Titanium substrate with a titanium surface pre-treated for gluing, coating or painting

Abstract

Described is a titanium substrate with a titanium surface pretreated for gluing, coating or painting, wherein the pretreatment comprises or consists of a surface modification, wherein the surface modification comprises a total proportion of titanium of 16 to 23 at.% and at the same time 1 to 3 at.% titanium at an oxidation state of 0, each measured by XPS and each based on the total number of atoms detected by XPS, and represents a microstructure, wherein the microstructure is characterised by mound-like circular or oval microstructuring elements.

Description

[0001] Titanium substrate with titanium surface pretreated for bonding, coating or painting

[0002] The present invention relates to a titanium substrate with a titanium surface pretreated for bonding, coating or painting, wherein the pretreatment comprises or consists of a surface modification, wherein the surface modification comprises a total titanium content of 16 to 23 atomic% and 1 to 3 atomic% titanium in the oxidation state 0, in each case measured by XPS and in each case based on the total number of atoms detected by XPS and represents a microstructuring, wherein the microstructuring is characterized by wall-like circular or oval microstructuring elements.

[0003] The invention further relates to the use of a corresponding surface modification on a titanium substrate and to a method for producing a titanium substrate according to the invention.

[0004] Titanium (and its various alloys) is a metal with unique and often superior material properties in terms of corrosion resistance, strength, Young's modulus, fatigue behavior, biological response, and density. It is therefore a preferred choice for many industrial applications in fields ranging from aerospace to medicine, for example, as an implant in the human body or as the cover sheet for the leading edge of an aircraft wing. In many applications, titanium is bonded to a second material, which can be titanium or another material, such as a fiber-reinforced plastic (CFRP). One example of such an application is the leading edge of an aircraft wing, which consists of an adhesive bond between a titanium cover bonded to the CFRP wing substructure.Bonding titanium requires surface preparation if reliable, long-lasting adhesive bonds with sufficient bond strength are required. Elevated temperatures and humidity (hydrothermal aging) are known to induce changes in the interphase of titanium bonds, which typically lead to deterioration (loss of bond strength) of the bond.

[0005] The objective of surface preparation is to (i) prevent or reduce this deterioration and (ii) create a surface suitable for bonding at the time of adhesive application. Typically, this means that surface preparation requires cleaning the titanium surface (for objective ii) and modifying or coating the titanium (oxide) surface (for objective i).

[0006] The technical challenge is therefore to find a surface preparation for titanium that enables its use for adhesive bonds when permanent adhesive bonds with sufficient bond strength (hereinafter referred to as "high-quality adhesive bonds" (HQAJ)) are desired.

[0007] In the narrower definition, a HQAJ means that the respective value / results of the adhesion strength measurement (e.g. peel strength, lap shear strength, fracture toughness) decrease by no more than 20% and the failure mode is 100% cohesive after a certain time of defined aging, e.g. 1000 h at 80 °C and 75 % RH.

[0008] For successful use in industrial production, the technical solution for surface preparation must also meet other criteria / constraints, such as cost, health and safety aspects, environmental concerns and regulations, and compatibility with the overall production and manufacturing process. Furthermore, the surface preparation should not have any adverse effects on the material properties of the titanium (e.g., mass, density, cohesive strength, or microstructure). Due to the different nature of the titanium (oxide) surface of different titanium alloys (e.g., titanium grade 2 versus grade 5), there may also be a high selectivity of the surface preparation for the specific titanium alloy.

[0009] The problems described for bonding apply in principle to coatings, especially paints. Here, too, a technical solution for surface preparation is required to improve the adhesion of a paint or other types of coatings to titanium substrates. In the current state of the art, titanium is treated to improve the adhesion of adhesives and other coatings by creating a porous oxide on the titanium surface through wet-chemical tank treatment.

[0010] Typical surface preparation for titanium can vary between different titanium alloys and depends on the bonding application and the required level of bonding performance.

[0011] For applications requiring HQAJ, surface preparation currently consists of a sequence of process steps. The typical surface preparation sequence for aerospace applications is outlined below. The process is referred to as N4 and is described at: https: / / onlinelibrary.wiley.eom / doi / epdf / 10.1002 / mawe.201600515

[0012] I) alkaline cleaning / etching in a chemical bath

[0013] II) electrochemical anodization process to produce a titanium oxide surface, e.g., to produce a porous / columnar TiO2 structure, where TiO2 is in rutile modification.

[0014] III) Applying a primer

[0015] After anodizing, a primer, usually containing chromium, is applied. The goal of the primer is to allow sufficient open time between surface preparation and bonding. Furthermore, it increases the overall performance of the bond.

[0016] Alternatives to the wet-chemical process using laser radiation have been described in the patent literature. For example, a process using laser radiation is known from DE102011121545 B4.

[0017] The document SPECHT, Uwe [et al.]: Stable titanium bonds through laser nanostructuring. January 9, 2014. URL: https: / / www.wotech-technical-media.de / womag / aus- gabe / 2014 / 01 / 10_specht_titanklebungen_01 j2014 / 10_specht_titanklebungen_01j2014.php [accessed March 26, 2024] discloses laser nanostructuring for preparing titanium for bonding. Larger structures are not created in this document.

[0018] The processes described in the literature produce a type of titanium oxide with a specific macro / micro-roughness or nanostructure, which is believed to result in stable adhesion to the titanium surface. Most prior art approaches are based on the idea of ​​creating a titanium oxide surface that is thermodynamically stable and, in particular, an oxide that does not undergo chemical and structural changes when subjected to hydrothermal aging.

[0019] Against this background, the object of the present invention was to provide a titanium substrate with an improved titanium surface pretreated for bonding, coating, or painting. In particular, it was desired that the pretreated titanium surface ensure good adhesion of a coating material, a paint, and / or an adhesive, largely without porous titanium oxide structures, and preferably achieve adhesive bonds of HQAJ quality.

[0020] This object is achieved by a titanium substrate with a titanium surface pretreated for bonding, coating or painting, wherein the pretreatment comprises or consists of a surface modification, wherein the surface modification a.) comprises a total Ti content of 16 - 23 at% and 1 - 3 at% Ti in the oxidation state 0, in each case measured by XPS and in each case based on the total number of atoms detected by XPS and b.) represents a microstructuring, wherein the microstructuring is characterized by wall-like, circular or oval microstructuring elements.

[0021] Strictly speaking, the processes of "bonding" and "painting" are also special cases of "coating." However, for the purposes of this text, "coating" applies to all other types of coating except bonding and painting, unless otherwise stated. Microstructuring, as defined in this text, is a topographical change to the (original) surface in the form of microstructuring elements, whereby these elements protrude or intrude in the Z direction (perpendicular to the plane of the surface) and have a height (in the Z direction) of > 1 pm and < 1 mm. The microstructuring elements can be present individually, adjacent to one another, or even overlapping. The microstructuring elements to be provided within the scope of the present invention are wall-like, circular, or oval microstructuring elements. It is preferred that these elements be simultaneously wall-like and rice-shaped or wall-like and oval.

[0022] “Wall-like” in the sense of this application means that the flanks of the microstructuring element rise substantially continuously perpendicular to the topographically highest circle or oval of the respective microstructuring element, whereby the gradient is not constant and preferably decreases towards the corresponding highest point.

[0023] “Oval” and “circular” means, for the shape of the microstructuring elements to be provided according to the invention, that of course a perfect circle or a perfect oval does not have to be present, but that the observer is given the (predominant) impression of the respective geometric shape.

[0024] Surprisingly, it has been found that the surface modification to be provided according to the invention is very well suited to provide a break-resistant bond with coatings, paint and adhesive without requiring a porous titanium oxide surface completely containing titanium in oxidation states > 0.

[0025] It has also been surprisingly shown that the provision of microstructuring contributes significantly to improved long-term stability and strength of a subsequent coating, and in particular, an adhesive bond. This can be attributed, for example, to improved mechanical bonding with the adhesive or coating materials compared to nanostructuring, as well as to an enlarged surface area, which supports increased specific adhesion (chemical, physical, and / or thermodynamically based adhesive forces). Furthermore, the microstructures provided according to the invention allow for the reduction / load absorption of mechanical stresses between the substrate(s) and the adhesive or coating material in an improved manner.These stresses result, for example, from curing-induced shrinkage or expansion of the respective polymer, or from extrinsic stresses due to applied loads, or from differences in the thermal expansion coefficient between the substrate and the coating or adhesive. Furthermore, a geographically larger structure (microstructure) compared to nanostructure allows for easier visual (quality) inspection (possibly under a light microscope or by means of gloss measurements), since the structures to be inspected are larger.

[0026] In this context, it is preferred that - assuming a suitable choice of adhesive - the bond achieves HQAJ quality.

[0027] In this context, it is preferred that the microstructures are formed from melted, rearranged and solidified substrate material.

[0028] Preferred titanium substrates are titanium grade 2 and titanium grade 5 and titanium grade 23 and titanium grade 7.

[0029] A titanium substrate according to the invention is preferred, wherein the surface modification has an oxygen content of < 1 atomic % deviating from the unmodified substrate surface, measured by XPS and based on the total number of atoms detected by XPS.

[0030] It is also preferred that the surface modifications have a specific oxygen depth concentration profile (determined by EDX line scan after FIB preparation, an oxygen concentration fluctuation within 5 pm from the surface of < 3%).

[0031] Each of the preferred features just mentioned, as well as individual combinations of these features or the combination of all these features, results in an even better adhesion behavior of the modified titanium substrate surface.

[0032] According to the invention, a titanium substrate according to the invention is preferred, wherein the microstructuring elements have a largest inner diameter D, measured at the average half height of the respective wall-like boundary parallel to the substrate surface, of 20 - 200 pm, preferably 35 - 150 pm and particularly preferably 50 - 100 pm, measured by means of laser confocal microscopy.

[0033] Also preferred according to the invention is a titanium substrate according to the invention, wherein the wall-like boundaries of the microstructuring elements have an average height h of 2 - 12 pm, preferably 3 - 10 pm and particularly preferably 4 - 8 pm and / or an average width d (FWHM) of 25 - 100 pm, preferably 30 - 90 pm and particularly preferably 40 - 60 pm, in each case measured by means of laser confocal microscopy.

[0034] FWHM means full width at half maximum.

[0035] It has been found that the preferred embodiments of the microstructuring elements to be provided according to the invention in turn contribute to improved adhesion behavior.

[0036] However, not only the shape and design of the microstructuring elements themselves are helpful for improved adhesion behavior, but also the arrangement of the microstructuring elements. Accordingly, a titanium substrate according to the invention is preferred, wherein the microstructuring elements are arranged in rows.

[0037] Further preferred in this context is that the first edge of the rows of microstructuring elements in the measuring direction is 20 - 200 pm, preferably 35 - 150 pm and particularly preferably 50 - 100 pm away from the first edge of the next row of microstructuring elements in the measuring direction.

[0038] This means that for this preferred distance determination, a row of the microstructuring to be provided according to the invention is always included in its width.

[0039] In addition to the features of the microstructuring to be provided according to the invention, as described above, it may also be advantageous for the surface modification to be provided according to the invention to comprise a nanostructuring in addition to the microstructuring.

[0040] Accordingly, a titanium substrate according to the invention is preferred, wherein the surface modification also comprises nanostructuring, preferably consisting of nanostructuring elements with a tuber-, particle-, or cauliflower-like shape and / or with a diameter of 2-50 nm and / or a height of 2 to 100 nm, preferably 44-64 nm, as determined by SEM. "Tuber-like," "particle-like," and "cauliflower-like" describe a macroscopic analogy of the observed nanoscale structures, with 2-20 nm primary structures clustering together to form larger conglomerates.

[0041] According to the invention, a titanium substrate according to the invention is preferred, wherein the surface modification comprises an oxygen content which is < 5 at%, preferably < 1 at% higher, than before the surface pretreatment, in each case measured by XPS and based on the total number of atoms detected by XPS.

[0042] Also preferred according to the invention is a titanium substrate according to the invention, wherein the surface modification comprises a carbon content of 35 - 40 at%, measured by XPS and based on the total number of atoms detected by XPS.

[0043] According to the inventors' experience, the above-mentioned substance components in the surface modification ensure particularly good adhesion to adhesives, but also to paints and coatings within the meaning of this text.

[0044] Also preferred according to the invention is a titanium substrate according to the invention, wherein the substrate in the area of ​​the surface modification after bonding has a crack propagation after 1000 h of < 0.5 mm, preferably < 0.25 mm, determined in the wedge test r with the adhesive FM300 K.

[0045] The wedge test is determined in accordance with DIN 65448 and ASTM D3762-03. In case of doubt, the procedure described in Example 1 is used.

[0046] It has been found that excellent crack propagation results can be achieved by means of the surface structuring provided according to the invention.

[0047] Part of the invention is also the use of a surface modification to be provided according to the invention as defined above for improving the durability of a paint, coating or adhesive on a titanium substrate.

[0048] It is understandable that the core of the present invention is to improve the adhesion quality of paint, coatings, and adhesives to titanium substrates using the surface modifications according to the invention. Also part of the invention is a method for producing a titanium substrate with a titanium surface pretreated for bonding, coating, or painting, comprising the steps of a) providing a titanium substrate and b) modifying the surface of the substrate using a laser to produce a surface modification as defined above.

[0049] It has been found that the modification to be provided according to the invention can be carried out in various ways, for example by means of lasers which emit in the far infrared range (5000 nm to 10600 nm), e.g. CO2 lasers, or lasers which emit in the optically visible range 400 to 800 nm.

[0050] However, the use of a near-IR laser with wavelengths between 800 nm and 5000 nm, preferably between 900 and 1500 nm and particularly preferably with a wavelength of 1064 nm has proven to be particularly suitable.

[0051] When using a near-infrared laser in particular, a focus diameter of 20 - 200 pm, preferably 35 - 150 pm and particularly preferably 50 - 100 pm is preferred.

[0052] Also preferred, especially when using a near-infrared laser, is a fluence of 2J / cm 2 up to 20 J / cm 2 , preferably 5 J / cm 2 up to 15J / cm 2 and particularly preferably 11 J / cm 2 .

[0053] When using a near-infrared laser, a pulse frequency of 10 kHz to 20,000 kHz, preferably 100 kHz to 1,000 kHz, is also preferred.

[0054] Furthermore, particularly when using a near-IR laser, an overlap of the focal areas on the surface in the scanning direction and orthogonal thereto of preferably 90% to minus 300%; more preferably 75% to minus 100%, particularly preferably 50% to 0%, and most preferably 25% is preferred. The pulse overlap is calculated from the difference of 100% subtracting the quotient of the focal areas or pulse spacing, divided by the focal diameter. A negative overlap then describes a relative distance between the focal areas on the surface, normalized to the focal diameter. The scanning speeds are between 1 and 2000 m / s, preferably between 2 and 50 m / s, depending on the focal diameter, the focal area overlaps, and the pulse repetition frequencies.

[0055] It has been found that the use of a Gaussian laser beam profile is particularly helpful for creating the wall-like structures provided according to the invention. This is therefore preferred. A Gaussian laser beam, with its intensity increase at the focal center on the substrate, causes a displacement of the laser-induced melt to the focal edge region, where these wall-like, circular, or oval structures solidify.

[0056] The skilled person will preferably ensure that the applied laser energy reliably prevents the melting enthalpy of the titanium substrate material from being exceeded. Furthermore, it is preferable that the laser pulses are arranged on the surface in such a way that the wall structures do not overlap to such an extent that they are re-leveled, so that the skilled person will select a suitable scanning strategy and laser parameter settings (average power), pulse duration, pulse energy, and focus diameter for the microstructures to be created.

[0057] In general, it should be noted that, in addition to superior bonding performance, the inventive solution offers a number of additional advantages: for example, it eliminates the need for chemical tanks for the previously predominantly wet-chemical process. Likewise, the use of chemicals such as etching solutions, anodizing solutions, and adhesion primers is no longer necessary, thus reducing occupational safety risks and environmental pollution.

[0058] Furthermore, when the method according to the invention is carried out with a laser, the process is faster since neither a special etching step nor an adhesion primer is required.

[0059] Furthermore, the method according to the invention is suitable for components that are particularly large or particularly small. Furthermore, the additional steps otherwise required after preparation for coating, painting, and / or bonding, such as primer, are no longer necessary. This is particularly advantageous given that many primers are now classified as carcinogenic. When using the laser process, care must be taken to ensure that the focal distances between the laser optics and the substrate are maintained within the tolerances to ensure a homogeneous surface treatment. Rapid scanning of the surface prevents the accumulation of heat, thus avoiding component damage. Furthermore, suction, preferably combined with a compressed air flow, helps to remove ablated contaminants from the substrate surface and enable homogeneous treatment.

[0060] Examples

[0061] Measurement example - Adhesion testing using crack propagation measurement

[0062] Adhesion was determined using a wedge test based on DIN 65448 and ASTM D3762-03. Titanium specimens with a thickness of 0.2 mm were bonded using the adhesive FM300 K. The specimens were then stored for 2000 hours at 70 °C and 85% relative humidity. An Al Doppler effect was then applied to prevent mechanical deformation of the thin Ti sheets during the test. A wedge was then driven in, and the crack propagation was determined after 24 hours. During the 24 hours, the specimen was again stored at 70 °C and 85% relative humidity.

[0063] Examples of implementation

[0064] In particular, a specific set of laser parameters, characterized by wavelength, pulse time, frequency, scan speed, and laser spot overlap (which leads to the fluence parameter), as well as the TEM mode, beam profile, and coherence, makes it possible to achieve even HQAJ.

[0065] Grade 2 titanium sheets with a thickness of 0.8 mm were treated with a 100 W IR, Nd:YAG laser, type CI 100 from cleanLasersysteme Herzogenrath, Germany, at a wavelength of 1064 nm using the parameter sets given in Table 1 to produce titanium substrates according to the invention.

[0066] Subsequently, two sheets were bonded together using the adhesive FM300 K, an epoxy film adhesive from Solvay, Belgium, and subjected to the adhesion test analogous to the measurement example.

[0067] However, measurements were taken not only for 2000 hours, but for 3000 hours.

[0068] As a result, all bonds demonstrated a durability of more than 3,000 hours in the aging test. Furthermore, under longer exposure conditions, no adhesion failure was observed between the bond and the sheets, but rather cohesive failure within the adhesive. This exceeds the bonding performance of other known pretreatment processes for titanium.

[0069] Comparison example

[0070] In the patent specification DE 10 2011 121 546 B4 cited above, titanium surfaces are also modified using lasers. However, these modifications involve modifications using nanostructures. For comparison, the data for the best measurement results (crack lengths) after corresponding bonding were extracted from Figure 14 of the aforementioned document using the Origin 2020 software. This is sample Ti-1 from DE 10 2011 121 546 B4. This showed that even the best sample according to this patent specification had a crack propagation of 0.7 mm after 24 hours and 2.8 mm after 1000 hours.

[0071] The following measurements were each performed analogously to the wedge test as described in the measurement example, with the measurement times adjusted. In contrast, the sample IR13, which was pretreated with the laser parameters shown in Table 1 (a 100 W laser from cleanLaser Systeme GmbH, Germany, type CL100, focus diameter 107 pm, focus area overlap 25% in the scanning direction and orthogonal to it, at a scanning speed of 8025 mm / s and a fluence of 11 J / cm) was used. 2 ), a crack propagation of 0.1 mm after 24 hours and a crack propagation of 0.1 mm after 1000 hours.

[0072] It is shown that the titanium substrates prepared according to the invention have significantly improved adhesion in terms of fracture mechanics tests.

Claims

Claims:

1. Titanium substrate with a titanium surface pretreated for bonding, coating or painting, wherein the pretreatment comprises or consists of a surface modification, wherein the surface modification a.) comprises a total Ti content of 16 - 23 at% and 1 - 3 at% Ti in the oxidation state 0, in each case measured by XPS and in each case based on the total number of atoms detected by XPS and b.) represents a microstructuring, wherein the microstructuring is characterized by wall-like, circular or oval microstructuring elements.

2. Titanium substrate according to claim 1, wherein the microstructuring elements have a largest inner diameter D, measured at the average half height of the respective wall-like boundary parallel to the substrate surface, of 20 - 200 pm, preferably 35 - 150 pm and particularly preferably 50 - 100 pm, measured by means of laser confocal microscopy.

3. Titanium substrate according to claim 1 or 2, wherein the wall-like boundaries of the microstructuring elements have an average height h of 2 - 12 pm, preferably 3 - 10 pm and particularly preferably 4 - 8 pm and / or an average width d (FWHM) of 25 - 100 pm, preferably 30 - 90 pm and particularly preferably 40 - 60 pm, in each case measured by means of laser confocal microscopy.

4. Titanium substrate according to one of the preceding claims, wherein the microstructuring elements are arranged in rows.

5. Titanium substrate according to claim 4, wherein the first edge of the rows of microstructuring elements is 20-200 pm, preferably 35-150 pm and particularly preferably 50-100 pm away from the first edge of the next row of microstructuring elements in the measuring direction.

6. Titanium substrate according to one of the preceding claims, wherein the surface modification also comprises a nanostructuring, preferably consisting of nanostructuring elements of tuber-, particle- or cauliflower-like shape and / or with a Diameter of 2 - 50 nm and / or a height of 2 - 100 nm, preferably 44 - 64 nm, determined by SEM.

7. Titanium substrate according to one of the preceding claims, wherein the surface modification comprises an oxygen content which is < 5 at%, preferably < 1 at% higher, than before the surface pretreatment, in each case measured by XPS and based on the total number of atoms detected by XPS.

8. Titanium substrate according to one of the preceding claims, wherein the surface modification comprises a carbon content of 35 - 40 at%, measured by XPS and based on the total number of atoms detected by XPS.

9. Titanium substrate according to one of the preceding claims, wherein the substrate in the area of ​​the surface modification after bonding has a crack thickness after 1000 h of < 0.5 mm, preferably < 0.25 mm with the adhesive FM300 K, determined in the wedge test.

10. Use of a surface modification on a titanium substrate as defined in any one of the preceding claims for improving the durability of a paint, coating or adhesive on a titanium substrate.

11. A method for producing a titanium substrate having a titanium surface pretreated for bonding or painting, comprising the steps of: a) providing a titanium substrate and b) modifying the surface of the substrate by means of a laser to produce a surface modification as defined in any one of claims 1-10.

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