Chemically Resistant Multilayered Coating for a Measuring Device Used in Process Engineering

Abstract

A field device used in process and/or automation engineering for monitoring at least one chemical or physical process variable of a medium in a component carrying a medium at least partially and temporarily and comprising at least an electronic unit and a sensor unit. At least one portion of at least one component of the sensor unit is in contact with the medium at least temporarily. The at least one portion of the component in contact with the medium is provided with a chemically resistant multilayered coating consisting of at least two layers, wherein a first layer is made of a material consisting of a densely packed atomic arrangement which provides a protection against corrosion by said medium, and a second layer consisting of a chemically resistant plastic material is arranged around the first layer and protects the first layer against outer damage and corrosion.

Claims

1-12. (canceled)

13. A field device used in process and/or automation engineering for monitoring at least one chemical or physical process variable of a medium in a component carrying a medium at least partially and temporarily, comprising: at least an electronics unit; and a sensor unit, wherein: at least one portion of at least one component of said sensor unit is in contact with the medium at least temporarily; at least the portion of said component in contact with the medium is provided with a chemically resistant multilayered coating consisting of at least two layers, wherein a first layer is made of a material consisting of a densely packed atomic arrangement and provides a protection against corrosion by said medium, wherein a second layer consisting of a chemically resistant plastic material is arranged around said first layer and protects the first layer against external damage and corrosion.

14. The field device according to claim 13, wherein: said second layer consists of PFA, PTFE, FEP, ECTFE, PEEK, or rubber.

15. The field device according to claim 13, wherein: said first layer consists of a metal—in particular, gold, platinum, silver, or tantalum—SiC, DLC, Al.sub.2O3, SiO.sub.2, or BN.

16. The field device according to claim 13, wherein: an elastic material—in particular, SiC or DLC, or a two-layer system made of SiC and DLC—is used for said first layer.

17. The field device according to claim 13, wherein: said first layer is produced using a galvanic deposition process.

18. The field device according to claim 13, wherein: said first layer is produced using a CVD method—in particular, using a CVD method in plasma under low temperature conditions.

19. The field device according to claim 13, wherein: said first layer is produced using a sol-gel method.

20. The field device according to claim 19, wherein: the surface energy of said first layer in the portion facing the medium is suitably adjusted—in particular, maximized—particularly by oxidation or doping.

21. The field device according to claim 20, wherein: said first layer is a hybrid structure.

22. The field device according to claim 13, wherein: the field device is a pressure-measuring cell; and said component in contact with the medium at least in one portion and at least partially is a membrane.

23. The field device according to claim 21, wherein: said first layer has a thickness of about 10 μm and is elastic; and said second layer (5) has a thickness of about 300 μm.

24. The field device according to claim 13, wherein: the field device is a fill state measuring device, and said sensor unit has a unit capable of oscillating which is said component in contact with the medium at least in one portion and at least partially, and which is provided in the portion facing the medium with a multilayered coating.

Description

[0033] The invention, as well as its advantages, are explained in more detail with reference to the following FIGS. 1 through 3. These show:

[0034] FIG. 1 a schematic drawing of a surface, which is coated with a multilayered coating according to the invention, of a component of a sensor unit in contact with the medium,

[0035] FIG. 2 a schematic drawing of a pressure-measuring cell, which is coated according to the invention, in a three-dimensional (a) and a two-dimensional (b) view, and

[0036] FIG. 3 a schematic drawing of a fill state measuring device (a), which is coated according to the invention, as well as a detailed schematic drawing of a unit (b), which is capable of oscillating and coated with a multilayered coating.

[0037] FIG. 1 shows a schematic drawing of a surface, which is coated with a multilayered coating 2 according to the invention, of a component of a sensor unit 1 in contact with the medium. For the sake of simplicity, the component 3 of the sensor unit in contact with the medium is illustrated as a rectangle. The multilayered coating 2 is composed of a first layer 4 and of a second layer 5 arranged thereon.

[0038] As already mentioned, the first layer can consist either of a metal, such as gold, platinum, silver, or tantalum, or of a so-called hard material, such as SiC, DLC, Al.sub.2O.sub.3, SiO.sub.2, or BN. Depending upon the application, different materials and, accordingly, also different coating methods are advantageous, such as galvanic vapor deposition, the physical vapor deposition processes (PVD), or even the CVD method. The underlying principles are known from a plurality of publications and are therefore not explained in more detail here.

[0039] In particular, the CVD method, in which a solid component is deposited from the gas phase onto a typically heated surface using a chemical reaction, offers the advantage of a conformal layer deposition. Thus, the CVD method is, in particular, suitable for complex, three-dimensionally formed surfaces.

[0040] Restrictions upon the method are, on the other hand, that a gaseous compound, from which the respective layer can be produced using the CVD method, does not exist for any desired material. In addition, the substrate, i.e., in this case, the at least one component of the sensor unit in contact with the medium, must be designed to withstand high temperatures. In some circumstances, however, a high temperature load can result in deformation of the sensor component, or even in diffusion processes within the sensor unit.

[0041] There are different variants of the PVC method, in which the temperature load of the substrate can be considerably reduced. One possibility is the plasma-enhanced CVD, or the plasma-enhanced, low-pressure CVD. In this case, an inductive or capacitive plasma is ignited above the substrate, which plasma excites the gas, breaking it down, used for the coating and can additionally provide for an increase in the deposition rate. Typical substrate temperatures for this method are in the range of approximately 200-500° C., whereas, without an enhancing plasma, substrate temperatures of up to 1000° C. are sometimes required.

[0042] Another advantage of the CVD method consists in the fact that heterogeneous coatings can be produced. For example, if the gas mixture used is changed continuously during a deposition process, the composition of the deposited layer also changes continuously over time, if a suitable composition of the gas mixture is used. In this way, layers produced can be oxidized and/or sealed in, for example, the area of their surface. The surface energy can, in particular, be specifically adjusted thereby.

[0043] Even though the layers produced in this way are characterized by a densely packed structure and already have an outstanding corrosion resistance at very low layer thicknesses, they are, nonetheless, unsuitable for continuous use in aggressive media. The reason lies in the already mentioned microscopic defects of the layers 6, which are typical for the CVD method. Examples are illustrated in FIG. 1. Even though the density of the defects is low, aggressive molecules can penetrate the layers at appropriate points and corrode the metal alloy. Another already mentioned problem with these layers consists in the layers being very delicate and easily damaged by scratches and/or impacts.

[0044] For this reason, the first layer 4 in FIG. 1 is surrounded according to the invention by a second layer 5, which consists of a chemically resistant plastic and which protects the first layer 4 against external damage. For this second layer 5, PFA, PTFE, FEP, ECTFE, PEEK, or rubber can, for example, be used. In this case, the list is also not exhaustive, and it goes without saying that other materials also fall under the invention. As mentioned above, for typical layer thicknesses, the diffusion resistance of a plastic coating is generally lower than with a metal and/or hard material. Nevertheless, the second layer 5 also effectively offers a resistance to diffusing molecules. To a greater extent, however, it is significantly more robust than the first layer 3 and accordingly provides for a protection of the first layer 4 against external damage.

[0045] FIG. 2a shows a pressure-measuring cell 7 according to the invention. Corresponding field devices are also produced and marketed in great variety by the applicant and are, for example, available under the designations CERABAR and DELTABAR. A measuring cell and a hermetic, hydraulic system 8 are closed by a membrane 9. In the present example, the membrane 9 is produced from a stainless-steel foil. It goes without saying, however, that other materials also fall under the present invention, such as Monel. The membrane is typically connected via a weld joint 11 (see FIG. 2b) with a flange 10, into which the chamber with a transmission fluid 8 is integrated. During the operation of the pressure-measuring cell 7, the membrane 9 respectively transmits the current process pressure to the measuring cell via the transmission fluid in the chamber 8. As a result of this functionality, the membrane 9 must be very flexible and thin (typically from 25 μm to 150 μm).

[0046] Corrosion basically has two consequences for pressure-measuring cells. On the one hand, the membrane 9 can completely corrode, so that the medium is contaminated by the transmission fluid of the pressure-measuring cell 7, and medium enters into the interior of the pressure-measuring cell 7. On the other hand, the membrane 9 and the container for the respective medium can form a galvanic element. For this reason, the membrane 9 is often provided with a coating 2 for protection against aggressive media.

[0047] Like the membrane 9 itself, a coating of the membrane 2 must also be thin and flexible, since the measurement performance of the pressure-measuring cell can otherwise be limited. A multilayered coating 2 according to the invention is, therefore, advantageous. This coating can be seen better in the two-dimensional view of the pressure-measuring cell in FIG. 2b. There, a first layer 4 and a second layer 5 are illustrated schematically.

[0048] Depending upon the material, a layer thickness of approximately 10 μm is already sufficient for the first layer 4. The second layer 5 made of plastic can, for example, be applied with a layer thickness of approximately 300 μm. The total thickness is thus considerably reduced compared to a purely elastic plastic coating with sufficient corrosion protection. It is advantageous, particularly in a pressure-measuring cell 7, if an elastic material—in particular, SiC, or even DLC—is also selected for the first layer. The flexibility and elasticity of the membrane 7 is thus limited as little as possible.

[0049] A second example of a field device with a multilayered coating according to the invention is the fill state measuring device 12 shown in FIG. 3a. This is a so-called vibronic sensor with a sensor unit 15 and an electronics unit 14. Corresponding field devices are produced and marketed in great variety by the applicant and are, for example, available under the designations LIQUIPHANT and SOLIPHANT. With this type of field device 12, the thickness and/or viscosity of a medium 16 in a component carrying the medium—in this case, a container 17—can also be determined in addition to the fill state. The underlying measuring principles are known from a plurality of publications and are, therefore, not explained in more detail here. The sensor unit comprises a unit 13 capable of oscillating and in contact with the medium, at least temporarily and partially. During measurement operation, the unit 13 capable of oscillating is caused to perform mechanical oscillations via an electrical excitation signal using an electromechanical transducer unit, which oscillations are converted into an electrical response signal and processed in the electronics unit 14. In order to be able to determine the fill state, the thickness, and/or the viscosity, the amplitude and/or phase of the oscillations, for example, are then evaluated. In such a fill state measuring device 12, it is also advantageous during operation in aggressive media to provide at least the unit capable of oscillating and in contact with the medium with a coating.

[0050] FIG. 3b shows a detailed, schematic drawing of a unit 13 capable of oscillating with a multilayered coating 2, which, again, consists of a first layer 4 and a second layer 5. In this case, the coating according to the invention also provides for a very good corrosion protection of the unit capable of oscillating and increases the service life of the measuring device accordingly.

[0051] In summary, a multilayered coating according to the invention for at least one component of the sensor unit in contact with the medium brings about a considerable prolongation of the service life of a corresponding field device in aggressive media. By the integration of a plastic as second layer 5, an electrical isolation between the at least one component and the medium is additionally achieved. The sensor unit is thus, where applicable, also protected against hydrogen diffusion in galvanic processes.

REFERENCE SYMBOLS

[0052] Surface, which comprises a multilayered coating, of a component of a sensor unit in contact with the medium [0053] 2 Multilayered coating [0054] 3 Component of the sensor unit in contact with the medium [0055] 4 First layer [0056] 5 Second layer [0057] 6 Microscopic defect within the first layer [0058] 7 Pressure-measuring cell [0059] 8 Chamber with transmission fluid [0060] 9 Membrane [0061] 10 Flange [0062] 11 Weld joint between membrane and flange [0063] 12 Fill state measuring device [0064] 13 Unit capable of oscillating [0065] 14 Electronics unit [0066] 15 Sensor unit [0067] 16 Medium [0068] 17 Component—in this case, container—carrying the medium