Method For Creating An Iridescent Visual Effect On The Surface Of A Material, Devices For Carrying Out Said Method, And Part Obtained Thereby

Abstract

Method for creating an iridescent visual effect on the surface of a part, using a laser beam having a pulse duration of less than a nanosecond sent onto said surface in the optical field of the focusing system of a device comprising also a laser source and a scanner, to apply wavelets having the same orientation to said surface over the pulse width. The scanner scans the surface using laser radiation along a series of consecutive lines, or a matrix of points using relative movement of said surface and the device, the width of each line or the dimension of each point of each matrix being equal to the pulse diameter. Between the carrying out of the scanning along two consecutive lines or two adjacent points the polarization of the laser beam is modified to create wavelets having different orientations on two consecutive lines or two adjacent points.

Claims

1. A method for creating an iridescence visual effect on the surface of a part, whereby a laser beam having a pulse duration of less than one nanosecond is sent onto said surface in the optical field of the focusing system of a device comprising a laser source a scanner and said focusing system, 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 scans said surface with said laser radiation along a series of consecutive 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 relative travel of said surface and device emitting said laser beam, wherein between the carrying out of the scanning along two consecutive lines or two adjacent points, the polarization of the laser beam is modified so as to create wavelets of different orientations on two consecutive lines or two adjacent points.

2. The method according to claim 1, wherein the polarization of the laser beam is modified according to a periodic pattern, said periodic pattern extending over M consecutive lines, M being equal to at least 2.

3. The method according to claim 1, character wherein two consecutive lines or two adjacent points have angles of polarization differing by at least 20° and at most 90°.

4. The method according to one of claim 1, wherein a laser beam, with a pulse duration of less than one nanosecond, is sent onto said surface in the optical field of the focusing system of a first device comprising a laser source, a scanner and said focusing system, in that a laser beam with a pulse duration of less than one nanosecond is sent onto said surface in the optical field of the focusing system of at least one second device comprising a laser source, a scanner and said focusing system, with the polarizations of two lines located in the extension of each other or of two adjacent points belonging to two adjacent fields being identical.

5. The method according to claim 1, wherein said relative travel 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. The method according to claim 1, wherein said relative movement of said surface of said part and of the device(s) emitting said laser beam(s) is carried out by placing the device(s) emitting said laser beam(s) on a mobile support.

7. The method according to one of claim 1, wherein said part is a sheet metal.

8. The method according to one of claim 1, wherein said surface of said part is three-dimensional

9. The method according to one of claim 1, wherein said part is a stainless steel.

10. An unit device for imparting an iridescent effect to the surface of a part through the formation of wavelets on said surface by the pulse of a laser beam, comprising a laser source generating a laser beam of pulse duration of less than 1 ns, an optical system shaping the beam, a scanner which enabling pulse of the beam, after it has passed through a focusing system, to scan an optical field on the surface of the part in the form of lines or a matrix of points, and means for creating 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, wherein said optical system comprises an optical polarizing system imparting determined polarization on said beam, and means for varying this polarization so that, on said surface, two lines or two contiguous points are produced with pulses of different polarizations.

11. The unit device according to claim 10, wherein said device allows the forming of two contiguous points with pulses whose polarizations differ by at least 20° and at most 90°.

12. The unit device according to claim 10, wherein it comprises 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 in order to maintain a constant pulse diameter and fluence on said surface, irrespective of said distance.

13. A device for imparting an iridescent effect on the surface of a part by the formation of wavelets on said surface by a laser beam pulse, wherein it comprises at least two unit devices according to claim 10, whose optical fields of the focusing systems overlap.

14. The device according to claim 10, wherein said means for creating a relative movement between said device and said part comprise a mobile support for the part.

15. The device according to one of claim 10, wherein said means for creating a relative movement between said device and said part comprise a mobile support for the unit device(s).

16. A part made of a material whose surface has an iridescent effect by means of a laser treatment, said treatment having formed wavelets on the surface of said part, wherein said wavelets have at least two orientations, distributed over the surface of said part.

Description

[0074] The invention will be better understood on reading the following description given with reference to following appended Figures:

[0075] FIG. 1, which shows, as mentioned in the introduction, the surface of a metal sheet on which an iridescent laser treatment has been carried out by a method according to the known prior art, by means of two contiguous laser devices of a known type, randomly forming lines located in the extension of each other with overlapping areas between two lines generated in the respective optical fields of the two devices, with the object of reducing the visibility of the overlapping areas of said lines;

[0076] FIG. 2, which shows the schematic diagram of a device according to the invention, allowing implementation of the method of the invention in the optical field of a laser treatment device, with the object of allowing observation of surface iridization of the metal sheet independently of observation angle;

[0077] FIG. 3, which shows the surface of a metal sheet resulting from implementation of a method improving the method used in the case of FIG. 1 by two contiguous laser treatment devices, and whose use may be cumulative with that of the method according to the invention.

[0078] As indicated, the iridescent effect obtained by treatment with an ultrashort pulse laser is related to the spontaneous formation on the surface of a periodic structure having a behavior similar to an optical network on surface-reflected light. As previously discussed, the formation mechanism of this wavelet structure distributed periodically over the treated surface has not yet been established by the scientific community.

[0079] However, it has been shown (see, for example, the paper “Control Parameters In Pattern Formation Upon Femtosecond Laser Ablation”, Olga Varlamova et al, Applied Surface Science 253 (2007) pp. 7932-7936), that the orientation of wavelets is chiefly related to the polarization of the laser beam irradiating the surface. Thus, the orientation of HSFLs is parallel to the polarization of the incident beam whereas LSFLs, which are subsequently formed when a greater amount of energy is delivered to the sheet surface, are oriented perpendicular to polarization of the incident beam.

[0080] For laser treatment by lines, it thus results that a surface treated without modification of polarization of the laser beam throughout the different passes thereof on a given line of said surface, would therefore result at the end of treatment in a structure composed of lines/wavelets all oriented in the same direction. This means that the “optical network” effect of the surface is also oriented.

[0081] Indeed, iridescent effect appears maximal if observation is made in transverse direction to the orientation of the wavelets and decreases as and when the orientation angle of observation aligns with the structure of the surface. Therefore, observation of the surface in the alignment of the wavelets does not cause any color to appear. This can be a disadvantage for the end product because the orientation of the wavelets must be chosen carefully at the start of treatment in order to obtain a product having the iridescent effect under the desired viewing conditions. Moreover, the end product only appears fully colored in one main viewing direction.

[0082] The invention makes it possible to avert this disadvantage, because the device used makes it possible to obtain a surface for which the iridescent effect is visible in an identical way in all directions of observation. If two consecutive fields, together forming the same line, have the same polarization along this line, the visual effect of 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°. Also, having polarizations that definitely differ sufficiently between two consecutive lines obviates the directionality of observation of the iridescent effect. The combination of these phenomena makes the iridescent effect of the treated sheet appear much more uniform, in all viewing directions than is the case where there is not this alternation of polarization between contiguous lines.

[0083] Where the treatment is performed “in lines”, with a distance separating the centers of the pulses slightly that is slightly smaller than the diameter of the pulse in the direction of fast scanning, to ensure that there are no zones not treated by the pulse, the solution according to the invention is to alternate lines for which wavelet orientation is modified from one line to another, via the action of a polarizer or any other type of polarizing optical device positioned on the optical pathway of the beam.

[0084] Therefore, either the treatment field is obtained with an automatic system allowing modification of the polarization of the incident beam between each line, or the treatment field is obtained in a number of times M equal to at least two, and preferably to at least three, M thus corresponding to the number of different orientations imparted to the wavelets by the periodically consecutive polarizations of the laser beam pulse forming these wavelets.

[0085] The principle of the invention is also valid when the treatment is carried out “by points” according to a matrix. Each point corresponding to a pulse impact has a different wavelet orientation than its neighbors. In two contiguous optical fields, points are generated according to matrices that extend each other.

[0086] FIG. 2 shows a typical architecture of a part of a unit device allowing implementation of the method of the invention, to treat at least part of a stainless steel sheet 1 on a given field. Of course, this device is controlled by automated means, allowing synchronization of the relative movements of the support 13 of the sheet 1 and of the laser beam 7, as well as to adjustment of the parameters of the laser beam 7 and its polarization, as required.

[0087] The device first comprises a laser source 6 of a type conventionally known to obtain iridescent effects on 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 typically being of the order of 30 to 40 μm, for example, as seen previously. The energy injected on the surface of the stainless steel by the pulse is to be determined experimentally, so as to generate LIPPS wavelets on the surface of the sheet 1, preferably of the LSFL type, and to prevent the formation of bumps, even more so of spikes, and the frequency and power of the laser beam 7 must be chosen accordingly, following criteria known for this purpose to those skilled in the art and having regard to 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 beam shaping system 8, which, in addition to its conventional components 9 allowing adjustment of the shape and dimensions of the beam 7, includes, according to the invention, a polarizing optical element 10 which makes it possible to confer a polarization, chosen by the operator or automations that manage the device, on the beam 7.

[0088] The laser beam 7 then passes through a scanning device (e.g. a scanner) 11 which, as is known, enables 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 as is conventional, there is a focusing system 12, such as a focusing lens, by means of which the laser beam 7 is focused in the direction of the sheet 1.

[0089] In the example shown, the sheet 1 is carried by a mobile support 13, allowing movement of the sheet along a plane or optionally in the three spatial dimensions relative to the device generating, polarizing and scanning the laser beam 7, so that the latter is able to process the surface of the sheet 1 along a new line of the treatment field of the illustrated device. But before this treatment of said new line, according to the invention, the optical polarization device 10 of the laser beam 7 has had its setting modified, so as to impart polarization to the laser beam 7 that differs from its previous polarization when treating the preceding line.

[0090] At least two different angles of polarization and preferably at least three are able to be obtained with the polarization optical device 10, and are alternated, preferably but not necessarily, periodically at each line change. Periodicity of the polarization pattern is not essential; it is sufficient, as mentioned, that the polarization angles of two adjacent lines 14, 15, 16 are different, preferably by at least 20° and at most 90°. However, periodicity of the pattern, for example as illustrated with polarization angles that are repeated every three lines 14, 15, 16, is preferred insofar as periodic programming of polarization change is simpler than random programming, in particular since two lines 14, 15, 16 belonging to two different fields and lying in the continuation of each other must have the same wavelet orientation.

[0091] A succession of random polarizations within a given optical field, preferably respecting the aforementioned minimum angular difference of 20° and the aforementioned maximum angular difference of 90°, would be acceptable, in particular if the facility were to be used to process relatively narrow sheets would only require a single field for this purpose and for which the question of polarization identity on two lines located in the extension of each other and generated in two contiguous fields does not arise.

[0092] The whole device for treatment the sheet 1 most typically comprises a plurality of unit devices such as just described, placed facing the sheet 1, and which are juxtaposed so that their respective treatment fields, i.e., the optical fields of the focusing systems 12 of the scanners 11, overlap slightly. This overlapping is typically about twice the size of the pulse, plus positional uncertainty related to the pulse feed period of the laser and the scanning speed of the laser along the fast axis. It must be verified experimentally that this overlap is sufficient to ensure that no untreated areas remain on the sheet at the end of the operation. Additionally, the lines generated by each of these fields must be in continuity with each other, and the settings of the unit devices must be identical, particularly in terms of shape, size, power and angle of polarization at an instant t of their respective laser beams 7, so that treatment is homogeneous over an entire line having the width of the sheet 1, and so that the alternation of the polarization angles of the laser beam 7 between two consecutive lines is identical over the whole width of the sheet.

[0093] The means controlling these unit devices are most typically means common to all the unit devices so that they operate in perfect synchronization with each other. They also control the movements of the support 13 of the sheet 1.

[0094] Of course, the mobile support 13 could be replaced by a fixed support, and the relative travel of the sheet 1 and the unit treatment devices could be ensured by placing them on a mobile support. Both variants could also be combined, in that the device of the invention would comprise both a mobile support 13 for the sheet 1 and another mobile support for the unit treatment devices, either one of the two possibly being actuated or both simultaneously by the control device as desired by the user.

[0095] The number M thus corresponds to the number of different orientations that one wants to give to the wavelets by ensuring a line spacing M times larger than conventional treatment and by offsetting the lines by conventional spacing between each field implementation. FIG. 3 shows an example of the effect of said creation with M=3.

[0096] The sheet 1 on its surface exhibits a periodic succession of lines 14, 15, 16 formed by two devices of the invention which allowed the creation of 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 lying in the continuation of lines 14, 15, 16 of the contiguous optical field.

[0097] The lines 14, 15, 16 in the pattern differ from each other by the effects of the different polarizations that the polarization device 10 applied to the laser beam 7 at the time of their formation.

[0098] As can be seen in the portion of FIG. 3 illustrating a magnified fraction of the surface, in the illustrated, nonlimiting example, the polarization imparted to the laser during 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 in relation 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 to obtain 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, the polarization of the laser beam 7 was modified so as to obtain an orientation of the wavelets at 45° of the orientation of the wavelets of the second line 15, hence at 90° of the orientation of the wavelets of the first line 14: the wavelets of the third line 16 are thus oriented parallel to the relative direction of movement 6 of the sheet 1 in relation to the laser treatment device.

[0099] In the junction zone of two contiguous fields, more energy is injected onto the surface of the sheet 1 than is injected onto the rest of the surface, just as in the prior art previously described. However, the fact that in this junction zone the lines 14, 15, 16 of each optical field that meet were produced with the same polarization of the laser beam 7 clearly attenuates deterioration of the visual iridescent effect of the surface which would be encountered if there were no controlled polarization of the laser beam 7. Lack of continuity of the orientation of the wavelets from one optical field to another would tend to increase the visibility of the junction zone of the fields on a given line 14, 15, 16, creating an area of heterogeneity on the surface. Care must simply be taken to ensure that the lines 14, 15, 16 of the two contiguous fields made with identical polarizations are in line with each other, but this precaution about the co-linearity of the lines 14, 15, 16 of contiguous fields was also to be taken in implementing methods of the prior art (see FIG. 1), and the equipment known for this purpose can be used in this variant of the invention. It only needs be ensured that the polarization changes of the laser beams 7 of the devices relating to each field are carried out with the same values for the joining lines of the fields.

[0100] The use of M=2 orientations of different polarizations offset for example by 90°, is already sufficient to obtain a visible iridescent effect along most viewing directions. However, the intensity of the iridescent effect still varies fairly substantially when viewing at an angle of 45°, and it can be considered that the problem of lack of directionality of the iridescent effect is still not solved in fully satisfactory manner. This is no longer visible as soon as M is higher than 2, preferably if the angles differ by more than 20° between two consecutive lines 14, 15, 16.

[0101] Therefore, by performing treatment with at least three different angles of polarization distributed between 0 and 90° and preferably having polarization differences of at least 20° between two consecutive lines 14, 15, 16, experience has shown that the iridescent effect of the surface is visible in all directions with similar intensity. It is possible to use a number of orientations M higher than 3, but care must then be taken to ensure that the polarization angles of two contiguous lines differ sufficiently from each other to avoid directionality of the desired iridescent effect.

[0102] The same condition of a polarization difference of at least 20° between two contiguous points should preferably be respected in the case of a point treatment.

[0103] It is evident, however, that the surface structure distribution in different orientations induces a decrease in the total intensity of the iridescent effect when compared with a surface treated in a single polarization direction and viewed at an optimal angle (transverse angle to the structure). A trade-off must therefore be found between the intensity of the visual iridescent effect perceived by an observer and the omnidirectional nature of this iridescent effect. However, three polarization directions (hence a periodicity of three lines of these directions, as illustrated in FIG. 3) already represent said good trade-off, at least in most cases.

[0104] Where the scanner allows treatment “in points”, according to a matrix, the wavelet orientation can be modified between the different points of a line and/or between consecutive 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 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 making M arrays of points, with M equal to at least 2 and preferably at least 3, each having a different wavelet orientation, in other words each having been made with a different polarization of the laser beam 7.

[0105] One could think of making differences in wavelet orientations not by optical means (the polarizer 10), but by mechanical means, by making modifications of the relative orientations of the support 13 of the sheet 1 and of the support of the laser scanner devices, typically by making the support 13 rotate by an angle equal to the desired difference in orientation for wavelets of a given line 14, 15, 16 in relation to that of the line 14, 15, 16 previously made. But this solution would not be ideal. Indeed, the precise creation of the wavelets would depend on possible polarization irregularities 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 facility intended to treat heavy and large objects. The use and control of a polarizer 10 is generally simpler to implement.

[0106] 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 is preferable to periodically alternate a single line of each orientation, with a width equal to or preferably slightly less than the diameter of the pulse (to ensure treatment of the entire surface of the sheet). In the case of spot treatment, it is preferable to periodically alternate the orientations on a square or rectangular pattern containing a number of spots equal to the number of different orientations possible for the polarization of the laser beams 7.

[0107] Of course, it would still be in the spirit of the invention to apply this method to a sheet whose relatively small width would require only one scanner to perform the structuring of its entire surface into lines of different polarizations in a periodic pattern. The main advantage of the invention is that the intensity of the iridescent effect does not depend on the angle the sheet is observed. If one only wants to treat such narrow sheets, one can then afford to do so with a facility that would include only one device according to FIG. 2.

[0108] It is also possible, on the same facility, to process both sheets of a relatively small width, less than or equal to that of a treatment field of a device according to FIG. 2, and sheets of larger width requiring the juxtaposition of several devices according to FIG. 2, each acting on a single treatment field. For this purpose, it is sufficient to activate only one of these devices when treating a narrow sheet. The fact that the method of the invention can be used for multiple sheet widths, with the same settings for each field taken individually, makes it possible to obtain sheets of identical effect irrespective of said width, and thus to homogenize the effect of the range of products of various widths that the manufacturer may wish to produce.

[0109] It is possible to process sheets 1 not having perfect planarity by including means in the treatment device to measure the distance between the focusing system 12 and the sheet 1, and by coupling these with the means for controlling the focusing system 12, so that the latter can guarantee that the diameter of the pulse and the fluence of the laser beam are substantially the same irrespective of the effective distance between the focusing system 12 and the sheet 1. The distance between the focusing system and the surface of the metal sheet 1 is also a parameter that can be influenced, if it can be adjusted in real time by appropriate mechanical means.

[0110] It is also possible to envisage the application of the method to materials other than planar metal sheets (for example to formed sheets, bars, tubes, parts generally comprising three-dimensional surfaces), by accordingly adapting the means for relative movement of the lasers and part to be treated, and/or the controls of the focusing means if differences in distance between the laser emitter and the surface are to be managed. For parts having substantially cylindrical surfaces (bars, tubes of circular section for example), one manner of proceeding would be to place the laser devices on a fixed support and to provide a support for the part allowing the part to be placed in rotation so that the surface of the part travels in the optical fields of the lasers.

[0111] Finally, it is recalled that while stainless steels are materials to which the invention is preferentially applicable, other metal and nonmetal materials on which an iridescent effect can be obtained on the surface thereof by laser treatment are also concerned by the invention.