Method for testing the properties of a thermal spray coating on a substrate

Abstract

Method for testing layer properties of a thermal spray layer (1) on a substrate (2), comprising the steps a) arranging an ultrasonic transmitter means (3) and an ultrasonic receiver means (4) spaced therefrom on the thermal spray layer (1), b) generating pulsed ultrasonic surface waves in the thermal spray layer (1) by means of the ultrasonic transmitter means (3), c) detecting the propagation times of the pulsed ultrasonic surface waves from the ultrasonic transmitter means (3) to the ultrasonic receiver means (4), d) comparing the measured propagation times with a reference propagation time of the pulsed ultrasonic surface waves in an undamaged thermal spray layer (1).

Description

[0001] The present invention relates to a method for testing layer properties of a thermal spray layer on a substrate.

[0002] Thermal spraying for coating surfaces has been used industrially for several decades. According to DIN EN 657, thermal spraying encompasses coating processes in which spray materials are melted, melted, or fused inside or outside of spraying devices and then centrifuged onto prepared substrate surfaces. The substrate surfaces are not melted during thermal spraying. The spray material is heated above its melting point and forms fine spray droplets that are sprayed onto the surface of the substrate to be coated. Due to the thermal and kinetic energy of the spray droplets, they bond to the substrate surface and to each other. Physical bonding is the primary mechanism that creates these bonds.

[0003] The thermal spray processes listed in the aforementioned standard include, in particular, molten bath spraying, arc spraying, plasma spraying, cold gas spraying, wire flame spraying, powder flame spraying, detonation spraying, high-velocity oxygen spraying, and laser spraying. Depending on the application, metallic, non-metallic, or composite materials can be used as spray materials. Typical examples of frequently used spray materials are titanium, zinc, stainless steels, ceramic materials, and thermoplastics. Important applications for thermal spray coatings include corrosion protection coatings, wear protection coatings, and sliding coatings.

[0004] Under certain circumstances, pores or oxide inclusions can occur in a thermal spray coating, which can lead to damage to the spray coating. For example, individual pores can form interconnected pore chains, which can lead to cracking within the thermal spray coating. Furthermore, the thermal spray coating may not adhere optimally to the substrate surface and detach from the substrate over time. This process is also known as delamination.

[0005] The quality of metallic spray coatings is often assessed using so-called metallographic sections. This is not a non-destructive testing method. Furthermore, the evaluation of metallographic sections has proven difficult and inaccurate. Interlaboratory tests show that the results are often neither reproducible nor comparable.

[0006] DE 197 52 574 A1 discloses a method for non-destructive testing of the properties of a coating applied to a substrate, particularly with regard to pores, adhesion, cracks, or layer thickness. An energy pulse is applied to the surface of the coating using radiation. The coating properties are determined by the reflected radiation, with the reflection of the energy pulse being measured as a function of time.

[0007] US 2008 / 0 038 477 A1 discloses a method for determining the properties of a thermal spray coating using ultrasound. The substrate coated with the thermal spray coating is placed in a water bath. An ultrasonic signal in the form of a pulse is generated by an ultrasonic transmitter and transmitted through the water bath to the substrate. The ultrasonic signal penetrates the coated substrate and is at least partially reflected. The reflected ultrasonic signal is transmitted through the water bath and received by the ultrasonic transmitter, which also forms an ultrasonic receiver, and processed by a signal processing device. A transmission technique, in which the ultrasonic signal can propagate from an ultrasonic transmitter through the coated substrate to an ultrasonic receiver, is also possible.This method can be used to detect, in particular, delamination of the thermal spray coating from the substrate. The resulting measurement data is evaluated using peak height analysis. In other words, the sound attenuation is measured and analyzed.

[0008] DE 196 40 859 A1 discloses a method for the non-destructive determination of the material condition in components, in particular for the detection of creep damage.

[0009] JP 2000 - 2 691 A discloses a storage device for rod-shaped articles.

[0010] JP 2009 - 150 692 A discloses a method for inspecting a sprayed coating, which enables the flaking of the sprayed coating to be checked by non-destructive testing using ultrasonic waves, in particular without contamination by a liquid medium for ultrasonic waves; and the provision of equipment for the method.

[0011] The present invention has for its object to provide a method for testing layer properties of a thermal spray coating on a substrate, which enables a fast, cost-effective and non-destructive analysis of different types of damage to the thermal spray coating.

[0012] The solution to this problem is provided by a method having the features of claim 1. An alternative solution to this problem is the subject of claim 6. The subclaims relate to advantageous developments of the invention.

[0013] According to claim 1, a method for testing layer properties of a thermal spray layer on a substrate according to a first embodiment of the present invention comprises the steps a) arranging an ultrasonic transmitter means and an ultrasonic receiver means spaced therefrom on the thermal spray layer, b) generating pulsed ultrasonic surface waves in the thermal spray layer by means of the ultrasonic transmitter means, c) detecting the travel times of the pulsed ultrasonic surface waves from the ultrasonic transmitter means to the ultrasonic receiver means, d) Comparing the measured travel times with a reference travel time of the pulsed ultrasonic surface waves in an undamaged thermal spray layer.

[0014] The method according to the invention is based on the idea that pores, oxide inclusions, cracks, or other defects in the thermal spray coating, as well as at least partial delamination of the spray coating from the substrate, have a direct influence on the propagation and thus also on the propagation time of the pulsed ultrasonic surface waves. By comparing the recorded propagation times with a reference propagation time of an ideal, undamaged thermal spray coating, it is possible to determine whether or not damage is present within the thermal spray coating. The extent of the changes in the propagation times also allows an estimate of the extent of the damage. The method according to the invention can be carried out relatively easily with little time and cost. Another advantage is that it is a non-destructive testing method that does not damage the thermal spray coating.The method is suitable for the examination of coated substrates of various shapes and sizes. Furthermore, it is applicable regardless of the chemical composition of the thermal spray coating. A stationary measurement setup is not required, allowing on-site measurements.

[0015] To obtain depth information, a preferred embodiment proposes that time-of-flight measurements be performed at different frequencies of the pulsed ultrasonic surface waves. Since a change in frequency also changes the penetration depth of the pulsed ultrasonic surface waves, the method described here makes it possible to determine the layer depth at which disturbances are present and how far they propagate into the interior of the layer. Delamination of the thermal spray layer from the substrate, at least in sections, can also be detected in this way.

[0016] The preferred procedure is to successively reduce the frequencies of the pulsed ultrasonic surface waves. This gradually increases the penetration depth from the surface of the thermal spray coating.

[0017] In a particularly advantageous embodiment, it is proposed that process steps c) and d) be carried out automatically using a measuring device. These process steps can thus advantageously be carried out without intervention by operating personnel.

[0018] Preferably, a plurality of reference runtimes can be stored in the measuring device's memory for retrieval. This facilitates automated evaluation of the measurement results. In particular, reference runtimes for different materials can be stored in the memory for retrieval.

[0019] According to claim 6, a method for testing layer properties of a thermal spray layer on a substrate according to a second embodiment of the present invention comprises the steps a) arranging an ultrasonic transmitter means and an ultrasonic receiver means spaced therefrom on the thermal spray layer, b) generating a continuous ultrasonic surface wave in the thermal spray layer by means of the ultrasonic transmitter means, c) detecting the oscillation of the continuous ultrasonic surface wave by means of the ultrasonic receiver means, d) changing and detecting a distance between the ultrasonic transmitter means and the ultrasonic receiver means, e) determining a phase shift between a continuous excitation oscillation of the ultrasonic transmitter means and an oscillation of the continuous ultrasonic surface wave detected by the ultrasonic receiver means, f) calculating a speed of sound from the detected distance between the ultrasonic transmitter means and the ultrasonic receiver means, the phase shift and a frequency of the continuous ultrasonic surface wave, g) Comparing the calculated sound velocity with a reference sound velocity of the continuous ultrasonic surface wave in an undamaged thermal spray layer.

[0020] This second variant of the method according to the invention is also based on the finding that pores, oxide inclusions, cracks, or other defects in the thermal spray layer, as well as at least partial delamination of the spray layer from the substrate, have a direct influence on the propagation and thus also on the speed of sound of a continuous ultrasonic surface wave. Unlike the method according to claim 1, the thermal spray layer is now exposed to a continuous ultrasonic surface wave and not to pulsed ultrasonic surface waves.The method is further based on the finding that changing the distance between the ultrasonic transmitter and the ultrasonic receiver by moving the ultrasonic receiver, or alternatively by moving the ultrasonic transmitter, changes the phase shift between the oscillations of the ultrasonic transmitter and the ultrasonic receiver. A phase shift of 2π (i.e., one complete phase revolution) corresponds to a shift by one wavelength λ of the ultrasonic surface wave. To increase measurement accuracy, the distance between the ultrasonic transmitter and the ultrasonic receiver is preferably changed until the total phase shift corresponds to a multiple of one complete phase revolution.By dividing the total measured displacement of the ultrasonic receiver or transmitter by the number of phase revolutions, an average wavelength of the continuous ultrasonic surface wave is obtained. The average wavelength λ obtained in this way can then be used to calculate the speed of sound c according to the well-known mathematical relationship. c=λ⋅f can be used, where f is the frequency of the continuous ultrasonic surface wave. By comparing the recorded speed of sound with a reference speed of sound of an ideal, undamaged thermal spray coating, this method can also be used to conclude that damage is present within the thermal spray coating. The extent of the changes in the speed of sound also allows an estimate of the extent of the damage. This variant of the method according to the invention can also be carried out relatively easily with little time and cost. A further advantage is that it is also a non-destructive testing method that does not damage the thermal spray coating. The method is suitable for examining coated substrates of different shapes and sizes. Furthermore, it can be used regardless of the chemical composition of the thermal spray coating.A stationary measurement setup is not required, so on-site measurements can also be carried out.

[0021] In order to obtain depth information about possible damage with this method as well, a preferred embodiment proposes varying the frequency of the continuous ultrasonic surface wave. As already explained above, a change in frequency also leads to a change in the penetration depth of the ultrasonic surface wave. Therefore, using the method described here, it is possible to determine, by simply changing the frequency and thus the wavelength, at what layer depth defects are present and how far they propagate into the layer. Delamination of the thermal spray layer from the substrate, at least in sections, can also be detected in this way.

[0022] Preferably, the frequency of the continuous ultrasonic surface wave can be successively reduced. This enables a systematic examination of the thermal spray coating from the outside inward toward an interface with the substrate.

[0023] In a particularly preferred embodiment, it is proposed that at least method steps e) to g) be carried out automatically using a measuring device. This advantageously reduces the measuring effort required by the operating personnel. Preferably, the distance between the ultrasonic receiver and the ultrasonic transmitter can be automatically changed and determined, for example, using a distance measuring device.

[0024] Preferably, a plurality of reference sound velocities can be stored in the measuring device's memory for retrieval. This can further simplify the evaluation of the measurement results. In particular, reference sound velocities for different materials can be stored in the memory for retrieval.

[0025] Further features and advantages of the present invention will become clear from the following description of preferred embodiments with reference to the accompanying drawings. Fig. 1 is a schematically highly simplified representation of a measuring setup which is designed to carry out a method for testing layer properties of a thermal spray layer on a substrate according to a first embodiment of the present invention, Fig. 2 a schematically highly simplified representation of a measuring setup which is set up to carry out a method for testing layer properties of a thermal spray layer on a substrate according to a second embodiment of the present invention.

[0026] With reference to Fig. 1 comprises a first measuring setup 100 for testing layer properties of a thermal spray layer 1 which has been applied to a substrate 2, an ultrasonic transmitter 3 and an ultrasonic receiver 4. The ultrasonic transmitter 3 is arranged at a first position on the thermal spray layer 1. The ultrasonic receiver 4 is arranged at a second position on the thermal spray layer 1. The distance d between the ultrasonic transmitter 3 and the ultrasonic receiver 4 is fixed.

[0027] The ultrasonic transmitter means 3 is designed to generate pulsed ultrasonic surface waves in the thermal spray layer 1. These pulsed ultrasonic surface waves propagate through the thermal spray layer 1 and are received by the ultrasonic receiver means 4. The propagation times t Puls The pulsed ultrasonic surface waves required for propagation from the ultrasonic transmitter means 3 to the ultrasonic receiver means 4 are determined by means of a measuring device 5. It is shown that, for example, pores, oxide inclusions or other types of disturbances present in the thermal spray layer 1 have an influence on the propagation times t Puls of the pulsed ultrasonic surface waves. The same applies to at least partial delamination of the thermal spray layer 1 from the substrate 2. This also affects the transit times t Pulsof the pulsed ultrasonic surface waves. Elastic and inelastic properties of the layer material, which are altered, for example, by the presence of pores, oxide inclusions, or delamination, have a measurable influence on the propagation of the ultrasound. The propagation times t Puls of the pulsed ultrasonic surface waves change compared to an ideal thermal spray coating 1, which does not exhibit any pores, oxide inclusions, delaminations, or other defects. By comparing the measured transit times t Puls The pulsed ultrasonic surface waves with a propagation time that would be expected in an ideal thermal spray coating 1 can be used to determine whether pores, oxide inclusions, or other defects are present in the thermal spray coating 1. Furthermore, it is possible to detect at least partial delamination of the thermal spray coating 1.

[0028] By changing the frequency of the pulsed ultrasonic surface waves, the penetration depth can be varied, since the frequency change also entails a change in wavelength. This makes it possible to examine the thermal spray coating 1 at different layer depths and to detect changes in the transit times t Puls of the pulsed ultrasonic surface waves in a depth-dependent manner. In this way, it is advantageously possible to determine at which layer depth pores, cracks, or oxide inclusions may be present and / or how far these extend into the depth. Knowing the thickness of the thermal spray layer 1, the interface between the thermal spray layer 1 and the surface of the substrate 2 can be examined by selecting a suitable frequency to detect possible delamination of the spray layer 1.

[0029] The method described above enables a simple and cost-effective inspection of the coating properties of the thermal spray coating 1, in particular with regard to damage caused by pores, cracks, oxide inclusions, or delamination. The method can be used regardless of the shape and size of the substrate 2 coated with the thermal spray coating 1 and has the particular advantage of being non-destructive. A stationary measuring setup 100 is not required. The measuring setup is also suitable for on-site measurements. This method provides sufficiently high measurement accuracy for numerous applications. The measuring device 5 can be designed such that it can perform and evaluate the measurements automatically. The reference transit times, with which the measured transit times are compared, are preferably stored in memory means of the measuring device 5 so that they can be retrieved.

[0030] With reference to Fig.2, a second measurement setup 200 will be explained in more detail below, which is configured to carry out a method for testing the coating properties of a thermal spray coating 1 on a substrate 2 according to a second exemplary embodiment of the present invention. This method is primarily characterized by the fact that the presence of pores, oxide inclusions, and possibly other defects in the thermal spray coating 1, or at least partial delamination of the thermal spray coating 1 from the substrate 2, can be detected with greater accuracy.

[0031] The measuring setup 200 again comprises an ultrasonic transmitter 6 and an ultrasonic receiver 7. The ultrasonic transmitter 6 is arranged in a stationary manner at a first position on the thermal spray layer 1. The ultrasonic receiver 7 is arranged at a second position on the thermal spray layer 1. Unlike in the first exemplary embodiment, however, the distance d between the ultrasonic transmitter 6 and the ultrasonic receiver 7 can be varied. For this purpose, the ultrasonic receiver 7 is connected to a displacement device 8, which is designed such that it can displace the ultrasonic receiver 7 either toward the ultrasonic transmitter 6 or away from the ultrasonic transmitter 6.Furthermore, a displacement measuring device 9 is provided, which is designed to measure the change in the distance d between the ultrasonic transmitter means 6 and the ultrasonic receiver means 7 caused by the displacement of the ultrasonic receiver means 7. The displacement measuring device 9 is integrated into the displacement device 8 in the present case. Alternatively, the displacement measuring device 9 and the displacement device 8 can also be two separate components of the measuring setup 200. In principle, it is also possible to reverse the setup and arrange the ultrasonic receiver means 7 stationary on the thermal spray layer 1 and to design the ultrasonic transmitter means 6 to be displaceable by means of the displacement device 8.

[0032] Unlike the first embodiment, the ultrasonic transmitter means 6 is designed to generate a continuous ultrasonic surface wave in the thermal spray layer 1. This continuous ultrasonic surface wave propagates through the thermal spray layer 1 and is received by the ultrasonic receiver means 7.

[0033] The measuring setup 200 comprises a measuring device 10 connected to the ultrasonic transmitter 6 and the ultrasonic receiver 7. The displacement measuring device 9 is connected to the measuring device 10. The displacement device 8 can optionally also be connected to the measuring device 10, in particular to enable automated displacement of the ultrasonic receiver 7, which can advantageously be controlled by the measuring device 10. The oscillations of the ultrasonic transmitter 6 and the ultrasonic receiver 7 form input variables for the measuring device 10 and are fed into it. The measuring device 10 is designed such that the phase relationship between the oscillations of the ultrasonic transmitter 6 and the ultrasonic receiver 7 can be detected.By changing the distance d between the ultrasonic transmitter means 6 and the ultrasonic receiver means 7 by moving the ultrasonic receiver means 7, the phase shift between the two oscillations changes.

[0034] A phase shift of 2π (i.e., one complete phase revolution) corresponds to a displacement of the ultrasonic receiver means 7, which can be detected by the displacement measuring device 9, by one wavelength λ of the ultrasonic surface wave. The measuring device 10 can advantageously comprise a display means 11 to visualize the oscillations of the ultrasonic transmitter means 6 and the ultrasonic receiver means 7 for a user.

[0035] To increase the measurement accuracy, the ultrasonic receiver means 7 is preferably displaced until the total phase shift corresponds to a multiple of a complete phase revolution. By dividing the total measured displacement distance of the ultrasonic receiver means 7 by the number of phase revolutions, an average value of the wavelength of the continuous ultrasonic surface wave is obtained. The average wavelength λ obtained in this way can be used to calculate the speed of sound c according to the known mathematical relationship c=λ⋅f can be used, where f is the frequency of the continuous ultrasonic surface wave.

[0036] Disturbances within the thermal spray layer 1, caused, for example, by pores, cracks, oxide inclusions, or at least partial delamination, have a direct influence on the sound velocity of the continuous ultrasonic surface wave. The experimentally determined sound velocity is then compared with a reference sound velocity, as expected for an ideal thermal spray layer 1 without pores, oxide inclusions, at least partial delamination, or other disturbances. In this way, it is very easy to determine whether corresponding pores, oxide inclusions, at least partial delamination, or other disturbances are present in the thermal spray layer 1.

[0037] To obtain depth information, it is advisable to repeat the measurements with different frequencies of the continuous ultrasonic surface wave. The lower the frequency selected, the greater the penetration depth of the ultrasonic surface wave into the interior of the thermal spray layer 1. For example, it is possible to scan the thermal spray layer from the outside to the inside by successively reducing the frequency of the continuous ultrasonic surface wave. In this process, an average wavelength is determined according to the principle explained above and calculated from the relationship c=λ⋅f The speed of sound is again determined. In this way, a depth profile of the speed of sound is obtained, from which, by comparison with a reference speed, it can be concluded where the thermal spray layer 1 exhibits damage, in particular in the form of pores or oxide inclusions. This makes it possible, for example, to determine where damage originates and how far individual damage extends into the depth of the thermal spray layer 1. The depth profile obtained in this way enables a clear distinction between at least partial delamination of the thermal spray layer 1 from the substrate 2 and pores or oxide inclusions.

[0038] This second variant of the method for testing the coating properties of a thermal spray coating 1 on a substrate 2 also enables non-destructive testing. Using this method, even large areas of the thermal spray coating 1 can be examined with minimal time and cost. The method can be performed regardless of the size and shape of the substrate 2 coated with the thermal spray coating 1. The method also advantageously enables on-site testing of the thermal spray coating 1. A stationary measurement setup 200 is not required.

[0039] Preferably, the measuring device 10 is designed to automatically perform and evaluate the measurements, in particular the depth profile measurements. In this case, the displacement measuring device 9 must be connected to the measuring device 10 so that information about the displacement of the ultrasonic receiver means 7 relative to the ultrasonic transmitter means 6 can be provided to the measuring device. The reference sound velocities with which the measured sound velocities are compared are preferably stored in memory means of the measuring device 10 for retrieval.

Claims

[1] Method for testing layer properties of a thermal spray layer (1) on a substrate (2), comprising the steps a) arranging an ultrasonic transmitter means (3) and an ultrasonic receiver means (4) spaced therefrom on the thermal spray layer (1), b) generating pulsed ultrasonic surface waves in the thermal spray layer (1) by means of the ultrasonic transmitter means (3), c) detecting the propagation times of the pulsed ultrasonic surface waves from the ultrasonic transmitter means (3) to the ultrasonic receiver means (4), d) comparing the measured propagation times with a reference propagation time of the pulsed ultrasonic surface waves in an undamaged thermal spray layer (1). [2] Method according to claim 1, characterized by that time-of-flight measurements are carried out at different frequencies of the pulsed ultrasonic surface waves. [3] Method according to claim 2, characterized by that the frequencies of the pulsed ultrasonic surface waves are successively reduced. [4] Method according to one of claims 1 to 3, characterized by that the process steps c) and d) are carried out automatically using a measuring device (5). [5] Method according to claim 4, characterized by that a plurality of reference running times are stored in storage means of the measuring device (5) in a retrievable manner. [6] Method for testing layer properties of a thermal spray layer (1) on a substrate (2), comprising the steps a) arranging an ultrasonic transmitter means (6) and an ultrasonic receiver means (7) spaced therefrom on the thermal spray layer (1), b) generating a continuous ultrasonic surface wave in the thermal spray layer (1) by means of the ultrasonic transmitter means (3), c) detecting the oscillation of the continuous ultrasonic surface wave by means of the ultrasonic receiver means (6), d) changing and detecting a distance between the ultrasonic transmitter means (6) and the ultrasonic receiver means (7), e) determining a phase shift between a continuous excitation oscillation of the ultrasonic transmitter means (6) and an oscillation of the continuous ultrasonic surface wave detected by the ultrasonic receiver means (7), f) calculating a speed of sound from the detected distance between the ultrasonic transmitter means (3) and the ultrasonic receiver means (7), the phase shift and a frequency of the continuous ultrasonic surface wave, g) comparing the calculated sound velocity with a reference sound velocity of the continuous ultrasonic surface wave in an undamaged thermal spray layer (1). [7] Method according to claim 6, characterized by that the frequency of the continuous ultrasonic surface wave is varied. [8] Method according to claim 7, characterized by that the frequency of the continuous ultrasonic surface wave is successively reduced. [9] Method according to one of claims 6 to 8, characterized by that at least the process steps e) to g) are carried out automatically using a measuring device (10). [10] Method according to claim 9, characterized by that a plurality of reference sound velocities are stored in storage means of the measuring device (10) in a retrievable manner.

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