DE102017103001A1 - Improved adhesive bond by microstructuring a surface - Google Patents

Abstract

The present invention relates to 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 portion 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 Furthermore, the present invention relates to a sensor unit for a vibronic sensor, in which an adhesive bond is produced by means of the method according to the invention and a vibronic sensor with a sensor unit according to the invention.

Description

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, for example, from the WO2014 / 094729A2 a method for structuring a non-conductive workpiece surface has become known in order to achieve a selective and adherent metallization of the same. 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, for example, the article "High-rate laser processing of metals using high-average power ultra short pulse lasers" by the authors 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. Furthermore, statistical structures, which are also referred to as cone-like-protrusions, CLPs for short, are 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 of a medium. In the case of level measuring devices, they have at least one mechanically oscillatable unit, such as a tuning fork, a monobloc or a membrane. This is excited during operation by means of a drive / receiving unit, often in the form of an electromechanical transducer unit to mechanical vibrations, which in turn may be, for example, a piezoelectric actuator or an electromagnetic drive. However, in the case of flowmeters, the mechanically oscillatable unit can also be designed as a vibratable tube through which the respective medium flows, 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 principles of measurement are known in principle from a variety of publications. The drive / receiving unit excites the mechanically oscillatable unit by means of an electrical pickup signal to mechanical vibrations. Conversely, the drive / receiving unit can receive the mechanical vibrations of the mechanically oscillatable unit and convert it into an electrical reception 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 to be guaranteed. 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, from the prior art various solutions, both analog and digital methods, such as those in the documents DE102006034105A1 . DE102007013557A1 . DE102005015547A1 . DE102009026685A1 . DE102009028022A . DE102010030982A1 or DE00102010030982A1 described. In addition to a, in particular predeterminable, level of a medium in a container, which is detectable by a change in a resonant frequency or an amplitude of the mechanically oscillatable unit, vibronic sensors are also suitable for determining the density and / or viscosity, such as in DE10050299A1 . DE102007043811A1 . DE10057974A1 , or DE102015102834A1 described.

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 sensitively on the adhesive bond. 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.

• - Strukturieren zumindest eines ersten Teilbereichs einer ersten Oberfläche des ersten Körpers mittels eines Ultrakurzpulslasers, und
• - Herstellen einer Klebeverbindung zumindest zwischen dem ersten Teilbereich der ersten Oberfläche des ersten Körpers und zumindest eines zweiten Teilbereichs einer zweiten Oberfläche des zweiten Körpers vermittels eines Klebemittels.

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 method steps:

• - Structure of at least a first portion of a first surface of the first body by means of a ultrashort pulse laser, and
• - Producing an adhesive bond at least between the first portion of the first surface of the first body and at least a second portion 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 surface available for the production of the adhesive bond can be achieved. Furthermore, a targeted modification of the surface is 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 is fundamental for the largest 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 with a power of about 50-200μJ 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 microns.

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. The fine structure preferably has a periodicity in the range of <1 μm.

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 25 μm, preferably 2-20 μm. 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 comprises processes according to at least one of the preceding claims, 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 bond between the oscillatable unit and the steatite disk significantly influences 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 the 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.

1. 1) Einstellung der mechanischen Schwingungsgüte der mechanisch schwingfähigen Einheit durch Änderung der Formgebung bzw. Geometrie (Membrandicke, Übergänge zwischen Membran und schwingfähiger Einheit) und/oder des Materials (u. a. beispielsweise die Steifigkeit);
2. 2) Vermeidung oder Verringerung von Energieverlusten, z. B. durch einen symmetrischen Aufbau und/oder die Schwingungsübertragungen zwischen Antriebs-/Empfangseinheit, Membran und schwingfähiger Einheit auftreten.
3. 3) Reduzierung der Anzahl von Bauteilen;
4. 4) Vermeidung von mechanischen Verspannungen;
5. 5) Gegebenenfalls Einstellung der Dicke einer isolierenden Scheibe zwischen Membran und Antriebs-Empfangseinheit, wie beispielsweise in der EP0985916B1 beschrieben;
6. 6) Einstellung des Kopplungsfaktors der Antriebs-/Empfangseinheit;und
7. 7) Optimierung von Klebungen und Kontaktierungen.

Basically, in the case of a vibronic sensor, the distance between the resonant frequency and the antiresonant frequency may be affected by various measures, e.g. B .:

1. 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. 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. 3) reducing the number of components;
4. 4) avoidance of mechanical tension;
5. 5) Optionally adjusting the thickness of an insulating disc between the membrane and the drive-receiving unit, such as in the EP0985916B1 described;
6. 6) Setting the coupling factor of the drive / receiver unit; and
7. 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 average structural heights adversely affect the mechanical vibration quality of the vibronic sensor.

It is also advantageous if an adhesive used for the production of the adhesive bond is an adhesive having a glass transition temperature, which glass transition temperature is outside a working range of the sensor.

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 steatite disk 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.

• 1 zwei mittels einer Klebeverbindung gefügte Bauteile, wobei das erste Bauteil ein Edelstahl ist, und wobei das zweite Bauteil aus einem nichtmetallischen Material besteht,
• 2 (a) und (b) zwei Abbildungen zweier erfindungsgemäß strukturierter Oberflächen,
• 3 eine schematische Skizze (a) eines vibronischen Sensors gemäß Stand der Technik, und (b) einer Schwinggabel mit einer daran befestigten Steatitscheibe.

The invention and its advantageous embodiments will be described below with reference to FIGS 1 - Fig. 3 described in more detail. It shows:

• 1 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) two images of two surfaces structured according to the invention,
• 3 a schematic sketch (a) of a vibronic sensor according to the prior art, and (b) a tuning fork with a Steatitscheibe attached thereto.

In 1a are a first body 1 made of stainless steel and a second body 2 made of a non-metallic material, which by means of the adhesive 3 along the surfaces 1a and 2a connected to each other. To ensure a 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 in 1b shown. The structuring 4 is made up of a superstructure 5 and one of these superstructures 5 superimposed fine structure 6 together, 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 a periodicity p ₁ in the micrometer range, while the periodicity p ₂ (not shown) of the fine structure 6 in the nanometer range. Besides, it is beneficial if the superstructure 5 an average structure height h ₁ (peak-to-valley) of up to 25 μm, preferably 2-20 μm.

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.

2 shows two stainless steel surfaces which have been structured according to the invention. For structuring, a picosecond laser with a wavelength of 1064 nm at a power of 150 μJ was used. The feed (lateral velocity along the surface) was for the in 2a surface shown 600mm / s and for the in 2 B surface shown 200mm / s. At the top right of each image are enlarged sections of the sample surface for a more detailed view.

The periodicity p _{1 of} the superstructure for the in 2a shown surface is about 18μm, and the average structural height h ₁ = 8 microns. In contrast, at the in 2 B shown surface the periodicity p ₂ about 80μm, and the average structure height is h ₂ = 60 microns. The fine structures are similar for both figures with a periodicity p2 of <1 .mu.m and an average structure height of about 0.8 .mu.m. While the surface is off 2a is surprisingly hydrophobic, is that according to 2 B 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 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 3a is finally a vibronic sensor 7 with a sensor unit 8th comprising an oscillatable unit 9 shown in the form of a tuning fork, which partially in a medium 10 dips, which is in a container 11 located. The oscillatable unit 9 is by means of the pickup / receiver unit 12 excited 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 represented, by means of which the signal detection, evaluation and / or supply is carried out.

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

The tuning fork 9 includes two to a membrane 14 molded forks 15a, 15b, which each consist of a vibrating rod and a paddle formed thereon. Before the drive-receiving unit 12 in the sensor 7 is integrated, in the area of the membrane 17 a steatite slice 18 by means of an adhesive connection to the oscillatable unit 9 attached.

LIST OF REFERENCE NUMBERS

1 First body 1a Surface of the first body 2 Second body 2a Surface of the second body 3 adhesive 4 structuring 5 superstructure 6 fine structure 7 Vibronic sensor 8th sensor unit 9 Oscillating unit, tuning fork 10 medium 11 container 12 Driver / receiver unit 13 electronics unit 14 membrane 15a, 15b forks 16 Steatitscheibe p, p ₁ , p ₂ Periodicity of structuring h, h ₁ , h ₂ average structure height

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2014/094729 A2 [0003]
• DE 102006034105 A1 [0007]
• DE 102007013557 A1 [0007]
• DE 102005015547 A1 [0007]
• DE 102009026685 A1 [0007]
• DE 102009028022 A [0007]
• DE 102010030982 A1 [0007]
• DE 00102010030982 A1 [0007]
• DE 10050299 A1 [0007]
• DE 102007043811 A1 [0007]
• DE 10057974 A1 [0007]
• DE 102015102834 A1 [0007]
• EP 0985916 B1 [0030]

Claims (15)

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 portion 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). Method according to Claim 1 wherein the second body (2) consists at least partially of steatite. Method according to Claim 1 or 2 wherein the laser is operated in a burst mode. Method according to at least one of the preceding claims, wherein the laser is operated with a power of 50-200μJ and / or a scanning speed is operated parallel to a longitudinal direction of the first surface of 0.1-1 cm / s. Method according to at least one of the preceding claims, wherein a structuring (4) in the form of a superstructure (5) and a fine structure (6) superimposed on the superstructure (5) is produced at least in the first subregion of the first surface (1a). Method according to Claim 5 in which the superstructure (5) is a periodic structure with a periodicity (p ₁ ) in the micrometer range, in particular in the range of up to 50 μm. Method according to claim 5 or 6, wherein the fine structure (6) is a periodic structure with a periodicity (p ₂ ) in the nanometer range. Method according to at least one of Claims 5 - 7 , wherein in the structuring (4) so-called cone-like-protrusions (CLPS) are generated. Method according to at least one of Claims 5 - 8th , wherein the superstructure (5) has a mean structural height (h ₁ ) of up to 25 μm. Method according to at least one of the preceding claims, wherein at least the first portion of the first surface (1a) of the first body (1) is rendered hydrophobic. Method according to at least one of the preceding claims, wherein at least the first portion of the first surface (1a) after structuring by means of the laser has an adhesive tensile strength of at least 20 MPa. 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 is produced by a method according to one of the preceding claims is .. Sensor unit (8) after Claim 12 , wherein the adhesive bond is made such that an anti-resonance of the oscillatory unit (9) is at a maximum 600Hz. Sensor unit (8) after Claim 12 or 13 in which an adhesive (3) used for the production 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). Vibronic sensor (7) comprising at least one sensor unit (8) according to at least one of Claims 12 - 14 ,

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