Patent Application
20110286873
The present invention refers to specific techniques for manufacturing finished products (pieces) and semi-finished products (several articles), conformed from a composition of particulate materials (in the form of metallic and non-metallic powders) and which are designed to be sintered, said products comprising, besides the elements constitutive of the metallic structural matrix of the product to be formed during the sintering step, a precursor phase of a solid lubricant, in the particulate form and which, by dissociation during the sintering step, generates precipitates of the solid lubricant in the volume of the metallic matrix, leading to the formation of the micro-structure of a self-lubricating product presenting a continuous metallic matrix and which is capable of imparting, to the sintered products, a low coefficient of friction allied to high mechanical strength and high hardness of the sintered piece or product. The invention refers to said metallurgical composition for forming the self-lubricating material “in situ” during the sintering, to the pieces or products in sintered steel obtained from said composition, as well as to the specific alternative techniques or processes for obtaining said pieces or products by powder metallurgy.
Given the advanced stage of technological development, there is a need for developing functional materials with high performance, which are specifically designed for each particular group of applications. In several mechanical engineering applications, a need exists for materials that have, at the same time, high mechanical strength and high wear strength allied to a low coefficient of friction.
It is estimated that about 35% of the whole mechanical energy produced in the planet is lost by lubrication deficiency and is converted in heat by friction. Apart from the energy loss, the generated heat impairs the performance of the mechanical system due to the heating. Thus, maintaining a low coefficient of friction in mechanical pieces under friction is highly important, not only for energy economy, but also to increase the durability of said pieces and of the mechanical systems in which they operate, besides contributing to environment preservation due to decrease of discarding material.
The way being used to reduce wear and friction between surfaces in relative movement is to maintain these surfaces separated, interleaving a lubricating layer therebetween. Among possible lubricating ways, the hydrodynamic (fluid lubricants) is the most used. In the hydrodynamic lubrication there is formed an oil film which separates completely the surface in relative movement. However, it should be pointed out that the use of fluid lubricants is usually problematic, as in applications at very high or very low temperatures, in applications in which the fluid lubricant may chemically react and when the fluid lubricant may act as a contaminant. Besides, in situations of limit lubrication resulting from cycle stops, or in situations in which it is impossible to form a continuous oil film, there occurs the contact between the pieces, consequently causing wear to the latter.
The dry lubrication, that is, the one using solid lubricants, is an alternative to the traditional lubrication, since it acts by the presence of a solid lubricant layer, which prevents the contact between the component surfaces, but without presenting rupture of the formed layer.
The solid lubricants have been well accepted in problematic lubrication areas. They can be used in extreme temperatures, under high-load conditions and in chemically reactive environments, where conventional lubricants cannot be used. Moreover, dry lubrication (solid lubricants) is an environmentally cleaner alternative.
The solid lubricant may be applied to the components of a tribological pair, in the form of films (or layers) that are deposited or generated on the surface of the components or incorporated to the volume of the material of said components, in the form of second-phase particles. When specific films or layers are applied and in case they suffer wear, there occurs the metal-metal contact and the consequent and rapid wear of the unprotected confronting surfaces and of the relatively movable components. In these solutions in which films or layers are applied, it should be further considered the difficulty in replacing the lubricant, as well as the oxidation and degradation of the latter.
Thus, a more adequate solution which allows increasing the lifetime of the material, that is, of the components, is to incorporate the solid lubricant into the volume of the material constitutive of the component, so as to form the structure of the component in a composite material of low coefficient of friction. This is possible through the powder metallurgy techniques, that is, by the conformation of a powder mixture by compaction, including pressing, rolling, extrusion and injection molding, followed by sintering, in order to obtain a continuous composite material, usually already in the final geometry and dimensions (finished product) or in geometry and dimensions close to the final ones (semi-finished product).
Self-lubricating mechanical components presenting low coefficient of friction, such as self-lubricating bushings, are produced by powder metallurgy techniques from metallic powders which form the metallic structural matrix of the sintered piece and which are mixed with solid-lubricant powders. Said components have been used in diverse household appliances and small equipment, such as: printers, electric shavers, drills, blenders, and the like. Most of the well-known prior art solutions for the structural matrix use bronze, copper, silver, and pure iron. There are used as solid lubricant: molybdenum disulfide (MoS2), silver (Ag), polytetrafluoroethylene (PTFE) and molybdenum diselenide (MoSe2). Bushings with these types of self-lubricating materials, mainly with bronze and copper matrix containing, such as solid lubricant particles, graphite powder, selenium and molybdenum disulfide and low melting point metals, have been produced and used for decades in several engineering applications.
However, these pieces do not present high mechanical strength, as a function of its high volumetric content (from 25% to 40%) of solid lubricant particles, which results in a low degree of continuity of the matrix phase, which is the micro-structural element responsible for the mechanical strength of the piece. This high content of solid lubricant has been considered necessary for obtaining a low coefficient of friction in a situation in which both the mechanical properties of the metallic matrix (strength and hardness) and the micro-structure parameters, such as the size of the solid lubricant particles dispersed in the matrix and the average free path between these particles in the formed composite material, are not optimized for applications in which the piece is required to have high mechanical strength. The high volumetric percentage of solid lubricant, which has an intrinsic low strength to shearing, does not contribute to the mechanical strength of the metallic matrix. Furthermore, the solid lubricant particles shear easily and alter their shape, as a function of shearing forces that occur during the steps of mechanically homogenizing the powder mixture (carried out in mixers) and compacting the mixture, reducing even more the degree of continuity of the metallic structural matrix of the formed self-lubricating composite. Moreover, the low hardness of the metallic matrix allows a gradual obstruction of the solid lubricant particles to occur on the contact surface of the sintered material or product. Thus, in order to maintain a sufficiently low coefficient of friction, there has been traditionally used a high volumetric percentage of solid lubricant in the composition of dry self-lubricating composite materials.
A partially differentiated and more developed scenario, as compared with that previously described, is disclosed in U.S. Pat. No. 6,890,368A, which proposes a self-lubricating composite material to be used at temperatures in the range between 300° C. and 600° C., with a sufficient traction resistance (Rm≧400 MPa) and a coefficient of friction lower than 0.3. This document presents a solution for obtaining pieces or products of low coefficient of friction, sintered from a mixture of particulate material which forms a metallic structural matrix and including, as solid lubricant particles in its volume, mainly hexagonal boron nitride, graphite or a mixture thereof, and states that said material is adequate to be used at temperatures in the range between 300° C. and 600° C., with a sufficient traction resistance (Rm≧400 MPa) and a coefficient of friction smaller than 0.3.
As described in the Brazilian patent application (provisional number 018080057518) filed on Sep. 12, 2008, in the name of the same applicants of the present invention, pieces or products obtained from the consolidation of a powder mixture simultaneously presenting the structural matrix powders and the solid lubricant powders, such as for example, hexagonal boron nitride and graphite, have low mechanical strength and structural fragility after sintering.
The deficiency cited above results from the inadequate spread (dispersion), by shearing, of the solid-lubricant phase between the powder particles of the structural matrix during the steps of mixing and conforming (densification) the pieces or products to be produced. The solid lubricant spreads, by shearing, between the particles of the structural matrix phase, and tends to surround said particles during the mixing and conforming steps, which submit said solid lubricant to stresses which surpass its low shearing stress.
On the other hand, the presence of the solid lubricant layer between the particles (of the powder) of the structural matrix, formed by shearing, impairs the formation of metallic contacts between these particles which form the structural matrix of the composite during the sintering; this contributes to a reduction of the degree of continuity of the structural matrix phase of the composite material, structurally fragilizing the material and the obtained products.
Such problems can be mostly solved through solutions proposed in the prior Brazilian patent application mentioned above, resulting in obtaining composite materials with mechanical strength greater than that of the prior art solutions.
However, in the solution proposed in said prior patent application of the same applicants of the present invention, the non-metallic particulate solid lubricant, for example hexagonal boron nitride, graphite or both, has to be mixed to the metallic materials which form the structural matrix of the composite product to be sintered, further requiring the addition of at least one particulate alloy element, so as to form, during the sintering step of the conformed metallurgical composition, a liquid phase between the particulate material which forms the structural matrix and the non-metallic particulate solid lubricant, in order to agglomerate the latter in discrete particles and prevent the non-metallic particulate solid lubricant from spreading, by shearing, between the particles of the structural matrix phase, tending to surround them during the steps of mixing and conforming (densification) the pieces or products to be produced, fragilizing the latter.
In face of the drawbacks cited above, it is desirable to provide a solution which does not require the previous mixing of the solid lubricant particles in the metallurgical composition to be sintered, nor the addition of an alloy element in the metallurgical composition to form a liquid phase in the latter during its sintering.
It is, therefore, an object of the present invention to provide a composition of particulate materials for forming sintered steels, comprising a metallic structural matrix which permits, per se and during sintering thereof, forming a finished or semi-finished product in self-lubricating sintered steels with a high degree of continuity of the structural matrix and presenting high mechanical strength and high hardness, with a fine distribution of a solid lubricant phase generated in the sintering.
It is likewise an object of the present invention to provide a product in self-lubricating sintered steel, obtained from a conformation by powder compaction via pressing, rolling and others or by injection molding, followed by sintering the composition defined above, and which presents a high degree of continuity of the metallic structural matrix, a low coefficient of friction and high mechanical strength and high hardness, with a fine distribution of a solid lubricant phase of graphite generated in the sintering.
It is also another object of the present invention to provide a process for obtaining products in self-lubricating sintered steel, such as defined above, from said composition of particulate materials, said process including neither the previous mixture of solid lubricant particles in the metallurgical composition to be sintered, nor the addition of an alloy element in the metallurgical composition to form a liquid phase in the latter during its sintering.
In a first aspect of the present invention, the objects cited above are attained through a composition of particulate materials for the manufacture of products in self-lubricating sintered steel, previously conformed by one of the operations of compacting and injection molding said composition which comprises: the iron as the main particulate metallic material; at least one particulate alloy element, with the function of hardening the iron, forming therewith a ferrous structural matrix; and a non-metallic compound, precursor of a solid lubricant phase of graphite to be formed in the product during the sintering.
In a way of carrying out the invention, the non-metallic particulate compound is a compound of the carbide or carbonate type including a chemical element which stabilizes the iron alpha phase of the ferrous structural matrix. In another way of carrying out the present invention, the non-metallic particulate compound is deprived of any chemical element which stabilizes the iron alpha phase, thus being necessary to include, in the metallurgical composition, an additional particulate alloy element which has the function of stabilizing the iron alpha phase.
In the present invention, there occurs the formation of graphite particles by dissociation of a precursor phase during the sintering step of the pieces or products. As examples of precursor phases for carrying out the invention, it can be cited: silicon carbide (SiC), molybdenum carbide (Mo2C), chromium carbide (Cr3C2), and the like. In the step of preparing the powder mixture which will constitute the new composite material, carbides in the form of fine powder particles (preferably from 5 to 25 μm) are mixed to the iron powder (major component) and other powders of alloy elements that are present in the powder mixture. The most indicated carbides to cause precipitation of graphite nodules in ferrous matrix, forming a self-lubricating sintered steel, are those which have in their formula a chemical element which can strongly stabilize the iron alpha phase, as for example, the element Si present in the silicon carbide (SiC). During the sintering step, that is, at the sintering temperature of the pieces or products, the silicon carbide (SiC) dissociates and the chemical element silicon becomes a solid solution in the iron, that is, in the ferrous structural matrix. As the dissociation of the SiC progresses, the amount of solubilized Si increases in the ferrous matrix in the surroundings of the SiC particles which are in dissociation. As can be verified in the iron-silicon equilibrium diagram, the chemical element silicon strongly stabilizes the iron alpha phase; the vertex of the loop α⇄ (α+γ) in Fe—Si diagram occurs for values of 2.15% by weight (4.2% at) of Si. Thus, during the sintering of the sintered steel, carried out typically between 1125° C. and 1250° C., when the concentration of silicon solubilized in iron, around the SiC particle in dissociation, reaches the solubility limit of the gamma-phase, there occurs a transformation of the gamma-iron into alpha-iron. In the first instants of the SiC dissociation process, while the Si concentration does not reach the value required to stabilize the alpha-phase around the SiC particle in dissociation, the carbon resulting from the dissociation also becomes a solid solution and spreads to the interior of the matrix, but as soon as the ferrous matrix around the SiC particulate in dissociation is transformed into alpha-phase, the process for solubilizing carbon is interrupted because the solubility of carbon in the iron alpha phase is very low (maximum value is of 0.022% by weight at 727° C.). Thus, the carbon released, as a consequence of the carbide dissociation, forms graphite nodules, which are surrounded by a layer of alpha-iron, although the remainder of the matrix can continue presenting the gamma phase.
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:
As already previously mentioned, one of the objects of the invention is to provide a composition of particulate materials, which can be homogeneously mixed and conformed (densified) by compaction (pressing, rolling) or by extrusion or injection molding of powders, 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 composition comprises: a main particulate metallic material which is preponderant in the formation of the composition, and at least one particulate alloy element with the function of hardening the preponderant material, these components being responsible for the formation of a structural matrix in the composite product, in steel, to be sintered; and a precursor particulate material which allows obtaining solid lubricant nodules upon its dissociation during the sintering.
According to the invention and as illustrated in
The alloy element with the function of hardening the ferrous structural matrix is defined, for example, by one of the elements selected from chrome, molybdenum, carbon, silicon, phosphorus, manganese and nickel, but it should be understood that one can use other elements, such as vanadium and copper, which carry out the same function in the structural matrix, as well as more than one alloy element at the same time. It should be noted that the invention requires the provision of an alloy hardening element which may carry out the function of hardening the ferrous structural matrix to be formed during the sintering, by interdiffusion of the components (chemical homogenization), but this aspect should not be limited to the alloy elements exemplified herein.
The parallel addition of other alloy elements, which stabilize the iron alpha phase in the powder mixture which will form the composite, accelerates the rising of the alpha phase in the matrix during the sintering operation, increases the tendency to generate graphite nodules 20 by dissociation of carbide particles mixed in the volume of the material.
The alloy elements which stabilize the iron alpha phase and coming from the carbide dissociation, besides preventing the carbon from solubilizing in the matrix, since they form a layer of alpha phase 12 around the particle 11 in dissociation, they also contribute to increase the hardness of the matrix when in solid solution; nevertheless, if the hardness increase reached by the presence of these alloy elements in solid solution in the iron is not sufficient, other alloy elements should be additionally added to the powder mixture, so as to be solubilized in the matrix during the sintering operation, aiming at achieving the hardness and mechanical strength necessary for the application.
Thus, in the present invention, the metallic structural matrix of the material is formed by iron automatically hardened by a solid solution with the alloy elements which stabilize the iron alpha phase, as for example, silicon and molybdenum dissolved in the ferrous matrix as a consequence of the dissociation of the carbides mixed to the iron powder in the processing of the material by powder metallurgy.
Besides these necessarily present stabilizing alloy elements, other alloy elements might be added to the powder mixture with the function of adjusting the mechanical strength and the hardness of the matrix, allowing reaching a high performance in relation to the tribological and mechanical behavior of the dry self-lubricating composite material generated during the sintering. As examples of other alloy elements, advantageously used in the present invention, to increase the mechanical strength and the hardness of the matrix, besides the Si, Mo, and P elements, which are strong stabilizers of the iron alpha phase, there can be cited the elements Cr, Ni, Mn, W, V, and C.
As to the types of carbides used, the powder mixture composition, which is formulated for the production of products by powder metallurgy in the present invention, is formed by two distinct alternatives:
Alternative 1: Iron powder+particles 11 of carbide powder which are formed by chemical elements which stabilize the iron alpha phase (mixed in a volumetric percentage ≦10%), which, at the sintering temperature, generate graphite nodules 20 upon dissociation thereof, +powder particles of other chemical elements called alloy elements, which have the function of increasing the hardness and the strength of the ferrous structural matrix 10;
Alternative 2: Iron powder+carbide powder particles which are not formed by chemical elements which stabilize the iron alpha phase (mixed in a volumetric percentage ≦10%), +powder of alloy elements which stabilize the iron alpha phase which has the function of stabilizing the alpha phase of the ferrous matrix, in order to prevent the carbon coming from the carbide dissociation from being dissolved by the ferrous matrix, +other alloy elements which are present for adjusting the mechanical properties of the structural matrix of the composite. Since the metallic ferrous structural matrix 10 is the sole micro-structural element of the composition that confers mechanical strength to the composite material to be formed, the higher the degree of continuity of the matrix of said composite, the higher will be the mechanical strength of the sintered article or piece produced with the material. The maintenance of the high degree of continuity of the metallic structural matrix of the dry self-lubricating sintered composite material requires, besides a low porosity, a low volumetric percentage of the solid lubricant phase, since the latter does not contribute to the mechanical strength of the material and, consequently, does not contribute to the mechanical strength of the sintered products. Besides, the solid lubricant which is present in the volume of the material should be dispersed in the form of discrete particles or nodules 20, uniformly distributed in the volume, that is, with a regular average free path “λ” in the interior of the ferrous structural matrix 10 (see
The metallic matrix of the material is required to be highly resistant to plastic deformation, in order to operate not only as a mechanical support with the necessary load capacity, but also to prevent the solid lubricant particles from being covered by plastic deformation of the structural matrix, upon operation of the piece (when frictioned in relative movement), preventing the solid lubricant from spreading in the interface where it should form a layer of solid lubricant.
According to the invention, the additional 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 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, in order to impair the carbon dissolution, and not in the fact that the alloy component(s) used are necessarily the ones exemplified herein.
When the composition of the invention is conformed by compaction, the main particulate metallic material (iron) presents, preferably, an average particle size lying between about 5 μm and about 90 μm. On its turn, the hardening element, with the function of hardening the structural matrix, and the precursor component of the solid lubricant phase (compound) should present a particle size preferably smaller than about de 45 μm; it should be further understood that the average particle size of the main particulate metallic material, that is, of the iron, should be always larger than the average particle size of the alloy elements and the precursor components (compounds) of the solid lubricant phase.
When the composition of the invention is conformed by injection molding, the main particulate metallic material (iron) presents, preferably, a particle size lying between about 5 μm and about 25 μm. In the same way, the alloy elements and the precursor components (compounds) of the solid lubricant phase present, preferably, a particle size also between about 5 μm and about 25 μm.
When the conformation of the 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 a 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.
The compositions described above are obtained by mixing, in any adequate mixers, predetermined quantities of the particulate materials selected for the formation of the composition and for the subsequent obtention of a self-lubricating sintered product.
The mixture of the different particulate materials is homogenized and submitted to a densification operation by compaction, that is, by pressing or rolling, or also by molding by extrusion or injection of powders, obtaining in this operation, not only the densification of the powder mass, but also the desired shape for the product to be obtained by sintering.
In case of conformation by powder molding by extrusion or injection, the mixture of the components containing the organic binder is homogenized at temperatures not inferior to the melting temperature of the organic binder, the thus homogenized mixture being granulated to facilitate its handling, storage and supply to an injection machine.
After conformation of the piece, this is submitted to the extraction of the organic binders, generally carried out in two steps, the first step being a chemical extraction process in solvents (for example, hexane) and the second step being an extraction process by thermal degradation, or a CD plasma assisted thermal process.
With the composition proposed herein, it is possible to obtain self-lubricating sintered pieces or products with hardness from 230 HV to 700 HV, a coefficient of friction μ≦0.15, a mechanical traction resistance from 350 to 750 MPa (depending on the alloy elements which are present and on the processing parameters used) and also with a dispersion of amorphous carbon nodules with the inner structure in the form of skins with nanometric thickness, which facilitates the spreading of the graphite in the interface of the movable surfaces, forming a solid lubricant layer.
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.
Thus, the desired self-lubricating product can be constituted, as illustrated in
When the compaction of the composition 40 is carried out by extrusion, as it occurs, for example, in the formation of the bars 2, 3 and 4 of
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.
While the invention has been presented herein by means of some examples of possible 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, in discrete particles, of the solid lubricant in the structural matrix, and also 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| PI0805606-4 | Dec 2008 | BR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/BR2009/000411 | 12/9/2009 | WO | 00 | 7/15/2011 |
