FR2876390A1 - Aluminium alloy component ion implantation system for nitriding uses electronic cyclotronic resonance source producing multi-energy ions - Google Patents

Abstract

The nitrogen ion implantation system for an aluminium component (5) uses ions emitted by an electronic cyclotronic resonance source producing multi-energy ions that are implanted at a temperature below 12 degrees C. In addition to regulators (7 - 11) controlling the initial ion beam (f1') the system has a second set of regulators (1, 4, 12) for the relative positions of the component and the ion source (6), one of which (12) is a holder that enables the component to be moved during treatment, equipped with a cooler (12) to remove heat generated in the component during the process.

Description

2876390 1

  FIELD OF THE INVENTION The invention relates to a method of nitriding a metal part by ion implantation of this part. This process consists in implanting nitrogen atoms in the crystal structure of a metal part, thanks to a beam of ions emitted from an ion source. The invention also relates to a device for implementing this method.

  The invention has applications in the technical fields where it is sought to perform a surface treatment of a metal part, in particular in the field of plastics, for the production of metal molds for the mass production of parts. molded plastics.

  STATE OF THE ART In the field of plastics, most plastic parts are made by molding in metal molds. Currently, most of these molds are steel. Indeed, steel is a solid material having good mechanical strength over time. Each steel mold thus makes it possible to produce a large number of plastic parts of the order of 500,000 to 1,000,000 units. The number of plastic parts per steel mold is in the range of 500,000 to 1 million. However, steel is a difficult material to process which, therefore, does not allow a quick setting. of production on the market. It also does not allow great flexibility of shape, while the current trend is to frequently change the shape of plastic parts and, therefore, the shape of the injection molds. For these reasons, the machining and time cost of a steel mold is relatively high.

  It is therefore increasingly sought in the field of plastics to produce injection molds in a metal other than steel. Aluminum alloys are one of these metals. Indeed, the aluminum alloy has the advantage of having excellent machinability, that is to say to allow machining at high speed. The aluminum alloy also has a high heat exchange capacity, which leads to a faster 2876390 2 cooling of the plastic part, as well as a great lightness, thus easier handling. The aluminum alloy has, at equal volume, a cost substantially comparable to that of steel. However, the aluminum alloy molds have a mechanical strength limited in time, resulting in a low production capacity compared to that of steel. The number of plastic parts made by aluminum alloy mold is of the order of 1000 units. In addition, on aluminum alloy molds, phenomena of erosion of the molding surface, matting of the joint plane or corrosion appear faster than on steel molds.

  Manufacturers of aluminum alloy injection molds seek to solve this problem by improving the surface mechanical strength of these molds. For this, they seek to increase the wear resistance by increasing the surface hardness and lubrication (lower coefficient of friction) and strengthening the corrosion resistance, mainly due to chlorine attacks.

  Various chemical processes are known to improve the mechanical strength of aluminum molds.

  One of these processes consists of anodizing the aluminum mold. Anodizing is an electrolytic process for thickening the natural alumina layer (Al 2 O 3) to thicknesses of the order of 20 microns. This layer of alumina is hard but very brittle (a toughness substantially identical to that of glass), has a high coefficient of thermal expansion and has a sensitivity to chlorine attacks, resulting in great fragility with regard to thermal fatigue and corrosion.

  Another method is hard chrome plating. This process is an electrolytic treatment of aluminum molds that allows them to harden. However, this process poses problems of homogeneity of thickness on the edges of the molds. In addition, it requires a so-called surface-removal surface preparation (creation of micro-grip patches of 7 to 8 microns) whose quality depends on the know-how of the subcontractor, or a bad reputation among the molders.

  Another process is nickel plating. This process consists of a uniform deposition of a Teflon impregnated nickel layer to lubricate the surface.

  However, the impregnation of nickel with Teflon requires the maintenance of the mold 2876390 3 for several hours at a temperature of 250 C, fatal to the mechanical properties of aluminum alloys. Without teflon, so without lubrication, the nickel layer in turn presents risks of delamination.

  Another method is the vapor phase deposition of chromium nitride.

  This method poses a problem with respect to the adhesion of the chromium nitride layer, which is of poor quality due to the low permissible application temperature (beyond which the mechanical properties of the substrate are destroyed. ).

  There is, moreover, another problem related to the type of materials to be treated, namely aluminum alloys. Indeed, aluminum alloys contain hardening precipitates obtained by thermal incomes of between 120 and 150 C. Also, is it impossible to use aluminum alloys at temperatures above 120 C, either during treatment or industrial exploitation.

  Another known method is thermal nitriding. It consists in cementing with nitrogen a metal piece to obtain a great surface hardness. Generally, this nitriding is carried out thermally, that is to say that the metal part to be treated is heated to a temperature above 500 C in a stream of ammonia gas. At this temperature, the ammonia gas dissolves and diffuses to form nitrides. However, according to what has been said previously, this process is inapplicable to aluminum alloys, again for reasons of temperature.

  DISCLOSURE OF THE INVENTION The object of the invention is precisely to remedy the disadvantages of the techniques described above. To this end, the invention proposes to perform the nitriding of a low temperature metal part, in particular an aluminum alloy mold, by ion implantation. This nitriding by ion implantation involves implanting nitrogen ions, emitted by an ion source, into the crystal structure of the metal part. A beam of nitrogen ions emitted by an ion source makes it possible to selectively treat the surface of a room, only on selected zones, thereby reducing the duration of the treatment and eliminating any risk of heating ( related to implantation energy) above a maximum temperature of 120 C.

  More specifically, the invention relates to a nitriding treatment method of a metal part, characterized in that it consists in subjecting at least one zone of the metal part to a beam of nitrogen ions emitted by an ion source.

  This low temperature process is advantageously used for an aluminum alloy part.

  The process of the invention may also include one or more of the following characteristics: the nitrogen ion beam moves relative to the metal part at a constant or variable rate taking into account the angle incidence of the beam relative to the surface.

  the nitrogen ion beam is emitted with a constant flow rate and emission energy.

  the nitrogen ion beam has an implantation energy in the metal part, modulated as a function of the distance between the ion source and the metal part.

  the nitrogen ion beam is emitted with a variable emission rate and energy, controlled by the ion source.

  the nitrogen ions are implanted in the metal part at a variable depth, as a function of the implantation energy of the ion beam.

  The invention also relates to a device for nitriding a metal part which implements the method of the invention. This device is characterized in that it comprises: an ion source, means for adjusting the ion beam, and means for controlling the relative displacement between the ion beam and the metal part.

  This device may include one or more of the following characteristics: The ion source is an electron cyclotron resonance source.

  - The ion source is a cyclotron.

  The adjusting means comprise optical focusing means and a profitant.

  - The adjustment means comprise a mass spectrometer.

Brief description of the drawings

  FIG. 1 represents two examples of the same metal part treated by ionic nitriding with different implantation energies.

  Figure 2 shows a functional diagram of the device of the invention.

  Figure 3 shows examples of implantation distribution, in an aluminum part, by an ECR source distributing Ni-, N2 + and N3 + ions.

  DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION The method of the invention proposes to perform the nitriding of a metal part, in particular an aluminum part, by using nitrogen ions emitted by an ion source under the form of an ion beam. This process processes the aluminum part locally to enhance its hardness, lubricity and corrosion resistance characteristics.

  This treatment is carried out by implantation of nitrogen ions in the crystalline structure of the metal part.

  Nitrogen ions are produced by an ion source. This source of ions makes it possible to ionize atoms before accelerating them. There is thus obtained, at the output of the source, a type of ions, for example 06+ which is an oxygen atom to which 6 electrons were torn off. It is also possible to obtain nitrogen ions N +, N2 + or N3 +. The ions thus produced can be accelerated by an electric field.

  The ion source in the invention may be an accelerator, in particular a cyclotron, or an electron cyclotron resonance source, referred to as the ECR source. The ions produced by an ECR ion source have a high charge and, as a result, a higher energy for the same acceleration voltage.

  A source of RCE ions consists of two main elements: a magnetic field which confines the ions in a defined volume (situated inside the source) and a high frequency wave injected inside the source; the source and intended to heat the electrons which can then be ionized.

  The interior of the source contains a hot plasma, consisting of a mixture of ions and electrons magnetically confined. The ions can be extracted from the source through an orifice and then accelerated.

  For the production of gaseous ions (oxygen, nitrogen, neon etc.), the selected gas is introduced into the source in an amount sufficient to achieve the intensity of the ion beam required.

  The method of the invention therefore proposes to use nitrogen ions produced in the manner previously explained and to implant them in interstices of the crystalline structure of the metal part. The implantation of these nitrogen ions can be done at varying depths, depending on the needs and the shape of the room. This depth depends on the implantation energy of the ion beam; it can vary from 0 to about 3 pm.

  This implantation of nitrogen ions makes it possible to increase the hardness characteristics of aluminum. For example, for 10% of implanted ions, the hardness of the part is increased locally by a ratio of 200%. In the case of aluminum, an increased hardness of 200% corresponds approximately to an intermediate hardness between that of titanium and that of steel. For 20% of nitrogen ions implanted in the workpiece, the hardness of the workpiece increases by a ratio of 300%. In the case of aluminum, a hardness increased by 300% corresponds to a hardness equal to or even greater than that of steel.

  In an application to aluminum alloy injection molds, the method of the invention makes it possible to obtain molds having a surface hardness close to that of steel, while preserving the massive mechanical properties of the alloy. aluminum. The process of the invention also makes it possible to improve the anti-corrosion characteristic of these aluminum alloy molds. Thus, the production capacity of an aluminum alloy mold, treated with the ion implantation nitriding process of the invention, is greatly increased compared to a conventional aluminum alloy mold.

  According to the method of the invention, a metal part can be treated locally, that is to say zone by zone. Thus, several zones of the same metal part can be treated so as to obtain identical or different hardnesses. The choice of the areas to be treated and the duration of the treatment to be provided to them are a function of their functional specificity (for example the area of the inner contour of the mold joint plane, the area of the molding surface).

  In order to be able to treat the metal part region by zone, a relative displacement between the ion source and the metal part is implemented. In one embodiment of the invention, it is the workpiece that is displaced with respect to the ion source. In another embodiment, it is the source of ions that is displaced with respect to the workpiece; this last embodiment can be implemented when the workpiece is very large.

  Whether the workpiece or ion source is displaced, the relative speed of movement between these two elements can be constant or variable depending on the angle of incidence of the beam relative to the surface, at least during the processing time of the coin area. Speed management can be different for each area to be treated in the room. The speed depends on the flow rate of the beam, the concentration profile of the implanted ions and the number of passes that it is desired to perform. The speed may vary depending on the angle of incidence of the beam relative to the surface, to compensate for the weakness of the implantation depth by an increase in the number of implanted ions.

  As explained above, the method of the invention makes it possible to act on the penetration depth of the ions in the part. This depth of penetration varies according to the ion beam implantation energy, that is to say the input energy of the ions at the surface of the piece. For an implantation in the open air, this energy depends on two variables: the emission energy of the source and the distance between the ion source and the part. Indeed, the air thickness between the part and the ion source plays a role of energy retarder of the incident ions. This is why for the same emission energy, the greater the distance between the ion source and the piece to be treated, the lower the implantation energy is, so less implantation is deep.

  More precisely, taking into account that the emission energy of the ion beam is constant for a given treatment, i.e. for beam f1. The implantation depth (prof2) of the ions in the example P2 is therefore lower than in the example P1 (profl). The implantation zones of the beam f1 on the part 5 are represented in FIG. 1 by a succession of grid ovals. The implantation areas of the beam f2 on the part 5 are represented in FIG. 1 by a succession of hatched ovals.

  In the examples of FIG. 1, the passes P1 and P2 are superimposed so as to allow the implanted zones to merge side by side. More generally, several passes can be made, either with the same implantation energy, to increase proportionally to their number, the concentration of ions in the same implantation zone, or with different energies to allow the melting of zones. implantation at different depths.

  The implantation of these nitrogen ions in the crystal structure of the part 5 has the effect of blocking dislocation sliding planes and thus to hinder their mobility. In other words, the fact of implanting nitrogen ions in the interstices of the crystalline structure of the part 5 makes it possible to block the different crystals between them and thus to increase the hardness of the structure. Under the effect of the stresses applied to the metal part, the deformations, which are irreversible in nature, are made more difficult by the presence of nitrogen ions inserted into the structure. The material then becomes very resistant to wear.

  Moreover, in the application to aluminum alloy injection molds, the nitrogen present in the aluminum has the effect, since it is a base, of reducing the existing acidity in the pits initiated by the ions. chlorides from molded plastics. Thus, the corrosion associated with the propagation of pits is greatly reduced by the method of the invention.

  The method of the invention which has just been described can be implemented by a device, an example of which is shown in FIG. 2. This device is placed in a chamber 3 placed under vacuum by means of a vacuum pump 2. The aim of this vacuum is to prevent the beam from being intercepted by residual gases and to prevent contamination of the surface of the room by these same gases during implantation.

  This device comprises an ion source, for example, a source RCE 6. This source RCE can deliver nitrogen ions for a total current of about 10 mA (all loads N +, N2 +, etc.), under an extraction voltage of approximately 35 KV.

  This device comprises means for controlling the relative displacement between the ion beam and the workpiece. The relative displacement between the ion beam and the workpiece can be obtained by driving the workpiece or the ion source by a machine (for example a table, a lathe, etc.) that can be digitally controlled. .

  As mentioned above, it is possible to choose to move the workpiece or the ion source, depending on the applications envisaged. In the embodiment of Figure 2, it is the workpiece that is moved relative to the source RCE. The workpiece 5 is placed on a numerically controlled machine 4. The movements of the machine 4 are calculated according to one or more axes by a CAD / CAM system (computer-aided design and manufacturing) 1. The result of this calculation is presented in the form of a postprocessor understandable by the machine 4.

  The displacement of the part 5 takes into account: the external and internal contours of the zones to be treated.

  a constant or variable effective displacement speed as a function of the angle of the beam relative to the surface.

  a number of passes for each implantation energy.

  The ECR source 6 emits a beam of nitrogen ions f1 'towards beam control means. These beam adjustment means ensure the focusing and adjustment of the initial beam f1 'emitted by the source into an ion implantation beam f1.

  These adjustment means comprise, from the RCE source 6 to the workpiece 5, the following elements: - a mass spectrometer 7 capable of filtering the ions according to their charge and mass. This element is optional; indeed, in the case of nitriding, it is possible to recover all the monocharged and multicharged nitrogen ions produced by the source.

  lenses 8 whose role is to give the ion beam a chosen shape, for example cylindrical, with a chosen radius.

  a profitor 9 whose role is to analyze the intensity of the beam in a perpendicular section plane. This analysis instrument becomes optional as soon as the lenses 8 are definitively adjusted during the first implantation.

  a current transformer 10 which continuously measures the intensity of the beam without intercepting it. The main function of this instrument is to detect any interruption of the ion beam and to allow the recording of beam intensity variations during the treatment. This instrument can be replaced by a device for measuring the electric currents produced by the implantation of ions in the workpiece.

  a shutter 11 whose role is to interrupt the trajectory of the ions 10 at certain times, for example during a displacement without treatment of the part.

  Control information (inf1) is transmitted from the RCE source 6 to the numerical control machine 4. This control information relates to the state of the beam. In particular, the RCE source informs the machine 4 when the ion beam is ready to be sent. Other control information (inf2) is transmitted by the machine 4 to the shutter 11, to the source RCE 6 and possibly to one or more machines outside the device. This control information may be the values of the ion beam radius, its flow rate and any other known values of the machine 4.

  The operation of the device of the invention is as follows: the workpiece 5 is placed on the numerically controlled machine 4; the enclosure 3 enclosing the device is closed.

  - The vacuum pump 2 is started so as to obtain a high vacuum in the chamber 3.

  as soon as the vacuum conditions are reached, the production and adjustment of the ion beam is carried out by means of adjustment means 7 to 11.

  when the beam is adjusted, the shutter 11 is raised and the numerically controlled machine 4 is launched, which then carries out the displacement in position and speed of the part 5 in front of the beam in one or more passes.

  when the required number of passes is reached, the shutter 11 is lowered to cut the beam, the production of the beam is stopped, the vacuum is broken by opening the chamber with ambient air and the treated mechanical part is recovered.

  FIG. 3 represents an example of distribution of nitrogen ions N implanted in an aluminum piece. In this example, the ion source delivers N +, N2 +, and N3 + ions, all of which are extracted with a single extraction voltage of, for example, 35 KV. Thus the N + ions emitted by the ion source have an energy of 35 KeV, the N2 + ions have an energy of 70 KeV and the N3 + ions have an energy of 105 KeV.

  Given a 75% implantation dispersion for nitrogen ions in aluminum, the N + ions reach a depth of 0.3 μm + 10.22 μm. The N2 + ions reach a depth of about 0.6 pm + 1-0.44 pm and the N3 + ions a depth of about 0.9 pm + 1- 0.66 pm. The maximum distance reached by ions in this example is 1.56 μm.

  The specificity of an RCE ion source that delivers mono and multi-charged ions makes it possible to implant ions of several energies with a single extraction voltage, which makes it possible to obtain a more or less smooth implanting profile. . For example, if we consider an ECR source delivering a total current of 10 mA, for a piece of aluminum whose treated area is 1 cm 2, for about 10 sec, the implantation profile is approximately as follows: 23% of N between 0.08 and 0.05 pm, which corresponds to an increase of the hardness of 300% - 8% of N between 0.5 and 1 pm, which corresponds to a hardness increase of 200%, and - 2% of N between 1 and 1.5 pm, which corresponds to a hardness increase of 35%.

Claims (4)

13 Claims   Process for the nitriding treatment of a metal part (5), characterized in that it consists in subjecting at least one zone of the metal part (5) to a nitrogen ion beam emitted by a source of ions (6).   2 treatment process according to claim 1, characterized in that the metal part is an aluminum alloy part.   Process according to Claim 1 or 2, characterized in that the nitrogen ion beam moves relative to the metal part at a constant speed or a variable speed taking into account an angle incidence of the ion beam relative to a surface of the metal part.   4. Process according to any one of claims 1 to 3, characterized in that the nitrogen ion beam is emitted with a constant flow rate and emission energy.     A treatment method according to claim 4, characterized in that the nitrogen ion beam has an implantation energy in the modulated metal part depending on the distance between the ion source and the metal part.   6 - Process according to any one of claims 1 to 3, characterized in that the nitrogen ion beam is emitted with a variable flow and emission energy, controlled by the ion source.   7 - Process according to any one of claims 1 to 6, characterized in that the nitrogen ions are implanted in the metal part at a variable depth, this depth being a function of the energy of implantation of the beam of ion.   8 - Device for nitriding a metal part implementing the method according to any one of claims 1 to 7, characterized in that it comprises: - an ion source (6), - adjusting means ( 7-11) of the ion beam, and control means (1, 4) for the relative displacement between the ion beam and the metal part.   9 - nitriding device according to claim 8, characterized in that the ion source is a cyclotron.   - Nitriding device according to claim 8, characterized in that the ion source is a source RCE, said device being placed in a vacuum chamber.   11 - nitriding device according to any one of claims 8 to 10, characterized in that the adjusting means comprise optical focusing means (8) and a shutter (11).   12 - nitriding device according to any one of claims 8 to 11, characterized in that the adjusting means comprise a mass spectrometer (7) and / or a profiteer (9) and / or an intensity transformer (10).