EP3956096A1

EP3956096A1 - Method for creating an iridescent visual effect on the surface of a material, devices for carrying out said method, and part obtained thereby

Info

Publication number: EP3956096A1
Application number: EP19721852.2A
Authority: EP
Inventors: Ismaël GUILLOTTE; Baptiste LATOUCHE; Marcos Vinicius LOPES; Jean-Michel Damasse; Francis DIET
Original assignee: Aperam SA
Current assignee: Aperam SA
Priority date: 2019-04-16
Filing date: 2019-04-16
Publication date: 2022-02-23
Also published as: US20220274204A1; JP2022533908A; CA3133730A1; KR20210151217A; WO2020212729A1; MX2021012613A; CN113825587A

Abstract

Disclosed is a method for producing an iridescent visual effect on the surface of a part (1), in which method a laser beam (7) having a pulse duration of less than a nanosecond is sent onto said surface in the optical field of the focusing system (12) of a device comprising a laser source (6), a scanner (11) and said focusing system (12), so as to apply a structure in the form of wavelets having the same orientation to said surface over the width of said pulse, and said scanner (11) scans the surface by means of said laser radiation (7) along a series of consecutive lines (14, 15, 16), or a matrix of points by means of a relative movement of the surface and of the device emitting the laser beam, the width of each line (14, 15, 16) or the dimension of each point of each matrix being equal to the diameter of said pulse, characterized in that between the scan along two consecutive lines (14, 15, 16) or two adjacent points, the polarization of the laser beam (7) is modified so as to create wavelets having different orientations on two consecutive lines (14, 15, 16) or two adjacent points. The invention also relates to devices for carrying out said method and to a part obtained thereby.

Description

Method of producing a visual iridescent effect on the surface of a material, devices for its implementation and part thus obtained

The present invention relates to laser treatments of the surfaces of stainless steel sheets or other materials, intended to give these surfaces an iridescent appearance.

The iridescent treatment, also called "LIPPS" or "ripples" consists of irradiating the surface of a material with pulsed laser radiation of short pulse duration (less than one nanosecond). The diameter of each tap at its point of impact on the material to be treated is typically of the order of 10 to a few hundred µm. If the energy of the incident beam is high enough, this irradiation induces the modification of the structure and / or the reorganization of the surface of the material which will adopt a periodic structure. However, if the energy of the beam is too great, a phenomenon of vaporization / sublimation / shock wave ablation can take place, preferentially or in conjunction with the formation of the periodic surface structure. One can easily determine experimentally what range of energy quantity is to be used for a given material, in order to achieve the desired iridescent effect with or without alteration of surface finish or gloss.

Such a treatment is carried out, in particular, but not exclusively, on stainless steels of all types. The purpose of this treatment may be purely aesthetic, but it also allows to modify the wettability of the surface, and also its resistance to friction and to reduce bacterial adhesion. The treatment can be done directly on the surface of the object on which the passivation layer of stainless steel is located without the need for prior activation / depassivation.

Other materials on which this treatment is performed are, in particular, various metals, polymers such as PVC, ceramics, glass.

In the remainder of the text, the case of stainless steels will be privileged, it being understood that the invention is applicable to all metallic or non-metallic materials which are currently, or would be known in the future, in order to be able to present an iridescent appearance. to a laser treatment carried out as indicated, possibly by adapting the precise operating parameters of the installation (power and frequency of the lasers, etc.) which are known to play a role in obtaining the resulting iridescent appearance of the formation of a periodic surface structure.

Although the exact mechanism of formation of this periodic surface structure has not yet been determined, tests and characterizations carried out by different laboratories show that depending on the number of laser passes and / or pulse energy and / or scan parameters, the surface structure can have one of the following four structures, depending on the total irradiation energy per unit area, these structures being listed in order of energy croissant and their names being usual for those skilled in the art, even non-English speakers:

1) Structure known as "HSFL" (High Spatial Frequency LIPPS):

This structure is made up of small wavelets which, in the case of stainless steels, are oriented in the direction of the polarization of the incident laser beam. The spatial frequency of these wavelets is less than the wavelength of the laser used for the treatment.

2) Structure known as "LSFL" (Low Spatial Frequency LIPPS):

This structure is composed of wavelets larger than the previous ones oriented, in the case of stainless steels, in the direction perpendicular to the polarization of the incident beam. The spatial frequency of these wavelets is slightly less than, or greater than, or equal to the wavelength of the laser. For the treatment of a stainless steel surface with a laser with a wavelength of 1064 nm, the periodicity of the wavelets is of the order of 1 µm. It is still possible to see the HSFL structure in the hollows of the LSFL structure.

Note that for some materials, the respective orientations of the HSFL and LSFL structures can be reversed from what they are for stainless steels.

3) Structure known as "Grooves" or "Bumps":

This structure is made up of micrometric-sized bumps covering the entire treated surface. These bumps are organized in a structure that resembles a "snakeskin" appearance.

4) Structure in peaks or "spikes":

This structure is made up of peaks whose height ranges from a few micrometers to a few tens of micrometers. The distance between the peaks depends on the treatment parameters.

More details on these structures and the mechanism of their appearance can be found in the article “Evolution of nano-ripples on stainless Steel irradiated by picosecond laser puises”, Journal of Laser Applications 26, February 2014, by B. Liu et al. . In particular, it is said there that, for an equal number of pulses, an increase in the fluence of the irradiation leads to obtaining HSFLs rather than LSFLs (as we have just said), whereas for an equal fluence, a number higher picks lead to the creation of LSFL rather than HSFL, until the number of picks becomes too high for ripples to be observed. The exact configuration of the surface after irradiation therefore results from a mechanism bringing into play both the number of pulses received and the energy delivered by each of them, for a given material. This mechanism is complex, but for a given material, conditions for obtaining reliable one or the other of the configurations cited above can be determined experimentally by the user.

In general, in the first two cases, this periodic organization of the surface allows an induced phenomenon, well known to practitioners of laser surface treatments, which is the diffraction of light by the creation of an optical grating when the sample treated is placed under a light source. Depending on the orientations and positions of the user and the light, we can then observe the colors of the rainbow on the sample. This is called an "iridescent look".

This aspect no longer exists when the surface of the sample exhibits a pronounced appearance according to the third and fourth aforementioned cases, since, in these two cases, the energy supplied by the laser source on the surface of the sample has reaches too high a level, at least locally, resulting in deformations of the surface which no longer allow obtaining the iridescent appearance, because the structuring of the surface has lost its periodic character.

This iridescence should not be confused with the colorations of the surface of stainless steels which are obtained, voluntarily or involuntarily, by plasma treatments or surface oxidation due to passage in an oven or by the passage of a torch. The iridescent appearance is not the result of coloring itself, but of the appearance of colors on the surface, under certain viewing conditions. The absence of periodicity of the surface structure in the actual coloring processes is an essential difference between the iridescence of the surfaces to which the present invention relates and the coloring of stainless steel by plasma, passage in the oven or passage of a torch.

However, the observation or not of such iridescence is very directional, that is to say that the observation of this iridescence, and the intensity of the iridescence observed, are strongly dependent on the angle according to which we observe the surface of the material.

Another problem that practitioners of surface iridescence face is the following.

It is currently possible to produce homogeneous samples in the laboratory with an iridescent treatment by using either only a system coupling a laser and a scanner producing both a rapid axis of movement of the laser beam (via a polygonal wheel or a galvo mirror) and a slow axis of movement of the laser beam (via a galvo mirror), or a laser scanner coupled with a robotic arm moving the scanner along the slow axis.

The movement of the scanner can be replaced along the slow axis, by a movement of the sheet to be treated, facing a laser which remains fixed along the slow axis. Provision can also be made for the laser to remain stationary along the two axes (slow and fast), and for the object to be treated to be moved along the two axes.

The mechanism of formation of the structures that have been described depends on the total energy transferred to the surface of the material and on the spatial and temporal distribution of this energy. Thus, the "intensity" of the iridescence obtained thanks to the LSFL will increase between each new passage of the laser on the areas already treated, until reaching a maximum, then it will decrease when the LSFL will gradually turn into "Bumps ”Under the effect of the additional energy input.

This implies that there is an optimum of energy to be transferred to the surface of the material, optimum for which the effect of iridescence is the most intense, and that it is advantageous to determine this optimum and achieve it on the entire area concerned.

However, these samples are generally small in size and / or produced with low productivities.

The limitation in the size of the samples is mainly due to the limitation of the dimensions of the optical fields of the assemblies formed by the laser, the scanner and the focusing system, which can be, for example, a lens or a converging mirror. In fact, obtaining a homogeneous treatment requires perfect control of the treatment at all points of the surface. However, whatever the focusing systems used, they have an optical field on which they have a stable effect in an optimal zone, but as soon as one leaves this optimal zone, the system induces distortions and / or attenuations. of the power of the laser beam, which results in a non-homogeneous treatment between the optimal zone of the optical field and the zones which are situated beyond this optimal zone.

Thus, to treat large surfaces of stainless steel sheets, it would require wide field focusing systems, which would be very expensive and very sensitive. In addition, they should be able to be used in conjunction with high power ultra-short pulse duration lasers, which are not yet widely available on the market.

To remedy this double drawback, the known solutions are to use conventional focusing systems and lasers currently available on the market and either to place several devices side by side including these focusing systems and lasers in the case of treatment. in line of a scrolling band, or to perform the treatment in several stages (by cutting the surface into strips for a discontinuous system), or to combine these two solutions. However, this solution requires particularly careful management of the junction zones between the optical fields of two successive devices, which, if they are poorly produced, can cause a phenomenon called "stitching" by those skilled in the art, and that the we will describe later.

This mechanism therefore prevents having to resort to a significant overlap of the fields to join two consecutive laser treatment fields.

Indeed, if there is a significant overlap of the fields, which would be of the order of magnitude of the resolution of the human eye, this implies that the overlap area receives double the amount of energy transferred to the remaining surface. This doubling of the energy injected during the treatment induces a local change in the structure, and therefore in the surface appearance, compared to the areas which have received only the nominal amount of energy from the treatment, and this change is visible to the naked eye. It is this phenomenon that is commonly called "stitching", in that it makes visible the junction zone of the two fields.

Conversely, a separation of the laser treatment fields, which would undoubtedly make it possible to avoid this phenomenon of local doubling of the treatment and the resulting "stitching", would involve the formation of an untreated area, or less treated than normal, between the two fields. This area would also be visible to the naked eye.

It would therefore be necessary to achieve an almost perfect junction between the consecutive fields of laser treatment.

On the other hand, carrying out this type of high productivity treatment involves working at high frequency (from hundreds of kHz). The scanning systems used for this type of processing are, most typically, scanners having at least one polygonal wheel. At high frequency, these systems generally present synchronization problems between the electronics of the laser and that of the scanner. These synchronization differences induce a shift in the position of the first pulse of the line relative to its target position, and therefore of the entire line. Although this difference is predictable and calculable (because it results from the difference in the management frequencies of the two devices), it is experienced in most current systems, and can represent a difference of a few tens of micrometers between the start of the treatment lines. (lines which are due to the movement of the polygonal wheel). This difference is a function of the speed of rotation of the polygon and the natural frequency of the laser, and experience shows that an overlap of fields with such a difference is already sufficient for the area where the treatment has been doubled to influence the iridescent appearance of the sheet. Certain systems in development have an internal means of partially correcting this shift, by the action of an additional deflector mirror, called "galvo", operating in the manner of a galvanometer, located upstream of the polygon. For example, the firm RAYLASE presented the concept of such a system at the SLT 2018 congress in Stuttgart on June 5 and 6, 2018: “New Generation of High-Speed Polygon-Driven 2D Deflection Units and Controller for High-Power and High- Rep. Rate Applications ”(presentation by E. Wagner, M. Weber and L. Bellini). The enhancement alone is not, however, of sufficient quality that the unwanted effects of a field shift are assuredly gone. Indeed, the initial and final parts of each line run the risk of not being treated with the same energy input as the rest of the line.To resolve this local treatment deficit, we can think of increasing the energy input on the rest of the line, but there is then a risk of exceeding the maximum energy input suitable for creating LSFLs, and therefore of reducing or even eliminating iridescence. The use of a galvo mirror upstream of the polygon can alleviate this problem, but this material is still only at the experimental stage and if it succeeds commercially it will necessarily be more complex and more expensive than what exists. For all of the other systems, this lack of synchronization implies a need for a “virtual” overlap of the order of at least twice the dispersion of the positions of the beginnings of lines between the different optical fields. Thus, this overlap results in a heterogeneous band where there are no untreated areas between the fields, but where there may be an overlap of twice this dispersion in places.

If the edges of each field are defined as "straight", the overlap zone is then presented as a thin rectilinear strip, of width substantially equal to the width of the treatment lines, therefore substantially equal to twice the diameter of the pulse, on which the appearance of the treatment is not identical to the rest of the surface. Likewise, if the edges of the treatment field are defined by a periodic pattern, the latter will remain visible to the naked eye.

Several strategies are then possible to try to attenuate or hide the heterogeneity of the overlap area.

The first strategy consists in using a random offset between two lines which follow one another perpendicular to the scanning direction of the scanner, so that the junctions between the optical fields of two successive lines do not form, taken together, a linear or periodic pattern, and therefore that this pattern is less visible than if it constituted a substantially straight line or a periodic pattern. The goal is to achieve a treatment whose defects would not be easily detected by the human eye, which quickly identifies what is periodic and / or linear. In this case, if we consider that the optimal treatment of the surface of sheet 1 requires N passes, the random shift of the N series of superimposed lines is identical from one pass to another and from one field to another

FIG. 1 schematically shows such a configuration, produced on a sheet 1. It can be seen that, for series of two passages (scan bands) of the scanner corresponding to two successive fields located in the continuation of one another, the junctions 2 of the respective optical fields of the two series 3, 4 of lines are shifted in a non-linear fashion. In other words, the respective junctions 2 of lines 3, 4 do not form between them a straight line or a periodic pattern, but a broken line which is less easily discernible than a straight line would be. A certain periodicity of the offsets between successive junctions 2 may be acceptable, but the period must extend over a sufficient length (typically at least 10 times the maximum value of the offset between two junctions 2 of two successive lines 4, 5 depending on the direction 6 scanners) so that the pattern of this periodicity is not visible.

It should be noted that between two successive lines 4, 5 produced by the same optical field and, therefore, offset in the direction of progression 6 of the scanners (or in the direction of progression of the sheet 1 if it is this which is moving in this direction while the scanners are stationary), this problem does not generally arise with the same intensity, unless the overlap between the lines is really bad. Indeed, as has been said, the different lines 3, 4, 5 have widths substantially equal to the diameter of the pulse, ie, for example, approximately 30-40 μm, generally. This diameter depends on the lens and the diameter of the laser beam entering the lens. To ensure that no untreated areas remain on the surface of the sheet between two successive lines 4, 5 along the slow axis, it is possible to adjust the galvo of the scanner and / or the device for moving the scanner. the sheet so that two successive lines 4, 5 overlap. In other words, the lines 4, 5 are formed after an offset of the relative positions of the picks of each scanner and of the sheet 1 which is slightly less than the diameter of the picks. There may therefore be a double treatment of the surface of the sheet 1 in the areas of overlap of the lines 4, 5, but since the offset of the lines 4, 5 can be controlled with good precision, significantly better than the precision of the overlap. of neighboring optical fields, the width of these zones, if they exist, is in any case sufficiently small so that the double treatment does not visually result in a disturbance of the iridescent effect compared to what is obtained on the remaining surface of sheet 1.

It should be understood that, in Figure 1, each series of lines 3, 4 located in the extension of one another and meeting at the junction 2 is constituted itself from the superposition of N superimposed lines, with, for example, N = 3. The number of superimposed lines for a given optical field depends on the quantity of energy which it is necessary to bring to the surface of the sheet 1 to obtain the desired wavelet configuration, responsible for the iridescence of the surface. The higher this quantity, the higher the number of lines, for the same energy supplied by each pass of the laser.

As far as possible, this configuration exhibits an LSFL-type structure, which has been seen to be most suitable for providing this iridescence under conditions which are, however, dependent on the viewing angle. The energy provided along a given line must therefore be contained between a lower limit below which there would be no sufficiently pronounced wavelets, and an upper limit above which the probability of an excessive presence is increased too much. by Bumps. These limits are, of course, very dependent on multiple factors, in particular the precise material of the sheet 1, its surface condition, the energy provided by the pulses during each of the passes of the laser over a given area ... routine allow a person skilled in the art to define these limits as a function of the material at his disposal and the material to be treated.

Although this first approach makes it possible to significantly reduce the visibility of the overlap of two successive fields, depending on the material used and / or the desired effect, because the overlaps between fields are not done on a straight line, but on a broken line, which follows the offsets between the overlaps, it may, however, be insufficient to make the surface sufficiently homogeneous. In this case, it is possible to use the same approach, but changing the offset between the different passes of the laser. This further increases the randomness of the overlap positioning pattern compared to the previous case. In other words, the broken line which joins the successive overlaps and constitutes said pattern has a non-periodic or random character which is even less obvious. But it is still necessary to ensure that the juxtaposed treatment fields have the same offsets as the first on each pass, because it is necessary to avoid the local accumulation of passes to remain homogeneous, as it is necessary that any point of the surface receives the same amount of energy with the same distribution, the same number of pulses and passes.

The use of a random field edge pattern therefore makes it possible to distribute the points of heterogeneity without these forming a straight line which would undoubtedly be too visible to the naked eye. When the pattern they draw is the same for all the passes, these points are locations where the heterogeneity is strong, because the discontinuity of the line is marked with each pass. However, when this pattern is different with each pass (whether random or not), although the number of points of heterogeneity is multiplied by the number of passes N, these points have a less pronounced heterogeneity compared to the rest of the surface than in the previous case, because they received N-1 continuous passes and only one discontinuous pass.

This second approach allows efficient masking of the junction zone of the treatment fields. However, it requires careful control of the positions of the treatment fields relative to each other, whether in the direction of the laser lines (so that there is no overlap or untreated area) or either in the transverse direction (if the fields are shifted, the junctions will no longer be exact and this could lead to the formation of insufficiently treated areas or, on the contrary, excessively treated. In addition, depending on the parameters chosen, it is sometimes possible to perceive the lines or the periodicity of the treatment lines on the surface An elevation shift of these lines between juxtaposed fields tends to amplify the visibility of the junction due to the phase shift between the lines.

Carrying out the treatment in the form of lines makes it possible to take advantage of the high repetition frequency of the ultra-short duration lasers to increase the productivity of the treatment. Thus, in a single scan of the line by the scanner, the line could be irradiated N times if the distance between two successive pulses is equal to the diameter of the pulse on N. This therefore makes it possible to erase the effect that small power fluctuations on the homogeneity of the surface.

This mode of action has, however, the drawback of forming areas of heterogeneity at the ends of the lines over distances equivalent to the diameter of a pulse (a few tens of micrometers).

To avoid this, a possible solution would be to carry out the processing by drawing at the pulses a pattern in the form no longer of lines, but of a matrix of points, said points being comparable to pixels, and by executing as many matrices as necessary so that the surface of the sheet is, at the end of treatment, entirely covered by the impacts of the pulses which overlap only very slightly or not at all. Thus, the junction of the different fields (and the different taps of each field) does not form a continuous pattern of relatively large dimensions, and is, in principle, no longer visible. Each point has a shape and a dimension (for example circular for a Gaussian laser) comparable to those of the pulse.

However, the point approach is not yet possible with high productivity because of the synchronization problems between the laser and the scanner mentioned above. Indeed, for this approach to be valid and provide treatment to the final homogeneous aspect, it is necessary that the laser irradiates precisely each time the same zone (the same point) in order to have the cumulative effect necessary for the formation of the same level of intensity of the wavelets of the LSFL structure at each point . However, this lack of synchronization causing a random shift which can be of dimensions similar to those of the pulse, it is not possible to achieve the precision required for the irradiation.

This problem could be partially solved by the use of the new generation of scanners, these having an additional galvo for the correction and / or the anticipation of this shift which would be due to the bad synchronization. In this case, the precision of the juxtaposition of two fields would also be improved, and the overall homogeneity of the surface as well. However, the productivity of the process would remain unsatisfactory for the treatment of large surface parts.

Moreover, the principle of dot processing is not, in itself, capable of solving the problem of the inability to observe iridescence from all the desired viewing angles.

The aim of the invention is to provide a method of laser treatment with ultra-short pulses of a surface of a product such as a stainless steel sheet, but not only, making it possible to give it an iridescence appearing homogeneous to following a treatment according to at least most, and preferably all of the viewing angles, even if this iridescence is obtained by means of a plurality of juxtaposed fields.

Also, this process should, optimally, in the case of a treatment by lines, lead to making invisible to the naked eye the junction zone of several successive optical fields which would be arranged so that, taken together, they allow to treat a larger portion of the surface (typically all of it) than a single optical field could. This process should have a good productivity for it to be applicable to the treatment of large surface products.

To this end, the invention relates to a method of producing a visual iridescent effect on the surface of a part, according to which a laser beam is sent to said surface, with a pulse duration of less than one nanosecond, in the optical field of the focusing system of a device comprising a laser source, a scanner and said focusing system, so as to give said surface over the width of said pulses a structure in the form of wavelets having the same orientation, and we performs a scan by said scanner of said surface by said laser radiation along a series of successive lines, or a matrix of points, the width of each line or the dimension of each point of each matrix being equal to the diameter of said pulse, by means of '' a relative displacement of said surface and of the device emitting said laser beam, characterized in that between carrying out the scan along two consecutive lines or two neighboring points, the polarization of the laser beam is modified so as to create wavelets of different orientations on two successive lines or two neighboring points.

The polarization of the laser beam can be changed in a periodic pattern, said periodic pattern extending over M consecutive lines, M being equal to at least 2, preferably at least 3.

Two successive lines or two neighboring points preferably have polarization angles that differ by at least 20 ° and at most 90 °.

It is possible to send onto said surface a laser beam, with a pulse duration of less than one nanosecond, into the optical field of the focusing system of a first device comprising a laser source, a scanner and said focusing system, and send onto said surface a laser beam, with a pulse duration of less than one nanosecond, in the optical field of the focusing system of at least one second device comprising a laser source, a scanner and said focusing system, and the polarizations of two lines located in the extension of one another, or of two neighboring points, belonging to two neighboring fields, being identical.

Said relative displacement of said surface of said part and of the device (s) emitting said laser beam (s) can be achieved by placing said part on a movable support.

Said relative displacement of said surface of said part and of the device (s) emitting said laser beam (s) can be achieved by placing the device (s) emitting said laser beam (s) on a support mobile.

Said part may be a sheet.

Said surface of said part may be three-dimensional

Said part can be made of stainless steel.

The invention also relates to a unitary device for imposing an iridescent appearance on the surface of a part by forming wavelets on said surface by the pulse of a laser beam, comprising a laser source generating a laser beam with a pulse duration of less than 1 ns, an optical system for shaping the beam, a scanner which allows the pulse of the beam, after passing through a focusing system, to scan a field in the form of lines or a matrix of points optical on the surface of the part, and means for creating a relative movement between said device and said part so as to carry out the treatment on at least part of the surface of said part, characterized in that said optical system comprises a system polarization optics which confers a determined polarization on said beam, and means for varying this polarization so that, on said surface, two neighboring lines or points are produced with pui its of different polarizations. Preferably, said device makes it possible to produce two lines or two neighboring points with pulses of polarizations which differ by at least 20 °.

said device may include means for measuring the distance between the focusing system and the surface of the workpiece connected to means for controlling the focusing system and / or the distance between the focusing system and the surface of the workpiece in order to maintaining a constant pulse diameter and fluence over said surface regardless of said distance.

Said means for creating relative movement between said device and said part may comprise a movable support for the part.

The subject of the invention is also a device for imposing an iridescent appearance on the surface of a part by forming wavelets on said surface by the pulse of a laser beam, characterized in that it comprises at least two unitary devices of the preceding type, whose optical fields of the focusing systems overlap.

Said means for creating a relative movement between said device and said part can comprise a movable support for said unitary device (s).

A subject of the invention is also a part made of a material whose surface has an iridescence provided by means of a laser treatment, said treatment having formed wavelets on the surface of said part, characterized in that said wavelets have at least two orientations, preferably at least three orientations, distributed over the surface of said part, preferably in a periodic pattern.

As will be understood, the invention consists in eliminating, or at least very greatly reducing, the problems associated with the excessive directionality of the vision of the iridescence of the surface of a stainless steel treated by a device comprising a laser. scanner, by imposing a different polarization of the light emitted by the laser for the formation of LIPPS of two consecutive lines, or of neighboring points of two matrices of points, formed by the scanning of the laser beam according to the optical field of the focusing lens of the device. The use of at least three different polarizations, for a series of at least three consecutive lines, or of three dot matrices, is recommended to obtain the desired effect.

This process can also be used in conjunction with a process intended to make invisible or almost invisible the junctions between two lines facing each other and produced by the juxtaposition of two laser scanner devices whose fields overlap slightly to avoid the risk of non- treatment or sub-treatment of these junction areas. It will be noted that the invention is applicable, in its basic principle, both to line laser treatments and to point laser treatments, or to a treatment which would combine the two modes. Of course, one can choose to limit the treatment to a part of the surface of the object (for which a single laser and its optical field could possibly be sufficient), or to carry out the treatment on the entire surface of the object. object. To do this, it suffices to adapt the number and the extent of the optical field (s) of the focusing lenses of the laser device (s) and the extent of the relative movements between the treatment device and the object to be treated, so that it is possible to treat the entire affected area.

The invention will be better understood on reading the following description, given with reference to the following appended figures:

- Figure 1 which shows, as said in the introduction, the surface of a sheet on which an iridescence laser treatment has been performed by a method according to the known prior art, by means of two devices laser of known contiguous type, randomly forming lines located in the continuation of one another with overlap zones between two lines generated in the respective optical fields of the two devices, with the aim of reducing the visibility of areas of overlap of said lines;

FIG. 2 which shows the block diagram of a device according to the invention, allowing the implementation of the method according to the invention in the optical field of a laser treatment device, in order to make the observation of the iridescence of the surface of the sheet independent of the viewing angle;

FIG. 3 which shows the surface of a sheet resulting from the implementation of a method improving the method used in the case of FIG. 1 by two contiguous laser treatment devices, and the use of which can be combined with that of the method according to the invention.

As has been said, the iridescence effect obtained by treatment with an ultra-short pulse laser is linked to the spontaneous formation at the surface of a periodic structure having a behavior similar to an optical grating on the reflecting light. on the surface. As discussed previously, the mechanism of formation of this wavelet structure periodically distributed over the treated surface has not yet been established by the scientific community.

However, it has been shown (see, for example, the document "Control Parameters In Pattern Formation Upon Femtosecond Laser Ablation", Olga Varlamova et al., Applied Surface Science 253 (2007) pp. 7932-7936), that the orientation wavelets was mainly related to the polarization of the laser beam irradiating the surface. Thus, the HSFLs have an orientation parallel to the polarization of the incident beam while the LSFL which then form, when a greater quantity of energy has been brought to the surface of the sheet, have an orientation perpendicular to the polarization of the beam. incident.

In the case of a laser treatment by lines, it therefore follows that a surface treated without modification of the polarization of the laser beam during its various passages on a given line of said surface has, at the end of the treatment, a structure made up of lines / wavelets all oriented in the same direction. This implies that the “optical network” effect of the surface is also oriented.

Indeed, the iridescence appears to be maximum if the observation is made in a direction transverse to the orientation of the wavelets and it decreases as the angle of orientation of the observation aligns with the structure of the surface. Thus, an observation of the surface in the alignment of the wavelets does not reveal any color. This can be a drawback for the final product because it means choosing the orientation of the wavelets from the treatment to have a product on which iridescence appears under the desired observation conditions. In addition, the final product only appears very fully colored in one main direction of observation.

The invention overcomes this drawback, because the device used makes it possible to obtain a surface for which the iridescence is visible identically in all directions of observation. If two successive fields, together forming the same line, have the same polarization along this line, the visual effect of a double treatment of the junction zone between these two fields tends to be much less marked than if the two fields have different polarizations, with a difference in polarization angle preferably greater than or equal to 20 ° and less than or equal to 90 °. And having polarizations that are certainly sufficiently different between two successive lines removes the directionality of the observation of iridescence. The combination of these phenomena means that the iridescence of the treated sheet appears much more uniform, in all directions of observation, than in the case where we do not have this alternation of polarization between neighboring lines.

In the case where the treatment is carried out “in lines”, with a distance between the centers of the pulses slightly less than the diameter of the pulse in the direction of rapid scanning, so that there are assuredly no areas not treated by the pulse. draws, the solution according to the invention consists in alternating lines for which the orientation of the wavelets is modified, from one line to another, by the action of a polarizer or any other type of polarizing optical device, placed on the optical path of the beam. Thus, either the treatment field is produced with an automatic system making it possible to modify the polarization of the incident beam between each line, or the treatment field is produced in a number of times M equal to at least two, and preferably to at least three, M corresponding, therefore, to the number of different orientations provided to the wavelets by the periodically successive polarizations of the pulse of the laser beam which forms them.

The principle of the invention is also valid when the processing is carried out "by points" according to a matrix. Each point corresponding to a pulse impact has a wavelet orientation different from that (s) of its neighbors. In two neighboring optical fields, we generate points along matrices that extend from each other.

FIG. 2 schematically shows a typical architecture of a part of a unitary device allowing the implementation of the method according to the invention to treat at least a part of a stainless steel sheet 1 in a given field. Of course, this device is controlled by automated means, which make it possible to synchronize the relative movements of the support 13 of the sheet 1 and of the laser beam 7, as well as to adjust the parameters of the laser beam 7 and its polarization as required.

The device firstly comprises a laser source 6 of a type conventionally known for producing iridescence of metal surfaces, therefore, typically a source 6 generating a pulsed laser beam 7 of short pulse duration (less than one nanosecond), the diameter of each pulse being typically, for example, of the order of 30 to 40 μm as seen above. The energy injected on the surface of the stainless steel by the pulse is to be determined experimentally, so as to generate on the surface of the sheet 1 LIPPS wavelets, preferably of LSFL type and to avoid the formation of bumps, a fortiori of peaks, and the frequency and power of the laser beam 7 must be chosen accordingly according to the criteria known to those skilled in the art for this purpose and taking into account the precise characteristics of the other elements of the device and of the material to be treated. The laser beam 7 generated by the source 6 then passes through an optical system for shaping the beam 8, which, in addition to its conventional components 9 making it possible to adjust the shape and dimensions of the beam 7, comprises, according to the invention, a polarizing optical element 10 which makes it possible to give the beam 7 a polarization chosen by the operator or the automatisms which manage the device.

The laser beam 7 then passes through a scanning device (for example a scanner) 11 which, as is known, allows the beam 7 to scan the surface of the sheet 1 along a rectilinear path in a treatment field. At the output of the scanner 11, again in a conventional manner, there is a focusing system 12, such as a focusing lens, through which the laser beam 7 is focused in the direction of the sheet 1.

In the example shown, the sheet 1 is carried by a movable support 13 which makes it possible to move the sheet 1, in a plane or, optionally, in the three dimensions of space, with respect to the device for generating, polarizing and scanning the laser beam 7, so that the latter can treat the surface of the sheet 1 along a new line of the treatment field of the device shown. But before this treatment of said new line, according to the invention, the optical device 10 for polarization of the laser beam 7 has had its adjustment modified, so as to give the laser beam 7 a polarization different from that which it had during the treatment of the previous line.

At least two different polarization angles, and preferably at least three, are capable of being obtained by means of the optical polarization device 10, and alternate, preferably but not necessarily, periodically with each change of line. A periodicity of the polarization pattern is not essential, it suffices, as has been said, that the polarization angles of two neighboring lines 14, 15, 16 are different, preferably at least 20 ° and at most 90 °. But a periodicity of the pattern, for example, as shown, with polarization angles which repeat every three lines 14, 15, 16, is preferred, since periodic programming of the polarization change is simpler than random programming, in particular as two lines 14, 15, 16 belonging to two different fields and situated in the extension of one another must have the same orientation of wavelets.

A succession of random polarizations within a given optical field, preferably nevertheless respecting the aforementioned minimum angular deviation of 20 ° and the aforementioned maximum angular deviation of 90 °, would be acceptable, in particular if the installation had to be able to be used to treat relatively narrow sheets which would require for that only a single field and for which the question of the identity of polarization on two lines located in the extension of one the other and generated in two neighboring fields do not arise.

The entire device for processing the sheet 1 most typically comprises a plurality of unit devices such as the one just described, placed facing the sheet 1, and which are juxtaposed so that their processing fields respective, that is to say the optical fields of the focusing systems 12 of the scanners 11, overlap slightly. This overlap is typically of the order of twice the size of the pulse, and we can add a positional uncertainty which is linked to the period of supply of the laser with pulses and the scanning speed of the laser according to l 'fast axis. It should be verified experimentally that this overlap is sufficient to ensure that no untreated areas remain on the sheet at the end of the operation. Also, the lines generated by each of these fields must be in continuity with one another, and the settings of the unit devices must be identical, in particular in terms of shape, dimension, power and angle of polarization at a time t of their respective laser beams 7, so that the treatment is homogeneous over the whole of a line the width of the sheet 1, and that the alternation of the angles of polarization of the laser beam 7 between two consecutive lines is identical over the entire width sheet metal.

The control means of these unit devices are, most typically, means common to all the unit devices, so that they act in perfect synchronization with one another. They also control the movements of the support 13 of the sheet 1.

Of course, one could replace the mobile support 13 by a fixed support, and ensure the relative displacement of the sheet 1 and the unit processing devices by placing them on a mobile support. The two variants can moreover be combined, in that the device according to the invention would include both a movable support 13 for the sheet 1 and another movable support for the unitary treatment devices, the two supports being able to be actuated. either, or both simultaneously, by the controller, depending on the user's wishes.

The number M therefore corresponds to the number of different orientations that we want to give to the wavelets by ensuring a spacing M times greater than a conventional treatment and by shifting the lines by a conventional spacing between each realization of the field. Figure 3 shows an example of the appearance of such an embodiment with M = 3.

The sheet 1 has on its surface a periodic succession of lines 14, 15, 16 produced using two devices according to the invention which have made it possible to produce this periodic pattern of three kinds of lines 14, 15, 16 on two contiguous optical fields 17, 18, the lines 14, 15, 16 of a given field being in the extension of lines 14, 15, 16 of the neighboring optical field.

The lines 14, 15, 16 of the pattern are distinguished from each other by the effects of the different polarizations that the polarization device 10 has applied to the laser beam 7 at the time of their formation.

As can be seen in the part of FIG. 3 which represents an enlarged fraction of the surface, in the example shown which is not limiting, the polarization conferred on the laser during the generation of the first line 14 of the pattern leads to an orientation of the wavelets in the direction perpendicular to the relative direction of travel 6 of the sheet 1 with respect to the laser treatment device. Then, to generate the second line 15 of the pattern, the polarization of the laser beam 7 has been modified so as to obtain an orientation of the wavelets at 45 ° from the orientation of the wavelets of the first line 14. Finally, to generate the third line 16 of the pattern, we modified the polarization of the laser beam 7 so as to obtain an orientation of the wavelets at 45 ° from the orientation of the wavelets of the second line 15, therefore at 90 ° from the orientation of the wavelets of the first line 14: the wavelets of the third line 16 are therefore oriented parallel to the relative direction of travel 6 of the sheet 1 with respect to the laser treatment device.

In the junction zone of two neighboring fields, an energy greater than that which is injected into the remainder of the surface is injected into the surface of sheet 1, just as in the prior art described above. But the fact that in this junction zone the lines 14, 15, 16 of each optical field which meet have been produced with the same polarization of the laser beam 7 clearly attenuates the deterioration of the visual effect of iridescence of the surface. which is observed in the absence of controlled polarization of the laser beam 7. The absence of continuity of the orientation of the wavelets from one optical field to another would have the effect of increasing the visibility of the junction zone fields on a given line 14, 15, 16, creating an area of heterogeneity on the surface. It is simply necessary to ensure that the lines 14, 15, 16 of the two neighboring fields which have been produced with identical polarizations are indeed in the extension of each other, but this precaution on the collinearity of the lines 14, 15, 16 of neighboring fields were also to be taken in the execution of the methods of the prior art (see FIG. 1), and the known equipment for this purpose can be used within the framework of this variant of the invention. It suffices to make sure that the changes in polarization of the laser beams 7 of the devices concerning each field take place with the same values for the lines of the joining fields.

The use of M = 2 different polarization orientations, out of phase, for example, by 90 °, already allows to have an iridescent effect visible in most directions. However, the intensity resulting from the iridescence still varies quite significantly when observed at an angle of 45 °, and it can be judged that the problem of the lack of directionality of the iridescence effect would still not be. resolved completely satisfactorily. This is no longer visible as soon as M is greater than 2, preferably if the angles are spaced between 20 ° and 90 ° between two successive lines 14, 15, 16.

Thus, by carrying out a treatment with at least three distinct polarization angles distributed between 0 and 90 ° and preferably exhibiting polarization differences of at least 20 ° between two successive lines 14, 15, 16, experience shows that the iridescence of the surface is visible in all directions with an intensity similar. It is possible to use a number M of orientations greater than 3, but care must be taken that the polarization angles of two neighboring lines are sufficiently different from one another to obtain the absence of directionality of the desired iridescent effect.

The same condition of polarization difference of at least 20 ° between two neighboring points should preferably be observed, in the case of point processing.

It is however evident that the distribution of the structure of the surface according to different orientations induces a decrease in the total intensity of the iridescence in comparison with a surface treated according to a single direction of polarization and observed according to the optimal angle (the transverse angle to the structure). There would therefore be a compromise to be found between the intensity of the visual iridescent effect perceived by the observer and the omnidirectional nature of this iridescent effect. But three polarization directions (therefore a periodicity of three lines of these directions, as shown in FIG. 3) already represent, at least in the most common cases, such a good compromise.

In the case where the scanner allows a "point" treatment, according to a matrix, the orientation of the wavelets can be changed between the different points of a line and / or between successive lines. However, it remains important that each point is formed only by the accumulation of irradiations sharing the same polarization, if the energy injected to form a given point is to be injected by means of several passes of the laser beam 7. This can be achieved by changing the polarization of the irradiating beam between each point or by making M matrixes of points, with M equal to at least 2 and preferably at least 3, each having an orientation of different wavelets, in other words having each been carried out with a different polarization of the laser beam 7.

One could think of making the differences between the orientations of the wavelets not by optical means (the polarizer 10), but by mechanical means by making modifications to the relative orientations of the support 13 of the sheet 1 and of the support of the laser scanner devices. , typically by rotating the support 13 by an angle equal to the difference in orientation desired for the wavelets of a given line 14, 15, 16 with respect to that of the line 14, 15, 16 made previously. This solution would, however, not be ideal. Indeed, the precise realization of the wavelets would depend on the possible irregularities of polarization of the laser beam 7, and turning the support 13 with the necessary speed and angular precision would pose complex mechanical problems, in particular in the case of an industrial installation intended for to handle heavy and large objects. The use and control of a polarizer 10 are generally simpler to implement. Finally, to obtain the most homogeneous effect possible, it is recommended to alternate the orientations, preferably periodically, over the shortest possible distances. In the case of lines, for M different orientations, it will be preferable to periodically alternate a single line of each orientation, of equal width or, preferably (to ensure a treatment of the entire surface of the sheet) slightly less than the diameter of the nozzle. . In the case of point processing, it will be preferable to periodically alternate the orientations on a square or rectangular pattern containing a number of points equal to the number of different orientations possible for the polarization of the laser beams 7.

Of course, it would remain in the spirit of the invention to apply this method to a sheet whose relatively small width would only require a single scanner to achieve the structuring of its entire surface in lines of different polarizations according to a periodic pattern. . This would take advantage of the main advantage of the invention that the intensity of iridescence does not depend on the viewing angle of the sheet. If we only want to process such thin sheets, we can then afford to do so with an installation that would only include a single device according to Figure 2.

It is also possible to treat on the same installation both sheets of relatively small width, less than or equal to that of a treatment field of a device according to FIG. 2, and sheets of greater width requiring the juxtaposition of several devices according to FIG. 2 each acting on a single treatment field. To do this, it suffices to activate only one of these devices when processing a sheet of small width. The fact of being able to use the method according to the invention for multiple widths of sheets, and with the same settings for each field taken individually, makes it possible to obtain sheets of identical appearance regardless of said width, and thus to homogenize the aspect of the product line of various widths which the manufacturer may desire to produce.

It is possible to treat sheets 1 whose flatness is not perfect by including in the processing device means for measuring the distance between the focusing system 12 and the sheet 1, and by coupling them to the control means of the focusing system 12, so that the latter guarantees that the diameter of the pulse and the fluence of the laser beam are substantially the same regardless of the effective distance between the focusing system 12 and the sheet 1. The distance between the focusing system and the surface of sheet 1 is also a parameter that can be played, if it can be adjusted in real time by appropriate mechanical means.

It is also possible to envisage the application of the process to materials other than flat sheets (for example to shaped sheets, to bars, to tubes, to three-dimensional surfaces in general), by accordingly adapting the means of relative displacement of the lasers and of the surface, and / or the focusing means if it is necessary to manage differences in distance between the laser emitter and the surface. In the case of parts having substantially cylindrical surfaces (bars and tubes of circular section, for example), one way of proceeding would be to place the laser devices on a fixed support and to provide, for the part, a support allowing it to be placed. rotating to scroll the surface of the part in the optical fields of lasers.

Finally, it should be recalled that while stainless steels are materials to which the invention is particularly applicable, other materials, metallic or non-metallic, on which the effect of iridescence of the surface by means of a laser treatment can be obtained, are also concerned by the invention.

Claims

1.- A method of producing a visual iridescent effect on the surface of a part (1), according to which a laser beam (7) is sent to said surface, with a pulse duration of less than one nanosecond, in the optical field of the focusing system (12) of a device comprising a laser source (6), a scanner (1 1) and said focusing system (12), so as to give said surface over the width of said pulses a structure in the form of wavelets having the same orientation, and a scan is carried out by said scanner (1 1) of said surface by said laser radiation (7) along a series of successive lines (14, 15, 16), or a matrix of points , the width of each line (14, 15, 16) or the dimension of each point of each matrix being equal to the diameter of said pulse, by means of a relative displacement of said surface and of the device emitting said laser beam, characterized in that that between carrying out the scan along two consecutive lines (14, 15, 16) es or two neighboring points, the polarization of the laser beam (7) is modified so as to create wavelets of different orientations on two successive lines (14, 15, 16) or two neighboring points.

2.- Method according to claim 1, characterized in that the polarization of the laser beam (7) is modified according to a periodic pattern, said periodic pattern extending over M consecutive lines, M being equal to at least 2, preferably to at least 3.

3.- Method according to claim 1 or 2, characterized in that two successive lines (14, 15, 16) or two neighboring points have polarization angles which differ by at least 20 ° and at most 90 °.

4 Method according to one of claims 1 to 3, characterized in that sends on said surface a laser beam (7), pulse duration less than one nanosecond, in the optical field of the focusing system (12) d 'a first device comprising a laser source (6), a scanner (1 1) and said focusing system (12), in that a laser beam (7) is sent to said surface, with a pulse duration of less than one nanosecond, in the optical field of the focusing system (12) of at least a second device comprising a laser source (6), a scanner (1 1) and said focusing system (12), and the polarizations of two lines ( 14, 15, 16) located in the extension of one another, or of two neighboring points, belonging to two neighboring fields, being identical.

5.- Method according to one of claims 1 to 4, characterized in that said relative displacement of said surface of said part and of the device (s) emitting said laser beam (s) is carried out by placing said laser beam (s). part on a mobile support.

6.- Method according to one of claims 1 to 5, characterized in that said relative displacement of said surface of said part and of the device (s) emitting said laser beam (s) is carried out by placing the or the device (s) emitting said laser beam (s) on a mobile support.

7.- Method according to one of claims 1 to 6, characterized in that said part is a sheet.

8.- Method according to one of claims 1 to 7, characterized in that said surface of said part is three-dimensional

9.- Method according to one of claims 1 to 8, characterized in that said part is made of stainless steel.

10.- Unit device for imposing an iridescent appearance on the surface of a part

(I) by forming wavelets on said surface by the pulse of a laser beam, comprising a laser source (6) generating a laser beam (7) with a pulse duration of less than 1 ns, an optical system (8) for setting in the form of the beam (7), a scanner

(II) which allows the pulse of the beam (7), after its passage through a focusing system (12), to scan in the form of lines or a matrix of points an optical field on the surface of the part (1), and means for creating a relative movement between said device and said part (1) so as to carry out the treatment on at least part of the surface of said part (1), characterized in that said optical system (8) comprises a system polarization optics (10) which confers a determined polarization on said beam (7), and means for varying this polarization so that, on said surface, two neighboring lines or points are produced with pulses of different polarizations.

1 1 .- Unit device according to claim 10, characterized in that said device makes it possible to produce two lines or two neighboring points with polarizations which differ by at least 20 ° and at most 90 °.

12.- Unit device according to claim 10 or 1 1, characterized in that it comprises means for measuring the distance between the focusing system (12) and the surface of the workpiece (1) connected to control means of the focusing system (12) and / or the distance between the focusing system (12) and the surface of the workpiece (1) to maintain a constant pulse diameter and fluence over said surface, regardless of said distance.

13.- Device for imposing an iridescent appearance on the surface of a part (1) by forming wavelets on said surface by the pulse of a laser beam, characterized in that it comprises at least two devices units according to claim 10 or 1 1, the optical fields of the focusing systems of which overlap.

14.- Device according to one of claims 10 to 13, characterized in that said means for creating relative movement between said device and said part (1) comprise a movable support (13) for the part (1).

15.- Device according to one of claims 10 to 14, characterized in that said means for creating a relative movement between said device and said part (1) comprise a movable support (13) for said unit (s). (s).

16.- Part (1) made of a material whose surface has an iridescence provided by means of a laser treatment, said treatment having formed wavelets on the surface of said part (1), characterized in that said wavelets have at at least two orientations, preferably at least three orientations, distributed over the surface of said part (1), preferably in a periodic pattern.