Patent Grant
7147726
The subject matter of the subject invention is related to application Ser. No. 10/343,009 filed on May 5, 2003, in the names of Jian LU and Ke LU, “METHOD FOR GENERATING NANOSTRUCTURES AND DEVICE FOR TREATING NANOSTRUCTURES”, the subject matter of which is incorporated herein in its entirety.
1. Field of the Invention
The present invention concerns a mechanical method for generating nanostructures on metal pieces and a mechanical device for generating nanostructures.
Nanocrystalline materials are characterized by ultrafine grains, typically with at least one dimension of less than 100 nm. These materials are produced using known methods such as, for example, IGC (inert gas condensation and consolidation), SPD (severe plastic deformation), etc. These methods have the drawback of generating materials that are neither porosity-free nor contamination-free, nor of sufficient size for industrial applications.
The purpose of the method of the invention is to create on the surface of the material a layer of this same material having constituent grains of several tens of nanometers, forming what is commonly called a layer of nanometric microstructures, or nanostructures.
There are conventional peening methods that are known in the prior art. Peening the surface of a material, for example a metal, consists of shooting projectiles of small size, for example balls, at speeds between 5 and 100 mps. In this prior art, the balls are projected by means of a blast of compressed air. In this method of peening, the balls are not immediately reused, and pass through a recycling device before being fed back to the blast nozzle, or in the absence of a recycling device, the method requires a large quantity of balls. Moreover, each blast that is incident on the piece is unidirectional, at a given angle for a given surface. Furthermore, a continuous sweep of the piece is required during the peening in order to obtain a homogeneous surface. In addition, the results obtained show that the surface of the treated piece comprises few or no nanostructures. The only advantage of the conventional peening process resides in the fact that it is possible to obtain higher ball speeds than in the generation of nanostructures using ultrasound. In essence, projecting balls using ultrasound makes it possible to obtain ball speeds between 5 and 20 mps, whereas pneumatic blast nozzle peening makes it possible to obtain ball speeds between 10 and 100 mps.
2. Description of the Related Art
There is also a method for ultrasonic hardening of metal pieces known from French patent application 2,689,431 or Russian patent application 1,391,135, which consists of setting balls in motion inside an enclosed volume for a given amount of time, using an ultrasound generator. In the method of the French patent application, it is possible to obtain, as a function of the speed, either a given roughness, or a given hardened layer depth. In order to obtain a uniform treatment, the travel speed of the transmitter must conform to a certain value, below which there is strain hardening of the surface and above which the treatment is no longer uniform, meaning that some point on the surface will not have been hit even once. The speeds envisaged in this patent application are no more than several tens of centimeters per second, and the amplitude of the transmitter is 100 μm. Thus, the known operating mode does not make it possible to create a hardened layer without obtaining a nanometric structure to a significant depth.
The object of the present invention is to eliminate the drawbacks of the prior art by offering a method for generating nanostructures that makes it possible to obtain a nanostructured layer in a given thickness of the surface of a piece to be treated by means of a mechanical device using a limited quantity of balls in an enclosed volume. This method is also called surface nanocrystallization by ball milling.
This first object is achieved using a mechanical method for generating nanostructures to obtain on a surface of a metal piece a nanostructured layer of defined thickness, characterized in that it comprises:
a step for setting in a circular motion a quantity of perfectly spherical balls (22) disposed in a chamber that is closed for the size of the balls, at least one of whose walls supports or constitutes the piece to be treated (10);
a step for imparting a vibrating motion, in a direction perpendicular to the plane of the circulation motion of the chamber supporting or constituting the piece to be treated; the speed of the circular motion and the frequency and amplitude of the vibrating motion being determined based on the physical properties of the balls so as to communicate to the latter sufficient kinetic energy to create nanostructures on the material of the piece to be treated.
Another object of the invention consists of offering a mechanical device for generating nanostructures that makes it possible to obtain a nanostructured layer in a given thickness by means of a mechanical device using a limited quantity of balls in an enclosed volume. This object is achieved by means of the mechanical device for generating nanostructures in a metal piece comprising at least one chamber that is closed for the size of the balls, containing a given quantity of perfectly spherical balls of given dimensions, a means for linking the chamber to means for generating a vibration communicated to the chamber, the assembly of the chamber and vibrating means being mounted by shock absorbing means (31) on a plate (30) that rotates at a given speed.
The invention and its characteristics and advantages will emerge more clearly with the reading of the description given in reference to the attached drawings, in which:
The principle of the invention is to perform a treatment of the surface of a metal piece in order to modify the mechanical properties of the metal piece and to modify the diffusion properties in the superficial layer of the treated surface.
In the prior art, the mechanical properties of nanometric microstructures, or nanostructures, are well known. In essence, the smaller the size of the metal grains, the greater the mechanical strength of the piece. Thus, current research is seeking to develop methods of fabrication for obtaining pieces constituted only of nanostructures. The subject of the invention is entirely different; it consists of using a process for generating nanostructures (described below) to create a superficial nanostructured layer that gives the surface of the piece the desired properties, for example mechanical properties, this being enough to guarantee the properties desired for the piece (fatigue strength, abrasion or friction resistance, corrosion resistance, etc.).
In order to obtain a nanostructure, it is necessary to reduce the size of the metal grains on the surface of the piece. Initially, for a piece made, for example, of pure iron, the grains have a dimension on the order of 100 μm. After the treatment according to the invention, the size of the grains is on the order of only several tens of nanometers.
In order to reduce the size of the grains on an entire surface, it is necessary to create on the surface of the material a plastic deformation in all directions in a random fashion, according to the processes and with the devices defined in connection with
The balls (22) chosen to strike the surface (10) to be treated are perfectly spherical and of high quality. For example, the balls (22) chosen are ball bearing balls. Given their quality, they are used in a given quantity. The use of perfectly spherical steel, glass or ceramic balls (22) makes it possible to prevent the localized accumulation of stress which could cause damage to the material from the impact of the ball. This perfect sphericity makes it possible to generate a plastic deformation of the surface of the material during the process for forming the nanostructured layer. Repeated multidirectional plastic deformations cause a fractionation of the grains of the metal or alloy of the piece to be treated, and hence a reduction in their size. An exemplary embodiment of the invention represented in
In a variant of embodiment, it is possible to attach the piece to any other wall of the bowl, and possibly to have the piece play the role of the bowl if its geometry lends itself to that. Likewise, it is possible to provide a space between the bowl and the cover, as long as it does not exceed the size of the balls. Thus, the chamber is considered to be closed for the size of the balls.
In another variant of embodiment of the invention, the inertial system (33, 34, 35) can be replaced by a sonotrode that communicates to the arm (32) a vibration of sufficient amplitude and frequency. In this case, it would be quite possible to use frequencies that are outside the ultrasonic spectrum, the frequency of the vibration being determined based on the rotation speed of the assembly of the mass and on the size of the balls in order to allow the communication of sufficient kinetic energy to the balls.
Lastly, when wishing to obtain a nanostructured thickness from several tens to several hundred microns, the surface of the piece to be treated can be placed under stress.
According to a variant of embodiment represented in
The invention is not limited to the embodiments described, but encompasses any embodiment that makes it possible to apply mechanical stress to one or more places on a piece. Thus, several rods can be provided in order to apply different stresses in several places so as to obtain different nanostructured thicknesses, proportional to the value of the stresses applied at the respective points.
In the embodiment of the stressing device represented in
In another variant of embodiment, the stress applied can be thermal. Thus, the surface to be treated is heated, either completely so as to obtain a uniform thickness of nanocrystalline structures on the entire surface of the piece subjected to the ball bombardment, or locally so as to obtain variations in the thickness of nanocrystalline structures. In this case, means for heating by radiation, conduction or convection are installed in the bowl, on the piece, or in the acoustic chamber of the machine.
In addition, it is possible to combine the mechanical stress with a heating of the surface to be treated in order to obtain the desired result. The purpose of applying stress and/or raising the temperature is to allow the generation of the plastic deformation in the underlying layer and in all directions, so as to promote the fractionation of the grains of material located at a depth.
Tests currently performed by treating an unstressed piece have made it possible to produce nanostructured layers running from 20 μm to 50 μm by applying stress. Nanostructures are thereby obtained in a thickness of several hundred microns. The increase in the thickness of the nanostructured layer can be produced by finding a compromise between the value of the stress and the increase in the temperature. The choice of the various parameters involved in the method for generating nanostructures is also important.
Thus, experience has shown that the larger the diameter of the balls used, within a range of dimensions on the order of a few hundred microns to a few millimeters, the larger the nanostructured layer obtained. Likewise, the treatment time is involved in determining the thickness of the nanostructure. It has been noted that up to a given time value, which is different depending on the size of the balls, the more the time increases, the more the thickness of the nanostructured layer increases up to a time that corresponds to saturation and allows no further modification of the thickness of the layer. This given value is obtained either through experience, or from a mathematical model for a given material. However, when the time becomes greater than the given value, the thickness of the nanostructured layer decreases. This phenomenon is due to the fact that the impact of the balls on the surface to be treated generates an emission of heat, which heats up the material. Beginning at a certain threshold, the result of the heat is to increase the size of the metal grains.
The general principle for choosing the parameters of the method for generating nanostructures according to the invention is that the greater the kinetic energy of the balls, the greater the level of stress generated in the underlying layer. The upper limit of the kinetic energy is defined particularly by the heating caused by the release of this kinetic energy during the impact on the surface to be treated, and by the mechanical strength of the balls and of the material constituting the piece. This drawback can be mitigated or eliminated by cooling the chamber or the piece with a cooling system. In essence, as explained above, a temperature increase tends to enlarge the metal grains, and the material must not crack.
Other parameters may be acted on in order to obtain larger nanostructured layers or to reduce the treatment time. For example, the hardness of the balls plays a role, particularly in the transfer of kinetic energy from the ball to the surface of the piece. Likewise, when an ultrasonic generator is used to set the balls in motion, the acoustic pressure generated by the sound waves also influences the nanostructure generating process. Likewise, according to the invention, the ultrasonic generation of nanostructures the projection of streams of balls can be achieved in a medium containing a specific given gas that modifies the mechanical behavior or the chemical composition of the surface of the material during the impacts of the balls.
For example, in order to obtain a nanostructured layer of about 20 μm, it is necessary to expose the surface to be treated to an ultrasonic generation of nanostructures for 2 to 3 minutes using balls of 3 mm in diameter. Likewise, in order to obtain a nanostructured layer of about 10 μm, it is necessary to expose the surface to be treated to an ultrasonic generation of nanostructures for about 400s with balls of 300 μm in diameter. Likewise, experience has shown that the treatment time for generating nanostructures is between 50 and 1300s for conventional metal materials (Fe, Ti, Ni, Al, Cu, etc.). The total time required may be prolonged or reduced depending on the material. The diameter of the balls used is between 300 μm and 3 mm. In essence, for a given ball size and a given material, the nanostructure generating time is determined based on the nanostructured thickness desired by the user.
Finally, if necessary for acoustical or security reasons, the entire mechanism can be disposed inside an acoustic chamber (25), making it possible to reduce the noise so as to make it compatible with the acceptable standards for this work. This chamber (25) can be sealed and provided with diffusing or vaporizing means (26) (represented by dotted lines), making it possible to perform one or more of the chemical or thermochemical treatments described below. In this case, the bowl, because of its circulation channel (204a, 204b) allows the penetration of chemical or thermochemical treatments. Thus, depending on the physicochemical properties of the piece to be treated, it may be useful to treat it once either in a vacuum, or in an inert atmosphere, for example in order to prevent oxidation, then again with the diffusion of specific chemical compounds that are advantageous for the piece. The generation of nanostructures on the treated surface of the piece (10) causes a modification of the law of diffusion in the treated area by multiplying the number of boundaries between the grains, these boundaries constituting nanometric channels that allow the diffusion of chemical compounds having a size on the order of several atoms. This allows a better penetration of the chemical compounds.
Thus, all of the surface treatment methods involving the diffusion of compounds into the surface of a metal piece are modified when the piece has been previously subjected to the method for generating nanostructures according to the invention, or is subjected to the method for generating nanostructures at the same time.
As an example,
The curve represented in
For the piece pre-treated by generating nanostructures according to the invention, the level of nitrogen is 17 times greater than the level in the piece whose surface is untreated. After that, the level of nitrogen gradually decreases in the thickness of the piece comprising the nanostructure, and ends up being equal to the level obtained according to the nitriding process of the prior art where the layer of the piece does not include any nanostructures.
It should be noted that the nitriding process according to the prior art can only be performed beginning at a certain temperature, near 550° C. for a piece made of steel or carbon. It is therefore clear that pre-treating the piece makes it possible not only to obtain a good structure on the surface of a piece, but also to lower the treatment temperature while retaining, in the case of the treatment at 350° C., a level of nitrogen greater than the level obtained without treatment by generating nanostructures according to the invention.
Thus, with the lowering of the treatment temperature, it becomes possible to perform nitriding on pieces which, in the prior art, could not be subjected to nitriding. In fact, nitriding must be performed at a temperature of approximately 550° C., although at this temperature a metal piece is necessarily subject to deformation. For pieces in which geometric precision is essential, such deformation is unacceptable, and consequently prohibits any nitriding according to the method of the prior art. By applying the method for generating nanostructures according to the invention prior to the nitriding, it is possible to lower the treatment temperature and hence to reduce or eliminate the deformation of the piece. As a result, precision pieces can be subjected to nitriding, which was impossible in the prior art.
Likewise, the pre-treatment according to the method of the invention for generating nanostructures also makes it possible to reduce the treatment time. In fact, the presence of nanostructures, and in particular nanometric diffusion channels, allows faster diffusion of the compounds into the superficial layer of the piece.
That which is explained above for nitriding is also true for any surface treatment or physicochemical surfacing method that depends on the law of diffusion in the superficial layer of a piece. Thus, methods for case hardening or ion catalysis or storage in a metal structure are modified when the piece has been previously subjected to the process for generating nanostructures according to the invention, i.e., when it comprises a layer of nanometric microstructures in a thickness of ten or several tens of microns.
Thus, the mechanical method for generating microstructures according to the invention is characterized by the fact that it comprises:
a step for setting in a circular motion a quantity of perfectly spherical balls (22) disposed in a chamber that is closed for the size of the balls, at least one of whose walls supports or constitutes the piece to be treated (10);
a step for imparting a vibrating motion in a direction perpendicular to the plane of the circulating motion of the chamber supporting or constituting the piece to be treated; the speed of the circular motion and the frequency and amplitude of the vibrating motion being determined based on the physical properties of the balls so as to communicate to the latter sufficient kinetic energy to create nanostructures on the material of the piece to be treated.
In another embodiment, the method includes a step for placing the metal piece (10) to be treated under mechanical and/or thermal stress.
In another embodiment, the method includes a step for treatment by diffusion of chemical compounds and by formation of new phases of materials of different composition in the nanostructured layer generated, either during the generation of the nanostructures or after the generation of same.
In another embodiment, the treatment time is between several seconds and 10 hours.
In another embodiment, the size of the balls varies from 3 to 10 mm.
In another embodiment, the treatment step is a nitriding comprising a placement of the piece (10) to be treated in a nitrogen atmosphere, at a given temperature between 350° and 550° C., for a given amount of time between 30 min. and 10 h.
In another embodiment, the treatment step is a case hardening or an ion catalysis or storage in the metallic structure of the piece.
In another embodiment, the step for imparting a vibrating motion is performed by means of an electronic vibrator whose waves cause the chambers to move in the desired direction.
In another embodiment, the vibrator is an ultrasonic generator.
In another embodiment, the diameter of the perfectly spherical balls (22) is between 300 μm and 3 mm, depending on the desired thickness of the nanostructured layer.
In another embodiment, for a given ball size, a given material constituting the piece (10) and a given machine configuration, the projection time is determined based on the nanostructured thickness desired by the user.
In another embodiment, the projection time of the balls (22) is between 30 and 1300s.
In another embodiment, the method includes a step for cooling the piece to be treated.
In another embodiment, the speed of the balls is between 5 and 100 mps.
Finally, the mechanical device for generating nanostructures in given thickness of a metal piece (10) comprising at least one chamber that is closed for the size of the balls containing a given quantity of perfectly spherical balls of given dimensions, a means for linking the chamber with means for generating a vibration communicated to the chamber, the assembly of the chamber and vibrating means being mounted by shock absorbing means (31) on a plate (30) that rotates at a given speed.
In another embodiment, the device includes means for adjusting the rotation speed of the plate and means for adjusting the frequency and the amplitude of the means for adjusting the vibration.
In another embodiment, the vibration generating means is an ultrasonic generator.
In another embodiment, the vibration generating means is constituted by an inertial assembly (34) driven in rotation around a shaft (33) perpendicular to the axis of rotation of the plate, the inertial assembly being mechanically linked to the means (32) for linking with the chamber (20a, 20b).
In another embodiment, the device includes means for placing the metal piece (10) under stress and/or means for heating the piece (10).
In another embodiment, the device includes means for adjusting the distance (d) between the ball emitting source and the piece to be treated.
In another embodiment, the distance is on the order of 4 to 40 mm.
In another embodiment, the device includes means for adjusting the emission time of the balls and their speed.
In another embodiment, the balls are of a quantity such that, when the means for setting them in motion using ultrasound are inactive, they occupy a surface area greater than 30% of the surface of the sonotrode.
In another embodiment, the speed is between 5 and 100 mps.
In another embodiment, the device includes means for performing a local cooling of the treated area of the piece.
In another embodiment, the device includes means for treatment by diffusion of chemical compounds in the nanostructured layer generated, either during the generation of the nanostructures or after the generation of same.
In another embodiment, the device includes means for placing the piece (10) to be treated in a nitrogen atmosphere, at a given temperature between 350 and 550° C., for a given amount of time between 30 min and 10 h.
In another embodiment, the device includes means for case hardening, carbonitration and other thermochemical treatments.
In another embodiment, the device is enclosed in an acoustic isolation chamber (25).
It should be clear to those skilled in the art that the present invention allows for embodiments in many other specific forms without going beyond the scope of application of the invention as claimed. Consequently, the present embodiments should be considered illustrative, but can be modified within the range defined by the scope of the attached claims, and the invention should not be limited to the details given above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 00 09949 | Jul 2000 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR01/02483 | 7/27/2001 | WO | 00 | 6/2/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/10463 | 2/7/2002 | WO | A |
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| 2460657 | Robinson | Feb 1949 | A |
| 4628834 | McKelvie | Dec 1986 | A |
| 5532495 | Bloomquist et al. | Jul 1996 | A |
| Number | Date | Country |
|---|---|---|
| WO 95 23876 | Sep 1995 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20040038626 A1 | Feb 2004 | US |
