Method and system for surface layer removal

Abstract

A method for removing a layer of material 3 from a substrate 4, comprising irradiating the material to be removed 3, and the substrate 4 with a pulse of laser radiation 1. The material has an electronic energy bandgap larger than 1.0 eV, and the pulse of laser radiation comprises photons with an energy less than the bandgap of the material. The photons may have an energy between 0.54 and 0.9 eV, with a pulse duration of 500 fs to 800 ns, the pulse may consist of two or more pulses of radiation emitted in parallel from different laser sources. The laser source may be a solid-state laser doped with chromophores e.g., Nd, or Er, whilst the material for removal may take the form of an iron oxide e.g., FeO (Wüstite), Fe2O3 (haematite), or Fe3O4 (magnetite), with the substrate potentially iron or steel.

Description

The present disclosure relates to methods and systems of removing surface layers such as oxides from substrates such as iron or steel. More specifically the present disclosure relates to methods and systems of removing iron oxides and iron oxide layers from the surface of iron or steel articles using a laser of appropriate wavelength that transmits a large proportion of optical energy through the oxide layer and interacts with the oxide to metal interface. The steel industry passes material through various forms of processing, either thermal or compression or ductile, until the material is given its final shape, temper and finish. Iron and steel are subject to rigorous oxidation, either by exposure to ambient atmospheric conditions or by elevated temperatures. Some of these conditions generate an oxide layer that is unwanted and many times inconsistent and thus needs to be removed as a contaminant before the next process takes place. To remove the oxide layer, the industry often utilises large installations which subject the metal articles to an elevated temperature acidic environment, either liquid or gas, which removes the oxide layer from the steel surface. The acid process for removing the oxide is typically referred to as pickling. The oxide layer being removed can often be referred to as scale. Laser cleaning can be applied to remove rust and other contamination from metal substrates, and the industry has evaluated the use of commercially available pulsed lasers of Nd:YAG and Yb or Nd doped fibre to remove the oxide layers predominantly by ablation. In the present disclosure, a laser-based solution with highly increased efficiency is provided. The inventors have identified that selecting a wavelength that can transmit through the oxide layer instead of being absorbed by it, significantly increases process efficiency. This is achieved by delivering the laser energy to the interface of the oxide layer and the metal substrate instead of primarily to the oxide layer. Summary In an aspect, a method for removing a layer of material from a substrate comprises irradiating the layer and the substrate with a pulse of laser radiation, wherein the pulse of laser radiation comprises photons with an energy between 0.54 and 0.9 eV, and wherein the layer comprises material that has an electronic energy bandgap larger than 1.0 eV. As would be understood by the skilled person, photon energy is interchangeable with photon wavelength. The photon energy may be less than 0.8 eV, less than 0.7 eV or less than 0.6 eV. The photon energy may be more than 0.6 eV, more than 0.7 eV or more than 0.8 eV. In another aspect, a method for removing a layer of material from a substrate, comprises irradiating the layer and the substrate with a pulse of laser radiation, wherein the layer comprises material that has an electronic energy bandgap larger than 1.0 eV, and wherein the pulse of laser radiation comprises photons with an energy that is less than the bandgap of the material. The photon energy my be 0.1, 0.2 or 0.5 eV less than the bandgap of the material. Alternatively, the photon energy may be 95%, 90%, 80% or 50% of the bandgap energy. The bandgap of the material may be larger than 1.5 eV or larger than 2.0 eV. The following optional features relate to all of the above aspects. The electronic energy bandgap of the material on the substrate may be less than 3 eV or less than 2.5 eV. The pulse of laser radiation may carry or have energy less than 2.0 J, 1.50 J, 1.00 J, 0.40 J, 0.10 J, or 0.04 J and / or more than 0.05 J, 0.10 J, 0.20 J, 0.50 J or 0.90 J. The pulse of laser radiation may have a duration between 500 fs and 800 ns, preferably more than 1000 fs, more preferably more than 10 ns, yet more preferably more than 30 ns and yet more preferably more than 100 ns and / or preferably less than 400 ns and more preferably less than 200 ns. The pulse of laser radiation may be two or more pulses of radiation. The two or more pulses of radiation may be emitted in parallel from different laser sources. The two or more pulses of radiation may impinge in parallel on different areas of the layer and substrate. The pulse of laser radiation may be focussed on the layer of material, the interface of the layer of material and substrate, or both the layer and substrate (given the depth of focus of the radiation). A beam profile of the pulse of laser radiation may be shaped into a line, rectangle, oval or other shape with one dimension being longer than the other, using optics. A beam profile of the pulse of laser radiation may be coupled into an optical fibre cable and transmitted via the optical fibre cable. A method according to any preceding claim, further comprising rastering or scanning a beam profile of the pulse of laser radiation over the layer and substrate using moving mirrors or prisms. Accordingly, a plurality of pulses may be rastered or scanned over the layer and substrate. The layer may comprise more than one type of material in sublayers or grains or a mixture of sublayers and grains. The layer may not be homogeneous or consistent in its constitution or distribution across the area of the substrate being irradiated with the laser radiation. The substrate may be a rod, rectangle, sheet, or i-beam. The substrate may be steel or iron. The layer may comprise ferrous oxide, Wustite or iron (II) oxide. The layer may comprise a plurality of materials and at least one but not all of the materials in the layer is in liquid form. The layer may comprise iron oxide, preferably in the form of Wustite, Hematite, Magnetite or a mixture thereof. The layer may comprise an oil, a lubricant, water, a sulphide, dust, sand, moisture, a silicate, metal powder, or a surfactant. The layer may comprise an oxide, a nitride, a carbide or a sulphide of iron or another metal, wherein the oxide, nitride, carbide or sulphide has an electronic energy bandgap larger than 0.9 eV, preferably larger than 1.0 eV, more preferably larger than 1.2 eV and yet more preferably larger than 1.5 eV. The laser radiation may be emitted by a solid-state laser comprising one of the following (or a hybrid configuration of) a fibre laser amplifier, a YAG laser amplifier, and an YV04 laser amplifier. The laser amplifying medium may be doped with one or more of the following chromophore elements: erbium, neodymium, thulium, ytterbium, samarium, and holmium. The method may further comprise, before irradiating the layer and substrate, processing the layer using a mechanical means such as a brush to remove a part of the layer. The method may further comprise, before irradiating the layer and substrate, using compressed gas or a waterjet to remove a part of the layer. The method may further comprise, before irradiating the layer and substrate, using a scale breaker with successive rollers to remove a part of the layer, wherein the rollers preferably have a diameter between 5 and 70 cm. In accordance with aspects of the disclosure there is provided a method for removing oxide layer from the surface of (optionally iron or steel) articles such as sheet metal, rod, rail beam, I-beam and other profiles, using a laser beam, wherein the laser beam wavelength is selected to ensure that the photon energy of the laser beam is smaller than the bandgap of the (optionally) iron oxide layer intended to be removed. In accordance with aspects of the disclosure the aforementioned tuning of the emitted laser wavelength and as a result photon energy in relation to the bandgap results in the optical transmission of the laser pulse through the oxide intended to be removed. Advantageously, this allows a large proportion of the photonic energy to transmit through the (oxide) layer and interact with the (oxide) layer-metal interface. The interaction of the laser light with the interface promotes a disruption of the interface, allowing the layer to detach from the substrate surface. In accordance with aspects of the disclosure, the laser beam reaching and disrupting the (optionally oxide to iron or oxide to steel) interface can promote the evaporation of mater local to the interface. The evaporated mater at the interface has the tendency to expand due to its elevated temperature and is consequently acting to separate the layer form the substrate. The method may comprise using a solid state laser source such as fibre, YAG or YV04, doped with lanthanides such as erbium, thulium, holmium, ytterbium, neodymium and samarium or combinations of these elements thereof. This is one way for the laser to achieve an advantageous emission wavelength in the region of 1400 nm and 2300 nm, optionally with a photon energy less than 1.01 eV. The laser of this wavelength transmits through the (optionally iron oxide) layer, for example comprising primarily a mixture of iron oxides, that has a bandgap larger than the corresponding photon energy of the laser wavelength. The laser beam may be shaped in a line or rectangle profile via a combination of optics. The laser beam may be scanned across a line or area on the surface of the iron or steel article, using a set of reflecting mirrors or optics. In other aspects, a system comprises a layer of material on a substrate and a laser, wherein the system is configured to perform the method of any preceding aspect. Detailed description Exemplary arrangements of the disclosure shall now be described with reference to the drawings in which: Figure 1 depicts the ablation of a portion of material [5] belonging to the layer intended for removal [3] by pulsed laser emission [1], on a pulse by pulse basis, where most of the photonic energy is absorbed by the layer intended for removal (i.e. not in accordance with the new method of the present disclosure). Figure 2 depicts the present method of oxide layer removal by laser [1] where most of the photonic energy transmits through the oxide layer [3] and interacts with the interface of the oxide layer [3] and the substrate [4], generating high pressure gaseous products [2] that force the layer away from the substrate. Figure 3 depicts the photon energy of typical Nd:YAG emitted laser light

[19] and photon energy of Er fibre or Er:YAG laser light

[18] in comparison with the conduction energy band levels of iron oxides, Magnetite

[17] , Hematite

[16] and Wustite

[15] . Figure 4 depicts a possible implementation of the disclosure comprising sheet steel [7] being rolled with rollers [8] under multiple laser beams [1], shaped and emitted via a series of optical tools [1] containing beam re-imaging optics [9] and beam shaping optics

[13] , where the beams are emitted by a set of laser sources

[12] and transmitted to the optical tools via a corresponding set of fibre optical cables

[11] . Figure 5 shows an example of beam scanning optics [6], moving the laser beam [1] on the surface of an iron or steel piece [4] to remove oxide layers [3], Throughout the description and the drawings, like reference numerals refer to like parts. Hereinafter the present disclosure is described in more detail with reference to the drawings. The descriptions include one or more exemplary embodiments of the disclosure, with the intention of aiding comprehension of the present disclosure, however the scope of the present disclosure is not reduced or limited by the exemplary embodiments. Iron and steel articles and products such as sheet, construction i-beam, reinforcing bars, rods, wires, rails, and others tend to grow oxide layers over their surfaces unless a protective coating or other form of surface passivation is applied to them in order to stop further oxidation from taking place. Oxidation can also take place in between manufacturing processes, during transport of intermediate products, or as a result of certain thermal processes during production. The oxide can degrade the value of the articles or cause problems in further downline processes, by contributing to abrasion and wear of machinery, surface inconsistencies, corrosion seeding and weld defect seeding. Consequently, the industry seeks to remove such oxides from the surface of these articles during processing. One common methodology is pickling, where the articles are exposed to an acidic environment. Other established methodologies involve brushing or abrading the material. The oxides are primarily iron oxides and commonly evolve as Wustite FeO, Hematite Fe2Os and Magnetite Fe3O4. The oxide layer can contain grains or sublayers of each of these oxides or mixtures of these. A laser beam [1] focussed on the surface of iron or steel [4] which is covered by a layer of oxide or mixed oxides being predominantly iron based oxides [3] can force a certain volume forming part of the layer [5] being subjected to the influence of laser radiation to transition from solid phase to vapour phase and thus be removed from the layer as gas. This is a typical laser machining, surface texturing, scribing, drilling and in may cases marking process taking place. In many cases it is executed using a pulsed laser and more commonly a ns pulsed laser, where laser energy stored in the gain media is released in individual pulses and are therefore delivering much higher peak power, than the steady state operation of the laser at its designated average power. This process benefits from the high peak power emitted within these laser pulses, that can more easily induce evaporation of the material being processed, as the energy becomes concentrated in the time domain and also in the space domain if focussing the beam to a small spot also. In the present disclosure, a pulsed laser beam [1] of selected wavelength

[18] is used to remove the oxide layer [3] from iron or steel article substrates [4] by transmitting through the oxide layer [3] and interacting at the interface of the oxide layer and iron or steel substrate to evaporate matter [2] and detach the oxide layer above the irradiated area. The laser beam has a selected wavelength or photon energy lower than 1.00 eV

[18] (e.g. 0.8 eV in the present example) which is smaller than the energy bandgap of all three variations of iron oxide present in the oxide layer and can thus transmit through the layer. Typical industrial laser sources today being DPSS Nd:YAG or Yb fibre emit photons with energy between 1.17 eV and 1.13 eV

[19] which is above the conduction band of at least one of the iron oxide variants, namely Wustite measures at 1.01 eV

[15] , An exemplary embodiment of the present disclosure comprises a ns pulsed Er doped fibre laser source emitting at 1550 +1- 100 nm, or 0.8 + 0.055 / - 0.0485 eV, which is lower than 2.2 eV for magnetite, 1.9 eV for hematite and 1.01 eV for Wustite. The intended method of removing the iron oxide layer from iron and steel is significantly more efficient because it only evaporates a volume of a few thousand cubic nm of material per pulse, rather than having to spend energy to evaporate a volume which could be 1000 times greater in the case of a Wustite containing layer. In an exemplary embodiment of the present disclosure, the laser is generated at one or multiple laser source devices

[12] and is transmitted by one or multiple fire optical cables

[11] to one or multiple optical processing tools

[10] which expand and refocus the beam using an arrangement of transmissive optics [9] and reshape the beam using single or multiple optic arrangements

[13] . The laser beam or beams [1] emerging from the optical processing tools

[10] then impinge on the iron or steel article surface [7] to induce detachment of the oxide layer [3], In an alternative exemplary embodiment of the present disclosure, the laser source emits pulses between 2 and 700 ns by q-switching or between 10 fs and 500 ps by mode locking, or between 500 ps and 2 ns by gain switching. In alternative exemplary embodiments of the present disclosure, different laser sources are used to emit photons with energy lower than the oxide bandgap, where in these lasers, the gain medium is a crystal such as YAG or orthovanadate (YV04) or fibre, doped with other chromophores or combinations of chromophores such as Thulium, Samarium, Holmium and possibly comprising also Neodymium or Ytterbium co-dopants. In a further exemplary embodiments of the present disclosure, the iron or steel article [7] is in sheet form and is continuously or intermittently fed and positioned under the laser beam or under multiple laser beams [1] using rollers [8] or other mechanised motion and positioning equipment. In an alternative exemplary embodiment of the above, the iron or steel article is in the form of a rod or an i-beam profile or a train rail profile, or other profile, which can be continuously fed under the laser beam [1] or past the laser beam [1] and where the laser beam [1] is accordingly shaped and directed by the processing tools

[10] to irradiate part or the whole of the profile’s perimeter surface. In a further exemplary embodiment of the present disclosure, the iron or steel article [7] is subjected to a precursor processing stage which can include a scale breaker by bending or vibrating the article, brushes, abrasion wheels or compressed air or water, to remove part of the oxide layer [3] or loosen up loose parts of the oxide layer [3]. In a further exemplary embodiment of the present disclosure, the layer intended to 5 be removed [3] comprises an iron compound with nitrogen or sulphur or carbon, or phosphorous or a combination of these elements. In such an embodiment, the laser emission photon energy

[18] will be selected to be lower than the bandgap of this compound defined as the energy difference between the conduction band energy level

[15] and the valance band energy level

[14] ,

Claims

1. A method for removing a layer of material from a substrate, the method comprising irradiating the layer and the substrate with a pulse of laser radiation, wherein the pulse of laser radiation comprises photons with an energy between 0.54 and 0.9 eV, andwherein the layer comprises material that has an electronic energy bandgap larger than 1.0 eV.

2. A method for removing a layer of material from a substrate, the method comprising irradiating the layer and the substrate with a pulse of laser radiation, wherein the layer comprises material that has an electronic energy bandgap larger than 1.0 eV, andwherein the pulse of laser radiation comprises photons with an energy that is less than the bandgap of the material.

3. A method according to any preceding claim, wherein the pulse of laser radiation carries energy less than 2.0 J, 1.50 J, 1.00 J, 0.40 J, 0.10 J, or 0.04 J and / or more than 0.05 J, 0.10 J, 0.20 J, 0.50 J or 0.90 J.

4. A method according to any preceding claim, wherein the pulse of laser radiation has a duration between 500 fs and 800 ns, preferably more than 1000 fs, more preferably more than 10 ns and yet more preferably more than 30 ns and / or preferably less than 400 ns and more preferably less than 200 ns.

5. A method according to any preceding claim, wherein the pulse of laser radiation is two or more pulses of radiation.

6. A method according to claim 5, wherein the two or more pulses of radiation are emitted in parallel from different laser sources.

7. A method according to claim 5 or 6, wherein the two or more pulses of radiation impinge in parallel on different areas of the layer and substrate.

8. A method according to any preceding claim, wherein the pulse of laser radiation is focussed on the layer of material or both the layer and substrate.

9. A method according to any preceding claim, wherein a beam profile of the pulse of laser radiation is shaped into a line, rectangle, oval or other shape with one dimension being longer than the other, using optics.

10. A method according to any preceding claim, wherein a beam profile of the pulse of laser radiation is coupled into an optical fibre cable and is transmitted via the optical fibre cable.

11. A method according to any preceding claim, further comprising rastering or scanning a beam profile of the pulse of laser radiation over the layer and substrate using moving mirrors or prisms.

12. A method according to any preceding claim, wherein the layer comprises more than one type of material in sublayers or grains or a mixture of sublayers and grains.

13. A method according to any preceding claim, wherein the layer is not homogeneous or consistent in its constitution or distribution across the area of the substrate being irradiated with the pulse of laser radiation.

14. A method according to any preceding claim, wherein the substrate is a rod, rectangle, sheet, or i-beam.

15. A method according to any preceding claim, wherein the layer comprises ferrous oxide, Wustite or iron (II) oxide.

16. A method according to any preceding claim, wherein the layer comprises a plurality of materials and at least one but not all of the materials in the layer is in liquid form.

17. A method according to any of claims 1 to 14 or 16, wherein the layer comprises iron oxide, preferably in the form of Wustite, Hematite, Magnetite or a mixture thereof.

18. A method according to any preceding claim, wherein the layer comprises an oil, a lubricant, water, a sulphide, dust, sand, moisture, a silicate, metal powder, or a surfactant.

19. A method according to any preceding claim, wherein the layer comprises an oxide, a nitride, a carbide or a sulphide of iron or another metal, wherein the oxide, nitride, carbide or sulphide has an electronic energy bandgap larger than 0.9 eV.

20. A method according to any preceding claim, wherein the laser radiation is emitted by a solid-state laser comprising one of the following or a hybrid configuration of a fibre laser amplifier, a YAG laser amplifier, and an YV04 laser amplifier.

21. A method according to any preceding claim, wherein the laser amplifying medium is doped with one or more of the following chromophore elements: erbium, neodymium, thulium, ytterbium, samarium, and holmium.

22. A method according to any preceding claim, further comprising, before irradiating the layer and substrate, processing the layer using a mechanical means such as a brush to remove a part of the layer.

23. A method according to any preceding claim, further comprising, before irradiating the layer and substrate, using compressed gas or a waterjet to remove a part of the layer.

24. A method according to any preceding claim, further comprising, before irradiating the layer and substrate, using a scale breaker, optionally with successive rollers, to remove a part of the layer, wherein the rollers preferably have a diameter between 5 and 70 cm.

25. A system comprising a layer of material on a substrate and a laser, wherein the system is configured to perform the method of any preceding claim.

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