EP2349634A1

EP2349634A1 - Method for smoothing the surface of a substrate, using a laser

Info

Publication number: EP2349634A1
Application number: EP09777921A
Authority: EP
Inventors: Oliver Nöll
Original assignee: Surcoatec International AG
Current assignee: SURCOATEC AG
Priority date: 2008-08-19
Filing date: 2009-08-17
Publication date: 2011-08-03
Also published as: WO2010020387A1; DE102008038395B3

Abstract

The invention relates to a method for smoothing the surface (1) of a substrate (2), the output beam of a laser (4) being directed onto at least a section of the surface (1), and the surface (1) being smoothed in a first step by roughing and in a second step by polishing, the polishing being carried out at a pulse repetition frequency of the laser (4) of > 45 kHz. The method according to the invention allows the production of an optimum or perfect smooth surface even of extremely hard materials in a simple, reliable and reproducible manner while ensuring a very gentle treatment of the material of the substrate surface and the removal of impurities.

Description

SURCOATEC INTERNATIONAL AG

Schleefstr. 2e 44287 Dortmund

PROCESS FOR SMOOTHING THE SURFACE OF A SUBSTRATE USING A LASER

The invention relates to a method for smoothing the surface of a substrate using a laser, wherein the surface is smoothed in a first step by roughing and in a second step by polishing.

Coatings can be homogeneously applied to almost all basic materials and to various forms. It is known that a diamond-like coating with a corresponding hardness of up to 10,000 Vickers can be applied in a low-temperature process at, for example, 80 ° C. In particular, it is also possible to successfully carry out the inner coating of hollow bodies with high adhesion and homogeneity.

For this purpose, from a gaseous carbon source, such as methane, with the aid of a plasma, i. by means of an electrically excited gas, and by process parameters that are matched to the coating, such as pressure, power, type of feed, gas flow, etc., a diamond-like carbon layer (hereinafter referred to as DLC layer designated) generated on the substrate. A DLC layer is due to its characteristic properties u.a. optimally suited for the coating of objects exposed to high friction and abrasive forces. These can be, for example, pistons or other engine parts, thread formers and other tools for metalworking.

BESTATIGUNGSKOPIE A DLC layer can be modified in such a way that, on the one hand, it obtains considerably better adhesion to the substrate and, on the other hand, permits high elongation without damage. By a homogeneous process, a uniform layer structure can be ensured, so that, for example, a highly polished hardmetal surface creates an equally glossy and smooth surface. Such a DLC layer reduces friction, even with the sparing use of coolants, auxiliaries or lubricants.

It is extremely problematic to ensure perfect polishing of extremely hard materials. For example, the use of lasers creates "edge drops" when engraving grooves in a slide ring intended for a mechanical seal / such "contaminants" of the surface must be removed to ensure proper functioning of such a seal.

For example, an impurity can also be a glass grain in the material used. Since a sliding ring usually comprises a hard metal, the use of conventional polishing is very problematic, because an optimally or homogeneously smoothed surface of a substrate can not be achieved with it.

In the prior art, a sandblast blower is usually used for this purpose. This is a technical device based on the jet pump, which blows blasting material such as sand onto compressed air by means of compressed air to rid it of rust, paint, unevenness and impurities or the like, or roughen it. The application of this device is also called "sand blasting." Sandblasting, however, gives unsatisfactory results when used to remove the "edges", because the use of sandblasting causes "pitting", so that the blasted surface is not homogeneous Since many blasting agents are hygroscopic, ie absorb moisture or humidity, sandblasting and blasting furnaces require dry air, so compressed air supplied by a compressor can only be used for sandblasting when They cooled and before was dried. Sandblasting creates a high dust load for the blasting staff as well as for the environment. As a result, there is the danger of a health impairment for the beam personnel.

As mentioned above, alternatively, the material processing may be performed by a laser. The removal of material from a surface of a substrate under pulsed laser radiation is referred to as laser ablation. With laser pulses in the nanosecond range, the energy of the laser leads to a heating of the surface during the laser pulse, in the sense of thermal movements of the atoms. Since the heat conduction allows only a slow energy transport into the volume, the radiated energy is concentrated on a very thin layer, for example to about 1 micron at 10 ns. Pulse length. Therefore, the surface reaches very high temperatures and it comes to the sudden evaporation of the material. By ionization, such as thermally, by the laser light or by electron collisions, a plasma of electrons and ions of the ablated material arises at high power density of the laser. The ions can be accelerated to energies up to more than 100 eV in this plasma. Laser ablation can be used for targeted removal of materials, for example, instead of mechanically engraving hard materials or drilling very small holes. Laser ablation can also be used to remove thin layers of various contaminants. Only the topmost layer, which is a few nanometers to several micrometers thick, is heated to a high temperature, keeping the workpiece as a whole cold.

From DE 101 44 008 Al a method and an apparatus for producing a bore in a workpiece with laser radiation are known. In this case, with a first laser in a first step, a bore having a first diameter is drilled, which is less than the final diameter to be achieved. This is then expanded with a second laser in a second step or in further steps to the diameter to be reached. The bore is formed in a first step by roughing and in a second step or in further steps by sizing and / or polishing. Here, a laser with a pulse repetition frequency between 1 and a maximum of 100 Hz is used. From DE 21 21 155 a device for tearing the surface of objects is also a known. Here, a YAG laser emitting at a wavelength of 1064 nm is used. The object to be applied is a semiconductor chip.

A problem in the prior art is to ensure an optimally smoothed surface of a substrate, especially if it involves extremely hard materials, such as hard metals. The material of the surface should remain undamaged or at least spared.

In general, the removal of material with a large chip volume is called roughing in machining production processes. Roughing methods are usually used for turning and milling and serve to approximate the workpiece as far as possible within the shortest possible machining time. The roughing process usually leaves rough surfaces with low dimensional accuracy.

When polishing, which basically involves spreading, but not the removal of material, the smoothing effect is usually achieved with two mechanisms. On the one hand, roughness peaks of the surface structure are plastically and partially plastically deformed and thus leveled. On the other hand, depending on the type of polish is a smallest to no material removal. The resulting surface is often shiny due to the smoothness. In summary, polishing can be referred to as a smoothing finish process that can be used for a wide variety of materials.

It can be stated that it is very problematic and requires a lot of effort to provide a perfectly smoothed surface of a substrate. In particular, the emergence of Kantenaufwürfen when engraving grooves in slip rings made of carbide is problematic because a homogeneously smooth surface can not be guaranteed with the conventional roughing and polishing.

It is the object of the invention to provide a way to rid the surface of substrates of impurities and to provide an optimally smoothed surface. This object is achieved by providing a method for smoothing the surface of a substrate, according to which the output beam of a laser is directed onto at least a partial area of the surface and the smoothing of the surface is carried out in a first step by roughing and in a second step by polishing with polishing taking place at a pulse repetition frequency of the laser of> 45 kHz. Thus, a perfectly smooth substrate surface is achieved even extremely hard materials.

The partial area of the surface is preferably a specific area on the substrate surface. The subregion may correspond only to one point on the surface or may comprise several points on the entire surface.

The roughing is preferably carried out at a pulse repetition frequency of the laser of> 4 kHz. As a result, the surface of the substrate is brought as close as possible to the final state, leaving a rough surface. According to other preferred embodiments, the roughing occurs at> 5.5 kHz. The upper limit of the pulse repetition frequency of the laser is preferably 12 kHz. According to a preferred embodiment, the polishing following the roughing takes place at> 58 kHz.

The maximum output power of the laser is preferably at least 20 watts and the adjusted output power is preferably reduced or increased during roughing and / or polishing. The change in the adjusted output power is preferably in both steps, i. polishing and smoothing, but can also be done in one step or none of the steps, i. Once the output power has been set, it remains unchanged for the entire time in this last case.

According to another preferred embodiment of the invention, the substrate comprises at least one of silicon nitride, silicon carbide, hardened steel and a cemented carbide. Preferably, the cemented carbide comprises tungsten nitride or tungsten carbide. According to a further preferred embodiment of the invention, the laser is moved relative to the surface of the substrate in space. The laser remains on all the time, making this technique "on-the-fly." Alternatively, the laser is sequentially timed relative to the surface of the substrate, beginning with a first axis direction and ending with a last axis direction, for example, third axis direction, moved in space.

By "depositing" and "starting" the laser, "trenches" can be formed in the material, usually by the fact that the laser typically overlaps at least in a width of the laser beam with respect to a partial area on the surface of the substrate when restarting, wherein the width of the laser Laser beam is also referred to as "Dotpoint". In this case, the double removal of the substrate would take place in the dotpoint and lead to trenches in the material.

Such trenches are avoided according to the preferred embodiment by the on-the-fly technique. This technique works as follows: All axes in the room work preferably in programmed relation to each other. Thus, a divider or a rotation axis is preferably mounted on a travel table, wherein the travel table is preferably movable in a plane in space. The traversing table is also referred to as a "cross-support unit" and preferably controls the axis of rotation while rotating the dividing apparatus into the focus of the laser.The laser per se is preferably also mounted on a second traversing table, wherein the second traversing table is preferably in space As a result, all axes to each other and / or all axes can be controlled individually even with extremely fine and coherent laser work, so that it is not necessary to set off and / or switch off the laser.

According to a further preferred embodiment of the invention is in a first

Step the surface of the substrate to be treated scrubbed. In this case, a preset value for the focus of the laser is preferably adopted. In a second step, preferably with the same focus of the laser as in the first step and with others Parameters of the laser, such as the pulse repetition frequency, the surface of the substrate polished. According to other preferred embodiments, the surface of the substrate is further ground in a third step. The times between the first and second step or between the second and third step may be a few milliseconds to days. In other words, the quality of the smoothed surface is independent of these times. The second step or the polishing step is preferably carried out immediately after the first step or the roughing step, ie, there is preferably no noticeable pause between the two steps. According to other preferred embodiments, there is a noticeable pause between the first two steps. During this pause, the starting position of the laser or the substrate is set. The search for the starting position of all the axes relative to each other is also referred to as "zero phase." Preferably, all the axes are included in the "zero point" defined in this way. Thereafter, according to a preferred embodiment, only the polishing step is performed.

During the entire process, the laser is preferably operated continuously. This means that the laser can stay on all the time and / or stay on without interruption. The laser is preferably designed as a pulsed solid-state laser. The pulsed solid-state laser is preferably designed further as a YAG laser or as a fiber laser. In this case, the YAG laser is preferably flash lamp pumped or diode pumped or the fiber laser preferably operated diode pumped and the pulsing is preferably carried out by mode coupling or by Q-switching.

In a further preferred embodiment of the invention, the laser has a mode aperture. Preferably, the mode aperture has an opening of> 0.35 mm and <2 mm. In this case, the opening of the mode aperture is preferably polished.

Preferably, the substrate is arranged on a mounting unit. The mounting unit serves to hold the substrate and is preferably movable in space. The laser is preferably mounted on a cross-support unit. The cross-support unit is also designed as a cross table or traversing table according to the International Organization for Standardization (ISO). Nannt. The turning or mounting unit, which is preferably mounted on the movement or cross table, is also referred to as a dividing attachment or as a rotation axis.

The inventive method ensures an optimally smoothed surface of a substrate. Typically, after use of the method according to the invention, the smoothed surface is completely or nearly homogeneous, moreover the material of the substrate is treated so gently that neither holes nor fusions on the surface arise.

The invention will be explained in more detail below with reference to a preferred embodiment with reference to the drawings.

Fig. 1 shows a laser whose output beam is directed to the surface of a substrate, according to an embodiment of the invention; and

Fig. 2 shows schematically the individual steps of the method according to the invention according to an embodiment of the invention.

As can be seen in FIG. 1, the output beam of a laser 4 is directed onto at least a portion of the surface 1 of a substrate 2. According to this preferred embodiment of the invention, roughing occurs at 5.5 kHz with 60% laser utilization, ie the maximum output power of the laser is not set, but a 40% lower value. The maximum output power of the laser in this embodiment is 20 W, so that a value of 12 W is set. The substrate 2 is arranged according to this embodiment on a mounting unit 3, which is fixed to a wall. However, according to other preferred embodiments of the invention, the mounting unit 3 may be arranged on the floor. The mounting unit 3 for holding the substrate 2 is movable in a plane of the space according to this embodiment. The laser 4 is mounted on a cross-support unit 5 according to this embodiment. According to this preferred embodiment of the invention, in the second step, the polishing takes place at a pulse repetition frequency of the laser 4 of 58 kHz. The set value of the laser output power is now 8 W, ie a 60% lower value than the output power of the laser. 4

Fig. 2 shows schematically the steps performed by the method according to the invention according to a preferred embodiment of the invention. In a first step 6, the surface 1 of the substrate 2 of FIG. 1 is scrubbed with the laser 4. During this roughing step 6, the surface 1 of the substrate 2 is so far freed of impurities that the surface 1 is approached almost to its final state. According to this preferred embodiment, the material of the substrate 2 comprises silicon carbide. After the roughing step 6, the surface 1 of the substrate 2 is relatively rough. In a subsequent second step 7, the polishing with the laser 4 is now carried out, so that the material is evenly distributed on the surface 1 of the substrate 2. According to this preferred embodiment of the invention, after the polishing step 7, the surface 1 is homogeneously smoothed. According to this preferred exemplary embodiment of the invention, only a partial area of the surface 1 of the substrate 2 is approached by the laser 4 in the roughing step 6 and in the polishing step 7, since there is contamination or unevenness on the surface 1 only in this partial area. The output beam of the laser 4 is thus directed only at this subarea. According to another preferred embodiment, the entire surface of a substrate is irradiated with laser radiation. According to yet another preferred embodiment, only one point on the substrate surface is approached.

As a result, an optimally smoothed surface of a substrate can be achieved in a simple, reliable and secure manner. The surface of the substrate is treated so gently that neither holes nor merging can occur. Not only in soft materials but also in hard or extremely hard materials, such as hard metals, the process gives very good results.

In the mentioned embodiment of the invention, the laser 4 is configured as a diode-pumped YAG laser. According to other preferred embodiments of the invention is the Laser designed as a fiber laser. Generally, a laser 4 is used, which corresponds to a pulsed solid-state laser. Typically, 30 laser pulses per μm of cross section are used for a groove to be engraved in a sliding ring. However, it is also possible to use more or fewer laser pulses. According to the preferred embodiment of the invention, the YAG laser on a so-called mode aperture with a polished opening of greater than 0.35 mm. The polished aperture of the laser is up to 2 mm in accordance with other preferred embodiments of the invention. It is noteworthy that the laser responds very insensitive to its adjusted output power to provide an optimally smoothed surface. Especially important here are the other parameters, such as the pulse repetition frequency of the laser, which is typically set to a value of 45 kHz or above. Due to the homogeneity of the process, lapping of the surface of the substrate is not necessary.

Claims

claims

A method of smoothing the surface (1) of a substrate (2), wherein the output beam of a laser (4) is directed to at least a portion of the surface (1), and smoothing the surface (1) in a first step by roughing and in a second step by polishing, wherein the polishing takes place at a pulse repetition frequency of the laser (4) of> 45 kHz.

2. The method of claim 1, wherein the roughing occurs at a pulse repetition frequency of the laser (4) of> 4 kHz.

3. The method according to any one of the preceding claims, wherein the maximum output power of the laser (4) is at least 20 W and the output line of the laser (4) is reduced or increased.

4. The method according to any one of the preceding claims, wherein the substrate (2) comprises at least one material of silicon nitride, silicon carbide, hardened steel and a hard metal.

5. The method according to any one of the preceding claims, wherein the laser (4) relative to the surface (1) of the substrate (2) is simultaneously temporally moved in all three axis directions in space or the laser (4) relative to the surface (1) of the substrate (2) is moved in space one after the other, beginning with a first axis direction and ending with a last axis direction.

6. The method according to any one of the preceding claims, wherein the laser (4) is operated continuously.

7. The method according to any one of the preceding claims, wherein the laser is designed as a pulsed solid-state laser.

8. The method of claim 7, wherein the pulsed solid-state laser is further configured as a YAG laser or as a fiber laser.

9. The method of claim 8, wherein the YAG laser is flash lamp pumped or diode pumped or the fiber laser is diode pumped and the pulsing is done by mode coupling or by Q-switching.

10. The method according to any one of the preceding claims, wherein the laser (4) has a mode aperture.

11. The method of claim 10, wherein the mode aperture has an opening of> 0.35 mm and <2 mm.

12. The method of claim 11, wherein the aperture of the mode aperture is polished.