Thin-film sensor with improved sensitivity

Abstract

The present invention relates to a coating system for use as a thin film sensor, comprising a piezoresistive sensor element, which is a layer integrated in the coating system, the coating system being deposited on a surface of a substrate, preferably the substrate being a component or tool or part of a component or tool, the piezoresistive sensor element comprising at least one hydrogen-free tetrahedral amorphous carbon coating doped with X, i.e. a ta-C:X coating layer, where X is one or more chemical elements selected from element groups 13 and 15 of the Periodic Table of the Elements, preferably X is one or more from B, Al, Ga, In, Ti, N and P, and the ta-C:X coating layer exhibits an anisotropic gauge factor.

Description

[Technical field]

[0001] The present invention relates to a thin-film sensor, understood as a coating system comprising a piezoresistive material sensor layer based on a hydrogen-free tetrahedral amorphous carbon material doped with a chemical element X, i.e. a ta-C:X layer, where X is one or more chemical elements selected from element groups 13 and 15 of the periodic table of the elements, in particular boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), nitrogen (N) and phosphorus (P), in order to improve the sensor properties, in particular to improve the sensor sensitivity. Likewise, the present invention relates to a component or part of a component comprising a thin-film sensor according to the invention, the material of the substrate surface of the component or part of a component to which the thin-film sensor is applied preferably comprises a polymer, a ceramic and / or a composite material. [Background technology]

[0002] Technical Field Sensors have been used for decades in many applications, today with a very strong presence in the semiconductor segment. Their demand is becoming more and more dramatic due to the emergence of new technologies such as the Internet of Things (loT), the need and possibility to process big data to understand and predict the behavior of complex systems, the introduction of Industry 4.0 standards, as well as for monitoring fully automated devices containing mechanical and electronic components.

[0003] A sensor is the interface between the real, analogue and digital worlds, it allows converting all measurable quantities such as light intensity, temperature, force, acceleration, movement, magnetic fields, etc. into electrical signals that can then be processed and analyzed digitally.

[0004] In the context of this specification, a thin film is a layer of material ranging in thickness from just a few nanometers to a few micrometers, to which any suitable technique for the deposition of thin films can be applied, and thin film deposition is the act of applying a thin film to the surface of a substrate or to the surface of a previously deposited layer.

[0005] The term embedded sensor is typically used to refer to two types of sensors:

[0006] 1. A sensor element integrated on a semiconductor chip, or contained in an Integrated Circuit (IC), and placed in an electronic device to measure different conditions of a larger device or machine, that means, for example, to determine the position of a car, or the acceleration of a robotic arm, or the cooling temperature of a heating system, etc.

[0007] 2. Directly implemented sensors, which means that they are applied directly to the surface of a component or are integrated into the component that needs to be monitored with regard to its properties and / or external effects on the component itself.

[0008] It is known that several sensors of the same or different types can be incorporated at different locations in a component in order to monitor one or more measurable properties of said component.

[0009] The term component in the context of this specification includes any part of a component or article or tool.

[0010] In particular, in the context of this specification, a component may be (but is not limited to) a tool, a machine element, a heating part, an automotive part (e.g., a piston, a bearing, a shaft, an axle, a valve, a battery, a part of an aircraft, or an aircraft (e.g., a turbine engine component, landing gear, a housing, or a part of an industrial machine).

[0011] Typical properties monitored by sensors in such applications are, for example, strain, force, pressure, vibration, and temperature (eg, the temperature at the surface of the component).

[0012] For simplicity, at least some portions of this specification will focus on strain gauge sensors and temperature sensors, however, this focus should not be understood as a limitation of the present invention.

[0013] Some known sensors that can be integrated into components include metal or semiconductor foil strain gauges, as well as resistance temperature detectors (RTDs), such as platinum-based PT100 sensors or negative temperature coefficient thermistors (NTCs). Such types of sensors, due to their smaller geometry, can be easily attached to the components that need to be monitored (i.e., from which some property needs to be measured). Additionally, bulkier sensors, such as magnetoresistive torque sensors and classical pressure sensors, are also known and can be considered for use as embedded sensors.

[0014] However, there are several limitations to the use of the known sensors mentioned above, for example metal foil strain gauges, in particular due to their high length to cross section ratio, as well as their nature and shape, which makes them difficult to incorporate into components with complex shapes.

[0015] Furthermore, the sensitivity of such known sensors is inherently very low, so that if a high sensitivity for a more accurate measurement is required, their setup becomes more complicated.

[0016] One further drawback of the known sensors mentioned above is that they are also prone to damage after long operating times.

[0017] Semiconductor film strain gauges also have deficiencies because they are brittle, expensive to manufacture, and more difficult to integrate into metallic components due to high temperature processing.

[0018] Platinum-based sensors are complex to set up, especially when precise measurements are required. As for metal foil strain gauges, they have a high length to cross-section ratio, which makes them difficult to integrate into complex surfaces. They are also very sensitive to chemically harsh environments and mechanical stresses, which require additional shielding. These sensors are already bulky relative to the size of the components they measure, and additional shielding would significantly increase their mass, resulting in an unfavorable decrease in their response time.

[0019] Thermistors, as well as magnetoresistive torque sensors, are bulky, have slow response times, require specialized materials for the components used as part of the machine as the base sensor, and are expensive to fabricate.

[0020] Pressure sensors are limited in the pressure and temperature ranges they measure, they have poor long term stability of the measurement, they are limited by the media resistance, and the measurement is strongly dependent on the temperature of the sensor.

[0021] In recent years, so-called thin-film sensors have been developed that comprise at least one coating layer to perform the sensor function and can be used as sensors to measure desired properties on any surface of a component. Such coating layers can be deposited on the surface in an easy way instead of requiring complex implementations of wires or foils to achieve the sensor function. Such recently developed thin-film sensors are now considered to be an established technology and also part of a family of coating solutions called "smart coatings".

[0022] Conventional thin film deposition methods (also called coating techniques) can be used to manufacture such thin film sensors, so that a thin film can be deposited on a substrate surface and used as a sensor element.

[0023] Some of such thin film deposition methods are, for example, physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma-assisted chemical vapor deposition (PA-CVD), thermal spraying, and the like.

[0024] The coating techniques described above are particularly suitable for depositing thin film sensors on components or portions of components having complex geometries.

[0025] In the context of this specification, a thin film sensor is also called a thin coating film sensor or simply a coating sensor.

[0026] Strain gauges with thin-film sensors have been proven to be very accurate and have long-term stability with respect to torque and force measurements, and can provide continuous measurements of force and torque during operation. In this known strain gauge, the thin-film sensor is usually a metal coating, in particular a coating made of Cr or Ni-based metals, which is deposited on an insulating layer if the substrate of the component on which the thin-film sensor is to be deposited is made of a metallic material. In such cases, the material used to form the insulating layer is, for example, Al2O3, CrO, AICrO, or an amorphous carbon-based coating. Furthermore, the metal coating can optionally be covered by a further insulating layer.

[0027] However, even if thin film sensors contribute to significant improvements and accuracy of measurements compared to more conventional embedded sensors, they still have the drawback of lacking the sensitivity required to achieve more accurate measurements and have the further drawback that their response time to transient measurements is not fast enough. Summary of the Invention [Problem to be solved by the invention]

[0028] Objective of the invention The main object of the present invention is to provide a thin film sensor which overcomes the drawbacks of the prior art. In particular, the thin film sensor according to the invention should make it possible to achieve a higher sensitivity to enable more accurate measurements and should preferably also allow faster response times for transient measurements. [Means for solving the problem]

[0029] Description of the invention The object of the present invention is achieved by providing a coating system as claimed in claim 1. Preferred embodiments of the invention or preferred aspects of the invention are defined in the dependent claims 2-15.

[0030] A coating system according to the present invention is a coating system for use as a thin film sensor, comprising a piezoresistive sensor element, which is a layer integrated in the coating system, the coating system being deposited on a surface of a substrate, preferably the substrate being a component or tool or part of a component or tool, the piezoresistive sensor element comprising at least one hydrogen-free tetrahedral amorphous carbon coating layer doped with X, i.e. a ta-C:X coating layer, where X is one or more chemical elements selected from element groups 13 and 15 of the Periodic Table of the Elements, preferably X is one or more from B, Al, Ga, In, Ti, N and P, and the ta-C:X coating layer exhibits an anisotropic gauge factor.

[0031] The anisotropic gauge factor of a ta-C:X coating layer should be understood as the anisotropy in the resistance response to strain of a ta-C:X coating layer, such that the magnitude of the response of the resistance change to strain is different when comparing measurements when strain is applied along different orthogonal axes of the piezoresistive sensor element, e.g., the longitudinal and perpendicular directions.

[0032] In other words, when strain is applied longitudinally, along the length of the sensor element, or perpendicular to the surface of the sensor element, different gauge factors are obtained, the difference being at least a factor of 10. Preferably, the gauge factor in the perpendicular direction is at least a factor of 10 higher than the gauge factor in the longitudinal direction.

[0033] For example, in a thin film sensor having a structure constructed as described in Figure 1. The thin film sensor can be strained, for example, by stretching or squeezing the substrate a) along the surface, i.e., along the length or width of the sensor element, or b) perpendicular to the surface, i.e., across the thickness of the sensor element. If component 1 was a beam with a rectangular cross section, it could be easily strained in the length direction by bending the beam and in the normal direction by squeezing it between two flat surfaces.

[0034] In one example where a ta-C:N coating layer is used as the ta-C:X layer, and the elastic modulus and hardness measured using nanoindentation technique are about 380 GPa and 36 GPa, respectively, by applying the method described above, in case a), the response of the longitudinal strain is 100 microstrain (10 -4 (relative dimensional change of 0.045-0.055% of the resistance), which was measured on the sample as a change in resistance of 0.045-0.055%, while in case b) the response for a normal strain of 2 microstrain was in the range of 0.03-0.09%. As sensitivity, this corresponds to a gauge factor in the range of 4.5-5.5 in the longitudinal direction (case a)) and a gauge factor in the range of 150-450 in the normal direction (also called the vertical direction in this context and corresponding to the measurements in case b).

[0035] Preferably, the inventive ta-C:X coating layer according to the present invention is designed to have an anisotropic gauge factor corresponding to a longitudinal gauge factor in the range of 4-20 and a perpendicular gauge factor (perpendicular gauge factor to be understood as the gauge factor in the normal direction as explained above) in the range of 100-450, for example a longitudinal gauge factor in the range of 4-6 and a perpendicular gauge factor in the range of 295-305.

[0036] At least one ta-C:X coating layer preferably comprises: - a hardness HIT determined by the nanoindentation technique of at least 29 GPa, preferably in the range of 30 GPa to 45 GPa, - an elastic modulus EIT determined by the nanoindentation technique of at least 300 GPa, preferably in the range of 330 GPa to 430 GPa; - a carbon content, determined by SIMS analysis, of at least 90 at.%, preferably in the range of 91 at.% to 98 at.%, and - a content determined by SIMS analysis of at least 2 at.%, preferably in the range of 2 at.% to 20 at.% Represents.

[0037] X - Nitrogen (N), or - Phosphorus (P), or - Nitrogen (N) and Phosphorus (P) Particularly good results have been achieved when

[0038] According to particularly preferred embodiments of the present invention, where X comprises nitrogen (N), the N content in the at least one ta-C:X coating layer is in the range of 2 at.% to 10 at.%, preferably in the range of 6.5 at.% to 8.5 at.%.

[0039] According to a preferred variant of the above-mentioned particularly preferred embodiment of the coating system of the invention, X is nitrogen, so that ta-CX is ta-C:N.

[0040] A particularly advantageously defined anisotropic gauge factor was observed when X=N, for example when the N content in the ta-C:N coating layer was in the range of 6.5 at.% to 7.5 at.%.

[0041] According to a preferred embodiment of the present invention, the piezoresistive sensor elements are contacted by a conductive connector such that the piezoresistive sensor elements provide an electrical signal as a function of strain and / or temperature from the substrate surface.

[0042] When the substrate is electrically conductive, the coating system is designed with an underlying electrically insulating layer disposed between the substrate surface and the piezoresistive sensor element, which can be, for example, an Al2O3 layer deposited using a thermal spray technique.

[0043] Depending on the type of substrate and use, the coating system can also be designed with an adhesive layer, preferably provided directly on the substrate surface, to ensure adhesion of the coating system to the substrate surface.

[0044] Depending on the type of use, the coating system of the invention can be designed with an upper electrically insulating layer that is applied directly onto the piezoresistive sensor element.

[0045] Depending on the type of use, it may also be beneficial to design the coating system with at least one functional layer.

[0046] The functional layer may preferably be a ta-C layer or any DLC coating (hydrogen-free or hydrogen-containing DLC ​​layer) or any other coating layer exhibiting specific tribological properties depending on the use.

[0047] The DLC layer is a so-called diamond-like coating layer. - hydrogen-free and undoped, e.g. aC and ta-C, or - does not contain hydrogen and is doped, for example aC:Me or ta-C:Me, where the doping element is a metal and metal is abbreviated as Me, or - hydrogenated and undoped, e.g. aC:H, or - Hydrogenated and doped, e.g. aC:H:Me where the doping element is a metal and metal is abbreviated as Me The amorphous carbon layer may also be referred to as an amorphous carbon layer.

[0048] Thin film sensors comprising a coating system according to the present invention are particularly useful for determining temperature and / or strain.

[0049] Thin film sensors according to the present invention are particularly useful for incorporation into automotive components to determine temperature and / or strain.

[0050] The invention therefore also relates to a component, preferably an automotive component, but also to a tool comprising a thin-film sensor according to the invention, integrated in the component or in the tool for determining temperature and / or strain. The thin-film sensor according to the invention is preferably manufactured by using a method comprising a step in which at least one ta-C:X coating layer is deposited using a PVD or PA-CVD technique.

[0051] A preferred method for manufacturing a coating system according to the invention or, respectively, for manufacturing a thin film sensor (i.e. also a thin film integrated into a component or tool as described above) comprises a step in which at least one ta-C:X coating layer is deposited using PVD or PA-CVD techniques.

[0052] Although the thin film sensor according to the present invention can be applied to any type of substrate, such thin film sensor is particularly suitable to be applied to a component or part of a component as an embedded sensor, preferably the material of the substrate surface of the component or part of a component to which the thin film sensor is applied preferably comprises a polymer, a ceramic and / or a composite material.

[0053] Surprisingly, by studying the electrical effects of nitrogen-doped hydrogen-free tetrahedral amorphous carbon coatings in a commonly used photovoltaic experiment, the inventors detected that slight movements occurring in the same room in which the photovoltaic sample was located (e.g. vibrations caused by a person walking in the room) induced very significant and measurable changes in the resistivity of the photovoltaic sample (the change in the film's resistivity was measured as the difference in resistance induced between visible and UV light).

[0054] The inventors were inspired to implement the observed features of the studied ta-C:N coatings in thin film sensors used as embedded sensors, which involved the use of a doped tetrahedral amorphous carbon (doped ta-C) layer as the piezoresistive material layer, preferably a nitrogen doped tetrahedral amorphous carbon (ta-C:N) coating layer, or a phosphorus doped tetrahedral amorphous carbon (ta-C:P) coating layer, or a nitrogen and phosphorus doped tetrahedral amorphous carbon (ta-C:N:P) coating layer, instead of the commonly used metal layer.

[0055] The thin film sensor of the present invention thus created is capable of responding to fast strain changes (fast strain changes should be understood as vibrations of several tens of kHz, e.g., higher than 40 kHz) and is a very sensitive and accurate temperature sensor (high sensitivity of the sensor in this context should be understood as a beta coefficient value of more than 900 in use as an NTC thermistor).

[0056] Moreover, the deposition of thin film sensors can be combined with laser structuring or patterning of the surface to create electrical paths on the surface to obtain optimized resistivity tailored according to the application requirements.

[0057] Considering that today many components are manufactured that include surfaces coated with coating systems that comprise functional coating layers for producing desired properties on their surfaces (e.g., functional coating layers for achieving heat barrier properties, corrosion resistance, wear resistance, friction reduction, etc.), the inventors believe that it would be clearly advantageous to integrate thin film sensors and such functional coating layers in the same coating system.

[0058] According to a preferred embodiment of the thin film sensor according to the invention, at least one embedded piezoresistive material sensor layer (hereinafter also referred to as embedded piezoresistive sensor element) is provided in a coating system applied to the surface of the component or tool, the at least one embedded piezoresistive sensor element comprising a ta-C:X coating layer. Preferably, the at least one embedded piezoresistive sensor element comprises a nitrogen-doped tetrahedral amorphous carbon coating layer (ta-C:N).

[0059] As mentioned above, the embedded piezoresistive sensor element may comprise or consist of a layer of X-doped tetrahedral amorphous carbon (ta-C:X), where X is one or more chemical elements selected from element groups 13 and 15 of the periodic table of the elements, in particular B, Al, Ga, In, Ti, N and P, to enhance sensor properties, in particular to enhance sensor sensitivity.

[0060] For the use of the thin film sensor according to the invention, the inventors have developed a method that makes it possible to measure at least the temperature, strain, pressure and / or vibration acting on the component surface to which the thin film sensor is applied.

[0061] According to a preferred embodiment of the present invention, there is provided at least one integrated piezoresistive sensor element which is a layer comprised in the coating system, whereby a functional layer, in particular a layer exhibiting desired tribological properties (also called tribological layer), is also comprised in the coating system, preferably as the outermost layer of the coating system (manufactured on top of the coating system).

[0062] Detailed Description of the Invention In order to facilitate the understanding of the present invention, some examples of the present invention are described in more detail below. However, the examples and figures used below should not be understood as limitations of the present invention, but only as showcases or preferred embodiments of the present invention.

[0063] Figures used: [Brief description of the drawings]

[0064] [Figure 1] 1 shows a construction of a preferred coating system according to the invention with a piezoresistive sensor element 5 included in the coating system, which is applied to the surface of a component. [Diagram 2] FIG. 2 is a further preferred configuration of a coating system according to the invention, comprising a piezoresistive sensor element 5 integrated into the coating system, which is applied to the surface of a component. [Diagram 3] FIG. 3 is a schematic diagram of the arrangement of the different layers included in the coating system structure shown in FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] A preferred embodiment of a coating system structure (also referred to as a coating system architecture) into which piezoresistive sensor elements are incorporated according to the present invention is shown in FIG.

[0066] A further preferred embodiment of a coating system architecture (also referred to as coating system architecture) in which a piezoresistive sensor element is integrated according to the invention and in which a functional layer 7 is deposited as functional layer is shown in FIG.

[0067] FIG. 3 shows a schematic diagram of the arrangement of the different layers included in the coating system structure shown in FIG. 2. This structure allows the measurement of torque, strain, force, pressure or vibration acting on the component 1 and / or its temperature. In this coating system structure, the piezoresistive sensor element 5 is provided on the surface of the component 1 as a thin film (also called thin film coating layer or thin film layer or simply coating layer). The coating system can be deposited to partially or completely cover the component 1. The piezoresistive sensor element 5 comprises a tetrahedral amorphous carbon layer doped with at least one chemical element, in particular nitrogen-doped tetrahedral amorphous carbon (ta-C:N), the nitrogen doping concentration of which is preferably in the range of 0.1 at.% to 10 at.%. Due to this choice of material, the gauge factor, i.e. the sensitivity of the strain measurement, is much higher than in conventional sensors (prior art metal thin films used as thin film sensors have a gauge factor of 2 to 4) and is anisotropic, thus reacting more precisely to small modifications of the resistivity of the sensor due to small deformations of the component. The layer thickness of the piezoresistive sensor element is preferably within a range of 0.1 μm to 5 μm, and more preferably within a range of 0.2 μm to 2 μm.

[0068] This type of thin film sensor according to the present invention allows better coverage of components compared to larger sensors (e.g. Pt100 for temperature measurements and MEMS sensors for pressure measurements), even when the components have complex geometries.

[0069] When the component is made of a metallic material or at least the substrate of the component being monitored (measured) is made of a metallic material, high electrical resistivity (high electrical resistivity in this context is 10 9An additional insulating layer 3 having a resistivity (which should be understood as a value of electrical resistivity equal to or greater than 0.01 μm in ohms*m) must be provided between the piezoresistive sensor element 5 and the component 1, the insulating layer being preferably a metal oxide layer, in particular an Al2O3 layer, a CrO layer, an AICrO layer or an amorphous carbon-based coating, the amorphous carbon-based coating layer having a different chemical element composition than the nitrogen-doped tetrahedral amorphous carbon layer (ta-C:N) deposited as the piezoresistive sensor element 5.

[0070] The thickness of the insulating layer 3 is preferably within a range of 0.1 μm to 5 μm, and more preferably within a range of 0.2 μm to 2 μm.

[0071] The insulating layer 3 may finally be an amorphous carbon layer containing one or more dopant elements. However, if the dopant element is nitrogen, the nitrogen concentration of the insulating layer 3 is lower than that of the piezoresistive sensor element 5. The concentration of the one or more dopant elements in the doped amorphous carbon layer may vary, for example describing a gradient, which means that one or more dopant elements, for example nitrogen, may be present in the insulating layer 3 as dopant elements such that the nitrogen concentration increases in the direction from the component to the piezoresistive sensor element 5, for example varying from zero to the nitrogen concentration of the piezoresistive sensor element 5.

[0072] To improve adhesion between component 1 and insulating layer 3, an optional adhesion layer 2 can be deposited directly on component 1 or, in the absence of the required insulating layer 3 (e.g. when the substrate is not metallic), between component 1 and the piezoresistive sensor element. The adhesion layer 3 preferably comprises a metal and / or a nitride, in particular chromium (Cr) or chromium nitride, or titanium (Ti) or a combination thereof, and the layer thickness is preferably in the range of 0.1 μm to 1 μm, more preferably in the range of 0.1 μm to 0.5 μm.

[0073] Further, one or more conductive connectors 4 are provided in contact with the piezoresistive sensor element 5 for conducting electrical signals from the sensor element to an electronic device which collects the electrical signals and converts them into digital signals which are processed in real time and stored in memory for later analysis or transmitted by wireless means.

[0074] Optionally, an upper insulating layer 6 can be deposited on at least a portion of the piezoresistive sensor element 5 and / or on a portion of the electrical connector 4, the composition of the upper insulating layer 6 being a metal oxide, in particular (e.g., 10 9 The upper optional insulating layer 6 is preferably an Al2O3, CrO, AICrO, or amorphous carbon based coating having a high electrical resistivity (in the order of magnitude of ohm*m or higher). The thickness of the upper optional insulating layer 6 is preferably in the range of 1 μm to 30 μm, more preferably in the range of 2 μm to 10 μm.

[0075] The coating system for forming a thin film sensor according to the present invention may further (but only optionally) comprise a functional layer 7 deposited as a top layer (the outermost layer of the coating system) to improve the surface properties of the component, e.g. to improve the corrosion resistance, wear resistance, friction properties and / or any other tribological properties of the component, or also to reduce thermal conductivity, e.g. by acting as a thermal barrier coating layer.

[0076] The substrate material comprising the surface of the component to be coated can include, for example, metals, metal oxides, carbides, flexible materials, especially steel, stainless steel, aluminum, copper, Ni-based superalloys, Al2O3, ZrO2, SiC, ceramic matrix composites (CMC), polymers, etc. The component can be flat, cylindrical or have a complex geometric shape, and the thin film sensor comprising the piezoresistive sensor element according to the invention partially or completely covers the surface of the component.

[0077] According to another preferred embodiment of the invention, illustrated by Fig. 1, a piezoresistive sensor element 5 is provided on the component 1, at least partially or completely covering it. The piezoresistive sensor comprises a doped ta-C material, in particular nitrogen-doped tetrahedral amorphous carbon (ta-C) having a high hardness (high hardness in this context should be understood as a HIT hardness of 30 GPa or more, preferably 35 GPa or more, measured by using nanoindentation techniques, for example by using a Fischerscope device). The layer thickness of the piezoresistive sensor element is preferably in the range of 0.1 μm to 5 μm, more preferably 0.2 μm to 2 μm.

[0078] If the component includes a metallic material as the surface to which the thin film sensor is to be applied, a thin film sensor may be applied between the piezoresistive sensor element 5 and the component 1 (e.g., 10 9 An additional insulating layer 3 is provided having a high electrical resistivity (in the order of magnitude of Ohm*m or more), which is preferably a metal oxide layer, in particular a layer comprising or consisting of an Al2O3, CrO, AICrO or amorphous carbon-based coating that is undoped or doped, i.e. includes a gradient with respect to the concentration of nitrogen, increasing the nitrogen concentration from the substrate to the piezoresistive sensor element 5 from zero to the value of the nitrogen concentration of the piezoresistive sensor element 5. The layer thickness of the insulating layer is preferably in the range of 0.1 μm to 5 μm, more preferably 0.2 μm to 2 μm. One or more conductive connectors 4 are provided on the piezoresistive sensor element 5 to convey the electrical signal from the sensor to an electronic device, which collects the electrical signal and converts it into a digital signal that is processed in real time and stored in a memory for later analysis or transmitted by wireless means. The piezoresistive sensor element is deposited to partially or completely cover the conductive connectors 4.

[0079] According to another embodiment (for example shown in FIG. 1 ), a piezoresistive sensor element 5 is manufactured which also functions as a tribological functional layer and is therefore placed on top of a coating system deposited on component 1, the composition of the piezoresistive sensor element comprising a doped ta-C material, in particular tetrahedral amorphous carbon doped with nitrogen (ta-C:N), having a high hardness (high hardness in this context should be understood as a HIT hardness of 30 GPa or more, preferably 35 GPa or more, measured by using nanoindentation techniques, for example by using a Fischerscope device).

[0080] According to an additional embodiment, the coating system with the piezoresistive sensor element 5 is repeated at least twice and provided for each component forming the stacked structure, such as stacked battery elements, to measure the pressure difference on the battery elements and / or the temperature change on the casing during operation, as well as the hardness measured by nanoindentation of the housing of the stacked components or the housing of the battery, exceeding 36 GPa. Pressure measurement of the individual battery elements or monitoring of the temperature of the battery and / or the housing allows for early safe operation to prevent damage to the battery elements and devices or when the battery is installed in an electric vehicle.

[0081] According to a further embodiment, the coating system comprising the piezoresistive sensor element 5, in particular the piezoresistive sensor element, can be configured, for example, in a specific pattern in order to adjust the resistivity of the sensor material by using laser patterning methods.

[0082] Overall, the coatings provided according to the present invention can be produced by the high power impulse magnetron sputtering process, also called the HIPIMS process, the radio frequency sputtering process, DC magnetron sputtering, cathodic arc discharge, plasma-assisted chemical vapor deposition (PA-CVD) or reactive physical vapor deposition (PVD).

[0083] A brief description of the two main sensing functions of the present invention (strain and temperature) is presented in the following paragraphs.

[0084] For strain measurements, strain is sensed as a change in the ohmic resistance of the sensor element. The gauge factor determines the sensitivity of the strain gauge sensor element according to the following equation:

[0085] GF=ΔR / Ro / ε Where: GF: Gauge factor DR: Resistance difference of strain gauge Ro: Unstrained resistance of the strain gauge ε=ΔL / L is the distortion, where ΔL is the absolute difference in length and L is the original undistorted length.

[0086] The temperature is measured through the change in ohmic resistance of the sensor element according to the following formula: R = Ro·exp[(β(1 / (T-1)) / To)] Where: Ro: Unstrained resistance of the strain gauge ε=ΔL / L is the distortion, where ΔL is the absolute difference in length and L is the original undistorted length.

[0087] In the present invention, the piezoresistive tetrahedral amorphous carbon material, especially when it is doped with nitrogen, provides a much higher gauge factor (e.g., up to 10 times) than conventional sensors. The relatively high resistivity of the sensor (e.g., sensor resistance varying from 10 kOhms to 500 kOhms) allows for simple measurement setup with low power consumption (e.g., power consumption at the level of 60 μW). Unlike other embedded thin-film sensors that use different materials as sensor layers, the response time is extremely fast (e.g., response time of less than 6 ms in temperature measurement), allowing for accurate measurement of transient effects (e.g., detecting camera flash light). The ta-C materials, in particular the doped tetrahedral amorphous carbon coatings (doped ta-C), offer a high sensitivity for temperature measurements comparable to those obtained by negative temperature coefficient (NTC) thermistors (e.g. corresponding to a thermistor beta value of more than 900), but with the advantage of being easy to incorporate into coating systems and thus easy to apply to components or parts of components (usually the beta value of NTC thermistors is between 3000 and 5000). Unlike metal-based sensors known from the prior art (e.g. thin-film sensors with a metal layer as a piezoresistive material sensor layer), the sensors provided by the invention are more robust, chemically inert and can be applied in applications with a long life span. The materials used in the coating system according to the invention are also biocompatible, making it possible to extend the use of such sensors to medical applications such as medical instruments, embedded sensors for implants, etc. Furthermore, the materials used in the invention have a high modulus of elasticity and high wear resistance. Moreover, components made from flexible materials such as polyamides as well as from materials with high rigidity can be provided with thin-film sensors according to the invention.

[0088] Free of hydrogen in the context of the present invention corresponds to a hydrogen content in the range of 0 at.% to 1.5 at.% when the hydrogen content is determined using a technique of type SIMS analysis. Preferably, the hydrogen content thus determined is in the range of 0 at.% to 1.4 at.%. Thus, the hydrogen content determined using SIMS analysis in hydrogen-free tetrahedral amorphous carbon coatings doped with X(ta-C:X) according to the present invention can be measured to be up to 1.5 at.%, preferably up to 1.4 at.%.

[0089] In the context of this specification, a hydrogen-free tetrahedral amorphous carbon coating layer is also referred to simply as a tetrahedral amorphous carbon coating layer.

Claims

1. 1. A coating system for use as a thin film sensor, comprising a piezoresistive sensor element, said piezoresistive sensor element being a layer integrated in said coating system, said coating system being deposited on a surface of a substrate, preferably said substrate being a component or tool or part of a component or tool, said piezoresistive sensor element comprising at least one hydrogen-free tetrahedral amorphous carbon coating doped with X, i.e. a ta-C:X coating layer, characterized in that X is one or more chemical elements selected from element groups 13 and 15 of the Periodic Table of the Elements, preferably X is one or more from B, Al, Ga, In, Ti, N and P, said ta-C:X coating layer exhibiting an anisotropic gauge factor.

2. 2. The coating system of claim 1, wherein the ta-C:X coating layer has an anisotropic gauge factor comprising a normal gauge factor and a longitudinal gauge factor, the normal gauge factor being at least 10 times higher than the longitudinal gauge factor, preferably the longitudinal gauge factor being in the range of 4-20 and the normal gauge factor being in the range of 100-450.

3. The at least one ta-C:X coating layer comprises: a hardness HIT determined by the nanoindentation technique of at least 29 GPa, preferably in the range of 30 GPa to 45 GPa; an elastic modulus EIT determined by the nanoindentation technique of at least 300 GPa, preferably in the range of 330 GPa to 430 GPa; - a carbon content, determined by SIMS analysis, of at least 90 at. %, preferably in the range of 91 at. % to 98 at. %, and - a content determined by SIMS analysis of at least 2 at. %, preferably in the range of 2 at. % to 20 at. % 2. The coating system of claim 1, characterized in that it represents:

4. X, Nitrogen N, or - phosphorus P, or - Nitrogen N and phosphorus P The coating system of claim 1 , comprising:

5. 5. Coating system according to claim 4, characterized in that the nitrogen content in the at least one ta-C:X coating layer is in the range of 2 at. % to 10 at. %, preferably in the range of 6.5 at. % to 8.5 at. %.

6. 5. The coating system of claim 4, wherein X is nitrogen, i.e. ta-C-X is ta-C:N.

7. 5. The coating system of claim 4, wherein the N content in the ta-C:N coating layer is in the range of 6.5 at. % to 7.5 at. %.

8. 10. The coating system of claim 1, wherein the piezoresistive sensor element is contacted by a conductive connector such that the piezoresistive sensor element provides an electrical signal as a function of strain and / or temperature from the substrate surface.

9. The coating system is disposed between the substrate surface and the piezoresistive sensor element.

10. The coating system of claim 1, further comprising an underlying electrically insulating layer.

10. The coating system of claim 1 , wherein the coating system comprises an adhesion layer disposed directly on the substrate surface.

11. 2. The coating system of claim 1, further comprising an upper electrical insulating layer disposed directly on the piezoresistive sensor element.

12. 2. The coating system according to claim 1, characterized in that the coating system comprises at least one functional layer, preferably said at least one functional layer being a DLC coating layer or any other coating layer exhibiting specific tribological properties depending on the use.

13. A thin film sensor comprising the coating system of claim 1 for use in determining temperature and / or strain.

14. 14. A component, preferably an automotive component, or a tool, comprising a thin-film sensor according to claim 13, said thin-film sensor being integrated into said component or tool for determining temperature and / or strain.

15. A method for manufacturing a coating system according to any of the preceding claims 1 to 12 or for manufacturing a thin film sensor according to claim 13 or 14, wherein the at least one ta-C:X coating layer is deposited using a PVD or PA-CVD technique.

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