Powder metallurgical composition and process for manufacturing nanofiber reinforced powder metallurgy product from the same

Abstract

 According to the invention a powder metallurgical powder composition is provided that comprises as a base material non-agglomerating metallic and/or metal oxide and/or metal carbide grains with a mean grain size of 0.01-5 mm and a high melting point lubricant admixed to the base material in powdery form in the amount of 0.01-10% calculated per unit mass of the base material. The melting temperature of said high melting point lubricant is at least 550 degrees Celsius, but lower than the melting temperature of the grains that have the lowest melting temperature in the base material. The high melting point lubricant comprises as a major component inorganic silica (SiO₂) and additionally at least one further oxide selected from the group of inorganic alkali metal oxides, inorganic alkali-earth metal oxides and inorganic semimetal oxides differing from silica.

Description

[0001] The present invention relates to a powder composition that can be used in powder metallurgy, a sintering process of such a powder composition and a nanofiber reinforced powder metallurgy product obtained by the sintering process.

[0002] Traditional powder metallurgical technology is widely known and used in the field of manufacturing parts from powdery semiproducts, mainly metal powders. According to the burden of the process, grains of a metallic (mainly iron) powder are mixed with a low melting point lubricant, generally zinc stearate, cellulose, graphite, or with a low melting point soft metal (see e.g. the teaching of U.S. Pat. No. 2,289,569), or with metallic alloys (as is disclosed e.g. in EP-0,846,782 A1 or U.S. Pat. No. 4,452,756). Appropriate amount of the thus obtained mixture is filled into a mould, compacted in a die machine, and then the low-strength compacted part ("green part") removed from the mould is baked at a high temperature for a relatively long time in order to achieve appropriate strength. If the lubricant also contains volatile substances, before baking, said green part is kept at a temperature that is much lower than the baking temperature to evaporate said volatile substances from the green part. The green part attains its adequate strength through the process of coming adjoining grains into contact over larger and larger surfaces at the temperature high enough and over a longer period of time due to the continuous material transport in the vicinity of the contact points of said grains. This process eventually results in a partial diffusion bonding of grain surfaces with one another, this gives the mechanical strength of the powder metallurgical parts, and in pores partially encompassed by grain surfaces not contacting one another that account for the typical porous structure of the powder metallurgical parts.

[0003] Traditional powder metallurgical technology exhibits many limitations. No shapes of complex geometry can be produced by it as the green part cannot be removed from a mould of complex inner structure without being damaged: due to the low strength, a green part of more complexity breaks even if it is subjected to an isostatic compacting. Although, the porous structure is advantageous in some applications, e.g. in the field of self-lubricating sliding bearings, as far as the mechanical properties (e.g. strength) are concerned, in general, it is disadvantageous. Furthermore, as the pore size distribution of green parts fluctuates in a broad range, besides variability of strength related properties of green parts, it also severely affects the dimensional accuracy of the products/parts manufactured by the technology. Moreover, the high intensity of specific compression force required to produce the green part limits both the strength of the green part and the geometrical dimensions of the components that can be produced in this way.

[0004] A great number of disadvantages of traditional powder metallurgical technology is alleviated or eliminated by the technique of Metal Injection Moulding (MIM). This technology is classified as the most modern and effective one amongst powder metallurgical technologies. According to its burden, iron powder grains are mixed up with a binder and then the thus obtained homogeneous powder composition is moulded to the desired shape in a way similar to injection moulding process of plastics. The binder used in MIM technology is a common thermoplast added in an amount of 20 to 40 percentage by weight, and it is chosen in such a way that it could be easily and cheaply dissolved from the injection moulded component by a solvent. For this purpose the most appropriate binder is, for example, a polyolefin polymer that can be dissolved by halohydrocarbons, as U.S. Pat. No. 6,790,252 teaches, or an alcohol or water soluble polymeric material according to the disclosure of U.S. Pat. No. 6,171,360. After dissolving the binders utilized, the final product is prepared by a usual heat treatment, performed in a furnace, of the green part consisting of merely the metallic powder.

[0005] By this technique, products with spatial shapes of much more complexity can be manufactured, however, due to the pores remaining behind when the binder is removed, a product of much higher porosity occurs that, compared to the former technology, can only be consolidated through a much more intensive heat treatment or an additional compacting followed by a heat treatment. Moreover, to obtain a decrease in final porosity and also an increase in mechanical strength, in general, a further ductile shaping is required that is usually completed by forging or repeated compacting.

[0006] As to the known powder metallurgical technologies, one can briefly conclude that poreless parts and consequently high mechanical strengths cannot be achieved through their application, since on the one hand due to the removal of lubricants or binders from the green part, that are indispensable for processing the powder materials, void spaces are left over within the green part, and on the other hand the irregularly shaped powder grains cannot be packed together continuously even under high pressures, and although the dimension of the pores can be decreased eventually by compacting, said pores can not be eliminated to the full extent.

[0007] Nowadays, attempts are made at compensating for the inadequate mechanical strength of powder metallurgical final products due to pores through adding nanofibers to the metallic powder to be compressed. To this end, said nanofibers have to be produced beforehand that takes place generally via an independent process and far away from the place where said metallurgical powder technologies are actually practiced.

[0008] The material of the nanofibers utilized varies over a broad scale. The most well-known nanofibrous material is carbon nanotubes (CNT), the greatest amount of which is produced by well-known processes. For example, U.S. Pat. No. 7,629,553 discloses a production method of metal oxide nanoparticles through vaporizing metallic salts in plasma arcs and by condensing. The production of metallic nanofibers from metallic salts is disclosed e.g. by U.S. Pat. No. 7,648,554. Furthermore, U.S. Pat. No. 7,531,155 teaches the production of silicon nanofibers by means of ultrasonic treatment from powdered silicon made porous by etching. When said nanofibers are used in powder metallurgical processes, independently of their way of production, they are simply mixed with metallic powders and the thus obtained composition of nanofibers and metallic powder is processed then by powder metallurgical techniques known per se.

[0009] The thus obtained powder metallurgical parts also containing nanofibers exhibit obviously improved mechanical properties compared to those of powder metallurgical products prepared traditionally. Their properties concerned, however, fall behind the properties that could be expected in view of the properties of the nanofibers. The major reason for this is that the nanofibers mixed with the metallic powder before processing do not become integrated into the surface structure of the metallic grains, instead they, like ropes, loosely surround and only anchor said individual metal grains to one another.

[0010] The aim of the present invention is to provide a powder metallurgical powder composition and process, as possible alternatives to the above discussed prior art powder metallurgical powder compositions and methods, that eliminate or at least alleviate disadvantages of the known techniques. In particular, an objective of the present invention is to define a powder composition, by means of which final products of much lower porosities, in certain cases with no pores at all, as well as with much greater strengths could be obtained when compared to respective properties of the powder metallurgical products manufactured by former processes. A further objective of the invention is to provide a powder metallurgical process by means of which the advantageous properties of the nanofibers within the powder metallurgical product, e.g. their strength enhancing effect, prevail to an extent which is in harmony with the expectations. A yet further objective of the invention is to enhance dimensional stability/precision of the powder metallurgical product by means of the process.

[0011] Upon our studies we concluded that the above objectives can be achieved if, in the form of a dry powder, a granulated lubricant of high melting point is mixed with the granulated material forming the base material of the powder metallurgical product; said lubricant gets melted during the sintering process and coats the surfaces of the grains of the granulated material forming the base material of the product and thereby, on the one hand, it joins said grains together via acting as brazing material and, on the other hand, due to the heat treatment applied, it transforms into nanostructures, in particular nanofibers, in situ within the volume of the granulated material forming the base material of the product and by becoming integrated into the surface structure of said grains in this form it improves the joining forces acting amongst the grains. Said joining force improved due to the in situ created nanostructures/nanofibers increases the mechanical strength of the whole powder metallurgical product.

[0012] In light of the above, the aimed objectives are achieved by a powder metallurgical powder composition according to Claim 1. Preferred embodiments of the powder composition are represented by the compositions according to Claims 2 to 11. Furthermore, said objectives are achieved by providing powder metallurgical processes in accordance with Claims 12 and 13. A preferred further variant of said processes is set forth by Claim 14.

[0013] The burden of the high melting point lubricant used within the powder mixture provided by the inventive powder composition is that it, on the one hand, acts as lubricant amongst the grains of the base material only at the high temperature of compression (650 to 800 degrees Celsius) and, on the other hand, during the compression and the possible additional heat treatment it does not escape from the part, instead it joins the individual grains via acting as brazing material by getting cooled and covering the surfaces of the grains of the base material in a layer of the order of nanometer in thickness. The melting temperature of the high meting point lubricant utilized is at least 550 degrees Celsius, preferably at least 600 degrees Celsius, and said lubricant reaches an adequate viscosity between about 700 to 800 degrees Celsius. Said high melting point lubricant is a multicomponent eutectic or it exhibits a composition close to that of a multicomponent eutectic. Consequently, its practical melting point, as well as the temperature boundaries mentioned before can be set to the above or any other values as required by the process according to the invention by carefully selecting the relative ratios of the involved components. On the one hand, at the temperature concerned, the melt of the lubricant dissolves at least partially the oxides on the grain surfaces and hence becomes integrated into the surface structure of the grains, thus the sliding/shear forces acting in the step of compression cannot separate said lubricant from the grain surfaces. On the other hand, when compression-moulding to a desired shape, the molten lubricant flattens into a thin layer between the grains, while said grains pass through a plastic deformation on one another and the accidental void spaces, microcavities between the grains get filled with the molten lubricant. Therefore, after said compression-moulding no elastic reflexion takes place and the porosity of the green blank remains at a low level.

[0014] The powder metallurgical powder compositions according to the invention can be comprised of various grains as base material. The grains used can equally be metallic grains, to mention only a few examples e.g. iron, copper or nickel grains, various metal oxides of high melting point, to mention only a few examples e.g. alumina (Al₂O₃), zirconia (ZrO₂) or titania (TiO₂) grains and any other grains similar to these, or various high-strength metal carbide grains, to mention only a few examples e.g. tungsten carbide (WC), titanium carbide (TC) or silicon carbide (SiC) grains and any other grains similar to these. A composition or a mixture of the above listed various kinds of grains in any ratios can also be used as base material for the powder metallurgical compositions. The mean grain size of the grains of the base material ranges between 0.01 mm and 5 mm, thus it is much larger than that of the high melting point lubricant.

[0015] The mean grain size of the grains of the high melting point lubricant utilized in the powder metallurgical powder compositions according to the invention is preferably 10 to 700 nm, more preferably 50 to 500 nm. The high melting point lubricant is admixed to the base material in dry and at least partially crystalline form in an amount of 0.01 to 10%, more preferably 0.05 to 9%, and even most preferably 0.1 to 8%; here the % values are calculated per unit mass of the base material.

[0016] If metallic grains are used as the base material in the powder compositions according to the invention, silica (SiO₂) that forms major component of the high melting point lubricant is typically present in an amount of 30-60% by weight (of the total powder composition), preferably 35-55% by weight. Silica content of the high melting point lubricant is typically 30-85% by weight, preferably 35-80% by weight if a metal oxide and/or a metal carbide grain based base material is used. When the base material is of composite type, that is, it contains both metallic grains and metal oxide or metal carbide grains, silica that forms major component of the high melting point lubricant is typically present in an amount of 30-60% by weight, preferably 35-55% by weight.

[0017] Inorganic alkali metal oxides, inorganic alkali-earth metal oxides and inorganic semimetal oxides differing from silica forming at least one further component of the high melting point lubricant are present in various amounts compared to one another depending on the type of the base material. In the high melting point lubricant of the powder metallurgical compositions according to the invention, as inorganic alkali metal oxide, sodium oxide (Na₂O) in an amount of at most 15% by weight, preferably at most 10% by weight alone and at least partially in crystalline form, as well as potassium oxide (K₂O) in an amount of at most 20% by weight, preferably at most 15% by weight alone and also at least partially in crystalline form can be used highly preferably. In the high melting point lubricant of the powder metallurgical compositions according to the invention, as inorganic alkali-earth metal oxide, calcium oxide (CaO) in an amount of at most 30% by weight, preferably at most 25% by weight alone and at least partially in crystalline form, as well as barium oxide (BaO) in an amount of at most 20% by weight, preferably at most 15% by weight alone and also at least partially in crystalline form can be used highly preferably. In the high melting point lubricant of the powder metallurgical compositions according to the invention, as inorganic semimetal oxide differing from silica, boron oxide (B₂O₃) with glass forming capability in an amount of at most 25% by weight, preferably at most 20% by weight alone and at least partially in crystalline form can be used highly preferably.

[0018] It is noted that composition of the high melting point lubricant used in the powder composition highly depends on the chemical nature of the grains selected for the base material of said powder composition. Based on the above teaching and general knowledge, however, a person skilled in the art can easily determine specific compositions for the high melting point lubricant depending on the chemical nature of the grains to be utilized.

[0019] Moreover, in the powder metallurgical process according to the invention the compression-moulding or injection moulding takes place at such temperatures of the mixture comprising the grains of the base material and the powder of the high melting point lubricant that are higher than the melting temperature of the lubricant, but lower than the melting temperature of said grains. That is, the powder metallurgical process according to the invention differs from the injection moulding technique of molten metals in that here injection moulding takes place not above the melting temperature of the metal but in a state heated to at least the melting temperature of the high melting point lubricant of the powder mixture and an afterpressure is also applied. This state is suitable for the injection moulding, because under mechanical loads, the powdery mixture exhibits liquid-like properties when its motion, mixing and space filling are considered, but it behaves like a two-phase material when its compressibility is considered, and thus an afterpressure must be applied in order to remove the intergranular gas. That is, apart from the shrinkage due to cooling, the green part undergoes no further changes in its geometrical dimension contrary to traditional powder metallurgical processes wherein the shrinkage induced by the heat treatment can be as high as several tens percents. In this way, parts with much higher geometrical precision can be manufactured by the powder metallurgical process according to the invention. A further advantage of the inventive powder metallurgical process is that the technology of processing the powdery mixture into a final product fits excellently into the known and used technologies, and can be performed by exploiting the production devices of those technologies. An intermediate result of the inventive process is a part that has much lower porosity and much higher mechanical strength than a traditional one.

[0020] In a possible further variant of the powder metallurgical process according to the invention, the process can be preferably combined with traditional cold pressing. To this end, a further low melting point lubricant, preferably e.g. zinc stearate, must be added/admixtured to the powder metallurgical powder composition according to the invention comprising the high melting point lubricant unable to exercise its lubricating/sliding effect at temperatures below about 550 degrees Celsius. The melting temperature of said low melting point lubricant is at most 250 degrees Celsius, preferably at most 200 degrees Celsius. In cold pressing, the low melting point lubricant (in particular zinc stearate) acts as lubricating material, the high melting point lubricant remains ineffective. Said high melting point lubricant will exercise its effect at the temperature of sintering. This temperature falls between at least the melting temperature of the high melting point lubricant and at most the melting temperature of the grains in the base material that have the lowest melting temperature. The actual effect of said high melting point lubricant is that, on the one hand, it gets molten over the grain surfaces and thereby fully or partially fills up the intergranular pores and joins the individual grains together by acting as brazing material, and, on the other hand, upon heat treatment it partially transforms into nanofibers, preferably a part of which becomes integrated into the surface structure of the grains. In the traditional processing, cold pressing is completed and zinc stearate is evaporated from the green part at a first temperature, then sintering is performed at a second temperature (being preferably much) higher than said first temperature, in particular at a temperature close to the melting temperature of a surface oxide layer forming on the surfaces of the grains of the base material when said base material is mixed with the high melting point lubricant, i.e. preferably at about 650 to 840 degrees Celsius and for a period of a few minutes. The significant difference between the structural properties of the powder metallurgical products obtained in this way and by the traditional technologies accrues from the fact that mechanical strength and microstructure of the part sintered by the inventive process develop not because of diffusion bonding of individual grains, but mainly due to the presence of nanostructures, in particular nanofibers, that form from the nanosized grains of the high melting point lubricant that coats the grains of the base material when admixed thereto. Molten oxides of the lubricant fill up the place that had been occupied by the low melting point lubricant (e.g. zinc stearate) before it was evaporated, as well as other intergranular void spaces, and said oxides also stay there after being solidified, thereby porosity of the part decreases or cancels. The compression strength developing due to the cohesive forces that form upon solidification and the nanofibers penetrating and becoming integrated into the surface structure of the grains exceeds that of the base metals in many cases; the improved mechanical strength of the parts obtained by this latter variant of the powder metallurgical process according to the invention derives just from this property.

[0021] Any traditional lubricant known by the skilled person in the art can be used as the low melting point lubricant. As far as the inventive solutions are concerned, the usage of zinc stearate (the melting temperature of which is 130 degree Celsius) is highly preferred.

[0022] Better understanding of the invention will be facilitated by exposing two simple embodiments illustrative of the present invention, as well as by analyzing several high-definition scanning electron microscope (SEM) photos. In the appended drawings:

• Figures 1 to 3 show SEM photos of fractures of samples obtained by performing consecutive steps of the inventive process from a powder metallurgical powder composition, according to Example 1 discussed below in detail, containing iron grains as the base material; and
• Figures 4 to 5 show SEM photos of the exposed outer surfaces of samples produced from a powder metallurgical powder composition containing copper grains as the base material.

Example 1

[0023] An iron-based powder metallurgical part is produced using an iron powder with the mean grain size of 80-100 micron as the base material, produced and made available by Hoegenas under the code of NC 24.100. To this end, at first the high melting point lubricant is prepared by admixing preferably 45% by weight SiO₂, 15% by weight B₂O₃, 10% by weight CaO, 5% by weight K₂O, also 5% by weight Na₂O and 20% by weight BaO. The thus obtained crystalline oxide mixture is then ground until a mean grain size of at least 0.5 micron is reached. The mill product is added to the iron powder in an amount of 5% calculated per unit mass of the iron powder and the thus obtained mixture is then homogenized through mixing in a ball-grinder. If the obtained powder metallurgical powder composition is to be cold pressed, zinc stearate, as a low melting point lubricant, is also added to the powder composition in an amount of 0.8% calculated per unit mass of said powder composition. By proper agitation, the zinc stearate is dispersed within the powder composition essentially in a homogeneous manner.

[0024] Then, said powder mixture is filled into a mould and pressed to a desired shape at room temperature and 500 MPa pressure. The green part is removed from the mould and is heated to about 200 degrees Celsius within a controlled-atmosphere furnace and maintaining the green part at this temperature, the zinc stearate is evaporated from it. After this, the furnace and the green part arranged therein are heated to about 960 degrees Celsius and kept at this temperature for 2 hours. The green part undergone the heat treatment is finally cooled or allowed to cool down to room temperature.

Example 2

[0025] Transformation of the powder metallurgical powder composition detailed in Example 1 into a powder metallurgical product is accomplished by hot pressing. In such a case, no low melting point lubricant is added to the powder mixture, instead the mixture of the iron powder and the high melting point lubricant mill product is heated to about 750 degrees Celsius and compression-moulding is performed under 100 MPa pressure. After this, the thus obtained green part is kept under pressure for 2 minutes and then is removed from the mould. The part at issue is basically ready now, however, its mechanical properties can be significantly improved by means of a heat treatment that takes place for further 2 hours at about 960 degrees Celsius.

[0026] In addition to simply brazing the grains of the base material together, the powder metallurgical process according to the invention, independent of the fact whether the heat treatment/sintering step forming part of said process takes place according to Example 1 or Example 2, exploits a yet further mechanism to improve the strength of the powder metallurgical product. This mechanism is the ― at least partial ― transformation of the oxide mixture used as the high melting point lubricant to nanostructures upon heat treatment. The course of transforming can be seen in the SEM photos (see Figures 1 to 3) of the sample fractures of powder metallurgical products containing iron grains as the base material. In Figure 1, an iron grain 1 of the initial powder composition is shown before pressing, said grain became coated by the oxide mixture powder during the admixture. As is clear, the surface of the iron grain 1 is covered by oxide mixture grains that are partially ordered into filaments 2 and partially agglomerated into tiny gobs 3. Figure 2 illustrates a fracture surface of the iron grain based green part obtained by pressing and heat treatment. In Fig. 2, one can clearly see nanofibers 4 nearly perpendicular to the fracture surface, as well as nanofibers 5 lying in the plane of the fracture surface. Figure 3 shows a different portion of the fracture surface of the green part, wherein upon melting due to the heat treatment, the oxide mixture forming the tiny gobs 3 of Fig. 1 was transformed partially into formations of plaques 7 and partially into a mixed formation 6 (which is practically a combination of the nanofiber 4 and the plaque 7 formations) besides the nanofibers located on the grain surface 8. The high melting point lubricant in excess that has been not transformed into nanostructures covers the surfaces of the grains of the base material in a thin layer and partially also hides the nanofibers. The mass ratio of nanofibers and other nanosized formations, as well as their dimension and composition are of statistical nature; starting from the same powder metallurgical powder composition and performing the same powder metallurgical technique, these characteristic properties will vary from product to product. The nanofibers enhancing the strength of the product, however, are present in each product prepared form the powder metallurgical powder composition according to the invention by the inventive powder metallurgical process.

[0027] Figures 4 and 5 illustrate the coverage of the grains of the powder composition by the molten high melting point lubricant; said figures are SEM photos taken on the exposed outer surfaces of the base material samples made of granulated powder of electrolytical copper coded CH-L10, as the base material, with a mean grain size of about 10 micron. The samples show the nanostructure formations on copper base material produced upon heat treatment from the high melting point lubricant with a composition discussed in Example 1. As it can be seen in Fig. 4, the surface of the sample is covered by a melt 13 that spreads mainly in a continuous manner. Uncovered surface portions are provided partially by pores 12 between copper grains and partially by copper grain surfaces 9, 11 projecting out of the surface covered by the melt 13. Surfaces covered by the melt 13 are partially covered by elements 14 forming a second layer in a discontinuous manner. This coverage can be clearly observed only on the exposed outer surface in extensive domains, however, it is equally present on the surfaces of the grains buried within the base material sample. The discontinuous coverage is illustrated in Figure 5, which shows a fragment of Fig. 4 in an enlarged view. In Fig. 5, one can clearly recognize, on the one hand, nanofibers 15 located parallel to the surface covered by the melt 13 and nanofibers 15 perpendicular to this surface and, on the other hand, cracked nanofibers 16 and plaques 17. The major effect of the heat treatment of the base material mixed up with the high melting point lubricant and being compression-moulded is that it brazes the nanofibers created in situ from the lubricant upon heat treatment to the copper grain surfaces covered by the melt 13 layer with a thickness that falls into the range of nanosizes. It should be here noted that a yet further effect of said high melting point lubricant is that it joins the grains of the base material mechanically to one another in an electrically insulated manner.

[0028] Without going into theoretical discussions in more detail, it is noted that at the temperature of the heat treatment, the at least two components forming the high melting point lubricant are at least partially fluidic and due to their different partial vapour pressures, different amounts thereof will escape from the metal body via material transport of the vapour of their own constitutional water. Upon increase in the temperature, vapour pressures of the released constitutional waters of the component that firstly gets molten and also of the further component(s) force the melt of the component that firstly gets molten into the open pores and thus decrease their penetrability or block them. Simultaneously with the occurrence of high temperatures, high vapour/gas pressures build up within the closed microcavities forming in this way from the pores, which is a fundamental requirement for the formation of nanostructures. As e.g. in Examples 1 and 2, the partial pressure of silicon dioxide is the lowest among that of the other components within the added oxide mixture, this preserves in the greatest amount compared to the initial equilibrium composition. Therefore, silicon dioxide will give the largest mass portion of the precipitations, that is, the nanostructures/nanofibers formed will be majorly based on silicon. In the powder metallurgical products manufactured by the process according to the invention heat treatment interconnects neighbouring grains with a plurality of nm sized fibers by the brazing material provided by the molten high melting point lubricant covering the grain surfaces or by penetrating through the layer of said brazing material and becoming integrated thereinto. Consequently, the mechanical properties of the obtained product will be significantly better than those of the products prepared by traditional powder metallurgical technologies.

[0029] The powder metallurgical process according to the invention is also suitable for processing mixtures of metallic and non-metallic (such as e.g. oxide or carbide) grains, that is, so-called composite materials, since the high melting point lubricants used in the initial composite powder compositions of such substances get fused over the surfaces of the non-metallic grains and thereby join the grains of the base material together similarly, as if they were metallic grains.

[0030] It is also noted that the inventive process results in powder metallurgical products with improved dimensional accuracy at significantly lower energy consumption and tooling costs, along with more advantageous physical properties compared to those of similar products manufactured by the known prior art solutions.

[0031] Furthermore, the technology according to the invention is a waste-free and clean, environment-friendly technology in contrast with the metallurgical, plastic shaping or machining technologies widely used nowadays.

Claims

1. A powder metallurgical powder composition comprising as a base material non-agglomerating metallic and/or metal oxide and/or metal carbide grains with a mean grain size of 0.01-5 mm and a high melting point lubricant admixed to the base material in powdery form in the amount of 0.01-10% calculated per unit mass of the base material, wherein the melting temperature of said high melting point lubricant is at least 550 degrees Celsius, but lower than the melting temperature of the grains that have the lowest melting temperature in the base material, wherein said high melting point lubricant comprising as a major component inorganic silica (SiO₂) and additionally at least one further oxide selected from the group of inorganic alkali metal oxides, inorganic alkali-earth metal oxides and inorganic semimetal oxides differing from silica.

2. The powder metallurgical powder composition of Claim 1, wherein the mean grain size of the powdery high melting point lubricant is 10-700 nm, preferably 50-500 nm.

3. The powder metallurgical powder composition of Claim 1 or 2, wherein the powdery high melting point lubricant is admixed in dry form.

4. The powder metallurgical powder composition of any one of Claims 1 to 3, wherein the alkali metal oxide is at least one of sodium oxide (Na₂O) and potassium oxide (K₂O) in at least partially crystalline form.

5. The powder metallurgical powder composition of Claim 4, wherein sodium oxide content of said high melting point lubricant is at most 15% by weight, preferably at most 10% by weight and potassium oxide content of said high melting point lubricant is at most 20% by weight, preferably at most 15% by weight.

6. The powder metallurgical powder composition of any one of Claims 1 to 5, wherein the alkali-earth metal oxide is at least one of calcium oxide (CaO) and barium oxide (BaO) in at least partially crystalline form.

7. The powder metallurgical powder composition of Claim 6, wherein calcium oxide content of said high melting point lubricant is at most 30% by weight, preferably at most 25% by weight and barium oxide content of said high melting point lubricant is at most 20% by weight, preferably at most 15% by weight.

8. The powder metallurgical powder composition of any one of Claims 1 to 7, wherein the semimetal oxide is boron oxide (B₂O₂) in at least partially crystalline form.

9. The powder metallurgical powder composition of Claim 8, wherein boron oxide content of said high melting point lubricant is at most 25% by weight, preferably at most 20% by weight.

10. The powder metallurgical powder composition of any one of Claims 1 to 9, wherein the base material contains metallic grains and silica content of said high melting point lubricant is at most 30-60% by weight, preferably at most 35-55% by weight.

11. The powder metallurgical powder composition of any one of Claims 1 to 9, wherein the base material consists of metal oxide grains and/or metal carbide grains and silica content of said high melting point lubricant is at most 30-85% by weight, preferably at most 35-80% by weight.

12. A process for manufacturing a powder metallurgical product, comprising the steps of
preparing a powder metallurgical powder mixture by mixing as a base material non-agglomerating metallic and/or metal oxide and/or metal carbide grains with a mean grain size of 0.01-5 mm and a high melting point lubricant in powdery form in the amount of 0.01-10% calculated per unit mass of the base material, said high melting point lubricant comprising as a major component inorganic silica (SiO₂) and additionally at least one further oxide selected from the group of inorganic alkali metal oxides, inorganic alkali-earth metal oxides and inorganic semimetal oxides differing from silica, the melting temperature of said high melting point lubricant falling between at least 550 degrees Celsius and the melting temperature of the grains that have the lowest melting temperature in the base material;
filling the thus obtained powder mixture into a mould;
heating said powder mixture within said mould to a temperature falling between the melting temperature of the high melting point lubricant and the melting temperature of the grains that have the lowest melting temperature in the base material, moulding said powder mixture into a green part at said temperature under pressure, performing sintering for a given period of time so as, on the one hand, to fill up pores of the green part with a melt obtained through melting said lubricant and to join said grains of the base material in the green part together and, on the other hand, to form in situ nanofibers reinforcing joints of said grains;
subjecting said green part to an afterpressure for a short period of time, then cooling it down so as to obtain a nanofiber reinforced powder metallurgical product.

13. A process for manufacturing a powder metallurgical product, comprising the steps of
preparing a powder metallurgical powder mixture by mixing as a base material non-agglomerating metallic and/or metal oxide and/or metal carbide grains with a mean grain size of 0.01-5 mm and a high melting point lubricant in powdery form in the amount of 0.01-10% calculated per unit mass of the base material, said high melting point lubricant comprising as a major component inorganic silica (SiO₂) and additionally at least one further oxide selected from the group of inorganic alkali metal oxides, inorganic alkali-earth metal oxides and inorganic semimetal oxides differing from silica, the melting temperature of said high melting point lubricant falling between at least 550 degrees Celsius and the melting temperature of the grains that have the lowest melting temperature in the base material;
admixing a low melting point lubricant to the powder mixture, the melting temperature of said low melting point lubricant being at most 250 degrees Celsius;
filling the thus obtained powder mixture into a mould;
form pressing said powder mixture within the mould at a given pressure so as to obtain a green part;
heating the green part to a first temperature being lower than the melting temperature of the high melting point lubricant, thereby evaporating said low melting point lubricant from said green part; then
heating the green part to a second temperature falling between the melting temperature of the high melting point lubricant and the melting temperature of the grains that have the lowest melting temperature in the base material, performing sintering for a given period of time at said second temperature, so as, on the one hand, to fill up pores of the green part with a melt obtained through melting said lubricant and to join said grains of the base material in the green part together and, on the other hand, to form in situ nanofibers reinforcing joints of said grains;
cooling down said green part so as to obtain a nanofiber reinforced powder metallurgical product.

14. The powder metallurgical process of Claim 12 or 13, wherein a powder metallurgical powder composition according to any one of Claims 2 to 11 is used as the powder mixture.

Drawing