Method and system for testing the coating adhesion of a coating

Abstract

The invention relates to a method for testing the coating adhesion of a coating of a material sample body. According to one exemplary embodiment, the method comprises the following: bringing the coated material sample body into contact with a sonotrode, wherein the sonotrode comprises a coupling element having a flat contact surface which touches a surface of the sample body; exciting a bending vibration of the material sample body in that the sonotrode carries out a longitudinal vibration, wherein, due to the bending vibration in the plane of the contact surface, a frictional force acts between the coupling element and the material sample body; and examining the surface of the material sample body in order to determine the extent of damage to the coating.

Description

Method and system for testing coating adhesion A COATING TECHNICAL FIELD

[0001] The invention described here relates to the field of materials testing, in particular to a method for testing the adhesion of a coating to a coated material specimen. BACKGROUND

[0002] The durability of coated parts depends heavily on the interfacial adhesion properties of the coating on the substrate. Debonding / delamination at the interface between the substrate and the coating has been recognized as a potential cause of failure in such systems. The strength of the interface can be assessed using a variety of qualitative and quantitative test methods based on relevant standards.

[0003] The quality of coatings on metals is typically assessed through scratch and abrasion tests, which can provide qualitative information about layer adhesion. These tests damage the surface of the coating, and adhesion strength is determined, for example, from the resulting detachment of the coating. While such methods can be used to evaluate layer adhesion, they do not provide quantitative measurements of adhesion strength.

[0004] Micro-indentation tests also allow a qualitative assessment of the bond strength by visually evaluating the cracks created during penetration. Quantitative scratch resistance testing with a diamond indenter is a common method used in production and research. In a scratch test, a Rockwell diamond tip moves at a constant speed across the surface of a coated substrate. The tip causes stress at the interface between the coating and the substrate, leading to delamination of the coating. The onset of delamination and the critical load can be determined by observing friction changes. Examination of the sample with a light microscope and measurement of scratch depths (e.g., with an atomic force microscope) can be determined. However, the coating's response to scratch loading is only the response of a system to a specific test condition. Therefore, such tests do not provide a defined physical value for adhesion strength.

[0005] Quantitative measurement and evaluation of interfacial adhesion or delamination can also be achieved using static or quasi-static testing methods. These tests, based on the principles of fracture mechanics, provide information on the growth of a crack in a (e.g., two- or multi-layer) material subjected to quasi-static loading and can be used to measure the (related) parameters stress intensity factor and strain energy release rate. Common methods include double-cantilevered beam (DCB) tests, end-notched flexure (ENF) tests, and four-point bending (4PB) tests. Depending on the type of delamination test and the desired loading regime, pre-notched specimens with defined geometries are used.

[0006] While the values ​​obtained from quasi-static tests are useful and important for evaluating bond strength, these characterization methods do not reflect the cyclic loading conditions that can lead to failure of layered structures under service conditions. In this case, delamination fatigue tests are required to better understand the response of multilayer structures under cyclic loading.

[0007] One object underlying the invention described here can be seen in describing a test method for coated material samples which enables rapid testing and quantitative evaluation of the coating adhesion. SUMMARY

[0008] The above-mentioned object is achieved by the methods according to claims 1 and 15 and by the devices according to claims 13 and 14. Different embodiments and further developments are the subject of the dependent claims.

[0009] The following describes a method for testing the adhesion of a coating on a material sample. According to one embodiment, the method comprises the following: contacting the coated material sample with a sonotrode, wherein the sonotrode comprises a coupling element with a planar contact surface that contacts a surface of the sample; exciting a bending vibration of the material sample by causing the sonotrode to perform a longitudinal vibration, wherein a friction force acts between the coupling element and the material sample due to the bending vibration in the plane of the contact surface; and examining the surface of the material sample to determine the extent of damage to the coating.

[0010] According to a further embodiment, the method comprises contacting the coated material sample body with a sonotrode, wherein the sonotrode comprises a coupling element which contacts a surface of the sample body; exciting a bending vibration of the material sample body by the sonotrode performing a longitudinal vibration, wherein the material sample body is mounted and a bearing has a contact surface on which the material sample body rests, such that a frictional force acts between the bearing and the material sample body in the plane of the contact surface; and examining the surface of the material sample body to determine the extent of damage to the coating.

[0011] Furthermore, devices suitable for carrying out the method are described. SHORT DESCRIPTION OF THE DRAWINGS

[0012] The invention is explained in more detail below using the examples shown in the figures. The illustrations are not necessarily to scale, and the invention is not limited to the aspects shown. Rather, emphasis is placed on illustrating the principles underlying the invention. The figures show:

[0013] Figure 1 illustrates, using a schematic sketch, an exemplary test setup according to an embodiment for the cyclic friction loading of a material sample.

[0014] Figure 2 is a top view of an exemplary material sample with a textured surface.

[0015] Figure 3 is a cross-sectional view of the material sample from Fig. 2, showing various damage patterns that can occur during material testing.

[0016] Figures 4-6 are exemplary scanning electron microscope images of a material sample after a different number of loading cycles.

[0017] Figure 7 shows an experimental setup according to an alternative embodiment. DETAILED DESCRIPTION

[0018] The embodiments described here relate to a method for testing the adhesion of coated material samples, wherein the material sample bodies are subjected to alternating bending loading with a friction component superimposed on the bending movement. The alternating bending loading can be high-frequency (e.g., in the ultrasonic range of 20 kHz and above). In some embodiments, the (transverse) excitation of the material sample occurs at or near a resonant frequency of the material sample.

[0019] Test systems according to the exemplary embodiments described here allow the application of (quasi-)static and dynamic loads in combination with pressure and friction. Depending on the selected number of cycles and / or the load amplitude, the coatings can exhibit damage in the form of (micro-)cracks, chipping, and full-surface delamination. The components (material samples) can consist of a substrate and a single- or multi-layer coating. The thickness of the coating can range from a few nanometers to several hundred micrometers. In a practical application, the layer thickness is between 1 and 10 pm. The concept described here allows investigations on both continuous and, in particular, structured coatings, as well as on flat and profiled substrates.

[0020] Fig. 1 illustrates, using a schematic sketch, an exemplary test setup according to an embodiment for cyclic friction loading of a material sample. The material sample body 10 is a plate with a structured surface. The surface of the material specimen 10 exhibits raised structures, such as those commonly found on printing plates or other coated components such as rolling bearings. The structured surface of the plate is coated. In the example shown, the material specimen 10 is supported at two locations (bearing locations 11 and 12), allowing the material specimen 10 to undergo a bending vibration when excited perpendicular to the plate plane.

[0021] The excitation is provided by a sonotrode. This comprises an ultrasonic transducer 21, a mechanical converter 22, and a coupling element 23. The coupling element 23 has a flat contact surface A with which the sonotrode contacts the structured surface of the sample body 10 during the material test. The sample body 10 is excited to vibrate by the sonotrode performing a longitudinal vibration, which is transmitted to the sample body 10. This induces a bending vibration of the sample body. Due to this bending vibration and the flat design of the contact surface A, a frictional force acts in the plane of the contact surface A between the coupling element 23 and the sample body 10.

[0022] In one embodiment, the coupling element 23 comprises an element in the shape of a flattened spherical segment. In this case, the contact surface A is formed by the circular, flattened part of the spherical segment (see Figs. 1 and 2, diameter d). Alternatively, the coupling element 23 can also comprise a cylindrical element, with the contact surface being formed by the circular end face of the cylinder. It is understood that the contact surface does not necessarily have to be circular, even if this may be advantageous for symmetrical loading of the specimen.

[0023] The friction loads the coating of the sample body 10. After a predefined number of vibration cycles (e.g. 10 6 up to 10 7Cycles), the surface of the specimen 10 can be examined to determine the extent of damage to the coating 11. For this purpose, the specimen can be inspected optically (i.e., by imaging) (e.g., under a microscope or an electron microscope), and the extent of the damage can be characterized (qualitatively and quantitatively). To quantify the damage to the coating, one or more parameters can be determined. For example, the length of all observed cracks in the coating of the specimen 10 can be determined. A useful parameter for characterizing In this case, the measure of surface damage is the crack length per unit area. Other parameters, such as the (average or maximum) crack width and the area of ​​chipping in the coating, can also be determined to quantify coating damage.

[0024] Fig. 2 is a plan view of an exemplary material sample with a structured surface. In the example shown, the surface of the sample body 10 has a regular structure, namely regularly (equistantly) arranged elevations (structural elements 100) with a square base. It is understood that this does not necessarily have to be the case. The surface structure can be arbitrary and does not have to consist of regularly arranged structural elements or have a specific shape. However, regardless of the shape and arrangement of the surface structural elements, in practice an average structural size a can be specified. In the example shown in Fig. 2, the structural size corresponds to the distance between two adjacent, square structural elements.

[0025] The extent of the contact area A of the coupling element 23 should be larger than the average structural size a of the structures on the surface of the test body 10. In the case of a circular contact area A, this means that the diameter d of the contact area is larger than the average structural size, in particular larger than twice the average structural size. In the exemplary embodiments shown here, the area A is at least large enough to cover several structural elements 100. In one exemplary embodiment, the test body is made of brass and the average structural size a is in the range of 50-150 pm. The height of the structural elements 100 can be, for example, 20-60 pm. The coating (e.g. a hard layer) can be a few micrometers thick (e.g. 3-5 pm).

[0026] In one embodiment, the sonotrode performs a resonant longitudinal oscillation with an amplitude in the range of 5-15 pm, whereby the sonotrode can exert an average force in the range of 5-20 Newtons on the surface of the material sample body 10. The sonotrode can be designed such that only elastic strains occur during the bending oscillation (no plastic deformation), although smaller plastic deformations cannot be ruled out depending on the material.

[0027] Fig. 3 illustrates, by way of example, possible damage to the coating 101 of the specimen 10. Fig. 3 is a cross-section, not to scale, through the structural elements 100 from Fig. 2. As a result of the alternating frictional load, delamination of the coating (i.e., the coating detaches locally from the underlying material), spalling (i.e., a portion of the coating breaks off), and the formation of cracks can occur. Crack formation, in particular, is of interest in practice. As mentioned, this can be quantified, for example, by determining the sum of the crack lengths per unit area as a function of the number of cycles.

[0028] Figs. 4, 5, and 6 show micrographs of the surface of a specimen after one million (see Fig. 4), five million (see Fig. 5), and ten million (see Fig. 6) cycles of alternating friction loading of the specimen, which can be generated using the device from Fig. 1. The micrographs show the surface of a single structural element 100. It is immediately apparent that the number of cracks, and consequently the cumulative crack length, increases with the number of cycles. The maximum width of the cracks also increases with the increasing number of cycles.

[0029] Fig. 7 shows an alternative to the test setup from Fig. 1. The test setup is very similar to that in Fig. 1. As in Fig. 1, a material sample body 10 is excited to vibrate using a sonotrode (transducer 21, converter 22 and coupling element 23), wherein the sonotrode performs a resonant longitudinal oscillation and excites a bending oscillation of the material sample body 10. Unlike in Fig. 1, the material sample body 10 is turned upside down, ie the coupling element 23 touches the underside of the material sample body 10 and the coated, structured upper side is the side facing away from the sonotrode.

[0030] As in Fig. 1, the material specimen 10 is supported at two locations (bearing points 11 and 12), so that the material specimen 10 can undergo a bending vibration when excited perpendicular to the plate plane. At least one of the bearing points (bearing point 12 in Fig. 7) is formed by a flat support surface A (external dimension d) that extends over several structural elements 100. Due to the bending vibration, an alternating frictional force load acts in the flat contact surface A between the coated surface of the material specimen 10 and the component 13, similar to what is the case in Fig. 1 in the contact surface A of the coupling element 23.

[0031] Unlike the example shown in Fig. 1, in the variant according to Fig. 7, the coupling element 23 does not have a flattened contact surface. In the example according to Fig. 7, the coupling element 23 has a hemispherical bulge which (due to its spherical shape) theoretically only touches the material specimen 10 at one point. Thus, the contact point between the coupling element 23 and the specimen 10 is not subjected to frictional stress as much as possible. Instead, alternating frictional loading of the specimen occurs in the contact surface A of the support 13. After a certain number of vibration cycles, the damage pattern can be examined as explained above, and the damage to the coating can be quantified.

Claims

PATENT CLAIMS 1. A method for testing the adhesion of a coating (11) on a material sample body (10); the method comprises Contacting the coated material sample body (10) with a sonotrode, wherein the sonotrode comprises a coupling element (23) with a flat contact surface (A) which touches a surface of the sample body (10); Exciting a bending vibration of the material sample body (10) by the sonotrode performing a longitudinal vibration, wherein a frictional force acts between the coupling element (23) and the material sample body (10) due to the bending vibration in the plane of the contact surface; Examination of the surface of the material sample body (10) to determine the extent of damage to the coating (11).

2. The method according to claim 1, wherein the surface of the material sample body (10) has raised structural elements (100) and wherein the extent of the contact surface (A) of the coupling element (23) is greater than an average structural size (a) of the structures on the surface of the sample body (10).

3. The method according to claim 1, wherein the surface of the material sample body (10) has raised structural elements (100) with an average structural size in the range of 50-150 pm.

4. The method according to claim 1, wherein the surface of the material sample body (10) has raised structural elements (100) and wherein the extent of the contact surface (A) of the coupling element (23) is so large that several structural elements (100) are covered by the contact surface (A).

5. The method according to any one of claims 1 to 4, wherein the sonotrode performs a resonant oscillation with an amplitude which is small enough that no plastic deformations occur in the material sample body (10).

6. The method according to one of claims 1 to 4, wherein the sonotrode performs a resonant oscillation with an amplitude in the range of 5-15 pm, and / or wherein the sonotrode exerts an average force in the range of 5-20 Newtons on the surface of the material sample body (10).

7. The method according to one of claims 1 to 6, wherein the coating (11) of the material sample body (10) is approximately 1-10 pm thick.

8. The method according to one of claims 1 to 7, wherein the coupling element (23) comprises an element having the shape of a flattened spherical segment and the contact surface (A) is formed by the circular, flattened part of the spherical segment.

9. The method according to one of claims 1 to 7, wherein the coupling element (23) comprises a cylindrical element, and the contact surface (A) is formed by the circular end face of the cylinder.

10. The method according to one of claims 1 to 9, wherein the examination of the surface of the material sample body (10) comprises the following: imaging inspection of the material sample body (10) after a certain number of vibration cycles and determining at least one parameter for characterizing the damage to the surface of the material sample body (10).

11. The method according to claim 10, wherein the parameter for characterizing the damage to the surface is a crack length per unit area, and / or wherein the parameter for characterizing the damage to the surface is an average or maximum crack width, and / or where the parameter for characterizing the surface damage represents an area of ​​flaking.

12. The method according to one of claims 1 to 11, wherein the material sample body (10) is a coated plate, in particular a printing plate.

13. A device for material testing, comprising: a material sample body (10) with a structured, coated surface, a sonotrode comprising a coupling element (23) with a flat contact surface (A) which contacts a surface of the sample body (10); wherein the material sample body (10) is mounted such that it can perform a bending oscillation when the sonotrode performs a longitudinal oscillation, so that a frictional force acts between the coupling element (23) and the material sample body (10) in the plane of the contact surface (A).

14. A device for materials testing, comprising: a material sample body (10) with a structured, coated surface, a sonotrode having a coupling element (23) which contacts a surface of the sample body (10); wherein the material sample body (10) is mounted such that it can perform a bending oscillation when the sonotrode performs a longitudinal oscillation, wherein a bearing (13) has a contact surface (A) on which the material sample body rests, such that a frictional force acts between the bearing (13) and the material sample body (10) in the plane of the contact surface.

15. A method for testing the adhesion of a coating (11) to a material sample (10); the method comprises: Contacting the coated material sample body (10) with a sonotrode, wherein the sonotrode comprises a coupling element (23) which contacts a surface of the sample body (10); Exciting a bending vibration of the material sample body (10) by the sonotrode performing a longitudinal vibration, wherein the material sample body (10) is mounted and a bearing (13) has a contact surface (A) on which the material sample body rests, so that in the plane of the contact surface (A) a frictional force acts between the bearing (13) and the material sample body (10); and Examination of the surface of the material sample body (10) to determine the extent of damage to the coating (11).

Images