Composition of particulate materials and process for obtaining self-lubricating sintered products

Abstract

The metallurgical composition comprises a main particulate metallic material, for example iron or nickel, and at least one alloy element for hardening the main metallic material, which form a structural matrix; a particulate solid lubricant, such as graphite, hexagonal boron nitride or mixture thereof; and a particulate alloy element which is capable of forming, during the sintering of the composition conformed by compaction or by injection molding, a liquid phase, agglomerating the solid lubricant in discrete particles. The composition may comprise an alloy component to stabilize the alpha-iron matrix phase, during the sintering, in order to prevent the graphite solid lubricant from being solubilized in the iron. The invention further refers to the process for obtaining a self-lubricating sintered product.

Claims

1. A process for obtaining self-lubricating sintered products from a metallurgical composition of particulate materials, the process comprising: mixing, in predetermined quantities, particulate materials which define a metallurgical composition; homogenizing the particulate material mixture; compacting the particulate material mixture, so as to provide the mixture with the shape of the product to be sintered; sintering the compacted and conformed mixture, at temperatures from about 1125 C. to about 1250 C., forming, during the sintering, a liquid phase with the particulate alloy element and thus promoting the agglomeration of the particulate solid lubricant in discrete particles dispersed in the volume of a structural matrix; wherein the metallurgical composition of particulate materials comprises, a main particulate metallic material, in the form of a main chemical element, wherein the main particulate metallic material is iron, and at least one particulate hardening element, which form the structural matrix in the composite product to be sintered wherein the particulate hardening element has the function of hardening the structural matrix; a non-metallic particulate solid lubricant, at least partially soluble in the structural matrix, wherein the particulate solid lubricant is a mixture consisting of graphite and hexagonal boron nitride; an alloy component, with the function of stabilizing an iron alpha phase and of preventing the solubilization of the particulate solid lubricant in the iron; and at least one particulate alloy element capable of forming, during a sintering of a conformed metallurgical composition, a liquid phase between the main particulate metallic material which forms the structural matrix and the particulate solid lubricant, agglomerating the latter in discrete particles; wherein the particulate solid lubricant represents a volumetric percentage lower than or equal to 15% of the volume of the composite material to be formed and the particulate hardening element, the particulate alloy element and the alloy component are silicon, at contents from 2% to 5% by weight of the metallurgical composition; and wherein the main particulate metallic material of iron presents an average particle size lying between about 10 m and about 90 m, the hardening element with the function of hardening the structural matrix, and the particulate alloy element with the function of forming the liquid phase and agglomerating the particulate solid lubricant, during the sintering of the metallurgical composition conformed by compaction, presenting an average particle size smaller than about 45 m.

2. The process, as set forth in claim 1, further comprising: stabilizing the iron alpha phase of the structural matrix, so as to prevent the dissolution of the portion of the solid lubricant defined in graphite, in the iron structural matrix.

3. The process, as set forth in claim 1, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises rolling the latter in the form of a plate or strip to be subsequently sintered.

4. The process, as set forth in claim 1, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises rolling the latter on at least one of the opposite faces of a structural substrate in the form of a plate or strip of particulate material compatible with the main particulate metallic material which forms the structural matrix.

5. The process, as set forth in claim 3, further comprising, after sintering the particulate materials, the additional step of cold rolling the plate or strip for reducing the residual porosity.

6. The process, as set forth in claim 1, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises the extrusion in one of the shapes defined by a bar and a tube.

7. The process, as set forth in claim 1, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises the co-extrusion of the latter in the form of a surface layer around a structural core in the form of a bar of particulate material compatible with the main particulate metallic material which forms the structural matrix, so as to form a composite bar.

8. The process, as set forth in claim 1, wherein the metallurgical composition comprises an organic binder to be thermally removed from the product, before the sintering step.

9. A process for obtaining self-lubricating sintered products from a metallurgical composition of particulate materials, the process comprising: mixing, in predetermined quantities, particulate materials which define the metallurgical composition; homogenizing the particulate material mixture, at a temperature not inferior to that of melting the organic binder; granulating the composition to facilitate its handling, storage and supply into an injection machine; injection molding the particulate material mixture, so as to provide the mixture with the shape of the product to be sintered; extracting the organic binder from the molded piece; and sintering the conformed mixture, at temperatures from about 1125 C. to about 1250 C., forming, during the sintering, a liquid phase with the particulate alloy element and thus promoting the agglomeration of the solid lubricant in discrete particles dispersed in the volume of a structural matrix, and stabilizing an iron alpha phase of the structural matrix, so as to prevent the dissolution of the portion of the solid lubricant, defined in graphite, in the structural matrix; wherein the metallurgical composition of particulate materials comprises, a main particulate metallic material, in the form of a main chemical element, wherein the main particulate metallic material is iron, and at least one particulate hardening element, which form the structural matrix in the composite product to be sintered wherein the particulate hardening element has the function of hardening the structural matrix; a non-metallic particulate solid lubricant, at least partially soluble in the structural matrix, wherein the particulate solid lubricant is a mixture consisting of graphite and hexagonal boron nitride; an alloy component, with the function of stabilizing the iron alpha phase and of preventing the solubilization of the particulate solid lubricant in the iron; and at least one particulate alloy element capable of forming, during a sintering of a conformed metallurgical composition, a liquid phase between the main particulate metallic material which forms the structural matrix and the particulate solid lubricant, agglomerating the latter in discrete particles; wherein the particulate solid lubricant represents a volumetric percentage lower than or equal to 15% of the volume of the composite material to be formed and the particulate hardening element, the particulate alloy element and the alloy component are silicon, at contents from 2% to 5% by weight of the metallurgical composition; and wherein the main particulate metallic material of iron presents an average particle size lying between about 10 m and about 90 m, the hardening element with the function of hardening the structural matrix, and the particulate alloy element with the function of forming the liquid phase and agglomerating the particulate solid lubricant, during the sintering of the metallurgical composition conformed by compaction, presenting an average particle size smaller than about 45 m.

10. A process for obtaining self-lubricating sintered products from a metallurgical composition of particulate materials, the process comprising: mixing, in predetermined quantities, particulate materials which define the metallurgical composition; homogenizing the particulate material mixture; compacting the particulate material mixture, so as to provide the mixture with the shape of the product to be sintered; and sintering the compacted and conformed mixture, at temperatures from about 1125 C. to about 1250 C., forming, during the sintering, a liquid phase with the particulate alloy element and thus promoting the agglomeration of the particulate solid lubricant in discrete particles dispersed in the volume of a structural matrix; wherein the metallurgical composition of particulate materials comprises, a main particulate metallic material, in the form of a main chemical element, wherein the main particulate metallic material is nickel, and at least one particulate hardening element, which form the structural matrix in the composite product to be sintered; a non-metallic particulate solid lubricant, wherein the particulate solid lubricant is selected from graphite, hexagonal boron nitride or from a mixture of both in any proportion; and at least one particulate alloy element capable of forming, during the sintering of the conformed metallurgical composition, a liquid phase between the main particulate metallic material which forms the structural matrix and the particulate solid lubricant, agglomerating the particulate solid lubricant in discrete particles, wherein the particulate hardening element, with the function of hardening the nickel matrix, and the particulate alloy element, with the function of forming the liquid phase and agglomerating the solid lubricant in discrete particles, are a mixture consisting of silicon, phosphorus and chrome, at contents from 2% to 8% by weight of the metallurgical composition.

11. The process, as set forth in claim 10, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises rolling the latter in the form of a plate or strip to be subsequently sintered.

12. The process, as set forth in claim 10, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises rolling the latter on at least one of the opposite faces of a structural substrate in the form of a plate or strip of particulate material compatible with the main particulate metallic material which forms the structural matrix.

13. The process, as set forth in claim 11, wherein the process further comprises, after sintering the particulate materials, the additional step of cold rolling the plate or strip for reducing the residual porosity.

14. The process, as set forth in claim 10, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises the extrusion in one of the shapes defined by a bar and a tube.

15. The process, as set forth in claim 10, wherein the step of compacting the particulate material mixture, which defines the metallurgical composition, comprises the co-extrusion of the latter in the form of a surface layer around a structural core in the form of a bar of particulate material compatible with the main particulate metallic material which forms the structural matrix, so as to form a composite bar.

16. The process, as set forth in claim 14, wherein the metallurgical composition comprises an organic binder to be thermally removed from the product, before the sintering step.

17. The process, as set forth in claim 16, further comprising the steps of: homogenizing the particulate material mixture, at a temperature not inferior to that of melting the organic binder; granulating the composition to facilitate its handling, storage and supply into an injection machine; injection molding the particulate material mixture, so as to provide the mixture with the shape of the product to be sintered; extracting the organic binder from the molded piece; and sintering the conformed mixture, at temperatures from about 1125 C. to about 1250 C., forming, during the sintering, a liquid phase with the particulate alloy element and thus promoting the agglomeration of the solid lubricant in discrete particles dispersed in the volume of the structural matrix.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described below, with reference to the enclosed drawings, given by way of example of embodiments of the invention and in which:

(2) FIG. 1A schematically represents a portion of the micro-structure of the prior art composition of particulate material, comprising a structural matrix and a solid lubricant containing hexagonal boron nitride and/or graphite, before submitted to the operations of mechanically homogenizing the mixture of the particulate materials and of conforming (densification) the piece, before sintering;

(3) FIG. 1B is similar to FIG. 1A, but illustrating the micro-structure of the same prior art composition of particulate material, after having been homogenized and conformed, with the formation of a solid lubricant layer between the particles of the structural matrix;

(4) FIG. 2A schematically represents a portion of the micro-structure of the composition or mixture of the particulate material of the metallic structural matrix, with the material of the particulate solid lubricant having a particle size similar (the same magnitude order) to that of the metallic structural matrix, favoring the degree of continuity of the latter;

(5) FIG. 2B schematically represents a micro-structure portion of the composition of the particulate material of the structural matrix, with the solid lubricant having particle size much smaller than that of the metallic structural matrix, whereby much finer particles of the solid lubricant tend to form a relatively continuous layer between the particles of the metallic structural matrix, even in the absence of shearing stresses during the processing steps previous to sintering;

(6) FIG. 3 schematically shows the solid lubricant in the form of discrete particles uniformly distributed, with a regular average free path therebetween, in a portion of the micro-structure of the composition of particulate material of the present invention;

(7) FIG. 4 represents a picture of the micro-structure of the self-lubricating sintered product whose structural matrix is a ferrous alloy, evidencing the graphite and hexagonal boron nitride particles and the provision of the liquid phase dispersed in the particulate material of the structural matrix during the sintering;

(8) FIG. 5 schematically represents in a simplified diagram, an example of compaction in the formation of a piece or product to be posteriorly sintered, said compaction being made so as to provide a self-lubricating layer in two opposite faces of the product to be sintered;

(9) FIGS. 6A, 6B and 6C represent examples of products whose conformation is obtained by compaction carried out by extrusion, respectively, of a bar in a self-lubricating composite material, of a tube in a self-lubricating composite material, and of a bar with a core in metallic alloy coated with an outer layer of self-lubricating material; and

(10) FIG. 7 schematically represents, in a simplified diagram, an example of compaction in the formation of a piece or product, to be posteriorly sintered, said compaction being made by rolling a self-lubricating composite material on the opposite faces of a plate or strip in metallic alloy.

DESCRIPTION OF THE INVENTION

(11) As already previously mentioned, one of the objects of the invention is to provide a metallurgical composition of particulate materials, which can be homogeneously mixed and conformed (densified) by compaction (pressing, rolling, extrusion) or by injection molding, so that it may assume a defined geometry (piece) to be submitted to a sintering operation, in order to obtain a product which presents high hardness, mechanical strength and reduced coefficient of friction in relation to the products obtained by the prior art teachings. The present metallurgical composition comprises a main particulate metallic material which is preponderant in the formation of the composition, and at least one particulate hardening alloy element, these components being responsible for the formation of a structural matrix 10 in the composite product to be sintered.

(12) According to the invention, the main particulate metallic material is usually iron or nickel, defining a ferrous structural matrix or a nickel-based structural matrix. In the composition which uses iron as the main particulate metallic material, the particulate hardening alloy element, with the function of hardening the matrix, is defined, for example, by at least one of the elements selected from chrome, molybdenum, carbon, silicon, manganese and nickel, but it should be understood that one can use other elements which carry out the same function in the structural matrix 10. It should be noted that the invention requires the provision of an alloy hardening element which may carry out the function of hardening the structural matrix 10 to be formed, but this aspect should not be limited to the exemplified alloy elements presented herein.

(13) Besides the components which form the structural matrix 10, the present composition comprises a non-metallic particulate solid lubricant 20 which is preferably, but not exclusively, defined by hexagonal boron nitride, graphite or also by a mixture of both in any proportion, said particulate solid lubricant 20 representing a volumetric percentage lower than or equal to about 15% the volume of the composite material to be formed, said percentage being much lower than the usual 25% to 40% of the prior art, relevantly contributing to a higher degree of continuity of the structural matrix 10 and, consequently, to a higher mechanical strength of the sintered product to be obtained.

(14) As already mentioned in the prior art discussion and as illustrated in FIGS. 1A, 1B, 2A and 2B, due to the low shearing stress of the non-metallic particulate solid lubricant used in the formation of the composition and, posteriorly, of the sintered composite product, during the step of mixing the particulate materials of the composition and the step of conforming the composition, by compaction or by injection molding, the stresses applied to the solid lubricant 20 cause the latter to spread between the particles which form the structural matrix 10 phase, tending to surround them in a film or layer 21, impairing the formation of the sintering necks between the particles which form the metallic structural matrix 10, in case the particulate solid lubricant 20 is insoluble in the material of the structural matrix 10, as it occurs with the hexagonal boron nitride in relation to a ferrous or nickel-based structural matrix 10.

(15) In order to avoid the deficiency mentioned above, the composition of the present invention further comprises at least one particulate alloy element which is capable of forming, at the sintering temperatures of the conformed metallurgical composition, a liquid phase between the particulate material which forms the structural matrix 10 and the particulate solid lubricant 20, forcing the latter to agglomerate in discrete particles that are homogeneously dispersed in the material of the structural matrix 10, as illustrated in FIG. 3. The formation of the liquid phase and its action on the particulate solid lubricant 20 allow obtaining a high degree of continuity of the structural matrix 10 in the sintered composite product to be obtained.

(16) When the metallurgical composition of the invention is conformed by compaction and uses a ferrous structural matrix, the main particulate metallic material of iron presents, preferably, an average particle size lying between about 10 m to about 90 m. On its turn, the hardening element, with the function of hardening the structural matrix 10, and the particulate alloy element, with the function of forming the liquid phase and agglomerating the particulate solid lubricant 20, during the sintering of the conformed metallurgical composition by compaction (densification), present an average particle size smaller than about 45 M. It should be understood that the average particle size of the main particulate metallic material of iron should preferably be larger than the average particle size of the hardening element and alloy element.

(17) The metallurgical composition with an iron-based structural matrix 10, described above and conformed by compaction or by injection molding, can be completed with the hardening element and with the alloy element when the particulate solid lubricant 20 is of the insoluble type in said ferrous structural matrix 10, for example the hexagonal boron nitride, since the particulate solid lubricant 20 does not react with the material which forms the structural matrix 10 at the sintering temperatures from about 1125 C. to about 1250 C. The reaction of the particulate solid lubricant 20 with the structural matrix 10 causes the former to partially or completely disappear in the material of the latter, impairing or even eliminating the self-lubricating characteristic of the sintered product to be obtained.

(18) However, in case the structural matrix 10 is, for example, iron-based and the particulate solid lubricant 20 is at least partially soluble in the structural matrix 10, at the sintering temperatures of the conformed metallurgical composition by compaction or by injection molding, as it occurs, for example, with the graphite or a mixture consisting of graphite and hexagonal boron nitride, the present metallurgical composition should further comprise at least one alloy component capable of stabilizing the iron alpha phase, during the sintering of the metallurgical composition, and thus preventing the occurrence of solubilization and incorporation of the particulate solid lubricant 20 in the iron of the structural matrix 10.

(19) According to the invention, the alloy component, which stabilizes the iron alpha phase, is defined by at least one of the elements selected from phosphorus, silicon, cobalt, chrome and molybdenum. Although these elements are considered the most adequate to separately or jointly act in stabilizing the iron alpha phase at the sintering temperatures (about 1125 C. to about 1250 C.), it should be understood that the invention resides in the concept of stabilizing the iron alpha phase and not in the fact that the alloy component(s) used are necessarily the ones exemplified herein.

(20) Preferably, for the composition having an iron-based structural matrix 10 with a particulate solid lubricant 20, at least partially soluble in the structural matrix 10 and which is constituted, for example, by graphite or by a mixture consisting of graphite and hexagonal boron nitride, the particulate hardening alloy element, with the function of hardening the structural matrix 10, the particulate alloy element, with the function of forming the liquid phase and agglomerating the particulate solid lubricant 20, and the alloy component, with the function of stabilizing the iron alpha phase, are defined, for example, by an element selected from silicon, at contents from about 2% to about 5% by weight of the metallurgical composition, and from a mixture of silicon, manganese and carbon, at contents from about 2% to about 8% by weight of the metallurgical composition.

(21) When the metallurgical composition of the invention is conformed by injection molding and uses a ferrous structural matrix, the main particulate metallic material of iron presents, preferably, a particle size lying between about 1 m to about 45 m. In the same way, the hardening element, with the function of hardening the structural matrix 10, the particulate alloy element, with the function of forming the liquid phase and agglomerating the particulate solid lubricant 20, during the sintering of the metallurgical composition conformed by injection molding and the particulate solid lubricant, present a particle size also from about 1 m to about 45 m.

(22) As already mentioned, the structural matrix 10 of the metallurgical composition may be nickel-based and, in this case, any of the particulate solid lubricants 20, exemplified herein as graphite, hexagonal boron nitride or a mixture thereof in any proportion, will have the characteristic of being insoluble in the nickel structural matrix 10, at the sintering temperatures of the metallurgical composition, from about 1125 C. to about 1250 C., dispensing the use of the particulate alloy component which stabilizes the iron alpha phase.

(23) In the metallurgical composition with a nickel structural matrix 10, the required particulate alloy element, with the function of forming the liquid phase and agglomerating the particulate solid lubricant 20, during the sintering of the metallurgical composition, is defined, for example, by at least one of the elements selected from chrome, phosphorus, silicon, iron, carbon, magnesium, cobalt and manganese.

(24) When the metallurgical composition of the invention uses a nickel-based structural matrix 10, the main particulate metallic material of nickel presents, upon the conformation by compaction, an average particle size preferably lying between about 10 m and about 90 m, the hardening element, with the function of hardening the structural matrix 10, and the particulate alloy element, with the function of forming the liquid phase and agglomerating the particulate solid lubricant 20, during the sintering of the metallurgical composition conformed by compaction (densification), presenting an average particle size equal to or smaller than about 45 m. When the conformation of the composition is made by injection molding, it is preferred that the main particulate metallic material of nickel and the hardening element, with the function of hardening the structural matrix 10, and also the particulate alloy element, with the function of forming the liquid phase, present a particle size preferably lying between about 1 m and about 45 m.

(25) Considering the metallurgical composition with a nickel-based structural matrix 10, the hardening element, with the function of hardening the nickel matrix, and the particulate alloy element, with the function of forming the liquid phase and agglomerating the particulate solid lubricant 20 in discrete particles, are defined by an element selected from silicon, phosphorus and chrome, at contents from about 2% to about 5%, or from a mixture consisting of silicon, phosphorus and chrome, at contents from about 2% to about 8% by weight of the metallurgical composition.

(26) When the conformation of the metallurgical composition, previous to the sintering, is carried out by extrusion or by injection molding, the composition should further comprise at least one organic binder selected preferably from the group consisting of paraffin and other waxes, EVA, and low melting point polymers in proportion generally ranging from about 15% to about 45% of the total volume of the metallurgical composition, upon the conformation by extrusion, and from about 40% to 45%, upon the conformation by injection molding. The organic binder is extracted from the composition after the conformation step, for example by evaporation, before the conformed product is conducted to the sintering step.

(27) The metallurgical compositions described above are obtained by mixing, in any adequate mixer(s), predetermined quantities of the particulate materials selected for the formation of the composition and for the subsequent obtention of a self-lubricating sintered product.

(28) The mixture of the different particulate materials is homogenized and submitted to a densification operation by compaction, that is, by pressing, rolling or extrusion, or also by injection molding, so that it can be conformed in a desired shape for the product to be obtained by sintering.

(29) In case of conformation by injection molding, the mixture of the components containing the organic binder is homogenized at temperatures not inferior to that of melting the organic binder, the thus homogenized mixture being granulated to facilitate its handling, storage and supply to an injection machine.

(30) After conformation of the composition, the conformed piece is submitted to a step of extracting the organic binder, generally by a thermal process.

(31) The homogenized and conformed metallurgical composition can be then submitted to a sintering step, at temperatures from about 1125 C. to about 1250 C. Considering that both the metallurgical compositions, with an iron-based or nickel-based structural matrix 10, comprise at least one particulate alloy element with the function of forming the liquid phase, it is formed, during the sintering, said liquid phase by the particulate alloy element, and promoted the agglomeration of the particulate solid lubricant 20 in discrete particles dispersed in the volume of the structural matrix 10.

(32) When the metallurgical composition comprises a particulate solid lubricant at least partially soluble in an iron-based structural matrix, as it occurs with the graphite and its mixture with the hexagonal boron nitride, the homogenized and conformed metallurgical composition further comprises at least one alloy component, already previously defined and which is capable of, during the step of sintering the metallurgical composition, stabilizing the iron alpha phase of the structural matrix 10, preventing the dissolution of the portion of the solid lubricant portion, defined by graphite, in the iron structural matrix.

(33) With the metallurgical composition proposed herein, it is possible to obtain self-lubricating sintered pieces or products, from particulate materials which do not require previous treatment for the non-metallic particulate solid lubricant, said pieces or products presenting: in case of using an iron structural matrix 10, a Hardness HV230, a coefficient of friction 0.15, a mechanical traction resistance .sub.t450 MPa and also a dispersion of discrete particles of solid lubricant 20 with average particle size between about 10 m and about 60 m for the products conformed by compaction and between about 2 m and about 20 m for the products conformed by injection molding; and, in case of a nickel-based structural matrix 10, a Hardness HV240, a coefficient of friction 0.20, a mechanical traction resistance .sub.t350 MPa and also a dispersion of discrete particles of solid lubricant 20 with average particle size between about 10 m and about 60 m for the products conformed by compaction, and between about 2 m and about 20 m for the products conformed by injection molding.

(34) FIGS. 5, 6A, 6B, 6C and 7 of the enclosed drawings have the purpose of exemplifying different possibilities of conforming the present metallurgical composition, by compacting a certain predetermined quantity of the metallurgical composition to any desired shape, which can be that of the self-lubricating sintered final piece or product desired to be obtained, or a shape close to that desired final one.

(35) However, in a large number of applications, the self-lubricating characteristic is necessary only in one or more surface regions of a mechanical component or piece, to be submitted to a friction contact with other relatively movable element.

(36) Thus, the desired self-lubricating product can be constituted, as illustrated in FIG. 5, by a structural substrate 30 preferably conformed in a particulate material and receiving, in one or two opposite faces 31, a surface layer 41 of the metallurgical composition 40 of the present invention. In the illustrated example, the structural substrate 30 and the two opposite surface layers of the metallurgical composition 40 are compacted in the interior of any adequate mold M, by two opposite punches P, forming a compacted and conformed composite product 1, which is posteriorly submitted to a sintering step. In this example, only the two opposite faces 31 of the structural substrate 30 will present the desirable self-lubricating properties.

(37) FIGS. 6A and 6B exemplify products in the form of a bar 2 and a tube 3, respectively, obtained by extrusion of the metallurgical composition 40 in an adequate extrusion matrix (not illustrated). In this case, the conformation by compaction of the metallurgical composition 40 is carried out in the extrusion step of the latter. The bar 2 or tube 3 can then be submitted to the sintering step, for the formation of the iron-based or nickel-based structural matrix 10 and incorporating dispersed discrete particles of the particulate solid lubricant 20, as schematically represented in FIGS. 3 and 4.

(38) FIG. 6C illustrates another example of product formed by a composite bar 4, comprising a structural core 35, in a particulate material and which is circumferentially and externally surrounded by a surface layer 41 formed from the metallurgical composition 40 of the invention. Likewise in this case, the conformation and the compaction (densification) of the structural core 35 and the outer surface layer 41 in the metallurgical composition 40 are obtained by co-extrusion of the two parts of the composite bar 4, which is then submitted to the sintering step.

(39) When the compaction of the metallurgical composition 20 is carried out by extrusion, as it occurs, for example, in the formation of the bars 2, 3 and 4 of FIGS. 6A, 6B and 6C, said composition can further comprise an organic binder which is thermally removed from the composition, after the conformation of the latter and before the sintering step, by any of the known techniques for said removal.

(40) The organic binder may be, for example, any one selected from the group consisting of paraffin and other waxes, EVA, and low melting point polymers.

(41) FIG. 7 represents, also schematically, another way to obtain a sintered composite product, presenting one or more surface regions having self-lubricating characteristics. In this example, the product 5 to be obtained presents a structural substrate 30 formed in a particulate material, previously conformed in the form of a strip, it being noted that, on at least one of the opposite faces of the structural substrate 30, in the form of a continuous strip, is rolled a surface layer 41 of the metallurgical composition 40 of the present invention. The composite product 5 is then submitted to a sintering step.

(42) While the invention has been presented herein by means of some examples of possible metallurgical compositions and associations with different structural substrates, it should be understood that such compositions and associations can suffer alterations that will become evident to those skilled in the art, without departing from the inventive concept of controlling the distribution, of the solid lubricant, in discrete particles, in the structural matrix, and of the eventual tendency of said solid lubricant to dissolve in said matrix, during the sintering step, as defined in the claims that accompany the present specification.