of the test piece before and after impregnation had been thus measured, an oil-impregnated rate Oc (unit: vol %) was calculated on the basis of the following Equation 3. In Equation 3, V represents the volume of the green compact (unit: cm.sup.3), and ρ represents the density of the lubricating oil (unit: g/cm.sup.3).

[00004]$\mathrm{Oc}=\frac{W.Math..Math.2-W.Math..Math.1}{\rho \times V}\times 100$

[0047] In this test, judgment criteria for the oil-impregnated rate were defined as described below. That is, the oil-impregnated rate (unit: vol %) is classified into three levels, i.e., less than 12, 12 or more and less than 15, and 15 or more, and respective corresponding evaluations are represented by Symbols “x”, “∘” , and “⊚” in order starting from the lowest value.

[0048] Next, the evaluation results are described on the basis of Table 2. Herein, a test piece having a radial crushing strength of 130 MPa or more and an oil-impregnated rate of 12 vol % or more was comprehensively judged as “∘”, and a test piece that did not satisfy at least one of the above-mentioned conditions was comprehensively judged as “x”.

TABLE-US-00002 TABLE 2 Radial crushing Oil-impregnated Comprehensive strength [MPa] rate [vol %] judgment Example 1 ◯ ⊚ ◯ Example 2 ◯ ◯ ◯ Comparative Δ ◯ X Example 1 Comparative ⊚ X X Example 2 Comparative ⊚ X X Example 3 Comparative ⊚ X X Example 4

[0049] First, with regard to the radial crushing strength, as shown in Table 2, all the evaluated test pieces (Examples 1 and 2, and Comparative Examples 1 to 4) showed a value at a level of more than 100 MPa. Specifically, Comparative Example 1 showed only a value of less than 130 MPa, whereas Examples 1 and 2 each showed a value of 130 MPa or more.

[0050] In addition, with regard to the oil-impregnated rate, Example 1 showed a value of 15 vol % or more, and Example 2 and Comparative Example 1 each showed a value of 12 vol % or more, whereas Comparative Examples 2 to 4 each showed only a value or less than 12 vol %.

[0051] In summary of the foregoing, it has been revealed that, according to the green compact using the metal powder (in this embodiment, the pure iron powder) having the shape showing a circularity R at a cumulative frequency of 80% of 0.75 or more, and/or showing a jaggedness C at a cumulative frequency of 80% of less than 2.90, an oil-impregnated rate of 12 vol % or more can be achieved while a radial crushing strength of 130 MPa is secured.

[0052] While one embodiment of the present invention has been described above, needless to say, the green compact and the method of producing the same according to the present invention may adopt any modes within the scope of the present invention without being limited to the mode exemplified above.

[0053] For example, in the above-mentioned embodiment, the case of using the pure iron powder produced by the water atomizing method has been described as Examples. Needless to say, however, the production method is not limited thereto. That is, as described above, the green, compact, according to the present invention has a feature in shape of the metal powder serving as a material for the green compact, and hence is not limited by its production method. Admittedly, there may be such an aspect that its shape (in terms of circularity R or jaggedness C, the magnitude thereof) may be determined to some degree by the product ion method, but even pure iron powder produced by a product ion method other than that of Examples (e.g., a gas atomizing method) may be used as the metal powder according to the present invention as long as its shape satisfies the criteria according to the present invention (a circularity R of 0.75 or more, or a jaggedness of less than 2.90). Needless to say, the same applies to the case of using metal powder other than pure iron powder.

[0054] In addition, in the above-mentioned embodiment, the case of using the pure iron powder as the metal powder serving as the main raw material for the taw material powder has been described. Needless to say, however, iron-based powder other than the pure iron (including alloy powder) may also be used, and metal powder containing two or more kinds of metal powders (e.g., pure iron powder and copper powder) may also be used. In that case, at least one kind of the metal powders only needs to be metal powder serving as the constituent particles of the green compact, and the remaining metal powder may be, for example, metal powder (e.g., tin powder) functioning as a binder between the constituent particles by being melted during the heat treatment (e.g., steam treatment) for forming the oxide film after the compaction-molding. Needless to say, the size of each powder (particle size) may also be any size as long as the compaction-molding can be performed, and the size is not limited to that in the above-mentioned embodiment.

[0055] In addition, in the above-mentioned embodiment, the case of using an organic lubricant as raw material powder other than the main raw material has been described. Needless to say, however, a lubricant other than the organic lubricant may also be used. In addition, one kind or two or more kinds of various additives for imparting functions other than a lubricating function in the compaction-molding to the green compact may be blended with the main raw material.

[0056] In addition, in the above-mentioned embodiment, the case where the raw material powder having blended thereinto the amide wax-based lubricant is compaction-molded, subjected to the degreasing treatment, and then subjected to the steam treatment has been exemplified. Needless to say, however, when the lubricant remaining in a finished product does not cause a problem in terms of function, the steam treatment may be performed without the degreasing treatment.

[0057] In addition, in the above-mentioned embodiment, the case of using the uniaxial pressing as a compaction-molding technique for the green compact has been exemplified. Needless to say, however, other molding techniques may also be adopted. For example, various molding techniques, such as multiaxial pressing with a CNC press or the like, and injection molding (MIM), may be adopted as the molding technique for the green compact.

[0058] In addition, the green compact according to the foregoing description is suitably applicable not only to a slide bearing, for example, a cylindrical oil-impregnated bearing (e.g., a perfectly circular fluid bearing or a fluid dynamic bearing capable of rotatably support a shaft through the intermediation of an oil film of a lubricating oil), but also to other kinds of sliding parts utilizing the seeping of a lubricating oil. Needless to say, the green compact according to the present invention may be applied to a machine part other than the sliding parts.