Method for modifying the appearance of a surface

Abstract

A process for modifying the appearance of a surface is provided. The process includes a stage of spraying particles exhibiting a maximum size of less than or equal to 500 m. The sprayed particles exhibit a relative density of greater than 90%, more than 5% and less than 80% by volume of the sprayed particles being particles exhibiting a salient sharp edge. The salient sharp edge is referred to as notching particles.

Claims

1. Process for modifying the appearance of a surface, the said process comprising a stage of spraying particles exhibiting a maximum size of less than or equal to 500 m, said particles exhibiting a maximum size of less than or equal to 500 m being referred to as sprayed particles, the sprayed particles exhibiting a relative density of greater than 90%, more than 5% and less than 80% by volume of the said sprayed particles exhibiting a salient sharp edge, said sprayed particles exhibiting a salient sharp edge being referred to as notching particles, said sprayed particles not exhibiting a salient sharp edge being referred to as non-notching particles.

2. Process according to claim 1, in which the group of the sprayed particles comprises more than 20% and less than 60%, by volume, of notching particles.

3. Process according to claim 1, in which the group of the sprayed particles exhibits a maximum size of less than 400 m and exhibits a minimum size of greater than 15 m.

4. Process according to claim 3, in which the group of the sprayed particles exhibits a maximum size of less than 200 m and a minimum size of greater than 30 m.

5. Process according to claim 4, in which the group of the sprayed particles exhibits a maximum size of less than 150 m.

6. Process according to claim 1, in which the ratio of a mean dimension of the notching particles to a mean dimension of the non-notching particles is less than 3.

7. Process according to claim 1, in which the sprayed particles exhibit a relative density of greater than 96%.

8. Process according to claim 1, in which the group of the notching particles exhibits a mean circularity squared of less than 0.9 and the group of the non-notching particles exhibits a mean circularity squared of greater than 0.7.

9. Process according to claim 1, in which the mean number of facets of the notching particles is greater than 3 and less than 30.

10. Process according to claim 9, in which the mean number of facets of the notching particles is less than 15.

11. Process according to claim 1, in which the sprayed particles are made of a ceramic material.

12. Process according to claim 11, in which the sprayed particles are made of a ceramic material, chosen from oxides, nitrides, carbides, borides, oxycarbides, oxynitrides and mixtures of oxides, nitrides, carbides, borides, oxycarbides, and oxynitrides.

13. Process according to claim 12, in which the group of the notching particles exhibits a composition such that Al.sub.2O.sub.3+ZrO.sub.2+SiO.sub.2>80%, as percentage by weight on the basis of the oxides.

14. Process according to claim 12, in which the group of the non-notching particles exhibits a composition such that Al.sub.2O.sub.3+ZrO.sub.2+SiO.sub.2>80%, as percentage by weight on the basis of the oxides.

15. Process according to claim 12, in which the sprayed particles are composed, for more than 80% of their weight, of silicon carbide.

16. Process according to claim 12, in which the group of the notching particles is composed, for more than 80% of its weight, of silicon carbide.

17. Process according to claim 12, in which the group of the non-notching particles is composed, for more than 80% of its weight, of silicon carbide.

18. Process according to claim 12, in which the sprayed particles exhibit a composition such that Al.sub.2O.sub.3+ZrO.sub.2+SiO.sub.2>80%, as percentage by weight on the basis of the oxides.

19. Process according to claim 18, in which the sprayed particles: exhibit a composition such that, as percentage by weight on the basis of the oxides: 70%Al.sub.2O.sub.3, Al.sub.2O.sub.3 constituting the remainder to 100%, 3%ZrO.sub.2+HfO.sub.220%, with HfO.sub.21%, 1%SiO.sub.210%, 0.3%CaO+MgO5%, other constituents <5%.

20. Process according to claim 18, in which the sprayed particles exhibit a composition such that, as percentage by weight on the basis of the oxides: Al.sub.2O.sub.310%, 60%ZrO.sub.2+HfO.sub.270%, with HfO.sub.21%, 25%SiO.sub.235%, other constituents <5%.

21. Process according to claim 18, in which the sprayed particles exhibit a composition such that, as percentage by weight on the basis of the oxides: Al.sub.2O.sub.310%, 65%ZrO.sub.2+HfO.sub.280%, with HfO.sub.21.5%, 10%SiO.sub.220%, 4%Y.sub.2O.sub.38%, other constituents <3%.

22. Process according to claim 18, in which the sprayed particles exhibit a composition such that, as percentage by weight on the basis of the oxides: 90%Al.sub.2O.sub.3, other constituents <10%.

23. Process according to claim 18, in which the sprayed particles are composed, for more than 80% of their weight, of zirconia which is at least partially stabilized, preferably at least partially stabilized with yttrium oxide.

24. Process according to claim 18, in which the sprayed particles exhibit a composition such that, as percentage by weight on the basis of the oxides: 70%Al.sub.2O.sub.380%, 20%ZrO.sub.2+HfO.sub.230%, with HfO.sub.21%, other constituents 3%.

25. Process according to claim 18, in which the group of the notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: 70%Al.sub.2O.sub.3, Al.sub.2O.sub.3 constituting the remainder to 100%, 3%ZrO.sub.2+HfO.sub.220%, with HfO.sub.21%, 1%SiO.sub.210%, 0.3%CaO+MgO5%, other constituents <5%.

26. Process according to claim 18, in which the group of the notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: Al.sub.2O.sub.310%, 60%ZrO.sub.2+HfO.sub.270%, with HfO.sub.21%, 25%SiO.sub.235%, other constituents <5%.

27. Process according to claim 18, in which the group of the notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: Al.sub.2O.sub.310%, 65%ZrO.sub.2+HfO.sub.280%, with HfO.sub.21.5%, 10%SiO.sub.220%, 4%Y.sub.2O.sub.38%, other constituents <3%.

28. Process according to claim 18, in which the group of the notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: 90%Al.sub.2O.sub.3, other constituents <10%.

29. Process according to claim 18, in which the group of the notching particles is composed, for more than 80% of its weight, of zirconia which is at least partially stabilized.

30. Process according to claim 18, in which the group of the notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: 70%Al.sub.2O.sub.380%, 20%ZrO.sub.2+HfO.sub.230%, with HfO.sub.21%, other constituents 3%.

31. Process according to claim 18, in which the group of the non-notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: 70%Al.sub.2O.sub.3, Al.sub.2O.sub.3 constituting the remainder to 100%, 3%ZrO.sub.2+HfO.sub.220%, with HfO.sub.21%, 1%SiO.sub.210%, 0.3%CaO+MgO5%, other constituents <5%.

32. Process according to claim 18, in which the group of the non-notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: Al.sub.2O.sub.310%, 60%ZrO.sub.2+HfO.sub.270%, with HfO.sub.21%, 25%SiO.sub.235%, other constituents <5%.

33. Process according to claim 18, in which the group of the non-notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: Al.sub.2O.sub.310%, 65%ZrO.sub.2+HfO.sub.280%, with HfO.sub.21.5%, 10%SiO.sub.220%, 4%Y.sub.2O.sub.38%, other constituents <3%.

34. Process according to claim 18, in which the group of the non-notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: 90%Al.sub.2O.sub.3, other constituents <10%.

35. Process according to claim 18, in which the group of the non-notching particles is composed, for more than 80% of its weight, of zirconia which is at least partially stabilized.

36. Process according to claim 18, in which the group of the non-notching particles exhibits a composition such that, as percentage by weight on the basis of the oxides: 70%Al.sub.2O.sub.380%, 20%ZrO.sub.2+HfO.sub.230%, with HfO.sub.21%, other constituents 3%.

37. Process according to claim 1, in which the sprayed articles are sprayed on said surface along a direction forming a spraying angle with the surface, the spraying angle being greater than 45.

38. Process according to claim 37, in which the spraying angle is greater than 50.

39. Process according to claim 37, in which the particles are sprayed by passing through a nozzle situated at a distance referred to as spraying distance, from the treated surface, the said spraying distance is greater than 10 cm and less than 25 cm.

40. Process according to claim 37, in which the particles are sprayed onto the surface by being carried by a fluid, the pressure of which is greater than 1 bar and less than 3 bar.

41. Process according to claim 37, in which the particles are sprayed with a degree of coverage of greater than 150% and of less than 250%.

42. Process according to claim 1, in which the surface is made of a metal material, the surface being devoid of a coating.

43. Process according to claim 1, in which the notching particles are mixed with the non-notching particles before being sprayed.

44. Process according to claim 1, in which, before the stage of spraying particles, the surface is polished so that its roughness Ra is less than or equal to 1 m.

45. Process according to claim 1, in which the surface is a surface of a product selected from the group formed by a jewel, a watch, a bracelet, a necklace, a ring, a broach, a tiepin, a handbag, a piece of furniture, a household utensil, a handle, a button, a veneer, a visible part of a consumer goods device, a part of a spectacle frame, a piece of crockery or a frame.

46. Process according to claim 1, in which the particles are sprayed by passing through a nozzle situated at a distance, referred to as spraying distance, from the treated surface, the said spraying distance being greater than 5 cm and less than 30 cm.

47. Process according to claim 1, in which the particles are sprayed onto the surface by being carried by a fluid, the pressure of which is greater than 0.5 bar and less than 4 bar.

48. Process according to claim 1, in which the particles are sprayed with a degree of coverage of greater than 100% and with a degree of coverage of less than 300%.

49. Process according to claim 1, in which the surface is made of stainless steel, of aluminium or of titanium, the surface being devoid of a coating.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other characteristics and advantages of the invention will become more apparent on reading the detailed description which will follow and on examining the appended drawing, in which:

(2) FIGS. 1 and 2 represent photographs of the sprayed particles (a) used in the process of Comparative Example 1 and of the sprayed particles (c) used in the process of Example 3 according to the invention, respectively, and

(3) FIGS. 3 and 4 represent photographs of surfaces treated in a process conventionally using spherical beads in accordance with Comparative Example 1 and according to the process of Example 3 according to the invention, respectively.

(4) In the figures, identical references are used to denote identical or analogous elements.

DETAILED DESCRIPTION

(5) The known techniques for cosmetic finishing treatment by spraying may be employed, using particles as described above.

(6) The surface to be treated may be subjected, before treatment by spraying, to a pretreatment, for example a polishing, so that the surface to be treated exhibits a roughness Ra of less than or equal to 1 m, preferably less than or equal to 0.8 m, preferably less than or equal to 0.5 m, preferably less than or equal to 0.3 m, preferably less than or equal to 0.2 m. The polishing can, for example, be of mirror type.

(7) In one embodiment, the surface onto which the particles are sprayed does not comprise a coating. In one embodiment, only particles exhibiting a maximum size of less than or equal to 500 m and a relative density of greater than 90% are sprayed in order to modify the appearance of the surface to be treated, more than 5% and less than 80% by volume of the said sprayed particles being notching particles.

(8) Preferably again, throughout the treatment of the surface to be treated, the amount by volume of notching particles in the group of the sprayed particles is substantially constant, whatever the moment considered. Preferably, the variation in the amount by volume of notching particles in the group of the sprayed particles, measured between the beginning and the end of the treatment, is less than 20%, preferably less than 10%, preferably less than 5%, on the basis of the said amount at the beginning of the treatment.

(9) Preferably, the sharp edges of the notching particles employed in a process according to the invention are capable of resulting from breakages of particles of larger origin. In one embodiment, they result from such breakages. In particular, the notching particles may be obtained by grinding larger particles, for example beads, for example by grinding using a roll mill.

(10) Preferably, the notching particles exhibit at least one substantially flat face.

(11) Preferably, the substantially flat surfaces cover more than 70%, more than 80%, more than 90%, indeed even substantially 100%, of the surface of the notching particles.

(12) The non-notching particles may be prepared by any technique known to a person skilled in the art which makes it possible to obtain non-notching particles, in particular beads, for example by atomization, by lapping, by granulation or by a process of gelling droplets of a suspension.

(13) In one embodiment, the group of the notching particles and the group of the non-notching particles exhibit substantially the same chemical analysis. Preferably, if the content of a constituent in a first group is greater than 10%, it preferably differs by less than 6%, preferably by less than 5%, preferably by less than 3%, as absolute percentage, from the corresponding content in the second said group. Preferably, if the content of a constituent in a first group is greater than 0.5% and less than or equal to 10%, it preferably differs by less than 40%, preferably by less than 30%, preferably by less than 20%, from the corresponding content in the second said group.

(14) In a preferred embodiment, the process comprises the following stages, preceding the spraying of the particles onto the surface to be treated: a) preparation of a powder formed of notching particles and of a powder formed of non-notching particles, b) mixing the powder formed of notching particles and the powder formed of non-notching particles.

(15) In stage a), the powder formed of notching particles may be prepared by any technique known to a person skilled in the art which makes it possible to obtain notching particles, for example by grinding, preferably using a roll mill. In stage b), the mixing of the powder formed of notching particles and of the powder formed of non-notching particles may be carried out according to any technique known to a person skilled in the art, for example using a mixer.

(16) Notching particles and non-notching particles are preferably mixed in an amount such that the volume of the notching particles represents more than 5%, preferably more than 10%, preferably more than 20%, preferably more than 30%, and less than 80%, preferably less than 70%, more preferably less than 60%, of the volume of the mixture.

(17) For the implementation of the invention, a compressed air blasting machine, preferably a pressurized blasting machine and preferably a Venturi-effect blasting machine is preferably used.

(18) The spray nozzle of the blasting machine preferably exhibits a diameter of greater than 6 mm, preferably greater than 7 mm, and/or of less than 10 mm, preferably less than 9 mm, preferably of approximately 8 mm.

(19) A process according to the invention makes it possible to maintain, indeed even to reduce, the Almen intensity, that is to say the energy deposited on the surface treated. Advantageously, this result makes it possible to limit the risks of deformation of the surface.

(20) A process according to the invention may in particular be carried out in order to reduce the gloss of a surface. To this end, from a first test, it is possible: to increase the volume of notching particles, and/or to increase the number of sharp edges, in particular of facets, of the notching particles, and/or to reduce the size of the sprayed particles, and/or to reduce the dimension of the notching particles.

(21) The gloss of a metal surface, in particular made of aluminium, may be thus reduced by more than 10%, indeed even by more than 30%, indeed even by more than 70%, without increasing the Almen intensity of the said surface, indeed even while reducing it.

(22) If after a first test, the gloss obtained is too low, in order to obtain a surface exhibiting a greater gloss starting from the same original surface, it is possible: to reduce the volume of notching particles, and/or to reduce the number of sharp edges, in particular of facets, of the notching particles, and/or to increase the size of the sprayed particles, and/or to increase the dimension of the notching particles.

(23) A process according to the invention may in particular be carried out in order to reduce the lightness L of a surface. To this end, starting from a first test, it is possible: to increase the volume of notching particles, and/or to reduce the size of the sprayed particles, and/or to decrease the dimension of the notching particles.

(24) The lightness L of a metal surface, in particular made of aluminium, may be thus reduced by more than 10%, indeed even by more than 20%, indeed even by more than 30%.

(25) If, after a first test, the lightness L obtained is too low, in order to obtain a surface exhibiting a greater lightness L starting from the same original surface, it is possible: to reduce the volume of notching particles, and/or to increase the size of the sprayed particles, and/or to increase the dimension of the notching particles.

(26) The surface obtained, preferably exhibiting an area of greater than 1 mm.sup.2, than 1 cm.sup.2, than 10 cm.sup.2, is covered, for more than 80%, preferably for more than 90%, preferably for 100%, with cavities, more than 90% by number of the said cavities exhibiting a size of less than 300 m and being a mixture of cavities existing in the form of scales and of cavities existing in the form of notches. The cavities existing in the form of a notch are mainly created by the impact of the notching particles sprayed onto the surface, whereas the cavities existing in the form of scales are mainly created by the impact of the non-notching particles.

(27) The following nonlimiting examples are given with the aim of illustrating the invention.

(28) The following particles were tested: Group of particles (a) of Comparative Example 1: Powder formed of Microblast B170 beads sold by Saint-Gobain Zirpro exhibiting the following characteristics: chemical analysis: Al.sub.2O.sub.3: 6%, ZrO.sub.2: 63%, SiO.sub.2: 30%, others: 1%, particles obtained by melting-solidification, passing through the square-meshed sieve with an opening equal to 90 m and not passing through the square-meshed sieve with an opening equal to 45 m, median size: 74 m, relative density of the particles, measured on the group of the said particles: 98%, bulk density of the particles, measured on the group of the said particles: 3.90 g/cm.sup.3, mean circularity squared of the group of the particles: 0.97, amount of notching particles: <1% by volume. Powder formed of notching particles used in the groups of particles (b) to (d), and (f), of Examples 2 to 4, and 6, respectively: Powder formed of Zirgrit F grains sold by Saint-Gobain Zirpro exhibiting the following characteristics: chemical analysis: Al.sub.2O.sub.3: 6%, ZrO.sub.2: 63%, SiO.sub.2: 30%, others: 1%, particles obtained by melting-solidification, then grinding, median size: 50 m, relative density of the particles, easured on the group of the said particles: 98%, bulk density of the particles, measured on the group of the said particles: 3.90 g/cm.sup.3, mean circularity squared of the group of the particles: 0.83, amount of notching particles: >99% by volume. Powder formed of notching particles used in the group of particles (e) of Example 5: a powder formed of Sika ABR F150 silicon carbide grains sold by Saint-Gobain, sieved so as to recover the part passing through the square-meshed sieve with openings equal to 125 m and not passing through the square-meshed sieve with an opening equal to 45 m, and exhibiting, after sieving, the following characteristics: chemical analysis: SiC>99% by weight, median size: 72 m, relative density of the particles, measured on the group of the said particles: 99%, bulk density of the particles, measured on the group of the said particles: 3.19 g/cm.sup.3, mean circularity squared of the group of the particles: 0.75, amount of notching particles >99% by volume. Powder formed of notching particles (g) used in Example 7: powder formed of abrasive alumina/zirconia grains, exhibiting the following characteristics: chemical analysis by weight: Al.sub.2O.sub.3: 57%, ZrO.sub.2: 40%, SiO.sub.2: 0.44%, Y.sub.2O.sub.3: 0.45%, TiO.sub.2: 1.61%, others: 0.5%, particles obtained by melting-solidification, then grinding, median size: 106 m, relative density of the particles, measured on the group of the said particles: 99%, bulk density of the particles, measured on the group of the said particles: 4.6 g/cm.sup.3, amount of notching particles >99% by volume.

(29) The notching particles were subsequently mixed, in the proportions by volume shown in Table 1, with the particles (a) of Comparative Example 1 in order to obtain the groups of particles (b) to (f) of Examples 2 to 6 respectively according to the invention.

(30) The characteristics of the groups of particles (a) to (f) of Examples 1 to 6 respectively appear in Table 1.

(31) The groups of particles (a) to (f) were subsequently used to treat the surface of a plate made of 6063 aluminium, exhibiting, before treatment, the following characteristics: a lightness L equal to 70, a gloss G equal to 100.

(32) The said treatment was carried out using a DUP suction blast machine with the following parameters: diameter of the nozzle: 8 mm, pressure: 2 bar, spraying distance: 15 cm, spraying angle: 85, degree of coverage: 100%.

(33) Example 7 consists of a first spraying of a powder formed of particles (a) of Comparative Example 1, followed by a second spraying of a powder formed of notching particles (g), the characteristics of which appear in Table 1. The sprayings are thus sequential.

(34) The treated surface exhibited, before the first spraying, the following characteristics: a lightness L equal to 70, a gloss G equal to 100.

(35) The first spraying was carried out by spraying the powder formed of particles (a) of Comparative Example 1 over the surface using a DUP suction blast machine with the following parameters: diameter of the nozzle: 8 mm, pressure: 2 bar, spraying distance: 15 cm, spraying angle: 85, degree of coverage: 100%.

(36) Then, the second spraying was carried out by spraying, over the surface obtained after the first spraying, the powder formed of notching particles (g), the second spraying being carried out using a DUP suction blast machine under the following conditions: diameter of the nozzle: 8 mm, pressure: 2 bar, spraying distance: 15 cm, spraying angle: 85, degree of coverage: 100%.

(37) The gloss G is measured using a Multi Gloss 268Plus device from Konica Minolta ith an angle equal to 60.

(38) The lightness L is measured with a Mini Scan XE Plus of the HunterLab brand according to Standard ASTM E308-01 Standard practice for computing the colors of objects by using the CIE system.

(39) The impact strength of each group of particles (a) to (e) is estimated using the following test: 100 g of particles are sprayed by means of the said blast machine onto a surface made of stainless steel for 5 minutes with a spraying angle, with respect to the surface, equal to 90, a spraying distance equal to 10 cm, a pressure equal to 2 bar and a diameter of the nozzle equal to 8 mm.

(40) Before the test, the weight W.sub.1 of the particles passing through the meshwork of a 45 m sieve is determined. The threshold of 45 m is well suited to demonstrating an enrichment in fine particles for the groups of particles tested.

(41) The test particles subsequently undergo recirculation for 5 min and are thus sprayed several times onto the surface.

(42) After the test, the weight W.sub.2 of the particles passing through the meshwork of a 45 m sieve is determined. The difference between the weights W.sub.1 and W.sub.2 corresponds to the amount of fine particles created during the test. This amount of fine particles generated, or reject rate, is expressed as percentage of the weight of particles before the test. The higher the reject rate, the lower the impact strength of the particles.

(43) It is considered that a reject rate of greater than 25% results in accelerated wear of the blast machine. Preferably, the reject rate is less than 20%, preferably less than 15%, preferably less than 10%.

(44) The Almen intensity is determined according to Standard NF L06-832 (Grenaillage conventionnel destin la mise en contrainte de compression superficielle de pices mtalliques [Conventional shot blasting machine intended to place metal parts under surface compressive stress]), on a test specimen of N type, on a DUP suction blast machine, with a degree of coverage equal to 100%, with a spraying angle, with respect to the surface, equal to 85, a spraying distance equal to 15 cm, a pressure equal to 2 bar and a diameter of the nozzle equal to 8 mm.

(45) For the sake of simplicity, the circularity squared, the area and the dimension of the particles and also the mean circularity squared, the total area and the mean dimension of the groups of particles (a) to (g) are evaluated on the source powders of the said particles, in other words on the group of particles (a), on the powder formed of Zirgrit F grains, on the powder formed of silicon carbide grains and on the powder formed of abrasive alumina/zirconia grains, by the following method:

(46) 11 mm.sup.3 of a sample of particles are poured into the dispersion unit (Sample dispersion unit) provided for this purpose of a Morphologi G3S device sold by Malvern. The dispersing of the sample over the glass plate is carried out using a pressure of 4 bar (Pressure) applied for 10 ms (Setting time), the dispersion unit remaining on the glass plate (Setting time) for 60 seconds. The magnification chosen is defined so as to be able to observe between 25 and 50 particles on the glass plate, in a region located in the centre of the disc of dispersed particles, so as to promote the observation of individual particles, that is to say particles which are not joined to other particles. An image analysis is subsequently carried out of the photographs produced, in a sufficient number so as to count a total number of particles of greater than 250.

(47) The device provides an evaluation of the circularity squared (HS circularity) of the area (Area) and of the dimension (CE diameter) of the particles counted, the said particles being counted by number. The mean circularities squared, total areas and mean dimensions of the groups of particles may then be calculated.

(48) The notching particles were faceted particles.

(49) The number of facets of the notching particles is evaluated by the following method: Photographs of the particles are taken using a scanning electron microscope, so as to have between 15 and 30 notching particles entirely visible per photograph. Photographs are taken so as to be able to count a minimum of 200 notching particles. The number of visible facets of each notching particle is determined. The mean number of facets of the notching particles is the arithmetic mean of the number of facets of each notching particle.

(50) The chemical analyses were carried out by X-ray fluorescence as regards the constituents for which the content is greater than 0.5%. The content of the constituents present in a content of less than 0.5% was determined by AES-ICP (Atomic Emission Spectroscopy-Inductively Coupled Plasma).

(51) The size of the particles and also the median size and the maximum size of a group of particles were determined using a Partica LA-950 laser particle sizer from Horiba.

(52) The results obtained appear in the following Table 1:

(53) TABLE-US-00001 TABLE 1 Example 7 - Example 7 - Example Example Example Example Example Example first second 1 2 3 4 5 6 spraying spraying Particles Particles Particles Particles Particles Particles Particles Particles (a) (b) (c) (d) (e) (f) (a) (g) % by volume of notching particles <1 10 50 75 50 85 <1 >99 Median size of the group of the sprayed 74 72 61 56 72 51 74 106 particles (m) Maximum size of the group of the sprayed 92 170 170 170 133 170 90 225 particles (m) Relative density of the sprayed particles 98 98 98 98 99 98 98 99 (%) Mean dimension of the notching particles n.d. 41 41 41 68 41 n.d. 102 (m) Mean dimension of the non-notching 65 65 65 65 65 65 65 n.d. particles (m) Ratio of the mean dimension of the n.d. 0.63 0.63 0.63 1.05 0.63 n.d. n.d. notching particles to the mean dimension of the non-notching particles Mean circularity squared of the group of n.d. 0.83 0.83 0.83 0.75 0.83 n.d. 0.73 the notching particles Mean circularity squared of the group of 0.97 0.97 0.97 0.97 0.97 0.97 0.97 n.d. the non-notching particles Mean number of facets of the notching n.d. 7 7 7 5 7 n.d. 5 particles Initial surface Gloss G 100 20 12 4 2 5 2 20 2 Lightness L 70 86 73 58 57 66 53 86 73 Almen intensity FN 8.8 8.1 7.4 5.5 4.7 5 8.8 8.8 (in hundredths of a mm) Reject rate (%) 5 7 16 23 17 27 5 30 n.d.: not determined

(54) Comparative Example 1 results in a darkening and in a reduction in the gloss, that is to say in a dark and matt rendering.

(55) In comparison with Example 1, Example 2 according to the invention results in a reduction in the gloss and also in a reduction in the lightness, with a low reject rate and a reduction in the Almen intensity. The efficiency (high powder consumption) and the productivity (frequent shutdowns of the blast machine in order to replace the powder) are thus low.

(56) In comparison with Examples 1 and 2, Example 3 according to the invention results in a reduction in the gloss and also in a reduction in the lightness and in the Almen intensity, with a moderate reject rate, without accelerated wear of the blast machine.

(57) In comparison with Examples 1 to 3, Example 4 according to the invention results in a reduction in the gloss and also in a reduction in the lightness and in the Almen intensity, with an acceptable reject rate and without accelerated wear of the blast machine.

(58) In comparison with Example 1, Example 5 according to the invention results in a reduction in the gloss and also in a reduction in the Almen intensity, with a moderate reject rate, without accelerated wear of the blast machine. Example 5 according to the invention illustrates the possibility of using notching particles which are not in the form of oxide(s), such as silicon carbide particles.

(59) Example 6, which is outside the invention, shows that the desired compromise is not achieved with a mixture comprising 85% by volume of notching particles: the reject rate is too high, which brings about accelerated wear of the blast machine.

(60) Example 7, which is outside the invention, shows that a first spraying of the powder formed of beads (a), followed by a second spraying of the powder formed of notching particles (g), does not make it possible to achieve the desired compromise: while the gloss is indeed reduced, the Almen intensity and the reject rate obtained after the second spraying are too high. It is thus important to spray a group of notching particles and of non-notching particles.

(61) As represented in FIG. 4, a visual examination of the surface obtained after the treatment of Example 3 according to the invention shows that it is covered with cavities 10 in the form of scales corresponding to the impression resulting from the spraying of the beads (non-notching particles) and with notches 20 corresponding to the impression resulting from the spraying of the notching particles.

(62) The comparison with FIG. 3 makes it possible to clearly distinguish the presence of notches.

(63) Of course, the invention is not limited to the embodiments described, which are provided by way of illustration and without implied limitation.