SENSORS AND PROCESS FOR PRODUCING SENSORS

Abstract

A method for producing a sensor on the surface of a functional layer, in which suitable sensor material in the form of powder or a wire is melted in a laser beam by way of a method similar to laser cladding and subsequently is applied to the surface of the functional layer. There is provided a considerably improved method for producing sensors, and in particular in-situ sensors, wherein the sensors can also be deposited onto a functional layer that, in part, is very coarse, without having to employ complex masks, as has previously been customary. The ease of adapting the method parameters ensures broad use both with respect to the sensor to be produced and the functional layer to be detected. The sensors thus produced are used, in particular, to detect components that are subject to high temperatures or the functional layers thereof. The sensors that can be produced in accordance with the invention include, in particular, temperature, pressure or voltage sensors, as well as acceleration sensors.

Claims

1. A method for producing a sensor on the surface of a functional layer, wherein the sensor material is at least partially melted in a laser beam using a method similar to laser cladding and is subsequently applied onto the surface of the functional layer, wherein, during the application of the sensor material, the surface temperature of the functional layer is established so as to be lower than the melting temperature of the functional layer.

2. The method according to claim 1, wherein the establishing of the surface temperature of the functional layer is achieved by limiting the heat input by shielding the process laser by way of the delivery rate of the sensor material.

3. The method according to claim 1, wherein a ceramic thermal barrier coating, an insulating layer, an oxidation (or corrosion) protective layer or an environmentally stable (thermal) protective layer is used as the functional layer.

4. A method according to claim 1, wherein the sensor material is applied under a protective gas atmosphere.

5. The method according to the claim 1, wherein argon is used as the protective gas.

6. A method according to claim 1, wherein powder having a mean particle diameter between 1 and 200 μm, and in particular between 2 and 50 μm, is used as the sensor material.

7. A method according to claim 1, wherein the structure cross-sections of the applied sensor are small compared to the dimensions of the functional layer.

8. A method according to claim 1, wherein Alumel®, Chromel®, platinum, iron, copper nickel alloys, platinum rhodium alloys, nickel chromium alloys, tungsten rhenium alloys, CrNi steel, nickel, Ni-20Cr, Cu-45Ni, Pd-13Cr, Cu-12Mn-2Ni, barium titanate or lead zirconate titanate ceramics (PZT), quartz, tourmaline, gallium phosphate or lithium niobate are used as the sensor material.

9. A method according to claim 1, wherein a temperature, pressure, stress or acceleration sensor is produced.

10. A method according to claim 1, wherein the sensor applied to the surface of the functional layer is at least partially embedded by applying a further layer.

11. The method according to claim 1, wherein a further functional layer is applied as the further layer.

12. A sensor wherein the sensor is disposed on the surface of a functional layer and having been produced by a method according to claim 1.

13. The sensor according to claim 12, wherein this is a temperature, pressure, stress or acceleration sensor.

14. The sensor according to claim 12, wherein the applied sensor material is designed to be uninterrupted and pore-free.

Description

IN THE DRAWINGS

[0064] FIG. 1 shows a schematic illustration of the material supply in a device for laser cladding according to the prior art (a) and a schematic illustration of an exemplary material supply and laser focusing with the process control according to the invention (b);

[0065] FIG. 2 shows a schematic sectional drawing (a) and top view (b) onto a functional layer comprising a temperature sensor applied thereto in accordance with the invention and a further layer optionally applied thereto;

[0066] FIG. 3 shows a schematic top view onto a functional layer comprising a strain sensor applied thereto in accordance with the invention and a further layer applied thereto;

[0067] FIG. 4 shows a top view onto a functional layer comprising a K-type temperature sensor disposed thereon in accordance with the invention;

[0068] FIG. 5 shows a diagram of the temperatures ascertained by the temperature sensors as a function of the heating time, and a comparison between a sensor according to the invention and a reference sensor;

[0069] FIG. 6 shows a surface profile of a functional layer comprising a temperature sensor (a) applied thereto in accordance with the invention and a further layer (b) optionally applied thereto;

[0070] FIG. 7 shows schematic cross-sections of conductor tracks applied onto a functional layer, and a comparison between a conductor track applied in accordance with the invention and a conductor track applied by way of conventional methods; and

[0071] FIG. 8 shows a cross-sectional view of a conductor track applied onto a functional layer in accordance with the invention and embedded therein.

[0072] FIG. 1 schematically illustrates the application of the sensor material, as it can also be employed in the present invention. The sensor material is supplied to a laser beam, in a manner similar to that in a device for laser cladding (FIG. 1a).

[0073] The powder to be applied (sensor material) (2), which later forms the sensor, can typically be provided via a powder nozzle disposed on the side (laterally), via multiple powder nozzles disposed on the side (radially), or via a powder nozzle disposed concentrically (coaxially).

[0074] Several millimeters, such as 7 mm, can be selected as a typical distance between the functional layer and the laser.

[0075] The supplied powdery material (2), which is initially melted in the laser beam (1), is deposited on the surface of the functional layer (4), for example a ceramic insulating layer, as a coating (3), such as a metallic linear conductor, after compaction. The functional layer (4) is disposed on the metallic component (substrate) (6), such as a turbine blade, optionally via a further intermediate layer (5) (bond coat).

[0076] The control of the process can be achieved, according to the invention, for example, by selecting the focus cross-section of the powder supply (2) at the substrate level so as to be greater than the cross-section of the focused laser beam (1) (FIG. 1(b)), so that a sufficiently high powder supply rate results in considerable shielding of the substrate from the laser, and furthermore by adapting the displacement speed so that the portion of the process powder melted in the central region of the laser spot is deposited as a continuous trace having good adhesion onto the substrate, which may be rough.

[0077] As an alternative to application by way of a powder, it is also possible to supply a prefabricated wire made of the sensor material directly to the laser beam, or alternatively this can also already be disposed on the surface of the functional layer, and be melted by way of a laser to serve as a corresponding coating/conductor track (not shown in FIG. 1).

[0078] FIG. 2a shows a schematic cross-section through the functional layer (4) and the sensor (3) applied thereto in accordance with the invention. The component and an optional additional intermediate layer are not illustrated in this figure. Furthermore, a further (second) functional layer (7) is shown as one embodiment of the invention, which covers portions of the sensor and of the first functional layer, for example, while leaving the region of the contacting of the sensor (9) exposed.

[0079] FIG. 2b schematically shows the corresponding top view for the cross-section illustrated in FIG. 2a. The temperature sensor comprising the two conductor tracks (3a, 3b) is applied in accordance with the invention onto the functional layer (4) in the form of two thin sensor coatings/conductor tracks made of differing materials. The two conductor tracks are electrically conductively connected to one another at a contact area (8). The region (9) intended for external contact with the compensating lines, which is to say the ends of the two conductors of the sensor, is not covered by the further functional layer (7). Thus, they remain freely accessible,

[0080] FIG. 3 schematically shows the top view onto a strain sensor produced in accordance with the invention. The strain sensor comprising the strain-sensitive conductor track (3c), which is disposed in a meander shape, and the electrical connecting lines (3d, 3e), is applied onto the functional layer (4) in accordance with the invention in the form of thin sensor coatings/conductor tracks made of a suitable material. The conductor tracks are electrically conductively connected to one another at the contact areas (8). The region (9) intended for the external contact, which is to say the ends of the two connecting lines of the sensor, is not covered by the further functional layer (7). Thus, they remain freely accessible.

[0081] In one embodiment of the invention, the sensor was produced in accordance with the invention as a type K thermocouple (see FIG. 4 in this regard). To this end, two conductor tracks made of differing materials, in this case made of Alumel® (12) and Chromel® (13), were applied at right angles with respect to one another, on the functional layer (11) of a metallic substrate by way of a method similar to laser cladding. The two conductor tracks are electrically connected to one another via a connecting point (contact area) (19). For the conductors, appropriate powders having a particle diameter of 2.6 to 20 μm (d.sub.50=7.4 μm) for Alumel® and of 3.5 to 35 μm (d.sub.50=12.1 μm) for Chromel® were used.

[0082] The sensor was applied onto the functional layer (11) by way of a laser, wherein the aforementioned powders were each supplied to the laser beam via a coaxial supply system. The laser used was a neodymium-doped yttrium aluminum garnet laser (Nd:YAG) in the lower power range.

[0083] To check the functional capability of the temperature sensor and to determine the Seebeck coefficient, the thermal and electrical data of the thermocouple produced in accordance with the invention were measured at temperatures between room temperature and 500° C. To this end, the negative conductor made of Alumel® (12) was contacted with a NiAl compensating line (14), and the positive conductor made of Chromel® (13) was contacted with a NiCr compensating line (15). For contact, the compensating lines (compensation wires) were each pressed onto the positive conductor and the negative conductor (contact areas 16, 17), at some distance from the contact point, and were each fixed by way of a small glass plate and metal clamps. The other ends of the compensation wires were connected to the measuring and recording device (temperature measuring device) (18), which includes a measuring transducer/transmitter. The contact point (19) is formed by the measuring point having the measuring temperature, while the measuring device (18) integrates the comparison point having the reference temperature, which typically is the room temperature.

[0084] In addition, a commercially available type K thermocouple was disposed adjacent to the contact area (19) of the thermocouple produced in accordance with the invention and likewise connected to the measuring and recording device (18).

[0085] The sample, comprising at least the functional layer and the two thermocouples disposed thereon and connected to the measuring device (18), was heated in a furnace under an argon protective gas atmosphere at a heating rate of 5 K/min. During the experiment, voltages generated by the two thermocouples were continuously detected and evaluated by the measuring device.

[0086] FIG, 5 indicates the temperature values detected by the measuring device during the experiment over the time of the experiment. The line marked by closed symbols represents the results of the thermocouple produced in accordance with the invention, and the line marked by open symbols represents the results of the reference thermocouple. Temporary interruptions in the measuring value sequence, in particular for the thermocouple produced in accordance with the invention, are caused on the fluctuating contact resistance which, in the present experiments, are compensating lines pressed on only by means of metallic clamps.

[0087] By evaluating the ascertained voltages, it was possible to confirm that, with the sensor produced in accordance with the invention, the generated voltage has a substantially linear dependence on the temperature in the analyzed temperature range. With the aid of a linear adjustment, a Seebeck coefficient could be determined from the detected thermal EMFs as a measure of the thermoelectric sensitivity of 41.2 μV/K at a regression factor of 0.9999. In contrast, commercially available thermocouples have a nominal Seebeck coefficient of 41.1 μV/K. The comparison shows that the voltages generated by way of the thermocouple produced in accordance with the invention can be rated as very trustworthy. This experiment impressively demonstrates that the sensor according to the invention even now can be used as an excellent temperature sensor.

[0088] After this test series, a further layer, which in the present case corresponded to a ceramic functional layer, was applied by way of atmospheric plasma spraying. In this way, the sensor (3) disposed on the first functional layer (4) could be completely embedded.

[0089] In this case, the sensor is present embedded in the two functional layers. The free line ends of the sensor produced in accordance with the invention were connected, via appropriate compensating lines, to a measuring and recording device, which was able to detect the electrical signals generated during the experiment.

[0090] The layer thickness of the second applied functional layer was approximately 200 μm. The two functional layers were produced by way of plasma spraying and had a surface roughness of approximately 40 μm. Experiments and powerful optical measuring methods (white light interferometry) for ascertaining height profiles of an embedded sensor clearly demonstrated that, at a sensor height (height of the applied conductor track) of approximately 90 μm and a layer thickness of the second functional layer of likewise approximately 200 μm, an excess height for the second functional layer of only approximately 40 μm is apparent at the location of the conductor tracks. This demonstrates that the sensor is embedded well in a relatively thin second functional layer (see also FIG. 6 in this regard).

[0091] It has been found that it is easily possible for a person skilled in the art to distinguish a sensor applied or deposited onto a functional layer by way of the method according to the invention from one obtained by way of previously customary application methods, such as plasma spraying, using masks. A comparison of the different cross-sections using the example of a conductor track is schematically illustrated in FIG. 7.

[0092] The characteristic cross-section of a conductor track (3) applied in accordance with the invention shows an overall arcuate progression, while a conductor track that was applied with the aid of a mask and using conventional plasma spraying, by comparison, shows a flattened cross-section (3.sub.SdT) extending almost parallel to the surface of the functional layer in the central region and has considerably steeper edges. This is due to the fact that plasma spraying, in principle, is a method for the planar, parallel application of material, while the method according to the invention is advantageously suitable for applying lines or spots. Depending on the surface properties of the functional layer and the wettability, an undercut can also be generated in the method according to the invention.

[0093] FIG. 8 shows a cross-sectional view of a conductor track applied onto a functional layer in accordance with the invention and embedded therein.

[0094] In summary, it can be stated that the invention provides a considerably improved method for producing sensors, and in particular also of in-situ sensors, wherein the sensors can also be deposited onto a functional layer that, in part, is very coarse, without having to employ the previously customary complex masks. The ease of adapting the method parameters ensures broad use both with respect to the sensor to be produced and the functional layer to be detected.