EP1681116B1 - Sinterable metal powder mixture for the production of autolubricant parts - Google Patents

Description

The invention relates to a new self-lubricating solid material; a process for producing such a material from a mixture of powders; said powder mixture; and mechanical parts made of this new material.

Solid self-lubricating materials, dry, are generally used for the manufacture of mechanical parts such as bushings, ball joints or pivots, subjected to significant friction while their operating conditions make it impossible to use liquid lubricants such as oil or fat. This is the case of bushings used to protect the variable-pitch blade roots, in aircraft jet engine compressors.

These bushings are generally mounted tightly in orifices formed in the stator housing of the compressor. They receive the feet of the variable-pitch blades of the compressor.

The sleeve / blade root assemblies are subjected to numerous friction related to the pivoting of the blades inside the bushes or to the vibrations caused by the operation of the turbojet engine. The bushings are made of a material "softer" than that of the pivots so that they wear out in priority and thus protect them.

In order to limit the wear of said bushings (and thus their replacement frequency), it is advantageous to reduce the friction at the contact surfaces between these bushings and the blade pivots. For this reason, these sleeves are made of a self-lubricating solid material, by sintering from an intimate mixture of powders.

Such a mixture generally comprises a powder of a metal alloy, precursor of the matrix of the self-lubricating material and particles of a solid lubricant, stable at the temperatures of production and use of the material so that they do not react with said metal alloy and remain intact so as to exert their lubricating action. Of course, the greater the proportion of these particles in the mixture, the better the self-lubricating properties of the final material (by final material means the material made from said mixture of powders).

However, the plaintiff company has been able to note that beyond a certain proportion of this type of solid lubricant in the intimate mixture, evaluated at 10% by volume, densification problems appear and the mixture of powders is more difficult to sinter. In practice, it is necessary to increase the temperature and duration of the sintering or to use more complex pressing techniques, such as hot isostatic pressing, in order to densify the powder mixture, which results in an increase in the cost price of the parts. manufactured. In all cases, the final material has a high porosity and its mechanical properties are affected.

Moreover, beyond a limit proportion of lubricant evaluated at 15% by volume, it is generally found that it becomes very difficult or impossible to sinter the powder mixture.

• une poudre d'un alliage métallique, comme par exemple du fer, du cuivre ou du bronze,
• des particules d'un premier lubrifiant solide comme par exemple du polytétrafluoroéthylène (PTFE), et
• des particules d'un deuxième lubrifiant solide comme par exemple du bisulfure de molybdène (MoS₂).

Document UA-2 964 476 discloses a sinterable powder mixture, comprising:

• a powder of a metal alloy, such as iron, copper or bronze,
• particles of a first solid lubricant such as polytetrafluoroethylene (PTFE), and
• particles of a second solid lubricant such as molybdenum disulfide (MoS ₂ ).

The object of the present invention is to provide a mixture of powders which can be sintered easily and which makes it possible to produce a material having good self-lubricating properties.

To achieve this object, the invention firstly relates to a mixture of powders capable of being sintered to form a self-lubricating solid material, characterized in that it comprises a powder of a metal alloy, a precursor of the matrix of said solid self-lubricating material. particles of a first solid lubricant for insertion into said matrix without interacting with said metal alloy during sintering of the powder and particles of a second solid lubricant for reacting with a component of said metal alloy during sintering powder to form a lubricating phase.

The invention therefore lies in the use of two types of solid lubricants, having different modes of integration in the matrix of the final material. Due to this difference, it has been found that a mixture comprising x% of first solid lubricant and y% of second solid lubricant is more easily sinterable than a mixture comprising only one of the two types of lubricant in a proportion of x + y%. .

Advantageously, to facilitate the sintering of said mixture, the proportion of the first solid lubricant in this mixture is of the order of or less than 15% by volume and, preferably, of the order of or less than 10% by volume. Likewise, the proportion of the second solid lubricant in said mixture is of the order of or less than 15% by volume and, preferably, of the order of or less than 10% by volume.

Advantageously, to obtain good self-lubricating properties for the final material, the sum of the proportions of the first and second solid lubricants is greater than 10% by volume and, preferably, greater than 15% by volume.

Thus, it may be advantageous to choose proportions of first and second solid lubricant each between 5 and 10% by volume, the sum of said proportions being greater than 10% by volume, or even 15% by volume.

A second object of the invention is a method for producing a self-lubricating solid material, characterized in that it comprises the steps of: producing a mixture of powders of the type of the one previously described, according to the first object; intimately mixing said mixture (i.e. making a very uniform mixture); and sinter the intimate mixture obtained.

Advantageously, to facilitate the agglomeration of the particles of the powder mixture, a binder is added to said intimate mixture.

The intimate mixture thus produced can then be molded by pressing or injection into a mold so as to form a blank of the part that it is desired to manufacture. This blank is then extracted from the mold, then the binder is discharged in a conventional manner during a catalytic or thermal debinding step, and said blank is finally densified by sintering. This method makes it possible to mass-produce very complex shape parts from the mixture of powders of the invention, and thus makes it possible to reduce the cost price of said parts.

A third subject of the invention is a self-lubricating solid material comprising a metal alloy matrix and particles of a solid lubricant, inserted in said matrix, characterized in that it further comprises a lubricating phase comprising a sulfurized compound. with hexagonal structure.

A fourth object of the invention is a mechanical part, characterized in that it is made of a material of the type previously described, according to the third object.

Advantageously, this mechanical part is a socket intended to receive a jet engine airfoil compressor blade, variable pitch.

The invention is defined in claims 1, 11, 12 and 15 and its advantages will be better understood on reading the detailed description which follows. This description refers to Figures 1 and 2 appended which respectively represent a first and a second type of assembly comprising a bushing for turbojet compressor blade.

It is possible to choose as metal alloy, precursor of the matrix of the final self-lubricating solid material, an iron, nickel or cobalt base alloy. As an example of a nickel base alloy, mention may be made of an alloy of the Astroloy® (registered trademark) type and more particularly a grade comprising 17.3% of cobalt, 14.3% of chromium, 4% of aluminum and 3.5 % of titanium. Another example of a base iron alloy is an alloy of the TY355® type (registered trademark) comprising 1.23% carbon, 4.05% vanadium, 4.68% chromium, 4.45% molybdenum and 5.46% tungsten. These two alloy examples are chosen for their resistance to oxidation at high temperatures and their mechanical properties, in particular their hardness greater than 400 HV.

Cerium trifluoride CeF ₃ is chosen as the first solid lubricant. CeF ₃ is a rare earth byproduct that exhibits good wear behavior, particularly due to its hexagonal lamellar structure. In addition, the CeF ₃ has good behavior at high temperatures, up to 1000 ° C, which makes the mixture of powder (or the self-lubricating solid material made from this mixture) particularly suitable for the production of mechanical parts subjected at high operating temperatures, such as turbojet compressor blade sockets.

To ensure that the first solid lubricant particles fit easily into the metal matrix of the final material, the average particle size of the first solid lubricant is selected based on the average size of the metal alloy particles.

In order for the powder mixture to be molded by pressing or injection into a mold, the average size of the metal alloy particles is preferably between 5 and 100 micrometers. In this case, the average particle size of the first solid lubricant is chosen to be less than 50 microns, so as to allow the particles of first lubricant solid to form agglomerates of different sizes capable of being inserted into said matrix.

According to a third aspect of the powder mixture of the invention, one can choose as the second solid lubricant tungsten bisulfide WS ₂ or molybdenum disulfide MoS ₂ .

These compounds belong to the family of dichalcogenides and have a hexagonal lamellar structure. They react with the precursor metal alloy of the matrix to give rise to at least one lubricating phase comprising at least one hexagonal-structured sulfur compound.

In the case of an alloy comprising chromium, it can be seen, in particular by X-ray diffraction analysis, that a predominant self-lubricating phase of Cr ₇ S ₈ chromium sulphide is formed. In the particular case of an Astroloy® type alloy, titanium and cobalt sulphides are also formed but in a smaller quantity than Cr ₇ S ₈ . In the case of a TY355® type alloy, chromium and vanadium sulfides are formed.

The greater the proportion of the second solid lubricant in the mixture, the more hexagonal-structured compounds are formed and the better the self-lubrication properties of the final material.

From the point of view of the particle size of the second solid lubricant, good results are obtained when the latter remains less than 50 micrometers.

In the particular case of a mixture comprising an Astroloy® type alloy powder, CeF ₃ as the first solid lubricant and WS ₂ as the second solid lubricant, it is found, in accordance with what has been previously described, that above 10% by volume of CeF ₃ in the mixture, difficulties in sintering and densification become important. As these difficulties become tangible from 7%, it may be preferable to keep the proportion of CeF ₃ and more generally the proportion of first solid lubricant, below 7% by volume.

On the other hand, it is also found that above 10% by volume of WS ₂ in the mixture, difficulties of sintering and densification appear.

The self-lubricating properties of the final solid material can be evaluated by measuring the coefficient of friction between this final material and a reference material. These properties are interesting from the at which time the sum of the proportions of CeF ₃ and WS ₂ exceeds 10% by volume and, advantageously, 15% by volume.

Thus, good sintering and lubrication results are observed with a powder mixture comprising: 5 to 10% or 5 to 7% by volume) of CeF ₃ ; from 5 to 10% by volume of WS ₂ ; and such that the sum of the proportions of CeF ₃ and WS ₂ exceeds 10% or even 15% by volume.

On the other hand, since the lubricating properties of the first solid lubricant and the lubricating phase are temperature dependent, it is possible to ensure that the temperature ranges under which these lubricating properties are optimal are not superimposed. To illustrate this, in the above example the lubricating properties of the Cr ₇ S ₈ phase are optimal at temperatures of the order of or less than 250 ° C, while the lubricating properties of CeF ₃ are optimal at temperatures In this manner, the solid material made from the powder mixture has sufficient self-lubricating properties regardless of the temperature at which it is used.

It is also possible to ensure that these self-lubricating properties are substantially constant over a large temperature range, for example from 100 ° C to 400 ° C.

The composition of the mixture of powders according to the invention and the self-lubricating material obtained from such a mixture being well understood, we will now describe an example of a mechanical part that can be made from this material, with reference to Figures 1 and 2 .

These figures represent a variable-pitch vane 3 on an aircraft turbojet compressor stator housing 5.

The so-called "stationary" vanes 3 of the stator are arranged radially at regular intervals inside the casing 5. They are fixed to the casing 5 by their feet 7 with a certain wedging angle which determines the direction of passage of the casing. air through the compressor. The blades 3 are called variable pitch because they can rotate around their foot 7 so as to vary the wedging angle.

Openings 9 are formed in the housing 5 to receive the blade roots 7, these openings 9 and these feet 7 being cylindrical. To limit the friction between each foot 7 and the casing 5, there is between these two elements of the sockets 11 made of a self-lubricating solid material according to the invention.

It is possible to use one or two sockets 11 per opening 9, as shown in the figures. These bushings are mounted tight inside the opening 9 so as to remain integral with the housing 5 even when the latter expands at high temperature. Each bushing 11 has a flange 11a which borders the opening 9 on the inner or outer face of the housing 5.

The bushings 11 are intended to protect the housing 5 and the blade roots 7, by wearing instead of these elements and when these sleeves 11 are too worn, they are replaced.

As shown figure 2 it is also possible to arrange around the foot 7 a ring 13 on which the sleeve 11 will come to rub. This ring 13 is tightly mounted around the foot 7 and aims to protect the latter.

The friction forces involved in the previous montages depend, of course, on the pairs of materials in the presence. When the bushing is made of a material according to the preceding examples (alloy metal matrix of the Astroloy® or TY355® type, solid lubricant CeF ₃ and self-lubricating phase Cr ₇ S ₈ ) the blade roots 7 can be made of an alloy base metal iron, nickel or titanium, and the rings 13, if any, a metal alloy base iron, nickel or cobalt.

Claims (16)

1. A powder mixture suitable for sintering to form a self-lubricating solid material, comprising a powder of a metal alloy that is a precursor for the matrix of said self-lubricating solid material, particles of a first solid lubricant for insertion in said matrix without reacting with said metal alloy during sintering of the powder, and particles of a second solid lubricant for reacting with a component of said metal alloy during sintering of the powder in order to form a lubricating phase, characterized in that the particles of the first solid lubricant are particles of cerium trifluoride CeF₃.
2. A powder mixture according to claim 1, characterized in that the content of the first solid lubricant in said mixture is equal or less than about 15% volume.
3. A powder mixture according to claim 2, characterized in that the content of the first solid lubricant in said mixture lies in the range 5% to 10% by volume.
4. A powder mixture according to any one of claims 1 to 3, characterized in that the content of the second solid lubricant in said mixture is equal to or less than about 15% by volume.
5. A powder mixture according to claim 4, characterized in that the content of the second solid lubricant in said mixture lies in the range 5% to 10% by volume.
6. A powder mixture according to any one of claims 1 to 5, characterized in that the sum of the contents of the first and second solid lubricants is greater than 10% by volume.
7. A powder mixture according to any one of claims 1 to 6, characterized in that said second solid lubricant is tungsten disulfide WS₂ or molybdenum disulfide MoS₂.
8. A powder mixture according to any one of claims 1 to 7, characterized in that said lubricating phase comprises at least one sulfur compound of hexagonal structure.
9. A powder mixture according to any one of claims 1 to 8, characterized in that said metal alloy is an alloy based on iron, nickel, or cobalt.
10. A powder mixture according to any one of claims 1 to 9, characterized in that said component of said metal alloy is chromium and said lubricating phase comprises Cr₇S₈.
11. A method of preparing a self-lubricating solid material, characterized in that it comprises the steps consisting in: making a powder mixture according to any preceding claim; intimately mixing said mixture; adding a binder to the resulting intimate mixture; molding the intimate mixture by pressing or injection in a mold in order to form a blank; extracting the molded blank from the mold; evacuating said binder; and densifying said blank by sintering.
12. A self-lubricating solid material comprising a matrix of metal alloy and particles of cerium trifluoride CeF₃ as a solid lubricant inserted in said matrix, the material being characterized in that it further comprises a lubricating phase comprising a sulfur compound of hexagonal structure.
13. A self-lubricating solid material according to claim 12, characterized in that said metal alloy is an alloy based on iron, nickel, or cobalt.
14. A self-lubricating solid material according to claim 12 or claim 13, characterized in that said sulfur compound of hexagonal structure is Cr₇S₈.
15. A mechanical part, characterized in that it is made from a material according to any one of claims 12 to 14.
16. A mechanical part according to claim 15, characterized in that it consists of a bushing (11) for receiving a root (7) of a variable-pitch vane (3) of an airplane turbojet compressor.

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