WO2018149574A1 - Improved adhesive connection by microstructuring a surface by means of a laser - Google Patents

Abstract

The invention relates to a method for producing an adhesive connection between a first body (1), consisting at least partially of a stainless steel, and a second body (2), comprising the following method steps: structuring (4) at least one first sub-region of a first surface (1a) of the first body (1) by means of an ultrashort pulse laser; and producing an adhesive connection at least between the first sub-region of the first surface (1a) and the first body (1) and at least one second sub-region of a second surface (2a) of the second body (2) by means of an adhesive agent (3). The invention also relates to a sensor unit for a vibronic sensor, wherein an adhesive connection is produced by means of the method according to the invention, as well as a vibronic sensor having a sensor unit according to the invention.

Description

 IMPROVED ADHESIVE CONNECTION BY MICROSTRUCTURING A SURFACE WITH A LASER

The invention relates to a method for producing an adhesive bond between a first body which consists at least partially of stainless steel and a second body. Furthermore, the present invention relates to a sensor unit for a vibronic sensor, which is produced by the method according to the invention, and a vibronic sensor with a corresponding sensor unit.

Joining processes for joining two components made of different materials are becoming increasingly important today. Depending on the materials used, some completely different problems can be considered, and proven methods can not easily be applied to other material classes. Typical joining methods involve gluing, or thermal processes such as laser welding. It is known that the quality of the respective joint decisively depends on the surface quality of the respective components. To ensure adequate adhesion, at least one of the surfaces to be joined of one of the two components is suitably pretreated in many cases, in particular surfaces are roughened in order to increase the contact surface of the components to be bonded. In the case of plastics, for example, various chemical processes are known, while for metals in particular structuring of the respective surface, for example by means of a variety of grinding or blasting processes, in particular sandblasting, offers.

Furthermore, it has become known to structure a surface by means of a suitable laser process. For this purpose, the respective surface is frequently irradiated with pulsed laser light, whereby self-organized structures can be produced on the surface. In this regard, WO2014 / 094729A2, for example, discloses a method for structuring a non-conductive workpiece surface in order to achieve a selective and adherent metallization thereof. The document also deals with the use of ultra-short pulse lasers, which are particularly suitable when the surface to be structured does not withstand high thermal stress. With regard to metallic surfaces, again reference is made, for example, to the article "High-rate laser processing of metals using high-average power ultra-short pulse lasers" by J. Schille, L. Schneider, L. Hartwig and U. Loeschner, published in the paper No. 3932 - 38th MATADOR Conference.

The resulting structures on the respective surface are fundamentally sensitive to the particular laser used and the parameters used in each case for its operation. Typical laser-induced structures are, for example, various periodic trench and grating structures, which are also referred to as corrugations. There are also statistical Structures, which are also referred to as cone-like-protrusions, CLPs short, observable. The latter consist of a superstructure in the micrometer range, which is superimposed on a nanostructure in the nanometer range. The respective structure on the surface has a considerable influence on the respective joining process.

Vibronic sensors are widely used in process and / or automation technology and serve to determine and / or monitor at least one process variable

Medium. In the case of level measuring devices, they have at least one mechanical

oscillatory unit, such as a tuning fork, a monobloc or a membrane. This is in operation by means of a drive / receiving unit, often in the form of a

electromechanical transducer unit excited to mechanical vibrations, which in turn may be, for example, a piezoelectric actuator or an electromagnetic drive. In the case of flowmeters, however, the mechanically oscillatable unit can also be designed as a vibratable tube which is flowed through by the respective medium, for example in a measuring device operating according to the Coriolis principle.

Corresponding field devices are manufactured by the applicant in great variety and distributed in the case of level measuring devices, for example under the name LIQUIPHANT or SOLIPHANT. The underlying measurement principles are in principle made of a variety of

Publications known. The drive / receiver actuation stimulates the mechanically oscillatable unit to generate mechanical vibrations by means of an electrical signal. Conversely, the drive / receiving unit, the mechanical vibrations of the mechanical

receive oscillatable unit and convert it into a received electrical signal. In this case, the drive / receiving unit is in many cases part of a feedback electrical resonant circuit, by means of which the excitation of the mechanically oscillatable unit to mechanical vibrations takes place. For example, for a resonant oscillation, the resonant circuit condition according to which the amplification factor is> 1 and all phases occurring in the resonant circuit are multiples of 360 ° must be satisfied. To excite and satisfy the resonant circuit condition, a certain phase shift between the excitation signal and the received signal must be ensured. For this reason, a predefinable value for the phase shift, that is to say a setpoint value for the phase shift between the excitation signal and the received signal, is frequently set. For this purpose, very different solutions, both analogous and digital methods, such as, for example, the documents DE102006034105A1, DE102007013557A1, DE 102005015547A1, DE102009026685A1, are known from the prior art.

DE102009028022A, DE102010030982A1 or DE00102010030982A1 described. In addition to a, in particular specifiable, level of a medium in a container, which reference a change in a resonant frequency or an amplitude of the mechanically oscillatable unit is detected, vibronic sensors are also suitable for determining the density and / or viscosity, as described for example in DE10050299A1, DE102007043811A1, DE10057974A1, or DE102015102834A1.

To the mechanically oscillatable unit of a vibronic sensor, which is often made of stainless steel, a steatite disk is usually glued before the drive / receiver unit is integrated. The quality of the sensor depends on the sensitive

Adhesive connection from. Straight adhesive bond between a stainless steel and a second, non-metallic component, however, are difficult to implement, as stated above.

Starting from the prior art, the present invention seeks to provide a way to produce a high quality adhesive bond between a stainless steel and a non-metallic component.

This object is achieved by the method according to the invention as claimed in claim 1 and by the sensor unit according to claim 12 and the vibronic sensor according to claim 15.

Advantageous embodiments are specified in the dependent claims.

With regard to the method, the object according to the invention is achieved by a method for producing an adhesive bond between a first body which at least partially consists of a stainless steel and a second non-metallic body, comprising the following

Steps:

 - Structure at least a first portion of a first surface of the first

Body by means of an ultrashort pulse laser, and

 Producing an adhesive bond at least between the first subregion of the first surface of the first body and at least one second subregion of a second surface of the second body by means of an adhesive.

Stainless steel is a chemically resistant material with a passive surface. Adhesives therefore adhere to stainless steel comparatively poorly. Due to the surface structuring by means of an ultrashort pulse laser on the one hand an enlargement of the one for the production of the

Adhere adhesive bond to the available surface. Furthermore, a targeted

Modification of the surface possible. The surface is preferably structured such that a substantially complete wetting of the enlarged surface can be realized or achieved by means of the adhesive. This allows a uniform flow of the adhesive, which essential for the greatest possible adhesion and, accordingly, crucial for the reproducibility and long-term stability of the adhesive bond.

Another advantage of using an ultrashort pulse laser is that the respective structured surface has fewer so-called melting artifacts or even spikes. Due to the short pulse durations in the range of picoseconds or femtoseconds can be achieved in conjunction with a targeted spatial focusing of the laser beam, that the heat introduced sufficient to evaporate material in a given region of the surface (removal) without a larger heat affected zone can form ,

According to the invention, only one surface of one of the two bodies is structured by means of the laser. This leads to an increased reproducibility of the splice. As the second body has a substantially smooth surface, lateral displacements between the first and second surfaces of the first and second bodies do not result in altered geometry around the joint.

In a preferred embodiment, the second body consists at least partially of steatite or soapstone. The realization of high quality joining of stainless steel and steatite is generally difficult to realize. For example, the adhesion properties of both materials are usually very different with respect to different adhesives.

In a further preferred embodiment, the laser is operated in a burst mode. In burst mode, the energy of a single laser pulse is split into a group of individual pulses of different frequencies. This allows a precise adjustment of the laser fluence and, consequently, a particularly gentle possibility of surface structuring. It can be achieved that the respectively structured surface is substantially free of artifacts, in particular melts or spikes.

An embodiment of the method includes that the laser is operated at a power of about 50-200μϋ and / or a scanning speed parallel to a longitudinal direction of the first surface of about 0.1-1cm / s. Preferred pulse lengths are 5-30ps. Furthermore, a laser with a frequency of 100-1000 kHz is preferably used. Frequencies <500 kHz are particularly preferred since so-called shielding effects can be avoided at these frequencies. In the text which follows, a shielding effect is understood to mean that an incident laser pulse is absorbed or scattered on plasma and / or material vapor clouds generated by pulses preceding this pulse. A further embodiment includes that structuring in the form of a superstructure and a fine structure superimposed on the superstructure is produced at least in the first subregion of the first surface. The parameters used for the operation of the laser are thus set appropriately.

For this embodiment, it is advantageous if the superstructure is a periodic structure with a periodicity in the micrometer range, in particular in the range of up to 50 μm. Particularly preferred are structures having a periodicity in the range of 5-30 μιη. Likewise, it is advantageous if the fine structure is a periodic structure with a periodicity in the nanometer range. The fine structure is characterized in particular by a comparatively low aspect ratio. This in turn leads to a significant increase in the wettable surface by means of the adhesive. Preferably, the fine structure has a periodicity in the range of <1 μιη.

It is furthermore advantageous if so-called cone-like protrusions (CLPS) are generated in the structuring.

Moreover, it is advantageous if the superstructure has an average structure height (peak-to-valley) of up to 25μιη, preferably 2-20 μιη. In contrast, the average structure height of the fine structure is preferably in a range of about 300-1500 nm.

In comparison to conventional methods for laser structuring, the structures produced according to the invention have comparatively low average structural heights and, consequently, a low aspect ratio, both for the superstructure and also for the fine structure. This is surprisingly particularly advantageous with respect to the wetting of the surface with the adhesive. Although the surface increases with increasing mean structure height.

However, in the case of larger average structural heights, it may no longer be possible to ensure that the surface can be wetted completely with the adhesive, since the adhesive can not flow completely into the structures. The transition here is fluid and depends in particular on the viscosity of the adhesive. In addition to the average structure height, the periodicity of the structures plays a decisive role in this respect. The larger the periodicity, the greater the average structure height can be selected. However, with increasing periodicity, the effect of surface augmentation may be lower than at lower periodicity with lower average struc- tural height. In a further preferred embodiment of the method, at least the first subregion of the first surface of the first body, in particular by means of the laser, is rendered hydrophobic. The structuring thus leads to a hydrophobization of the surface. Surprisingly, a hydrophobic surface leads to improved wetting with, preferably heat-curing, adhesives in the form of epoxy resins. Such adhesives are cured at temperatures> 100 ° C and have at the beginning of the thermally induced curing usually comparatively low mixing viscosities.

 In a further preferred embodiment, at least the first subregion of the first surface is structured in such a way that the adhesive bond produced in each case

Method according to at least one of the preceding claims, having an adhesive tensile strength of at least 20 MPa. The adhesive tensile strength represents a measure of the quality of the adhesive bond.

The object according to the invention is also achieved by a sensor unit for a vibronic sensor, comprising at least one mechanically oscillatable unit made of a stainless steel and a steatite disk fastened to the sensor unit by means of an adhesive connection, wherein the adhesive connection is produced by means of a method according to one of the preceding claims. In the case of a vibronic sensor, the adhesive connection between the

oscillatory unit and the steatite disc significantly the vibration characteristics of the sensor. In particular, the adhesive bond has a great influence on the rigidity of the oscillatable unit, from which the resonant frequency of the oscillatable unit directly depends.

The structuring of a surface of the oscillatable unit also influences its rigidity, so that structuring with a low average structural height is advantageous here in two respects. On the one hand, as already mentioned, a substantially complete wetting of the surface can be achieved by the adhesive used. On the other hand, due to the low structural height, it is ensured that the rigidity of the oscillatable unit is influenced as little as possible.

In a preferred embodiment, the adhesive connection is produced in such a way that an anti-resonance frequency of the oscillatable unit is at a maximum of 600 Hz. The location of

Antiresonance is determined inter alia by the mechanical coupling between the oscillatable unit and the drive-receiving unit, which in turn depends on the adhesive bond between the oscillatory unit and the steatite disc. Basically, in the case of a vibronic sensor, the distance between the

Resonance frequency and the anti-resonant frequency can be influenced by various measures, for. B .:

1) adjustment of the mechanical vibration quality of the mechanically oscillatable unit by changing the shape or geometry (membrane thickness, transitions between membrane and oscillatable unit) and / or the material (including, for example, the stiffness);

 2) prevention or reduction of energy losses, eg. B. by a symmetrical structure and / or the vibration transmission between drive / receiving unit, membrane and oscillatory unit occur.

 3) reducing the number of components;

 4) avoidance of mechanical tension;

 5) Optionally, adjusting the thickness of an insulating disk between the diaphragm and the drive-receiving unit, as described for example in EP0985916B1;

 6) Setting the coupling factor of the drive / receiver unit; and

 7) Optimization of bonds and contacts.

Advantageously, according to the invention, the bond between the oscillatable unit and steatite disk can be optimized without having to accept a loss in terms of mechanical vibration quality. Conventional laser structures with larger mean heights of structure adversely affect the mechanical vibration quality of the vibronic sensor.

It is also advantageous if it is one for the production of the adhesive bond

used adhesive to an adhesive having a glass transition temperature, which

Glass transition temperature is outside a working range of the sensor, is.

Other important properties of the adhesive used concern the viscosity and

Surface tension of the adhesive, as well as the adhesion properties with respect to the

Steatitscheibe whose surface is not processed according to the invention.

 Finally, the present invention relates to a vibronic sensor, comprising at least one sensor unit produced according to the invention.

It should be pointed out that the embodiments mentioned in connection with the method according to the invention are mutatis mutandis also applicable to the sensor unit according to the invention and the vibronic sensor according to the invention, and vice versa.

The invention as well as its advantageous embodiments will be described in more detail below with reference to FIGS. 1 to 3. It shows: 1 shows two components joined by means of an adhesive connection, wherein the first component is a stainless steel, and wherein the second component consists of a non-metallic material,

2 (a) and (b) show two images of two surfaces structured according to the invention, and FIG. 3 shows a schematic sketch (a) of a vibronic sensor according to the prior art, and (b) a tuning fork with a steatite disk attached thereto.

In Fig. 1 a are a first body 1 made of stainless steel and a second body 2 of a

shown non-metallic material, which are connected together by means of the adhesive 3 along the surfaces 1 a and 2a. To ensure the most stable and durable adhesive bond, the first surface has a structuring 4, which was produced by means of a method according to the invention.

A more detailed view of a preferred embodiment of the achieved structuring is shown in Fig. 1 b. The structuring 4 is composed of a superstructure 5 and a superstructure 6 superimposed on this superstructure 5, which is shown only in a partial area. For this purpose, the parameters used for the operation of the laser are thus set appropriately. Advantageously, the superstructure 5 has a periodicity pi in the micrometer range, while the

Periodicity p2 (not shown) of the fine structure 6 is in the nanometer range. Moreover, it is advantageous if the superstructure 5 has an average structure height hi (peak-to-valley) of up to 25 μιη, preferably 2-20 μιη. The aspect ratio is generally understood to mean the ratio of depth or height h of a structure in comparison to its lateral extent, that is to say in comparison with its periodicity p. In the case where an adhesive bond is to be made, a comparatively low aspect ratio h / p is desirable, as this generally results in a significant increase in the surface wettable by the adhesive.

Fig. 2 shows two stainless steel surfaces, which were structured according to the invention. to

μιη ist. Dagegen beträgt bei der in Fig. 2b gezeigten Oberfläche die Periodizität p2 etwa 80μιη, und die mittlere Strukturhöhe liegt bei h2=60 μιη. Die Feinstrukturen sind für beide Figuren ähnlich mit einer Periodizität p2 von <1 μιη und einer mittleren Strukturhöhe von ca. 0,8 μιη. Während die Oberfläche aus Fig. 2a überraschenderweise hydrophob ist, ist diejenige gemäß Fig. 2b hydrophil. Zwar wurde bezüglich der Haftung von Klebemitteln bezüglich der beiden unterschiedlichen Oberflächenstrukturen kein signifikanter Unterschied festgestellt. Jedoch hat sich herausgestellt, dass bei einer Strukturierung entsprechend dem Beispiel der Probe gemäß Fig. 2a eine erhöhte Qualität der Klebeverbindung erzielt werden konnte. So wurden beispielsweise durchgängig Haftfestigkeiten der beiden jeweils gefügten Körper von >50MPa erreicht. Structuring was a picosecond laser with a wavelength of 1064nm at a power of 150μϋ used. The feed (lateral velocity along the surface) was 600mm / s for the surface shown in Fig. 2a and 200mm / s for the surface shown in Fig. 2b. At the top right of each image are enlarged sections of the sample surface for a more detailed view. The periodicity pi of the superstructure for the surface shown in Fig. 2a is about 18μιη, and the average structure height

μιη is. In contrast, in the surface shown in Fig. 2b, the periodicity p2 is about 80μιη, and the average structural height is h2 = 60 μιη. The fine structures are similar for both figures with a periodicity p2 of <1 μιη and an average structure height of about 0.8 μιη. While the surface of Fig. 2a is surprisingly hydrophobic, that of Fig. 2b is hydrophilic. Although no significant difference was found with respect to the adhesion of adhesives with respect to the two different surface structures. However, it has been found that in a structuring according to the example of the sample according to FIG. 2a, an increased quality of the adhesive bond could be achieved. Thus, for example, consistently adhesive strengths of the two respectively joined bodies of> 50 MPa were achieved.

In Fig. 3a, finally, a vibronic sensor 7 is shown with a sensor unit 8 comprising a vibratable unit 9 in the form of a tuning fork, which partially immersed in a medium 10, which is located in a container 11. The oscillatable unit 9 is excited by the exciting / receiving unit 12 to mechanical vibrations, and may be, for example, a piezoelectric stack or bimorph drive. However, it goes without saying that other embodiments of a vibronic sensor 7 fall under the invention. Furthermore, an electronic unit 13 is shown, by means of which the signal detection, evaluation and / or - power supply takes place.

In Fig. 3b is a more detailed view of a sensor unit 8 for a vibronic sensor 7 is shown, which also has an oscillatable unit 9 in the form of a tuning fork, as they are integrated, for example, in the marketed by the applicant under the name LIQUIPHANT vibronic sensors 1.

The tuning fork 9 comprises two prongs 15a, 15b which are integrally formed on a membrane 14 and which each consist of a vibrating rod and a paddle formed thereon. Before the

Drive receiving unit 12 is integrated into the sensor 7, a Steatitscheibe 18 is attached by means of an adhesive bond to the oscillatable unit 9 in the region of the diaphragm 17.

LIST OF REFERENCE NUMBERS

1 first body

 1 a Surface of the first body 2 Second body

 2a surface of the second body

3 adhesives

4 structuring

5 superstructure

6 fine structure

7 Vibronic sensor

8 sensor unit

9 Oscillating unit, tuning fork 10 Medium

1 1 container

12 drive / receiver unit

13 electronics unit

14 membrane

 15a, 15b forks

16 steatite slice p, pi, P2 Periodicity of structuring h, hi, h.2 mean height of structure

Claims

claims 1. A method for producing an adhesive bond between a first body (1) which consists at least partially of a stainless steel and a second body (2), comprising the following method steps:  Structuring (4) at least a first subregion of a first surface (1a) of the first body (1) by means of an ultrashort pulse laser, and  Producing an adhesive bond at least between the first portion of the first surface (1a) of the first body (1) and at least a second portion of a second surface (2a) of the wide body (2) by means of an adhesive (3). 2. The method according to claim 1,  wherein the second body (2) consists at least partially of steatite. 3. The method according to claim 1 or 2,  the laser being operated in a burst mode. 4. The method according to at least one of the preceding claims,  wherein the laser is operated with a power of 50-200μϋ and / or one  Grid speed is operated parallel to a longitudinal direction of the first surface of 0.1-1 cm / s. 5. The method according to at least one of the preceding claims,  wherein at least in the first subregion of the first surface (1a) a structuring (4) in the form of a superstructure (5) and a superstructure (5) superimposed fine structure (6) is generated. 6. The method according to claim 5,  wherein the superstructure (5) is a periodic structure having a periodicity (pi) in the micrometer range, in particular in the range of up to 50μιη, is. 7. The method according to claim 5 or 6,  wherein the fine structure (6) is a periodic structure with a periodicity (P2) in the nanometer range. 8. The method according to at least one of claims 5-7, wherein structuring (4) produces cone-like protrusions (CLPS). 9. The method according to at least one of claims 5-8,  wherein the superstructure (5) has a mean structural height (hi) of up to 25 μιη. 10. The method according to at least one of the preceding claims,  wherein at least the first portion of the first surface (1 a) of the first body (1) is rendered hydrophobic. 1 1. A method according to at least one of the preceding claims,  wherein at least the first portion of the first surface (1 a) after structuring by means of the laser has an adhesive tensile strength of at least 20 MPa. 12. Sensor unit (8) for a vibronic sensor (7), comprising at least one mechanically oscillatable unit (9) made of a stainless steel and a steatite disk (17) fastened to the sensor unit by means of an adhesive connection, wherein the adhesive connection by means of a method according to one of the preceding Claims is made ..  13. Sensor unit (8) according to claim 12, wherein the adhesive bond is made such that an antiresonance of the oscillatable unit (9) is a maximum of 600Hz. 14. Sensor unit (8) according to claim 12 or 13, wherein an adhesive (3) used for the preparation of the adhesive bond is an adhesive having a glass transition temperature, which glass transition temperature is outside a working range of the sensor (7). 15. Vibronic sensor (7) comprising at least one sensor unit (8) according to at least one of claims 12-14.