Iron-based sintered sliding member and production method therefor

Abstract

An iron-based sintered sliding member consists of, by mass %, 0.1 to 10% of Cu, 0.2 to 2.0% of C, 0.03 to 0.9% of Mn, 0.52 to 6.54% of S, and the balance of Fe and inevitable impurities. The iron-based sintered sliding member satisfies the following First Formula in which [S %] represents mass % of S and [Mn %] represents mass % of Mn in the overall composition. The iron-based sintered sliding member exhibits a metallic structure in which pores and sulfide particles are dispersed in the matrix that includes a martensite structure at not less than 50% by area ratio in cross section. The sulfide particles are dispersed at 3 to 30 vol. % with respect to the matrix.
[S %]=0.6[Mn %]+0.5 to 6.0First Formula

Claims

1. An iron-based sintered sliding member comprising a composition consisting of: 0.1 to 10 mass % of Cu, 0.2 to 2.0 mass % of C, 0.03 to 0.9 mass % of Mn, 0.52 to 6.54 mass % of S, optionally at least one of not more than 10 mass % of Ni and not more than 10 mass % of Mo, and a balance of Fe and inevitable impurities, wherein: the iron-based sintered sliding member satisfies a First Formula:
[S %]=0.6[Mn %]+0.5 to 6.0 where [S %] represents a mass % of S and [Mn %] represents a mass % of Mn based on a total mass % of the composition, the iron-based sintered sliding member exhibits a metallic structure in which pores and sulfide particles are dispersed in a matrix that contains a martensite structure at not less than 60% by area ratio in a cross section, the sulfide particles being dispersed at 3 to 30 vol. % with respect to the matrix; and a majority of the sulfide particles in volume percent is iron sulfide that precipitated in the matrix.

2. The iron-based sintered sliding member according to claim 1, wherein 60% or more of the sulfide particles have maximum particle sizes of 10 m or more.

3. The iron-based sintered sliding member according to claim 1, wherein the iron-based sintered sliding member is usable in a sliding condition in which not less than 20 MPa of surface pressure is applied.

4. The iron-based sintered sliding member according to claim 1, wherein [S %]0.06[Mn %]=1.0 to 6.0.

5. The iron-based sintered sliding member according to claim 1, wherein S is added in a form of at least one metallic sulfide powder selected from the group consisting of an iron sulfide powder and a copper sulfide powder.

6. A production method for the iron-based sintered sliding member according to claim 1, the method comprising: preparing an iron powder, a copper powder, a graphite powder, and at least one sulfide powder of an iron sulfide powder and a copper sulfide powder, the iron powder consisting of 0.03 to 1.0 mass % of Mn and the balance of Fe and inevitable impurities; forming a raw powder by mixing the copper powder, the graphite powder, and the sulfide powder with the iron powder so as to consist of, by mass %, 0.1 to 10% of Cu, 0.2 to 2.0% of C, 0.03 to 0.9% of Mn, 0.52 to 6.54% of S, and the balance of Fe and inevitable impurities; compacting the raw powder into a green compact with a predetermined shape; sintering the green compact at a temperature of 1000 to 1200 C.; and quench hardening and tempering the sintered compact.

7. The production method for the iron-based sintered sliding member according to claim 6, wherein at least one of a nickel sulfide powder and a molybdenum disulfide powder is added to the raw powder in addition to the sulfide powder, or instead of a part of an amount or the entirety of the sulfide powder, so that the raw powder further includes at least one of not more than 10 mass % of Ni and not more than 10 mass % of Mo.

8. The production method for the iron-based sintered sliding member according to claim 6, wherein Mo is added in the form of an iron alloy powder by adding Mo to the iron powder, or Mo is added to the raw powder in the form of a molybdenum powder in addition to or instead of the iron alloy powder, so that the raw powder further includes not more than 10 mass % of Mo.

9. The production method for the iron-based sintered sliding member according to claim 6, wherein Ni is added in the form of an iron alloy powder by adding Ni to the iron powder, or Ni is added to the raw powder in the form of a nickel powder in addition to or instead of the iron alloy powder, so that the raw powder further includes not more than 10 mass % of Ni.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a photograph example of a metallic structure of an iron-based sintered sliding member of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

(2) The iron-based sintered sliding member and the production method therefor of the present invention will be described in detail hereinafter. It should be noted that the following preferred embodiments are examples, and the present invention is not limited thereto.

(1) First Embodiment

(3) An iron powder, a copper powder, a graphite powder, and at least one kind of an iron sulfide powder and a copper sulfide powder, are prepared. The iron powder consists of 0.03 to 1.0 mass % of Mn and the balance of Fe and inevitable impurities. The copper powder, the graphite powder, at least one kind of the iron sulfide powder and the copper sulfide powder at the amount for satisfying the First Formula, are mixed with the iron powder, whereby a raw powder is formed. In this case, the amount of each powder is adjusted so that the raw powder consists of, by mass %, 0.1 to 10% of Cu, 0.2 to 2.0% of C, 0.03 to 0.9% of Mn, 0.52 to 6.54% of S, and the balance of Fe and inevitable impurities. The raw powder is compacted into a green compact with a predetermined shape. The green compact is sintered at a temperature in the range of 1000 to 1200 C. Next, the sintered compact is quench hardened by heating to a temperature of 820 to 1000 C. in a nonoxidizing atmosphere and then by rapidly cooling in oil or water. Then, the sintered compact is tempered by heating to a temperature in the range of 150 to 280 C. and cooling to room temperature. Thus, an iron-based sintered sliding member having the above-described composition and having a metallic structure in which pores and sulfide particles are dispersed in a matrix is obtained. The matrix includes a martensite structure at not less than 50% by area ratio in a cross section. The sulfide particles are dispersed at 3 to 30 vol. % with respect to the matrix.

(4) In this iron-based sintered sliding member, not less than 50% of the matrix is made of martensite, and a large amount of iron sulfides, and small amounts of manganese sulfides and copper sulfides, are dispersed in the matrix. Therefore, the iron-based sintered sliding member has superior sliding characteristics. In particular, when the amount of Cu is not less than 3.5 mass % in the raw powder, a greater amount of the copper phase is dispersed in the matrix, whereby the wear characteristics with respect to a mating member are further decreased. Since liquid-phase sintering occurs, and diffusion among the raw powder particles is smoothly performed, the strength of the iron-based matrix is improved, and the wear resistance of the iron-based matrix is improved. The solid lubricant is uniformly dispersed in the matrix in addition to the pores and the powder grain boundaries and is firmly fixed to the matrix. Accordingly, the sliding characteristics and the strength of the matrix are improved, and the wear resistance is improved.

(2) Second Embodiment

(5) In order to improve strength of the matrix, at least one metallic sulfide powder of a nickel sulfide powder and a molybdenum disulfide powder is added to the raw powder so as to satisfy the First Formula in the First Embodiment. The nickel sulfide powder and the molybdenum disulfide powder are used instead of the entirety or a part of the amount of the iron sulfide powder and the copper sulfide powder. The amount of each of the nickel sulfide powder and the molybdenum disulfide powder is set so that each of Ni and Mo is not more than 10 mass % in the overall composition of the raw powder. Then, compacting, sintering, and a heat treatment (quench hardening and tempering) are performed as in the case of the First Embodiment, whereby an iron-based sintered sliding member is produced.

(6) In this case, the iron-based sintered sliding member has an overall composition in which at least one alloy element of Ni and Mo is added at not more than 10 mass % to the overall composition in the First Embodiment. The nickel sulfide powder and the molybdenum disulfide powder are decomposed in the sintering and generate Ni and Mo, respectively. These alloy elements are solid solved in the iron-based matrix, whereby the strength of the iron-based matrix is improved. Part of the amounts of Ni and Mo form sulfides. Therefore, the iron-based sintered sliding member has a metallic structure in which a large amount of iron sulfides and small amounts of manganese sulfides, copper sulfides, sulfides of at least one of Ni and Mo, are dispersed in a matrix that contains martensite at not less than 50%.

(3) Third Embodiment

(7) In order to improve strength of the matrix, Mo is further added to the raw powder in the First Embodiment. Mo is used in the form of an iron alloy powder by adding Mo to the iron powder. Alternatively, Mo may be used in the form of a molybdenum powder in addition to, or instead of the iron alloy powder. The amount of the powder containing Mo is adjusted so that Mo is not more than 10 mass % in the overall composition of the raw powder. Then, compacting, sintering, and a heat treatment (quench hardening and tempering) are performed as in the case of the First Embodiment. Thus, an iron-based sintered sliding member in which Mo is further added at not more than 10 mass % in the overall composition in the First Embodiment is obtained. This iron-based sintered sliding member has a metallic structure similar to that in the Second Embodiment. In the metallic structure, a large amount of iron sulfides, and small amounts of metallic sulfides such as manganese sulfides, copper sulfides, molybdenum sulfides, and the like, are dispersed. Since Mo is added, the strength of the iron-based matrix is improved, and the area ratio of the martensite structure is increased because the martensite structure is more easily obtained, compared with the case of the First Embodiment.

(4) Fourth Embodiment

(8) Whereas Mo is used in the Third Embodiment, Ni may be used in order to improve strength of the matrix. Ni is used in the form of an iron alloy powder by adding Ni to the iron powder. Alternatively, Ni may be used in the form of a nickel powder in addition to, or instead of the iron alloy powder. The amount of the powder containing Ni is adjusted so that Ni is not more than 10 mass % in the overall composition of the raw powder. Then, compacting, sintering, and heat treatment (quench hardening and tempering) are performed as in the case of the First Embodiment. Thus, an iron-based sintered sliding member in which Ni is further added at not more than 10 mass % in the overall composition in the First Embodiment is obtained. The iron-based sintered sliding member has a metallic structure in which a large amount of iron sulfides, and small amounts of metallic sulfides such as manganese sulfides, copper sulfides, nickel sulfides, and the like, are dispersed. Since Ni is added, the strength of the iron-based matrix is improved, and the area ratio of the martensite structure is increased because the martensite structure is more easily obtained, compared to the case of the First Embodiment.

EXAMPLES

(9) The iron-based sintered sliding member of the present invention will be described in further detail by way of examples hereinafter.

First Example

(10) An iron powder containing 0.3 mass % of Mn, an iron sulfide powder containing 36.48 mass % of S, a copper powder, and a graphite powder, were prepared. The iron sulfide powder in the amounts shown in Table 1, 1.5 mass % of the copper powder, and 1.0 mass % of the graphite powder, were mixed with the iron powder, whereby a raw powder was obtained. The raw powder was compacted at a compacting pressure of 600 MPa, and a larger green compact and a smaller green compact were formed. The larger green compact had a ring shape with an outer diameter of 25.6 mm, an inner diameter of 20 mm, and a height of 15 mm. The smaller green compact had a ring shape with an outer diameter of 18 mm, an inner diameter of 10 mm, and a height of 10 mm. Next, these green compacts were sintered at 1150 C. in a nonoxidizing gas atmosphere, and they were maintained at 850 C. in a carburizing gas atmosphere and were oil quenched. Then, these sintered compacts were tempered at 180 C., whereby samples of sintered members of samples Nos. 01 to 14 were formed. The overall compositions of these samples are also shown in Table 1. In addition, the value of 0.6[Mn %] which represents the amount of S combining with Mn, and the value of [S %]0.6[Mn %] which represents the amount of S combining with elements other than Mn, are also shown in Table 1. In this case, the symbol [Mn %] represents the amount of Mn, and the symbol [S %] represents the amount of S, in the overall composition, respectively.

(11) In each of these samples, a cross sectional structure was observed, and an area of sulfides was measured by using image analyzing software (WinROOF produced by Mitani Corporation). Moreover, an area of sulfides with maximum particle sizes of not less than 10 m was measured, and the ratio of this area to the area of the overall sulfides was calculated. Similarly, a ratio of an area of martensite was measured. These results are shown in Table 2.

(12) The sintered member formed of the larger green compact was subjected to a sliding test by using a ring on disc frictional wear testing machine, and a frictional coefficient was measured. In the sliding test, a heat treated steel of SCM435H specified by Japanese Industrial Standards (JIS) was used as a mating material. The sliding test was performed at a circumferential speed of 400 rpm by applying a load of 20 MPa. As lubricating oil, an engine oil equivalent to 10W-30 was used. This result is also shown in Table 2.

(13) Moreover, the sintered member formed of the smaller green compact was tested by using an AUTOGRAPH manufactured by Shimadzu Corporation, and compressive strength was measured. This result is also shown in Table 2.

(14) TABLE-US-00001 TABLE 1 Mixing ratio mass % Iron Sintering Overall composition [S %] Iron sulfide Copper Graphite temperature mass % [Mn %] [Mn %] No. powder powder powder powder C. Fe Cu C Mn S 0.6 0.6 01 Bal. 0.75 1.5 1.0 1150 Bal. 1.5 1.0 0.29 0.27 0.2 0.1 02 Bal. 1.02 1.5 1.0 1150 Bal. 1.5 1.0 0.29 0.37 0.2 0.2 03 Bal. 1.30 1.5 1.0 1150 Bal. 1.5 1.0 0.29 0.47 0.2 0.3 04 Bal. 1.57 1.5 1.0 1150 Bal. 1.5 1.0 0.29 0.57 0.2 0.4 05 Bal. 1.84 1.5 1.0 1150 Bal. 1.5 1.0 0.29 0.67 0.2 0.5 06 Bal. 3.21 1.5 1.0 1150 Bal. 1.5 1.0 0.28 1.17 0.2 1.0 07 Bal. 4.57 1.5 1.0 1150 Bal. 1.5 1.0 0.28 1.67 0.2 1.5 08 Bal. 5.94 1.5 1.0 1150 Bal. 1.5 1.0 0.27 2.16 0.2 2.0 09 Bal. 8.66 1.5 1.0 1150 Bal. 1.5 1.0 0.27 3.16 0.2 3.0 10 Bal. 11.39 1.5 1.0 1150 Bal. 1.5 1.0 0.26 4.15 0.2 4.0 11 Bal. 14.12 1.5 1.0 1150 Bal. 1.5 1.0 0.25 5.15 0.2 5.0 12 Bal. 16.85 1.5 1.0 1150 Bal. 1.5 1.0 0.24 6.15 0.1 6.0 13 Bal. 22.30 1.5 1.0 1150 Bal. 1.5 1.0 0.23 8.14 0.1 8.0

(15) TABLE-US-00002 TABLE 2 Amount Ratio of sulfides Amount of of with sizes martensite Compressive sulfides of not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 01 1.2 0 93 0.26 720 02 1 2 92 0.24 720 03 2 10 90 0.22 710 04 2 40 90 0.20 710 05 3 60 90 0.14 700 06 5 67 88 0.12 690 07 7 72 84 0.11 600 08 10 85 80 0.10 550 09 15 95 75 0.10 480 10 20 97 70 0.10 450 11 26 99 60 0.12 440 12 30 99 50 0.14 420 13 40 99 40 0.20 320

(16) As shown in Tables 1 and 2, according to the increase of the amount of the iron sulfide powder, the amount of S in the overall composition was increased, and a larger amount of sulfides was precipitated. The sulfides with maximum particle sizes of not less than 10 m hardly existed when the amount of S was small in the overall composition. According to the increase of the amount of S, the ratio of the sulfides with maximum particle sizes of not less than 10 m was increased, and most of the sulfides had maximum particle sizes of not less than 10 m. In addition, according to the increase of the amount of S, the area ratio of the martensite structure was decreased.

(17) In the samples of the samples Nos. 01 to 04, since the amounts of the iron sulfide powder were small, the amounts of S were small. In these samples, the values of [S %]0.6[Mn %] were less than 0.5, and the amounts of the sulfides were less than 3% by area ratio, whereby the frictional coefficients were large. On the other hand, in the samples of the samples Nos. 05 to 12, the values of [S %]0.6[Mn %] were 0.5 or higher, whereby the amounts of the sulfides were not less than 3% by area ratio, and the ratios of the sulfides with maximum particle sizes of not less than 10 m were not less than 60%. In addition, the ratios of the martensite structure were not less than 50%. In these samples, sulfides with sizes that are large enough to function as a solid lubricant were sufficiently precipitated. Therefore, the frictional coefficients were decreased by not less than 40% with respect to that of the sample of the sample No. 01. The frictional coefficient was decreased with the increase of the precipitated amount of the sulfides.

(18) In the sample of the sample No. 13, the amount of the sulfides was greater than 30%, and the sulfides were coarsened. This was because according to the increase of the amount of S, the sulfides were excessively precipitated, and plural sulfide particles grew at the same site and became a composite particle. Since the sulfides were large, distances between the sulfide particles were extended, and area of the matrix without the sulfides was increased. Therefore, the frictional coefficient was slightly increased. According to the increase of the amount of the iron sulfide powder, generation of the liquid phase was increased. As a result, in the sample of the sample No. 13 in which the value of [S %]0.6[Mn %] was greater than 6.0, the liquid phase was excessively generated, whereby the sample lost its shape.

(19) On the other hand, the compressive strength was decreased with the increase of the amount of the iron sulfide powder. According to the increase of the amount of the iron sulfide powder, since S is a ferrite-stabilizing element, the sample became difficult to be austenitized even by the heat treatment at the same temperature, whereby the ratio of martensite was decreased. In addition, the metallic sulfides were increased at the same time, which also decreased the ratio of martensite. Therefore, the compressive strength was decreased. In particular, in the sample of the sample No. 13, the liquid phase was substantially generated, whereby the compressive strength was greatly decreased.

(20) Accordingly, it is necessary to set the value of [S %]0.6[Mn %] to be 0.5 to 6.0. Thus, the sizes and the amount of the sulfides dispersed in the matrix are appropriately adjusted. In this case, the sulfide particles are obtained at 3 to 30% by area ratio, and the sulfides with maximum particle sizes of not less than 10 m are obtained at a ratio of not less than 60%. In addition, the martensite phase is obtained at not less than 50% by area ratio. By controlling the sizes and the amount of the sulfides as described above, an iron-based sintered sliding member having superior sliding characteristics, high wear resistance, and high compressive strength, is obtained.

(21) A metallic structure of the iron-based sintered sliding member of the sample No. 09 is shown in FIG. 1 as an example. As shown in FIG. 1, the matrix is made primarily of a martensite structure, and a small amount of pearlite (black structure in FIG. 1) is observed. The martensite matrix increases the hardness of the material and increases the compressive strength. On the other hand, sulfide particles (gray portions in FIG. 1) are dispersed in the matrix, and they are precipitated from the matrix. Since the sulfide particles had predetermined sizes as shown in FIG. 1, they functioned as a solid lubricant and decreased the frictional coefficient. The metallic structure includes pores (black portions in FIG. 1) that have relatively round shapes due to generation of FeS liquid phase.

Second Example

(22) An iron powder containing Mn at the ratio shown in Table 3, an iron sulfide powder containing 36.48 mass % of S, a copper powder, and a graphite powder, were prepared. The iron sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0. Moreover, 1.5 mass % of the copper powder and 1.0 mass % of the graphite powder were also mixed with the iron powder, whereby a raw powder was obtained. Then, the raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 14 to 20 were formed. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 4. The value of 0.6[Mn %] and the value of [S %]0.6[Mn %] are also shown in Table 3. In addition, the values of the sample of the sample No. 08 in the First Example are also shown in Tables 3 and 4.

(23) TABLE-US-00003 TABLE 3 Mixing ratio mass % Iron powder Iron Sintering Overall composition [S %] Mn sulfide Copper Graphite temperature mass % [Mn %] [Mn %] No. mass % powder powder powder C. Fe Cu C Mn S 0.6 0.6 14 Bal. 0.03 5.53 1.5 1.0 1150 Bal. 1.5 1.0 0.03 2.02 0.0 2.0 15 Bal. 0.05 5.56 1.5 1.0 1150 Bal. 1.5 1.0 0.05 2.03 0.0 2.0 16 Bal. 0.10 5.63 1.5 1.0 1150 Bal. 1.5 1.0 0.09 2.06 0.1 2.0 17 Bal. 0.20 5.78 1.5 1.0 1150 Bal. 1.5 1.0 0.18 2.11 0.1 2.0 08 Bal. 0.30 5.94 1.5 1.0 1150 Bal. 1.5 1.0 0.27 2.16 0.2 2.0 18 Bal. 0.50 6.23 1.5 1.0 1150 Bal. 1.5 1.0 0.46 2.27 0.3 2.0 19 Bal. 0.96 6.91 1.5 1.0 1150 Bal. 1.5 1.0 0.87 2.52 0.5 2.0 20 Bal. 1.20 7.26 1.5 1.0 1150 Bal. 1.5 1.0 1.08 2.65 0.6 2.0

(24) TABLE-US-00004 TABLE 4 Amount Ratio of sulfides Amount of of with sizes martensite Compressive sulfides of not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 14 9 95 78 0.12 570 15 9 95 78 0.12 560 16 9 90 80 0.11 560 17 10 90 80 0.10 550 08 10 85 80 0.10 550 18 10 80 82 0.10 520 19 11 70 82 0.10 480 20 12 60 84 0.10 350

(25) As shown in Tables 3 and 4, when the amount of Mn in the iron powder was increased, and the amount of Mn in the overall composition was increased, the precipitated amount of the sulfides was increased. According to the increase of the amount of Mn, since fine manganese sulfides were increased, the ratio of the sulfides with maximum particle sizes of not less than 10 m was decreased, and the sulfides were decreased in sizes.

(26) Even when the amount of Mn was increased, until the amount of Mn in the overall composition was not more than 0.9%, a sintered member with low frictional coefficient was obtained by adjusting the amount of S so as to include an appropriate amount of the sulfides with predetermined sizes.

(27) When the amount of Mn was increased, the area ratio of the martensite phase was also increased. However, the compressive strength was decreased. This was because according to the increase of the amount of Mn, the area ratio of the sulfides was increased, and the bonding strength between the metallic particles was decreased. In addition, according to the increase of the amount of Mn in the iron powder, the iron powder was hardened, and the compressibility was decreased, whereby the compressive strength was decreased. In general, Mn is one of elements that improve the hardenability. Nevertheless, since the amount of S was supersaturated with respect to the amount of Mn, Mn for improving the hardenability was hardly obtained, whereby the effect of Mn for improving the strength was not obtained. Therefore, in the sample of the sample No. 20, the compressive strength was greatly decreased. Accordingly, the amount of Mn in the overall composition is set to be 0.03 to 0.9 mass %.

(28) The frictional coefficient and the compressive strength were superior when the amount of S was 0.67 to 6.15 mass % in the overall composition in the First Example. However, in view of the First Formula and the results of the Second Example, the amount of S can be set so as to be 0.52 to 6.54 mass %.

Third Example

(29) An iron powder containing 0.3 mass % of Mn, an iron sulfide powder containing 36.48 mass % of S, a copper powder, and a graphite powder, were prepared. The iron sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0. Moreover, the copper powder in the amounts shown in Table 5, and 1.0 mass % of the graphite powder, were also mixed with the iron powder, whereby a raw powder was obtained. Then, the raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 21 to 30 were formed. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 6. The value of 0.6[Mn %] and the value of [S %]0.6[Mn %] are also shown in Table 5. In addition, the values of the sample of the sample No. 08 in the First Example are also shown in Tables 5 and 6.

(30) TABLE-US-00005 TABLE 5 Mixing ratio mass % Iron Sintering Overall composition [S %] Iron sulfide Copper Graphite temperature mass % [Mn %] [Mn %] No. powder powder powder powder C. Fe Cu C Mn S 0.6 0.6 21 Bal. 5.94 0.0 1.0 1150 Bal. 0.0 1.0 0.28 2.17 0.2 2.0 22 Bal. 5.94 0.1 1.0 1150 Bal. 0.1 1.0 0.28 2.17 0.2 2.0 23 Bal. 5.94 0.5 1.0 1150 Bal. 0.5 1.0 0.28 2.17 0.2 2.0 24 Bal. 5.94 1.0 1.0 1150 Bal. 1.0 1.0 0.28 2.17 0.2 2.0 08 Bal. 5.94 1.5 1.0 1150 Bal. 1.5 1.0 0.27 2.16 0.2 2.0 25 Bal. 5.93 2.0 1.0 1150 Bal. 2.0 1.0 0.27 2.16 0.2 2.0 26 Bal. 5.93 2.5 1.0 1150 Bal. 2.5 1.0 0.27 2.16 0.2 2.0 27 Bal. 5.92 5.0 1.0 1150 Bal. 5.0 1.0 0.26 2.16 0.2 2.0 28 Bal. 5.91 7.5 1.0 1150 Bal. 7.5 1.0 0.26 2.15 0.2 2.0 29 Bal. 5.89 10.0 1.0 1150 Bal. 10.0 1.0 0.25 2.15 0.1 2.0 30 Bal. 5.87 15.0 1.0 1150 Bal. 15.0 1.0 0.23 2.14 0.1 2.0

(31) TABLE-US-00006 TABLE 6 Amount Ratio of sulfides Amount of of with sizes of martensite Compressive sulfides not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 21 11 85 70 0.16 390 22 11 85 72 0.14 460 23 11 85 74 0.12 500 24 10 85 78 0.12 520 08 10 85 80 0.10 550 25 10 85 80 0.12 560 26 10 85 80 0.12 560 27 10 85 78 0.10 560 28 9 85 76 0.10 510 29 9 85 75 0.10 470 30 8 85 60 0.10 390

(32) As shown in Tables 5 and 6, when the amount of Cu in the overall composition was increased by increasing the amount of the copper powder, the amount of the iron sulfide powder was decreased, whereby the amount of the sulfides was decreased. In addition, a part of the amount of Cu formed sulfides, but the specific weights of the copper sulfides are greater than those of the iron sulfides, whereby the area ratio of the sulfides was decreased. Nevertheless, when the amount of Cu was in the range described in the Third Example, sufficient amount of sulfides were obtained, and frictional coefficients were low.

(33) According to the increase of the amount of the copper powder, the compressive strength was greatly increased. In the sample of the sample No. 21, the frictional coefficient was 0.16 and was low, but the compressive strength was lower than 400 MPa. According to the observation of the metallic structure of this sample, a great amount of sulfides was precipitated at interfaces between powder particles, which caused a decrease in strength. On the other hand, in the samples of the samples Nos. 22 to 29, the sulfides were dispersed in the matrix. In these samples, since Cu tends to precipitate separately in the matrix, the iron sulfides were precipitated around Cu from the matrix, whereby bonding between the powder particles was strengthened. In addition, according to the increase of the copper powder, the area ratio of the martensite was increased because Cu improved the hardenability of the steel.

(34) When the amount of Cu was greater than 5 mass %, a free copper phase was increased, whereby the amount of the iron-based matrix containing the martensite structure was decreased, and the compressive strength was decreased. Therefore, when the present invention is applied to a sliding member that must have high strength, the amount of Cu is preferably set to be not more than 10 mass %.

Fourth Example

(35) An iron powder containing 0.3 mass % of Mn, an iron sulfide powder containing 36.47 mass % of S, a copper powder, and a graphite powder, were prepared. Then, 1.5 mass % of the copper powder, the graphite powder in the amounts shown in Table 7 were added to the iron powder. Moreover, the iron sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0, whereby a raw powder was obtained. The raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 31 to 41 were formed. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 8. The values of the sample of the sample No. 08 in the First Example are also shown in Tables 7 and 8.

(36) TABLE-US-00007 TABLE 7 Mixing ratio mass % Iron Sintering Overall composition [S %] Iron sulfide Copper Graphite temperature mass % [Mn %] [Mn %] No. powder powder powder powder C. Fe Cu C Mn S 0.6 0.6 31 Bal. 5.94 1.5 0.0 1150 Bal. 1.5 0.0 0.28 2.17 0.2 2.0 32 Bal. 5.94 1.5 0.2 1150 Bal. 1.5 0.2 0.28 2.17 0.2 2.0 33 Bal. 5.94 1.5 0.4 1150 Bal. 1.5 0.4 0.28 2.17 0.2 2.0 34 Bal. 5.94 1.5 0.6 1150 Bal. 1.5 0.6 0.28 2.17 0.2 2.0 35 Bal. 5.94 1.5 0.8 1150 Bal. 1.5 0.8 0.28 2.17 0.2 2.0 08 Bal. 5.94 1.5 1.0 1150 Bal. 1.5 1.0 0.27 2.16 0.2 2.0 36 Bal. 5.93 1.5 1.2 1150 Bal. 1.5 1.2 0.27 2.16 0.2 2.0 37 Bal. 5.93 1.5 1.4 1150 Bal. 1.5 1.4 0.27 2.16 0.2 2.0 38 Bal. 5.93 1.5 1.6 1150 Bal. 1.5 1.6 0.27 2.16 0.2 2.0 39 Bal. 5.93 1.5 1.8 1150 Bal. 1.5 1.8 0.27 2.16 0.2 2.0 40 Bal. 5.93 1.5 2.0 1150 Bal. 1.5 2.0 0.27 2.16 0.2 2.0 41 Bal. 5.93 1.5 2.2 1150 Bal. 1.5 2.2 0.27 2.16 0.2 2.0

(37) TABLE-US-00008 TABLE 8 Amount Ratio of sulfides Amount of of with sizes of martensite Compressive sulfides not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 31 11 88 0 0.22 350 32 11 88 60 0.18 480 33 10 86 70 0.16 500 34 10 86 75 0.14 530 35 10 85 80 0.12 540 08 10 85 80 0.10 550 36 10 85 78 0.12 560 37 10 85 75 0.12 520 38 10 85 70 0.14 500 39 10 85 65 0.16 450 40 9 84 62 0.18 400 41 9 84 60 0.22

(38) As shown in Tables 7 and 8, when the amount of C in the overall composition was increased by increasing the amount of the graphite powder, the amount of the iron sulfide powder was decreased. Therefore, the amount of the sulfides was slightly decreased, but the sizes of the sulfides were not greatly changed. In this case, the area ratio of the martensite was changed. In the sample of the sample No. 31, C was not contained, whereby the martensite structure was not obtained, and the hardness was decreased. As a result, the sample of the sample No. 31 was worn away, and the frictional coefficient was high. In addition, the compressive strength was low. On the other hand, when the amount of C was not less than 0.2 mass % in the overall composition, not less than 60% of the martensite structure was obtained, whereby the frictional coefficient was decreased, and the compressive strength was increased. By forming not less than 50% of the matrix so as to be made of the martensite structure, the wear resistance was improved, and the samples were not easily worn even under high surface pressure.

(39) On the other hand, when the amount of C was greater than 1.0 mass % in the overall composition, the area ratio of the martensite was decreased. In this regard, the frictional coefficient was increased, and the compressive strength was decreased, because cementite started to precipitate and residual austenite tended to be generated according to the increase in the amount of C. The cementite is hard and thereby wore the mating material, whereby the frictional coefficient was increased. When the amount of the graphite powder was greater than 2.0 mass %, a great amount of cementite was generated and decreased the melting point, whereby the liquid phase was excessively generated. As a result, the sample of the sample No. 41 lost its shape. Accordingly, the amount of C is set to be not less than 0.2 mass % and not more than 2 mass %.

Fifth Example

(40) An iron powder containing 0.3 mass % of Mn, an iron sulfide powder containing 36.47 mass % of S, a copper powder, and a graphite powder, were prepared. Then, 1.5 mass % of the copper powder and 1.0 mass % of the graphite powder were added to the iron powder. Moreover, the iron sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0, whereby a raw powder was obtained. The raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 42 to 46 were formed. In this case, the sintering was performed at the sintering temperature shown in Table 9. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 9. The values of the sample of the sample No. 08 in the First Example are also shown in Table 9.

(41) TABLE-US-00009 TABLE 9 Sintering [S %] Amount of Ratio of sulfides Amount of Compressive temperature [Mn %] [Mn %] sulfides with sizes of not martensite phase Frictional strength No. C. 0.6 0.6 area % less than 10 m % area % coefficient MPa 42 950 0.2 2.0 10 50 80 0.20 350 43 1000 0.2 2.0 10 65 80 0.18 420 44 1100 0.2 2.0 10 80 80 0.13 530 08 1150 0.2 2.0 10 85 80 0.10 550 45 1200 0.2 2.0 10 90 80 0.11 560 46 1250 0.2 2.0

(42) As shown in Table 9, the ratio of the sulfides with sizes of not less than 10 m was increased with the increase of the sintering temperature. In this regard, the frictional coefficient was correspondingly decreased. In this case, when the sintering temperature was less than 1000 C., sulfides with sufficient sizes were not obtained, whereby the frictional coefficient was high. In addition, sintering was not sufficiently performed, whereby sufficient compressive strength was not obtained. On the other hand, when the sintering temperature was 1250 C., the liquid phase was excessively generated, whereby the shape of the sample of the sample No. 46 could not be maintained and was damaged. Accordingly, the sintering temperature must be set at a temperature in the range of 1000 to 1200 C.

Sixth Example

(43) An iron powder containing 0.3 mass % of Mn, an iron sulfide powder containing 36.47 mass % of S, a copper powder, a graphite powder, and a nickel powder, were prepared. Then, 1.5 mass % of the copper powder, 1.0 mass % of the graphite powder, and the nickel powder in the amounts shown in Table 10, were added to the iron powder. Moreover, the iron sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0, whereby a raw powder was obtained. The raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 47 to 51 were formed. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 11. The values of the sample of the sample No. 08 in the First Example are also shown in Tables 10 and 11.

(44) TABLE-US-00010 TABLE 10 Mixing ratio mass % Iron Sintering Overall composition [S %] Iron sulfide Copper Nickel Graphite temperature mass % [Mn %] [Mn %] No. powder powder powder powder powder C. Fe Cu Ni C Mn S 0.6 0.6 08 Bal. 5.94 1.5 0.0 1.0 1150 Bal. 1.5 0.0 1.0 0.27 2.2 0.2 2.0 47 Bal. 5.93 1.5 1.0 1.0 1150 Bal. 1.5 1.0 1.0 0.27 2.2 0.2 2.0 48 Bal. 5.92 1.5 3.0 1.0 1150 Bal. 1.5 3.0 1.0 0.27 2.2 0.2 2.0 49 Bal. 5.91 1.5 5.0 1.0 1150 Bal. 1.5 5.0 1.0 0.26 2.2 0.2 2.0 50 Bal. 5.89 1.5 10.0 1.0 1150 Bal. 1.5 10.0 1.0 0.24 2.1 0.1 2.0 51 Bal. 5.86 1.5 15.0 1.0 1150 Bal. 1.5 15.0 1.0 0.23 2.1 0.1 2.0

(45) TABLE-US-00011 TABLE 11 Amount Ratio of sulfides Amount of of with sizes of martensite Compressive sulfides not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 08 10 85 80 0.10 550 47 10 85 84 0.12 560 48 10 85 86 0.11 580 49 10 85 82 0.10 580 50 9 85 75 0.12 540 51 9 85 70 0.22 490

(46) As shown in Tables 10 and 11, by increasing the amount of the nickel powder, the area ratio of the martensite was increased because Ni improved the hardenability. Therefore, the compressive strength was increased. On the other hand, when the amount of the nickel powder was greater than 3.0 mass %, a soft nickel-rich phase was increased, whereby the area ratio of the martensite was decreased, and the compressive strength was decreased. Moreover, when the amount of the nickel powder was greater than 10 mass %, a great amount of the soft nickel-rich phase was generated, whereby wear proceeded from the nickel-rich phase, and the frictional coefficient was increased. Accordingly, the amount of Ni is preferably not more than 10 mass % in the overall composition.

Seventh Example

(47) An iron powder containing 0.3 mass % of Mn, a molybdenum disulfide powder containing 40.06 mass % of S, a copper sulfide powder containing 33.54 mass % of S, a copper powder, and a graphite powder, were prepared. Then, one of the molybdenum disulfide powder and the copper sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0. Moreover, 1.5 mass % of the copper powder and 1.0 mass % of the graphite powder were also added to the iron powder, whereby a raw powder was obtained. The raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 52 and 53 were formed. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 13. The values of the sample of the sample No. 08 in the First Example are also shown in Tables 12 and 13.

(48) TABLE-US-00012 TABLE 12 Mixing ratio mass % Sintering Overall composition [S %] Iron Sulfide Copper Graphite temperature mass % [Mn %] [Mn %] No. powder powder powder powder C. Fe Cu Mo C Mn S 0.6 0.6 08 Bal. Iron sulfide 1.5 1.0 1150 Bal. 1.5 1.0 0.27 2.16 0.16 2.00 powder 5.94 52 Bal. Molybdenum 1.5 1.0 1150 Bal. 1.5 3.24 1.0 0.28 2.17 0.17 2.00 disulfide powder 5.41 53 Bal. Copper 1.5 1.0 1150 Bal. 5.8 1.0 0.27 2.16 0.16 2.00 sulfide powder 6.45

(49) TABLE-US-00013 TABLE 13 Amount Ratio of sulfides Amount of of with sizes of martensite Compressive sulfides not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 08 10 85 80 0.10 550 52 10 84 85 0.11 580 53 11 85 83 0.12 560

(50) As shown in Tables 12 and 13, even when the kind of the metallic sulfides was changed, the sizes and the amount of the sulfides were not greatly changed, and the frictional coefficient was approximately constant, as long as the amount of S was not greatly changed. Since the molybdenum disulfide powder and the copper sulfide powder include Mo and Cu, respectively, which improve the hardenability, the strength of the matrix was improved, and the compressive strength was high.

Eighth Example

(51) An iron alloy powder containing Mn at 0.3 mass % and Mo in the amounts shown in Table 14, an iron sulfide powder containing 36.47 mass % of S, a copper powder, and a graphite powder, were prepared. Then, 1.5 mass % of the copper powder and 1.0 mass % of the graphite powder were added to the iron alloy powder. Moreover, the iron sulfide powder was added to the iron powder so that the value of [S %]0.6[Mn %] would be 2.0, whereby a raw powder was obtained. The raw powder was compacted, sintered, and heat treated in the same manner as in the First Example, whereby samples of samples Nos. 54 to 58 were formed. As in the case of the First Example, the area of the sulfides, the ratio of the sulfides with maximum particle sizes of not less than 10 m to the total area of the sulfides, the area ratio of the martensite structure, the frictional coefficient, and the compressive strength, were measured. These results are shown in Table 15. The values of the sample of the sample No. 08 in the First Example are also shown in Tables 14 and 15.

(52) TABLE-US-00014 TABLE 14 Mixing ratio mass % Iron alloy Iron sulfide powder powder Sintering Overall composition [S %] Mo Mn S Copper Graphite temperature mass % [Mn %] [Mn %] No. mass % mass % mass % powder powder C. Fe Mo Cu C Mn S 0.6 0.6 08 Bal. 0.0 0.30 5.94 36.47 1.5 1.0 1150 Bal. 0.0 1.5 1.0 0.27 2.16 0.16 2.00 54 Bal. 1.0 0.30 5.94 36.47 1.5 1.0 1150 Bal. 0.9 1.5 1.0 0.27 2.16 0.16 2.00 55 Bal. 3.0 0.30 5.94 36.47 1.5 1.0 1150 Bal. 2.7 1.5 1.0 0.27 2.16 0.16 2.00 56 Bal. 5.0 0.30 5.94 36.47 1.5 1.0 1150 Bal. 4.6 1.5 1.0 0.27 2.16 0.16 2.00 57 Bal. 10.0 0.30 5.94 36.47 1.5 1.0 1150 Bal. 9.2 1.5 1.0 0.27 2.16 0.16 2.00 58 Bal. 15.0 0.30 5.94 36.47 1.5 1.0 1150 Bal. 13.7 1.5 1.0 0.27 2.16 0.16 2.00

(53) TABLE-US-00015 TABLE 15 Amount Ratio of sulfides Amount of of with sizes of martensite Compressive sulfides not less phase Frictional strength No. area % than 10 m % area % coefficient MPa 08 10 85 80 0.10 550 54 10 85 86 0.11 570 55 10 85 88 0.11 590 56 10 85 88 0.10 600 57 10 85 88 0.11 610 58 10 85 88 0.11 610

(54) As shown in Tables 14 and 15, according to the increase of the amount of Mo in the iron alloy powder, the area ratio of the martensite was increased because Mo improved the hardenability. Therefore, the compressive strength was also improved. According to the sample of the sample No. 58, the compressive strength was not further improved even by adding Mo at greater than 10 mass %. In addition, Mo is an expensive alloy element. Accordingly, the amount of Mo is preferably not more than 10 mass % in the overall composition.