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 pulse at its place 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 sufficiently high, 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 high, a phenomenon of ablation by vaporization/sublimation/shock wave can take place, preferentially or jointly with the formation of the periodic surface structure.

Such treatment is practiced, in particular, but not exclusively, on stainless steels of all types. The purpose of this treatment can 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 stainless steel passivation layer is located without the need for prior activation/depassivation.

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

In the rest of the text, the case of stainless steels will be preferred, 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, to be able to present an iridescent appearance continued 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 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 scanning parameters, the structure of the surface may exhibit one of the following four structures, depending on the total irradiation energy per unit area, these structures being ranked in order of energy crescent and their denomination being usual for the person skilled in the art, even non-English speaking:

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

This structure is composed 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 lower than the wavelength of the laser used for the treatment.

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

This structure is composed of larger wavelets 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 lower, or higher, or equal to the wavelength of the laser. For the treatment of a stainless steel surface with a laser of wavelength 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 depressions of the LSFL structure.

It will be noted that for certain materials, the respective orientations of the HSFL and LSFL structures can be reversed with respect to what they are for the stainless steels.

3) Structure called “Grooves” or “Bumps”:

This structure is made up of bumps of micrometric dimensions covering the entire treated surface. These bumps are organized according to a structure resembling a "snakeskin" appearance.

4) Structure in peaks or “spikes”:

This structure is composed of peaks whose height ranges from a few micrometers to a few tens of micrometers. The distance between the peaks depends on the processing 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 pulses", Journal of Laser Applications 26, February 2014, by B. Liu et al. . It is notably 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 pulses leads to the creation of LSFL rather than HSFL, until the number of pulses becomes too high for ripples to be observed. The exact configuration of the surface after irradiation therefore results from a mechanism involving both the number of pulses received and the energy delivered by each of them, for a given material.

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 network when the sample treated is placed under a light source. It is then possible to observe, depending on the orientations and positions of the user and the light, the colors of the rainbow on the sample. This is called an “iridescent appearance”.

This appearance no longer exists when the surface of the sample has a pronounced appearance according to the third and fourth cases mentioned above, because, in these two cases, the energy supplied by the laser source to the surface of the sample has reaches too high a level, at least locally, resulting in deformations of the surface which no longer allow the iridescent appearance to be obtained, 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 oxidations due to a passage in an oven or by the passage of a blowtorch. The iridescent aspect does not result from a coloring strictly speaking, but from the appearance of colors on the surface, under certain conditions of observation. 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 steels by plasma, baking or passing a blowtorch.

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

Another problem faced by surface iridescence practitioners is the following.

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

The displacement of the scanner can be replaced along the slow axis, by a displacement 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 fixed along the two axes (slow and fast), and for it to be the object to be treated which is moved along the two axes.

The formation mechanism 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 LSFLs will increase between each new pass of the laser on the areas already treated, until it reaches a maximum, then it will decrease when the LSFLs will gradually turn into “Bumps ” as a result of the additional energy input.

This implies that there is an optimum of energy to be transferred to the surface of the material, an optimum for which the iridescence effect 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 and/or made with low productivities.

The size limitation 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. Indeed, 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 result in an uneven treatment between the optimal zone of the optical field and the zones which are located beyond this optimal zone.

Thus, to treat large surfaces of stainless steel sheets, wide-field focusing systems would be required, which would be very expensive and very sensitive. In addition, it should be possible to use them 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 a treatment line of a moving strip, either to carry out 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 made, can cause a phenomenon called "stitching" by those skilled in the art, and that the will be described later.

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

Indeed, if there is a significant overlapping of the fields, which would be of the order of magnitude of the resolution of the human eye, this implies that the overlapping zone receives twice the quantity of energy transferred to the remainder of the 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 zones which have received only the nominal quantity 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 certainly make it possible to avoid this phenomenon of local doubling of the treatment and the "stitching" which would result therefrom, would involve the formation of an untreated area, or less treated than normal, between both 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 processing 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 problems of synchronization between the electronics of the laser and that of the scanner. These synchronization deviations induce a shift in the position of the first pulse of the line with respect 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 suffered in most current systems, and can represent a difference of a few tens of micrometers between the beginnings of the processing lines (lines which are due to the movement of the polygon wheel). This deviation is a function of the rotational speed of the polygon and the natural frequency of the laser, and experience shows that an overlapping of the fields with such a deviation is already sufficient for the area where the treatment has been doubled to be able to influence the iridescent appearance of sheet metal.

Some systems under development have an internal means of partially correcting this shift, by the action of an additional deflecting mirror, called “galvo”, operating like 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 improvement is, however, not by itself of sufficient quality for the undesirable effects of a shift in the fields to disappear with certainty. 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, one can consider increasing the energy input on the rest of the line the line, but there is then a risk of exceeding the maximum energy input suitable for the creation of LSFLs, and therefore of reducing or even eliminating the 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 inevitably be more complex and more expensive than what exists. For all 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.

If the edges of each field are defined as "straight", the overlap zone then appears as a thin rectilinear band, 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. Similarly, if the edges of the processing field are defined by a periodic pattern, the latter will remain visible to the naked eye.

Several strategies are then possible to attempt to attenuate or mask the heterogeneity of the overlap zone.

La première stratégie consiste à utiliser un décalage aléatoire entre deux lignes qui se succèdent perpendiculairement à la direction de balayage du scanner, afin que les jonctions entre les champs optiques de deux lignes successives ne forment pas, prises ensemble, un motif linéaire ou périodique, et donc que ce motif soit moins visible que s’il constituait une ligne sensiblement droite ou un motif périodique. Le but est de réaliser un traitement dont les défauts ne seraient pas détectés facilement par l’œil humain, qui repère rapidement ce qui est périodique et/ou linéaire. Dans ce cas, si l’on considère que le traitement optimal de la surface de la tôle 1 nécessite N passages, le décalage aléatoire des N séries de lignes superposées est identique d’un passage à l’autre et d’un champ à l’autre

FIG. 1 schematizes such a configuration, produced on a sheet 1 . It can be seen there that, for series of two passages (scan strips) of the scanner corresponding to two successive fields located in the extension 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,

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 mobile in this direction while the scanners are fixed), this problem does not generally arise with the same intensity, unless the overlap between the lines is frankly bad. Indeed, as has been said, the various 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 there are no untreated areas remaining 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 sheet so that two successive lines 4, 5 overlap. In other words, the lines 4, 5 are formed after a shift in the relative positions of the pulses of each scanner and of the sheet 1 which is slightly less than the diameter of the pulses. There may therefore well be a double treatment of the surface of the sheet 1 in the overlapping zones of the lines 4, 5, but since the offset of the lines 4, 5 is controllable with good precision, much better than the precision of the overlap neighboring optical fields, the width of these zones, if they exist,

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 level of the junction 2 is itself made up of 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 surface iridescence. 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 has a structure of the LSFL type, which has been seen to be the most suitable for providing this iridescence under conditions which are, however, dependent on the viewing angle. The energy brought along a given line must therefore be contained between a lower limit below which there would not be sufficiently pronounced wavelets, and an upper limit above which the probability of an excessive presence is increased too much. of 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 supplied by the pulses during each passage of the laser over a given zone, etc.

Although this first approach makes it possible to significantly reduce the visibility of the overlapping of two successive fields, depending on the material used and/or the intended effect, due to the fact that the overlaps between fields are not made on a straight line, but on a broken line, which follows the offsets between the overlaps, it may, however, prove to 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 passages of the laser. This makes it possible to further increase the random character of the positioning pattern of the overlaps with respect to the previous case. In other words, the broken line which joins the successive overlaps and constitutes said pattern has an even less obvious non-periodic or random character.

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 doubtless be too visible to the naked eye. When the pattern they draw is identical for all the passes, these points are localizations where the heterogeneity is strong, because the discontinuity of the line is marked with each pass.

However, when this pattern is different at each pass (whether random or not), although the number of heterogeneity points 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 effective masking of the junction zone of the treatment fields. However, it requires rigorous 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 may lead to the formation of insufficiently treated or, on the contrary, excessively treated zones. In addition, depending on the parameters chosen, it is sometimes possible to perceive the lines or the periodicity of the processing lines on the surface.A shift in altitude of these lines between juxtaposed fields tends to amplify the visibility of the junction due to the phase difference between the lines.

Performing the treatment in the form of lines makes it possible to take advantage of the high repetition frequency of lasers with an ultra-short pulse duration 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).

Pour éviter cela, une solution envisageable serait de réaliser le traitement en faisant dessiner aux puises un motif en forme non plus de lignes, mais de matrice de points, lesdits points étant assimilables à des pixels, et en exécutant autant de matrices que nécessaire pour que la surface de la tôle soit, en fin de traitement, entièrement recouverte par les impacts des puises qui ne se recouvrent que très faiblement ou pas du tout. Ainsi, la jonction des différents champs (et des différents puises de chaque champ) ne forme pas de motif continu de relativement grandes dimensions, et n’est, en principe, plus visible. Chaque point a une forme et une dimension (par exemple circulaire pour un laser gaussien) comparables à celles du puise.

L’approche par points n’est cependant pas encore possible avec une haute productivité à cause des problèmes de synchronisation entre le laser et le scanner cités précédemment. En effet, pour que cette approche soit valable et fournisse un traitement à l’aspect final homogène, il faut que le laser irradie précisément chaque fois la même zone (le même point) afin d’avoir l’effet cumulé nécessaire à la formation du même niveau d’intensité des vaguelettes de la structure LSFL en chaque point. Or ce manque de synchronisation entraînant un décalage aléatoire pouvant être de dimensions similaires à celles du puise, il n’est pas possible d’atteindre la précision demandée pour l’irradiation.

Ce problème pourrait être partiellement résolu grâce à l’utilisation de la nouvelle génération de scanners, ceux-ci possédant un galvo additionnel pour la correction et/ou l’anticipation de ce décalage qui serait dû à la mauvaise synchronisation. Dans ce cas, la précision de la juxtaposition de deux champs s’en trouverait également améliorée, et l’homogénéité globale de la surface également. Toutefois, la productivité du procédé demeurerait insatisfaisante pour le traitement de pièces de grande surface.

De plus, le principe du traitement par points n'est pas, en lui-même, capable de résoudre le problème de l’impossibilité d’observer l’irisation selon tous les angles de vision souhaités.

Le but de l’invention est de proposer un procédé de traitement au laser à puises ultra-courts d’une surface d’un produit tel qu’une tôle en acier inoxydable, mais pas seulement, permettant de lui conférer une irisation paraissant homogène à la suite d’un traitement selon au moins la plupart, et de préférence tous les angles d’observation, même si cette irisation est obtenue au moyen d’une pluralité de champs juxtaposés.

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

To this end, the subject of the invention is a method for producing a visual effect of iridescence on the surface of a part, according to which a laser beam is sent onto 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 pulse a structure in the form of wavelets having the same orientation, and carries out a scanning by said scanner of said surface by said laser radiation according to 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 performing 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 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.

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

It is possible to 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 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 prolongation 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 mobile support.

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

Said part can 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 ripples 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 system on the surface of the part, and means for creating a relative movement between the said device and the said part so as to carry out the treatment on at least part of the surface of the said part, characterized in that the said optical system comprises a system polarization optics which confers a determined polarization to said beam,and means for varying this polarization so that, on said surface, two neighboring lines or points are produced with pulses of different polarizations.

Preferably, said device makes it possible to produce two neighboring lines or points with polarization pulses which differ by at least 20°.

said device may comprise means for measuring the distance between the focusing system and the surface of the part connected to means for controlling the focusing system and/or the distance between the focusing system and the surface of the part for maintain a constant pulse diameter and fluence on said surface, regardless of said distance.

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

The invention also relates to a device for imposing an iridescent appearance on the surface of a part by forming ripples on said surface by the pulse of a laser beam, characterized in that it comprises at least two unitary devices of the previous 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 mobile support for said unitary device(s).

L’invention a également pour objet une pièce réalisée en un matériau dont la surface présente une irisation ménagée au moyen d’un traitement laser, ledit traitement ayant formé des vaguelettes à la surface de ladite pièce, caractérisée en ce que lesdites vaguelettes présentent au moins deux orientations, de préférence au moins trois orientations, réparties sur la surface de ladite pièce, de préférence selon un motif périodique.

Comme on l’aura compris, l’invention consiste à supprimer, ou au moins très fortement atténuer, les problèmes liés à la directionnalité excessive de la vision de l’irisation de la surface d’un acier inoxydable traité par un dispositif comprenant un laser scanner, en imposant une polarisation différente de la lumière émise par le laser pour la formation des LIPPS de deux lignes consécutives, ou de points voisins de deux matrices de points, formés par le balayage du faisceau laser selon le champ optique de la lentille de focalisation du dispositif. L’utilisation de trois polarisations différentes au moins, pour une série d’au moins trois lignes consécutives, ou de trois matrices de points, est conseillée pour obtenir l’effet recherché.

Ce procédé peut aussi être utilisé en conjonction avec un procédé destiné à rendre invisibles ou quasiment invisibles les jonctions entre deux lignes se faisant face et réalisées par la juxtaposition de deux dispositifs à laser scanner dont les champs se recouvrent légèrement pour éviter le risque de non-traitement ou de sous-traitement de ces zones de jonction.

On notera que l’invention est applicable, dans son principe de base, aussi bien aux traitements laser par lignes qu’aux traitements laser par points, ou qu’à un traitement qui combinerait les deux modes. Bien entendu, on peut choisir de limiter le traitement à une partie de la surface de l’objet (pour laquelle un laser unique et son champ optique pourrait éventuellement être suffisant), ou de réaliser le traitement sur la totalité de la surface de l’objet. Pour ce faire, il suffit d’adapter le nombre et l’étendue du ou des champs optiques des lentilles de focalisation du ou des dispositifs à laser et l’ampleur des déplacements relatifs entre le dispositif de traitement et l’objet à traiter, de façon à ce qu’il soit possible de traiter toute la surface concernée.

L’invention sera mieux comprise à la lecture de la description qui suit, donnée en référence aux figures annexées suivantes :

- la figure 1 qui montre, comme on l’a dit dans l’introduction, la surface d’une tôle sur laquelle on a exécuté un traitement laser d’irisation par un procédé selon l’art antérieur connu, au moyen de deux dispositifs à laser de type connu contigus, formant de manière aléatoire des lignes situées dans le prolongement l’une de l’autre avec des zones de recouvrement entre deux lignes générées dans les champs optiques respectifs des deux dispositifs, dans le but de réduire la visibilité des zones de recouvrement desdites lignes ;

la figure 2 qui montre le schéma de principe d’un dispositif selon l’invention, permettant la mise en oeuvre du procédé selon l’invention dans le champ optique d’un dispositif de traitement laser, dans le but de rendre l’observation de l’irisation de la surface de la tôle indépendante de l’angle d’observation ;

- la figure 3 qui montre la surface d’une tôle résultant de la mise en oeuvre d’un procédé améliorant le procédé utilisé dans le cas de la figure 1 par deux dispositifs de traitement laser contigus, et dont l’utilisation peut se cumuler avec celle du procédé selon l’invention.

Comme on l’a dit, l’effet d’irisation obtenu par traitement avec un laser à puises ultra-courts est lié à la formation spontanée en surface d’une structure périodique ayant un comportement analogue à un réseau optique sur la lumière se réfléchissant sur la surface. Comme discuté précédemment, le mécanisme de formation de cette structure en vaguelettes réparties périodiquement sur la surface traitée n’a pas encore été établi par la communauté scientifique.

Cependant, il a été montré (voir, par exemple, le document « Control Parameters In Pattern Formation Upon Femtosecond Laser Ablation », Olga Varlamova et al., Applied Surface Science 253 (2007) pp. 7932-7936), que l’orientation des vaguelettes était

principalement liée à la polarisation du faisceau laser irradiant la surface. Ainsi, les HSFL ont une orientation parallèle à la polarisation du faisceau incident alors que les LSFL qui se forment ensuite, lorsqu’une plus grande quantité d’énergie a été apportée en surface de la tôle, ont une orientation perpendiculaire à la polarisation du faisceau incident.

Dans le cas d’un traitement laser par lignes, il en résulte donc qu’une surface traitée sans modification de la polarisation du faisceau laser au cours de ses différents passages sur une ligne donnée de ladite surface présente, en fin de traitement, une structure constituée de lignes/vaguelettes toutes orientées dans la même direction. Ceci induit que l’effet“réseau optique” de la surface est également orienté.

Indeed, the iridescence appears as 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 constitute a drawback for the final product because it requires the orientation of the wavelets to be carefully chosen from the processing stage in order to have a product on which the iridescence appears under the desired observation conditions. In addition, the final product only appears very fully colored in one main viewing direction.

The invention makes it possible to eliminate this drawback, since the device used makes it possible to obtain a surface for which the iridescence is visible in the same way 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 sufficiently different polarizations between two successive lines removes the directionality of the observation of the iridescence.

In the case where the treatment is carried out “in lines”, with a distance separating the centers of the pulses slightly less than the diameter of the pulse in the direction of rapid scanning, so that there are certainly no zones not treated by the pulses, 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 in 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 pull impact has a wavelet orientation different from that of its neighbours. In two neighboring optical fields, points are generated according to matrices which extend one another.

FIG. 2 schematizes a typical architecture of part of a unitary device allowing the implementation of the method according to the invention to treat at least part of a sheet 1 of stainless steel over 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 according to requirements.

The device firstly comprises a laser source 6 of a type conventionally known for producing iridescence on metallic surfaces, therefore, typically a source 6 generating a pulsed laser beam 7 with a 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 previously. The energy injected onto 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 the LSFL type and to avoid the formation of bumps, a fortiori of peaks, and the frequency and the 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 allowing the shape and dimensions of the beam 7 to be adjusted, comprises, according to the invention, a polarizing optical element 10 which makes it possible to impart to the beam 7 a polarization chosen by the operator or the automations 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 straight 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, thanks to 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 mobile support 13 which makes it possible to move the sheet 1, in a plane or, possibly, in the three dimensions of space, with respect to the device for generating, polarizing and scanning the laser beam 7, so that it can treat the surface of the sheet 1 according to a new line of the treatment field of the device shown. But before this processing of said new line, according to the invention the optical polarization device 10 of the laser beam 7 had its adjustment modified, so as to give the laser beam 7 a different polarization from that which it had during the processing of the previous line.

At least two different angles of polarization, and preferably at least three, are capable of being obtained thanks to the optical polarization device 10, and alternate, preferably but not necessarily, periodically at each line change. A periodicity of the polarization pattern is not essential, it suffices, as has been said, that the angles of polarization of two neighboring lines 14, 15, 16 be different, preferably by at least 20° and at most 90°. But a periodicity of the pattern, for example, as shown, with angles of polarization which are repeated every three lines 14, 15, 16, is preferred, insofar as a periodic programming of the change of polarization is simpler than a random programming, especially like two lines 14, 15,

Une succession de polarisations aléatoires à l’intérieur d’un champ optique donné, respectant de préférence néanmoins le minimum d’écart angulaire précité de 20° et le maximum d’écart angulaire précité de 90°, serait acceptable, en particulier si l’installation devait pouvoir être utilisée pour traiter des tôles relativement étroites qui ne nécessiteraient pour cela qu’un champ unique et pour lesquelles la question de l’identité de polarisation sur deux lignes situées dans le prolongement l’une de l’autre et générées dans deux champs voisins ne se pose pas.

L’ensemble du dispositif de traitement de la tôle 1 comporte le plus typiquement une pluralité de dispositifs unitaires tels que celui qui vient d’être décrit, placés face à la tôle 1 , et qui sont juxtaposés de façon à ce que leurs champs de traitement respectifs, c’est-à-dire les champs optiques des systèmes de focalisation 12 des scanners 1 1 , se chevauchent légèrement. Ce chevauchement est, typiquement, de l’ordre de deux fois la taille du puise, et on peut y ajouter une incertitude de position qui est liée à la période d’alimentation du laser en puises et à la vitesse de balayage du laser selon l’axe rapide.

On doit vérifier expérimentalement que ce chevauchement est suffisant pour assurer qu’il ne subsiste pas sur la tôle de zones non traitées à la fin de l’opération. Egalement, les lignes générées par chacun de ces champs doivent être dans la continuité les unes des autres, et les réglages des dispositifs unitaires doivent être identiques, en particulier en termes de forme, dimension, puissance et angle de polarisation à un instant t de leurs faisceaux laser 7 respectifs, pour que le traitement soit homogène sur l’ensemble d’une ligne de la largeur de la tôle 1 , et que l’alternance des angles de polarisation du faisceau laser 7 entre deux lignes consécutives soit identique sur toute la largeur de la tôle.

Les moyens de commande de ces dispositifs unitaires sont, le plus typiquement, des moyens communs à tous les dispositifs unitaires, pour qu’ils agissent en parfaite synchronisation les uns avec les autres. Ils commandent aussi les déplacements du support 13 de la tôle 1.

Bien entendu, on pourrait remplacer le support mobile 13 par un support fixe, et assurer le déplacement relatif de la tôle 1 et des dispositifs unitaires de traitement en plaçant ceux-ci sur un support mobile. Les deux variantes peuvent d’ailleurs être combinées, en ce que le dispositif selon l’invention comporterait à la fois un support mobile 13 pour la tôle 1 et un autre support mobile pour les dispositifs unitaires de traitement, les deux supports pouvant être actionnés l’un ou l’autre, ou les deux simultanément, par le dispositif de commande, selon les souhaits de l’utilisateur.

Le nombre M correspond donc au nombre d’orientations différentes que l’on veut donner aux vaguelettes en assurant un interligne M fois plus grand qu’un traitement classique et en décalant les lignes d’un interligne classique entre chaque réalisation du champ. La figure 3 montre un exemple de l’aspect d’une telle réalisation avec M = 3.

La tôle 1 présente sur sa surface une succession périodique de lignes 14, 15, 16 réalisées à l’aide de deux dispositifs selon l’invention qui ont permis la réalisation de ce motif périodique de trois sortes de lignes 14, 15, 16 sur deux champs optiques 17, 18 contigus, les lignes 14, 15, 16 d’un champ donné étant dans le prolongement de lignes 14, 15, 16 du champ optique voisin.

Les lignes 14, 15, 16 du motif se distinguent les unes des autres par les effets des polarisations différentes que le dispositif de polarisation 10 a appliquées au faisceau laser 7 au moment de leur formation.

Comme on peut le voir sur la partie de la figure 3 qui représente une fraction agrandie de la surface, dans l’exemple représenté qui n’est pas limitatif, la polarisation conférée au laser lors de la génération de la première ligne 14 du motif conduit à une orientation des vaguelettes dans la direction perpendiculaire à la direction relative de défilement 6 de la tôle 1 par rapport au dispositif de traitement laser. Puis, pour générer la deuxième ligne 15 du motif, on a modifié la polarisation du faisceau laser 7 de façon à obtenir une orientation des vaguelettes à 45° de l’orientation des vaguelettes de la première ligne 14. Enfin, pour générer la troisième ligne 16 du motif, on a modifié la polarisation du faisceau laser 7 de façon à obtenir une orientation des vaguelettes à 45° de l’orientation des vaguelettes de la deuxième ligne 15, donc à 90° de l’orientation des vaguelettes de la première ligne 14 : les vaguelettes de la troisième ligne 16 sont donc orientées parallèlement à la direction relative de défilement 6 de la tôle 1 par rapport au dispositif de traitement laser.

In the junction zone of two neighboring fields, an energy greater than that injected into the rest of the surface is injected into the surface of the sheet 1, just as in the prior art previously described. 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 alteration of the visual effect of iridescence of the surface. that 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 a zone of heterogeneity on the surface. You just have to make sure that lines 14, 15, 16 of the two neighboring fields which were carried out 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 was also to be taken in the execution of the methods of the prior art (see FIG. 1), and the equipment known for this purpose can be used in the context of this variant of the invention. It suffices to ensure 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 fields which join. 16 of neighboring fields was also to be taken in the execution of the methods of the prior art (see FIG. 1), and the equipment known for this purpose can be used in the context of this variant of the invention. It suffices to ensure 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 fields which join. 16 of neighboring fields was also to be taken in the execution of the methods of the prior art (see FIG. 1), and the equipment known for this purpose can be used in the context of this variant of the invention. It suffices to ensure 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 fields which join.

The use of M=2 different polarization orientations, phase-shifted, for example, by 90°, already makes it possible to have a visible iridescence effect 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 satisfactorily. This is no longer visible as soon as M is greater than 2, preferably if the angles are separated by between 20° and 90° between two successive lines 14, 15, 16.

Thus, by carrying out a treatment with at least three distinct angles of polarization distributed between 0 and 90° and presenting, preferably, differences in polarization of at least 20° between two successive lines 14, 15, 16, experience shows that the iridescence of the surface is visible from 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 angles of polarization of two neighboring lines are sufficiently different from each other to obtain the desired absence of directionality of the iridescent effect.

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

It is however obvious that the distribution of the structure of the surface according to different orientations induces a reduction 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 angle transverse to the structure). There would therefore be a compromise to be found between the intensity of the visual iridescence effect perceived by the observer and the omnidirectional nature of this iridescence effect. But three directions of polarization (thus a periodicity of three lines of these directions, as shown in Figure 3) already represent, at least in the most common cases, such a good compromise.

In the case where the scanner allows processing “in points”, according to a matrix, the orientation of the wavelets can be modified between the different points of a line and/or between successive lines. It remains however important that each point is formed solely by the accumulation of irradiations sharing the same polarization, if the energy injected to form a given point must 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 producing 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 each having been produced with a different polarization of the laser beam 7.

One could think of realizing the differences between the orientations of the wavelets not by optical means (the polarizer 10), but by mechanical means by operating modifications of the relative orientations of the support 13 of the sheet 1 and of the support of the devices to be scanned laser , typically by rotating the support 13 through 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, however, would not be ideal. Indeed, the precise realization of the wavelets would depend on the possible irregularities of polarization of the laser beam 7, and to rotate 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 to process 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, it will be preferred for M different orientations to periodically alternate a single line of each orientation, of equal width or, preferably (to ensure treatment of the entire surface of the sheet) slightly less than the diameter of the pulse. . In the case of point processing, it will be preferred to periodically alternate the orientations on a square or rectangular pattern containing a number of points equal to the number of different possible orientations for the polarization of the laser beams 7.

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

There is also the possibility of treating 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. For this, it suffices to activate only one of these devices when treating a sheet of small width. The fact of being able to use the process 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 independently of said width, and thus to homogenize the aspect of the range of products of various widths that the manufacturer may wish to produce.

It is possible to process 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 the sheet 1 is also a parameter which can be played on, 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), adapting accordingly the means of relative displacement of the lasers and 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 with 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 in rotation to scroll the surface of the part in the optical fields of the lasers.

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

CLAIMS

1.- Method for producing a visual effect of iridescence on the surface of a part (1), according to which a laser beam (7) is sent onto 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 pulse a structure in the form of wavelets having the same orientation, and scanning is carried out by said scanner (1 1 ) of said surface by said laser radiation (7) according to 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 the said surface and of the device emitting the said laser beam, characterized in that between carrying out the scanning along two consecutive lines (14, 15, 16) or two neighboring points, the polarization of the beam is modified laser (7) so as to create ripples 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 by at most 90°.

4 Method according to one of claims 1 to 3, characterized in that a laser beam (7) is sent onto said surface, with a pulse duration of 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 onto said surface, with a pulse duration of less than a 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) situated 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 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) onto 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.- Unitary 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 in the form of the beam (7), a scanner

(II) which allows the pulse of the beam (7), after passing through a focusing system (12), to scan in the form of lines or a matrix of points in 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 .- Unitary device according to claim 10, characterized in that said device makes it possible to produce two neighboring lines or points with polarization pulses which differ by at least 20° and by at most 90°.

12.- unitary 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 of the distance between the focusing system (12) and the surface of the workpiece (1) to maintain a constant pulse diameter and fluence on said surface, whatever said distance.

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

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